Biomedical Engineering Reference
In-Depth Information
Table 6.1: Binding and dissociation rate coefficients and fractal dimensions for the binding and
the dissociation phase for Escherichia coli in solution by two different biosensors.
Analyte in Solution/
Receptor on Surface
k
k d
D f
D fd
10 7 CFU/ml E. coli
with lysozyme/
immunosensor with PAPG
0.4647 0.0520
na
0.5270 0.06676
na
10 7 CFU/ml E. coli without
lysozyme/immunosensor
with PAPG
0.8083
0.0709
na
1.6998
0.0839
na
10 8 CFU/ml E. coli/
conjugated magnetic
beads
20.063 2.197
3.0656 0.1817 2.0082 0.0760
1.9878 0.0618
out that aptamers can play the role of capture molecules like antibodies in different types of
biosensor schemes ( Liu and Lu, 2006 ; Bang et al., 2008). Lee et al. (2009) report an addi-
tional advantage of using aptamers vis- ` -vis antibodies in the sense that the PCR may be used
to increase the detection sensitivity ( Zhang et al., 2006 ).
Lee et al. (2009) point out that though microorganism-specific aptamers to enhance the
detection of microorganisms is limited, several aptamers for the detection of E. coli are avail-
able ( So et al., 2008 ). They selected an RNA aptamer for the detection of E. coli . They cap-
tured the target E. coli on antibody-conjugated magnetic beads, and the aptamers were bound
to the surface of the captured E. coli by a sandwich method.
Figure 6.2 shows the binding of 10 8 CFU/ml E. coli in solution to the antibody-conjugated
magnetic beads. The aptamers were amplified by PCR ( Lee et al., 2009 ). A single-fractal
analysis is adequate to describe the binding and dissociation kinetics. The binding rate coef-
ficient, k , and the fractal dimension D f , for a single-fractal analysis, and the dissociation rate
coefficient, k d , and the fractal dimension for dissociation, D fd , are given in Table 6.1 .
The aim of this chapter is to compare the binding and the dissociation (if applicable) rate
coefficients and the corresponding fractal dimensions for the same analyte on different bio-
sensor surfaces. It is hoped that further physical insights may be gained by such an analysis.
It may be noted that the binding of E. coli to both the immunosensor with PAPG
( Abu-Rabesh et al., 2009 ), and to the antibody-conjugated magnetic beads is described by
a single-fractal analysis. This would indicate that the binding (and dissociation) mechanism
is not complex. There is an order of magnitude change in the E. coli concentration in solution
as one goes from the immunosensor with PAPG (10 7 CFU/ml) to the antibody-conjugated
magnetic beads (10 8 CFU/ml) ( Lee et al., 2009 ). Also, no dissociation is exhibited during
the binding of 10 7 CFU/ml E. coli in solution to the immunosensor. This would indicate that
either the binding in this case is strong enough to prevent the dissociation or the E. coli
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