Graphene nanoflakes (GNF) of diameter ca. 30 nm and edge-terminated with carboxylic acid (COOH) or amide functionalities were characterised electrochemically after drop-coating onto a boron-doped diamond (BDD) electrode. In the presence of the outer-sphere redox probe ferrocenemethanol there was no discernible difference in electrochemical response between the clean BDD and GNF-modified electrodes. When ferricyanide or hydroquinone were used as redox probes there was a marked difference in response at the electrode modified with COOH-terminated GNF in comparison to the unmodified BDD and amide-terminated GNF electrode. The response of the COOH-terminated GNF electrode was highly pH dependent, with the most dramatic differences in response noted at pH < 8. This pH range coincides with partial protonation of the carboxylic acid groups as determined by titration. The acid edge groups occupy a range of bonding environments and are observed to undergo deprotonation over a pH range ca. 3.7 to 8.3. The protonation state of the GNF influences the oxidation mechanism of hydroquinone and in particular the number of solution protons involved in the reaction mechanism. The voltammetric response of ferricyanide is very inhibited by the presence of COOH-terminated GNF at pH < 8, especially in low ionic strength solution. While the protonation state of the GNF is clearly a major factor in the observed response, the exact role of the acid group in the redox process has not been firmly established. It may be that the ferricyanide species is unstable in the solution environment surrounding the GNF, where dynamic protonation equilibria are at play, perhaps through disruption to ion pairing.
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