Fluorescent Labeling Strategies for Tracking Micro- and Nanoplastics in Biological Systems.

Micro- and nanoplastic (MNP) contamination is a global health issue, with growing concerns regarding human exposure and potential health impacts. Understanding their biodistribution, cellular uptake, and toxicological effects in biological systems requires sensitive analytical tools capable of detecting and localizing particles across multiple scales and matrix compositions. Techniques such as Fourier-transform infrared spectroscopy, Raman spectroscopy, and mass spectrometry provide valuable chemical information but face limitations in nanoscale detection, sensitivity, tissue penetration, and spatial localization within biological matrices. Fluorescent labeling techniques have emerged as a powerful complementary approach, offering high sensitivity, real-time imaging capability, and broad compatibility with in vitro and in vivo platforms. This review summarizes the principal fluorescent labeling strategies used for MNPs, including adsorption-based staining, swelling-diffusion methods, covalent conjugation, and polymerization-based incorporation of fluorescent probes. We also examine the imaging modalities used to visualize and quantify fluorescent MNPs in biological contexts, including fluorescence microscopy, confocal microscopy, flow cytometry, and whole-body optical imaging. Applications in cellular uptake studies, biodistribution in animal models, transport across biological barriers, and cumulative accumulation measurements are highlighted. Persistent challenges such as dye leaching, biological autofluorescence, photobleaching, and polymer-dependent labeling efficiency are addressed, alongside emerging opportunities in near-infrared fluorescence imaging and multimodal detection strategies. Continued development of fluorescent labeling approaches will enhance our ability to track MNPs across biological systems and inform our understanding of their toxicological consequences.