Organo-Modified Montmorillonite Enhanced Chemiluminescence via

Jan 17, 2014 - In this work, it was found that cationic surfactant cetyltrimethylammonium bromide (CTAB)-modified montomorillonite (MMT) can significa...
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Organo-Modified Montmorillonite Enhanced Chemiluminescence via Inactivation of Halide Counterions in a Micellar Solution Shuang Chen,† Wenjuan Zhou,† Yuqing Cao, Congcong Xue, and Chao Lu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: In this work, it was found that cationic surfactant cetyltrimethylammonium bromide (CTAB)-modified montomorillonite (MMT) can significantly amplify chemiluminescence (CL) from the peroxynitrite system. Transmission electron microscopy (TEM), scanning electron microscope (SEM), powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ζ potential measurements and differential thermogravimetry (DTG) analyses confirmed the geometrical configuration of CTAB at the surface of MMT. In addition, the enhancement mechanism of CTAB-modified MMT on the peroxynitrite CL was discussed by the CL spectrum, the scavengers of reactive oxygen species, and various controlled experiments. The results indicated that the electrostatic attraction between CTA+ cations and MMT could form a CTA+ monolayer at the surface of MMT, resulting in the effective removal of halide counterions in the CTAB micellar solution. Therefore, the inactivation of the halide counterion-quenched CL could generate the intense CL emissions. The success of this work has explored a novel route to the applications of clay nanocomposites in CL field. density and interlayer cation mobility.8−11 Therefore, MMT has been applied extensively in catalysts, ion exchangers, and adsorbents.12−15 The introduction of cationic surfactants into MMT (also called organo-modified MMT) leads to the formation of alternating inorganic−organic assemblies with unique nanocomposite structure, expanding the interlayer distance and rendering the surface hydrophobicity.16−18 Recently, organo-modified MMT has been a subject of considerable interest with extensive application as catalysts, adsorbents, and optical and electrical devices.19−21 Undoubtedly, halide counterions attached to the hydrophilic headgroup of cationic surfactants can be easily separated from organomodified MMT. It is expected that this interesting phenomenon can break the bottleneck of halide counterion-quenched CL signals. However, there have been no reports on the CL amplification induced by organo-modified MMT. In this contribution, we used commercially available Ca2+MMT and cationic surfactant cetyltrimethylammonium bromide (CTAB) to prepare the CTAB-modified MMT by the ion exchange method.22,23 The peroxynitrite CL was selected as a model system to investigate the CL effect of the CTABmodified MMT in a static injection CL setup (Figure S1, Supporting Information). Inspiringly, we observed the strong CL emission of the peroxynitrite in the presence of CTAB-

1. INTRODUCTION In comparison to anionic surfactant-enhanced chemiluminescence (CL), cationic surfactant-amplified CL is a common technique for a variety of assays. The fascinating CL properties from micellar microenvironments are attributed to their many unique and advantageous features as follows: solubilize, concentrate, and organize reactants; alter electronic microenvironments; alter chemical and photophysical pathways and rates; and facilitate energy transfer.1−4 Unfortunately, the inert bromide or chloride counterions in a micellar solution can compete with or displace reactive counterions (e.g., hydroxide ion) from the micellar surface, leading to a decrease in reaction rate. In addition, as a kind of scavenger for reactive oxygen species, halide ions in a cationic micellar solution can quench hydroxyl radicals.5−7 These negative effects of halide ions can quench the CL emission. Therefore, it seems to be significant to effectively eliminate halide counterions in a micellar solution. However, to the best of our knowledge, no attention has been focused on the removal of halide counterions in cationic surfactant-enhanced CL reactions. Such a knowledge gap may impede further development of cationic surfactant-enhanced CL emission. Montmorillonite (MMT) is a large family of natural host− guest layered materials consisting of negatively charged sheets (one aluminum octahedral layer between two silicon tetrahedral layers) that are compensated by interlayer cations like Ca2+. MMT exhibits a well-defined layered structure with relatively large surface area, high porosity, high layer charge © 2014 American Chemical Society

Received: November 17, 2013 Revised: January 6, 2014 Published: January 17, 2014 2851

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deionized water. The pH was adjusted with aqueous NaOH solution. 2.2. Synthesis of CTAB-Modified MMT. CTAB-modified MMT was prepared by simple ion exchange method. Typically, a 1.0 g portion of Ca2+-MMT was mixed with 100 mL of deionized water. Then, 0.3645 g of CTAB was added in the MMT solution. The ion exchange was carried out under stirring for 1 h at 60 °C. The delamination occurred after ion exchange process and obtained 10 mL CTAB-modified MMT in the lower layer of the solution. 2.3. Apparatus. ζ potential data were measured at 25.0 ± 0.1 °C by a Zetasizer DTS Nano (Malvern Intrument). DTG was analyzed on a TGA/DSC 1/1100 SF (Mettler, Toledo, OH, USA) in N2 at a heating rate of 10 °C/min. XRD patterns were carried out on a Bruker D8 Advance (Bruker AXS GmbH) X-ray diffractometer under Cu/Kα radiation (λ = 1.5406 Ǻ ). The 2θ angle of the diffractometer was stepped from 2° to 10° at a scan rate of 10°/min. TEM image was obtained with a Tecnai G220 TEM (FEI Co., Netherlands) at an accelerating voltage of 200 kV. A S-4700 field-emission scanning electron microscope was used for obtaining the SEM images. FT-IR spectra were collected with a Nicolet 6700 FT-IR spectrometer (Thermo, America). Fluorescence spectra were performed using a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan). The CL detection was conducted on a biophysics chemiluminescence (BPCL) luminescence analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China). The CL spectrum of this system was measured with high-energy cutoff filters from 490 to 640 nm between the flow CL cell and the photomultiplier tube (PMT). The centrifugation was operated on a TGL-16B centrifugal machine (Shanghai Anting Scientific Instrument Factory, Shanghai, China). 2.4. CL Measurements. For the preparation of 0.1 mM peroxynitrous acid (ONOOH) solution, a mixture solution of 0.05 M H2O2 (0.02 M HCl) and 0.01 M NaNO2 was freshly prepared. Then, 200 μL of CTAB-modified MMT with a certain amount of NaOH was mixed with the ONOOH solution by the injection of 100 μL of 0.1 mM ONOOH solution. The CL signals were monitored by a PMT adjacent to the CL quartz vial. The data 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 CL signals were imported to the computer for data acquisition.

modified MMT. A series of characterization methods, including transmission electron microscopy (TEM) image, scanning electron microscope (SEM), powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ζ potential measurements, and differential thermogravimetry (DTG) analyses, were used to confirm the geometrical configuration of CTAB at the surface of MMT. Moreover, the enhancement mechanism of the CTAB-modified MMT on the peroxynitrite CL was demonstrated by the CL spectrum, the scavengers of reactive oxygen species, and various controlled experiments. The origin of CL enhancement was ascribed to the successful separation of bromide counterions from the hydrophilic headgroup of CTAB, making the occurrence of the peroxynitrite CL more effective in CTAB-modified MMT microenvironments without the negative effects of bromide counterions (Figure 1). The success of this work has broken the bottleneck of halide counterion-quenched CL, and developed a novel route to the applications of clay nanocomposites in CL field.

Figure 1. Possible mechanism of the peroxynitrite CL enhancement induced by CTAB-modified MMT.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Solutions. All the reagents were of analytical grade, and all solutions were prepared with deionized water (18.2 MU cm, Milli Q, Millipore, Barnstead, CA, USA). CTAB, cetyltrimethylammonium hydroxide (CH3(CH2)15N(OH)(CH3)3, CTAOH) and sodium bromide (NaBr) were purchased from Beijing Chemical Reagent Company (Beijing, China) without further purification. Natural Ca2+-MMT clay with 91 mmol/100 g of cation exchange capacity (CEC) (the raw clay has a basal (001) spacing of 15.4 Å) was obtained from Inner Mongolia. DABCO (1,4-diazabicyclo[2.2.2]octane), ascorbic acid (AA), and sodium azide (NaN3) were purchased from Beijing Chemical Reagent Company (Beijing, China). Thiourea was purchased from Tianjin Fuchen Chemical Reagents Factory. Nitro blue tetrazolium chloride (NBT) was purchased from Nacalai Tesque Inc. (Tokyo, Japan). Uranine was purchased from Acros. A 0.1 M nitrite stock solution was prepared by dissolving NaNO2 (Tianjin Chemical Reagent Company, Tianjin, China) in deionized water. Working solutions of nitrite were freshly prepared by diluting the nitrite stock solution with deionized water. A mixed working solution of 0.05 M H2O2 and 0.02 M HCl was freshly prepared by volumetric dilution of commercial 30% (v/v) H2O2 (Beijing Chemical Reagent Company, Beijing, China) and 36% (v/v) HCl (Beijing Chemical Reagent Company, Beijing, China) with

3. RESULTS AND DISCUSSION 3.1. Characterization of CTAB-Modified MMT. The CTAB-modified MMT was synthesized via ion exchange method. The structural characterization of the CTAB-modified MMT was confirmed by TEM, SEM, and XRD. The TEM images of natural MMT and CTAB-modified MMT (Figure 2A,B) exhibited a structure composed of stacked layers. In comparison to SEM of MMT (Figure 2C), the morphologies of the CTAB-modified MMT were similar to those of MMT except that the particles seemed to be swollen (Figure 2D).24,25 In addition, the XRD patterns of MMT and CTAB-modified MMT were depicted. Generally, the degree change of silicate layers dispersion within the range 2−10° indicated the formation of an ordered intercalated system with alternating cationic layers.26 As shown in Figure 2E, the degree of silicate layers of Ca2+-MMT was 6° with the 15.0 Å interlayer spacing value (d001). However, the interlayer distance of the CTABmodified MMT increased to 21.2 from 15.0 Å, and the degree 2852

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Figure 3. CL intensity of the peroxynitrite in the presence of MMT, 0.1 M CTAB, and CTAB-modified MMT, respectively.

surfaces of MMT particles can be exchanged by CTA+ ions. Which CTAB molecules, adsorbed outside the MMT particles or in the MMT interlayers, are responsible for the remarkable CL amplification from the CTAB-modified MMT? Herein, the CTAB molecules at the surface of the MMT particles were completely removed after several centrifuging-washing processes. The FTIR spectrum of the CTAB-modified MMT in Figure S6 (Supporting Information) confirmed the presence of CTAB molecules at the surface of the MMT particles. The C− H bending vibration band at 1464 cm−1 and the bands at 2918 and 2849 cm−1 corresponded to CH2 symmetric and asymmetric stretching vibrations of the alkyl chains, respectively.30−32 Figure 4A showed that the CTAB-modified MMT after centrifugation cannot increase the CL intensity of the peroxynitrite. After the washed CTAB-modified MMT was dispersed in aqueous solution of 0.01 M CTAB for 30 min, the peroxynitrite CL could be restored to the same value as that of the original CTAB-modified MMT. These significant findings demonstrated that CTAB molecules at the surface of the MMT particles rather than in the MMT interlayers contributed to the enhanced CL. 3.3. Geometrical Configuration of CTAB at the Surface of MMT. The specific adsorption of surfactants at the particle surface can have a dramatic effect on the ζ potential of the particle dispersion.33,34 Herein, ζ potential measurements were used to investigate the geometrical configuration of CTAB at the surface of MMT particles. Figure S7 (Supporting Information) showed that the ζ potential of Ca2+-MMT was −22.6 mV due to the negative charge of the MMT surface, whereas the CTAB-modified MMT increased close to zero, which was ascribed to the absorption of CTAB on the MMT surface could neutralize the electronegativity of MMT. These results indicated that a CTA+ monolayer was formed at the MMT surface by electrostatic attraction. Interestingly, the ζ potential of the washed CTAB-modified MMT decreased to −10.2 mV, meaning the occurrence of partial desorption of the CTA+ monolayer, which was ascribed to a decrease in the CL intensity. The self-assembly of the CTA+ monolayer at the surface of MMT was further verified by thermogravimetric analysis (TGA) analysis (Figure 4B). Four weight loss steps appeared in the DTG curve of the CTAB-modified MMT. The first weight loss at 100 °C was obtained from the water molecules at the surface of the MMT particles and in the MMT interlayers,

Figure 2. (A) TEM image of MMT, (B) TEM image of CTABmodified MMT, (C) SEM image of MMT, (D) SEM image of CTABmodified MMT, and (E) powder XRD patterns of MMT and CTABmodified MMT.

of silicate layers decreased to 4°, meaning the successful intercalation of CTA+ ions into the interlayers of MMT.27,28 3.2. CTAB-Modified MMT-Amplified Peroxynitrite CL. A series of experiments were optimized to obtain the strongest CL intensity of the peroxynitrite. First, the concentration of CTAB was studied (Figure S2, Supporting Information). The results showed that the CL intensity was strongest when the dosage of CTAB was one time of the cation exchange capacity (CEC, i.e., the concentration of CTAB was about 0.01 M). Note that the XRD data of the CTAB-modified MMT indicated that one CEC can result in well-formed crystallites with a narrow range of crytallite sizes, facilitating the occurrence of the CL.29 Next, the effects of reaction temperature and reaction time for the synthesis of the CTAB-modified MMT, as well as the pH for the CL reaction were investigated (Figures S3−S5, Supporting Information). Under the optimum experimental conditions, we investigated the influence of CTAB-modified MMT on the peroxynitrite CL in a static injection CL setup. As shown in Figure 3, the CTAB-modified MMT can induce a significant increase in the CL intensity of the peroxynitrite. However, a slight CL enhancement was observed in aqueous micellar solution of CTAB. In contrast, a weak CL enhancement was observed in the presence of MMT, possibly due to the heterogeneous characteristics of MMT nanomaterials.29 These results indicated that the geometrical configuration of CTAB in the organo-modified MMT colloidal solution played an important role in enhancing the peroxynitrite CL. During the production of the CTAB-modified MMT, the inorganic cations in the interlayer spaces and on the external 2853

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no weight loss between 200 and 300 °C, indicating the disappearance of the CTA+ monolayer. These findings were in accordance with those obtained by ζ potential measurements (please see Figure S7, Supporting Information). 3.4. Inactivation of Halide Counterions-Induced CL Enhancement. The decomposition of peroxynitrite to form singlet oxygen (1O2) has been extensively investigated.37,38 In the present system, except that the inherent properties of cationic micelles induced a remarkable enhancement in the CL emissions,1−4 Both the removal of the inert bromide counterions in the CTAB-modified MMT and the generation rate of reactive oxygen species could contribute to the increase of the peroxynitrite CL observed. At first, the CL kinetic in the presence of MMT, CTAB, and CTAB-modified MMT were studied. As shown in Figure 5A, the CL intensity of the peroxynitrite system in the presence of CTAB-modified MMT was about 20 times that in the presence of MMT and CTAB; however, the shape of the kinetics curve was almost same (maximum at 4.2 s). These results demonstrated that the rate of the generation of reactive oxygen species had little effect on the observed CL enhancement. On the other hand, the fluorescence intensity of uranine was greatly decreased in the presence of CTAB-modified MMT in comparison to CTAB and MMT (Figure 5B), indicating the formation of the largest amount of hydroxyl radicals (•OH) in the presence of CTABmodified MMT.39 More interestingly, the addition of 0.01 M NaBr into the CTAB-modified MMT could decrease the CL intensity of the peroxynitrite. In contrast, the CL intensity of the peroxynitrite in aqueous micellar solution of 0.01 M CTAB micelles was weaker than that in 0.01 M cetyltrimethylammonium hydroxide (CTAOH) micelles (Figure 6). These results demonstrated that the removal of the inert halide counterions had a great impact on the CL amplification induced by organomodified MMT. In addition, the CL spectrum from the peroxynitrite in the presence of CTAB-modified MMT using high-energy cutoff filters from 400 to 640 nm demonstrated that the maximum CL wavelength was about 480 nm (insert of Figure S8, Supporting Information), indicating the formation of the excited singlet oxygen molecules (1O2)2* species.40−42 Furthermore, the scavengers of various reaction oxygen species were used to further confirm the emitting species, as shown in Figure S8 (Supporting Information). Thiourea is an effective radical scavenger for •OH, the strong inhibition on the CL after

Figure 4. (A) CL intensity of the peroxynitrite system mixed with CTAB-modified MMT (1), washed CTAB-modified MMT (2), and washed CTAB-modified MMT with 0.01 M CTAB (3), respectively. (B) TGA and DTG plots for CTAB-modified MMT (green line) and washed CTAB-modified MMT (purple line).

following by the decomposition of the CTA+ monolayer at the MMT surface from 200 to 300 °C, and in the MMT interlayer from 300 to 400 °C. The last weight loss between 400 and 500 °C was corresponded to the loss of the polyhydroxyaluminum.35,36 In comparison to the original CTAB-modified MMT, the DTG curve of the washed CTAB-modified MMT showed

Figure 5. (A) Kinetic CL intensity−time profile from MMT-peroxynitrite, CTAB-peroxynitrite, and CTAB-modified MMT-peroxynitrite. (B) Fluorescence spectra of uranine in the peroxynitrite system in the absence/presence of MMT, 0.1 M CTAB, and CTAB-modified MMT. 2854

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system and CL spectrum for the CTAB-modified MMTperoxynitrite CL system. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*C. Lu: e-mail, [email protected]; fax/tel, +86 10 64411957. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program, 2014CB932103), the National Natural Science Foundation of China (21375006), and the Program for New Century Excellent Talents in University (NCET-110561). We also thank Prof. Xue Duan, Beijing University of Chemical Technology, for his valuable discussions.

Figure 6. CL intensity of the peroxynitrite system mixed with different solutions including 0.01 M CTAB, 0.01 M CTAOH, CTAB-modified MMT, and CTAB-modified MMT with 0.01 M Br−.

the addition of thiourea indicated that •OH was involved in the CL reaction.43 The nitro blue tetrazolium (NBT) was frequently used for the detection of superoxide ion radical (•O2−), and the significant decrease of CL intensity upon addition of NBT showed the generation of •O2− in the present CL system.44 Also, the results showed the CL intensity was decreased in the presence of NaN3, whereas the CL intensity was increased after the addition of 1,4-diazabicyclo[2.2.2.2]octane (DABCO), meaning the generation of (1O2)2* species.45 These results indicated that the (1O2)2* species were still responsible for the emitting species in the present CL system, and MMT acted as support materials for the CL amplification (inset of Figure 5).

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4. CONCLUSIONS In summary, cationic surfactant-amplified CL acts as a common technique for a variety of assays. However, the inert bromide or chloride counterions in a micellar solution can compete with or displace reactive counterions from the micellar surface, as well as be a kind of scavenger for reactive oxygen species, leading to an obvious decrease in the CL emissions. Herein, we improved the CL enhancement effect of cationic micellar solution as a result of the inactivation of halide counterions using the CTABmodified MMT. The geometrical configuration of CTAB at the surface of MMT and the enhancement mechanism of the CTAB-modified MMT on the peroxynitrite CL were further confirmed. The success of this work has broken the bottleneck of halide counterion-quenched CL signals and thus would broaden the application of cationic surfactant-amplified CL. Further research into applications of the CTAB-modified MMT in CL assays is ongoing.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Schematic diagram of a static CL setup; effects of the concentration of CTAB modified with MMT, the CTABmodified MMT synthesis temperature, the CTAB-modified MMT synthesis time, and the reaction pH on the peroxynitrite CL intensity; FT-IR spectra of the MMT, CTAB, and CTABmodified MMT; ζ potential measurements of CTAB, MMT, CTAB-modified MMT, and CTAB-modified MMT after centrifugation, respectively; effects of scavengers for reactive oxygen species on the CTAB-modified MMT-peroxynitrite 2855

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dx.doi.org/10.1021/jp411290z | J. Phys. Chem. C 2014, 118, 2851−2856