Biomedical Engineering Reference
In-Depth Information
electrodes with immobilized or coimmobilized enzymes belonging to two different
classes: oxidases (peroxidase and tyrosinase) and hydrolases (acetylcholinesterase (AChE)
and butyrylcholinesterase (BuChE)). The results showed that when used in array, the
biosensor system could provide complementary information about the composition of the
sample, although the electrochemical signals of one enzyme electrode might be affected by
the substrate of the other enzyme used in the same array. Schmid et al. (6,7) reported a sim-
ilar screen-printed electrodes array combined with artificial neural networks (ANNs) for
simultaneous and selective determination and discrimination of organophosphorus and
carbamate pesticides in mixtures. The sensor is based on the inhibition of AChE by pesti-
cides. While both organophosphorus and carbamate pesticides inhibit the AChEs, dis-
crimination was possible by using recombinant enzymes, specifically modified to possess
different sensitivities toward a particular pesticide. To construct the sensor array, four
types of native and genetically modified AChEs from different sources have been
deposited onto four screen-printed working electrode surfaces. The array was then con-
nected to a commercial four-channel potentiostat, which applies a constant potential to all
four electrodes and the resulting currents were recorded on a multichannel recorder for
further processing, evaluation, and crossvalidation with the ANNs method. The opti-
mized sensor was tested using solutions of pesticides separately or in mixture, while sam-
ple discrimination in real environmental matrices was carried out on spiked samples
using several sets of modified electrodes.
Some other multibiosensor systems have been reported as a combination of ampero-
metric, potentiometric, and conductometric sensors coupled with multichannel transduc-
ers. For instance, Arkhypova et al. (5) reported a three-enzyme (urease, AChE, and
BuChE)-based multibiosensor coupled with potentiometric pH-sensitive transistors and
conductometric thin-film interdigitated electrodes for the detection of two different classes
of toxic analytes: pesticides and heavy metals. Silber et al. (20) developed a seven-channel
enzyme and ion-selective multibiosensor containing 14 working and counter electrodes
fabricated by screen-printing technique and a reference Ag/AgCl electrode for dual
amperometric-potentiometric measurements of ions (K
+
, Li
+
), glucose, urea, and lactate
simultaneously (20).
19.3.2
Electronic Nose Technology
In the electronic nose technology, cross-reactive chemical-sensing arrays are coupled with
an appropriate sample collection, transduction system, and special computational pro-
grams and function as an effective analytical tool for detection, quantification, and classi-
fication of volatile or semivolatile organics based on their odors (21). These less
conventional sensing systems are referred to as electronic noses. A general overview of
this technology, the chemistry and fabrication of the sensor arrays, and a description of
their characteristics and applications have been extensively discussed in numerous
reviews (8-10,22). Most of these sensors are based on quartz crystal piezoelectric (23),
metal oxide semiconductor, and conducting polymers technologies (9,24).
The principle of a gas sensor is based on the changes in the electrical resistance when a
volatile chemical is absorbed onto its surface. This change in electrical resistance is char-
acteristic to each analyte and can be monitored and used to create a distinct pattern. The
samples are collected using a sample collector system, equilibrated, and exposed to the
sensor array under controlled temperature, pH, and pressure. In addition to the analyte
samples, reference air samples are passed through the sensor array to produce a baseline
signal. Finally, the changes in resistance versus time are monitored and recorded on each
channel. We have recently used this technology for the detection and classification of OP
nerve agents: paraoxon, parathion, malathion, dichlorvos, trichlorfon, and diazinon (4). In
