The Novel CuInS₂/ZnS@Fe Fluorescent Probe With Turn-on Response For H₂S Sensor And Bioimaging

Objective:As gaseous signaling molecule,hydrogen sulfide（H₂S）has attracted great attention.It can pass through the cell membrane quickly and freely,transmit rapidly,produce continuously,exist in various tissues and organs of the human body,and thus participate in a variety of physiological processes.Therefore,it is of great scientific significance to develop highly selective and sensitive tools for monitoring H₂S level in complex biological systems.Recently,molecular imaging technology based on semiconductor quantum dot fluorescent probes has shown great application potential in the field of biomolecular detection due to their tunable optical properties,multimodal imaging and easy modification.Herein,we developed a"turn-on"type CIS/ZnS@Fe fluorescent probe for H₂S detection,which consists of CuInS₂/ZnS（CIS/ZnS）quantum dots with excellent optical performance,low toxicity and red emission as fluorophores,and Fe³⁺as the recognition group.The probe design strategy was as follows:Fe³⁺was introduced to turn off the fluorescence signal due to its effect of photoelectron transfer and aggregation-induced quenching on CIS/ZnS quantum dots.In the presence of H₂S,Fe³⁺in the probe is reduced and further forms insoluble precipitation,releasing free CIS/ZnS quantum dots and turning on the probe fluorescence signal.The response ability of CIS/ZnS@Fe to H₂S was evaluated and the application potential of CIS/ZnS@Fe in H₂S bioimaging was investigated as wellMethods:（1）CIS/ZnS quantum dots was prepared by colloid synthesis method.First,cationic precursors（Cu Cl₂·2H₂O,In Cl₃·4H₂O）and surfactants（GSH）were dissolved in deionized water,after adjusting p H anionic precursor（Na₂S·9H₂O）were added to the reactive system.Then,the above mixture was transferred to a three-mouth bottle and heated under magnetic stirring for a period to obtain CIS nuclear nanocrystals.Next,the shell precursors Zn（Ac）₂ and Na₂S·9H₂O were added to the reaction system at a certain molar ratio,and the heating was continued for a period.Finally,the crude reaction solution was centrifuged,washed and purified,and then dispersed into deionized water for further use.The structure and optical properties of the crude products were characterized by XRD,TEM,XPS,IR,Uv-vis and PL,respectively.（2）Based on the strong chelating ability of Fe³⁺,the CIS/ZnS@Fe fluorescent probe was obtained by direct modification of Fe³⁺on CIS/ZnS quantum dots which possesses rich carboxyl anions and amino groups on their surface,and then the fluorescence properties and feasibility of the as-designed probe were evaluated.On this basis,the response ability of CIS/ZnS@Fe to H₂S was investigated as follows:（i）the quantitative analysis of H₂S was performed from the relationship between the fluorescent signal of the probe and the analytical concentration,and then the detection limit was calculated.（ii）The reaction rate constant was calculated from the kinetic equation which fitted based on the change of the fluorescence signal of the probe with H₂S response time.（iii）The H₂S selectivity of the probe was investigated by comparing the fluorescence signal and detection performance of the probe in the presence and absence of various common interferences.（3）CCK-8 assay was used to evaluate the cytotoxicity of CIS/ZnS quantum dots.Subsequently,the CIS/ZnS@Fe probe was used for fluorescence imaging of exogenous and endogenous H₂S in MCF-7 living cells.Results:（1）ZnS can be enriched on the CIS surface to form CIS/ZnS quantum dots with core-shell structure.The prepared CIS/ZnS quantum dots have high purity,single crystal,relatively uniform morphology and good dispersibility,and the average particle size is about 3.12±0.3 nm.The spectral analysis results show that the band gap value of CIS/ZnS quantum dots is 2.22 e V,which confirms the existence of quantum confinement effect.The emission peak of CIS/ZnS quantum dots was about 628 nm,whose intensity enhanced significantly compared with the CIS core attributing to the existence of ZnS shell.（2）With the increase of Fe³⁺concentration in CIS/ZnS@Fe,the fluorescence signal of the probe gradually decreased.When Fe³⁺was greater than 160μM,over 90%fluorescent intensity of CIS/ZnS quantum dots can be quenched,and the quenching mechanism can be found to change from photoinduced electron transfer to aggregation-induced quenching at the same time.In the presence of H₂S,Fe³⁺in the representative CIS/ZnS@Fe-160,CIS/ZnS@Fe-180 and CIS/ZnS@Fe-200 probes could be reduced to Fe²⁺and then formed precipitate,so that the fluorescence signal was turned on and showed a two-stage linear response to the amount level of H₂S.Among them,the optimal detection limit can reach 0.51μM,and the highest response rate constant is0.0612 min^-1.In addition,the CIS/ZnS@Fe probe has the ability to recognize H₂S specifically.（3）The viability of MCF-7 cells incubated with CIS/ZnS quantum dots below 0.28mg/m L for 48 hours was still more than 85%.The CIS/ZnS quantum dots can label the cell for bioimaging.Moreover,CIS/ZnS@Fe probe could be used for fluorescence imaging of exogenous and endogenous H₂S in MCF-7 living cells,and the fluorescence signal was enhanced with the increase of exogenous H₂S.Conclusion:In this study,CIS/ZnS quantum dots with high quality and red emission were prepared by simple colloid synthesis method.The CIS/ZNS quantum dots were used as fluorophore moiety to design a turn-on type CIS/ZnS@Fe probe for the detection of gaseous signaling molecule H₂S.The results showed that the probe possessed lower detection limit and higher selectivity for H₂S,and the potential to be applied in the field of visual qualitative and quantitative analysis for H₂S in biological system.