Chemiluminescence Arising from the Decomposition of

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Chemiluminescence Arising from the Decomposition of Peroxymonocarbonate and Enhanced by CdTe Quantum Dots Hui Chen,†,‡ Ling Lin,‡ Zhen Lin,† Guangsheng Guo,‡ and Jin-Ming Lin*,† Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China, and State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China ReceiVed: May 4, 2010; ReVised Manuscript ReceiVed: August 7, 2010

CdTe quantum dots (QDs) capped with mercaptoacetic acid were applied to the hydrogen peroxide-sodium hydrogen carbonate chemiluminescence (CL) system. The CL emission intensity was significantly enhanced by different sizes of CdTe QDs. Peroxymonocarbonate (HCO4-) was formed in the reaction of hydrogen peroxide and sodium hydrogen carbonate, which was a reactive oxygen species. Decomposition of HCO4generated superoxide ion radical ( · O2-) and hydroxide radical ( · OH). The enhanced CL was induced by the excited CdTe QDs, which could be produced from the combination of hole (oxidized QDs (h+)) and electron (reduced QDs (e-)) injected QDs. Radical scavengers and organic reagents such as nitro blue tetrazolium chloride (NBT), cytochrome c, sodium azide, ascorbic acid, thiourea, tert-butanol, and dimethyl sulphoxide were used to study the emitting species. The intermediate hydroxide radical and superoxide ion were key species for producing hole and electron-injected QDs. Four emitters such as 1O2, (O2)2*, (CO2)2* and CdTe* were detected in the CL system. The mechanism was discussed based on the CL emission spectra, electron spin resonance spectra, fluorescence spectra, and UV-vis absorption spectra. The CL properties of CdTe QDs will be helpful to study semiconductor nanocrystals and will open new avenues for the application of QDs in many fields, such as chemistry, biology, microbiology, and biochemistry. Introduction Peroxymonocarbonate (HCO4-) is an oxidant whose existence in equilibrium with hydrogen peroxide and bicarbonate has been known since the 1980s (Reaction 1).1,2 HCO3 + H2O2 h HCO4 + H2O

(1)

The use of a more soluble source of bicarbonate (NH4HCO3) and an alcohol cosolvent can shift the equilibrium of Reaction 1 to the right with the increasing formation of HCO4-.3 This reaction occurs rapidly at 25 °C near neutral pH in aqueous solution and alcohol/water mixtures. The value of Eθ (HCO4-/ HCO3-) is 1.8 ( 0.1 V (vs NHE).4 It offers a simple, inexpensive, and relatively nontoxic alternative to other oxidants.5,6 It is a true peroxide with structure HOOCO2- and can be classified as an anionic peracid.7 Peroxymonocarbonate is a reactive oxygen species. Chemiluminescence (CL) was generated during the decomposition of peroxymonocarbonate. It was the precursor of carbonate radical. In our previous work, we had observed the chemiluminescence resonance energy transfer (CRET) between peroxymonocarbonate and lanthanide inorganic coordinate complexes.8,9 Subsequently, the effect of Co2+ on carbonate/bicarbonate and hydrogen peroxide system, and the CL mechanism were reported.10 (O2)2* and (CO2)2* were the emitters. The CRET can take place between HCO4- and eosin Y catalyzed by gold nanoparticles.11 But the CL intensity of the hydrogen peroxidesodium hydrogen carbonate system was not strong enough, as * To whom correspondence should be addressed. E-mail: jmlin@ mail.tsinghua.edu.cn. † Tsinghua University. ‡ Beijing University of Chemical Technology.

well as the half-life of the generated HCO4- is only about 300 s. The CL was fast and ultraweak. It is of great importance to study HCO4- because of the ubiquity of CO2/HCO3- buffers in biology and the well-known production of H2O2 in both normal metabolism and the immune response.12 Peroxymonocarbonate is the main species responsible for biothiol peroxidation in the presence of bicarbonate.13 Thus, a better understanding of its formation and reactions in aqueous buffer is required; to enhance the CL intensity and vary the reaction process to prolong the CL time of carbonate/ bicarbonate and hydrogen peroxide system are goals of great significance. Colloidal semiconductor nanocrystals, or quantum dots (QDs), have attracted much attention due to their unique and excellent size-dependent optical and electronic properties, and have been widely used as multicolored photoluminescent probes14 and biological luminescent labels15 since two works were reported in 1998.16,17 The fluorescence18 or chemiluminescence resonance energy transfer19 and electrochemical analytical techniques20 coupled with QDs have been rapidly developed. The application is based on the light emission of the excited state of QDs, which can be produced by photoexcitation, electron injection, electron impact,21 or chemical reaction of QDs as chemiluminescent emitters.22 The CdSe/ZnS core/shell QDs have been used in fluorescence resonance energy transfer for monitoring purposes.23 Electrochemiluminescence (ECL) of silicon QDs was first observed by Bard in 2002.24 The ECL processes of TOPOcapped CdSe25 and CdTe QDs, and a CdSe/ZnSe core-shell26 structure have also been investigated. Chemical or physical interaction between analytes and CdS QDs can change their surface charges or components, then affect the efficiency of the core electron-hole recombination and thus the luminescent emission. This principle has been used to detect copper cation27 and cyanide anion.28

10.1021/jp104060x  2010 American Chemical Society Published on Web 08/25/2010

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Figure 1. Schematic diagram for chemiluminescent detection. (a) The batch method for CL detection. (b) The flow system for CL spectra recording. PMT, photomultiplier tube; P1 and P2, peristaltic pump; M, mixing coil; F, flow cell; W, waste.

Nowadays, high quality nanocrystals of different II-VI and III-V semiconductor materials can be easily synthesized. CdTe QDs prepared organometallically with very high photoluminescence quantum efficiencies of 65% at room temperature have been reported.29 Since the aqueous synthesis of mercaptoethanoland thioglycerol-capped CdTe QDs was reported in 1996,30 sufficient progress has been made in the preparation and the design of the surface properties of CdTe QDs. Luminescence of CdTe QDs is very stable and covers almost the entire visible spectral range (500-730 nm), which is decided by the particle size.31,32 The chemiluminescence resonance energy transfer from the excited oxidation product of luminol to CdTe QDs has also been investigated.33 Wang et al. reported the oxidized CL of CdTe QDs/H2O2 system and its size-dependent and surfactantsensitizing effects in aqueous solution in detail.34 This work explored the effect of CdTe QDs on the chemiluminescence of hydrogen peroxide-sodium hydrogen carbonate system. We found CdTe QDs can significantly enhance the CL intensity of the NaHCO3-H2O2 system. The CL behavior was studied, and a possible mechanism was discussed based on the CL spectra, ultraviolet-visible (UV-vis) absorption, and the effect of radical scavengers and organic reagents on CL intensity. This result demonstrated QDs as a potential alternative for traditional CL emitters. Moreover, the size-dependent properties of QDs would provide the emitter promising behaviors for CL application. We hope it will be helpful in the development of QDs materials and the applications in the realms of chemistry, biology, microbiology, and biochemistry. Experimental Section Chemicals. All chemicals used in this experiment were of analytical grade. Mercaptoacetic acid (MAA) and hydrogen peroxide (H2O2, 30%) were obtained from Alfa Aesar, a Johnson Matthey Company (Heysham, UK). Sodium hydrogen carbonate (NaHCO3) and cadmium chloride (CdCl2) were obtained from Beijing Chemical Reagent Co. (Beijing, China). Tellurium and sodium borohydride were obtained from Tianjin Chemical Reagent Co. (Tianjin, China). CdS and CdSe quantum dots (emission wavelength: 549 and 607 nm) were purchased from Wuhan Jiayuan Quantum Dots Co. Ltd. (Wuhan, China). Nitro blue tetrazolium (NBT) and cytochrome c were purchased from Nacalai Tesque Inc. (Tokyo, Japan). 1,4-Diazabicyclo [2,2,2]octane (DABCO) was obtained from Amresco (Ohio, USA). 2-Methyl-6-(4-methoxyphenyl)-3,7-dihydroimidazo[1,2R]pyrazin3-onehydrochloride (MCLA) and 5,5-dimethyl-1-pyrroline Noxide (DMPO) were purchased from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan). 2,2,6,6-Tetramethyl-4-piperidine was obtained from Sigma-Aldrich (St. Louis, USA). All water was

freshly deionized using an ultraviolet ultrapurewater system (18.3 MΩ · cm, Barnstead, IO, USA). Apparatus. Batch chemiluminescence experiments were carried out with a BPCL luminescence analyzer (Institute of Biophysics, Chinese Academy of Sciences, Beijing, China). Two peristaltic pumps (Baoding Longer Precision Pump Co. Ltd., China) were used to deliver three flow lines at a flow rate of 4.0 mL min-1 in a flow system. Absorption spectra were collected using a UV-vis spectrophotometer (UV-2100s, Shimadzu, Japan). Emission spectra were measured with a fluorescence spectrophotometer (F-7000, Hitachi, Japan). Transmission electron microscopy (TEM) images were recorded by a Hitachi H-7500 electron microscope operating at 80 kV. Electron spin resonance (ESR) spectra were measured on a Bruker ESP-300 E spectrometer (Bruker, Germany). HCO3--H2O2 Chemiluminescence System. Light-producing reactions were carried out in the glass cuvette by a batch method, and the detection was performed on a BPCL luminescence analyzer. A 50 µL H2O2 and 50 µL NaHCO3 mixed solution was added to the cuvette, then 100 µL CdTe QDs were injected (see Figure 1a). The CL intensity was displayed and integrated with 0.1 s interval. In a flow system for the detection of CL spectra, NaHCO3 and H2O2 in two lines were mixed first by a three-way piece, then CdTe QDs or H2O in another line was mixed with them in a flow cell for the detection. Spectra Measurements. Absorption spectra were obtained using a Shimadzu UV-vis spectrophotometer. Fluorescence experiments were performed using a Hitachi F-7000 spectrophotometer and the CL spectrum of this system was measured by using it when the Xe lamp was turned off. The emission slit width was opened maximally to 20 nm during the recording of the CL spectra in a flow analysis apparatus, which was consisted of two peristaltic pumps, a CL detector, and a flow cell placed inside the cell holder of the fluorescence spectrophotometer (see Figure 1b). All optical measurements were performed at room temperature under ambient conditions. Results and Discussion Characterization of CdTe QDs. Figures 2a and 2b show typical absorption and room-temperature fluorescence spectra of a size series of CdTe nanocrystals. The spectra were measured on a prepared CdTe colloidal solution that was taken from the refluxing reaction mixture at different intervals and diluted with water to provide the appropriate optical densities for fluorescent measurements. All samples showed a well-resolved absorption maximum of the first electronic transition indicating a sufficiently narrow size distribution of the CdTe QDs, which shifted to the longer wavelengths with the increasing size of nano-

Chemiluminescence from Peroxymonocarbonate

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Figure 2. Characterization of CdTe QDs. UV-vis absorption (a), fluorescence spectra with particle size (b), and TEM image (c) of MAA-capped CdTe QDs obtained at different heating times. The absorption peaks occur at 477, 482, 488, 502, 514, and 531 nm, and the fluorescence spectra emission peaks are at 523, 525, 529, 531, 537, and 545 nm (λex ) 350 nm) for the heating times of 1, 3, 5, 13.5, 17, and 20 h, respectively. In TEM image of one CdTe QDs sample with the heating time of 13.5 h, the particles were transferred from water to ethanol in order to achieve better separation on the TEM grids.

crystals as a consequence of the quantum confinement. According to literature, the CdTe particle size of synthesized CdTe was calculated in virtue of the following expression:35

D ) (9.8127 × 10-7)λ3 - (1.7147 × 10-3)λ2 + 1.0064λ - 194.84 where D (nm) was the size of a given nanocrystal, and λ (nm) was the wavelength of the first excitonic absorption peak of the corresponding nanocrystal. The position of the fluorescent maximum of the smallest (1.55 nm) luminesceing CdTe QDs was located at 523 nm (green emission), whereas the largest (3.00 nm) CdTe nanocrystals obtained emit with a fluorescent maximum at 545 nm (see Figure 2b). The entire spectral range between these two wavelengths was covered by the intermediate sizes of CdTe QDs. The room-temperature photoluminescence (PL) quantum efficiency (QE) of CdTe QDs has been obtained by comparison with rhodamine 6G in ethanol assuming its PL QE as 95%.36 The PL QEs of as-synthesized CdTe QDs lay typically between 3 and 40%. TEM image of one CdTe sample with the heating time of 13.5 h was shown in Figure 2c. It reveals that the CdTe QDs were sufficiently dispersed and well separated with semblable size in aqueous solution. Kinetic Aspect. The kinetic study was performed to provide more details that might lead to an explanation of reaction mechanism. Different mixing orders of reagents were studied to obtain the highest intensity of CL signal by a batch method.

The results were shown in Figure 3. In the NaHCO3-H2O2 system, H2O2 was injected to NaHCO3, and two CL peaks were obtained. The CL intensity got first maximum value of 225 in 0.2 s and was then quenched quickly. After 60 s, the second maximum value of 30 was recorded, and the CL lasted for about 90 s (see Figure 3a). This phenomenon was due to the formation of three emitters such as singlet oxygen (1O2), singlet oxygen molecular pair [(O2)2*], and excited triplet dimers of two molecules ((CO2)2*). The mechanism can be explained as follows. HCO4- was formed from the reaction of bicarbonate and hydrogen peroxide (Reaction 1), and it was a unstable compound, which could be converted into · CO3- and · OH (Reaction 2).37 HCO4 f · CO3 + · OH

(2)

The formed · CO3- reacted with excess H2O2 and generated HO2 · radical (Reaction 3). HO2 · radical decomposed to yield · O2- (Reaction 4).38 H2O2 + · CO3 f HCO3 + HO2 ·

(3)

HO2 · f H+ + · O2

(4)

These oxygen radicals took part in the following reactions and formed the singlet oxygen molecules (Reactions 5-7).39

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Figure 3. Kinetic curves of NaHCO3-H2O2 system (a) and NaHCO3-H2O2-CdTe QDs system (b). The solution conditions were 0.5 mol L-1 H2O2, 1.0 mol L-1 NaHCO3, and 1.2 × 10-6 mol L-1 CdTe QDs. 1 · O2 + · OH f O2 + OH

(5)

1 · O2 + HO2 · f O2 + HO2

(6)

HO2 · + HO2 · f 1O2 + H2O2

(7)

O2 f 3O2 + hV

(8)

1

Singlet oxygen had higher energy than the ground-state triple oxygen, which was one emitter of the CL emission (Reaction 8).40 Recombination of the · O2- radical caused the formation of oxygen dimer. The CL was also due to the formation of singlet oxygen molecular pair [(O2)2*], which could lose its excess energy by luminescence (Reactions 9 and 10).41-44 2 · O2 + 2 · O2 + 4H2O f (O2)* 2 + 2H2O2 + 4OH

(9) (O2)*2 f 2O2 + hV

(10)

light signal remained for about 450 s before quenched utterly (see Figure 3b). That means the CL intensity and lifetime were obviously enhanced. This CL was due to the semiconductor properties and nanoscale effect of CdTe QDs. In the above discussion, there were · O2- and · OH radicals in the HCO3--H2O2 system. According to related reports about CL of semiconductor and common CL reactions,52,53 · O2- was stable in high pH aqueous solution. It directly injected one electron into the 1Se quantumconfined orbital of CdTe QDs to form reduced CdTe (e-1Se) (Reaction 14). At the same time, the formed · OH could inject a hole in the 1Sh quantum-confined orbital of CdTe QDs to form cation radicals CdTe (h+1Sh) (Reaction 15).33,52 CdTe + · O2 f CdTe(e1Se) + O2

(14)

+ CdTe + · OH f CdTe(h1Sh ) + OH-

(15)

Then, the hole (CdTe (h+1Sh)) and electron (CdTe (e-1Se)) injected QDs recombined and produced the excited CdTe (CdTe*, Reaction 16).33,52

Many investigations have confirmed that HCO3- was a luminous species when it was presented with a strong oxidant in basic solution.45 When · OH radical reacts with excess HCO3-, bicarbonate radicals ( · HCO3) were produced (Reaction 11).46 The recombination of · HCO3 could generate an intermediate as (CO2)2*, which was unstable and decomposed to CO2 releasing energy to generate light (Reactions 12 and 13).46,47-51

Simultaneously, injection of a hole into the CdTe (e-1Se) from H2O2 and · OH, and the injection of electrons from · O2- into CdTe (h+1Sh), might have additionally existed (Reactions 17-19).54

· OH + HCO3 f OH + · HCO3

(11)

CdTe(e1Se ) + H2O2 f CdTe* + 2OH-

(17)

2 · HCO3 f (CO2)*2 + H2O2

(12)

CdTe(e1Se ) + · OH f CdTe* + OH-

(18)

(CO2)*2 f 2CO2 + hV

(13)

+ CdTe(h1Sh ) + · O2 f CdTe* + O2

(19)

According to literature, the lifetime of 1O2 was only 3.1 µs in H2O. We presumed the first CL peak might be due to 1O2 and (O2)2*, and the second CL peak was due to (CO2)2* in HCO3--H2O2 system. In NaHCO3-H2O2-CdTe QDs system, H2O2 and NaHCO3 were mixed first in the glass cuvette, and then CdTe QDs were injected to this mixture. Only one CL peak was observed. The CL intensity reached the maximum value of 2766 rapidly. This

+ CdTe(e1Se ) + CdTe(h1Sh ) f CdTe*

(16)

Both injection processes led to the same CdTe*. When CdTe* returned to the ground-state accompanied with photon irradiation, the CL emission occurred (Reaction 20). CdTe* could be considered as the emitter to produce 1Se-1Sh transition emission.

CdTe* f CdTe + hV

(20)



Figure 4. Effects of pH (a) and concentration of H2O2 (b) on the CL intensity by batch method. The solution conditions were 1.0 mol L-1 NaHCO3, 1.2 × 10-6 mol L-1 CdTe and different concentrations of 0.01, 0.05, 0.1, 0.5, 1.0, and 2.0 mol L-1 of H2O2. Different pH values of 2.89, 6.83, 7.14, 8.20, 9.73, 10.45, 11.11, 11.35, 12.30, and 13.21 were achieved by varying the ratio of 0.1 mol L-1 NaHCO3, Na2CO3, and 1 mol L-1 NaOH.

Effects of Reaction Conditions. The CL intensity and the shape of the kinetic profile were greatly affected by the reaction pH. Experiments in different pH were performed, which were achieved by varying the ratio of 0.1 mol L-1 NaHCO3, Na2CO3, and 1 mol L-1 NaOH. As shown in Figure 4a, the CL intensity was increased with increasing pH value, reaching a maximum at pH 11.11. With the increasing pH, the CL system contained more OH-, the CdTe QDs surface was more negatively charged, and the electron transfer from the conduction band of CdTe was easier. Furthermore, the · O2- species was more stable in high pH solution, which benefited the CL emission and lead to the increasing CL intensity. But when the pH value was higher than 11.11, too many anions loaded on the surface of CdTe QDs, which would suppress the approach of negatively charged · O2- to the CdTe surface, and make the decrease of CL intensity. When 1.0 mol L-1 or a higher concentration of NaOH was added, the CL emission was totally quenched. Therefore, the pH value of 11.11 was selected in this CL system. The concentration of H2O2 played an important role in the CL reaction. H2O2 reacted with NaHCO3 to generate HCO4ions that lead to the formation of · OH radical. Simultaneously, H2O2 reacted with the reactive intermediate and yielded · O2radicals. Excessively low concentration of H2O2 did not yield enough HCO4- ions. High concentration could cause rapid decomposition, producing air bubbles in the reaction that affected the system light emitting and stability. As shown in Figure 4b, the CL intensity changed with the increasing concentration of H2O2 in the range of 0.01-2.0 mol L-1 and reached the maximum at the concentration of 0.5 mol L-1. The effect of NaHCO3 concentration on CL intensity was investigated, and the results indicated that a higher concentration could give a stronger CL intensity. NaHCO3 solution with a concentration of 1.0 mol L-1 was selected since the high concentration could arouse solubility saturation effects. CL Enhanced by QDs. Size effect was a basic characteristic of semiconductor nanocrystals. It was found that the CL intensity gradually increased with the increasing particle size of CdTe QDs. Figure 5a showed that the highest CL intensity was obtained by the CdTe QDs with 2.40 nm in diameter. According to CL energy match theory, the energy band gap of semiconductor QDs decreased with the increasing particle size.35 The CL intensity increased with the decreasing band gaps that lead to a faster electron injection to the surface states of QDs. But when QDs were larger than 2.40 nm in diameter, it would lead to the relative small surface-to-volume ratio, which reduced the contact area of QDs and thus the efficient electron transfer.

The kinds of QDs also affected the CL intensity. CdS and CdSe QDs with emission wavelength of 549 and 607 nm were compared with CdTe QDs (see Figure 5b). CdTe QDs could greatly enhance the CL intensity when using larger size than that of CdS and CdSe QDs at the same concentration. The energy band gaps of CdS, CdSe, and CdTe QDs are 2.5 eV, 1.74 and 1.50 eV at 300 K, respectively.55 It may be attributed to the different energy matching degree between the chemical energy generated during CL redox reaction and the required excitation energy for the formation of different excited state of luminophor. The chemical energy generated during the chemical reaction between H2O2 and NaHCO3 matches the smallest energy band gap of CdTe QDs in these three kinds of QDs. The more chemical energy matches the excitation energy needed, the stronger the CL intensity was, and the higher efficiency could be obtained. Solutions of different concentrations of CdTe QDs were added to the CL system; they enhanced the CL intensity in different degrees (Figure 5c). Concentrations of CdTe QDs were estimated from the first adsorption peaks in UV-visible spectra and several empirical equations reported previously.35 The CL intensity can be gradually intensified when the concentration of CdTe QDs was lower than 1.2 × 10-6 mol L-1. The energy generated during the chemical reaction between H2O2 and NaHCO3 was limited. It could excite a certain amount of CdTe QDs. When the concentration of CdTe QDs was higher than 1.2 × 10-6 mol L-1, the chemical energy could not satisfy the distribution to all QDs and affected the CL efficiency. Furthermore, the CdTe QDs molecule collision chance was increased in the dense reactive solution and the light quenching was enhanced. So the CL intensity was decreased. The CdTe QDs of 2.4 nm at the concentration of 1.2 × 10-6 mol L-1 gave the most sensitive response and were selected as the CL emitter in this study. Emitting Species Studies. To further study the mechanism of the CL system, effects of different active oxygen radical scavengers on the CL intensity of NaHCO3-H2O2 and NaHCO3-H2O2-CdTe QDs systems were investigated. Results are shown in Table 1. NBT Reduction, Cytochrome c, and MCLA Reaction for · O2- Radical. NBT was frequently used for the detection of · O2- radicals.42 · O2- can reduce NBT to its deep-blue diformazan form.56 When NBT was added to NaHCO3 or CdTe QDs, there was no color change. The color change from yellow to blue was obvious when 1.0 × 10-5 mol L-1 NBT was mixed with NaHCO3-H2O2 or NaHCO3-H2O2-CdTe QDs solution.

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Figure 5. Effect of QDs on CL intensity. CL of different sizes of CdTe QDs (a), different kinds of QDs (b), and different concentrations of CdTe QDs (c). CdTe QDs were injected to the NaHCO3-H2O2 mixture solution in the same volume of 100 µL by a batch method. The solution conditions were 0.5 mol L-1 H2O2, 1.0 mol L-1 NaHCO3, and 1.2 × 10-6 mol L-1 CdTe, CdS, and CdSe QDs.

TABLE 1: Effects of Radical Scavengers on NaHCO3-H2O2-CdTe QDs System Intensitya radical scavengers H 2O NBT cytochrome c MCLA DABCO NaN3 thiourea ascorbic acid t-butanol DMSO

concentration (mol L-1) 1.0 × 10-5 4.0 × 10-8 5.0 × 10-6 1.0 × 10-2 1.0 × 10-5 1.0 × 10-5 1.0 × 10-5 0.01% (V/V) 0.01% (V/V)

+ · O2 + · O2 + 2H f H2O2 + O2

CL intensity 600 80 5000 23 000 65 120 70 200 100 50

a Solution conditions were 1.0 mol L-1 NaHCO3, 0.5 mol L-1 H2O2 and 1.2 × 10-6 mol L-1 CdTe QDs. The volume of NaHCO3H2O2 mixed solution, H2O, QDs, or radical scavengers was 100 µL.

After adding NBT to NaHCO3-H2O2-CdTe QDs solution, the absorption peak of NBT at 259 nm changed to 580 nm, and the absorbance increased quickly. Then it began to decrease after 29 min of reaction, and the absorption location changed to 660 nm (Figure 6a). As an assistant detection method, we speculated that · O2- ions existed in the mixing solution. During the experiment, we found NBT of different concentrations could increase or inhibit the CL intensity because it generated reactive oxygen species (ROS) in the reversible reactions (Reactions 21 and 22).57

+ NBT2+ + · O2 h · NBT + O2

(22)

(21)

Cytochrome c was also a special compound for detecting · O2-.58,59 It could be reduced by · O2- radical and changed its color from red to colorless (Reaction 23). Cytochrome c had a maximum absorption at 410 nm in aqueous solution. It did not react with NaHCO3, H2O2, or CdTe QDs. However, the color changed to yellowish after injecting cytochrome c to NaHCO3H2O2 or NaHCO3-H2O2-CdTe QDs solution in the reaction cuvette. When 4.0 × 10-8 mol L-1 of cytochrome c was added to NaHCO3-H2O2-CdTe QDs solution, the absorption at 410 nm was decreased rapidly (Figure 6b) and the CL intensity was enhanced greatly (about 9 times, Table 1). Cytochrome c could enhance the CL intensity about 20 times in the higher concentration of 4.0 × 10-5 mol L-1. These results validated that · O2- was produced during the CL process.57 2+ Cyt c - Fe3+ + · O+ O2 2 f Cyt c - Fe

(23)

MCLA could react with reactive oxygen species and emit strong CL light at 465 nm by forming a high-octane dioxetanone intermediate.60 It had been used as a CL probe for the determination of · O2- and 1O2.61 At the concentration of 5.0 × 10-6 mol L-1, MCLA enhanced CL intensity about 40 times in NaHCO3-H2O2-CdTe QDs system (Table 1). It further confirmed that · O2- and 1O2 were generated and participated in the energy-transfer CL reaction. DABCO, D2O, CCl4, and NaN3 Experiments for 1O2. 1,4Diazabicyclo[2,2,2]octane (DABCO), deuterium oxide (D2O), and CCl4 were also used to study the characterization of the generation of 1O2 in this CL reaction. DABCO was known to



Figure 6. Effect of NBT and cytochrome c on NaHCO3-H2O2-CdTe QDs system. Absorption spectra of NBT (a) and cytochrome c (b) in NaHCO3-H2O2-CdTe QDs solution. Times in the figure were the mixing time of NBT or cytochrome c with NaHCO3-H2O2-CdTe QDs solution.

TABLE 2: Effect of DABCO on the CL Intensity in NaHCO3-H2O2-CdTe QDs Systema compound H 2O DABCO

concentration (mol L-1)

CL intensity (count)

1.0 × 10-8 1.0 × 10-7 1.0 × 10-6 1.0 × 10-5 1.0 × 10-4 1.0 × 10-3 1.0 × 10-2 1.0 × 10-1 1.0 × 10°

600 140 125 115 101 98 90 65 22 8

a Solution condition was the same as Table 1. The volume of NaHCO3-H2O2 mixed solution, H2O, QDs, or DABCO was 100 µL.

be a quencher of 1O2.62 O2 (1Σg+) deactivation by DABCO was mainly an electronic-to-vibrational (e-v) process.63 Electronicto-vibrational energy transfer was a general deactivation process of O2 (1Σg+) and O2 (1∆g), which converted electronic excitation energy of the O2 molecule into vibration of O2 and quencher and occurred with any di- or polyatomic collider in gas or liquid phase.64 As shown in Table 2, the CL intensity of NaHCO3-H2O2CdTe QDs solution was decreased in the presence of DABCO. The CL emission decreased with the increasing DABCO concentration. The lifetime of 1O2 in D2O was about 10 fold longer than that in water.65 The lifetime of 1O2 was as much as 59 ms in CCl4 but was only 3.1 µs in H2O.44 It was due to the different vibrations of O-D, C-Cl, and O-H in solvent. D2O and CCl4 were used to confirm the generation of 1O2 by comparing with H2O. As shown in Figure 7, the CL profiles of NaHCO3H2O2-CdTe QDs reacting in D2O, CCl4, and H2O media were different. Both the peak height and area of the CL signals from mixing in D2O and CCl4 solutions were bigger than those results in H2O. For example, the peak height detected in D2O was 25% higher than that in H2O. This phenomenon evidenced the fact that 1O2 was produced during the reactions of HCO4-. Sodium azide (NaN3) was a scavenger for 1O2.66,67 The generation of 1O2 in the examined system was also confirmed with the observed quenching effect. NaN3 was a physical quencher for 1O2 (Reaction 24). The CL intensity was effectively inhibited by 1.0 × 10-5 mol L-1 NaN3 (Table 1), and the CL

Figure 7. Effects of D2O, CCl4, and H2O on the CL intensity in NaHCO3-H2O2-CdTe QDs system. Solution condition was the same as that in Figure 3.

intensity decreased with the increasing concentration. It further indicated that 1O2 was involved in the CL reaction.68 1 3 N3 + O2 f N3 + O2

(24)

Thiourea, t-Butanol, and DMSO Experiments for · OH. Hydroxide radical was considered to be one of the most potent oxidizers. Thiourea was an effective radical scavenger for · OH.69 It had a significant inhibition in this CL system at the concentration of 1.0 × 10-5 mol L-1 (about 9 times, Table 1), and the quenching effect increased with the increasing concentration. It implied that · OH was released in the reaction. To further characterize the generation of · OH in this CL reaction, some organic reagents were also studied. Tert-butanol and dimethyl sulphoxide (DMSO)70 were introduced into the CL system, respectively. The reaction constant of t-butanol with hydroxyl radicals amounts to 6 × 108 mol L-1 s-1.71 The mechanism of DMSO with · OH was shown in Reactions 25-27. The CL intensity decreased greatly by t-butanol or DMSO with each injection content of 0.01% (v/v, 6 times and 12 times, Table 1). At the content of 0.05%, the CL intensity decreased to about 8.0%. It further indicated that · OH existed in the CL system.71

(CH3)2SO + · OH f CH3SO2H + · CH3

(25)

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Figure 8. ESR spectra of nitroxide radicals generated by reaction of TEMP (a) and ESR spectra of DMPO-OH (b) in NaHCO3-H2O2 system and NaHCO3-H2O2-CdTe QDs system. Conditions: Receiver gain, 1.00 ×105; modulation amplitude, 1G; microwave power, 1.00 × 101 mW.

· CH3 + O2 f CH3OO ·

(26)

2CH3OO · f HCHO + CH3OH + O2

(27)

Ascorbic acid was a common free radical scavenger.72 As a classical reducing agent, it was dehydrogenized by reactive oxygen species to form dehydroascorbic acid, which had strong reducibility. It could further inhibit the CL intensity by interacting with oxygen in the solution and form diketogulonic acid. 1.0 × 10-5 mol L-1 ascorbic acid was added to the CL system, and considerable inhibition of the CL intensity was observed (3 times, Table 1). It confirmed that the CL system was a radical reaction. ESR Spin-Trapping with 2,2,6,6-Tetramethyl-4-piperidine and 5,5-Dimethyl-1-pyrroline N-Oxide. Room-temperature electron spin resonance (ESR) spectroscopy was used to detect the free radical intermediates. 2,2,6,6-Tetramethyl-4-piperidine (TEMP) was a specific target molecule of 1O2. It could react with 1O2 to give the adduct 2,2,6,6-tetramethyl-4-piperidine-Noxide (TEMPO), which was a stable nitroxide radical with a characteristic spectrum.12 Figure 8a (black line) showed the specific signals of TEMPO, which supported the formation of 1 O2 in NaHCO3-H2O2 CL system. When TEMP was added to the mixture solution of CdTe QDs, NaHCO3 and H2O2, the signal intensity decreased (red line, Figure 8a). It reveals that singlet oxygen was consumed in this solution. Because the formation of CdTe* QDs came from the collision of radicals, such as · O2- and · OH radicals, it decreased the generation of 1 O2. The reaction rate was approximately that of TEMP. The detection of · OH by ESR spectroscopy was also performed in this work. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO), a specific target molecule of · OH,73 was used. Figure 8b (black line) presented the production of DMPO-OH adduct. The result confirmed the generation of · OH in the NaHCO3-H2O2 CL system. When DMPO was added into the mixture solution of 1.0 mol L-1 NaHCO3, 0.5 mol L-1 H2O2, and 1.2 × 10-6 mol L-1 CdTe QDs, the ESR signal disappeared at this optimized concentration (red line, Figure 8b). This phenomenon further proved that · OH existed and reacted with CdTe QDs as Reactions 15 and 18. On the basis of the above experimental results, · OH, · O2-, and 1O2 were all involved in NaHCO3-H2O2-CdTe QDs system. Spectra of CL Systems. To identify the emitting species in NaHCO3-H2O2-CdTe QDs system, the CL spectra were measured with a fluorescence spectrometer. Reaction reagents were

pumped through three separate lines into a flow cell (Figure 1b). The flow rates of the solutions were 4.0 mL min-1. The operation was described in the Spectra Measurements section. There were four peaks in the range of 350-700 nm in NaHCO3-H2O2 CL spectrum (Figure 9a). The peak at 434.6 nm corresponded to the decomposition of excited double (CO2)2*. The decomposition energy of the (CO2)2 dimer was calculated by the EHMO method and was found to be 132 kcal mol L-1, which was high enough to promote emission at a wavelength higher than 220 nm.74 The CL spectrum of 430-460 nm band was suggested as the result of this energy transportation.75,76 The peak at 580 nm resulted from the formation of a single oxygen molecular pair (O2)2*.43,77 The peak at 634 nm resulted from the emission of 1O2.46 They had higher energy and could lose excess energy by luminescence. When CdTe QDs solution were added to the NaHCO3-H2O2 flow system, the CL spectrum had peaks with maximums located at 381, 452.6, 556.6, 690.6, and 768.6 nm, respectively (Figure 9b). The peak at 381 nm was ascribed from the emission of 1 O2. The peak at 452.6 nm resulted from the reaction of · CO3radicals and was the same as that obtained from the reactions of NaHCO3-H2O2 solution.78 The CL intensity was significantly decreased after the addition of CdTe QDs. It was attributed to the transfer of energy from (CO2)2* to CdTe QDs. The peak at 556.6 nm was from the excited CdTe QDs. The CL spectrum (350-500 nm, Figure 9a) of NaHCO3-H2O2 system covered a broad range, which overlapped well with the absorption of CdTe QDs (λm ) 502 nm) used in this experiment. CdTe QDs could be excited by this CL system. To further ascertain the emission species, UV-vis absorption spectroscopy and fluorescence spectroscopy were also studied to characterize the change of CdTe QDs before and after the CL reaction, respectively. Detailed information about UV-vis spectra (Figure 9c) showed that when CdTe QDs were added to the HCO4- system, the absorbance of QDs (502 nm) disappeared and the absorbance of HCO4- (350 nm) significantly increased, which suggested the reaction of CdTe QDs. The fluorescence spectra showed significant differences between the one before and the one after reaction. Three bands (410 nm, 450 and 556 nm) appeared in the fluorescence spectra of reaction for 5 min (Figure 9d). Fluorescence spectrum displayed an emission band similar to the CL spectra with peak wavelength of 556 nm. The emissive species of the observed CL was rather possible for the excited state of CdTe QDs. They were generated in situ during the chemical reactions. Phenomenon of the characteristic fluorescence spectrum emission peak (531 nm) and UV-vis absorption peak (502 nm) of these CdTe



Figure 9. Research of NaHCO3-H2O2 system with and without CdTe QDs. Emission spectra of NaHCO3-H2O2 (a) and NaHCO3-H2O2-CdTe QDs (b) CL systems, absorption (c) and fluorescence spectra (d) of NaHCO3-H2O2-CdTe QDs system before and after reaction.

QDs disappeared after the CL reactions demonstrated that the CdTe QDs were oxidized completely after reaction. CdTe QDs CL Emission in NaHCO3-H2O2 System. On the basis of above results, the mechanism for the CL of CdTe QDs in NaHCO3-H2O2 solution was shown in Figure 10. When NaHCO3 was mixed with H2O2, HCO4- was yielded. It is a highly active species and easily decomposed, which generated · O2- and · OH radicals in solution. The products of the simultaneous processes decomposition and radical recombination were molecules of 1O2, (O2)2*, and (CO2)2*, which released the excess energy immediately and produced CL emission. The supports for this proposition were confirmed by the CL spectra and radical scavenger’s reactions. The lifetime of · O2- ions was approximately 1 min in high pH aqueous. They could easily donate one electron to the 1Se quantum-confined orbital of CdTe QDs to form electron injected CdTe (e-1Se) QDs. At the same time, · OH ions injected a hole in the 1Sh quantum-confined orbital of CdTe QDs to form hole injected CdTe (h+1Sh) QDs. An electron-transfer reaction between CdTe (e-1Se) and CdTe (h+1Sh) for direct electron-hole recombination produced the excited CdTe. Simultaneously, injection of holes into the CdTe (e-1Se) from · OH, and injection of electrons from · O2- into CdTe (h+1Sh) also existed to form CdTe*. The CL emission occurred when CdTe* returned to the ground-state. In the HCO3--H2O2-CdTe QDs system, reactions of · O2and · OH radicals with CdTe QDs played important roles. The formation of 1O2, (O2)2*, and (CO2)2* were competed with the generation of CdTe*. The CL intensity significantly depended on the rates of generation and annihilation of CdTe*. Conclusions This work elucidated the CL process of CdTe QDs in NaHCO3-H2O2 aqueous system. Four emitters such as 1O2,

Figure 10. Principle of chemiluminescence in NaHCO3-H2O2-CdTe QDs system.

(O2)2*, (CO2)2*, and CdTe* were formed in the CL system. A mechanism of the exciton CL is proposed. The CL evolved from the 1Se-1Sh transition emission of CdTe QDs and possessed

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size-dependent effect. It provided an alternative for traditional CL light emitters and gave the ideal to further improve the efficiency of nanocrystal CL and discovery of a suitable CL system constitution. Acknowledgment. This work was supported by National Natural Science Foundation of China (No. 20935002). Supporting Information Available: The synthesis method of CdTe QDs and reaction schemes of NBT, MCLA, and thiourea with · O2-, 1O2, or · OH. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Flanagan, J.; Jones, D. P.; Griffith, W. P.; Skapski, A. C.; West, A. P. J. Chem. Soc., Chem. Commun. 1986, 20–21. (2) Jones, D. P.; Griffith, W. P. J. Chem. Soc., Dalton Trans. 1980, 2526–2532. (3) Richardson, D. E.; Yao, H. R.; Frank, K. M.; Bennett, D. A. J. Am. Chem. Soc. 2000, 122, 1729–1739. (4) Yao, H. R.; Richardson, D. E. J. Am. Chem. Soc. 2003, 125, 6211– 6221. (5) Deon, A. B.; Yao, H. R.; Richardson, D. E. Inorg. Chem. 2001, 40, 2996–3001. (6) Ho, K. P.; Wong, K. Y.; Chan, T. H. Tetrahedron 2006, 62, 6650– 6658. (7) Adam, A.; Mehta, M. Angew. Chem., Int. Ed. 1998, 37, 1387– 1388. (8) Liu, M.; Zhao, L.; Lin, J.-M. J. Phys. Chem. A 2006, 110, 7509– 7514. (9) Liu, M.; Cheng, X.; Zhao, L.; Lin, J.-M. Luminescence 2006, 21, 179–185. (10) Liang, S.; Zhao, L.; Zhang, B.; Lin, J.-M. J. Phys. Chem. A 2008, 112, 618–623. (11) Lin, J.-M.; Liu, M. J. Phys. Chem. B 2008, 112, 7850–7855. (12) Chance, B.; Sies, H.; Boveris, A. Physiol. ReV. 1979, 59, 527– 605. (13) Trindade, D. F.; Cerchiaro, G.; Augusto, O. Chem. Res. Toxicol. 2006, 19, 1475–1482. (14) Perroy, J.; Pontier, S.; Charest, P. G.; Aubry, M.; Bouvier, M. Nat. Methods 2004, 1, 203–208. (15) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538–544. (16) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013–2016. (17) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016–2018. (18) Fernańdez-Arguëlles, M. T.; Yakovlev, A.; Sperling, R. A.; Luccardini, C.; Gaillard, S.; Medel, A. S.; Mallet, J. M.; Brochon, J. C.; Feltz, A.; Oheim, M.; Parak, W. J. Nano Lett. 2007, 7, 2613–2617. (19) Wang, H. Q.; Li, Y. Q.; Wang, J. H.; Xu, Q.; Li, X. Q.; Zhao, Y. D. Anal. Chim. Acta 2008, 610, 68–73. (20) Hansen, J. A.; Wang, J.; Kawde, A.; Xiang, Y.; Gothelf, K. V.; Collins, G. J. Am. Chem. Soc. 2006, 128, 2228–2229. (21) Rodriguez-Viejo, J.; Jensen, K. F.; Mattoussi, H.; Michel, J.; Dabbousi, B. O.; Bawendi, M. G. Appl. Phys. Lett. 1997, 70, 2132–2134. (22) Poznyak, S. K.; Talapin, D. V.; Shevchenko, E. V.; Weller, H. Nano Lett. 2004, 4, 693–698. (23) Medintz, I. L.; Trammell, S. A.; Mattoussi, H.; Mauro, J. M. J. Am. Chem. Soc. 2004, 126, 30–31. (24) Ding, Z.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Science 2002, 296, 1293–1297. (25) Myung, N.; Ding, Z.; Bard, A. J. Nano Lett. 2002, 2, 1315–1319. (26) Myung, N.; Bae, Y.; Bard, A. J. Nano Lett. 2003, 3, 1053–1055. (27) Chen, Y. F.; Rosenzweig, Z. Anal. Chem. 2002, 74, 5132–5138. (28) Jin, W. J.; Fernańdez-Arguëlles, M. T.; Costa-Fernańdez, J. M.; Pereiro, R.; Sanz-Medel, A. Chem. Commun. 2005, 883–885. (29) Talapin, D. V.; Haubold, S.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. J. Phys. Chem. B 2001, 105, 2260–2263. (30) Rogach, A. L.; Katsikas, L.; Kornowski, A.; Su, D.; Eychmu¨ller, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1772. (31) Rockenberger, J.; Tro¨ger, L.; Rogach, A. L.; Tischer, M.; Grundmann, M.; Eychmu¨ller, A.; Weller, H. J. Chem. Phys. 1998, 108, 7807– 7815. (32) Kapitonov, A. M.; Stupak, A. P.; Gaponenko, S. V.; Petrov, E. P.; Rogach, A. L.; Eychmu¨ller, A. J. Phys. Chem. B 1999, 103, 10109–10113.

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Chemiluminescence Arising from the Decomposition of

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