lead as own luminescent sensor for determination

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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 Pb 2+ solution of N,N -dimethylformamide (DMF), the highly emissive cluster Pb 4 Br 11 3can be formed and the fluorescence intensity of the formed cluster is proportional to the concentration of Pb 2+ , based on which, a novel, simple approach that uses the emission from itself as the sensor for determination of Pb 2+ 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; Pb 2+ ; Sensor; Tetrabutyl ammonium bromide 1. Introduction Pb 2+ is a harmful metal ion, which causes adverse envi- ronmental and health problems. A wide variety of symptoms such as memory loss, irritability, anemia, muscle paralysis, and mental retardation have been attributed to lead poi- soning [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 Pb 2+ , therefore, is of great importance and urgency in environmental and medical fields. Numerous analytic methods can be applied to determine Pb 2+ , such as atomic absorption spectrometry [4–6], induc- tively 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 be- ing 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 sen- sitivity and selectivity and simplicity, fluorescence method attracts more and more interests for the trace determination. Corresponding author. Tel.: +86-27-87218734; fax: +86-27-87647617. E-mail address: [email protected] (Z. He). Recently, more and more attention is paid to lumines- cent sensors for various ions and molecules, particularly for heavy metals ions such as Pb 2+ [1,19,20]. Previously, a host–guest strategy was used as sensor molecules [1]. Herein, an alternative approach that uses the emission from pb 2+ itself as the sensor is proposed. By adding bromide to Pb 2+ solution of N,N -dimethylformamide (DMF), the highly emissive cluster Pb 4 Br 11 3can be formed [21,22]. The fluorescence intensity of the formed cluster is propor- tional to the concentration of Pb 2+ , based on which, the fluorescence method is set up. Under the optimum condi- tions, 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 sam- ples with satisfactory results. The mechanism of the reac- tion is also studied. In this method, no chromogenic agent is needed which has complex structure and is not easy to pre- pare, which simplifies the determination method. As there does not exist a competitive host–guest relationship, there- fore, the determination is absolutely sensitive and specific. 2. Experimental 2.1. Apparatus Fluorescence intensity was measured with a Perking Elmer LS 55 Luminescence Spectrometer with a quartz 1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2003.08.010

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Page 1: 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 ofN,N′-dimethylformamide (DMF), the highly emissive clusterPb4Br11

3− can be formed and the fluorescence intensity of the formed cluster is proportional to the concentration of Pb2+, based on which, anovel, simple approach that uses the emission from itself as the sensor for determination of Pb2+ is proposed. Under the optimum conditions, thelinear range and detection limit is 1.0× 10−7 to 1.0× 10−5 mol l−1 (correlation coefficientr = 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 thesynthetic 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 envi-ronmental and health problems. A wide variety of symptomssuch as memory loss, irritability, anemia, muscle paralysis,and mental retardation have been attributed to lead poi-soning[1]. The threshold limit value in the environment is4.83 × 10−7 mol l−1 [2] and according to the US Centerfor Disease Control (CDC), the limit value of blood lead is1.16× 10−6 mol l−1 [3]. The determination of trace amountof Pb2+, therefore, is of great importance and urgency inenvironmental and medical fields.

Numerous analytic methods can be applied to determinePb2+, such as atomic absorption spectrometry[4–6], induc-tively coupled plasma atomic emission spectrometry[7,8],inductively coupled plasma mass spectrometry[9–11],X-ray absorption spectroscopy[12], atomic fluorescencespectrometry[13,14], etc. For expensive instruments and be-ing complex to use, the above methods are limited to widelyuse; other methods, such as spectrophotometric[15,16],electroanalysis[17], and stripping voltammetry[18] are notsatisfactory enough due to low sensitivity. With high sen-sitivity and selectivity and simplicity, fluorescence methodattracts more and more interests for the trace determination.

∗ Corresponding author. Tel.:+86-27-87218734;fax: +86-27-87647617.

E-mail address:[email protected] (Z. He).

Recently, more and more attention is paid to lumines-cent sensors for various ions and molecules, particularlyfor 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 frompb2+ itself as the sensor is proposed. By adding bromideto Pb2+ solution of N,N′-dimethylformamide (DMF), thehighly emissive cluster Pb4Br11

3− can be formed[21,22].The fluorescence intensity of the formed cluster is propor-tional to the concentration of Pb2+, based on which, thefluorescence method is set up. Under the optimum condi-tions, the linear range and detection limit is 1.0 × 10−7

to 1.0 × 10−5 mol l−1 (correlation coefficientr = 0.9997)and 7.6× 10−9 mol l−1, respectively. This method has beenapplied to the determination of lead in the synthetic sam-ples with satisfactory results. The mechanism of the reac-tion is also studied. In this method, no chromogenic agent isneeded which has complex structure and is not easy to pre-pare, which simplifies the determination method. As theredoes not exist a competitive host–guest relationship, there-fore, the determination is absolutely sensitive and specific.

2. Experimental

2.1. Apparatus

Fluorescence intensity was measured with a PerkingElmer LS 55 Luminescence Spectrometer with a quartz

1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.saa.2003.08.010

Page 2: Lead as own luminescent sensor for determination

1448 H. Duan et al. / Spectrochimica Acta Part A 60 (2004) 1447–1451

cell (1 cm× 1 cm cross-section). UV-Vis absorption spectrawere recorded on a Shimadzu UV-1601 Spectrophotometerusing 1 cm path length cells.

2.2. Reagents

A stock Pb2+ solution (0.1 mol l−1) was prepared by dis-solving 3.3185 g of lead nitrate (the second chemical regentfactory of Shenyang, China) inN,N′-dimethylformamide(Shanghai chemical reagent Co. Ltd., China) in 100 mlstandard flask, working solution was prepared from thisstock solution by appropriate dilution with DMF. The stocksolution of bromide was prepared by dissolving 6.4474 gof tetrabutyl ammonium bromide (China medicine (group)Shanghai chemical reagent corporation, China) in DMF in100 ml standard flask.

All the chemicals used are of analytical-reagent grade.

2.3. Procedure

Samples containing appropriate concentration of Pb2+were transferred into a calibrated 10 ml test tube, thenadding 0.2 mol l−1 tetrabutyl ammonium bromide to themark. The fluorescence intensity was measured with thefollowing 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 inFig. 1. Comparingthe curves, it was found that after adding Pb2+, two new

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Fig. 1. Absorption spectra of Pb2+, bromide, and Pb4Br113− 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.

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Fig. 2. Fluorescence spectra of Pb4Br113− in DMF. Curves 1 and 2 are

excitation and emission spectrum, respectively.

peaks (at 308 and 350 nm, respectively) appear, which is dueto the newly formed cluster. The common peak at 276 nmis the absorption of the solvent and 350 nm is the maximumabsorption wavelength. Based on the two absorptions, thefurther fluorescence experiments are carried out. The resultshows that the maximum absorption peak (350 nm) and itsrelative fluorescence peak (550 nm) have higher intensityand are more suitable for determination (the fluorescencespectrum is shown inFig. 2). But the fluorescence peakwhose excitation wavelength is at 308 nm has some influ-ence on the fluorescence spectrum, and to make the spectrumsimple, synchronous fluorescence spectrum instead of fluo-rescence spectrum usually used in fluorescence determina-tion is proposed.Fig. 3shows the synchronous fluorescencespectrum. From the spectrum, the fluorescence intensity of350 nm (λex/λem = 350/550) greatly increases after theaddition of Pb2+, which is used for the determination.

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

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H. Duan et al. / Spectrochimica Acta Part A 60 (2004) 1447–1451 1449

3.2. Optimization of experimental conditions

3.2.1. Choice of different solventsPb2+ and bromide can form the highly emissive clus-

ter Pb4Br113− [21,22] in aprotic polar organic solvents,

such as acetonitrile,N,N′-dimethylformamide, etc. Proticsolvents such as water and alcohol do not work, whichcan be attributed to the inhibition of the cluster forma-tion [22]. The comparison of different solvents suggestsN,N′-dimethylformamide is the most suitable solvent whichis due to its high solubility to many lead salts and bromide,so it is employed to determine Pb2+.

3.2.2. Effect of the concentration of tetrabutylammonium bromide

The effect of the concentration of tetrabutyl ammoniumbromide ranged from 0.06 to 0.50 mol l−1 is demonstratedin Fig. 4. It is shown that the fluorescence intensity increaseswith the increase of the concentration of tetrabutyl ammo-nium bromide. The higher concentration of bromide meansPb2+ can react more completely and more emissive clustercan be formed. Considering the consumption and solubilityof tetrabutyl ammonium bromide, 0.20 mol l−1 is used forthe further experiments.

3.2.3. Effect of temperatureIt is obvious that the emissive cluster is temperature

dependent. With the increase of the temperature, the fluo-rescence intensity drops quickly. As shown inFig. 5, thefluorescence intensity drops more quickly when the temper-ature is above 25◦C, so the temperature of 15◦C is chosenas the experiment temperature.

3.2.4. Effect of reaction timeThe effect of time to form stable cluster is investigated

and the result shows that the fluorescence intensity of thecluster will reach the peak immediately after adding bro-

0.0 0.1 0.2 0.3 0.4 0.5100

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

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Fig. 5. Effect of temperature on the fluorescence intensity of Pb2+ inpresence 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 declineunder continuous determination in almost 100 min, and 12and 24 h later the fluorescence intensities only decrease 1.26and 2.04%, respectively. All these show that the cluster,Pb4Br11

3−, is photochemically stable, which is accordedwith the conclusion of Jones and Aikens[21] and Dutta andPerkovic[22].

3.3. Linear range and detection Limit

Under the optimum conditions, when the concentrationof Pb2+ ranges from 1.0 × 10−7 to 1.0 × 10−5 mol l−1,the fluorescence intensity is linearly increased. The linearregression equation isIf = 10.14+ 51.16C (10−6 mol l−1)and the correlation coefficientr = 0.9997. The detectionlimit is 7.6 × 10−9 mol l−1. The precision of the proposedmethod is evaluated by determining 10 samples containing1.0 × 10−6 mol l−1 Pb2+. The relative standard deviation(R.S.D.) is 1.64%.

3.4. Influence of coexisting substances

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 in-tensity of the cluster. Adding small amount of Cu2+ orFe3+ into the system, the solution turns fuscous and thefluorescence of the cluster has a great change, whichis due to Cu2+ and Fe3+ being transitional mental ionsand their chelation withN,N′-dimethylformamide. Blueshift can be observed in the fluorescence spectrum af-ter the addition of Hg2+, which is due to another clusterformed by Hg2+ and tetrabutyl ammonium bromide inDMF (the excitation wavelength and the emission wave-length are 352 and 431 nm, respectively). The interference

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Table 1Tolerance of foreign substances

Foreign substance Concentration(1.0 × 10−6 mol l−1)

Change of�If (%)

Na+(NaNO3) 1000 −0.68NH4

+(NH4NO3) 1000 0.32K+(KNO3) 1000 0.48Al3+(Al(NO3)3) 500 −3.72Ni2+(Ni(NO3)2) 100 −3.97Ca2+(Ca(NO3)2) 100 −1.01Ag+(AgNO3) 100 −1.39Mg2+(MgSO4) 100 −1.65Zn2+(ZnCl2) 100 −2.47Mn2+(MnSO4

.H2O) 50 −3.98Cl−(NaCl) 50 −2.25SO4

2−(Na2SO4) 10 −1.59Co2+(CoCl2·6H2O) 10 −1.49

CPb2+ = 5 × 10−6 mol l−1.

caused by those ions can be easily eliminated by priorextraction.

3.5. Analysis of the synthetic sample

The standard addition method is used for the determi-nation of Pb2+ in synthetic samples. Synthetic samples ofPb2+ containing metal ions, based on the tolerance of co-existing species, are analyzed and the results are given inTable 2. It can be seen that Pb2+ in synthetic samples canbe determined with satisfactory results.

3.6. The possible mechanism

As the Pb–halide complex was first discovered by theincreased solubility of PbBr2 in propylene carbonate causedby addition of Br− [21]. The formation of the cluster by thefollowing equations is assumed.

PbBr2 + Br− = PbBr3− (1)

4PbBr2 + 3Br− = Pb4Br113− (2)

The former predominated in dilute Br− solution (<0.03mol l−1) and the latter in concentrated Br- solution, basedon which, the conclusion is drawn that the formation of thecluster is followed by the second equation.

Pb belongs to the IVA group; it has d10s2 electron configu-ration, the s2 complex has been characterized by low-energymetal-centered s–p transitions[23,24]. The emission of thecluster (Pb4Br11

3−) is 550 nm and it exhibits large shifts

Table 2Recoveries 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 50.00 48.55 2.32 97.12 80.00 78.59 1.58 98.23 150.00 147.83 2.02 98.5

from the absorption maximum (350 nm), which is attributedto strong distortions between the ground- and excited-stategeometries. It is reasonable to assign the 550 nm emissionto charge transfer states and states delocalized over thebromide-bridged lead ions.

4. Conclusion

Using a heavy metal as its own luminescent sensor forthe determination is a new, excellent method that discardsthe traditional host–guest approach usually employed inthe analysis and determination. Pb2+ and bromide canform highly emissive cluster, which can be used its flu-orescence to determine trace amount of Pb2+. This is asimple, sensitive, high selective method that can be wildlyused.

Acknowledgements

The authors acknowledge the supports from National Nat-ural Science Foundation of China (20275028) and the Na-tional Key Basic Research and Development Program (973Program 2002CB2118).

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