Optimal geometry parameter for plasmonic sensitivities of individual Au nanopoarticle sensors.

The shape, aspect ratio and size are key parameters governing the plasmon sensitivities of individual Au nanoparticle bio/chemical sensors. It is crucial to unveil the general geometry parameters to optimize their corresponding sensitivity applications. In this work, the geometry-dependent refractive-index sensitivity factor (S) and figure of merit (FOM) of individual Au nanoparticle sensors (including a nanodisc, nanorod, nanoellipsoid and hexagonal nanoplate) are numerically investigated by discrete dipole approximation (DDA). S is revealed to increase quadratically/linearly with aspect ratio, while FOM reaches a maximum at an optimized aspect ratio of about 3.0/8.0 for the studied prolate/oblate nanoparticles, respectively, reflecting their shapes and aspect ratios and, hence, their size effects. However, their responses to shape factors are shown to follow nearly the same trend regardless of their different detailed geometries, demonstrating that their shape factors provide the general geometry parameters governing the plasmon sensitivities of the concerned individual Au nanoparticle sensors. This can be analytically explained well under dipolar localized surface plasmon resonance (LSPR) conditions. Their optimal FOM is predicted to be about 12.5 RIU-1 at a shape factor of 10.5; the underlying reason for this is analytically discussed as well. The obtained results in this work are believed to hold great promise for choosing appropriate nanoparticle geometry parameters for individual Au nanoparticle LSPR-based bio/chemical sensor design and applications as well as to access the corresponding optimal geometry parameters and FOM simultaneously.