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are very similar—thus poor reproducibility and selectivity plagued earlier attempts. Further on this subject, graphene modified electrodes have shown superior bio-sensing performance over SWCNTs toward dopamine detection in the presence of another common inter- fering agent, serotonin,56 and it has also been shown that adenine and guanine can be simultaneously detected using a graphene based Nafion composite film modified GCE; with a separation between the two oxidation peaks of 0.364 V, and low mM limits of detection for both analytes being reported.57 Sensitive detection of disease-related proteins (immunopro- teins) is critical to many areas of modern biomedical and biochemical research.56 Immunosensors show great promise for early disease detection and reliable diagnosis, for example cancer or tumour biomarkers may be present, that if detected early enough, effective pro-active treatment can be undertaken long before the disease has taken hold—increasing the patients’ chances of recovery. Since the introduction of graphene there have already been some exciting enhancements, these have been highlighted below. Zhong and co-workers58 have demonstrated how graphene can be utilised within an immunosensor. The authors developed a highly sensitive electrochemical immunosensor that was designed to quantify carcinoembryonic antigens (CEAs) as a tumour marker using nanogold–enwrapped graphene nano- composites (NGGNs) as trace labels in clinical immunoassays. The device consisted of a GCE coated in Prussian Blue (PB), on whose surface the NGGNs were electrochemically deposited, and then further modified with the specific analyte-capturing molecule, anti-CEA antibodies. Their results indicated that the method using the anti-CEA–NGGN–PB–GCE as detection antibodies showed high signal amplification and exhibited a dynamic working range of 0.05–350 ng mL1, with a low limit of detection at 0.01 ng mL1. These results are more sensitive, and exhibited a higher amplified signal, than when the same analysis was conducted using an anti-CEA–nanogold–PB–GCE for detection. Thus the use of graphene within methods as such, promises to provide ultrasensitive assay sensors for clinical applications within the future. Furthermore, Du et al.56 have reported a novel electrochemical immunosensor for the sensitive detection of the cancer biomarker a-fetoprotein (AFP) utilising a GNS sensor platform and functionalised carbon nanosphere (CNS) labelling. The developed immunosensor showed a 7-fold increase in detection signal compared to the immunosensor without graphene modi- fication and CNS labelling. Graphene was found to greatly enhance the sensitivity for the cancer biomarker, and the author’s method could respond to 0.02 ng mL1 AFP with a linear range of 0.05–6 ng mL1. Again this is a promising platform on which future enhancements will no doubt develop. Following on from the ‘wiring’ of graphene with bio-molecules for analytical sensing purposes, this approach is technologically important in the development of biofuel cells. Enzymatic biofuel cells The most compelling advancement and use of graphene is its application in a membraneless biofuel cell. Recently, there has been substantial interest towards the development of enzymatic biofuel cells (EBFCs) as they can be employed as an ‘in vivo’ power source for implantable medical devices such as pace- makers.14 The most striking feature of the EBFC is that they can utilise glucose or other carbohydrates copiously present in the human body as a fuel. However, low power densities and the poor stability of the EBFC are major challenges to be rectified. The low power density of the EBFC in comparison with conventional inorganic fuel cells is due to the location of the active site of the enzyme buried deep under the protein shell; hindering the electron transfer pathway between the enzyme’s active site and the electrode. Previously researchers have employed CNTs to improve elec- tron transfer to sites ‘buried’ deep within the enzyme. Other approaches involve the covalent binding of the enzyme, for example, GOx for use in biofuel cells with glucose as a fuel. However, a complex chemical treatment process of CNTs has to be performed in order to create active binding sites on the edge of the CNTs. Such a process hinders the mass production of this electrode.14 Furthermore, a number of redox mediators are widely used to boost electron transfer rates between the species involved. Due to graphene’s large surface area, excellent conductivity, ballistic electron mobilities at room temperature,14 and other unique properties (as discussed above); it can be thought of as an optimal replacement and starting platform for further research, where high performance EBFCs are expected soon. It is also worth mentioning that graphene can be syn- thesised to possess a number of surface active functional moieties such as carboxylic, ketonic, quinonic and C]C. Of these, the carboxylic and ketonic groups are reactive and can easily bind covalently with GOx. The presence of extended C]C conjuga- tion in graphene is also expected to shuttle electrons.14 The use of GNSs within the construction of membraneless EBFC has been reported by Liu et al.14 The authors employed graphene to fabricate the anode and cathode in the biofuel cell; Fig. 5 depicts their experimental set-up. The anode of the biofuel cell consisted of a gold electrode on which the authors co- immobilised graphene with GOx using silica sol–gel matrix; the cathode was constructed in the same manner except they employed bilirubin oxidase (BOD) as the cathodic enzyme. Voltammetric measurements were conducted to quantitatively evaluate the suitability and power output of employing a GNS as an electrode dopant and its performance was compared with a similar EBFC system constructed using SWCNTs. Upon comparison, the graphene based biofuel cell exhibited a maximum power density of 24.3 4 mW at 0.38 V (load 15 kU), which is nearly two times greater than that of the SWCNTs EBFC (7.8 1.1 mW at 0.25 V (load 15 kU)), the maximum current density of this graphene based EBFC was found to be 156.6 25 mA cm2, and for the SWCNT based EBFC 86.8 13 mA cm2. Thus it is evident that the graphene based electrode is better suited for applications in EBFCs. To evaluate the stability of the graphene based EBFC, the system was stored in pH 7.4 phosphate buffer solution at 4 C and tested every day with a 15 kU external load. After the first 24 hours, it had lost 6.2% of its original power output. Later, the power output was found to decay slowly and became 50% of its original power output after 7 days; which is substantially longer than other EBFC devices, and outperforms the SWCNT based EBFC.14 The authors stated that the enhanced performance was based upon the larger surface This journal is a The Royal Society of Chemistry 2010 Analyst, 2010, 135, 2768–2778 | 2775 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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