Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 241

a heterostructure of Au/ o -aminobenzenethiol layer/covalent-coupling antibody/elec-
trode for the direct detection of the antibody-antigen interaction by capacitance meas-
urements [91]. A capacitive immunoassay based on antibody-embedded ultra-thin
alumina sol-gel fi lms ( 20 to 40 nm) was reported and used for direct determination of

antigens with a detection limit as low as

∼

1 ng mL 1 [92]. Fernandez-Sanchez et al.
reported a successful integration of the lateral fl ow immunoassay format and imped-
ance detection for prostate-specifi c antigen of tumor marker, where the electrochemi-
cal transducer was coated with a pH-sensitive polymer layer [93].
Although capacitance and impedance immunosensors can directly be utilized to
investigate the antibody-antigen interaction without the need of other reagents and a
separation step, their analytical sensitivity is limited in clinical applications [14]. In
order to amplify the capacitance or impedance response to immunoreaction for the sen-
sitive detection of various clinical markers, different labels have been used including
enzymes, fl uorophores, and metal chelates [103-104]. Ruan et al. developed an immu-
nosensor based on enzyme-stimulated precipitation for the detection of Escherichia
coli O157:H7 using an electrochemical impedance spectroscopy [103]. Another illus-
trative example was the sensitized immunosensor proposed by Chen et al. [104]. In
their study, a receptor protein was directly adsorbed on a porous nanostructure gold
fi lm to perform a sandwich immunoreaction with the precipitation of insoluble product
on the electrode. The impedance signals so amplifi ed showed good linearity with the
content of IgG in the range 0.011-11 ng mL 1 with a detection limit of 0.009 ng mL 1 .
A new strategy of signal amplifi cation was also introduced for highly sensitive imped-
ance measurements using biotin-labeled protein-streptavidin network complex [105].
Amperometric immunosensors , as the most popular immunosensing formats, are
based on the measurement of the currents resulting from the electrochemical oxidation
or reduction of electroactive species at a certain constant voltage. This kind of immu-
nosensor usually uses a complex three-electrode measuring system consisting of a
working electrode (e.g. gold, glassy carbon, or carbon paste), a reference electrode (e.g.
Ag/AgCl), and a conducting auxiliary electrode (e.g. platinum). Since most antibodies
and antigens are not electrochemically active, there are only a few applications availa-
ble for direct amperometric sensing. Therefore, most amperometric immunosensors are
indirect ones which can detect mainly the redox currents associated with electroactive
or catalytic labels [25-26, 28, 106-116]. Aizawa et al. fi rst developed an amperometric
immunosensor for the determination of human chorionic gonadotropin using an amper-
ometric oxygen electrode [106]. Among the labels used, enzymes are the most popu-
lar ones in different types of immunoassays, such as horseradish peroxidase (HRP) or
glucose oxidase. An immunosensor was designed for determining isopentenyl adenos-
ine based on the electro-polymerization of polypyrrole and poly(m-phenylenediamine)
entrapped with HRP on the glassy carbon electrode [108]. A design strategy of reagen-
tless immunosensor was reported for the detection of carcinoma antigen-125 antibod-
ies by direct HRP-labeled electrochemistry [109]. Due to the high sensitivity inherent
in these transducers by enzymatic catalysis, amperometric immunosensors can obtain
a much higher sensitivity than the classical ELISA. For the immunosensors used in
clinical applications, their surfaces should be capable of renewal. Yu et al. developed