Electrochemical And Sensitive Properties Of Polyoxoanions In Different Medium

Transition metal substituted polyoxometalate, especially the Keggin-type polyoxoanions [XW₁₁MoO₄₀]^n- (X=P, Si, etc. M=transition metal), have attracted special attention over the last two decades because of their wide range of potential applications. Their activity and selectivity depend on the type of substate, experiment conditions (oxidant, solvent, temperature), and on the substituting metal. Studies on the electrochemistry of such polyoxoanions in different solvents may help in understanding their behaviour as oxidative catalysts. Recently, one promising synthesis approach to the organic functionalization of POMs by covalent linkage has emerged, in which hexamolybdate could be covalently bonded with aromatic amine by reacton in acetonitrile. However, while the approach is extended to Keggin-type hetropolymolybdate system, the reactant of aromatic amine is readily oxidated by the strong oxidbillity of Keggin-type hetropolymolybdate anion, so that the main reaction can not proceed. Since the synthesis approach only depends on the activation of Mo=O bond instead of W=O bond, it could be anticipated that Mo-substituted polyoxotungstates [XW₁₁MoO₄₀]^n- (X=P, Si, Ge) may be promising candidates to successfully process such synthesis approach. Also, it was found that Mo-substituted polyoxotungstates could stimulate the oxidative polymerization of pyrrole. Therefore, the comparative studies on the electrochemical behavior of Keggin-type Mo-substituted polyoxotungstates in aqueous solution, organic solvent and film will provide valuable information for realizing those important synthesis reaction with POMs. So, we systematically investigate the electrochemical behavior of Keggin anions [XW₁₁MoO₄₀]^n- (X=P, Si, Ge) in aqueous solution, N,N-dimethyformamide (DMF) and multilayer films by cyclic voltammetry. Thus, the difference of oxidative properties for these Keggin anions will arise from different central hetero atoms and different mediums. Electrochemical sensor was widely used in the analytical determination. Recently the study of chemical modified electrode (CME) which act as electrochemical sensor was plentiful. The chemical modified carbon paste electrode (CMCPE) which was fabricated by mingling modifier with carbon paste was applied more often. The CPE has the characteristic easily modified, wide using range, fabricated easily, innocuity, long life span etc. Because modified special substance CMCPE has special function, it is studied more and more by electrochemical worker now. Polyoxometalates (POM) are very attractive in electrode modification due to their excellent electrochemical and electrocatalytic activity. There are some methods used commonly to immobilize POM on electrode surface, for example, electrodeposition of POMs on the electrode surface from a polyoxometalates solution under constant potential, self-assembly and layer-by-layer deposition, entrapment POM into polymers matrics and sol-gel method. There are some drawbacks using above methods. For instance, the electrode surface can not be renewed in the case of contamination and stability of electrode is short-term. We carry out an electrochemical study of the behavior of phosphomolybdate-polypyrrole composite bulk-modified carbon paste electrode as a sensor for the determination of nitric oxide (NO) in aqueous solution. This sensor, which maintained the good conductivity of the polymeric matrix and presented splendid electrochemical activity of POM, exhibited excellent reproducibility of surface, long-term stability and fast response. Also we studied exploringly polyoxometalates modified electrode as a sensor for the determination of dye and carbon dioxide.