Facile Synthesis Of Functional Nanomaterials(ZnS,Au,Ag)and Their Application In The Detection Of Heavy Metal Ions

Heavy metal ions, released from both natural and industrial sources, have severe adverse effects on human health and environment even at low concentrations. The rapid, economical, sensitive and selective detection of trace heavy metal ions in the environment is extremely important. Up to date, different analytical techniques including atomic absorption spectroscopy, cold vapor atomic fluorescence spectrometry, inductively coupled plasma mass spectrometry, electrochemical methods, high performance liquid chromatography and gas chromatography have been developed to detect heavy metal ions. They generally require expensive equipment or complicated sample preparation processes. Alternatively, nanomaterial-based optical sensors including metal nanoparticles (NPs) and semiconductor quantum dots have been demonstrated to be the cost-effective, simple and quick approach for detecting heavy metal ions. Quantum dots (QDs), also referred to as colloidal semiconductor nanocrystals, have unique optical and electrical properties. In comparison with dyes, quantum dots have high fluorescence quantum yields, good photostability and negligible photobleaching, biological compatibility and stability. QDs as a novel fluorescent nanomaterial has been widely applied in the detection of heavy metal ions. Metal nanoparticles (Au and Ag) have high extinction coefficients and possess strong surface plasmon resonance (SPR) absorption properties which depend on their size, shape, dielectric properties of the surrounding medium and interparticle distances. The color-change behavior depending on the interparticle distance of metal nanoparticles (NPs) provides the basis for the colorimetric sensing, in which the analytes can be easily monitored by the naked eye. At the same time, metal NPs can be used as surface-enhanced Raman scattering (SERS)-active substrates which can exhibit high sensitivity reaching the single molecule level. The metal NPs-based SERS sensor as an alternative to commonly used optical sensor has been widely applied in biological and environmental detection. In this paper, ZnS QDs, Au and Ag nanoparticles are synthesized in an aqueous environment and they are used as ion probes to detect heavy metal ions. The main contents can be summarized as follows:1. We described an investigation of a novel eco-friendly fluorescence sensor for Hg2+ions based on N-acetyl-L-cysteine (NAC)-capped ZnS quantum dots (QDs) in aqueous solution. Some QDs-based sensors for Hg2+ions in aqueous medium have been developed. Although these developed QDs-based sensors for Hg2+provide low detection limits, nearly all of them were based on Cd-chalcogenide QDs (CdS, CdSe, CdTe). Considering the high toxicity of Cd, ZnS QDs modified by NAC were easily synthesized in aqueous medium via a one-step method. The used stabilizer N-acetyl-L-cysteine is a non-toxic and cost-effective biomolecule. The prepared products were water-soluble, biocompatible and low-toxic. The obtained ZnS QDs were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), infrared (IR) spectra, ultraviolet-visible (UV-vis) absorption spectra and photoluminescence (PL) spectra. To obtain high-quality ZnS QDs, the influences of various synthesis parameters including the precursor concentration, the molar ratio of NAC to Zn2+, the pH of the reaction solution, the reaction time and different kinds of stabilizers on the fluorescence intensity of ZnS QDs were investigated. ZnS QDs with relatively high fluorescence (QY=10%) were prepared at90℃for2h, with the precursor concentration of2mM, at NAC to Zn2+molar ratio of4:1and pH10.8. The ZnS QDs was used as fluorescent probe for the quantitative detection of Hg2+ions, based on fluorescence quenching of ZnS QDs. To optimize the detection conditions, we discussed the effect of reaction time and mixing sequence, pH and buffer solution as well as ZnS QDs concentration on the detection of Hg2+ions with NAC-capped ZnS QDs. The results indicated that the best order was to mix ZnS QDs, buffer solution first and then Hg2+. The optimal buffer solution was citric acid-Na2HPO4, and the best buffer volume was0.35mL. The fluorescence signals of the system were recorded after the reaction lasted for15min. The maximum F0/F was achieved when the concentration was1.6×10-4mol L-1. Under optimal conditions, its response was linearly proportional to the concentration of Hg2+ions in a range from0to2.4×10-6mol L-1with a detection limit of5.0×10-9mol L-1. Most of common physiologically relevant cations and anions did not interfere with the detection of Hg2+. The proposed method was applied to the trace determination of Hg2+ions in water samples. The possible quenching mechanism was also examined by fluorescence and UV-vis absorption spectra. The fluorescence quenching of ZnS QDs, with no spectra shift in both absorption and emission peaks in the presence of Hg2+, was in accordance with the literature. Meanwhile, XRD and EDS of ZnS QDs after addition of Hg2+ions were performed. The diffraction patterns before and after addition of Hg2+were identical and there was no Hg element found in EDS data, indicating HgS particles were not formed in our system. The quenching mechanism was assumed to be the effective electron transfer from surface traps of ZnS QDs to Hg2+recombination of excited electrons (e-) in the S2--vacancy-related band and holes (h+) in the valence band. To further confirm the quenching mechanism, the fluorescence decay profiles of ZnS QDs at different concentrations of Hg2+were recorded, The fluorescence lifetimes of ZnS QDs were decreased with addition of Hg2+,which may result from the electron transfer from surface traps of ZnS QDs to Hg2+.2. We designed a colorimetric and SERS dual-signal sensor for detecting Hg2+in aqueous solution by facilely mixing Bismuthiol Ⅱ and AuNPs, without dye tag. Bismuthiol Ⅱ was chosen as a multibridging molecular probe associated with AuNPs for the following reasons:(1) Bismuthiol Ⅱ with rich sulphur atoms could bind to AuNPs and Hg2+.(2) Bismuthiol Ⅱ with a large scattering cross section could be used as a Raman reporter for SERS detection.(3) The commercial Bismuthiol Ⅱ was highly voluble in water, allowing the detection to be performed in aqueous solution. The addition of Bismuthiol Ⅱ to the gold nanoparticles (AuNPs) solution led to the aggregation of AuNPs with a color change from red to blue. As a result, hot spots were formed and strong surface-enhanced Raman scattering (SERS) signal of Bismuthiol Ⅱ was observed. However, the Bismuthiol Ⅱ-induced aggregation of AuNPs could be inhibited by Hg2+in the system due to Bismuthiol Ⅱ had stronger affinity with Hg2+than that with AuNPs, accompanied by a remarkable color change from blue to red. As evidenced by UV-vis and SERS spectroscopy, the variation in absorption band and SERS intensity was strongly dependent on the concentration of Hg2+, suggesting a colorimetric and SERS dual-signal sensor for Hg2+. The factors (the pH and buffer volume, the concentration of Bismuthiol Ⅱ and AuNPs, as well as the size of AuNPs) affecting the detection of Hg2+were tested. The ratio of A780/A530in the absence and in the presence of Hg2+reached the maximum at pH4.4. The highest ratio of A780/A530in the absence and presence (240nM) of Hg2+was obtained at80μL of buffer solution. Therefore,80μL of citric acid-sodium citrate buffer solution (100mM, pH4.4) was selected for use in this work. When the concentration of AuNPs was fixed, the aggregation degree of AuNPs increased with increasing Bismuthiol Ⅱ concentration. While under the same concentration of Bismuthiol Ⅱ, the aggregation degree of AuNPs decreased with increasing the concentration of AuNPs. Considering the sensitivity,0.15nM AuNPs and0.50μM Bismuthiol Ⅱ were utilized in the dual-signal sensing system. In our work, two sizes of AuNPs (30nm and15nm) were employed. Much more amounts of Bismuthiol Ⅱ were needed to induce the same aggregation of15nm AuNPs compared to that of30nm AuNPs. Besides, the15nm Bismuthiol Ⅱ-AuNPs aggregates showed weak SERS spectrum of Bismuthiol Ⅱ. Considering both the sensitivity and the strong SERS signal,30nm AuNPs was chosen for use. Under the optimal conditions, the method showed high sensitivity, low detection limits of2nM and30nM could be achieved by UV-vis spectroscopy and by SERS spectroscopy, respectively. Other environmentally relevant metal ions did not interfere with the detection of Hg2+. The method was successfully applied to detect Hg2+in water samples. It was simple, rapid and cost-effective without any modifying or labeling procedure.3. A simple, cost-effective and rapid colorimetric method has been developed for detection of Ni2+and Co2+using6-Thioguanine-modified silver nanoparticles (Ag NPs). The6-Thioguanine-AgNPs displayed the most obvious response to Ni2+in an aqueous solution based on the aggregation-induced color change of AgNPs. We discussed the sensing mechanism, the effect of solution pH and the selectivity. The addition of Ni2+and Co2+to AgNPs led to the aggregation of AgNPs. The color of the AgNPs solution changed from orange to red, and a new absorption band at ca.530nm was formed. With increasing Ni2+and Co2+concentrations, the absorbance at404nm decreased while that at530nm increased. The ratio between the absorbance at530nm and404nm (A530/A404) was used for the quantitative analysis of Ni2+and Co2+. The results indicated that the AgNPs were stable under alkaline environment. We tested the effect of environmentally relevant metal ions including Co2+, Ni2+, Cd2+, Fe3+, Cu2+, Hg2+, Ag+, Zn2+, Na+, Mg2+, Ca2+on the value of A53o/A404.The presence of Ni2+displayed the most obvious increase of A530/A404value compared to that of the blank. The results indicated that other tested metal ions did not interfere with the detection, which showed that the method had good selectivity. For Ni2+, a linear range of0-300nM was obtained with a detection limit of5nM. For Co2+, a linear range of60-600nM was obtained with a detection limit of25nM. The method showed high sensitivity.