Ratiometric fluorescence detection of mercuric ion based on the nanohybrid of fluorescence carbon dots and quantum dots

Analytica Chimica Acta 786 (2013) 146–152

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Ratiometric fluorescence detection of mercuric ion based on the nanohybrid of fluorescence carbon dots and quantum dots Benmei Cao a,b , Chao Yuan a,b , Bianhua Liu b,∗ , Changlong Jiang b , Guijian Guan b , Ming-Yong Han b,c a

Department of Chemistry, University of Science & Technology of China, Hefei, Anhui 230026, China Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, China c Institute of Materials Research and Engineering, A*STAR, 117602 Singapore, Singapore b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• The simplicity of the preparation of the ratiometric fluorescence probe.

• Visual fluorescence detection of Hg2+ which could be observed with the naked eye. • High selectivity and low detection limit for aqueous Hg2+ sensing.

a r t i c l e

i n f o

Article history: Received 22 December 2012 Received in revised form 3 May 2013 Accepted 6 May 2013 Available online 20 May 2013 Keywords: Quantum dots Carbon dots Ratiometric fluorescence Mercuric ion

a b s t r a c t A novel nanohybrid ratiometric fluorescence probe comprised of carbon dots (C-dots) and hydrophilic CdSe@ZnS quantum dots (QDs) has been developed by simply mixing the blue-emission C-dots with red-emission carboxylmethyldithiocarbamate modified CdSe@ZnS QDs (GDTC-QDs). The nanohybrid ratiometric fluorescence probe exhibits dual emissions at 436 nm and 629 nm under a single excitation wavelength. Due to the strong chelating ability of GDTC on the surface of QDs to mercuric ion (Hg2+ ), the fluorescence of the GDTC-QDs in the nanohybrid system could be selectively quenched in the presence of Hg2+ while the fluorescence of the C-dots remained constant, resulting in an obviously distinguishable fluorescence color evolution (from red to blue) of the nanohybrid system. The detection limit of this method was found to be as low as 0.1 ␮M. Furthermore, the recovery result for Hg2+ in real samples including tap water and lake water by this method was satisfying, suggesting its potential application for Hg2+ sensing. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Mercuric ion (Hg2+ ), one of the most deleterious heavy metals in the environment, could cause serious damage to the central nervous and endocrine systems, due to its easy passing through biological membranes [1]. Therefore, the detection of Hg2+ plays an important role in environmental protection and human

∗ Corresponding author. Tel.: +86 551 65591156; fax: +86 551 65591156. E-mail address: [email protected] (B. Liu). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2013.05.015

health. Except for the traditional detection methods including cold-vapor atomic fluorescence spectrometry (CV-AFS) [2], coldvapor atomic absorption spectrometry (CV-AAS) [3], inductively coupled plasma-mass (ICPMS) [4], ultraviolet–visible spectrometry and X-ray absorption spectroscopy [5], there are several other ones including luminescent [6–12], electrochemical [13–16], colorimetric [17–21], and surface enhanced Raman scattering (SERS)-based [22,23] analytical methods for Hg2+ detection that have been reported in the past two decades. Many of these methods are sensitive and selective, however, the sample preparation for most of them are time-consuming, laboratory-based, require expensive

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instrumentation. Therefore, it is an important goal to obtain new Hg2+ detection methods that are cost-effective, rapid, facile and applicable to the environmental and biological milieus. Ratiometric fluorescence strategy, which compares fluorescence intensities of two different wavelengths before and following analyte recognition, has become a powerful tool for the detection of trace amounts of analyte because of its simplicity and sensitivity. To the best of our knowledge, there are many novel research work involving ratiometric fluorescence detection of Hg2+ has been reported in the literature and displayed the potential in practical application in the past few years [24–28]. For example, Nolan and co-workers [26] have developed a water-soluble organic molecule containing a seminaphthofluorescein chromophore and a thioether-rich metal-binding unit to ratiometric detection Hg2+ in aqueous solution with high selectivity and sensitivity (the LOD could reach 50 nM at the probe concentration of 500 nM). Zhang and co-workers [27] have developed a naphthalimide–porphyrin hybrid sensing system for ratiometric detection of Hg2+ in aqueous solution with the LOD of 20 nM and imaging of Hg2+ in living cells with satisfying resolution. However, the probes for Hg2+ sensing in most of the work are made of organic molecules, which always suffer from narrow excitation profile, low quantum yield and photobleaching. Semiconductor quantum dots (QDs), since its discovery three decades ago, has attracted intense interest from the scientists and achieved a variety of application in the community of analytical chemistry, due to their high fluorescence quantum efficiency, size-dependent broad absorption, high extinction coefficients, readily size-tunable narrow emission as well as resistance to photobleaching [29–31,17,32–36]. Carbon dots (Cdots), as a new form of carbon nanomaterials emerged in recent years, has also being studied extensively by the scientists due to their chemical inertness, a lack of optical blinking, low photobleaching, low cytotoxicity and excellent biocompatibility [37–41]. In the present work, we develop a selective and sensitive fluorescence sensor for ratiometric detection of Hg2+ by simply mixing blue-emission fluorescence C-dots and red-emission carboxylmethyldithiocarbamate modified CdSe@ZnS QDs (GDTC-QDs). The hydrophobic TOPO stabilized CdSe@ZnS QDs was transfer to water through surface ligand exchange by GDTC as recently reported in the literature [42]. The GDTC molecule containing carboxyl group and bidentate thiol group could afford the stability and hydrophilic feature of the QDs in water. Due to the high chelating ability of GDTC to Hg2+ , the fluorescence of the GDTC-QDs could be selectively and sensitively quenched in the presence of Hg2+ , while all of the metal ions tested have no obvious quenching effect on the fluorescence property of the water-soluble C-dots. By taking advantage of this phenomenon, a ratiometric fluorescence strategy was developed by simply mixing the GDTC-QDs and C-dots in aqueous solution with appropriate proportion, and the fluorescence color of the probe solution gradually changed to blue from red upon the increasing concentration of Hg2+ added which could be obviously observed by the naked eye. The limit of detection of this method for Hg2+ could reach 0.1 ␮M in aqueous solution.

2. Experimental

147

without further purification. Ultrapure water (18 M) was produced using Millipore purification system and used for all solution preparation. 2.2. Synthesis of hydrophilic C-dots High-quality C-dots were synthesized according to the reported method with minor modification [43]. In a typical synthesis, 1.125 g of glycine was dissolved in 15 mL of water and added into a 40 mL autoclave tube. The solution was then sealed and treated at 200 ◦ C for 4 h. The resulting brown solution was cooled to room temperature naturally and centrifuged at 3000 rpm for 10 min to remove large or agglomerated particles. To further purify the C-dots, the brown solution was then subjected to dialysis against pure water through a membrane (MWCO = 3.5–5 kDa, Float-A-Lyzer G2, Spectrum Laboratories, Rancho Dominguez, CA, USA) for 3 h. After repeating the previous procedure three times, a clear yellowbrown aqueous solution containing surface passivated C-dots was obtained and stored at 4 ◦ C for further use. 2.3. Synthesis of hydrophobic TOPO-QDs Hydrophobic core-shell CdSe@ZnS QDs were synthesized according to the reported method with minor modification [44,45]. Briefly, 0.064 g of CdO, 5.91 g of TOPO and 6.02 g of HDA were added to a flask. The mixture was kept under vacuum and then slowly heated to ∼100 ◦ C for 2 h followed by filling with nitrogen. 0.064 g of selenium powder was dissolved in 6.0 mL of TOP in a schlenk flask and then pumped for 2 h at room temperature. The cadmium precursor solution was heated at ∼340 ◦ C until a colorless solution formed. The solution was then cooled to 300 ◦ C and stabilized for 2 h. At this temperature, selenium precursor solution was rapidly injected into the cadmium solution to form CdSe nanoparticles followed by rapidly reducing the temperature to 270 ◦ C in a few seconds. 2 mL of ZnEt2 in hexane (1 M) and 0.45 mL of (TMS)2 S were dissolved in 6 mL of TOP in a schlenk flask. The ZnS precursor solution was added dropwise to the crude CdSe QDs solution at ∼230 ◦ C to form a ZnS shell over CdSe cores. The reaction mixture was naturally cooled to about 80 ◦ C followed by addition of about 10 mL of toluene in order to prevent solidification at room temperature. To purify the TOPO-QDs, 1 mL of the as-prepared CdSe@ZnS QDs was diluted in 1 mL of toluene and 2 mL of methanol was then added. The mixture was vigorously stirred with vortex for 1 min followed by centrifugation at 8000 rpm for 5 min. The supernatant was discarded and the precipitate was dissolved again in 2 mL of toluene to form a clear solution. Methanol was added dropwise to the toluene solution until the solution became cloudy. The mixture was thoroughly vortexed followed by centrifugation at 8000 rpm for 5 min. The supernatant was discarded and the precipitate was dissolved in 2 mL of toluene. After repeating the previous procedure one more time, the purified TOPO-QDs were obtained at the bottom of the centrifugation tube. 10 mL of chloroform was added to the tube to form a clear quantum dots solution. The UV–vis spectrum was recorded to estimate the concentration of CdSe@ZnS QDs based on an empirical equation by Peng and co-workers [46], and the TOPO-QDs chloroform solution was kept in the dark for future use.

2.1. Materials 2.4. Surface complexation of QDs with GDTC Trioctylphosphine oxide (TOPO, 90%), trioctylphosphine (TOP, 99%), selenium fine powder (99.9%), Cadmium oxide powder (99.9%), hexadecylamine (HDA, 99%), diethyzinc (ZnEt2 ) (1.0 M in hexane), tetramethylammonium hydroxide pentahydrate and bis(trimethylsilyl)sulfide (TMS)2 S were purchased from Sigma–Aldrich. Glycine, carbon disulfide (99%), and all the metal salts were obtained from Sinopharm Chemical Reagent and used

The hydrophilic GDTC-QDs were prepared by ligand exchange and surface complexation using TOPO-QDs as starting materials [42]. Briefly, 100 ␮L of crude CdSe@ZnS QDs was purified and redispersed in 10 mL of mixture of methanol and chloroform (volume ratio; 1:1) to form a homogeneous solution. In a separate flask, 40 mg of glycine and 192 mg of tetramethylammonium hydroxide

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pentahydrate were added to 10 mL of the same mixture of solvents. 32 ␮L of pure carbon disulfide liquid was then dropwise added into the above solution in an ice bath and the mixture was vigorously stirring for 10 min to form light yellow solution. To the latter was added the solution of QDs and the mixture was stirred at room temperature for 12 h in the dark. After the reaction was stopped, 5 mL of diethylether was then added to the reaction solution to precipitate the GDTC-QDs by centrifugation (5000 rpm, 10 min). The supernatant was discarded and the GDTC-QDs was dissolved in 5 mL of methanol. To purify the GDTC-QDs, 10 mL of THF was added to the GDTC-QDs methanol solution, and the mixture was vigorously stirred with vortex prior to centrifugation (5000 rpm, 10 min). After repeating the procedure two more times, the purified red solid of GDTC-QDs was obtained and could be readily dispersed in water. The UV–vis spectrum was recorded to estimate the concentration of CdSe@ZnS QDs based on an empirical equation by Peng and co-workers [46]. 2.5. Fluorescence quenching of the nanohybrid system by Hg2+ Briefly, 1 mL of the purified GDTC-QDs aqueous solution was firstly mixed with 2 mL of water and 10 ␮L of C-dots aqueous solution was then added to the mixture. For aqueous Hg2+ detection, 2 ␮L of a known concentration of Hg2+ aqueous solution was injected into the above probe solution containing GDTC-QDs and C-dots. The mixture was shaken thoroughly at room temperature prior to fluorescence measurement. The fluorescence spectra were recorded using a 365 nm excitation wavelength. All fluorescence measurements were performed at room temperature under ambient conditions. All the fluorescence intensities were an average of three independent measurements. 2.6. Instrumentation Infrared spectra of the dried TOPO-QDs, GDTC-QDs and C-dots dispersed in KBr pellets were recorded on a Thermo-Fisher Nicolet iS10 FTIR spectrometer. UV–vis absorption and fluorescence spectra were recorded at room temperature on a Shimadzu UV-2550 spectrometer and Perkin-Elmer LS-55 luminescence spectrometer. Photographs were taken with a Canon 350D digital camera. The TEM samples were prepared by dropping an aqueous solution containing GDTC-QDs or C-dots onto a holey carbon film of copper grids and air drying prior to the image collection, respectively. The sizes and morphologies of GDTC-QDs and C-dots were observed using a JEOL 2010 transmission electron microscope operating at an accelerating voltage of 200 kV. 3. Results and discussion 3.1. Characterization of the as-prepared GDTC-QDs It is reported that dithiocarbamate (DTC) ligands-modified QDs could be readily prepared by simply mixing them with an equimolar ratio of carbon disulfide and primary or secondary amines, and that the solubility of the QDs in various solvents could be spontaneously obtained depending on the intrinsic nature of the substituents borne by the amine group [42]. Herein, hydrophobic TOPO-capped CdSe@ZnS QDs with emission wavelength of 629 nm was used as the starting material to prepare carboxylmethyldithiocarbamate capped CdSe@ZnS QDs (GDTCQDs) through ligand exchange reaction by GDTC according to the method reported [42]. Fig. 1 illustrates the successful replacement of TOPO molecule on the surface of the QDs by GDTC. As can be clearly seen from the fluorescence images in Fig. 1b, the hydrophobic CdSe@ZnS QDs dispersed in cyclohexane were totally transferred to water, due to the presence of the carboxyl group

in the GDTC molecule. In addition, the FTIR spectra was also collected and the results showed that the strong vibration bands at 2925 cm−1 and 2852 cm−1 (asymmetric and symmetric stretch of methylene group) and the bands at 1462 cm−1 and 1061 cm−1 (deformation of methylene groups next to the phosphorus and P O stretching) from the TOPO-QDs [47] were nearly disappeared (Fig. S1a in the Supporting Information), indicating that most of the TOPO molecules were disassociated from the QDs surface during the ligand exchange procedure by GDTC. The appearance of a new bands at 1113 cm−1 and 1630 cm−1 (vibration of the dithiocarbamate moieties and C O stretching vibration) and the bands in the region of 3200–3550 cm−1 (stretching vibration of hydroxy group) further confirmed the existence of GDTC on the surface of the QDs [43,47]. The optical properties of the QDs were retained after the ligand exchange, which was shown by comparing the UV–vis absorption and PL spectra of the TOPO-QDs dispersed in chloroform and GDTC-QDs in water (Fig. 1b). It can be seen that the GDTC-QDs have similar UV–vis spectra, emission peaks and half width at half-maxima as that of TOPO-QDs, suggesting the ligand exchange process has no obvious effect on the fluorescence property of the QDs. It is reported that the decrease of the fluorescence quantum yields of the QDs following transfer into aqueous solution has been commonly observed. The relative fluorescence quantum yields measured for the as-prepared GDTC-QDs were about 50% as compared to the native TOPO-QDs (Fig. S2 in the Supporting Information), which was probably due to the formation of surface defects during the surface ligand exchange process. In addition, the TEM image illustrates that the hydrophilic GDTC-QDs after the phase transfer showed well dispersibility in water, further confirming the successful phase transfer of the QDs (Fig. S3 in the Supporting Information). The stability of the as-prepared GDTCQDs was also investigated. The result shows that the GDTC-QDs dispersed in water could be stored at least 3 months at 4 ◦ C in the dark (Fig. S4 in the Supporting Information).

3.2. Characterization of the as-prepared C-dots The water-soluble photoluminescent C-dots were prepared through a simple one-pot hydrothermal approach from inexpensive materials glycine. Fig. 2a shows the fluorescence property of the as-prepared C-dots. As can be seen in Fig. 2a, the as-prepared C-dots shows excitation-wavelength-dependent fluorescence emission spectra, and exhibits strong blue fluorescence at the excitation wavelength of 365 nm (inset in Fig. 2a). Fig. 2b shows the transmission electron microscopy (TEM) image of the as-prepared C-dots. As can be seen in Fig. 2b, the diameter of the as-prepared C-dots is less than 5 nm, which is consistent with the previous report [43]. The fluorescence quantum yield of the asprepared C-dots was measured to be 5% through a comparison with quinine sulfate [48–50]. In addition, the FTIR spectrum of the asprepared C-dots was also collected. The peaks at 3200–3550 cm−1 (O H stretching vibration) and 1610 cm−1 (C O stretching vibration) indicate the existence of carboxyl group in the surface of the C-dots. In addition, the peaks at 1510, 1560 and 1630 cm−1 (assigned to the amide II- and amide I stretching vibrations), and 1455 cm−1 (amide III C N stretch vibration) indicate the existence of amide group (Fig. S1b in the Supporting Information), which was further confirmed by acid–base titration method in the literature [43]. The as-prepared C-dots could be readily dispersed in polar solvents such as water, methanol and ethanol, due to the existence of carboxyl and amino groups on the surface of it. Moreover, the as-prepared C-dots exhibits excellent stability and could be stored at room temperature in water for more than one year (Fig. S5 in the Supporting Information).

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Fig. 1. (a) Schematic illustration of the preparation of GDTC-QDs using TOPO-QDs through surface ligands replacement. (b) UV–vis absorption and fluorescence spectra of the TOPO-QDs in chloroform and the GDTC-QDs in water. The inset images are their corresponding fluorescence images.

3.3. Sensitive response of the nanohybrid system of GDTC-QDs and C-dots to Hg2+ Inspired by the fact that most of the synthesized C-dots are chemical inertness in the presence of heavy metal ions and in harsh condition (strong acidic or alkali condition), and that the fluorescence of our prepared GDTC-QDs could be selectively quenched by Hg2+ due to its strong chelating ability to GDTC on the surface of the QDs [42], the nanohybrid system of those two fluorescence materials has been preliminarily investigated for Hg2+ detection by simply mixing the GDTC-QDs and C-dots aqueous solution. The nanohybrid system exhibited two well-resolved emission peaks at 436 nm and 629 nm under a single wavelength excitation (365 nm), which were emitted from the blue C-dots and red GDTC-QDs, respectively. Prior to Hg2+ detection, the interaction between GDTC-QDs and Cdots in the hybrid system was firstly investigated by monitoring the fluorescence intensity changes of those two fluorescence materials (Fig. 3). Similar to our recently reported work [28], both the fluorescence intensity of GDTC-QDs and C-dots gradually decreased upon the increasing addition of the other one (see Fig. 3), which could be

probably attributed to their competitive absorption of the excitation light. Moreover, the results that both of the absorption spectra of the GDTC-QDs and C-dots overlapped the 365 nm excitation wavelength further supported the possibility of the presumption (Fig. S6 in the Supporting Information). To obtain a ratiometric fluorescence probe with appropriate color ratio, a mixture solution containing GDTC-QDs (0.15 ␮M) and C-dots (0.05 mg mL−1 ) was prepared as the probe solution for the Hg2+ detection. Fig. 4 is the fluorescence response of the nanohybrid system to the presence of different metal ions. As can be seen in Fig. 4a, all of the metal ions tested had no effect on the fluorescence property of C-dots. However, like the sole system of GDTC-QDs, all of the metal ions tested had no effect on the fluorescence properties of the GDTC-QDs, except for Hg2+ , indicating the excellent selectivity of this nanohybrid system for Hg2+ detection. The selectivity observed for Hg2+ over other metal ions was remarkably high, and the fluorescence intensity of the GDTC-QDs in the nanohybrid system was quenched by about 80% in the presence of Hg2+ at the concentration of 1.8 ␮M (see Fig. 4b). In addition, the fluorescence of the GDTC-QDs in the nanohybrid system was quenched by about 94%

Fig. 2. (a) Fluorescence spectra of the C-dots prepared from glycine under varied excitation wavelengths. The inset is the fluorescence image of the as-prepared C-dots under the excitation of 365 nm. (b) TEM image of the as-prepared C-dots. Scale bar = 10 nm.

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Fig. 3. Fluorescence spectra of (a) the C-dots (0.05 mg mL−1 ) upon the addition of GDTC-QDs and (b) the GDTC-QDs (0.1 ␮M) upon the addition of C-dots. The PL spectra were recorded with excitation at 365 nm.

in two minutes after the addition of Hg2+ , and remained unchanged with further increasing of reaction time, while the fluctuation of the fluorescence intensity of the C-dots was small all the time, indicating that it was very fast to reach equilibrium for the interaction between Hg2+ and the nanohybrid system (Fig. S7 in the Supporting Information). The selectivity of this probe to Hg2+ detection could be ascribed to the fact that only Hg2+ could dissociate the GDTC molecules on the surface of the GDTC-QDs to form more stable mercuric dithiocarbarmate, which is the most stable among other transition metal-dithiocarbamate complexes [42]. In addition, the UV–vis spectra also indicated that GDTC-Cd and GDTC-Zn complexes could be broken in the presence of Hg2+ (Fig. S8 in the Supporting Information). To evaluate the sensitivity of the nanohybrid system to Hg2+ sensing, the fluorescence intensities of the assay was measured after the addition of various concentrations of Hg2+ (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0 ␮M). As can be seen from Fig. 5a, with the addition of Hg2+ to the nanohybrid system, the fluorescence intensity of the GDTC-QDs continuously decreased with the increasing of Hg2+ concentration, while there was no obvious change on the fluorescence intensity of the C-dots. Fig. 5b shows the plot of the fluorescence quenching percentage against the concentration of Hg2+ . As can be seen in Fig. 5b, at the Hg2+ concentration range of 0.2 ␮M to 2 ␮M, the fluorescence intensity of the GDTC-QDs was closed related to the amount of Hg2+ added to the nanohybrid system, which could be used for the quantification of Hg2+ with correlation coefficient of 0.9919. The results indicate that Hg2+ can be analyzed to a detection limit (3) of 0.1 ␮M.

3.4. Visual detection of Hg2+ by the nanohybrid system Fig. 6 shows the fluorescence response of the nanohybrid system to Hg2+ . As can be seen in Fig. 6a, the intensity of the red emission from the GDTC-QDs was gradually decreased by the addition of Hg2+ , whereas the intensity of the blue emission from the Cdots still remained constant. The changes in the intensity leaded to an obviously distinguishable fluorescence color from the origin background with the naked eye. The advantages of the ratiometric fluorescence probe for visual detection could be confirmed by the comparison with the single fluorescence quenching experiment of GDTC-QDs. As can be seen in Fig. 6b, unlike the ratiometric probe, the fluorescence images of the single red GDTC-QDs were hard to distinguish among the other images with the naked eye. The comparison clearly shows that the ratiometric fluorescence method was more sensitive and reliable for visual detection of Hg2+ than a single fluorescence quenching method, although the intensity of the red emission decreased at the same level. 3.5. Spike and recovery test Due to the selectivity of the nanohybrid system of GDTC-QDs and C-dots for Hg2+ detection, the practical application of the designed ratiometric fluorescence sensor was evaluated by determination of the recovery of the spiked Hg2+ in natural medial including tap water and real lake water from a local lake. All samples collected were simply filtered through 0.45 ␮M Supor filters and showed that no detectable Hg2+ was present based on the

Fig. 4. (a) Fluorescence responses of the nanohybrid system to different metal ions (2 ␮M for all the metal ions, excitation at 365 nm). (b) Selectivity of the nanohybrid system toward various metal ions. The black bars represent the addition of an excess of metal ions (0.1 mM for K+ , Na+ , Ca2+ and Mg2+ ; 1 ␮M for other cations). The red bars represent the subsequent addition of 1.8 ␮M Hg2+ to the hybrid solution. I436 and I629 are the fluorescence intensities at the wavelengths of 436 nm and 629 nm in the nanohybrid system solution, respectively.

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Fig. 5. (a) Fluorescence quenching of the nanohybrid system upon the addition of Hg2+ . (b) Plot of fluorescence quenching efficiency of the nanohybrid system solution as a function of the Hg2+ concentration.

Fig. 6. Fluorescence responses of the ratiometric probe solution (a) and the single GDTC-QDs solution (b) upon the addition of different amounts of Hg2+ . The images in the bottom are their corresponding fluorescence images.

Table 1 Recovery test of Hg2+ spiked in tap water and real lake water.a Spiked concentration (␮M)

0.6 1.2 1.8 a

Tap water

Lake water

Founded (␮M)

Recovery (%)

Founded (␮M)

Recovery (%)

0.56 1.27 1.78

93.3 ± 2.4 105.8 ± 1.7 98.8 ± 2.2

0.62 1.29 1.84

103.3 ± 3.1 107.5 ± 1.9 102.2 ± 3.5

Values shown were the calculated mean Hg2+ concentration for each sample and were determined from three replicates.

inductively coupled plasma-mass (ICPMS) method. The recovery study was carried out with three concentrations (0.6 ␮M, 1.2 ␮M and 1.8 ␮M) of spiked Hg2+ in each real sample. Each measurement was done in triplicate and the average was presented with standard deviation. The analytical result was showed in Table 1. As can be seen in Table 1, the results obtained in the two real samples show good recovery values, which suggested that the impurities in the real samples did not cause serious interferences to our sensor for Hg2+ detection, and that the proposed sensor was applicable for practical detection in real samples with other potentially competing species coexisting.

solution. Due to the strong chelating ability of GDTC on the surface of the QDs to Hg2+ , the fluorescence of the GDTC-QDs in the nanohybrid system could be selectively quenched in the presence of Hg2+ while the fluorescence of the C-dots remained constant, resulting in a continuous fluorescence color evolution from red to blue upon the increasing addition of Hg2+ . The detection limit of this method was found to be as low as 0.1 ␮M. This strategy reported herein shows a simple but effective way for the visual detection of aqueous Hg2+ and could be extended to the ratiometric detection of other metal ions through functionalizing the QDs with proper ligands. Acknowledgements

4. Conclusion A ratiometric fluorescence sensor for Hg2+ sensing has been developed by simply mixing C-dots and GDTC-QDs in aqueous

Thanks for the supports from National Key Technology R&D Program (2012BAJ24B02) and Natural Science Foundation of China (nos. 21277145, 21077108, 21275145).

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