Carbon Nitride Quantum Dots: A Novel Chemiluminescence System

Mar 24, 2014 - Carbon Nitride Quantum Dots: A Novel Chemiluminescence System for Selective Detection of Free Chlorine in Water ... Tel/Fax: +86-28-854...
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Carbon Nitride Quantum Dots: A Novel Chemiluminescence System for Selective Detection of Free Chlorine in Water Yurong Tang,† Yingying Su,‡ Na Yang,† Lichun Zhang,† and Yi Lv*,† †

Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China ‡ Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, China S Supporting Information *

ABSTRACT: A facile one-step microwave-assisted approach for the preparation of strong fluorescent carbon nitride quantum dots (g-CNQDs) by using guanidine hydrochloride and EDTA as the precursors was developed. Strong chemiluminescence (CL) emission was observed when NaClO was injected into the prepared g-CNQDs, and a novel CL system for direct detection of free chlorine was established. Free residual chlorine in water was sensitively detected with a detection limit of 0.01 μM and had a very wide detection range of 0.02 to 10 μM. On the basis of CL spectral, UV−visible absorption spectral, and electron spin resonance (ESR) spectral studies, as well as investigations on the effects of various free radical scavengers, a possible CL mechanism was proposed. It was suggested that the radiative recombination of oxidant-injected holes and electrons in the g-CNQDs accounted for the CL emission. Meanwhile, 1O2 on the surface of g-CNQDs, generated from some reactive oxygen species in the g-CNQDs-NaClO system, could transfer energy to gCNQDs and thus further enhance the CL emission. The CL system is highly sensitive and differentiable, opening a new field for the development of novel CL-emitting species, but also expanding the conventional optical utilizations of g-CNQDs.

C

carbon dots in the presence of a strong alkaline solution without the presence of any CL reagent [KMnO4, NaIO4, Ce(IV), H2O2−NaHSO3]6 was also reported.7 The possible CL mechanism involving the radiative recombination of the injected electrons by “chemical reduction” of carbon dots with thermally excited generated holes was proposed, which shed new light on the characteristics of carbon dots.7b Intense research still focused on exploring the new CL systems and obtaining new insight into the interaction between CL reagents. Carbon nitride as an organic semiconductor consists of carbon and nitrogen, which exhibits structural similarity to graphene but different from the property point of view, has received great attention.8 Apart from the use of carbon nitride as a metal-free catalyst for photocatalysis, nanoscale carbon nitrides, such as fluorescent graphitic carbon nitride (g-C3N4) nanosheets and graphitic carbon nitride quantum dots (gCNQDs), have been explored as fluorescent probes for biological and environment detection due to their distinct optical features.9 Recently, we found g-C3N4 possessed an intensively long-persistent luminescence property, so we applied it as a turn-on persistent luminescence probe for imaging detection of biothiols in biological fluids.10 Similar to

hemiluminescence (CL), with the advantages of simple instrumentation, high sensitivity, wide linear range, no interference from background scattering light, and versatility for the determination of a wide variety of species, is of extensive interest and has been developed to be a powerful tool in analytical fields over the past several decades.1 Traditionally, the CL systems are limited to molecular systems, such as luminol, lucigenin, tris(2,2-bipyridine)ruthenium(II), and peroxyoxalate, which unfortunately suffer from expensive and/or poisonous reagents or poor selectivity.1a,c Therefore, it is urgent to develop new CL systems to improve the status and expand the practical application. With the promising optical, catalytic, and biocompatible properties, nanomaterials have attracted tremendous attention, and offer new opportunities to CL and their applications.2 Cui’s group reported gold nanoparticle-catalyzed luminol CL and applied it to the determination of various organic compounds based on their use as CL inhibitors.3 Wang et al. found that CdTe quantum dots (QDs) capped with thioglycolic acid could be directly oxidized by oxidants and produce CL emission and, consequently, established a series of semiconductor nanoclusters-based CL sensors.4 Fluorescent carbon dots were first demonstrated to have the ability of CL in the presence of H2O2 and NaNO2 by Lin’s group. They proposed that the reaction between ONOOH related species and carbon dots is the main pathway for the CL.5 More recently, chemiluminescence (CL) behavior of fluorescent © 2014 American Chemical Society

Received: February 2, 2014 Accepted: March 24, 2014 Published: March 24, 2014 4528

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Scheme 1. Schematic Illustration for the Preparation of g-CNQDs by Microwave-Assisted Method and the CL Mechanism of the g-CNQDs-NaClO System

advances, both methods were conducted in the traditional luminol system, which was easily interrupted by transition metal in water samples. Herein, we developed a facile one-step microwave-assisted approach for the preparation of strong fluorescent g-CNQDs, by using guanidine hydrochloride and EDTA as the precursors. Interestingly, a strong CL emission was observed when NaClO was injected into the prepared g-CNQDs. To the best of our knowledge, it is the first example of investigation the CL properties of g-CNQDs and the CL mechanism of the gCNQDs-NaClO system. Furthermore, a highly sensitive and selective measurement of free chlorine-based g-CNQDsNaClO CL system was established. This novel CL system not only opened the field of development of novel CL-emitting species but also shed new light on the optical properties of the g-CNQDs.

carbon dots, g-CNQDs have the advantages of bright fluorescence, good stability, water solubility, low cytotoxicity, and excellent biocompatibity, making them good candidates in place of traditional quantum dots (which involve heavy metals and induce serious health and environmental problems).11 As a consequence, it will be a growing trend to study intrinsic photoluminescence mechanisms of g-CNQDs and explore its new application potentials in the fields of material and analytical sciences. Chlorine is a powerful oxidizing agent and has been extensively used in water disinfection, blanching, and other numerous manufacturing processes.12 In water treatment, the concentration of free residual chlorine [the sum of dissolved chlorine (Cl2), hypochlorous acid (HClO), and hypochlorite (ClO−)] must be strictly controlled. Free residual chlorine with too low a level cannot kill pathogenic bacteria and cause many hazards of an insufficient disinfection, and meanwhile, free residual chlorine with too high a level may produce a large number of undesirable byproducts, especially trihalomethane,13 harmful to human beings and animals.14 Therefore, the measurement and control of chlorine concentration in drinking water, swimming pool water, and wastewater for nonpotable reuse is extremely necessary. Up to now, many analytical methods based on different principles, including colorimetric method,15 electrochemistry,16 liquid chromatographic,17 and fluorescence,18 have been developed. However, these methods suffer from their respective disadvantages, such as the requirement for many types of reagents that may produce strong toxicity, low sensitivity, poor selectivity, and complicated performance. The CL detection for chlorine can be carried out by hypochlorite-oxidized luminol to generate CL in aqueous solution.19 In previous studies, the microfluidic injection system and reusable CL test strip have been fabricated for online and real-time analysis of chlorine in water.19b Lin et al.19f also developed a microfluidic chlorine gas sensor based on gas− liquid interface absorption and CL detection. In spite of these

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EXPERIMENTAL SECTION

Preparation of g-CNQDs. Guanidine hydrochloride [0.095 g (1.00 mmol)] and EDTA [0.048 g (0.16 mmol)] were added into 10 mL of H2O, and the pH value of the mixture was adjusted by HCl (1 M) or NaOH (1M) and homogenized by stirring. Then the mixture solution was heated in a domestic microwave oven for two minutes. After centrifugation (12000 rpm, 10 min) to remove byproducts of large particles, the lightbrown supernatant containing g-CNQDs was collected and exhibited strong fluorescence under UV irradiation. A dialysis membrane (MWCO: 1 kDa; pore size: ca. 1.0 nm) was then used to separate the g-CNQDs from any residual unreacted species. The concentration of g-CNQDs was calculated based on the guanidine hydrochloride precursor concentration. In the control experiments, 0.095 g of guanidine hydrochloride and 0.048g of EDTA in 10 mL of H2O were treated by microwave for two minutes, respectively. Both of the two reactants showed no obvious change, and the products exhibited no fluorescence emission. 4529

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Figure 1. (a) TEM image of the as-prepared g-CNQDs. (b) Size distributions histogram of the g-CNQDs. (c) AFM image of synthetic g-CNQDs deposited on a mica substrate. (d) The corresponding height image of three dots.

CL from g-CNQDs-NaClO System. To investigate the gCNQDs-NaClO CL system, CL kinetic curves were obtained by the static injection CL analysis, which was carried out in a 2 mL quartz cuvette. One hundred microliters of NaClO (1 μM) was quickly injected into the 100 μL g-CNQDs (0.01 M) solution by a microliter syringe from the upper injection port. The CL profiles were displayed and integrated at intervals of 0.1 s at −880 V. The flow injection analysis (FIA) system for determination of chlorine concentration consisted of two flow lines. The gCNQDs solution was injected through a six-valve injector and mixed sodium hypochlorite standards or samples within the mixing coil in front of the PMT. The flow rates were 2.5 mL min−1 for both g-CNQDs and NaClO solution. The CL signals were detected and recorded with the BPCL luminescence analyzer. The peak height of the signal recorded was measured relative to CL intensity. Analysis of Real Sample. The local tap water samples were obtained on two consecutive days, and part of the water was boiled to obtain the boiling water samples. Two swimming water samples were collected from adult pool and children pool in Sichuan Province natatorium, respectively. The samples were analyzed for free chlorine via both the DPD colorimetric procedure12 and the proposed g-CNQDs-NaClO CL procedure. DPD analyses were run on samples without dilution, while for the proposed CL analyses, samples were run after 2fold dilution to ensure the concentration of chlorine in the linear range.

reported. We chose EDTA as the capping and stabilizing reagent since it contains ample carboxyl motifs and can control the product size effectively. The g-CNQDs were prepared by a microwave-assisted method with the advantages of simultaneous, homogeneous, and fast heating, leading to a uniform size distribution of quantum dots. As illustrated in Scheme 1a, the water-solubility g-CNQDs were synthesized by the microwaveassisted treatment of guanidine hydrochloride with EDTA in one step. Figure 1a showed the transmission electron microscopy (TEM) image of the monodispersed g-CNQDs. As displayed in Figure 1b, the g-CNQDs diameters were mainly distributed in the narrow range of 3.2−6.5 nm, with an average size of 5 nm. The corresponding AFM image (Figure 1c) revealed a typical topographic height of ∼1.7 nm (Figure 1d), suggesting that the g-CNQDs consist of a few layers of C−N sheets.8a The XRD pattern of the prepared g-CNQDs, shown in Figure S1a of the Supporting Information, demonstrated two characteristic peaks at 27.3° and 13.1°, which are consistent with the previous reports about graphitic carbon nitride.10 XPS is a powerful tool to characterize the chemical composition and structure of carbon-based materials. Figure S1b of the Supporting Information showed that the XPS survey scan spectrum of the g-CNQDs. The binding energy peaks at 288.5, 399.5, and 531.1 eV corresponded to C1s, N1s, and O1s, respectively. A large amount of oxygen was measured in the obtained gCNQDs when compared to the previous report.9e,g,11 The C1s spectra in Figure S1c of the Supporting Information revealed four peaks centering at 284.6, 286.1, 288.0, and 289.02 eV, while the binding energy at 284.6 eV was ascribed to sp2 C−C bonds and at 288.0 eV was attributed to sp2 N−CN bonds. The binding energies at 286.07 and 289.02 eV were identified as C−O and O−CO bonds, respectively, indicating the existence of oxygen-rich groups. The N1s spectrum (Figure S1d of the Supporting Information) demonstrated three peaks at 398.8, 399.8, and 400.4 eV, which were associated with CN− C, C−N−C, and N−(C)3 groups, respectively. These spectra as well as the FTIR spectroscopy (Figure S2 of the Supporting

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RESULTS AND DISCUSSION Synthesis and Characterization of g-CNQDs. Guanidine hydrochloride was used as a precursor due to its low-cost, abundance, and the nitrogen-rich nature. So far, bulk graphitic carbon nitrides are usually achieved by thermal heating guanidine hydrochloride at high temperatures.10 However, to the best of our knowledge, the synthesis of highly fluorescent gCNQDs from guanidine hydrochloride have never been 4530

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indicating the existence of an unsaturated bond. It was reported that a peak at ∼250 nm was due to π−π* electronic transitions for carbon nitrides containing s-triazine rings.20 The narrow and symmetrical fluorescence peak centered at 453 nm was observed when the aqueous solution of g-CNQDs was excited at 360 nm (Figure S3a of the Supporting Information). The digital picture of the aqueous dispersions further showed that the obtained g-CNQDs were water-soluble and exhibited strong blue fluorescence under 365 nm UV light (inset of Figure S3a of the Supporting Information). Notably, the quantum yield (λex = 360 nm) was measured to be 35%, using quinine sulfate as a standard.21 Similar to most of luminescent carbon dots and graphene quantum dots,22 the as-prepared gCNQDs exhibited λex-dependent emission FL behavior. As shown in Figure S3a of the Supporting Information, the emission peak of g-CNQDs was red shifted from 442 to 550 nm with the excitation wavelength changing from 320 to 480 nm, which was possibly attributed to the optical selection of different-size nanoparticles (quantum effect) and different emissive energy trap sites on the surface of g-CNQDs.23 Many parameters which include microwave power, heating time, the guanidine hydrochloride concentration and pH might affect the growth of the g-CNQDs. As illustrated in Figure S4 of the Supporting Information, microwave heating of 120 s at 595 W, the guanidine hydrochloride of 0.01 M, the molar ratio between guanidine hydrochloride and EDTA of 6:1, and the neutral solution were used as the optimal reaction conditions.

Information) indicated that the basic substructure of g-CNQDs was the heptazine heterocyclic ring units. The remarkable optical properties of the as-obtained gCNQDs were confirmed by the UV−vis absorption and the steady-state fluorescent spectra. The UV−visible spectrum of gCNQDs was shown in Figure 2a, exhibiting a peak at 260 nm,

Figure 2. The CL kinetic file obtained when the NaClO solution was added into the control solution (guanidine hydrochloride or EDTA upon microwave treatment) (black line) and g-CNQDs solution (red line). Conditions: the concentration of g-CNQDs and NaClO was 0.01 M and 1 μM. The inset is the CL spectrum of the g-CNQDsNaClO system obtained by a fluorescence spectrometer with the xenon lamp turned off.

Figure 3. (a) Flow injection signals and (b) standard curve for the determination of free chlorine concentration in the range from 2.0 × 10−8 to 1.0 × 10−5 M (various concentrations of chlorine: 0.02, 0.05, 0.1, 0.5, 1, 3, 5, 7, and 10 μM). Conditions: g-CNQDs with a concentration of 0.01 M. (c) Selectivity of this CL sensor for free chlorine over other ions: concentrations of g-CNQDs and free chlorine were 0.01 M and 1 μM, respectively; concentrations of other common ions were all 1 mM; concentrations of heavy metal ions were 100 μM; and the concentration of Hg2+ and Cu2+ was 10 μM. 4531

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Table 1. Determination of Free Chlorine in Tap Water, Boiling Tap Water, and Swimming Water Samples sample tap water 1 tap water 2 boiling tap water 1 boiling tap water 2 swimming pool water for adults swimming pool water for children

proposed methoda (1 × 10−6 M) 9.56 10.49 5.83 6.64 11.78 10.27

± ± ± ± ± ±

DPD methodb (1 × 10−6 M)

[Cl] spiked (1 × 10−6 M)

± ± ± ± ± ±

2.00 2.00 2.00 2.00 2.00 2.00

0.03 0.02 0.03 0.02 0.03 0.02

9.48 10.52 5.75 6.78 11.73 10.32

0.04 0.03 0.02 0.02 0.02 0.02

founda (1 × 10−6 M) 11.64 12.51 7.76 8.74 13.72 12.24

± ± ± ± ± ±

0.04 0.02 0.02 0.02 0.01 0.03

recovery (%) 104 101 96 106 97 98

± ± ± ± ± ±

3 2 2 2 2 2

a Optimum conditions: g-CNQDs with a concentration of 0.01 M with a flow rate of 2.5 mL min−1; and sample carrier (H2O) with a flow rate of 2.5 mL min−1. Values include mean values of three determinations and the standard deviation. bData taken from ref 21.

shown in Figure S6c of the Supporting Information, there was a weak CL when the pH was below 9.0. The CL intensity was increased with increasing pH and reached a maximum at pH 11.0 in the flow-injection system. Free chlorine existed mainly as hypochloric acid (HClO) in neutral and weakly alkaline solutions (pH 3.0−9.0) and as a hypochlorite anion (ClO−) in strong alkaline solutions (pH > 9.0).25,18b Only the ClO− ion was effective in producing the luminescing species,19a which would benefit the CL emission and lead to the increasing CL intensity. Therefore, pH 11.0 was selected in the CL system. Under these conditions, the flow-injection CL signals for free chlorine with sharp peaks and low noises were obtained, as shown in Figure 3a. Analytical Performance. Under the optimum experimental conditions, the calibration plot showed a good linear relationship between the CL intensity and the chlorine concentration in the range from 2.0 × 10−8 to 1.0 × 10−5 M, with a correlation coefficient of 0.9979 (Figure 3b). The limit of detection for chlorine was 1.0 × 10−8 M (S/N = 3).The relative standard deviation (RSD) values for 10 parallel measurements (intra-assay) of 1.0 μM free chlorine concentrations was 1.9%. The RSD for six parallel measurements (interassay) of 1.0 μM free chlorine using six different batches of g-CNQDs was 5.3%. In addition, the detection limit of this CL system for free chloride determination has been compared with the other sensors, such as a DPD colorimetric sensor15 and GQD fluorescent sensor18b with detection limits of 1.40 μM (or equivalently 49.70 μg L−1) and 0.05 μM, respectively. These results indicated that the proposed CL method had good linearity and relatively high sensitivity and precision. Interference Studies. The selectivity of this sensing method for free chlorine was evaluated before its application in a real sample. The effects of coexisting ions in water on the CL detection of 1 μM of free chlorine were investigated. As shown in Figure 3c, the heavy metal ions, for example, 10-fold Hg2+ and Cu2+ and 100-fold Fe3+, Pb2+, Cd2+, Ag+, and Cr3+ had nearly no effect on CL intensity. Other common ions, such as 1000-fold Mg2+, Mn2+, Ca2+, Al3+, Zn2+, As3+, Ni2+, Ca2+, Co2+, NH4+, Br−, NO3−, Ac−, Cl−, I−, SO42−, SO32−, CO32−, and PO43− had no influence on the free chlorine detection. Apparently, the present sensing system showed lower interference for free chlorine determination than the luminol CL system.19a These results demonstrated that the proposed method was suitable for the determination of free chlorine in real samples. Analysis of Real Samples. On the basis of the above results, the applicability of this proposed method for detection of free chlorine in water was demonstrated. As shown in Table 1, the results obtained with the two methods were in good agreement for the sample determination, indicating the

Moreover, the photoluminescence of g-CNQDs was also pHdependent. As depicted in Figure S5 of the Supporting Information, fluorescence intensity increased with the increase of pH value from 3 to 9, but the change of fluorescence intensity was comparatively small in the pH range of 9 to 12, as compared to other pH regions. The pH sensitive fluorescence behavior for g-CNQDs was similar to carbon dots22a and graphene quantum dots24 with the basic sites, which was partially ascribed to the presence of free zigzag sites. But, it is worth noting that such observations were exactly opposite to that observed for other carbon nitride dots which passivated by the N-containing polymer or had acidic sites. 9e,g,11b,c Furthermore, the ζ-potential of the g-CNQDs dispersion was −18.7 ± 0.81 mV, further confirming that the presence of negative functional groups (such as carboxyl groups) were decorated on the surface of g-CNQDs and the electrostatic repulsions were responsible for the formation of stable dispersion. CL from g-CNQDs-NaClO System. A strong CL emission was observed after addition of NaClO solution into g-CNQDs (Figure 2). The CL spectrum with an emission peak centered at 555 nm was obtained by a fluorescence spectrometer with the xenon lamp turned off (inset of Figure 2). No CL emission was observed for the control solution, including the product of guanidine hydrochloride or EDTA upon microwave treatment, indicating the CL reaction of NaClO with g-CNQDs solution rather than with associated species present in the synthesis process. To establish the optimal conditions for the analysis of chlorine, the effects of g-CNQDs concentration, flow rate, and pH value on the CL analysis were investigated. The concentration of g-CNQDs played an important role in the CL reaction. As shown in Figure S6a of the Supporting Information, the CL intensity for 0.05−2.0 μM chlorine increased with the increasing concentration of g-CNQDs in the range of 0.001−0.01 M. Higher concentrations of gCNQDs were not desired, since the quenching effect would be enhanced due to molecule collision of g-CNQDs in high concentration. The g-CNQDs concentration of 0.01 M was selected to provide a strong CL signal. The effect of flow rates of the carrier was also investigated (Figure S6b of the Supporting Information). The low flow rate caused a broad CL signal, which decreased the CL intensity. Flow rates that were too high not only increased the pressure in the flow line, but also decreased the reagent passing time in the spiral CL detection cell, which reduced the CL signal. Considering the analytical precision and solution consumption, the most suitable flow rates for g-CNQDs and NaClO were both 2.5 mL min−1. To study the pH effect on the CL, experiments at different pH values (from 6.0 to 12.0) were performed. As 4532

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Figure 4. EPR spectra of (a) g-CNQDs and g-CNQDs oxidized by NaClO, and (b) singlet oxygen generated via the reaction of 2,2,6,6-tetramethyl4-piperidine (TEMP) probe in the NaClO and g-CNQDs-NaClO systems. Experimental conditions: the concentration of TEMP, g-CNQDs, and NaClO solution was 0.05 M, 0.1 M, and 10 μM, respectively.

which revealed the change of single occupied orbitals in gCNQDs after the CL reaction. The result suggested that electron transfer from the g-CNQDs to NaClO happened and NaClO could inject holes into the g-CNQDs.6 On the other hand, EPR was also utilized to confirm the existence of radicals in this CL system. Figure 4b (black line) confirmed the generation of 1O2 in NaClO solution using 2,2,6,6-tetramethyl-4-piperidine (TEMP) as the specific detection reagent for 1O2.29 The mixture with g-CNQDs could increase the production of 1O2 greatly (red line, Figure 4b). The result indicated that g-CNQDs did show excellent catalytic activity to generate a high yield of 1O2 on their surface. Furthermore, sodium azide (NaN3), which is a scavenger for 1 O2,30 had an obvious inhibition of the CL in the system. As shown in Figure S8a of the Supporting Information, the intensity-inhibited percentage increased when the concentration of NaN3 was increased. For example, only 0.1 mM NaN3 could quench 86% of the original CL intensity. On the basis of these experimental data, it was speculated that 1O2 was generated in high efficiency, and then the reaction of 1O2 with g-CNQDs might also be responsible for the enhanced CL emission centered at 555 nm. It was reported that the formation of 1O2 was the result of an interaction of the reactive oxygen species (ROS).31 To study the origin of 1O2 and explore the CL phenomena in the present study, different active oxygen radical scavengers were used to identify other intermediates of the CL reaction. As illustrated in Figure S8b of the Supporting Information, the CL intensity was effectively quenched upon addition of ascorbic acid (AA), which was widely accepted as an effective active oxygen free radical (•OH and O2•−) scavenger. Therefore, we considered that the free radical reaction should take place during the CL reaction. Thiourea was commonly used as an •OH radical scavenger.32 The CL emission decreased ∼72% when 0.1 mM thiourea was added to the proposed CL system (Figure S8c of the Supporting Information), revealing that OH· was produced in the mixing solutions and might greatly contribute to the CL. Superoxide dismutase (SOD) was frequently used for the detection of O2•−.33 The generation of O2•− was confirmed with the significant quenching effects upon the addition of SOD (Figure S8d of the Supporting Information). However, when DMPO as the specific spin trap reagent34 for •OH was introduced into the g-CNQDs-NaClO system, no such characteristic peak of the typical DMPO−OH· adduct was observed. This phenomenon proved that OH· was formed and reacted with O2•− to produce 1O2 in the presence of g-CNQDs,

measurements were comparable and acceptable. The concentration of free chlorine in boiling tap water was relatively lower than that in fresh tap water, indicating boiling water can decrease chlorine concentration and reduce the harm to the human body. Recovery tests were performed to evaluate the accuracy of this method. The recoveries for sample determination were in the range of 96%−106%. CL Mechanism. To explain the CL phenomenon, emitting species and radicals which were generated in the g-CNQDsNaClO CL system were investigated with the CL spectrum, ESR spectrum, and radical scavengers. The cut off filters (400− 625 nm) were used to determine the CL emission spectrum of g-CNQDs-NaClO. As shown in Figure S6d of the Supporting Information, a wide peak with a maximum of the peak appeared at 555 nm in the spectrum of the g-CNQDs-NaClO system, which was consistent with that obtained by a fluorescence spectrometer (see the inset of Figure 3). Note that the CL spectrum was similar to the FL spectra of g-CNQDs but redshifted in comparison to the intense PL. The red-shift most likely resulted from the smaller energy separations of the gCNQDs surface states, compared with the energy for the most intense PL.26 Hence, this CL may be attributed to the various surface energy traps existing on the g-CNQDs. Interestingly, with the addition of NaClO into the g-CNQDs solution, the color of the g-CNQDs aqueous solution changed from yellow-brown to light yellow. The absorption of gCNQDs at 260 nm decreased upon the addition of NaClO solution and another absorption peak at 265 nm appeared in the UV−vis spectrum (Figure S7a of the Supporting Information). Changes in the surface structure of the gCNQDs were thought to be the cause of the new absorption feature.27 With the addition of NaClO, the maximum FL emission intensity decreased obviously, while a slight blue shift of the maximum emission wavelength was observed (Figure S7b of the Supporting Information). Therefore, it was reasonable to believe that the strongly oxidizing NaClO affecting g-CNQDs surface states induces CL. The ground-state properties of luminescent species in the gCNQDs were investigated by EPR. The g-CNQDs showed an EPR signal at g = 1.99942 (Figure 4a), which revealed a singly occupied orbital in ground-state g-CNQDs.22a The single occupied orbital indicated that the g-CNQDs could be electron donors or electron acceptors during the reaction.28 The EPR spectra of the g-CNQDs after their reaction with NaClO was presented in Figure 4a. The g-value for the g-CNQDs was reduced from 1.99942 to 1.99908 after oxidation by NaClO, 4533

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was possibly due to the radiative recombination of oxidantinjected holes and electrons of g-CNQDs. It should be noted that the formation of 1O2 from reactive oxygen species such as O2•− and •OH in the CL system could transfer energy to gCNQDs and thus enhance the CL emission. This method not only expanded the CL application but also provided new insight into the optical characteristics of the g-CNQDs.

which was similar to the effects of graphene oxide as a catalyst to accelerate electron-transfer processes and the 1O2 generation on its surface.35 On the basis of the above study, the CL mechanism for the gCNQDs-NaClO system was illustrated, as shown in Scheme 1b. Due to the redox property of the discrete electron and hole states of g-CNQDs, a hole can be injected into the valence band by NaClO as a strong oxidizer and convert g-CNQDs to oxidized-state g-CNQDs (g-CNQDs •+) (reactions 1 and 2). The single orbital detected by ESR spectra could serve as the hole traps. And then the thermally excited electrons in the high energy band annihilate with the oxidant injected holes to produce the CL.

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Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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g‐CNQDs + ClO− + H 2O

AUTHOR INFORMATION

Corresponding Author

→ g‐CNQDs•+ + •Cl + 2OH−

*E-mail: [email protected]. Tel/Fax: +86-28-8541-2798.

(1)

g‐CNQDs + •Cl → g‐CNQDs•+ + Cl‐

Notes

The authors declare no competing financial interest.

(2)

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On the other hand, the g-CNQDs acting as electron donors in the redox reaction could also transfer an electron to oxygen in the alkaline system, producing oxidized-state g-CNQDs (gCNQDs •+) and O2•− (reaction 4).36 Apart from the NaClO decomposition, a fraction of O2•− may come from the oxygen dissolved in the solution because there were some changes in CL intensity when N2 was bubbled into the reactant solutions for 30 min before the CL reaction. O2•− is unstable in aqueous solution initiating the reactive oxygen chain reaction, which leads to the generation of the reactive oxygen radical (such as HO2• and •OH).7a Meanwhile, OH− can be oxidized by the oxidized-state g-CNQDs (g-CNQDs •+) to •OH (reaction 5).4,37 Then on the surface of g-CNQDs, the radical recombination reactions between the as-formed oxygen-related radicals would take place to form abundant 1O2.36 2ClO− → 2Cl− + O2

ACKNOWLEDGMENTS Authors gratefully acknowledge financial support for this project from the National Natural Science Foundation of China [Grants 21375089 and 21105067]. The authors also thank Dr. Haijun Yang of the Analytical & Testing Center of Tsinghua University for his generous help in EPR analysis.

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(3)

g‐CNQDs + O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ g‐CNQDs•+ + O2•−

(4)

g‐CNQDs•+ + OH− → g‐CNQDs + •OH

(5)

The generated dimol singlet-oxygen ( O2)2* is believed to cause light emission at the bands of 381, 477, and 578 nm,38 which overlaps the excitation wavelength for g-CNQDs. Hence, the energy from the (1O2)2* could transfer to g-CNQDs and generate excited-state g-CNQDs (g-CNQDs*), which returned to the ground-state and released photons (reaction 8). This route can enhance the CL emission. 1

1

O2 + 1O2 → (1O2 )2 *

(6)

(1O2 )2 * + g − CNQDs → g − CNQDs* + 2O2

(7)

g − CNQDs* → g + CNQDs + hv

(8)

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donate electron

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ASSOCIATED CONTENT

S Supporting Information *

CONCLUSIONS In conclusion, g-CNQDs with high fluorescence quantum yield (35%) were synthesized by using an economical, green, facile, and effective microwave pyrolysis approach with guanidine hydrochloride and EDTA as the precursors. The as-prepared gCNQDs were first demonstrated to exhibit the CL ability in the presence of NaClO, and a novel flow-injection CL method for free chlorine in water was developed with excellent sensitivity. The possible CL mechanism for the g-CNQDs-NaClO system 4534

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