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Electrochemical Sensors

As indicated in previous chapters, the top two most popular transducers for biosensors are optical and electrochemical, followed by piezoelectric and thermal. We have already learned a great deal of optical transducers and equipment for biosensor applications in Chaps. 8 and 9. In this chapter, we will learn another very popular transducer for biosensors: electrochemical. Electrochemical sensors are essentially electrochemical transducers, where the concentrations of ions or chemicals are converted into electrical voltage ( potentiometric), electrical current (amperometric), or electrical resistance/conductance (conductometric). If electrochemical sensors are used together with bioreceptors (enzymes or antibodies), they become electrochemical biosensors. In this chapter, we will learn the basic electrochemistry (electrochemical cells), which involves: 1. Ion-selective electrodes including pH electrode as examples of potentiometric electrochemical sensors. 2. Electrochemical glucose sensors as an example of amperometric electrochemical biosensor. 3. Conductometric electrochemical biosensor.

10.1

Electrolytic and Electrochemical Cells

An electrolytic cell decomposes ionic chemical compounds by applying voltage to its solution. Figure 10.1 shows a typical electrolytic cell, where two electrodes (metal rods) are inserted into a solution of metal salts (electrolytes). Electrons are taken from metal ions at one electrode (oxidation) and are released to metal ions at the other electrode (reduction). All together, the whole reaction is called redox (reduction + oxidation) reaction. The current that flows between the electrodes depends not only on the voltage that is applied, but also on the electrical properties of the solution. J.-Y. Yoon, Introduction to Biosensors: From Electric Circuits to Immunosensors, 161 DOI 10.1007/978-1-4419-6022-1_10, # Springer Science+Business Media New York 2013

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10

Electrochemical Sensors

Fig. 10.1 An electrolytic cell

Fig. 10.2 Electrochemical cells: Galvanic (left) and Daniell (right) cells

An electrochemical cell is a device that generates electrical voltage and current. Two electrodes are inserted into electrolytes that are separated by a salt bridge (e.g., Galvanic cell) or a semi-permeable membrane that replaces the function of a salt bridge (e.g., Daniell cell) (Fig. 10.2). The above cells are essentially identical to each other. Each side of the above cells is referred to as a half-cell. To facilitate the notation, the above cell is described as follows: Cu(v) j CuSO4 ðaqÞ jj ZnSO4 ðaqÞ j Zn(s) The following chemical formulas describe the equilibrium of both metal ions: Cu2þ þ 2e ! Cu Zn2þ þ 2e ! Zn

(10.1)

10.1

Electrolytic and Electrochemical Cells

163

Combining these two formulas yields: Cu2þ þ Zn ! Cu þ Zn2þ

(10.2)

For this case, a forward reaction is spontaneous; in other words, the change in Gibbs free energy is negative or DG < 0. DG is defined as: DG ¼ nFDE

(10.3)

where n ¼ number of electrons, F ¼ Faraday constant ¼ 96,487 C