@article{bai_xu_zhang_liu_rojas_2015, title={Dynamics of Cyclodimerization and Viscoelasticity of Photo-Crosslinkable PVA}, volume={53}, ISSN={["1099-0488"]}, DOI={10.1002/polb.23634}, abstractNote={ABSTRACT}, number={5}, journal={JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS}, author={Bai, Huiyu and Xu, Jing and Zhang, Yanxia and Liu, Xiaoya and Rojas, Orlando J.}, year={2015}, month={Mar}, pages={345–355} }
 @article{langdon_mirhossaini_mabry_sriram_lajmi_zhang_rojas_schwartz_2015, title={Single-Molecule Resolution of Protein Dynamics on Polymeric Membrane Surfaces: The Roles of Spatial and Population Heterogeneity}, volume={7}, ISSN={["1944-8252"]}, DOI={10.1021/am507730k}, abstractNote={Although polymeric membranes are widely used in the purification of protein pharmaceuticals, interactions between biomolecules and membrane surfaces can lead to reduced membrane performance and damage to the product. In this study, single-molecule fluorescence microscopy provided direct observation of bovine serum albumin (BSA) and human monoclonal antibody (IgG) dynamics at the interface between aqueous buffer and polymeric membrane materials including regenerated cellulose and unmodified poly(ether sulfone) (PES) blended with either polyvinylpyrrolidone (PVP), polyvinyl acetate-co-polyvinylpyrrolidone (PVAc-PVP), or polyethylene glycol methacrylate (PEGM) before casting. These polymer surfaces were compared with model surfaces composed of hydrophilic bare fused silica and hydrophobic trimethylsilane-coated fused silica. At extremely dilute protein concentrations (10(-3)-10(-7) mg/mL), protein surface exchange was highly dynamic with protein monomers desorbing from the surface within ∼1 s after adsorption. Protein oligomers (e.g., nonspecific dimers, trimers, or larger aggregates), although less common, remained on the surface for 5 times longer than monomers. Using newly developed super-resolution methods, we could localize adsorption sites with ∼50 nm resolution and quantify the spatial heterogeneity of the various surfaces. On a small anomalous subset of the adsorption sites, proteins adsorbed preferentially and tended to reside for significantly longer times (i.e., on "strong" sites). Proteins resided for shorter times overall on surfaces that were more homogeneous and exhibited fewer strong sites (e.g., PVAc-PVP/PES). We propose that strong surface sites may nucleate protein aggregation, initiated preferentially by protein oligomers, and accelerate ultrafiltration membrane fouling. At high protein concentrations (0.3-1.0 mg/mL), fewer strong adsorption sites were observed, and surface residence times were reduced. This suggests that at high concentrations, adsorbed proteins block strong sites from further protein adsorption. Importantly, this demonstrates that strong binding sites can be modified by changing solution conditions. Membrane surfaces are intrinsically heterogeneous; by employing single-molecule techniques, we have provided a new framework for understanding protein interactions with such surfaces.}, number={6}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Langdon, Blake B. and Mirhossaini, Roya B. and Mabry, Joshua N. and Sriram, Indira and Lajmi, Ajay and Zhang, Yanxia and Rojas, Orlando J. and Schwartz, Daniel K.}, year={2015}, month={Feb}, pages={3607–3617} }
 @article{zhang_carbonell_rojas_2013, title={Bioactive Cellulose Nanofibrils for Specific Human IgG Binding}, volume={14}, ISSN={["1526-4602"]}, DOI={10.1021/bm4007979}, abstractNote={Bioactive films were produced by conjugation of a short peptide onto modified cellulose nanofibrils (CNF). Specifically, a hydrophilic copolymer, poly(2-aminoethyl methacrylate hydrochloride-co-2-hydroxyethylmethacrylate) (poly(AMA-co-HEMA)), was grafted via surface initiated polymerization from an initiator coupled to CNF. The poly(AMA-co-HEMA) was used as a spacer and support layer for immobilization of the peptide, acetylated-HWRGWVA, which has specific affinity with human immunoglobulin G (hIgG). Two methods for peptide grafting were compared: modification of CNF in aqueous suspension followed by assembly into a bioactive film and peptide grafting on a preformed CNF film. The CNF-based networks were examined on solid supports via atomic force microscopy (AFM) and extreme resolution imaging with ultralow electron landing energies (scanning low energy electron microscopy). The specific binding capability of hIgG and nonspecific protein resistance of the resultant peptide-modified CNF were evaluated by using quartz crystal microgravimetry (QCM). The effects of initiator concentration and thickness of poly(AMA-co-HEMA) layer on hIgG adsorption were investigated in the developed systems, which exhibited high signal-to-noise response.}, number={12}, journal={BIOMACROMOLECULES}, author={Zhang, Yanxia and Carbonell, Ruben G. and Rojas, Orlando J.}, year={2013}, month={Dec}, pages={4161–4168} }
 @article{zhang_islam_carbonell_rojas_2013, title={Specific Binding of Immunoglobulin G with Bioactive Short Peptides Supported on Antifouling Copolymer Layers for Detection in Quartz Crystal Microgravimetry and Surface Plasmon Resonance}, volume={85}, ISSN={["1520-6882"]}, DOI={10.1021/ac302874s}, abstractNote={A new peptide-based system supported on copolymer brushes grafted from gold sensors and with resistance to nonspecific adsorption is reported for selective binding of human immunoglobulin G (IgG). A random copolymer rich in primary amines, poly(2-aminoethyl methacrylate hydrochloride-co-2-hydroxyethyl methacrylate) (poly(AMA-co-HEMA)) was first grafted from initiator-coated gold substrates via activators regenerated by electron transfer-atom transfer radical polymerization (ARGET-ATRP), followed by immobilization of acetylated-HWRGWVA peptide, which has specific binding affinity with IgG. The peptide ligands covalently linked to the soft copolymer layer were characterized by X-ray photoelectron spectroscopy (XPS), water contact angle, ellipsometry, and atomic force microscopy (AFM). The extent of binding, binding affinity, and selectivity for target IgG molecules as well as the capability to minimize nonspecific interactions with other proteins were examined by fluorescence imaging, surface plasmon resonance (SPR), and quartz crystal microgravimetry (QCM). The effect of copolymer molecular composition and analyte concentration was elucidated in order to design systems based on immobilized peptides for high signal-to-noise response and detection limits that meet the requirements for IgG biosensing in fluid matrixes.}, number={2}, journal={ANALYTICAL CHEMISTRY}, author={Zhang, Yanxia and Islam, Nafisa and Carbonell, Ruben G. and Rojas, Orlando J.}, year={2013}, month={Jan}, pages={1106–1113} }
 @article{zhang_islam_carbonell_rojas_2013, title={Specificity and Regenerability of Short Peptide Ligands Supported on Polymer Layers for Immunoglobulin G Binding and Detection}, volume={5}, ISSN={["1944-8244"]}, DOI={10.1021/am4021186}, abstractNote={We demonstrate the specificity, regenerability, and excellent storage stability of short peptide-based systems for detection of immunoglobulin G (IgG). The bioactive component consisted of acetylated-HWRGWVA (Ac-HWRGWVA), a peptide with high IgG binding affinity, which was immobilized onto copolymer matrixes of poly(2-aminoethyl methacrylate hydrochloride-co-2-hydroxyethyl methacrylate) (poly(AMA-co-HEMA)). Surface plasmon resonance (SPR) and quartz crystal microgravimetry (QCM) were utilized with other complementary techniques to systematically investigate interfacial activities, mainly IgG binding performance as a function of the graft density and degree of polymerization of the poly(AMA-co-HEMA) support layer. Results from sodium dodecyl sulfate polyacrylamide gel electrophoresis and fluorescence microscopy indicate that the bioactive system is highly specific to IgG and resistant to nonspecific interactions when tested in mixed protein solutions.}, number={16}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Zhang, Yanxia and Islam, Nafisa and Carbonell, Ruben G. and Rojas, Orlando J.}, year={2013}, month={Aug}, pages={8030–8037} }