Dielectric Sensing with Deposited Gold Bipyramids

Colloidal gold bipyramids with a narrow ensemble plasmon resonance in the near-infrared are adhered on glass or silicon substrates using polyelectrolytes. Atomic force microscopy and scanning electron microscopy show a monolayer of the deposited colloids that remains nonaggregated, with an optical density of similar to 0.1 at the peak plasmon resonance. The substrates can be repeatedly immersed in various solvents. The near-infrared resonance shifts with the optical index of the solvent by similar to-0.62 eV/refractive index unit. The figure of merit for the ensemble absorption shift is comparable to the best values reported for single metallic colloidal particles.