Investigation of the correlation between the bulk and surface sensing performance in plasmonic crystals

Abstract

In the investigation of the sensing performance based on localized surface plasmon resonance (LSPR), the bulk sensitivity, which is the spectral shift per refractive index unit upon the change of the surrounding index to spatial infinite, is often measured and considered as the indication of the sensing ability. To the contrary, in biosensing applications only the refractive index in a confined region close to the metal nanostructure surface is altered due to the attachment of the recognition and target molecules. The correlation between the bulk and surface sensitivity is nevertheless ambiguous, especially in strongly coupled plasmonic systems, and rarely discussed in the literature. In this paper, we examine the bulk and surface sensing performance of periodic Au nanodisk arrays on quartz substrates. By means of diffractive coupling of LSPR in the periodic arrays, the bulk sensitivity and the figure of merit (FoM, as high as ~30) could be varied by engineering the coupling strength and adjusting the size of the Au nanodisk and the array pitch. The surface sensing performance is also explored by sequential atomic layer deposition of Al2O3, and the electric field decay from the Au nanodisk surface could thus be extracted using an exponential function. It is demonstrated that in these substrates the surface and bulk sensitivity have an opposite dependence on the coupling strength. In spite of the high bulk sensitivity and FoM in systems with strong coupling, a low surface sensitivity is demonstrated due to the large electric field decay length. Therefore, detailed and careful design of the coupling strength and the surface electric field to match the size of the target biomolecule is very critical for the best sensing performance. Moreover, we provide a more reliable biosensor design protocol based on the surface sensing performance.