Letter pubs.acs.org/ac
Dual-Peak Electrogenerated Chemiluminescence of Carbon Dots for Iron Ions Detection Pengjia Zhang,† Zhenjie Xue,‡ Dan Luo,‡ Wei Yu,‡ Zhihui Guo,*,† and Tie Wang*,‡ †
Key Laboratory of Analytical Chemistry for Life Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, P. R. China ‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: Carbon dots (CDs) have rigorously been investigated on their unique fluorescent properties but rarely their electrogenerated chemiluminescence (ECL) behavior. We are here to report a dual-peak ECL system of CDs, one at −2.84 V (ECL-1) and the other at −1.71 V (ECL-2) during the cyclic sweep between −3.0 and 3.0 V at scan rate of 0.2 V s−1 in 0.1 M tetrabutyl ammonium bromide (TBAB) ethanol solution, which is more efficiency to distinguish metallic ions than single-peak ECL. The electron transfer reaction between individual electrochemically reduced nanocrystal species and coreactants led to ECL-1, in which the electron injected to the conduction band of CDs in the cathodic process. Ion annihilation reactions induced direct formation of exciplexes that produced another ECL signal, ECL-2. ECL-1 showed higher sensitivity to the surrounding environment than ECL-2 and thus was used for ECL detection of metallic ions. Herein, we can serve as an internal standard method to detect iron ions. A linear relationship of the intensity ratio R of ECL-1 and ECL-2 to iron ions was observed in the concentration extending from 5 × 10−6 to 8 × 10−5 M with a detection limit of 7 × 10−7 M.
C
in an organic phase. Greatly different from previous works of single-peak ECL system that cannot obviously provide a complete long-term stability of the detection due to their inherent instability, we report here for the first time a dual-peak ECL system for detection of iron ions. The ECL intensity of one peak changes with the concentration of iron ions in a certain range, and the other one is stable, which is similar to internal standard method. So, the accuracy and sensitivity of the detection of iron ions are greatly improved comparing to singlepeak ECL. To study the ECL behavior and mechanism of CDs, different supporting electrolytes with various length of alkyl chain were added to the ECL system. The two ECL peaks present distinct sensitivity to the surrounding environment. Therefore, the dual-peak ECL provides more information for detecting metallic ions, which is more efficiency to distinguish iron ions from other metallic ions than single-peak ECL. CDs were synthesized by the microwave treatment according to the reported method with a slight modification.22 In a typical synthesis, an appropriate amount of poly(ethylene glycol) (PEG-200, 1.125 g/mL) and ascorbic acid (0.08 g/mL) were added to distilled water to form a transparent homogeneous solution. Then the solution was heated in a microwave oven
arbon nanoparticles, namely, carbon dots (CDs) have generated much excitement because of their superiority in chemical inertness, low cost, low cytotoxicity, environmentally friendly, ease of functionalization and resistance to photobleaching.1 CDs with tunable emission are considered to be the next generation of green nanomaterials and are promising candidates for numerous exciting applications,2 such as photocatalysis,3 biomedicine, targeted drug delivery,4,5 medical imaging and biosensing,6−12 and photovoltaic devices.13 Electrogenerated chemiluminescence (ECL), a phenomenon in which one or more of the reagents is generated in situ in an electrolytic process, has grown significantly as a highly sensitive and selective analytical and diagnostic method in recent years.14 As the light emitting species are generated in situ close to electrode surfaces, ECL has a near zero background and allows temporal and spatial control over the reaction.15 ECL from semiconductor nanoparticle has been observed in many systems, but heavy metals as the essential elements in quantum dots have raised serious health and environmental concerns.16,17 Additionally, the electrochemically formed semiconductor nanocrystal species have low instability.18,19 To maintain an environment, low-toxicity silicon and carbon nanostructures are preferred.20,21 A single-peak ECL was observed in the presence of coreactants. Considering the potential window for the electrochemical oxidation and reduction of water is too narrow, we employ the CDs ECL © 2014 American Chemical Society
Received: April 1, 2014 Accepted: May 23, 2014 Published: May 23, 2014 5620
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about 2 min until a clear yellow-brown aqueous solution was obtained. Purified CDs were obtained by further dialysis and ultrafiltration of the initial yellow-brown solution. Transmission electron microscopy (TEM) image (Figure 1a) demonstrates that these CDs are nearly monodispersed with a
Figure 1. (a) Low-resolution and (b) high-resolution TEM images of the as-synthesized CDs. The inset in part b shows the hexagonal structure of the CDs.
Figure 2. ECL curves of CDs in ethanol containing 0.1 M TBAB. (a) The reproducibility of ECL at a continuous scan mode. (b) Enlargement of one cycle highlighted with a rectangle in part a.
diameter of 2.3 ± 0.5 nm. The high-resolution TEM image in Figure 1b represents that the CDs have hexagonal structures. For a simple hexagonal crystal, d-spacing at the (100) plane is around 0.22 nm. So a lattice constant is near to 0.25 nm, which implies the graphitic (sp2) cluster is a major component of CDs.23,24 As with previous CDs, the peak at 264 nm in UV−vis absorption is attributed to the n−π* transition of the CO band and a π−π* transition of the conjugated CC band. The fluorescence emission has a red shift as the excitation wavelength is increasing. The maximum emission fluorescence centered at 450 nm is excited by a laser of 373 nm (Figure S1 in the Supporting Information). The addition of TBAB into the CDs solution causes no change in the UV−vis absorption of the CDs and slightly varies in fluorescent spectra. There is only a 2 nm red shift in emission and excitation spectra compared to the CDs solution in the absence of TBAB (Figure S1 in the Supporting Information). These results show that TBAB has a negligible influence on the fluorescent properties of CDs, indicating that TBAB absorbed on the surface of the CDs could not change the band gap of the core. The ECL of CDs in organic solution was studied in detail at a glassy carbon electrode (GCE). The evident ECL signal of the CDs in the presence of 0.1 M TBAB was observed when the applied potential was cycled between −3.0 and 3.0 at 0.2 V s−1 (Figure 2). Cyclic voltammograms (CV) and ECL curves were recorded synchronously as shown in Figure 3. There are three reduction peaks at 0.14 V (Rc1), −0.63 V (Rc2), and −1.39 V (Rc3), and one oxidation peak at 0.50 V (Oa1). The corresponding ECL peaks located at −2.84 V (ECL-1) and −1.71 V (ECL-2), respectively. Oa1 and Rc2 are assigned to the redox of dissolved oxygen, as they disappear after the solution is bubbled by high-purity nitrogen for 15 min. The Rc1 and Rc3 peaks have a slightly negative shift when the dissolved oxygen is removed from the solution with no change in the peak position of ECL-1 and ECL-2 (Figure 3a). Thus, dissolved oxygen or its reduced product OOH− does not participate in the electrode process of ECL reactions. ECL-1 and ECL-2 have a different luminescent mechanism. In order to clarify that, a series of controlled trials was ran. Two of the molecules that have different alkyl chains and similar structure were selected to replace TBAB (Scheme 1 and Figure
Figure 3. (a) ECL-potential curves and (b) corresponding cyclic voltammograms of CDs in ethanol containing 0.1 M TBAB before (red) and after (blue) removing oxygen. Inset: enlargement of the area highlighted with a rectangle. Scan rate: 0.2 V s−1.
S2 in the Supporting Information). The electrochemical and ECL data of three molecules are summarized in Table S1 in the Supporting Information. ECL-1 is more sensitive to the surrounding environment than ECL-2. ECL-1 results from the reaction between coreactants and CDs reduced nanocrystal species, which is related to Rc3.25 The ECL-1 is based on the electron injection to the conduction band of CDs, and subsequently, the electron in the conduction band annihilates with the hole in the valence band (formed by reacting with R• radical) to produce the luminescence. The ECL-1 process is proposed as follows: 2R + e → R− + R• 5621
(1)
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Scheme 1. (a) Proposed Mechanism of ECL-1 and (b) Schematic Representation of the Coreactant Process between CDs and Different Organic Molecules
[CDs]CB + e → [CDs]CB−
(2)
[CDs]VB + R• → [CDs]VB+ + R−
(3)
Figure 4. Effects of metal ions on the ECL-1 (a) and ECL-2 (b) intensity of the CDs/TBAB system. (c) Linear calibration plot for iron ions detection. R indicates the intensity ratio of ECL-1 and ECL-2.
[CDs]CB− + [CDs]VB+ → [CDs]CB + [CDs]VB + hv
Cd2+, Ca2+, Mg2+, Zn2+, Cu2+, Co2+, Ni2+), each at a concentration of 1 mM. Totally they can be separated into three groups: first, slightly effecting on two of ECL peaks (Al3+, Mn2+, Cd2+, Ca2+, Mg2+, Cu2+, Zn2+); second, quenching on both of ECL peaks (Co2+, Ni2+); third, quenching on ECL-1 and mildly enhancing on ECL-2 (Fe2+, Fe3+). Because Fe2+ is oxidized into Fe3+ in positive potential, the ECL behavior of Fe2+ is the same as that of Fe3+. The ECL-1 presented higher sensitivity to the surrounding environment than ECL-2 because of the direct electron transfer during the ECL-1 procedure. Detailed experiments indicate the increase of the concentration of iron ions causes a gradual decrease of ECL-1, while there is a mild increase of ECL-2 (Figure S5 in the Supporting Information). It is interesting to note that the intensity of ECL-1 is not linear to decrease at a function of the concentration of iron ions (Figure S6 in the Supporting Information), which is consistent with the fact that single-peak ECL detection has low accuracy determined by its inherent instability. To improve ECL accuracy, the intensity ratio R of ECL-1 and ECL-2 is selected to detect the concentration of iron ions, in which ECL-1 is an analyte signal and ECL-2 is an internal standard signal, respectively. As depicted in Figure 4b, when the concentration of iron ions changes from 5 × 10−6 to 8 × 10−5 M, R has good linearity with the iron ions concentration with a correlation coefficient R2 = 0.993. Detection limitation was calculated to be 7 × 10−7 M. In conclusion, the electrochemical and ECL behaviors of CDs are studied in detail in this work. ECL of carbon nanoparticles in ethanol with coreactants exhibits two peaks upon a negative potential scan, which corresponds to two different luminescent mechanisms. Dissolved oxygen does not participate in the electrode process and ECL reactions. ECL-1 results from the electron-transfer reaction between coreactants and reduced nanoparticles generated at cathodic scan potentials. ECL-2 is an annihilation process of CDsR. On the basis of the dual-peak ECL of CDs, the concentration of iron ions is detected by the internal standard method, showing a linear response at a range of micromoles with good sensitivity
(4)
Rc1 is a reduction peak associated with ECL-2. Although the peak intensity of ECL-2 has a little change after the addition of different organic molecules, the value of Rc1 is almost constant. The related potential data of different organic molecule systems listed in Table S1 in the Supporting Information. Ethanol, TBAB, and CDs themselves cannot produce ECL signal under the experimental conditions, as well as the redox peak was not found in the corresponding cyclic voltammograms (Figure S3 in the Supporting Information). These phenomena indicate that ECL-2 is a result of the interaction between CDs and TBAB. Exciplexes (CDsR) were directly formed by ion−ion annihilation reactions of CDs and TBAB, which lead to the appearance of ECL-2 signal. The ECL-2 mechanism can involve exciplexes formation as follows: [CDs] − e → [CDs]•+
(5)
R + e → R•−
(6)
[CDs]•+ + R•− → [CDsR]*
(7)
[CDsR]* → [CDsR] + hv
(8)
The accurate and rapid determination of iron ions are of practical importance in living systems. Although iron ions can be detected by many methods, even a nanomolar detection limit could be achieved based on the ECL of the o-CDs/ K2S2O8 system,26 the single-peak ECL system cannot obviously provide a complete long-term stability of the detection due to their inherent instability. In a dual-peak ECL system, the intensity of ECL-1 emission changes with the concentration of iron ions in a certain range, while ECL-2 has no change, which is similar to internal standard method. On the basis of these mechanisms, the accuracy and sensitivity of the detection of iron ions are improved. Figure 4a,b shows the ECL quenching value of the CDs/ TBAB system with various metal ions (Fe2+, Fe3+, Al3+, Mn2+, 5622
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(18) Ding, Z.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Science 2002, 296, 1293−1297. (19) Myung, N.; Ding, Z. F.; Bard, A. J. Nano Lett. 2002, 2, 1315− 1319. (20) Zheng, L. Y.; Chi, Y. W.; Dong, Y. Q.; Lin, J. P.; Wang, B. B. J. Am. Chem. Soc. 2009, 131, 4564−4565. (21) Sun, Y. P.; Zhou, B.; Lin, Y.; Wang, W.; Shiral Femando, K. A.; Pathak, P.; et al. J. Am. Chem. Soc. 2006, 128, 7756−7757. (22) Zhu, H.; Wang, X. L.; Li, Y. L.; Wang, Z. J.; Yang, F.; Yang, X. R. Chem. Commun. 2009, 5118−5120. (23) Zhu, S. J.; Meng, Q. N.; Wang, L.; Zhang, J. H.; Song, Y. B.; Jin, H.; Zhang, K.; Sun, H. Y.; Yang, B. Angew. Chem., Int. Ed. 2013, 52, 3953−3957. (24) Kwon, W.; Lee, G.; Do, S.; Joo, T.; Rhee, S. W. Small 2014, 10, 506−513. (25) Zou, G. Z.; Ju, H. X. Anal. Chem. 2004, 76, 6871−6876. (26) Xu, Y.; Wu, M.; Feng, X. Z.; Yin, X. B.; He, X. W.; Zhang, Y. K. Chem.Eur. J. 2013, 19, 6282−6288.
and accuracy. This work provides an alternative method to avoid the inherent instability of ECL for the detection of metal ions. Furthermore, the CDs could be utilized as a biosensor reagent capable of detecting iron ions in biosystems.
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ASSOCIATED CONTENT
* Supporting Information S
UV−vis absorption and PL emission spectra of CDs and CDs/ TBAB system; ECL-potential curves and cyclic voltammograms of CDs/TMAB and CDs/TOAB; electrochemical and ECL data in various supporting electrolytes; ECL curves and cyclic voltammograms of ethanol, TBAB, and CDs; and intensity variations of ECL-1 and ECL-2 with the increase of the concentration of iron ions. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Phone: +86-10-62562042. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA09020100), the Institute of Chemistry (Grant Nos. Y31Z0C1BZ1, Y329751261), and the National Natural Science Foundation of China (Grant No. 20905044)
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REFERENCES
(1) Li, H.; Kang, Z.; Lin, Y.; Lee, S. T. J. Mater. Chem. 2012, 22, 24230−24253. (2) Cao, L.; Mezinai, M. J.; Sahu, S.; Sun, Y. P. Acc. Chem. Res. 2013, 46, 171−180. (3) Li, H. T.; He, X. D.; Kang, Z. H.; Huang, H.; Liu, Y.; Liu, J. L.; Lian, S. Y.; Tsang, C. H. A.; Yang, X. B.; Lee, S. T. Angew. Chem., Int. Ed. 2010, 49, 4430−4434. (4) Zheng, G. F.; Gao, X. P. A.; Lieber, C. M. Nano Lett. 2010, 10, 3179−3183. (5) Zheng, G. F.; Daniel, W. L.; Mirkin, C. A. J. Am. Chem. Soc. 2008, 130, 9644−9645. (6) Kong, B.; Zhu, A. W.; Ding, C. Q.; Zhao, X. M.; Li, B.; Tian, Y. Adv. Mater. 2012, 24, 5844−5848. (7) Liu, R. L.; Wu, D. Q.; Liu, S. H.; Koynov, K.; Knoll, W.; Li, Q. Angew. Chem., Int. Ed. 2009, 48, 4598−4601. (8) Chen, T.; Wu, C. S.; Jimenez, E.; Zhu, Z.; Dajac, J. G.; You, M.; Han, D.; Zhang, X.; Tan, W. Angew. Chem., Int. Ed. 2013, 52, 2012− 2016. (9) Huang, P.; Lin, J.; Wang, X. S.; Wang, Z.; Zhang, C. L.; He, M.; Wang, K.; Chen, F.; Li, Z. M.; Shen, G. X.; Cui, D. X.; Chen, X. Y. Adv. Mater. 2012, 24, 5104−5110. (10) Zhu, A.; Qu, Q.; Shao, X.; Kong, B.; Tian, Y. Angew. Chem., Int. Ed. 2012, 51, 7185−7189. (11) Zhu, A.; Luo, Z.; Ding, C.; Li, B.; Zhou, S.; Wang, R.; Tian, Y. Analyst 2014, 139, 1945−1952. (12) Ding, C.; Zhu, A.; Tian, Y. Acc. Chem. Res. 2014, 47, 20−30. (13) Li, Y.; Hu, Y.; Zhao, Y.; Shi, G. Q.; Deng, L. E.; Hou, Y. B.; Qu, L. T. Adv. Mater. 2011, 23, 776−780. (14) Deng, S. Y.; Ju, H. X. Analyst 2013, 138, 43−61. (15) Fahnrich, K. A.; Pravda, M.; Guilbault, G. G. Talanta 2001, 54, 531−559. (16) Bae, Y.; Myung, N.; Bard, A. J. Nano Lett. 2004, 4, 1153−1161. (17) Myung, N.; Bae, Y.; Bard, A. J. Nano Lett. 2003, 3, 1053−1055. 5623
dx.doi.org/10.1021/ac5011734 | Anal. Chem. 2014, 86, 5620−5623
Dual-Peak Electrogenerated Chemiluminescence of Carbon Dots for Iron Ions Detection Pengjia Zhang,† Zhenjie Xue,‡ Dan Luo,‡ Wei Yu,‡ Zhihui Guo,*,† and Tie Wang*,‡ †
Key Laboratory of Analytical Chemistry for Life Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, P. R. China ‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: Carbon dots (CDs) have rigorously been investigated on their unique fluorescent properties but rarely their electrogenerated chemiluminescence (ECL) behavior. We are here to report a dual-peak ECL system of CDs, one at −2.84 V (ECL-1) and the other at −1.71 V (ECL-2) during the cyclic sweep between −3.0 and 3.0 V at scan rate of 0.2 V s−1 in 0.1 M tetrabutyl ammonium bromide (TBAB) ethanol solution, which is more efficiency to distinguish metallic ions than single-peak ECL. The electron transfer reaction between individual electrochemically reduced nanocrystal species and coreactants led to ECL-1, in which the electron injected to the conduction band of CDs in the cathodic process. Ion annihilation reactions induced direct formation of exciplexes that produced another ECL signal, ECL-2. ECL-1 showed higher sensitivity to the surrounding environment than ECL-2 and thus was used for ECL detection of metallic ions. Herein, we can serve as an internal standard method to detect iron ions. A linear relationship of the intensity ratio R of ECL-1 and ECL-2 to iron ions was observed in the concentration extending from 5 × 10−6 to 8 × 10−5 M with a detection limit of 7 × 10−7 M.
C
in an organic phase. Greatly different from previous works of single-peak ECL system that cannot obviously provide a complete long-term stability of the detection due to their inherent instability, we report here for the first time a dual-peak ECL system for detection of iron ions. The ECL intensity of one peak changes with the concentration of iron ions in a certain range, and the other one is stable, which is similar to internal standard method. So, the accuracy and sensitivity of the detection of iron ions are greatly improved comparing to singlepeak ECL. To study the ECL behavior and mechanism of CDs, different supporting electrolytes with various length of alkyl chain were added to the ECL system. The two ECL peaks present distinct sensitivity to the surrounding environment. Therefore, the dual-peak ECL provides more information for detecting metallic ions, which is more efficiency to distinguish iron ions from other metallic ions than single-peak ECL. CDs were synthesized by the microwave treatment according to the reported method with a slight modification.22 In a typical synthesis, an appropriate amount of poly(ethylene glycol) (PEG-200, 1.125 g/mL) and ascorbic acid (0.08 g/mL) were added to distilled water to form a transparent homogeneous solution. Then the solution was heated in a microwave oven
arbon nanoparticles, namely, carbon dots (CDs) have generated much excitement because of their superiority in chemical inertness, low cost, low cytotoxicity, environmentally friendly, ease of functionalization and resistance to photobleaching.1 CDs with tunable emission are considered to be the next generation of green nanomaterials and are promising candidates for numerous exciting applications,2 such as photocatalysis,3 biomedicine, targeted drug delivery,4,5 medical imaging and biosensing,6−12 and photovoltaic devices.13 Electrogenerated chemiluminescence (ECL), a phenomenon in which one or more of the reagents is generated in situ in an electrolytic process, has grown significantly as a highly sensitive and selective analytical and diagnostic method in recent years.14 As the light emitting species are generated in situ close to electrode surfaces, ECL has a near zero background and allows temporal and spatial control over the reaction.15 ECL from semiconductor nanoparticle has been observed in many systems, but heavy metals as the essential elements in quantum dots have raised serious health and environmental concerns.16,17 Additionally, the electrochemically formed semiconductor nanocrystal species have low instability.18,19 To maintain an environment, low-toxicity silicon and carbon nanostructures are preferred.20,21 A single-peak ECL was observed in the presence of coreactants. Considering the potential window for the electrochemical oxidation and reduction of water is too narrow, we employ the CDs ECL © 2014 American Chemical Society
Received: April 1, 2014 Accepted: May 23, 2014 Published: May 23, 2014 5620
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about 2 min until a clear yellow-brown aqueous solution was obtained. Purified CDs were obtained by further dialysis and ultrafiltration of the initial yellow-brown solution. Transmission electron microscopy (TEM) image (Figure 1a) demonstrates that these CDs are nearly monodispersed with a
Figure 1. (a) Low-resolution and (b) high-resolution TEM images of the as-synthesized CDs. The inset in part b shows the hexagonal structure of the CDs.
Figure 2. ECL curves of CDs in ethanol containing 0.1 M TBAB. (a) The reproducibility of ECL at a continuous scan mode. (b) Enlargement of one cycle highlighted with a rectangle in part a.
diameter of 2.3 ± 0.5 nm. The high-resolution TEM image in Figure 1b represents that the CDs have hexagonal structures. For a simple hexagonal crystal, d-spacing at the (100) plane is around 0.22 nm. So a lattice constant is near to 0.25 nm, which implies the graphitic (sp2) cluster is a major component of CDs.23,24 As with previous CDs, the peak at 264 nm in UV−vis absorption is attributed to the n−π* transition of the CO band and a π−π* transition of the conjugated CC band. The fluorescence emission has a red shift as the excitation wavelength is increasing. The maximum emission fluorescence centered at 450 nm is excited by a laser of 373 nm (Figure S1 in the Supporting Information). The addition of TBAB into the CDs solution causes no change in the UV−vis absorption of the CDs and slightly varies in fluorescent spectra. There is only a 2 nm red shift in emission and excitation spectra compared to the CDs solution in the absence of TBAB (Figure S1 in the Supporting Information). These results show that TBAB has a negligible influence on the fluorescent properties of CDs, indicating that TBAB absorbed on the surface of the CDs could not change the band gap of the core. The ECL of CDs in organic solution was studied in detail at a glassy carbon electrode (GCE). The evident ECL signal of the CDs in the presence of 0.1 M TBAB was observed when the applied potential was cycled between −3.0 and 3.0 at 0.2 V s−1 (Figure 2). Cyclic voltammograms (CV) and ECL curves were recorded synchronously as shown in Figure 3. There are three reduction peaks at 0.14 V (Rc1), −0.63 V (Rc2), and −1.39 V (Rc3), and one oxidation peak at 0.50 V (Oa1). The corresponding ECL peaks located at −2.84 V (ECL-1) and −1.71 V (ECL-2), respectively. Oa1 and Rc2 are assigned to the redox of dissolved oxygen, as they disappear after the solution is bubbled by high-purity nitrogen for 15 min. The Rc1 and Rc3 peaks have a slightly negative shift when the dissolved oxygen is removed from the solution with no change in the peak position of ECL-1 and ECL-2 (Figure 3a). Thus, dissolved oxygen or its reduced product OOH− does not participate in the electrode process of ECL reactions. ECL-1 and ECL-2 have a different luminescent mechanism. In order to clarify that, a series of controlled trials was ran. Two of the molecules that have different alkyl chains and similar structure were selected to replace TBAB (Scheme 1 and Figure
Figure 3. (a) ECL-potential curves and (b) corresponding cyclic voltammograms of CDs in ethanol containing 0.1 M TBAB before (red) and after (blue) removing oxygen. Inset: enlargement of the area highlighted with a rectangle. Scan rate: 0.2 V s−1.
S2 in the Supporting Information). The electrochemical and ECL data of three molecules are summarized in Table S1 in the Supporting Information. ECL-1 is more sensitive to the surrounding environment than ECL-2. ECL-1 results from the reaction between coreactants and CDs reduced nanocrystal species, which is related to Rc3.25 The ECL-1 is based on the electron injection to the conduction band of CDs, and subsequently, the electron in the conduction band annihilates with the hole in the valence band (formed by reacting with R• radical) to produce the luminescence. The ECL-1 process is proposed as follows: 2R + e → R− + R• 5621
(1)
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Scheme 1. (a) Proposed Mechanism of ECL-1 and (b) Schematic Representation of the Coreactant Process between CDs and Different Organic Molecules
[CDs]CB + e → [CDs]CB−
(2)
[CDs]VB + R• → [CDs]VB+ + R−
(3)
Figure 4. Effects of metal ions on the ECL-1 (a) and ECL-2 (b) intensity of the CDs/TBAB system. (c) Linear calibration plot for iron ions detection. R indicates the intensity ratio of ECL-1 and ECL-2.
[CDs]CB− + [CDs]VB+ → [CDs]CB + [CDs]VB + hv
Cd2+, Ca2+, Mg2+, Zn2+, Cu2+, Co2+, Ni2+), each at a concentration of 1 mM. Totally they can be separated into three groups: first, slightly effecting on two of ECL peaks (Al3+, Mn2+, Cd2+, Ca2+, Mg2+, Cu2+, Zn2+); second, quenching on both of ECL peaks (Co2+, Ni2+); third, quenching on ECL-1 and mildly enhancing on ECL-2 (Fe2+, Fe3+). Because Fe2+ is oxidized into Fe3+ in positive potential, the ECL behavior of Fe2+ is the same as that of Fe3+. The ECL-1 presented higher sensitivity to the surrounding environment than ECL-2 because of the direct electron transfer during the ECL-1 procedure. Detailed experiments indicate the increase of the concentration of iron ions causes a gradual decrease of ECL-1, while there is a mild increase of ECL-2 (Figure S5 in the Supporting Information). It is interesting to note that the intensity of ECL-1 is not linear to decrease at a function of the concentration of iron ions (Figure S6 in the Supporting Information), which is consistent with the fact that single-peak ECL detection has low accuracy determined by its inherent instability. To improve ECL accuracy, the intensity ratio R of ECL-1 and ECL-2 is selected to detect the concentration of iron ions, in which ECL-1 is an analyte signal and ECL-2 is an internal standard signal, respectively. As depicted in Figure 4b, when the concentration of iron ions changes from 5 × 10−6 to 8 × 10−5 M, R has good linearity with the iron ions concentration with a correlation coefficient R2 = 0.993. Detection limitation was calculated to be 7 × 10−7 M. In conclusion, the electrochemical and ECL behaviors of CDs are studied in detail in this work. ECL of carbon nanoparticles in ethanol with coreactants exhibits two peaks upon a negative potential scan, which corresponds to two different luminescent mechanisms. Dissolved oxygen does not participate in the electrode process and ECL reactions. ECL-1 results from the electron-transfer reaction between coreactants and reduced nanoparticles generated at cathodic scan potentials. ECL-2 is an annihilation process of CDsR. On the basis of the dual-peak ECL of CDs, the concentration of iron ions is detected by the internal standard method, showing a linear response at a range of micromoles with good sensitivity
(4)
Rc1 is a reduction peak associated with ECL-2. Although the peak intensity of ECL-2 has a little change after the addition of different organic molecules, the value of Rc1 is almost constant. The related potential data of different organic molecule systems listed in Table S1 in the Supporting Information. Ethanol, TBAB, and CDs themselves cannot produce ECL signal under the experimental conditions, as well as the redox peak was not found in the corresponding cyclic voltammograms (Figure S3 in the Supporting Information). These phenomena indicate that ECL-2 is a result of the interaction between CDs and TBAB. Exciplexes (CDsR) were directly formed by ion−ion annihilation reactions of CDs and TBAB, which lead to the appearance of ECL-2 signal. The ECL-2 mechanism can involve exciplexes formation as follows: [CDs] − e → [CDs]•+
(5)
R + e → R•−
(6)
[CDs]•+ + R•− → [CDsR]*
(7)
[CDsR]* → [CDsR] + hv
(8)
The accurate and rapid determination of iron ions are of practical importance in living systems. Although iron ions can be detected by many methods, even a nanomolar detection limit could be achieved based on the ECL of the o-CDs/ K2S2O8 system,26 the single-peak ECL system cannot obviously provide a complete long-term stability of the detection due to their inherent instability. In a dual-peak ECL system, the intensity of ECL-1 emission changes with the concentration of iron ions in a certain range, while ECL-2 has no change, which is similar to internal standard method. On the basis of these mechanisms, the accuracy and sensitivity of the detection of iron ions are improved. Figure 4a,b shows the ECL quenching value of the CDs/ TBAB system with various metal ions (Fe2+, Fe3+, Al3+, Mn2+, 5622
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Analytical Chemistry
Letter
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and accuracy. This work provides an alternative method to avoid the inherent instability of ECL for the detection of metal ions. Furthermore, the CDs could be utilized as a biosensor reagent capable of detecting iron ions in biosystems.
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ASSOCIATED CONTENT
* Supporting Information S
UV−vis absorption and PL emission spectra of CDs and CDs/ TBAB system; ECL-potential curves and cyclic voltammograms of CDs/TMAB and CDs/TOAB; electrochemical and ECL data in various supporting electrolytes; ECL curves and cyclic voltammograms of ethanol, TBAB, and CDs; and intensity variations of ECL-1 and ECL-2 with the increase of the concentration of iron ions. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Phone: +86-10-62562042. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA09020100), the Institute of Chemistry (Grant Nos. Y31Z0C1BZ1, Y329751261), and the National Natural Science Foundation of China (Grant No. 20905044)
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