@article{neupane_chase_zhao_wang_he_2021, title={Optical properties of segmented Ag-Au wire at single particle level studied with a home-built micro-spectrometer}, ISSN={["2577-8196"]}, DOI={10.1002/eng2.12439}, abstractNote={Nanomaterials having hetero‐metallic junctions are being explored for sensing, catalytic, and biomedical applications. Insight into the bimetallic junction at nanoscale is important from both fundamental and application perspectives. In this study, we synthesized segmented Au–Ag wire by sequentially electroplating Au and Ag in cylindrical pores in anodic alumina membrane filters. We probed the one photon, two photon, and second harmonic signal in Au–Ag wire at single particle level by focusing laser beam to a diffraction limited spot using a home‐built sample scanning type micro‐spectrometer. When exciting the Au–Ag junctions into the mid‐UV range using two‐photon excitation, we observed two luminescence peaks at 455 and 520 nm, respectively, possibly because of the surface plasmon resonances supported by the sharp boundary and granular nanostructures on the Au–Ag interface. Interestingly, we also observed intense second harmonic generation (SHG) signal from the junction with a yield more than two orders of magnitude higher than that from the Au tips. The SHG intensity as a function of excitation wavelength showed a similar trend to the two‐photon excited luminescence emission spectrum, indicating that the SHG signal was enhanced in the presence of optical resonances. The observation of enhanced radiative properties of the bimetallic junction in the suggested that the Au–Ag wire may serve as an excellent imaging probe or single particle sensors.}, journal={ENGINEERING REPORTS}, author={Neupane, Bhanu B. and Chase, Thomas E. and Zhao, Luyang and Wang, Gufeng and He, Lin}, year={2021}, month={Jul} }
 @article{neupane_zhong_wang_2020, title={Study on self-assembly of colloidal particles at high ionic strength with stimulated emission depletion microscopy}, volume={2}, ISSN={["2577-8196"]}, DOI={10.1002/eng2.12233}, abstractNote={Understanding the spatial organization of nano‐ and micro‐sized particle is very important in the fabrication of complex structures having unprecedented properties. Study on self‐assembly of submicroscopic colloidal particles at high ionic strength solution at single particle resolution can provide new insight into the nanoscale interactions. In this study, we studied the self assembly behavior of negatively charged 0.2 and 1 μm colloidal particles at high ionic strength on glass‐solution interface that is, in situ environment. The self‐assembled 0.2 μm particles could not be resolved with conventional confocal and epi‐fluorescent microscopy, so a home‐built continuous wave stimulated emission depletion (STED) microscope was used for the study. We found that particles self‐assemble into ordered and disordered structures at higher and lower ionic strength solution, respectively. The optical imaging methods allowed us to measure inter‐particle gap at second energy minimum directly. Interestingly, we found that the inter‐particle gap in the wet self‐assembly higher than the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory predicted. The in situ investigation of particle self‐assembly at high ionic strength will provide more insight for the understanding nanoscale interactions.}, number={9}, journal={ENGINEERING REPORTS}, author={Neupane, Bhanu B. and Zhong, Yaning and Wang, Gufeng}, year={2020}, month={Sep} }
 @article{zhong_wang_2020, title={Three-Dimensional Single Particle Tracking and Its Applications in Confined Environments}, volume={13}, ISSN={["1936-1335"]}, DOI={10.1146/annurev-anchem-091819-100409}, abstractNote={Single particle tracking (SPT) has proven to be a powerful technique in studying molecular dynamics in complicated systems. We review its recent development, including three-dimensional (3D) SPT and its applications in probing nanostructures and molecule-surface interactions that are important to analytical chemical processes. Several frequently used 3D SPT techniques are introduced. Especially of interest are those based on point spread function engineering, which are simple in instrumentation and can be easily adapted and used in analytical labs. Corresponding data analysis methods are briefly discussed. We present several important case studies, with a focus on probing mass transport and molecule-surface interactions in confined environments. The presented studies demonstrate the great potential of 3D SPT for understanding fundamental phenomena in confined space, which will enable us to predict basic principles involved in chemical recognition, separation, and analysis, and to optimize mass transport and responses by structural design and optimization. Expected final online publication date for the Annual Review of Analytical Chemistry, Volume 13 is June 12, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.}, journal={ANNUAL REVIEW OF ANALYTICAL CHEMISTRY, VOL 13}, author={Zhong, Yaning and Wang, Gufeng}, year={2020}, pages={381–403} }
 @article{jin_garcia-lopez_chiang_kuwahara_tour_wang_2019, title={Enhancing Photostability of Fluorescent Dye-Attached Molecular Machines at Air-Glass Interface Using Cyclooctatetraene}, volume={123}, ISSN={["1932-7447"]}, DOI={10.1021/acs.jpcc.8b11592}, abstractNote={Single-molecule fluorescence microscopy at an air–solid interface severely suffers from photobleaching. In this study, we evaluated using a triplet-state quencher cyclooctatetraene (COT) group to enhance the photostability of cy5-attached molecular machines. For single-dye-modified nanocars, the photobleaching lifetime of the dye was extended by 2.1 times after the attachment of the COT group. For two-COT-two-dye-attached unimolecular submersible nanomachine (USN) molecules, both one-step and two-step photobleaching of the dyes were observed, similar to those cy5-USNs without COT protection. The fraction of one-step photobleaching events was nearly a constant under different laser powers, indicating that one-step photobleaching is a single-photon process and that the product of the first photobleaching event destroys the second dye. The COT-cy5-USNs showed a larger fraction of two-step photobleaching events as compared to cy5-USNs, indicating that the COT group provides further protection for the second d...}, number={5}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Jin, Tao and Garcia-Lopez, Victor and Chiang, Pinn-Tsong and Kuwahara, Shunsuke and Tour, James M. and Wang, Gufeng}, year={2019}, month={Feb}, pages={3011–3018} }
 @article{kumar_o'donnell_slang_maggard_wang_2019, title={Harnessing Plasmon-Induced Hot Carriers at the Interfaces with Ferroelectrics}, volume={7}, ISSN={["2296-2646"]}, DOI={10.3389/fchem.2019.00299}, abstractNote={This article reviews the scientific understanding and progress of interfacing plasmonic particles with ferroelectrics in order to facilitate the absorption of low-energy photons and their conversion to chemical fuels. The fundamental principles of hot carrier generation and charge injection are described for semiconductors interfaced with metallic nanoparticles and immersed in aqueous solutions, forming a synergistic juncture between the growing fields of plasmonically-driven photochemistry and semiconductor photocatalysis. The underlying mechanistic advantages of a metal-ferroelectric vs. metal-nonferroelectric interface are presented with respect to achieving a more optimal and efficient control over the Schottky barrier height and charge separation. Notable recent examples of using ferroelectric-interfaced plasmonic particles have demonstrated their roles in yielding significantly enhanced photocurrents as well as in the photon-driven production of molecular hydrogen. Notably, plasmonically-driven photocatalysis has been shown to occur for photon wavelengths in the infrared range, which is at lower energies than typically possible for conventional semiconductor photocatalysts. Recent results thus demonstrate that integrated ferroelectric-plasmonic systems represent a potentially transformative concept for use in the field of solar energy conversion.}, number={299}, journal={Frontiers in Chemistry}, author={Kumar, V and O'Donnell, S.C. and Slang, D.L. and Maggard, P.A. and Wang, G.}, year={2019}, month={May}, pages={1–19} }
 @article{kumar_o'donnell_zoellner_martinez_wang_maggard_2019, title={Interfacing Plasmonic Nanoparticles with Ferroelectrics for Hot-Carrier-Driven Photocatalysis: Impact of Schottky Barrier Height}, volume={2}, ISSN={["2574-0962"]}, DOI={10.1021/acsaem.9b01682}, abstractNote={Emergent strategies for efficient solar energy conversion have focused on ways to harness photons in the lower-energy range of sunlight that cannot be utilized by conventional semiconductor photocatalyst systems. Recent research has demonstrated that interfaced plasmonic–ferroelectric particles represent a promising strategy for the utilization of near-infrared (NIR) light owing to the possibility of the more efficient injection of hot charge carriers from noble metal nanoparticles. Described herein, platinum-end-capped gold nanorods (AuNRs) function as antennae to absorb low-energy NIR photons to generate hot electrons that can be injected into ferroelectric PbZrxTi1–xO3 (PZT; x = 0.48, 0.50, 0.52, 0.54, 0.56, and 0.60) and drive the reduction of water to molecular hydrogen at its surfaces. As an aqueous suspension, the interfaced AuNR-PZT particles exhibited maximal photocatalytic rates for hydrogen formation under a 976 nm diode laser (powder density = 2.0 W cm–2) for the 52% Zr (x = 0.52) composition ...}, number={10}, journal={ACS APPLIED ENERGY MATERIALS}, author={Kumar, Vineet and O'Donnell, Shaun and Zoellner, Brandon and Martinez, Jhon and Wang, Gufeng and Maggard, Paul A.}, year={2019}, month={Oct}, pages={7690–7699} }
 @article{liu_garcia-lopez_gunasekera_nilewski_alemany_aliyan_jin_wang_tour_pal_2019, title={Near-Infrared Light Activates Molecular Nanomachines to Drill into and Kill Cells}, volume={13}, ISSN={["1936-086X"]}, DOI={10.1021/acsnano.9b01556}, abstractNote={Using two-photon excitation (2PE), molecular nanomachines (MNMs) are able to drill through cell membranes and kill the cells. This avoids the use of the more damaging ultraviolet light that has been used formerly to induce this nanomechanical cell-killing effect. Since 2PE is inherently confocal, enormous precision can be realized. The MNMs can be targeted to specific cell surfaces through peptide addends. Further, the efficacy was verified through a controlled opening of synthetic bilayer vesicles using the 2PE excitation of MNM that had been trapped within the vesicles.}, number={6}, journal={ACS NANO}, author={Liu, Dongdong and Garcia-Lopez, Victor and Gunasekera, Richard S. and Nilewski, Lizanne Greer and Alemany, Lawrence B. and Aliyan, Amir and Jin, Tao and Wang, Gufeng and Tour, James M. and Pal, Robert}, year={2019}, month={Jun}, pages={6813–6823} }
 @article{ortiz_zoellner_kumar_janelli_tang_maggard_wang_2018, title={Composite Ferroelectric and Plasmonic Particles for Hot Charge Separation and Photocatalytic Hydrogen Gas Production}, volume={1}, ISSN={["2574-0962"]}, DOI={10.1021/acsaem.8b00772}, abstractNote={Plasmonic nanoparticles are excellent light absorbers for harvesting solar energy, resulting in hot electrons that can be utilized in photocatalytic hydrogen production. However, the hot electrons generated in a localized surface plasmon resonance process have a very short lifetime and are challenging to use efficiently. Herein, using near IR light irradiation, we show that by combining gold nanorods (AuNRs) with ferroelectric PbTiO3 particles that possess a large remanent electric dipole moment, hot charges generated on plasmonic particles can be injected into ferroelectric materials and drive the photocatalysis reaction. Compared to metallic Pt-end-capped AuNRs, the efficiency of using hot electrons for photocatalytic reactions is enhanced for the composite catalyst, which improves the light-to-chemical energy conversion efficiencies by about 1 order of magnitude for the same amount of plasmonic particles being used.}, number={9}, journal={ACS APPLIED ENERGY MATERIALS}, author={Ortiz, Nathalia and Zoellner, Brandon and Kumar, Vineet and Janelli, Tara and Tang, Shuli and Maggard, Paul A. and Wang, Gufeng}, year={2018}, month={Sep}, pages={4606–4616} }
 @article{zhong_li_zhou_wang_2018, title={Developing Noise-Resistant Three-Dimensional Single Particle Tracking Using Deep Neural Networks}, volume={90}, ISSN={["1520-6882"]}, DOI={10.1021/acs.analchem.8b01334}, abstractNote={Three-dimensional single particle tracking (3D SPT) is a powerful tool in various chemical and biological studies. In 3D SPT, z sensitive point spread functions (PSFs) are frequently used to generate different patterns, from which the axial position of the probe can be recovered in addition to its xy coordinates. Conventional linear classifier-based methods, for example, the correlation coefficient method, perform poorly when the signal-to-noise ratio (S/N) drops. In this work, we test deep neural networks (DNNs) in recognizing and differentiating very similar image patterns incurred in 3D SPT. The training of the deep neural networks is optimized, and a procedure is established for 3D localization. We show that for high S/N images, both DNNs and conventional correlation coefficient-based method perform well. However, when the S/N drops close to 1, conventional methods completely fail while DNNs show strong resistance to both artificial and experimental noises. This noise resistance allows us to achieve a camera integration time of 50 μs for 200 nm fluorescent particles without losing accuracy significantly. This study sheds new light on developing robust image data analysis methods and on improving the time resolution of 3D SPT.}, number={18}, journal={ANALYTICAL CHEMISTRY}, author={Zhong, Yaning and Li, Chao and Zhou, Huiyang and Wang, Gufeng}, year={2018}, month={Sep}, pages={10748–10757} }
 @article{jin_garcia-lopez_kuwahara_chiang_tour_wang_2018, title={Diffusion of Nanocars on an Air-Glass Interface}, volume={122}, ISSN={["1932-7447"]}, DOI={10.1021/acs.jpcc.8b05668}, abstractNote={At room temperature, four-adamantane-wheeled nanocars thermally diffuse on an air–glass interface. A line-scan imaging method was developed to improve the time resolution in tracking their surface movement. The fast imaging technique disclosed that the four-wheeled nanocars diffuse on glass surfaces in a quasi-random two-dimensional (2D) diffusion manner. That is, they have a high tendency to keep a linear diffusion trajectory at a short time scale, which is consistent with the wheel-rolling mode diffusion. The nanocar molecules lose the directionality over time, indicating that other diffusion modes, e.g., pivoted movement, may also contribute to their thermal diffusion at room temperature. The characteristic linear movement time for the two types of nanocar molecules in this study was ∼1.2 s, from which the activation energy for the nanocars to pivot away from the original direction was estimated to be ∼65 kJ mol–1. Finally, it was shown that using the line-scanning method the diffusion coefficient of q...}, number={33}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Jin, Tao and Garcia-Lopez, Victor and Kuwahara, Shunsuke and Chiang, Pinn-Tsong and Tour, James M. and Wang, Gufeng}, year={2018}, month={Aug}, pages={19025–19036} }
 @article{kumar_zoellner_maggard_wang_2018, title={Effect of doping Ge into Y2O3:Ho,Yb on the green-to-red emission ratio and temperature sensing}, volume={47}, ISSN={["1477-9234"]}, DOI={10.1039/c8dt02216j}, abstractNote={A series of Ge-doped monophase Y2O3:Ho,Yb phosphor materials has been synthesized using solid state reactions. The addition of Ge to the Y2O3 host decreases the Ho green emission (5F4/5S2 → 5I8) and increases the red emission (5F5 → 5I8), providing a new means to tune the green-to-red emission intensity ratio. It is proposed that the Ge-induced multiphonon relaxation process enhances the transition from the intermediate state 5I6 to 5I7, which tunes the green and red emission intensities. Most importantly, with the addition of Ge, the non-thermally coupled Ho green and red emitting levels are associated together, and the red-to-green emission intensity ratio becomes sensitive to environmental temperature change. The absolute thermal sensitivity is enhanced by a factor of >5 times that in the absence of Ge. The matched green and red emission intensities, as well as the high thermal sensitivity, make Y2O3:Ho,Yb,Ge an ideal probe for optical temperature sensing at the single particle level in live biological samples. This study demonstrates a new mechanism to channel non-thermally coupled energy levels to achieve high temperature sensitivity.}, number={32}, journal={DALTON TRANSACTIONS}, author={Kumar, Vineet and Zoellner, Brandon and Maggard, Paul A. and Wang, Gufeng}, year={2018}, month={Aug}, pages={11158–11165} }
 @article{zhong_wang_2018, title={Three-Dimensional Heterogeneous Structure Formation on a Supported Lipid Bilayer Disclosed by Single-Particle Tracking}, volume={34}, ISSN={["0743-7463"]}, DOI={10.1021/acs.langmuir.8b01690}, abstractNote={Three-dimensional (3D) single-particle tracking was employed to study the lipid membrane morphology change at different pHs on glass supported lipid bilayers (SLBs) [1,2-dioleoyl- sn-glycero-3-phosphoethanolamine/1,2-dioleoyl- sn-glycero-3-phospho-l-serine (sodium salt)/1,2-dioleoyl- sn-glycero-3-phosphocholine = 5:3:2]. Fluorescently tagged, carboxylated polystyrene nanoparticles (of 100 nm) were used as the probes. At neutral pHs, the particles' diffusion was close to two-dimensional Brownian motion, indicating a mainly planar structure of the SLBs. When the environmental pH was tuned to be basic at 10.0, transiently confined diffusions within small areas were frequently observed. These confinements had a lateral dimension of 100-200 nm. Most interestingly, they showed 3D bulged structures protruding from the planar lipid bilayer. The particles were trapped by these 3D structures for a short period of time (∼0.75 s), with an estimated escape activation energy of ∼4.2 kB T. Nonuniform distribution of pH-sensitive lipids in the membrane was proposed to explain the formation of these 3D heterogeneous structures. This work suggests that the geometry of the 3D lipid structures can play a role in tuning the particle-lipid surface interactions. It sheds new light on the origin of lateral heterogeneity on the lipid membrane.}, number={39}, journal={LANGMUIR}, author={Zhong, Yaning and Wang, Gufeng}, year={2018}, month={Oct}, pages={11857–11865} }
 @article{kumar_wang_2018, title={Tuning green-to-red ratio of Ho3+/Yb3+ activated GdPO4 upconversion luminescence through Eu3+ doping}, volume={199}, ISSN={["1872-7883"]}, DOI={10.1016/j.jlumin.2018.03.037}, abstractNote={The upconverison photoluminescence of co-doped Ho and Eu in GdPO4:Yb was studied. GdPO4:Ho3+(1%),Yb3+(10%) phosphors with varying Eu3+ fractions from 0% to 5% were synthesized using solid-state reaction and characterized using X-ray diffraction, diffuse reflectance, and upconversion (UC) photoluminescence. The co-doped Ho and Eu showed negligible energy transfer between their visible emission states (Ho 5F4/5S2 and Eu 5D0) so the two luminophores can be selectively excited. However, the presence of 5% Eu in the host lattice enhances the green-to-red emission ratio by 25 times from 0.04 to 1.0 under near infrared (NIR) excitation. It is suggested that the energy transfer from Ho intermediates states to Eu ground state (from Ho 5I6 and 5I7 to Eu 7F6) tunes the Ho emission G/R ratio. These phosphor particles are suitable for long-term single particle tracking and fluorescent labels.}, journal={JOURNAL OF LUMINESCENCE}, author={Kumar, Vineet and Wang, Gufeng}, year={2018}, month={Jul}, pages={188–193} }
 @article{ortiz_hong_fonseca_liu_wang_2017, title={Anisotropic Overgrowth of Palladium on Gold Nanorods in the Presence of Salicylic Acid Family Additives}, volume={121}, ISSN={["1932-7447"]}, DOI={10.1021/acs.jpcc.6b12024}, abstractNote={We explored the use of salicylic acid (SA) and its derivatives 5-formylsalicylic acid (FSA) and 5-sulfosalicylic acid (SSA) as organic additives to cetyltrimethylammonium bromide (CTAB) in synthesizing gold nanorods (AuNRs) followed by palladium (Pd) capping at the ends of AuNRs. In the AuNR synthesis step, SA family additives in the presence of low concentration of CTAB (50 mM) serve as both the prereducing agent and the cofactor in nanorod growth. At an optimum additive/CTAB ratio (0.1–0.2), AuNRs grow to the longest length. At low additive concentrations, the gold seeds do not grow. At high concentrations, the longitudinal growth of AuNRs is disrupted because the excessive additive disturbs the ligand structure, leading to more isotropic growth. In the Pd overgrowth step, Pd starts to grow from both ends for AuNRs synthesized at optimum additive/CTAB ratios. Feeding more Pd grows the particles into a core–shell structure, possibly because there lacks a tight ligand layer on Pd that favors the longitudi...}, number={3}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Ortiz, Nathalia and Hong, Soung Joung and Fonseca, Francini and Liu, Yang and Wang, Gufeng}, year={2017}, month={Jan}, pages={1876–1883} }
 @article{chen_gu_sun_dong_wang_fan_xia_fang_2017, title={Characteristic rotational behaviors of rod-shaped cargo revealed by automated five-dimensional single particle tracking}, volume={8}, ISSN={2041-1723}, url={http://dx.doi.org/10.1038/S41467-017-01001-9}, DOI={10.1038/S41467-017-01001-9}, abstractNote={We report an automated single particle tracking technique for tracking the x, y, z coordinates, azimuthal and elevation angles of anisotropic plasmonic gold nanorod probes in live cells. These five spatial coordinates are collectively referred to as 5D. This method overcomes a long-standing challenge in distinguishing rotational motions from translational motions in the z-axis in differential interference contrast microscopy to result in full disclosure of nanoscale motions with high accuracy. Transferrin-coated endocytic gold nanorod cargoes initially undergo active rotational diffusion and display characteristic rotational motions on the membrane. Then as the cargoes being enclosed in clathrin-coated pits, they slow down the active rotation and experience a quiet period before they restore active rotational diffusion after fission and eventually being transported away from the original entry spots. Finally, the 3D trajectories and the accompanying rotational motions of the cargoes are resolved accurately to render the intracellular transport process in live cells.Distinguishing rotational motions from translational motions in the z-axis has been a long-standing challenge. Here the authors develop a five-dimensional single particle tracking method to detect rotational behaviors of nanocargos during clathrin-mediated endocytosis and intracellular transport.}, number={1}, journal={Nature Communications}, publisher={Springer Nature}, author={Chen, Kuangcai and Gu, Yan and Sun, Wei and Dong, Bin and Wang, Gufeng and Fan, Xinxin and Xia, Tian and Fang, Ning}, year={2017}, month={Oct} }
 @article{ortiz_zoellner_hong_jo_wang_liu_maggard_wang_2017, title={Harnessing Hot Electrons from Near IR Light for Hydrogen Production Using Pt-End-Capped-AuNRs}, volume={9}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.7b05064}, abstractNote={Gold nanorods show great potential in harvesting natural sunlight and generating hot charge carriers that can be employed to produce electrical or chemical energies. We show that photochemical reduction of Pt(IV) to Pt metal mainly takes place at the ends of gold nanorods (AuNRs), suggesting photon-induced hot electrons are localized in a time-averaged manner at AuNR ends. To use these hot electrons efficiently, a novel synthetic method to selectively overgrow Pt at the ends of AuNRs has been developed. These Pt-end-capped AuNRs show relatively high activity for the production of hydrogen gas using artificial white light, natural sunlight, and more importantly, near IR light at 976 nm. Tuning of the surface plasmon resonance (SPR) wavelength of AuNRs changes the hydrogen gas production rate, indicating that SPR is involved in hot electron generation and photoreduction of hydrogen ions. This study shows that gold nanorods are excellent for converting low-energy photons into high-energy hot electrons, which can be used to drive chemical reactions at their surfaces.}, number={31}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Ortiz, Nathalia and Zoellner, Brandon and Hong, Soung Joung and Jo, Yue and Wang, Tao and Liu, Yang and Maggard, Paul A. and Wang, Gufeng}, year={2017}, month={Aug}, pages={25962–25969} }
 @article{zhong_zhao_tyrlik_wang_2017, title={Investigating Diffusing on Highly Curved Water-Oil Interface Using Three-Dimensional Single Particle Tracking}, volume={121}, ISSN={["1932-7447"]}, DOI={10.1021/acs.jpcc.7b01721}, abstractNote={Diffusion on highly curved surfaces is important to many industrial and biological processes. Despite the progress made in theoretical studies, how diffusion is affected by the curvature is unclear due to experimental challenges. Here, we measured the trajectories of polystyrene nanoparticles diffusing on highly curved water-silicone oil interface, where the oil droplet diameter ranges from several μm to as small as ∼400 nm. To analyze the diffusion coefficients on curved surface, an analytical solution developed by Castro-Villarreal containing an infinite series can be used. Through Monte Carlo simulations, we simplified the Castro-Villarreal equation and defined the conditions that satisfy corresponding approximations. For the experiments, unexpectedly, we found that the diffusion slows down significantly when the oil droplet becomes smaller. Possible reasons were discussed, and a diffusion-induced droplet deformation and interface fluctuation model is consistent with the experimental results. This stud...}, number={14}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Zhong, Yaning and Zhao, Luyang and Tyrlik, Paul M. and Wang, Gufeng}, year={2017}, month={Apr}, pages={8023–8032} }
 @article{kumar_bullis_wang_2017, title={Investigation of NIR-to-red upconversion luminescence mechanism in Y2O3:Er3+, Yb3+ and the effect of co-doping Zn in the matrix}, volume={192}, ISSN={["1872-7883"]}, DOI={10.1016/j.jlumin.2017.08.039}, abstractNote={We studied composite ZnO:Y2O3 matrix for upconversion luminophore pair Er:Yb using near IR light excitation. Uni-phase crystalline nanoparticles were produced up to a 4% co-doped ZnO in Y2O3. In ZnO:Y2O3 co-host matrix, Er(2%):Yb(10%) pair emits strong red luminescence. Doping 4% ZnO into the matrix enhances the luminescence by ~ 2.5 times. The mechanisms for the predominant red emission in the presence and absence of Zn ions were studied. An energy back transfer (EBT) process from Er to Yb is the main pathway for the intense red emission under NIR excitation although other mechanisms, e.g. Er-Er cross relaxation (CR), may also contribute to the red emission. The Er-Yb EBT process brings the Er ion from the green emission state to the intermediate 4I13/2 state, which further absorbs a near IR photon and jumps to the red emission state. The introduction of ZnO into Y2O3 matrix does not interfere with this process significantly. Thus, the enhancement of luminescence is mainly caused by the breaking of the symmetry around Er3+ ions due to the Y-to-Zn substitution, which favors the f-f electric dipole transitions that are forbidden in centrosymmetric environment.}, journal={JOURNAL OF LUMINESCENCE}, author={Kumar, Vineet and Bullis, Grant and Wang, Gufeng}, year={2017}, month={Dec}, pages={982–989} }
 @article{garcía-lópez_chen_nilewski_duret_aliyan_kolomeisky_robinson_wang_pal_tour_2017, title={Molecular machines open cell membranes}, volume={548}, ISSN={0028-0836 1476-4687}, url={http://dx.doi.org/10.1038/NATURE23657}, DOI={10.1038/NATURE23657}, abstractNote={Beyond the more common chemical delivery strategies, several physical techniques are used to open the lipid bilayers of cellular membranes. These include using electric and magnetic fields, temperature, ultrasound or light to introduce compounds into cells, to release molecular species from cells or to selectively induce programmed cell death (apoptosis) or uncontrolled cell death (necrosis). More recently, molecular motors and switches that can change their conformation in a controlled manner in response to external stimuli have been used to produce mechanical actions on tissue for biomedical applications. Here we show that molecular machines can drill through cellular bilayers using their molecular-scale actuation, specifically nanomechanical action. Upon physical adsorption of the molecular motors onto lipid bilayers and subsequent activation of the motors using ultraviolet light, holes are drilled in the cell membranes. We designed molecular motors and complementary experimental protocols that use nanomechanical action to induce the diffusion of chemical species out of synthetic vesicles, to enhance the diffusion of traceable molecular machines into and within live cells, to induce necrosis and to introduce chemical species into live cells. We also show that, by using molecular machines that bear short peptide addends, nanomechanical action can selectively target specific cell-surface recognition sites. Beyond the in vitro applications demonstrated here, we expect that molecular machines could also be used in vivo, especially as their design progresses to allow two-photon, near-infrared and radio-frequency activation.}, number={7669}, journal={Nature}, publisher={Springer Science and Business Media LLC}, author={García-López, Víctor and Chen, Fang and Nilewski, Lizanne G. and Duret, Guillaume and Aliyan, Amir and Kolomeisky, Anatoly B. and Robinson, Jacob T. and Wang, Gufeng and Pal, Robert and Tour, James M.}, year={2017}, month={Aug}, pages={567–572} }
 @article{jin_garcia-lopez_chen_tour_wang_2016, title={Imaging Single Molecular Machines Attached with Two BODIPY Dyes at the Air-Solid Interface: High Probability of Single-Step-Like Photobleaching and Nonscaling Intensity}, volume={120}, ISSN={["1932-7447"]}, DOI={10.1021/acs.jpcc.6b07518}, abstractNote={Single-molecule fluorescence microscopy (SMFM) is a powerful technique in monitoring single molecular machine actions at ambient conditions. To improve the fluorescence intensity and photostability, one strategy is to attach multiple dyes to the same single molecular machine. However, it is unclear how the fluorescence property of the dyes will change when multiple dyes are compacted into the same molecule within a distance between them of ∼2 nm. In this study, we investigated the photophysics of two types of single molecular machines that are each equipped with two BODIPY dyes with different distances. We found that at the air–glass interface, single molecules attached with two dyes have a high tendency to show a single-step-like photobleaching, making them to appear like a single dye. We propose that the product of the first photobleaching event, possibly a superoxide anion radical, is involved in the destruction of the neighboring dye. In addition, the fluorescence intensity of the two-dye system does ...}, number={46}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Jin, Tao and Garcia-Lopez, Victor and Chen, Fang and Tour, James and Wang, Gufeng}, year={2016}, month={Nov}, pages={26522–26531} }
 @article{chen_neupane_li_su_wang_2016, title={Investigating axial diffusion in cylindrical pores using confocal single-particle fluorescence correlation spectroscopy}, volume={37}, ISSN={["1522-2683"]}, DOI={10.1002/elps.201600158}, abstractNote={We explored the feasibility of using confocal fluorescence correlation spectroscopy to study small nanoparticle diffusion in hundred‐nanometer‐sized cylindrical pores. By modeling single particle diffusion in tube‐like confined three‐dimensional space aligned parallel to the confocal optical axis, we showed that two diffusion dynamics can be observed in both original intensity traces and the autocorrelation functions (ACFs): the confined two‐dimensional lateral diffusion and the unconfined one‐dimensional (1D) axial diffusion. The separation of the axial and confined lateral diffusion dynamics provides an opportunity to study diffusions in different dimensions separately. We further experimentally studied 45 nm carboxylated polystyrene particles diffusing in 300 nm alumina pores. The experimental data showed consistency with the simulation. To extract the accurate axial diffusion coefficient, we found that a 1D diffusion model with a Lorentzian axial collection profile needs to be used to analyze the experimental ACFs. The diffusion of the 45 nm nanoparticles in polyethyleneglycol‐passivated 300 nm pores slowed down by a factor of ∼2, which can be satisfactorily explained by hydrodynamic frictions.}, number={15-16}, journal={ELECTROPHORESIS}, author={Chen, Fang and Neupane, Bhanu and Li, Peiyuan and Su, Wei and Wang, Gufeng}, year={2016}, month={Aug}, pages={2129–2138} }
 @article{zhao_zhong_wei_ortiz_chen_wang_2016, title={Microscopic Movement of Slow-Diffusing Nanoparticles in Cylindrical Nanopores Studied with Three-Dimensional Tracking}, volume={88}, ISSN={["1520-6882"]}, DOI={10.1021/acs.analchem.5b04944}, abstractNote={To study slow mass transport in confined environments, we developed a three-dimensional (3D) single-particle localization technique to track their microscopic movements in cylindrical nanopores. Under two model conditions, particles are retained much longer inside the pores: (1) increased solvent viscosity, which slows down the particle throughout the whole pore, and (2) increased pore wall affinity, which slows down the particle only at the wall. In viscous solvents, the particle steps decrease proportionally to the increment of the viscosity, leading to macroscopically slow diffusion. As a contrast, the particles in sticky pores are microscopically active by showing limited reduction of step sizes. A restricted diffusion mode, possibly caused by the heterogeneous environment in sticky pores, is the main reason for macroscopically slow diffusion. This study shows that it is possible to differentiate slow diffusion in confined environments caused by different mechanisms.}, number={10}, journal={ANALYTICAL CHEMISTRY}, author={Zhao, Luyang and Zhong, Yaning and Wei, Yanli and Ortiz, Nathalia and Chen, Fang and Wang, Gufeng}, year={2016}, month={May}, pages={5122–5130} }
 @article{chen_garcia-lopez_jin_neupane_chu_tour_wang_2016, title={Moving Kinetics of Nanocars with Hydrophobic Wheels on Solid Surfaces at Ambient Conditions}, volume={120}, ISSN={["1932-7447"]}, DOI={10.1021/acs.jpcc.6b01249}, abstractNote={Motivated by “driving” nanoscopic nanocars on solid substrate surfaces at ambient conditions, we studied the moving kinetics of nanocars on differently modified surfaces. Single molecule fluorescence imaging was used to track the nanocar movement so that the molecules were minimally perturbed. On freshly cleaned, hydroxylated glass surfaces, nanocars with hydrophobic adamantane wheels can diffuse with a relatively large diffusion coefficient of 7.6 × 10–16 m2/s. Both the number of moving molecules and the mobility of the moving molecules decreased over time when the sample was exposed in the air. Similar declinations in movement were observed on a poly(ethylene glycol) (PEG)-modified glass surface, but the declination rate was lowered. The slowing of molecular surface diffusion is correlated to the hydrophobicity of the surface and is likely caused by the adsorption of hydrophobic molecules from the air. A proposed sticky-spots model explains the decreasing apparent diffusion coefficient of the hydrophobi...}, number={20}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Chen, Fang and Garcia-Lopez, Victor and Jin, Tao and Neupane, Bhanu and Chu, Pin-Lei E. and Tour, James and Wang, Gufeng}, year={2016}, month={May}, pages={10887–10894} }
 @article{neupane_chen_wei_fang_ligler_wang_2016, title={Nanosecond Time-Resolution Study of Gold Nanorod Rotation at the Liquid-Solid Interface}, volume={17}, ISSN={["1439-7641"]}, DOI={10.1002/cphc.201600174}, abstractNote={Early studies showed that the adsorption of nanorods may start from a special "anchored" state, in which the nanorods lose translational motion but retain rotational freedom. Insight into how the anchored nanorods rotate should provide additional dimensions for understanding particle-surface interactions. Based on conventional time-resolution studies, gold nanorods are thought to continuously rotate following initial interactions with negatively charged glass surfaces. However, this nanosecond time-resolution study reveals that the apparent continuous rotation actually consists of numerous fast, intermittent rotations or transitions between a small number of weakly immobilized states, with the particle resting in the immobilized states most of the time. The actual rotation from one immobilized state to the other happens on a 1 ms timescale, that is, approximately 50 times slower than in the bulk solution.}, number={14}, journal={CHEMPHYSCHEM}, author={Neupane, Bhanu and Chen, Fang and Wei, Yanli and Fang, Ning and Ligler, Frances S. and Wang, Gufeng}, year={2016}, month={Jul}, pages={2218–2224} }
 @article{su_tang_li_wang_xiao_li_huang_gu_lai_zhang_2016, title={New dinuclear ruthenium arene complexes containing thiosemicarbazone ligands: synthesis, structure and cytotoxic studies}, volume={45}, ISSN={["1477-9234"]}, DOI={10.1039/c6dt03306g}, abstractNote={A series of mononuclear ruthenium arene complexes with thiosemicarbazone (TSC) ligands (A-type, 1-8) and their corresponding di-nuclear analogues (B-type, 9-16) were synthesized and characterized by NMR, elemental analysis and HR-ESI-mass spectrometry. The molecular structures of 1, 2, 6, 9-11 and 13-16 were determined using single-crystal X-ray diffraction analysis. The Gibbs free energy of the two examples of the two types of complexes (1 and 9) and the bonding order in their single-crystals were studied using density functional theory (DFT) calculations. The compounds were further evaluated for their in vitro antiproliferative activities against CNE-2 human nasopharyngeal carcinoma, KB human oral epithelial carcinoma, SGC-7901 human gastric carcinoma, HepG2 human liver carcinoma, HeLa human cervical carcinoma and HEK-293T noncancerous cell lines. Furthermore, the interactions between the compounds and DNA were studied by electrophoretic mobility spectrometry studies.}, number={48}, journal={DALTON TRANSACTIONS}, author={Su, Wei and Tang, Zhaofeng and Li, Peiyuan and Wang, Gufeng and Xiao, Qi and Li, Yuchun and Huang, Shan and Gu, Yunqiong and Lai, Zefeng and Zhang, Yuexing}, year={2016}, pages={19329–19340} }
 @article{pacardo_neupane_rikard_lu_mo_mishra_tracy_wang_ligler_gu_2015, title={A dual wavelength-activatable gold nanorod complex for synergistic cancer treatment}, volume={7}, ISSN={["2040-3372"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000357805700034&KeyUID=WOS:000357805700034}, DOI={10.1039/c5nr01568e}, abstractNote={A multifunctional gold nanorod (AuNR) complex is described with potential utility for theranostic anticancer treatment. The AuNR was functionalized with cyclodextrin for encapsulation of doxorubicin, with folic acid for targeting, and with a photo-responsive dextran-azo compound for intracellular controlled drug release. The interaction of a AuNR complex with HeLa cells was facilitated via a folic acid targeting ligand as displayed in the dark-field images of cells. Enhanced anticancer efficacy was demonstrated through the synergistic combination of promoted drug release upon ultraviolet (UV) light irradiation and photothermal therapy upon infrared (IR) irradiation. This multifunctional AuNR-based system represents a novel theranostic strategy for spatiotemporal delivery of anticancer therapeutics.}, number={28}, journal={NANOSCALE}, author={Pacardo, Dennis B. and Neupane, Bhanu and Rikard, S. Michaela and Lu, Yue and Mo, Ran and Mishra, Sumeet R. and Tracy, Joseph B. and Wang, Gufeng and Ligler, Frances S. and Gu, Zhen}, year={2015}, pages={12096–12103} }
 @article{pacardo_neupane_wang_gu_walker_ligler_2015, title={A temperature microsensor for measuring laser-induced heating in gold nanorods}, volume={407}, ISSN={["1618-2650"]}, DOI={10.1007/s00216-014-8222-9}, number={3}, journal={ANALYTICAL AND BIOANALYTICAL CHEMISTRY}, author={Pacardo, Dennis B. and Neupane, Bhanu and Wang, Gufeng and Gu, Zhen and Walker, Glenn M. and Ligler, Frances S.}, year={2015}, month={Jan}, pages={719–725} }
 @article{wei_chen_li_shuang_dong_wang_2015, title={An exonuclease I-based label-free fluorometric aptasensor for adenosine triphosphate (ATP) detection with a wide concentration range}, volume={63}, ISSN={["1873-4235"]}, DOI={10.1016/j.bios.2014.07.064}, abstractNote={A novel aptamer-based label-free assay for sensitive and selective detection of ATP was developed. This assay employs a new aptamer/fluorescent probe system that shows resistance to exonuclease I (Exo I) digestion upon binding to ATP molecules. In the absence of ATP, the complex between the ATP-binding aptamer (ATP–aptamer) and a DNA binding dye, berberine, is digested upon the addition of exonuclease I, leading to the release of berberine into solution and consequently, quenched berberine fluorescence. In the presence of ATP, the ATP-binding aptamer folds into a G-quadruplex structure that is resistant to Exo I digestion. Accordingly, berberine is protected in the G-quadruplex structure and high fluorescence intensity is observed. As such, based on the fluorescence signal change, a label-free fluorescence assay for ATP was developed. Factors affecting the analysis of ATP including the concentration of ATP-binding aptamer, reaction time, temperature and the concentration of Exo I were comprehensively investigated. Under optimal conditions, the fluorescence intensity of the sensing system displayed a response for ATP in a wide range up to 17.5 mM with a detection limit of 140 nM.}, journal={BIOSENSORS & BIOELECTRONICS}, author={Wei, Yanli and Chen, Yanxia and Li, Huanhuan and Shuang, Shaomin and Dong, Chuan and Wang, Gufeng}, year={2015}, month={Jan}, pages={311–316} }
 @article{neupane_jin_mellor_loboa_ligler_wang_2015, title={Continuous-wave stimulated emission depletion microscope for imaging actin cytoskeleton in fixed and live cells}, volume={15}, number={9}, journal={Sensors (Basel, Switzerland)}, author={Neupane, B. and Jin, T. and Mellor, L. F. and Loboa, E. G. and Ligler, F. S. and Wang, G. F.}, year={2015}, pages={24178–24190} }
 @article{garcia-lopez_chiang_chen_ruan_marti_kolomeisky_wang_tour_2015, title={Unimolecular Submersible Nanomachines. Synthesis, Actuation, and Monitoring}, volume={15}, ISSN={["1530-6992"]}, DOI={10.1021/acs.nanolett.5b03764}, abstractNote={Unimolecular submersible nanomachines (USNs) bearing light-driven motors and fluorophores are synthesized. NMR experiments demonstrate that the rotation of the motor is not quenched by the fluorophore and that the motor behaves in the same manner as the corresponding motor without attached fluorophores. No photo or thermal decomposition is observed. Through careful design of control molecules with no motor and with a slow motor, we found using single molecule fluorescence correlation spectroscopy that only the molecules with fast rotating speed (MHz range) show an enhancement in diffusion by 26% when the motor is fully activated by UV light. This suggests that the USN molecules give ∼9 nm steps upon each motor actuation. A non-unidirectional rotating motor also results in a smaller, 10%, increase in diffusion. This study gives new insight into the light actuation of motorized molecules in solution.}, number={12}, journal={NANO LETTERS}, author={Garcia-Lopez, Victor and Chiang, Pinn-Tsong and Chen, Fang and Ruan, Gedeng and Marti, Angel A. and Kolomeisky, Anatoly B. and Wang, Gufeng and Tour, James M.}, year={2015}, month={Dec}, pages={8229–8239} }
 @article{wei_li_hao_chen_dong_wang_2015, title={beta-Cyclodextrin functionalized Mn-doped ZnS quantum dots for the chiral sensing of tryptophan enantiomers}, volume={6}, ISSN={["1759-9962"]}, DOI={10.1039/c4py00618f}, abstractNote={Hydrothermally prepared mono-6-SH-β-cyclodextrin capped Mn-doped ZnS quantum dots (β-CD-Mn-ZnS QDs) exhibited dual photoluminescence (PL) at 430 nm and 598 nm, respectively, upon excitation at 315 nm. The PL intensity of β-CD-Mn-ZnS QDs responded to tryptophan enantiomers differently: the D-isomer showed little effect while L-tryptophan displayed a large time-dependent enhancement in the PL intensity of QDs. This chiral selectivity originated from the β-cyclodextrin coating on the surface of Mn-ZnS QDs, which shows different inclusion constants for tryptophan enantiomers. β-CD-Mn-ZnS QDs further selectively hydrolyse L-tryptophan, forming a product that greatly enhances the QD photoluminescence. Based on these findings, a photoluminescence chiral-assay for tryptophan enantiomers was developed. L-Tryptophan can be detected in the presence of its stereoisomer with a detection limit of 5.4 nM in a linear range of 0–6.0 μM.}, number={4}, journal={POLYMER CHEMISTRY}, author={Wei, Yanli and Li, Huanhuan and Hao, Hongye and Chen, Yanxia and Dong, Chuan and Wang, Gufeng}, year={2015}, pages={591–598} }
 @misc{neupane_ligler_wang_2014, title={Review of recent developments in stimulated emission depletion microscopy: Applications on cell imaging}, volume={19}, number={8}, journal={Journal of Biomedical Optics}, author={Neupane, B. and Ligler, F. S. and Wang, G. F.}, year={2014} }
 @article{gu_wang_fang_2013, title={Simultaneous Single-Particle Superlocalization and Rotational Tracking}, volume={7}, ISSN={["1936-086X"]}, DOI={10.1021/nn305640y}, abstractNote={Superlocalization of single molecules and nanoparticles has become an essential procedure to bring new insights into nanoscale structures and dynamics of biological systems. In the present study, superlocalization is combined with the newly introduced differential interference contrast (DIC) microscopy-based single-particle orientation and rotational tracking. The new technique overcomes the difficulty in localization of the antisymmetric DIC point spread function by using a dual-modality microscope configuration for simultaneous rotational tracking and localization of single gold nanorods with nanometer-scale precision. The new imaging setup has been applied to study the steric hindrance induced by relatively large cargos in the microtubule gliding assay and to track nanocargos in the crowded cellular environment. This technique has great potential in the study of biological processes where both localization and rotational information are required.}, number={2}, journal={ACS NANO}, author={Gu, Yan and Wang, Gufeng and Fang, Ning}, year={2013}, month={Feb}, pages={1658–1665} }
 @misc{stender_marchuk_liu_sander_meyer_smith_neupane_wang_li_cheng_et al._2013, title={Single Cell Optical Imaging and Spectroscopy}, volume={113}, ISSN={["1520-6890"]}, DOI={10.1021/cr300336e}, abstractNote={In his 1665 treatise, Micrographia, Robert Hooke described the many observations he had made using a microscope, including compartment-like structures in cork samples that he termed ‘cells’.1 In the three and a half centuries since Hooke’s day, both the microscope and our understanding of the cell have been vastly improved upon, and the current outlook suggests that the symbiotic relationship between the microscope and the cell will continue to flourish into the foreseeable future. The cell is a basic yet complicated ‘unit’ of interest to biology, just as the atom is to chemists. Ultimately, scientists want to ‘see to believe’ when it comes to an explanation of the complex inner workings of cells, but therein lies a complication. Seeing is not always a possibility in biological systems. Size, speed, sensitivity, and additional concerns plague the microscopist who wants to peek inside of a cell. Enter a variety of molecular and nanoparticle probes that are capable of tagging and pinpointing the location of biological components that would otherwise be invisible under the microscope. Advances in laser, camera, and imaging processing technologies have also played a crucial role in the burgeoning field of single cell imaging, because they have brought into view the fast processes that would normally escape the human eye. 
 
The purpose of this review is to highlight the key advances that have occurred in the past several years in the field of single cell optical imaging. It is not our intent to provide a comprehensive review of the types of experiments or the areas of cell research that are ongoing. Reviews with a distinctly biological flavor have been published recently, and these alternative reviews focus on specific details of the cell and the processes that occur within.2-7 Likewise, exceptional review papers that have discussed the full spectrum of nanoparticle probes and their properties have appeared recently.6-12 This review is designed to give an overview of the tools that are being specifically used to accomplish single cell imaging. As such, much of our emphasis in the first several sections of this paper is on imaging platforms, with a focus on design details that are important to single cell imaging experiments. Next we emphasize specific imaging experiments that highlight the types of findings that are possible at the nexus of microscopy, nanoprobes, and live cells. Particular attention is paid to the emerging orientation and rotational tracking of single probes linked to mechanistic functions and differentiated structures of biological interest. Finally, we provide a brief, yet rather complete, summary of single cell manipulation techniques.}, number={4}, journal={CHEMICAL REVIEWS}, author={Stender, Anthony S. and Marchuk, Kyle and Liu, Chang and Sander, Suzanne and Meyer, Matthew W. and Smith, Emily A. and Neupane, Bhanu and Wang, Gufeng and Li, Junjie and Cheng, Ji-Xin and et al.}, year={2013}, month={Apr}, pages={2469–2527} }
 @article{neupane_chen_sun_chiu_wang_2013, title={Tuning donut profile for spatial resolution in stimulated emission depletion microscopy}, volume={84}, ISSN={0034-6748 1089-7623}, url={http://dx.doi.org/10.1063/1.4799665}, DOI={10.1063/1.4799665}, abstractNote={In stimulated emission depletion (STED)-based or up-conversion depletion-based super-resolution optical microscopy, the donut-shaped depletion beam profile is of critical importance to its resolution. In this study, we investigate the transformation of the donut-shaped depletion beam focused by a high numerical aperture (NA) microscope objective, and model STED point spread function (PSF) as a function of donut beam profile. We show experimentally that the intensity profile of the dark kernel of the donut can be approximated as a parabolic function, whose slope is determined by the donut beam size before the objective back aperture, or the effective NA. Based on this, we derive the mathematical expression for continuous wave (CW) STED PSF as a function of focal plane donut and excitation beam profiles, as well as dye properties. We find that the effective NA and the residual intensity at the center are critical factors for STED imaging quality and the resolution. The effective NA is critical for STED resolution in that it not only determines the donut shape but also the area the depletion laser power is dispersed. An improperly expanded depletion beam will have negligible improvement in resolution. The polarization of the depletion beam also plays an important role as it affects the residual intensity in the center of the donut. Finally, we construct a CW STED microscope operating at 488 nm excitation and 592 nm depletion with a resolution of 70 nm. Our study provides detailed insight to the property of donut beam, and parameters that are important for the optimal performance of STED microscopes. This paper will provide a useful guide for the construction and future development of STED microscopes.}, number={4}, journal={Review of Scientific Instruments}, publisher={AIP Publishing}, author={Neupane, Bhanu and Chen, Fang and Sun, Wei and Chiu, Daniel T. and Wang, Gufeng}, year={2013}, month={Apr}, pages={043701} }
 @article{neupane_zhao_wang_2013, title={Up-Conversion Luminescence of Gold Nanospheres When Excited at Nonsurface Plasmon Resonance Wavelength by a Continuous Wave Laser}, volume={13}, ISSN={["1530-6992"]}, DOI={10.1021/nl401505p}, abstractNote={We show that, when gold nanospheres are excited at the red side of the surface plasmon resonance (SPR) wavelength at 592 nm by a continuous wave (CW) laser, they give substantial up-converted luminescence in the SPR wavelength range. The luminescence intensity scales as a second-order function of the excitation power, with a quantum yield ~1/50 of down-conversion luminescence when illuminated at a power of 30 MW/cm(2). The luminescence spectrum is completely different than the SPR profile, indicating a new emission mechanism possibly involving interband transitions coupled with phonons or localized vibration of neighboring gold atoms. Such luminescence is also observed to be substantial for short gold nanorods with an aspect ratio of ~2 but weak for bulk gold. This study provides new insight to the understanding of gold nanoparticle luminescence and opens a new detection scheme for gold nanoparticle-based biological imaging.}, number={9}, journal={NANO LETTERS}, author={Neupane, Bhanu and Zhao, Luyang and Wang, Gufeng}, year={2013}, month={Sep}, pages={4087–4092} }
 @article{fang_sun_gu_wang_won ha_2012, title={Deciphering Orientation and Rotational Information of Cargoes at Pauses During Axonal Transport}, volume={102}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2011.11.238}, DOI={10.1016/j.bpj.2011.11.238}, abstractNote={The study of axonal transport is essential in learning motor protein working mechanisms and neuronal functions. Direct visualization of the transport events in living neurons has led to the current understanding of the mechanism of axonal transport. However, the mechanism for generating the pauses in the transit of cargoes is still largely unknown, mainly because the molecular or nanoparticle probes used in the previous studies could not reveal the orientation and rotational information of the cargo at the pausing moments. Recently, we introduced the single particle orientation and rotational tracking (SPORT) technique to follow the rotational motion of plasmonic gold nanorods (References: 1. J. Am. Chem. Soc., 2010, 132, 16417. 2. ACS Nano, 2010, 4, 7667. 3. J. Am. Chem. Soc., 2011, 133, 5720.). In the present work, we acquired for the first time the orientation and rotational information of the cargo during both the moving (directional transport) and pausing stages of axonal transport with a high temporal resolution of 2 ms. The lateral and rotational motions of cargos during directional transport and pausing stages are elucidated, and their relationship with motor protein competition and regulation is discussed. The simultaneous tracking of lateral movement and rotational motions of nano-objects also shadows a new insight into the dynamic function of living cells.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Fang, Ning and Sun, Wei and Gu, Yan and Wang, Gufeng and Won Ha, Ji}, year={2012}, month={Jan}, pages={38a–39a} }
 @inproceedings{wang_fang_2012, title={Detecting and tracking nonfluorescent nanoparticle probes in live cells}, volume={504}, booktitle={Imaging and spectroscopic analysis of living cells: optical and spectroscopic techniques}, author={Wang, G. F. and Fang, N.}, year={2012}, pages={83–108} }
 @article{xiao_ha_wei_wang_fang_2012, title={Determining the Full Three-Dimensional Orientation of Single Anisotropic Nanoparticles by Differential Interference Contrast Microscopy}, volume={51}, ISSN={1433-7851}, url={http://dx.doi.org/10.1002/anie.201202340}, DOI={10.1002/anie.201202340}, abstractNote={Keeping track: By combining differential interference contrast (DIC) image pattern recognition with DIC polarization anisotropy, the exact full three-dimensional angular information of individual tilted gold nanorods positioned in the focal plane of the objective lens can be readily determined. The angular rotational modes and kinetics of individual in-focus gold nanorods can thus be resolved dynamically.}, number={31}, journal={Angewandte Chemie International Edition}, publisher={Wiley}, author={Xiao, Lehui and Ha, Ji Won and Wei, Lin and Wang, Gufeng and Fang, Ning}, year={2012}, month={Jun}, pages={7734–7738} }
 @article{han_wang_qi_ma_yeung_2012, title={Electrophoretic Migration and Axial Diffusion of Individual Nanoparticles in Cylindrical Nanopores}, volume={116}, ISSN={1932-7447 1932-7455}, url={http://dx.doi.org/10.1021/jp303855d}, DOI={10.1021/jp303855d}, abstractNote={Membranes with straight, vertical nanopores have found widespread applications in chemical and biological sciences, including separation, detection, catalysis, and drug delivery. They can also serve as a model system to understand molecular behavior and fundamental mechanisms of separation, bridging the gap between conventional model systems such as flat surfaces and real chromatographic stationary phases such as micrometer-sized porous particles. We recently found that the axial motion of individual biomolecules inside nanopores can be significantly slower than in bulk solution. This suggests that either chromatographic adsorption was present and/or the viscosity inside the nanopores was unusually high. In this study, we measured the electrophoretic motion as well as the axial diffusion of individual nanoparticles in cylindrical alumina nanopores. We found that the electrophoretic mobilities and the diffusion coefficients of polystyrene nanoparticles were both substantially smaller compared to bulk solution independent of particle size or pore diameter. The results imply that the apparent solution viscosity in nanodomains is anomalous.}, number={34}, journal={The Journal of Physical Chemistry C}, publisher={American Chemical Society (ACS)}, author={Han, Rui and Wang, Gufeng and Qi, Shengda and Ma, Changbei and Yeung, Edward S.}, year={2012}, month={Aug}, pages={18460–18468} }
 @article{stender_augspurger_wang_fang_2012, title={Influence of Polarization Setting on Gold Nanorod Signal at Nonplasmonic Wavelengths Under Differential Interference Contrast Microscopy}, volume={84}, ISSN={["1520-6882"]}, DOI={10.1021/ac301072f}, abstractNote={Researchers rely on a variety of microscopic techniques for observing and tracking anisotropic nanoparticles in real time experiments. This technical note focuses on the optical behavior exhibited by gold nanorods at nonplasmonic wavelengths under differential interference contrast microscopy (DIC). Intense diffraction patterns appear at nonplasmonic wavelengths, and the behavior of these patterns can be altered by adjusting the surrounding medium or the polarizer setting. Such patterns are absent when linear and crossed polarizations are utilized. Making polarization adjustments is important in DIC microscopy, because it affects bias retardation and image contrast. The nonplasmonic diffraction bands that were observed could potentially be exploited for rotational tracking, but more importantly, researchers should exhibit care in selecting a nanorod sample and the polarization setting when working with DIC microscopy.}, number={12}, journal={ANALYTICAL CHEMISTRY}, author={Stender, Anthony S. and Augspurger, Ashley E. and Wang, Gufeng and Fang, Ning}, year={2012}, month={Jun}, pages={5210–5215} }
 @article{gu_sun_wang_zimmermann_jernigan_fang_2012, title={Revealing Rotational Modes of Functionalized Gold Nanorods on Live Cell Membranes}, volume={9}, ISSN={1613-6810}, url={http://dx.doi.org/10.1002/smll.201201808}, DOI={10.1002/smll.201201808}, abstractNote={A full understanding of cell mechanics requires knowledge of both translational and rotational dynamics. The single particle orientation and rotational tracking (SPORT) technique is combined here with correlation analysis to identify the fundamental rotational modes: in-plane rotation and out-of-plane tilting, as well as other more complex rotational patterns, from the vast image data captured at a temporal resolution of 5 ms for single gold nanorod probes in live cell imaging experiments. The unique capabilities of visualizing and understanding rotational motions of functional nanoparticles on live cell membranes allow correlation of the rotational and translational dynamics in unprecedented detail and provide new insights into complex membrane processes. Particles with functionalized surfaces, which interact with the membrane in fundamentally different ways, can exhibit distinct rotational modes and are, for the first time, directly visualized, and these show the early events for membrane approach and attachment.}, number={5}, journal={Small}, publisher={Wiley}, author={Gu, Yan and Sun, Wei and Wang, Gufeng and Zimmermann, Michael T. and Jernigan, Robert L. and Fang, Ning}, year={2012}, month={Nov}, pages={785–792} }
 @article{gu_sun_wang_jeftinija_jeftinija_fang_2012, title={Rotational dynamics of cargos at pauses during axonal transport}, volume={3}, ISSN={2041-1723}, url={http://dx.doi.org/10.1038/ncomms2037}, DOI={10.1038/ncomms2037}, abstractNote={Direct visualization of axonal transport in live neurons is essential for our understanding of the neuronal functions and the working mechanisms of microtubule-based motor proteins. Here we use the high-speed single particle orientation and rotational tracking technique to directly visualize the rotational dynamics of cargos in both active directional transport and pausing stages of axonal transport, with a temporal resolution of 2 ms. Both long and short pauses are imaged, and the correlations between the pause duration, the rotational behaviour of the cargo at the pause, and the moving direction after the pause are established. Furthermore, the rotational dynamics leading to switching tracks are visualized in detail. These first-time observations of cargo's rotational dynamics provide new insights on how kinesin and dynein motors take the cargo through the alternating stages of active directional transport and pause.}, number={1}, journal={Nature Communications}, publisher={Springer Science and Business Media LLC}, author={Gu, Yan and Sun, Wei and Wang, Gufeng and Jeftinija, Ksenija and Jeftinija, Srdija and Fang, Ning}, year={2012}, month={Jan} }
 @article{gu_di_sun_wang_fang_2012, title={Three-dimensional super-localization and tracking of single gold nanoparticles in cells}, volume={84}, DOI={10.1021/ac300249d}, abstractNote={We introduce a precise three-dimensional (3D) localization method of spherical gold nanoparticle probes using model-based correlation coefficient mapping. To accomplish this, a stack of sample images at different z-positions are acquired, and a 3D intensity profile of the probe serving as the model is used to map out the positions of nanoparticles in the sample. By using this model-based correlation imaging method, precise localization can be achieved in imaging techniques with complicated point spread functions (PSF) such as differential interference contrast (DIC) microscopy. We demonstrated the localization precision of 4-7 nm laterally and 16 nm axially for 40-nm gold nanospheres at an imaging rate of 10 frames per second. The 3D superlocalization method was applied to tracking gold nanospheres during live endocytosis events.}, number={9}, journal={Analytical Chemistry}, author={Gu, Y. and Di, X. W. and Sun, W. and Wang, G. F. and Fang, N.}, year={2012}, pages={4111–4117} }
 @article{wang_lipert_jain_kaur_chakraboty_torres_batra_brand_porter_2011, title={Detection of the Potential Pancreatic Cancer Marker MUC4 in Serum Using Surface-Enhanced Raman Scattering}, volume={83}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac102829b}, DOI={10.1021/ac102829b}, abstractNote={Pancreatic cancer (PC) is one of the most lethal malignancies. It has a 5-year survival rate of only 6%, owing in part to the lack of a reliable tumor marker for early diagnosis. Recent research has shown that the mucin protein MUC4 is aberrantly expressed in pancreatic adenocarcinoma cell lines and tissues but is undetectable in normal pancreas and chronic pancreatitis. Thus, the level of MUC4 in patient sera has the potential to function as a diagnostic and prognostic marker for PC. However, the measurement of MUC4 in sera using conventional test platforms (e.g., enzyme linked immunosorbent assay (ELISA) and radioimmunoassay (RIA)) has been unsuccessful. This has prevented the assessment of the utility of this protein as a possible PC marker in sera. In addressing this obstacle, the work herein examines the potential to create a simple diagnostic test for MUC4 through the development of a surface-enhanced Raman scattering (SERS)-based immunoassay, which was then used to demonstrate the first ever detection of MUC4 in cancer patient serum samples. Importantly, these measurements showed that sera from patients with PC produced a significantly higher SERS response for MUC4 compared to sera from healthy individuals and from patients with benign diseases. These results indicate that a SERS-based immunoassay can monitor MUC4 levels in patient sera, representing a much needed first step toward assessing the potential of this protein to serve as a serum marker for the early stage diagnosis of PC. This paper details these and other findings (i.e., the detection of the mucin protein CA19-9), which demonstrate that our SERS assay outperforms conventional assays (i.e., RIA and ELISA) with respect to limits of detection, readout time, and required sample volume.}, number={7}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Wang, Gufeng and Lipert, Robert J. and Jain, Maneesh and Kaur, Sukhwinder and Chakraboty, Subhankar and Torres, Maria P. and Batra, Surinder K. and Brand, Randall E. and Porter, Marc D.}, year={2011}, month={Apr}, pages={2554–2561} }
 @article{won ha_sun_wang_fang_2011, title={Differential interference contrast polarization anisotropy for tracking rotational dynamics of gold nanorods}, volume={47}, ISSN={1359-7345 1364-548X}, url={http://dx.doi.org/10.1039/c1cc11679g}, DOI={10.1039/c1cc11679g}, abstractNote={We describe differential interference contrast (DIC) polarization anisotropy for tracking rotational dynamics of gold nanorod (AuNR) probes. DIC polarization anisotropy enabled us to reveal the unidirectional clockwise circular translocation of an AuNR attached to a kinesin-driven microtubule and to precisely determine the real-time orientation of the AuNR during the dynamic process.}, number={27}, journal={Chemical Communications}, publisher={Royal Society of Chemistry (RSC)}, author={Won Ha, Ji and Sun, Wei and Wang, Gufeng and Fang, Ning}, year={2011}, pages={7743} }
 @article{sun_gu_wang_fang_2011, title={Dual-Modality Single Particle Orientation and Rotational Tracking of Intracellular Transport of Nanocargos}, volume={84}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac202824v}, DOI={10.1021/ac202824v}, abstractNote={The single particle orientation and rotational tracking (SPORT) technique was introduced recently to follow the rotational motion of plasmonic gold nanorod under a differential interference contrast (DIC) microscope. In biological studies, however, cellular activities usually involve a multiplicity of molecules; thus, tracking the motion of a single molecule/object is insufficient. Fluorescence-based techniques have long been used to follow the spatial and temporal distributions of biomolecules of interest thanks to the availability of multiplexing fluorescent probes. To know the type and number of molecules and the timing of their involvement in a biological process under investigation by SPORT, we constructed a dual-modality DIC/fluorescence microscope to simultaneously image fluorescently tagged biomolecules and plasmonic nanoprobes in living cells. With the dual-modality SPORT technique, the microtubule-based intracellular transport can be unambiguously identified while the dynamic orientation of nanometer-sized cargos can be monitored at video rate. Furthermore, the active transport on the microtubule can be easily separated from the diffusion before the nanocargo docks on the microtubule or after it undocks from the microtubule. The potential of dual-modality SPORT is demonstrated for shedding new light on unresolved questions in intracellular transport.}, number={2}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Sun, Wei and Gu, Yan and Wang, Gufeng and Fang, Ning}, year={2011}, month={Dec}, pages={1134–1138} }
 @article{fang_wang_sun_2011, title={Rotational Motions of Endocytic Cargos Revealed by Single Particle Orientation and Rotation Tracking (Sport)}, volume={100}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2010.12.2407}, DOI={10.1016/j.bpj.2010.12.2407}, abstractNote={Single-particle tracking (SPT) is a powerful approach to probe biological processes at molecular level in live cells. However, conventional SPT techniques are still having difficulties in reporting the orientation and rotational motion of the probe thus the target molecules of interest. Herein, we present an integrated imaging platform based on the combination of birefringent gold nanorod probes and differential interference contrast (DIC) microscopy that can achieve single particle orientation and rotation tracking (SPORT). This new technique allows us to resolve translational and rotational motions of gold nanorod probes at each stage throughout the clathrin-mediated endocytosis process, leading to the disclosure of their binding status on the cell membrane and characteristic rotational motions generated by protein modules.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Fang, Ning and Wang, Gufeng and Sun, Wei}, year={2011}, month={Feb}, pages={405a–406a} }
 @article{gu_sun_wang_fang_2011, title={Single Particle Orientation and Rotation Tracking Discloses Distinctive Rotational Dynamics of Drug Delivery Vectors on Live Cell Membranes}, volume={133}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja200603x}, DOI={10.1021/ja200603x}, abstractNote={Engineered nanoparticles have emerged as potentially revolutionary drug and gene delivery vectors. Using rod-shaped gold nanoparticles as a model, we studied for the first time the rotational dynamics of nanoparticle vectors on live cell membranes and its impact on the fate of these nanoparticle vectors. The rotational motions of gold nanorods with various surface modifiers were tracked continuously at 200 frames/s under a differential interference contrast microscope. We found that the rotational behaviors of gold nanorod vectors are strongly related to their surface charges. Specific surface functional groups and the availability of receptors on cell membranes also contribute to the rotational dynamics. The study of rotational brownian motion of nanoparticles on cell membranes will lead to a better understanding of the mechanisms of drug delivery and provide guidance in designing surface modification strategies for drug delivery vectors under various circumstances.}, number={15}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Gu, Yan and Sun, Wei and Wang, Gufeng and Fang, Ning}, year={2011}, month={Apr}, pages={5720–5723} }
 @article{luo_sun_liu_wang_fang_2011, title={Superlocalization of Single Molecules and Nanoparticles in High-Fidelity Optical Imaging Microfluidic Devices}, volume={83}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac201056z}, DOI={10.1021/ac201056z}, abstractNote={Superlocalization of single molecules and nanoparticles with a precision of subnanometer to a few tens of nanometers is crucial for elucidating nanoscale structures and movements in biological and chemical systems. A novel design of ultraflat and ultrathin glass/polydimethylsiloxane (PDMS) hybrid microdevices is introduced to provide almost uncompromised optical imaging quality for on-chip superlocalization and super-resolution imaging of single molecules and nanoparticles under a variety of microscopy modes. The performance of the high-fidelity (Hi-Fi) optical imaging microfluidic device was validated by precisely mapping micronecklaces made of fluorescent microtubules and 40 nm gold nanoparticles and by demonstrating the activation and excitation cycles of single Alexa Fluor 647 dyes for direct stochastic optical reconstruction microscopy in PDMS-based microchannels for the first time. Furthermore, the microdevice's feasibility for multimodality microscopy imaging was demonstrated by a vertical scan of live cells in epi-fluorescence and differential interference contrast (DIC) microscopy modes simultaneously.}, number={13}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Luo, Yong and Sun, Wei and Liu, Chang and Wang, Gufeng and Fang, Ning}, year={2011}, month={Jul}, pages={5073–5077} }
 @article{gu_sun_wang_fang_2011, title={Understanding Nanoparticle Drug Delivery from Rotational Dynamics and Behaviors of Functionalized Gold Nanorods on Live Cell Membranes}, volume={100}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2010.12.2771}, DOI={10.1016/j.bpj.2010.12.2771}, abstractNote={Gold nanoparticles with distinctive surface properties share different interactions with the cell membranes, thus show different activities in drug delivery. We used gold nanoparticles themselves as non-blinking, non-photobleaching nanoprobes to study nanoparticle drug delivery. Unlike most single particle tracking experiments that studied the translational motions of particles, we tracked the real-time rotational behaviors of single gold nanorods on live A549 cell membranes. The rotations of differently functionalized gold nanorods are tracked constantly under a Differential Interference Contrast (DIC) Microscope at 200 frames per second. The time series of DIC intensities of the images reflect the rotational dynamics of the gold nanorods. The in-plane and out-of-plane rotations were characterized by calculating the correlation coefficients between bright part DIC intensities and dark part DIC intensities of the gold nanorod. The autocorrelations of the image contrast time-series were calculated, and the rotation characteristic times over time of observation were derived by a non-linear fitting. We found that on one hand, the rotational behaviors of gold nanorod probes are strongly related to the surface charges of the gold nanorods, such as positively charged gold nanorods (PEIs modified, TAT modified, and CTAB stabilized gold nanorods) show a stronger interaction with cell membranes than negative ones (tranferrin modified and carboxylic gold nanorods). On the other hand, specific surface functional groups and availabilities of receptors on cell membranes also contribute to the rotational dynamics of the gold nanorods (such as shown in the differences between transferring modified gold nanorods and carboxylic gold nanorods). This study of nanoparticle rotational diffusion on cell membranes will lead to better understanding of the mechanisms of drug delivery and provide guidance in designing the modification strategies for drug delivery agents under different circumstances.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Gu, Yan and Sun, Wei and Wang, Gufeng and Fang, Ning}, year={2011}, month={Feb}, pages={473a} }
 @article{sun_xu_marchuk_wang_fang_2011, title={Whole-Cell Scan Using Automatic Variable-Angle and Variable-Illumination-Depth Pseudo–Total Internal Reflection Fluorescence Microscopy}, volume={16}, ISSN={2211-0682}, url={http://dx.doi.org/10.1016/j.jala.2010.10.001}, DOI={10.1016/j.jala.2010.10.001}, abstractNote={An automatic calibration and angle-scanning prism-type total internal reflection fluorescence microscope (TIRFM) was modified to function in both TIRFM and pseudo-TIRFM modes. When the incident angle of the excitation laser beam was controlled to be larger than the critical angle, the instrument served as a variable-angle TIRFM. A homemade computer program automatically calibrates the laser illumination spot in the sample to overlap with the center of the microscope's field of view. Then, by measuring the fluorescence intensities at different incident angles, the z-positions of fluorescent nanospheres close to the cell basolateral membrane can be extracted. When the incident angle is reduced to be in the subcritical range, the instrument works as a pseudo-TIRFM. The whole cell body from bottom to top can be imaged in a vertical scan process. Furthermore, the illumination field depth in the pseudo-TIRFM can be controlled by changing the incident angle or the horizontal position of the laser spot.}, number={4}, journal={Journal of Laboratory Automation}, publisher={SAGE Publications}, author={Sun, Wei and Xu, Aoshuang and Marchuk, Kyle and Wang, Gufeng and Fang, Ning}, year={2011}, month={Aug}, pages={255–262} }
 @article{sun_marchuk_wang_fang_2010, title={Autocalibrated Scanning-Angle Prism-Type Total Internal Reflection Fluorescence Microscopy for Nanometer-Precision Axial Position Determination}, volume={82}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac902789z}, DOI={10.1021/ac902789z}, abstractNote={An automatic calibration and scanning-angle prism-type total internal reflection fluorescence microscope (TIRFM) was constructed and tested for the highest vertical resolution. The angle of the incident laser beam can be changed automatically and reliably from subcritical angles to nearly 90 degrees with intervals smaller than 0.2 degrees, and the laser illumination spot in the sample can be calibrated to automatically overlap with the center of the microscope's field of view. By scanning through a wide range of incident angles with different evanescent-field layer thicknesses, the fluorescence intensity decay curves of randomly distributed fluorescent nanospheres in agarose gel were obtained and fitted with the theoretical decay functions to determine their vertical positions. The best axial resolution was demonstrated to be better than 10 nm under the rigorous statistical analysis of confidence levels and by the Monte Carlo simulation. The new setup was further utilized to determine the tilting angle of the microtubules buried in agarose gel and to find the precise surface plasmon resonance (SPR) angle for gold film enhanced TIRFM. We demonstrate the new microscope's unique capability to find the best illumination configuration for complex systems automatically and reproducibly.}, number={6}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Sun, Wei and Marchuk, Kyle and Wang, Gufeng and Fang, Ning}, year={2010}, month={Mar}, pages={2441–2447} }
 @article{stender_wang_sun_fang_2010, title={Influence of Gold Nanorod Geometry on Optical Response}, volume={4}, ISSN={1936-0851 1936-086X}, url={http://dx.doi.org/10.1021/nn102500s}, DOI={10.1021/nn102500s}, abstractNote={As noble metal nanoparticles are deployed into increasingly sophisticated environments, it is necessary to fully develop our understanding of nanoparticle behavior and the corresponding instrument responses. In this paper, we report on the optical response of three important gold nanorod configurations under dark field and differential interference contrast (DIC) microscopy after first establishing their absolute geometries with transmission electron microscopy (TEM). The observed longitudinal plasmon wavelengths of single nanorods are located at wavelengths consistent with previously developed theory. A dimer is shown exhibiting a multipole plasmon at wavelengths that are consistent with the dipole plasmon of single nanorods in the sample. DIC can also distinguish a single nanorod from a pair of uncoupled nanorods with an interparticle distance below the diffraction limit. The experimental observations are consistent with simulated DIC images using a DIC point spread function. The findings herein are a critical step toward being able to characterize nanorods in dynamic environments without the use of electron microscopy.}, number={12}, journal={ACS Nano}, publisher={American Chemical Society (ACS)}, author={Stender, Anthony S. and Wang, Gufeng and Sun, Wei and Fang, Ning}, year={2010}, month={Nov}, pages={7667–7675} }
 @article{wang_stender_sun_fang_2010, title={Optical imaging of non-fluorescent nanoparticleprobes in live cells}, volume={135}, ISSN={0003-2654 1364-5528}, url={http://dx.doi.org/10.1039/b916395f}, DOI={10.1039/b916395f}, abstractNote={Precise imaging of cellular and subcellular structures and dynamic processes in live cells is crucial for fundamental research in life sciences and in medical applications. Non-fluorescent nanoparticles are an important type of optical probe used in live-cell imaging due to their photostability, large optical cross-sections, and low toxicity. Here, we provide an overview of recent developments in the optical imaging of non-fluorescent nanoparticle probes in live cells.}, number={2}, journal={The Analyst}, publisher={Royal Society of Chemistry (RSC)}, author={Wang, Gufeng and Stender, Anthony S. and Sun, Wei and Fang, Ning}, year={2010}, pages={215–221} }
 @article{wang_sun_luo_fang_2010, title={Resolving Rotational Motions of Nano-objects in Engineered Environments and Live Cells with Gold Nanorods and Differential Interference Contrast Microscopy}, volume={132}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja106506k}, DOI={10.1021/ja106506k}, abstractNote={Gold nanorods are excellent orientation probes due to their anisotropic optical properties. Their dynamic rotational motion in the 3D space can be disclosed with Nomarski-type differential interference contrast (DIC) microscopy. We demonstrate that by using the combination of gold nanorod probes and DIC microscopy, we are able to resolve rotational motions of nano-cargos transported by motor proteins at video rate not only on engineered surfaces but also on cytoskeleton tracks in live cells.}, number={46}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Wang, Gufeng and Sun, Wei and Luo, Yong and Fang, Ning}, year={2010}, month={Nov}, pages={16417–16422} }
 @article{luo_sun_gu_wang_fang_2010, title={Wavelength-Dependent Differential Interference Contrast Microscopy: Multiplexing Detection Using Nonfluorescent Nanoparticles}, volume={82}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac101336d}, DOI={10.1021/ac101336d}, abstractNote={The wavelength dependence of plasmonic nanoparticles' contrasts in differential interference contrast (DIC) microscopy has been exploited previously for unambiguous identification and dynamic tracking of these nanoprobes in complex environments (Anal. Chem. 2009, 81, 9203-9208). In the present study, the suitability of multiplexing detection in DIC microscopy was investigated systematically with 19 kinds of nanoparticles of different materials and/or sizes. A unique DIC contrast spectrum was found for each kind of nanoparticle. Multiplexing detection was accomplished by measuring DIC contrasts at a minimum of two specific illumination wavelengths. The main advantages of DIC microscopy for multiplexing detection over other nonfluorescence techniques, such as dark field microscopy and surface-enhanced Raman scattering, were demonstrated by differentiating four kinds of nanoparticles on the cell membrane while providing high-contrast images of both the nanoprobes and cell features.}, number={15}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Luo, Yong and Sun, Wei and Gu, Yan and Wang, Gufeng and Fang, Ning}, year={2010}, month={Aug}, pages={6675–6679} }
 @article{wang_driskell_porter_lipert_2009, title={Control of Antigen Mass Transport via Capture Substrate Rotation: Binding Kinetics and Implications on Immunoassay Speed and Detection Limits}, volume={81}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac900704t}, DOI={10.1021/ac900704t}, abstractNote={In conventional heterogeneous immunoassays, assay speed is usually limited by the rate of mass transport, i.e., diffusion of antigen to an antibody-coated surface. We previously demonstrated that assay speed can be significantly increased, without losing analytical sensitivity, by rapidly rotating the capture substrate, which decreases the thickness of the diffusion layer. In this work, we raised the rotation speed and observed that the capture of antigens deviates from the mass transport-limited assumption. To examine this issue, a general equation was derived for the rate of immuno-reaction on a rotating capture surface that takes into account both diffusion and the rate of reaction between antigen and antibody, which applies over a wide range of rotation rates. Results show that by vigorously rotating the substrate, the binding of antigens reaches a regime of intermediate binding kinetics, for which mass transport is comparable to the reaction rate. With this general solution, we are able to determine the two important binding kinetics parameters: the diffusion coefficient and the reaction rate constant. Then, using porcine parvovirus as an example, we use these parameters to investigate the limit of the assay speed and the limit of detection achievable on a practical time scale through numerical simulations of the kinetic binding curves for various assay conditions.}, number={15}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Wang, Gufeng and Driskell, Jeremy D. and Porter, Marc D. and Lipert, Robert J.}, year={2009}, month={Aug}, pages={6175–6185} }
 @article{wang_park_lipert_porter_2009, title={Mixed Monolayers on Gold Nanoparticle Labels for Multiplexed Surface-Enhanced Raman Scattering Based Immunoassays}, volume={81}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac901711f}, DOI={10.1021/ac901711f}, abstractNote={This paper describes a new approach, based on self-assembled mixed monolayers, to the design and preparation of extrinsic Raman labels (ERLs). ERLs function as spectroscopic tags for the readout of sandwich-type immunoassays using surface-enhanced Raman scattering (SERS). They are created by coating gold nanoparticles with Raman reporter molecules and antibodies specific for the target analyte. Mixed-monolayer ERLs are formed by covering gold nanoparticles with a mixture of two different thiolates. One thiolate serves to covalently bind antibodies to the particles, imparting biospecificity to the ERLs, while the other thiolate produces a strong Raman signal. Mixed-monolayer ERLs can be prepared in a few relatively simple steps using readily available materials. The SERS intensity of each type of ERL can be tuned to match other ERLs by adjusting the mixed monolayer composition, greatly facilitating the generation of sets of ERLs for multiplexed applications. The work herein not only describes the new pathway for ERL production, but also demonstrates the simultaneous qualitative and quantitative multiplexed detection using a set of four mixed-monolayer ERLs.}, number={23}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Wang, Gufeng and Park, Hye-Young and Lipert, Robert J. and Porter, Marc D.}, year={2009}, month={Dec}, pages={9643–9650} }
 @article{sun_wang_fang_yeung_2009, title={Wavelength-Dependent Differential Interference Contrast Microscopy: Selectively Imaging Nanoparticle Probes in Live Cells}, volume={81}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac901623b}, DOI={10.1021/ac901623b}, abstractNote={Gold and silver nanoparticles display extraordinarily large apparent refractive indices near their plasmon resonance (PR) wavelengths. These nanoparticles show good contrast in a narrow spectral band but are poorly resolved at other wavelengths in differential interference contrast (DIC) microscopy. The wavelength dependence of DIC contrast of gold/silver nanoparticles is interpreted in terms of Mie's theory and DIC working principles. We further exploit this wavelength dependence by modifying a DIC microscope to enable simultaneous imaging at two wavelengths. We demonstrate that gold/silver nanoparticles immobilized on the same glass slides through hybridization can be differentiated and imaged separately. High-contrast, video-rate images of living cells can be recorded both with and without illuminating the gold nanoparticle probes, providing definitive probe identification. Dual-wavelength DIC microscopy thus presents a new approach to the simultaneous detection of multiple probes of interest for high-speed live-cell imaging.}, number={22}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Sun, Wei and Wang, Gufeng and Fang, Ning and Yeung, Edward S.}, year={2009}, month={Nov}, pages={9203–9208} }
 @article{porter_lipert_siperko_wang_narayanan_2008, title={SERS as a bioassay platform: fundamentals, design, and applications}, volume={37}, ISSN={0306-0012 1460-4744}, url={http://dx.doi.org/10.1039/b708461g}, DOI={10.1039/b708461g}, abstractNote={Bioanalytical science is experiencing a period of unprecedented growth. Drivers behind this growth include the need to detect markers central to human and veterinary diagnostics at ever-lower levels and greater speeds. A set of parallel arguments applies to pathogens with respect to bioterrorism prevention and food and water safety. This tutorial review outlines our recent explorations on the use of surface enhanced Raman scattering (SERS) for detection of proteins, viruses, and microorganisms in heterogeneous immunoassays. It will detail the design and fabrication of the assay platform, including the capture substrate and nanoparticle-based labels. The latter, which is the cornerstone of our strategy, relies on the construction of gold nanoparticles modified with both an intrinsically strong Raman scatterer and an antibody. This labelling motif, referred to as extrinsic Raman labels (ERLs), takes advantage of the well-established signal enhancement of scatterers when coated on nanometre-sized gold particles, whereas the antibody imparts antigenic specificity. We will also examine the role of plasmon coupling between the ERLs and capture substrate, and challenges related to particle stability, nonspecific adsorption, and assay speed.}, number={5}, journal={Chemical Society Reviews}, publisher={Royal Society of Chemistry (RSC)}, author={Porter, Marc D. and Lipert, Robert J. and Siperko, Lorraine M. and Wang, Gufeng and Narayanan, Radha}, year={2008}, pages={1001} }
 @article{wang_gao_geng_2006, title={Analysis of heterogeneous fluorescence decays in proteins. Using fluorescence lifetime of 8-anilino-1-naphthalenesulfonate to probe apomyoglobin unfolding at equilibrium}, volume={1760}, ISSN={0304-4165}, url={http://dx.doi.org/10.1016/j.bbagen.2006.02.019}, DOI={10.1016/j.bbagen.2006.02.019}, abstractNote={The solvatochromic fluorescent dye 8-anilino-1-naphthalenesulfonate (ANS) is one of the popular probes of protein folding. Folding kinetics is tracked with ANS fluorescence intensity, usually interpreted as a reflection of protein structure—the hydrophobicity of the binding environments. Such simplistic view overlooks the complicated nature of ANS–protein complexes: the fluorescence characteristics are convoluted results of the ground state populational distribution of the probe–protein complex, the structural changes in the protein and the excited state photophysics of the probe. Understanding of the interplay of these aspects is crucial in accurate interpretation of the protein dynamics. In this work, the fluorescence decay of ANS complexed with apomyoglobin at different conformations denatured by pH is modeled. The fluorescence decay of the ANS–apomyoglobin complex contains information on not only apomyoglobin structure but also molecular populational distributions. The challenge in modeling fluorescence decay profiles originates from the convolution of heterogeneous binding and excited-state relaxation of the fluorescent probe. We analyzed frequency-domain fluorescence lifetime data of ANS-apomyoglobin with both maximum entropy methods (MEM) and nonlinear least squares methods (NLLS). MEM recovers a model of two expanding-and-merging lifetime distributions for ANS–apomyoglobin in the equilibrium transition from the native (N) through an intermediate (I-1) to the acid-unfolded state UA. At pH 6.5 and above, when apomyoglobin is mostly populated at the N-state, ANS–apomyoglobin emits a predominant long-lifetime fluorescence from a relaxed charge transfer state S1,CT of ANS, and a short-lifetime fluorescence that is mainly from a nascent excited-state S1,np of ANS stabilized by the strong ANS–apomyoglobin interaction. Lowering the pH diminishes the contribution from the S1,np state. Meanwhile, more protein molecules become populated at the UA state, which exhibits a short lifetime that is not distinguishable from the S1,np state. At pH 3.4, when the population of the UA becomes significant, the short-lifetime fluorescence comes predominantly from ANS binding to the UA. Further lowering the pH leads to more exposure of the bound ANS. The long lifetime shifts toward and finally merges with the short lifetime and becomes one broad distribution that stands for ANS binding to the UA below pH 2.4. The above expanding-and-merging model is consistent with F-statistic analysis of NLLS models. The consistency of this model with the knowledge from the literature, as well as the continuity of the decay parameters changing upon experimental conditions are also crucial in drawing the conclusions.}, number={7}, journal={Biochimica et Biophysica Acta (BBA) - General Subjects}, publisher={Elsevier BV}, author={Wang, Gufeng and Gao, Yi and Geng, M. Lei}, year={2006}, month={Jul}, pages={1125–1137} }
 @article{wang_gao_geng_2006, title={Generalized Two-Dimensional Heterocorrelation Analysis of Spectrally Resolved and Temporally Resolved Fluorescence of the 8-Anilino-1-naphthalenesulfonate−Apomyoglobin Complex with pH Perturbation}, volume={110}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/jp0555293}, DOI={10.1021/jp0555293}, abstractNote={We demonstrate two-dimensional heterocorrelation analysis between spectrally resolved and temporally resolved fluorescence to investigate the decay dynamics of the 8-anilino-1-naphthalenesulfonate- (ANS-) apomyoglobin complex. The dynamic changes of the lifetime components are disclosed across the emission spectrum with an external pH-perturbation. Two different fluorescence lifetime schemes of the ANS-apomyoglobin complex are revealed. From pH 8.5 to 4.5, the transition of protein conformation from the native state to the folding intermediate, a short lifetime component is found to correlate with a short-wavelength emission whose population diminishes with decreasing pH. The lifetime components reflect the excited-state populations of the nascent and the charge-transfer species. From pH 4.2 to 1.0, the transition from the folding intermediate to the acid-unfolded state, the short lifetime is responsible for a long-wavelength emission and the fraction of this component increases when the solution becomes more acidic. In this pH range, the decay components reflect the ground-state populations of microenvironments. The relative decay dynamics across the emission spectrum are revealed without collecting decays at each wavelength. More importantly, these conclusions are reached without the necessity of statistical fitting of the decay data with an a priori decay model.}, number={16}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Wang, Gufeng and Gao, Yi and Geng, M. Lei}, year={2006}, month={Apr}, pages={8506–8512} }
 @article{zhong_wang_geng_2006, title={Probing strong adsorption of individual solute molecules at solid/liquid interfaces with model-based statistical two-dimensional correlation analysis}, volume={799}, ISSN={0022-2860}, url={http://dx.doi.org/10.1016/j.molstruc.2006.04.031}, DOI={10.1016/j.molstruc.2006.04.031}, abstractNote={Single-molecule spectroscopy is a powerful approach to studying adsorption phenomena at solid/liquid interfaces that are responsible for peak tailing in chromatography. For complete understanding of adsorption properties, a statistically significant number of strong adsorption events of single molecules need to be identified and their corresponding desorption times be determined. In this paper, model-based statistical two-dimensional (2D) correlation analysis is used to search/identify individual strong adsorption events from large data sets of single-molecule photon bursts. It is accomplished by correlating the single molecule photon-burst data with a model adsorption function. The desorption times are evaluated by varying the model size and moving the model function along time axis in 2D correlation analysis. It is shown that 2D correlation analysis can reliably determine strong adsorption events where many other molecules are diffusing through the laser probe volume during the adsorption of one molecule, and/or the adsorbed molecule experiences complex photophysical behaviors such as triplet blinking and spectral diffusion.}, number={1-3}, journal={Journal of Molecular Structure}, publisher={Elsevier BV}, author={Zhong, Zhenming and Wang, Gufeng and Geng, M. Lei}, year={2006}, month={Nov}, pages={204–210} }
 @article{skvortsova_wang_geng_2006, title={Statistical two-dimensional correlation coefficient mapping of simulated tissue phantom data: Boundary determination in tissue classification for cancer diagnosis}, volume={799}, ISSN={0022-2860}, url={http://dx.doi.org/10.1016/j.molstruc.2006.04.005}, DOI={10.1016/j.molstruc.2006.04.005}, abstractNote={Abstract Statistical correlation coefficient mapping has proven to be a useful technique in tissue classification for cancer diagnosis. The classification is achieved by comparing the correlation coefficients for an unknown to a set of selected tissue samples with known pathological conditions. Currently, the correlation coefficient threshold in the classification is empirically determined. In this paper, boundaries of statistical significance between different tissue pathological conditions are established through Bayesian analysis on the Fisher’s z-transformed Pearson’s correlation coefficients between tissue samples. Moreover, probability values are provided in assigning a tissue sample to a specific tissue clinical condition, which is more appreciable in clinical practices. The methodology is examined with a simulated tissue-phantom data set, yielding satisfactory diagnostic results.}, number={1-3}, journal={Journal of Molecular Structure}, publisher={Elsevier BV}, author={Skvortsova, Yulia and Wang, Gufeng and Geng, M. Lei}, year={2006}, month={Nov}, pages={239–246} }
 @article{wang_karnes_bunker_lei geng_2006, title={Two-dimensional correlation coefficient mapping in gas chromatography: Jet fuel classification for environmental analysis}, volume={799}, ISSN={0022-2860}, url={http://dx.doi.org/10.1016/j.molstruc.2006.04.006}, DOI={10.1016/j.molstruc.2006.04.006}, abstractNote={We demonstrate the feasibility of using two-dimensional correlation coefficient mapping to classify gas chromatograms of environmental hazards. Correct identification and classification of the contaminants is the prerequisite for their appropriate treatment and containments. A data set consisting of 76 gas chromatograms of eight types of jet fuels, which are common sources of hydrocarbon contamination in ground water, is examined with two-dimensional statistical sample–sample correlation coefficients. Analysis demonstrates that jet fuel samples of the same type correlate strongly with each other but less significantly with other jet fuel classes. According to the magnitude of the correlation coefficients between each pair of the samples, jet fuel types of each sample in the data set can be assigned with an accuracy of 100% through a leave-one-out cross validation (LOOCV) procedure. Correlation coefficient mapping is thus a promising method to classify samples of environmental importance.}, number={1-3}, journal={Journal of Molecular Structure}, publisher={Elsevier BV}, author={Wang, Gufeng and Karnes, John and Bunker, Christopher E. and Lei Geng, M.}, year={2006}, month={Nov}, pages={247–252} }
 @article{wang_geng_2006, title={Unfolding of apomyoglobin studied with two-dimensional correlations of tryptophan, 8-anilino-1-naphthalenesulfonate, and pyrene fluorescence}, volume={799}, ISSN={0022-2860}, url={http://dx.doi.org/10.1016/j.molstruc.2006.04.007}, DOI={10.1016/j.molstruc.2006.04.007}, abstractNote={Acid-induced unfolding of horse apomyoglobin is studied with fluorescence of tryptophan residues and extrinsic probes 8-anilino-1-naphthalenesulfonate (ANS) and pyrene. Two-dimensional statistical correlation coefficient analysis of tryptophan, ANS and pyrene fluorescence reveals that apomyoglobin unfolds in two steps via a folding intermediate. Both tryptophan and ANS fluorescence show microenvironment-sensitive emission maximum and fluorescence intensity. Hetero-spectral 2D correlation analysis reveals that the “red” tryptophan fluorescence correlates with the “red” ANS fluorescence, and the “blue” correlates with the “blue” in the acid-induced unfolding process. Correlation curves are established between the hydrophobicity of the probe-binding site(s) as disclosed by ANS and pyrene fluorescence and the folding extent of apomyoglobin as indicated by tryptophan fluorescence. Two anion-induced refolding intermediates of apomyoglobin: Cl−-induced A-1, and trichloroacetate (TCA)-induced I-2 do not follow the correlation trajectory in the acid-induced unfolding. This suggests that the two anion-induced intermediates do not belong to the low-salt acid-unfolding pathway of apomyoglobin.}, number={1-3}, journal={Journal of Molecular Structure}, publisher={Elsevier BV}, author={Wang, Gufeng and Geng, M. Lei}, year={2006}, month={Nov}, pages={177–187} }
 @article{wang_lowry_zhong_geng_2005, title={Direct observation of frits and dynamic air bubble formation in capillary electrochromatography using confocal fluorescence microscopy}, volume={1062}, ISSN={0021-9673}, url={http://dx.doi.org/10.1016/j.chroma.2004.11.052}, DOI={10.1016/j.chroma.2004.11.052}, abstractNote={Confocal fluorescence microscopy has been used to study the capillary electrochromatography (CEC) frits and dynamic air bubble formation under real chromatographic conditions. Confocal fluorescence microscopy provides a nondestructive way to view the three-dimensional structure of the frits with high spatial resolution. Frits prepared with four different procedures were studied: (1) sintering bare silica beads with sodium silicate; (2) sintering bare silica beads wetted with water; (3) sintering C18 beads wetted with water; and (4) sintering C18 beads wetted with water and then surfaced-recovered with C18. Frits prepared with sintering silicate-wetted beads have a high degree of heterogeneity, while the other three types of frits have similar, more homogeneous packing structures. Confocal fluorescence microscopy also provides sufficient temporal resolution for in situ observation of the dynamic processes in air bubble formation. In this study, air bubble formation is imaged during the reorganization process of the packing bed and is shown to occur close to the border between the packing bed and the outlet frit. Confocal fluorescence microscopy opens a new avenue in studying dynamic processes in situ in CEC separations.}, number={2}, journal={Journal of Chromatography A}, publisher={Elsevier BV}, author={Wang, Gufeng and Lowry, Mark and Zhong, Zhenming and Geng, Lei}, year={2005}, month={Jan}, pages={275–283} }
 @article{zhong_lowry_wang_geng_2005, title={Probing Strong Adsorption of Solute onto C18-Silica Gel by Fluorescence Correlation Imaging and Single-Molecule Spectroscopy under RPLC Conditions}, volume={77}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac048290f}, DOI={10.1021/ac048290f}, abstractNote={Understanding molecular adsorption at a chromatographic interface is of great interest for addressing the tailing problem in chemical separations. Single-molecule spectroscopy and confocal fluorescence correlation imaging are used to study the adsorption sites of C(18) silica beads under RPLC chromatographic conditions. The experiments show that cationic molecule rhodamine 6G laterally diffuses through the chromatographic interface of a C(18) hydrocarbon monolayer and acetonitrile with occasional reversible strong adsorptions. Fluorescence correlation imaging extracts the rare strong adsorption events from large data sets, revealing that the strong adsorption sites are randomly distributed throughout the silica beads. Virtually every imaging pixel of silica beads adsorbs molecules. Single-molecule spectroscopy of the 584 strong adsorption events observed indicates that the strong adsorptions persist on the time scales from several milliseconds to seconds, having an average desorption time of 61 ms. The strong adsorption events are rare, comprising 0.3% of the total observation time. The sizes of strong adsorption sites are within the optical resolution of confocal imaging.}, number={8}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Zhong, Zhenming and Lowry, Mark and Wang, Gufeng and Geng, Lei}, year={2005}, month={Apr}, pages={2303–2310} }
 @article{wang_geng_2005, title={Statistical and Generalized Two-Dimensional Correlation Spectroscopy of Multiple Ionization States. Fluorescence of Neurotransmitter Serotonin}, volume={77}, ISSN={0003-2700 1520-6882}, url={http://dx.doi.org/10.1021/ac0492362}, DOI={10.1021/ac0492362}, abstractNote={Fluorescence spectra of neurotransmitter serotonin are analyzed with generalized and statistical two-dimensional correlation spectroscopy. A comparison is provided for these two emerging data analysis techniques. Both methods reveal correlations between spectral variables and demonstrate enhanced sensitivity in detecting the dynamic spectral changes over conventional one-dimensional spectroscopy. Both statistical and generalized 2D correlation analysis emphasize simultaneous spectral changes in response to external perturbations. Generalized 2D correlation spectroscopy further reveals the difference in rates of these dynamic changes. Using 2D correlation analysis, a third ionization species of serotonin is identified using pH and excitation wavelength perturbation. This species is a doubly deprotonated serotonin with very low fluorescence quantum yield, confirmed by using a laser excitation at longer wavelength and at higher pH. Taking advantage of the spectral differences between excitation of serotonin and tryptophan, as low as 3.8 nM serotonin can be detected in the presence of 20 microM tryptophan, with long-wavelength excitation. This represents the sensitive detection of serotonin in 5000-fold excess of tryptophan.}, number={1}, journal={Analytical Chemistry}, publisher={American Chemical Society (ACS)}, author={Wang, Gufeng and Geng, Lei}, year={2005}, month={Jan}, pages={20–29} }