2.3 ±0.0

2.0 ±0.1

the reported fcbind value is 8.43 + 0.22. The 95% confidence limit indicates that 95% of the febind values will lie between 8.21 and 8.65. This indicates that the values are precise and significant. The curves presented in the figures are theoretical curves.

Figure 7.7b, 7.7c, and 7.7d show the binding of 250, 500, and 1000 nM SJA in solution to a lactose-immobilized surface as well as the dissociation of the SJA agglutinin-lactose complex from the surface. In each case, a single-fractal analysis is sufficient to describe both the binding and the dissociation kinetics. The values of the binding and dissociation rate coefficients are given in Table 7.4. Note that as the SJA concentration in solution increases from 125 to 1000 nM both febind (or feads) and kdiss (or kdes) exhibit increases.

Since there is no nonselective adsorption of the SJA, our analysis does not include this nonselective adsorption. We do recognize that, in some cases, this may be a significant component of the adsorbed material and that this rate of association, which is of a temporal nature, would depend on surface availability. If we were to accommodate the nonselective adsorption into the model, there would be an increase in the degree of heterogeneity on the surface since by its very nature nonspecific adsorption is more heterogeneous than specific adsorption. This would lead to higher fractal dimension values since the fractal dimension is a direct measure of the degree of heterogeneity that exists on the surface.

Table 7.4 and Fig. 7.8a indicate that febind increases as the SJA concentration in solution increases. Clearly, fcbind varies with SJA concentration in solution in a nonlinear fashion. Table 7.4 and Fig. 7.8b indicate that kdiss increases as the SJA concentration in solution increases. Again, fedlss varies with SJA concentration in solution in a nonlinear fashion.

The ratio K = fediss/febind is important since it provides a measure of affinity of the receptor for the analyte, in this case the lactose-immobilized surface for SJA in solution. Table 7.4 indicates that K decreases as the SJA concentration in solution increases from 125 to 1000 nmol in solution. Figure 7.8c shows the decrease in K as the SJA concentration in solution increases. At the lower SJA concentrations, the K value is higher. Thus, if the affinity is of concern and one has the flexibility of selecting the analyte concentration to be analyzed, then one should utilize lower concentrations of SJA. This is true, at least, in the 125-1000 nmol SJA concentration range.

Table 7.4 and Fig. 7.8d indicate that Dfdiss decreases as the SJA concentration in solution increases. Dfdiss is rather insensitive to the SJA concentration. Table 7.4 and Fig. 7.8e indicate that an increase in the fractal dimension for dissociation, Df diss, leads to a decrease in the binding rate coefficient, kdlss. At the outset, it may not appear to be appropriate to relate the binding rate coefficient to the fractal dimension. However, the topology of the surface, in our case the biosensor surface, does influence the binding rate

200 400 600 800 SJA concentration, nmol

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