Understanding the Toxicity of Carbon Dots: The Role of Synthesis Variability, Surface Chemistry, and Biological Context.

Since their initial discovery in 2003, carbon quantum dots (CDs) have attracted significant attention due to their unique optical properties and potential biomedical applications. This review critically examines the past 20 years of research on CDs, with a particular focus on cytotoxicity studies from the last decade. CDs, typically less than 10 nm in size, have been synthesized from various organic and inorganic precursors using multiple methods, including hydrothermal, microwave, and chemical reduction techniques. Their properties can be finely tuned by modifying synthesis parameters and incorporating dopants. The preliminary studies on the biological effects of CDs were published in 2013, highlighting their antibacterial properties and low toxicity in certain contexts. Subsequent research has explored their bioactivity, including their application in drug delivery, bioimaging, and photothermal therapy. However, the cytotoxicity of CDs remains a critical area of investigation. Further studies have demonstrated that surface functional groups, charge, concentration, and size significantly influence their interaction with biological systems. For instance, CDs with positive surface charges exhibit higher cellular uptake and greater cytotoxicity compared to their negatively charged counterparts. In vivo studies utilizing animal models such as zebrafish, mice, and planarians have provided valuable insights into the potential toxicological impacts of CDs. The results indicate that while CDs generally exhibit low toxicity at certain concentrations, high doses can lead to adverse effects, including oxidative stress, organ damage, and disrupted cellular functions. Notably, the route of administration (oral, intravenous, or intraperitoneal) also affects the observed toxicity profiles. The goal of this review is to integrate the results of various studies to provide a balanced perspective on the potential risks and benefits of CDs, guiding future research and applications in nanomedicine. This review underscores the necessity for standardized and comprehensive toxicological evaluations of CDs to fully understand their safety and efficacy for biomedical applications.