A novel strategy to construct a sensitive amperometric sensor of H2O2 was described. Horseradish peroxidase (HRP) wassuccessfully immobilized on nanometer-sized Au colloids, which were supported by thiol-tailed groups of cysteaminemonolayer. The thiol-tailed groups were formed through the covalent binding of glutaraldehyde on a cysteamine-modifedgold electrode. With the aid of the catechol mediator in the solution, HRP-labeled Au colloids displayed excellentelectrocatalytical response to the reduction of H2O2, which increased with decreasing size of Au colloids. The sensorsresponded to H2O2 in the concentration range of 0.39 mM-0.33 mM, and reached 95% of the steady-state current in less than5 s with the detection limit of 0.15 mM. The response showed a Michaelis-Menten behavior at larger H2O2 concentrations.The KappM values for the sensors based on different sized HRP-labeled Au colloids (colloid A-D) were found to be 0.11, 0.094,0.054 and 0.064 mM, respectively. The low KappM values demonstrated that HRP immobilized on Au colloids exhibited a highaf

Direct electron transfer of immobilized horseradishperoxidase on gold colloid and its application as a biosensorwere investigated by using electrochemicalmethods. The Au colloids were associated with a cysteaminemonolayer on the gold electrode surface. A pairof redox peaks attributed to the direct redox reaction ofhorseradish peroxidase (HRP) were observed at theHRP/Au colloid/cysteamine-modi®ed electrode in 0.1 Mphosphate buffer (pH 7.0). The surface coverage of HRPimmobilized on Au colloid was about 7.6×10-10 mol/cm2.The sensor displayed an excellent electrocatalytic responseto the reduction of H2O2 without the aid of anelectron mediator. The calibration range of H2O2 was 1.4mMto 9.2mMwith good linear relation from 1.4mMto 2.8mM. A detection limit of 0.58 mM was estimated at asignal-to-noise ratio of 3. The sensor showed good reproducibilityfor the determination of H2O2. The variationcoef®cients were 3.1 and 3.9% (n＝10) at 46 mM and 2.8mM H2O2, respectively. The response showed a Michaelis－Menten behavior at higher H2O2 concentrations. TheKMapp value for the H2O2 sensor was found to be 2.3 mM.

A chemically modified microelectrode has been prepared successfully by means of electrodepositing nickel hexacyanoferrate(II-III)on a microdisk platinum electrode. This modified electrode could catalyze the oxidation of dopamine and, moreover, eliminate theelectrode poisoning caused by the electrochemical polymerization of dopamine at the bare platinum electrode. However, the oxidation ofascorbic acid could also be catalyzed at this modified electrode, thus interfering with the electrochemical response of dopamine. When themodified electrode was further covered with Nation

Hemoglobin (Hb) was immobilized successfully on nanometer-sized gold colloid particles associated with a cysteaminemonolayer on a gold electrode surface, and was characterized by atomic force microscopy (AFM), UV–vis spectroscopy(UV–vis), surface enhanced Raman spectroscopy (SERS) and electrochemical impedance spectroscopy (EIS). The immobilizedHb was shown to keep its biological activity well. Direct electron transfer (eT) between Hb and the modified electrode wasachieved without the aid of any electron mediator. In pH 7.2 phosphate buffer solution, the formal potential (E°[1]) of Hb was−0.051 V (vs. SCE) and the eT rate constant was 0.49 s−1. The average surface coverage of Hb immobilized on the gold colloidwas about 6.71×10−11 mol cm−2. The immobilized Hb displayed the features of a peroxidase and gave an excellentelectrocatalytic response to the reduction of H2O2. The Michaelis–Menten constant (Km) was 1.2×10−4 M. The currents wereproportional to the H2O2 concentration from 3.6×10−7 to 8.6×10−4 M and the detection limit was as low as 1.2×10−7 M(S/N=3).

A stable electroactive thin film of cobalt hexacyanoferrate (CoHCF) was electrochemically deposited on the surface of a microbandgold electrode. The cyclic voltammograms of CoHCF film indicate the presence of two redox peaks corresponding to the hexacyanofer-rate(ll/lII) redox couple. The electrochemical behaviour of CoHCF is related to the concentration of supporting electrolyte andcounter-ions. The modified electrode shows excellent electrocatalytic activity towards the oxidation of reduced nicotinamide adeninedinucleotide (NADH) in phosphate buffer solution (pH 7.0), with an overpotential ca.310-370 mV lower than that of the bare microbandgold electrode. The catalytic peak current is proportional to NADH concentration in the range 0.5-6.0 mM with a correlation coefficientof 0.98. The catalytic rate constant of the modified electrode for the oxidation of NADH is determined using a rotating-disc electrode.The mechanism of the oxidation of NADH catalysed by the electrode is discussed.

Nickel hexacyanoferrate films have been deposited on the surface of a microband gold electrode by cyclic voltammetry.Cyclic voltammograms of the resulting nickel hexacyanoferrate films indicate the presence of a couple of redox peaks whichcorrespond to the hexacyanoferrate(II/III) redox couple. The nickel hexacyanoferrate modified microband gold electrodeshows an excellent electrocatalytic activity for oxidation of reduced nieotinamide adenine dinueleotide (NADH) inphosphate buffer solution (pH 7.0), with an overpotential ca. 340mV lower than that of the bare electrode. The steady-statecurrent increases linearly with NADH concentration in the range of 0.5-8.0 mmolx. The catalytic rate constant of themodified electrode for the oxidation of NADH is determined using a rotating disk electrode. The further experimental resultsshow that the rate constant decreases with increasing NADH concentration. The mechanism of the oxidation of NADHcatalyzed by the nickel hexacyanoferrate modified gold electrode is discussed.

Multi-walled carbon nanotube (MWCNT) is successfully immobilized on the surface of platinum electrode by mixing with DNA.The DNA/MWCNT modified electrodes are characterized by electrochemical impedance spectroscopy and cyclic voltammetry.Further research indicates that cytochrome c can strongly adsorbed on the surface of the modified electrode, and forms an approximatemonolayer. The immobilized MWCNT can promote the redox of horse heart cytochrome c which gives reversible redoxpeaks with a formal potential of 81 mV vs SCE.

The formation of an inclusion complex of methylene blue with b-cyclodextrin is testi®ed by voltammetry and spectroscopyand its stable constant is determined to be 4.4×103M－1. The interactions of methylene blue and the inclusion complex withDNA have been investigated by means of voltammetry. The binding constants of methylene blue and the inclusion complex toDNA are obtained through the changes of peak currents and their values are 1.1×104M-1 and 7.2×103M－1, respectively.The experimental results reveal that the inclusion complex does not decompose when it interacts with DNA, and bcyclodextrinitself does not interact with DNA.Their binding numbers (n) are 1 both methylene blue and the inclusioncomplex. According to the experimental data, it can be inferred that the binding model of methylene blue to DNA may be`electrostatic binding' under our experimental conditions. This can clearly explain the experimental phenomena.

Simultaneous determination of purine bases, ribonucleosides and ribonucleotides was achieved by coupling capillary electrophoresis (CE) with wall-jet amperometric detection. A 200um diameter copper disk electrode was applied at working potential, +0.65 V vs. saturated calomel electrode. The current response of high sensitivity and stability was obtained in strong basic solutions which were suitable for satisfactory CE separations. The calibration curve was linear over 2-3 orders of magnitude and the limits of detection for adenine, guanine, xanthine, uric acid, adenosine, guanosine, adenosine-5-monophosphate and guanosine-5'-monophosphate were below 9 fmol (S/n=3). The use of this method for the separation and detection of compounds present in human plasma samples was reported.

A new type of sol /gel/organic hybrid composite material based on the cross-linking of natural polymer chitosan with (3-aoryloxypropyl) dimethoxymethylsilane was developed for the fabrication of an amperometric H2O2 biosensor. The composite filmwas used to immobilize horseradish peroxidase (HRP) on a gold disk electrode. The properties of sol-gel/chitosan and sol-gel/chitosan-HRP films have been carefully characterized by atomic force microscopy and Fourier transform infrared. By usingfluorescent label, a protein density on sol /gel/chitosan has been calculated to be 3.14×1012 molecules cm 2. With the aid ofcatechol mediator, the biosensor had a fast response of less than 2 s with linear range of 5.0×10-9-1.0×10-7 mol-1 and adetection limit of 2×10-9 mol-1. Its current response shows a typical Michaelis-Menten mechanism. The apparent Michaelis-Menten constant KappM is found to be 1.30 mmol 1. The activation energy for enzymatic reaction is calculated to be 8.22 kJ mol 1.The biosensor retained approximately 75% of its original activity after about 60 days of storage in a phosphate buffer at 4℃.