Diamond microelectrodes

Compared to conventional (macroscopic) electrodes discussed hitherto, microelectrodes are known to possess several unique properties, including reduced IR drop, high mass transport rates and the ability to achieve steady-state conditions. Diamond microelectrodes were first described recently diamond was deposited on a tip of electrochemically etched tungsten wire. The wire is further sealed into glass capillary. The microelectrode has a radius of few pm [150]. Because of a nearly spherical diffusion mode, voltammograms for the microelectrodes in Ru(NHy)63 and Fe(CN)64- solutions are S-shaped, with a limiting current plateau (Fig. 33a), unlike those for macroscopic plane-plate electrodes that exhibit linear diffusion (see e.g. Fig. 18). The electrode function is linear over the micro- and submicromolar concentration ranges (Fig. 33b) [151]. 

As recently as 2007, few reports on boron-doped diamond microelectrodes in biological tissue had been published. This mainly arises from the difficulties in making boron-doped diamond microelectrodes with very small tips that are less invasive of tissue. The diameter of reported boron-doped diamond microelectrodes (10-30 pm) is still too big for applying them to in vivo detection, when a diameter of 10 pm with a length of 25-500 pm is generally required for minimal tissue damage[10],... 

Peckova, K. and Barek, J. (2011) boron doped diamond microelectrodes and microelectrode arrays in organic electrochemistry. Current Organic Chemistry, 15, 3014-3028. 

Lawrence, N.S., Pagels, M., Meredith, A. et al. (2006) Electroanalytical applications of boron-doped diamond microelectrode arrays. Talanta, 69, 829. 

Ward-Jones, S Banks, C.E., Simm, A.O. etal. (2005) An in situ copper plated boron-doped diamond microelectrode array for the sensitive electrochemical detection of nitrate. Electroanalysis, 17,1806. 

Cvacka, J., Quaiserova, V., Park, J.W. et al. (2003) Boron-doped diamond microelectrodes for use in capillary electrophoresis with electrochemical detection. Anal. 

Carabehi, V., Gosso, S., Marcantoni, A. et al. 2010. Nanocrystahine diamond microelectrode arrays fabricated on sapphire technology for high-time resolution of quantal catecholamine secretion from chromafhn cells. Biosens. Bioelectron. 26 92-98. 

The diamond microline electrode was prepared by sandwiching freestanding diamond CVD films between two glass slides with UV adhesive. In order to obtain a structure with dimensions appropriate for the inner diameter of the fused silica electrophoresis capillaries, the cross section (W50 x L300 500 pm) of the diamond thin film was exposed as an electrode surface area by polishing the glass-diamond-glass sandwich structure. The separation efficiency and analytical performance of the diamond microelectrode in end-column CE-ED was evaluated for the determination of a catecholamine mixture (see Table 14.1). 

Fig. 14.2 illustrates the determination of an equimolar low concentration (0.1 pM) of three neurotransmitters with the diamond-based CE detector. The very low and stable noise levels in the background current enabled us to attain the lowest detection limits from the diamond-based electrochemical detector. Under the same CE separation conditions, the diamond microhne electrode showed lower noise levels (0.5 - 1 pA), and a more stable background current, than that of the carbon fiber electrode (whose minimum noise level was 2 pA), even though its surface area was 25 times larger. Variations in the background current (low-frequency noise) were also much smaller and less irregular than those for the carbon fiber microelectrodes. In addition, the diamond microelectrodes exhibited greater stability in the amperometric response compared to that for the carbon fiber microelectrodes. 

The double layer capacitance of a diamond microelectrode is calculated based on the equation... 

Microelectrodes, due to their unique properties, such as small size, non-planar diffusion, small capacitance, and small current, have provided a major breakthrough in electrochemistry. Taking advantage of the superior properties of diamond, the diamond microelectrode is a promising new device for several applications in electrochemistry. Furthermore, modification of the diamond microelectrode with appropriate metal nanoparticles increases the quality of the measurements, in the terms of sensitivity, stability, and selectivity. 

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