The Impact of Pesticides - Academy Publish

Amperometric biosensorThis is the most common electrochemical detection method used in biosensors. Thebasic principle of amperometric sensors is based on electrode reactions. It is a threeelectrode system consisting of a working electrode, a reference electrode and anauxiliary electrode. The working electrode, a polarizable electrode of a noble metalor carbon is used, where as Ag/AgCl electrode or calomel electrode (nonpolarizable)is usually used as reference electrode. The auxiliary electrode should bemuch larger than the working electrode to make the reaction on the workingelectrode rate limiting. Amperometric biosensors operate by applying a constantpotential and monitoring the current associated with the reduction or oxidation of anelectroactive species involved in the recognition process. The current generated islinearly related to the analyte concentration. The solution in which the electrodes areimmersed must contain sufficient amount of supporting electrolyte to minimize theOhmic drop between the electrodes. The sensor potential is set at a value where theanalyte, directly or indirectly, produces a current at the electrode.Potentiometric sensorThe potentiometric sensors consist of an ion-selective membrane and somebioactive material, e.g. an enzyme and an internal reference electrode. The nature ofthe ion selective membrane determines the selectivity of the electrode it is alsocalled as working electrode or ion selective electrode. Many types of electrodes areavailable and are classified by the properties and nature of the membrane.Representative membranes include pressed single crystals, sparingly soluble saltspressed into a pellet, and solvent polymeric membranes. For the polymericmembranes, polyvinylchloride (PVC) has been used as supporting matrix otherpolymers are also used such as polystyrene, polyamide etc. A plasticizer and anionophore (ion-exchange) compound are incorporated there the potential differenceacross the ion-selective electrode is measured using an external reference electrodeunder the condition of zero current. The potential of the ion selective electrode (ISE)measured with respect to the reference electrode is linearly dependent on thelogarithm of the activity or concentration of the analyte. Since potentiometry yieldsa logarithmic concentration response, the technique enables the detection ofextremely small concentration changes. The widely used ion selective electrode isglass electrode for pH measurement.VoltammetryVoltammetric techniques involve the application of a potential (E) to an electrodeand the monitoring of the resulting current (i) flowing through the electrochemicalcell. It consists of three electrodes system working electrode, reference electrodeand auxiliary electrode. The current will pass through in between working andauxiliary electrode and measured in between working and reference electrode. Inmany cases the applied potential is varied or the current is monitored over a periodof time (t). Thus, all voltammetric techniques can be described as some function ofE, i, and t. The analytical advantages of the various voltammetric techniques includeexcellent sensitivity with a very large useful linear concentration range for bothAcademyPublish.org - The Impact of Pesticides395