Toxicity Of Endogenous CdS Quantum Dots To Synthetic Cells During The Biosynthesis Process

In the recent years,as a kind of promising nanomaterial,semiconductor quantum dots(QDs)have gradually and widely used in the fields of clinical medicine,biology and pharmacology due to their tiny particle size and unique optical and electrical properties.Compared with other semiconductor QDs,CdS QDs are a typical II-VI elemental semiconductor,which can be widely used in various fields such as biomarkers,medical imaging,and photocatalysis.The chemical preparation methods are relatively mature,but due to the poor biocompatibility,its application has certain limitations in the biological field.The biological process of synthesizing quantum dots has a simple synthesis process,which can be performed under normal conditions.The entire synthesis process can be controlled by microorganisms,so it has broad development prospects.In our study,we used E.coli CD-2 as the target stain to biosynthesize CdS QDs inside its cells.Cadmium chloride and L-cysteineusing were used as precursor salts.The composition,structure and morphology of the CdS QDs were investigated by TEM?EDS?XRD?fluorescence spectrum analysis.The results showed that the biosynthesis CdS QDs had a high dispersion in cells with a diameter of about 4 nm;EDS analysis showed that the ratio of Cd/S in the cells was 3:2 which indicated that not all the Cd2+ions in the cells had been converted to CdS QDs;XRD analysis confirmed that the crystal type of the CdS quantum dots were cubic crystal form.The fluorescent spectra of the biosynthesized QDs were obtained by exciting at 365 nm.It could be observed that the Cd-treated E.coli cells displayed a strong fluorescence emission peak at 476 nm.The contents of extracelluar Cd2+,intracelluar Cd2+and intracelluar CdS QDs were determined during the whole CdS biosynthesis process by using atomic absorption spectroscopy analysis.The antimicrobial effects of Cd2+ions and CdS QDs,the activities of superoxide dismutase(SOD)and catalase were evaluated during the QDs biosynthesis process.Moreover,the degree of protein oxidation was observed in Western-Blot immunological experiments.The results showed that the final cadmium conversion rate was 82.80%,which was consistent with the energy spectrum analysis.The intracellular CdS QDs did not have fatal cytotoxicity to bacterial cells.The SOD and catalase activities increased during the CdS biosynthesis process to eliminate ROS inside the cells.However,the biosynthesis cells still faced severe oxidative stress induced both by Cd2+ and CdS QDs.Quantitative real-time PCR(qRT-PCR)analysis was used to detect the changes in gene expression during the CdS QDs biosynthesis process.The results showed that compared to control cells,all the ROS-related genes were up-regulated during the CdS biosynthesis process in the bacterial cells.Thus,the biosynthesis cells had effective ways to eliminate ROS to ensure the cells to maintain the basic metabolic vitality for CdS QDs biosynthesis.The four genes which play important roles in protein biosynthesis,bacterial cell division,DNA repairation,protein misfolding reversing,respectively were up-regulated during the QDs biosynthesis process.It indicated that CdS might have severe cytotoxicity to the biosynthesis cells.Thus,the cells had to over-express some related genes to response to the QDs induced stress.