Biosensors based on immobilization of biomolecules in sol-gel matrices - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 520

In competitive immunoassays antigen has also been immobilized in sol-gel matri-
ces for the development of immunosensors. Gong et al. [135] immobilized Schistosoma
japonicum antigens (SjAg) in TMOS along with BSA and graphite powder, fi nally
squeezing the mixture into a PVC tube with a screw thread at one end. Through the
screwing up and polishing, the electrode surface could be easily regenerated. Using
competitive immunoassay between HRP-SjAb, SjAb, and SjAg entrapped on the elec-
trode surface, the immunosensor was able to detect SjAb up to 4.5 ng ml 1 through the
fl uorescence detection method. They then designed an amperometric immunosensor by
encapsulating the new castle disease antigen (NDAg) in TEOS sol-gel [136]. In this
investigation antigen was entrapped in two different matrices, i.e. paraffi n-ND Ag-BSA
and sol-gel-ND Ag-BSA. Sol-gel-NDAg-BSA had good sensitivity in determination of
NDAb present in the rabbit serum, and the antigen retained antigenic properties very
effectively in sol-gel. Du et al. [137] reported an immunosensor for the detection of
carcinoma antigen (CA) 19-9 by immobilizing the antigen in titania sol-gel matrix.
The incubation of the immunosensor in a solution containing horseradish peroxidase
(HRP)-labeled CA19-9 antibody led to the binding of HRP-labeled antibody with the
immobilized antigen. The immobilized HRP catalyzed the oxidation of catechol by
H 2 O 2 and provided a competitive method for the measurement of serum CA19-9. The
current decrease of the immunosensor was proportional to CA19-9 concentration in
the range 3-20 U/ml with a detection limit of 2.68 U/ml. By taking advantage of the
direct electrochemistry of HRP, Ju and coworkers [138, 139] developed two reagent-
less immunosensors for CA125, and human serum chorionic gonadotrophin (HCG).
These immunosensors were prepared by immobilizing CA125 or HCG with titania sol-
gel on glassy carbon electrodes by the vapor deposition method. The incubation of the
immunosensors in PBS including HRP-labeled CA125 or HRP-labeled HCG antibody
led to the formation of HRP-modifi ed surface (Fig. 16.9). The immobilized HRP dis-
played its direct electrochemistry. Under optimal conditions, the current decrease was
proportional to CA125 or HCG concentration ranging from 2 to 14 units ml 1 or 2.5
to 12.5 mIU ml 1 with a detection limit of 1.29 units ml 1 or 1.4 mIU ml 1 using a
competition mechanism and differential pulse voltammetric determination method (Fig.
16.10). The CA125 immunosensor showed good accuracy and acceptable precision and

and

in 0.1 M pH 7.0 PBS

CA125
HRP labeled
CA125 antibody

Immunosensor
for CA125

Immunosensor
for CA125 assay

FIGURE 16.9 Principle of reagentless amperometric immunosensor based on immobilized antigen,
competitive immunological reaction, and direct electrochemistry of HRP label (adapted from [138]).