Detection of Hg2+ by a Dual-Fluorescence Ratio Probe Constructed with Rare-Earth-Element-Doped Cadmium Telluride Quantum Dots and Fluorescent Carbon Dots.

Quantum dots (QDs) and carbon quantum dots (CDs) are classes of zero-dimensional materials whose sizes can be ≤10 nm. They exhibit excellent optical properties and are widely used to prepare fluorescent probes for qualitative and quantitative detection of test objects. In this article, we used cerium chloride as the cerium source and used the in situ doped cerium (rare-earth element) to develop cadmium telluride (CdTe) quantum dots following the aqueous phase method. CdTe: Ce quantum dots were successfully synthesized. The solution of CdTe:Ce QDs was mixed with the CD solution prepared following the green microwave method to form a ratio fluorescence sensor that can be potentially used for the selective detection of mercury ions (Hg2+). We used transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and other microscopy and spectral characterization techniques to validate that Ce had been successfully doped. The test results on the fluorescence performance revealed that Ce doping enhances the predoped fluorescence performance of the CdTe QDs. We have quantitatively detected Hg2+ using a ratiometric fluorescence sensor to show that in the range of 10-60 nM, the fluorescence quenching efficiency increases linearly with the increase in Hg2+ concentration. The linear correlation coefficient R 2 = 0.9978, and its detection limit was found to be 2.63 nM L-1. It was observed that other interfering ions do not significantly affect the fluorescence intensity of the probe. According to the results of the blank addition experiment, the developed proportional fluorescence probe can be used for the detection of Hg2+ in actual samples.

Introduction

Hg2+ is a stronpan>g carcinogen. Excessive exposure to this ionpan> canpan> cause adverse physiological reactionpan>s (such pan> class="Chemical">as vomiting, diarrhea, headache, and nausea). Prolonged exposure can lead to kidney failure, liver poisoning, and damage to the human central nervous system.[1−5] With the advancement of industrial technology, increasing amounts of Hg2+ are being used, which inevitably pollute water sources and soil.[6−9] Therefore, it is important to detect Hg2+ present in the environment.[10−14] At present, atomic absorption spectroscopy, atomic fluorescence spectrometry, and inductively coupled plasma mass spectrometry (ICP-MS) techniques are used to detect Hg2+.[15−20] Although these instruments are very accurate and sensitive, they are expensive and complex.[21] These factors limit their widespread use. Developing a detection method for Hg2+, which involves relatively low-cost and simple instruments, is a major research focus.[22−26]

Fluorescent probes are widely used for detectionpan> anpan>d anpan>alysis pan> class="Chemical">as they are simple, highly sensitive, and precise.[27−30] Quantum dots (QDs) are tiny semiconductor materials that are widely used as fluorescent probes as they exhibit excellent optical properties. However, the toxicity of the Cd element limits the application of Cd QDs as fluorescent probes. The emergence of doped QDs has provided a barrier around this issue.[31−35] Doped QDs have several advantages, such as high stability and the generation of the tunable emission spectrum.[36−38] Doping transition elements to CdTe QDs or rare-earth metal ions does not only reduce the Cd content but also effectively modifies the surface defects and enhances the fluorescent properties of QDs.[39−43] Doped QDs have a longer emission lifetime than undoped QDs.[44] Due to their high fluorescence stability and low toxicity, carbon quantum dots (CDs) have received extensive attention in the field of fluorescent probes.[45] CDs can be directly used as probes to interact with the target analyte or can be combined with other fluorescent probes to form a ratio fluorescent probe that can be used to detect the target analyte. The ratio fluorescent probe combines two fluorescent materials with different emission wavelengths. The ratio of the intensity of the two fluorescent materials is linearly related to the concentration of the target analyte. It is a detection method with strong anti-interference ability. An internal standard is established, and the ratio fluorescent probe exhibits a self-regulating function that reduces the interference from other factors to obtain more accurate data.

In this paper, CdTe QDs are pan> class="Chemical">doped with rare-earth Ce ions to form CdTe:Ce QDs, which reduce the content of Cd on the one hand and enhance the emission peak intensity of CdTe QDs on the other. The solution of CdTe:Ce QD and the solution of CD are mixed to produce the dual-emission ratio fluorescence sensor that can be used for the selective detection of Hg2+ in the solution. Dual-emission ratio fluorescent probes can effectively reduce the influence of excitation light and concentration changes and improve the ability of the probe to resist interference. We have thoroughly studied the interaction mechanism of CdTe:Ce QDs with Hg2+ and applied the probe to determine the amount of Hg2+ present in tap water.[46] QDs are widely used for heavy metal ion detection. However, the traditional Cd-based QDs exhibit a high Cd content, which exerts a negative impact on the environment. Traditional single-emitting QDs exhibit a limited ability to resist interference.[47]

CdTe: Characterization of Ce QDs

CdTe: Fluorescence Characterization of Ce QDs

Figure shows the fluorescence spectra recorded with CdTe QDs anpan>d pan> class="Chemical">CdTe:Ce QDs. At an excitation wavelength of 300 nm, the concentrations of CdTe QD and CdTe:Ce QD solutions are similar. The fluorescence emission peak intensity of CdTe:Ce QDs is significantly higher than that of the undoped compound. Ce doping minimizes the surface defects of CdTe QDs, fills the trap state, and enhances the radiation rate. The fluorescence intensity of CdTe:Ce QDs is observed to be higher than that of CdTe QDs. It also helps reduce the Cd content in the QDs, thereby reducing the quantum dot toxicity.

Figure 1

CdTe:Ce QD fluorescence spectrum and CdTe QD fluorescence spectra.

n class="Chemical">CdTe:Ce QD fluorescenpan>ce spectrum anpan>d pan> class="Chemical">CdTe QD fluorescence spectra.

Characterization of CdTe:Ce QDs Using X-ray Photoelectron Spectroscopy (XPS) Technique

We used the X-ray photoelectron spectroscopy technpan>ique to depan> class="Chemical">termine the elemental composition of CdTe:Ce QDs. Characteristic peaks corresponding to the rare-earth Ce were observed (Figure A). This proved the successful doping of Ce. Ce 3d was split into two peaks: Ce 3d3/2 (904.08 eV) and Ce 3d5/2 (886.08 eV). Te 3d was split into four peaks: Te 3d3/2 (586.68 eV), Te 3d3/2 (582.68 eV), Te 3d5/2 (576.08 eV), and Te 3d5/2 (572.28 eV). Cd 3d was split into two peaks: Cd 3d3/2 (411.78 eV) and Cd 3d5/2 (404.98 eV). Ce doping was proved when the combined Te and Ce energy was analyzed. The results indicate that simple mixing or coating does not occur.

Figure 2

XPS survey spectra of CdTe:Ce QDs (A), Ce 3d (B), Te 3d (C), and Cd 3d (D).

XPS survey spectra of CdTe:pan> class="Chemical">Ce QDs (A), Ce 3d (B), Te 3d (C), and Cd 3d (D).

X-ray Diffraction (XRD) Patterns Recorded for CdTe:Ce QDs

Figure shows that the CdTe:pan> class="Chemical">Ce QDs exhibit three major diffraction peaks. The diffraction angles were recorded to be 24.56, 40.04, and 47.46°, which corresponded to the (111), (220), and (311) crystal planes, respectively. Thus, the successful synthesis of CdTe:Ce QDs was proved. The XRD patterns of CdTe:Ce QDs and CdTe QDs were compared, and it was observed that the doping of Ce elements did not hamper the structure of the CdTe QDs.

Figure 3

XRD patterns of CdTe:Ce QDs.

XRD patternpan>s of pan> class="Chemical">CdTe:Ce QDs.

Characterization of the Samples Using Transmission Electron Microscopy (TEM) Technique

Figure shows the TEM images of pan> class="Chemical">CdTe:Ce QDs. The images revealed that the particle sizes of the CdTe:Ce QDs were approximately 4 nm. It can be seen from the figure that the samples exhibited good dispersion properties and were uniform in size.

Figure 4

TEM images of the CdTe:Ce QDs.

n class="Chemical">TEM images of the n class="Chemical">CdTe:Ce QDs.

Characterization of CDs

Fluorescence Characterization of CDs

Figure shows the fluorescence spectrum of CDs. Under the excitationpan> wavelenpan>gth of 300 nm, the emissionpan> peak of the fluorescenpan>ce spectrum of pan> class="Chemical">CDs was located at 445 nm.

Figure 5

CDs fluorescence spectrum.

n class="Chemical">CDs fluorescenpan>ce spectrum.

X-ray Photoelectron Spectroscopy (XPS) Technique for the Characterization of CDs

We used the XPS technpan>ique to anpan>alyze the elemental compositionpan> of the pan> class="Chemical">CDs. Figure reveals that the CDs are composed of three elements: C, N, and O. Figure B reveals that the high-resolution XPS spectrum of C 1s consists of three peaks, namely, C=C (284.6 eV), CN (285.4 eV), and C=O (289.0 eV).[48]Figure C reveals three fitting peaks in the N 1s high-resolution XPS spectrum. This indicates that they are primarily present in the form of CNC (398.3 eV), CN (399.7 eV), and NH (400.1 eV).[49] The high-resolution energy spectrum of O 1s reveals that there are C–OH (531.2 eV) and C=O (533.2 eV) bonds in the CDs (Figure D).[50]

Figure 6

XPS survey spectra of CDs (A), C (B), N 2p (C), and O (D).

XPS survey spectra of CDs (A), C (B), pan> class="Chemical">N 2p (C), and O (D).

Characterization of C QDs and CdTe/C QDs Using the Hybrid Transmission Electron Microscopy Technique

Figure shows the TEM images of the pan> class="Chemical">CDs and CdTe/C QDs. It could be seen from the figure that the particle size of CDs was approximately 5 nm. When CdTe/C QDs and CDs were mixed, the morphology was not hampered and uniform dispersion was achieved.

Figure 7

TEM images of the CdTe/C QDs and CDs.

TEM images of the pan> class="Chemical">CdTe/C QDs and CDs.

Results and Discussion

Effect of Hg2+ on the Ratio Fluorescent Probe Composed of CdTe:Ce QDs and CDs

We found that Hg2+ could significanpan>tly quench the fluorescence inpan> class="Chemical">tensity of the CdTe:Ce QDs (quenching of the peak intensity at 599 nm; Scheme ). This ion exerted little effect on the fluorescence peak at 445 nm corresponding to the CDs. According to the Fajans rule, ions that can form insoluble or insoluble substances with the ions that make up the crystal are preferentially adsorbed. The solubility product of HgS was significantly smaller than that of CdS, indicating that CdTe:Ce preferentially adsorbs Hg2+. A part of CdS was converted to HgS, which caused MPA to change from CdTe:Ce quantum. The dot surface falls off under these conditions, resulting in a decrease in the fluorescence intensity. Thus, fluorescence quenching was observed. The ion did not exert a significant effect on the fluorescence intensity of the carbon dots. The relationship between the fluorescence quenching efficiency of the ratio fluorescent probe composed of the CdTe:Ce QDs and CDs and the concentration of Hg2+ was studied. A ratio fluorescence sensor was proposed for the detection of Hg2+. Other interfering ions did not significantly impact the fluorescence intensity of the fluorescent probe sensor composed of CdTe:Ce QDs and CDs. This indicated that the sensor was selective toward Hg2+.

Scheme 1

Principle Scheme of the Developed Hg2+ Detection Strategy

A ratio fluorescence probe sensor composed of CdTe:pan> class="Chemical">Ce QDs and CDs was used to quantitatively detect Hg2+. The experimental conditions were optimized (including the pH value of the ratio fluorescence probe sensor solution and the ratio fluorescence probe). Conditions such as the reaction time of the needle sensor and Hg2+ were also optimized. The method is simple and has a strong anti-interference ability. This method can be popularized.

Effect of Ce Doping on the Synthesis of CdTe:Ce QDs

The introduction of the doped rare-earth element Ce usually does not chanpan>ge the specific lattice structure of the bulk quanpan>tum dots. However, inpan> class="Chemical">terference effect on the electronic energy level of the bulk was observed, and new electronic energy levels were generated. The characteristics of the quantum dots were improved. It was observed that the maximum fluorescence intensity of the quantum dots was achieved under conditions of n(Cd):n(Ce) = 1:0.2 (Figure ). The ratio of Cd to Ce was selected to be 1:0.2 for the synthesis of CdTe:Ce.

Figure 8

Selection of rare-earth element Ce doping content.

Selection of rare-earth element Ce doping conn class="Chemical">tent.

Effect of pH on the Ratio of CdTe:Ce QDs and CDs to Fluorescent Probes

Different pH buffer solutions exerted different effects onpan> the fluorescence inpan> class="Chemical">tensity of the ratio fluorescent probe sensor. Figure depicts the effect of pH (pH = 5–9) on the CdTe:Ce QDs and CDs in the presence and absence of Hg2+. The effect of ratio fluorescence probe fluorescence. It can be seen from the figure that the ratio fluorescent probe composed of the same concentration of CdTe:Ce QDs and CDs exhibit the maximum fluorescence intensity when the pH is 7. Significant fluorescence quenching was observed when Hg2+ was added. Therefore, Hg2+ was added at pH 7 for conducting the tests.

Figure 9

Fluorescence responses of the ratio fluorescence sensor in the absence and presence of Hg2+ at different pH values.

Fluorescence responses of the ratio fluorescence sensor in the absence and presence of n class="Species">Hg2+ at differenpan>t pH values.

Determination of the Reaction Time for the Reaction between Hg2+ and the Ratio Fluorescent Probe System

The effect of reaction time on the ratio of fluorescence inn class="Chemical">tensity of fluorescenpan>t probes wpan> class="Chemical">as studied at room temperature, and the results are shown in Figure . Experimental results show that the ratio of fluorescence intensity to the fluorescent probe decreases rapidly in the presence of Hg2+ and reached equilibrium after 5 min. The ratio of fluorescence intensity can be stable for 30 min. Therefore, the reaction time of this experiment was selected as 5 min.

Figure 10

Determination of the reaction time of Hg2+ and ratio fluorescent probe system.

Den class="Chemical">terminpan>ationpan> of the reactionpan> time of pan> class="Species">Hg2+ and ratio fluorescent probe system.

CdTe: The Ratio of Ce QDs to CDs Constitutes the Anti-interference Ability of the Fluorescent Probe Sensors

The fluorescence intensity of the probe wpan> class="Chemical">as significantly reduced in the presence of 60 nM of Hg2+. The concentrations of the other interfering ions at 60 nM caused a very slight change in the fluorescence intensity ratio (Figure ). This result indicated that the fluorescent probe (ratio) was selective toward Hg2+. The effect of fluorescence quenching of the ratio fluorescent probe under the coexistence of some interfering ions and Hg2+ was further studied. As shown in the figure, even if Hg2+ coexisted with interfering ions, the ratio fluorescent probe could still selectively detect Hg2+.

Figure 11

Ratio fluorescence sensor selectively detects Hg2+.

Ratio fluorescence sensor selectively den class="Chemical">tects pan> class="Species">Hg2+.

CdTe:Ce QDs and CDs to Detect Hg2+ by Ratio Fluorescent Probe

We observed that the fluorescence intensity of the pan> class="Chemical">CdTe:Ce QDs gradually decreased under conditions of varying Hg2+ concentrations. As shown in Figure , the fluorescence quenching efficiency (F4450/F5990–F445/F599) linearly increased with the Hg2+ concentration in the range of 10–60 nM. The linear equation used was F4450/F5990–F445/F599 = 0.0292C-0.0515, and the correlation coefficient R2 was 0.9978. F4450/F5990 represents the ratio of the fluorescence intensity in the absence of Hg2+. F445/F599 represents the ratio of fluorescence intensity in the presence of Hg2+. The detection limit (LOD = 3σ K–1) was 2.63 nM L–1. The present detection method was compared with the Hg2+ detection methods developed using the state-of-the-art research technique. Our dual-emission ratio fluorescence sensor composed of CdTe:Ce QDs, CDs can detect low levels of Hg2+ and can reduce the toxicity of CdTe QDs to a certain extent. The sensor can reduce the influence of other interference factors, thereby improving the accuracy of the probe.

Figure 12

(A) Fluorescence spectra of dual-emitting CdTe:Ce QDs as a probe for the detection of Hg2+ (10–60 nM). (B) Best-fitting linear relationship for Hg2+ detection (in the range of 10–60 n mol L–1).

(A) Fluorescence spectra of dual-emitting CdTe:pan> class="Chemical">Ce QDs as a probe for the detection of Hg2+ (10–60 nM). (B) Best-fitting linear relationship for Hg2+ detection (in the range of 10–60 n mol L–1).

Blank Spike Recovery Experiment

A ratio fluorescent probe composed of CdTe:pan> class="Chemical">Ce QDs and CDs was used to detect the Hg2+ levels in water (Table ). The results are shown in the table. The sample recovery rate was 91–114%, and the relative standard deviation (RSD) was less than 10%. This indicates that the accuracy and precision of the established method for the detection of Hg2+ in an actual sample.

Table 1

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

sample	spiked (nM)	found (nM)	recovery (%)	RSD (%)
purified water	10.0	11.4	114	5.2
	20.0	18.2	91	6.4
	30.0	32.4	108	3.5
	40.0	42.6	106	7.1
	50.0	47.7	95	8.3
	60.0	57.9	97	4.6

Selective Detection of Hg2+ Using Fluorescent Probe Sensors: Use of CdTe:Ce QDs and CDs

It can be hypothesized that n class="Species">Hg2+ destroys the pan> class="Chemical">Cd–S bond formed by Cd and 3-MPA and quenches the fluorescence of CdTe:Ce QDs.[51−54] As the formation of metal sulfides is determined by their Ksp value, the Ksp value can potentially be a key factor to determine the extent of fluorescence quenching (of quantum dots) in the presence of metal ions.[55−59] The Ksp value of the Hg–S bond (6.3 × 10–36) was much smaller than the Ksp value of Cd–S (8.0 × 10–27).[60] When Hg2+ was added to the vector dot solution, Hg2+ combined with S in MPA and replaced Cd. Thus, fluorescence fragmentation could be achieved.

Conclusions

In short, quantum dots doped with pan> class="Chemical">CdTe:Ce quantum dots were synthesized following the hydrothermal method. They were mixed with carbon dots synthesized following the microwave method to form a ratio fluorescent probe for the quantitative determination of Hg2+. The Hg2+ can effectively quench the fluorescence of CdTe:Ce quantum dots without significantly affecting the fluorescence intensity of the carbon dots. A dual-emission ratio fluorescence sensor with CdTe:Ce quantum dots and carbon dots was developed to quantitatively detect Hg2+. When Ce quantum dots were mixed with CDs, fluorescent probes exhibiting strong anti-interference properties were produced. This ratio fluorescent probe exhibited a good fluorescence response to low concentrations of Hg2+ and can be used for sensitive and selective detection of low concentrations of Hg2+. Several detection conditions have been optimized. Selectivity and interference experiments were carried out to explore the selectivity of the fluorescent probes. The results showed that the selectivity of the probe was very good. The linear range of detection was 10–60 nM, and the LOD was 2.63 nM L–1. The mixed ratio fluorescent probe can be synthesized following a simple method that is convenient to operate. The probe can be used for the detection of Hg2+ in actual water samples. Finally, the mechanism of fluorescence fracture has been discussed.

Experimental Section

Experimental Materials

The chemicals used were Te (anpan>alytically pure), pan> class="Chemical">NaBH4 (analytically pure), CdCl2 (analytically pure), C3H6O2S (analytically pure), CeCl3 (analytically pure), NaOH (analytically pure), C3H6O2 (analytically pure), CH4N2O (analytically pure), CH3CH2OH, HgSO4, Cd(CH3COOH)2 (analytically pure), FeCl3 (analytically pure), Ca(NO3)2 (analytically pure), Pb(NO3)2 (analytically pure), CoCl2 (analytically pure), A1Cl3, Zn(NO3)2·6H2O, CuSO4·5H2O (analytically pure), MnCl2 (analytically pure), Na2HPO4 (analytically pure), NaSO4 (analytically pure), NaNO3 (analytically pure), KCl (analytically pure), Na2CO3 (analytically pure), and CH3COONa (analytically pure).

Experimental Instrument

The instruments used were fluorescence spectrometer (Shimadzu), X-ray photoelectronpan> spectrum anpan>alyzer (Shimadzu), X-ray diffractomepan> class="Chemical">ter (malvernpanalytical), and transmission electron microscope (FEI).

Preparation of NaHTe Precursor

Tellurium powder (0.06 g) anpan>d 10 mL of ultrapure pan> class="Chemical">water were added to a three-necked flask. After deoxygenating with nitrogen, 0.04 g of NaBH4 was added and the mixture stirred and heated at 80 °C for 30 min to obtain the NaHTe precursor.

Preparation of CdTe:Ce QDs

In a 250 mL three-necked flask, 0.55 g of pan> class="Chemical">CdCl2 was dissolved in 100 mL of deionized water, N2 was passed to remove oxygen, 0.08 g of mercaptopropionic acid was added, and 1.0 mol L–1 NaOH solution was used to adjust the pH to 11.8 to obtain the Cd precursor. The newly prepared NaHTe aqueous solution was added to the CdCl2 solution under the protection of N2, and the color of the solution became orange and then transparent. The prepared mixture was heated under the reflux, and 10 mL of CeCl3 solution of different concentrations was added to synthesize CdTe:Ce QDs doped with different concentrations of the rare-earth element Ce.

Preparation of Carbon Dots

The citric acid (1.5 g) and 1.5 g of n class="Chemical">urea were weighed anpan>d 5 mL of ultrapure pan> class="Chemical">water was added for ultrasonic dissolution; the mixture was heated by microwave heating for 10 min at a power of 1 kW and then for 10 min; 50 mL of ultrapure water was added, dissolved again by ultrasonication, centrifuged at high speed, and the supernatant taken. The solution was diluted to obtain the required cyan carbon dot solutions (CDs).

Preparation of Mixed CdTe QDs and CDs

CdTe QDs solutionpan> (9.7 mL) wpan> class="Chemical">as mixed with 0.3 mL of carbon dot solution to obtain mixed CdTe QDs and CDs.

Detection of Hg2+

We prepared a CdTe:Ce QD suspensionpan> with a conpan>centrationpan> of 0.8 g L–1 in the pan> class="Chemical">PBS buffer with pH 7, added the same volume of CdTe:Ce QDs solution (9.8 mL) to the test tube, and then added it to each test tube. We added 100 μL of Hg2+ solution of different concentrations to each test tube. The reaction was mixed at room temperature for 5 min, and the fluorescence emission spectrum was recorded at an excitation wavelength of 300 nm.

Interference Experiment

In the n class="Chemical">water phpan> class="Chemical">ase, we added interfering ions with a concentration of Hg2+ 10 times to the ratio fluorescent probe system and measured the fluorescence.

Detection of Hg2+ in Actual Samples

The actual sample comes from tap n class="Chemical">water inpan> the laboratory, anpan>d the specific depan> class="Chemical">termination method is the same as the operation step 1 in Section .