Zhijun Cao

A third-generation hydrogen peroxide biosensor based on horseradish peroxidase immobilized in a tetrathiafulvalene-tetracyanoquinodimethane/multiwalled carbon nanotubes film.

A new third-generation biosensor for H(2)O(2) assay was developed on the basis of the immobilization of horseradish peroxidase (HRP) in a nanocomposite film of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)/multiwalled carbon nanotubes (MWCNTs) modified gold electrode. The prepared HRP/TTF-TCNQ/MWCNTs/Au electrode was used for the bioelectrocatalytic reduction of H(2)O(2), with a linear range from 0.005 to 1.05 mM and a detection limit of 0.5 microM for amperometric sensing of H(2)O(2). In addition, a novel method on the basis of electrochemical quartz crystal microbalance (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 TTF-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.

Highly sensitive and surface-renewable electrochemical quartz crystal microbalance assays of heparin and chondroitin sulfate based on their effects on the electrodeposition of neutral red

The electrochemical quartz crystal microbalance (EQCM) technique was used to investigate the electrochemistry of neutral red (NR) in phosphate buffer solution (PBS) and the effects of coexisting heparin (Hep) or chondroitin sulfate (CS) for the first time. The pH dependence of the electrochemistry of NR was examined, and a V-shaped frequency response (versus time) was observed during the cyclic voltammetric experiment of NR in a nearly neutral medium (pH ca. 6.10-7.00), being due to the electrodeposition and stripping of the poorly soluble reduced product of NR (NR(Red)) at these pH values. The effects of potential scan rate, the concentration of NR, and several supporting electrolytes were examined at pH 6.80. The V-shaped response to the redox switching of NR was weakened by the introduction of Hep or CS, being due to the increased inhibition of the NR(Red) electrodeposition probably via the electrostatic interaction of the NR and especially the NR(Red) with Hep or CS. The height of the V-shaped response decreases with the increase of Hep or CS concentration, with limits of detection down to 3 nmol L(-1) for Hep and 2 nmol L(-1) for CS, respectively. The novel and surface-regenerable EQCM assay protocol based on the electrochemically switchable deposition of a dye is highly recommended for wide biosensing applications.

Electrodeposition of the Charge-Transfer Complex Generated during Electrooxidation of o-Tolidine and the Effects of Coexisting Chondroitin Sulfate

Abstract The electrochemical quartz crystal microbalance (EQCM) technique was used to investigate the electrodeposition of the charge-transfer complex (CTC) generated during electrooxidation of o-tolidine (o-TD) in Britton-Robinson buffers and the effects of coexisting chondroitin sulfate (CS). A V-shaped frequency response to the cyclic voltammetric switching of o-TD indicated the precipitation and dissolution of the poorly soluble CTC, an oxidation intermediate, formed at the Au electrode during the redox switching of o-TD in a neutral or a weakly acidic medium (pH=4.07–6.50). The effects of potential scan rate, solution pH, and several supporting electrolytes were examined. The depth of the V-shaped frequency curves (–Δf0V) was related to the supporting electrolyte used, with a decreasing sequence for −Δf0V as 0.20 mol·L−1 NaNO3 > 0.20 mol·L−1 NaClO4 > 0.10 mol·L−1 Na2SO4. The −Δf0V response to the redox switching of the CTC/o-TD “couple” was enhanced by the introduction of CS because of the formation of the CTC-CS adduct, as also characterized and supported by UV-Vis and FTIR spectrophotometry. The molar ratio (x) of the CTC to CS in the adduct and the electrode-collection efficiency of the CTC (η) were estimated using EQCM. The values of −Δf0V increased with the increase in CS concentration, with a linear range from 0.75 to 15.2 μmol·L−1, and a detection limit down to 50 nmol·L−1. The new method proposed for CS assay was characterized by a dynamically renewed surface of the detection electrode.

Electrochemical Quartz Crystal Impedance and Fluorescence Quenching Studies on the Binding of Carbon Nanotubes (CNTs)-Adsorbed and Solution Rutin with Hemoglobin

Electrochemical quartz crystal impedance (QCI) technique was utilized to monitor in situ the adsorption of rutin (RT) onto a carbon nanotubes (CNTs)‐modified gold electrode and to study the binding process of solution hemoglobin (Hb) to RT immobilized on the electrode. Time courses of the QCI parameters including crystal resonant frequency were simultaneously obtained during the RT adsorption and Hb‐RT binding. In contrast to the negligible RT adsorption at a bare gold electrode, the modification by CNTs notably enhanced the amount of adsorption, and almost all of the adsorbed RT molecules were found to be electroactive. On the basis of the frequency response from the binding of adsorbed RT to solution Hb and the diminished electroactivity of adsorbed RT after the formation of the electrochemically inactive RT‐Hb adduct, the average binding molar ratio of adsorbed RT to Hb was estimated to be 23.9:1, and the association constant (Ka) for the binding was estimated to be 2.87 × 106 (frequency) and 3.92 × 106 (charge) L mol−1, respectively. Comparable results were obtained from fluorescence quenching measurements in mixed solutions containing RT of fixed concentration and Hb of varying concentrations, demonstrating that the interfacial RT here behaved equivalently in the RT‐Hb binding activity compared to that in solution. This work may have presented a new and general protocol involving CNTs to study many other electroactive natural antioxidants or drugs that are at the interface or in solution, their binding with proteins or other biomolecules, and changes of their antioxidant activity after the binding.