This article explores the cytotoxicity of semiconductor quantum dots (QDs), specifically cadmium selenide (CdSe)-core QDs, and their potential for use in biological labeling. The study highlights that while CdSe QDs are promising alternatives to organic dyes due to their bright fluorescence and photostability, their cytotoxicity depends on processing conditions, surface coatings, and exposure to ultraviolet (UV) light. The research shows that under certain conditions, such as exposure to air or UV light, CdSe QDs can become toxic due to the release of free cadmium ions from the QD surface. This toxicity is linked to the oxidation of the QD surface, which leads to the degradation of the CdSe lattice and the release of Cd²⁺ ions, which can bind to thiol groups in mitochondria, causing oxidative stress and cell death. The study also demonstrates that appropriate surface coatings, such as ZnS and bovine serum albumin (BSA), can significantly reduce the cytotoxicity of CdSe QDs by preventing surface oxidation. These coatings act as a physical barrier to oxygen, minimizing the release of free cadmium ions and thus reducing toxicity. The findings suggest that surface modification is crucial for the safe use of QDs in biological applications, both in vitro and in vivo. While QDs can be used for long-term live cell labeling in vitro without adverse effects on cell viability, migration, or function, their use in vivo requires careful consideration due to the potential for Cd release over time, which could lead to chronic toxicity. The study provides a framework for evaluating the biocompatibility of QDs and highlights the importance of designing QDs with stable surface coatings to minimize heavy metal toxicity in biological systems. Overall, the research underscores the need for careful synthesis, processing, and surface coating strategies to ensure the safe and effective use of QDs in biological applications.This article explores the cytotoxicity of semiconductor quantum dots (QDs), specifically cadmium selenide (CdSe)-core QDs, and their potential for use in biological labeling. The study highlights that while CdSe QDs are promising alternatives to organic dyes due to their bright fluorescence and photostability, their cytotoxicity depends on processing conditions, surface coatings, and exposure to ultraviolet (UV) light. The research shows that under certain conditions, such as exposure to air or UV light, CdSe QDs can become toxic due to the release of free cadmium ions from the QD surface. This toxicity is linked to the oxidation of the QD surface, which leads to the degradation of the CdSe lattice and the release of Cd²⁺ ions, which can bind to thiol groups in mitochondria, causing oxidative stress and cell death. The study also demonstrates that appropriate surface coatings, such as ZnS and bovine serum albumin (BSA), can significantly reduce the cytotoxicity of CdSe QDs by preventing surface oxidation. These coatings act as a physical barrier to oxygen, minimizing the release of free cadmium ions and thus reducing toxicity. The findings suggest that surface modification is crucial for the safe use of QDs in biological applications, both in vitro and in vivo. While QDs can be used for long-term live cell labeling in vitro without adverse effects on cell viability, migration, or function, their use in vivo requires careful consideration due to the potential for Cd release over time, which could lead to chronic toxicity. The study provides a framework for evaluating the biocompatibility of QDs and highlights the importance of designing QDs with stable surface coatings to minimize heavy metal toxicity in biological systems. Overall, the research underscores the need for careful synthesis, processing, and surface coating strategies to ensure the safe and effective use of QDs in biological applications.