# 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  
The second main strategy for the detection and quantification of MNPs is based on the use of highly sensitive SERS-active devices. The general idea is to develop a reproducible nanoplatform based on a SERS-efficient metal substrate. Over the last few years, several research groups have developed many SERS-substrates employing very different synthetic approaches and starting materials, resulting in promising sensitive devices for plastic pollutants. Different shapes and sizes of pure noble-metal nanomaterials have been investigated and employed with very promising results. A good example of this approach is the functionalization of glass slides with spherical gold nanoparticles (AuNPs), creating a simple SERS substrate [84]. After being cleaned in piranha solution, glass microscopy slides were soaked in poly allylamine hydrochloride solution, rinsed with water, and finally soaked in a AuNP dispersion for 4 h, resulting in Au-functionalized glass slides. Both 14 and 46 nm AuNPs were synthetized to obtain the substrate and to carry out the detection tests for MNPs. The above-mentioned device was capable of detecting both PS and PET nanoparticles, reaching the lowest LOD of 10 ug/mL by employing 46 nm AuNPs to detect 161 nm of PS nanoparticles [84]. A SEM image of the 161 nm PS nanoparticles on the gold substrate is reported in Figure 5a. [PERSON] et al. [94] also employed AuNPs with an average diameter of 23 nm as a hotspot source. Filter paper supported AuNPs were used to capture and detect PET microplastics. The substrate preparation is very simple: 80 uL of gold colloidal solution is dropped onto a square of filter paper, then dried in an oven at 35 \({}^{\circ}\)C. The pores of the fiber filter paper could trap the PET microplastics employed in this study (10, 15, and 20 um), allowing their identification down to concentrations of 100 ug/mL. Real sample analyses were carried out to find out whether the filter paper doped with AuNPs was capable of detecting microplastics in real water samples. Interestingly, with spiked PET microplastics of 20 um, the same LOD obtained in standard solutions of microplastics (100 ug/mL) was achieved, making this device suitable for the analysis of real samples. [PERSON] et al. [79] reported another gold SERS-substrate, but in this case, it was obtained by sputtering a thin layer of gold on glass slides. This method is fast, cheap, and simple, and the obtained device is capable of detecting nanosized PS particles of 100, 300, 600, and 800 nm, with a very low detection limit calculated to be around 0.10 ug/mL. The enhancement factor employing \(p\)-ATP as a Raman probe (reported in Table 3) was evaluated to identify the best thickness of sputtered gold, which proved to be 32 nm. Figure 5: (**a**) Representative SEM image of the 161 nm PS nanoparticles after their addition on the glass slides functionalized with AuNPs, adapted from [84]. Examples of plastic particles are highlighted with red arrows. Creative Commons 4.0 [[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)) (accessed on 12 August 2023. (**b**) FE-SEM images of AgNPs/RC film (surface density 0.99 mg/cm\({}^{2}\), thickness 3.6 μm) on the left and cross-sectional FE-SEM images of the same substrate on the right. Adapted from [85]. Copyright 2021, Elsevier Ltd. All rights reserved with permission from Elsevier. Other papers have reported the use of gold in different applications and methods to achieve a more reproducible and robust substrate. [PERSON] et al. [86] employed a 3D crossed gold nanowire substrate that enabled the detection of PS particles smaller than 1 \(\upmu\)m. Another reported example is the use of a Klarite substrate, an ordered, dense grid structure of cavities (or \"pits\") with an inverted pyramid shape [88] capable of detecting nanoplastics down to 360 nm.