Electrochemically induced disintegration of layer-by-layer-assembled thin films composed of 2-iminobiotin-labeled poly(ethyleneimine) and avidin

A layer-by-layer assembly composed of avidin and 2-iminobiotin-labeled poly(ethyleneimine) (ib-PEI) was prepared on the surface of a platinum (Pt) film-coated quartz resonator, and an electrochemically induced disintegration of the avidin - ib-PEI assembly was studied using a quartz crystal microbalance. The resonance frequency of a five-bilayer (avidin - ib-PEI)₅ film-coated quartz resonator was increased upon application of an electric potential to the Pt layer of the quartz resonator, suggesting that the mass on the quartz resonator was decreased as a result of disintegration of the (avidin - ib-PEI)₅ film, due to a pH change in the vicinity of the surface of the Pt-coated quartz resonator. It may be that the (avidin - ib-PEI)₅ film assembly was decomposed by acidification of the local pH on the surface of the Pt layer, which in turn was induced through electrolysis of water on Pt, because ib-PEI forms complexes with avidin only in basic media. In pH 9 solution, the (avidin - ib-PEI)₅ film was decomposed under the influence of an applied potential of 0.6-1.0 V versus Ag/AgCl. The (avidin - ib-PEI)₅ film was decomposed almost completely within a minute in a low concentration buffer (1 mM, pH 9), while the decomposition was slower in 10 and 100 mM buffer solutions at the same pH. The decomposition of the assembly was rapid when the electrode potential was applied in pH 9 solutions, while the response was relatively slow in pH 10 and 11 solutions. All the results are rationalized on the basis of an electrochemically induced acidification of the local environment around the (avidin - ib-PEI)₅ film on the Pt layer.