Etude de différentes méthodes de biofonctionnalisation pour la ...

III. Etudes du film Lan gmuir-Blodgett enzymatique
In this study, the stabilization of the fatty amine monolayers with enzyme was expected. In addition,
the strong interaction between the enzyme globally negatively charged and the amine, positively
charged, would contribute to favor the enzyme adsorption.
4.2. Surface pressure for mixed enzyme/ODA film: from O to 26 mN m (cf. Figure 7a)
lt was generally accepted that the transport of the protein to the interface was entirely governed by
diffusion21. However a lot of studies have shown that the rate of diffusion of proteins to the air-water
interface deviated significantly from the bulk diffusion coefficients of proteins. So different theories
have been proposed for better explaining the protein adsorption, by including for example in the
diffusion model, a thermal convection phenomenon22. In a different approach, it has been proposed
that the slower rate of adsorption could be attributed
to the dominating repulsive dispersion
interactions23. The aim of this paper is not to validate one particular theory, but rather to show the
influence of the amphiphile and of the enzyme concentration in the isotherm adsorption so that the
biosensor construction could be more controlled.
From Figure 1, we can see that whatever the ODA concentration chosen, an equilibrium at 26 mN
m1 between adsorption and desorption is reached, even if for the highest concentration (8 mg 11) the
surface pressure increases firstly up to 29.4 mN m' for decreasing in the following 600 min back to 26
mN m1. lt is likely that at this surface pressure, proteins are present at the air/water interface, some of
them already in contact with ODA.
It will be assumed, that at this value, the protein adsorption at the air-water interface is maximum,
and all the isotherms could start after this surface pressure be reached. Furthermore, in the following
experiment, it will not be necessary to chose an enzyme concentration as high as 8 mg í, since after
600 minutes an equivalent surface pressure is reached with only 4 mg 11.
Figure 1 shows that the adsorption of the protein is also controlled by the nature of the amphiphilic
molecules since a faster adsorption with ODA than with SAc occurred.
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