Simulation analysis of a multi-hot-spot system for SERS detection of the nanoplastics

In recent years, nanoplastics have attracted increasing attention due to their widespread presence in the environment and potential harm to living organisms. To provide a theoretical basis for using surface-enhanced Raman spectroscopy (SERS) mechanism to detect nanoplastics of different sizes, this work employed lasers to irradiate the substrate composed of a bowl-shaped particle-in-cavity structure and nanoplastics. Then, the electric field distribution was obtained using the finite-difference time-domain (FDTD) method. By altering the curvature of the Ag nanobowl and the size of gold nanoparticles (AuNPs), the electric field enhancement capability of this SERS structure can be ameliorated. It is found that when the diameter of AuNPs is 30 nm, the larger the nanoplastics, the more suitable the structure with smaller curvature as a substrate. But when the diameter of AuNPs is 40 nm, the larger the nanoplastics, the more suitable the structure with larger curvature as a substrate. AuNPs with a size of 40 nm are generally superior to those with a size of 30 nm. To verify the feasibility of this SERS structure for detecting various nanoplastics, we tested a range of nanoplastic materials. The results prove that the materials of nanoplastics will not have a significant impact on the detection. Moreover, a multi-hot-spot system is analysed to reveal the SERS signal enhancement mechanism. A laser of 785 nm can produce stronger ‘localised hot spots’ (LHSs) and weaker ‘volume hot spots’ (VHSs) than a laser of 532 nm. The issue of nanoplastic detection is optimistically poised for resolution, as the hot spots within the bowl-shaped particle-in-cavity structure can effectively approach and surround nanoplastics, stimulating highly intense SERS signals that demonstrate their promising application in nanoplastic detection.