Revealing Carbon Nanodots As Coreactants of the Anodic

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Revealing Carbon Nanodots As Coreactants of the Anodic Electrochemiluminescence of Ru(bpy)32+ Yan-Min Long, Lei Bao, Jing-Ya Zhao, Zhi-Ling Zhang, and Dai-Wen Pang* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, State Key Laboratory of Virology, and Wuhan Institute of Biotechnology, Wuhan University, Wuhan, 430072, P. R. China S Supporting Information *

ABSTRACT: Recently, research on carbon nanodots (C-dots), a new type of luminescent nanoparticles with superior optical properties, biocompatibility, and low cost, has been focused on exploring novel properties and structure-related mechanisms to extend their scope. Herein, electrochemiluminescence, a surface-sensitive tool, is used to probe the unrevealed property of carbon nanodots which is characterized by surface oxygencontaining groups. Together with chemiluminescence, carbon nanodots as the coreactants for the anodic electrochemiluminescence of Ru(bpy)32+ are demonstrated for the first time. During the anodic scan, the benzylic alcohol units on the C-dots surface are oxidized “homogeneously” by electrogeneratedRu(bpy)33+ to form reductive radical intermediate, which further reduce Ru(bpy)33+ into Ru(bpy)32+* that produces a strong ECL emission. This work has provided an insight into the ECL mechanism of the C-dots-involved system, which will be beneficial for in-depth understanding of some peculiar phenomena of C-dots, such as photocatalytic activity and redox properties. Moreover, because of the features of C-dots, the ECL system of Ru(bpy)32+/C-dots is more promising in the bioanalysis.

C

intermediate evolved from a coreactant (coreactant mechanism).19,20 ECL is an ideal tool for probing the surface structure and related mechanism of nanoparticles. In recent work, C-dots in the ECL system have been studied only as nanoemitters by similar approaches for fluorescent semiconductor nanoparticles,12,13 regardless of their unique surface structure of oxygen-containing units. Meanwhile, most of oxygen-containing compounds (e.g., alcohols) have been reported to serve as the coreactants in ECL.21,22 As discussed, it is essential to take oxygen-containing units on the surface into account and review the behaviors of C-dots in the ECL process. Herein, C-dots are introduced into the classic Ru(bpy)32+-ECL system,20 and their part as coreactants for the anodic ECL of Ru(bpy)32+ has been reported for the first time. The specific mechanism has been investigated in detail and the benzylic alcohol units on the Cdots surface are proposed to be responsible for the coreactant activity. This work has provided an insight into the ECL mechanism of C-dots-involved system, which will be beneficial for in-depth understanding of some peculiar phenomena of Cdots, such as photocatalytic activity and redox properties. The C-dots used in this work were prepared by the electrooxidation method (see the Supporting Information).5,6,23 Highresolution transmission electron microscopy (HRTEM) images (Figure S-1a in the Supporting Information) clearly exhibited that the C-dots were monodisperse and spherical nanoparticles

arbon nanodots (C-dots), characterized by surface oxygen-containing units, have recently exhibited a great potential in biomedicine and photophysics for their superior optical properties, excellent biocompatibility, small size, and low cost.1−4 The recent work including our research has revealed that surface oxide-related states5,6 on the C-dots can capture the excitons under light excitation to give off photoluminescence (PL). Besides PL,7−9 chemiluminescence (CL)10,11 and electrochemiluminescence (ECL)12,13 can be produced with chemical and electrochemical excitations. The sensitive and selective analysis for pollutants and biomolecules have been achieved on the basis of the luminescence.10,14,15 In addition to radiative recombination, the electrons and holes trapped by the surface states can transfer to the external electron acceptors and donors,16−18 resulting in redox reactions, based on which many photocatalysts and electrocatalyst with C-dots have been constructed.4 Overall, novel properties related with surface states of C-dots are worth intensive exploring for further applications. Moreover, most related work suggests that the properties of C-dots are predominantly attributed to the surface oxide-related states, but little reseach has been done on what the exact surface oxides or surface oxygen-containing units of C-dots are and how they dictate the surface, resulting in corresponding properties, which are crucial for deep understanding and development of C-dots. ECL, combining advantages of both electrochemical and CL techniques, is sensitive to the surface states of nanoparticles and the key step is the electron transfer between the emitter ions (annihilation mechanism) or between the emitter ion and the © 2014 American Chemical Society

Received: May 5, 2014 Accepted: July 21, 2014 Published: July 21, 2014 7224

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electrochemistry of Ru(bpy)32+, whereas Ru(bpy)32+ or C-dots alone exhibited weak ECL signals (Figure 1a, inset). Furthermore, the normalized ECL spectrum of the mixture at 615 nm agreed with the PL spectrum of Ru(bpy)32+ at 605 nm but distinguished from the ECL spectrum of C-dots alone at 535 nm (Figure 1b). These results demonstrated that the anodic ECL of Ru(bpy)32+/C-dots mixture originated from Ru(bpy)32+ assisted by C-dots. This conclusion was further confirmed by the severely suppressed ECL signal of Ru(bpy)32+/C-dots by O2 (Figure S-4 in the Supporting Information) via an energy-transfer quench of the excited state Ru(bpy)32+*.27,28 UV−vis absorption and PL emission were determined to explore the reason for C-dots assisting the anodic ECL of Ru(bpy)32+. In the range of 300−750 nm, the mixture of Ru(bpy)32+/C-dots exhibited a rising absorption with a broad peak at 453 nm (Figure 1c, solid line), which was a merging of absorptions from separated C-dots (dashed line) and Ru(bpy)32+ (dotted line). Likewise, Ru(bpy)32+ and C-dots in the mixture also keep their PL properties (Figure 1d). These suggested that no reaction occurred between the excited-state or ground-state C-dots and Ru(bpy)32+ molecule.29 Thus, the light emission should result from the reaction between the electrogenerated Ru(bpy)33+ and C-dots. Since Ru(bpy)32+ in the mixture was identified as the emitter, C-dots in the mixture probably acted as coreactants, which can be verified by the linearly increasing ECL signal of Ru(bpy)32+ with increasing the concentration of C-dots from 0.02 to 0.5 mg mL−1 (Figure 2a). Since the solution of C-dots became dark brown at a high concentration, the ECL intensity of Ru(bpy)32+/C-dots leveled off at the concentration above 0.5 mg mL−1 from the inner filter effect of C-dots. As a coreactant in an ECL process, it should be capable of forming reductive or oxidative species during a unidirectional voltammetric scan.19,20 In the present case, C-dots as coreactants should be able to produce a reductive intermediate to reduce the electrogenerated Ru(bpy)33+ to the excited state Ru(bpy)32+* that can emit light. To demonstrate this, the CL technique was used herein, since Ru(bpy)33+ could give off the CL emission when reacting with a reductive agent.30 The CL signal of Ru(bpy)33+/C-dots were monitored on a flow injection analysis system by injecting C-dots into a continuous flow of 0.05 mol L−1 H2SO4 solution containing 100 μmol L−1 Ru(bpy)33+ obtained by chemically oxidizing Ru(bpy)32+ with PbO2.25,31 As shown in Figure 2b,c, strong CL emissions were detected and the intensity linearly increased with increasing the concentration of C-dots from 0.002 to 0.1 mg mL−1, suggesting C-dots actually functioned as reductive intermediates in the CL process of Ru(bpy)33+. In a typical CL process of Ru(bpy)33+, the other agent usually undergoes a chemical reaction (decomposition or deprotonation) after one-electron oxidation by Ru(bpy)33+ and further form reductive intermediates that can reduce Ru(bpy)33+ to generate the excited state Ru(bpy) 2+ 25,26 Therefore, in this CL process, C-dots also can be 3 *. converted to the reductive intermediate after one-electron oxidation by Ru(bpy)33+. In the ECL of Ru(bpy)32+/C-dots, Cdots, in a similar way, can be converted to the reductive intermediate via a “homogeneous” oxidation by electrogenerated Ru(bpy)33+. This can be further confirmed by voltammetry of Ru(bpy)32+/C-dots. Instead of a pair of reversible redox peaks (Figure 1a, inset), Ru(bpy)32+ in the mixture with C-dots showed an enhanced oxidation peak at +1.12 V and a weakened reduction peak at +1.00 V, implying

with a lattice spacing of 2.03 Å, which corresponded to the 101 facet of graphite (PDF no. 65-6212). And the graphitestructured C-dots had a narrow size distribution of 2.4 ± 0.3 nm (Figure S-1b in the Supporting Information). FT-IR and XPS characterization of C-dots demonstrated that rich oxygencontaining groups (e.g., −OH, CO, and −COOH) capped the surface (Figure S-2a in the Supporting Information, curve I, and Figure S-2b in the Supporting Information), which made C-dots water dispersible. Besides, some unsaturated groups, such as CC, were also detected on their surface. C-dots showed a continuously rising absorption with a defined peak at 230 nm, which was assigned to the π−π* transition of the conjugation system within C-dots (Figure S-3a in the Supporting Information).24 A series of shifting emissions of C-dots showed and the optimal emission wavelength was 449 nm when excited at 330 nm (Figure S-3b in the Supporting Information). Herein, C-dots were introduced into Ru(bpy)32+-ECL system and their electrochemical and ECL signals were recorded at a glass carbon (GC) electrode while the potential was cycled between +0.20 and +1.40 V (Figure 1a). As shown in the

Figure 1. (a) Cyclic voltammographs (bottom) and ECL curves (top) of 0.2 mg mL−1 (solid) and 1.0 mg mL−1 (dotted) C-dots mixed with 20 μmol L−1 Ru(bpy)32+ at the GC electrode in 0.2 mol L−1 pH 7.2 phosphate buffer. Inset: cyclic voltammographs (bottom) and ECL curves (top) of Ru(bpy)32+ (blue) and C-dots (black) alone. The cyclic voltammograph of Ru(bpy)32+ was shifted negatively for a better view. (b) The normalized PL spectrum of Ru(bpy)32+ (hollow circle) and ECL spectra of Ru(bpy)32+/C-dots (solid circle) and C-dots (solid square). (c and d) The UV−vis absorption and PL spectra of C-dots (dashed), Ru(bpy)32+ (dotted), and Ru(bpy)32+/C-dots (solid), respectively.

voltammographs, Ru(bpy)32+ gave a reversible redox at a formal potential of +1.07 V (Figure 1a, inset). While, upon addition of C-dots, the oxidation peak of Ru(bpy)32+ was enhanced and the reduction peak weakened, which was more obvious with increasing the concentration of C-dots from 0.2 mg mL−1 to 1.0 mg mL−1. This indicated a catalytic oxidation of C-dots by Ru(bpy)32+. The similar catalytic oxidations were also observed in the coreactant ECL systems of Ru(bpy)32+/C2O42−25 and Ru(bpy)32+/tripropylamine (TPrA),26 where C2O42− or TPrA was first catalytically oxidized to C2O4−• or TPrA+• by electrogenerated Ru(bpy)33+. The ECL curves showed that Ru(bpy)32+/C-dots mixture gave a strong ECL signal rising at +1.00 V and peaking at +1.15 V, which was consistent with the 7225

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Figure 2. (a) Linear fitting of ECL intensity of Ru(bpy)32+/C-dots against the concentration of C-dots. (b) Chemiluminescence signals of Ru(bpy)33+ in 0.05 mol L−1 H2SO4 by injecting C-dots. Inset: a linear fitting of the chemiluminescence intensity versus the concentration of C-dots injected. (c) Chemiluminescence spectrum of Ru(bpy)33+/C-dots in 0.05 mol L−1 H2SO4. (d) Dependence of catalytic oxidation current on the concentration of C-dots.

Figure 3. (a) Structure model of benzylic alcohol units exsiting on the C-dots. (b) Cyclic voltammographs (bottom) and ECL curves (top) of 20 μmol L−1 Ru(bpy)32+ in the presence (solid) and absence (dashed) of 0.15 mol L−1 benzyl alcohol in 0.2 mol L−1 pH 7.2 phosphate buffer. Inset: cyclic voltammograph (bottom) and ECL curve (top) of 0.15 mol L−1 benzyl alcohol alone. (c) The anodic ECL curves of Ru(bpy)32+/C-dots solutions with C-dots as-prepared (solid), deoxygenized with H2 (dotted), and reduced by NaBH4 (dashed), respectively. (d) pH effect on the ECL intensity of Ru(bpy)32+/C-dots solution.

that C-dots were involved in the catalytic route during the anodic scan. Electrogenerated Ru(bpy)33+ chemically oxidized C-dots to produce Ru(bpy)32+ at the electrode surface, which would subsequently be electro-oxidized to Ru(bpy)33+ as the oxidant for the next cycle. Ru(bpy)33+/2+ acted as a redox mediator, resulting in an enhanced Ru(bpy)32+ oxidation peak together with a weakened reduction peak. Also, the electro-

catalytic oxidation current showed a linear dependence on the concentration of added C-dots from 0.02 to 1.0 mg mL−1 (Figure 2d), demonstrating a “homogeneous” oxidation of Cdots by electrogenerated Ru(bpy)33+. Therefore, the linearly increased signals of ECL, CL, and electrocatalytic current with increasing concentration of C-dots suggested that the ECL of Ru(bpy)32+/C-dots was mainly carried out in a catalytic 7226

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reducing Ru(bpy)33+ to Ru(bpy)32+* (E0(Ru(bpy)33+/Ru(bpy)32+*) = −0.86 V vs NHE38). The directly obtained ESR signal of intermediate [C6H5CHOH]• by Gilbert et al. in aqueous solution39 suggested the sufficient stability of this intermediate to complete the reduction of Ru(bpy)33+. Therefore, the benzyl alcohol was indeed capable of being the coreactant for the ECL of Ru(bpy)32+ during the anodic scan. As for the experimental proof, an anodic ECL emission, corresponding to the catalytic oxidation of benzyl alcohol by Ru(bpy)32+ at +1.15 V, was detected from the mixture of Ru(bpy)32+/benzyl alcohol (Figure 3b). Accordingly, it is reasonable to deduce that the similar structure on the surface, benzylic alcohol units can make C-dots as coreactants for the anodic ECL of Ru(bpy)32+. To further prove the key role of benzylic alcohol units, the anodic ECL signals from Ru(bpy)32+ together with C-dots treated in different manners were determined. As shown in Figure 3c, when using the deoxygened C-dots with a few alcohol units (Figure S-2a in the Supporting Information, curve II) instead of as-prepared C-dots, the anodic ECL emission of Ru(bpy)32+/Cdots nearly disappeared (Figure 3c, dotted). On the contrary, the anodic ECL intensity increased a lot (Figure 3c, dashed) by selectively reducing the C-dots with NaBH4 to form more alcohols on the surface (Figure S-2a in the Supporting Information, curve III).40 Moreover, the ECL intensity increased sharply with pH increasing from 6 to 10 (Figure 3d), indicating that deprotonation of α-H in benzylic alcohol is a key step in the ECL process. Therefore, a mechanism for Ru(bpy)32+/C-dots ECL reactions was proposed in Scheme 1. During the anodic scan, the benzylic alcohol units on C-dots are oxidized by electrogenerated Ru(bpy)33+, turn to the related reductive intermediate upon a subsequent deprotonation, and further reduce Ru(bpy)33+ to Ru(bpy)32+*, producing an anodic ECL emission. Meanwhile, the direct electro-oxidation of Cdots also may contribute to the anodic ECL of Ru(bpy)32+. For C-dots are provided with many virtues, and the novel ECL system of Ru(bpy)32+/C-dots would have a great potential in bioanalytical applications. The anodic ECL exhibited a robust stability during the 15 continuous cycles (Figure 4a) and an excllent 1-week repeatibility with a relative standard deviation of 1.1%, providing a premise for bionanlysis. To further demonstrate this potency, dopamine (DA), an essential neurotransmitter closely associated with many disorders of nervous system such as Parkinson’s disease,41 was used as a model molecule to be quantitatively determined. As shown in Figure 4b, with the addition of 10 μmol L−1 DA, the ECL

Scheme 1. Proposed Pathway for Anodic ECL of Ru(bpy)32+ Coreacted by C-Dots

route.25,26 On the other side, the involvement of direct electrooxidation of C-dots in the anodic ECL process of Ru(bpy)32+/ C-dots cannot be absolutely excluded. As discussed, C-dots can act as coreactants in the Ru(bpy)32+/C-dots-ECL system to be converted to reductive intermediates via chemical oxidation by electrogeneratedRu(bpy)33+, which played a quite similar role to that of TPrA in anodic ECL of Ru(bpy)32+/TPrA.26 As nanomaterials, Cdots are far more complicated due to their complex structures.1 Thus, it is crucial to determine the structure that makes C-dots as coreactants in the ECL. C-dots could be taken as the assembly of multilayered graphene oxide32 of a few nanometers, which was proposed to contain many benzylic alcohol functional units33 (Figure 3a). Considering that alcohols could be converted to alkoxide radical after one-electron oxidation followed by deprotonation and the alkoxide radical would then reduce electrogenerated Ru(bpy)33+ to form the excited state Ru(bpy)32+*,21,34 oxygen-containing units of benzylic alcohol units on the C-dots were expected to serve as the coreactant sites. Herein, the simplest benzylic alcohol, benzyl alcohol, was taken as the model to assess this possibility. Benzyl alcohol can be converted to the reductive intermediate of α-hydroxybenzylic radical [C6H5CHOH]• upon oneelectron abstraction by Ru(bpy)33+ followed by a fast deprotonation and further to corresponding aldehyde C6H5CHO by subsequent oxidation.35,36 The standard potential for [C6H5CHOH]•/C6H5CHO was calculated to be −1.25 V (vs NHE) according to the E0 (C6H5CHO¯•/ C6H5CHO) and thermodynamic parameters reported by Saveant et al.,37 indicating that benzyl alcohol can be converted to the reductive intermediate that has enough reducibility for

Figure 4. (a) ECL signals of Ru(bpy)32+/C-dots monitored during 15 cycles. (b) The ECL signals of Ru(bpy)32+/C-dots collected before (solid) and after successive addition of 10 μmol L−1 DA (dashed) and 200 μmol L−1 β-mercaptoethanol (dotted). Inset: a linear fitting of I0/I versus the concentration of DA. 7227

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intensity of Ru(bpy)32+/C-dots decreased a lot, while βmercaptoethanol, an antioxidation reagent for DA, efficiently retrieved the ECL signal quenched by DA, which suggested a quenching mechnism involving energy-transfer between benzoquinone species (oxidation product of DA) and excited state Ru(bpy)32+*.42,43 On the basis of the mechanism, the DA molecules were quantitatively determined in the range of 0.5− 20 μmol L−1 (Figure 4b, inset), and the linear dependence of I0/I on the concentration of DA followed the Stern−Volmer equation, I0/I = 1 + Ksv[Q], with the constant Ksv of 1.0 × 105 M−1 (R = 0.998). The limit of determination was 0.3 μmol L−1, and the relative standard deviation for four separated measurments of 10 μmol L−1 DA was 3.6%. In summary, C-dots have been demonstrated to be the coreactants for the anodic ECL of Ru(bpy)32+ for the first time. The related mechanism was also put forward with the aid of CL, PL, absorption, and multiple treatments. It has been found that the benzylic alcohol units on the C-dots are responsible for their capability as coreactants in the anodic ECL process of Ru(bpy)32+/C-dots. C-dots possess many merits, for example, excellent water dispersibility, low toxicity, large specific surface area, and ease of biofunctionalization; thus, the system of Ru(bpy)32+/C-dots would be much promising in bioanalysis, which was exemplified by the quantitative detection of DA molecules. This work has deepened and broadened the knowledge of C-dots, which will be quite beneficial for further development of C-dots.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of C-dots preparation, electrochemical, ECL, CL measurements, and reduction treatments; characterizations of prepared C-dots; and effect of atmosphere on the ECL. 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]. Fax: (+86) 27-68754685. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program, Grant No. 2011CB933600) and the 111 Project (Grant 111-2-10).

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REFERENCES

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