## 4 Methods for Detecting of Microplastic Particles
The method requires the prior construction of a reference library by analyzing known polymer types [22]. By design, thermo-analytical methods do not allow for the determination of particle number, size, or shape [101]. However, they enable the assessment of the total mass of the polymer and the identification of its type. A novel approach for detecting nanoplastics may lie in the application of biosensors, including those focusing on measuring electrochemical properties [103]. Commonly, this type of sensor consists of three main components: a receptor, a transducer, and a reading device. The receptor can be a peptide, a protein (including enzymes), an antibody, or even a whole cell. It interacts specifically with the analyte, in this case a microplastic particle. With the help of the transducer, these interactions generate electrochemical or optical responses. An example of a sensor from this group is a system with functionalized gold nanoparticles designed to detect PET MPs [104]. In this approach, gold nanoparticles were intended to be linked with a synthetic peptide exhibiting high affinity for PET monomers and PET polymeric ligands. The authors proposed a concept for developing a tool for the instantaneous colorimetric evaluation of microplastic presence. However, despite its promising potential, this concept requires practical validation, as the study was based solely on computational in silico simulations. The microplastic detection methods described in this section are summarized in Table 1.  
The degradation of plastic can also be assessed by examining the migration of low molecular weight substances into water. These undesirable compounds include, among others, antimony trioxide, a catalyst used in PET production, as well as formaldehyde and acetaldehyde, which result from the thermo-mechanical and thermo-oxidative degradation of PET [19]. Instrumental analytical techniques, such as liquid chromatography coupled with mass spectrometry, can be employed to detect these substances [105]. The significance of this issue is recognized, particularly in the case of recycled PET [39]. Summarizing this brief overview of MP detection methods, we would like to draw attention to an important issue. Unexpectedly, data from [31] suggest that polyethylene MPs may, in some cases, be more abundant than PET in these products. However, other reports present different findings [30; 32]. This highlights the importance of keeping an open mind, as research findings may not always align with initial expectations. The methods discussed here focused primarily on analytical techniques for direct detection of microplastics and other degradation products. However, determining the MP content is a broader challenge that encompasses several critical aspects. These include, among others: separation [106; 107], concentration, and collection of MPs, matrix removal (for example, H\({}_{2}\)O\({}_{2}\) digestion [31], Fenton reaction, or enzymatic treatments [101]) and mitigation of cross-contamination. Filters used for MP collection can themselves introduce microparticles. To ensure the reliability of results, performing appropriate negative and positive controls is absolutely necessary. Ultimately, addressing the issue of microplastic content determination requires a comprehensive approach that covers all stages of the process, rather than focusing solely on the detection step itself.