*5.5. Potentiometric Biosensors*

A potentiometric biosensor works on the principle of potential difference between the working electrode and the reference electrode. The measured analytes are not consumed in the same way as in the amperometric biosensor. In this biosensing method, two electrodes galvanic cells immersed in the electrolyte solution generate the electromotive force (e.m.f.) measured by a high impedance voltmeter [127]. One electrode is used as a working electrode, and another is used as a reference electrode. The e.m.f. value is determined by the potential difference between the two electrodes. The analyte's concentration and the potential difference is measured by the Nernst equation [128], which is explained as follows:

$$E\_{cell} = E\_0 - \frac{RT}{zF} \ln Q \tag{2}$$

where *Ecell* is the e.m.f., *E*<sup>0</sup> is the potential of the standard electrode, *R* is the gas constant, *T* is the temperature in Kelvin, *z* is no of charge of the electrode reaction, *F* is the Faraday constant, and *Q* is ion concentration ratio of the anode to cathode.

Huang, Mei Rong et al. [129] have reported a membrane based on semi-conducting poly(phenylenediamine) microparticles for Pb2+ detection in natural water. The electrode is selective towards the Pb2+ with the concentration range 3.16 × <sup>10</sup>−<sup>6</sup> to 0.0316 M with a high sensitivity displaying a near-Nernstian slope of 29.8 mV decade−1. The proposed electrode showed a long lifetime of 5 months, where the short response time was 14 s. Thayyath S. Anirudhan, and S. Alexander [130] have developed a modified multiwalled carbon nanotube (MWCNT) based imprinting polymer for the determination of pesticide 2,4-D (2,4-dichlorophenoxyacetic acid) in natural water. The sensor responds in the range of 1 × <sup>10</sup><sup>−</sup>9–1 × <sup>10</sup>−<sup>5</sup> M, where the detection limit is 1.2 × <sup>10</sup>−<sup>9</sup> M. The developed sensor is stable and can be reusable many times in the first 3 months. Mashhadizadeh, Mohammad Hossein et al. [131] have reported a newly modified carbon electrode to determine the Cu2+ with the presence of other interfering ions. The proposed potentiometric sensor showed a Nernstian slope of 30 (±0.5) mV/decade over a concentration range from 1.0 × <sup>10</sup><sup>−</sup>8– 1.0 × <sup>10</sup>−<sup>2</sup> mol L<sup>−</sup>1. The LOD was 7.0 × <sup>10</sup>−<sup>9</sup> mol L−<sup>1</sup> and the response time was 30 s, which can be used for at least 3 months without sacrificing any quality of the sensor's response without any considerable divergence in responses.
