Ultrasensitive determination of lead and chromium contamination in well and dam water based on fluorescence quenching of CdS quantum dots

Abstract

We report lead (Pb²⁺) and chromium (Cr³⁺) determination in several water samples obtained from Nahand dam, Karkaj and Azarshahr well, tab and mineral waters to study their contamination in these systems. Determination method is based on the characteristic aggregation and fluorescence quenching of synthesized CdS quantum dots (QDs) at 555 nm by considered cations. Water-soluble and biocompatible CdS QDs modified with thioglycolic acid have been synthesized in one step and characterized by fluorescence and absorption spectroscopy. The fluorescence intensity of CdS QDs is linearly proportional to Pb²⁺ and Cr³⁺ concentration from 0.8 to 100 and 3 to 400 ng mL⁻¹ with a detection limit of 0.3 and 1.2 ng mL⁻¹, respectively. Other potentially interfering ions such as iron, sodium, potassium, calcium, and magnesium ions did not affect the luminescence. The method showed lower detection blank and higher sensitivity.

Introduction

Pollution by heavy metal ions, particularly lead (Pb²⁺) and chromium (Cr³⁺), causes a serious hazard to human life and the environment. Pb²⁺, the heaviest stable element, is one of the four metals that have the greatest effects on human health. Due to its characteristics as a very toxic poison to humans, even at low concentrations, Pb²⁺ has been studied extensively. Many diseases are spread because of the toxic effects of Pb²⁺, such as respiratory diseases like asthma and allergies, lowered IQ and other nervous system disorders, impaired biosynthesis of hemoglobin and anemia, high blood pressure, kidney damage and brain [1]. The use of Pb²⁺ in electronics has been restricted by European Parliament to reduce unsafe chemical waste leakage to the groundwater [2]. Also Environmental Protection Agency (EPA) set the allowed limit of Pb²⁺ in drinking water as 15 ng mL⁻¹. Unfortunately, Pb²⁺ is still used in refineries to increase gasoline octane number. Cigarette smoke has also a small amount of Pb²⁺. Also, Pb²⁺ enters drinking water and soil due to corrosion of water lead pipes.

Non-degradable and bio-fermentation Cr³⁺ in nature as well as its extensive usage is the main reason that it is considered a serious environmental pollutant. Cr³⁺ is one of the main pollutants in water, because of its lower storage requirements in some industrial activities. Chromium is known to be toxic to plants and animals because of its potentially carcinogenic effect duo to permeability of cell membranes.

Therefore, due to health concerns and legal restrictions, rapid and sensitive methods are critical for evaluation of heavy metals. Several methods have been reported for Pb²⁺ and Cr³⁺ determination based on electrothermal atomic absorption spectrometry [3, 4], X-ray fluorescence [5], chromatography [6], electrochemical methods [7–9], mass spectroscopy [10, 11] and photometric methods [12]. These methods often require complex and expensive equipment, skilled labor, and high time for preparation of samples. Also, it is necessary to further improve their sensitivity and selectivity. In the meantime, fluorescence recently has attracted much attention, because of its high sensitivity.

Several fluorescent probes based on organic luminescent dyes [13–16] have been reported for detection of Pb²⁺ or Cr³⁺. These methods generally displayed relatively high detection limits. In contrast, quantum dots (QDs) displayed superior luminescence properties and stability in aqueous solutions [17, 18]. There are several reports for ion detection based on the fluorescence intensity of quantum dots. Since, Chen and Rosenzweig showed that fluorescence intensity of thioglycerol-coated CdS QDs was reduced selectively in the presence of Cu²⁺ [19], several research groups have applied different types of quantum dots as luminescence probe for Hg²⁺ [20–24], Cu²⁺ [25–29], Ag⁺ [25, 30], Pb²⁺ [31–33] based on their fluorescence quenching effect. All the reports show the interaction between the analyte and quantum dots which change their physical and photochemical features.

This paper describes a sensitive method for the determination of Pb²⁺ and Cr³⁺ contaminations in several environmental water samples, based on fluorescence quenching of thioglycolic acid (TGA) capped cadmium sulfide (CdS) quantum dots (QDs). The quenched intensity of fluorescence was proportional to the concentration of Pb²⁺ and Cr³⁺ in different conditions. The procedure is simple, convenient and sensitive with a detection limit of 0.3 and 1.2 ng mL⁻¹ for Pb²⁺ and Cr³⁺, respectively. It does not need any pretreatments for samples. The proposed method was successfully applied to the determination of Pb²⁺ and Cr³⁺ in real samples including Nahand dam, Karkaj and Azarshahr well, tab and mineral waters.

Experimental

Apparatus

Fluorescence measurements were performed on a RF-5301 spectrofluorophotometer (Shimadzu, Japan) equipped with a xenon lamp source, using 1.0 cm quartz cell with a cell holder kept in a constant-temperature water circulating device, thermo bath TB-85. The widths of both excitation and emission slits were set at 5 nm. The optimum excitation and emission wavelengths for CdS QDs were found to be 427 and 555 nm, respectively. Absorption spectra were recorded by a UV-1800 spectrophotometer (Shimadzu, Japan).

Materials

All experiments were carried out with analytical grade chemicals and solvents. Doubly distilled deionized water was used for the preparation of all solutions. Cadmium chloride (CdCl₂·5H₂O), sodium sulfide (Na₂S·9H₂O) and thioglycolic acid (TGA) were all purchased from sigma Aldrich and were used without further purification. Buffer solutions of acetate and phosphate (0.5 mol L⁻¹) were prepared by dissolving the appropriate amount of sodium acetate (Merck) or NaH₂PO₄·2H₂O (Merck) in double distilled water, respectively. The pHs of these buffer was adjusted to 5 and 6, respectively. Stock standard solutions of 100 mg L⁻¹ Pb²⁺ and Cr³⁺ were prepared in water from lead nitrate (Merck) and chromium (III) nitrate (Merck), respectively.

Synthesis of TGA-capped CdS quantum dots

TGA-capped CdS QDs were synthesized in aqueous solution using a simple reported procedure [28]. Briefly, 100 ml of TGA (0.05 mol L⁻¹) solution and 100 ml of CdCl₂ (0.02 mol L⁻¹) solution were mixed in a round bottom flask. The pH of this solution was adjusted in 8 by NaOH (1 mol L⁻¹) solution. White residue, appear in the solution, was dissolved by adding NaOH solution and finally, the clear solution was obtained. This was probably due to formation of Cd-thioglycolic complexes with different structures at different pH values because of different dissociation of Carboxylate and sulfhydryl group. The reaction mixture was then heated up to 90 °C in the argon atmosphere. Then 50 ml of Na₂S (0.02 mol L⁻¹) was added. After refluxing for 2 min, the flask was submerged in the ice-water bath until it cooled to room temperature. After purification, QDs were used as fluorescent nanoparticles.

General procedure

For Pb²⁺ determination, 75 µL synthesized QDs solution and 200 µL phosphate buffer (pH = 5) were transferred to 5 mL volumetric flask. Then an appropriate volume of sample or standard Pb²⁺ solution was added and the final volume was made to 5 mL with distilled water. The fluorescence intensity of the solution was recorded as analytical signal.

For Cr³⁺ determination, 100 µL synthesized QDs solution and 200 µL acetate buffer (pH = 5) were transferred to 5 mL volumetric flask. Then an appropriate volume of sample or standard Cr³⁺ solution was added and the final volume was made to 5 mL with distilled water. The fluorescence intensity of the solution was recorded as analytical signal.

Results and discussion

Characterization of thioglycolic acid-capped CdS quantum dots

Synthesized thioglycolic acid-capped CdS QDs (TGA-capped CdS QDs) are optically characterized by UV–vis absorption spectroscopy and fluorometry. The TGA-capped CdS QDs show long emission wavelengths and a high separation between the excitation and emission wavelengths (ca. 125 nm) simplifying fluorescence measurements. The absorption spectrum shows well-defined excitonic absorption peak at wavelength of 427 nm, and a sharp band edge emission peak (λ = 555 nm), which was independent of the excitation wavelength, was obtained in the FL spectrum (Fig. 1).

Fig. 1

a Absorption, b Fluorescence spectra and c TEM image of synthesized aqueous TGA-capped CdS QDs

Also TEM (transmission electron microscopy) image of Synthesized QDs is shown in Fig. 1c.

Effect of cations on the fluorescence of QDs

The fluorescence intensity (λ_ex = 427 nm) of quantum dots drastically decreases with some cations in a certain condition. It is observed that synthesized QDs have high tendency for Pb²⁺ and Cr³⁺. The more the Pb²⁺ or Cr⁺³ concentration, the lower was the FL intensity obtained. This decrease was proportional to each cation concentration (Fig. 2).

Fig. 2

Fluorescence spectra of aqueous TGA-capped CdS QDs in the presence of different concentrations of a Cr³⁺ and b Pb²⁺. a QDs 75 µL, phosphate buffer 0.01 M, pH = 6 (1–7: 0, 25, 50, 100, 200, 300, 400 ng mL⁻¹). b QDs 100 µL, acetate buffer 0.01 M, pH = 5 (1–7: 0, 1, 2.5, 5, 10, 50, 75 ng mL⁻¹)

Optimization of condition

It is clear that pH influences the fluorescence intensity of the complex significantly. To investigate the effects of pH on determinations, the pH ranges between 4 and 9. At lower pHs, there is no fluorescence intensity because of the protonation of the thiol groups and aggregation of the QDs [35]. The maximum value of F₀/F was obtained when pH was 6 and 5 for Cr³⁺ and Pb²⁺, respectively (Fig. 3). (F₀ and F are the fluorescence intensity of the aqueous CdS QDs without and at a given analyte concentration). Therefore, these optimal pH values were chosen to be applied in further experiments.

Fig. 3

Effects of pH on the F₀/F for a Cr³⁺ 10 ng mL⁻¹ (QDs = 100 µL, phosphate buffer 0.05 M) and b Pb²⁺ 5 ng mL⁻¹ (QDs = 75 µL, acetate buffer 0.05 M)

It was found that the concentration of QDs affected not only the fluorescent intensity but also the sensitivity of the assay. As shown in Fig. 4, high concentration of aqueous QDs decreases the sensitivity and causes self-quenching of the QDs fluorescence. Also, very low concentration of QDs gives very weak fluorescence intensity, which may result in narrow linear range. Thus, 100 and 75 µL of QDs was chosen for Cr³⁺ and Pb²⁺ determinations, respectively. In selected optimal values of the quantum dots, wide linear range and low detection limit concentrations are achieved.

Fig. 4

Effects of QDs amount on the F₀/F for a Cr³⁺ 10 ng mL⁻¹ (phosphate buffer 0.05 M, pH = 6) and b Pb²⁺ 5 ng mL⁻¹ (acetate buffer 0.05 M, pH = 5)

The effect of buffer solution on fluorescence intensity system was discussed. To do so, different buffer solutions such as phosphate, tris and acetate were investigated. The results showed that phosphate and acetate have the greatest effect on the fluorescence quenching of TGA-capped CdS QDs in the presence of Cr³⁺ and Pb²⁺, respectively. Hence, these buffers were selected in all experiments. Optimization of buffer concentration was carried out and the result is showed in Fig. 5. Maximum signal was obtained in 0.01 M for both states.

Fig. 5

Effects of buffer concentration on the F₀/F for a Cr³⁺ 10 ng mL⁻¹ (QDs = 100 µL, phosphate buffer pH = 6) and b Pb²⁺ 5 ng mL⁻¹ (QDs = 75 µL, acetate buffer pH = 5)

In results, the best optimal conditions for the measurement of Cr³⁺ are: QDs = 100 µL, phosphate buffer 0.01 M, pH = 6, and for Pb²⁺ are: QDs = 75 µL, acetate buffer 0.01 M, pH = 5.

Quantitative characteristics

Under optimal condition, the ratio of FL intensity (F₀/F) in the absence (F₀) and presence of analyte (F) was plotted versus the concentration of each analyte in ng mL⁻¹. Calibration graphs and corresponding FL spectra are shown in Fig. 6. Obtained analytical characteristics are indicated in Table 1.

Fig. 6

Calibration curve of the a Cr³⁺ and b Pb²⁺ in optimum condition

Table 1 Analytical characteristics of the method for the determination of Cr³⁺ and Pb²⁺

The quenching ability of Cr³⁺ and Pb²⁺ can be expressed by Stern–Vollmer equation:

$$\frac{F_0}{F}=1+K_{\text{SV}}\left[C\right]$$

where F and F₀ are fluorescence quantum yield or fluorescence intensity of quantum dots with (F) and without (F₀) extinguishing or analyte. K_sv is the Stern–Vollmer constant. Linear equations between concentration and relative fluorescence intensity obtained for Cr³⁺ and Pb²⁺ (y = 0.0024x + 1.065 and y = 0.0098x + 1.057, respectively) are consistent with Stern–Vollmer equation.

Interference of coexisting foreign substances

In order to investigate the effect of coexisting substances on Cr³⁺ and Pb²⁺ determination in real samples, various concentrations of interfering ions are added to determination solution (1 μg mL⁻¹ analyte) and, F/F₀ value is measured versus interference ion concentration. Interference limit is considered 5 % deviation from the initial F/F₀ value (Table 2). The results show that the method for analysis of Cr³⁺ and Pb²⁺ is free from any interference.

Table 2 Interfering effects of some coexisting ions on Cr³⁺ and Pb²⁺ determination

Analysis of real samples

The represented method was successfully applied to the quantitative determination of Cr³⁺ and Pb²⁺ in the real samples of various compositions such as Nahand dam, Karkaj and Azarshahr well, tab and mineral waters. The real samples were spiked with standard solution of each analyte, and then were analyzed by standard addition method. In Tables 3 and 4, the obtained recovery values in real samples are listed.

Table 3 Results for the determination of Pb²⁺ in the real samples

Table 4 Results for the determination of Cr³⁺ in the real samples

The recoveries were between 97 and 104 %, showing that the proposed procedure is applicable to the determination of Cr³⁺ and Pb²⁺ in environmental samples.

On the other hand, the results were compared with those obtained by a standard method using electrothermal atomic absorption spectroscopy. Statistical analysis using Student t test showed that there are no significant differences between the results of two methods.

Conclusion

A simple, sensitive and rapid method for determination of Cr³⁺ and Pb²⁺ contamination has been offered based on the quenching of the fluorescence of TGA-capped CdS quantum dots. The preparation of quantum dots is very simple, fast and economical. Also the method does not need any pretreatment steps in analysis. The procedure possesses the advantage of wide dynamic range, selectivity and high sensitivity (expressed by the detection limits) which may be an incentive to other workers to consider it for determination of Cr³⁺ and Pb²⁺ in traces. Finally, this procedure was successfully applied to determine Cr³⁺ and Pb²⁺ in environmental real samples.