Dopamine (DA) is a very important neurotransmitter in the mammalian central nervous system. Because low levels of DA have been found in patients with Parkinson's disease, 1 its detection with high selectivity and sensitivity is of great significance in investigating its physiological functions and the diagnose of nervous diseases resulting from an abnormal metabolite. When solid electrodes are used to detect DA, the main and foremost difficulty is the interference of ascorbic acid (AA), which is oxidized at almost the same potential as DA.2 It is thus very important to develop an electrochemical method of high selectivity and sensitivity for the determination of DA.Chitosan, which is derived from chitin, has excellent biologic compatibility and can be used as a modifier because it has many amino and hydroxyl groups.3-8 Besides, using a natural polymer as a dispersant is a requirement of environment protection. Carbon nanotubes have attracted much attention because they have the ability to promote electron transfer reactions when used as electrode materials, 9,10 but carbon nanotubes are very stable and insoluble in most solvents, which restrained its application in electroanalysis. Thus, in this study, a multiwall carbon nanotube (MWNT)-chitosan modifier was obtained by putting MWNT into a chitosan solution (0.5%). The MWNTchitosan modified electrode (MC/GCE) has many advantages with regard to a low detection limit, a fast response, excellent reproducibility for the detection for DA and AA simultaneously. Experimental ApparatusAll of the electrochemical measurements were carried out with a bioanalytical system CV-50W electrochemical analyzer. The three-electrode system consisted of a modified GCE, a saturated calomel reference electrode (SCE), and a platinum auxiliary electrode. All potentials were referred to the SCE. The transmission electron microscopy image was done with a TEM-100CXII. ReagentsDopamine, ascorbic acid (Fluka Corp.). A chitosan solution (0.5%) was prepared by dissolving 0.05 g of chitosan in 10 ml acetic acid (2 mol l -1 ). 11 The MWNT used in this work (obtained from the Institute of Nanometer, Central China Normal University) was synthesized by catalytic pyrolysis and purified followed the method given in a reference. 12 Preparation for a MWNT-chitosan modified electrodeUpon putting 1 mg of MWNT into 5 mL of a chitosan solution (0.5%), ultrasonic agitation for a few minutes gave 0.2 mg ml -1 of a black suspension. Before being modified, the GCE was polished with 0.3 µm and 0.05 µm aluminum slurry, rinsed thoroughly with redistilled water, then ultrasonically rinsed with alcohol, redistilled water for 1 min each, and dried under an infrared lamp. After the GCE was cooled, it was smeared evenly with 10 µl of a MWNT-chitosan (0.5%) solution by a micro-syringe, and then dried under an infrared lamp for 10 min. After cooling, the MC/GCE could be used. Experimental procedureA 0.1 mol l -1 phosphate buffer solution (pH = 7.2) was used as the supporting electrolyte for the determination of DA and AA...

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Selected from a series of structurally related heteroaromatic thiols, a newly synthesized reagent 2-amino-5-mercapto-[1,3,4] triazole (MATZ) was used to fabricate self-assembled monolayers (SAMs) on gold electrode for the first time. The MATZ/Au SAMs was characterized by electrochemical methods and scanning electronic microscopy (SEM). In 0.04 mol/L Britton -Robinson buffer solution (pH 5), the electrochemical behavior of dopamine showed a quasireversible process at the MATZ/Au SAMs with an electrode kinetic constant 0.1049 cm/s. However, the electrochemical reaction of uric acid at the SAMs electrode showed an irreversible oxidation process, the chargetransfer kinetics of uric acid was promoted by the SAMs. By Osteryoung square-wave voltammetry (OSWV), the simultaneous determination of dopamine and uric acid can be accomplished with an oxidation peak separation of 0.24 V, the peak current of dopamine and uric acid were linearly to its concentration in the range of 2.5 Â 10 À6 -5.0 Â 10 À4 mol/L for dopamine and 1 Â 10 À6 -1 Â 10 À4 mol/L for uric acid with a detection limit of 8.0 Â 10 À7 mol/L for dopamine and 7.0 Â 10 À7 mol/L for uric acid. The MATZ/Au SAMs electrode was used to detect the content of uric acid in real urine and serum sample with satisfactory results.

A novel chemically modified electrode based on the chitosan-multiwall carbon nanotube (MWNT) coated glassy carbon electrode (GCE) is described, which exhibited an attractive ability to determine uric acid (UA) and norepinephrine (NE) simultaneously. The responses of UA and NE merged into a large peak at a bare GCE, but yielded two well-defined oxidation peaks at the chitosan-MWNT modified GCE (MC/GCE). The experimental parameters were optimized, and a direct electrochemical method for the simultaneous determine for UA and NE is proposed. The MC/GCE showed good sensitivity, selectivity and stability.

Hemoglobin was entrapped in composite electrodeposited chitosan-multiwall carbon nanotubes (MCNTs) film by assembling gold nanoparticles and hemoglobin step by step. In phosphate buffer solution (pH 7), a pair of welldefined and quasireversible redox peaks appeared with formal potential at À 0.289 V and peak separation of 100 mV. The redox peaks respected for the direct electrochemistry of hemoglobin at the surface of chitosan-MCNTs-gold nanoparticles modified electrode. The parameters of experiments have also been optimized. The composite electrode showed excellent electrocatalysis to peroxide hydrogen and oxygen, the peak current was linearly proportional to H 2 O 2 concentration in the range from 1 Â 10 À6 mol/L to 4.7 Â 10 À4 mol/L with a detection limit of 5.0 Â 10 À7 mol/L, and this biosensor exhibited high stability, good reproducibility and better selectivity. The biosensor showed a MichaelisMenten kinetic response as H 2 O 2 concentration is larger than 5.0 Â 10 À4 mol/L, the apparent Michaelis -Menten constant for hydrogen peroxide was calculated to be 1.61 mmol/L.