Study On The Detection And Degradation Methods Of Microcystin-LR Based On Upconversion Nanoparticles

Algae pollution caused by eutrophication has become a common problem in the world.The algal toxins in water threaten the safety of drinking water and increase the toxin level in aquatic products,resulting in a potential safety hazard to human health.At present,the conventional detection methods for microcystin-LR（MC-LR）have limitations such as long detection time,complex operation,high detection limit and expensive instruments,which run counter to the high sensitive and rapid detection of toxins in water.Besides,conventional degradation technology with long degradation time can not remove MC-LR completely,and even produce disinfection by-products.Therefore,it is important to establish a rapid,sensitive and accurate strategy for MC-LR detection and to constructe a highly efficient,thorough and clean method for MC-LR degradation.Upconversion nanoparticles have a wide application in biological analysis for its strong photochemical stability,low interference of background fluorescence,large Stokes shift,high signal-to-noise ratio and low toxicity.Here,we successfully synthesized upconversion nanoparticles by thermal decomposition.Combined with MoS₂ and TiO₂,a highly sensitive MC-LR detection method and an efficient MC-LR degradation method were successfully constructed,respectively.The details are as follows:Firstly,upconversion nanoparticles grafted molybdenum disulfide nanosheets platform was fabricated for microcystin-LR sensing.In this study,NaYF₄:Yb,Tm@NaYF₄:Yb upconversion nanparticles were prepared by layer by layer method.The fluorencence spectra revealed that the fluorencence intensity of NaYF₄:Yb,Tm@NaYF₄:Yb enhanced 7 times than NaYF₄:Yb,Tm NPs.We then constructed a simple and homogenous aptasensor for MC-LR using the quenching property of single-layer MoS₂,specific recognition of aptamer and enhanced fluorescence of NaYF₄:Yb,Tm@NaYF₄:Yb nanoparticles.Under the optimal condition,this assay specifically determined MC-LR in the linear range of 0.01⁵0 ng/mL（R²=0.9942）with a limit of detection（LOD）of 0.002 ng/mL.The specific experiments showed that the sensor has high specificity and strong interference resistance.The standard recovery experiments indicated that this assay owns well enough reliability and feasibility to allow the determination of MC-LR in real samples.Secondly,enhanced photocatalytic degradation of microcystin-LR with upconversion-assisted titanium dioxide was realized in this study.The photocatalyst（NaYF₄:Yb,Tm@TiO₂）can make highly efficient use of the large portion of NIR energy as well as the UV energy.Photocatalytic degradation of MC-LR was carried out in UV,NIR and simulated sunlight,respectively.The results indicated that NaYF₄:Yb,Tm@TiO₂ exhibited significantly enhanced photocatalytic activity than P25 under NIR and simulated sunlight.While under UV light,NaYF₄:Yb,Tm@TiO₂ exhibited similar degradation efficiency to P25.Under full spectrum lighting,the degradation efficiency of NaYF₄:Yb,Tm@TiO₂ and P25 are 83% and 61% within 30 min,respectively.The optimum conditions were obtained with the initial photocatalyst concentration of 0.4 mg/m L,the system at pH 4.0,and the initial MC-LR concentration of 10 μg/m L.Kinetic study of degradation process showed that all the experiment data fitted the first-order kinetic model well.Under the optimum conditions,the apparent rate constant of the NaYF₄:Yb,Tm@TiO₂ photocatalyst is 3.1 times than P25.Thirdly,we researched the degradation mechanism of microcystin-LR with NaYF₄:Yb,Tm@TiO₂ photocatalyst.On the basis of the enhanced photocatalytic degradation efficiency,the mechanism of photocatalytic degradation of MC-LR,the intermediates and the possible pathways of degradation were systematicly analyzed.Results showed that the degradation mechanism of MC-LR is the generation of ·OH,which acted as oxidant to attack the MC-LR molecular structure to achieve the purpose of degradation.The process produced seven intermediates: m/z 1029.6,795.4,1009.6,1011.6,1027.6,835.4 and 781.4.Three possible pathways were inferred:（1）hydroxylation of conjugated double bonds of Adda group;（2）hydrogen abstraction of methoxyl group of Adda group;（3）hydroxylation of benzene on Adda group.The above three pathways suggested that the Adda group in the MC-LR was destroyed.According to the previous work,the degradation of MC-LR by NaYF₄:Yb,Tm@TiO₂ can achieve the effect of detoxification.