Shape-Dependent Refractive Index Sensitivities of Gold Nanocrystals with the Same Plasmon Resonance Wavelength

Gold nanocrystals in seven different shapes were prepared, including large nanorods, nanobipyramids, oxidized nanobipyramids, oxidized nanorods, dog-bone-like nanorods, peanut-like nanorods, and small nanorods. Their longitudinal plasmon resonance wavelengths were all synthetically controlled to be around 730 nm. Their refractive index sensitivities were measured by dispersing them in water-glycerol mixtures of varying compositions. The index sensitivities were found to be dependent on the shapes of the gold nanocrystals, with the large nanorods exhibiting the highest index sensitivity of 326 nm/RIU (refractive index unit) and the small nanorods exhibiting the lowest sensitivity of 156 nm/RIU. Finite-difference time-domain calculations were performed to obtain the electric field intensity enhancement contours around these gold nanocrystals. The index sensitivity was found to exhibit an approximately linear dependence on the product of the nanocrystal polarizability and end curvature.