Macroelectrodes

1.5.1.1 Introduction

The working electrode (WE) represents the most important component of an electrochemical cell. It is at this interface between the WE and the solution that electron transfers of greatest interest occur. The selection of a working electrode material is critical to the experiments success. So far a variety of electrode substrates have been exploited. Noble metals, mercury, carbon and semi- conductor materials are commonly used electrode materials. Some composite materials with excellent characteristics have also been employed for electrode substrates, such as tantalum and niobium [45-46]. Several important factors need to be considered before choosing which electrode material would best suit any given experiment. The first factor is that the material used must exhibit favourable redox behaviour with the analyte you intend to test and ideally fast with reproducible electron transfer without fouling. Another factor to be considered is the potential window over which the electrode performs in a given electrolyte. This potential window should be as wide as possible to allow for the highest degree of analyte characterisation. Additional considerations include the cost of the electrode material, its ability to be fabricated or formed into useful geometries, toxicity and the ease of surface renewal following a measurement [47].

1.5.1.2 Metal Working Electrodes

Noble metals such as platinum, gold and silver have all been widely used as electrode substrates. These metal electrodes can offer very favourable electron transfer kinetics and a wide potential range. The cathodic potential window of these electrodes is usually restricted due to the low hydrogen overvoltage. However, the formation of surface oxide or adsorbed hydrogen layers may lead to

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high background currents, which can strongly affect the kinetics of the electrode reaction. To rectify this issue, a pulse potential cycle should be carried out before electrochemical experiments [48]. Several typical metal electrodes will now be discussed briefly.

The chemical properties of gold and platinum electrodes are both very stable and can be easily manufactured thus allowing them to become the most popular metal electrodes. Silver is also a valuable electrode substrate, which is usually used for the preparation of chemically modified electrodes (CMEs) in various electrochemical experiments [49-50]. Moreover, some proteins such as cytochrome c may exhibit the capability of direct electron transfer on silver electrode material, so silver substrate has also been used for protein analysis [51]. Besides noble metal electrodes, some other metals have also been employed as electrode substrates. For example, copper electrodes and nickel electrodes have been used for the detection of carbohydrates or amino acids in alkaline media. In comparison to gold or silver electrodes, these two kinds of electrodes possess a stable response for carbohydrates at constant potentials [52]. In addition alloy electrodes like platinum-ruthenium and nickel-titanium electrodes have also been reported, and these are often used for the preparation of fuel cells, owing to their bi functional catalytic mechanism [53].

1.5.1.3 Mercury Electrodes

Mercury is a classic electrode material. It displays an excellent potential window in the cathodic direction, but is severely limited in the anodic direction due to its ease of oxidation. Mercury electrodes possess highly reproducible, renewable and smooth surfaces, which are very beneficial in electrochemical analysis. A variety of mercury electrodes including the dropping mercury electrode, hanging mercury drop electrode and mercury film electrode have all been developed [54-56]. Among these mercury electrodes, the dropping mercury electrode (DME) is the most commonly used. A dropping mercury electrode is one in which drops are formed and fall off repeatedly during a potential scan, being replaced by a ‘fresh’ electrode every second or so. The main advantage of this is that the electrode is

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self-renewing, so it does not need to be cleaned or polished before each experiment. Moreover, each drop of mercury has an uncontaminated and uniform surface. This was commonly used in past years however the toxicity of mercury has lead to a limited use in the present day experiments. Instead mercury films are formed on the surface of solid electrodes rather than the pure mercury metal [57- 58].

1.5.1.4 Carbon Electrodes

A family of carbon materials have been widely used as electrode substrates to make various electrodes. The surface of these electrodes can be easily renewable for electron exchange due to the soft properties of carbon. This material allows for a broad potential window, low background current, rich surface chemistry and comparative chemical inertness. Carbon also has the advantage of having a very low cost in comparison to other substrate materials and therefore these electrodes are currently widely used. The most commonly used carbon electrodes include pyrolytic graphite electrode (PGE), glassy carbon electrode, carbon paste electrode, carbon fiber electrode and screen-printed electrode [59-63]. The work in this thesis uses the latter of these, the screen-printed electrode which will now be discussed in more detail.

1.5.1.5 Screen-Printed Electrodes

Production of screen-printed electrodes (SPE) involves several steps, including placement of an ink (both conducting and insulating) onto a patterned screen or stencil followed by forcing it through the screen with the aid of a squeegee onto a planar substrate (plastic or ceramic), and then drying/curing the printed patterns. Such a process yields mass-producible (uniform and disposable) electrodes of different shapes or sizes. The electrochemical reactivity and overall performance of screen-printed electrodes are dependent on the composition of the ink employed and on the printing and curing conditions (temperature, pressure, etc.).

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The composition of the SPE is very important as it defines the general characteristic of the sensor. If the SPE are not uniform and reproducible then they are useless for analytical purposes. Vigorous quality control must be maintained and a batch system operated if error and defective electrodes are kept to a minimum. Figure 1.21 shows the shape of the working SPE used in this work. The fabrication method of this electrode will be further discussed in Chapter 4.

Figure 1.21: Working SPE design