Piezoelectric Electrochemical Studies On Polymer Modified Electrode For Bioanalysis

The area of polymers confined to different electrode surfaces has received great attention since its inception in 1970's. Polymer-modified electrodes (PMEs), in contrast to metallic or carbon substrate electrodes, can be designed through polymer screening to provide tremendous improvements in sensitivity and selectivity to detect variety of analytes. These electrodes have been demonstrated to be useful for improving ionic permselectivity, the stability and reproducibility of the electrode response, and for favorably realizing electrocatalytic chemical analysis. Also, the electroactive films have been used to entrap different biomolecules at the electrode surface, but without obvious loss of their bioactivity, for the development of biosensors.Electrochemical quartz crystal microbalance (EQCM) is a useful and powerful tool to study polymer-modified electrodes, e.g., the growth process of polymer, ions transport in polymer film. Compared with the conventional electrochemical quartz crystal microbalance that generally records information on the oscillation frequency of a piezoelectric quartz crystal (PQC), piezoelectric quartz crystals impedance (PQCI) can be used to synchronously obtain multiple chemical/physical parameters and study materials characteristics during an electrochemical perturbation, such as electrode-mass changes down to the nanogram level, the elasticity of modified films and the solution viscosity and density. The studies in this thesis are summarized as follows.1. The recent progress of the polymer modified electrodes, electrochemical quartz crystal researches and scanning electrochemical microscopy are reviewed.2. The combination of the scanning electrochemical microscopy (SECM) with piezoelectric quartz crystals impedance (PQCI) as a novel multi-parameter method for investigating cyclic voltammetric growth of poly(o-phenylenediamine) (PoPD) thin films at Au electrodes in aqueous solutions of various pH values, and the potentiostatic micro-etching (localized degradation) of these films in 0.10 mol L^-1 aqueous H₂SO₄ for comparative examinations on polymer porosity and stability. Two potential-sweep ranges, -0.4 to 0.9 (â… ) and 0 to 0.9 (â…¡) V vs SCE, and four solutions, acidic (A: 0.20 mol L^-1 H₂SO₄+0.10 mol L^-1 Na₂SO₄; B: 0.10 mol L^-1 H₂SO₄+0.20 mol L^-1 Na₂SO₄), neutral (C: 0.10 mol L^-1 PBS+0.20 mol L^-1 Na₂SO₄, pH 7.2) and alkaline (D: 0.20 mol L^-1 NaOH+0.20 mol L^-1 Na₂SO₄) aqueous solutions, were selected for PoPD growth. The pH increase for the polymerization solution increased the molar percentage of polyaniline-like chains in PoPD, as quantified from the current peaks at ca. 0.6 V vs SCE for oxidation of-NH₂ groups in and the film mass (EQCM measurements) of as-prepared PoPD (grown from solutions C and D) during their redox switching in 0.10 mol L^-1 aqueous H₂SO₄ for the first time. The unusual PQC impedance responses observed at negative potentials (potential rangeâ… ) in the first several potential cycles during cyclic voltammetric growth of PoPD in acidic and neutral solutions have been reasonably explained as due to the precipitation/dissolution of the poorly soluble phenazinehydrine charge-transfer complexes developed during redox switching of oligomers for the first time, which brought about much less compact PoPD films and their higher degradability than those grown in the same solution but over potential rangeâ…¡. SECM, scanning electron microscopy (SEM) and PQC frequency were used to estimate the sizes of etched micro-scale spots. In addition, the x-, y- or z-axis movement of a Pt microelectrode of 25-μm diameter near the PQC electrode was found to influence negligibly the PQCI responses in 1.0 mol L^-1 aqueous Na₂SO₄ containing K₄Fe(CN)₆ up to 0.10 mol L^-1, and a new protocol of dynamically electrodepositing silver microwires via chemical-lens method was proposed for examining the local mass-sensitivity distribution on the PQC surface.3. The combination of reflectance UV-Vis spectroelectrochemistry with electrochemical quartz crystal microbalance (EQCM) and separate reflectance FTIR characterization were used to investigate the structural interconversion for poly(o-phenylenediamine) (PoPD) between its ladder structure with phenazine units and polyaniline-like linear chains. The poly(o-phenylenediamine) films were potentiostatically (0.8 V vs SCE) grown on Au electrodes from 0.20 mol L^-1 H₂SO₄ (PoPD₁) or 0.40 mol L^-1 NaOH (PoPD₂) aqueous solution containing 0.20 tool L^-1 Na₂SO₄ + 0.10 mol L^-1 o-phenylenediamine. By considering the mass of deposited PoPD₂ film obtained from the EQCM data and the charge consumed under the current peak at ca. 0.6 V vs SCE for oxidation of-NH₂ groups in as-prepared PoPD₂ during potential cycling in 0.10 mol L^-1 aqueous H₂SO₄, the molar percentage of the polyaniline-like chains was estimated to be 19% (relative to total phenylenediamine units), being in agreement with the result obtained from a formaldehyde-combination experiment through the aminocarbonyl reaction. After 40-cycle potential sweeps between 0.2 and 0.8 V vs SCE the polyaniline-like chains in PoPD2 could be completely converted via intramolecular cyclization into the ladder structure with phenazine units. However, PoPD₁ was found to be perfectly composed of the ladder structure with phenazine units, and after 40-cycle potential sweeps between -0.4 and 0.1 V vs SCE only 2.5% in molar percentage of PoPD's ladder structure could be converted into polyaniline-like chains, suggesting that the ladder structure with phenazine units is thermodynamically more stable due to its possessing higher conjugation.4. The electrochemical piezoelectric quartz crystal (PQC) impedance analysis was used to investigate the effects of several additives on the polymer chain structure and the precipitation of phenazinehydrine charge-transfer complexes (CTC) during the cyclic voltammetric deposition of poly(o-phenylenediamine) (PoPD) thin films at Au electrode. Two parent solutions, acidic (0.10 mol L^-1 H₂SO₄, mainly for examining the CTC precipitation) and neutral (0.1 mol L^-1 phosphate buffer, pH 7.0, mainly for examining the polymer structure) aqueous solutions, and the concentration modulations for Na₂SO₄, K₂SO₄, sodium gluconate, tannic acid, sodium heparin, or glucose oxidase (GOD) as additives, were chosen for such purposes. The results show the concentration increases for all anions (especially those of large sizes) weakened the CTC precipitation, similar anionic effects on weakening the CTC precipitation were found during redox switching of phenazine too. In comparison with the absence of the large sized anions, the pendant amino groups (reflecting the polyaniline-like chain structure) in as-prepared PoPD thin films became doubled or tripled in the neutral polymerization bath containing sub-g L_-1 sodium heparin or GOD. The anionic effects on the PoPD structure and related CTC precipitation have been reasonably explained via the protonation at nitrogen atoms of the monomer/oligomers/polymers, induced anion incorporation and steric hindrance in related reactions/processes. The pedant amino groups in PoPD multiplied simply by the anionic effect of GOD were active in the aminocarbonyl reaction with glutaraldehyde, and thus the glutaraldehyde-assisted immobilization of more GOD molecules in PoPD was briefly demonstrated here for glucose biosensing with satisfactory results.5. The combination of piezoelectric quartz crystals impedance (PQCI) with scanning electrochemical microscopy (SECM) was proposed as a novel multi-parameter method for investigating the releasing/collection of electroactive drug.6. The electrochemical quartz crystal impedance (EQCI) method was used to study the overoxidation of polypyrrole (PPy)-multiwalled carbon nanotubes (MWCNT) nanocomposite film in neutral and alkaline solutions. The values of molar mass per electron transferred (M/n) obtained during the overoxidation of PPy in 0.10 mol L^-1 Na₂SO₄ and 0.20 mol L^-1 NaOH aqueous solutions were estimated to be ca. 17 and 22 g mol^-1, respectively, suggesting the nucleophilic attack of solution OH^- to the pyrrole units during the overoxidation, and the possible partial formation of carboxylic groups after the overoxidation in the NaOH solution. Also, the overoxidized PPy-CNT composite film prepared in the NaOH solution showed a notably larger affinity to dopamine (DA) dissolved in a neutral phosphate buffer than that prepared in the Na₂SO₄ solution. The modification of the overoxidized nanocomposite film improved substantially the sensitivity for DA assay in a neutral phosphate buffer, as compared with the modification of overoxidized PPY or MWCNT alone. At a -6 kHz (201-nm thickness) nanocomposite film prepared in a polymerization bath containing 1.0 mg mL^-1 MWCNT and overoxidized in 0.20 tool L^-1 aqueous NaOH, the peak current response from differential pulse voltammetric assay of DA was linear with DA concentration from 4.0×10s to 1.4×10^-6 mol L^-1, with a lower limit of detection of 1.7 nmol L^-1, good anti-interferent ability, as well as good stability and reproducibility.7. A novel nanocomposite of quinone-amine polymer and multiwalled carbon nanotubes was synthesized from iodate-oxidation/Michael addition reaction of 1,2-dihydroxybenzene with o-phenylenediamine, which was characterized by TEM, FTIR and UV-VIS spectra. The nanocomposite modified Au electrode with well-defined quinone redox peaks effectively mediated the oxidation of NADH in pH 7.0 phosphate buffer, with an overpotential decrease by ca. 470 mV (vs. bare Au), a limit of detection of 6.4 nmol L^-1 and good anti-interferent ability.8. A novel quinone-amine polymer (QAP) was synthesized from iodate-oxidation/Michael addition reaction of 1,2-dihydroxybenzene with 4,4'-diaminodiphenylmethane. The QAP-MWCNTs hybrid film modified GC electrode with well-defined quinone redox peaks effectively mediated the oxidation of cysteine in pH 7.0 phosphate buffer, with an overpotential decrease by ca. 140 mV (vs. bare GC), a limit of detection of 75 nmol L-1. cysteine can be detected in the presence of human urine.