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Application of Electrochemical Methods and Biosensors in Food Forensics

Introduction

Electrochemical methods of analysis have attractive features which include selectivity, sensitivity, speed and also involve simple procedures and relatively simple/inexpensive instrumentation as well as simplicity of the analytical procedure, which involves less sample preparation procedures/steps (Bard and Faulkner, 1980). Electrochemical methods are capable of both qualitative and quantitative measurements of samples and there are several variants that have been applied in the analysis or detection of food components, food contaminants, etc.

Examples of electrochemical methods that are frequently applied in the analysis of food additives and food ingredients, include sugars (mainly glucose), vitamins, antioxidants (i.e. ascorbic acid), as well as other food and beverage components such as ethanol. Among these electrochemical techniques, applications in food analyses are those based on voltammetry, such as stripping voltammetry, an anodic stripping voltammetry technique that can be useful in the trace analysis of metal cations where the cation (analyte) is deposited onto the working electrode during a deposition step, and then oxidized during the stripping step when the current is measured.

Another voltammetric stripping technique is known as cathodic stripping voltammetry, which is useful in the trace analysis of anions whereby a positive potential is applied, oxidizing the working electrode such as a mercury electrode, thus forming insoluble precipitates of the anions. A negative potential then strips the deposited film into solution. Other voltammetric methods include linear sweep voltammetry, staircase voltammetry, square wave voltammetry and cyclic voltammetry. Cyclic voltammetry is useful in the determination of diffusion coefficients, as well as half‐cell reduction potentials of the analyte. Another voltammetric method is adsorptive stripping voltammetry, which can be useful in the trace analysis of analytes. In this technique, the analyte is deposited by adsorption on the electrode surface and then electrolyzed in order to generate a measurable signal. In addition to these variants of voltammetric methods of analysis, there are other variants that make use of mercury as a working electrode and they are commonly known as polarography. Others make use of rotated electrodes and are useful for studying the kinetics and electrochemical reaction mechanism for a half reaction. Other variants of voltammetry include normal pulse voltammetry, differential pulse voltammetry and chronoamperometry.

Of these voltammetric electroanalytical techniques, stripping voltammetric methods have been widely applied in the analysis of foods and food ingredients important for quality authentication such as vitamins, aromatic flavors, alkaloids, organic acids, etc. (Pisoschi, 2013). Stripping voltammetric techniques have been known for their high sensitivity over other voltammetric methods due to the fact the analytes of interest are accumulated and preconcentrated onto the working electrode by means of controlled potential electrolysis prior to the stripping step, which involves the dissolution of the deposit that will either oxidize or reduce the analyte when a linear ramp is applied to the electrode and thus generate a measurable signal at the electrode surface (Pisoschi, 2013).

From the signal generated, which is normally plotted as the current‐potential plot known as the current‐potential voltammogram, the shape of this plot as generated during the initial step is vital in terms of providing information about the sample. For example, the value/magnitude of the peak potential is specific and a unique characteristic of the analytes and can thus be useful qualitatively. The peak current associated with the process is very quantitative, as it is proportional to the concentration of the analyte. Moreover, since the magnitude of the stripping peaks for different analytes occur at uniquely characteristic potentials, it is then possible to have several analyte species in the same matrix/food and analyze them all simultaneously (Branina, 1974 ; Brainina and Neyman, 1993 ; Vydra et al., 1976 ; Wang, 1985, 1994).

In anodic stripping voltammetry, which has been used for the trace analysis of heavy metals in foods, the working electrode that has been applied includes a hanging mercury drop electrode (HMDE) or mercury film electrode (MFE), whereby the cations/metal species deposited or preconcentrated cathodically at a controlled time and potential onto the mercury electrode are dissolved electrolytically. After preconcentration and dissolution, the potential is then scanned anodically and linearly such that during the anodic scan process, the amalgamated metallic species are stripped out of the electrode and are then reoxidized and dissolved in the sample solution. The magnitude of the anodic current associated with this process is then recorded as a function of the applied voltage. In cases where the anodic reaction generates insoluble products such as mercury salts or mercury complexes, it is then plausible to use the cathodic stripping voltammetric technique (CSV) in which the oxidation of the analyte is used for its preconcentration step as an insoluble film on the electrode, and then the concentrated reduced analyte species is measured during the negative scan.

In cases where the analyte species cannot be preconcentrated by electrolysis processes at the electrode surface, then the preconcentration of the analyte can be achieved using adsorption processes (adsorptive stripping voltammetry (AdSV)), whereby the accumulation of the analyte takes place through physical adsorption instead of electrolytic deposition processes.

The use of HMDE or MFE have been discouraged in many laboratories due to the fact that they are a health concern and also not environmentally friendly. Therefore other electrode systems have been suggested to replace them. These include carbon paste electrodes (CPE) (Shams and Torabi, 2006), polymer film electrode (PFE) (Shelton and Chambers, 1991), surface‐bound crown ethers electrodes (SBCEE) (Ijeri and Srivastava, 2001), carbon nanotube electrodes (CNE) (Gong et al., 2005), boron‐doped diamond electrodes (BDDE) (Spataru et al., 2007), bismuth film electrodes (BFE) (Baldo et al., 2003) and screen printed electrodes (SPE) (Crew et al., 2008).

Sample preparation procedures for samples to be subjected to stripping voltammetry involve mainly acid digestion using mixtures of strong acids to decompose the sample or the use of microwave digestion approaches for elemental analysis, solvent extraction and solid phase extraction procedures.

The application of stripping voltammetry in foods has been reported in the analysis of toxic food contaminants of metallic origin, such as mercury (Capar et al., 1982 ; Sancho et al., 2001), lead (Satzger et al., 1983 ; Zink et al., 1983), cadmium (Capar et al., 1982), arsenic (Sancho et al., 1998), antimony (Locatelli and Torsi, 2004) and uranium (El‐Maali and El‐Hady, 1999). In another development, a simultaneous analysis of lead and cadmium in food crops, using differential pulse anodic stripping voltammetry, has also been reported (Golimowski et al., 1979 ; Satzger et al., 1982).

Differential pulse anodic stripping voltammetry has been reported in the simultaneous determination of lead and cadmium in food crops (Satzger et al., 1984), while stripping voltammetry was reported in the determination of lead and cadmium in vegetables (Matloob, 2003), also in wheat and rice (Ogorevc et al., 1987), in table salt (Ali, 1999), in liver and fish (Adeloju et al., 1983), in infant formulas (Esteve et al., 1994) and in canned soft drinks (Sabry and Wahbi, 1999).

Agrochemical contaminants residues analysis in foodstuffs including water is also one of the application areas for stripping voltammetric techniques (Ibrahim et al., 2001 ; Mainisankar et al., 2005 ; Pedrero et al., 1993). For example, organophosphorus pesticides such as fenthion (Diaz et al., 2008) and pyridafenthion (Sampedro et al., 1998) have been determined in olive oil and wine respectively using the adsorptive stripping voltammetric technique, while other organophosphorus insecticides such as phosalone and carbophos (malathion) have also been determined in potato and tomato with this same technique (Ulakhovich et al., 1998).

Certain types of carbamate insecticides have been detected in rice using anodic stripping voltammetry (Mathew et al., 1996). These are only a few examples of the application of stripping voltammetric techniques with application in food analysis, but there are many reports on the technique’s application in the analysis of various agrochemical residues in different types of foodstuffs and drinks.

Apart from agrochemical residues, stripping voltammetric techniques have been widely applied in the determination of many pharmaceutical drug residues as well as veterinary drug residues in foods/feeds (Abu Zuhri et al., 1998 ; Alghamdi, 2002 ; Gratteri et al., 1992 ; Vire et al., 1989). In other developments, anodic stripping approaches have been reported in the determination of fertilizer residues in foods such as meat and other foodstuffs (Guanghan et al., 1997 ; Santos et al., 2009). Voltammetric/polarographic methods have been developed for contaminants such as heavy metals, food additives such as coloring compounds and organic compounds in foods (Alghamdi, 2010).

Biosensors for Food Analysis

Biosensors are artificial devices fabricated to sense or detect specific analytes and they are composed of three components (Figure 19.1) namely:

  1. a biological recognition element (tissue, microbial cell, cell receptor, cell organelle, nucleic acids, enzymes, antibodies, protein, aptamers, synthetic oligonucleotides, etc.), commonly known as a bioreceptor. A bioreceptor is the biomimetric component that functions to bind and recognize the analyte being investigated or measured. This implies that when the target analyte interacts with the bioreceptor, it generates a signal that is measured by the detector (transducer);
  2. a biotransducer, a component that plays the detection function in various ways, e.g. physical, optical, piezoelectric, electrochemical, etc. and through these mechanisms the signal of the recognized analyte is transformed or transduced into another measurable signal, e.g. electrical signal (current, voltage, etc.);
  3. an electronic system that may incorporate signal amplifier, processor and display platform. In some cases, the transducer component and the electronic system component are combined together.
Image described by caption and surrounding text.

Figure 19.1 Schematic depiction of a biosensor.

The type of bioreceptor normally gives the name to the biosensor, for example if an enzyme is used as the bioreceptor to specifically convert the substrate analyte which acts like a reactant molecule into a product, then this biosensor is known as an enzyme‐biosensor. In enzyme‐biosensors, it is common to incorporate cofactors which are other molecules or ions that assist in the reaction in such a way that they become catalytically changed, chemically producing physico‐chemical effects that can play important roles in the monitoring and detection of the enzymatic process. Antibodies also can be used as bioreceptors for some specific molecules (antibody‐biosensor) and if DNA is used as a bioreceptor then we will have a DNA‐biosensor and all these bioreceptors can be used to specifically recognize and bind any kind of analyte present in the food matrix for the purpose of providing the evidence required for the specific issue. The analytes may be drugs residues, toxic molecules, etc.

Biosensors can also be grouped based on the type of biotransducer used for that particular biosensor. There are several classes of biotransducers that have been fabricated and used in biosensor systems and they include electrochemical biosensors, piezoelectric biosensors, optical biosensors, electronic biosensors, gravimetric biosensors and pyroelectric biosensors.

There classes of biosensors, irrespective of the type of classification, have been used widely in food analysis. For example, antibody‐coated optics are more often used in the food industry for the detection of pathogens or food toxins, whereby fluorescence is used as the preferred light source due to its advantage to significantly amplify the signal.

Electrochemical Biosensors in Food Analyses

Generally, electrochemical biosensors have been widely used in food analysis and are based on redox mediated enzymatic catalysis of bio‐reactions that either generate or consume electrons, which are detected using a set of three electrodes (working electrode, reference electrode and a counter electrode). During the measurement process, the analyte is brought to the surface of the active working electrode where there will be a reaction with the bioreceptor that will either cause electrons to be transferred across the double layer and thus generate current or can be added to the double layer potential implying that voltage (potential) will be produced. The current or voltage produced can allow both qualitative and quantitative measurement. For example, it is possible to measure the current (the rate of flow of electrons), which will be proportional to the analyte concentration when the current measurement is done at a fixed potential (Lud et al., 2006). Where the potential produced is measured at zero current to give a logarithmic response with a high dynamic range, such a biosensor is generally known as the potentiometric biosensor.

One attractive feature about biosensors is that they involve very minimal sample preparation due to the fact that the biological sensing component (the bioreceptor) is fabricated to have high selective properties for the analyte being measured. The electrochemical as well as physical changes (e.g. ionic strength, pH, hydration and redox reactions) are the ones to account for the signal that is produced at the conducting polymer layer at the surface of the transducer/sensor.

For example, other electrochemical methods are based on the potential vs. pH change measurements, which occur due to changes in the ion concentration in the sample matrix, i.e. potentiometric techniques. Other techniques involve the use of sensors such as potentiometric biosensors, which are fabricated with a sensing bio‐based transducer that monitors biochemical reactions involving variations in terms of ion concentration variation. Examples of transducers that may be incorporated into potentiometric biosensors include enzymes coupled with a glass‐pH electrode such that enzymatic reactions with target analytes like sugars or food contaminants (i.e. pesticides or pharmaceutical residues) can occur, leading to either generation of hydrogen ions or reactions that may use the generated hydrogen ions and this will be measured by the pH sensor (Blum and Coulet, 1991 ; Scheller and Schubert, 1992).

Among the various types of electrochemical biosensors there are voltammetric sensors, which provide a measure of the concentration effect of the analytes on the current potential characteristics of either the reduction or oxidation of the reaction (Bakker, 2004). Another class of electromechanical sensors are the amperometric biosensors, which are actually a subclass of the voltammetric biosensors and are based on the passing of a fixed voltage/potential to the electrochemical cell, thus generating a signal in the form of the current due to either oxidation or reduction reaction processes, and this current is proportional to the concentration of the analytes being investigated (Bakker, 2004 ; Viswanathan and Radecki, 2008 ; Wang, 2005). Amperometric biosensors make use of conventional detectors to detect and measure the metabolic substrate or product of the analyte under investigation (Patel, 2002).

Potentiometric biosensors are another type of electrochemical‐based biosensor, which investigate the magnitude of the potential difference measurement between the working electrode and the reference electrode as a function of the redox reaction of the species being investigated. The potentiometric biosensors are useful in cases where the accumulation of charge at zero current as created by selective binding at the electrode surface needs to be known (Bakker, 2004). The application of potentiometric biosensors has been greatly enhanced by the development of the ion‐selective electrodes including the glass electrode, which measures pH of solution, and ion‐selective field effect transistors (ISFETs), which incorporate an ion‐sensitive surface (Castellarnau et al., 2007). In ion selective electrodes, the surface electrical potential is governed by the ions interacting with the semiconductor surface, which results in a measurable potential difference.

Other classes of biosensors such as piezoelectric biosensors, employ crystals which undergo an elastic deformation when an electrical potential is applied to them such that the process leads to the generation of a standing wave in the crystal at a specific characteristic frequency that is highly dependent on the elastic properties of the crystal. For example, if a crystal is coated with a biological recognition element, the binding of a (large) target analyte to a receptor will generate a change in the resonance frequency, which results in the generation of a binding signal. The application of other classes of biosensors, such as thermometric biosensors and magnetic‐based biosensors are rare in food analysis applications.

Generally, all these classes of electrochemical biosensors have been used in applications related to immobilization of biomolecules, electrode design and signal transduction for food samples as well as other applications. Currently, the advent of nanotechnology has also revolutionized the technology related to fabrication of electrochemical sensors where nanoparticles, for example gold nanoparticles and nanostructures such as nanotubes, nanofibers, nanorods, nanoparticles and thin films, have been incorporated to enhance sensitivity and selectivity of the detection of specific analytes (Bonnemann and Richards, 2001 ; Niemeyer, 2001).

For example, various nanoparticles have been found to enhance the signals of electrochemical biosensors when they are incorporated/bound to biological molecules such as antibodies, peptides, proteins, nucleic acids, etc. (Hernández‐Santos et al., 2002). The ability of metal nanoparticles to enhance the amount of immobilized biomolecules in fabrication of biosensors lies in their high surface area, which is very central to even lowering detection limit of the biosensor (Cai et al., 2001). Apart from metallic nanoparticles, inorganic nanocrystals such as zinc sulfide, cadmium sulfide and lead sulfide have also been utilized for a multi‐target electronic detection of DNA or proteins. Moreover, the application of magnetic nanoparticles, which are generally prepared in the form of either single domain or superparamagnetic (Fe3O4), greigite (Fe3S4), maghemite (g‐Fe2O3), and various types of ferrites (MeO‐Fe2O3, where Me = Ni, Co, Mg, Zn, Mn, etc.), as diagnostic tools in biosensors has also been known to separate or enrich the analyte being investigated. Other biosensors have been fabricated containing modified surfaces of various functional monolayers or thin films. Electrodes have also been fabricated with functionalized redox active components, thus enhancing the sensitivity of the biosensors, for application in food analysis (Wang, 2002a, b).

Other recent developments in the area of biosensors for food application include the incorporation of one‐dimensional nanostructures, which include carbon nanotubes, polypyrrole nanotubes, conducting polymer or semiconductor nanowires that are characterized by having high surface area to volume ratios as well as excellent electron transport characteristics and strong electronic conductance (Baughma et al., 2001 ; Miao et al., 1999). The application of these nanostructures in biosensor fabrication is attractive due to the fact that they make it possible for sensing elements to be packed in high numbers within a miniaturized device (Siwy et al., 2005).

In other developments, liposomes, which are biological microstructures made of layers of phospholipids, have been used as the supporting substrate for immobilizing the biorecognition molecules and for the enhancement of electrochemical signals in applications related to the detection of organophosphorus agrochemical residues (Baeumner et al., 2003 ; Vamvakaki and Chaniotakis, 2007). Moreover, the concept of using artificial sensory mappers, such as the electronic nose and electronic tongue, has received considerable attention in the recent past as sensors for various volatile compounds. These kinds of sensors are capable of discriminating a number of volatile species based on their electronic responses such as voltage, resistance, conductivity, etc. that come from the different gas sensors, normally metal‐oxide chemosensors. For example, Jonsson et al. (1997) reported the application of the electronic nose to classify cereal grains, for example to discriminate between mouldy, weakly musty and strongly musty oat samples. The same kind of sensor was able to predict ergosterol levels and fungal colony‐forming units in wheat (Jonsson et al., 1997 ; Olsson et al., 2002). Olsson and colleagues also used the sensor to predict deoxynivalenol and ochratoxin A levels in barley, to indicate ochratoxin A, citrinin and ergosterol production in wheat, while other researchers used the same biosensor system to indicate mycotoxin formation by Fusarium strains (Falasconi et al., 2005). On the other hand, the electronic tongue has been used to discriminate between natural and artificial water, different sorts of beverages, natural and artificial mineral waters, brands of coffee, sweeteners, etc. (Rudnitskaya et al., 2002).

Conclusions

Electrochemical methods and biosensors present a cheap and rapid means of analysis. In most cases they may serve as screening methods (the absence and/or presence of adulterants, etc.). Biosensors can be fabricated as miniaturized devices that may be taken to the scene where food poisoning has taken place and they only need minute amounts of the sample to provide data that can provide a clue to the evidence needed.

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