# Plasmonic Nanomaterials for Micro- and Nanoplastics Detection
## 1 Introduction  
The global plastics production was estimated to be over 350 million metric tons per year in 2021, and is still growing [1]. In parallel, the market value of these materials is constantly increasing. Most of the industrially produced plastics are discarded into the environment, and for the majority of this waste, the destiny is still uncertain: 8% is discarded in landfills, 9% is recycled, and 12% is used for energy recovery. The rest, more than two thirds, is released and accumulated in the environment, with rivers and oceans being the main destination [1]. Released plastics can have different sizes, ranging from bulk objects down to the nanometric scale. Objects in the micrometric and nanometric size range are commonly classified as micro- and nanoplastics, respectively (MNPs). The upper and lower limits of these categories are not universally recognized [2]. Regarding microplastics, the commonly accepted upper limit is 5 mm, even if some authors set 1 mm as the superior limit of this category. The European Chemical Agency (ECHA) defines microplastics as particles below 5 mm in size, which also includes fiber-like particles that are between 5 and 15 mm in length [3]. The same limit is set by the National Oceanic and Atmospheric Administration in the U.S. [4]. The definition of nanoplastics is even more uncertain: according to the European Food Safety Authority (EFSA) panel on contaminants in the food chain (CONTAM), nanoplastics range from 1 to 100 nm in size [5], in compliance with the ISO definition of nanomaterials, which states that a nanomaterial is a material with any external dimension in the nanoscale or having an internal structure or surface structure in the nanoscale. The nanoscale is defined as ranging from approximately 1 to 100 nm (0.001-0.1 \(\upmu\)m), but some papers have set the upper size limit for nanoplastics to 1 \(\upmu\)m [6]. On the other hand, in order to establish a regulation, it may be necessary to raise the lower size limit of nanoplastics to 5 nm, since no techniques are currently available to detect smaller particles. The origins of MNPs dispersed in the environment are different: the so called primary MNPs are intentionally produced polymeric particles under 5 mm in size, like in the case of personal care products or precursors of plastic products. Secondary MNPs, on the other hand, are generated by the fragmentation and erosion of bigger plastic wastes. Thus far, EU regulations have mainly targeted the use of primary MNPs in order to reduce their production and commercialization: in January 2019, the ECHA proposed restricting intentionally added microplastics [7]. In order to reduce the impact of petroleum-derived plastics to the environment, the use of bioplastics is continuously increasing, since bioplastics can be used as substitutes for traditional plastics in several fields [8; 9], or even as materials for MNP removal [10]. Only recently (2020) has the control of microplastics in drinking water become a priority for the European Union (EU directive 2020/2184) \"on the quality of water intended for human consumption\". This directive demands the adoption of a methodology to measure microplastics by January 2024, since no standard procedure has been set thus far. While MNPs were initially thought to accumulate mainly in seawater, in the last years, evidence has been reported for their presence in almost every area of the environment: fresh water [11; 12], tap water [13; 14], soil [15], air [16], plants [1; 17], ice [18], etc. Another crucial aspect that needs to be addressed in the next few years is the lack of data on the exposure and risk for human health [19]. At present, a complete human health risk assessment (HRA) is not possible due to the lack of data on human exposure to MNPs, but a recent study showed the presence of microplastics in human blood [20], increasing the concerns on the persistence of this pollutant in the body. Recent data have also reported on the presence of microplastics in human placenta [21].