The Mechanism Of The Interfacial Interaction Between ZnO Nanoparticles And Algal Cells Disturbed By Co-existing Pollutants

The rapid development of nanotechnology has increased the ecological risk of engineered nanomaterials.The interaction between the surface of nanoparticles（NPs）and the surface substances of cells（such as proteins,polysaccharides,phospholipids,etc.）forms the NPs-cell interface,which depends on the colloid force（long-range DLVO,short-range steric and solvation forces,etc.）,biological,physical,and chemical interactions.These interactions ultimately determine the environmental fate and biological effects of NPs.However,the natural water environment is a complex system containing various co-existing pollutants.To evaluate the ecological risk of NPs scientifically and truly,it is necessary to clarify the effects and mechanisms of co-existing pollutants on NPs-cell interface interaction.In this paper,zinc oxide nanoparticles（ZnO NPs）were selected as an example of NPs,and Chlorella vulgaris,a common primary producer in the aquatic environment,was used as the test organism.The effects and mechanisms of two typical organic pollutants with different charges and groups,molecular（surfactant）and granular（polystyrene microplastics,PS MPs）pollutants,on the interaction between NPs and algal cells were investigated.Firstly,the effect of molecular surfactants on the dissolution of ZnO NPs was investigated,and the dissolution equilibrium of ZnO NPs in the presence of granular PS MPs was investigated employing the Noyes-Whitney dissolution equation and Langmuir adsorption equation.Meanwhile,the adhesion of nanoparticles on the cell surface was observed.Then,the effects of dissolved Zn²⁺and ZnO NPs-algal interface interaction on NPs cytotoxicity and bioaccumulation were determined.Secondly,Two in-situ methods of microscale thermophoresis（MST）and ZnO NPs probe force spectroscopy based on atomic force microscopy（AFM）were extended to the direct determination of the interaction between NPs and algal cells.Combined with the extended Derjaguin-Landau-Verway-Overbeek（XDLVO）theory,the effects and mechanisms of differently charged surfactants(positively charged cetytrimethyl ammonium chloride（CTAC）,negatively charged sodium dodecyl benzene sulfonate（SDBS）,and uncharged 2-（2-[4-（1,1,3,3-Tetramethylbutyl）phenoxy]ethoxy（TX-100））on NPs-cell interface interaction were further investigated.Thirdly,the homoaggregates kinetics of ZnO NPs and PS MPs with different groups（negatively charged unmodified PS,negatively charged carboxyl modified PC,negatively charged amino-modified PN–and positively charged amino-modified PN+）were studied and the critical coagulation concentration was calculated.On this basis,the heteroaggregates and co-deposition between ZnO NPs and PS MPs,or between cells and micro-nano particles under the influence of aqueous solution chemistry were studied.The mechanism of MPs-NPs-cells interfacial interaction was analyzed by the XDLVO model.The main results are as follows:（1）The negative co-existing pollutants（anionic SDBS,negatively charged PS,PC,and PN–）had low cytotoxicity to algae.And their combined toxicity with ZnO NPs depended on the bioaccumulation of Zn,which mainly relied on the interface interaction between granular ZnO NPs and algal cells.Because positive co-existing pollutants（CTAC and PN+）had high cytotoxicity to algae,their combined toxicity with ZnO NPs was mainly determined by the bioaccumulation of Znand interface interaction between co-existing pollutants and algal cells.（2）About effects of surfactants on the interaction between ZnO NPs and algal cells,cationic CTAC（K_d=136.6±23.5μM,adhesion force was 1.9 n N）and anionic SDBS（K_d=69.7±9.2μM,adhesion force was 2.2 n N）increased while TX-100（K_d=935.8±124.1μM,adhesion force was 1.3 n N）decreased the adhesion of ZnO NPs to algae cells（Kd=597.8±49.7μM,adhesion force was 1.6 n N）.However,the force signature exhibited a smooth single retracted peak at short distances in the SDBS-and TX-100-treated groups,distinguished from the’see-saw’pattern peak in the CTAC-treated and control groups.XDLVO model calculation further confirmed that negatively charged SDBS and uncharged TX-100 mainly disturbed the short-range hydration on the NPs-cell interface.Conversely,positively charged CTAC reduced the long-range electrostatic repulsion,which was mainly attributed to the compressed electrical double-layer brought by adsorption of cationic CTAC on the negatively charged surface of NPs and algal cells.Furthermore,there was an excellent linear correlation between Znbioaccumulation and two parameters characterizing the NPs-cells interface interaction,i.e.,K_d and adhesion force,confirming that the Znbioaccumulation was associated with granular ZnO NPs-cell interfacial interaction.（3）During the processes of PS MPs-ZnO NPs-algal cells interface interaction,ZnO NPs were easy to agglomerate in the water environment,which was controlled by the polar interactions.The surface functional groups of PS MPs had significant effects on their stability.The critical flocculation concentrations（CCC）of PS and PC in Na Cl and Ca Cl₂ solution followed the Schulze-Hardy rule,confirming that electrostatic interactions were the dominant stabilization mechanism.However,the hydrophilic interactions played the main role in the main stabilization mechanism of PN–and PN+.Due to the strong surface hydrophobicity of PS and PC,the heteroaggregation of them with ZnO NPs or algal cells was mainly controlled by hydrophobic and electrostatic interactions.The ZnO NPs in the MPs-NPs-algal cells ternary system tended to bind with PS or PC.While the heteroaggregation of PN+and PN–with ZnO NPs or algal cells was mainly determined by the hydration and Van der Waals interactions.In addition,the content of dissolved Zn(Zn²⁺)was controlled by the dissolution of ZnO NPs and the adsorption of PS MPs to it.In addition,the interfacial interaction between PS or PC with ZnO NPs and algae cells was more likely to occur under acidic conditions.While PN–-ZnO NPs-algae cell ternary system was always relatively stable under the water environment conditions in this study.