The Environmental Behavior,Toxic Effect And The Related Mechanism Of Cadmium-based Quantum Dots

Due to their excellent photophysical properties,cadmium-based quantum dots（Cd-QDs）are not only increasingly used in consumer products such as light-emitting diodes,solar cells and inkjet printing,but also have great potential in biomedical fields such as sensor detection,drug targeting and in vivo imaging.Therefore,Cd-QDs will inevitably enter the environment and cause potential harm to the environment and human health.At present,it has attracted wide attention for the research on the environmental behavior and toxic effects of Cd-QDs.However,it is not clear for the effects of complex water environment on the environmental behavior of Cd-QDs;it is also required for further discussing the formation of Cd-QDs protein corona and its toxic effects;it is insufficient for studying the effects of doping on the environmental behavior and toxic effects of Cd-QDs.Based on these backgrounds,cadmium telluride quantum dots（CdTeQDs）and zinc doped cadmium telluride quantum dots（Zn-QDs）with different doping ratios were synthesized and characterized in this study.Using these two kinds of Cd-QDs as research objects,the impact of different water composition,divalent metal cations(Ca²⁺and Mg²⁺)and typical natural organic matter（NOM）on the environmental behaviors,including agglomeration,dissolution and sedimentation,and the underlying mechanisms of Cd-QDs were investigated under the condition of simulating natural water.This study also investigated the formation of Cd-QDs protein corona and the related molecular and cellular toxicity as well as their underlying mechanisms.And the effects of Zn doping on the environmental behavior and toxicity of CdTeQDs were compared and analyzed.The specific research contents and conclusions are as follows:（1）CdTeQDs coated with N-acetyl-L-cysteine and Zn-QDs with doping ratio of 1%,5%and 10%were synthesized by one-pot method.The morphology,elemental composition and optical properties of Cd-QDs were characterized by high resolution transmission electron microscopy,X-ray diffraction,inductively coupled plasma-mass spectrometry（ICP-MS）,UV-vis absorption spectra and fluorescence spectra.The results showed that the particle size of the prepared Cd-QDs were about 3.4 nm.The synthesized spherical Cd-QDs were of high purity and good crystallization.The original crystal structure of CdTeQDs did not change with the doping of Zn.The total Cd content of 5%Zn-QDs was the highest and1%Zn-QDs was the lowest.Zn doping effectively increased the fluorescence quantum yield（FLQY）of CdTeQDs（41%）.And the FLQY value of 10%Zn-QDs increased to 59%.These contents could help to provide theoretical data for innovating the design route and synthesis method of metal-doped Cd-QDs.（2）The environmental behaviors of CdTeQDs and 10%Zn-QDs were studied by conducting the hydration diameter andζpotential,absorbance and ICP-MS measurement.Using humic acid（HA）,fulvic acid（FA）and bovine serum albumin（BSA）as research objects,the binding mechanism of NOM and Cd-QDs was analyzed by adsorption capacity measurement,isothermal calorimetric titration（ITC）and multi-spectroscopy.The results showed that the ion composition and divalent cations could affect the agglomeration,dissolution and sedimentation behavior of the two kinds of Cd-QDs.The two kinds of Cd-QDs showed the same trend on these behaviors when impacted by the above factors.The degree of agglomeration,dissolution and sedimentation of Cd-QDs in seawater were stronger than that in freshwater.Ca²⁺and Mg²⁺enhanced the agglomeration,dissolution and sedimentation of Cd-QDs in freshwater,but inhibited the dissolution of Cd-QDs in seawater.The sedimentation degree of Cd-QDs was positively correlated with the ionic strength of water.Except for seawater(Ca²⁺and Mg²⁺)-HA systems,the addition of NOM inhibited the agglomeration of Cd-QDs.Except for freshwater-HA and freshwater-BSA systems,the addition of NOM promoted the dissolution of Cd-QDs.The treatment of HA and FA aggravated the sedimentation of the two kinds of Cd-QDs in all aqueous phase systems,while BSA delayed the process.In artificial water without bivalent cations and NOM,Zn doping improved the colloid,structural and suspension stability of CdTeQDs.In freshwater(Ca²⁺and Mg²⁺)systems,both Cd-QDs and NOM bound spontaneously through hydrophobic interaction.And 10%Zn-QDs adsorbed higher amount of NOM.The influence of nano-effect of Cd-QDs on HA was stronger than that of free Cd²⁺.Cd-QDs mainly bound to the aromatic ring skeleton of HA and quenches its fluorescence.These results were helpful to correctly evaluate the environmental safety of Cd-QDs,and provide scientific methods and new ideas for the study of the behavior and mechanism of Cd-QDs in water.（3）Using the typical plasma protein BSA as the research object,the effect of protein corona formation on the physicochemical properties of Cd-QDs was studied by ICP-MS,hydration diameter andζpotential measurement.The formation mechanism of Cd-QDs protein corona and its toxic effect on protein molecules and cells were investigated by ITC,esterase-like activity determination,multi-spectroscopy,CCK-8,reactive oxygen species（ROS）and Cd²⁺fluorescence probe.The binding between BSA and CdTeQDs was an endothermic process dominated by hydrophobic interaction.The number of binding sites（N）and the binding constant（K）was 1.79 and 4.07×10~4 M^-11,respectively.The interaction of BSA with 10%Zn-QDs was exothermic with van der Waals force and hydrogen bond as the main binding force.The number of binding sites（N）and the binding constant（K）was3.08 and 3.51×10~4 M^-1,respectively.The formation of Cd-QDs protein corona caused the loosening of BSA skeleton,the decrease of fluorescence intensity,the increase ofα-helix content and other secondary structure content changes as well as the inhibition of esterase-like activity.The influence of 10%Zn-QDs on the structure and function of BSA was less than that of CdTeQDs.The regulatory effect of BSA on the cytotoxicity of Cd-QDs was similar to that of fetal bovine serum,and BSA had a significant effect on the cytotoxicity of Cd-QDs.Compared with CdTeQDs,cells exposed to 10%Zn-QDs showed lower cell viability and higher ROS level.The cell viability and oxidative stress level of mouse kidney cells exposed to Cd-QDs were not proportional to the total intracellular Cd content,but positively correlated with the intracellular Cd²⁺content.These results could help to clarify the formation mechanism of QDs protein corona and its toxic effects at the molecular and cellular level.And this work was helpful to the establish a new research system to jointly evaluating the biotoxicity of NPs from the molecular and cellular level.