Dual-Peak Electrogenerated Chemiluminescence of Carbon Dots forIron Ions DetectionPengjia 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 ofChemistry, The Chinese Academy of Sciences, Beijing 100190, China

*S Supporting Information

ABSTRACT: Carbon dots (CDs) have rigorously beeninvestigated on their unique fluorescent properties but rarelytheir 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) duringthe cyclic sweep between −3.0 and 3.0 V at scan rate of 0.2 Vs−1 in 0.1 M tetrabutyl ammonium bromide (TBAB) ethanolsolution, which is more efficiency to distinguish metallic ionsthan single-peak ECL. The electron transfer reaction betweenindividual electrochemically reduced nanocrystal species andcoreactants led to ECL-1, in which the electron injected to theconduction band of CDs in the cathodic process. Ion annihilation reactions induced direct formation of exciplexes that producedanother ECL signal, ECL-2. ECL-1 showed higher sensitivity to the surrounding environment than ECL-2 and thus was used forECL detection of metallic ions. Herein, we can serve as an internal standard method to detect iron ions. A linear relationship ofthe 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 Mwith a detection limit of 7 × 10−7 M.

Carbon nanoparticles, namely, carbon dots (CDs) havegenerated much excitement because of their superiority in

chemical inertness, low cost, low cytotoxicity, environmentallyfriendly, ease of functionalization and resistance to photo-bleaching.1 CDs with tunable emission are considered to be thenext generation of green nanomaterials and are promisingcandidates for numerous exciting applications,2 such asphotocatalysis,3 biomedicine, targeted drug delivery,4,5 medicalimaging and biosensing,6−12 and photovoltaic devices.13

Electrogenerated chemiluminescence (ECL), a phenomenonin which one or more of the reagents is generated in situ in anelectrolytic process, has grown significantly as a highly sensitiveand selective analytical and diagnostic method in recent years.14

As the light emitting species are generated in situ close toelectrode surfaces, ECL has a near zero background and allowstemporal and spatial control over the reaction.15 ECL fromsemiconductor nanoparticle has been observed in manysystems, but heavy metals as the essential elements in quantumdots have raised serious health and environmental con-cerns.16,17 Additionally, the electrochemically formed semi-conductor nanocrystal species have low instability.18,19 Tomaintain an environment, low-toxicity silicon and carbonnanostructures are preferred.20,21 A single-peak ECL wasobserved in the presence of coreactants. Considering thepotential window for the electrochemical oxidation andreduction of water is too narrow, we employ the CDs ECL

in an organic phase. Greatly different from previous works ofsingle-peak ECL system that cannot obviously provide acomplete long-term stability of the detection due to theirinherent instability, we report here for the first time a dual-peakECL system for detection of iron ions. The ECL intensity ofone peak changes with the concentration of iron ions in acertain range, and the other one is stable, which is similar tointernal standard method. So, the accuracy and sensitivity of thedetection of iron ions are greatly improved comparing to single-peak ECL. To study the ECL behavior and mechanism of CDs,different supporting electrolytes with various length of alkylchain were added to the ECL system. The two ECL peakspresent distinct sensitivity to the surrounding environment.Therefore, the dual-peak ECL provides more information fordetecting metallic ions, which is more efficiency to distinguishiron 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 typicalsynthesis, an appropriate amount of poly(ethylene glycol)(PEG-200, 1.125 g/mL) and ascorbic acid (0.08 g/mL) wereadded to distilled water to form a transparent homogeneoussolution. Then the solution was heated in a microwave oven

Received: April 1, 2014Accepted: May 23, 2014Published: May 23, 2014

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about 2 min until a clear yellow-brown aqueous solution wasobtained. Purified CDs were obtained by further dialysis andultrafiltration of the initial yellow-brown solution.Transmission electron microscopy (TEM) image (Figure 1a)

demonstrates that these CDs are nearly monodispersed with a

diameter of 2.3 ± 0.5 nm. The high-resolution TEM image inFigure 1b represents that the CDs have hexagonal structures.For a simple hexagonal crystal, d-spacing at the (100) plane isaround 0.22 nm. So a lattice constant is near to 0.25 nm, whichimplies the graphitic (sp2) cluster is a major component ofCDs.23,24 As with previous CDs, the peak at 264 nm in UV−visabsorption is attributed to the n−π* transition of the COband and a π−π* transition of the conjugated CC band. Thefluorescence emission has a red shift as the excitationwavelength is increasing. The maximum emission fluorescencecentered at 450 nm is excited by a laser of 373 nm (Figure S1 inthe Supporting Information). The addition of TBAB into theCDs solution causes no change in the UV−vis absorption of theCDs and slightly varies in fluorescent spectra. There is only a 2nm red shift in emission and excitation spectra compared to theCDs solution in the absence of TBAB (Figure S1 in theSupporting Information). These results show that TBAB has anegligible influence on the fluorescent properties of CDs,indicating that TBAB absorbed on the surface of the CDs couldnot 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 ofthe CDs in the presence of 0.1 M TBAB was observed whenthe applied potential was cycled between −3.0 and 3.0 at 0.2 Vs−1 (Figure 2). Cyclic voltammograms (CV) and ECL curveswere recorded synchronously as shown in Figure 3. There arethree 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). Thecorresponding ECL peaks located at −2.84 V (ECL-1) and−1.71 V (ECL-2), respectively. Oa1 and Rc2 are assigned to theredox of dissolved oxygen, as they disappear after the solution isbubbled by high-purity nitrogen for 15 min. The Rc1 and Rc3peaks have a slightly negative shift when the dissolved oxygen isremoved from the solution with no change in the peak positionof ECL-1 and ECL-2 (Figure 3a). Thus, dissolved oxygen or itsreduced product OOH− does not participate in the electrodeprocess 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. Twoof the molecules that have different alkyl chains and similarstructure were selected to replace TBAB (Scheme 1 and Figure

S2 in the Supporting Information). The electrochemical andECL data of three molecules are summarized in Table S1 in theSupporting Information. ECL-1 is more sensitive to thesurrounding environment than ECL-2. ECL-1 results fromthe reaction between coreactants and CDs reduced nanocrystalspecies, which is related to Rc3.

25 The ECL-1 is based on theelectron injection to the conduction band of CDs, andsubsequently, the electron in the conduction band annihilateswith the hole in the valence band (formed by reacting with R•

radical) to produce the luminescence. The ECL-1 process isproposed as follows:

+ → +− •2R e R R (1)

Figure 1. (a) Low-resolution and (b) high-resolution TEM images ofthe as-synthesized CDs. The inset in part b shows the hexagonalstructure 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.

Figure 3. (a) ECL-potential curves and (b) corresponding cyclicvoltammograms of CDs in ethanol containing 0.1 M TBAB before(red) and after (blue) removing oxygen. Inset: enlargement of the areahighlighted with a rectangle. Scan rate: 0.2 V s−1.

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+ → −[CDs] e [CDs]CB CB (2)

+ → +• + −[CDs] R [CDs] RVB VB (3)

+ → + +− + hv[CDs] [CDs] [CDs] [CDs]CB VB CB VB(4)

Rc1 is a reduction peak associated with ECL-2. Although thepeak intensity of ECL-2 has a little change after the addition ofdifferent organic molecules, the value of Rc1 is almost constant.The related potential data of different organic molecule systemslisted in Table S1 in the Supporting Information. Ethanol,TBAB, and CDs themselves cannot produce ECL signal underthe experimental conditions, as well as the redox peak was notfound in the corresponding cyclic voltammograms (Figure S3in the Supporting Information). These phenomena indicatethat ECL-2 is a result of the interaction between CDs andTBAB. Exciplexes (CDsR) were directly formed by ion−ionannihilation reactions of CDs and TBAB, which lead to theappearance of ECL-2 signal. The ECL-2 mechanism can involveexciplexes formation as follows:

− → •+[CDs] e [CDs] (5)

+ → •−R e R (6)

+ → *•+ •−[CDs] R [CDsR] (7)

* → + hv[CDsR] [CDsR] (8)

The accurate and rapid determination of iron ions are ofpractical importance in living systems. Although iron ions canbe detected by many methods, even a nanomolar detectionlimit could be achieved based on the ECL of the o-CDs/K2S2O8 system,26 the single-peak ECL system cannot obviouslyprovide a complete long-term stability of the detection due totheir inherent instability. In a dual-peak ECL system, theintensity of ECL-1 emission changes with the concentration ofiron ions in a certain range, while ECL-2 has no change, whichis similar to internal standard method. On the basis of thesemechanisms, the accuracy and sensitivity of the detection ofiron ions are improved.Figure 4a,b shows the ECL quenching value of the CDs/

TBAB system with various metal ions (Fe2+, Fe3+, Al3+, Mn2+,

Cd2+, Ca2+, Mg2+, Zn2+, Cu2+, Co2+, Ni2+), each at aconcentration of 1 mM. Totally they can be separated intothree groups: first, slightly effecting on two of ECL peaks (Al3+,Mn2+, Cd2+, Ca2+, Mg2+, Cu2+, Zn2+); second, quenching onboth of ECL peaks (Co2+, Ni2+); third, quenching on ECL-1and mildly enhancing on ECL-2 (Fe2+, Fe3+). Because Fe2+ isoxidized into Fe3+ in positive potential, the ECL behavior ofFe2+ is the same as that of Fe3+. The ECL-1 presented highersensitivity to the surrounding environment than ECL-2 becauseof the direct electron transfer during the ECL-1 procedure.Detailed experiments indicate the increase of the concentrationof iron ions causes a gradual decrease of ECL-1, while there is amild increase of ECL-2 (Figure S5 in the SupportingInformation). It is interesting to note that the intensity ofECL-1 is not linear to decrease at a function of theconcentration of iron ions (Figure S6 in the SupportingInformation), which is consistent with the fact that single-peakECL detection has low accuracy determined by its inherentinstability. To improve ECL accuracy, the intensity ratio R ofECL-1 and ECL-2 is selected to detect the concentration ofiron ions, in which ECL-1 is an analyte signal and ECL-2 is aninternal 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 concentrationwith a correlation coefficient R2 = 0.993. Detection limitationwas 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 carbonnanoparticles in ethanol with coreactants exhibits two peaksupon a negative potential scan, which corresponds to twodifferent luminescent mechanisms. Dissolved oxygen does notparticipate in the electrode process and ECL reactions. ECL-1results from the electron-transfer reaction between coreactantsand reduced nanoparticles generated at cathodic scanpotentials. ECL-2 is an annihilation process of CDsR. On thebasis of the dual-peak ECL of CDs, the concentration of ironions is detected by the internal standard method, showing alinear response at a range of micromoles with good sensitivity

Scheme 1. (a) Proposed Mechanism of ECL-1 and (b)Schematic Representation of the Coreactant Processbetween CDs and Different Organic Molecules

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 ironions detection. R indicates the intensity ratio of ECL-1 and ECL-2.

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and accuracy. This work provides an alternative method toavoid the inherent instability of ECL for the detection of metalions. Furthermore, the CDs could be utilized as a biosensorreagent capable of detecting iron ions in biosystems.

■ ASSOCIATED CONTENT*S Supporting InformationUV−vis absorption and PL emission spectra of CDs and CDs/TBAB system; ECL-potential curves and cyclic voltammogramsof CDs/TMAB and CDs/TOAB; electrochemical and ECLdata in various supporting electrolytes; ECL curves and cyclicvoltammograms of ethanol, TBAB, and CDs; and intensityvariations of ECL-1 and ECL-2 with the increase of theconcentration of iron ions. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: zhguo@snnu.edu.cn.*E-mail: wangtie@iccas.ac.cn. Phone: +86-10-62562042.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was financially supported by the “StrategicPriority Research Program” of the Chinese Academy ofSciences (Grant No. XDA09020100), the Institute ofChemistry (Grant Nos. Y31Z0C1BZ1, Y329751261), and theNational Natural Science Foundation of China (Grant No.20905044)

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