Gold Nanostructures for Plasmonic Enhancement of Hyper-Raman Scattering

Surface-enhanced hyper-Raman scattering (SEHRS) is very useful for the vibrational characterization of organic and biological molecules and their interaction with noble metal nanostructures. Many potential applications should ideally make use of gold nanostructures in order to enhane both the excitation and the weak hyper-Raman light, rather than silver nanostructures. Here, we report high SEHRS enhancement from spherical gold nanoparticles with different particle diameters ranging from 30 to 70 nm and from gold nanorods. SEHRS data of the two molecules crystal violet and rhodamine 6G obtained at an excitation wavelength of 1064 nm, absorbance spectra, and finite-difference time domain simulations of the electromagnetic field enhancement provide evidence that the SEHRS enhancement relies on the formation of nanoaggregates, with higher SEHRS signals yielded with increasing size of the nanoparticles in the aggregates. Gold nanorods and their aggregates are shown to provide optical properties that are specifically suited to support enhancement of SEHRS. The reported results suggest that plasmon resonances at the excitation wavelength, as well as enhancement due to the lightning rod effect, can contribute significantly to the total SEHRS enhancement. From the different concentration dependence of the signals of the different molecules as well as from comparison with salt-induced aggregation, it is concluded that the specific analyte-induced aggregation determines the specific gold nanoaggregates' geometry, arrangement, and interparticle distances. Understanding the influence of the nanoaggregate properties therefore is crucial for exploiting gold SEHRS nanosensors in future applications.