# Plasmonic Nanomaterials for Micro- and Nanoplastics Detection
## 4 SERS-Based MNPs Detection
### Detection of MNP Using SERS-Based Techniques
#### 4.2.2 SERS Detection Employing Nanostructured Devices
With different EF, both the PS and PMMA particles could be identified, but it should be mentioned that to achieve the best Raman signal enhancements, the location of the plastics onto the substrate is critical. Being aware of this, when plastics reach the center of the pits, distinct Raman peaks are displayed. Although it is possible to obtain ordered and homogeneous structures with gold, it is well-known that silver provides the highest SERS enhancements in the visible range. A lot of interest has thus been dedicated to silver-based SERS devices. The work of [PERSON] et al. [85] compared the SERS signals of regenerated cellulose (RC) hydrogel films with AuNPs and silver nanowires (AgNWs). As expected, it can be observed from the EF listed in Table 3 that the AgNPsNWs substrate was more sensitive than the one with AuNPs. This was also confirmed by the detection limits achieved from both substrates for the identification of PS nanoparticles. With the gold substrate, the characteristic Raman peak at 1002 cm\({}^{-1}\) was visible at a concentration of 500 \(\upmu\)g/mL for all three different PS studied (84, 444, 630 nm). On the other hand, for the silver substrate, nanoplastics could be detected at a concentration of 100 \(\upmu\)g/mL, confirming silver's greater ability to enhance the weak Raman signals. Furthermore, to improve the SERS performance, many researchers have managed to prepare highly ordered noble metal substrates to achieve a greater homogeneity and a more densely packed arrangement of metal nanostructures. A popular support for the construction of SERS devices is anodic aluminum oxide (AAO), chosen mainly for its highly uniform nanoporous structure. [PERSON] et al. [81] obtained V-shaped nanopores on AAO with deposited AuNPs through magnetron ion sputtering. This approach generated several hotspots with which PS and PMMA microplastics have been successfully identified by SERS. Using 1, 2, and 5 \(\upmu\)m microplastics as the target, the obtained EFs allowed for some interesting observations. First of all, the Raman signals of the PS microspheres obtained with both ion and magnetron sputtering, compared with the silicon wafer, were significantly enhanced due to the presence of AuNPs. Moreover, with both PS and PMMA microspheres, it was possible to observe the single-particle Raman signal, proving that a AuNP@V-shaped AAO SERS substrate is suitable for the detection of different plastics. According to the calculation results of EFs reported in Table 3, the strongest SERS signals were exhibited when the magnetron sputtering method was employed to prepare the SERS substrate, probably because it provides a more uniform \"hotspot\" array. It must also be pointed out that the AuNP@V-shaped AAO SERS substrate was more sensitive for PS than for PMMA, according to the weaker Raman cross section of the latter. With the common aim of obtaining internal hot spots in the AAO structure, [PERSON] et al. [87] inserted silver-coated gold nanostars (AuNSs@Ag) into the nanopores by ultrasonication-induced self-assembly. The anisotropic shape of the nanostars enables a great SERS enhancement and makes this nanoplatform suitable for MNPs detection. The AuNSs@Ag dimer arrays exploited high homogeneity in self-assembling into the AAO nanopores and the SERS substrate obtained was employed to study the detection of PS nanoparticles of assorted sizes in several concentrations in distilled water, as represented in the SEM images reported in Figure 6a. Thanks to the nanosized gap, when the small nanoplastics interact with the branches of the AuNSs@Ag, a great SERS enhancement is achieved, if compared with the bare AAO structure. In this work, [PERSON] et al. investigated the size-dependent detection of PS in an aqueous environment with different concentrations.