# Plasmonic Nanomaterials for Micro- and Nanoplastics Detection
## 3 Plasmonic Nanomaterials for MNPs Detection
### Colorimetric Sensors  
Plasmonic metal nanoparticles are commonly employed as colorimetric sensors for a wide variety of analytes and applications: food safety [35], environmental pollutants [36], cancer diagnostics [37], and the sensing of pathogens [38]. Colorimetric methods are simple and low-cost, and the response of the sensor can often be evaluated by the naked eye, without the need for complex instrumentation. Metal nanoparticle-based colorimetric sensors usually exploit an aggregation of the colloid, leading to a dramatic change in the LSPR maximum absorption wavelength [39]. To our knowledge, only one paper in literature has reported the use of gold nanoparticles for the colorimetric sensing of polystyrene (PS) nanoplastics [40]. The method is based on a partial dissolution of PS in acetone. PS chains inhibit the acetone-induced aggregation of AuNPs, leading to different colors of the dispersion in the presence and absence of nanoplastics, as shown in Figure 1. In other words, non-aggregated AuNPs maintain their typical purple-red color, signaling the presence of plastics. Aggregated AuNPs, on the other hand, shift to a blue color, diagnostic of particle aggregation and of a consequent change in the maximum absorption wavelength of the plasmon. This method is promising and provides easy to read results, but it has some major limitations, also acknowledged \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Technique**} & \multicolumn{1}{c}{**Size Range**} & \multicolumn{1}{c}{**Composition**} & \multicolumn{1}{c}{**Concentration**} & \multicolumn{1}{c}{**Limitations**} \\ \hline Spectroscopy & FTIR & 50–500 μm & YES & \(\backslash\backslash\backslash\backslash\backslash\backslash\backslash\) & Not suitable for nanoplastic \\ \cline{2-6} \multirow{2}{*}{[26]} & Raman & \(>\)100 nm & YES & \(\backslash\backslash\backslash\backslash\backslash\backslash\backslash\) & identification \\ \hline \multirow{6}{*}{Fluorescence microscopy} & NILE RED [27] & 20 μm-1 mm & NO & \(\backslash\backslash\backslash\backslash\backslash\backslash\) & Not suitable for less hydrophobic \\ \cline{2-6}  & PDI [28] & \(\backslash\backslash\backslash\backslash\backslash\backslash\backslash\) & Selective for PVC & \(\backslash\backslash\backslash\backslash\backslash\backslash\backslash\) & \\ \cline{2-6}  & Fluorogenic polymer & & & & \\ \cline{2-6}  & [29] & 0.1–100 μm & YES & \(\backslash\backslash\backslash\backslash\backslash\backslash\backslash\) & \\ \hline \multirow{6}{*}{MS} & Py-GC/MS [30] & \(\backslash\backslash\backslash\backslash\backslash\backslash\backslash\) & YES & LOD: 4 mg/L & Complex sample pre-treatment \\ \cline{2-6}  & MALDI TOF MS [31] & \(\backslash\backslash\backslash\backslash\backslash\backslash\backslash\backslash\) & YES & LOD: 25 mg/L & Thermal pre-treatment \\ \cline{2-6}  & Single particle & \multirow{2}{*}{135 nm–1 μm} & \multirow{2}{*}{YES} & 8.4 \(\times\) 10\({}^{5}\) NP/L & NPs need to be carboxylated for \\  & ICP-MS [32] & & & (LOQ) & their identification \\ \hline \multirow{6}{*}{Microscopy} & SEM [33] & \(>\)100 nm & NO & \(\backslash\backslash\backslash\backslash\backslash\backslash\) & No information available about \\ \cline{2-6}  & TEM [33] & \(>\)100 nm & NO & \(\backslash\backslash\backslash\backslash\backslash\backslash\) & other techniques. \\ \cline{2-6}  & optical & \(>\)0.5 μm & NO & \(\backslash\backslash\backslash\backslash\backslash\backslash\backslash\) & Not suitable for nanoplastic \\ \cline{1-1} \cline{2-6}  & & & & & \(\backslash\backslash\backslash\backslash\backslash\backslash\) & identification. Not possible to \\ \cline{1-1} \cline{2-6}  & & & & & & confirm the chemical composition \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the main analytical techniques currently used for MNPs detection and quantification. by the authors, which need to be overcome in order to use it in a real application.