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Advances in techniques for monitoring pH in complex fluids could havesignificant impact on analytical and biomedical applications ranging from waterquality assessment to in vivo diagnostics. We developed flexible graphenemicroelectrodes (GEs) for rapid (< 5 seconds), very low power (femtowatt)detection of the pH of complex biofluids. The method is based on real-timemeasurement of Faradaic charge transfer between the GE and a solution at zeroelectrical bias. For an idealized sample of phosphate buffer solution (PBS),the Faradaic current varied monotonically and systematically with the pH withresolution of ~0.2 pH unit. The current-pH dependence was well described by ahybrid analytical-computational model where the electric double layer derivesfrom an intrinsic, pH-independent (positive) charge associated with thegraphene-water interface and ionizable (negative) charged groups described bythe Langmuir-Freundlich adsorption isotherm. We also tested the GEs in morecomplex bio-solutions. In the case of a ferritin solution, the relativeFaradaic current, defined as the difference between the measured currentresponse and a baseline response due to PBS, showed a strong signal associatedwith 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 reproduciblerapid (<20s) response to pH. By combining the Faradaic current and real timecurrent variation, the methodology is potentially suitable for use to detecttumor-induced changes in extracellular pH.

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