Study On A New Method Of Heavy Metal Detection Based On The Oxidase-Mimicking Activity Of Manganese Oxide Nanomaterials

With the development of human society and the change of natural environment,the degree and scope of heavy metal pollution is increasing more and more,which has been a global hot spot.Heavy metal ions in the environment can enter animals and plants through bioaccumulation,and enter the human body through the food chain,resulting in a variety of human diseases,even carcinogenesis,seriously threatening human health.Therefore,it is necessary to detect it in time to protect the natural environment and human health.Usually traditional methods are usually time-consuming and expensive,which is not conducive to the promotion of on-site detection.Consequently,it is of great significance to establish a rapid and simple method to realize the visual detection mode of heavy metal ions.Recently,a variety of nanomaterials with simulated natural enzyme activity have been used to establish effective detection methods for heavy metal ions.Researchers found that manganese oxide nanoparticles have a variety of enzyme activities,but there are few reports for the detection of heavy metal ions.In this paper,we focus on manganese oxide nanomaterials,researched its oxidase-mimicking catalytic activity,and established the new colorimetric detection methods for mercury,cadmium and arsenic.The main works of this article are as follows:?1?Herein we report a novel strategy to construct a colorimetric assay for the determination of heavy metal ions is presented that is based on the regulation of the oxidase-mimicking activity of Mn₃O₄ nanoparticles?Mn₃O₄NPs?by oligonucleotides.Nanozymes like Mn₃O₄ NPs have attracted extensive attention because of their distinguished catalytic advantages,but their poor specificity restricts the future application in the field of analysis and diagnosis.Several Mn₃O₄ NPs with different shapes were synthesized and characterized by TEM,SEM,FT-IR and XPS.Their size,morphology,element composition and surface chemistry properties were analyzed.It is confirmed that Mn₃O₄ NPs with different shapes have been successfully synthesized,and they all have the oxidase-mimicking catalytic activity.The chromogenic agent tetramethylbenzidine?TMB?can be thoroughly oxidized by Mn₃O₄ NPs to generate yellow products in acid condition,with a characteristic absorption peak at 450 nm.Notably,the catalytic reaction of Mn₃O₄ NPs does not require hydrogen peroxide,so the color of sensing solution is stable and intuitive,which makes it an ideal signal output material for sensors.Oligonucleotides can absorb on the regular surface of Mn₃O₄ NPs and temporarily inhibit its catalytic activity,leading to a significant decrease in absorbance and make sensing solution light green.The results testified that purine bases in oligonucleotides play a key role in their regulation on the catalytic activity of Mn₃O₄ NPs,and such regulatory effect is agreed with noncompetitive inhibition mechanism.On adding target heavy metals like Hg?II?or Cd?II?,the inhibition effect on catalytic activity of Mn₃O₄ NPs is relieved due to the specific binding of heavy metals and thymine bases,and changes the color of sensing solution from light green to yellow.The increase in absorbance at450 nm is related to the amount of heavy metal ions present.The method allows Hg?II?and Cd?II?to be determined visually in concentrations as low as 20?g?L^-1.The detection limit of the colorimetric assay is 3.8 and 2.4?g?L^-1 of Hg?II?and Cd?II?,respectively.The assay displays good selectivity over other heavy metal ions.The method was successfully validated by analyzing several water samples.?2?The adsorption of arsenic significantly enhances the oxidase-mimicking catalytic activity of octahedral Mn₃O₄ NPs,which enable us to construct a novel colorimetric chemosensor for arsenic determination.Studies have found that Mn-based nanomaterials can be used to adsorb and remove arsenic in water environment.However,there are few reports about the effect of arsenic adsorption on the catalytic activity of manganese oxide nanomaterials.Therefore,the arsenic adsorption performance of Mn₃O₄ NPs in water was firstly studied by carrying out the adsorption using different shape of Mn₃O₄ NPs,and the octahedral Mn₃O₄ NPs displays the strongest adsorption capacity for arsenic.A series of characterization techniques were employed to confirm the successful adsorption of arsenic on the surface of octahedral Mn₃O₄ NPs.The results indicate that arsenic adsorption can change surface morphology of octahedral Mn₃O₄ NPs and further release Mn²⁺to generate sufficient active sites,which can catalyze the reduction of dissolved oxygen to form large quantity of oxygen radicals?O₂?^-/OH??.Consequently,the chromogenic substrate?TMB?molecules were thoroughly oxidized to lose two electrons and make the solution change to yellow,with a characteristic absorption peak at 450 nm.The limit of detection?LOD?of such colorimetric chemosensor for arsenic detection was determined as 1.32?g?L^-1.The chemosensor allows arsenic to be determined visually at the concentrations as low as 10?g?L^-1,and displays good selectivity against other metal ions.Moreover,the chemosensor was successfully validated by analyzing several actual environmental and biological samples,indicating the excellent prospect of octahedral Mn₃O₄ NPs in the application of arsenic detection and removal.In conclusion,manganese oxide nanomaterials were used as sensing probes in this paper,and some new colorimetric methods for the detection of heavy metal ions were established.These established methods were simple,reliable,low cost and do not need complex modification.In addition,it is expected to be used in the field detection of heavy metal pollution in actual samples,and has certain application potential for the purification of pollutants in the environment.