# 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
Concentration-dependent SERS spectra were obtained for all sizes of PS particles, measuring a LOD of 1 \(\upmu\)g/mL for 0.1 \(\upmu\)m PS particles, one of the lowest presented in this section. They also conducted a SERS characterization of the substrate using Rhodamine 6G as the probe molecule, resulting in an enhancement factor of approximately 5.4 \(\times\) 10\({}^{5}\), which was higher than those in the other previous works. The second work is the one of [PERSON] et al. [78], where they carried out a rapid in situ Raman detection of PS nanoplastics based on dielectrophoresis (DEP) and AC electro-osmosis (ACEO), known as DEP-ACEO-Raman tweezer (DART). This technology, outlined in Figure 4a, allows one to obtain Raman signals in real-time from PS-underwater nanoplastics with ultralow concentrations, exploiting a field-induced active collection of 30 nm PSNPs and gold nanoparticles toward the optical sensing area of the DART substrate from remote areas. This system, besides a rapid aggregation, allows one to simultaneously achieve the preconcentration, separation, identification, and on-site detection of target molecules. Moreover, thanks to the addition of plasmonic nanostructures that support Raman amplification, this strategy can overcome the diffusion limits and low Raman scattering cross-section of nanoplastics. With this method, for 30 nm PSNPs, the lowest LOD of all the devices (0.00117 \(\upmu\)g/mL) based on colloidal NP detection was obtained, even lower than the works with silver nanostructures, which are known to have a higher SERS amplification effect than the gold ones. In addition, tests were also carried out on the target-selective clustering and separation of two different nanoplastics by using a mixture of PMMA-NPs and PSNPs: SERS spectra of PS and PMMA particles were individually registered by applying two different AC signals by the lapse of time. In summary, the last two works by [PERSON] et al. [125] and [PERSON] et al. [78] were the two with the lowest LODs in this paragraph, and they both reported a SERS characterization of their respective gold nanorods and an evaluation of their enhancement factors, lacking instead in detection tests on real samples. Having discussed colloidal substrates based on the aggregation of noble metal nanoparticles, the final section of this paragraph addresses the only work based on simple gold nanoparticle colloids [124]. [PERSON] and [PERSON] used 50 nm gold nanourchins (AuNUs) as the SERS substrate, and 100 nm PS nanoplastics as the probe analyte. The goal of this work was to evaluate the minimum number of AuNUs required for PS detection at a single-particle level, instead of determining the limit of detection of the PSNPs (see Figure 4b). The PS concentration was kept constant, while the concentration of AuNUs in terms of particle number was varied. Samples were then air-dried on a flat surface of aluminum and the SERS spectra were collected, recording an increase in the intensity of PS signals as the number of AuNUs rises. However, this work does not provide information on the SERS characterization of the substrate and the limit of detection of PS nanoplastics, neither in the pure water nor real samples. The last paper to be discussed in this section is actually on the borderline between this paragraph and the next on the SERS detection of micro- and nanoplastics employing nanostructured devices. [PERSON] et al. [82] were able to detect 100 nm PS nanoplastics at a concentration of 200 \(\upmu\)g/mL by using spherical gold nanoparticles with an average diameter of 62 nm. For the SERS measurements, the substrates were prepared just by drying AuNP drops on a silicon wafer. The PSNP samples with different concentrations were placed on the substrate. The authors tested different concentrations of PSNP aqueous solutions ranging from 1600 \(\upmu\)g/mL to 200 \(\upmu\)g/mL. Besides determining the LOD, they also registered a strange behavior in the SERS spectra of the PS samples with concentrations in the range of 1000-1600 \(\upmu\)g/mL.