Reversible dispersion and aggregation of Ag2S nanoparticles capped with azobenzene-derivatized alkanethiols

Ag2S nanoparticles (NPs) of 3.1 +/- 0.6, 8.1 +/- 1.0, and 10.2 +/- 1.6 nm in diameter capped with a long-chain amidoamine derivative (C18AA) were synthesized by a modified Brust method. Ag2S NPs capped with two types of azobenzene-derivatized alkanethiol differing in chain length (2AM10SH and 8AM5SH) were obtained by the ligand exchange method. The trans to cis photoisomerization conversion of 2AM10SH and 8AM5SH on Ag2S NPs dispersed in toluene was above 95%. 2AM10SH-capped Ag2S NPs of 8.1 nm or more in toluene were found to show reversible dispersion-aggregation behavior under alternating irradiation with UV and visible lights, i.e., Ag2S NPs capped with trans- and cis-2AM10SH were in dispersed and aggregated states, respectively. However, Ag2S NPs of 3.1 nm capped with 2AM10SH and Ag2S NPs of 8.1 nm capped with 8AM5SH were always in a dispersed state regardless of whether 2AM10SH and 8AM5SH were in the trans or cis conformation.