Letter pubs.acs.org/ac
Enhancement of the Carbon Dots/K2S2O8 Chemiluminescence System Induced by Triethylamine Hui Zhang,†,‡ Xiaowei Zhang,†,‡ and Shaojun Dong*,†,‡ †
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, Jilin 130022, PR China ‡ University of Chinese Academy of Sciences, Beijing, 100049, PR China S Supporting Information *
ABSTRACT: Triethylamine (TEA), a common coreactant for electrochemiluminescence (ECL), is first utilized as a coreactant for chemiluminescence (CL). The CL intensity of carbon dots/K2S2O8 could be increased by ∼20 times in the presence of TEA. On the basis of this fascinating phenomenon, a room temperature operated senor is constructed for the fast, selective, and sensitive determination of TEA. A wide linear relationship between CL intensity and TEA concentration from 1 μM to 1000 μM (R2 = 0.9995) was found with the detection limit down to 1 μM. The enhancement mechanism of TEA to this CL system is carefully investigated. Experimental results reveal that the forming of TEA free radical is what indeed induced the enhancement of the CL efficiency of CDs.
C
efficiency of the nanoparticles-based ECL system,18,19 and several TEA probes with low operating temperature have been reported on the ECL technique.20,21 However, the ECL technique also has distinct deficiencies (namely, energyconsuming) when compared with CL. No article to date has reported that TEA can be used as a coreactant for CL. In this work, we reported for the first time that TEA could dramatically enhance the CL intensity of the CDs/K2S2O8 system. The CL signal of the CDs/K2S2O8 system could be increased by ∼20 times after addition of TEA. This phenomenon opens up a new way for designing facile, inexpensive, quick, and sensitive TEA sensors that can be operated at room temperature.
arbon dots (CDs) have drawn enormous attention in recent years due to exceptional advantages such as chemical stability, easy functionalization, and low toxicity.1−4 Those features make CDs well behaved in sensors, bioimaging, catalysis, and medical diagnosis.5−7 Most of this research focuses on the fluorescence of CDs; while fluorescence analysis suffers from relatively expensive instruments, leading to limitation for the application of CDs. Chemiluminescence (CL) has been widely used in analytical fields over the past several decades, because it promises simple instruments, high sensitivity, wide linear range, and no interference from background scattering light.8−12 Unfortunately CL analysis about CDs is still rare.13−15 For better satisfaction of future applications, multifarious CL activity of CDs needs to be further exploited. Triethylamine (TEA) is a common reagent encountered in the chemical industry, especially organic synthesis, functioning as a significant intermediate for manufacturing medicines, pesticides, and other chemicals. Developing efficient TEA senor is necessary and significant since TEA is volatile, toxic, inflammable, and explosive.16,17 Traditional TEA detection methods (such as a gas chromatograph) require an expensive instrument, complex manipulation, and long response time. Previous TEA sensors mostly based on metal oxide lead to a higher cost. Even worse, TEA is usually detected at high temperature, resulting in energy waste and risk of TEA volatilization and explosion. New TEA sensors operated at room temperature are invaluable. It has been reported that TEA can effectively enhance the electrochemiluminescence (ECL) © XXXX American Chemical Society
■
EXPERIMENTAL SECTION Materials. N,N-Dipropyl-1-propanamine were purchased from Alfa Aesar. Citric acid, ethylenediamine, triethylamine (TEA), potassium persulfate, and sodium hydroxide were from Beijing Chemical Reagents Company (Beijing, China). All other reagents were of analytical reagent grade and used without further purification. Aqueous solutions were prepared with ultrapure water from a water purifier (Sichuan Water Purier Co., Ltd., China). Synthesis of CDs. The photoluminescent CDs was fabricated according to a published method2,22 with some Received: July 8, 2015 Accepted: October 26, 2015
A
DOI: 10.1021/acs.analchem.5b02562 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry modifications. Briefly, ethylenediamine (500 μL) was added to citric acid (0.1 g/mL, 10 mL) under vigorous stirring. Then the clear solution was placed on a domestic microwave oven (500 W) and heated for 3 min and repeated 2 times after naturally cooled to room temperature. Pure water (1 mL) and ethanol (4 mL) were introduced. The brownish CDs were collected by means of centrifugation at 10 000 rpm for 10 min and then subjected to centrifugation purification with the use of ethanol. Procedure of CL Measurements. The CL experiments were carried out in a glass cuvette containing NaOH (0.30 M), TEA (0.10 M), K2S2O8 (0.07 M), and CDs (0.62 mg/mL). TEA and K2S2O8 were premixed half an hour before the experiment. The CDs were injected finally. CL intensities were monitored on a model MPI-A capillary electrophoresis ECL system (Xi’an Remax Electronics Co., Ltd., China). The peak height of the signal was recorded relative to CL intensity.
leads to a more reproducible output signal. Then, the impact of solution pH on the CL intensity was evaluated. The CL signal increased with the pH in the range from 2 to 12. Sodium hydroxide (NaOH) was utilized to adjust the system pH and the optimal concentration was 0.30 M (Figure 2A). Further
■
RESULTS AND DISCUSSION Characterization of Carbon Dots. The CDs were formed from ethylenediamine and citric acid by facile microwave treatment according to a published procedure2,22 with some modifications. Excitation of the CDs at 360 nm yielded an emission spectrum that peaked at 454 nm (Figure S1A). The CDs emitted bright blue fluorescence under a hand-held UV lamp (Figure S1B). The high-resolution transmission electron microscopy (HRTEM) image of CDs clearly revealed that CDs are spherical and monodispersed with an average size of 1.7 nm (Figure S1D). The surface composition and element analysis for CDs were investigated by X-ray photoelectron spectroscopy (XPS). The XPS (Figure S1E) described three peaks at 285.0, 400.0, and 532.0 eV, which were attributed to C1s, N1s, and O1s, respectively. A typical X-ray diffraction (XRD) pattern of the CDs presented a broad diffraction peak centered at 18.9°, as a result of amorphous carbon (Figure S1F). CL of CDs/K2S2O8/TEA System. The prepared CDs generated faint CL signal when mixed with K 2 S 2 O 8 . Surprisingly, the CL signal exhibited extraordinary enhancement after adding TEA to CDs/K2S2O8 (Figure 1A). TEA, a
Figure 2. Effects of the concentration of (A) NaOH, (B) CDs, (C) K2S2O8, and (D) the ratio of K2S2O8 and TEA on the CL detection system. Conditions: the concentration of NaOH, K2S2O8, TEA, and CDs was 0.30 M, 0.07 M, 0.10 M, and 0.62 mg/mL.
improvements in the CL properties of the system were focused on optimizing the concentration of CDs, K2S2O8, and TEA. Results showed that the best CDs and K2S2O8 concentrations were 0.62 mg/mL (Figure 2B) and 0.07 M (Figure 2C), respectively. The highest CL signal could be found when the concentration ratio of K2S2O8 and TEA was 0.7:1 (Figure 2D). The optimized microwave heating time for CDs preparation was 9 min (Figure S2). Besides, the reaction temperature had significant influence on the CL efficiency (Figure S3). Although the CL signal became larger at a higher temperature, the CL behavior became unstable when the temperature was higher than 20 °C. When the temperature was high, free radicals became highly active, and the CL output signal became stronger. That is to say, the CL output signal becomes quite sensitive to the temperature when the temperature was higher than 20 °C. A slight change of the temperature would result in a big change of the CL signal. Thus, all the following experiments were performed at 20 °C. Under the optimized conditions, the CL signal exhibits sharp peak and high signal-to-noise ratio, which is very favorable for the following detection of TEA. As shown in Figure 3A, the increased CL signals were directly related to the concentration of TEA. A wide linear relationship between CL intensity and TEA concentration from 1 μM to 1000 μM (R2 = 0.9995) was found with the detection limit down to 1 μM. In addition, the relative standard deviation (RSD) values (n = 3) of the analysis were 4.4%, 2.6%, and 4.1% for TEA concentrations of 100, 500, 1000 μM, respectively. To demonstrate the selectivity of the sensing platform, the CL response in the presence of two TEA analogues (N,Ndipropyl-1-propanamine and ethylenediamine), three anions (ClO−, SO32−, NO2−) and 11 cations (Na+, K+, Mg2+, Ca2+, Zn2+, Fe3+, Cu2+, Cd2+, Pb2+, Cr3+, Hg+) were investigated (Figure 3B). All species had no obvious effect on TEA detection when a selective masking of Cu2+ and Hg+ was achieved by using ethylenediaminetetraacetic acid (EDTA) as a
Figure 1. (A) Difference of CL intensity between the CDs/K2S2O8 system and the CDs/TEA/K2S2O8 system. (B) CL kinetic curves of the CDs/TEA/K2S2O8 system with different reagent mixing orders (the black curve, injecting K2S2O8 into TEA and CDs; the red curve, injecting TEA finally; the blue curve, injecting CDs finally).
common coreactant for ECL, has not been employed as a coreactant for CL before. Next, we developed a facile sensing CL strategy for the determination of TEA at room temperature. Several critical parameters, including the mixing order, pH, the reagent concentration, the reaction temperature, and the microwave heating time of the CDs were optimized to obtain the best performance for the TEA analysis. It was found that injecting the CDs at the last could get a larger signal (Figure 1B). Premixing K2S2O8 and TEA prior to introduction of CDs B
DOI: 10.1021/acs.analchem.5b02562 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry
Figure 3. (A) Standard curve for the determination of TEA concentration in the range from 0.001 to 1 mM (various concentrations of TEA, 0.001, 0.01, 0.1, 0.5, and 1 mM). (B) Selectivity of this CL sensor for TEA over other ions: concentrations of other ions in blue were all 10 mM; concentrations of ions in red were 2 mM; concentrations of ions in black were 0.5 mM; concentrations of ions in green were 4 mM; concentrations of ions in yellow were 1 mM. Conditions: the concentration of NaOH, K2S2O8, TEA, EDTA, and CDs was 0.08 M, 0.04 M, 0.0005 M, 0.01 M and 0.62 mg/mL.
Figure 4. (A) Absorption spectra of the reagent in the CL reaction. S stands for K2S2O8. (B) The variation of dissolved oxygen during the reaction.
TEA, an absorption peak at 250−300 nm arised. The new peak was attributed to n−π* transitions of NO,13 indicating TEA was oxidized by K2S2O8. The CDs alone revealed an absorption peak at 300−400 nm, which was attributed to n−π* transitions of CO.26 The absorption peak at 300−400 nm increased while CDs were injected to K2S2O8/TEA. The variation of this peak confirmed the fact that CDs were transformed in the process. We further provided insight into the mechanism by recording the variation of dissolved oxygen (DO) over the course of time (Figure 4B). The YSI 5000 oxygen dissolving meter was employed to monitor the DO directly. TEA did not affect the measurement. The DO of TEA only was 6.85 mg/L, and it did not change with time (first point in Figure 4B, and the experiment has been repeated). The DO of the K2S2O8/TEA system declined quickly, and there was almost no DO 13 min later. Still no DO was observed after CDs were introduced to K2S2O8/TEA. Moreover, the CL intensity fell ∼60% when N2 was bubbled into K2S2O8/TEA for 20 min prior to the CL analysis. Thus, the DO was consumed during the premixing of K2S2O8 and TEA, and a product that could greatly enhance the CL efficiency of CDs/K2S2O8 system was produced during this process. This probably explained why premixing K2S2O8 and TEA for a moment prior to injection of CDs could achieve a more reproducible signal. An earlier review summarized that ketones efficiently catalyzed the decomposition of peroxy acids (K2S2O8 is a peroxy acid) in alkaline aqueous solution (NaOH) and substantially enhanced the CL intensity.27 Besides, the enhancement mechanism of the ECL by an aliphatic tertiary amine (TEA is an aliphatic tertiary amine) has also been reported.28,29 A. J. Bard28 reported that [Ru(bpy)3]2+ could be reduced by tri-n-propylamine (TPrA) cationic radicals to generate [Ru(bpy)3]2+* and then exhibit ECL behavior. Inspired by these works and the observed experimental facts, a similar mechanism is rationally proposed for CDs/K2S2O8/ TEA (Figure 5). In this way, TEA is first oxidized by K2S2O8 to form oxidized and cationic radicals TEA (reaction 1). Oxygen is also involved but not necessary in the process (reaction 2). Reaction 2 would not happen without oxygen and TEA only (reaction 1) could enhance the CL of CDs/K2S2O8. Under alkaline conditions, TEA cationic radicals are subsequently deprotonated to TEA free radicals (reaction 3). From the EPR spectra (Figure S5), a new peak generated after K2S2O8 was added to TEA, indicating the formation of free radicals. This is the direct evidence for the generation of TEA free radicals. The CDs are reduced by TEA free radicals and generate the reduced and excited CDs (reaction 4) which decay into reduced CDs (rCDs) with CL emission (reaction 5). Researchers have explored the difference of emission between the reduced CDs
strong metal ion chelator. It was beyond our expectation that the tolerance concentrations of these oxidizing anions (ClO−, SO32−, NO2−) were at least 8 times that of TEA. Therefore, the CDs/K2S2O8/TEA system is remarkably tolerant of interference and highly specific toward TEA. The applicability of this approach in real samples (tap water and lake water) was demonstrated (Table S2). The recoveries for sample determination were in the range of 93.6% to ∼96.8%, indicating this strategy was accurate and acceptable. Compared with traditional TEA sensors (Table S3), the design was comparable and benefited from good sensitivity, high selectivity, fast response, room temperature operation, simple manipulation, and no need for an expensive instrument, which made it a promising platform for TEA detection. CL Mechanism. Finally, several control experiments have been performed to clarify the principle of the interesting phenomenon. The solution comprised of K2S2O8 and TEA did not display any CL behavior at all, while the solution only containing K2S2O8 and CDs generated weak CL signal. It is concluded that CDs are the main emitters. More K2S2O8 and TEA were added to the residual solution (CDs/K2S2O8/TEA), and no sharp CL signal enhancement was observed. It was first deduced that the CDs might be damaged. Nevertheless, the HRTEM images showed little differences between the CDs before and after the reaction. Thus, we hypothesized it was the surface structure of the CDs that was changed. To gain more insight into the origin of the CL behavior, some other oxidant (H2O2, KMnO4, and NaClO) were employed instead of K2S2O8, owing to that K2S2O8 is also a coreactant for ECL or CL.10,23−25 The CDs/TEA/oxidants (H2O2, KMnO4 and NaClO) systems could still exhibit CL behavior. However, the CL signal was not so strong as that of the CDs/TEA/K2S2O8 system. TEA did not remarkably increase the CL intensity of CDs/other oxidant. The CDs synthesized by other methods were tested (Figure S4). It is found that not all CDs were active in the reaction, which further confirmed that the surface structure of the obtained CDs was characteristic and played a crucial role in the CDs CL system. Besides, TEA analogues could not interfere with the platform as the interference test above showed. All these facts confirm that this CL activity is highly specific. The absorption spectra were also utilized to explore the principle of the unusual CL enhancement of CDs/K2S2O8 caused by TEA (Figure 4A). When K2S2O8 was mixed with C
DOI: 10.1021/acs.analchem.5b02562 Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Letter
■
ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21375123), the 973 project (Grant No. 2011CB911002), and the Ministry of Science and Technology of China (Grant No. 2013YQ170585).
■
Figure 5. Schematic illustration of the CL mechanism of the CDs/ K2S2O8/TEA system.
and oxidized CDs.23 They found that CDs with different oxidation levels displayed disparate ECL activity. This is why the CL signal would not be recovered by adding more K2S2O8 and TEA to the CDs/K2S2O8/TEA residual solution. The CDs were transformed into rCDs after the reaction and rCDs scarcely exhibited CL behavior. 2TEA + K 2S2 O8 → 2TEA•+ + K 2SO4 + SO4 2 − K 2S2O8,O2
TEA ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ TEA•+ OH
■
(1) (2)
−
TEA•+ ⎯⎯⎯⎯→ TEA•
(3)
TEA• + CDs → TEA fragment + rCDs*
(4)
rCDs* → rCDs + hv
(5)
CONCLUSIONS In conclusion, we suggested a new CL coreactant TEA, which exhibited extraordinary enhancement to the CDs/K2S2O8 CL system. Experimental results reveal that the forming of TEA free radical is what indeed induced the enhancement of the CL efficiency of CDs. Also, a CL platform based on CDs/K2S2O8 for the detection of TEA was proposed, which opens up the opportunity to develop fast response TEA sensors operated at room temperature. This study not only expanded the application related to CL but also provided clues for the origin of the CDs luminescence.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02562. Detailed characterization of the CDs; conditions optimization of the CDs microwave heating time, temperature, CDs synthesized by different methods; determination of TEA in real samples; and comparison of probes for the detection of TEA (PDF)
■
REFERENCES
(1) Lim, S. Y.; Shen, W.; Gao, Z. Q. Chem. Soc. Rev. 2015, 44, 362− 381. (2) Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Angew. Chem., Int. Ed. 2013, 52, 3953−3957. (3) Baker, S. N.; Baker, G. A. Angew. Chem., Int. Ed. 2010, 49, 6726− 6744. (4) Lai, T.; Zheng, E.; Chen, L.; Wang, X.; Kong, L.; You, C.; Ruan, Y.; Weng, X. Nanoscale 2013, 5, 8015−8021. (5) Liu, S.; Tian, J.; Wang, L.; Zhang, Y.; Qin, X.; Luo, Y.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Adv. Mater. 2012, 24, 2037−2041. (6) Wang, D. M.; Gao, M. X.; Gao, P. F.; Yang, H.; Huang, C. Z. J. Phys. Chem. C 2013, 117, 19219−19225. (7) Son, Y. W.; Cohen, M. L.; Louie, S. G. Nature 2006, 444, 347− 349. (8) Ding, Z. F.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Science 2002, 296, 1293−1297. (9) Chen, H.; Lin, L.; Li, H. F.; Lin, J. M. Coord. Chem. Rev. 2014, 263−264, 86−100. (10) Zheng, L. Y.; Chi, Y. W.; Dong, Y. Q.; Lin, J. P.; Wang, B. B. J. Am. Chem. Soc. 2009, 131, 4564−4565. (11) Zhu, H.; Wang, X. L.; Li, Y. L.; Wang, Z. J.; Yang, F.; Yang, X. R. Chem. Commun. 2009, 34, 5118−5120. (12) Tang, Y.; Su, Y.; Yang, N.; Zhang, L.; Lv, Y. Anal. Chem. 2014, 86, 4528−4535. (13) Lin, Z.; Xue, W.; Chen, H.; Lin, J. M. Anal. Chem. 2011, 83, 8245−8251. (14) Teng, P.; Xie, J. X.; Long, Y. J.; Huang, X. X.; Zhu, R.; Wang, X. L.; Liang, L. P.; Huang, Y. M.; Zheng, H. Z. J. Lumin. 2014, 146, 464− 469. (15) Lin, Z.; Xue, W.; Chen, H.; Lin, J. M. Chem. Commun. 2012, 48, 1051−1053. (16) Perkins, F. K.; Friedman, A. L.; Cobas, E.; Campbell, P. M.; Jernigan, G. G.; Jonker, B. T. Nano Lett. 2013, 13, 668−673. (17) Auvray, S.; Derycke, V.; Goffman, M.; Filoramo, A.; Jost, O.; Bourgoin, J. P. Nano Lett. 2005, 5, 451−455. (18) Collinson, M. M.; Novak, B.; Martin, S. A.; Taussig, J. S. Anal. Chem. 2000, 72, 2914−2918. (19) Qi, W. J.; Xu, M.; Pang, L.; Liu, Z. Y.; Zhang, W.; Majeed, S.; Xu, G. B. Chem. - Eur. J. 2014, 20, 4829−4835. (20) Sreedhar, M.; Lin, Y. W.; Tseng, W. L.; Chang, H. T. Electrophoresis 2005, 26, 2984−2990. (21) Chiang, M. T.; Lu, M. C.; Whang, C. W. Electrophoresis 2003, 24, 3033−3039. (22) Zhai, Y. L.; Zhu, Z. J.; Zhu, C. Z.; Ren, J. T.; Wang, E. K.; Dong, S. J. J. Mater. Chem. B 2014, 2, 6995−6999. (23) Xu, Y.; Wu, M.; Feng, X. Z.; Yin, X. B.; He, X. W.; Zhang, Y. K. Chem. - Eur. J. 2013, 19, 6282−6288. (24) Li, J. Z.; Wang, N. Y.; Tran, T. T.; Huang, C. A.; Chen, L.; Yuan, L. J.; Zhou, L. P.; Shen, R.; Cai, Q. Y. Analyst 2013, 138, 2038−2043. (25) Poznyak, S. K.; Talapin, D. V.; Shevchenko, E. V.; Weller, H. Nano Lett. 2004, 4, 693−698. (26) Hu, S.; Trinchi, A.; Atkin, P.; Cole, I. Angew. Chem., Int. Ed. 2015, 54, 2970−2974. (27) Adam, W.; Kazakov, D. V.; Kazakov, V. P. Chem. Rev. 2005, 105, 3371−3387. (28) Hu, T.; Liu, X. F.; Liu, S. Q.; Wang, Z. L.; Tang, Z. Y. Anal. Chem. 2014, 86, 3939−3946. (29) Miao, W. J.; Choi, J. P.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 14478−14485.
AUTHOR INFORMATION
Corresponding Author
*Phone: +86 431 85692886. Fax: +86 431 85689711. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.analchem.5b02562 Anal. Chem. XXXX, XXX, XXX−XXX
