Construction Of Sulfur Quantum Dot Switched Fluorescence System And Its Detection Application In Environment And Biological Matrices

Luminescent quantum dots(QDs)have been widely used in the fields of optoelectronic devices,biomarkers and biomedicine due to their unique physical and chemical properties.Metal-based quantum dots pose a threat to human health,and their preparation costs are relatively high.In the past decade,researchers focused on the development of low-cost,heavy metal-free quantum dots,especially pure element quantum dots such as phosphorous,carbon,sulfur and silicon quantum dots.Sulfur quantum dots(SQDs)have excellent water dispersibility,light stability,environmental friendliness and inherent antibacterial properties,and thus are considered as one type of the promising nanomaterials.Herein,we used the "assembly-fission" top-down method to synthesize SQDs that are metal-free,low-toxic and have good biocompatibility.SQDs were used to construct a fluorescent sensor based on their strong fluorescence characteristics,biocompatibility and internal internal filtration effect(IFE)mechanism.The main contents are summarized as follows:(1)By employing sublimed sulfur as a precursor and PEG-400 as a preserving agent,stable SQDs were fabricated by a simple synthesis method on the basis of the "assembly-fission" principle.A series of analytical instruments were used to characterize the morphology,composition and optical properties of SQDs,which included high-resolution transmission electron microscopy(HRTEM),infrared spectroscopy(FT-IR),X-ray photoelectron spectroscopy(XPS),ultraviolet-visible absorption spectroscopy(UV-Vis)and molecular fluorescence spectroscopy.Addition of Cr(?)could effectively quench the fluorescence of SQDs.The IFE-based quenching mechanism was evidenced by the changes in fluorescence lifetime(FLs)of SQDs and the overlap of Cr(?)absorption and excitation.Owing to the IFE principle,the formation of SQD/Cr(?)complexes effectively quenched the fluorescence of SQDs by Cr(?).As a result,an“ON-OFF” platform based on SQDs was constructed for the detection of Cr(?)in the environment and drinking waters,yielding a linear range of 0.005-1.5 m M and a LOD of1.5 ?M.Compared to previously reported nanoprobes,the newly developed fluorescence platform shows a wide linear range,high selectivity to Cr(?),and minimal matrix interference.(2)The reduction effect of ascorbic acid(AA)was used to convert Cr(?)to Cr(?);subsequently,an “OFF-ON” platform was constructed by employing the SQDs/Cr(?)/AA mixing system.When AA was introduced into the SQDs/Cr(?)system,the fluorescence of SQDs was restored due to the reduction of Cr(?)induced by AA to Cr(?).For the detection of AA in human serum and vitamin C tablets,the as-constructed fluorescence platform gave a linear range of 0.01-5.5 m M,and a LOD of 3 ?M.Additionally,it presented a high selectivity to AA and a little matrix interference.The SQDs-based fluorescent probe showed good bioimaging effects in He La cell lines and zebrafish embryos/larvae.The yolk of zebrafish embryos and the yolk and head of the larvae showed obviously blue fluorescence.Benefiting from the above application,this nanosensor based on SQDs have broad application prospects in water quality monitoring and biomedicine fields.(3)A new method was developed for the determination of 4-aminophenol(p-AP)using SQDs as a fluorescent probe.By employing the catalyzing reaction of tyrosinase(Tyr)to p-AP(oxidation to 4-amino-1,2-benzoquinone),we constructed a new type of nano-fluorescence sensor.The addition of p-AP could effectively quench the fluorescence of SQDs.Through the analyses of fluorescence lifetime,Zeta potential and UV-Vis absorption spectra overlapping,it was demonstrated that the quenching mechanism of the SQDs/Tyr/p-AP system was attributable to the IFE mechanism.In the presence of Tyr,the detection of p-AP provided a linear range of 0.5-120 ?M and a detection limit of 0.15 ?M,which was comparable with other previous methods and also showed a wider linear range with a smaller matrix interference.Finally,the sensor system was successfully applied to the detection of p-AP in environmental waters with satisfactorily fortified recoveries.