Solvent Effects on the Optical Properties of PEG-SH and CTAB Capped Gold Nanorods

Cetyltrimethylammonium bromide (CTAB) and (11-mercaptoundecyl) tetra(ethylene glycol) (PEG-SH) capped gold nanorods were prepared and dispersed in water and dimethyl sulfoxide (DMSO). Transmission electron microscopy images reveal that changing the solvent from water to DMSO cause that nanoparticles tend to organize (PEG-SH ligand) or aggregate (CTAB ligand). UV-vis absorbance spectra reveal that ligand as well as solvent exchange cause positive solvatochromic shifts and changes in the relative extinction values. After the transfer of nanorods from a solvent of lower to higher refraction index a red shift of the longitudinal surface plasmon resonance band is observed. This effect is more pronounced in the case of PEG-SH capped nanorods. Time resolved pump-probe measurements revealed that both ligand and solvent exchange influence the excited state relaxation times, however, a more pronounced change is induced by the ligand exchange. Two-photon excited fluorescence spectra of PEG-SH covered nanorods showed a slight intensity increase when moving from water to DMSO solvent.