Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 242

a renewable amperometric immunosensor for the determination of Schistosoma japo-
nium (Sj) antibody by using the paraffi n graphite-Sj antigen biocomposite paste elec-
trodes which might be regenerated by polishing the surface [25]. Ionescu and his
collaborators have developed two similar amperometric immunosensors for cholera
antitoxin immunoglobulins, where the cholera toxin biorecognition entities were bound
to a biotinylated polypyrrole fi lm or pyrrole-biotin and lactitobionamide electropo-
lymerized copolymer [117-118]. Moreover, nano-sized particles or sol-gel matrixes
have also been increasingly employed in the design of amperometric immunosensors
with enhanced analytical performance [23, 119-120]. For example, an electrochemical
immunosensor has been developed for probing complement III (C 3 ) by use of nano-
gold particle monolayer as the sensing interface [119]. With the coupling of sol-gel
and screen-printing technologies, a sensitive thick fi lm immunosensor was fabricated
by dispersion of rabbit immunoglobulin G, graphite powder, and a binder in the sol-
gel solution [23]. A new HRP-labeled amperometric immunosensor for determination
of chorionic gonadotrophin in human serum was constructed by immobilizing HCG
within titania sol-gel on a glassy carbon electrode [120]. Anodic stripping voltamme-
try as an electrochemical assay technique has been well adopted for sensitive measure-
ments of heavy metals such as copper and silver, which may also offer an attractive
way of sensitive immunosensor development [121-122]. An immunosensor was
designed by coupling immunoassay with the square wave anodic stripping voltamme-
try technique involving copper ion-labeled antigen in the competitive immunoreaction
[121]. This immunosensor might allow rapid, accurate, and inexpensive detection of
gibberellin acid with a concentration as low as 1 g mL 1 . Chu et al. designed a silver-

enhanced colloidal gold metalloimmunoassay for the determination of Schistosoma
japonicum antibody (SjAb) in rabbit serum [122]. In their study, after the immunore-
action of SjAb target with immobilized Sj antigens, colloidal gold-labeled secondary
antibody was introduced to favor the silver enhancement process. An acidic solution
was further used to dissolute silver metal atoms, followed by the sensitive determi-
nation of dissolved silver ions using anodic stripping voltammetry. In addition, many
immunoreaction signal-amplifi ed methods or processes have also been adopted for the
development of sensitive amperometric immunosensors. Willner's group reported an
amplifi ed immunosensing scheme of chronopotentiometry and Faradaic impedance
spectroscopy by way of a bio-catalyzed precipitation of the insoluble product onto the
gold electrode [123]. They also designed a variation of this scheme with signal ampli-
fi cation by employing liposomes labeled with biotin and HRP as a probe to amplify
the sensing of antigen-antibody interactions [124]. In this case, the electrode with the
antigen-antibody complex was exposed to the biotinylated anti-IgG antibody, and fur-
ther the biotin-labeled HRP-liposomes through an avidin bridge to achieve the biocata-
lyzed precipitation of an insoluble product on the conductive support.

µ

9.3.2 Optical immunosensors
Since almost all optical phenomena at sensing surfaces (e.g. adsorption, fl uorescence,
luminescence, scatter or refractive index, etc.) can be used for biochemical sensing