Molecular Sensitivities of Substrate-Supported Gold Nanocrystals

The sensitivity of plasmonic sensors in response to the refractive index changes of the surrounding medium is highly dependent on the shape of metal nanocrystals. In this work, the bulk refractive index sensitivities of dispersed and immobilized Au nanocrystals in different shapes (nanospheres, nanorods, nanobipyramids, and nanoplates) are investigated. The Au nanoplates exhibit the largest bulk refractive index sensitivity in solutions. The substrate effect, which is correlated with the fractional particle surface area in contact with the substrate, makes the Au nanoplates fall behind the nanorods and nanobipyramids because of their larger interfacial contact area. The single-particle dark-field scattering technique is employed for the detection of the spectral shift upon molecular adsorption. The oxygen plasma treatment is proved to be incapable of improving the molecular sensitivity because of the structural damage to the nanocrystals. The Au nanobipyramids possess the highest molecular sensitivity among the four types of the Au nanocrystals. The distinction in the tendency of the differently shaped Au nanocrystals between the bulk and molecular sensitivities is systematically studied by electrodynamic simulations. The molecular sensitivity measurements with short-chain molecules further verify the sensitivity difference of the differently shaped Au nanocrystals. Taken together, our study and understanding of the effect of the shape on the sensitivities of plasmonic metal nanocrystals will be valuable for the design of highly sensitive plasmonic sensors as well as for the development of plasmon-enabled spectroscopies.