Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 243

designs, optical immunosensors are considered as one of the most promising alterna-
tives to the traditional immunoassays in clinic diagnosis and environmental analysis.
In recent years, there has been an increased trend in the use of optical transduction
techniques in immunosensor technologies due to the advantages of applying visible
radiation, non-destructive operation mode, and the rapid signal generation and reading
[1, 125-126]. The optical immunosensors may be divided into two types of approaches:
direct optical immunosensors and indirect immunosensors depending upon the use of
labeled signaling molecules.
Surface plasmon resonance (SPR) as a direct and reliable optical transducer is com-
monly based on the evanescent wave, in which a thin gold layer is generally deposited
on a prism serving as an optically rarer medium [127-128]. Not requiring additional
labels and separation steps, the direct SPR immunosensors have been proven to be
powerful analytical tools for rapid real-time monitoring the immunological targets.
Schofi eld and Dimmock developed a SPR system in combination with the fl ow sys-
tem for detection of infl uenza virus by use of carboxylated dextran polymer matrix
to couple monoclonal antibody of HC10 [129]. In order to validate the feasibility of
SPR immunosensor as a tool for diagnosing type I diabetes, Choi et al. modifi ed mixed
SAMs onto the optical substrate achieving the immuno-response detection for mono-
clonal antibodies of anti-glutamic acid decarboxylase [130]. Moreover, the fatty acid-
binding protein assay has an application potential in clinical analysis for diagnosis of
myocardial infarction. A direct optical immunosensor based on SPR was developed
for detecting the human heart-type fatty acid binding protein with a detection limit of
200 ng mL 1 [131]. Highly sensitive SPR-based immunosensors using self-assembled
protein G have also been successfully applied for the detection of microbes such as
Salmonella typhimurium and Legionella pneumophila [132-133]. More importantly,
several instrument systems using SPR technology have been commercially available,
such as the BIAcore ™ system from Pharmacia Biosensor, the Iasys ™ system from
Affi nity Sensors, and so on. Nevertheless, at present, there are still some unsolved
problems for these SPR devices, such as non-specifi c adsorption and poor analytical
sensitivity to analytes of low molecular weight.
Fluorescence immunosensors , as the total internal refl ection fl uorescence devices,
continue to prove themselves as another promising type of sensitive and selective
optical immunoassay technique, in which labels are sometimes used [134]. When the
fl uorescence-labeled antibodies or antigens are attached to the transducer surface and
enter the evanescent fi eld, the incident light will excite fl uorescent molecules produc-
ing a fl uorescent evanescent wave signal to be detected. The optic-fi ber immunosen-
sor system by fl uorescence enhancement or quenching is separation-free, reagentless
and applicable to the determination of various proteins by antigen-antibody reactions
[134-138]. Maragos et al. described the development of a fl uorescence polarization-
based competition immunoassay for fumonisins in maize using fumonisin-specifi c
monoclonal antibodies [135]. A fl uorescence-based immunosensor array for simulta-
neous determination of multiple clinical analytes was developed by Rowe et al. [137].
In their study, the patterned array of recognition elements was immobilized onto the
planar waveguide to “capture” the analytes from the samples to be quantifi ed by means