RESUMO
Self-assembled monolayers (SAMs) comprised from n-alkanethiols and terminal alkynes were subjected to solutions containing ferrocene-terminated thiol, thioacetate, and terminal alkyne. The rate and extent of chemical exchange were monitored by scanning tunneling microscopy (STM). In several cases, a rate constant for exchange could be obtained by fitting to a model for exchange. In each case where this could be accomplished, a different rate model gave the best fit to the data, suggesting that the mechanism of exchange depended on either or both the original SAM and the incoming molecule. In scenarios where the rate of exchange was slow, directed exchange was accomplished via STM tip-induced lithographic patterning (replacement lithography). The extent of exchange was independent of the incoming molecule, suggesting that tip-induced desorption was the limiting factor in this process.
RESUMO
Reversible addition-fragmentation chain transfer polymerization is employed here to allow detector-free visualization of specific DNA sequences for which dynamic polymer growth is used in signal amplification. In particular, surface-initiated polymer growth was regulated by the immobilization of chain transfer agents on the Au surface where DNA hybridization occurred. A linear polymer growth was observed as a function of the reaction time, characteristic of "living" polymer reactions. Significant improvement in assay sensitivity was realized in comparison to the previously reported polymerization-based sensing method by enhancing polymer growth rate and reducing background noises caused by nonspecific adsorption. Direct visualization of fewer than 2,000 copies of a short oligonucleotide sequence was demonstrated in a detector-free fashion.
