Tuning the miscibility of gold nanoparticles dispersed in liquid crystals via the thiol-for-DMAP reaction

Abstract

Ligand exchange reactions using 4–5 nm 4-(N,N-dimethylamino)pyridine (DxMAP)-capped gold nanoparticles (AuNPs) formed the basis for synthesizing a family of liquid crystal (LC)-capped NPs for a rationalized miscibility in liquid crystal matrices. NPs with ligand capping layers composed of CH₃(CH₂)_mSH (m = 5, 11) or 4′-(n-mercaptoalkyloxy)biphenyl-4-carbonitriles (CBO(CH₂)_nSH, n = 8, 12, 16) and their binary mixtures were prepared. The miscibility of the NPs in liquid crystals is found to be sensitive to the ligand chain length and the density of the LC ligands within the capping layers. Polarized optical microscopy and UV-vis data show that the NPs with only CH₃(CH₂)_mSH ligands are either immiscible or only partially disperse in the isotropic phases of 4-n-pentyl-4′-cyanobiphenyl (5CB) and 4-n-octyl-4′-cyanobiphenyl (8CB). NPs with CBO(CH₂)_nSH (n = 8, 12, 16) ligands or mixed CH₃(CH₂)₅SH/CBO(CH₂)₁₂SH ligand shells containing 28% or 70% CBO(CH₂)₁₂SH ligand content partly disperse. However, NPs with a 1 : 1 CH₃(CH₂)₅SH/CBO(CH₂)₁₂SH ratio are completely miscible in isotropic 5CB up to at least 25 wt% Au. In general, the derivatization methodology developed here for mesogenic ligands provides in a complementary approach to thiol-for-thiol exchange for designing bifunctional AuNPs, offering the advantages of high reproducibility, access to a wide composition range and no need for large excesses of valuable functionalized ligand.