Comparative toxicity assessment of novel Si quantum dots and their traditional Cd-based counterparts using bacteria models Shewanella oneidensis and Bacillus subtilis

Quantum dots are crystalline semiconductor nanoparticles with unique optical properties due to quantum confinement effects. They have several advantages compared to traditional organic fluorescent dyes, such as high fluorescent brightness, photostability, and tunable emission wavelengths, dependent upon particle size. Their unique optical properties have led to an increased use in a variety of devices, including diode lasers and television displays, as well as in biomedical research. The most commonly used quantum dots (QDs) are made of cadmium selenide (CdSe) and have cadmium selenide core with zinc sulfide shell (CdSe/ZnS), containing inherently toxic cadmium. This work focuses on comparison of the toxic effects of conventional CdSe and CdSe/ZnS QDs and silicon QDs, which are emerging as a potentially benign alternative, using bacteria as model organisms. The bacteria models used for our studies are Shewanella oneidensis MR-1, a Gram-negative bacterium, and Bacillus subtilis SB 491, a Gram-positive bacterium. This research assesses changes in cell viability, respiration pattern, and cell membrane integrity in the presence of the nanoparticles using colony counting, respirometry and membrane integrity assays, respectively. The association of the QDs with bacterial cell membranes was investigated using transmission electron microscopy (TEM). Results indicate that the silicon QDs are benign to the bacteria considered, and they do not associate with the cell membranes. The CdSe cores exhibit significant toxicity to the bacterial cells, whereas the CdSe/ZnS QDs are comparatively less toxic.