Piezoelectric Electrochemical Studies On Several Charge Transfer Complexes And Chitosan-Silica Gel Films For Bioanalysis

In the past decades,the chemically modified electrode(CME)has been one of the most active fields in electrochemistry and electroanalytical chemistry.A charge-transfer complex(CTC)is formed via the charge transfer effect between an electron donor(D)and an electron acceptor(A), which can be modulated reversibly by potential switching of the A and/or the D under certain conditions,and thus we expect that the CTC-based rapid and reversible renewal of the electrode surface may find some new and interesting applications in quantitative analyses including bioassay and biosensing.Silica sol-gel materials have been widely used as a kind of matrices for immobilization of chemical or bioactive molecules(such as dyes, enzymes,etc.)to construct biosensors,due mainly to their chemical inertness,thermal stability and bio-compatibility.A silica-based film can be electrodeposited on an electrode surface,however,the common electrodeposition protocol generally involves organic solvents and extreme potential or current,which limit its widespread applications in biotechnology such as biosensing.Therefore,it is interesting and important to develop new electrodeposition protocols for silica-based bio-purpose films.The electrochemical quartz crystal microbalance(EQCM)is an useful and powerful tool to simutaneously obtain multiple chemical/physical parameters during an electrochemical perturbation,such as electrode-mass changes,the elasticity of modified films and the solution viscosity and density.The studies in this dissertation are summarized as follows.1.The electrooxidation of o-tolidine(o-TD)was investigated via the electrochemical quartz crystal microbalance(EQCM)technique.The formation and breakage of the poorly soluble charge-transfer complex (CTC)occurred during the redox switching of o-TD,and the CTC precipitation on and its removal from the electrode surface led to a V-shaped frequency response to the cyclic voltammetric switching of o-TD.The V-shaped frequency response and the electrode-collection efficiency of the CTC precipitate were notably enhanced by the introduction of sodium heparin due to the formation of the CTC-heparin adduct.FTIR and UV-Vis characterizations also supported the interaction between the CTC and heparin.The stiochiometry for the CTC-heparin adduct was estimated to be between 31.5 and 36.5,close to the anticipated molar ratio of the CTC to heparin,37.5,for the full electrical neutralization in the adduct.An EQCM-based biosensor featured by a dynamically renewed surface of the detection electrode was proposed for heparin assay,with a limit of detection of 0.05 unit mL^-1(S/N=3)in pH 6.0 Britton-Robinson(B-R) solution containing a 10-fold diluted blood serum.This method is convenient in operation and highly free from the interference from coexisting substances including proteins.The new and intriguing CTC-based biosensing concept is featured by a dynamically renewed surface of the detection electrode,and it is recommended for wide biosensing and electroanalytical applications.2.The EQCM technique was used to investigate the electrochemistry of three benzidine derivatives,o-tolidine(o-TD),3,3′,5, 5′-tetramethyl-benzidine(TMB)and o-dianisidine(o-DA),in Britton-Robinson(B-R)buffer solutions with and without coexisting dextran sodium sulphate(DSS),respectively.The mechanism for the CTC-DSS interaction is briefly discussed from EQCM,FUR and UV-Vis data.The CTC-based EQCM determination of DSS,which is featured by a dynamically renewed surface of the detection electrode, was thus proposed,with a linear range from 0.002 to 1.6μmol L^-1and a detection limit down to 0.7 nmol L^-1(o-TD system).3.A new third-generation biosensor for H₂O₂ assay was developed based on the immobilization of horseradish peroxidase(HRP)in a nanocomposite film of tetrathiafulvalene-tetracyanoquinodimethane (TIT-TCNQ)/multiwalled carbon nanotubes(MWCNTs)modified gold electrode.The prepared HRP/TTF-TCNQ/MWCNTs/Au electrode was used for the bioelectrocatalytic reduction of H₂O₂,with a linear range from 0.005 to 1.05 mmol·L^-1and a detection limit of 0.5μmol·L^-1for amperometric sensing of H₂O₂.In addition,a novel method on the basis of EQCM measurements was proposed to determine the effective enzymatic specific activity(ESA)of the immobilized HRP for the first time,and the ESA was found to be greater at the TTF-TCNQ/MWCNTs/Au electrode than that at the MWCNTs/Au or TTT-TCNQ/Au electrode,indicating that the TTF-TCNQ/MWCNTs film is a good HRP-immobilization matrix to achieve the direct electron transfer between the enzyme and the electrode.4.H₂O₂ electroreduction-induced deposition of thickness-controllable SiO₂-chitosan-MWCNTs nanocomposite films is proposed to immobilize glucose oxidase(GOD)and develop enzyme electrodes with good performance.The EQCM technology was used to monitor the deposition processes on the Au electrode.The effects of experimental parameters on the sensor performance were examined.At an optimal potential of 0.7 V vs SCE,the current response of the biosensor in selected phosphate buffer(pH 7.0)was linear with glucose concentration from 0.001 to 3.5 mmol L^-1,with a lower limit of detection of 0.5μmol L^-1(S/N=3)and short response time(within 6 s).The Michaelis constant(K_m^app)was estimated to be 4.05 mmol L^-1. In addition,the electrodepostion of methyltrimethoxysilane(MTMOS) and 3-aminopropyltriethoxysilane(APTEOS)was also investigated, indicating that the electrodeposition protocol for the CS-silica gel films induced by the electroreduction of an oxidant is rather universal in preparing other inorganic-organic hybrid films.