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As we have highlighted above, graphene beholds inimitable properties that are superiorly different from other carbon allo- tropes of various dimensions and from any other electrode material for that matter. Depending on the particular problem at hand, graphene’s prospects can be sometimes superior and others inferior.4 However, essentially graphene precludes previous problems such as impurities, production costs and large scale manufacturing associated with CNTs and other electrode materials while outperforming their properties in many ways, making graphene the ideal replacement electrode material in many electrochemical applications. The future is bright for gra- phene and its application holds great promise. Graphene for sensing Electro-catalysis of graphene The ‘electro-catalysis’ of graphene has been widely reported. For example, Kang et al.24 have reported the electro-catalytic sensing of paracetamol using graphene created from graphite oxide, producing ‘wrinkled’ graphene sheets functionalised with hydroxyl and carboxylic groups. Fig. 2 depicts the voltammetric profiles where a quasi-reversible redox process of paracetamol at the graphene-modified glassy carbon electrode was obtained showing that the over-potential of paracetamol was significantly decreased when compared to that of the same analysis upon an unadulterated glassy carbon electrode (GCE), meaning that the graphene-modified electrode exhibited excellent ‘electro-cata- lytic’ activity towards paracetamol. The graphene modified GCE outperformed the unadulterated GCE consistently, and at a low concentration and a scan rate of 50 mV s1, where the unadul- terated GCE showed an irreversible behaviour with relatively weak redox current peaks, the graphene-modified GCE exhibited a pair of well-defined redox waves. The enhancement of the graphene based electrode compared to the unadulterated GCE is claimed to be due to the nanocomposite film of graphene ‘accelerating’ the electrochemical reaction, and that the large background current exhibited by the graphene modified electrode in Fig. 2 is due to the large surface area of graphene. Thus, their graphene based electrochemical sensor shows excel- lent performance for detecting paracetamol; with a detection limit of 3.2 108 M, a linear range of 0.1–20 mM, and a reproducibility of 5.2% RSD. This sensor demonstrated that the simple, sensitive, and quantitative detection and screening of paracetamol are possible with graphene based electrodes. Another notable example is the electrochemical sensing of hydrazine, highly important in fuel cells, rocket propellants, insecticides and explosives.38 Hydrazine usually exhibits irre- versible kinetics on graphite surfaces, such that the electro- chemical signal occurs at high potentials close to the end of the accessible voltammetric window, but is more electrochemically reversible on metallic surfaces.33,39 Graphene oxide has been shown to possess excellent ‘electro-catalytic’ activity towards the sensing of hydrazine as compared to the underling electrode substrate, glassy carbon.38 While the analytical utility was not examined in much depth, the smallest addition was 1 mM which was linear up to 25 mM.38 There are many reports of graphene and related structures exhibiting enhanced analytical perfor- mances and ‘electro-catalysis’; Table 1 overviews a selection of recent literature reports of graphene utilised in many areas of sensing. Interestingly in the above examples and of many others (see Table 1), graphene is compared only to the underlying electrode and not to other relevant carbon materials. This was a similar problem when CNTs were first introduced into electrochemistry, with ‘electro-catalysis’ of the CNTs claimed when only compared to the underlying electrode material, usually glassy carbon.34 To gain an insight into the electrochemical reactivity of graphene, Mao and co-workers40 compared the electrochemical activity of four kinds of carbon materials (single-walled carbon nanotubes, pristine graphene oxide nanosheets (GONs), chemically reduced GONs, and electrochemically reduced GONs) with potassium ferricyanide, b-nicotinamide adenine dinucleotide (NADH), and ascorbic acid as redox probes. It was found that the electron transfer kinetics of the redox probes depend on the kind of material used, of which the redox processes of the probes at SWCNTs and electrochemically reduced GONs are faster than those at the pristine and chemically reduced GONs. The authors believe that the synergetic effects of the surface chemistry (such as the C/O ratio, presence of quinone-like groups, surface charge, and surface cleanness) and the conductivity of the materials contribute. In essence, Mao’s paper highlights that the electron transfer kinetics of the redox probes employed are essentially dependent on the kind of carbon-based materials studied in this study.40 Other important work towards understanding the electro- chemical reactivity of graphene is from Tsai and co-workers36 who compared graphene ‘film’ modified basal and edge plane pyrolytic graphite electrodes for the electro-catalytic oxidation of hydrogen peroxide and NADH with that of bare basal and edge plane pyrolytic graphite electrodes. They observed that the application of graphene to the electrode surface has the ability to lower the electro-oxidation potentials of hydrogen peroxide and NADH in comparison with bare basal and edge plane pyrolytic graphite electrodes.36 However, a problem with this approach is that the graphene utilised in their study36 contained on average 4–6+ layers; which is approaching that of graphite! The electrochemical sensing of 100 mM paracetamol at a bare glassy carbon electrode (a) and compared with graphene modified GCE with (b) 20 mM paracetamol and without paracetamol (c) in the buffer of 0.1 M NH3$H2O–NH4Cl, pH 9.3, scan rate: 50 mV s1. Reproduced with permission from ref. 24. Fig. 2 This journal is a The Royal Society of Chemistry 2010 Analyst, 2010, 135, 2768–2778 | 2771 View Article Online Published on 04 October 2010. Downloaded by Manchester Metropolitan University on 18/07/2015 16:37:37.PDF Image | Graphene Electrochemistry
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