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 ∼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 ∼−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.