Carbon microelectrodes

Suaud-Chagny and Gonon [3] presented a new procedure for protein immobilization adapted to carbon microelectrode characteristics. The principle of this method of immobilization is based on the association of the protein with an inert porous film immobilized around the active tip of the electrode. For this purpose the carbon was coated with an inert, electrochemically obtained protein sheath (bovine serum albumin, BSA) a few micrometers thick. Then the sheath around the fiber was impregnated with lactate dehydrogenase (LDH), which could be immobilized onto the electrode and resulted in an electrode sensitive to pyruvate. 

Figure 10.1 Differential pulse voltammetry (DPV) of guanine at pH 4.5 in 0.2 M acetate buffer at a glassy carbon microelectrode, (a) 0.5 mM guanine and (b) 50 pM guanine at a scan rate of 5 mV/s. 5 (Solid line fifth scan dotted line first scan dashed fine second scan.) Reprinted from Ref. 5a with permission from Elsevier.

Voltammetry has been adapted to HPLC (when the mobile phase is conducting) and capillary electrophoresis (CE) as a detection technique for electroactive compounds. In this usage, the voltammetric cell has to be miniaturised (to about 1 pi) in order not to dilute the analytes after separation. A metal or carbon microelectrode has a defined potential (vs the reference electrode) depending on the substances to be detected (ions or molecules) and the mobile phase flows through the detection cell (Fig. 19.5). This method of amperometric detection in the pulsed mode is very... 

Fig. 2.8 Experimental setup for the separate monitoring of oxidation and reduction reactions occurring on the Ti02-IT0 film using a carbon microelectrode.

M. L. Kovarik, M.W. Li and R.S. Martin, Integration of a carbon microelectrode with a microfabricated palladium decoupler for use in microchip capillary electrophoresis/ electrochemistry, Electrophoresis, 26 (2005) 202-210. 

M.L. Kovarik, N.J. Torrence, D.M. Spence and R.S. Martin, Fabrication of carbon microelectrodes with a micromolding technique and their use in microchip-based flow analyses, Analyst, 129 (2004) 400-405. 

Fig. 2.18 Chronoamperometric profiles showing oxidative faradaic transients of gold nanoparticles at potentials of (a) 0.8 V and (b) 1.1 V at a Glassy Carbon microelectrode of 11 pm of radius. Reproduced from reference [62] with permission...

Carbon microelectrodes decorated with enzyme were prepared with two different methods (Horrocks et al. 1993). Type A electrode (Fig. 3.15a) was prepared by heat sealing carbon fibers, with diameter of 11 or 8 p,m, in 2-mm outer diameter (OD) Pyrex capillaries. The resulting tip geometry was an inlaid microdisk electrode. The carbon microdisk was then coated with the electrically wired enzyme by soaking the... 

Fig. 3.15 Schematic shape of two different types of carbon microelectrodes decorated with enzyme (after Horrocks et al. 1993)...

Nagy et al. (1982) employed an ascorbate oxidase membrane to eliminate the oxidation current caused by ascorbic acid during the microelectrochemical mesurement of catecholamines in brain. The membrane was attached to a carbon microelectrode and was able to completely oxidize the penetrating ascorbic acid to electrochemically inert dehydroascorbic acid whereas the catecholamines could diffuse to the electrode. The sensor was called an eliminator electrode . 

In this chapter, several methods for the fabrication of different types of amperometric tips suitable for SECM are described. We have also suggested some methods for microelectrode fabrication, which have not yet been tested for SECM but may provide alternative ways for its tip preparation. Section II.A describes the techniques for the preparation of various metallic microelectrodes, including Pt, Ir-Pt, Au, Hg, and W. The manufacture of carbon microelectrodes is presented in Section II.B. Most of these tips are encapsulated in or supported with glass capillaries. Other coating materials and techniques are treated in Section II.C. 

In the last 10 years, a lot of work has been done on the design of carbon microelectrodes, and several reviews have been devoted to this large amount of work (for reviews see Refs. 32 and 33). The purpose of this section is therefore only to review the techniques useful for the preparation of SECM tips based on the criteria presented earlier defining a good SECM tip. Some of our own work about the use of pyrolytic carbon electrodes as SECM probes will also be discussed. 

FIG. 11 Top Schematic diagram of the capillary carbonization setup. Bottom Schematic picture of the pyrolytic carbon microelectrode made from pulled quartz capillaries using this setup. 

FIG. 12 Cyclic voltammogram of a pyrolytic carbon microelectrode in a 5 mM aqueous solution of hexaamineruthenium(III) chloride and 1 M KC1 electrolyte, (a) Scan rate 0.1 V/s. (b) Scan rate 10 V/s. 

FIG. 13 Experimental approach curve obtained with a 0.4 /xm apparent diameter pyrolytic carbon microelectrode in a 5 mM aqueous solution of hexaamineruthen-ium(III) chloride and 1 M KC1 electrolyte. The substrate is a 100 /xm platinum disk. (Upper dashed curve) Theoretical positive feedback approach curve. (Lower dashed curve) Theoretical negative feedback approach curve. (Inset) Schematic picture of the very end of the carbonized capillary compatible with the experimental approach... 

This relationship can be used to calculate the applied potential required for the electrolysis of the test ion at the microelectrode. Suppose, for example, we place a 10 M solution of cadmium nitrate in the test cell with a carbon microelectrode and impress a voltage difference between the working and auxiliary electrodes, making the microelectrode negative relative to the SCE. The electrode reaction will be... 

Fig. 1. Cyclic voltammograms obtained with a carbon microelectrode in the cerebro spinal fluid of an anaethetized rat (a) before and (b) after electrical stimulation. (After Wightman et al. [3].)...

Fig. 15. Subtracted amperometric responses at +450mV versus SSCE in PBS/NaOH (pH 10.5) using the same platinized carbon microelectrode. The dotted curves were obtained in the absence of [Mn(ll)(pyane)Cl2], whereas the dashed and solid curves were obtained in the presence of 25 and 100 iM of the complex, respectively. Region I corresponds to the period prior to ONOO addition, while regions II—IV correspond to the initial ONOO concentrations of 12.8, 25.0, and 36.6 i M, respectively. The arrows and shaded areas correspond to injections of ONOO and mixing of the solution. Taken from Filipovic et al. 44).

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