Criteria for visual inspection are the following: there should be no cellular elements among the identified particles; the particles are not shiny, do not break when stretched; the fibers are not twisted and of the same thickness; the particles have a uniform color. Clear or white particles should be examined by high-resolution microscopy or fluorescence microscopy to rule out an organic origin [177], [198]. To separate microplastics from other materials, the so-called hot needle test can facilitate selection [203]. Particles smaller than 500 µm can only be identified with a microscope. Light microscopes are most used, but stereo microscopes and higher resolution fluorescence and scanning electron microscopes are also mentioned in some studies [177]. The latter can also be used to study surface structure and can be used to obtain a particularly high-resolution image of microplastics, but its use has not been widespread because of the expensive instrument and the time required for sample preparation and coating [195]. In fluorescence microscopy, the sample is often labeled with the dye Nile red. The fluorescent dye Nile red binds selectively to hydrophobic particles. The labeled particles fluoresce when illuminated with blue light and can be examined with a fluorescence microscope by measuring the emission. The method is fast, and the number of particles can be determined with relative accuracy. On the other hand, the disadvantage is that not only microplastics, but also other organic hydrophobic materials are stained, and some types of polymers (PVC, PUR, PET) and degraded particles are less colored, which must be considered when determining the number of particles. The dye Nile red is solvatochromatic, its color changes when the polarity of the solvent changes, so polar and hydrophobic microparticles can be separated [205], [206]. Another staining method uses Rose Bengal dye to separate microplastics from other materials and facilitate visual identification. Chitin, mineral and synthetic materials remain colorless, while organic material is stained with it [207].

Large-scale examination of nanoplastics: Described methods for characterizing particle size and morphology: laser diffraction (LD), light scattering, most commonly dynamic light scattering and nanoparticle tracking analysis; imaging techniques, including scanning and transmission electron microscopy; scanning probe techniques, such as atomic force microscopy and scanning tunneling microscopy [178]. Techniques for size characterization and imaging of plastic sub- and nanoparticles (information, size intervals, advantages, disadvantages) are analyzed. Laser diffraction is a static light scattering technique that can be used in a very wide particle size range; from nanometers to millimeters. The incident laser beam passes through the dispersed particle sample and the angular dependence of the intensity of the scattered light is measured. The scattering body is asymmetrical, large particles scatter the light more intensely at a small angle to the laser beam, and small particles do so at large angles. The angle-dependent intensity data are evaluated using Mie theory. The particle size is given as volume equivalent sphere diameter.

Dynamic light scattering (DLS) is a commonly used method to determine particle size and size distribution in the 1-3000 nm range [208]. Dynamic light scattering measurement, also known as photon correlation spectroscopy (PCS), detects and analyzes the time dependence of the fluctuation of the scattered light intensity. The intensity fluctuation is caused by the motion of the dynamic units (scattering centers) of the sample (e.g. translational diffusion). The digital correlator calculates the correlation function from the signal. By mathematical analysis of the correlation function, the particle size and size distribution can be calculated. In the simplest case, if the intensity fluctuation is caused by translational diffusion of monodisperse particles, the correlation function can be fitted with a simple exponential function, from the exponent of which the diffusion coefficient can be calculated. From this, the hydrodynamic radius can be determined based on the Stokes-Einstein equation. When studying non-monodisperse systems, the correlation function can be fitted with the sum of exponential terms containing the diffusion coefficients of particles of different sizes, from which the diffusion coefficients and size distribution curves can be calculated. For polydisperse samples, cumulant analysis is most used to determine the Z-average size of the sample particles and the polydispersity index (PDI - a number between 0 and 1 that denotes the width of the distribution).

In nanoparticle tracking analysis (NTA), the suspended particles in the sample chamber are illuminated with a specially shaped laser beam. The laser light scattered by the particles is observed with a digital camera through the lens of a light microscope with 20x magnification, and the Brownian motion of the particles is recorded on a video. The NTA software analyzes multiple particles individually and simultaneously, then calculates their hydrodynamic size using the Stokes-Einstein equation. The method can be used to measure the particle size distribution of colloidal materials in the size range of 10-1000 nm, such as nanoplastics [209].

Among imaging techniques, scanning electron microscopy (SEM) is suitable for determining the size and shape of nanoparticles, but it requires expensive and complicated sample preparation, which makes it an unattractive method. In addition, this technique cannot confirm the presence of plastic, unless the option of energy dispersive X-ray spectroscopy (EDS or EDX - energy dispersive X-ray spectroscopy) is also available, which can image the elemental composition of the sample at micrometer resolution or even higher. Transmission electron microscopy (TEM) is not effective for visualizing nanoplastics; contrast enhancement, heavy metal surface treatment (staining, painting) is necessary. Among the scanning probe methods, atomic force microscopy is sometimes used to determine the size, morphology, and state of aggregation [178].