Stimuli-sensitive thin films prepared by a layer-by-layer deposition of 2-iminobiotin-labeled poly(ethyleneimine) and avidin.

Layered thin films composed of avidin and 2-iminobiotin-labeled poly(ethyleneimine) (ib-PEI) were prepared by a layer-by-layer deposition of avidin and ib-PEI on a solid surface, and the disintegration induced by changing environmental pH and adding biotin in the solution was studied. The avidin/ib-PEI layered film could be deposited only from the solutions of pH 10-12. The film did not form in pH 9 or more acidic media because of a low affinity of protonated 2-iminobiotin residues in ib-PEI to avidin. The avidin/ib-PEI layered films were stable in pH 8-12 solutions, while in pH 5-7 media the film decomposed spontaneously as a result of the protonation to 2-iminobiotin residues in ib-PEI. The avidin/ib-PEI films were disintegrated also upon addition of biotin and analogues in the solution owing to the preferential binding of biotin or analogues to the binding site of avidin. The decomposition rate was arbitrarily controlled by changing the type of stimulant (biotin or analogues) and its concentration. The avidin/ib-PEI films were disintegrated rapidly by addition of 10(-)(5) M of biotin or desthiobiotin, while the rate was slower upon adding the same concentration of lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid. On the other hand, the film was fully decomposed within 1 min in the 10(-)(3) M lipoic acid or 2-(4'-hydroxyphenylazo)benzoic acid solution. Thus, the decomposition rate is highly dependent on the concentration of the stimulants. It was observed that the stimuli-induced decomposition of the films is slow at pH 12, in contrast to a rapid decomposition in pH 8 medium due to a low affinity of the protonated 2-iminobiotin to avidin. The present system may be useful for constructing stimuli-sensitive devices that can release drug or other functional molecules.