Aqueous Synthesis And Characterization Of Near-Infrared-Emitting Metal Sulfide Nanocrystals And Their Applications

Near-infrared?NIR?-emitting semiconductor nanocrystals?NCs?have drawn much attention for their potential application in optical amplifier media for telecommunications systems,in biotechnology as promising fluorescent probe and in electroluminescent devices,photodetectors and photovoltaics.Until now,high-quality NIR NCs are genereally synthesized by organic methods,however,aqueous synthesis of NIR NCs recevied limitted success.Mercury chalcogenides NCs are one of attractive among the NCs with NIR-to-IR emission owing to their bulk material with unique narrow band gap closing to semimetal.Despite great efforts have been made for the synthesis of HgS NCs via diverse synthetic strategies,the obtained HgS NCs all exhibited weak emissions with maximum photoluminescence quantum yield of 5%and poorly thermal stability.In this dissertation,we focus on the aqueous synthesis of HgS nanocrystals with near-infrared photoluminescence;highly fluorescent NIR-emitting Cd:HgS/CdS NCs and HgS/ZnS core/shell NCs with optical and bioimaging applications;NIR-emitting Ag2S NCs and application of HgS/ZnS NCs for sensitive dection of Cu²⁺.The major contents of this dissertation are described as follows:1.Aqueous synthesis and characterization of glutathione-stabilized ?-HgS nanocrystals with near-infrared photoluminescenceA simple,rapid and green aqueous approach to near-infrared-emitting ?-HgS nanocrystals was firstly demonstrated by using glutathione?GSH?as the stabilizer at room temperature.The resulting HgS NCs with zinc blend structure exhibited strong quantum size effect,and the emission peak could be tuned in a wide NIR region from ca.775 to 1041 nm.As compared with early achievements,the emission intensity of GSH-stabilized HgS NCs enhanced with the maximum quantum yield reaching to?2.8%.It was also found that the stability of the GSH-HgS NCs was improved noticeably,the PL peak red-shifting only 9 nm and 23 nm after stored at 4 ? for 4 months and 25 ? for 7 days,respectively.The better stability of the HgS NCs was elucidated by FT-IR due to the multiple coordination of GSH molecule to surface Hg of the NCs.The emission range of GSH-stabilized HgS NCs was located between the visible region?500-800 nm?and IR region?1000-1600 nm?of HgS NCs as reported previously,extending the emission region of HgS nanomaterial.Therefore,the continuous emission from visible to IR spectral ranges provided HgS material more potential applications.2.Highly fluorescent,near-Infrared emitting Cd²⁺-tuned HgS nanocrystals with optical applicationsBulk HgS itself has proven to be a technologically important material,however,the poor stability and weak emission of HgS nanocrystals has greatly hindered their promising applications.Presently,a critical problem is the uncontrollable growth of HgS NCs and their intrinsic surface states which are susceptible to the local environment.Here,we address the issue by an ion-tuning approach to fabricate stable,highly fluorescent Cd:HgS/CdS NCs for the first time,which efficiently tuned the band-gap level of HgS NCs,pushing their intrinsic states far away from the surface,reducing the strong interaction of environment with surface states and hence drastically boosting excitons transition.As compared with bare HgS NCs,the obtained Cd:HgS/CdS NCs exhibited tunable luminescence peaks in 724-825 nm with unprecedentedly high quantum yield up to 40%at room temperature,and excellent thermal and photo-stability.Characterized by TEM,XRD,XPS and AAS,the resultant Cd:HgS/CdS NCs possessed a zinc blend structure and composed by a homogenous alloyed HgCdS structure overcoated with a thin layer of CdS shell.The formation mechanism of Cd:HgS/CdS NCs was proposed.This bright and stable HgS-based NCs presented promising applications as fluorescent ink for anti-counterfeit,and as excellent light converter when coated on a blue light emitting diode.3.Ultra-bright near-infrared-emitting HgS/ZnS core/shell nanocrystals for in vitro and in vivo imagingNear-infrared-emitting nanocrystals have enormous potential as an enabling technology for applications ranging from tunable infrared lasers to biological labels.Mercury chalcogenides NCs are one of attractive NCs with NIR emission,however,the potential toxicity of Hg restricts their diverse applications.Herein,we devised tosynthesize a low-toxic,highly luminescent and stable GSH-capped HgS/ZnS core/shell NCs by aqueous route for the first time.The core/shell structure was characterized by using TEM,XRD and XPS which provide evidence for the shell growth.After successfulgrowth of a proper ZnS shell around HgS NCs,poorly luminescent HgS NCs converted into ultra-bright HgS/ZnS NCs,substantially increasing photoluminescence quantum yield up to 43.8%at room temperature.The fluorescence peak of HgS/ZnS NCs was successfully tuned in a wide NIR window ranged from 785 nm to 1060 nm with highemission efficiency by controlling the synthetic pH values.Significantly,in vitro cytotoxicity study clearly demonstrated that the HgS/ZnS NCs exhibited good biocompatibility as evidenced by the cell viability retained above 80%at a dose of HgS/ZnS NCs up to 150 ?g·mL-1.More importantly,the low-toxic NIR-emitting HgS/ZnS NCs have proved to be an effective fluorescent label in in vitro and in vivo imaging.The penetration depth reached 2 cm in a nude mouse with distinct separation of autofluorescence and NCs' fluorescence,giving excellent contrast at all depth.The novel highly-luminescent NIR-emitting HgS/ZnS NCs raise new possibilities in highly-sensitive,highly-spectrally resolved and multicolor imaging in biomedical application.4.Aqueous synthesis and characterization of near-infrared-emitting Ag2S nanocrystalsBy employing glutathione as a stabilizer,AgNO3 as silver precursor,thioacetamide?TAA?as a sulfide source,we synthesized water-soluble Ag2S NCs with near-infrared fluorescence.After carefully examined the effect of sulfide source,monomer concentration,Ag/S molar ratio,ligand,reaction temperature and heating time on the optical property of Ag2S NCs,the optimal Ag2S NCs emitted at 915 nm.Remarkably,as compared with early achievements,the emission intensity of GSH-stabilized Ag2S NCs enhanced greatly with the maximum quantum yield reaching to?4.8%.Powder X-ray diffraction and transmission electron microscopy were performed to show that the prepared Ag2S nanocrystals were monoclinic ?-Ag2S with size distribution about 5 nm.The biocompatible Ag2S NCs developed by our simple,rapid and green aqueous approach were suitable for bioimaging.5.Sensitive Cu²⁺ sensing by highly-fluorescent,near-infrared-emitting HgS/ZnS nanocrystalsThe near-infrared-emitting HgS/ZnS NCs capped with glutathione were applied for sensitive Cu²⁺ sensing for the first time.The sensing approach was based on the fluorescence of the HgS/ZnS NCs selectively quenched in the presence of Cu²⁺.Experimental results showed that the normal foreign ions,such as Na+,Mg²⁺,Al3+ et al.,had a low interference for the detection of Cu²⁺.The response of the NIR optical sensor was linearly proportional to a wide concentration of Cu²⁺ ranging from 1.25×10-7 to 1.25×10-5 mol·L-1 with a correlation coefficient of 0.9962.Furthermore,it has been successfully applied to the detection of Cu²⁺ in alcohol samples with satisfactory results.Since biomolecules have low effect on the fluorescence due to the minimized interferences in NIR region,the ultra-birght NIR-emitting HgS/ZnS NCs opens new possiblities for bioananlysis.