Plasmonic and sensing properties of vertically oriented hexagonal gold nanoplates
Plasmonic metal nanocrystals, owing to their high sensitivity to the dielectric changes in the surrounding environment, can allow for the direct probing and monitoring of molecular binding on their surfaces. Anisotropic Au nanoplates possess high refractive index sensitivities, with their nanoscale sensing volumes located at their sharp tips and edges. One of the main challenges in molecular detection based on localized plasmon resonance is to increase the detection capability at the single-particle level. Vertically oriented Au nanoplates are more attractive candidates for developing ultrasensitive plasmonic sensors than horizontally oriented ones, as vertical Au nanoplates allow for more analyte molecules to access their sharp tips and edges. However, few reports have studied the sensing performance of vertically oriented, elongated, individual metal nanocrystals. Herein we report on the deposition of vertically oriented, hexagonal Au nanoplates on substrates and the study of their plasmonic and sensing properties. The vertically aligned nanoplates are compared with the horizontally oriented ones both experimentally and through numerical simulations. The vertically oriented nanoplates possess shorter plasmon resonance wavelengths and narrower peak widths than the horizontally oriented ones of similar sizes. The shorter plasmon wavelengths and smaller peak widths are also confirmed by knocking down the vertical Au nanoplates through mild perturbation and performing the scattering measurements beforehand and afterwards. Further scattering measurements of the Au nanoplates upon the adsorption of positively charged polyelectrolyte reveal that the vertical Au nanoplates are more sensitive to the polyelectrolyte molecules than the horizontal ones.