Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 239

methodology for antibodies through their native thiol (-SH) groups, which were liber-
ated after the splitting of the intact IgG into two antibody fragments [72-73]. Karyakin
et al. reported a site-oriented immobilization strategy of antibodies on the gold elec-
trode surfaces by use of native sulfi de groups of IgG fragments obtained by reduction
of intact IgG [72]. They found that antibodies immobilized by this procedure showed
an antigen binding capacity 20-30 times higher than that of non-specifi cally adsorbed
intact ones traditionally used.
In general, an ideal immobilization should have the following characteristics:
(i) a suffi cient loading amount of active antigens or antibodies at the transducer sur-
face; (ii) the immobilized antigens or antibodies staying stable during the measurement
process; (iii) the immobilization process having no infl uence to the sensing behavior of
the transducer; and (v) the ability of sensor regeneration. An effective dissociation of
the antigen and regeneration of antibody, i.e. by using Gly-HCl buffer (pH 2.3), for
cost effectiveness is of practical interest in real immunosensor applications [74].
9.3 MAJOR TYPES OF IMMUNOSENSORS
There are mainly three types of transducers used in immunosensors: electrochemical,
optical, and microgravimetric transducers. The immunosensors may operate either as
direct immunosensors or as indirect ones. For direct immunosensors, the transducers
directly detect the physical or chemical effects resulting from the immunocomplex
formation at the interfaces, with no additional labels used. The direct immunosensors
detect the analytes in real time. For indirect immunosensors, one or multiple labeled
bio-reagents are commonly used during the detection processes, and the transducers
should detect the signals from the labels. These indirect detections used to need several
washing and separation steps and are sometimes called immunoassays. Compared with
the direct immunosensors, the indirect immunosensors may have higher sensitivity and
better ability to defend interference from non-specifi c adsorption.
9.3.1 Electrochemical immunosensors
The majority of known immunosensor devices belong to the group of electrochemical
immunosensors. Electrochemical immunosensors may possess several advantages, for
example high sensitivity, low cost, and portable design. The principle of their opera-
tion is based on the electrochemical detection of the labeled immunoagents or mark-
ers such as enzymes, metal ions, or other electroactive compounds, thus providing an
opportunity to analyze complex multicomponent mixtures for diagnosing diseases or
monitoring the status of patients [75]. The kinds of detection transducer for electro-
chemical immunosensors can be mainly subdivided into potentiometric, conductomet-
ric, capacitive, impeditive, and amperometric (metal and graphite electrodes) devices.
Potentiometric transducers now belong to the most mature transducers with
numerous commercial products. For potentiometric transducers, a local equilibrium
is established at the transducer interface at near-zero current fl ow, where the change