The increased environmental presence of micro-/nanoplastics (MNPLs) and the potential health risks associated with their exposure classify them as environmental pollutants with special environmental and health concerns. Consequently, there is an urgent need to investigate the potential risks associated with secondary MNPLs. In this context, using “true-to-life” MNPLs, resulting from the laboratory degradation of plastic goods, may be a sound approach. These non-commercial secondary MNPLs must be labeled to track their presence/journeys inside cells or organisms. Because the cell internalization of MNPLs is commonly analyzed using fluorescence techniques, the use of fluorescent dyes may be a sound method to label them. Five different compounds comprising two chemical dyes (Nile Red and Rhodamine-B), one optical brightener (Opticol), and two industrial dyes (Amarillo Luminoso and iDye PolyPink) were tested to determine their potential for such applications. Using commercial standards of polystyrene nanoplastics (PSNPLs) with an average size of 170nm, different characteristics of the selected dyes such as the absence of impact on cell viability, specificity for plastic staining, no leaching, and lack of interference with other fluorochromes were analyzed. Based on the overall data obtained in the wide battery of assays performed, iDye PolyPink exhibited the most advantages, with respect to the other compounds, and was selected to effectively label “true-to-life” MNPLs. These advantages were confirmed using a proposed protocol, and labeling titanium-doped PETNPLs (obtained from the degradation of milk PET plastic bottles), as an example of “true-to-life” secondary NPLs. These results confirmed the usefulness of iDye PolyPink for labeling MNPLs and detecting cell internalization. Environmental ImplicationsEnvironmental micro-/nanoplastics (MNPLs) are emerging pollutants of special concern, and determining their potential risks is essential. Although a large dataset has been obtained using pristine nano-polystyrene, it is not considered representative of the secondary MNPLs present in the environment. Consequently, obtaining true-to-life MNPLs resulting from the laboratory degradation of plastic goods seems to be an appropriate alternative to fill this gap. Notably, the use of such noncommercial MNPLs requires labeling to confirm their cell/tissue internalization. Although different dyes have been proposed for this purpose, there is a lack of comparative studies showing the advantages and disadvantages of each dye. In this study, after using a wide variety of assays to demonstrate their advantages and disadvantages, iDye PolyPink was proposed as the most suitable dye for labeling secondary MNPLs in studies aimed at identifying their internalization. Such labeling can be used in both in vivo/in vitro models to determine potential environmental and health hazards.