Anisotropic Plasmonic Sensing of Individual or Coupled Gold Nanorods

We perform a theoretical investigation of individual and coupled gold nanorods as plasmonic nanosensors using the finite-difference time-domain method. Key features of single-nanorod sensors are discussed. The sensitivity distribution of an individual nanorod is anisotropic. The characteristic sensitivity decay length of a single-nanorod sensor is comparable to its diameter. Plasmonic sensing abilities are additive, so analyte-detection sensitivity is not affected by substrates or surface treatments; shifts caused by analytes are only determined by their positions relative to the sensor. Coupled nanorods enhance and concentrate plasmonic sensitivities, and the sensitivity within the gap can be over an order of magnitude higher than that at the nanorod cylinder. The sensitivities of coupled nanorods are only higher than those of individual nanorods when the analytes are anchored within the gaps between nanorods. The calculations show that a single biological molecule can be detected by optimizing nanostructure design and surface treatments to anchor analytes locally on high-sensitivity areas of the sensor surface. Our simulation results assist the design and optimization of plasmonic nanosensors, using single or coupled nanorods.