# Plasmonic Nanomaterials for Micro- and Nanoplastics Detection
## 4 SERS-Based MNPs Detection
### Detection of MNP Using SERS-Based Techniques
#### 4.2.4 Influence of MNP and Plasmonic Nanomaterial Size on SERS Response and Enhancement Factor  
It is interesting to observe that, despite the use of the same SERS substrate/colloid approach in most of the papers previously cited, different MNP sizes influenced the SERS response. As can be seen from Table 3, several of the listed papers reported the quantitative analysis of MNPs of different sizes with different results in the related EFs and LODs. For example, [PERSON] et al. [76] studied the SERS enhancement of PS particles in a range from 50 nm to 500 nm by applying different SERS strategies. The best LOD value was obtained for the 100 nm sized PS, while PS particles with a larger size showed an extremely lower intensity of the characteristic Raman peak (1002 cm\({}^{-1}\)) compared to the intensities obtained on smaller nanoplastic particles. Recalling that the electromagnetic enhancement is practically negligible for distances above 10-20 nm, this behavior can be explained since micrometer-sized particles possess a smaller specific surface area than nanometric ones; in fact, only a slight surface contact between microplastics and the SERS active substrate (here AgNPs) occurs, resulting in a poor SERS response. Clearly, not only is the size of the target plastics important for obtaining a strong signal, but also the size, shape, and material of the plasmonic nanoparticles. The crucial importance of using colloids of appropriate shape and material has already been discussedin the previous paragraphs. In brief, sharper morphologies can increase the presence of hotspots (consequently leading to higher EFs) as well as the use of silver (as the core nanomaterial or as a coating on different metals), which can scatter light more efficiently than gold. Regarding the size of the metal nanoparticles, it can be observed, for example, in the work of [PERSON] et al. (2018), that using SERS substrates with smaller (14 nm) or larger (46 nm) gold nanoparticles generates a different signal intensity. In fact, by employing 46 nm AuNPs, PS and PET nanoplastics could be detected at lower concentrations compared to the use of 14 nm AuNPs. If the plasmonic particles employed are indeed too small, the electronic scattering on the surface decreases, and consequently, the radiated field. Of course, it needs to be kept in mind that there is also an upper limit in dimensions, otherwise, the localized surface plasmons cannot be excited. While SERS is a relatively recent application, its use for the determination of micro- and nanoplastics is still at a primordial stage. If what can be observed is a tendency for the size of the plastics to increase as the SERS response decreases, more study and investigation is still needed on the subject. Certainly, the difference between a colloidal and a substrate application, the attractive and repulsive forces between plastic pollutants and plasmonic particles, the influence of the sample preparation method, etc. are key points on which much work is still needed. Challenges, Current Limitations, and Future Perspectives for the Use of Plasmonic and SERS-Based Techniques  
The use of SERS is one of the most recent and promising developments in the field of micro- and nanoplastics detection. This approach was tested in order to overcome the limitations encountered with other analytical techniques such as difficult sample pre-treatment, expensive and time-consuming analysis, etc. The combination of the diagnostic Raman fingerprints of different polymers and the great enhancement of signals thanks to the proximity of plasmonic nanostructures makes SERS an interesting and viable replacement to other techniques that cannot usually provide simultaneous quantitative and qualitative information on MNP samples.