# Plasmonic Nanomaterials for Micro- and Nanoplastics Detection
## 4 SERS-Based MNPs Detection
### Detection of MNP Using SERS-Based Techniques
#### 4.2.2 SERS Detection Employing Nanostructured Devices
PS nanospheres with sizes of 200, 500, 800 nm, and 1 \(\upmu\)m were analyzed on the optimized SiO\({}_{2}\) PC@80 nm Ag. Despite the particle size, the LODs all reached 0.5 \(\upmu\)g/mL (0.00005%) in deionized water, observing that the intensity of the characteristic of PS (1003 cm\({}^{-1}\)) increased with the concentration until it reached 0.1%. The analysis of real samples was also performed, reaching a 5 \(\upmu\)g/mL LOD with spiked PS nanoplastics in bottled water, tap water, and river water. With the common purpose of producing a SERS device capable of detecting nanoplastics with the help of cavity-structure, [PERSON] et al. developed a nanowire-in-bowl-shaped piezoelectric cavity structure [89]. In particular, the bowl-shaped structure was built on a BaTiO\({}_{3}\)@PVDF film and then CuO nanowires (NW) were formed inside it, obtaining a CuO NW/BaTiO\({}_{3}\)@PVDF bowl-shaped structure. In addition, to confer SERS activity to the substrate, 30 nm of silver was deposited after evaluating the variation in the SERS activity with R6G by varying the silver thickness, as can be observed in Figure 7f. The main advantage of this device is the optical field amplification, thanks to the large specific surface area and light focusing properties of the bowl-shaped structure. Additionally, the dense CuO NWs into the structure create a multi-dimensional SERS hotspot, thus achieving an excellent enhancement factor, equal to 2.7 \(\times\) 10\({}^{8}\). Moreover, being able to bind the nanoplastics, this bowl-shaped substrate was employed to successfully detect PS nanoplastics up to 20 nm in size. Compared with larger MNPs (100 and 300 nm), the 20 and 50 nm PS nanospheres showed higher Raman intensity, demonstrating that this SERS substrate had directional detection ability for smaller sized nanoplastics. Beyond the classical so-called SERS chips, in recent years, some other interesting devices for MNPs detection have been reported in the literature. For example, a wearable SERS sensor based on ultrathin, flexible biointegratable gold nanomesh [96]. This particular device was demonstrated to be highly scalable, easy to fabricate, low cost, flexible, and adhesive, very important features for a wearable sensor prepared with the purpose of detecting different substances such as microplastics. This sensor is made of gold-coated biocompatible polyvinyl alcohol (PVA) nanofibers capable of sticking onto virtually any surface such as human skin. In this work, [PERSON] et al. optimized the dimension of the substrate in order to obtain SERS capabilities while  
Figure 7: SEM images of the Cu/BaTiO\({}_{3}\)@PVDF bowl-shaped substrates after soaking in NaClIO solution for different times in order to obtain CuO NWs: subfigures (**a–c**) show samples prepared at 10s, 20s and 30s times, respectively. (**d**) Raman spectra of R6G 10\({}^{-6}\) M on the SERS substrate after soaking in NaClIO solution for different times (unsoaked, 10, 20, and 30 s). (**e**) UV–Visible spectra of the Cu/BaTiO\({}_{3}\)@PVDF bowl-shaped substrates after soaking in NaClIO solution for different times (10, 20, and 30 s). (**f**) Raman spectra of R6G on the Ag/CuO NW/BaTiO\({}_{3}\)@PVDF bowl-shaped substrate with different thicknesses of the Ag layer. Reproduced from [89]. Copyright 2023, Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement. maintaining the features listed above. The number of hotspots in a unit volume was optimized by decreasing the diameter of the nanowires, and the final setup was reached by selecting 490 nm for the nanowire diameter and a deposited gold thickness of 150 nm. For a proof-of-concept, by employing R6G as the Raman probe, an enhancement factor of 108 was obtained, and a \(10\times 10^{-9}\) M concentration of R6G was the lowest detectable. Then, to demonstrate the capability of the sensor for health monitoring, [PERSON] et al. successfully detected human sweat biomarkers (urea and ascorbic acid) and some abuse drugs.