# Plasmonic Nanomaterials for Micro- and Nanoplastics Detection
## 3 Plasmonic Nanomaterials for MNPs Detection
### Surface Plasmon Resonance  
Surface plasmon resonance (SPR) sensing is a powerful analytical technique used to detect and analyze molecular interactions in real-time [41]. It is a label-free, non-destructive technique that can be used to study a wide range of molecular, ionic, and biomolecular interactions including protein interactions [42], protein-DNA interactions [43], and antibody-antigen [44] binding. SPR sensing relies on the phenomenon of surface plasmon resonance, which occurs when a thin layer of metal (typically gold or silver) or a layer of nanoparticles is grafted on a surface and excited with light. This excitation causes a collective oscillation of free electrons, known as SPR, at the metal surface. When biomolecules are immobilized on the metal surface, any changes in their mass, conformation, or refractive index can alter the SPR, which can be detected as a change in the reflected light intensity or in the maximum absorption wavelength. SPR sensing has become an important tool in drug discovery, diagnostics, and biotechnology, and its applications continue to expand as new technologies and detection methods are developed. Although SPR sensing is most commonly associated with biosensing applications, it has also found applications in other areas. One notable example is the use of SPR in the field of materials science. SPR can be used to study the properties of thin films and coatings [45]. The study of the interactions of SPR with gases [46] and liquids has led to the development of SPR-based sensors for gas detection, environmental monitoring, and food safety [47]. SPR sensors based on gold nanostructures can be easily integrated in optic fibers, useful for the creation of compact and easy to use devices for bio-chemical detection and pollutant analysis [48,49]. In 2016, [PERSON] et al. [50] proposed a method based on a commercial SPR setup for biosensing. A commercial SPR chip, made of a 50-nm thin film of gold, was used as the sensing chip, operating at 670 nm. The method was tested on 100 nm, 300 nm, and 460 nm nominal diameter polystyrene. The method was based on the measurement of the diameter and particle number concentration of the PS nanoparticles by fitting the measured effective refractive index of polystyrene colloids (Figure 2). Data obtained from the SPR measurements were compared to the SEM size analysis, and concentrations were compared to the concentration of the commercial standards. Figure 1: Colorimetric detection of MNPs. PS nanoparticles in acetone can be detected via the PS induced aggregation of AuNPs. Non-aggregated AuNPs show the typical deep ref colour, while aggregated colloids turn blue, as shown in the pictures in figure. Reprinted with permission from ref. [40]. Copyright Elsevier 2022. Fitting of the effective refractive index was conducted using the coherent scattering theory (CST), and data obtained for the three standard samples were coherent with what were obtained from the validation methods. For the 100 nm sample, the measured refractive index values were higher than the prediction carried out with CST, probably due to the adsorption of the particles on the Au surface. Some problems also emerged in the size measurement of the 300 nm particles, with an SPR measured diameter lower than the diameter obtained by means of SEM microscopy. The method is thus promising, but still needs some improvement and a better understanding of the data. The feasibility of a simultaneous measurement of the diameter and concentration has still to be demonstrated, but the method shows good results in this direction. With a similar approach, [PERSON] et al. [51] prepared an SPR chip functionalized with estrogen receptors (ER) for the real-time detection of microplastics. With chromatography experiments, the authors demonstrated the possibility of distinguishing different microplastics (polystyrene--PS, polyvinyl chloride--PVC, polyethylene--PE with average size of 20 \(\upmu\)m) based on their surface charge.