Study On Electrochemical Sensing Systems Based On Advanced Cabon Materials And Room Temperature Ionic Liquids

The electrochemical method has wide applications because of its easy preparation,low cost, high sensitivity, excellent selectivity, stability and reproducibility. Since thecharacteristics of the electrochemical biosensors greatly rely on the materials modifiedon the electrodes, in order to improve the performance of the biosensor, developingsuitable electrode materials is one of the most interesting projects. Novel carbonmaterials (such as active carbon, carbon nanotubes, B/N doped carbon, graphene et al.),due to unique physiochemical properties including excellent conductivity,morphology-controllable synthesis and easy functionalization, have been widely usedin electrochemical biosensor. On the other hand, room temperature ionic liquids(RTILs) can be used for both of solvent and electrolyte due to its intrinsic ionicconductivity, extremly low vapor potential, wide electrochemical window, which havebeen attracted wide consideration in electrochemical sensors. In this t hesis, a fewelectrochemical sensors based on carbon materials and RTILs have been developed, ofwhich the main points are summarized as follows:(1) Polymerized ionic liquid-wrapped carbon nanotubes (PIL-CNTs) were firstlydesigned for direct electrochemistry and biosensing of redox proteins. The CNTs werecoated successfully with polymerized ionic liquid (PIL) layer, as verified bytransmission electron microscopy (TEM), thermogravimetric analysis (TGA) andFourier transform infrared (FT-IR) spectroscopy. The PIL-CNTs were dispersed betterin water and showed superior electrocatalysis toward O2and H2O2comparing topristine CNTs and the mixture of IL monomer and CNTs. With glucose oxidase (GOD)as a protein model, the direct electrochemistry of the redox protein was investigated onthe PIL-CNTs modified glassy carbon (GC) electrode and excellent directelectrochemical performance of GOD molecules was observed. The proposedbiosensor (GOD/PIL-CNTs/GC electrode) displayed good analytical performance forglucose with linear response up to6mM, response sensitivity of0.853μA mM-1, goodstability and selectivity.(2) A novel one-dimensional (1-D) caterpillar-like manganese dioxide-carbon(MnO2-C) nanocomposite has been synthesized by a direct redox reaction betweencarbon nanotubes and permanganate ions for the first time. The as-preparednanostructured MnO2-C composite mainly consisting of-MnO2nanoflakes had a unique microstructure, high specific surface area (200m2/g) and favourableconductivity. The MnO2-C composite, added as a modification to the glassy carbon(GC) electrode via a direct electrochemical co-deposition process with a chitosanhydrogel, was found to exhibit excellent catalytic activity toward L-cysteineelectro-oxidation because the specific interaction between the-SH group of L-cysteineand solid MnO2occurred to form surface complexes. A determination of L-cysteine atthe MnO2-C/chitosan/GC electrode was carried out by amperometric measurement.Under the optimum experimental conditions, the detection response for L-cysteine wasfast (within7s). The logarithm of catalytic currents shows a good linear relationshipwith that of the L-cysteine concentration in the range of0.5-680mM, with a lowdetection limit of22nM. The MnO2–C/Chit/GC electrode exhibited excellent stability(without any decrease of the response signal after1month) and admirable resistanceagainst interference like glutathione and other oxidizable amino acids (tryptophan,tyrosine, L-lysine and methionine).(3) Hollow nitrogen-doped carbon microspheres (HNCMs) as a novel carbonmaterial have been prepared and the catalytic activities of HNCMs-modified glassycarbon (GC) electrode towards the electrooxidation of uric acid (UA), ascorbic acid(AA) and dopamine (DA) have also been investigated. Comparing with the bare GCand carbon nanotubes (CNTs) modified GC (CNTs/GC) electrodes, the HNCMsmodified GC (HNCMs/GC) electrode has higher catalytic activities towards theoxidation of UA, AA and DA. Moreover, the peak separations between AA and DA,and DA and UA at the HNCMs/GC electrode are up to212and136mV, respectively,which are superior to those at the CNTs/GC electrode (168and114mV). Thus thesimultaneous determination of UA, AA and DA was carried out successfully. In theco-existence system of UA, AA and DA, the linear response range for UA, AA and DAare5-30μM,100-1000μM and3-75μM, respectively and the detection limits (S/N=3)are0.04μM,0.91μM and0.02μM, respectively. Meanwhile, the HNCMs/GCelectrode can be applied to measure uric acid in human urine, and may be useful formeasuring abnormally high concentration of AA or DA. The attractive features ofHNCMSprovide potential applications in the simultaneous determination of UA, AAand DA.(4) CNTs wrapped with nitrogen-doped carbon (CNx) layer (CNx-CNTs) werepyrolyzed from Polydopamine-CNTs composite (PDA-CNTs). Using CNx-CNTs assupport, PtNPs with high dispersion and small particle size were successfully anchoredon CNT surface and as-prepared Pt/CNX-CNTs nanohybrids were fabricated in amperometirc enyzme sensor for glucose detection. The micrographs of Pt/CNX-CNTsnanohybrids were characterized by TEM. The result shows that the CNTs surface wasuniformly coated with a CNXlayer with a thickness of ca.9nm. Pt NPs with anaverage diameter of ca.1.7±0.5nm were highly dispersed on CNX-CNTs surface.Comparing Pt/CNTs, Pt/CNX-CNTs nanocomposite shows much better electrocatalyticactivity toward H2O2electrooxidation. Based on these results, we succeed inconstructing a glucose amperometric biosensor with admirable pH tolerant, highsensitivity (66.51μA mM-1cm-2), low detection limit (0.4μM) and excelent stability.(5) The full spectrum of properties associated with RTILs is exploited to assess theviability of this platform, thus revealing the correlation between the redox propertiesand the physiochemical parameters of the species involved. This includes theevaluation of (1) the variation of redox responses toward analytes with similarmolecular structures or functionalities of RTILs;(2) the influence in terms of physicalcriteria of the system such as viscosity and conductivity as well as chemical structureof RTILs;(3) the sustainability in harsh conditions (high temperature or humidity) andinterferences. The principle is exemplified via trinitrotoluene (TNT) and dinitrotoluene(DNT) with inherent redox activity as analytes and IL membranes as solvents andelectrolytes using glassy carbon (GC) electrodes. A discrete response pattern isgenerated that is analyzed through linear discriminant analysis (LDA) leading to100%classification accuracy even for the mixture of analytes. Quantitative analysis throughsquare wave voltammetry (SWV) gave rise to the detection limits in liquid phase of190and230nM for TNT and DNT, respectively, with a linear range up to100μM.Gas-phase analysis shows strong redox signals for the estimated concentrations of0.27(1.2nM) and2.05ppm (11.3nM) in the gas phase for TNT and DNT, respectively,highlighting that RTILs adopt a role as a preconcentrator to add on sensitivity withenhanced selectivity coming from their physiochemical diversity, thus addressing themajor concerns usually referred to most sensor systems.(6) A simple online electrochemical cell design, consisting of Au quartz crystalworking electrode and incorporating ionic liquids (RTILs) as electrolytes, has beensuccessfully applied for the amperometric sensing of oxygen. In addtion, the ionicliquid electrochemical system has also been investigated by the real time EQCMintergrated technique, which provides sensitive measure of interfacial dynamicprocesses that other techniques can not. The obtained analytical parameters were foundto be strongly dependent on the choice of cation and anion. A limit of detection foroxygen as low as0.05vol.%, linearity over an oxygen partial pressure between0%and 20%, with a stable practical analytical response shown over the examined period of60days with no obvious fouling of the electrode surface.