Changes in pH in biofluids such as urine, sweat, and blood can reveal pathophysiological conditions like tumor metastasis, microbial infection, acidosis, cystic fibrosis, wound healing, etc. Thus, monitoring pH could provide insights on those patho-physiological changes. Among others, sweat analysis provides convenient and non-invasive way to track personal health. Sweat pH changes can reveal information about atopic dermatitis, fungus infections, and cystic fibrosis. Thus, sweat pH measurements could aid in accurately identifying certain health issues. Although solution pH can be measured by various methods including electrochemical, colorimetric and fluorescence measurements, optical pH measurements have gained popularity in recent decades due to versatile material choices and new imaging technologies. Semiconducting single-walled carbon nanotubes (SWCNTs) are emerging molecular optical probes and sensors due to their outstanding photophysical properties, which include photo-stable near-infrared (nIR) fluorescence and sensitive molecular recognition. Here, we report that SWCNT-based sensors can reliably detect pH changes in a model biofluid - artificial sweat. The sensor exhibited significant responses within biologically relevant pH ranges within minutes through changes in the nIR fluorescence. The nanotubes' optical response to pH changes provides new opportunities to develop optical pH sensors for healthcare, bioengineering, environmental sciences, and chemistry.