# 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
Compared to the previous works based on the aggregation of silver nanoparticles for plastics detection, the proposed method exhibited high sensitivity, with a LOD very close to the one in the work by [PERSON] et al. Good repeatability, linear relationships for all nanoplastic sizes, interference resistance, and quantitative analysis capacity were reported. Silver is not the only metal that can provide a good SERS effect: the use of gold with a similar particle aggregation approach has also been reported in the literature. In 2022, [PERSON] et al. [71] detected PS nanoplastics using a filter paper-based method with gold nanoparticles. Specifically, they relied on gold nanoparticles with an average size of 23.2 \(\pm\) 4.2 nm, obtained via a reversed Turkevich synthesis, for the detection of polystyrene nanoplastics with different sizes (20 nm and 200 nm) and surface functionalization (carboxylic acid and amidine-modified, PS(\(-\)) and PS(+) respectively). Prior to deposition onto filter paper, AuNPs and PSNPs were pre-mixed in equal parts, then the colloidal mixture was added dropwise to filter paper and left to dry in air. This system allowed for the detection of positively charged 20 nm PSNPs up to 10 \(\upmu\)g/mL, whereas for larger PSNPs(+) (200 nm), the LOD was significantly reduced to 1 \(\upmu\)g/mL. They also tested this substrate in the presence of NaCl to simulate the ionic strengths of biological fluids and seawater, lowering the LOD to 100 \(\upmu\)g/mL for both 20 nm PSNPs(+) and 200 nm PSNPs(+). For the negatively charged PSNPs, on the other hand, no enhancement effect was reported. This is probably due to the electrostatic repulsion between PSNPs and AuNPs, both negatively charged, preventing PSNPs from being in the SERS hotspot of AuNPs. In summary, this filter paper-based SERS system is an efficient method for the detection of 20 nm and 200 nm positively charged PSNPs. This paper is a good starting point for the detection of 20 nm and 200 nm PSNPs. The detection of 2  
study of the detection of PSNPs in real samples. The same substrate could also be tested for negatively charged PSNPs by considering the use of positively charged AuNPs. [PERSON] et al. [77] used a similar approach, starting from the synthesis of gold nanospheres of different dimensions (average sizes of 33 nm, 67 nm, and 94 nm) and gold nanorods with shorter and longer diameters of 23.5 nm and 35.5 nm (see Figure 3b). All of these nanostructures were subsequently tested for the detection of 350 nm PS nanoplastics to select the one with the best SERS response, which were the gold nanorods (AuNRs) with a LOD of 6.25 \(\upmu\)g/mL. More tests were conducted to find the optimal conditions for the SERS measurements, from refining the volume ratio between the gold colloid and PS sample, to the choice of the aggregating agent and its optimal concentration. In contrast to earlier findings with 350 nm PSNPs, such promising results were not obtained for the polyethylene microplastics (1-4 \(\upmu\)m), whose SERS signals were difficult to obtain, probably because the PE plastics were not completely covered with AuNRs. In summary, the results of [PERSON] et al. are comparable with the data from [PERSON] et al. with gold nanoparticles. The same can be said for the results published with silver nanoparticles for PSNPs from [PERSON] et al. and [PERSON] et al. The major limitation of the former work is the lack of detection in real samples. In addition to the works based on the aggregation of gold nanoparticles with MNPs by the addition of salts, two other papers [78; 125] reported on NP aggregation achieved through different approaches. [PERSON] et al. [125] worked with on-site 3D assemblies of colloids ranging from nano- to microparticles for sensitive SERS detection without specific binding by exploiting the photothermally driven convective flow. With this method, the authors obtained co-assemblies of CTAB-stabilized gold nanorods (10 nm \(\times\) 36 nm) with larger PS microparticles of various sizes (0.1, 0.5, and 1 \(\upmu\)m).