The Application Of Novel Nanostructural Materials In Electrochemical Sensors

Nanomaterials are the materials possessing one or more dimensional features having a length on the order of1nm to less than100nm, exhibiting new or enhanced size-dependent properties compared with larger particles of the same material. Studies of novel materials on the nanometer scale have been the most important and challenging topics in modern materials science, which involve nanoparticles, nanowires, nanotubes and their potential applications in catalytic, microelectronics devices, magnetic materials, new energy materials and sensors.Today's human health management requires to be dealt with from a holistic "One Health" perspective that acknowledges the systemic interconnections of human, animal and environmental health in close relation with food safety and security. However, the environmental pollution is becoming worse and worse. Therefore, developing simple and effective methods for trace analysis of environmental pollutants is of great significance in both clinical and industrial applications. Among a variety of analytical methods, electrochemical methods have received considerable attentions due to unique advantages, e.g., simple operation, fast response, low cost and high detection sensitivity. The nanostructures with large specific surface area can provide an important and feasible platform for separation and sensing. Therefore, the fabrication of a new platform of miniaturized devices for high sensitive analyses is a great challenge that will be applied in diverse fields including clinical diagnosis, food analysis, process control and environmental monitoring in the near future.In this paper, we aim at utilizing simple and novel methods to fabricate novel nanostructured materials and and studying their applications in electrochemical sensors. The main contents of this paper include:(1) S-containing amino acids, e.g., L-cysteine (CySH), homocysteine and methionine have played a crucial role in both bio-and environmental chemistry, and can be applied to many biochemical processes and to diagnosis of disease states, for example, as prospective radiation protector and cancer indicator. In particular, CySH provides a modality for the intramolecular crosslinking of proteins through disulfide bonds to support their secondary structures and functions. Therefore, developing simple and effective methods for trace analyzing CySH is of great significance in both clinical and industrial applications.Nanoporous gold (NPG), simply prepared by selective dissolution of Ag from Au42Ag58alloy samples in HNO3solutions, possesses uniform pore sizes and ligaments, open bicontinuous metal-void phase, high surface-to-volume ratio and great porosity, which are suitable for acting as a electrochemical sensor material. NPG electrodes exhibit a higher electrocatalytic activity towards the oxidation of CySH than other electrodes. The mechanism for the electrochemical reaction of CySH at the modified electrode has been discussed. Interestingly, if the operating potential is fixed at0.65V, a strong current is observed and the interferences by tryptophan and tyrosine are avoided. The calibration plot is linear in the concentration range from1μM to400μM (R2=0.994), and the quantification limit is as low as50nM.(2) Nitrophenols are widely used in some manufacturing industries (e.g., dyes, pesticides, wood preservatives, explosives and leather treatments) and known to be seriously toxic to humans, animals and plants. In particular, p-NP is a toxic derivative of the parathion insecticide and among the priority toxic pollutants'list given by US Environmental Protection Agency (EPA). Therefore, monitoring and selectively determining p-NP in environmental samples are extremely significant. Many analytical methods, such as UV-visible spectroscopy and liquid chromatography, have been employed to detect p-NP. Due to the great advantages, e.g. simple operation, good selectivity and sensitivity, electrochemical methods have received considerable attention in the determination of p-NP.An unsupported nanoporous gold (us-NPG) was applied as a novel electrochemical sensor material for the selective detection of p-nitrophenol (p-NP) among its isomers in acid solution based on the distinct cyclic voltammetric behavior. To ascertain the ascription of each peak in cyclic voltammograms (CVs) of three nitrophenol isomers (NPs) on the us-NPG, contrast voltammetric studies of NPs were conducted at an ultrathin NPG film supported on glassy carbon substrate (NPG/GC) and a well-defined Au(111) electrode, respectively. Effects of potential scan rate and rotation speed of the electrode on the redox reaction of NPs were investigated to further confirm the reaction mechanisms. Interestingly, unlike those obtained on NPG/GC and Au(111) electrodes, the CVs of both o-nitrophenol (o-NP) and m-nitrophenol (m-NP) on us-NPG presented a new pair of redox waves separated very well with the redox waves of p-NP, indicating an obviously different mechanism involved in the redox process. Based on the voltammetric behavior, trace level determination of p-NP can be achieved on us-NPG electrode in the presence of o-NP and m-NP. Thus, the fast electron transfer, high selectivity and good sensitivity make us-NPG act as an ideal electrochemical sensor material in the selective determination of p-NP in aqueous solutions.(3) Graphene sheet, a two-dimensional monolayer graphite, has attracted enormous attentions in recent years not only for its scientific significance but also for its applicable future. Large specific surface area, good electrocatalytic activity and excellent electrical conductivity make graphene as a good choice for electrode materials in electrochemical super-capacitors, fuel cells and field-effect transistors. Particularly, graphene is an ideal candidate as chemical sensors, and has been used to fabricate pH sensors, gas sensors and biosensors. However, the lack of functional groups on surface leads to the poor dispersion of graphene in solvent and narrows its application areas. In order to overcome these shortcomings, some functional groups have been employed to modify the surfaces of graphene, for example, sulfonates and cyclomaltoheptaose. All the functionalized and defective graphene sheets are more hydrophilic and can be easily dispersed in solvents with long-term stability.Cyclodextrins (CDs), as an important host in supramolecular chemistry, are oligosaccharides composed of six, seven or eight glucose units, which are toroidal in shape with a hydrophobic inner cavity and a hydrophilic exterior. CDs can be attached on the surface of graphene sheet by the strong hydrogen bonding to make graphene more hydrophilic. As a result, CDs-functionalized graphene sheets can be used as electrochemical sensor.β-CD modified reduced graphene oxide (RGO) sheets have been prepared and characterized by TEM, AFM, IR, EIS and CVs. In comparison with bare glass carbon electrode (GCE) and RGO modified GCE, CD-RGO/GCE showed much higher peak currents to the reduction of nitrophenol isomers (NPs), attributed to the larger specific surface area of RGO and high quantities of host-guest recognition sites. Three pairs of redox peaks are observed on the CVs of CD-RGO for p-NP (0.3V), o-NP (-0.2V) and m-NP (0.05V), separating well with each other. Under the optimized condition, the anodic peak currents were linear over ranges around1-10mg dm-3for p-NP,1-9mg dm-3for o-NP and1-6mg dm-3for m-NP, with the detection limits of0.05mg dm-3,0.02mg dm-3and0.1mg dm-3, respectively. Thus, the CD-RGO modified electrode is a kind of promising electrochemical sensor material for detecting trace NPs in waste water.