Food Biochemistry and Food Processing (2 edition)

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45 Biosensors for Sensitive Detection of Agricultural Contaminants, Pathogens and Food-Borne Toxins 869

array, and analytes were measured in this matrix to enhance the

enzymatic responses over analyte ranges of 0–7 mM. Interfer-

ences normally related to electrochemically active compounds

present in fruits were minimised by including a membrane made

out of cellulose acetate.

Electrochemical assay formats may be further enhanced by

electrochemiluminescence (ECL). ECL reactions are of interest

because of their versatility for a range of different types of im-

munoassay. The principle of ECL is the co-oxidation of luminol

and a substrate (called an enhancer) by H 2 O 2 in the presence of

the enzyme horseradish peroxidase (HRP). The resulting amper-

ometric signal is detected by the transducer and a quantifiable

electrical signal is generated. ECL as a detection system has

advantages over other methods, including high sensitivity and

a reduced assay time. In experiments carried out by Rubtsova

et al. (1998), specific antibodies against atrazine were covalently

immobilised on photo-activated nylon. A chemiluminescence-

based assay was subsequently used to measure 2,4-D with a

detection limit of 0.2μg/L.

Portable electrochemical biosensors are also applicable for

the detection of herbicide and pesticide residues. A portable and

disposable immunomembrane-based electrochemical biosensor

was developed for the detection of picloram in spiked lettuce,

rice and water samples, and the latter extracted from paddy

fields. The competitive assay format developed here by Tang

et al. (2008) utilised a polyclonal anti-picloram antibody, puri-

fied from serum extracted from immunised rabbits, and had a

sensitivity of 5 ng/mL.

Alternative Biosensor Formats

Other types of biosensors have also been used for post-harvest

analysis. Pogaˇcnik and Franko (2003) developed a photother-

mal biosensor to measure organophosphate and carbamate com-

pounds in salads, lettuce and onions. This approach used thermal

lens spectrometry, a technique that depends upon the adsorp-

tion of optical radiation in the sample generating heat. This

introduces a change in the RI, and, through this analysis, con-

centrations can be determined. Paraoxon was detected in all of

the samples tested by this protocol. Recently, a piezoelectric

antibody-based sensor for the detection of pesticide residues

in fruit juice matrices was described by March et al. (2009).

Piezoelectric, or mass-based, sensors operate on the principle

that a biorecognition event, such as the interaction between an

antibody and an agricultural contaminant, results in a change

in mass. This is conducive to a change in, for example, reso-

nance frequency that can readily be detected by the end-user.

Piezoelectric sensors typically use quartz crystals as transduc-

ers, and are a cost-effective alternative to optical sensors, which

are often expensive (Byrne et al. 2009, Conroy et al. 2009).

The immunosensor described by March et al. was based on the

implementation of a quartz-crystal microbalance for detecting

carbaryl and 3,5,6-trichloro-2-pyridinol (TCP) residues. Here,

a rapid (20 minute) inhibition assay format was adopted, us-

ing antigen-specific monoclonal antibodies for biorecognition,

and the piezoelectric sensor had good detection limits for car-

baryl (11μg/L) and TCP (7μg/L) in spiked fruit juice samples,

although the authors suggested that assay sensitivity could be

improved by incorporating crystals with enhanced resonance fre-

quencies. However, this example does emphasise the efficiency

of using biosensors for detecting analytes of interest in com-

plex and coloured sample matrices without the need for sample

pre-preparation, which significantly reduces analysis times.

PATHOGENS

Bacterial Pathogens

Several bacterial strains are of great significance for the food

and horticultural industries, includingL. monocytogenes,S. ty-

phimuriumandE. coliO157:H7, and Table 45.6 show some

of the earlier examples of using SPR instruments to detect the

presence of bacterial contaminants in food. The bacterial strain

E. coli0157:H7 is a major pathogen of interest in fruit and veg-

etables, as it causes severe illness and can be fatal in the infants,

the elderly and the immunocompromised (Byrne et al. 2009).

Hence, the detection and enumeration of this and other bacterial

pathogens are absolute requirements for the food industry. A dis-

posable conductometric electrochemical immunosensor, based

on a lateral flow strip connected to an ohmmeter, was described

by Muhammad-Tahir and Alocilja (2004). Anti-E. coli0157:H7

antibodies, labelled with polyaniline (PANI), were immobilised

onto the nitrocellulose strip and a sample was allowed to migrate

up the strip. PANI is an excellent conducting polymer frequently

selected for use in chemical sensor platforms. A drop in resis-

tance, proportional to the concentration ofE. coli0157:H7 cells

binding to the antibodies, indicated a decrease in the electron

transfer from the PANI-conjugated antibody.

L. monocytogenesis a highly infectious pathogen that has very

serious implications if present in food or food products due to the

Table 45.6.Examples of Surface Plasmon Resonance-Based Analysis of Bacterial Pathogens

Analyte Limit of Detection Reference

Salmonella enteritidis,Listeria monocytogenes 106 cells/mL Koubov ́a et al. 2001.

Salmonellagroups B, D and E 1.7×103 CFU/mL Bokken et al. 2003.

Staphylococcalenterotoxin B 0.5 ng/mL Homola 2003.

Staphylococcalenterotoxin B 1.0 ng/mL Nedelkov et al. 2000.

Salmonella typhimurium 102 –10^9 CFU/mL Oh et al. 2004.

CFU, colony forming unit.