ARTICLE pubs.acs.org/JPCC
Enhancement of Ultraweak Chemiluminescence from Reaction of Hydrogen Peroxide and Bisulfite by Water-Soluble Carbon Nanodots Wei Xue,†,‡ Zhen Lin,‡ Hui Chen,‡ Chao Lu,† and Jin-Ming Lin*,‡ † ‡
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Beijing Key Laboratory of Microanalysis and Instrumentation, Department of Chemistry, Tsinghua University, Beijing 100084, China
bS Supporting Information ABSTRACT: In this work, carbon nanodots were synthesized through a novel solvothermal route, and the effects of carbon nanodots on the ultraweak chemiluminescence (CL) reaction of hydrogen peroxide (H2O2) and sodium bisulfite (NaHSO3) were explored for the first time. It was found that the CL emission intensity of H2O2HSO3 was significantly enhanced by carbon nanodots: about 60-fold increase in the CL intensity was obtained. The enhanced CL was induced by the excited-state carbon nanodots (CD*), which could be produced from the electron-transfer annihilation of positively charged carbon nanodots (CD•+) and negatively charged carbon nanodots (CD•). Radical scavengers such as nitro blue tetrazolium chloride (NBT), sodium azide, thiourea, 5,5-dimethyl-1-pyrroline Noxide, and ascorbic acid were used to study the intermediate species. The intermediate radicals generated during the reaction of H2O2 and NaHSO3, such as hydroxide radical (•OH), sulfate anionic radical (SO4•), superoxide anionic radical (•O2), and sulfur trioxide anionic radical (•SO3), were key species for producing CD•+ and CD•. The CL enhancement mechanism was proposed based on the results of the CL emission spectra, fluorescence spectra, and electron spin resonance (ESR) spectra. The CL properties of carbon nanodots will provide a new route to study the novel materials and broaden the use of them in many fields, such as chemistry, biology, microbiology, and biochemistry.
1. INTRODUCTION Fluorescent semiconductor quantum dots (QDs) have been widely investigated during the past decade for their variety of potential applications in optical bioimaging and beyond.1 However, a serious disadvantage with these popular QDs is that they contain heavy metals, such as cadmium, whose significant toxicity and environmental hazard are well-documented.24 Recently, emerging photoluminescent carbogenic nanoparticles with high quantum yields appear to be a promising alternative to semiconductor QDs in many fields of applications and have attracted much attention from a large number of researchers. The particles were called “carbogenic” because they were not of pure carbon composition like carbon nanotubes or carbon nanodiamonds but proved to be oxygen-containing carbon nanodots.5 Carbon nanodots are discrete nanoparticles of near spherical geometry with sizes below 10 nm, and they inherently fluoresce in the visible upon light excitation.6 Compared to fluorescent semiconductor nanocrystals, photoluminescent carbon nanodots are superior in chemical stability, biocompatibility, and low toxicity.7 Intense research has focused on the synthesis of carbon nanodots since the first report on the preparation of these versatile materials.8 Much research concerns the preparation of carbon nanodots in different ways, such as the laser ablation method,9 electrochemical release or exfoliation from a graphitic source,10 separation of combusted carbon soot,11,12 thermal oxidation of suitable molecular precursors,13,14 dehydration of carbohydrates using concentrated sulfuric acid,15 and one-step synthesis of highly luminescent carbon nanodots in r 2011 American Chemical Society
noncoordinating solvents.16 However, there are only a few reports on the fluorescence5,11 and electrochemiluminescence (ECL)14,17 studies of carbon nanodots, and no article to date has reported the chemiluminescence (CL) properties of carbon nanodots. CL is the production of electromagnetic radiation by a chemical reaction between at least two reagents in which an electronically excited intermediate or product is obtained and subsequently relaxes to the ground state with emission of light.18 Additionally, CL has proved to be a useful phenomenon in the laboratory, finding ever increasing applications in analytical chemistry for its high sensitivity, wide linear range, simple instrumentation, and lack of background scattering light interference.19 The classical reagents, such as luminol, lucigenin, peroxalate, potassium permanganate, and Ce (IV) have been widely studied and employed in analytical chemistry due to their highly CL intensity.20 Unfortunately, these CL methods suffered from expensive or poisonous reagents, poor selectivity, or narrow linear range. Meanwhile, the development of CL was limited in other CL reaction systems because the intensity of many reactions was not strong enough for detecting demand. Therefore, it has become a crucial problem to develop new CL systems with relatively cheap and green reagents. What is more, it is of significant value to enhance the intensity of the proposed CL Received: August 6, 2011 Revised: September 30, 2011 Published: October 04, 2011 21707
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The Journal of Physical Chemistry C systems for the purpose of better analytical performance. Recently, the study of CL has been extended to nanoparticles. For example, gold nanoparticles of different sizes react with the KIO4Na2CO3/NaOH system to yield CL as a nanosized reaction platform,21 CdTe QDs enhance the CL arising from the decomposition of peroxymonocarbonate,22 and the application of molecularly imprinted polymer (MIP)-capped Mn-doped ZnS QDs to a KIO4H2O2 CL system improves the selectivity and sensitivity of the CL method.23 The H2O2HSO3 CL phenomenon was discovered by Stauff in 1975.24 However, the CL mechanism of the reaction was not further investigated owing to its ultraweak luminescence, not to mention the application of this CL reaction. In this work, we chose the H2O2HSO3 CL reaction as a model system and explored the effects of carbon nanodots on the CL for the first time. To the best of our knowledge, this is the first paper about the use of carbon nanodots in CL, which not only broadened the application of the novel materials but also provided a new route to study the properties of carbon nanodots. It was found that carbon nanodots could enhance the CL from the H2O2HSO3 system. The enhanced mechanism of carbon nanodots on the H2O2HSO3 CL system was investigated based on the CL emission spectra, fluorescence spectra, electron spin resonance (ESR) spectra, and the effects of radical scavengers on CL intensity. The results demonstrated that carbon nanodots are potential alternatives to traditional CL emitters. We hope the investigation of the CL mechanism of the carbon nanodots will be valuable both for the understanding of these versatile materials and for the acceleration of the use of ultraweak CL systems in analytical fields.
2. EXPERIMENTAL SECTION 2.1. Reagents. All chemicals used were of analytical grade. Hydrogen peroxide (H2O2, 30%) was obtained from Alfa Aesar A Johnson Matthey Company (Heysham, UK). Sodium hydrogen sulfite (NaHSO3), sodium hydroxide (NaOH), sulfuric acid (H2SO4, 98%), sodium azide (NaN3), thiourea, and glycerin were from Beijing Chemical Reagent Co. (Beijing, China). Citric acid anhydrate and poly(ethylene glycol) average MW 1500 (PEG1500) were purchased from Merck Company (Darmstadt, Germany). Nitro blue tetrazolium chloride (NBT) was purchased from Nacalai Tesque Inc. (Tokyo, Japan). 5,5-Dimethyl1-pyrroline N-oxide (DMPO) was bought from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan). 2,2,6,6-Tetramethyl-4-piperidine (TEMP) was from SigmaAldrich (St. Louis, MO, USA). All chemicals were used as received without further purification, and all solutions were prepared fresh daily in pure water. The water used in the experiments was freshly deionized using an ultraviolet ultrapure water system (18.3MΩ cm, Barnstead, IO, USA). 2.2. Apparatus. Batch CL experiments were carried with a BPCL luminescence analyzer (Institute of Biophysics, Chinese Academy of Sciences, Beijing, China). Absorption spectra were collected by a UVVis spectrophotometer (UV-3900, Hitachi, Japan). Emission spectra were measured with a fluorescence spectrophotometer (F7000, Hitachi, Japan). ESR spectra were measured on a Bruker spectrometer (ESP-300E, Bruker, Germany). High-resolution transmission electron microscopy (HRTEM) images were recorded by a electron microscope operating at 100 kV (JEM-2010, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) spectra were obtained by scanning X-ray microprobes (PHI Quantera, UlvacPHI, Inc., Japan) for determining the composition and chemical bonding configurations.
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Scheme 1. Preparation Procedure of Luminescent Carbon Nanodots
Figure 1. HRTEM image of the aqueous-dispersible nanoparticles. The inset shows magnification of a single nanoparticle. The corresponding size histograms are given aside.
2.3. Synthesis of Water-Soluble Carbon Nanodots. Carbon nanodots were prepared via the solvothermal route proposed by Liu et al.25 with some modifications. Briefly, 15 mL of glycerin, 1.0 g of PEG1500 and 1.0 g of citric acid anhydrate were placed into a 100 mL Teflon equipped stainless steel autoclave, the solution was heated solvothermally at 260 °C for 6 h using a muffle oven, and then cooled down to ambient temperature naturally (the concentration of the carbon nanodots solution was calculated according to the concentration of carbon atom in the carbon source). Aliquots were taken at different time intervals and diluted with water for the optical and morphology measurements. The products were purified by dialyzing against Milli-Q water with a cellulose ester membrane bag (Mw = 3500), during which excess PEG1500 and glycerin were removed. 2.4. H2O2HSO3 CL System. Light-producing reactions were carried out in the glass cuvette placed in the cell under ambient conditions by the batch method, and the detection was performed on a BPCL luminescence analyzer. The CL profile and intensity were displayed and integrated for a 0.1 s interval while the voltage of the photomultiplier tube (PMT) was set at 1.2 kV. In a typical experiment, 50 μL of carbon nanodot solution was added to 100 μL of NaHSO3 solution in a cuvette first, and then 100 μL of H2O2 was injected by a microliter syringe from the upper injection port. 2.5. Spectra Measurements. The CL spectra of this system were measured on a BPCL luminescence analyzer with high-energy cutoff filters from 400 to 640 nm between the flow CL cell and the PMT as described elsewhere.26 Fluorescence spectra, UVvis spectra, and ESR spectra were measured on the F-7000 fluorescence spectrophotometer, a UV-3900 UVvis recording spectrophotometer, and a Bruker ESP-300 E spectrometer, respectively. 21708
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3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Carbon Nanodots. The process of synthesis was inspired by the previous methods proposed by Wang16 and Liu.25 First, citric acid anhydrate served as the source of carbon, while PEG1500 served as the passivation agent (see Scheme 1).27 Then, the route underwent a depolymerization, decomposition, and pyrolysis process.25 It is noteworthy that we used the solvothermal route to synthesize carbon nanodots instead of the refluxing route.16 The carbon nanodots obtained by the solvothermal route had higher quantum yield (21%) than those synthesized by the refluxing route (17%). HRTEM analysis (Figure 1) showed the formation of near spherical but less monodisperse nanoparticles with an average size of 5 nm. The exact structure of carbon nanodots is still under investigation. It was suggested that the cores consist of carbonized intermediates with a highly defected structure of coexisting aromatic and aliphatic regions, similar to graphite oxide or the interrelated carbogenic networks of lignite, coal, and humic substances.6 Additionally, the XPS spectra of the as-prepared carbon nanodots were measured and are shown in the Supporting Information. The survey XPS scan indicated the presence of 66.52% carbon (C1s) atoms in the nanoparticle structures, in addition to 33.48% oxygen (O1s) (Figure S1a in the Supporting Information). The high-resolution XPS results showed that the carbon nanodots were mainly composed of graphitic carbon (sp2) (Figure S1b in the Supporting Information). The emission and absorption spectra of the carbon nanodots solution in water were also measured and shown in the Supporting Information (Figures S2, S3, and S4 in the Supporting Information), which were comparable to those previously reported.1416 There are several mechanisms explaining the unique optical characteristics of carbon nanodots, such as the size distribution of the carbon nanodot particles within the sample,11,12 a distribution of different emissive trap sites,11,12 and the pyrolytic formation of several different polyaromatic fluorophores within the carbon nanodots.13 Among these, the distribution of different emissive trap sites theory is most favorable. According to the theory, the enhanced luminescence in carbon nanodots was due to the passivation that stabilized the surface energy traps of carbon nanodots and made them emissive. Different heating times resulted in the carbon nanodots with different distributions of the surface energy trap, which not only accounted for the multicolor photoluminscence of carbon nanodots but also for the different responses to excitation wavelengths mentioned above.25 The carbon nanodots synthesized by our group exhibited excellent stability, especially their pH independence. The fluorescence intensity of carbon nanodots were relatively stable within a wide pH range of 213 (see Figure S5), which showed significant advantage over the conventional QDs. 3.2. CL Enhanced by Carbon Nanodots for the H2O2 NaHSO3 System. The kinetic study was performed to provide more details that might lead to an explanation of the reaction mechanism. It was observed in Figure 2a that the H2O2NaHSO3 reaction was a fast CL reaction. There was a weak luminescence when 100 μL of 0.5 M H2O2 was injected into 100 μL of 0.1 M NaHSO3 solution. The intensity reached a maximum value of 145 in 0.2 s, then quenched quickly. The mixing order of the reagent solutions played a critical role in the light emission. In order to explore the effect of carbon nanodots on the H2O2NaHSO3 system, batch CL experiments were used to evaluate the different orders of injection solutions. No CL signal was recorded when
Figure 2. CL kinetic curves of (a) H2O2NaHSO3 system and (b) H2O2NaHSO3carbon nanodots system with different mixing orders: (1) carbon nanodots injected into the mixture of H2O2 and NaHSO3; (2) NaHSO3 injected into the mixture of carbon nanodots and H2O2; (3) H2O2 injected into the mixture of carbon nanodots and NaHSO3. The solution conditions were 100 μL of 0.5 M H2O2, 100 μL of 0.1 M NaHSO3, and 50 μL of 0.35 M carbon nanodots. High voltage: 1200 V; interval time was set for 0.1 s.
carbon nanodots were mixed with H2O2 or NaHSO3 alone. As for the reaction of NaHSO3, H2O2, and carbon nanodots, three different mixing orders were compared in Figure 2b. It showed that the injection of H2O2 solution into the NaHSO3carbon nanodots mixed solution generated the strongest CL emission. About 60-fold increase in the CL intensity was obtained. The CL intensity reached the maximum value rapidly. The light remained for 30 s before being utterly quenched. The CL intensity and lifetime were obviously enhanced. As was mentioned above, the fluorescence intensity of carbon nanodots was dependent on the reaction time of the solvothermal method. In order to examine the CL responses of carbon nanodots, the solutions of carbon nanodots synthesized with different reaction time were added to the H2O2NaHSO3 system. The reaction-time-dependent phenomenon was also observed (see Figure S6a in the Supporting Information). The CL intensity of carbon nanodots increased as the reaction time increased, and reached a plateau after 6 h. It was reported that the surface of carbon nanodots continuously evolved and resulted in changes at the emissive sites over the reaction time.25 This led to our speculation that the carbon nanodots after 6 h solvothermal treatment had more suitable surface structure for CL reaction and thus had better enhancing ability. Solutions of different concentrations of carbon nanodots were added to the CL system, and they enhanced the CL intensity to 21709
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Table 1. Effects of Radical Scavengers on H2O2NaHSO3 and H2O2NaHSO3Carbon Nanodots Systemsa CL intensity
CL intensity of
radical
concentration
of
H2O2NaHSO3-carbon
scavengers
(M)
H2O2NaHSO3
nanodots
H2O
230
8000
30
1000
1.0 102
90
4400
thiourea
1.0 104
105
4600
DMPO
0.01% (v/v)
103
2100
ascorbic acid
1.0 103
48
6152
NBT
1.0 102
NaN3
a
Solution conditions were 0.1 M NaHSO3, 0.5 M H2O2, and 0.35 M carbon nanodots. The volume of H2O2, NaHSO3, carbon nanodots, H2O, or radical scavengers was 100 μL.
different degrees (Figure S6b). The CL intensity could be gradually intensified when the concentration of carbon nanodots was lower than 0.35M. The energy generated during the chemical reaction between H2O2 and NaHSO3 was limited. It could excite a certain amount of carbon nanodots. When the concentration of carbon nanodots was higher than 0.35M, the chemical energy could not satisfy the distribution to all carbon nanodots and affected the CL efficiency. The carbon nanodots after 6 h solvothermal treatment at the concentration of 0.35 M gave the most sensitive response and were selected to study the CL mechanism in this study. 3.3. Intermediates Study by Active Oxygen Radical Scavengers. The liquid-phase oxidation of HSO3 by H2O2 has been investigated in detail by several groups.2830 Although Dasgupta31 and Flockhart32 argued in favor of a radical pathway, a great many researchers suggested that the major part of the reaction probably proceeded via a nucleophilic substitution by H2O2 on HSO3 to form a peroxomonosulfite intermediate (HSO4) which then underwent a rate-determining rearrangement to form SO42.30 In their opinions, contribution of a free-radical pathway in the overall reaction seemed unlikely but could not be definitely excluded since radical intermediates have been detected in the reaction of SO2 and H2O2. For example, •SO3 anionic radical and •OH radical were detected using ESR and high-performance liquid chromatography (HPLC) by Shi in the reaction of acidified sulfite and H2O2.33 Ozawa and Hanaki also detected •SO3 anionic radical in the nonenzymatic reaction of H2O2 with sulfite by ESR methods.34 The CL emission during the oxidation of sulfite by other oxidants such as KMnO4 or Ce(IV) in acidic solutions has been attributed to the formation of excited sulfur dioxide molecules (SO2*), which radiate during deexcitation,35,36 and the CL spectrum of SO2* is within the range of 450600 nm.37 On the basis of the research results of Shi and Ozawa,33,34 we presumed that the CL emitters SO2* and 1O2 might be produced during the simultaneous processes of decomposition and radical recombination in the H2O2 NaHSO3 CL reaction, which further proved the contribution of free radical pathway to the reaction. As for the H2O2NaHSO3 carbon nanodots system, there are mainly three possible mechanisms explaining the enhancing effect of carbon nanodots: (1) Carbon nanodots acted as the catalyst in the H2O2NaHSO3 CL reaction, which facilitated the radical generation and the formation of SO2* and 1O2.38 (2) SO2* and 1O2 transferred the energies to fluorescent carbon nanodots, which acted as emitters in the enhanced CL reaction system. (3) The intermediate radicals generated in the H2O2NaHSO3 reaction such as •OH radical, SO4•, •O2, and •SO3 anionic radicals first reacted with carbon
nanodots to produce postively (CD•+) and negatively (CD•) charged carbon nanodots, and then excited-state carbon nanodots (CD*) were formed through the electron-transfer annihilation of CD•+ and CD•. Finally, the enhanced CL emission occurred when CD* returned to the ground-state. The proposition of the third mechanism was inspired by the ECL mechanism of carbon nanodots proposed by Chi et al.17 In the H2O2NaHSO3 carbon nanodots system, reactions of •O2, •SO3, and SO4• anionic radicals and •OH radical with carbon nanodots played important roles. The formation of SO2* and 1O2 competed with the generation of the CD*. To study the intermediates generated in the CL systems, the effects of different active oxygen radical scavengers on the CL intensity of H2O2NaHSO3 and H2O2NaHSO3carbon nanodots systems were investigated. The results are shown in Table 1. NBT was frequently used for the detection of •O2 anionic radical.39 NBT could be reduced to its deep-blue diformazan form by •O2.40 When 1.0 102 M NBT was mixed with H2O2NaHSO3 or H2O2NaHSO3carbon nanodots solutions, there was an obvious color change from yellow to blue. Meanwhile, we found that NBT could inhibit the CL intensity (Table 1). As an assistant detection method, we speculated that •O2 anionic radical existed in the mixing solutions and reacted with NBT (reaction 1). NBT2þ þ •O2 f •NBTþ þ O2
ð1Þ 1
41,42
Sodium azide (NaN3) was a scavenger for O2. Reaction 2 shows that the quenching of 1O2 by NaN3 was through a physical process. The CL intensity of the two systems was effectively quenched by 1.0 102 M NaN3 (Table 1), which was a strong indication that 1O2 was generated in the examined systems. N3 þ 1 O 2 f N3 þ 3 O 2
ð2Þ
•OH radical was considered to be one of the most potent oxidizers. It was presumed that •OH played an important role in the CL of H2O2NaHSO3 and H2O2NaHSO3carbon nanodots systems. Thiourea was an effective radical scavenger for •OH.43 The generation of •OH in the examined systems was confirmed with the observed quenching effects upon the addition of thiourea. The CL intensity was effectively inhibited by 1.0 104 M thiourea. SO4• anionic radical was known to be as strong an oxidant as the •OH radical and was assumed to be one of the products of the chain reactions initiated by O2 and •SO3. It was reported that SO4• and •SO3 could react with DMPO.44 The mechanisms of DMPO with SO4• and •SO3 were shown in reactions 3 and 4. When DMPO was added to H2O2NaHSO3 and H2O2 NaHSO3carbon nanodots systems, the CL intensity decreased greatly with injection content of 0.01% (v/v, Table 1). This indicated that SO4• and •SO3existed in the CL systems. DMPO þ SO4• f DMPO•þ þ SO4 2
ð3Þ
DMPO þ •SO3 f DMPO=•SO3
ð4Þ 45
Ascorbic acid was a common free radical scavenger. As a classical reducing agent, it was dehydrogenized by reactive oxygen species to form dehydroascorbic acid.22 The effects of ascorbic acid on the CL of H2O2NaHSO3 and H2O2NaHSO3carbon nanodots systems were studied in our work. It significantly inhibited the CL intensity of the above two CL systems at the concentration of 1.0 103 M, which further confirmed that the CL reactions proceeded via a radical pathway. 21710
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nanodots, thus lead to the decrease in fluorescence intensity. Meanwhile, the CL spectrum was measured using high-energy cutoff filters of various wavelengths. As shown in Figure 4d, one peak located at 490 nm was observed from the reaction of H2O2NaHSO3carbon nanodots, which was different from the CL spectra of SO2* and 1O2.35,22 Additionally, the ESR results also indicated that free radicals did reduce during the reaction (Figure 3). Therefore, the enhancing effect of carbon nanodots on the H2O2NaHSO3 system could not be attributed to the catalytic mechanism. In view of the above results, we speculated that the CL peak located at 490 nm might be attributed to the emission from carbon nanodots, which was identical to the fluorescence spectra of carbon nanodots excited at 400 nm. 3.6. Mechanism of the CL System. Based on the above results, the mechanism of the CL reaction enhanced by carbon nanodots is summarized in Figure 5. HOOSO2 is formed from the reaction of HSO3 and H2O2 (reaction 5), and it is an unstable compound, which could be converted into •SO3 anionic radical and •OH radical (reaction 6).
Figure 3. ESR spectra of (a) nitroxide radicals generated by the reaction of TEMP and 1O2 and (b) DMPOOH in (1) H2O2NaHSO3 system and (2) H2O2NaHSO3carbon nanodots system. Conditions: modulation amplitude, 2mT; microwave power, 4 mW; sweep width, 5mT.
3.4. ESR Spin-Trapping with TEMP and DMPO. Room-temperature ESR spectroscopy has been used to detect free radical intermediates (Figure 3a and b).46 TEMP is a specific target molecule of 1O2. It can react with 1O2 to give the adduct 2,2,6,6tetramethyl-4-piperidine-N-oxide (TEMPO), which is a stable nitroxide radical with a characteristic spectrum. In this research, the specific signals of TEMPO were observed, which supported the existence of 1O2 in the NaHSO3H2O2 system. DMPO, a specific target molecule of •OH, was also used. Figure 3b presented the production of the DMPOOH adduct in the NaHSO3H2O2 CL system. It confirmed the generation of •OH.33 However, when TEMP and DMPO were added into the mixture solution of carbon nanodots, NaHSO3 and H2O2, the ESR signal decreased. These phenomena proved that 1O2 and •OH existed and reacted with carbon nanodots. 3.5. Study of Emitting Species. To further ascertain the emission species, fluorescence spectroscopy studies were used to characterize carbon nanodots before and after the CL reaction. The fluorescence spectra of carbon nanodots and the mixture of NaHSO3H2O2carbon nanodots were shown in Figure 4a,b. It was found that the fluorescence intensity of carbon nanodots decreased to some extent after the reaction, while the fluorescence maxima did not change. Control experiments (Figure 4c) showed that the decrease of fluorescence intensity was not mainly due to the change in pH (from 5.84 to 1.90) after the reaction. In view of these phenomena, we could come to the conclusion that some carbon nanodots did vanish during the CL reaction. It was assumed that some strong oxidizing free radicals generated during the CL reaction of NaHSO3H2O2 might consume part of carbon
HOSO2 þ H2 O2 f HOOSO2 þ H2 O30
ð5Þ
HOOSO2 f •SO3 þ •OH33
ð6Þ
The formed •SO3 anionic radical reacts with excess H2O2 and generates HO2• radical (reaction 7). Radical HO2• decomposes to yield •O2 anionic radical (reaction 8), and the combination of HO2• generates 1O2 (reaction 9). 1O2 is of higher energy than the ground-state 3O2, which is one emitter of the CL emission (reaction 10). H2 O2 þ •SO3 f HSO3 þ HO2 •
ð7Þ
HO2 • f Hþ þ •O2
ð8Þ
HO2 • þ HO2 • f 1 O 2 þ H2 O2 47
ð9Þ
1
O 2 f 3 O 2 þ hv 48
ð10Þ
When •OH radical reacts with excess HSO3, •SO3 anionic radical is produced (reaction 11).49 After the formation of •SO3 anionic radical, chain reactions are initiated (reactions 1215).44 The major reaction product of •SO3 with oxygen is proposed to be O3SOO• anionic radical (reaction 12) while the minor product is •O2 anionic radical (reaction 13). Subsequent formation of •SO3 anionic radical may then proceed autocatalytically via two possible propagation steps (reactions 14 and 15). •OH þ HSO3 f H2 O þ •SO3 49
ð11Þ
•SO3 þ O2 f O 3 SOO•
ð12Þ
•SO3 þ O2 f SO3 þ •O2
ð13Þ
ð14Þ
O 3 SOO• þ SO3 2 f SO4 • þ SO4 2
SO4 • þ SO3 2 f •SO3 þ SO4 2 21711
ð15Þ
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Figure 4. Fluorescence spectra of (a) carbon nanodots (pH = 5.84), (b) H2O2NaHSO3carbon nanodots system (pH = 1.90), (c) carbon nanodots (pH = 1.90), and (d) CL spectrum of H2O2NaHSO3carbon nanodots system.
•OH radical, SO4•, •SO3, and •O2 anionic radicals were generated,33,44 which could react with carbon nanodots to produce CD•+ and CD• (reactions 1922). The electron-transfer annihilation of CD•+ and CD• could form excited-state CD*, which acted as the final emitter in the system (reaction 23).17,22
Figure 5. CL reaction mechanism for the H2O2NaHSO3carbon nanodots system.
Finally, the recombination of •SO3 anionic radicals could generate an intermediate as SO2*, which is unstable and decomposes to SO2, releasing energy to generate light (reactions 1618).3537 2•SO3 f S2 O6 2
ð16Þ
S2 O6 2 f SO2 þ SO4 2
ð17Þ
SO2 f SO2 þ hv
ð18Þ
CD þ •OH f CD•þ þ OH
ð19Þ
CD þ SO4 • f CD•þ þ SO4 2
ð20Þ
CD þ •O2 f CD• þ O2
ð21Þ
CD þ •SO3 f CD• þ SO3
ð22Þ
CD• þ CD•þ f CD17;22
ð23Þ
Simultaneously, injection of a hole into CD• from H2O2 or •OH radical and the injection of electrons from •O2 or •SO3 anionic radicals into CD•+ might have additionally existed (reactions 2427).22
In H2O2NaHSO3carbon nanodots system, NaHSO3 and carbon nanodots were mixed first in a glass cuvette, and then H2O2 was injected to the mixture. When H2O2 was injected, 21712
CD• þ •OH f CD þ OH
ð24Þ
CD• þ H2 O2 f CD þ 2OH
ð25Þ
CD•þ þ •O2 f CD þ O2
ð26Þ
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ð27Þ
Both injection processes lead to the same CD*. When CD* returns to the ground-state accompanied with photon irradiation, the CL emission occurs (reaction 28). CD f carbon nanodots þ hv
ð28Þ
Meanwhile, the CL spectrum (380640 nm) of the H2O2 NaHSO3 system covers a broad range, which overlaps well with the absorption of carbon nanodots; in other words, the carbon nanodots could be excited directly by 1O2 and SO2*. Therefore, the direct energy transfer from 1O2 and SO2* to the carbon nanodots mechanism may also be possible for this system.
4. CONCLUSIONS Highly fluorescent carbon nanodots were synthesized successfully by the solvothermal route. The products had unique optical properties and pH inertness. This work also elucidated the CL enhancement mechanism of carbon nanodots in the H2O2 NaHSO3 aqueous system. •OH radical, SO4•, •SO3, and •O2 anionic radicals were generated during the reaction of HSO3 and H2O2. Carbon nanodots could react with these free radicals to form CD• and CD•+, the electron-transfer annihilation of CD• and CD•+ formed the excited-state CD*, which acted as the final emitter in the system. The H2O2NaHSO3 CL system is a simple, inexpensive, and relatively nontoxic alternative to conventional CL systems. The investigation of the CL properties of carbon nanodots for the first time not only gained a better understanding of the unique surface property and chemical reactivity of carbon nanodots but also brought a broad prospect for the application of the H2O2NaHSO3 CL system. ’ ASSOCIATED CONTENT
bS
Supporting Information. The XPS spectra of carbon nanodots, the optical properties of carbon nanodots synthesized by the solvothermal route, and the optimization of carbon nanodot solution conditions for CL reaction. This information is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone/Fax: +86-10-62792343.
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