Biomedical Engineering Reference
In-Depth Information
A recent development in the field of immunosensors is the use of antibodies labeled
with paramagnetic particles (23; 24). With the antigen immobilized, the binding event can
be detected by measuring the magnetic field induced by the magnetic particles. A major
advantage of this method is the ability to permanently record the signal, which could, in
principle, be remeasured at any time.
The last category is optical immunosensors, for which direct and indirect detection
methods exist. Direct methods, requiring no labeling of the antibody or antigen, include
internal reflectance spectroscopy, evanescent wave sensing, and SPR, which will be exam-
ined in detail in Section 20.2.
The above-mentioned methods have also been applied to indirect immunosensing, in
which a label, usually fluorescent, is used to monitor the binding event (21). This approach
improves the sensitivity and selectivity of the assay, given that the quantitative signal
arises only from the presence of the label, thereby eliminating nonspecific binding inter-
ference.
20.1.5.1.2 Other Types of Affinity Biosensors
Other types of affinity-based biosensors include deoxyribonucleic acid (DNA) biosensors,
also known as gene chips. These biosensors are used for recognition and quantitation of
target DNA sequences, based on hybridization of complementary strands to immobilized
single-stranded (ss) DNA (20; 21). Electrochemical and optical transduction methods have
been applied, using electroactive or fluorescent labels specific for binding to hybridized
sequences. Labelfree electrochemical detection is also possible through the direct oxida-
tion of guanine bases (25; 26). Fluorescent tags are still the labeling method of choice for
DNA biosensors and were first introduced by Affymetrix with the GeneChip, in 1996 (21).
Piezoelectric DNA biosensors, capable of detecting the mass change associated with
hybridization, have also been reported (27), as have electrochemical sensors, such as those
from Nanogen.
Molecularly imprinted polymers are artificial ligands which possess steric and chemical
memory for a template, which they can bind with specificity similar to that of an anti-
body-antigen interaction (28). They have been employed as nonbiological alternatives to
antibodies in competitive binding assays, and applications for analytes such as cortisol,
theophylline, and morphine have been investigated, suggesting that MIPs are a promising
technique for use in affinity sensors (19).
Lastly, aptamers are also a very promising technology for the use in immunosensors,
and will be discussed in detail in Section 20.3.
20.1.6
Recent Work on Immunoglobulin E Biosensors
Early work performed by Su et al. (29) made use of antihuman IgE antibodies immobilized
on a piezoelectric sensor for the detection of total IgE in serum samples. The immunosen-
sor proved to have a sensitivity of 12 µg/L and could be regenerated five times without
appreciable loss of activity. Further work was performed by Kreuzer et al. (30), who
described an amperometric immunosensor for the detection of IgE. The sensor used a dis-
posable screen-printed carbon electrode and had a detection limit of 90 ng/L of IgE in
whole blood, with an analysis time of 30 min.
More recently, Liss et al. (31) reported on work performed using a quartz crystal biosen-
sor and compared the performance of a specifically designed aptamer and an antibody for
the detection of IgE. The authors compared a well-established, published aptamer, which
they subsequently improved on during experimentation, with a commercially available
monoclonal anti-IgE antibody. Their work demonstrated an equal specificity and sensitivity
