Biosensors for Pesticide Detection: New Trends

Audrey Sassolas, Beatriz Prieto-Simón, Jean-Louis Marty

Laboratoire IMAGES EA 4218, Université de Perpignan via Domitia, Perpignan, France.
Nanobioengineering Group, Institute for Bioengineering of Catalonia, Barcelone, Spain.

DOI: 10.4236/ajac.2012.33030                

Abstract

Due to the large amounts of pesticides commonly used and their impact on health, prompt and accurate pesticide analysis is important. This review gives an overview of recent advances and new trends in biosensors for pesticide detection. Optical, electrochemical and piezoelectric biosensors have been reported based on the detection method. In this review biosensors have been classified according to the immobilized biorecognition element: enzymes, cells, antibodies and, more rarely, DNA. The use of tailor-designed biomolecules, such as aptamers and molecularly imprinted polymers, is reviewed. Artificial Neural Networks, that allow the analysis of pesticide mixtures are also presented. Recent advances in the field of nanomaterials merit special mention. The incorporation of nanomaterials provides highly sensitive sensing devices allowing the efficient detection of pesticides.

Keywords

Biosensors; Pesticides

1. Introduction

In agriculture, farmers use numerous pesticides to protect crops and seeds before and after harvesting. Pesticide is a term used in broad sense for organic toxic compounds used to control insects, bacteria, weeds, nematodes, rodents and other pests. The pesticide residues may enter into the food chain through air, water and soil. They affect ecosystems and cause several health problems to animals and humans. Pesticides can be carcinogenic and cytotoxic. They can produce bone marrow and nerve disorders, infertility, and immunological and respiratory diseases.

Detection of pesticides at the levels established by the Environmental Protection Agency (EPA) remains a challenge. Chromatographic methods coupled to selective detectors have been traditionally used for pesticide analysis due to their sensitivity, reliability and efficiency. Nevertheless, they are time-consuming and laborious, and require expensive equipments and highly-trained technicians. Over the past decade, considerable attention has been given to the development of biosensors for the detection of pesticides as a promising alternative. A biosensor is a self-contained device that integrates an immobilized biological element (e.g. enzyme, DNA probe, antibody) that recognizes the analyte (e.g. enzyme substrate, complementary DNA, antigen) and a transduction element used to convert the (bio)chemical signal resulting from the interaction of the analyte with the bioreceptor into an electronic one. According to the signal transduction technique, biosensors are classified into electrochemical, optical, piezoelectric and mechanical biosensors. Electrochemical transducers have been widely used in biosensors for pesticides detection due to their high sensitivity [1-3]. Additionally, their low cost, simple design and small size, make them excellent candidates for the development of portable biosensors [4-8]. According to the biorecognition element, enzymatic, whole cell, immunochemical, and DNA biosensors have been developed for pesticides detection.

This review presents a state-of-the-art update in pesticide biosensors. To clearly report the last advances, biosensors have been classified according to the immobilized recognition element. New trends in the field of pesticide analysis are also reviewed. Aptamers are shown as good candidates to replace the conventional antibodies and, thus, to be the biorecognition elements in more robust and stable biosensors for pesticide detection. Due to exceptional characteristics, molecular imprinted polymers (MIPs) are innovative affinity-based recognition elements that are exploited for the development of environmental sensors. The use of Artificial Neural Networks (ANNs) coupled with a sensor array could substantially improve the selectivity and allow exact identification of pesticides present in a sample. Recent reports on the properties of nanomaterials show nanoparticles and nanotubes as promising tools to improve the efficiency of biosensors for the detection of pesticides.

2. Enzyme Biosensors

Enzyme biosensors for pesticide detection are based on measurements of enzyme inhibition or on direct measurements of compounds involved in the enzymatic reaction.

2.1. Inhibition-Based Biosensors

2.1.1. Cholinesterase-Based Biosensors

Enzymatic detection of pesticides is mainly based on cholinesterase (ChE) inhibition [6-10]. Organophosphate and carbamate insecticides are the main ChE inhibitors (Table 1). Other compounds, such as heavy metals, fluoride, nerve gas or nicotine, can also inhibit ChE enzyme. Although this lack of selectivity, ChE-based biosensors are shown as powerful tools when a rapid toxicity screening is required.

2.1.1.1. Mono-Enzymatic Biosensors

Two types of natural ChE enzymes are known: acetylcholinesterase (AChE) and butyrylcholine-sterase (BChE). These enzymes have different substrates: AChE preferentially hydrolyzes acetyl esters, such as acetylcholine (Equation (1)), whereas BChE hydrolyzes butyrylcholine (Equation (2)):

(1)

(2)

The pH variation produced by the acid formation can be measured using electrochemical methods, such as potentiometry [11]. This pH change can also be measured using pH-sensitive spectrophotometric indicators [12,13] or pH sensitive fluorescence indicators [14].

Artificial substrates, acetylthiocholine for AChE and butyrylthiocholine for BChE, have been also used. The enzymatic hydrolysis of these substrates produces electroactive thiocholine (Equations (3) and (4)).

(3)

(4)

(5)

This system has two advantages over the bi-enzymatic ChE/ChOD biosensors. First, it has a simpler design. Secondly, the detection potential is lower than the one used for the oxidation of H₂O₂.

La Rosa et al. proposed the use of 4-aminophenyl acetate as alternative ChE substrate [15,16]. They oxidize the enzymatic product 4-aminophenol at +250 mV vs SCE. Electrochemical biosensors for pesticide detection based on the use of this substrate avoid interferences from the oxidation of other electroactive compounds [15- 18]. However, 4-aminophenyl acetate is not commercially available and its use involves a laborious and timeconsuming synthesis. Moreover, this substrate is unstable and requires special storage conditions (nitrogen atmosphere, below 0˚C).

2.1.1.2. Bi-Enzymatic Biosensors

In most cases, ChE is coupled to choline oxidase (ChOD) [6]. AChE hydrolyzes its natural substrate to choline and acetic acid (Equation (1)). Since choline is not electrochemically active, ChOD is used to produce H₂O₂, which can be oxidized onto the platinum electrode at around + 0.7 V vs Ag/AgCl (Equations (7) and (8)). However, an over-potential is required, favouring the oxidation of interfering electroactive species present in real samples. To overcome this drawback, different approaches have been proposed, such as the use of nanomaterial-modified electrodes. A biosensor for the detection of pesticides and nerve agents was developed by immobilizing AChE and ChOD onto Au-Pt bimetallic NPs [19]. The synergistic effect of these nanoparticles increased the surface area and facilitated the electron transfer process, reducing the applied potential for the detection of H₂O₂. Alternatively to H₂O₂ oxidation, ChE inhibition can be followed using a Clark electrode able to measure the oxygen consumed by the ChOD catalyzed reaction (Equations (6)-(8)) [20].

(1)

(6)

(7)

(8)

AChE was also coupled to tyrosinase [18]. In this case, AChE enzymatic hydrolysis of phenyl acetate produces phenol compounds, characterized by a high oxidation potential. For this reason, tyrosinase enzyme was used to convert the phenol to quinone, compound that can be electrochemically reduced to catechol at –150 mV vs Ag/AgCl.

2.1.1.3. Tri-Enzymatic Biosensors

Peroxidase may be added to the bi-enzyme system to develop a tri-enzymatic biosensor. Karousos et al. used a Quartz Crystal Microbalance (QCM) sensor based on three enzymes for the determination of organophosphorus and

Table 1. Characteristics of electrochemical cholinesterase-based biosensors for pesticide detection.

carbamate pesticides [21]. Acetylcholine was converted to choline by AChE and then, choline was converted to hydrogen peroxide by choline oxidase. In the presence of HRP, H₂O₂ oxidized 3,3’-diaminobenzidine to an insoluble product that precipitated out and adsorbed on the crystal surface causing a decrease in the resonant frequency of the crystal. AChE inhibition caused by pesticides reduced the amount of QCM-detectable precipitate produced. This QCM-enzyme sensor system allowed detecting carbaryl and dicholorvos concentrations down to 1 ppm.

2.1.1.4. ChE Sources

The enzyme source has an important effect on the biosensor performance. Several AChE enzymes are available from different sources, such as Electric eel, Bovine or Human erythrocytes, Horse serum and Human blood. Generally, ChE enzymes isolated from insects are more sensitive than those extracted from other sources. The use of recombinant ChE enzymes also allows improvements on the sensitivity of biosensors [22]. As an example, Valdes-Ramirez and co-workers compared the use of three AChEs in biosensors for the detection of dichlorovos in a sample of apple skin [23]. The use of genetically modified AChE decreased four orders of magnitude the detection limit found for the use of AChE from wild type Drosophila melanogaster and Electriceel.

2.1.2. Tyrosinase-Based Biosensors

Tyrosinase oxidizes monophenols in two consecutive steps: first, the enzyme catalyzes the o-hydroxylation of monophenol to o-diphenol (cresolate activity, Equation (9)) which, in a second step, is oxidized to its corresponding o-quinone (catecholase activity, Equation (10)):

(9)

(10)

Tyrosinase is inhibited by different compounds, such as carbamate pesticides and atrazine. Numerous electrochemical biosensors based on the inhibition of tyrosinase activity have been reported [24-29] (Table 2).

Tyrosinase biosensors suffer from poor specificity since many substrates and inhibitors can interfere. The enzyme is inherently unstable, reducing the lifetime of the tyrosinase-based biosensors. However, tyrosinase can stand high temperatures and the organic solvents used to dissolve the pesticides.

2.1.3. Alkaline Phosphatase (ALP)-Based Biosensors

Alkaline phosphatase catalyses the following reaction:

(11)

ALP is inhibited by different compounds. Several ALPbased biosensors for the detection of pesticides have

Table 2. Characteristics of electrochemical inhibition-based biosensors using tyrosinase for pesticide detection.

been developed using different enzyme substrates depending on the transduction method.

Ayyagari et al. described a chemiluminescent ALPbased biosensor for the detection of paraoxon [30]. The biosensor was based on the measurement of the intensity of the light generated by ALP-catalyzed dephosphorylation of a chemiluminescent substrate, chloro 3-(4-methoxy spiro [1,2-dioxetane-3-2’-tricyclo-[3.3.1.1]-decan]-4- yl) phenyl phosphate.

A fluorescent ALP-based biosensor for the detection of organochlorine, pesticides (carbamate and fenitrothion), heavy metals and CN^– was also described [31]. ALP enzyme catalyzed the hydrolysis of 1-naphthyl phosphate to fluorescent 1-naphthol.

Mazzei and co-workers developed electrochemical ALP-based biosensors for the detection of malathion and 2,4-dichlorophenoxyacetic acid (2,4-D) by using 3-indoxyl phosphate, phenyl phosphate or ascorbate-2- phosphate as enzyme substrates [32]. Another electrochemical ALP-based biosensor was also described for the screening of several environmental pollutants. The biosensor was based on the entrapment of ALP in a hybrid sol-gel/chitosan film, deposited on the surface of a screen-printed electrode [33]. The substrate ascorbic acid 2-phosphate was catalyzed by the enyme to produce ascorbic acid, which was monitored by amperometry.

2.1.4. Peroxidase-Based Biosensors

Peroxidase molecules can be first oxidized by H₂O₂ and then reduced by phenolic compounds. This process involves two enzyme intermediates: compounds I and II (Figure 1). Phenolic compounds are thus oxidized to quinones or free radical products, able to be electrochemically reduced on the electrode surface. Several organic and inorganic compounds have been reported to inhibit the enzyme activity of peroxidase by coordinating compound I. A biosensor based on the inhibition of peroxidase was described for the detection of thiodicarb, a carbamate pesticide [34]. HRP was covalently bound on a gold electrode. In the presence of hydrogen peroxide, hydroquinone was oxidized by peroxidase to p-benzoquinone which could be electrochemically reduced to hydroquinone at a potential of –0.072 V vs Ag/AgCl. The presence of inhibitor compounds induced a decrease of the biosensor current response.

2.1.5. Acid Phosphatase

Acid phosphatase (AP) is reversibly inhibited by some pesticides. AP has been used with glucose oxidase (GOD) to develop a bienzymatic biosensor for the electrochemical detection of Malathion, methyl parathion and paraoxon [37]. Both enzymes were coupled on a commercial H₂O₂ sensing electrode. This system is based on the following reactions:

Figure 1. Scheme of the reactions occurring at the surface of a peroxidase-modified electrode. ox: oxidized form, red: reduced form [35,36].

2.2. Catalytic Biosensors

2.2.1. Organophosphorus Hydrolase (OPH)

OPH is an enzyme that hydrolyzes organophosphorus pesticides [38], such as parathion, methyl parathion [39] or paraoxon [40,41]. This enzyme hydrolyzes P-O, P-S and P-CN bonds generating two protons, able to be electrochemically detected, and an alcohol, which in many cases is chromophoric and/or electroactive.

However, these biosensors show lower sensitivity values and higher detection limits than cholinesterase-based biosensors. Moreover, they can only detect some organophosphorus (OP) compounds.

Table 3 summarizes the performances of some OPHbased biosensors reported in the literature.

2.2.2. Glutathion-S-Transferase

Glutathion-S-transferase (GST) was used to develop a fiber-optic biosensor for the detection of atrazine [42]. The enzyme was immobilized by cross-linking on a membrane that was supported on an inner glass disk by means of an intermediate binder sol-gel layer. Bromcresol green was incorparated in the sol-gel as pH indicator. GST catalyzed the nucleophile attack of GSH on atrazine, releasing H⁺. This pH variation was optically measured by colour changes of bromcresol green.

3. Whole Cell Biosensors

3.1. Microbial Biosensors

To develop a microbial biosensor, microorganisms have to be immobilized onto a transducer using different chemical (e.g. cross-linking) or physical techniques (e.g. entrapment) [43]. Microorganisms have several advantages

Table 3. Characteristics of hydrolase-based biosensors.

as sensing elements in the development of biosensors. They are able to metabolise a wide range of chemical compounds. The use of whole cells, as a source of intracellular enzymes, avoids expensive protocols of enzyme purification. The enzyme is maintained in its natural environment improving its stability and activity. The main limitation of the use of whole cells is the diffusion of substrate and products through the cell wall resulting in a slow response as compared to enzyme-based biosensors. To overcome this drawback, cells can be permeabilised [44].

3.1.1. Electrochemical Microbial Biosensors

3.1.1.1. Amperometric Detection

Amperometric microbial biosensors have been widely developed for the determination of biochemical oxygen demand (BOD) in order to measure biodegradable organic pollutants in aqueous samples. Most of BOD biosensors consist of a microbial film sandwiched between a porous cellulose membrane and a gas-permeable membrane. Organic substrates, present in wastewater samples, diffuse through the dialysis membrane and are assimilated by the immobilized microbial population, increasing the bacterial respiration rate. Therefore, less dissolved oxygen diffuses through the gas-permeable Teflon membrane to be detected by a Clark oxygen electrode [45]. Different microbial strains were used as biosensing element such as Arxula adeninivorans [46], Bacillus subtilis [47], Serratia marcescens [48] or yeast [49]. Single microorganisms metabolize a limited range of organic pollutants, which may result in an inaccurate estimation of BOD values. To overcome this problem, mixed cultures (e.g. Bacillus subtilis and Trichosporon cutaneum [50]) or activated sludges [51] were used.

Mulchandani’s group developed amperometric microbial biosensors for the determination of organophosphate pesticides with p-nitrophenyl substituent (e.g. paraoxon, methyl parathion, parathion, fenitrothion and ethyl pnitrophenol thiobenzenephosphonate (EPN)) [52]. These biosensors were based on the co-immobilization of microorganisms and OPH (free or expressed on the cell surface of other microorganisms). OPH hydrolyzes the pesticide and releases p-nitrophenol. Released p-nitrophenol can be oxidized by some microorganisms, such as or Pseudomonas putida JS444. Two detection strategies were used:

• OPH hydrolyzes the organophosphorus compounds to produce p-nitrophenol. Released p-nitrophenol was degraded by some bacteria, such as Pseudomonas putida JS444. This degradation resulted in electroactive compounds, amperometrically detected [53,54].

• The degradation of p-nitrophenol by some microbes, such as Arthrobacter sp. JS443, consumes oxygen. A Clark oxygen electrode was used to measure oxygen concentration changes [55-57].

3.1.1.2. Potentiometric Detection

Conventional potentiometric microbial biosensors have been developed using ion-selective electrodes (e.g. pH, ammonium) or gas sensing electrodes (e.g. pCO₂) coated with an immobilized microbial layer. Assimilation of substrates by microbes causes changes in potential due to ion accumulation or depletion [43].

A potentiometric biosensor for the direct detection of paraoxon was based on the immobilization of recombinant E. Coli on a glass pH electrode. Bacteria was engineered to contain the opd gene that encodes the OPH enzyme [58]. Entrapped OPH-active bacteria hydrolyzed OP compounds producing two protons. The quantity of released H⁺ was correlated to the concentration of hydrolyzed paraoxon.

3.1.2. Optical Microbial Biosensors

Optical microbial biosensors allowing the detection of pollutants such as phenols and heavy metals have been developed [52,59]. However, only few optical microbial biosensors allowing the detection of pesticides have been reported. A disposable colorimetric microbial biosensor for the detection of methyl parathion pesticide was described [60]. Whole cells of Flavobacterium sp. were immobilized on a glass fiber filter paper. The OPH activity of Flavobacterium sp. hydrolyzed methyl parathion into p-nitrophenol that can be detected at 410 nm.

3.2. Plant Tissue and Photosynthesis-Based Biosensors

3.2.1. Plant Tissue-Based Biosensors

The use of plant tissue is an attractive alternative to enzymatic biosensors. Tissue that acts as enzyme source presents many advantages [61]:

• High stability and activity resulting from the maintenance of the enzyme in its natural environ-ment;

• Long lifetime of biosensors;

• High reproducibility of the experimental results;

• Availability and low price of a wide range of plant tissues;

• Avoidance of tedious and time-consuming enzyme extraction and purification steps;

• Presence of the required cofactors in the used tissue.

Planktonic algae have been widely used to develop biosensors for pollutants present in the aquatic ecosystems. Biosensors based on immobilized Chlorella vulgaris microalgae were reported [62-64]. Those biosensors were based on the inhibition of enzymes located on the external membrane, such as alkaline phosphatases and esterases, by heavy metals and pesticides.

3.2.2. Photosynthesis-Based Biosensors

Different types of photosynthetic materials were used as recognition element for the development of biosensors: whole cells (e.g. microalgae), chloroplasts or thylakoids and photosystem II (PS II) [65]. PS II is a supramolecular pigment-protein complex located in the thylakoid membrane. It catalyzes the light-induced transfer of electrons from water to plastoquinone in a process that evolved oxygen. The activity of PS II can be inhibited by several groups of herbicides and heavy metals [66].

The measurement of oxygen evolution using a Clarktype electrode is a standard procedure for the determination of the photosynthetic activity [61,67,68]. The incorporation of several types of photosystem II specific artificial electron acceptors as electroactive mediators allows to maximize the photosynthetic activity. Other biosensors are not based on the use of a Clark-type electrode. In these cases, alga were immobilized on the surface of ITO electrode [69] or SPE [70].

Optical photosynthesis-based biosensors have also been described based on the fluorescence induced by chlorophyll a. The light absorbed by chlorophyll molecules of PSII may be assimilated into the light reactions of the photosynthesis or may be released as fluorescence or thermal energy. Herbicides inhibit photosynthetic electron flow by blocking the PSII quinone binding site causing an increase in the chlorophyll fluorescence emission. Based on this principle, herbicide biosensors based on the measurement of the algal chlorophyll fluorescence at 682 nm (under 469 nm excitation light) were developed [71-73]. Recently, three microalgae species (Dictyosphaerium chlorelloides, Scenedesmus intermedius and Scenedesmus sp.) were entrapped within a silica matrix and the increase in the amount of chlorophyll fluorescence signal was used to quantify simazine [74].

4. Immunosensors

Immunosensors are characterized by the highly selective affinity interactions between immobilized antibodies (Ab) or antigens (Ag), on the transducer surface, and their specific analytes, Ag or Ab respectively [75-77]. Unlike enzyme-based biosensors, able to evaluate total toxicity, immuno-sensors are specific for a molecule.

Several immunosensors for pesticides detection have been described, based on electrochemical, optical, piezoelectric and mechanical transduction methods.

4.1. Electrochemical Immunosensors

Table 4 presents performance characteristics of some electrochemical immunosensors.

Conflicts of Interest

The authors declare no conflicts of interest.

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