Confinement Effect in Layered Double Hydroxide Nanoreactor

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Confinement Effect in Layered Double Hydroxide Nanoreactor–Improved Optical Sensing Selectivity Liqing Song, Wen Ying Shi, and Chao Lu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02000 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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Analytical Chemistry

Confinement

Effect

in

Layered

Double

Hydroxide

Nanoreactor–Improved Optical Sensing Selectivity

Liqing Song, Wenying Shi and Chao Lu*

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Fax/Tel.: +86 10 64411957. E−mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: Confinement effect in the layered double hydroxide (LDH) nanoreactors can control the reaction rate and disperse the fluorescence guest, which are promising to be introduced in optical sensing systems. In this work, an optical sensor has been fabricated by combining the confinement effect from the LDH nanoreactors with advantageous sensing performances of graphene quantum dots (GQDs). The mechanism indicated the LDHs with two dimension (2D) confined space provided a stable microenvironment and acted as the disperse matrix to control the distribution of intercalated GQDs. Such a confinement effect may decrease the diffusion rate of hydroxyl radicals (•OH), and thus •OH with the short lifetime (10-9 s) is annihilated during the diffusing process into the LDH interlayer galleries. As a result, the inherently existing interference from •OH for detection of NO2 was eliminated. Furthermore, a rapid and portable fluorescent paper sensor coated with the as-prepared GQD-LDHs for visual detection of NO2 gas was successfully developed. Our work provides a feasible method to remarkably improve the selectivity by virtue of confinement effect.

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INTRODUCTION Nanoreactors (hollow inorganic nanoparticles,1 microemulsions,2 block copolymer,3 layer inorganic nanomaterials,4 et al.) possessed a confined space in the nano-size scale, which could control both the chemical reaction process (rate and selectivity) and product morphology (shapes and sizes).5-7 In particular, some nanoreactors have the tunability and functionality, rendering them attractive in chemical sensing.8,9 The nanoreactor-based chemical sensors show some advantages: 1) the interior void space of nanoreactors can prevent the encapsulated nanomaterials from aggregating, subsequently increasing exposed active surface of nanomateials;10,11 2) the synergetic effect between the unique microstructure and the encapsulated nanomaterials in the nanoreactors can effectively improve the sensing performances.12-14 Among chemical sensors, optical sensors are powerful tools in analytical application owing to their high sensitivity, fast response and easy operation.15,16 However, the optical sensing units often suffer from instability, interference and quenching.17,18 Thus, it is the key to design a kind of nanoreactor-based optical sensor through encapsulating and dispersing sensory units into nanoreactors to achieve advanced optical performances. Layered double hydroxides (LDHs) exhibit the typical two dimension (2D) confined space in nanoscale, tunable charge distribution on layer and interlayer distance, as well as excellent stability. Thus, they not only disperse guest species in nano-interlayer by the electrostatic host-gust interaction,19-21 but also control reaction process.22 The confined LDH space can act as the disperse matrix of fluorescent molecules, avoiding the aggregation quenching and enhancing optical/thermal stability and fluorescence lifetime.23,24 On the other hand, LDHs can act as the attractive nanoreactors with large versatility in controlling stereochemistry, reaction rate and product morphology.22 In this study, we strived to design an optical sensor by introducing the fluorescence 3



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probes into LDH nanoreactors, so as to realize the controllability of the reaction rate and selectivity of optical sensor. Graphene quantum dots (GQDs) own excellent photostability, low cost, eco-friendly,25,26 and have a diverse range of promising applications in sensors.27,28 Therefore, we chose GQDs as model fluorescence probes to evaluate the performance of LDH nanoreactors. Unfortunately, the chemical structure and fluorescent center of GQDs are easily destroyed by hydroxyl radical (•OH)29,30 which exists extensively in the atmosphere.31 This disadvantage of GQDs could greatly degrade the fluorescent performances (intensity, efficiency, selective detection, etc.). Therefore, we designed an optical sensor by confining GQD florescence probes in LDH nanoreactors to inhibit the interference of •OH, achieving the selective recognition of NO2. It has been well recognized that NO2 is harmful to the environment as a source of acid rain and atmospheric particulate matter (such as PM2.5).32,33 As a notorious gas, NO2 can penetrate into the sensitive lung tissue and damage it when it reacts with the moisture and other compounds to form the related particles.34 Furthermore, the inhaled NO2 gas can reach the epithelium of the trachea or the bronchi, leading to destroy the human respiratory system.35 Note that NO2 may cause harm to life at concentrations higher than 20 ppm according to the National Institute for Occupational Safety and Health.36 Therefore, the sensitive monitoring of NO2 is extremely important to public health and the environment. In addition, the GQD-LDHs loaded indicator paper served as the rapid and convenient sensor in practical application when it was exposed to the NO2 gas. The mechanism study showed LDHs provided several advantages: 1) a stable microenviroment of LDHs could control distribution of GQDs between the adjacent layers; 2) confinement effect of LDH layers decreased •OH diffusion rate and most of the •OH is annihilated during the diffusion process into the LDH interlayer galleries; 3) LDH layers tailored the contact degree between •OH and GQDs, improving selectivity of GQD-LDH optical sensors (Figure 1). Therefore, this work provides a new method in improving the selectivity of sensors via 4


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confinement effect in inorganic LDH nanoreactors.

Figure 1. Discrimination of NO2 from •OH by fluorescence sensor based on confinement effect in LDH nanoreactors.

EXPERIMENTAL SECTION Chemicals and Solutions. All chemicals used were of analytical grade without any further purification. The deionized water from a Millipore water purification system was used throughout the experiments. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH, KO2, NaClO, FeSO4, HNO3, NaNO2, NaNO3, H2O2, NaSO3, HCl, citrate, ammonia were purchased from Beijing Chemical Reagent Company (Beijing, China). •OH was prepared from on-line Fenton reaction (Fe2+/H2O2=10) and its final concentration was determined by H2O2.37 Superoxide anion (O2•−) was prepared by dissolving KO2 in the anhydrous dimethyl sulfoxide solution and the final concentration was determined by the UV-vis absorbance at 550 nm (ε = 21.6 mM−1cm−1).38 Peroxynitrite (ONOO−) was produced on-line by the reaction of H2O2−HCl and NaNO2 solution, and the final concentration was determined by the UV-vis absorbance at 302 nm (ε = 1670 M−1cm−1).37 ClO− was derived from diluting NaClO solution in deionized water and the final concentration was determined by the UV-vis absorbance at 292 nm (ε = 350 M−1cm−1).37 Singlet oxygen (1O2) was provided by the reaction between H2O2 and NaClO and the final concentration was determined by H2O2.39

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Apparatus. The fluorescence spectra were measured using a F-7000 fluorescence spectrophotometer (Hitachi, Japan) at a slit of 5.0 nm. The UV-vis absorption spectra were carried out using a Shimadzu UV-3600 spectrophotometer (Tokyo, Japan). Transmission electron microscopy (TEM) photographs, high resolution transmission electron microscopy (HRTEM) were measured on a Tecnai G220 TEM (FEI Company, USA). Atomic force microscopy (AFM) was carried out on a NanoScope IIIa (Digital Instruments Co., Santa Barbara, CA, USA) instrument with tapping-mode. The fluorescence lifetime was measured with an Edinburgh FLS 980 Spectrometer. The fluorescence lifetime values were obtained upon the reconvolution fit analysis (Edinburgh F980 analysis software). The absolute quantum yield was measured on Edinburgh FLS 980 using an integrating sphere. X-ray photoelectron spectroscopy (XPS) measurements were acquired on an ESCALAB-MKII 250 photo electron spectrometer (Thermo, USA). The powder X-ray diffraction (XRD) data were recorded by a Bruck (Germany) D8 Advance X-ray diffractometer equipped with graphite-monochromatized Cu/Kα radiation (λ = 1.54178 Ǻ). The 2θ angle of the diffractometer was stepped in the range of 5-70° with a scan rate of 10 °/min. Thermal gravimetric analysis (TGA) experiment was performed on a TA instrument Q5000 (TA, America) under nitrogen atmosphere. 5.0 mg sample was heated in the temperature range of 25-800 ˚C at a heating rate of 10 ˚C/min. The sample was dried in vacuo at 40 ˚C for 24 h to remove free water. Raman spectrum was carried out on LabRAM ARAMIS Raman System (HORIBA Jobin Yvon, Japan) with 532 nm laser radiation source. Fourier transform infrared (FTIR) spectrum was measured using a Nicolet 6700 FTIR spectrometer (Thermo, America). Synthesis of GQD-LDHs. GQD-LDHs were synthesized according to our reported method.40 The precursors of citrate-intercalated LDHs was synthesized by a co-precipitation method, then they were hydrothermally carbonized to form GQD-LDHs. The prepared GQD-LDHs were dispersed in 10 mM PBS buffer (pH 7.0) as the work solution with a final 6


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concentration of 0.5 mg mL-1. The intercalated GQDs were separated from the GQD-LDHs by chemical etching with HCl for further characterization (denoted as GQDsLDHs). The ratio of GQD to LDH in the synthesized GQD-LDHs was about 11% (w/w). Synthesis of GQDs. 0.2 g citrate, 2.25 mL aqueous ammonia and 12.25 mL deionized water were sealed into a Teflon-equipped stainless steel autoclave and heated at 180 ˚C for 8 h. The product was purified by dialyzing, and then dispersed in 10 mM PBS buffer (pH 7.0). Synthesis of GQD@LDHs. The GQD@LDHs were obtained by adsorbing GQDs on the surface of NO3-LDHs. NO3-LDHs were synthesized by the co-precipitation method.41 The mixed salt solution containing Mg(NO3)2·6H2O (0.045 mol) and Al(NO3)3·9H2O (0.015 mol) dissolved in 60 mL of CO2-free water and 0.12 mol NaOH solution was added dropwise to the flask under N2 atmosphere at room temperature at pH 10. After aged for 24 h, the obtained precipitate was separated by centrifugation and washed thoroughly with the CO2-free water for

three

times.

The

corresponding

chemical

composition

of

NO3-LDHs

was

[Mg2+0.74Al3+0.26(OH)2]0.26+(NO3)−0.24(CO3)2−0.01.42 Subsequently, the NO3-LDH colloidal solution (100 mg) was dispersed into the GQD solution (0.5 mg mL-1) under vigorous stirring at room temperature until the equilibration. The product was separated by centrifugation to obtain the well-dispersed GQD@LDH solution. Concentration Calibration of NO2. NO2 gas was slowly bubbled into absolute ethanol until the solution turned yellow. The ABTS method was used to calibrate the concetration of saturated NO2 solution (15 µL). Based on the absorbance at 660 nm (ε = 12000 M-1 cm-1) of ABTS radical, which was oxidized by NO2 in PBS buffer (50 mM, pH 7.0),43,44 the NO2 stock solution was calibrated to 1.0 mM (Figure S1). Fluorescence Responses to NO2. NO2 solution was injected into the solution of GQD-LDHs (0.5 mg mL-1) in the PBS buffer (pH 7.0). Then, the resulting solution was kept for 30 s before recording its fluorescence spectra. 7



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Paper Sensor for Visual Detection of Gaseous NO2. 2.0 µL of the probe solution was dropped on the filter paper. The fluorescent paper sensor was obtained after dried under vacuum for 20 min at 60 ˚C to form a 3 mm spot. Then, the indicating paper was fixed on the inner wall of a glass container for visual detection of NO2. Subsequently, the different amounts of NO2 gaseous samples were injected into the container. After 10 min, the fluorescence change of the indicating paper under 365 nm UV lamp was recorded by digital camera.

RESULTS AND DISCUSSION Confined Space of GQD-LDH Nanoreactors. The GQD-LDH composite materials were synthesized by carbonization of citrate in the confined space of LDHs in situ. The confined interlayer galleries of LDH as nanoreactors controlled the growth of GQDsLDHs. As shown in Figure S2, the XRD pattern of GQD-LDHs corresponding to a series of characteristic reflections of LDH structure indicated that the basal spacing was about 1.2 nm.40 It was evident that nanoscale compartments in LDHs were prerequisite to construct the nanoreactor systems. The GQD-LDHs showed a maximum UV-vis absorption at 365 nm and fluorescence (FL) emission wavelength at 425 nm (Figure 2A) with strong blue fluorescence under 365 nm UV light (inset of Figure 2A). The TEM image (Figure 2B) revealed that the as-prepared GQDsLDHs had a uniform size with 2.0 nm diameter. In addition, the AFM image (Figure S3) showed that the average thickness of GQDsLDHs was ca. 0.7 nm, indicating that they were consisted of a monolayer graphene sheet.45 The 0.2 nm lattice parameter shown in inset of Figure 2B was consistent with the (1100) lattice fringes of graphene,27 implying that the GQDsLDHs kept the similar crystallinity with graphene. The Raman spectrum (Figure S4) showed that the ID/IG value was 0.7, indicating that GQDsLDHs exhibited relatively high quality of graphitic structure.25 These results further confirmed that LDHs could be served as 8


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a kind of excellent nanoreactors for achieving morphology-controlled nanoparticles. In addition, FTIR and XPS were used to characterize the structure and configuration of GQDs in the interlayer of LDHs. The FTIR spectrum (Figure S5) and XPS results (Figure S6) showed that the as-prepared GQDs in the interlayer of LDHs had some functional groups (carboxyl, hydroxyl) and a certain amount of pyridine-like N.25 The fluorescence quantum yield of GQD-LDHs was as high as 40%. Furthermore, the corresponding lifetime of GQD-LDHs was prolonged up to 10.77 ns in comparison with the pure GQDs (6.37 ns) (Figure 2C). This enhancement can be mainly attributed to the reason that the LDH confined space provided a homogenous distribution of GQDs to eliminate the π-π stacking and suppress the thermal vibration.46,47

Figure 2. (A) The UV-vis absorption (dotted line) and fluorescence emission spectra (solid lines) of the GQD-LDHs, inset: photographs of the GQD-LDHs under (left) daylight and 365 nm UV light (right); (B) TEM image of GQDsLDHs, inset: HRTEM image of the GQDLDHs; (C) The fluorescence decay curves of GQD-LDHs (red) and pure GQDs (black).

Selective Fluorescence Response of GQD-LDHs towards NO2. As shown in Figure 3A, there were no distinct changes in the spectra of the GQD-LDHs among some ROS (ONOO−, ClO−, 1O2, H2O2, •OH, and O2•−), and some ions (SO32-, HNO3, NO2-, and NO3-) and some gases (NO, CO, and O3) at concentration of 10 µM, respectively. NO2 can be adsorbed on the surface of LDHs and pass into the interlayer space because of the large surface areas and micropore surface of LDHs.48 Accordingly, a significantly quenched fluorescence (about 70%) 9



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was found after the addition of NO2 in GQD-LDHs (Figure 3A). The good selectivity clearly indicated that GQD-LDHs could be used to detect NO2 efficiently. Note that HNO3 and NO2did not quench the fluorescence of GQD-LDHs. The XPS characterizations of GQD-LDHs reacted with HNO3 and NO2- showed no evident change in the N 1s spectra after the reaction of HNO3 and NO2- (Figure S7). In order to evaluate the confinement effect of LDHs, the control experiments were carried out. Firstly, the pure GQDs with a diameter of 3.0 ± 0.5 nm (Figure S8A), and the fluorescence emission at 440 nm (Figure S8B) were used to detect the tested ROS and ions (Figure 3B and S9A). It was obvious that •OH could quench the fluorescence of GQDs and interfere the detection of NO2 owing to its strong oxidation property and high reactivity.49,50 In contrast, other ROS with weaker oxidation capability had nearly no effect on the fluorescence signals of GQDs. Secondly, the GQD@LDHs were prepared by adsorbing GQDs on the surface of NO3-LDHs. As shown in Figure S10, the interlayer spacing was calculated to be 0.85 nm, which was corresponding to the spacing of NO3− in hydroxide layers.41 The unchanged XRD patterns (Figure S10) and appearance of fluorescence emission spectra (Figure S11) indicated that GQDs were only adsorbed on the surface of NO3-LDHs. As showed in Figure S9B, the fluorescence intensities of GQD@LDHs changed obviously in the presence of •OH and NO2. These results demonstrated that NO2 could not be distinguish from •OH in the absence of the confinement effect of LDH layers. Therefore, it can be concluded that the confinement effect of LDH layers could improve the sensing selectivity towards NO2 (Figure 3B).

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Figure 3. (A) Fluorescence emission spectra of GQD-LDHs in the presence of different ROS, ions and gases (each 10 µM, excitation at 365 nm); (B) The fluorescence intensities of GQDs, GQD@LDHs and GQD-LDHs after adding of NO2 (10 µM) and •OH (10 µM).

Mechanism of Selective Fluorescence Response of GQD-LDHs towards NO2. XPS spectra were used to provide the chemical information of the GQD-LDHs and interpret the selective sensing mechanism of the GQD-LDHs towards NO2. The remarkable changes were observed in the XPS spectra of GQD-LDHs before and after exposure to NO2. As shown in Figure 4A, after exposure to NO2, the additional peak presented in the N 1s spectra corresponding to nitro groups at 405-410 eV,51 because NO2 acted as the nitrating agent.52-54 This result indicated that the appearance of nitro groups may lead to the fluorescence quenching of GQDsLDHs due to their strong electron-withdrawing effect.55,56 The kinetic processes of the •OH reacting with pure GQDs and GQD-LDHs were also monitored to explain the differences in the confined and unconfined space. The time-dependent fluorescence changes of GQD-LDHs and GQDs in the presence •OH were compared. As shown in Figure 4B, the fluorescence intensity of GQD-LDHs kept constant in the presence of •OH within 30 s. It was reported that solid materials would not affect the intrinsic property of •OH.57,58 However, •OH had a short lifetime (~10-9 s) and diffusion length,31,59,60 and thus it may be annihilated during the diffusing process.61 Therefore, the produced •OH could not diffuse to the LDH interlayer space. As a result, •OH would not react 11



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with the confined GQDs. In contrast, this reaction could occur immediately in the unconfined solution (Figure 4B). These results confirmed that the confined space of LDHs played a key role in avoiding the quenching of GQDs by •OH within 30 s.

Figure 4. (A) The N 1s XPS spectra of the GQD-LDHs in the absence and presence of NO2; (B) Time-dependent fluorescence changes of GQD-LDHs and pure GQDs in the presence of •OH. The concentrations of •OH, GQDs and GQD-LDHs were 10 µM, 0.5 mg mL-1, 0.5 mg mL-1, respectively.

GQD-LDH Sensor for NO2 Detection. Based on the advantageous properties of GQD-LDHs, we explored the feasibility by using the as-prepared composite materials for NO2 detection. After adding different concentrations of NO2 to the GQD-LDH solution, the fluorescence spectra of GQD-LDH were monitored (Figure 5A). Obviously, the fluorescence intensity of GQD-LDHs decreased gradually with the increasing of NO2 concentration. Inset of Figure 5A depicted the fluorescence intensity changes of the GQD-LDHs versus the concentration of NO2. Good linear correlation (R2 = 0.997) was obtained over the concentration of NO2 in the range from 0.1 to 10 µM, and the limit of the detection was estimated to be 90 nM at a signal-to-noise ratio of 3, which was comparable with the other reported methods (Table S1).33,38,62,63 The proposed sensor was used to detect NO2 on the dual carriageway trunk roads of Beijing at 9:00 A.M. and 3:00 P.M., respectively. Table S2 showed 12


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that the proposed method was applicable to the quantification of NO2 in the environment. The results of the proposed method were in good agreement with those obtained by a standard absorbance

method

by

the

reaction

of

N-(1-naphthyl)ethylenediamine

dihydrochloride-sulfanilamideand with NO2 (Figure S12).38 It was reported that •OH would react with high concentration of NO2.31 However, NO2 is relative stable in the environment in comparison to the short lifetime of •OH. It is impossible that a large amount of •OH and NO2 can coexist in the environment. Therefore, the proposed GQD-LDH sensor can monitor the net content of NO2 in the environment. We tried to develop a simple and rapid paper sensor to detect NO2. Figure 5B showed the fluorescence images of the indicating paper, after exposed to different concentrations of NO2 gas for 10 min under 365 nm UV light. Clearly, the fluorescence intensity of the spots gradually decreased in a dose-dependent manner with increasing the concentration of NO2 gas from 0 to 50 ppm. These results demonstrated that the as-prepared GQD-LDH indicating paper was highly desirable to serve as a direct, sensitive and convenient visual sensor of NO2.

Figure 5. (A) Fluorescence emission spectra of GQD-LDHs with different concentrations of NO2 (λex = 365 nm). (From top to bottom: 0, 0.1, 0.5, 1, 2, 4, 6, 8 and 10 µM), inset: fluorescence intensity of GQD-LDHs vs the concentration of NO2; (B) Visual detection of NO2 gas. NO2 concentrations were 0, 5, 10, 20, 30, 50 ppm. The images were taken under 365 nm UV lamp. 13



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Stability of GQD-LDHs. The long term stability is one of the crucial parameters for any sensor. It is necessary to investigate photo-stability and thermostability of the GQD-LDHs. After irradiation with UV light for 180 min, the fluorescence intensity of GQD-LDHs showed almost no change, demonstrating the excellent resistance to photo-bleaching (Figure 6A). Moreover, GQD-LDHs could store for one month under ambient conditions without the loss of fluorescence (Figure 6B), further indicating their favorable photo-stability. In addition, the thermal stability of GQD-LDHs was investigated by TGA under a N2 atmosphere and the corresponding TGA cures were showed in Figure 6C. The reduction in mass between 100 and 250 °C (11%, w/w) corresponded to the water loss from the surface adsorbed and interlayer of LDH. The rapid and major mass loss from 250 to 450 °C was attributed to the dehydroxylation of the inorganic layers and decomposition of intercalated GQDsLDHs (25%, w/w).64 These results indicated that the GQD-LDH composite materials with high performances in optical stability and thermal stability may be the promising candidates for the construction of the effective sensing platform.

Figure 6. (A) Effect of photoirradiation time on the fluorescence intensity of the GQD-LDHs; (B) Fluorescence emission spectra of fresh and one-month-stored of GQD-LDHs; (C) TGA plots of GQD-LDHs.

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CONCLUSIONS In summary, we have developed a new GQD-LDH sensor for NO2 with high selectivity and sensitivity by virtue of the confinement effect of LDH nanoreactors. As the attractive nanoreactors, LDHs could control the distribution of GQDs between the adjacent layers and tailor the contact degree between •OH and intercalated GQDs. In comparison with the pure GQDs and the adsorbed product of GQD@LDHs, the interference from •OH could be ruled out, and the selectivity in detection of NO2 was efficiently improved using the as-prepared GQD-LDH sensor. In addition, a simple and portable fluorescent paper sensor was fabricated by immobilizing GQD-LDHs on the paper for visual detection of NO2. It can be anticipated that the sensing units confined in LDH interlayers will open a new method to markedly improve the selectivity of optical sensor.

ASSOCIATED CONTENT Supporting Information Available UV-vis absorption spectra of ABTS to NO2 in the PBS solution, XRD pattern of GQD-LDHs, and AFM image of GQDsLDHs, Raman spectrum of GQDsLDHs, FTIR spectrum of GQDsLDHs, C 1s XPS spectra and N 1s XPS spectra of GQDsLDHs, N 1s XPS spectra of GQD-LDHs before and after the reaction with HNO3 and NO2-, TEM image of pure GQDs, UV-Vis absorption and fluorescence emission spectra of GQDs, fluorescence emission spectra of pure GQDs with different ROS and ions, and fluorescence emission spectra of the GQD@LDHs with different ROS and ions, XRD patterns of NO3-LDHs and GQD@LDHs, fluorescence emission spectra of NO3-LDHs and GQD@LDHs, the comparison of this work with some other reported methods for NO2 detection, determination results of NO2 gas samples with proposed method and standard method, the UV-vis absorption intensities of the reaction solution of NO2 with N-(1-naphthyl)ethylenediamine dihydrochloride and sulfanilamide 15



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solution vs the concentrations of NO2 at 545 nm. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E−mail: [email protected]. Fax/Tel.: +86 10 64411957. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (973 Program, 2014CB932103), the National Natural Science Foundation of China (21375006 and 21575010), and the Innovation and Promotion Project of Beijing University of Chemical Technology.

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For TOC only

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Confinement Effect in Layered Double Hydroxide Nanoreactor

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