Electrochemical characterisation of graphene nanoflakes with functionalised edges

Abstract

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.

Use of Redox Probes for Characterization of Layer-by-Layer Gold Nanoparticle-Modified Screen-Printed Carbon Electrodes

“…The only significant differences between the bare and modified SPCEs were found when Fe(CN) 6 3− was used as the redox probe, with bare SPCEs exhibiting larger E p and smaller i p than PDDA-and AuNP/PDDAmodified SPCEs. Similarly large E p values for the Fe(CN) 6 3−/4− redox couple have been previously reported for commercially available SPCEs 13,25,29,36 as well as HOPG 21 and graphene-modified 22,23 electrodes. The response of boron-doped diamond electrodes modified with carboxylate-functionalized graphene was found to depend on electrolyte pH with pH <8 corresponding to larger E p and smaller i p for the Fe(CN) 6 3-/4-redox couple.…”

“…41,42 Recent studies have drawn similar conclusions about the surface sensitivity of the electrochemical response of Fe(CN) 6 3−/4− on HOPG and graphene-modified electrodes. [21][22][23] Here, we report evidence that suggests caution must also be practiced when characterizing SPCEs using Fe(CN) 6 3− . Previous work with AuNP/PDDA-modified carbon electrodes 17,19 did not seem to address the possible role of PDDA in the observed peak current increase and attributed the enhancement in voltammetric signal over bare electrodes entirely to the presence of AuNPs.…”

“…24 Variations in the response of the Fe(CN) 6 3−/4− redox couple on HOPG and graphene-modified electrodes have been used to help explain the wide disparities in reported electrochemical behaviors of carbon nanotubes and graphene as well as the perceived dependence of electrochemical properties of carbon materials on edge and basal plane surfaces and defects. [21][22][23] Such observations suggest that care must be taken when employing Fe(CN) 6 3-as a redox probe to characterize the electrochemical properties of carbon-based electrodes. 21 While the electrochemical characteristics of SPCEs have been widely reported, many of these studies have relied on interpretation of SPCE response with the Fe(CN) 6 3−/4− redox couple.…”

“…CV studies of graphenemodified electrodes using Fe(CN) 6 3− , Ru(NH 3 ) 6 3+ , and ferrocene methanol (FcMeOH) produced similar results, with Fe(CN) 6 3− exhibiting sensitivity to supporting electrolyte solution and electrode surface conditions that were not observed with either Ru(NH 3 ) 6 3+ or FcMeOH. 22 Overall these studies suggest that the Fe(CN) 6 3−/4− redox couple may be a poor choice for the electrochemical characterization of some carbon-based electrodes due to its somewhat unique surface sensitivity…”

“…17 Recent studies with highly ordered pyrolytic graphite (HOPG) and graphene-modified electrodes have indicated that characterization of carbon electrodes through use of the Fe(CN) 6 3−/4− redox couple can result in misinterpretation of their electrochemical properties. [21][22][23] For example, the cyclic voltammetric (CV) response of HOPG electrodes using 1 mM Fe(CN) 6 4− in 0.1 M KCl was found to depend on time. 21 Freshly cleaved HOPG surfaces exhibited voltammetric peak-to-peak separation ( E p ) values near 59 mV as expected for an electrochemically reversible process, while aged HOPG surfaces produced much larger E p values and lower peak currents for the Fe(CN) 6 4-/3− redox couple that are suggestive of poorer electron transfer kinetics.…”