Lead as own luminescent sensor for determination

Spectrochimica Acta Part A 60 (2004) 1447–1451

Lead as own luminescent sensor for determination Hongying Duan, Xinping Ai, Zhike He∗ College of Chemistry & Molecular Sciences, Wuhan University, Wuhan 430072, PR China Received 22 May 2003; received in revised form 22 May 2003; accepted 21 August 2003

Abstract In our experiments, it was observed that adding bromide to Pb2+ solution of N,N -dimethylformamide (DMF), the highly emissive cluster Pb4 Br11 3− can be formed and the fluorescence intensity of the formed cluster is proportional to the concentration of Pb2+ , based on which, a novel, simple approach that uses the emission from itself as the sensor for determination of Pb2+ is proposed. Under the optimum conditions, the linear range and detection limit is 1.0 × 10−7 to 1.0 × 10−5 mol l−1 (correlation coefficient r = 0.9997) and 7.6 × 10−9 mol l−1 , respectively. Foreign substrates effects were also investigated. The proposed method has been successfully applied to the determination of lead in the synthetic samples. The mechanism of the reaction is also studied. © 2003 Elsevier B.V. All rights reserved. Keywords: Fluorescence; Pb2+ ; Sensor; Tetrabutyl ammonium bromide

1. Introduction Pb2+ is a harmful metal ion, which causes adverse environmental and health problems. A wide variety of symptoms such as memory loss, irritability, anemia, muscle paralysis, and mental retardation have been attributed to lead poisoning [1]. The threshold limit value in the environment is 4.83 × 10−7 mol l−1 [2] and according to the US Center for Disease Control (CDC), the limit value of blood lead is 1.16 × 10−6 mol l−1 [3]. The determination of trace amount of Pb2+ , therefore, is of great importance and urgency in environmental and medical fields. Numerous analytic methods can be applied to determine Pb2+ , such as atomic absorption spectrometry [4–6], inductively coupled plasma atomic emission spectrometry [7,8], inductively coupled plasma mass spectrometry [9–11], X-ray absorption spectroscopy [12], atomic fluorescence spectrometry [13,14], etc. For expensive instruments and being complex to use, the above methods are limited to widely use; other methods, such as spectrophotometric [15,16], electroanalysis [17], and stripping voltammetry [18] are not satisfactory enough due to low sensitivity. With high sensitivity and selectivity and simplicity, fluorescence method attracts more and more interests for the trace determination.

Recently, more and more attention is paid to luminescent sensors for various ions and molecules, particularly for heavy metals ions such as Pb2+ [1,19,20]. Previously, a host–guest strategy was used as sensor molecules [1]. Herein, an alternative approach that uses the emission from pb2+ itself as the sensor is proposed. By adding bromide to Pb2+ solution of N,N -dimethylformamide (DMF), the highly emissive cluster Pb4 Br11 3− can be formed [21,22]. The fluorescence intensity of the formed cluster is proportional to the concentration of Pb2+ , based on which, the fluorescence method is set up. Under the optimum conditions, the linear range and detection limit is 1.0 × 10−7 to 1.0 × 10−5 mol l−1 (correlation coefficient r = 0.9997) and 7.6 × 10−9 mol l−1 , respectively. This method has been applied to the determination of lead in the synthetic samples with satisfactory results. The mechanism of the reaction is also studied. In this method, no chromogenic agent is needed which has complex structure and is not easy to prepare, which simplifies the determination method. As there does not exist a competitive host–guest relationship, therefore, the determination is absolutely sensitive and specific. 2. Experimental 2.1. Apparatus

∗

Corresponding author. Tel.: +86-27-87218734; fax: +86-27-87647617. E-mail address: [email protected] (Z. He). 1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2003.08.010

Fluorescence intensity was measured with a Perking Elmer LS 55 Luminescence Spectrometer with a quartz

H. Duan et al. / Spectrochimica Acta Part A 60 (2004) 1447–1451

cell (1 cm × 1 cm cross-section). UV-Vis absorption spectra were recorded on a Shimadzu UV-1601 Spectrophotometer using 1 cm path length cells. 2.2. Reagents A stock Pb2+ solution (0.1 mol l−1 ) was prepared by dissolving 3.3185 g of lead nitrate (the second chemical regent factory of Shenyang, China) in N,N -dimethylformamide (Shanghai chemical reagent Co. Ltd., China) in 100 ml standard flask, working solution was prepared from this stock solution by appropriate dilution with DMF. The stock solution of bromide was prepared by dissolving 6.4474 g of tetrabutyl ammonium bromide (China medicine (group) Shanghai chemical reagent corporation, China) in DMF in 100 ml standard flask. All the chemicals used are of analytical-reagent grade.

600

Fluorescence Intensity

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1

2

500

400

300

200

100

0 300

350

400

450

500

550

600

650

Wavelength (nm)

Fig. 2. Fluorescence spectra of Pb4 Br11 3− in DMF. Curves 1 and 2 are excitation and emission spectrum, respectively.

2.3. Procedure Samples containing appropriate concentration of Pb2+ were transferred into a calibrated 10 ml test tube, then adding 0.2 mol l−1 tetrabutyl ammonium bromide to the mark. The fluorescence intensity was measured with the following settings of the spectrofluorometer: λ = 200 nm (excitation wavelength (λex ), 350 nm; emission wavelength (λem ), 550 nm); excitation slit (EX), 7.5 nm; emission slit (EM), 10 nm.

3. Results and discussion 3.1. Spectra characteristics The absorption spectra are shown in Fig. 1. Comparing the curves, it was found that after adding Pb2+ , two new

peaks (at 308 and 350 nm, respectively) appear, which is due to the newly formed cluster. The common peak at 276 nm is the absorption of the solvent and 350 nm is the maximum absorption wavelength. Based on the two absorptions, the further fluorescence experiments are carried out. The result shows that the maximum absorption peak (350 nm) and its relative fluorescence peak (550 nm) have higher intensity and are more suitable for determination (the fluorescence spectrum is shown in Fig. 2). But the fluorescence peak whose excitation wavelength is at 308 nm has some influence on the fluorescence spectrum, and to make the spectrum simple, synchronous fluorescence spectrum instead of fluorescence spectrum usually used in fluorescence determination is proposed. Fig. 3 shows the synchronous fluorescence spectrum. From the spectrum, the fluorescence intensity of 350 nm (λex /λem = 350/550) greatly increases after the addition of Pb2+ , which is used for the determination.

0.5

240

3

220

Fluorescence Intensity

0.4

ABS

0.3

0.2

0.1

1

2

200 180

2

160 140 120 100 80 60 40

1

20

0.0 300

400

Wavelength (nm)

Fig. 1. Absorption spectra of Pb2+ , bromide, and Pb4 Br11 3− in DMF. Curves 1, 2, and 3 are of Pb2+ , tetrabutyl ammonium bromide, and tetrabutyl ammonium bromide in presence of Pb2+ , respectively. CPb2+ = 5 × 10−6 mol l−1 , Cbromide = 0.2 mol l−1 .

0 200

250

300

350

400

450

Excitation wavelength (nm)

Fig. 3. Synchronous fluorescence spectra of tetrabutyl ammonium bromide (1) and tetrabutyl ammonium bromide in presence of Pb2+ (2). CPb2+ = 5 × 10−6 mol l-1 , Cbromide = 0.2 mol l-1 .

H. Duan et al. / Spectrochimica Acta Part A 60 (2004) 1447–1451 200

3.2.1. Choice of different solvents Pb2+ and bromide can form the highly emissive cluster Pb4 Br11 3− [21,22] in aprotic polar organic solvents, such as acetonitrile, N,N -dimethylformamide, etc. Protic solvents such as water and alcohol do not work, which can be attributed to the inhibition of the cluster formation [22]. The comparison of different solvents suggests N,N -dimethylformamide is the most suitable solvent which is due to its high solubility to many lead salts and bromide, so it is employed to determine Pb2+ .

180

3.2.2. Effect of the concentration of tetrabutyl ammonium bromide The effect of the concentration of tetrabutyl ammonium bromide ranged from 0.06 to 0.50 mol l−1 is demonstrated in Fig. 4. It is shown that the fluorescence intensity increases with the increase of the concentration of tetrabutyl ammonium bromide. The higher concentration of bromide means Pb2+ can react more completely and more emissive cluster can be formed. Considering the consumption and solubility of tetrabutyl ammonium bromide, 0.20 mol l−1 is used for the further experiments. 3.2.3. Effect of temperature It is obvious that the emissive cluster is temperature dependent. With the increase of the temperature, the fluorescence intensity drops quickly. As shown in Fig. 5, the fluorescence intensity drops more quickly when the temperature is above 25 ◦ C, so the temperature of 15 ◦ C is chosen as the experiment temperature.

Fluorescence Intensity

3.2.4. Effect of reaction time The effect of time to form stable cluster is investigated and the result shows that the fluorescence intensity of the cluster will reach the peak immediately after adding bro-

Fluorescence Intensity

3.2. Optimization of experimental conditions

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160

140

120

100

80 0

10

20

30

40

50

60

temperature (ºC)

Fig. 5. Effect of temperature on the fluorescence intensity of Pb2+ in presence of tetrabutyl ammonium bromide. CPb2+ = 5 × 10−6 mol l-1 , Cbromide = 0.2 mol l−1 .

mide to Pb2+ . The fluorescence intensity does not decline under continuous determination in almost 100 min, and 12 and 24 h later the fluorescence intensities only decrease 1.26 and 2.04%, respectively. All these show that the cluster, Pb4 Br11 3− , is photochemically stable, which is accorded with the conclusion of Jones and Aikens [21] and Dutta and Perkovic [22]. 3.3. Linear range and detection Limit Under the optimum conditions, when the concentration of Pb2+ ranges from 1.0 × 10−7 to 1.0 × 10−5 mol l−1 , the fluorescence intensity is linearly increased. The linear regression equation is If = 10.14 + 51.16C (10−6 mol l−1 ) and the correlation coefficient r = 0.9997. The detection limit is 7.6 × 10−9 mol l−1 . The precision of the proposed method is evaluated by determining 10 samples containing 1.0 × 10−6 mol l−1 Pb2+ . The relative standard deviation (R.S.D.) is 1.64%.

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3.4. Influence of coexisting substances

300

The effects of different foreign substrates are discussed (Table 1). It is found that, except for Cu(II), Fe(III), and Hg(II), most ions, such as Na+ , NH4 + , K+ , Ca2+ , Co2+ , Mg2+ , etc., have no effect on the fluorescence intensity of the cluster. Adding small amount of Cu2+ or Fe3+ into the system, the solution turns fuscous and the fluorescence of the cluster has a great change, which is due to Cu2+ and Fe3+ being transitional mental ions and their chelation with N,N -dimethylformamide. Blue shift can be observed in the fluorescence spectrum after the addition of Hg2+ , which is due to another cluster formed by Hg2+ and tetrabutyl ammonium bromide in DMF (the excitation wavelength and the emission wavelength are 352 and 431 nm, respectively). The interference

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100 0.0

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the concentration of Br

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Fig. 4. Effect of tetrabutyl ammonium bromide concentration on the fluorescence intensity of cluster. CPb2+ = 5 × 10−6 mol l-1 .

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H. Duan et al. / Spectrochimica Acta Part A 60 (2004) 1447–1451

Table 1 Tolerance of foreign substances Foreign substance

Concentration (1.0 × 10−6 mol l−1 )

Change of If (%)

Na+ (NaNO3 ) NH4 + (NH4 NO3 ) K+ (KNO3 ) Al3+ (Al(NO3 )3 ) Ni2+ (Ni(NO3 )2 ) Ca2+ (Ca(NO3 )2 ) Ag+ (AgNO3 ) Mg2+ (MgSO4 ) Zn2+ (ZnCl2 ) Mn2+ (MnSO4 . H2 O) Cl− (NaCl) SO4 2− (Na2 SO4 ) Co2+ (CoCl2 ·6H2 O)

1000 1000 1000 500 100 100 100 100 100 50 50 10 10

−0.68 0.32 0.48 −3.72 −3.97 −1.01 −1.39 −1.65 −2.47 −3.98 −2.25 −1.59 −1.49

from the absorption maximum (350 nm), which is attributed to strong distortions between the ground- and excited-state geometries. It is reasonable to assign the 550 nm emission to charge transfer states and states delocalized over the bromide-bridged lead ions.

4. Conclusion Using a heavy metal as its own luminescent sensor for the determination is a new, excellent method that discards the traditional host–guest approach usually employed in the analysis and determination. Pb2+ and bromide can form highly emissive cluster, which can be used its fluorescence to determine trace amount of Pb2+ . This is a simple, sensitive, high selective method that can be wildly used.

CPb2+ = 5 × 10−6 mol l−1 .

caused by those ions can be easily eliminated by prior extraction. 3.5. Analysis of the synthetic sample The standard addition method is used for the determination of Pb2+ in synthetic samples. Synthetic samples of Pb2+ containing metal ions, based on the tolerance of coexisting species, are analyzed and the results are given in Table 2. It can be seen that Pb2+ in synthetic samples can be determined with satisfactory results. 3.6. The possible mechanism As the Pb–halide complex was first discovered by the increased solubility of PbBr2 in propylene carbonate caused by addition of Br− [21]. The formation of the cluster by the following equations is assumed. PbBr 2 + Br − = PbBr3 −

(1)

4PbBr 2 + 3Br − = Pb4 Br 11 3−

(2)

The former predominated in dilute Br− solution (<0.03 mol l−1 ) and the latter in concentrated Br- solution, based on which, the conclusion is drawn that the formation of the cluster is followed by the second equation. Pb belongs to the IVA group; it has d10 s2 electron configuration, the s2 complex has been characterized by low-energy metal-centered s–p transitions [23,24]. The emission of the cluster (Pb4 Br11 3− ) is 550 nm and it exhibits large shifts Table 2 Recoveries of Pb2+ from synthetic samples Sample

Pb2+ added (10-8 mol l−1 )

Pb2+ found (10-8 mol l−1 )

R.S.D (%)

Recovery (%)

1 2 3

50.00 80.00 150.00

48.55 78.59 147.83

2.32 1.58 2.02

97.1 98.2 98.5

Acknowledgements The authors acknowledge the supports from National Natural Science Foundation of China (20275028) and the National Key Basic Research and Development Program (973 Program 2002CB2118).

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