Insertion of individual dendrimer molecules into self-assembled monolayers on gold:: A mechanistic study

The insertion process of individual dendrimer sulfide 1 molecules into self-assembled monolayers (SAMs) of 11-mercapto-1-undecanol has been investigated by atomic force microscopy (AFM), wettability, and electrochemical measurements. Immersion of 11-mercapto-1-undecanol SAMs in solutions of increasing concentrations of 1 leads to an increase in the number of dendrimer molecules inserted into the thiol layer. Similarly, longer contact times of the original SAM with the solvent (dichloromethane) also lead to a higher number of inserted dendrimers. However, 11-mercapto-1-undecanol monolayers prepared by immersion into 1 mM solutions of the thiol for 2, 5, 10, 30, 60, 120, and 180 min, followed by a 60 min exposure to a solution of dendrimer 1 did not show significant differences in the number of inserted dendrimers. This indicates that the initial layer quality is not a determining factor for the insertion process. A mechanism consisting of rapid dissociation of surface thiols, possibly as disulfides, followed by slow dendrimer adsorption, is proposed. The rate-determining step of the process is the insertion of the individual dendrimer molecules. For the dissociation step, first-order kinetics for the fraction of covered substrate versus time of exposure to solvent was found, with a rate constant, k, of (8.0 +/- 1.5) x 10(-4) min(-1).