The Need for Properly Designed Synthesized Micro- and Nanoplastics with Core–Shell Structure

Abstract

While there are a number of available reference and testing materials for micro- and nanoplastic (MNP) studies in toxicology, they are not well-characterized and do not cover all major polymer types that may potentially pollute the environment. This review article will address the question of why we need properly designed synthesized micro- and nanoplastics with a core–shell structure (with organic–inorganic units) and provide researchers with a scientific basis for the design of synthesized MNP particles. It will include a list of commercially available MNPs, an overview of the theoretical background to polymer particle synthesis, and an analysis of the advantages and disadvantages of MNP preparation methods, namely, fragmentation and synthesis, along with examples of synthesized MNP particles. The current study will demonstrate that polystyrene is one of the most prevalent MNP particle types among reference materials from certification bodies and among testing particles synthesized by chemical scientists. Nevertheless, the global industrial production of polystyrene represents approximately 5% of the total, and it is not a dominant plastic type in the textile or packaging industries. In contrast to mechanically fragmented MNP particles, the synthesis approach offers the potential to control the physico-chemical properties, enabling the more selective detection and quantification, as well as a greater comparability of the results amongst toxicological studies.

1. Introduction

Today, polymers, their products, and their impact on our daily lives are undergoing significant developments. We are entering a period where fundamental questions about the structure, existence, and characterization of polymers have been resolved and replaced by new questions about microplastics (MPs), sustainability, recycling, and health [1,2]. Today’s research challenges are being addressed by scientists from a wide range of disciplines—biology, analytical chemistry, polymer chemistry, colloidal chemistry, physics, materials science, and medicine. It is often said that all plastics are polymers, but not all polymers are plastics. The term “microplastic” is defined by the ISO 4484-2:2023 standard as a material consisting of a solid polymer containing particles, to which additives or other substances may have been added, and where a weight fraction of ≥1% particles have: (1) all sizes 100 nm ≤ x ≤ 5 mm; and, (2) for fibers, a length of 300 nm ≤ x ≤ 15 mm and a length/diameter ratio > 3 [3]. The size definition of microplastics is not internationally consistent and has been used in different ways. Another standard, ISO/TR 21960:2020, published a few years ago, classifies particles between 1 µm and 1000 µm (=1 mm) as microplastics, those between 1 mm and 5 mm as large microplastics, and those smaller than 1 µm as nanoplastics (NPs) [4]. It is interesting to note that the nanoplastics category was recently defined in response to studies on microplastics, which showed that microplastics smaller than 1 µm behave so differently from larger microplastics (1 µm–5 mm) that they had to be classified as a completely separate category [5,6,7]. While smaller plastic particles have a higher potential to cross the biological membrane than larger plastic particles, in situ and ex situ studies of micro- and nanoplastics (MNPs) have been hampered by the lack of methods for controlled experiments. As a result, the threat posed by MNPs to human health is still largely unknown [8,9,10,11,12,13]. One of the optimal solutions is to use an MNP particle with a core–shell structure. If the core of the MNP contains an inorganic moiety, such as a metal tracer, this can facilitate the detection of the MNP [14]. If the MNP core with an inorganic unit is covered by a polymeric shell, e.g., polystyrene [14], this may help to avoid contact between chemical residues (other than the polymeric organic unit) and the exposure medium of the toxicological experiment. The lack of harmonized protocols is affected by the availability of commercial reference MNP particles and the representativeness of these MNP, which do not reflect the particles found in the environment. Many studies have shown the differences in the characteristics of commercial MNP particles (e.g., size range and size deviation) together with the limited variety of commercial MNP (polystyrene MNP is still dominant) and the limitations of existing non-commercial production methods of individual research groups (such as particle morphology, good size fractionation over a wide size range, and no modification of the plastic surface) [15,16]. The use of MNP particles with incoherent physico-chemical properties and non-harmonized protocols in both environmental and toxicological studies leads to the possible misinterpretation of the data obtained and the conclusions drawn [17].

This review article provides researchers with a scientific basis for the design of synthesized micro- and nanoplastic particles. It includes a list of commercially available MNPs, an overview of the theoretical background to polymer particle synthesis, an analysis of advantages and disadvantages of MNP manufacturing methods, and examples of synthesized MNP particles including those with organic and inorganic units in the particle’s structure. This information is intended to facilitate the transfer of useful knowledge to researchers engaged in the design and development of engineered MNP particles.

2. The Motivation for the Appropriate Design of Synthesized MNPs

There is a clear need for well-characterized and selectively detectable MNP particles for ecotoxicological and human toxicological studies, along with approaches for the sampling and separation of MNP particles from environmental matrices for method development and method validation. Different routes for obtaining MNP particles for experimental studies are shown below.

2.1. The Availability of Reference MNP Particles from Certification Bodies and Other Providers

The application and research areas of plastic-derived particles can be represented as a global chain. The representative chain starts with the production of plastics. Plastics are produced for a multitude of sectors within the economy, including packaging (e.g., plastic bags), construction (e.g., paints and pipes), transport (e.g., car parts), electronics (e.g., mobile phones), textiles (e.g., T-shirts), medicine (e.g., syringes), and household (e.g., sponges). Once plastic products have been manufactured, they become a source of both primary and secondary microplastics and nanoplastics. Primary microplastics and nanoplastics are directly released into the environment from plastic products (e.g., cosmetics), while secondary microplastics and nanoplastics originate from large plastic objects (e.g., fishing nets) [18,19]. Due to their low density, MNPs can be easily transported over long distances in the aquatic environment and remain on the water surface for a long time [20]. Furthermore, due to their small size, MNPs are preferred by aquatic organisms as a food source. The global chain of MNPs is completed when they enter the human body through ingestion via the food chain, inhalation (polluted air), or skin contact (e.g., implants and hygiene products). Thus, over the last 50 years, research has been conducted on the accumulation of plastic particles in water, air, soil, and aquatic organisms [21], animals (shrimps [22]), and human food (milk [23] and sugar [24]) and drinking water [25]. Numerous toxicological studies on animal and human biological samples, e.g., feces [26,27], placenta [28,29], lung [30], and liver [31], have described the uptake of MPs into tissues, blood [32], and breast milk [33]. In addition to the need for MNP accumulation databases, MNP research is attracting analytical chemistry scientists to solve measurement problems. A key challenge here is the small size of the particles (the nanometer range means 10⁻⁹ m) and the small variety of chemical elements in hydrocarbons (mainly just two chemical elements: carbon and hydrogen) [34].

MNP studies rely on commercially available particles as reference materials. There are currently reference materials commercially available from two material certification bodies, a German Federal Institute for Materials Research and Testing (BAM) and American National Institute of Standards and Technology (NIST). A corresponding list of materials is given in

Table 1. Available reference materials from two certification bodies, the German Federal Institute for Materials Research and Testing (BAM) and the American National Institute of Standards and Technology (NIST).

Plastic Type	Index	Certificate Details	Particle Size	Particle Use
PE	BAM-P201	Artificially aged polyethylene (PE). Particle size distribution (by laser diffraction), carbonyl index (by FTIR), and enthalpy (by DSC).	17.9 µm 61.2 µm 156.6 µm	Validation of sampling, sample preparation and detection of microplastics in the field of ecotoxicology or human toxicology, and pollutant transport and agglomeration behavior.
PS	BAM-P202	Polystyrene (PS) powder. Particle size distribution (by laser diffraction), and average value spectrum (by FTIR and DSC).	91 µm 206 µm 311 µm
PET	BAM-P206	Polyethylene terephthalate (PET) powder. Equivalent particle diameter with standard deviation (by laser diffraction), SEM image of particles, and ATR-FTIR spectrum. DSC spectrum. Average values for glass transition and melting temperatures.	30.5 µm 62.6 µm 107 µm
PS	SRM 1691	Number average particle determined by laser diffraction.	0.3 µm	Primary particle size reference standard for the calibration of particle-size-measuring instruments including electron microscopes.
PS	SRM 1961	Number of average particles measured in air by center distance finding, and optical technique related to array sizing.	30 µm	Primary particle size reference standard for the calibration of particle-measuring instruments including flow-through counters, and optical and electron microscopes.
PS	SRM 1963a	Size probability distribution.	100 nm	Calibration/validation of particle-sizing instruments, including electron microscopes, differential mobility analyzers, scanning surface inspection systems, and other light scattering instruments.
PVP	RM 8017	Polyvinylpyrrolidone (PVP)-coated silver nanoparticles. Particle size (by dynamic light scattering, ultra-small-angle X-ray scattering, and transmission electron and atomic-force microscopy (TEM and AFM)). Ultraviolet–visible absorbance spectrum. Values from isotope dilution mass spectrometry, asymmetric-flow field flow fractionation, and zeta potential and pH of suspension.	75 nm	Benchmark and investigative tool for the evaluation of potential environmental, health, and safety risks that may be associated with manufactured nanomaterials during their product lifetime.

(data valid for the period of manuscript preparation). The BAM provides certificates for three reference microplastic powders, polyethylene (PE), polystyrene (PS) and polyethylene terephthalate (PET), in three size categories [35,36,37]. The NIST provides certificates for two microplastic materials, PS and polyvinylpyrrolidone (PVP). The NIST certifies particles of PS in four size categories, 100 nm, 0.3 µm, 1 µm, and 30 µm, and PVP with a particle size of 75 nm [38,39,40,41,42]. The certified documents for the reference particles contain a range of information, dependent on the product. This may include particle size distribution, as determined by laser diffraction or measured in air by the center-distance-finding method, as well as an FTIR spectrum, a differential scanning calorimetry (DSC) spectrum, a SEM image, the glass transition temperature, the melting temperature, an expiry date, information on recommended use, safety, transport, and storage and handling, as well as references to the literature used in the preparation of the certificate. When procuring the reference material, it is of the utmost importance to peruse the information displayed on the packaging label and to gain an understanding of the information presented in the product certificate. This is to ensure that the material is suitable for the intended purpose.

As shown in Table 1, there is an obvious lack of available well-characterized and selectively detectable reference particles, which has long been a limitation to systematic MNP studies. On the one hand, research on microplastics started many years ago, which could logically lead to the need for commercially available reference particles. On the other hand, the necessity for a particular type of reference particle can only be determined by the demands of research studies [43]. It could be argued that the current necessity for engineered MNP particles with well-controlled parameters for environmental and toxicological studies is timely. In the following chapters, a selection of studies on synthesized (with an emphasis on those with organic and inorganic units in the particle’s design) and mechanically fragmented microplastics will be discussed. The objective of this study is to summarize the requirements for specific parameters of MNP, which will be useful for those involved in the design and manufacturing of synthesized particles.

Senfter et al. [15] has shown the differences in characteristics for commercial particles, which can be very valuable for other future studies. They tested three physical parameters (shape, particle density, and particle size distribution) of ten MP powders (high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyamide (PA), PET, polypropylene (PP), PS, polyurethane (PUR), and polyvinyl chloride (PVC)). A comparison of the test results with label claims showed that the particle densities were close to the manufacturer’s claims. However, there were significant gaps in the particle size information, highlighting the need for an accurate description of the particle size distribution, e.g., D50 or D90 [15].

The focus of Biale et al. [44] was to compare the chemical composition of leachates obtained from commercial reference materials and those collected in the environment. They carried out the accelerated photo-oxidative ageing of four reference microplastics (HDPE, LDPE, PP, and PS) directly in artificial seawater. The chemical composition of the leachate was determined and compared with that of plastic debris collected in a natural environment. The differences observed between the reference and the environmental plastic leachates mainly concerned the relative abundances of the chemical species detected, with the environmental samples showing higher amounts of dicarboxylic acids and oxidized species [44].

In the study by Ramsperger et al. [43], two identical PS microplastic particles were found to differentially affect particle–cell interactions and cellular responses. The adverse effects were explained after the characterization of the particles; it was evident that particles from two different manufacturers differed in monomer content, ζ-potentials, and surface charge densities. In another toxicological study [45], in addition to the size and surface charge, the shape of microplastics was proposed as a possible driver of MP toxicity. Thus, the studies pointed out that the polymer type and size class are not sufficient for describing the particles employed for monitoring the effects of microplastics and that more details on the other important properties of microplastics are needed.

The above examples illustrate the reality that reference particles are generally expected to be accurate, consistent, homogeneous, pure, stable, and well-characterized for all possible physico-chemical parameters.

2.2. The Preparation of MNP Particles through a Mechanical Degradation Process (Also Known as “Top-Down”)

It is important to have well-characterized and selectively detectable MNP particles for ecotoxicological and human toxicological studies. This is a foundation for sampling and separating MNP particles from environmental matrices for method development and method validation. The lack of commercially available, well-characterized particles has prompted researchers to develop their own testing materials. Two main approaches are known for the preparation of MNP particles, namely, “top-down” and “bottom-up”. The former includes laser ablation [46,47,48], photodegradation [49,50,51,52], ultra-sonication [53,54], and mechanical fragmentation [55,56,57,58], while the latter involves a chemical synthesis of MNP particles [59]. The following two examples illustrate the “top-down” approach.

The study [58] focused on the production of true-to-life nanoplastics, following the protocol previously published [57]. Particles were produced by the cryogenic mechanical fragmentation of everyday plastic materials, including PET water bottles, polyamide 66 (PA66) fabrics, and PS disposable drinking cups. Briefly, the plastics were manually cut with scissors and fragmented using a mixer mill and a centrifugal mill under cryogenic conditions, then the plastic pieces were pre-treated in liquid nitrogen to make them brittle, and, finally, they were separated from the obtained MP powders using a protocol of sequential centrifugations. Compared to PS samples (mostly spherical), PET spherical particles show a broader size distribution with heights most concentrated below 100 nm and diameters ranging from 10 nm to 1100 nm. The median values of the size distribution are 20.3 nm and 195.3 nm for the height and diameter, respectively. The PA66 samples were characterized by AFM. In this dimensional analysis, spherical nanoparticles were considered, while fiber-like nanoparticles were excluded. The size distribution shows that most of the heights are concentrated below 50 nm, while the diameters range from 10 nm to 250 nm. The authors aimed to develop more realistic testing materials for NPs that have broad environmental relevance and applicability [58]. In their previous work, they produced particles by the mechanical fragmentation of PS-based everyday products and demonstrated the different interaction with biomolecules compared to PS nanobeads [57].

The study by Parker et al. [16] demonstrated a method for the preparation, fractionation, and characterization of PP/talc and PVC micro- and nanoplastic testing particles (particle density was >1 g/cm³ for sedimentation in aqueous solution). The initial size of commercial powders or pellets was reduced to 500 μm using a centrifugal mill under cryogenic conditions. Further ball milling was performed to achieve the final particle sizes. Fractionation into size ranges <1, 1–5, 5–10, 10–30, 90–180, and 180–300 μm was performed by sedimentation and filtration. Characterization showed the desired particle size and <1% contamination. The particles produced were stable in bovine serum albumin solution for nine months [16].

Mechanically fragmented MNP particles are known to be environmentally relevant and are sometimes referred to as “realistic particles” because they can satisfy the following conditions: (a) fragment or fiber particle morphology; (b) good size fractionation over a wide size range; and (c) no modification of the plastic surface (e.g., without surfactants, and metal-dopants) [16,58]. In contrast to fragmented particles produced by mechanical fragmentation, bottom-up approaches produce spherical particles, sometimes with certain markers (necessary for tracking and recovery purposes) and stabilizers (against agglomeration), which is considered less environmentally relevant. However, the advantages of bottom-up synthetic MNPs are the final yield of the produced material in the nanometer range and the possibility to control the physico-chemical parameters. The ability to control the parameters in a synthesized MNP approach is of significant value to other scientists, as it enables a higher degree of reproducibility in measurement results across laboratories. The theoretical background to polymer particle synthesis, along with examples of synthesized MNP particles, are presented below.

3. The Preparation of MNP Particles through a Chemical Synthesis (Also Known as “Bottom-Up”)

MNP particles produced by chemical synthesis are known in the field of microplastics research as a bottom-up approach. The choice of the chemical composition of the particles depends on the polymer of interest, the laboratory equipment suitable for the detection of organic and inorganic substances in the micro- and nano-size range, the chemical residues suitable for toxicological exposure studies, and the skills of the researcher. The theoretical background of polymer particle synthesis, the scientific disciplines involved in MNP research, the polymerization reactions, the physico-chemical properties together with the quality criteria of synthesized MNP particles, and the instruments suitable for the detection of MNP particles are presented in the following section, Section 3.1. Examples of the design of synthesized MNP particles with organic and inorganic units in the particle structure are presented in Section 3.2.

3.1. The Theoretical Background to Polymer Particle Synthesis

The production of MNPs requires a close link and exchange of knowledge between colloidal, polymer, and analytical chemistry. Colloidal chemistry is applicable here because the MNP produced is called a colloid, which is a mixture that has particles between 1 and 1000 nanometers in diameter and yet is able to remain uniformly distributed throughout the liquid media, e.g., water. These are also known as colloidal dispersions because the substances remain dispersed and do not settle to the bottom of the container. Experience in polymer chemistry is essential in the production of MNPs in order to select an appropriate synthesis method depending on the desired polymer type. Knowledge of analytical chemistry will help in the production of MNPs by selecting an inorganic substrate (necessary for new properties and features of the polymer matrix) and characterization methodology (e.g., a Raman marker or metal cation may be required for some measuring instruments). Therefore, these three areas will continue to dominate and be interlinked in the answers to current and future research questions on MNP particle synthesis.

Basically, polymerization can be divided into two types—chain polymerization (e.g., PE, PVC, acrylics, and PS) and step polymerization (e.g., polyesters, PA, PUR, and proteins). Chain polymerization is a process of the repeated addition of monomers with double or triple bonds, initiated by initiators, pressure, heat, or UV light. Initiators can be decomposed by heat or UV light to form free radicals, which then react with the monomer added to the homogeneous solution. In this way, initiators become part of the final polymer product. Step polymerization is a process of the repeated reaction between two different bi- or trifunctional monomers, producing a polymer and usually water [60].

Bulk polymers can be converted into MNP particles in a variety of ways. Macromolecules can be converted into colloidal structures directly during synthesis, for example, by suspension polymerization, a heterogeneous process that uses mechanical agitation to mix one or more monomers in a liquid phase as the monomers polymerize to form spheres [61] (e.g., PS, polymethyl methacrylate (PMMA), polyvinyl acetate, styrene–acrylonitrile copolymer, and acrylonitrile–butadiene-styrene copolymer). Emulsion polymerization is another type of heterogeneous polymerization where the monomer is soluble in the continuous phase, but the resulting polymer is not (examples of three categories: first category—synthetic rubber, styrene-butadiene, and polybutadiene; second category—plastics, some types of PVC, and PS; third category—polymer sold as aqueous dispersions, and polyvinyl alcohol). In addition, macromolecules can be converted into colloidal structures using certain other techniques, e.g., dispersion processes, where the process starts from a homogeneous solution of all the starting materials (monomer, initiator, stabilizer, additive, etc.) [62].

When synthesizing MNP particles, the following aspects must be taken into account: the control of particle size and size distribution (monodisperse and polydisperse particles), choice of well-defined initiator, choice of steric stabilizer (influences particle stability, final particle size, size distribution, and polymer molar mass), control of the reaction rate and yield (solid phase reactions are much slower and yields are generally lower compared to homogeneous reactions), control of contamination resulting from the different synthesis processes (residual precursors or solvents) [63], and control of homo- and heteroaggregation (colloidal stability of NPs, which is determined by size and shape, and this is a determining factor for their bioavailability and toxicity [64,65,66,67,68].

In Kokalj et al. [69], quality criteria for MNP studies are proposed and a review of 47 studies based on the developed checklist is shown. The quality criteria have been developed for primary and secondary nanoplastics, but the author has also considered the possibility of applying them to particles of a larger size. The proposed quality criteria for main nanoplastics characteristics included: the polymer chemical composition, origin of nanoplastics, protocol of nanoplastics production and collection, primary size, shape, specific surface area, surface charge and chemistry, density, porosity and defect density, crystal structure, impurities, chemical additives, other pollutants, and other relevant properties. The proposed checklist for nanoplastics in the exposure medium was extended to include the following criteria: the size distribution, surface charge and chemistry, concentration of nanoplastics, leaching of chemicals, and formation of biocorona and biofouling. After reviewing 47 ecotoxicological studies on MNPs using the proposed quality criteria, the author concluded that none of them were classified as very high quality, 36% of the studies were of high quality, 23% were of medium quality, and 41% were unacceptable. As the publication clearly shows, fully and properly characterized particles are used in toxicological studies and, therefore, special attention must be paid to the design by the manufacturers and the selection of particles by the users [69].

In general, microplastic identification is performed by vibrational spectroscopy (ATR-FTIR (down to a 20 µm particle size) [49,70,71] and Raman spectroscopy (down to a 1 µm particle size)) in combination with optical microscopy (Raman imaging (down to a 200 nm particle size)) [72,73], but this method becomes difficult as the particle size decreases towards the nanoscale [74,75]. In addition to FTIR and Raman, X-ray photoelectron spectroscopy (XPS) is also used to confirm the chemical structure of plastic particles [70,71,76]. Destructive mass spectrometry methods are also used for the identification of plastic particles. These include pyrolysis gas chromatography–mass spectrometry (Py-GC-MS) for complex environmental samples [77,78] and ICP–MS for metal-doped plastic particles [14,79,80]. Unlike spectroscopic methods, mass spectrometry must be used in combination with scanning electron microscopy (SEM) or transmission electron microscopy (TEM) techniques for morphological information. While measurement procedures for the analysis of microplastics are well-established, equivalent methods for nanoplastics are still in their infancy. In addition to a comprehensive and appropriate characterization, the selective identification of particles utilized in experiments can be achieved through the implementation of specific modifications. The addition of inorganic units to polymers makes it possible to modify the physical properties of polymers (improving the toughness and hardness of elastomeric and transparent materials [81]) and make them suitable for detection by, for example, ICP–MS [14], to extend their fields of application (optics, electronics, and surface coatings) and chemical disciplines (immobilization of enzymes, stabilization of colloids, adhesion in composite materials, catalysis, etc. [82]). Some examples of the design of synthesized MNP particles including those with organic and inorganic units in the particle’s structure are presented below.

3.2. Examples of the Design of Synthesized MNP Particles

3.2.1. Polyurethane (PUR)

The synthesis of uniform PUR particles by step growth polymerization in an organic dispersed medium is being developed using functional homopolymers such as ω–hydroxy–polystyrene [83]. The PUR particles had a size range of 0.2–5 µm with a narrow size distribution (span = 0.7). The particle size increased to up to 80% conversion. To attach polyurethane to the inorganic nanoparticle, diisocyanate (one of the main components of polyurethane) can be modified. For example, in the work of Ziegler [84], a coupling reaction between amine (the inorganic nanoparticle is first treated with an amino-functional silane coupling agent) and isocyanate is carried out to form a urea bond. A schematic diagram is shown in Figure 1. PUR is known to be in high demand in the mattress industry and in the medical sector (cardiovascular, dental, breast implants, and wound dressings) [85]. The effectiveness of antibacterial catheters has been investigated by Xiu et al. [86].

3.2.2. Polymethyl Methacrylate (PMMA)

A novel synthetic copolymer based on PS and poly(2–vinylpyridine) (P2VP) MNP is intercalated with gold (Au), platinum (Pt), or palladium (Pd) nanoparticles that can be capped with PMMA shells. The synthesis of the core material (copolymer of PS and P2VP) has been described elsewhere [87]. An emulsion polymerization approach was employed. Prior to gold ion loading (KAuCl₄), polymer cores are coated with the polymer of interest (here PMMA or PS) via a seeded emulsion polymerization approach [88]. If Pd or Pt nanoplastic tracers are desired instead, the above KAuCl₄ solution is replaced by KPdCl₆/HCl and KPtCl₆/HCl equivalents for ion loading. Photoreduction resulted in solid gold particles growing inside the NP core, ensuring that the gold does not leach into the solution. The MNPs were synthesized in the size range of 300–500 nm in diameter, the size distribution was characterized by SEM, DLS, and AFM, and monodisperse metal doping loading was confirmed. The sedimentation behavior study showed that the concentration of Au-doped particles decreased with the increasing salinity of a commercial mixture, simulating a typical ocean sample. The aim of the study is as follows: the development and application of nanoparticle–nanopolymer composite beads to study the behavior of microplastic particles in estuarine sediments and coastal waters. The metallic signatures of the tracers allow quantification by both bulk and single-particle ICP–MS [89].

The grafting kinetics of polymethyl methacrylate on 12 nm silica are described in [82]. The degree of polymerization increased with the increasing reaction time, but leveled off after 16 h, with free polymer still being formed, with a total conversion of around 100%. This work was used as a reference experiment for polymerization and grafting on micro-sized chalk particles in [90].

3.2.3. Polyvinyl Chloride (PVC)

Novel fluorescent and metal-doped (platinum octaethylporphyrin) PVC microplastics in the 100 µm size range were synthesized following a modification of the protocol described in [91]. Polyvinyl chloride was dissolved in tetrahydrofuran and mixed with platinum octaethylporphyrin. The custom experimental set-up was used for the final synthesis of the particles. The volume and number-based particle size distributions of PVC MPs were determined by laser diffraction (LD) and optical microscopy, respectively (62 ± 1 µm, 120 ± 5 µm, and 380 ± 80 µm). The aim of the study is as follows: PVC tends to sink and accumulate in sediments due to its density, making it available to benthic organisms. To facilitate the identification and quantification of microplastics ingested by mussels, a new MP tracer was synthesized for detectability in complex matrices [92].

PE and PVC spherical NP doped with 3 nm-diameter gold nanoparticles were synthesized in the narrow size range of 50–350 nm by an oil-in-water emulsion technique. Ultrasmall gold nanoparticles dispersed in toluene were synthesized by the standard two-phase liquid–liquid Brust method [93]. Colloidal stability was confirmed over three months. The aim of the study is as follows: the synthesis of doped NPs for rapid detection by ICP–MS and qualitative study of their cellular interactions and as models for floating and sedimenting NPs [94].

3.2.4. Polystyrene (PS)

The radical chain polymerization of styrene and its attachment to silica nanoparticles via the grafting of the azo initiator is reported in [95,96,97]. The polymer molecules produced under these conditions had an average molecular weight of 290,000, the radius of such a molecule was calculated to be 21.5 nm, and silica gel had a specific surface area of 285 ± 15 m²∙g⁻¹. The aim of the study is as follows: to investigate the kinetics and mechanism of a radical chain polymerization reaction. The application of azo compounds in polymer chemistry has been studied in the works of [98,99]. A schematic diagram is shown in Figure 2.

We found that 20 nm PS was formed by the surfactant-free emulsion polymerization of 14C–styrene in the presence of sodium dodecyl sulfate (SDS) in an emulsion of water and hexane. PS NPs had a size distribution in the range of 1–100 nm. Two sizes of unlabeled PS (20 and 250 nm) were synthesized with 12C–styrene to provide materials for TEM measurements (safety regulations prevented the use of radioactive PS for TEM). The aim of the study is as follows: to produce radio-labeled NPs that can be tracked in simulated environments to study the behavior of NPs in vivo in bivalve molluscs [100].

We found that 90.1 ± 4.9 nm PS nanoparticles were synthesized by the emulsion polymerization of the styrene monomer on the surface of an inorganic oxide, as reported by Lu et al. [101], with some modifications. One of the modifications was the replacement of the inorganic oxide magnetite Fe₃O₄ by Al₂O₃ nanoparticles with a diameter of 13 nm. In addition, a fluorescent tracer was added to the polymer matrix, which increased the average diameter of the NPs to 94.5 ± 3.9 nm. The aim of the study is as follows: to develop methods to quantify the amount of PS nanoparticles ingested by Daphnia magna (fluorescent monomer is added to NP for quantification by fluorescence light microscopy, and total aluminum is quantified by ICP–MS) [102].

Nanoparticles with a polyacrylonitrile (PAN) core material containing metal Pd and a crosslinked polystyrene shell were synthesized via emulsion polymerization. The average core particle diameter was 100–120 nm; after the shell addition, the diameter was 150–170 nm depending on the batch; the polydispersity index was <0.05; and the final weight percentage of Pd in nanoplastic was 0.49%. The diluted particle solution in deionized water was shaken end-over-end for 2 months and showed a good particle stability. The aim of the study is as follows: the synthesis of nanoplastic particles with a metal tracer, needed for a robust method to detect NPs more easily, accurately, and quantitatively by ICP–MS in complex media [14,79].

The europium chelate Eu–β–diketonate was synthesized for incorporation into the PS particles (PS–Eu) according to a previously reported procedure [103]. The PS particles were synthesized via an emulsion polymerization process [101]. The diameter of the synthesized PS–Eu and PS particles was 244 ± 12 nm and 260 ± 2 nm, respectively, and they were spherical, uniform in size, and highly monodisperse. PS–Eu particles were prepared by a combined swelling–diffusion technique. The aim of the study is as follows: polystyrene particles were doped with the europium chelate, which was used to quantify the uptake of PS–Eu particles by two crop plants grown hydroponically and in sandy soil using ICP–MS [104].

The previous examples refer to less complex designs of particles, comprising two main parts: inorganic and polymeric. A positive assessment of previously described examples of MNP synthesis is shown in

Table 2. Positive assessment of previously described examples of MNP particle synthesis.

MNP Type	MNP Size	Positive Assessment of the Study
PUR MNP	0.2–5 µm	Narrow size distribution [83,84]
PS NP	21.5 nm	High-molecular-weight polymer brushes with high graft density were successfully produced. Kinetic studies of the polymerization process showed that the initiation and propagation of the polymer at low conversion was similar to that of solution polymerization [95,96,97].
PS NP Radio–labeled PS NP	20 nm	One-step surfactant-free polymerization process. Radio-labeling approach provides valuable procedure. Behavior of synthesized polystyrene nanoplastic (PS NP) using a common initiator represents the behavior of PS NP found in the environment [100].
Metal-doped PS NP	150–170 nm	The chemical bond between acrylonitrile and palladium and the core–shell structure ensure that there is minimal leaching of metal from the particle (eight weeks of stability). The shell could be modified independently of the core (i.e., different styrene morphologies and/or different polymers could be added to the shell). The metal-doping tracer in NP has been designed for detection in complex matrices by inductively coupled plasma–mass spectrometry (ICP–MS) [14,79].
Fluorescent-labeled PS NP	94.5 nm	Two methods are proposed for the quantification of NP in organisms: the addition of fluorescent monomer to NP for fluorescence light microscopy and the addition of aluminum as a metal core of NP for ICP–MS. NP was functionalized with palmitic acid to simulate environmental conditions [101,102].
Metal-doped PS MNP Metal-doped PMMA MNP	300–500 nm	Photoreduction resulted in solid gold particles growing within the core of the NP, ensuring that the gold does not leach out into solution. Metal-doping tracers were developed with recognizable isotopic, metallic, fluorescent signatures for detection in complex matrices by ICP–MS: a single-particle ICP–MS. Synthetic approach produces well-defined and monodisperse materials and provides potential for multiplexing. Core–shell structures allow the environmental matrix to interact only with a shell polymer composition (besides PS and polymethyl methacrylate (PMMA), other vinyl-based polymers such as butyl rubber, natural rubber, and vinyl chloride can be used as shells). The ability to adjust the hydrophilicity of the cores by adjusting the pH of the solution may allow the inclusion of more hydrophilic polymers as the shell material [82,87,89].
Metal-doped PS MNP	244 nm	The synthesis is not complicated. In addition to PS, the method can be extended to other types of NPs found in the environment. The labeling technology used in this study could also be applied in microcosm or mesocosm experiments to increase the sensitivity of NP detection. Indirect quantification and visualization of NPs in plant tissues were performed by fluorescence of europium (Eu) element doping. Due to the time-dependent luminescence of Eu chelates, Eu chelate doping also allows background-free fluorescence imaging. PS–Eu particles prepared with Eu chelate were chosen as luminophore because of their high luminescence quantum yield, stability, and solubility in aqueous buffers [101,103,104].
Fluorescent metal-doped PVC MP	100 µm	This provides a benefit to the scientific community as polyvinyl chloride microplastic (PVC MP) is in the minority of particles studied compared to other types of plastics. Synthesis of a new MP tracer for detection in complex matrices (optical method, ICP–MS, and total reflection X-rays fluorescence). An important advantage is that the fluorescence of MP is not limited to a single excitation band (like most MPs used in the literature), but to two (blue and green light). This makes them an ideal material for identification studies of microplastics in natural auto-fluorescent samples, as they are perfectly distinguishable from any other endogenous or xenobiotic particles often present in the matrix (such as algae). Platinum octaethylporphyrin is a tracer suitable for following the bioaccumulation of plastics in organisms. The release of platinum octaethylporphyrin and platinum during analysis can be largely excluded as the dye is insoluble in aqueous media [92].
Metal-doped PVC NP Metal-doped PE NP	50–350 nm	Synthesized environmentally relevant NP. Gold nanoparticles were chosen as the dopant because they are considered to be chemically stable, relatively easy to obtain, interference-free for elemental analysis; and suitable for bio-applications and as a tracer for detection by ICP–MS. Successful metal doping distributed throughout the volume of the NP. Narrow size distribution. The presence of metal doping did not alter the Raman spectra of polyethylene (PE) and PVC due to the small size of the metal-doping particles. Metal-doping content in NP was sufficient to produce a detectable individual by single-cell ICP–MS [93,94].

. In addition, the design of a particle comprising four parts has been known for decades, namely, a surface-enhanced Raman scattering (SERS) particle. The typical four-part design of an SERS particle is as follows: a noble metal nanostructure which serves as the substrate for signal enhancement, a Raman reporter molecule, a coating shell for improving the biocompatibility and stability of the SERS sample, and a bio-recognition element such as an antibody, aptamer, or peptide. For example, gold nanostar particles with a Raman reporter molecule coated with a PS shell and numerous other studies can be found in the review article of Chen et al. [31].

4. Conclusions

While reference and testing MNP particles are available, they are not well-characterized and do not cover all major polymer types that may potentially pollute the environment. The current study has demonstrated that polystyrene is one of the most prevalent MNP particle types among reference materials from certification bodies and among testing particles synthesized by chemical scientists. Although PS represents approximately 5% of the global production, it is not a dominant plastic type in the textile or packaging industries [105]. Nevertheless, polystyrene is frequently cited in studies as one of the significant environmental micro- and nanoplastic pollutants, along with PE and PP [106]. There are numerous methods for manufacturing testing MNP particles with polymer types other than PS, each with its own unique benefits. It is known that mechanically fragmented MNP particles exhibit comparable characteristics to those observed in the environment. However, fragmented particles are only approximations of realistic particles and do not fully reflect the characteristics of particles found in the environment. Nevertheless, they satisfy three key conditions: (a) they exhibit a morphology consistent with that of a fiber or particle; (b) they exhibit good size fractionation over a wide size range; and c) they exhibit no modification of the plastic surface (e.g., without surfactant and metal-dopants) [16,58]. In contrast to mechanically fragmented MNP particles, the yield of synthesized MNP particles is larger in the nanometer range, the physico-chemical parameters can be controlled, and this approach is reproducible and complementary to the top-down approach [107].

It is challenging to reproduce under laboratory conditions all the potential degradation processes that could occur with MNP particles in the environment. However, the implementation of the core–shell synthesis approach in the manufacture of MNPs can facilitate the control of their physico-chemical properties, enabling the more selective detection and quantification (due to the presence of inorganic markers inside the polymeric shell) and a greater comparability of results across toxicological studies.