Biomedical Engineering Reference
In-Depth Information
(A) Direct competitive immunoassay
Affinity reaction
Incubation
A
Enz
S
Ag
Washing
Ag
Ag
Enz
Enz
Ag
Ag
Ag
Ag
Ag
Enz
Enz
P
Ag
Ag
(B) Indirect competitive
Ag
Ag
Ag
Enz
Ag
Detection
Ag
A
Enz
Ag
Ag
Ag
(C) Sandwich immunoassay
Enz
Enz
Enz
Detection
Enz
Enz
Enz
Ag
Ag
Antibody
Antigen
Enzyme-labeled
antigen
Enzyme-labeled
antibody
FIGURE 20.4
Immunosensor formats. (A) Direct competitive immunoassay. (B) Indirect competitive immunoassay. (C)
Sandwich immunoassay. (Modified from Turner APF, Laschi S and Mascini M. 2002.
Biosensors
:
Kirk-Othmer
Encyclopaedia of Chemical Technology
. Wiley.)
the ability to detect the label in turbid media, which is not possible with optical sensors,
and the ability to increase the surface area to increase sensitivity (21). Potentiometric
immunosensors, based on charge differences between antibody-antigen complexes and
the antibody or antigen alone, ion-selective, or gas-sensing electrodes, have also been
reported (21). They have, however, been less successful because of the lack of sensitivity
caused by nonspecific binding and other background interferences with the transducer
(21). They also require additional time due to the indirect nature of the configurations.
Piezoelectric immunosensors, the most common of which is the quartz crystal microbal-
ance, have also been widely applied to detect antibody binding to an immobilized antigen
(18). This technique is able to measure small changes in surface properties, such as bound
surface mass and surface viscosity, which can then be related to the concentration of the
analyte in the sample (18; 20). The major advantages of these devices include their small
size, high sensitivity and stability, simplicity of construction and operation, and low-
power requirement (20).
