Effects Of Extracellular Polymeric Substances On Silver Nanoparticle Transformation Behavior And Phytotoxicity

Ag-NPs have been widely used in various production and life field due to their excellent antibacterial properties and unique physical and chemical properties.Ag-NPs can cause potential harm to organisms when it released into the environment.Ag-NPs may release Ag⁺through oxidative dissolution after entering the environment,and will be mainly transformed to the corresponding low activity Ag₂S-NPs.The biological effects of Ag-NPs and Ag₂S-NPs are highly dependent on the form of Ag.Microbial extracellular polymers substances（EPS）,as natural organic matter that widely exists in the environment,can significantly affect the transformation behavior of nanoparticles in the environment,and then change the phytotoxicity effect of nanoparticles.Currently,the effect and mechanism of EPS on Ag-NPs and Ag₂S-NPs transformation behavior and phytotoxicity are still unclear.In this study,we investigated the effects of soil biogenic organic matter EPS（Bacillus subtilis）on the transformation behavior of Ag-NPs and Ag₂S-NPs and the subsequent changes of phytotoxicity.To reveal the molecular mechanism of EPS affecting Ag-NPs/Ag₂S-NPs transformation behavior and explore the key role of EPS in the change of phytotoxicity of different forms of Ag,the internalization concentration and oxidative damage level of different forms Ag in wheat（Triticum aestivum L.）tissues were investigated,combined with fourier-transform infrared spectroscopy,three-dimensional excitation-emission matrix and transmission electron microscopy,ultrafiltration technology.The main conclusions of this study are as follows:（1）Exposure to different Ag forms significantly affected the phytotoxicity of wheat.The median effect concentrations(EC₅₀)values of AgNO₃,Ag-NPs and Ag₂S-NPs on wheat root elongation were 0.1,0.3 and 12.0 mg·L^-1,respectively.At the same Ag exposure level(1.0 mg·L^-1),the Ag accumulation in wheat roots in AgNO₃ and Ag-NPs treatments were 19.4 and 3.4 times higher than that in the Ag₂S-NPs group.The results showed that the phytotoxicity effect of Ag on wheat was related to the exposure form and exposure concentration of Ag.The toxic effects:Ag⁺>Ag-NPs>Ag₂S-NPs.（2）EPS significantly alleviated the inhibition of AgNO₃ and Ag-NPs on wheat growth,but aggravated the toxic effect of Ag₂S-NPs on wheat.When the EPS concentration was 100 mg·L^-1,compared with the control,the total silver concentration in wheat roots in AgNO₃,Ag-NPs and Ag₂S-NPs groups decreased by 27.7%,61.2%and 22.4%,respectively.This may be due to EPS can significantly reduce the content of free Ag⁺in Ag-NPs and Ag₂S-NPs exposure solutions.The transfer factors of Ag:Ag⁺>Ag-NPs>Ag₂S-NPs,and the Ag transfer factors decreased with the increase of EPS concentration.（3）According to the analysis of fourier-transform infrared spectroscopy and 3D-EEM spectrum,it was found that the-NH,-OH,C=O and C-O functional groups in EPS reacted with Ag,and the protein in EPS was the main action substance.Transmission electron microscopy analysis showed that EPS complexed with nanoparticles to form a complex,thus reducing the bioavailability of nanoparticles and reducing phytotoxicity.