The demonstration of stable alkanethiolate-capped colloidal gold nanocrystals launched a wave of research on surface functionalization strategies to control the assembly of plasmonic metal nanocrystals (NCs). One basic degree of control is the ability to reverse the NC aggregation process using an external stimulus. Reversibility is interesting because it allows for dynamic tuning of the aggregates, real-time response to stimuli, resetting of the NC assembly to its initial state, and reuse of the NCs. Current approaches rely on non-covalent interactions, including hydrogen bonding, solvophobicity, electrostatic and van der Waals forces, and π-effects. A general method for reversible self-assembly of covalently cross-linked Au NCs has not been demonstrated.<br/><br/>In this talk, I present the reversible assembly and disassembly of dithiol-linked aggregates of Au NCs by selectively converting the strongly-bound thiolate anchoring groups to weakly-bound sulfonate groups using ozone. Ozone has been previously used to induce reversible shifts of the LSPR of Au NCs, and to oxidize organic ligands from colloidal and surface-supported Au NCs. We show here that ozone depolymerizes the NC precipitates by converting the dithiol linkers to weakly-bound disulfonates that exchange with free citrate in solution to yield dispersions of citrate-capped NCs. The process works with all dithiol linkers we tested, including oligo ethylene glycol and alkane dithiols of various lengths and water solubility. The NC size and shape distribution is preserved. Complete redispersion of “hard crashed” bulk NC precipitates occurs with shaking at room temperature or assistance from sonication. Disassembly of colloidal aggregates formed by adding a different amount of dithiol takes seconds to days. The precipitation-redispersion cycle can be repeated multiple times by sequential addition of dithiol linker and ozone. Ozone-mediated linker oxidation is a simple, clean, and general approach to the reversible assembly of covalently cross-linked Au NCs. Interesting future directions include using smaller, more controlled ozone doses to stop rather than reverse the aggregation of Au NCs and extending this approach to Ag NCs, which may require a milder oxidant than ozone.

Reversible aggregation of covalently cross-linked gold nanocrystals by linker oxidation

2019 Spring National Meeting

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