Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOSENSOR-BASED TECHNOLOGIES 433
electrochemical method, López et al. (1998) used a redox polymer that was coimmobilized
with an antibody embedded in hydrogel on the surface of an electrode to detect atrazine.
The assay format was competitive and incorporated a HRP-labeled antigen. They achieved
detection limits of 1 ppb (part per billion).
Electrochemically based 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 immunoassay. The principle of ECL is the cooxidation of luminol and a
substrate (called an enhancer) by hydrogen peroxide in the presence of the enzyme HRP.
The resulting amperometric signal is detected by the transducer, and an 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 immobilized on photoactivated nylon.
A chemiluminescence-based assay was then used to measure the herbicide 2,4-D with a
detection limit of 0.2 μg/L.
20.5 Biosensor arrays
In recent years there is a steadily increasing trend for the development of biosensors in
an array format, where multiple analytes can be measured simultaneously on a single
device. An enzyme-based three-electrode biosensor for detecting the presence of markers
of maturity and quality in tropical fruits was developed by Jawaheer et al. (2003) to measure
the analytes β-D-glucose, total D-glucose, sucrose, and ascorbic acid. These markers are
indicative of maturity and quality in selected fruit products and were assayed in pectin
(a natural polysaccharide present in plant cells). A fabrication format was developed that
permitted the integration of the individual sensors into a multisensor array, and analytes
were measured in this matrix to enhance the enzymatic responses over analyte ranges of
0–7 mM. Interferences normally related to electrochemically active compounds present in
fruits were minimized by including a membrane made out of cellulose acetate.
Several bacterial strains are of great significance for the horticultural industry.
Pathogenic bacteria in food account for 90% of food-borne illnesses in the United States.
The bacterial strain Escherichia coli 0157:H7 is a major pathogen of interest in fruits and
vegetables as it causes severe illness and can be fatal in infants, the elderly, and the immunocompromised.
A disposable conductometric electrochemical biosensor, based on a lateral
flow strip connected to an ohmmeter, was described by Muhammad-Tahir and Alocilja
(2004) to detect this strain and Salmonella spp. Anti-E. coli 0157:H7 antibodies, labeled
with polyaniline, were immobilized onto the nitrocellulose strip, and a sample was allowed
to migrate up the strip. A drop in resistance, proportional to the concentration of E. coli
0157:H7 cells binding to the antibodies, indicated a decrease in the electron transfer from
the polyaniline-conjugated antibody.
Listeria monocytogenes is a highly infectious pathogen that has very serious implications
if present in food or food products due to the associated risk of fatality. While the
natural ecosystem of this bacterial strain includes soil, water, plant material, and decaying
plant detritus (Suihko et al., 2002), numerous foods, such as raw vegetables, fruits,
and horticultural samples, are also prone to infection. The psychrophilic nature of this
strain, conferring the ability to grow at refrigeration temperature, and the ability of this
bacterium to withstand high salt concentrations and tolerate a wide pH range suggest that