# Plasmonic Nanomaterials for Micro- and Nanoplastics Detection
## 4 SERS-Based MNPs Detection
### Detection of MNP Using SERS-Based Techniques
#### 4.2.3 Hybrid SERS System Capable of Simultaneous Detection and Remediation of MNPs  
As mentioned in the previous paragraphs, SERS has attracted considerable attention in the last years for chemical sensing and environmental monitoring. With sensitivity up to the single molecule level and capability in the identification of molecules through their vibrational fingerprint, SERS is widely used to identify low concentrations of numerous chemical substances such as pollutants, biomolecules, and micro- and nanoplastics [77]. However, the detection limits of SERS are far from environmentally relevant concentrations [76], furthermore, SERS detection on targets without a specific adsorption group that do not present an affinity for the metallic nanostructure remains a significant challenge [127; 128]. Another limitation of this technique is the lack of selectivity for the analytes when found in complex matrices: it is therefore necessary to develop a method to separate and pre-concentrate micro- and nanoplastics. The two papers reported in this paragraph used membrane filtration for the pre-concentration, one of the most common methods in this regard. For the detection of microplastics in non-pre-treated water samples, [PERSON] et al. [74] used a sponge to locally concentrate microplastics, functionalizing it with a layer of 20 nm Au nanoparticles as the SERS substrate. The sponge (commercial, made of styrene butadiene latex) was used as a template to assemble AuNPs, and its adjustable size could allow for the distance between the hotspots on the substrate surface to be controlled. This makes it possible to assemble AuNPs with improved gaps and avoid NP aggregation, producing a more uniform SERS substrate. Moreover, the pores of the sponge allowed water to be rapidly separated from the sample, while their bowl-shaped structure effectively trapped microplastics (see the scheme in Figure 8a). The fabrication of the substrate is very simple: first of all, sponges were functionalized with PDDA (Poly(diallydimethylammonium chloride)), that is, the linkage between the sponge and AuNPs; after rinsing, sponges were soaked in the AuNP solution to be uniformly functionalized. A preliminary study was made to optimize the substrate, fabricating it with one to eight AuNP layers: to evaluate their SERS response, crystal violet (CV) was used as the probe molecule, registering an increase in the intensity of the CV characteristic peak between one and five layers, and a decrease beyond five layers. Hence, the substrate with five layers of AuNPs was used for subsequent measurements. Moving to the detection of plastics, the authors prepared aqueous solutions of different microplastics: polystyrene from Petri dishes, polyethylene terephthalate, polyethylene, polyvinyl chloride, polypropylene, and polycarbonate, all of them with dimensions of 80-150 \(\upmu\)m.  
First, they tested their substrate with PS-microparticles, noting that when the amount of the test solution changed, the amount of microplastics attached to the substrate also changed: for test solution volumes of 2 mL, 4 mL, and 6 mL, respectively, LODs of 50.0, 5.0, and 1.0 \(\upmu\)g/mL were found. To test the selectivity of this substrate, they also exposed it to the other microplastics, noting that all their signals did not overlap and that, therefore, the signal at 1002 cm\({}^{-1}\) was selective and characteristic of PS-microplastics. In addition, analyses of real samples were also made, specifically in river water, seawater, rainwater, and snow water, where a LOD of 50 \(\upmu\)g/mL was obtained. The second paper [67] reported a silver SERS substrate. More specifically, the authors worked with Ag nanowires with a diameter of approximately 60 nm and a length of about 15 \(\upmu\)m. The AgNPs were incorporated in a membrane prepared with the flow-through method: a commercial filter paper was connected to a syringe containing the AgNW solution that was flowed through the same filter paper under the action of pressing.