Double-Emission Ratiometric Fluorescent Sensors Composed of Rare-Earth-Doped ZnS Quantum Dots for Hg2+ Detection.

Quantum dots (QDs) are a class of zero-dimensional nanocrystal materials, whose lengths are limited to 2-10 nm. Their unique advantages such as wide excitation spectra, narrow emission spectra, and high quantum yield make their application possible in fluorescence sensing, wherein QDs such as CdSe, CdTe, and CdS are used. Indeed, QDs have a wide range of applications in fluorescence sensing, and there have been many reports of applications based on QDs as ion probes. The emission spectra of QDs can be adjusted by changing the size of the QDs or doping them with other ions/elements. However, the high toxicity of Cd and the poor anti-interference ability of single-emission fluorescent probes greatly limit the applications of QDs in many fields. In this paper, ZnS QDs are doped with the rare-earth element Ce to form a low-toxicity double-emission ratiometric fluorescent sensor, ZnS:Ce, for Hg2+ detection. The results of transmission electron microscopy (TEM), X-ray diffractometry, X-ray photoelectron spectroscopy, and optical spectroscopy show that ZnS:Ce QDs were successfully synthesized. Under the optimal conditions, the concentration of Hg2+ was in the range of 10-100 μM, which had a linear relationship with the fluorescence intensity of the ZnS:Ce QDs: the linear correlation coefficient was 0.998, and the detection limit was 0.82 μM L-1. In addition, the fluorescent sensor had good selectivity for Hg2+, and it was successfully applied to the detection of Hg2+ in laboratory water samples.

Introduction

Hg2+ is a strong carcinogen. Long-term exposure to Hg2+ can damage the urinary system, nervous system, reproductive system, and immune system of humans, thus leading to cancer, biological malformation, and gene mutations.[1] At present, there are many methods used to identify Hg2+, such as atomic absorption spectrometry,[2] inductively coupled plasma mass spectrometry,[3] electrochemical methods,[4] and ion chromatography.[5] The above methods have certain advantages in terms of their detection sensitivity and selectivity, but they are complicated to conduct, incur high costs, require large-scale instruments, and take a long time to yield detection. Therefore, it is of practical significance to develop a simple and efficient method to determine Hg2+ in water.

Quantum dots (QDs) are zero-dimensional semiconductor nanomaterials, and their particle sizes are smaller than, or close to, the Bohr radius of self-trapped excitons in three spatial dimensions.[6] QDs are ideal fluorescent materials with wide excitation spectra, narrow and symmetrical emission spectra, high quantum yields, and stable optical properties.[7,8] Based on these unique properties, QDs are widely used as fluorescent probes in the field of fluorescence sensing.[9,10]

The fluorescence emission spectra of QDs can be adjusted by changing the size of the QDs and by introducing doped ions, which can be used to prepare QDs for detecting specific targets.[11,12] Up to now, there have been many reports that Cu2+, Pb2+, Hg2+, and other heavy metal ions have been detected using the fluorescence characteristics of Cd QDs.[13−18] However, the fluorescence spectra of fluorescent probes constructed by a single fluorophore are easily affected by changes and impurities in the environment, which thus generate systematic errors and affect the detection results. Moreover, Cd-based QDs are mostly synthesized in organic phases, and because Cd is a heavy metal, its usage causes pollution.[19,20] Doped QDs can replace traditional Cd-containing QDs.[21−24] On the one hand, doped QDs have low toxicity, and on the other hand, doped QDs have double fluorescence emission and can form ratio-type fluorescent probes.[25−30]

A ratiometric fluorescent probe can obtain two different emission wavelengths under single wavelength excitation, and it can detect an object based on the ratio of the fluorescence intensities of two independent emission peaks.[31−36] The internal standard emission wavelength of a ratiometric fluorescent probe endows fluorescent probes with anti-interference capabilities, effectively weakening the influence caused by changes in the excitation light, environment, and probe concentration and improving the accuracy of fluorescence detection.[37−42] Therefore, in this work, rare-earth Ce ions are doped on the basis of low-toxicity ZnS QDs to form a double-emission ratiometric fluorescent sensor, ZnS:Ce, which is used as a fluorescent probe to detect Hg2+ in a pH = 7 phosphate buffer solution. The interaction mechanism between ZnS:Ce QDs and Hg2+ is subsequently discussed. The probe is then successfully applied to the determination of trace Hg2+ in laboratory tap water.

Characterization of ZnS:Ce QDs

Fluorescence Characterization of ZnS:Ce QDs

First, ZnS:Ce QDs were tested for fluorescence performance. Figure shows the fluorescence spectrum of ZnS:Ce QDs. At an excitation wavelength of 230 nm, it can be seen that the fluorescence spectrum of the ZnS:Ce QDs has two obvious emission peaks located at 459 and 689 nm, wherein the emission peak at 459 nm belongs to Zn2+ defect luminescence of deep electron wells and the emission peak at 689 nm belongs to doped Ce3+ luminescence.

Figure 1

Fluorescence spectrum of ZnS:Ce QDs.

X-ray Photoelectron Spectroscopy (XPS) Characterization of ZnS:Ce QDs

X-ray photoelectron spectroscopy experiments were carried out on ZnS:Ce QDs. As shown in Figure , Zn 2p has split into two peaks, Zn 2p3/2 (1021.58 eV) and Zn 2p1/2 (1044.68 eV); Ce 3d has divided into two peaks, Ce 3d3/2 (917.08 eV) and Ce 3d5/2 (882.48 eV); the valence of S is S2– (S 2p3/2, 161.28 eV). Based on the valence states of Zn, Ce, and S elements, the successful synthesis of ZnS:Ce QDs is proved.

Figure 2

XPS survey spectra of ZnS:Ce QDs for ZnS:Ce (A), Ce 3d (B), Zn 2p (C), and S 2p (D).

X-ray Diffraction Characterization of ZnS:Ce QDs

Figure shows the X-ray diffraction pattern of ZnS:Ce QDs. It can be seen from the figure that the diffraction angles of the XRD peaks of the ZnS:Ce QDs prepared in this experiment are 28.68, 47.84, and 56.56°, which are, respectively, consistent with the (111), (220), and (311) crystal planes of ZnS, which has a cubic sphalerite structure. This proved the successful synthesis of ZnS:Ce QDs.

Figure 3

X-ray diffraction (XRD) pattern of ZnS:Ce QDs.

TEM Characterization of ZnS:Ce QDs

The morphology of the synthesized ZnS:Ce QDs was characterized by transmission electron microscopy. As shown in Figure , the particle size of the composite is about 3.5 nm, and some quantum dots are polymerized.

Figure 4

Transmission electron microscopy (TEM) (inset shows high-resolution) images of ZnS:Ce QDs.

Results and Discussion

Effect of Hg2+ on the Fluorescence Intensity of ZnS:Ce QDs

In this study, it is found that Hg2+ can quench the fluorescence peak generated by Zn2+ defects in a deep electron trap at 459 nm and the fluorescence peak generated by the doped Ce at 689 nm in ZnS:Ce QDs as illustrated in Scheme . Therefore, a ratiometric fluorescent sensor is proposed to detect Hg2+ ions according to the change in the intensity ratio of the double-emission fluorescence peaks. The ratiometric fluorescent sensor was constructed based on measuring the ratio of fluorescence intensities of two independent emission peaks, which can effectively reduce the influence caused by changes in the excitation light, environment, and probe concentration and improve the accuracy of Hg2+ detection. Other metal ions (Cd2+, Pb2+, Fe3+, Co3+, Na+, Mn2+, Zn2+, K+, Al3+, Cu2+, and Mg2+) did not significantly affect the intensity of the double-emission fluorescence peaks of the sensor, indicating that the sensor has excellent selectivity for Hg2+.

Scheme 1

Principle Scheme of the Developed Hg2+ Detection Strategy

Therefore, in this experiment, the concentration of Hg2+ is detected by a ratiometric fluorescent sensor composed of ZnS:Ce QDs. The sensor is used for the quantitative detection of Hg2+, and experimental conditions such as the pH value of the solution and the reaction time with Hg2+ were optimized. The method is simple to operate and has a strong anti-interference capability.

Effect of pH on the Ratiometric Fluorescent Sensor

Different pH solutions will affect the sensitivity and selectivity of detection substances. Figure depicts the effect of a range of pH values, from 5 to 9, on the fluorescence intensity of ZnS:Ce QDs F459/F659 in the presence and absence of Hg2+. It can be seen from the figure that the fluorescence intensity ratio of the ZnS:Ce QDs with the same concentration is the strongest at pH = 7, indicating that fluorescence quenching is most obvious when Hg2+ is added. Therefore, is it suggested that a pH value of 7 should be used for Hg2+ detection.

Figure 5

Fluorescence responses of ZnS:Ce in the absence and presence of Hg2+ at different pH values.

Determination of Reaction Time between the Ratiometric Fluorescent Sensor and Hg2+

The effect of the reaction time on the fluorescence intensity was studied at room temperature, and the results are shown in Figure . The experimental results show that the fluorescence intensity of the ZnS:Ce QDs rapidly quenched in the presence of Hg2+ and reached equilibrium within 10 min, whereby the fluorescence signal stabilized for at least another 20 min. Therefore, the experiment was carried out after 10 min.

Figure 6

Fluorescence responses of ZnS in the absence and presence of Hg2+ at different incubation times.

Anti-Interference Ability of the Ratiometric Fluorescent Sensor

We investigated the effects of other interfering ions on ZnS:Ce QDs. In the presence and absence of Hg2+, an interfering substance with a concentration of 100 μM was added to ZnS:Ce QDs. Figure shows that in the presence of Hg2+ at a concentration of 100 μM, the fluorescence of ZnS:Ce QDs is significantly broken, while other interfering ions have little effect on the fluorescence of ZnS:Ce QDs in the absence of Hg2+. This fully illustrates the selectivity of this method for Hg2+.

Figure 7

Selective detection of Hg2+ by a ZnS:Ce ratiometric fluorescent sensor.

Detection of Hg2+ Using the Sensor

As shown in Figure , the experimental results demonstrate that the fluorescence intensity of the Zn:Ce QDs at 459 and 689 nm gradually decreased in the presence of Hg2+ with different concentrations. The fluorescence quenching efficiency (F4590/F6890–F459/F689) had a linear relationship with Hg2+ concentration in the range 10–100 μM. The best-fitting linear equation is F4590/F6890–F459/F689 = 0.0194C – 0.0024, with a correlation coefficient of R2 = 0.998. F4590/F6890 represents the fluorescence intensity ratio at 459 nm without Hg2+, and F459/F689 represents the fluorescence intensity ratio at 689 nm in the presence of Hg2+. The detection limit (limit of detection (LOD) = 3σ/K) is 0.82 μM L–1.

Figure 8

(A) Fluorescence spectra of dual-emitting ZnS:Ce QDs as a probe for the detection of Hg2+ (10–100 μM). (B) Best-fitting linear relationship for Hg2+ detection (in the range 10–100 μmol L–1).

Application to Actual Water Samples

To demonstrate the ability of our proposed ratiometric fluorescent probe, it was used to detect Hg2+ in laboratory tap water. The results are shown in Table . The recovery rate after the standard addition was 96–116%, and the relative standard deviation was less than 10%, which indicates the accuracy and precision of our method for determining Hg2+ concentrations in actual samples.

Table 1

Determination of Hg2+ in Real Samples (n = 3)

sample	spiked (nM)	found (nM)	recovery (%)	RSD (%)
tap water	20.0	23.2	116	4.5
	40.0	43.6	109	6.2
	60.0	62.4	104	8.3
	80.0	76.8	96	4.8
purifified water	20.0	21.9	110	3.7
	40.0	42.3	106	5.4
	60.0	61.7	103	6.6
	80.0	75.4	94	4.9

Mechanism of ZnS:Ce QDs for Selective Detection of Hg2+

The interaction mechanism between ZnS:Ce QDs and Hg2+ is discussed here. As shown in Figure A, Hg2+ has almost no ultraviolet absorption, so fluorescence energy resonance transfer is not the cause of fluorescence quenching of the ZnS:Ce QDs. As shown in Figure B, by monitoring the changing double-emission fluorescence peak intensity of the ZnS:Ce QDs solution, it is found that the fluorescence peak intensity at 459 nm obviously reduced, and the fluorescence peak intensity at 689 nm slightly reduced, after adding Hg2+.

Figure 9

(A) (a) Ultraviolet–visible (UV–vis) absorption spectra of Ce:ZnS and (b) UV–vis absorption spectra of Hg2+. (B) (a) Fluorescence emission spectrum of Ce:ZnS QDs and (b) fluorescence emission spectrum of Ce:ZnS QDs solution after adding 100 μM Hg2+.

According to Fajans’ rule, ions that can form insoluble or insoluble substances with ions that make up a crystal are preferentially adsorbed.[43−50] On the other hand, the solubility product of HgS is much smaller than that of ZnS, which indicates that the ZnS:Ce QDs preferentially adsorbed Hg2+, and some ZnS was converted into HgS, resulting in the aggregation and precipitation of the ZnS:Ce QDs and the change of surface structure, thus causing fluorescence quenching. The decrease of fluorescence intensity had a quantitative relationship with the concentration of Hg2+. Therefore, it can be predicted that the ratio change of the double-emission fluorescence peak intensities of the system has a quantitative relationship with the concentration of Hg2+.

Comparison with Other Sensors that Detect Hg2+ Ions

Compared with the Hg2+ detection methods used in other studies, as shown in Table , the prepared ZnS:Ce double-emission ratiometric fluorescent sensor not only has low toxicity but also can reduce the effects of interference caused by excitation light, the environment, and probe concentration changes, and it has improved the detection accuracy relative to other methods.

Table 2

Comparison of the Reported Methods for Hg Detection Using QDs

QDs	modification method	biological toxicity	whether ratiometric fluorescent probe	references
ZnS	doped Ce	iow toxicity	dual emission	this work
CdTe	bovine serum albumin (BSA)	highly toxic	single emission	(51)
CdTe	N-acetyl-l-cysteine	highly toxic	single emission	(52)
CdTe	Au nanoclusterâ, bovine serum albumin (BSA)	highly toxic	single emission	(53)
CdTe	2-mercaptoethanesulfonate (MES)	highly toxic	single emission	(54)
CdTe	thioglycolic acid	highly toxic	single emission	(55)
CdTe	cysteamine	highly toxic	single emission	(56)
CdTe/CdS	unmodified	highly toxic	dual emission	(57)

Conclusions

ZnS:Ce-doped QDs were synthesized using a hydrothermal method, which were then utilized as ratiometric fluorescent probes for the quantitative determination of Hg2+. The ZnS:Ce-doped QDs are not only less toxic than other QDs but also have the advantages of being very suitable ratiometric fluorescent probes. The proposed method has good stability and dispersibility in aqueous solutions (Cd2+, Pb2+, Fe3+, Co3+, Na+, Mn2+, Zn2+, K+, Al3+, Cu2+, and Mg2+), and they can realize the selective detection of Hg2+. Their detection sensitivity is high in the linear detection range of 10–100 μM, and the LOD was 0.82 μM L–1. The synthesis method of the probe is simple and cheap, and it can realize the detection of trace amounts of Hg2+ in actual water samples.

Experimental Section

Materials

(CH3COO)2Zn·2H2O, Na2S·9H2O, CeCl3, NaOH, CH3CH2OH, Ca(NO3)2, Cd(CH3COOH)2, FeCl3, Pb(NO3)2, CoCl2, NaCl, MnCl2, ZnCl2, KCl, A1Cl3, HgSO4, and Zn(NO3)2·6H2O were used.

Instruments

A fluorescence spectrometer, a UV spectrophotometer, an X-ray photoelectron spectroscopy analyzer, an X-ray diffractometer, a transmission electron microscope, and a scanning electron microscope were used.

Preparation of ZnS:Ce QDs

ZnS:Ce QDs were prepared according to the synthetic methods reported in the literature.[21−24] First, 2.201 g of zinc acetate was added to 20 mL of ultrapure water, which was stirred until the zinc acetate was completely dissolved. Then, 0.4732 g of cerium trichloride was added and stirred until the solution became clear and transparent. Next, the pH was adjusted to 10 with sodium hydroxide and the process of removing air with nitrogen was repeated three times. Under the protection of nitrogen, 20 mL of a solution containing 2.883 g of sodium sulfide nonahydrate was added dropwise and stirred for 20 min. The mixture was then stirred at 60 °C for 6 h. The obtained ZnS:Ce QDs were purified three times by anhydrous ethanol precipitation to remove impurities and unreacted precursors, and then the pure ZnS:Ce QDs were dried under vacuum at 60 °C for 12 h. After drying, they were ground into powder. Finally, the obtained ZnS:Ce QDs were re-dispersed in ultrapure water for further analysis and testing.

Hg2+ Detection

ZnS:Ce QDs with a concentration of 10 mg mL–1 were prepared with phosphate-buffered saline at pH = 7, and the same volume of ZnS:Ce QDs solution (0.9 mL) was added to a test tube and then added to additional test tubes containing 100 μL of different concentrations (10–100 μM) of Hg2+ solution. The reaction was mixed at room temperature for 15 min, and the fluorescence emission spectra were recorded at an excitation wavelength of 230 nm. The fluorescence quenching efficiency was calculated using the formula F4590/F6880–F459/F688, where F4590/F6880 represents the ratio of fluorescence intensity at 459 nm without Hg2+, and F459/F688 represents the ratio of fluorescence intensity at 688 nm in the presence of Hg2+.

Detection of Hg2+ in Actual Samples

The actual samples were laboratory tap water and ultrapure water. The specific measurement method is the same as that used in Section 4.4.