Green-Synthesis-Derived CdS Quantum Dots Using Tea Leaf Extract: Antimicrobial, Bioimaging, and Therapeutic Applications in Lung Cancer Cells

Low-dimensional (<10 nm) semiconductor quantum dots (QDs) have received great attention for potential use in biomedical applications (diagnosis and therapy) for which larger nanoparticles (>10 nm) are not suitable. Here, we demonstrate a green, biogenic synthesis route for making CdS QDs with 2-5 nm particle size using tea leaf extract (Camellia sinensis) as a toxic-free particle stabilizing agent. We explored the biological activity of these CdS QDs in different applications, namely, (a) antibacterial activity, (b) bioimaging, and (c) apoptosis of lung cancer cells. The antibacterial activity of the CdS QDs was studied against different types of bacteria growth, showing that CdS QDs effectively inhibit the bacterial growth and exhibit cytotoxicity toward A549 cancer cells when compared to a control (no QD treatment). We compared this cytotoxicity effect on A549 cancer cells with a standard drug, cisplatin, showing comparable results. Additionally, these CdS QDs produce high-contrast fluorescence images of A549 cancer cells indicating a strong interaction with the cancer cell. To further understand the role of CdS QDs in bioimaging and cytotoxicity effect in A549 cells, fluorescence emission and flow cytometry analyses were performed. The fluorescence emission of CdS QDs was recorded with λ_exc = 410 nm, showing concentration-dependent fluorescence emission centered at 670 nm. From the flow cytometry analysis, it is confirmed that the CdS QDs are arresting the A549 cell growth at the S phase of cell cycle, inhibiting further growth of lung cancer cell. The multifunctional advantages of C. sinensis extract-mediated green CdS QDs will be of widespread interest in implementing in vivo-based bioimaging and therapeutic cancer treatment applications.