5 years ago

pH Sensing Properties of Flexible, Bias-Free Graphene Microelectrodes in Complex Fluids: From Phosphate Buffer Solution to Human Serum

pH Sensing Properties of Flexible, Bias-Free Graphene Microelectrodes in Complex Fluids: From Phosphate Buffer Solution to Human Serum
Ramya Vishnubhotla, Alan T. Charlie Johnson, Carl H. Naylor, Amey Vrudhula, Jeffery G. Saven, Jacquelyn E. Blum, Zhaoli Gao, Jinglei Ping
Advances in techniques for monitoring pH in complex fluids can have a significant impact on analytical and biomedical applications. This study develops flexible graphene microelectrodes (GEs) for rapid (<5 s), very-low-power (femtowatt) detection of the pH of complex biofluids by measuring real-time 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 is varied monotonically and systematically with the pH, with a resolution of ≈0.2 pH unit. The current–pH dependence is 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. For ferritin solution, the relative Faradaic current, defined as the difference between the measured current response and a baseline response due to PBS, shows a strong signal associated with ferritin disassembly and the release of ferric ions at pH ≈2.0. For samples of human serum, the Faradaic current shows a reproducible rapid (<20 s) 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. Faradaic charge transfer is observed from a complex biofluid into a flexible graphene microelectrode in the absence of an applied bias. The Faradaic current responds sensitively and rapidly to pH variation. The results provide a detailed understanding of the electrical double layer above graphene, and pave the way for early cancer diagnosis via monitoring of extracellular pH.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/smll.201700564

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