The Allium cepa assay in nanotoxicology: A scoping review of its biological and toxicological applications

In recent years, the development of nanostructured systems, particularly in medicine and cosmetics, has expanded considerably. As the utilization of these technologies expands, there has been a growing discourse about their potential implications for human health and environmental safety. To address these issues while adhering to the 3Rs principle, alternative toxicological models have been explored in the early stages of nanocarrier development. Among these, Allium cepa has been extensively used to assess cytotoxic, genotoxic, and ecotoxicological effects. This scoping review mapped and analyzed 35 studies retrieved from PubMed, Scopus, and Web of Science databases, following the Joanna Briggs Institute's methodological guidelines and reported according to PRISMA-ScR. Most studies were published in 2024 (6/35) and 2023 (6/35), mainly from India (12/35) and Brazil (7/35). Inorganic nanocarriers, predominantly metal and metal oxide nanoparticles, accounted for 26/35 studies, while organic carriers, including lipidic, polymeric, and biomolecule-based systems, represented 9/35. Approximately half of the studies used a positive control group (17/35), and the standard exposure duration was generally 24 h (17/35). The Allium cepa assay demonstrates high sensitivity and reproducibility in detecting mitotic disruptions, chromosomal anomalies, and indicators of oxidative stress. Overall, inorganic nanocarriers induced more frequent cytogenotoxic and oxidative effects, while organic systems showed milder responses, often linked to higher biocompatibility and stability limitations. The findings reinforce Allium cepa as a reliable, ethical, and low-cost bioindicator for early nanotoxicological screening. However, significant methodological variability across studies underscores the need to standardize protocols and incorporate molecular endpoints to enhance the predictive and translational capabilities of this alternative nanotoxicology model.