Anodic Electrogenerated Chemiluminescence Behavior of Graphite

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Anodic Electrogenerated Chemiluminescence Behavior of GraphiteLike Carbon Nitride and Its Sensing for Rutin Changming Cheng,†,§ Ying Huang,†,§ Jun Wang,† Baozhan Zheng,‡ Hongyan Yuan,*,† and Dan Xiao*,†,‡ †

College of Chemical Engineering, Sichuan University, Chengdu 610065, PR China College of Chemistry, Sichuan University, Chengdu 610064, PR China

‡

S Supporting Information *

ABSTRACT: In this paper, the anodic electrogenerated chemiluminescence (ECL) behavior of graphite-like carbon nitride (g-C3N4) is studied using cyclic voltammetry with triethanolamine (TEA) as a coreactant. The possible anodic ECL response mechanism of the g-C3N4/TEA system is proposed. Furthermore, it is observed that the anodic ECL signal can be quenched efficiently in the presence of rutin, on the basis of which a facile anodic ECL senor for the determination of rutin is developed. This ECL sensor is found to have a linear response in the range of 0.20−45.0 μM and a low detection limit of 0.14 μM (at signal-to-noise of 3). These results suggest that semiconductor g-C3N4 has great potential in extending the application in the ECL field as an efficient luminophore.

order to expand their potential application in the ECL fields though the previous work usually focuses on one side (anodic ECL or cathodic ECL). However, to the best of our knowledge, there is no paper issued about the anodic ECL behavior of semiconductor g-C3N4 until now. As such, it is of great importance to expand the potential application of luminophore g-C3N4 in the fields of ECL and analytical chemistry. Herein, the anodic electrogenerated chemiluminescence using g-C3N4 as luminophore is investigated. The ECL measurements are carried out in 0.10 M phosphate buffer solution (PBS) containing 0.10 M KCl by cyclic voltammetry. The possible reaction mechanism in the anodic ECL process is proposed. The electro-oxidized g-C3N4 can readily react with the intermediate generated from coreactant triethanolamine (TEA) upon electrochemical oxidation, to produce excited state g-C3N4 which subsequently decays back to its ground state, emitting strong luminescence. The major contribution of the present work is the demonstration of the anodic ECL behavior of luminophore g-C3N4, which would open new avenues to construct anodic ECL sensing platforms based on the stable and cost-effective semiconductor g-C3N4. Rutin (vitamin P) is a kind of bioactive flavonoid glycoside. It exhibits broad range physiological activities, for instance, antiinflammatory, antibacterial, antitumor, antiviral, and antioxidant.32 In clinical chemistry, it has been used as therapeutic medicine.33 As such, the accurate detection of rutin is of practical importance. As a simple, fast, sensitive, and low-cost approach, ECL has been widely used in the field of analytical

D

uring the past several years, a lot of theoretical and experimental attention has been paid to graphite-like carbon nitride (g-C3N4) because of its unique structure and properties. As a chemically and thermally stable semiconductor, it has been used widely in the fields of catalysis,1−3 degradation,4,5 photoelectronic materials,6,7 sensors,8,9 and so forth. Recently, complement to its ultraviolet−visible adsorption and photoluminescence properties,7,8 the cathodic electrogenerated chemiluminescence behavior of g-C3N4 has been reported in our group, and an electrogenerated chemiluminescence (ECL) sensor based on the g-C3N4-modified carbon paste electrode has been used for the sensitive and selective determination of trace Cu2+, demonstrating that semiconductor g-C3N4 may be a new class of efficient and promising candidate luminophore for ECL sensing.10 In the previous ECL systems, Ru complex,11−14 luminol,15−17 gold nanocluster,18,19 and various semiconductors (i.e., CdS, 20 −22 CdSe, 23−2 5 CdTe, 26−28 PbSe 29 ) have been widely used as ECL luminophores, based on which numerous chemosensors and biosensors have been fabricated for the determination of H2O2, Pb2+, Cu2+, tripropylamine, ascorbic acid, dopamine, and DNA. However, it is still one of the key subjects in the ECL research to indentify innovative, stable, and highly efficient ECL luminophores.30,31 Meanwhile, g-C3N4 exhibits some charming advantages over the aforementioned conventional ECL luminophores. It is inexpensive, nontoxic, and chemically/ thermally stable and can be prepared by a simple thermal pyrolysis method. As is well-known, for the ECL luminophores, not only cathodic but also anodic ECL behaviors are of great importance, and their reaction mechanisms are completely different. In the past few years, researchers have investigated the anodic and cathodic ECL behaviors of various luminophores in © 2013 American Chemical Society

Received: November 9, 2012 Accepted: February 3, 2013 Published: February 3, 2013 2601

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0.00 and 1.90 at 0.30 V/s. A weak anodic ECL signal could be observed on the g-C3N4-modified carbon paste electrode in the buffer solution without coreactant TEA, as shown in the inset of Figure 1A; however, the ECL intensity could be greatly enhanced by introducing TEA into the buffer solution (curve b), indicating that coreactant TEA may play a crucial role in the ECL process. Analogous to anodic ECL pathways of other semiconductors37,38 and Ru complex39,40 in the presence of coreactants, the possible (electro)chemical reactions of g-C3N4 and TEA on the carbon paste electrode in the present work are summarized as follows:

chemistry. However, only a few ECL methods have been applied for the determination of rutin.34−36 It is found that rutin could quench the anodic ECL from g-C3N4 through an energy transfer process, based on which an ECL sensor is developed for rutin determination with a detection limit of 0.14 μM.

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EXPERIMENTAL SECTION Chemicals. Melamine (2,4,6-triamino-1,3,5-trazine, 95%) was purchased from Kelong Chemical Co., Ltd. (Chengdu, China). Triethanolamine (AR grade) was purchased from Beijing Chemical Factory (Beijing, China). Rutin (BR grade) was obtained from National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China). Rutin tablets, with the specified amount of 60 mg per tablet, were purchased from a local hospital (Chengdu, China). All other reagents of analytical grade were used as received without further purification. Double distilled water was used throughout. Synthesis and Characterization of g-C3N4. g-C3N4 was prepared by one-step thermal-induced self-condensation of melamine under atmospheric condition, and the anodic ECL behavior of g-C3N4 was investigated on a g-C3N4-modified carbon paste electrode, similar to that described in our previous work.10 The CV and ECL measurements were conducted at 298 K in a classical three-electrode configuration consisting of a g-C3N4-modified carbon paste working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode. The CV and ECL curves were recorded simultaneously using a MPI-E electrochemiluminescence analyzer system (Xi’an Remax Analysis Instrument Co., Ltd., Xi’an, China). The supporting electrolyte was 0.10 M PBS containing 0.10 M KCl. The anodic ECL emission and photoluminescence (PL) spectra were measured on a Hitachi F-4500 fluorescence spectrophotometer (Tokyo, Japan).

C3N4 − e− → C3N4•+

(1)

TEA − e− → TEA•+

(2)

TEA•+ − H+ → (EtO)2 N•CHCH 2OH (TEA•)

(3)

C3N4•+ + TEA• → C3N4* + TEA oxidant

(4)

C3N4* → C3N4 + hν

(5)

Both anodic and cathodic ECL behaviors of various semiconductors have been widely reported along with different reaction procedures, where the electro-oxidized (or electroreduced) semiconductors could react with radicals from coreactants to produce ECL. In our previous research, semiconductor g-C3N4 could be electro-reduced by injecting an electron into its conduction band to form the negatively charged g-C3N4.10 Similarly, the positively charged g-C3N4 (i.e., g-C3N4•+) may be produced from the electro-oxidation of gC3N4 if the applied potential on the working electrode is more positive than the valence band of g-C3N41 in the positive potential scan so that the electron transfer is energetically possible, as shown in eq 1. The analogous reaction schemes have been postulated in the anodic ECL behaviors of graphene oxide41 as well as CdTe quantum dots.42 Figure 1B depicts the CV curves of the g-C3N4-modified carbon paste electrodes in 0.10 M PBS (pH 7.4) containing 0.10 M KCl in the absence (curve c) and presence (curve d) of coreactant TEA. During the positive potential scanning, a broad anodic wave at the formal potential of ca. 0.80 V (vs Ag/AgCl) could be found in curve d. In contrast, the broad anodic wave disappears when no TEA is used, as shown in curve c. Similar CV scan was conducted on the pure carbon paste electrode in the presence of TEA, and a similar broad anodic wave could also be observed, as depicted in Figure S1, Supporting Information. We can get a conclusion that the broad anodic wave is attributed to the electro-oxidation of coreactant TEA. Similar to common ECL coreactants, such as tripropylamine,39,41,43 triethylamine,18,37 2-(dibutylamino)ethanol,38,44 etc., coreactant TEA used in the present work could be electro-oxidized to a cation (TEA•+), as shown in eq 2, and then, upon deprotonation of an α-carbon from one of the hydroxyethyl groups, TEA•+ would subsequently decompose to produce a radical of powerful reductive properties (TEA•),45 as shown in eq 3, which can react with the oxidized form of g-C3N4 to produce the excited state g-C3N4 (g-C3N4*) via electron transfer (eq 4). Finally, an intense emission is obtained when g-C3N4* decays back to the ground state g-C3N4 (eq 5). It is noted that, in the absence of coreactant TEA, the electro-oxidation current of g-C3N4 is too slight to be discernible, as shown in curve c. However, after introducing TEA into the buffer solution, no obvious peak attributed to the electro-oxidation process of g-C3N4 is

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RESULTS AND DISCUSSION Figure 1A displays the ECL curves of the g-C3N4-modified carbon paste electrode in the absence (curve a) and presence (curve b) of coreactant TEA by cycling the potential between

Figure 1. (A) ECL-potential (a, b) and (B) CV (c, d) curves of the gC3N4-modified carbon paste electrode in 0.10 M PBS (pH 7.4) containing 0.10 M KCl without (a, c) and with (b, d) 20 mM TEA. The scan rate is 0.30 V/s. The inset of A displays the enlarged view of curve a. 2602

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low in the present work. However, we observe herein that the ECL from the g-C3N4/triethanolamine system is higher than those in the g-C3N4/triethylamine and g-C3N4/tripropylamine systems, indicating that electron-withdrawing hydroxyethyl groups exhibit positive effect on ECL intensity. These results could be attributed to the fact that the hydroxyethyl groups in triethanolamine can catalyze the electro-oxidation of amines and substantially increase the ECL efficiency, similar to the previous reports.38,46,48 In addition, tertiary amines are more effective than secondary amines as ECL coreactants;46 therefore, the ECL from the g-C3N4/triethanolamine system is higher than that from the g-C3N4/diethanolamine system. Furthermore, TEA is much less toxic and more soluble than the popular coreactant TPrA in aqueous solution, demonstrating that TEA may be a very promising coreactant in the anodic ECL systems. In the previous work, a selective and sensitive Cu2+ sensor based on the cathodic ECL from g-C3N4 has been developed. Besides cathodic ECL, anodic ECL is of great importance for luminophores. It is our goal to expand the application of luminophore g-C3N4 in the ECL field. Thus, its anodic ECL behavior is studied in this work. First, the anodic ECL stability is investigated for the purpose of the potential application in the analytical chemistry field. Figure 3 displays the ECL

observed, since it is probably overshadowed by the relatively high oxidation current of TEA, as shown in curve d. The similar phenomenon has been observed in the CdTe/2(dibutylamino)ethanol ECL system.38 Figure S2, Supporting Information, reveals the anodic ECL emission spectrum of g-C3N4 in 0.10 M PBS (pH 7.4) containing 0.10 M KCl and 20 mM TEA, recorded on a fluorescence spectrophotometer. It exhibits a broad emission band at ca. 400−600 nm and an emission peak of ca. 470 nm, which matches well with spectra of the cathodic ECL10 and photoluminescence of g-C3N4 (Figure S2, Supporting Information), indicating that the same light-emitters, excited state g-C3N4 (g-C3N4*), are generated in the electrochemical reactions and photoexcitation process. The results demonstrate that semiconductor g-C3N4 could serve as an efficient luminophore in not only cathodic but also anodic ECL systems. Furthermore, a series of experimental parameters, including coreactant concentration, solution pH, g-C3N4/carbon powder ratio, and scan rate, are investigated in detail to acquire the optimal ECL performance, as depicted in Figure S3, Supporting Information. Under the optimized conditions, the ECL intensities of the g-C3N4-modified carbon paste electrode in 0.10 M PBS (pH 7.4) containing 20 mM TEA and the carbon paste electrode in a standard solution of 0.10 M PBS (pH 7.4) containing 1.0 mM Ru(bpy)32+ and 20 mM TEA are compared to estimate the relative ECL efficiency (Figure S4, Supporting Information). The integrated ECL intensity of g-C3N4 is only about 0.004 times of that of the standard solution, indicating the low emission efficiency of the anodic ECL. It is of great importance to select proper coreactants for ECL systems, since they would be transferred to oxidized (or reduced) species and help the light emission in a single direction potential sweep. In the past few years, tripropylamine (TPrA) has been exclusively used as the coreactant in the anodic ECL systems; however, it is toxic, volatile, and corrosive and has low electro-oxidation rate and poor solubility.45,46 Recently, some amine compounds have been developed as alternative ECL coreactants, for instance, triethylamine18,47 and triethanolamine38,48 as well as diethanolamine.46 In the present work, we investigate the effect of different amine coreactants on the anodic ECL intensity, as shown in Figure 2. It has been reported that carboxylic groups could decrease ECL intensity since they are electron-withdrawing groups.46 It is considerable that ECL from the g-C3N4/nitrilotriacetic acid system is very

Figure 3. Stability of anodic ECL intensities from the g-C3N4-modified carbon paste electrode.

intensities of the g-C3N4-modified carbon paste electrode under 10 repeated cyclic voltammetric scans in the potential window of 0.00−1.90 V (vs Ag/AgCl) in 0.10 M PBS (pH 7.4) containing 0.10 M KCl and 20 mM TEA at 0.30 V/s. The anodic ECL intensity from g-C3N4 is quite stable with a relative standard deviation of 0.23%, indicating its satisfying reversibility and reliability as a sensing signal. In this work, it is found that the anodic ECL signal from gC3N4 is effectively quenched in the presence of rutin, demonstrating its potential application for rutin determination. Figure 4 presents the effect of various concentration of rutin on the anodic ECL intensity. It is obvious that the ECL intensity decreases with the increase in rutin concentration. The Poisson statistics static quenching model10,49,50 is employed to linearly relate the decrease in anodic ECL intensity and concentration of rutin by plotting ln(Io/I) against the concentration of rutin as shown in the inset of Figure 4, where Io and I are the ECL intensities in the absence and presence of rutin. The curve has a linear range from 0.20 to 45.0 μM with a detection limit of 0.14 μM (S/N = 3), which is lower than or comparable with the

Figure 2. Anodic ECL intensities of the g-C3N4-modified carbon paste electrode with different kinds of amine coreactants with the same concentration of 20 mM. 2603

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good, consistent with the label value (60 mg/tablet), and the recovery experiments were carried out with acceptable results (96.5−102.3%), indicating that the analytical performance of our proposed ECL sensor is satisfactory and could be applied for the rutin determination in real samples.

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CONCLUSION In the present work, we have investigated the anodic ECL behaviors of semiconductor g-C3N4 in the presence of ECL coreactant TEA, expanding the potential application of g-C3N4 in the ECL field as an efficient luminophore. The possible response mechanism for the anodic ECL is proposed. Furthermore, a sensitive and selective ECL method for the determination of rutin is successfully developed. This study complements the research of the nontoxic, inexpensive, and cost-effective luminophore g-C3N4, which may provide an alternative for traditional ECL luminophores and open new avenues for the promising application of semiconductor g-C3N4 in the field of ECL sensing.

Figure 4. Effect of the concentration of rutin (a) 0.0, (b) 0.20, (c) 0.60, (d) 1.0, (e) 2.50, (f) 5.0, (g) 10.0, (h) 15.0, (i) 20.0, (j) 25.0, (k) 30.0, (l) 35.0, (m) 40.0, and (n) 45.0 μM on the ECL intensity of the g-C3N4-modified carbon paste electrode. The inset displays the plot of ln(Io/I) against the concentration of rutin, where Io and I are the ECL intensities in the absence and presence of rutin, respectively.

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previously reported ECL detection of rutin, as shown in Table S1, Supporting Information. Furthermore, the selectivity of the present ECL sensor was also examined. Figure 5 reveals that it has an excellent

ASSOCIATED CONTENT

* Supporting Information S

Supplementary figures of the CV curve, spectra of PL and anodic ECL emission, optimization of ECL working conditions, chronoamperometry and ECL-time curves in the measurement of relative ECL efficiency, and comparison of analytical performance of different ECL sensors for rutin. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.X.); [email protected] (H.Y.Y.). Author Contributions §

C.C. and Y.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (20927007, 21175094, and 21177090) and the Youth Foundation of Sichuan University (2010SCU11048).

Figure 5. Comparison of the quenching effect of rutin (15.0 μM) and other possible interfering agents (1.50 mM) on the g-C3N4-based ECL intensity. The quenching coefficient is determined in terms of (Io−I)/ Io, where Io and I are the ECL intensities in the absence and presence of analyst, respectively.

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