Biomedical Engineering Reference
In-Depth Information
Antibodies were immobilized onto oxide surface of the chip between the gold electrode
fingers. The change in impedance from 100 Hz to 10 MHz at an applied potential of 50 mV
upon cell binding was measured. The sensor showed a linear response to changes in cell
concentration, an analysis time of 10 min and a detection limit of 10 4 cfu/ml, and the abil-
ity to detect E. coli from real food samples. The system also performed well in the presence
of other microbes, giving no false positives.
A multianalyte immunosensor was developed using a commercially available micro-
electrode array chip (52). The chip contains 1000 individually addressable electrodes in a
1-cm 2 area. Short-oligonucleotide sequences are immobilized onto the electrodes allowing
for attachment of biomolecules conjugated to the complementary sequence. Using this
strategy a sandwich immunoassay was developed. Antibodies for five different targets,
including B. globigii spores, were conjugated to DNA sequences and immobilized onto
selected electrodes. Captured targets were then tagged with a secondary antibody conju-
gated to biotin. Finally to a horseradish peroxidase enzyme-avidin conjugate was added.
The product of the enzymatic reaction was then reduced at the platinum electrode surface
at 0.3 V. The system was successfully tested for the simultaneous detection of all five
different analytes using a checkerboard pattern on the electrodes. The limits of detection
for most analytes were in the attomolar range with an assay volume of 50
l. Assays could
be completed in 12 min, with maximum sensitivity occurring in 60 min.
A screen-printed array based on coulometry was developed to identify different strains
of E. coli (53). In this strategy, lectins, proteins that bind oligosaccharides with high affin-
ity and specificity, were immobilized onto porous membranes. It was shown that different
strains of E. coli would bind different lectins with specific patterns. Although ten different
lectins were used, only five were required to differentiate between the four strains of E. coli
tested. The membranes were incubated in the sample containing the cells as well as the
respiratory substrates succinate and formate and the redox mediator ferricyanide. The
membranes were then placed over the electrode arrays. Viable cells reduce the ferri-
cyanide, which is then oxidized at the electrode surface and measured coulometrically.
Computer software was used to analyze the lectin-binding pattern and correlate it to pat-
terns of known standards. This system shows the capability to capture a wide variety of
bacteria because antibodies are not used for capture. Additionally, only viable cells are
detected at the electrode making this an attractive method for food quality control.
An electronic tongue was used to recognize six microbial species, including bacteria,
mold, and yeast commonly found as food contaminates (54). The system is based on
voltammetry where a pulse is applied over an array of metal electrodes and the resulting
current is measured. Redox-active species, commonly found as metabolites of respiration,
are measured. Each species in the sample will have characteristic metabolites present at
different phases of the growth cycle. Sample data is analyzed using multivariate methods
and compared with stored data patterns obtained from standards of pure microbial
strains. The method addresses two important problems in food quality control: first,
because the measured analytes are products of metabolism, only viable microbes are
detected. Second, the system itself does not require a biological component, such as anti-
bodies for capture or detection. The work demonstrated that the method could identify
and differentiate between microorganisms by electrochemical detection of metabolites but
statistical measurements such as the limit of detection were not made.
Similarly, the electronic nose has been shown to be capable of identifying microbial
species based on the volatile compounds excreted from bacteria and yeast growing on
nutrient agar plates (55). The setup consisted of a 16-sensor array of electroconductive
polymers. The system correctly classified 228 out of 244 samples (93.4%) containing 12
different types of bacteria and 1 type of yeast. For seven species samples, all were classi-
fied correctly. Limits of detection were not established. The electronic nose was also used
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