Optical detection of pH changes in artificial sweat using near-infrared fluorescent nanomaterials

Abstract

Increasing demand for pH measurements for healthcare, food safety, environmental monitoring, and industrial treatments require molecular probes and sensors capable of measuring pH in specific environments. In biological systems, pH changes in tissues and biofluids including urine, sweat, and blood could indicate patho-physiological conditions such as tumor metastasis, microbial infection, acidosis, cystic fibrosis, wound healing, etc. Sweat analysis provides a non-invasive and convenient measure to monitor personal health. Thus, monitoring sweat pH could provide insights on patho-physiological changes. Optical pH measurements have gained significant interest in recent decades due to versatile material choices and new imaging technologies that allow exciting new applications. Single-walled carbon nanotubes (SWCNTs) are emerging molecular optical probes and sensors because of their outstanding photophysical properties, which include fluorescence in the near-infrared (nIR) spectral range, sensitivity to the molecular recognition, and non-photo bleaching nIR emission. Tunable surface functionalization on SWCNTs could produce targeted molecular probes that are responsive to environmental clues including changes in solution pH. This research describes the development of SWCNT-based optical probes to enable detection of pH changes in aqueous media and artificial sweat as a model biofluid. The sensors can reliably detect pH changes within a biologically relevant range and beyond, through changes in nIR emission. The sensors that can respond within this pH range are valuable investigational tools in certain disease conditions that experience extreme pH changes. We modulated the surface chemistry of nanotubes with a polymer and showed that the sensor response direction can be modulated upon surface chemistry changes (for example, brightening instead of quenching in acidic media). Ability to control sensor response direction is highly important to develop optimal sensors for targeted applications. The nanotubes' optical response to changes in solution pH is meaningful and provides new opportunities to develop optical pH sensors for wide applications including in healthcare, bioengineering, environmental sciences, and chemistry.