Electrochemical Biosensors for the Detection of Pesticides

22 The Open Electrochemistry Journal, 2010, 2, 22-42 Open Access Electrochemical Biosensors for the Detection of Pesticides Gamal A. E. Mostafa* Pharmaceutical Chemistry Department, College of Pharmacy, King Saud University, Riyadh, Saudi Arabia Abstract: Biosensors have been developed for the detection of pesticides using integrated enzymes, antibodies, cell and DNA-based biosensors. Enzymatic determination of pesticides is most often based on inhibition of the activity of selected enzymes such as cholinesterase, acid phosphatase, ascorbate oxidase, acetolactate synthase and aldehyde dehydrogenase. Enzymatic Biosensors were developed using various electrochemical signal transducers and different electrodes. Various immobilization protocols used for the formation of a biorecognition interface are also discussed: In addition, techniques of regeneration, single amplification, and miniaturization are evaluated for the development of immunosensor. Both batch and flow-injection analyses with enzyme biosensors are most intensively developed. It included that, in the future, com- pact, disposable and portable devices especially designed for in-field analysis with high sensitivity, selectivity; development of arrays and multiple sensors will continue another area of intensive research for biosensors. Keywords: Pesticides, electrochemical biosensors, detection. 1. INTRODUCTION [13-15] (Fig. 1). The biological recognition element (enzyme, antibody, microorganism or DNA), in this case, the Pesticides (herbicides, fungicides, insecticides) are biosensor is based on a reaction catalyzed by macromole- widely used in the agriculture and industry around the world cules, which are present in their biological environment, due to their high insecticidal activity [1, 2]. The presence of have been isolated previously or have been manufactured. pesticide residues and metabolites in food, water and soil Thus, a continuous consumption of substrate(s) is achieved currently represents one of the major issues for environ- by the immobilized biocatalyst incorporated into the sensor: mental chemistry. Pesticides are, in fact, among the most transient or steady-state responses are monitored by the inte- important environmental pollutants because of their increas- grated detector. ing use in agriculture [3-5]. The transducer part of the sensor serves to transfer the Among the pesticides, organophosphorus and carbamate signal from the output domain of the recognition system to, insecticides form an important class of toxic compounds; mostly, the electrical domain. The transducer part of a sensor their toxicity is based on the inhibition of acetylcho- is also called a detector, sensor or electrode, but the term linesterase (AChE). Organophosphate and carbamate pesti- transducer is preferred, to avoid confusion. Example of elec- cides toxicity can vary considerably, depending on the trochemical transducers (Potentiometry, amperometry, volt- chemical structure of the pesticide [6, 7]. Many methods are ammetry, surface charge using field effect transistors available for pesticide detection: chromatographic methods, (FETs), and conductometry) which are often used to measure such as gas chromatography (GC) and high performance the output signal from the biorecognition domain. liquid chromatography (HPLC) coupled with mass spec- trometry (MS). These methods are very sensitive and reliable 1.1. Sensing Mode but present strong drawbacks such as complex and time- consuming treatments of the samples, i.e. extraction of pesti- 1.1.1. Amperometry cides, extract cleaning, solvent substitution, etc. [8-12]. Amperometry is based on the measurement of current re- Moreover, they can only be performed by highly trained sulting from the electrochemical oxidation or reduction of an technicians and are not convenient for on-site or on in-field electroactive species. It is usually performed by maintaining detection. a constant potential at a Pt-, Au- or C-based working elec- Biosensors are potentially useful as they detected pesti- trode or an array of electrodes with respect to a reference cides quickly and have been active in the research area for electrode (two measuring electrode system without auxiliary some years. Biosensors have been defined as analytical de- electrode), if the current are low (10-9 to 10-6 A). vices which tightly combine bio-recognition elements with physical transducers for detection of the target compounds Amperometric immunosensors detect the concentration- dependent current, generated when an electroactive species is either oxidized or reduced at the electrode surface to which Ab-Ag binds specifically, it is held at a fixed electrical *Address correspondence to this author at the Pharmaceutical Chemistry potential. The current is directly proportional to specific Department, College of Pharmacy, King Saud University, Riyadh, Saudi Arabia; Tel: 00966-55-7117817; Fax: 00966-1-4667220; Ab-Ag binding. The current and bulk concentration of the E-mail: [email protected] detecting species can be approximated as: 1876-505X/10 2010 Bentham Open Electrochemical Biosensors for the Detection of Pesticides The Open Electrochemistry Journal, 2010, Volume 2 23 Fig. (1). Schematic representation of biosensors. (Anal. Chim. Acta, 2006, 568, 221). * I = Z F km C (1) 1.1.4. Conductometry where I is the current to be measured, Z and F are constants, The principle of the detection is based on the fact that * km is the mass transfer coefficient and C is the bulk many biochemical reactions in solution produce changes in concentration of the detecting species. the electrical resistance (reciprocal conductance). Conduc- 1.1.2. Potentiometry tance measurements involve the resistance determination of a sample solution between two parallel electrodes. Many en- Potentiometric measurements involve the determination zyme reactions, such as that of urea and many biological of potential difference between both an either indicator and a membrane receptors may be monitored by ion conductomet- reference electrode or two reference electrodes separated by ric or impedimetric devices, using interdigited microelec- a permselective membrane, when there is no significant cur- trode [19, 20]. In an immunosensor, there is an overall elec- rent flow between them. The most common potentiometric trical conductivity of the solution and capacity alteration due devices are pH electrodes; several other ions (F-, I-, CN-, to the Ab-Ag interaction at the electrode surface. + + + Na , K , Ca , NH4) or gas- (CO2, NH3) selective electrodes are available. 2. BIOSENSORS Potentiometric immunosensors are based on measuring Biosensors and bioanalytical methods appears well suited the changes in potential induced by the label used, which to complement, standard analytical methods for a number of occur after the specific binding of the Ab-Ag. They measure environmental monitoring applications. The definition for a the potential across an electrochemical cell containing the Ab biosensor is generally accepted in the literature as a self con- or Ag, usually by measuring the activity of either a product tained integrated device consisting of a biological recogni- or a reactant in the recognition reaction monitored. The tion element (enzyme, antibody, receptor, DNA or microor- measured potential is given by the Nernst equation: ganism) which is interfaced to a chemical sensor (i.e., analytical device) that together reversibly respond in a concen- E = constant ± RT ln a (2) nF tration-dependent manner to chemical species. The use of biosensors for environmental applications has been reviewed where E is the potential to be measured, R, T, F are con- in considerable detail [21]. Different recommendations were stants, n is the electron transfer number, and a is the relative postulated for defining and describing the characteristic ef- activity of the ion of interest. fect on biosensors performance. Some properties and charac- 1.1.3. Surface Charge Using Field-Effect Transistors teristic behaviours of ideal biosensors were evaluated, in (FETs) accordance with standard IUPAC protocols or definition [22- 24]. Which include selectivity, response time, linear range, Field effect transistor (FET) and particular ion sensitive limit of detection, reproducibility, stability and lifetime. FETs (ISFET), have to be presented as a basis for biosensor developments. The main part of an ISFET is ordinary metal 2.1. Enzyme-Based Biosensor oxide silicon FET (MOSFET) with the gate electrode re- placed by an ion selective membrane, a solution and a refer- Enzymes are organic catalysts produced by the living cell ence electrode. The nature of the membrane /insulator will, that act on substances called substrates. Like all other catalysts, enzymes only catalyse thermodynamically feasible then, give the ion specificity of the sensor (pH, NH3). The pH sensitive IFSETs are the most widely used sensors for the reactions. The enzyme-based sensors measure the rate of the biosensor developments, with a large range of possible insu- enzyme-catalyzed reaction as the basis for their response, any physical measurement which yields a quantity related to lators (SiO2, Al2O3 and Ta2O5) [16-18] and enzyme labels. When such ISFETs are coupled with a biocatalytical or bio- this rate can be used for detection. Several procedures have comlpexing layer, they become a biosensor, and are usually been devised for the monitoring of the activity of an enzyme called either enzyme (ENFETs) or immunological (IMFETs) using electrochemical transducers. The

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