pH sensing properties of flexible, bias-free graphene microelectrodes in complex fluids: from phosphate buffer solution to human serum

Abstract: Advances in techniques for monitoring pH in complex fluids could have significant impact on analytical and biomedical applications ranging from water quality assessment to in vivo diagnostics. We developed flexible graphene microelectrodes (GEs) for rapid (< 5 seconds), very low power (femtowatt) detection of the pH of complex biofluids. The method is based on real-time measurement of Faradaic charge transfer between the GE and a solution at zero electrical bias. For an idealized sample of phosphate buffer solution (PBS), the Faradaic current varied monotonically and systematically with the pH with resolution of ~0.2 pH unit. The current-pH dependence was well described by a hybrid analytical-computational model where the electric double layer derives from an intrinsic, pH-independent (positive) charge associated with the graphene-water interface and ionizable (negative) charged groups described by the Langmuir-Freundlich adsorption isotherm. We also tested the GEs in more complex bio-solutions. In the case of a ferritin solution, the relative Faradaic current, defined as the difference between the measured current response and a baseline response due to PBS, showed a strong signal associated with the disassembly of the ferritin and the release of ferric ions at pH ~ 2.0. For samples of human serum, the Faradaic current showed a reproducible rapid (<20s) response to pH. By combining the Faradaic current and real time current variation, the methodology is potentially suitable for use to detect tumor-induced changes in extracellular pH.