Excess Surface Energy at the Tips of Gold Nanospikes: From Experiment to Modeling

Physicochemical aspects of anisotropic gold nanostructures have been of considerable interest due to intrinsic shape-dependent phenomena that open up newer perspectives from nanoscale electromagnetism to basic thermodynamics since gold has a highly symmetric face-centered cubic (fcc) structure and usually tends to afford a spherical geometry to reduce surface free energy. The emergence of novel properties of these anisotropic structures could be attributed to the lack of symmetry at the interface or to the confinement of electrons that does not scale linearly with size. Based on these concepts, anisotropic gold nanospike has been chosen as a typical nanostructural system that possesses different surface energy around the nanostructure in comparison to isotropic gold nanosphere to quantify the precise surface energy at the tips of these intricate nanostructures. Cetyltrimethylammonium bromide-stabilized gold nanospheres and nanospikes have been synthesized in aqueous medium under ambient conditions. Since fluorescence spectroscopy is a very sensitive technique, fluorescent dye, alizarin red, has been employed as a local probe to elucidate the detailed spectral characteristics of the metalprobe hybrid assemblies. The influence of morphological anisotropy of these nanostructures has been, further, emphasized by following the temporal changes in the emission characteristics during the photoinduced conversion of gold nanospikes to nanospheres under NIR laser irradiation. Experimental realization of excess surface energy at the tips of the nanospikes has been calculated from theoretical perspectives.