Phosphopolyoxomolybdate absorbed on lipid membranes/carbon nanotube electrode

https://doi.org/10.1016/j.jelechem.2005.01.028Get rights and content

Abstract

Carbon nanotube (CNT) electrode was selected to support lipid membranes. From electrochemical impedance results, lipid membranes of didodecyldimethylammonium bromide (DDAB) were identified to be formed on CNT electrode. Phosphopolyoxomolybdate (P2Mo18) was immobilized on DDAB modified CNT electrode through electrostatic adsorption and the properties of P2Mo18 were investigated by cyclic voltammetry (CV), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Additionally, CNT/DDAB/P2Mo18 electrode shows good stability in strong acidic medium (0.5 M H2SO4) and excellent electrocatalytic ability for the reduction of bromate and nitrite.

Introduction

Polyoxometalates (POMs) are inorganic metal–oxygen cluster compounds that are unique in their topological and electronic versatility [1]. POMs with well-defined primary structures are currently attracting much attention as building units of novel inorganic materials that are useful in catalysis, solid state devices, photo and electrochromic displays, biochemistry, and medicine [1], [2], [3]. Recently, using POMs as inorganic surfactants to stabilize carbon and platinum nanoparticles have been reported [4], [5].

Immobilization of POMs on electrodes not only simplifies their electrochemical study but also facilitates their applications [6], [7]. In general, there are three main strategies for POMs immobilization: (I) Electrochemical deposition [8], [9], [10], only on metal electrode. (II) Immobilization as a dopant in conductive polymeric matrices [11], [12], [13], [14]. However, the polymer environment affects the electrochemical behavior and the electrocatalytic properties of the immobilized POMs [12], [15]. (III) Adsorption [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. It is a new kind of immobilization method for POMs based on electrostatic interaction between oppositely charged species and caused a variety of well-defined multi-layer assemblies of POMs with a precisely controlled thickness and layer sequence.

On the other hand, lipid membranes have been extensively used in recent due to the properties such as facilitating, tightly linking to substrates and easily controlling its thickness as well as retaining hydrophilicity. Studies on the lipid membranes have laid the foundations for its potential applications in the field of specific electrodes, such as biosensor devices, biomolecular electronic devices and solar energy transduction [29], [30], [31]. Usually, the lipid membranes can be prepared by casting aqueous vesicle of lipid onto the surface of electrodes [32], [33]. Casting membranes of surfactants onto electrodes seem a particularly easy method to make multi-layer coatings. Stable, ordered surfactant membranes have a wide range of potential application. Didodecyldimethylammonium bromide (DDAB), a synthetic lipid, is one of typical cationic surfactant. The incorporation of anionic metal phthalocyanines into cast DDAB membranes and the use of these stable multilayer membranes to catalyze reductive dechlorination have been reported [34], [35]. Additionally, it has also been reported that heme proteins (catalase, myoglobin and horseradish peroxidase) were entrapped in DDAB membrane and well-defined redox peaks and fast electron transfer rate could be observed [36], [37], [38].

In this paper, a kind of solid substrate, carbon nanotube (CNT) electrode, was utilized to support lipid membranes. On the surface of CNT electrode, DDAB was self-assembled firstly to membranes. One kind of phosphopolyoxomolybdate was introduced in the DDAB membranes by electrostatic adsorption. The properties of phosphopolyoxomolybdate immobilized in the DDAB membranes were studied by electrochemistry, scanning electron microscopy and energy dispersive X-ray spectroscopy in detail. The resulting electrodes have several attractive features, such as simple preparation, good stability and high catalysis for the reduction of bromate and nitrite.

Section snippets

Reagents and apparatus

DDAB was purchased from Acros (Belgium). 2:18 phosphomolybdic acid salt (NH4)6P2Mo18O62 · 14H2O(P2Mo18) was synthesized according to the literature [39]. All other reagents were of analytical grade and were used without further purification. Double-distilled water was used throughout.

All electrochemical experiments were carried out in a conventional three-electrode cell controlled by CHI 660A Electrochemical Work Station (CH Instruments, Inc.). CNT electrodes (0.12 cm2) were used as working

Electrochemical impedance measurements of the CNT electrode modified with DDAB membranes

Electrochemical impedance analysis is an effective method for probing the feature of the lipid membranes modified electrode [42], [43]. The complex impedance can be presented as the sum of the real, Zre, and imaginary, Zim, components that originate mainly from the resistance and capacitance of the cell, respectively. Fig. 1 illustrates the results of electrochemical impedance analysis on CNT electrodes without (a) and with DDAB membrane (b). It can be seen from Fig. 1(a) that the bare CNT

Conclusions

The immobilization of P2Mo18 in DDAB and the characteristics of the corresponding CNT/DDAB/P2Mo18 electrode have been investigated in this paper by SEM, EDS, electrochemical impedance analysis and cyclic voltammetry. In 0.5 M H2SO4, the CNT/DDAB/P2Mo18 electrode shows three reversible redox peaks with the formal potential of +0.46, +0.33, +0.18 V, respectively. The electrochemistry of the P2Mo18 electrode depends strongly on the pH value of the solutions. With the increase of the pH value, the

Acknowledgements

This work was financially supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (2001-498); the National Natural Science Foundation of China (No. 20275009) and the foundation of the Ministry of Science and Technology of China (2001-51).

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