  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.
