Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 235

hepatitis B surface antibody on a platinum electrode [11]. A piezoelectric immuno-
sensor was developed for the on-line detection of severe acute respiratory syndrome
(SARS)-associated coronavirus (SARS-CoV) in sputum in the gas phase. Compared
to other SARS detection techniques, this method can rapidly test SARS-CoV at low
cost [12]. Moreover, the determination of some tumor markers plays an important
role in diagnosing, screening, and determining the prognosis of a cancer disease. Such
tumor markers to be detected are often found in abnormally high amounts in the blood,
urine, or tissue of patients with certain types of cancers. The examples include carci-
noembryonic antigen (CEA), carbohydrate antigen 19-9 (CA19-9), carcinoma antigen
125 (CA125), alpha-fetoprotein (AFP), prostate specifi c antigen (PSA), CA15-3 and
human chorionic gonadotropin (HCG) [13-15]. Wilson proposed an electrochemical
immunosensor for the simultaneous detection of two tumor markers of CEA and AFP
[15]. An increasing number of immunosensors have been utilized to analyze a series
of biochemical targets for diagnosing infectious diseases, although there are still prob-
lems concerning the assay of analytes in real sample matrixes [1].
9.2 IMMOBILIZATION OF IMMUNOACTIVE ELEMENTS
Since immunosensors usually measure the signals resulting from the specifi c immu-
noreactions between the analytes and the antibodies or antigens immobilized, it is
clear that the immobilization procedures of the antibodies (antigens) on the surfaces of
base transducers should play an important role in the construction of immunosensors.
Numerous immobilization procedures have been employed for diverse immunosen-
sors, such as electrostatic adsorption, entrapment, cross-linking, and covalent bonding
procedures. They may be appropriately divided into two kinds of non-covalent interaction-
based and covalent interaction-based immobilization procedures.
9.2.1 Non-covalent interaction-based immobilization procedures
This type of immobilization of immunoactive entities is based on the non-covalent
interactions between the antibody or antigen molecules and the transducer substrates,
and usually refers to hydrophobic interaction, electrostatic interaction, van der Waals
force, and hydrogen bonding. One notices that besides pure physical adsorption, some
weak chemical interactions are also involved here. The non-covalent interactions may
vary from the different substrates of transducers. For a non-polarity sensing substrate,
the antibody or antigen molecules can be adsorbed through the hydrophobic interaction
and van der Waals force. Wenmeyer et al. attached anti-digoxin antibodies at the surfaces
of polystyrene microtubes by direct adsorption interaction, achieving the determination
of digoxin with a detection limit of 50 pg mL 1 [16]. While for the charged substrates,
the non-covalent interactions are mainly associated with the electrostatic interactions.
The most typical layer-by-layer technique of self-assembly has attracted considerable
attention in biomolecular immobilizations [17-21]. Caruso and coworkers assem-
bled polyallylamine hydrochloride/polystyrene sulfonate layers on the self-assembled