Synthesis Of Novel Keggin Polyoxometalates-based Compounds Used For L-cysteine Colorimetric Sensing

Objective Various novel Keggin polyoxometalates-based crystals were synthesized by selecting suitable Keggin polyoxometalates,metals and organic ligands,and the Keggin polyoxometalates-based nanocrystal/carbon nanotubes composite was further prepared.Furthermore,their peroxidase-like activities were evaluated for L-cysteine colorimetric sensing.Methods Novel Keggin polyoxometalates-based crystal compounds were synthesized by the self-assembly strategy,and the Keggin polyoxometalates-based nanocrystal/carbon nanotubes composite was further prepared by abrasive method,microwave method and periodic ultrasonic functionalization strategy.The composite structures were analyzed and characterized by X-ray single-crystal diffraction,element analysis,infrared spectrum,X-ray powder diffraction,scanning electron microscope,energy spectrum,Raman spectrum and transmission electron microscope.The X-ray powder diffraction of the compounds after acid and high-temperature treatment was tested for the stability study.Hydrogen peroxide and 3,3’,5,5’-tetramethylbenzidine were chosen as substrates,and the naked eye observation and UV-Vis spectrum were combined for the peroxidase-like activity evaluation and the colorimetric sensing of L-cysteine.Results Four novel Keggin polyoxometalates-based crystals were successfully synthesized:[Cu₆（m-pyttz）₂（H₂O）][HPMo₁₂O₄₀]（1）,[Ag₃（p-H₂pyttz）（p-Hpyttz）Cl][H₂PMo₁₂O₄₀]·6H₂O（2）,[Cu₁₈（trz）₁₂Cl₃（H₂O）₂][PW₁₂O₄₀]（3）and[Cu₂^ⅠCu₄^Ⅱ（trz）₈Cl₂][PW₁₂O₄₀]（4）.It was found that compounds 1,3 and 4 had strong stability and good peroxidase-like activity.Based on compound 3 with the best activity,the"on-off switch"colorimetric sensing of L-cysteine was carried out,obtaining a detection limit of 0.35μmol·L^-1.Due to the synergistic effect of the two components,the further prepared Keggin polyoxometalates-based nanocrystal/carbon nanotubes composite showed significantly enhanced peroxidase-like activity,which was highly dependent on the mass ratio of the two components.A simple and effective"on-off switch"L-cysteine colorimetric sensor based on the composite with excellent stability and the best activity was established with a detection limit of 0.103μmol·L^-1,which is the best among the reported L-cysteine colorimetric sensing materials.Conclusions In this work,various excellent L-cysteine colorimetric sensing materials have been obtained,and the classification of polyoxometalates-based crystal compounds and its application in colorimetric sensing have been expanded.It has been proved that the preparation of polyoxometalates-based nanocrystal/carbon nanotubes composite by combining nanocrystallization polyoxometalates-based crystal with carbon nanotubes is a good method to improve its colorimetric sensing performance,providimg important basis for later researchers to expand their research on polyoxometalates chemistry.