Fractal Plasmonic Molecule for Multi-Sensing: SERS Platform for SARS-CoV-2 Detection

In recent decades, there has been a strong and growing interest in developing strategies to engineer materials with tailored optical and plasmonic properties. In the plethora of geometries studied and proposed in the literature, nanostructures based on cluster arrangements known as "plasmonic molecules" (PMs) have shown several interesting properties and represent potential candidates to implement different types of opto-plasmonic applications. In this paper, we examine a fractal PM (FPM), analyzing its properties by numerical simulations based on the finite element method (FEM). Next, periodic arrangements of the FPM in two different configurations were fabricated by an electron beam lithography (EBL) process, and their plasmonic features were experimentally characterized. Our numerical simulations are in good agreement with the experimental results. Our study highlights noticeable features of the analyzed PM, such as multi-resonant and anisotropic properties, which can be exploited to design plasmonic devices with customized optical characteristics also compatible with multiple sensing applications. We tested the performance of our FPM for surface-enhanced Raman spectroscopy (SERS) sensing, realizing a functionalized system for the specific detection of the SARS-CoV-2. We tested different concentrations of the virus, obtaining a limit of detection (LOD) of 88 PFU/mL, lower than that of commercial rapid kits. Our results demonstrate that the proposed plasmonic nanopatterns are promising to develop integrable devices for a sensitive SERS detection in a portable point-of-care (POC) platform.