A Molecular and Thermodynamic View of the Assembly of Gold Nanoparticles in Nematic Liquid Crystal

JONATHAN MILETTE; VIOLETA TOADER; EZEQUIEL R. SOULÉ; R. BRUCE LENNOX; ALEJANDRO D. REY; LINDA REVEN

LANGMUIR

AMER CHEMICAL SOC

Lugar: Washington; Año: 2013 vol. 29 p. 1258 - 1263

0743-7463

The ligand dynamics and assembly of 4-5 nm gold nanoparticles (AuNPs) dispersed in the liquid crystal (LC) 4´-pentyl-4-cyanobiphenyl (5CB) was probed by solid-state 1H, 2H and 13C NMR. The mixed ligand shell with a 1:1 ratio of hexanethiol and 4’-(12-mercaptododecyloxy)biphenyl-4- carbonitrile, CBO(CH2)12SH, show high chain mobility as compared to a monolayer of CBO(CH2)12SH only, resulting in highly miscible nanoparticles in isotropic 5CB. Upon cooling to the isotropic-nematic phase transition, the AuNPs form a reversible, long range network with the topology controlled by the particle concentration, cooling rate and film thickness. Both the host LC (5CB-d2) and the ligand CBO(CH2)12SH (AuNP-d2) were selectively deuterated to separately follow the orientational order of the matrix and the NP ligands by variable temperature 2H NMR as a function of particle concentration. The 2H NMR spectrum of the mixed monolayer AuNP-d2 in isotopic 5CB consists of a sharp singlet. Below TNI, the AuNP-d2 spectrum transforms into a doublet superimposed on a broadened singlet, assigned to field aligned and conformationallydisordered ligands respectively. The matrix 5CB-d2 displays a nematicisotropic biphasic range that increases with particle concentration. The intensities of the AuNP-d2 signals for aligned and disordered ligands are strongly correlated with the signals for the aligned and isotropiccomponents of the host 5CB-d2. The NMR results were inputted into a mean field thermodynamic model to construct the phase diagrams of the AuNP-LC mixtures.