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detection limit of 5 ng L1.50 Clearly graphene has an excellent potential for use in trace metal detection, and appears to provide state-of-the-art analytical responses. Miscellaneous sensing Graphene has also been used for the rapid detection (after 10 seconds of exposure) of three main classes of chemical-warfare agents and explosives at parts-per-billion concentrations which were found to be superior to that previously reported using CNTs.51 Other uses of graphene are as a pH sensor, based on the capacitive charging of hydroxyl and hyroxonium ions, which exhibited a ‘supra-Nernstian’ response over the pH range of 2 and 12.52 The high electro-catalytic activity of graphene modified elec- trodes for methanol and ethanol has also been shown. Platinum and platinum–ruthenium nanoparticles have been synthesised on graphene sheets (based upon a method using graphene oxide powder dispersed in ethylene glycol) and compared to widely used carbon black catalyst supports. The graphene modified electrodes display an enhanced efficiency for both methanol and ethanol electro-oxidations with regard to oxidation potential, diffusion efficiency, forward oxidation peak current density, and the ratio of forward to reverse peak current density.53 For example, the forward peak current densities of methanol oxida- tion for graphene- and carbon black-supported Pt nanoparticles were 19.1 and 9.76 mA cm2 respectively, whereas for ethanol oxidation 16.2 and 13.8 mA cm2 were obtained respectively.53 Bio-sensing Bio-sensing involves the monitoring of substances that are usually apparent within biological systems and concerns electron transfer between redox enzymes.54 It is a vital utility within areas of biochemical and biophysical science. As we have shown above, graphene based electrode materials are beginning to revolutionise sensing whatever the application, and have been shown to exhibit improved selectivity and sensitivity, offer quicker response times, display wider dynamic ranges, as well as exhibit lower detection limits than other analytical methods available today.24 The application of graphene within bio-sensing is no exception to this trend. The advantages observed here are reported to be because of improvements in graphene’s conduc- tion and electrode kinetics, its larger surface area, thermal stability, flexibility, lack of metallic impurities, ability to act as a nanoconnector for electrical connection with electrode substrates, and the higher density of edge plane sites it offers. These properties have lead to graphene exhibiting a high enzyme surface load capacity, and in addition to this graphene offers a truly unique opportunity within bio-sensing, because unlike in most semiconductor systems, its two-dimensional electronic states are not buried deep under the surface,54 and can be accessed directly by tunnelling and other local probes, meaning that its enzyme catalytic properties can truly be realised;4 because of this, graphene is an ideal material for electrochemical bio- sensing. Zhu et al. have shown that GNSs are a novel and promising electrochemical sensing platform for detecting bio-molecules.13 They demonstrated that GNSs (of which were synthesised through reducing sugar) show electro-catalytic activity towards the most abundant catecholamines (dopamine, epinephrine, and norepinephrine), which act as neuromodulators in nervous systems or as hormones in the circulatory system.13 It was shown that the utilisation of a GNS modified GCE for the electro- catalysis detection of cholamines achieved much larger current densities, and exhibited better electrochemical activity than those of unadulterated GCEs. The improved electrochemical response was ascribed to many reasons similar to those discussed above, including graphenes capability of accumulation for various analytes, and it was proposed: GNSs have improved conduc- tivity and a greater surface area; the p–p interactions between the analytes and the basal planes of graphene were proposed to play an important role in electro-catalysis; and the electrostatic interactions between the GNS and the target molecules also play an important role in enhancing the electrochemical response.13 Graphene modified electrodes have also been used within bio- sensing as a glucose sensor. Kang et al.54 reported a study upon the direct electrochemistry of a glucose oxidase–graphene–chi- tosan nanocomposite (loaded upon a GCE), claiming that the immobilised enzyme retained its bioactivity and exhibited a surface confined, reversible two-proton and two-electron transfer reaction; exhibiting stability, activity, and a fast heterogeneous electron transfer rate. The authors commented that the use of a graphene based material showed a much improved enzyme loading capability than glassy carbon surfaces. Their biosensor exhibited a wider linear range of 0.08 to 12 mM, and had a lower glucose detection limit of 0.02 mM, in addition to a much higher sensitivity when compared to other nano- structured supports (including: SWCNTs, MWCNTs, and an unadulterated GCE), demonstrating that a glucose oxidase– graphene–chitosan nanocomposite film can be used as a sensitive and effective glucose detection strategy. Again, the excellent performance is attributed to the unique properties that graphene has to offer, particularly its large surface area, high conductivity and enhanced enzyme absorption promoting the mediator-less direct electron transfer between redox enzymes and the surface electrode which has distinct advantages. Other work55 on glucose sensing has reported that graphene oxide exhibits high sensi- tivity, with potential for graphene based glucose biosensors for clinical diagnosis in the future, their linear range was up to 28 mM for glucose.55 Recently, Kim et al.7 have developed a sensitive sensor for the determination of dopamine without the interference of ascorbic acid; since dopamine plays a significant role in the function of human metabolism, yet is always present with much higher quantities of ascorbic acid in biological samples. They reported the application of a graphene based electrode for the electro- chemical detection of dopamine, in which they modified a GCE with graphene and compared its performance to unmodified GCEs using cyclic voltammetry. They found that the difference between the two peak potentials of dopamine and ascorbic acid oxidation was well over 200 mV with use of the graphene modified electrode. The observed linear range for the detection of dopamine concentration was from 4 to 100 mM, and the detec- tion limit was shown to be 2.64 mM. This detection was much superior to conventional electrodes such as Au, Pt, and the bare GCE, where the interference from ascorbic acid causes an overlap in signal; as the oxidation potentials of the two analytes 2774 | Analyst, 2010, 135, 2768–2778 This journal is a The Royal Society of Chemistry 2010 View Article Online Published on 04 October 2010. 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