# 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
At 10 mg/mL and 1 mg/mL, all of the different investigated dimensions of plastics (400 \(\upmu\)m, 800 \(\upmu\)m, 2.3 \(\upmu\)m, and 4.8 \(\upmu\)m) were observed, while only the 400 \(\upmu\)m PS nanoplastics showed a detectable Raman signal at the concentration of 50 \(\upmu\)g/mL. These interesting results revealed that SERS detection should depend on the size of the PS MNPs. To complete the study, MNPsdetection in the environmental water sample were also performed in order to quantitatively monitor PS particles via SERS. The tap, river, and seawater samples were analyzed reaching 500 \(\upmu\)g/mL as the limit of detection for 400 \(\upmu\)m PS nanoplastics. This particle-in-cavity (PIC) structure was also employed to develop an AAO conical orifice with a 2D MoS\({}_{2}\) layer and 15 nm AgNPs [83]. [PERSON] et al. exploited the capacity of PIC structures to obtain a wide distribution of hotspots from inside the dielectric cavity to close to the sidewall, in order to detect water contaminants from ions to nanoplastics. More specifically, the substrate was composed of an AAO conical orifice, AgNPs, and a MoS\({}_{2}\) interlayer between the AAO and the AgNPs as both a chemical enhancer and internal standard for the PIC architecture. Employing R6G as the Raman probe, the authors found that Ag with a thickness of 15 nm had the highest intensity of the characteristic Raman peak. The limit of detection was achieved with a 10\({}^{-12}\) M solution of R6G, with a resulting EF equal to 1.95 \(\times\) 10\({}^{10}\). Beyond ions and molecules, this substrate also proved to be advantageous, thanks to the obconic cavity, for the detection of nanoplastics. Due to the curvature of cavities, MNPs inside the pores are entirely surrounded by AgNPs, thus enabling their recognition, which was confirmed by experimental tests of 100, 200, and 300 nm PS nanoparticles (see Figure 6b). [PERSON] et al. reported Raman spectra where the characteristic Raman peak of PS at 1002 cm\({}^{-1}\) was observed, even though they did not specify the concentration at which the spectra were acquired. Alongside the use of AAO-based structures, other interesting SERS devices have been proposed in the literature. [PERSON] et al., for example, developed a NWERS (nanowell-enhanced Raman spectroscopy) substrate composed of self-assembled SiO\({}_{2}\) sputtered with a silver film (SiO\({}_{2}\) PC@Ag) [80]. Their strategy was to exploit the coffee-ring effect and the gravity of nanoplastics to trap them exactly on the nanowells, thus enabling their detection in water environments. Through a simple two-step approach, the SiO\({}_{2}\) PC@Ag substrate was formed: first, using the interfacial assembly method, the structured SiO\({}_{2}\) nanospheres assemblies were fabricated, then silver was sputtered to obtain a thin shell to acquire the SERS effect. To optimize the SERS performance, the thickness of the sputtered silver was studied by observing the enhancement of \(p\)-ATP. With a shell thicker than 100 nm, the inter-nanosphere gaps disappeared, consequently, an 80 nm silver shell (SiO\({}_{2}\) PC@80 nm Ag) was chosen as the best substrate (see SEM images in Figure 7a). Prior to studying the performance of this substrate to detect PS MNPs, [PERSON] et al. collected different Raman spectra by employing three different laser sources, finally choosing a 633-nm laser as it matched the photonic bandgap and possessed better SERS performance. Figure 6: (**a**) SEM images of 400 nm PS nanoplastics (0.1% concentration) on the AuNS@Ag@AAO substrates (**top**) and a schematic representation of the plausible SERS detection mechanism of nanoplastics with the AuNS@Ag@AAO substrate (**bottom**), Reprinted from [87]. Copyright 2020, Elsevier B.V. All rights reserved with permission from Elsevier. (**b**) SEM images of AAO/MoS\({}_{2}\)/Ag containing PS nanoplastics (**up**) and the simulated electric fields in AAO/MoS\({}_{2}\)/Ag containing PS nanoplastics of different sizes. Adapted with permission from [83]. Copyright 2022, American Chemical Society.