Ultrafast thermal analysis of surface functionalized gold nanorods in aqueous solution

The thermal conductivity and heat capacity of surfactant and polyelectrolyte coatings of gold nanorods (GNRs) in aqueous solution are investigated by transient absorption, following femtosecond pumping of the longitudinal localized surface plasmons. Surfactant and polyelectrolyte layer thicknesses are measured by dynamic light scattering (DLS). The GNRs are initially coated with a bilayer of the quaternary ammonium surfactant cetyltrimethylammonium bromide (CTAB). The rate of change of the absorption of gold nanorods in aqueous solution varies with the probe laser wavelength due to the shift in the plasmon resonance created by heating of media around the particles. The cooling dynamics of gold nanorods are best measured by tuning the pump-probe laser wavelength to the absorption peak of the sample. The heat capacity of the surfactant layer is 2.0 ± 0.3 J cm^-3 K ^-1; the thermal conductivity of the surfactant layer drops from 0.24 to 0.18 W m^-1 K^-1 at solution concentrations above the CTAB critical micelle concentration (cmc). Layer-by-layer polyelectrolyte coatings using poly(acrylic acid) (PAA) and polyallyamine hydrochloride (PAH) increase the thermal conductivity and heat capacity of the surface layer. PAH-terminated layers have increased thickness, thermal conductivity, and heat capacity relative to PAA-terminated layers; this effect is attributed to greater water penetration into PAH-terminated surface layers.