- Email: [email protected]
A novel chemiluminescence method for determination of bisphenol Abased on the carbon dot-enhanced HCO3−–H2O2 system
Journal of Luminescence 158 (2015) 160–164
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A novel chemiluminescence method for determination of bisphenol A based on the carbon dot-enhanced HCO3 –H2O2 system Mohammad Amjadi n, Jamshid L. Manzoori, Tooba Hallaj Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 5166616471, Iran
art ic l e i nf o Article history: Received 23 July 2014 Received in revised form 22 September 2014 Accepted 23 September 2014 Available online 30 September 2014 Keywords: Carbon dots Chemiluminescence Bisphenol A H2O2
a b s t r a c t A simple and sensitive chemiluminescence (CL) method on the basis of carbon dot (C-dot) enhanced HCO3 –H2O2 system, is designed for the determination of bisphenol A (BPA). The very weak CL of the HCO3 –H2O2 system is enhanced by a factor of 100 in the presence of C-dots. Possible mechanisms that lead to the effect were elucidated by recording fluorescence and CL spectra and studying the effect of some radical scavengers. This enhancement is inhibited by BPA in the concentration range from 1.0 to 100 mg L 1. This is exploited for its trace determination with a detection limit (3 s) of 0.3 mg L 1. The established method was applied to the determination of BPA in baby bottle and water samples with satisfactory results. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Bisphenol A (BPA), 2,2-bis (4-hydroxyphenyl) propane, is a monomer used in the synthesis of polycarbonate and epoxy resins and some other plastics. They are widely used as food-storage or packaging materials, such as feeding bottles, water bottles and cans and tableware [1]. When these plastic or resin materials expose to heat, acid or base, their linking ester bonds of BPA monomers will be hydrolyzed, thus releasing BPA into the environment [2,3]. Due to the possible estrogenic and carcinogenic effects of BPA, this monomer is harmful for human health. Therefore, the analysis of BPA has appeared of great importance and various analytical methods including chromatography [4,5], electrochemistry [6,7], spectrophotometry [8], fluorescence [9], chemiluminescence (CL) [10–12] and electrochemiluminescence (ECL) [13,14] have been reported for this purpose. Compared to the other analytical techniques, CL methods exhibit several advantages such as high sensitivity, simplicity of instrumentation, low detection limits, large calibration ranges and short analysis time. However, due to very low quantum yield, applicability of some traditional CL systems, such as HCO3–H2O2 is limited. Therefore, it is necessary to enhance their CL intensity for applications in analytical chemistry. In recent years, nanomaterials have been extensively applied to enhance CL emission [15–17]. For example, Lin and coworkers have investigated the enhancing effect of some nanomaterials such as AuNPs [18], CdTe [19], CdSe/Cds [20],
n
Corresponding author. Tel.: þ 98 4133393109; fax: þ 98 4133340191. E-mail address: [email protected] (M. Amjadi).
http://dx.doi.org/10.1016/j.jlumin.2014.09.045 0022-2313/& 2014 Elsevier B.V. All rights reserved.
Cu/Ni metal NPs [21], NaYF4:Yb3 þ /Er3 þ NPs [22] and carbon nanospheres [23] on the very week HCO3–H2O2 CL reaction. These enhanced CL systems have been applied to the determination of L-ascorbic acid [20], ammonia [22] and hydrogen peroxide [23] in real samples. Also, Azizi et al. [24] reported that CdS has sensitizing effect on the HCO3–H2O2 system and applied it for the analysis of epinephrine in pharmaceutical formulation. Carbon dots (C-dots) are a fascinating class of luminescent carbon nanomaterials that comprise discrete, quasispherical nanoparticles with sizes below 10 nm. They have attracted tremendous attention in varies fields since discovered in 2004 because of their alluring properties such as size- and wavelength-dependent luminescence emission, excellent photostability, favorable biocompatibility, simplicity of synthesis, good water solubility [25,26]. In addition, C-dots could be promising alternative to semiconductor quantum dots duo to low toxicity, high chemical stability and low environmental hazard, [27,28]. Therefore, they have been applied in various fields such as bioimaging [29,30], photocatalysis [31] and fluorescence sensing [32–35]. More recently, the CL and ECL [36] behavior of C-dots has also been studied. They could participate in a CL reaction as emitting species, after direct oxidation [37–42]; as catalysts of a reaction involving others luminophores [43,44]; or as emitter, after CL energy transfer [41,42,44–46]. In the present work, we studied the effect of C-dots on the HCO3 –H2O2 CL reaction. It was found that C-dots could significantly intensify the weak CL emission of this reaction. Moreover, based on the diminishing effect of BPA on the HCO3 –H2O2–C–dots CL system, a new CL method has been designed for the analysis of trace levels of BPA in baby bottle and water samples.
M. Amjadi et al. / Journal of Luminescence 158 (2015) 160–164
161
2. Experimental 2.1. Apparatus The chemiluminescence signals were monitored by LUMAT LB 9507 chemiluminometer (Berthold; www.berthold.com). CL spectrum was recorded with RF-540 spectrofluorimeter (Shimadzu, Japan) using flow mode with the excitation light source being turned off. The fluorescence spectra were also recorded by the same instrument under normal conditions. UV–vis spectra were recorded on a Cary-100 Spectrophotometer (Varian; www.varia ninc.com). The size and shape of C-dots were characterized with transmission electron microscopy (TEM, Leo 906, Zeiss, Germany). 2.2. Reagents All reagents used were of analytical reagent grade. Doubly distilled deionized water (obtained from Ghazi Serum Co. Tabriz, Iran) was used throughout the experiment. Sodium bicarbonate, hydrogen peroxide, citric acid monohydrate and poly(ethylene glycol) with average molecular weight of 1000 were all purchased from Merck (Darmstadt, Germany, www.merck-chemicals.com). 2.3. Synthesis of C-dots C-dots were prepared by hydrothermal treatment according to the our previous work [37]. Briefly, 15 mL of glycerin, 1.0 g of poly(ethylene glycol) 1000 and 1.0 g of citric acid monohydrate were mixed. Then, the mixture was transferred into a 100 mL Teflon equipped stainless steel autoclave and heated at 220 1C for 12 h. The prepared C-dots were purified by dialysis membrane (with MW cutoff of 2000) against deionized water for 24 h. The concentration of the C-dots solution was calculated according to the concentration of carbon atom in the carbon source.
Fig. 1. (a) TEM image of the prepared C-dots. (b) Fluorescence spectra for C-dots excited at wavelengths of 300–425 nm, with increments of 25 nm (a–e).
2.4. General procedure for CL monitoring Chemiluminescence analyses were carried out in a 3 mL tube, in the static injection condition. An amount of 200 μL of C-dots (0.57 mol L 1) and 500 μL of HCO3 (1.0 mol L 1) were added into the cell. Then an appropriate volume of sample or standard solution was added and the final volume was reached to 900 μL with distilled water. After injection of 100 μL of 1.0 mol L 1H2O2 by an automatic injector, monitoring of CL signal versus time was started automatically. Maximum CL intensity was used as analytical signal. 2.5. Sample pretreatment A baby bottle made from polycarbonate plastic was prepared from local store. They were cut into small pieces (size 0.5 cm2). An accurately weighed amount of sample ( 5 g) was then transferred into a 50 mL flask filled with pure water. The flask was placed in the water bath kept at 50 1C to extract BPA. After 12 h, the content of BPA in the aqueous extracts was determined. Water samples were analyzed without any pretreatment.
3. Results and discussion 3.1. Characterization of C-dots In this study C-dots were synthesized by solvothermal route using citric acid as a carbon source and poly(ethylene glycol) 1000 as a passivation agent. Fig. 1 shows the TEM image of C-dots. The particles of C-dots are almost spherical and monodisperse with average sizes below 10 nm. The fluorescence spectra of C-dots are
shown in Fig. 1b. As can be seen, the maximum emission was obtained at about 455 nm with an excitation wavelength of 375 nm. Moreover, as expected, the emission wavelength red shifted with increasing excitation wavelength, which revealed a distribution of the different surface energy traps of the C-dots. 3.2. HCO3 –H2O2–C-dot CL reaction The effect of synthesized C-dots on the H2O2–HCO3 CL reaction was investigated. This system produces a very weak CL signal, and two peaks can be observed in its kinetic profile. The first peak that appeared 1 s after injection of the oxidant solution quenched quickly. After 15 s, the second maximum value was recorded, and the CL lasted for about 50 s. These peaks may be attributed to three emitting species of H2O2–HCO3 CL reaction, (1O2), (O2)n2 and excited (CO2)n2. According to literature, the lifetime of 1O2 was only 3.1 μs in H2O. Therefore, the first CL peak can be attributed to 1O2 and (O2)n2, and the second peak to (CO2)n2 [19]. Our studies indicated that the CL intensity of both peaks enhanced when a small volume of C-dot solution was added to this reaction (Fig. 2). However, the enhancement that observed for the second peak (about 100-fold) was much more remarkable than that of observed for the first peak (about 3-fold). Therefore, in this work, the CL intensity of the second peak was applied as analytical signal. It is interesting to compare this results with those obtained by Lin and coworkers using 100 nm carbon nanospheres [23]. The latter nanoparticles have been reported to enhance the first peak of the H2O2–HCO3 CL reaction much more than the second peak. Moreover, the overall enhancement by carbon nanospheres is much lower than the enhancement effect of C-dots (40-fold compared to 100-fold).
162
M. Amjadi et al. / Journal of Luminescence 158 (2015) 160–164
1500
150
CL Intesity a.u.
100
1000 50 0
500
0
0
0
100
200
200
300
Time(s)
CL Intensity a.u.
12000
8000
4000
0
Fig. 3. (a) CL spectrum of HCO3 –H2O2–C-dots system, obtained with continuous flow of reagents: H2O2 (0.2 M) in one line and C-dot solution (0.55 mol L 1) and HCO3 (1 mol L 1) in other line. (b) Fluorescence spectrum of 0.228 M C-dots at λex ¼450 nm.
0
200
400
600
Time (s)
Fig. 2. Kinetic curve for (a) HCO3 –H2O2, (b) H2O2–C-dots and (c) HCO3 –H2O2– C-dots CL reactions. Conditions: HCO3 , 0.5 M, H2O2, 0.1 M, C-dot, 0.114 M.
Thus, the enhancement of the first peak can be due to the direct CL of Cdots and the energy transfer from (1O2)n2 to C-dots. Based on these facts, the enhanced CL mechanism can be described as follows [19,22,23]: HCO3 þ H2O2-HCO4 þH2O
Although C-dots were purified by dialysis, a control experiment with the reagents used for the preparation of C-dots (the mixture of citric acid, poly(ethylene glycol) and glycerin) was carried out. It was found that this solution has no contribution to the enhancement effect. Furthermore, direct reaction of H2O2 with C-dots was examined. As can be seen from Fig. 3b, a weak CL is produced by this reaction just after injection of the oxidant solution.
HCO4 -CO3 þ dOH OH þHCO3 -dOH þ dHCO3
d
2HCO3-(CO2)n2 þH2O2 (CO2)n2 þC-dot-2CO2 þC-dotn C-dotn-C-dotþhν ( 510 nm)
3.3. Possible CL reaction mechanism In order to identify the emitting species in the CL reaction, the CL emission spectrum of HCO3–H2O2–C-dot system was obtained using a spectrofluorimeter in flow mode. The result (Fig. 3a) indicated that there was an emission band in the range of 400–600 nm and the CL peak was located at 510 nm. This CL spectrum is similar to the C-dot fluorescence spectrum with the excitation wavelength of 450 nm (Fig. 3b). Therefore, the CL may be reasonably attributed to the excited-state C-dots. As mentioned before, One of the emitting species for HCO3–H2O2 CL reaction is (CO2)n2, which was unstable and decomposed to CO2 releasing energy to generate light at a wavelength higher than 220 nm [19,23]. Therefore, the energy transfer process could occur between the excited (CO2)n2 molecules as donors and C-dots as acceptors. The excited-state C-dots (C-dotsn) finally returned to their ground states with production of CL emission. So, the observed great enhancement for the second peak may be attributed to this energy transfer process. Furthermore, the dO2 and dOH radicals obtained from HCO3–H2O2 reaction could serve as the hole and electron injectors to produce C-dotdþ and C-dotd . Then, the C-dotsn that result from annihilation of C-dotdþ and C-dotd return to the ground state by photon emission. Also, dO2 is unstable in aqueous solution initiating the reactive oxygen chain reaction, which leads to the generation of the (1O2)n2. Then, C-dots could accept the energy from (1O2)n2, generating Cdotn, which returned to the ground state and released photon [42,45].
H2O2 þCO3 -HCO3 þHO2 HO2 -H þ þ dO2 C-dot þ dO2 -C-dot þ þO2 C-dot þOH-C-dot þOH C-dot þ C-dot þ -C-dotn C-dotn-C-dotþhν ( 510 nm)
O2 þ OH-1O2 þOH
O2 þ 1O2-(1O2)n2
1
(1O2)n2 þ C-dot-C-dotn C-dotn-C-dotþhν ( 510 nm) To further confirm this pathway, the effect of some radical scavengers on the CL intensity of HCO3–H2O2–C-dot system was investigated (Table 1). Thiourea and DMSO, the hydroxide radical scavengers [47], had a remarkable negative effect on the CL intensity. Thus it was assumed that dOH is generated in the examined systems. Moreover, NaN3 which was an effective radical scavenger for 1O2 [48], quenched the CL of the system which provided a evidence that
M. Amjadi et al. / Journal of Luminescence 158 (2015) 160–164
Table 1 Effect of radical scavengers on HCO3 –H2O2–C-dots CL system. Radical scavenger
Concentration (M)
CL intensity
H2O DMSO Thiourea NaN3 Ascorbic acid
– 5 10 3 5 10 3 5 10 3 10 4
10,968 328 494 2381 100
CL Intesity a. u.
20000 16000 12000 8000
3.5. Analytical application of the CL system 0
0.2
0.4 0.6 [HCO3-]/M
0.8
1
10000
CL Intesity a. u.
The effect of HCO3 concentration on the CL intensity was examined (Fig. 4a). It was found that the CL intensity linearly increased with increasing the concentration of HCO3 in the range of 0.1–0.8 M. Since at high concentration the solubility of HCO3 decreases, 0.5 M HCO3 was selected as the optimum concentration for further work. Moreover, the influence of H2O2 concentration on the CL intensity over the range of 0.025–0.3 M was studied. According to the results (Fig. 4b), the maximum CL response was obtained for 0.1 M H2O2. Due to an important role of H2O2 in the formation of free radicals, at lower concentrations the number of excited intermediates is decreased and the CL intensity is diminished. Finally, the effect of C-dot concentration in the range of 0.014–0.228 M on the CL intensity was investigated (Fig. 4c). The CL intensity is increased by increasing the C-dot concentration up to 0.114 mol L 1 and then remained constant. Because the generated energy of HCO3 –H2O2 CL reaction was limited, just a certain amount of C-dots could be excited by the energy transfer process.
4000 0
8000
6000
4000
0
0.1
0.2 [H2O2]/M
0.3
0.4
16000 CL Intensity a. u.
163
12000
3.6. Study of interferences
8000 4000 0
It was found that low concentrations of BPA have a diminishing effect on the CL reaction. BPA has a reducing property and may compete with C-dots and HCO3 for H2O2, which leads to a reduction in the CL emission intensity. Based on this phenomenon, a sensitive method was developed for the determination of BPA. Under the optimum conditions described above, the analytical figures of merit for the determination of BPA was obtained. The CL response was found to be linear in the concentration range of 1.0–100 mg L 1 with a detection limit (3 s) of 0.3 mg L 1. The regression equation was logΔI¼ 2.7 Cþ 4.0, R2 ¼0.9973, where ΔI¼I I0 is the difference between the CL intensity in the presence of BPA (I) and in its absence (I0), and C is concentration of BPA in mg L 1. The relative standard deviation (RSD) was obtained to be 2.4% and 1.5% for five replicate determinations of 2.5 and 10.0 mg L 1 BPA, respectively. The results indicate that this CL system has good linearity, relatively high sensitivity and suitable precision. Comparison between the developed method and some other reported CL methods for BPA quantification is shown in Table 2. As can be seen, the developed method has better limit of detection than other methods except one.
0
0.05
0.1 0.15 [C-dot]/M
0.2
0.25
Fig. 4. Optimization of the CL reaction conditions. (a) Effect of HCO3 concentration: H2O2, 0.1 M, C-dot 0.057 M. (b) Effect of H2O2 concentration: HCO3 , 0.5 M; other conditions are as in a (c) effect of C-dot concentration: H2O2, 0.1 M; other conditions are as in c.
1
O2 contributes to the observed CL. Also, ascorbic acid as a common oxygen free radical scavenger [49] had a negative effect on the CL signal, which further indicated that the generation of a free radical is critical in the CL reaction. 3.4. Optimization of chemical conditions In order to obtain the maximum sensitivity for the reaction, the effects of several variables such as concentration of HCO3 , H2O2 and C-dots on the CL intensity were investigated.
In order to evaluate the selectivity of the developed method, the effects of some common ions on the determination of 10 mg L 1 BPA were investigated. The tolerable concentrations for interfering species in relative error of o5% were summarized in Table 3. These results demonstrate that the method possesses a good selectivity for the determination of BPA in real samples. Table 2 Comparison of the developed CL method for the determination BPA with some previously published CL methods. CL system
HCO4 –AuCl4 Luminol–K3Fe(CN)3 Luminol–KMnO4–AgNPs Luminol ECL Lucigenin–graphene oxide ECL Ru(bpy)23 þ /dibutylamino ethanol ECL HCO3 –H2O2–C-dots
LOD (mg L 1)
Linear range (mg L 1)
Ref.
12 70 0.001 80 0.068 0.01
45–12,000 180–2700 0.01–50 100–5000 0.22–22,800 0.05–25
[11] [12] [10] [50] [13] [14]
0.3
1.0–100
This method
164
M. Amjadi et al. / Journal of Luminescence 158 (2015) 160–164
Table 3 Tolerance limit of interferences on the determination of 10.0 mg L 1 BPA. The tolerable concentration ratios
Interfering species
10,000
Na þ , K þ , Zn2 þ , Ca2 þ , Hg2 þ , Pb2 þ ,Al3 þ NO3 , Cl , SO4 2 , PO4 3 Mg2 þ Fe3 þ , Cu2 þ
8000 500
Table 4 Results for the determination of BPA in real samples. Sample
Added (mg L 1)
Found (mg L 1)
Baby bottle
0.0 20.0 50.0 0.0 10.0 50.0
Well water
a b
a
Recovery (%)
t-Statisticb
9.9 70.1 30.6 71.2 60.4 71.3
– 102.2 7 4.1 100.8 7 2.5
1.7 0.65
36.7 71.4 46.3 71.3 88.2 71.9
– 96.9 7 2.9 101.6 7 2.1
– 1.8 1.3
Mean of three determinations 7 standard deviation. t-critical ¼4.3 for n¼ 2, P¼ 0.05.
3.7. Analysis of real samples The developed method was easily applied to the determination of BPA in baby bottle and water samples. Furthermore, known quantities of BPA were added into the samples, and then the samples were prepared and analyzed according to the general procedure. The obtained results are shown in Table 4. Statistical analysis of these results using Student t-test showed that there are no significant differences between added and found values. 4. Conclusion We described a new analytical method for the determination of BPA based on the C-dot- enhanced HCO3 –H2O2 CL system. In presences of C-dots, the CL intensity of H2O2–HCO3 system remarkably enhanced. A possible mechanism for the reaction was explained, which involves the CL energy transfer from (CO2)n2 as a donor to C-dot as an acceptor, along with a contribution from direct oxidation of C-dots. We found that with addition of even trace levels of BPA to this enhanced CL system, the CL intensity decreased. Based on this phenomenon, a simple and sensitive CL method was designed for the determination of BPA with good linearity, high selectivity and precision, which can be applied to the analysis of BPA in baby bottle and water samples. References [1] K.V. Ragavan, N.K. Rastogi, M.S. Thakur, Trends Anal. Chem. 52 (2013) 248. [2] A. Goodson, H. Robin, W. Summerfield, I. Cooper, Food Addit. Contam. 21 (2004) 1015.
[3] D.S. Lim, S.J. Kwack, K.-B. Kim, H.S. Kim, B.M. Lee, J. Toxicol, Environ. Health A 72 (2009) 1285. [4] E. Yiantzi, E. Psillakis, K. Tyrovola, N. Kalogerakis, Talanta 80 (2010) 2057. [5] J. López-Darias, M. Germán-Hernández, V. Pino, A.M. Afonso, Talanta 80 (2010) 1611. [6] P. Deng, Z. Xu, Y. Kuang, Food Chem. 157 (2014) 490. [7] H. Fan, Y. Li, D. Wu, H. Ma, K. Mao, D. Fan, Anal. Chim. Acta 711 (2012) 24. [8] Z. Mei, H. Chu, W. Chen, F. Xue, J. Liu, H. Xu, Biosens. Bioelectron. 39 (2013) 26. [9] K.V. Ragavan, L.S. Selvakumar, M.S. Thakur, Chem. Commun. 49 (2013) 5960. [10] X. Chen, C. Wang, X. Tan, J. Wang, Anal. Chim. Acta 689 (2011) 92. [11] C. Lu, J. Li, Y. Yang, J.-M. Lin, Talanta 82 (2010) 1576. [12] S. Wang, X. Wei, L. Du, H. Zhuang, Luminescence 20 (2005) 46. [13] H. Dai, S. Zhang, Y. Lin, Y. Ma, L. Gong, G. Xu, M. Fu, X. Li, G. Chen, Anal. Methods 6 (2014) 4746. [14] X. Jiang, W. Ding, C. Luan, Can. J. Chem. 91 (2013) 656. [15] Q. Li, L. Zhang, J. Li, C. Lu, Trends Anal. Chem. 30 (2011) 401. [16] D.L. Giokas, A.G. Vlessidis, G.Z. Tsogas, N.P. Evmiridis, Trends Anal. Chem. 29 (2010) 1113. [17] H. Chen, L. Lin, H. Li, J.-M. Lin, Coord. Chem. Rev. 86 (2014) 263. [18] J.-M. Lin, M. Liu, J. Phys. Chem. B 112 (2008) 7850. [19] H. Chen, L. Lin, Z. Lin, G. Guo, J.-M. Lin, J. Phys. Chem. A 114 (2010) 10049. [20] H. Chen, R. Li, L. Lin, G. Guo, J.-M. Lin, Talanta 81 (2010) 1688. [21] H. Chen, R. Li, H. Li, J.-M. Lin, J. Phys. Chem. C 116 (2012) 14796. [22] H. Chen, H. Li, J.-M. Lin, Anal. Chem. 84 (2012) 8871. [23] H. Chen, L. Lin, Z. Lin, C. Lu, G. Guo, J.-M. Lin, Analyst 136 (2011) 1957. [24] S.N. Azizi, M.J. Chaichi, P. Shakeri, A. Bekhradnia, J. Fluoresc. 23 (2013) 227. [25] J.C.G. Esteves da Silva, H.M.R. Gonçalves, TrAC Trends Anal. Chem. 30 (2011) 1327. [26] S.N. Baker, G.A. Baker, Angew. Chem. Int. Ed. 49 (2010) 6726. [27] R. Hardman, Health Perspect. 114 (2006) 165. [28] Y. Su, Y. He, H. Lu, L. Sai, Q. Li, W. Li, Biomaterials 30 (2009) 19. [29] Y.-P. Sun, B. Zhou, Y. Lin, W. Wang, K.A.S. Fernando, P. Pathak, J. Am. Chem. Soc. 128 (2006) 7756. [30] L. Cao, X. Wang, M.J. Meziani, F. Lu, H. Wang, P.G. Luo, J. Am. Chem. Soc. 129 (2007) 11318. [31] H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, Angew. Chem. Int. Ed. 49 (2010) 4430. [32] H. Gonçalves, P.A.S. Jorge, J.R.A. Fernandes, J.C.G. Esteves da Silva, Sens. Actuators B Chem. 145 (2010) 702. [33] W. Lu, X. Qin, S. Liu, G. Chang, Y. Zhang, Y. Luo, Anal. Chem. 84 (2012) 5351. [34] S.N.A. Mohd Yazid, S.F. Chin, S.C. Pang, S.M. Ng, Microchim. Acta 180 (2012) 137. [35] Y. Guo, Z. Wang, H. Shao, X. Jiang, Carbon 52 (2013) 583. [36] L. Sun, T.-H. Teng, M.d.H. Rashid, M. Krysmann, P. Dallas, Y. Wang, B.-R. Hyun, A.C. Bartnik, G.G. Malliaras, F.W. Wise, E.P. Giannelis, MRS Proc. (2011), http: //dx.doi.org/10.1557/opl.2011.650 (mrsf10-1284-c10-10). [37] W. Xue, Z. Lin, H. Chen, C. Lu, J.-M. Lin, J. Phys. Chem. C 115 (2011) 21707. [38] Z. Lin, W. Xue, H. Chen, J.-M. Lin, Anal. Chem. 83 (2011) 8245. [39] Z. Lin, W. Xue, H. Chen, J.-M. Lin, Chem. Commun. 48 (2012) 1051. [40] M. Amjadi, J.L. Manzoori, T. Hallaj, M.H. Sorouraddin, Spectrochim. Acta A 122 (2014) 715. [41] M. Amjadi, J.L. Manzoori, T. Hallaj, M.H. Sorouraddin, Microchim. Acta 181 (2014) 671. [42] J. Jiang, Y. He, S. Li, H. Cui, Chem. Commun. 48 (2012) 9634. [43] Z. Lin, X. Dou, H. Li, Q. Chen, J.-M. Lin, Microchim. Acta 181 (2014) 801. [44] Y. Zhou, G. Xing, H. Chen, N. Ogawa, J.-M. Lin, Talanta 99 (2012) 471. [45] X. Dou, Z. Lin, H. Chen, Y. Zheng, C. Lu, J.-M. Lin, Chem. Commun. 49 (2013) 5871. [46] J. Shi, C. Lu, D. Yan, L. Ma, Biosens. Bioelectron. 45 (2013) 58. [47] W. Wang, M.N. Schuchmann, H.-P. Schuchmann, W. Knolle, J. von Sonntag, C. von Sonntag, J. Am. Chem. Soc. 121 (1999) 238. [48] S. Hosaka, T. Itagaki, Y. Kuramitsu, Luminescence 14 (1999) 349. [49] D. Bagchi, A. Garg, R.L. Krohn, M. Bagchi, M.X. Tran, S.J. Stohs, Res. Commun. Mol. Pathol. Pharmacol. 95 (1997) 179. [50] J. Ya-Feng Zhuang, Chin. Chem. Soc. 55 (2013) 994.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A novel chemiluminescence method for determination of bisphenol A based on the carbon dot-enhanced HCO3 –H2O2 system Mohammad Amjadi n, Jamshid L. Manzoori, Tooba Hallaj Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 5166616471, Iran
art ic l e i nf o Article history: Received 23 July 2014 Received in revised form 22 September 2014 Accepted 23 September 2014 Available online 30 September 2014 Keywords: Carbon dots Chemiluminescence Bisphenol A H2O2
a b s t r a c t A simple and sensitive chemiluminescence (CL) method on the basis of carbon dot (C-dot) enhanced HCO3 –H2O2 system, is designed for the determination of bisphenol A (BPA). The very weak CL of the HCO3 –H2O2 system is enhanced by a factor of 100 in the presence of C-dots. Possible mechanisms that lead to the effect were elucidated by recording fluorescence and CL spectra and studying the effect of some radical scavengers. This enhancement is inhibited by BPA in the concentration range from 1.0 to 100 mg L 1. This is exploited for its trace determination with a detection limit (3 s) of 0.3 mg L 1. The established method was applied to the determination of BPA in baby bottle and water samples with satisfactory results. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Bisphenol A (BPA), 2,2-bis (4-hydroxyphenyl) propane, is a monomer used in the synthesis of polycarbonate and epoxy resins and some other plastics. They are widely used as food-storage or packaging materials, such as feeding bottles, water bottles and cans and tableware [1]. When these plastic or resin materials expose to heat, acid or base, their linking ester bonds of BPA monomers will be hydrolyzed, thus releasing BPA into the environment [2,3]. Due to the possible estrogenic and carcinogenic effects of BPA, this monomer is harmful for human health. Therefore, the analysis of BPA has appeared of great importance and various analytical methods including chromatography [4,5], electrochemistry [6,7], spectrophotometry [8], fluorescence [9], chemiluminescence (CL) [10–12] and electrochemiluminescence (ECL) [13,14] have been reported for this purpose. Compared to the other analytical techniques, CL methods exhibit several advantages such as high sensitivity, simplicity of instrumentation, low detection limits, large calibration ranges and short analysis time. However, due to very low quantum yield, applicability of some traditional CL systems, such as HCO3–H2O2 is limited. Therefore, it is necessary to enhance their CL intensity for applications in analytical chemistry. In recent years, nanomaterials have been extensively applied to enhance CL emission [15–17]. For example, Lin and coworkers have investigated the enhancing effect of some nanomaterials such as AuNPs [18], CdTe [19], CdSe/Cds [20],
n
Corresponding author. Tel.: þ 98 4133393109; fax: þ 98 4133340191. E-mail address: [email protected] (M. Amjadi).
http://dx.doi.org/10.1016/j.jlumin.2014.09.045 0022-2313/& 2014 Elsevier B.V. All rights reserved.
Cu/Ni metal NPs [21], NaYF4:Yb3 þ /Er3 þ NPs [22] and carbon nanospheres [23] on the very week HCO3–H2O2 CL reaction. These enhanced CL systems have been applied to the determination of L-ascorbic acid [20], ammonia [22] and hydrogen peroxide [23] in real samples. Also, Azizi et al. [24] reported that CdS has sensitizing effect on the HCO3–H2O2 system and applied it for the analysis of epinephrine in pharmaceutical formulation. Carbon dots (C-dots) are a fascinating class of luminescent carbon nanomaterials that comprise discrete, quasispherical nanoparticles with sizes below 10 nm. They have attracted tremendous attention in varies fields since discovered in 2004 because of their alluring properties such as size- and wavelength-dependent luminescence emission, excellent photostability, favorable biocompatibility, simplicity of synthesis, good water solubility [25,26]. In addition, C-dots could be promising alternative to semiconductor quantum dots duo to low toxicity, high chemical stability and low environmental hazard, [27,28]. Therefore, they have been applied in various fields such as bioimaging [29,30], photocatalysis [31] and fluorescence sensing [32–35]. More recently, the CL and ECL [36] behavior of C-dots has also been studied. They could participate in a CL reaction as emitting species, after direct oxidation [37–42]; as catalysts of a reaction involving others luminophores [43,44]; or as emitter, after CL energy transfer [41,42,44–46]. In the present work, we studied the effect of C-dots on the HCO3 –H2O2 CL reaction. It was found that C-dots could significantly intensify the weak CL emission of this reaction. Moreover, based on the diminishing effect of BPA on the HCO3 –H2O2–C–dots CL system, a new CL method has been designed for the analysis of trace levels of BPA in baby bottle and water samples.
M. Amjadi et al. / Journal of Luminescence 158 (2015) 160–164
161
2. Experimental 2.1. Apparatus The chemiluminescence signals were monitored by LUMAT LB 9507 chemiluminometer (Berthold; www.berthold.com). CL spectrum was recorded with RF-540 spectrofluorimeter (Shimadzu, Japan) using flow mode with the excitation light source being turned off. The fluorescence spectra were also recorded by the same instrument under normal conditions. UV–vis spectra were recorded on a Cary-100 Spectrophotometer (Varian; www.varia ninc.com). The size and shape of C-dots were characterized with transmission electron microscopy (TEM, Leo 906, Zeiss, Germany). 2.2. Reagents All reagents used were of analytical reagent grade. Doubly distilled deionized water (obtained from Ghazi Serum Co. Tabriz, Iran) was used throughout the experiment. Sodium bicarbonate, hydrogen peroxide, citric acid monohydrate and poly(ethylene glycol) with average molecular weight of 1000 were all purchased from Merck (Darmstadt, Germany, www.merck-chemicals.com). 2.3. Synthesis of C-dots C-dots were prepared by hydrothermal treatment according to the our previous work [37]. Briefly, 15 mL of glycerin, 1.0 g of poly(ethylene glycol) 1000 and 1.0 g of citric acid monohydrate were mixed. Then, the mixture was transferred into a 100 mL Teflon equipped stainless steel autoclave and heated at 220 1C for 12 h. The prepared C-dots were purified by dialysis membrane (with MW cutoff of 2000) against deionized water for 24 h. The concentration of the C-dots solution was calculated according to the concentration of carbon atom in the carbon source.
Fig. 1. (a) TEM image of the prepared C-dots. (b) Fluorescence spectra for C-dots excited at wavelengths of 300–425 nm, with increments of 25 nm (a–e).
2.4. General procedure for CL monitoring Chemiluminescence analyses were carried out in a 3 mL tube, in the static injection condition. An amount of 200 μL of C-dots (0.57 mol L 1) and 500 μL of HCO3 (1.0 mol L 1) were added into the cell. Then an appropriate volume of sample or standard solution was added and the final volume was reached to 900 μL with distilled water. After injection of 100 μL of 1.0 mol L 1H2O2 by an automatic injector, monitoring of CL signal versus time was started automatically. Maximum CL intensity was used as analytical signal. 2.5. Sample pretreatment A baby bottle made from polycarbonate plastic was prepared from local store. They were cut into small pieces (size 0.5 cm2). An accurately weighed amount of sample ( 5 g) was then transferred into a 50 mL flask filled with pure water. The flask was placed in the water bath kept at 50 1C to extract BPA. After 12 h, the content of BPA in the aqueous extracts was determined. Water samples were analyzed without any pretreatment.
3. Results and discussion 3.1. Characterization of C-dots In this study C-dots were synthesized by solvothermal route using citric acid as a carbon source and poly(ethylene glycol) 1000 as a passivation agent. Fig. 1 shows the TEM image of C-dots. The particles of C-dots are almost spherical and monodisperse with average sizes below 10 nm. The fluorescence spectra of C-dots are
shown in Fig. 1b. As can be seen, the maximum emission was obtained at about 455 nm with an excitation wavelength of 375 nm. Moreover, as expected, the emission wavelength red shifted with increasing excitation wavelength, which revealed a distribution of the different surface energy traps of the C-dots. 3.2. HCO3 –H2O2–C-dot CL reaction The effect of synthesized C-dots on the H2O2–HCO3 CL reaction was investigated. This system produces a very weak CL signal, and two peaks can be observed in its kinetic profile. The first peak that appeared 1 s after injection of the oxidant solution quenched quickly. After 15 s, the second maximum value was recorded, and the CL lasted for about 50 s. These peaks may be attributed to three emitting species of H2O2–HCO3 CL reaction, (1O2), (O2)n2 and excited (CO2)n2. According to literature, the lifetime of 1O2 was only 3.1 μs in H2O. Therefore, the first CL peak can be attributed to 1O2 and (O2)n2, and the second peak to (CO2)n2 [19]. Our studies indicated that the CL intensity of both peaks enhanced when a small volume of C-dot solution was added to this reaction (Fig. 2). However, the enhancement that observed for the second peak (about 100-fold) was much more remarkable than that of observed for the first peak (about 3-fold). Therefore, in this work, the CL intensity of the second peak was applied as analytical signal. It is interesting to compare this results with those obtained by Lin and coworkers using 100 nm carbon nanospheres [23]. The latter nanoparticles have been reported to enhance the first peak of the H2O2–HCO3 CL reaction much more than the second peak. Moreover, the overall enhancement by carbon nanospheres is much lower than the enhancement effect of C-dots (40-fold compared to 100-fold).
162
M. Amjadi et al. / Journal of Luminescence 158 (2015) 160–164
1500
150
CL Intesity a.u.
100
1000 50 0
500
0
0
0
100
200
200
300
Time(s)
CL Intensity a.u.
12000
8000
4000
0
Fig. 3. (a) CL spectrum of HCO3 –H2O2–C-dots system, obtained with continuous flow of reagents: H2O2 (0.2 M) in one line and C-dot solution (0.55 mol L 1) and HCO3 (1 mol L 1) in other line. (b) Fluorescence spectrum of 0.228 M C-dots at λex ¼450 nm.
0
200
400
600
Time (s)
Fig. 2. Kinetic curve for (a) HCO3 –H2O2, (b) H2O2–C-dots and (c) HCO3 –H2O2– C-dots CL reactions. Conditions: HCO3 , 0.5 M, H2O2, 0.1 M, C-dot, 0.114 M.
Thus, the enhancement of the first peak can be due to the direct CL of Cdots and the energy transfer from (1O2)n2 to C-dots. Based on these facts, the enhanced CL mechanism can be described as follows [19,22,23]: HCO3 þ H2O2-HCO4 þH2O
Although C-dots were purified by dialysis, a control experiment with the reagents used for the preparation of C-dots (the mixture of citric acid, poly(ethylene glycol) and glycerin) was carried out. It was found that this solution has no contribution to the enhancement effect. Furthermore, direct reaction of H2O2 with C-dots was examined. As can be seen from Fig. 3b, a weak CL is produced by this reaction just after injection of the oxidant solution.
HCO4 -CO3 þ dOH OH þHCO3 -dOH þ dHCO3
d
2HCO3-(CO2)n2 þH2O2 (CO2)n2 þC-dot-2CO2 þC-dotn C-dotn-C-dotþhν ( 510 nm)
3.3. Possible CL reaction mechanism In order to identify the emitting species in the CL reaction, the CL emission spectrum of HCO3–H2O2–C-dot system was obtained using a spectrofluorimeter in flow mode. The result (Fig. 3a) indicated that there was an emission band in the range of 400–600 nm and the CL peak was located at 510 nm. This CL spectrum is similar to the C-dot fluorescence spectrum with the excitation wavelength of 450 nm (Fig. 3b). Therefore, the CL may be reasonably attributed to the excited-state C-dots. As mentioned before, One of the emitting species for HCO3–H2O2 CL reaction is (CO2)n2, which was unstable and decomposed to CO2 releasing energy to generate light at a wavelength higher than 220 nm [19,23]. Therefore, the energy transfer process could occur between the excited (CO2)n2 molecules as donors and C-dots as acceptors. The excited-state C-dots (C-dotsn) finally returned to their ground states with production of CL emission. So, the observed great enhancement for the second peak may be attributed to this energy transfer process. Furthermore, the dO2 and dOH radicals obtained from HCO3–H2O2 reaction could serve as the hole and electron injectors to produce C-dotdþ and C-dotd . Then, the C-dotsn that result from annihilation of C-dotdþ and C-dotd return to the ground state by photon emission. Also, dO2 is unstable in aqueous solution initiating the reactive oxygen chain reaction, which leads to the generation of the (1O2)n2. Then, C-dots could accept the energy from (1O2)n2, generating Cdotn, which returned to the ground state and released photon [42,45].
H2O2 þCO3 -HCO3 þHO2 HO2 -H þ þ dO2 C-dot þ dO2 -C-dot þ þO2 C-dot þOH-C-dot þOH C-dot þ C-dot þ -C-dotn C-dotn-C-dotþhν ( 510 nm)
O2 þ OH-1O2 þOH
O2 þ 1O2-(1O2)n2
1
(1O2)n2 þ C-dot-C-dotn C-dotn-C-dotþhν ( 510 nm) To further confirm this pathway, the effect of some radical scavengers on the CL intensity of HCO3–H2O2–C-dot system was investigated (Table 1). Thiourea and DMSO, the hydroxide radical scavengers [47], had a remarkable negative effect on the CL intensity. Thus it was assumed that dOH is generated in the examined systems. Moreover, NaN3 which was an effective radical scavenger for 1O2 [48], quenched the CL of the system which provided a evidence that
M. Amjadi et al. / Journal of Luminescence 158 (2015) 160–164
Table 1 Effect of radical scavengers on HCO3 –H2O2–C-dots CL system. Radical scavenger
Concentration (M)
CL intensity
H2O DMSO Thiourea NaN3 Ascorbic acid
– 5 10 3 5 10 3 5 10 3 10 4
10,968 328 494 2381 100
CL Intesity a. u.
20000 16000 12000 8000
3.5. Analytical application of the CL system 0
0.2
0.4 0.6 [HCO3-]/M
0.8
1
10000
CL Intesity a. u.
The effect of HCO3 concentration on the CL intensity was examined (Fig. 4a). It was found that the CL intensity linearly increased with increasing the concentration of HCO3 in the range of 0.1–0.8 M. Since at high concentration the solubility of HCO3 decreases, 0.5 M HCO3 was selected as the optimum concentration for further work. Moreover, the influence of H2O2 concentration on the CL intensity over the range of 0.025–0.3 M was studied. According to the results (Fig. 4b), the maximum CL response was obtained for 0.1 M H2O2. Due to an important role of H2O2 in the formation of free radicals, at lower concentrations the number of excited intermediates is decreased and the CL intensity is diminished. Finally, the effect of C-dot concentration in the range of 0.014–0.228 M on the CL intensity was investigated (Fig. 4c). The CL intensity is increased by increasing the C-dot concentration up to 0.114 mol L 1 and then remained constant. Because the generated energy of HCO3 –H2O2 CL reaction was limited, just a certain amount of C-dots could be excited by the energy transfer process.
4000 0
8000
6000
4000
0
0.1
0.2 [H2O2]/M
0.3
0.4
16000 CL Intensity a. u.
163
12000
3.6. Study of interferences
8000 4000 0
It was found that low concentrations of BPA have a diminishing effect on the CL reaction. BPA has a reducing property and may compete with C-dots and HCO3 for H2O2, which leads to a reduction in the CL emission intensity. Based on this phenomenon, a sensitive method was developed for the determination of BPA. Under the optimum conditions described above, the analytical figures of merit for the determination of BPA was obtained. The CL response was found to be linear in the concentration range of 1.0–100 mg L 1 with a detection limit (3 s) of 0.3 mg L 1. The regression equation was logΔI¼ 2.7 Cþ 4.0, R2 ¼0.9973, where ΔI¼I I0 is the difference between the CL intensity in the presence of BPA (I) and in its absence (I0), and C is concentration of BPA in mg L 1. The relative standard deviation (RSD) was obtained to be 2.4% and 1.5% for five replicate determinations of 2.5 and 10.0 mg L 1 BPA, respectively. The results indicate that this CL system has good linearity, relatively high sensitivity and suitable precision. Comparison between the developed method and some other reported CL methods for BPA quantification is shown in Table 2. As can be seen, the developed method has better limit of detection than other methods except one.
0
0.05
0.1 0.15 [C-dot]/M
0.2
0.25
Fig. 4. Optimization of the CL reaction conditions. (a) Effect of HCO3 concentration: H2O2, 0.1 M, C-dot 0.057 M. (b) Effect of H2O2 concentration: HCO3 , 0.5 M; other conditions are as in a (c) effect of C-dot concentration: H2O2, 0.1 M; other conditions are as in c.
1
O2 contributes to the observed CL. Also, ascorbic acid as a common oxygen free radical scavenger [49] had a negative effect on the CL signal, which further indicated that the generation of a free radical is critical in the CL reaction. 3.4. Optimization of chemical conditions In order to obtain the maximum sensitivity for the reaction, the effects of several variables such as concentration of HCO3 , H2O2 and C-dots on the CL intensity were investigated.
In order to evaluate the selectivity of the developed method, the effects of some common ions on the determination of 10 mg L 1 BPA were investigated. The tolerable concentrations for interfering species in relative error of o5% were summarized in Table 3. These results demonstrate that the method possesses a good selectivity for the determination of BPA in real samples. Table 2 Comparison of the developed CL method for the determination BPA with some previously published CL methods. CL system
HCO4 –AuCl4 Luminol–K3Fe(CN)3 Luminol–KMnO4–AgNPs Luminol ECL Lucigenin–graphene oxide ECL Ru(bpy)23 þ /dibutylamino ethanol ECL HCO3 –H2O2–C-dots
LOD (mg L 1)
Linear range (mg L 1)
Ref.
12 70 0.001 80 0.068 0.01
45–12,000 180–2700 0.01–50 100–5000 0.22–22,800 0.05–25
[11] [12] [10] [50] [13] [14]
0.3
1.0–100
This method
164
M. Amjadi et al. / Journal of Luminescence 158 (2015) 160–164
Table 3 Tolerance limit of interferences on the determination of 10.0 mg L 1 BPA. The tolerable concentration ratios
Interfering species
10,000
Na þ , K þ , Zn2 þ , Ca2 þ , Hg2 þ , Pb2 þ ,Al3 þ NO3 , Cl , SO4 2 , PO4 3 Mg2 þ Fe3 þ , Cu2 þ
8000 500
Table 4 Results for the determination of BPA in real samples. Sample
Added (mg L 1)
Found (mg L 1)
Baby bottle
0.0 20.0 50.0 0.0 10.0 50.0
Well water
a b
a
Recovery (%)
t-Statisticb
9.9 70.1 30.6 71.2 60.4 71.3
– 102.2 7 4.1 100.8 7 2.5
1.7 0.65
36.7 71.4 46.3 71.3 88.2 71.9
– 96.9 7 2.9 101.6 7 2.1
– 1.8 1.3
Mean of three determinations 7 standard deviation. t-critical ¼4.3 for n¼ 2, P¼ 0.05.
3.7. Analysis of real samples The developed method was easily applied to the determination of BPA in baby bottle and water samples. Furthermore, known quantities of BPA were added into the samples, and then the samples were prepared and analyzed according to the general procedure. The obtained results are shown in Table 4. Statistical analysis of these results using Student t-test showed that there are no significant differences between added and found values. 4. Conclusion We described a new analytical method for the determination of BPA based on the C-dot- enhanced HCO3 –H2O2 CL system. In presences of C-dots, the CL intensity of H2O2–HCO3 system remarkably enhanced. A possible mechanism for the reaction was explained, which involves the CL energy transfer from (CO2)n2 as a donor to C-dot as an acceptor, along with a contribution from direct oxidation of C-dots. We found that with addition of even trace levels of BPA to this enhanced CL system, the CL intensity decreased. Based on this phenomenon, a simple and sensitive CL method was designed for the determination of BPA with good linearity, high selectivity and precision, which can be applied to the analysis of BPA in baby bottle and water samples. References [1] K.V. Ragavan, N.K. Rastogi, M.S. Thakur, Trends Anal. Chem. 52 (2013) 248. [2] A. Goodson, H. Robin, W. Summerfield, I. Cooper, Food Addit. Contam. 21 (2004) 1015.
[3] D.S. Lim, S.J. Kwack, K.-B. Kim, H.S. Kim, B.M. Lee, J. Toxicol, Environ. Health A 72 (2009) 1285. [4] E. Yiantzi, E. Psillakis, K. Tyrovola, N. Kalogerakis, Talanta 80 (2010) 2057. [5] J. López-Darias, M. Germán-Hernández, V. Pino, A.M. Afonso, Talanta 80 (2010) 1611. [6] P. Deng, Z. Xu, Y. Kuang, Food Chem. 157 (2014) 490. [7] H. Fan, Y. Li, D. Wu, H. Ma, K. Mao, D. Fan, Anal. Chim. Acta 711 (2012) 24. [8] Z. Mei, H. Chu, W. Chen, F. Xue, J. Liu, H. Xu, Biosens. Bioelectron. 39 (2013) 26. [9] K.V. Ragavan, L.S. Selvakumar, M.S. Thakur, Chem. Commun. 49 (2013) 5960. [10] X. Chen, C. Wang, X. Tan, J. Wang, Anal. Chim. Acta 689 (2011) 92. [11] C. Lu, J. Li, Y. Yang, J.-M. Lin, Talanta 82 (2010) 1576. [12] S. Wang, X. Wei, L. Du, H. Zhuang, Luminescence 20 (2005) 46. [13] H. Dai, S. Zhang, Y. Lin, Y. Ma, L. Gong, G. Xu, M. Fu, X. Li, G. Chen, Anal. Methods 6 (2014) 4746. [14] X. Jiang, W. Ding, C. Luan, Can. J. Chem. 91 (2013) 656. [15] Q. Li, L. Zhang, J. Li, C. Lu, Trends Anal. Chem. 30 (2011) 401. [16] D.L. Giokas, A.G. Vlessidis, G.Z. Tsogas, N.P. Evmiridis, Trends Anal. Chem. 29 (2010) 1113. [17] H. Chen, L. Lin, H. Li, J.-M. Lin, Coord. Chem. Rev. 86 (2014) 263. [18] J.-M. Lin, M. Liu, J. Phys. Chem. B 112 (2008) 7850. [19] H. Chen, L. Lin, Z. Lin, G. Guo, J.-M. Lin, J. Phys. Chem. A 114 (2010) 10049. [20] H. Chen, R. Li, L. Lin, G. Guo, J.-M. Lin, Talanta 81 (2010) 1688. [21] H. Chen, R. Li, H. Li, J.-M. Lin, J. Phys. Chem. C 116 (2012) 14796. [22] H. Chen, H. Li, J.-M. Lin, Anal. Chem. 84 (2012) 8871. [23] H. Chen, L. Lin, Z. Lin, C. Lu, G. Guo, J.-M. Lin, Analyst 136 (2011) 1957. [24] S.N. Azizi, M.J. Chaichi, P. Shakeri, A. Bekhradnia, J. Fluoresc. 23 (2013) 227. [25] J.C.G. Esteves da Silva, H.M.R. Gonçalves, TrAC Trends Anal. Chem. 30 (2011) 1327. [26] S.N. Baker, G.A. Baker, Angew. Chem. Int. Ed. 49 (2010) 6726. [27] R. Hardman, Health Perspect. 114 (2006) 165. [28] Y. Su, Y. He, H. Lu, L. Sai, Q. Li, W. Li, Biomaterials 30 (2009) 19. [29] Y.-P. Sun, B. Zhou, Y. Lin, W. Wang, K.A.S. Fernando, P. Pathak, J. Am. Chem. Soc. 128 (2006) 7756. [30] L. Cao, X. Wang, M.J. Meziani, F. Lu, H. Wang, P.G. Luo, J. Am. Chem. Soc. 129 (2007) 11318. [31] H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, Angew. Chem. Int. Ed. 49 (2010) 4430. [32] H. Gonçalves, P.A.S. Jorge, J.R.A. Fernandes, J.C.G. Esteves da Silva, Sens. Actuators B Chem. 145 (2010) 702. [33] W. Lu, X. Qin, S. Liu, G. Chang, Y. Zhang, Y. Luo, Anal. Chem. 84 (2012) 5351. [34] S.N.A. Mohd Yazid, S.F. Chin, S.C. Pang, S.M. Ng, Microchim. Acta 180 (2012) 137. [35] Y. Guo, Z. Wang, H. Shao, X. Jiang, Carbon 52 (2013) 583. [36] L. Sun, T.-H. Teng, M.d.H. Rashid, M. Krysmann, P. Dallas, Y. Wang, B.-R. Hyun, A.C. Bartnik, G.G. Malliaras, F.W. Wise, E.P. Giannelis, MRS Proc. (2011), http: //dx.doi.org/10.1557/opl.2011.650 (mrsf10-1284-c10-10). [37] W. Xue, Z. Lin, H. Chen, C. Lu, J.-M. Lin, J. Phys. Chem. C 115 (2011) 21707. [38] Z. Lin, W. Xue, H. Chen, J.-M. Lin, Anal. Chem. 83 (2011) 8245. [39] Z. Lin, W. Xue, H. Chen, J.-M. Lin, Chem. Commun. 48 (2012) 1051. [40] M. Amjadi, J.L. Manzoori, T. Hallaj, M.H. Sorouraddin, Spectrochim. Acta A 122 (2014) 715. [41] M. Amjadi, J.L. Manzoori, T. Hallaj, M.H. Sorouraddin, Microchim. Acta 181 (2014) 671. [42] J. Jiang, Y. He, S. Li, H. Cui, Chem. Commun. 48 (2012) 9634. [43] Z. Lin, X. Dou, H. Li, Q. Chen, J.-M. Lin, Microchim. Acta 181 (2014) 801. [44] Y. Zhou, G. Xing, H. Chen, N. Ogawa, J.-M. Lin, Talanta 99 (2012) 471. [45] X. Dou, Z. Lin, H. Chen, Y. Zheng, C. Lu, J.-M. Lin, Chem. Commun. 49 (2013) 5871. [46] J. Shi, C. Lu, D. Yan, L. Ma, Biosens. Bioelectron. 45 (2013) 58. [47] W. Wang, M.N. Schuchmann, H.-P. Schuchmann, W. Knolle, J. von Sonntag, C. von Sonntag, J. Am. Chem. Soc. 121 (1999) 238. [48] S. Hosaka, T. Itagaki, Y. Kuramitsu, Luminescence 14 (1999) 349. [49] D. Bagchi, A. Garg, R.L. Krohn, M. Bagchi, M.X. Tran, S.J. Stohs, Res. Commun. Mol. Pathol. Pharmacol. 95 (1997) 179. [50] J. Ya-Feng Zhuang, Chin. Chem. Soc. 55 (2013) 994.