br Materials and methods br Results and discussion br

Materials and methods
Results and discussion
Acknowledgments This research was supported in part by NSF through the Partnership for Research and Education in Materials grant (DMR-0611539). Core research facilities were supported by grants from the NSF (CHE-0840450) and NIH (NCRR 8G12MD007581).
Introduction Dopamine (DA), a simple organic chemical in the catecholamine family, is a monoamine neurotransmitter that plays an important role in the central nervous, renal, hormonal and cardiovascular systems [1,2]. Since its discovery, DA has attracted a great deal of attention in clinical fields because its abnormal levels in the human body indicate various diseases, such as Parkinson’s disease, senile dementia, and schizophrenia [3]. Monitoring the concentration of DA is essential for either nerve physiology study or diagnosis. Over the past several decades, tremendous effort has been made and various techniques have been developed for the DA detection, such as fluorimetry, chemiluminescence, capillary electrophoresis, mass-spectrography and ion chromatograph [4,5]. Compared with the above approaches, an electrochemical method has received considerable interest as it owns a series of advantages, such as fast detection, simplicity, reproducibility, impressive cost-effectiveness, non-destructive detection and facile operation [6,7]. However, the electrochemical determination of DA is hindered by the co-existence of interfering compounds, such as ascorbic AMG 925 (AA) and uric acid (UA) in neural biological environment [8]. AA, another electroactive species that plays an important role as an antioxidant in human metabolism, is the main interfering species that coexists with DA in the central nervous system [9]. Due to this interference, it results in poor selectivity and sensitivity towards DA detection. Moreover, AA is oxidized at almost the same potential as DA, hence resulting an overlapping voltammetric response for the oxidation of a mixture of DA and AA on a bare electrode [10]. In order to overcome this problem, two fundamental approaches have been often performed; (i) improve the electrocatalytic performance on the electrode surface to separate their oxidation potentials, and (ii) selective interaction with DA on the electrode surface without interferences [3,11]. Following these two strategies, various chemically modified electrodes have been fabricated using polymers [12], metal oxides [13], nanoparticles [14], and carbon based materials, example graphene [15–17]. As a new member of the family of carbon-based nanomaterials, graphene has attracted enormous interest in fundamental and applied science communities. Moreover, it shows specific characteristics and could be a promising candidate for aforementioned two strategies. Graphene have been most commonly utilized due to their high surface area, unique structures, outstanding charge-transfer characteristics and good chemical stability [18,19]. As a “rising-star” carbon material, it has great promise for potential applications in many fields such as nanoelectronics, nanophotonics, catalysis, sensors, nanomaterials, supercapacitors and so on due to its unique electronic, mechanical, and thermal properties [19,20]. Owing to its high specific surface area and excellent electric conductivity, graphene has been intensively employed as modification materials on the surface of glassy carbon [21]. Hybridization of graphene with a second component such as noble metal [22] or metal oxide [23,24] nanoparticles leads to a binary composite, which combines the merits of the two materials, providing superior properties over their single component in various applications including lithium ion batteries, ultracapacitors, solar cells, gas sensors, fuel cells, and electronic devices [21,25]. As an n-type semiconductor with a wide-band gap of 3.6eV, SnO2 has been extensively studied in different fields such as gas sensors and anode materials in batteries due to its diverse optical, electrical and electrochemical properties [26,27]. Recently, many efforts have been devoted to fabricate SnO2 nanostructures with different morphologies because their sensitivity and efficiency directly depend on the specific surface area and morphology [27,28].