pH Effects on Graphene Foam Capacitance Induced by Adsorption of 1-Pyrenemethylamine

The interfacial capacitance of graphene foam electrodes (Gii-Sens) in contact to aqueous media (determined by electrochemical impedance spectroscopy) is strongly affected by adsorption of 1-pyrenemethylamine (PMA). An order of magnitude increase in capacitance upon adsorption is ascribed predominantly to the quantum capacitance contribution (i.e. changes in the electronic density of states in graphene layers) in response to the cationic adsorbent. A change in capacitance (reversible) is observed as a function of pH. Although likely to be linked to the amine protonation, the change in measured capacitance occurs over a wide range of pH values (approx. linear from pH 2 to pH 12) and could provide a diagnostic capacitance-based tool for pH. Exploratory measurements in pure human serum (with pH adjustment) suggest that the capacitance effect is specific to protons and correlated to pH even in complex sensing media. However, the response of the graphene foam electrode surface is sensitive to the preparation and storage conditions and currently not fully understood.