Monitoring multilayer film growth with the atomic force microscope. Aluminum(III) alkanebisphosphonate multilayer films and DNA immobilization

The growth of aluminum(III) alkanebisphosphonate multilayer thin films on gold surfaces in aqueous solutions was investigated by probing the surface charge following alternate treatments with anionic phosphonate and cationic AI(III). This was accomplished by determining the force between a modified silica tip of an atomic force microscope (AFM) and the film-covered gold substrate. The AFM force measurements revealed that the formation of the films followed a regular layer-by-layer growth mechanism as evidenced by the occurrence of surface charge reversal with each adsorption step. However, the quantitative surface charge data, obtained by theoretical fits of the force data to solutions of the complete nonlinear Poisson-Boltzmann equation with a knowledge of the silica probe surface potential, indicated that the films became less ordered with an increase in the number of layers. The AFM force measuring technique was also employed to monitor the immobilization of both single-stranded (ss) and double-stranded (ds) DNA on positively charged surfaces (i.e., aluminum(III)- and ammonium-terminated surfaces) and their subsequent interactions with a transition metal chelate, Ru(phen)(3)(2+). The force measurement results showed that both the ss-DNA and ds-DNA could be immobilized on positively charged surfaces, while only the ds-DNA showed interaction with Ru(phen)(3)(2+).