# Plasmonic Nanomaterials for Micro- and Nanoplastics Detection
## 4 SERS-Based MNPs Detection
### Detection of MNP Using SERS-Based Techniques
#### 4.2.1 SERS Detection Based on Colloidal Noble Metal Nanoparticles  
One of the most popular strategies for the SERS detection of micro- and nanoplastics is a direct method based on colloidal noble metal nanoparticles without requiring any substrate fabrication. First of all, it should be emphasized that the LSPR, and in turn, the electromagnetic enhancement of the field, are strictly related to the nature and geometry of the SERS-active particles. In this regard, great efforts have been made in the last years to achieve high enhancement factors using different noble metal nanoparticles and, as will be discussed later, silver and gold nanostructures are the most widely used metals in SERS substrates due to their physical and LSPR properties [126]. Another important factor that strongly influences the SERS enhancement is the possible aggregation of colloidal nanoparticles, which could result in nanoparticle clusters with different plasmonic properties with respect to isolated NPs, and with enhancement efficiencies that exceed those of the latter. In most of the works reported in this section, the detection of micro- and nanoplastics was made by adding salts and/or acids to colloidal systems, causing their precipitation and aggregation, thus increasing the SERS hotspots. In 2020, [PERSON] et al. [123] used this exact approach, with spherical silver nanoparticles (average diameter of 56 \(\pm\) 0.82 nm, see Figure 3a) and sodium chloride as their aggregating agent [123]. Their paper describes the detection of three different types of MNPs by first making a comprehensive study of the best conditions in which to run the tests. The volume ratio of the PS sphere aqueous solution to silver colloid, NaCl concentration, and the concentration of the PS samples were optimized. The best result was obtained for the detection of PS spheres of 100 nm and 500 nm, for which a LOD of 40 \(\upmu\)g/mL was reached in both pure water and seawater. The SERS signals of PE and PP plastic powder with a size of 10 \(\upmu\)m were also recorded in both pure water and seawater, but the enhanced signal of these microplastics was not as strong as that of the PS nanoplastics, most likely due to the larger size of PE and PP. In summary, [PERSON] et al. provided an efficient SERS substrate for the quantitative and qualitative detection of PS nanoplastics, which, however, showed limitations in the identification of PE and PP micropowder. Another work based on the aggregation of silver nanoparticles was published by [PERSON] et al. in 2021 [68]. The authors used silver spheres with an average size of 56.7 \(\pm\) 14.1 nm, and aggregated them with PS nanoplastics that were 1 \(\upmu\)m and 50 nm in size using magnesium sulfate (MgSO\({}_{4}\)) as the coagulant agent. Specifically, AgNPs were mixed homogeneously, then the PS aqueous solution and MgSO\({}_{4}\) and 5 \(\upmu\)L of the resulting sample were placed on a clean silicon wafer and dried. SEM images were taken to investigate the morphology of the aggregates before analyzing them by SERS. A lower LOD than the previous work was reported, around 5 \(\upmu\)g/mL for both the 1 \(\upmu\)m and 50 nm PS particles. In spite of the interference peaks from the sample matrix, since detection was carried out n spiked river water samples, several characteristic PS peaks were obtained, demonstrating the power of SERS to probe nanoplastics. [PERSON] et al. [76] also worked with a similar system for quantitative nanoplastics analysis, in which potassium iodide was added to silver nanoparticles (50-60 nm), working both as an aggregating agent and as a cleaner to remove impurities from the surface of the silver nanoparticles, which is crucial for a quantitative investigation. Polystyrene nanoplastics of different sizes were detected in aqueous media, with concentrations ranging from 6.25 \(\upmu\)g/mL to 2000 \(\upmu\)g/mL, with LODs of 12.5 \(\upmu\)g/mL for 50 nm PSNPs, 6.25 \(\upmu\)g/mL for 100 nm PSNPs, 25 \(\upmu\)g/mL for 200 nm PSNPs, and 25 \(\upmu\)g/mL for 500 nm PSNPs.