Control of dispersion-coagulation behavior of Au nanoparticles capped with azobenzene-derivatized alkanethiol in a mixed chloroform-ethanol solvent

Self-assembled monolayers (SAMs) of azobenzene-derivatived alkanethiol, 5-[2-methyl-4-(4-octyl-phenylazo)-phenoxy]-pentane-1-thiol (8AM5SH), were prepared on flat Au substrates and Au nanoparticles. The photoisomerization from trans-8AM5SH to cis-8AM5SH was 22% for flat Au substrates and 60% for Au nanoparticles. Further Au nanoparticles capped with cis-8AM5SH were found to disperse in a mixed solvent of chloroform and ethanol (CHCl3:ethanol = 1:3), whereas those capped with trans-8AM5SH coagulated in the solvent. However, the Au nanoparticles dispersed in chloroform, regardless of whether 8AM5SH was in the trans or cis conformation.