Revisiting the Surface Sensitivity of Nanoplasmonic Biosensors

In nanoplasmonic sensing, the bulk refractive index sensitivity is often used as a metric for performance evaluation. However, for biosensing applications, which involve molecular binding events, only the refractive index in a confined region close to the metal surface is altered. The correlation between the bulk and the surface sensitivity strongly depends on the nanostructure geometry, especially in strongly coupled systems. In this paper, we thoroughly investigate the surface sensing performance of diffractively coupled plasmonic crystals using the atomic layer deposition of conformal Al2O3 layers with well-defined thickness and refractive index. It is demonstrated that the surface sensing capacity cannot be fully described by the bulk sensitivity. It not only shows opposite dependence on the coupling strength compared to the bulk sensitivity, but also the bulk sensitivity cannot reflect the fact that the surface sensitivity could be different in different thickness ranges on the metal surface. The reason rests on the different decay lengths of the plasmonic crystal arrays with different coupling strengths and can be well explained by the second order surface sensitivity that has recently been proposed. Furthermore, we provide a quantitative method to evaluate the surface sensing performance of specific target analyte. This method is generic and can be applied to other nanoplasmonic systems and a broad range of biomolecules with various sizes.