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Electrochemiluminescence of CdTe quantum dots capped with glutathione and thioglycolic acid and its sensing of pb2+
Electrochimica Acta 72 (2012) 28–31
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemiluminescence of CdTe quantum dots capped with glutathione and thioglycolic acid and its sensing of pb2+ Haiyan Wang ∗ , Qiongfang Chen, Zhian Tan, Xunxun Yin, Lun Wang Anhui Key Laboratory of Chemo-biosensing, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2012 Received in revised form 27 March 2012 Accepted 27 March 2012 Available online 4 April 2012 Keywords: Electrochemiluminescence CdTe quantum dots TGA-GSH Pb2+
a b s t r a c t Water-soluble and stable CdTe quantum dots (QDs) capped with glutathione (GSH) and thioglycolic acid (TGA) were synthesized (denoted as GSH-TGA-CdTe) and the electrochemiluminescence (ECL) behavior of the QDs was investigated. As a result of specific interaction between the GSH and Pb2+ , the ECL of the QDs was selectively reduced in the presence of Pb2+ . Due to the low ECL intensity of QDs capped with GSH alone, TGA as another capping agent was introduced to improve the sensitivity. On the basis of the quenching effect on the ECL of GSH-TGA-CdTe QDs, a sensitive and selective method for the determination of Pb2+ was developed and a detection limit of 0.26 nM was obtained, which was lower than previous report. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Semiconductor quantum dots (QDs), also known as nanocrystals (NCs), have gained a center stage of interest both in fundamental and industrial area as a consequence of the unique optical and electronic properties and they have shown great promise in molecular detection [1–3]. Since the first report on the electrochemiluminescence (ECL) of silicon QDs by Bard’s group [4], the ECL of the semiconductor QDs, especially the II–VI semiconductor (e.g. CdS, CdSe and CdTe) has been extensively studied [5–11]. The subsequent work of Bard el al. showed that the ECL emission was generated in an annihilation process in which the electrogenerated reduced species (QD−• ) are in collision with the oxidized species (QD+• ) [5,6]. Furthermore, the ECL of QDs has been applied for determination of various analytes. Ju et al. [7] have studied the anodic ECL of CdTe QDs on a tin-doped indium oxide (ITO) electrode and its analytical application for detection of catechol derivatives based on the energy transfer to analytes. Dong’s group [8] has applied the ECL of CdTe QDs for the sensitive and selective determination of Cu2+ . Han and co-workers reported the size-dependent ECL behavior of water-soluble CdTe QDs and selective sensing of l-cysteine [9] and they also investigated the direct ECL of CdTe QDs based on room temperature ionic liquid film and the highly sensitive sensing of gossypol [10]. Lead ion (Pb2+ ), a well-known chemical pollutant, possesses a serious threat to human health and the environment [11]. It is per-
∗ Corresponding author. Tel.: +86 553 3869303; fax: +86 553 3869303. E-mail address: [email protected] (H. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.03.146
sistent in the environment and can produce toxic effects in plants and animals as lead is nondegradable and lead-poisoning causes various neurotoxic effects particularly in children. Traditional methods such as inductively coupled plasma mass spectrometry often require expensive and sophisticated instrumentation as well as complicated sample preparation processes [12]. Therefore, it is of significance to develop simple and sensitive method for determination of Pb2+ . A novel method for the determination of Pb2+ was proposed by Han et al. [13] based on quenching of the fluorescence of thiol-capped CdTe QDs and a detection limit of 0.27 M was achieved. Recently, Ying et al. [12] have reported glutathione (GSH)-capped ZnCdSe and CdTe QDs as selective fluorescent probe for detection of Pb2+ with low detection limit (20 nM). GSH has two free COOH groups and a NH2 group and it has high affinity with Pb2+ , thus it played a crucial role in improving the selectivity. On the basis of the aforementioned results, we conclude that GSH-capped QDs may be utilized for the ECL detection of Pb2+ with high selectivity [14]. However, the ECL intensity of CdTe QDs capped with GSH alone was low and the stability was poor. Previous reports have shown that capping agent affected the optical properties of the QDs greatly [15]. CdTe QDs in the presence of both GSH and cysteine as stabilizers exhibited much higher photoluminescence quantum yield [16] and the quantum yield of CdTe QDs capped by GSH and thioglycolic acid (TGA) can reach 63% [17]. The above results suggest that the ECL of CdTe QDs may be improved by using two stabilizers. In this study, we present a simple, sensitive and selective ECL method for the detection of Pb2+ based on CdTe QDs capped with GSH and TGA (GSH-TGA-CdTe). It is found that the ECL intensity and the stability f the QDs were improved dramatically compared
H. Wang et al. / Electrochimica Acta 72 (2012) 28–31
with the QDs capped with GSH alone. With the as-prepared QDs, a liner range from 0.8 to 15 nM (R = 0.997) with a detection limit of 0.26 nM was achieved for the determination of Pb2+ .
0.10
29
A
60 min 120 min 180 min 220 min 280 min
2. Experimental 2.1. Reagents Sodium hydroxide (NaOH), tellurium powder, sodium borohydride (NaBH4 ) and thioglycolic acid (TGA) were purchased from Sinopharm Chemical Reagent Co., Ltd. Glutathione (GSH) was obtained from Hefei Bomei Biotechnology Co., Ltd. All other chemicals were of analytical reagent grade and used as received. Doubly distilled water was used in all experiments.
Absorbance / a.u.
0.08
0.06
0.04
0.02
0.00
2.2. Apparatus
450
500
550
600
650
700
Wavelength / nm
B
60 min 120 min 180 min 220 min 280 min
2000
PL intensity / a.u.
Electrochemical measurements were performed with a CHI 760C electrochemical workstation (Chenhua, Shanghai). Conventional three-electrode system was used with indium tin oxide glass (ITO, area: 0.4 cm2 ) as the working electrode, a Pt wire as the auxiliary electrode and an Ag/AgCl (saturated KCl) as the reference electrode. The ECL signal was detected and recorded by IFFM-E flow injection chemiluminescence analyzer (Xi’an Remax, China). Unless noted otherwise, the photomultiplier tube (PMT) was biased at 800 V. The UV–vis spectra were recorded on a UV–vis spectrophotometer (Hitachi U-3010, Japan). The photoluminescence (PL) spectra were obtained by a Hitachi F-4500 spectrofluorometer (Tokyo, Japan).
400
1600
1200
800
400
2.3. Preparation of GSH-TGA modified CdTe QDs CdTe QDs were prepared using CdCl2 and NaHTe as precursors according to the previous report [16,17]. Briefly, 37.6 mg of sodium borohydride, 1 ml H2 O and 16.2 mg of tellurium powder were added to a small flask to prepare NaHTe. 72.6 mg CdCl2 ·2.5H2 O, GSH or mixture of GSH and TGA (the molar ratio of GSH/TGA was 1:3) were dissolved in water and the pH of the mixture was adjusted to 11 with 1 M NaOH. After deaeration of the solution by N2 , the freshly prepared NaHTe solution was injected. The molar ratio of Cd2+ /stabilizer/HTe− was fixed at 1:1.2:0.4. The solution was refluxed and the reaction time was controlled to obtain different-sized QDs. The resulted QDs were denoted as GSH-CdTe and GSH-TGA-CdTe, respectively. 3. Results and discussion 3.1. Optical properties of the GSH-TGA-CdTe QDs UV–vis absorption and photoluminescence (PL) emission spectra are often used to monitor the size-dependent optical properties of the QDs. Fig. 1 shows the temporal evolution of the UV–vis and PL spectra of GSH-TGA-CdTe QDs. With prolonging the reflux time, the absorption peak and the PL emission peak shifts toward longer wavelength, indicating clearly the increase in QDs size. Based on the UV–vis absorption spectra, the particle size of the QDs is calculated to be 1.97, 2.45, 2.80, 2.95 and 3.04 nm, respectively, using the following empirical equation [18]: D = (9.8127 × 10−7 )3 − (1.7147 × 10−3 )2 + 1.0064 − 194.84 In which, D (nm) is the diameter of a given QDs, and (nm) is the wavelength of the first excitonic absorption peak of UV–vis absorption spectra.
0 400
450
500
550
600
650
700
Wavelength / nm Fig. 1. (A) UV–vis and (B) PL spectra of GSH-TGA-CdTe as the function of refluxing time.
3.2. Electrochemical and ECL behavior of GSH-TGA-CdTe QDs The electrochemical and ECL properties of the GSH-TGA-CdTe QDs were investigated in air-saturated 0.1 M HAc–NaAc solution (pH 7.0) by sweeping the potential in the range from 0 to 1.3 V at 100 mV/s. As shown in Fig. 2, GSH-TGA-CdTe QDs showed an intensive anodic ECL emission at the ITO electrode with the peak emission at +1.3 V and onset potential of +0.9 V. In the air-saturated solution, in the absence of QDs, ECL emission was not observed, suggesting that the anodic ECL emission resulted from the oxidation of the QDs, which led to the formation of ECL emitter. Meanwhile, when the detection solution was bubbled with N2 , the ECL emission decreased dramatically and gradually disappeared as prolonging the N2 purging time, indicating that dissolved oxygen played an important role in the ECL emission procedure [7]. The capping agent plays an important role in affecting the photoluminescence property of the QDs [15]. The ECL behavior of the GSH-TGA-CdTe QDs was also influenced by the capping agent. As shown in Fig. 3, the ECL intensity of the QDs capped with GSH alone was low. Moreover, the ECL signal decreased under continuous potential sweeping, indicating the stability was poor. However, when mixed capping agent (both TGA and GSH) was used, the ECL emission and the stability were improved greatly. The relative standard deviation (RSD) of the ECL intensity was 17.6% for GSH-CdTe and 2.5% for GSH-TGA-CdTe (n = 5), respectively, suggesting that the
30
H. Wang et al. / Electrochimica Acta 72 (2012) 28–31
3600
3000 40
a
3000
a
2500
ECL intensity / a.u.
2400
I /µ A
ECL intensity / a.u.
30
20
b 10
1800 0
1200
0.0
0.3
0.6
0.9
1.2
b
E / V vs. Ag/AgCl
2000
1500
1000
600
500
0
0
0.0
0.3
0.6
0.9
E / V vs. Ag/AgCl Fig. 2. ECL-potential curves of 5 M GSH-TGA-CdTe in 0.1 M HAc–NaAc buffer (pH 7.0) (a) without and (b) with 7.5 nM Pb2+ . The inset shows the corresponding cyclic voltammograms. Scan rate: 100 mV/s.
using of mixed capping agent could improve the ECL property of the QDs because of their different structures. GSH possesses long and branched chain while TGA is short carbon chain and more flexible than GSH and TGA can further cap the surface defects. The combination of the two capable of coordinating with the surface atoms more efficiently, resulting in enhancement of ECL intensity and stability [17]. Moreover, the molar ratio between GSH and TGA affect the ECL intensity and the maximum ECL was achieved at a molar ratio of 1:3 between GSH and TGA. Fig. 4 depicts the effect of the refluxing time on the ECL intensity. It is observed that the ECL intensity gradually increased with increasing the refluxing time. As prolonging the refluxing time, the size of the GSH-TGA-CdTe QDs increased, suggesting that the ECL intensity of the QDs was size-dependent, consistent with the previous report [9]. According to the ECL energy match theory, the ECL intensity depends on the yield of excited states of QDs. With the particle size increases, the band gap energy of QDs decrease. The energy generated from the ECL reaction more matches the
GSH-TGA-CdTe
ECL intensity / a.u.
3000
2000
1000 GSH-CdTe
0
100
100
200
Time / s Fig. 3. ECL curves of GSH-TGA-CdTe and GSH-CdTe.
300
150
200
250
300
Time / min Fig. 4. Effect of the refluxing time on the ECL intensity of the QDs (5 M). Inset: the UV–vis and PL spectra of GSH-TGA-CdTe obtained at refluxing time of 200 min.
chemical energy for the formation of excited state of the QDs, leading to high ECL emission. With further increasing the particle size, the ECL intensity decreased. This is because the increase in the size results in smaller surface area and reduction of the electron transfer [9]. Therefore, the QDs obtained at the refluxing time of 200 min were selected in the following experiments. In addition, the ECL of GSH-TGA-CdTe QDs was increased with increasing the pH, however, in order to avoid the precipitation of Pb2+ , pH 7.0 was chosen. Moreover, different buffers such as Tri-HCl, HAc-NaAc, phosphate and borate buffer were tested and it is found that HAc-NaAc could produce the highest ECL response. Furthermore, the effect of the concentration of the QDs was studied. The ECL intensity increases with increasing concentration of the QDs and reached a plateau when the concentration of QDs was 5 M. Therefore, the amount of the QDs was chosen to be 5 M. 3.3. Quenching effect and ECL detection of Pb2+ After addition of trace amount of Pb2+ to the solution, the ECL signal of GSH-TGA-CdTe QDs decreased dramatically, while the current changed a little (Fig. 2). The significant decrease in the ECL upon addition of Pb2+ may be ascribed to the competitive binding of GSH between the QDs and Pb2+ due to the high affinity of GSH with Pb2+ , which consequently changed the surface properties of the QDs, leading to decrease in the ECL intensity [12]. The above result was in agreement with the previous report that the surface states was the key factors determining the ECL of the QDs [5,6]. On the basis of the quenching effect on the anodic ECL of GSH-TGACdTe QDs, a simple method for Pb2+ detection could be developed. As shown in Fig. 5, the ECL signal decreased with increasing the concentration of Pb2+ . The dependence of the ECL intensity of the QDs on the Pb2+ concentration can be described by the following equation [8,19]: ln
0
50
1.2
I 0
I
= [Pb2+ ]/[QDs]
Here, I0 and I were the ECL emission in the absence and presence of Pb2+ , respectively, and was the number of CdTe molecules per colloidal particle, which is a constant when the concentration of the QDs was fixed. In addition, the RSD for 7.4 nM Pb2+ determination was 4.2% (n = 5). Under the optimal conditions, a liner range from 0.8 to 15 nM (R = 0.997) with a detection limit of 0.26 nM was achieved
H. Wang et al. / Electrochimica Acta 72 (2012) 28–31
31
Table 1 ECL determination of Pb2+ in three water samples with the GSH-TGA-CdTe QDs. Water samples
Original (nM)
Added (nM)
Found (nM)
Recovery (%)
A
1.49 1.52 1.38
3.7 3.7 3.7
5.06 5.42 5.28
97 105 105
B
1.39 1.42 1.42
3.7 3.7 3.7
5.24 5.21 4.97
104 103 96
C
1.56 1.42 1.42
3.7 3.7 3.7
5.37 5.10 5.31
103 99 105
A, tape water; B, Jinghu Lake water; C, Changjiang River water.
4. Conclusion 1.0
In summary, a new path to improve the ECL efficiency of CdTe QDs by using mixed capping agent of TGA and GSH was presented. Moreover, a sensitive and selective method for the determination of Pb2+ was developed based on the ECL of the as-prepared QDs and a detection limit of 0.26 nM was achieved.
a
0.8
ECL (I/I0)
0.6
h
Acknowledgments
0.4
We gratefully acknowledge the support of State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (SKLEAC201108), the National Natural Science Foundation of China (No. 20705002) and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
0.2
0.0 0
5
10
15
20
25
Time / s Fig. 5. ECL response of GSH-TGA-CdTe in the presence of various Pb2+ concentrations (a–h): 0.8, 3.1, 5.3, 7.4, 9.4, 11.3, 13.2 and 15 nM. The inset shows the resulting calibration curve.
for the determination of Pb2+ , which is lower than previous report (20 nM) [12]. To study the selectivity of the method, different cations (Na+ , + K , Mg2+ , Ca2+ , Ba2+ , Ni2+ , Zn2+ , Hg2+ , Fe2+ , Fe3+ , Al3+ , Cu2+ ) were added to the QDs system and the ECL signal was recorded. Only Hg2+ exhibited noticeable ECL quenching. In addition, it is reported that copper showed stronger fluorescence quenching of CdTe QDs than lead. However, when the same amount of Cu2+ and Pb2+ were added to the CdTe QDs, the ECL quenching of Cu2+ was less distinct compared with that of Pb2+ (the I/I0 was 0.7072 and 0.1795 for Cu2+ and Pb2+ , respectively), indicating high selectivity of the proposed method. The ECL of GSH-TGA-CdTe QDs was applied for the determination of Pb2+ in domestic water samples. As depicted in Table 1, the recovery obtained was satisfactory, indicating that the proposed method hold promise for the determination of Pb2+ in natural water samples.
References [1] Z.Y. Tang, N.A. Kotov, M. Giersig, Science 297 (2002) 237. [2] X. Wang, Y. Du, S. Ding, Q. Wang, G. Xiong, M. Xie, X. Shen, D. Pang, Journal of Physical Chemistry B 110 (2006) 1566. [3] B.Y. Han, J.P. Han, E.K. Wang, Analytical Chemistry 81 (2009) 5569. [4] Z. Ding, B.M. Quinn, S.K. Karam, L.E. Pell, B.A. Korgel, A.J. Bard, Science 296 (2002) 1293. [5] N. Myung, Z. Ding, A.J. Bard, Nano Letters 2 (2002) 1315. [6] Y. Bae, N. Myung, A.J. Bard, Nano Letters 4 (2004) 1153. [7] X. Liu, H. Jiang, J.P. Lei, H.X. Ju, Analytical Chemistry 79 (2007) 8055. [8] L.H. Zhang, L. Shang, S.J. Dong, Electrochemistry Communications 10 (2008) 1452. [9] L.J. Hua, H.Y. Han, X.J. Zhang, Talanta 77 (2009) 1654. [10] L.J. Hua, J.J. Zhou, H.Y. Han, Electrochimica Acta 55 (2010) 1265. [11] A.R. Flegal, D.R. Smith, Reviews of Environmental Contamination and Toxicology 143 (1995) 1. [12] E.M. Ali, Y.G. Zheng, H.H. Yu, J.Y. Ying, Analytical Chemistry 79 (2007) 9452. [13] H.M. Wu, J.G. Liang, H.Y. Han, Mikrochimica Acta 161 (2008) 81. [14] Q.Z. Wu, H.Q. Cao, Q.Y. Luan, J.Y. Zhang, Z. Wang, J.H. Warner, A.A.R. Watt, Inorganic Chemistry 47 (2008) 5882. [15] Y.H. Zhang, H.S. Zhang, M. Ma, X.F. Guo, H. Wang, Applied Surface Science 255 (2009) 4747. [16] Y.X. Yan, Y. Mu, G.D. Feng, L. Zhang, L.L. Zhu, L. Xu, R. Yang, Q.H. Jin, Chemical Research in Chinese Universities 24 (2008) 8. [17] J.N. Tian, R.J. Liu, Y.C. Zhao, Q. Xu, S.L. Zhao, Journal of Colloid and Interface Science 336 (2009) 504. [18] W.W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Chemistry of Materials 15 (2003) 2854. [19] K.W. Huang, C.J. Yu, W.L. Tseng, Biosensors and Bioelectronics 25 (2010) 984.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemiluminescence of CdTe quantum dots capped with glutathione and thioglycolic acid and its sensing of pb2+ Haiyan Wang ∗ , Qiongfang Chen, Zhian Tan, Xunxun Yin, Lun Wang Anhui Key Laboratory of Chemo-biosensing, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2012 Received in revised form 27 March 2012 Accepted 27 March 2012 Available online 4 April 2012 Keywords: Electrochemiluminescence CdTe quantum dots TGA-GSH Pb2+
a b s t r a c t Water-soluble and stable CdTe quantum dots (QDs) capped with glutathione (GSH) and thioglycolic acid (TGA) were synthesized (denoted as GSH-TGA-CdTe) and the electrochemiluminescence (ECL) behavior of the QDs was investigated. As a result of specific interaction between the GSH and Pb2+ , the ECL of the QDs was selectively reduced in the presence of Pb2+ . Due to the low ECL intensity of QDs capped with GSH alone, TGA as another capping agent was introduced to improve the sensitivity. On the basis of the quenching effect on the ECL of GSH-TGA-CdTe QDs, a sensitive and selective method for the determination of Pb2+ was developed and a detection limit of 0.26 nM was obtained, which was lower than previous report. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Semiconductor quantum dots (QDs), also known as nanocrystals (NCs), have gained a center stage of interest both in fundamental and industrial area as a consequence of the unique optical and electronic properties and they have shown great promise in molecular detection [1–3]. Since the first report on the electrochemiluminescence (ECL) of silicon QDs by Bard’s group [4], the ECL of the semiconductor QDs, especially the II–VI semiconductor (e.g. CdS, CdSe and CdTe) has been extensively studied [5–11]. The subsequent work of Bard el al. showed that the ECL emission was generated in an annihilation process in which the electrogenerated reduced species (QD−• ) are in collision with the oxidized species (QD+• ) [5,6]. Furthermore, the ECL of QDs has been applied for determination of various analytes. Ju et al. [7] have studied the anodic ECL of CdTe QDs on a tin-doped indium oxide (ITO) electrode and its analytical application for detection of catechol derivatives based on the energy transfer to analytes. Dong’s group [8] has applied the ECL of CdTe QDs for the sensitive and selective determination of Cu2+ . Han and co-workers reported the size-dependent ECL behavior of water-soluble CdTe QDs and selective sensing of l-cysteine [9] and they also investigated the direct ECL of CdTe QDs based on room temperature ionic liquid film and the highly sensitive sensing of gossypol [10]. Lead ion (Pb2+ ), a well-known chemical pollutant, possesses a serious threat to human health and the environment [11]. It is per-
∗ Corresponding author. Tel.: +86 553 3869303; fax: +86 553 3869303. E-mail address: [email protected] (H. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.03.146
sistent in the environment and can produce toxic effects in plants and animals as lead is nondegradable and lead-poisoning causes various neurotoxic effects particularly in children. Traditional methods such as inductively coupled plasma mass spectrometry often require expensive and sophisticated instrumentation as well as complicated sample preparation processes [12]. Therefore, it is of significance to develop simple and sensitive method for determination of Pb2+ . A novel method for the determination of Pb2+ was proposed by Han et al. [13] based on quenching of the fluorescence of thiol-capped CdTe QDs and a detection limit of 0.27 M was achieved. Recently, Ying et al. [12] have reported glutathione (GSH)-capped ZnCdSe and CdTe QDs as selective fluorescent probe for detection of Pb2+ with low detection limit (20 nM). GSH has two free COOH groups and a NH2 group and it has high affinity with Pb2+ , thus it played a crucial role in improving the selectivity. On the basis of the aforementioned results, we conclude that GSH-capped QDs may be utilized for the ECL detection of Pb2+ with high selectivity [14]. However, the ECL intensity of CdTe QDs capped with GSH alone was low and the stability was poor. Previous reports have shown that capping agent affected the optical properties of the QDs greatly [15]. CdTe QDs in the presence of both GSH and cysteine as stabilizers exhibited much higher photoluminescence quantum yield [16] and the quantum yield of CdTe QDs capped by GSH and thioglycolic acid (TGA) can reach 63% [17]. The above results suggest that the ECL of CdTe QDs may be improved by using two stabilizers. In this study, we present a simple, sensitive and selective ECL method for the detection of Pb2+ based on CdTe QDs capped with GSH and TGA (GSH-TGA-CdTe). It is found that the ECL intensity and the stability f the QDs were improved dramatically compared
H. Wang et al. / Electrochimica Acta 72 (2012) 28–31
with the QDs capped with GSH alone. With the as-prepared QDs, a liner range from 0.8 to 15 nM (R = 0.997) with a detection limit of 0.26 nM was achieved for the determination of Pb2+ .
0.10
29
A
60 min 120 min 180 min 220 min 280 min
2. Experimental 2.1. Reagents Sodium hydroxide (NaOH), tellurium powder, sodium borohydride (NaBH4 ) and thioglycolic acid (TGA) were purchased from Sinopharm Chemical Reagent Co., Ltd. Glutathione (GSH) was obtained from Hefei Bomei Biotechnology Co., Ltd. All other chemicals were of analytical reagent grade and used as received. Doubly distilled water was used in all experiments.
Absorbance / a.u.
0.08
0.06
0.04
0.02
0.00
2.2. Apparatus
450
500
550
600
650
700
Wavelength / nm
B
60 min 120 min 180 min 220 min 280 min
2000
PL intensity / a.u.
Electrochemical measurements were performed with a CHI 760C electrochemical workstation (Chenhua, Shanghai). Conventional three-electrode system was used with indium tin oxide glass (ITO, area: 0.4 cm2 ) as the working electrode, a Pt wire as the auxiliary electrode and an Ag/AgCl (saturated KCl) as the reference electrode. The ECL signal was detected and recorded by IFFM-E flow injection chemiluminescence analyzer (Xi’an Remax, China). Unless noted otherwise, the photomultiplier tube (PMT) was biased at 800 V. The UV–vis spectra were recorded on a UV–vis spectrophotometer (Hitachi U-3010, Japan). The photoluminescence (PL) spectra were obtained by a Hitachi F-4500 spectrofluorometer (Tokyo, Japan).
400
1600
1200
800
400
2.3. Preparation of GSH-TGA modified CdTe QDs CdTe QDs were prepared using CdCl2 and NaHTe as precursors according to the previous report [16,17]. Briefly, 37.6 mg of sodium borohydride, 1 ml H2 O and 16.2 mg of tellurium powder were added to a small flask to prepare NaHTe. 72.6 mg CdCl2 ·2.5H2 O, GSH or mixture of GSH and TGA (the molar ratio of GSH/TGA was 1:3) were dissolved in water and the pH of the mixture was adjusted to 11 with 1 M NaOH. After deaeration of the solution by N2 , the freshly prepared NaHTe solution was injected. The molar ratio of Cd2+ /stabilizer/HTe− was fixed at 1:1.2:0.4. The solution was refluxed and the reaction time was controlled to obtain different-sized QDs. The resulted QDs were denoted as GSH-CdTe and GSH-TGA-CdTe, respectively. 3. Results and discussion 3.1. Optical properties of the GSH-TGA-CdTe QDs UV–vis absorption and photoluminescence (PL) emission spectra are often used to monitor the size-dependent optical properties of the QDs. Fig. 1 shows the temporal evolution of the UV–vis and PL spectra of GSH-TGA-CdTe QDs. With prolonging the reflux time, the absorption peak and the PL emission peak shifts toward longer wavelength, indicating clearly the increase in QDs size. Based on the UV–vis absorption spectra, the particle size of the QDs is calculated to be 1.97, 2.45, 2.80, 2.95 and 3.04 nm, respectively, using the following empirical equation [18]: D = (9.8127 × 10−7 )3 − (1.7147 × 10−3 )2 + 1.0064 − 194.84 In which, D (nm) is the diameter of a given QDs, and (nm) is the wavelength of the first excitonic absorption peak of UV–vis absorption spectra.
0 400
450
500
550
600
650
700
Wavelength / nm Fig. 1. (A) UV–vis and (B) PL spectra of GSH-TGA-CdTe as the function of refluxing time.
3.2. Electrochemical and ECL behavior of GSH-TGA-CdTe QDs The electrochemical and ECL properties of the GSH-TGA-CdTe QDs were investigated in air-saturated 0.1 M HAc–NaAc solution (pH 7.0) by sweeping the potential in the range from 0 to 1.3 V at 100 mV/s. As shown in Fig. 2, GSH-TGA-CdTe QDs showed an intensive anodic ECL emission at the ITO electrode with the peak emission at +1.3 V and onset potential of +0.9 V. In the air-saturated solution, in the absence of QDs, ECL emission was not observed, suggesting that the anodic ECL emission resulted from the oxidation of the QDs, which led to the formation of ECL emitter. Meanwhile, when the detection solution was bubbled with N2 , the ECL emission decreased dramatically and gradually disappeared as prolonging the N2 purging time, indicating that dissolved oxygen played an important role in the ECL emission procedure [7]. The capping agent plays an important role in affecting the photoluminescence property of the QDs [15]. The ECL behavior of the GSH-TGA-CdTe QDs was also influenced by the capping agent. As shown in Fig. 3, the ECL intensity of the QDs capped with GSH alone was low. Moreover, the ECL signal decreased under continuous potential sweeping, indicating the stability was poor. However, when mixed capping agent (both TGA and GSH) was used, the ECL emission and the stability were improved greatly. The relative standard deviation (RSD) of the ECL intensity was 17.6% for GSH-CdTe and 2.5% for GSH-TGA-CdTe (n = 5), respectively, suggesting that the
30
H. Wang et al. / Electrochimica Acta 72 (2012) 28–31
3600
3000 40
a
3000
a
2500
ECL intensity / a.u.
2400
I /µ A
ECL intensity / a.u.
30
20
b 10
1800 0
1200
0.0
0.3
0.6
0.9
1.2
b
E / V vs. Ag/AgCl
2000
1500
1000
600
500
0
0
0.0
0.3
0.6
0.9
E / V vs. Ag/AgCl Fig. 2. ECL-potential curves of 5 M GSH-TGA-CdTe in 0.1 M HAc–NaAc buffer (pH 7.0) (a) without and (b) with 7.5 nM Pb2+ . The inset shows the corresponding cyclic voltammograms. Scan rate: 100 mV/s.
using of mixed capping agent could improve the ECL property of the QDs because of their different structures. GSH possesses long and branched chain while TGA is short carbon chain and more flexible than GSH and TGA can further cap the surface defects. The combination of the two capable of coordinating with the surface atoms more efficiently, resulting in enhancement of ECL intensity and stability [17]. Moreover, the molar ratio between GSH and TGA affect the ECL intensity and the maximum ECL was achieved at a molar ratio of 1:3 between GSH and TGA. Fig. 4 depicts the effect of the refluxing time on the ECL intensity. It is observed that the ECL intensity gradually increased with increasing the refluxing time. As prolonging the refluxing time, the size of the GSH-TGA-CdTe QDs increased, suggesting that the ECL intensity of the QDs was size-dependent, consistent with the previous report [9]. According to the ECL energy match theory, the ECL intensity depends on the yield of excited states of QDs. With the particle size increases, the band gap energy of QDs decrease. The energy generated from the ECL reaction more matches the
GSH-TGA-CdTe
ECL intensity / a.u.
3000
2000
1000 GSH-CdTe
0
100
100
200
Time / s Fig. 3. ECL curves of GSH-TGA-CdTe and GSH-CdTe.
300
150
200
250
300
Time / min Fig. 4. Effect of the refluxing time on the ECL intensity of the QDs (5 M). Inset: the UV–vis and PL spectra of GSH-TGA-CdTe obtained at refluxing time of 200 min.
chemical energy for the formation of excited state of the QDs, leading to high ECL emission. With further increasing the particle size, the ECL intensity decreased. This is because the increase in the size results in smaller surface area and reduction of the electron transfer [9]. Therefore, the QDs obtained at the refluxing time of 200 min were selected in the following experiments. In addition, the ECL of GSH-TGA-CdTe QDs was increased with increasing the pH, however, in order to avoid the precipitation of Pb2+ , pH 7.0 was chosen. Moreover, different buffers such as Tri-HCl, HAc-NaAc, phosphate and borate buffer were tested and it is found that HAc-NaAc could produce the highest ECL response. Furthermore, the effect of the concentration of the QDs was studied. The ECL intensity increases with increasing concentration of the QDs and reached a plateau when the concentration of QDs was 5 M. Therefore, the amount of the QDs was chosen to be 5 M. 3.3. Quenching effect and ECL detection of Pb2+ After addition of trace amount of Pb2+ to the solution, the ECL signal of GSH-TGA-CdTe QDs decreased dramatically, while the current changed a little (Fig. 2). The significant decrease in the ECL upon addition of Pb2+ may be ascribed to the competitive binding of GSH between the QDs and Pb2+ due to the high affinity of GSH with Pb2+ , which consequently changed the surface properties of the QDs, leading to decrease in the ECL intensity [12]. The above result was in agreement with the previous report that the surface states was the key factors determining the ECL of the QDs [5,6]. On the basis of the quenching effect on the anodic ECL of GSH-TGACdTe QDs, a simple method for Pb2+ detection could be developed. As shown in Fig. 5, the ECL signal decreased with increasing the concentration of Pb2+ . The dependence of the ECL intensity of the QDs on the Pb2+ concentration can be described by the following equation [8,19]: ln
0
50
1.2
I 0
I
= [Pb2+ ]/[QDs]
Here, I0 and I were the ECL emission in the absence and presence of Pb2+ , respectively, and was the number of CdTe molecules per colloidal particle, which is a constant when the concentration of the QDs was fixed. In addition, the RSD for 7.4 nM Pb2+ determination was 4.2% (n = 5). Under the optimal conditions, a liner range from 0.8 to 15 nM (R = 0.997) with a detection limit of 0.26 nM was achieved
H. Wang et al. / Electrochimica Acta 72 (2012) 28–31
31
Table 1 ECL determination of Pb2+ in three water samples with the GSH-TGA-CdTe QDs. Water samples
Original (nM)
Added (nM)
Found (nM)
Recovery (%)
A
1.49 1.52 1.38
3.7 3.7 3.7
5.06 5.42 5.28
97 105 105
B
1.39 1.42 1.42
3.7 3.7 3.7
5.24 5.21 4.97
104 103 96
C
1.56 1.42 1.42
3.7 3.7 3.7
5.37 5.10 5.31
103 99 105
A, tape water; B, Jinghu Lake water; C, Changjiang River water.
4. Conclusion 1.0
In summary, a new path to improve the ECL efficiency of CdTe QDs by using mixed capping agent of TGA and GSH was presented. Moreover, a sensitive and selective method for the determination of Pb2+ was developed based on the ECL of the as-prepared QDs and a detection limit of 0.26 nM was achieved.
a
0.8
ECL (I/I0)
0.6
h
Acknowledgments
0.4
We gratefully acknowledge the support of State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (SKLEAC201108), the National Natural Science Foundation of China (No. 20705002) and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
0.2
0.0 0
5
10
15
20
25
Time / s Fig. 5. ECL response of GSH-TGA-CdTe in the presence of various Pb2+ concentrations (a–h): 0.8, 3.1, 5.3, 7.4, 9.4, 11.3, 13.2 and 15 nM. The inset shows the resulting calibration curve.
for the determination of Pb2+ , which is lower than previous report (20 nM) [12]. To study the selectivity of the method, different cations (Na+ , + K , Mg2+ , Ca2+ , Ba2+ , Ni2+ , Zn2+ , Hg2+ , Fe2+ , Fe3+ , Al3+ , Cu2+ ) were added to the QDs system and the ECL signal was recorded. Only Hg2+ exhibited noticeable ECL quenching. In addition, it is reported that copper showed stronger fluorescence quenching of CdTe QDs than lead. However, when the same amount of Cu2+ and Pb2+ were added to the CdTe QDs, the ECL quenching of Cu2+ was less distinct compared with that of Pb2+ (the I/I0 was 0.7072 and 0.1795 for Cu2+ and Pb2+ , respectively), indicating high selectivity of the proposed method. The ECL of GSH-TGA-CdTe QDs was applied for the determination of Pb2+ in domestic water samples. As depicted in Table 1, the recovery obtained was satisfactory, indicating that the proposed method hold promise for the determination of Pb2+ in natural water samples.
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