Improved Chemiluminescence in Fenton-Like Reaction via

Article pubs.acs.org/JPCC

Improved Chemiluminescence in Fenton-Like Reaction via Dodecylbenzene-Sulfonate-Intercalated Layered Double Hydroxides Lijuan Zhang,† Zhiming Zhang,† Chao Lu,*,† and Jin-Ming Lin‡ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of Chemistry, Tsinghua University, Beijing 100084, China

‡

S Supporting Information *

ABSTRACT: The interlamellar dodecylbenzene sulfonate (DBS) in layered double hydroxides (LDHs) can amplify an ultraweak chemiluminescence (CL) from Fenton-like reaction (Co(II) + H2O2 + OH−). The CL enhancement mechanism of the intercalated DBS on the H2O2−Co(II) CL was investigated by CL spectrum, radical scavengers, powder X-ray diffraction measurements, scanning electron microscope images, transmission electron microscopy images, Fourier transform infrared spectroscopy, electronic spinning resonance measurements, and elemental analysis techniques. The hydrophobic microenvironment of the intercalated DBS in LDHs facilitated the formation of hydroxyl radical (•OH) by the reaction of Co(II) with H2O2 in alkaline medium. The abundant •OH radical can easily react with the intercalated DBS in LDHs to form sulfite radical (•SO3−). The recombination of •SO3− radical could generate an intermediate as the excited sulfur dioxide molecules (SO2*), which generated strong CL emission centered at ∼430 nm. Our experiments illustrated a powerful use of surfactant-intercalated LDHs in CL studies and could have potential applications in the quantitative determination of radical production. excited singlet oxygen dimol species, (O2)2*, and a lightamplifying substance, such as luminol, amino acid, DNA, and gold nanoparticles, was added to improve the emission intensity.8−12 However, the introduced CL enhancers increased the difficulties in confirming the reaction intermediates of Fenton-like processes. In recent years, microheterogeneous systems can solubilize, concentrate, and organize reactants as well as alter microenvironments of solubilized species.13 Therefore, they have been used frequently as CL media to amplify the quantum yield or CL intensity for various systems (e.g., luminol and lucigenin systems).13−15 However, there has not been any reported case involving microheterogeneous CL from Fenton-like reactions. Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, are a class of synthetic anionic layered clays with a lamellar structure and an exceptionally high anion exchange capacity. The structure of LDHs is based on positively charged metal hydroxide layers, which require the presence of interlayer anions to maintain overall charge neutrality.16 The intrinsic hydrophilic surface property of LDHs can be modified

1. INTRODUCTION Fenton reagent is a mixture of ferrous ion and hydrogen peroxide (H2O2). The generally accepted mechanism for Fenton process identifies hydroxyl radical (•OH) as the active oxidizing intermediate in the system. These reactions usually take place under acidic conditions due to the solubility limitations resulting in Fe (III) hydrous oxyhydroxides (Fe2O3•nH2O) at basic pH, which can significantly reduce the production of •OH radical from H2O2.1−3 Recently, modified Fenton processes have gained considerable attention, where chelating agents, such as EDTA, various amino acids, and porphyrin structures have been widely used to allow the production of oxidizing species to occur at basic pH levels while restricting the production of insoluble iron compounds.4 Moreover, others transition-metal-mediated Fenton-like reactions (e.g., Co(II)/H2O2 and Cu(II)/H2O2) can produce •OH radical.5 However, the exact mechanisms of Fenton-like reactions are extremely complex, and there is no conclusive answer for or against the involvement of •OH radical.6 Chemiluminescence (CL) is a highly sensitive, nondestructive, and continuous monitoring technique for oxygen radical generation.7 There have been some reports lately that an ultraweak CL has been observed by homogeneous Fenton-like processes in basic solutions as a result of the production of the © 2012 American Chemical Society

Received: April 30, 2012 Revised: June 9, 2012 Published: June 12, 2012 14711

dx.doi.org/10.1021/jp304147x | J. Phys. Chem. C 2012, 116, 14711−14716

The Journal of Physical Chemistry C

Article

atmosphere to exclude the aqueous CO2. The resulting slurry was centrifuged and washed with degassed and deionized water several times and dried in vacuo at 60 °C for 24 h. 2.3. Synthesis of Mg−Al-DBS LDHs. Mg−Al-DBS LDHs were prepared from a typical anion exchange procedure. The Mg−Al−NO3 LDH powder (1.0 g) was contacted with a 0.2 M DBS aqueous solution (25 mL) prepared by degassed and deionized water. After vigorous stirring at 80 °C for 24 h in a N2 atmosphere, the prepared Mg−Al-DBS LDHs suspension was stored at 4 °C for further use. 2.4. Characterization. The powder XRD measurements were recorded at a scanning rate of 10°/min from 2θ = 2 to 70° with a Bruker (Germany) D8 ADVANCE X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.5406 Ǻ ). The SEM macrographs were taken using S-4700 fieldemission scanning electron microscope (Hatachi, Japan). The particle sizes and external morphology of the samples were observed on a TEM (Tecnai G220, FEI Company). FTIR spectra were collected with a Nicolet 6700 FT-IR spectrometer (Thermo). ESR measurements were performed on a JEOL JESFA200 apparatus, using DMPO as a spin trap for radicals. The CL spectrum was obtained using a F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan) at a slit of 10 nm and at a scanning rate of 1200 nm/min. Elemental analyses were performed on an Vario EL cube elemental analysis in Germany. The CL detection was conducted on a BPCL luminescence analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China). 2.5. CL Measurements. We injected 200 μL of 0.1 mM Co(NO3)2 solution into the mixed solution containing 200 μL of Mg−Al-DBS LDHs suspension and 100 μL of 10 mM H2O2. The CL signals were monitored by a photomultiplier tube (PMT) adjacent to the CL quartz cell (inset of Figure 1). The data

through exchanging the interlayer anions with anionic surfactants, such as dodecylsulfate (DS) and dodecylbenzene sulfonate (DBS).17 The intercalation of anionic surfactants not only enlarges the gallery spacing of LDHs but also improves the hydrophobic nature of LDHs. In this sense, the intercalation of anionic surfactants in LDHs becomes a very interesting field of research and has potential applications as functional adsorbents for hydrophobic organic compounds.18−20 Our recent work demonstrated the adsorbed DBS on the external surface of DBS-modified LDHs can generate the higher CL intensity from IO4−−H2O2 system than DBS micelles, due to the concentrating of anionic reactants including IO4− and HO2− on the adsorbed surfactant layers of DBS-modified LDHs.21 However, the CL enhancement effect observed was not obvious enough (ca. 25-fold) because the intercalated DBS in LDHs cannot play any role in the CL enhancement. In this study, we investigated the effect of the DBS-intercalated LDHs on the Fenton-like CL reaction (Co(II) + H2O2 + OH−). Interestingly, it was found that the intercalated DBS in LDHs could significantly enhance an ultraweak CL from the H2O2− Co(II)−OH− system, whereas the DBS micelles do not amplify weak CL signals. The great enhancement mechanism of the intercalated DBS on the present CL system was investigated by CL spectrum, radical scavengers, powder X-ray diffraction (XRD) measurements, scanning electron microscope (SEM) images, transmission electron microscopy (TEM) images, Fourier transform infrared (FTIR) spectroscopy, electronic spinning resonance (ESR) measurements, and elemental analysis techniques. Our experiments are valuable for understanding the emitting species from Fenton-like processes. We may foresee that surfactant-intercalated LDHs would become a powerful tool in ultraweak CL studies.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Solutions. All chemicals were of analytical grade without further purification. Co(NO3)2·6H2O, Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Na2CO3, and NaOH were purchased from Beijing Chemical Reagent Company (Beijing, China). Thiourea and NaN3 were purchased from Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). Ascorbic acid was purchased from Beijing Aoboxing Biotech (Beijing, China). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was purchased from Tokyo Kasei Kogyo (Tokyo, Japan). Working solutions of H2O2 were prepared daily from 30% (v/v) H2O2 (Beijing Chemical Reagent Company, China). A 0.1 M Co(NO3)2 stock solution was prepared by dissolving Co(NO3)2 with deionized water (18.2 MΩ·cm, Milli Q, Millipore, Barnstead, CA). The working solutions of Co(NO3)2 were freshly prepared by diluting the stock solution with deionized water. DBS was purchased from Tokyo Chemical Industry (Tokyo, Japan). 2.2. Synthesis of Mg−Al−NO3 LDHs. The coprecipitation method was carried out to synthesize Mg−Al−NO3 LDHs. The precipitation process was taken under low supersaturation conditions at constant pH. The salt solution was prepared by dissolving Mg(NO3)2·6H2O (0.06 mol) and Al(NO3)3·9H2O (0.02 mol) in 80 mL of degassed and deionized water in which the ratio of Mg/Al was 3. The NaOH solution was prepared by dissolving NaOH (0.16 mol) in 80 mL of degassed and deionized water. The two solutions were added dropwise to a 250 mL fournecked flask under vigorous stirring maintaining pH 10 at room temperature. The resulting white precipitate was aged for 24 h at 65 °C. The whole procedure was performed under a nitrogen

Figure 1. CL intensity of H2O2−Co(II) CL system mixed with different solutions including NaOH, 0.2 M DBS, NO3-LDH, and DBS-LDH (inset: schematic diagram of the bath method CL detection system). Note that the pH of each solution was 9.5.

integration time of the BPCL analyzer was set at 0.1 s per spectrum, and a work voltage of −1000 V was used for the CL detection. The signal was imported to the computer for data acquisition.

3. RESULTS AND DISCUSSION 3.1. H2O2−Co(II) CL in Different Media. As shown in Figure 1, at pH 9.5 (the same pH as that for the LDH 14712



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Figure 2. (a) CL intensity of H2O2−Co(II) CL system mixed with DBS micellar solution, the as-prepared DBS-LDHs, the DBS-LDHs in the supernate, and the redispersion DBS-LDHs, respectively. (b) TEM image of the supernate solution. (c) TEM image of the redispersion DBS-LDHs. (d) SEM image of the redispersion DBS-LDHs.

Figure 3. (a) Effect of loading amounts of DBS in LDHs on the H2O2−Co(II) CL. (b) XRD patterns of DBS-intercalated LDHs with different loading amounts (inset: elemental analysis data of DBS-intercalated LDHs with different loading amounts).

Co(II) CL system was examined. It can be seen that the CL signal from the H2O2−Co(II) CL system can be dramatically enhanced in the presence of DBS-intercalated LDHs. These results demonstrated that DBS-intercalated LDHs took effect in the CL enhancement. Furthermore, we investigated the effect of other five metal ions (i.e., Fe (II), Cu(II), Ni(II), Mn(II), and Cr(III)) on the CL intensity of DBS-intercalated LDHscatalyzed Fenton-like processes. As shown Figure S1 in Supporting Information, Co(II) exhibited the strongest CL intensity of DBS-intercalated LDHs-catalyzed Fenton-like reactions. Therefore, Co(II) was used for a model metal ion in further experiments. 3.2. Interlamellar DBS in LDHs-Enhanced H2O2−Co(II) CL. Our previous papers found that both adsorbed and interlamellar anions in LDHs could catalyze and enhance the

preparation), weak CL was emitted during the decomposition of H2O2 in the presence of Co(II); however, the CL signals cannot be observed by a BPCL ultraweak luminescence analyzer.8 When DBS micelles at pH 9.5 were added to the H2O2−Co(II) system, there appeared to be a very slight increase in CL emission due to microenvironmental properties of DBS micelles. Note that the pH of DBS micellar solution decreased rapidly, and thus it is impossible to enhance the H2O2−Co(II) CL by using DBS micellar solution.22 Next, we investigated the effect of NO3intercalated LDHs on the H2O2−Co(II) CL system, and the results showed that a stronger CL enhancement was observed in the presence of NO3-intercalated LDHs, which may be ascribed to the strong buffering capacity of the LDHs,23,24 resulting in the production of the excited singlet oxygen dimol species.8 Finally, the catalytic effect of DBS-intercalated LDHs on the H2O2− 14713



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CL signals greatly.21,25 In this study, we need to verify whether it is interlamellar DBS in LDHs that is responsible for signal amplification. DBS-intercalated LDHs were centrifuged at 10 000 rpm for 5 min, and the sediment was redispersed in distilled−deionized water. We compared the emission intensity of three systems: redispersed LDHs, as-prepared DBS-LDHs, and DBS-LDHs in the supernate. As shown in Figure 2a, the CL intensity of DBS-intercalated LDHs-enhanced H2O2−Co(II) system was decreased by ∼60% after centrifugation (i.e., redispersion LDHs); however, the CL intensity of the supernate solutions was stronger than that of the uncentrifuged DBSintercalated LDHs. As shown in Figure 2b, DBS-intercalated LDHs in the supernate solutions exhibited a small crystallite size with a uniform diameter (3 ± 0.6 nm). In contrast, the crystallite size of the redispersion DBS-intercalated LDHs was nonuniform, as observed from the TEM and SEM images (Figure 2c,d). Note that the amount of DBS-LDHs in the supernate was much lower that of redispersed LDHs and as-prepared DBS-LDHs, and the amount of as-prepared DBS-LDHs was largest. It was found that the CL intensity increased steadily with increasing the amount of LDHs in three systems. Therefore, it was inferred that the amount of DBS-LDHs in the supernate may not influence the emission significantly, and the increased CL intensity in the supernate may be due to uniform crystallite sizes.25,26 Furthermore, the physisorption of DBS on the surface of the centrifuged DBS-intercalated LDHs for 30 min cannot increase the CL intensity (Figure S2 of the Supporting Information). These phenomena indicated that the CL enhancement of DBSintercalated LDHs on H2O2−Co(II) system was ascribed to the intercalated DBS other than the adsorbed DBS on the surface of DBS-modified LDHs. 3.3. Loading Amounts of DBS in LDHs. In this study, the effect of loading amounts of DBS (0.05 to 0.25 M) in LDHs on the H2O2−Co(II) CL was examined (Figure 3a). The results showed that the strongest CL intensity was obtained at a loading amount of 0.2 M DBS. However, the loading amounts of DBS higher than 0.2 M led to a decrease in CL intensity. This phenomenon is probably attributed to the abundant physisorbed DBS, inhibiting the concentrating of anionic CL reactants on the surface of the LDHs.21 With the purpose of ascertaining the effect of loading amounts of DBS on the CL intensity, the following experiments were carried out. The representative XRD patterns of DBS-intercalated LDHs with different loading amounts are displayed in Figure 3b. An enlargement of the interlayer distance was achieved as the loading amounts increased, indicating that the intercalated surfactant chains can transform from lying parallel to the surface of Mg−Al LDH layers at low loading amounts to lying perpendicular to the LDH interlamellar surface at high loading amounts (>0.15 M DBS).27 Moreover, along with 0.48 nm LDH brucite layer, the interlayer gallery height of DBS-intercalated LDHs at high loading amounts was 2.47, which is close to the theoretical calculated size of DBS of 2.42 nm.28,29 The presence of DBS was further supported by FTIR spectra (Figure S3 of the Supporting Information). These results indicated the possible arrangements for the monolayer DBS molecules, orienting in the perpendicular direction from the LDH surface. Elemental analysis data (inset of Figure 3b) showed that the concentrations of interlamellar DBS increased with increasing the loading amounts of DBS from 0.05 to 0.2 M. Note that 0.13 M DBS is theoretically interlamellar amount for DBS-intercalated LDHs preparation.29 However, the interlamellar DBS from elemental analysis was greater than the theoretical interlamellar amount

when the loading amount of DBS was 0.25 M, which was ascribed to higher physisorption amount in this case. From the above XRD and elemental analysis data, we can explain the effect of loading amounts of DBS in LDHs on the H2O2−Co(II) CL. In this study, the intercalated DBS resulted in the CL enhancement on H2O2−Co(II) system, and thus the CL intensity increased with increasing the intercalated DBS from 0.05 to 0.2 M. However, the higher physisorption at 0.25 M DBS loading amount may inhibit the concentrating of anionic reactants on the surface of the LDHs, and thus decreased the CL intensity. These results were in good agreement with the previous studies.25,30 3.4. Emitting Species. Some studies revealed that the homogeneous Fenton-like processes in basic solutions can emit an ultraweak CL as a result of the generation of the excited singlet oxygen dimol species, (O2)2*, corresponding to an emission band in the range of 490−550 nm.8,9 The CL spectrum of the DBS-intercalated LDHs-enhanced H2O2−Co(II) system was measured using an FL-7000 model spectrofluorimeter combined with a flow-injection system. As shown in Figure 4, the maximum

Figure 4. CL intensity of H2O2−Co(II) system resulted from the cointercalation of DBS with different carbonate contents (inset: CL spectrum of the DBS-intercalated LDHs-enhanced H2O2−Co(II) system).

emission for the present CL system was ∼430 nm, indicating that the emitting species may not ascribe to the excited singlet oxygen dimol species. Moreover, the emitting species was also confirmed by adding some radical scavengers into the present CL system (Figure S4 of the Supporting Information). The results showed that 3.0 mM NaN3, a well-known scavenger of singlet oxygen, did not quench the CL intensity, which also provided strong evidence that singlet oxygen did not contribute to the observed CL.31 However, the CL signal was greatly quenched by 0.1 mM ascorbic acid or 4.0 mM thiourea (scavengers of •OH radical), meaning the existence of •OH radical (Figure S4 of the Supporting Information).31 It was reported that the excited carbon dioxide dimer ((CO2)2*) and the excited sulfur dioxide molecules (SO2*) can generate light centered at approximately 430 nm.32−35 Identification of the emitting species responsible for the emission observed in the CL spectrum was achieved in the following experiments. First, carbonate anions in the range from 0 to 2.5 mmol were cointercalated with DBS. XRD data (Figure S5 of the Supporting Information) showed that the intercalated DBS in LDHs decreased as carbonate content increased. However, the CL intensity of the present system was significantly decreased when extremely low carbonate (0.5 mmol) displaced the intercalated DBS (Figure 4). Note that 7.5 mmol carbonate is theoretically intercalated for LDH preparation.25 Moreover, 14714



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room-temperature ESR spectroscopy was used to prove the existence of reaction intermediates (Figure 5). Herein, as a

S2 O6 2 − → SO4 2 − + SO2 *

(5)

SO2 * → SO2 + hv (λ = 430 nm)

(6)

4. CONCLUSIONS In summary, we have developed a novel CL pathway of Fentonlike reactions with the participation of the interlamellar DBS in LDHs. The hydrophobic microenvironment of DBS intercalated LDHs and improved the shortcomings of Fenton-like CL reaction, such as low emission efficiency and unstable radical species, facilitating the output of SO2* intermediates. This work is of great importance for understanding the emission origin of Fenton-like reactions. It also offers new insight into the wellknown CL enhancers of Fenton-like reaction. We may predict the potential applications of DBS-intercalated LDHs as biological luminescent labels.

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Figure 5. ESR spectra of hydroxyl radical from the DBS-intercalated LDHs-enhanced H2O2−Co(II) system. Experimental procedure: 10 μL of 0.01 M H2O2 + 20 μL of 0.2 M DMPO + 20 μL of 0.1 mM Co(II) + 20 μL of DBS-LDHs).

S Supporting Information *

FT-IR spectra of Mg−Al−NO3 LDHs and Mg−Al−DBS LDHs., effect of scavengers of radicals on the CL intensity, XRD data of DBS cointercalation with different carbonate contents, and ESR spectra of hydroxyl radicals from the DBS-intercalated LDHsenhanced H2O2−Co(II) system. This material is available free of charge via the Internet at http://pubs.acs.org.

specific target molecule of •OH radical, DMPO identified the formation of •OH radical during the CL reaction.34 These results demonstrated that sulfur dioxide molecules may be responsible for the observed emission centered at 430 nm. 3.4. Mechanism Discussion. On the basis of the above data, the mechanism of the present CL system was summarized in Scheme 1. H2O2 is a weak acid and dissociates to give the hydroperoxyl anion (HO2−) in alkaline environment (Reaction 1),36 which can easily concentrate on the surface of DBSintercalated LDHs.21 The hydrophobic microenvironment of the intercalated DBS in LDHs facilitated the output of •OH radical, which was obtained by the reaction of Co(II) with H2O2 (Reaction 2).37 The abundant •OH radical can easily react with the intercalated DBS in LDHs to form •SO3− radical (Reaction 3).38−40 H 2O2 + HO− ⇌ HO2− + H 2O

(1)

H 2O2 + Co(II) → HO • + Co(III) + OH‐

(2)

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AUTHOR INFORMATION

Corresponding Author

*Fax/Tel: +86 10 64411957. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21077008 and 20975010), the Program for New Century Excellent Talents in University (NCET-110561), and the Fundamental Research Funds for the Central Universities (ZZ1230). We also thank Prof. Xue Duan, Beijing University of Chemical Technology for his valuable discussions.

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REFERENCES

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The recombination of •SO3− radical could generate an intermediate as SO2*, which generated the CL emission centered at 430 nm when it returned to its ground state.34,35 These results fitted with the CL spectrum shown in Figure 4, which has been reported by many investigations (Reactions 4−6).31,34,35 •SO3− + •SO3− → S2 O6 2 − + H 2O

ASSOCIATED CONTENT

(4)

Scheme 1. Possible CL Mechanism for the DBS-Intercalated LDHs-Enhanced H2O2−Co(II) System

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Improved Chemiluminescence in Fenton-Like Reaction via

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