Showcasing collaborative research from Nano Bio Mater. Lab. (Prof. J. M. Oh) at the Department of chemistry and medical chemistry, Yonsei University and Nano Energy Mater. Lab. (Prof. S. M. Paek) at the Department of chemistry, Kyungpook National University.
As featured in: Volume 43 Number 27 21 July 2014 Pages 10261–10652
Dalton Transactions An international journal of inorganic chemistry www.rsc.org/dalton
Isomorphous substitution of divalent metal ions in layered double hydroxides through a soft chemical hydrothermal reaction We proposed a soft chemical reaction to isomorphously substitute aqueous Co(II) ions for Mg(II) in the lattice of layered double hydroxides (LDHs). This method was proven to introduce external metal ions into the crystal lattice without changing crystallinity, size and morphology of LDH nanomaterials.
Themed issue: Layered Inorganic Solids ISSN 1477-9226
PAPER B. Marler et al. Topotactic condensation of layer silicates with ferrierite-type layers forming porous tectosilicates
See Seung-Min Cao, DaltonPaek, Trans., Jae-Min 2014, 43, Oh 423. et al. Dalton Trans., 2014, 43, 10430.
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Isomorphous substitution of divalent metal ions in layered double hydroxides through a soft chemical hydrothermal reaction† Tae-Hyun Kim,a Won-Jae Lee,b Ji-Yeong Lee,a Seung-Min Paek*b and Jae-Min Oh*a We have successfully incorporated Co2+ ions into layered double hydroxides (LDHs) comprising Mg and Al hydroxides via isomorphous substitution utilizing a soft chemical hydrothermal reaction. The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis showed that the incorporation of Co2+ into an LDH was highly dependent on the dissolution of Mg2+. The X-ray diffraction (XRD) patterns showed that the crystalline phase, as well as the crystallinity of pristine LDH, was well preserved without the evolution of impurities during the substitution reaction. It was notable that the size (∼250 nm) and hexagonal plate-like morphology of LDHs did not change significantly upon Co2+ substitution. Trans-
Received 5th February 2014, Accepted 4th March 2014
mission electron microscopy-energy dispersive spectroscopy (TEM-EDS) exhibited homogeneous distribution of Co2+ in the LDH particles obtained by this substitution reaction. Solid-state UV-vis and X-ray
DOI: 10.1039/c4dt00373j
absorption spectroscopy (XAS) verified that the incorporated Co2+ ions were well stabilized in the
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octahedral sites of an LDH, which were formerly occupied by Mg2+ ions.
Introduction Layered materials comprising various chemical compositions of metal oxides or metal hydroxides have swelling properties, high specific surface areas, exchangeable interlayer substances, and modifiable external surfaces resulting from their characteristic 2D nanosheet-stacking structure.1,2 Therefore, they have been considered as versatile materials with high applicability in various fields, including catalyst, catalytic support, biomedical, and environmental fields.3–7 Among the various classes of layered materials, layered double hydroxides (LDHs), also known as anionic clays, along with layered hydroxy salts and hydroxy double salts, are unique materials that possess positively charged layers to accommodate anionic chemical species between the layers. The general chemical formula of an LDH is M(II)1−xM(III)x(OH)2(An−)x/n· mH2O (0 < x < 1; m and n are integers), where M(II) and M(III) stand for divalent and trivalent metal cations, respectively, and a Department of Chemistry and Medical Chemistry, College of Science and Technology, Yonsei University, Wonju, Gangwondo 220-710, Korea. E-mail: [email protected]; Fax: +82-33-760-2182; Tel: +82-33-760-2368 b Department of Chemistry, Kyungpook National University, Taegu 702-701, Korea. E-mail: [email protected]; Fax: +82-53-950-6330; Tel: +82-53-950-5335 † Electronic supplementary information (ESI) available: ICP-AES results of Co2+ substitution under room temperature, calculated lattice parameters of Co2+-sub2+ stituted LDHs, XRD patterns of Co -substituted LDHs with a high concentration 2+ of Co in solution, and SEM-EDS mapping results. See DOI: 10.1039/c4dt00373j
10430 | Dalton Trans., 2014, 43, 10430–10437
An− represents the anionic species.8 The structure of an LDH is based on that of Mg(OH)2 (brucite), where Mg(OH)6 octahedra are infinitely propagated in the direction of the xy-plane by sharing their edges.9 Partial substitution of Mg2+ with trivalent cations such as Al3+ produces permanently positively charged layers; this substitution is further compensated by interlayerexchangeable anions.10,11 It has been known that selected LDHs such as MgAl-LDH and ZnAl-LDH have low biological toxicity,12 and can be dissolved in ionic species under weakly acidic physiological conditions.13,14 Thus, LDHs have been attracting tremendous interest, especially in the field of biomedical applications. Furthermore, LDHs have found application as cellular delivery carriers for anionic drug molecules,15–18 because the plasma membranes are usually negatively charged.19 It has been reported that LDHs can effectively deliver not only anionic anticancer agents such as methotrexate and 5-fluorouracil but also therapeutic genes to enhance the medicinal efficacy.18,20–22 The main reason for the effective cellular delivery by LDHs is clathrin-mediated endocytosis.18 It has been demonstrated that the size and morphology of an LDH play key roles in cellular uptake and intracellular trafficking.23–25 Especially, the particle size of an LDH is essential in cellular delivery with a maximized cellular uptake efficiency of ∼200 nm particles.24 Although LDHs exhibit high cellular-delivery efficacy, it is difficult to label them with a traceable moiety for detection in biological systems. Thus, they cannot be, as yet, utilized in a
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practical drug delivery system. Recent studies on nanomedicine pursue theranosis, a simultaneous diagnosis and therapy strategy.26 Tracing of LDHs in a biological system would increase the possibility of LDHs serving as practical drug delivery systems for nanomedicine. There have been several reports on the labelling of LDHs with tracers. Fluorescent dyes, magnetic resonance contrasting Gd complexes, and radioisotopes have been incorporated into LDHs via intercalation or surface modification.24,27–30 However, the introduction of these tracers into the lattice of LDHs is required for robust labelling. Recently, Musumeci et al. have directly incorporated radioactive 57Co2+ and 67Ga3+ into the LDH lattice to develop radiolabelled LDHs.31 Byeon et al. have developed a new type of LDHs that consists of only rare earth metals having photoluminescence properties.32 To optimise the application of labelled-LDHs to nanomedicine, it is of utmost importance to obtain LDHs with wellcontrolled size and morphology. MgAl-LDH is one of the most extensively studied LDHs in terms of size and morphology control. It is possible to fabricate MgAl-LDHs with particle sizes ∼200 nm and plate-like shapes with relatively low aspect ratios (diameter/thickness), which are favourable properties for cellular uptake.24 In this context, we aimed to synthesise MgAl-LDHs with controlled size and morphology whilst incorporating Co2+ into the LDH lattice via isomorphous substitution under soft chemical reaction conditions. 57Co is a radioisotope detectable by positron emission tomography (PET) and can thus be employed as a radiolabel. In this study, we utilized non-radioisotopic Co2+ for incorporation into LDHs as a proof-of-concept experiment. The possibility that metal ions in solid LDHs can be isomorphously substituted by the ions in solution was first suggested by Komarneni et al.,33 possibly via diadochy, a wellknown cation-exchange process in calcite or hydroxyapatite.34,35 Richardson et al. observed a similar phenomenon by suspending LDHs in metal-ion-containing solutions for a very long period of 5 days.36 Infrared (IR) and diffuse reflectance visible-to-near infrared spectroscopies were utilized to confirm the isomorphous substitution in LDHs. However, considerable structural reorganization in the LDH lattice was also reported. Park et al. studied the reaction between LDHs and metal ions, Cu2+ and Pb2+, and observed the evolution of metal hydroxide precipitates on the surface of LDHs.37 In this study, our overarching goal was to incorporate Co2+ into MgAl-LDHs that possess a well-controlled size and morphology via isomorphous substitution under soft chemical hydrothermal treatment. To accomplish this, we desired to: (1) preserve the crystallinity, size, and shape of pristine LDH, (2) introduce Co2+ homogeneously into the lattice, (3) minimize the formation of surface precipitation impurities, and (4) carry out the substitution in a short time under simple reaction conditions. In this paper, we demonstrate the successful substitution of Mg2+ with Co2+ in MgAl-LDH utilizing both structural characterisation (X-ray diffraction (XRD), solid state UV-vis spectroscopy, and X-ray absorption spectroscopy (XAS)) and microscopic techniques (scanning electron microscopy (SEM)
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and transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS)).
Results and discussion The chemical formulae of pristine LDH and Co2+-substituted LDH were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Table 1). Co2+ was found to gradually incorporate over time into the LDH lattice upon exposure to a 0.02 M Co2+ solution under 150 °C hydrothermal conditions. The graph in Fig. 1 displays time-dependent changes in the Co2+ and Mg2+ molar ratio with regard to the net composition of divalent ions (Mg2+ + Co2+) in the LDH lattice. We could clearly observe the time-dependent increase in Co2+ and the corresponding decrease in Mg2+ through an S-type curve. The S-type curve of Co2+ substitution was different from the L-type Co2+ uptake reported by Komarneni et al.,33 suggesting that hydrothermal conditions required a minimum time of 3 h for effective metal substitution.
Table 1 Chemical formula, particle diameter, and thickness of pristine and Co2+-substituted LDHs ( particle diameter and thickness of LDH particles were calculated as the average of 50 randomly selected particles from SEM images). Samples are named in the format LDH-Co-n, where n stands for reaction time (in hours) of the LDH sample in 0.02 M Co2+ solution under 150 °C hydrothermal conditions
Sample
Chemical formula
Pristine LDH LDH-Co-1
Mg2.54Al1(OH)7.08(CO3)0.5· mH2O Mg2.02Co0.008Al1 (OH)6.06(CO3)0.5·mH2O Mg2.05Co0.025Al1 (OH)6.15(CO3)0.5·mH2O Mg1.65Co0.59Al1 (OH)6.48(CO3)0.5·mH2O Mg1.31Co0.97Al1 (OH)6.56(CO3)0.5·mH2O Mg1.25Co1.04Al1 (OH)6.58(CO3)0.5·mH2O
LDH-Co-3 LDH-Co-6 LDH-Co-12 LDH-Co-24
Diameter (nm)
Thickness (nm)
250 ± 20
80 ± 8
250 ± 22
80 ± 10
250 ± 20
80 ± 10
250 ± 20
90 ± 10
280 ± 27
110 ± 21
290 ± 30
140 ± 19
Fig. 1 Time-dependent change of Mg2+ (open circle, right-side y-axis) and Co2+ (solid circle, left-side y-axis) content in the LDH lattice. The y-axis is the molar ratio between Mg2+ or Co2+ and net divalent metal (Mg2+ + Co2+).
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Both curves for Co2+ increase and Mg2+ decrease were topto-bottom symmetrical, suggesting that Co2+ incorporation into the LDH lattice was closely related to Mg2+ dissolution into the aqueous environment, thus implying that the substitution of Mg2+ with Co2+ is isomorphous. Co2+ substitution reaches a plateau at ∼24 h (Fig. 1) at which time the Mg2+/Co2+ molar ratio was almost 1 (Table 1). At room temperature, substitution of Co2+ into MgAl-LDH does not occur to a significant extent, even at high Co2+ concentrations (0.8 M) after 24 h of reaction time (Table S1†). Whilst Co2+ could be incorporated into the LDH lattice upon dissolution of Mg2+, there still remained concern pertaining to the isomorphous nature of substitution between Mg2+ and Co2+. The aqueous Co2+ might not exist in the previously occupied Mg2+ site in LDHs, forming cobalt hydroxide precipitates similarly to the report by Park et al.37 Various cobalt hydroxide precipitates such as α-Co(OH)2 (JCPDS no. 741057),38 β-Co(OH)2 (JCPDS no. 30-0443)39 and Co2(OH)3Cl (JCPDS no. 73-2134)40 could form through hydrolysis of Co2+ under basic conditions. The powder XRD patterns for pristine and Co2+-substituted LDHs are shown in Fig. 2. Pristine
MgAl-LDH showed a well-crystallised and single-phased hydrotalcite-like (JPCDS no. 14-0191) structure (Fig. 2(a)). Sharp peaks corresponding to (003) and (006) revealed that the pristine LDH possessed well-organised 2D layer stacking. Other reflection peaks attributed to the lattice, such as (013), (015), (016), (018), (019), (110), and (113), were observed at 35.9°, 38.4°, 41.5°, 45.5°, 49.9°, 60.7°, and 62.0°, respectively.41 We found a slight shift in the peak position of (110) according to reaction time. The calculated lattice parameter a decreased until 3 h and increased afterwards within a small range of ±0.01 Å (Table S2†). This was attributed to the formation of stable crystals during hydrothermal treatment as reported by Tok et al.42 The XRD patterns for Co2+-substituted LDHs were nearly identical to those of pristine LDHs in terms of peak position and intensities; no significant impurity phase (corresponding to α-Co(OH)2, β-Co(OH)2 or Co2(OH)3Cl) was found. This result confirms that there was neither evolution of an impurity phase nor structural deformation during the substitution reaction. However, an impurity phase corresponding to Co2(OH)3Cl (JCPDS no. 73-2134) was found when Co2+ solutions of higher concentration (≥0.4 M) were treated with LDHs under hydrothermal conditions (Fig. S1†). Therefore, we concluded that 0.02 M Co2+ concentration was appropriate for isomorphous substitution at 150 °C. The overall particle size and morphology of MgAl-LDH, the most important factors in drug delivery application, were wellpreserved after the Co2+-substitution reaction, as proved by the SEM images of pristine and Co2+-substituted LDHs (Fig. 3). Pristine LDH (Fig. 3(a)) showed the typical shape of hexagonal hydrotalcite as prepared by hydrothermal methods.41,43 The average particle size and standard deviation of pristine LDH (determined by the examination of 50 randomly selected particles) were between ∼250 and ∼20 nm, demonstrating a narrow particle size distribution suitable for cellular uptake.24 Upon hydrothermal treatment in Co2+ solutions, no significant changes in the overall particle shape were observed and the hexagonal plate-like morphology of pristine LDH was preserved (Fig. 3(b)–(f ), Table 1).
Fig. 2 Powder X-ray diffraction patterns of (a) pristine and Co2+-substi2+ tuted LDHs in 0.02 M Co solution under 150 °C hydrothermal conditions: (b) LDH-Co-1, (c) LDH-Co-3, (d) LDH-Co-6, (e) LDH-Co-12, and (f ) LDH-Co-24.
Fig. 3 Scanning electron microscopic images of (a) pristine and Co2+-sub2+ stituted LDHs in 0.02 M Co solution under 150 °C hydrothermal conditions: (b) LDH-Co-1, (c) LDH-Co-3, (d) LDH-Co-6, (e) LDH-Co-12, and (f) LDH-Co-24.
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According to the Student’s t-test, LDH-Co-1, LDH-Co-3, and LDH-Co-6 were determined to have statistically the same particle diameter as that of pristine LDH, while LDH-Co-12 and LDH-Co-24 showed slightly increased particle sizes. It is believed that the LDH particles grew by an aging effect whilst exchanging with Co2+ (in solution) under hydrothermal conditions. However, this does not imply the evolution of a solid impurity phase through surface precipitation. No significant signals were observed in XRD patterns, thus confirming the absence of surface precipitation (Fig. 2). The distribution of Co2+ ions in the LDH lattice upon substitution with 0.02 M Co2+ solution (under hydrothermal conditions) was visualized with energy dispersive spectroscopy (EDS) mapping (Fig. 4). In order to observe the distribution of metal ions (Mg2+, Al3+, and Co2+) in a single particle, we magnified the images to contain a few particles and carried out EDS mapping experiments. Yellow, cyan, and magenta colours indicate the locations of Mg2+, Al3+, and Co2+, respectively. We could clearly observe that Co2+ was homogeneously distributed throughout the entire particle in both LDH-Co-1 (Fig. 4(b′)) and LDH-Co-24 (Fig. 4(c′)), while no cobalt was detected in pristine LDH (Fig. 4(a′)). Furthermore, the content of cobalt increased significantly with increasing reaction times, which was in good agreement with the ICP-AES results (Table 1). The elemental distribution mapping, at low magnification, was also carried out with SEM-EDS mapping and likewise showed a homogeneous distribution of the three different metal ions in the LDH-Co-24 sample (Fig. S2†). If the Co2+ substitution only occurred at the edge or surface of LDH particles or if cobalt hydroxide precipitates covered the LDH particles, the Co2+ distribution would be localized to only the outer parts of LDH particles. We therefore conclude that the successful replacement of Mg2+ with Co2+ occurs through isomorphous substitution as corroborated by the EDS mapping results. We next carried out solid-state UV-vis spectroscopic studies to verify that the substituted Co2+ ions were stabilized in the
Fig. 4 Transmission electron microscopic images (a–c) and corresponding energy dispersive spectroscopy (EDS) mapping images (a’–c’) of (a, a’) pristine LDH and Co2+-substituted LDHs in 0.02 M Co2+ solution under 150 °C hydrothermal conditions: (b, b’) LDH-Co-1 and (c, c’) LDH-Co-24. (Legend: yellow = Mg2+, cyan = Al3+, magenta = Co2+.)
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Fig. 5 Solid state UV-vis absorption spectra of Co2+-substituted LDHs: (a) LDH-Co-1, (b) LDH-Co-3, (c) LDH-Co-6, (d) LDH-Co-12, (e) LDH-Co-24, and those of conventionally prepared Co2+-containing LDHs: (f) MgCoAlLDH and (g) CoAl-LDH (solid line: observed peak, dotted line: fitted peaks, open circle: summation of fitted peaks).
octahedral sites of an LDH that were formerly occupied by Mg2+ ions (Fig. 5). For the comparative study, we prepared MgCoAl-LDH (Mg1.18Co1.12Al(OH)6.6·mH2O) and CoAl-LDH (Co2.23Al-(OH)6.46(CO3)0.5·mH2O) through conventional coprecipitation10,44 as reference samples. There are three possible spin-allowed d–d transitions in the octahedral symmetry of Co2+ ions: ν1 (4T1g(F) → 4T2g(F)), ν2 (4T1g(F) → 4A2g(F)) and ν3 (4T1g(F) → 4T1g(P)).45 The UV-vis spectra of all samples showed broad peaks ranging from 400 to 650 nm. To analyse the peak positions and relative intensities in detail, we separated the peaks by utilizing the Gaussian peak function in OrginPro® 8.0. As the ν1 transition is generally observed in the near-IR region and the ν2 and ν3 transitions are known to occur in the visible light region,46 we assigned the two peaks at ∼600 and 530 nm as ν2 and ν3, respectively. It is also known that the ν2 transition, a 2-electron process with very low probability, is rarely observed,46 which is in good agreement with our results showing reduced absorbance for ν2. The small peak observed around 450 nm was also attributed to the absorption of the
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Co(OH)6 octahedron.47 The strongest peak was attributed to ν3, indicating the existence of Co2+ ions in octahedral sites. The characteristic pink colour of Co(OH)6-containing compounds was clearly observed in all samples. We found that the ν3 transition shifted bathochromically upon increasing the cobalt content from 519.3 nm for LDH-Co-1 to 528.4 nm for LDH-Co24 (Table 2). This kind of red shift in cobalt transition may be attributed to the increasing Co2+ content according to substitution time.48,49 The Racah parameter (B) and octahedral splitting energy (Δo) were calculated from the UV-vis spectra and the Tanabe–Sugano diagram with a d7 configuration. As summarised in Table 2, the calculated B and Δo values of Co2+-substituted LDH lay in the regions 830–840 cm−1 and 9250–9350 cm−1, respectively. Although the values showed slight decreases in energy in correlation with increasing reaction time and Co2+ content, these values were fairly comparable with those of [Co(OH)6]4− (Δo = 9200 cm−1 and B = 825 cm−1), as previously reported.50 The UV-vis spectroscopic results suggested that all of the Co2+ ions in the LDH lattice were well stabilized in the octahedral sites formerly occupied by Mg2+ ions. The local structure and chemical environment around Co2+ in Co2+-substituted LDHs were also confirmed by XAS. As XAS is a very sensitive spectroscopic tool that reflects the subtle changes in symmetry and bond lengths in coordinate compounds, it would elucidate the precise chemical environment around the substituted Co2+. The Co K-edge X-ray absorption near-edge structure (XANES) of Co2+-substituted LDHs and reference compounds (MgCoAl-LDH and CoAl-LDH) showed intense white lines attributed to the 1s → 4p transition at ∼7720 eV (Fig. 6(A)). The main edge positions of the Co2+-substituted LDHs and reference compounds were nearly identical. Furthermore, we could not find any significant preedges that would correspond to Co in tetrahedral symmetry, corroborating that the Co2+ ions existed in octahedral sites. It was notable that the threshold energy of Co-containing LDHs (dotted line in Fig. 6(A)) was ∼3.02 eV higher than that of β-Co(OH)2 (dashed line in Fig. 6(A)). This chemical shift was attributed to the different effective nuclear charge of Co in the LDH phase as compared with that of Co in β-Co (OH)2. According to Sapre’s report,51 the chemical shift becomes larger with increasing effective nuclear charge. Mean-
Table 2 Peak positions of ν2 and ν3 transitions, calculated Racah parameters (B), and octahedral splitting energy (Δo) values for Co2+-substituted LDH compounds. (The Racah parameter and octahedral ligand field split7 ting energy were calculated on the basis of the d Tanabe–Sugano diagram.)
Sample
ν2 (Oh) (nm)
ν3 (Oh) (nm)
B (cm−1)
Δo (cm−1)
LDH-Co-1 LDH-Co-3 LDH-Co-6 LDH-Co-12 LDH-Co-24 MgCoAl-LDH CoAl-LDH
591.8 587.1 592.8 593.6 593.6 596.0 601.8
519.3 520.6 523.6 526.8 528.4 527.8 534.7
840 840 840 830 830 830 820
9350 9370 9300 9270 9250 9240 9140
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Fig. 6 (A) XANES spectra of (a) LDH-Co-1, (b) LDH-Co-3, (c) LDH-Co-6, (d) LDH-Co-12, (e) LDH-Co-24, (f ) MgCoAl-LDH, (g) CoAl-LDH, and (h) β-Co(OH)2. (B) Fourier transformed EXAFS spectra for (a) LDH-Co-1, (b) LDH-Co-3, (c) LDH-Co-6, (d) LDH-Co-12, (e) LDH-Co-24, (f ) MgCoAl-LDH, and (g) CoAl-LDH.
while, the shorter bond distances between neighbouring ions can trigger an inductive effect, which results in the larger ionicity and higher chemical shifts.52 Thus, Co2+ in an LDH (lattice parameter a = 0.306 nm) contains shorter Co–O bond lengths than with β-Co(OH)2 (a = 0.312 nm) and exhibits greater chemical shifts. Similar changes in the chemical shift between β-Co(OH)2 and Co2+-containing LDH in Co K-edge have been reported by Leroux et al.53 Therefore, it was concluded that all the Co2+ ions in the substituted LDHs were stabilized in octahedral sites of pure LDH structure, and cobalt hydroxide impurities did not develop. To compare the local structure around the Co2+ ion in substituted LDH with that of conventionally prepared reference LDHs (MgCoAl-LDH and CoAl-LDH), we analysed the Fourier-transforms (FTs) of the extended X-ray absorption fine structure (EXAFS) in Co K-edge by utilizing the UWXAFS code (Fig. 6(B)). The peaks near 1.7 Å (dashed line in Fig. 6(B)) and 2.7 Å (dotted line in Fig. 6(B)) in FTs (non-phase-shift-corrected) could be ascribed to the contribution of Co–O and Co– (Mg, Co, Al), respectively. The first shells for LDH-Co-1 and LDH-Co-3 were very broad, suggesting that LDH-Co-1 and LDH-Co-3 had poorly crystalline characteristics surrounding the Co2+ ions. Such results were in good agreement with the chemical formulae of LDH-Co-1 and LDH-Co-3 obtained from ICP-AES (Table 1), in which a very small amount of Co2+ was found in both LDH-Co-1 and LDH-Co-3. However, as the reaction time increased, the FTs for all the samples gradually resembled that of MgCoAl-LDH, in which the chemical formula was closer to LDH-Co-24. Furthermore, the spectral features of LDH-Co-12 and LDH-Co-24 were virtually identical to that of MgCoAl-LDH, clarifying that Co2+ ions were successfully incorporated into the LDH framework under proper hydrothermal treatment. The second shell intensities of the samples were relatively small owing to the nanocrystalline nature of Co2+-substituted LDHs.
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Experimental
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Materials Mg(NO3)2·6H2O, Al(NO3)3·9H2O, and NaHCO3 were purchased from Sigma-Aldrich Co., Ltd (USA); NaOH pellets and CoCl2 were obtained from Daejung Chemicals & Metals Co., Ltd (Korea). All chemicals were used without further purification. Synthesis of pristine LDH, MgCoAl-LDH, and CoAl-LDH For the preparation of pristine MgAl-LDH having controlled particle size, morphology, and homogeneous distribution, a mixed metal solution (0.1875 M of Mg2+ and 0.0973 M of Al3+) was titrated by an alkaline solution (0.75 M of NaOH and NaHCO3) until pH ∼9.5, followed by the hydrothermal treatment at 150 °C for 48 h.41 The final products were washed several times with deionized water. The pristine LDH was stored in a slurry state after centrifugation at 10 000 rpm. For the comparative study, we prepared MgCoAl-LDH and CoAl-LDH through a conventional coprecipitation method. A mixed metal solution (0.04685 M of Mg2+, Co2+, and Al3+ for MgCoAl-LDH; and 0.0937 M of Co2+ and 0.04685 M of Al3+ for CoAl-LDH, respectively) was titrated with an alkaline solution (0.375 M of NaOH and NaHCO3) until the pH reached ∼9.5. The precipitates were aged for 24 h at room temperature. The resulting precipitate was collected by centrifugation and washed several times with deionized water and then dried with a lyophilizer. Substitution of Co2+ under hydrothermal condition For the substitution of Co2+, an LDH was mixed with a CoCl2 solution (0.02 M) and directly subjected to hydrothermal treatment. First, pristine LDH was suspended in 5 mg mL−1 of deionized water and 80 mL of the suspension was mixed with 100 mL of a 0.02 M Co2+ solution. After vigorous stirring for ∼3 min, the mixtures were transferred to autoclaves and hydrothermally treated at 150 °C. At each time point (1, 3, 6, 12, and 24 h), the reaction was quenched and Co2+-substituted LDHs were collected by centrifugation, and washed with deionized water and lyophilized. The Co2+-substituted LDHs are denoted LDH-Co-n, where n stands for reaction time in hours. Characterisation The powder XRD patterns were obtained with a Bruker AXS D2 phaser (LYNXEYE™ detector) by utilizing Ni-filtered Cu-Kα radiation (λ = 1.5406 Å) with a 1 mm air-scattering slit and a 0.1 mm equatorial slit. Data were collected from 3° to 70° (2θ) with time step increments of 0.02° and 0.5 s per step, respectively. ICP-AES (Perkin Elmer Optima-7300DV) was utilized to determine the chemical composition of pristine LDH, Co2+-substituted LDH (LDH-Co-1–LDH-Co-24), and Co2+-containing LDHs (MgCoAl-LDH, CoAl-LDH). The powdered sample was completely dissolved in hydrochloric acid and diluted with ultrapure deionized water to obtain a metal concentration within 10–100 ppm. The ICP-AES measurement was repeated three times and the average values were utilized.
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The particle size and morphology of pristine and Co2+-substituted LDHs were measured with SEM on Quanta 250 FEG. For the SEM measurements, the powdered samples were sus−1 pended in deionized water (∼0.4 mg mL ) and sonicated for 3 min to disperse the particles. One drop of the suspension was placed on a piranha-solution-treated silicon wafer. After vacuum drying at 40 °C, the sample was coated with Pt/Pd plasma for 50 s and the images were collected by a 30 kV accelerated electron beam. To obtain the average particle size and sample thickness, 50 particles from at least five different sites were randomly selected for measurement. To visualize the distribution of metal ions in Co2+-substituted LDHs, TEM and EDS mappings were carried out with a Tecnai F20 instrument at the Korean Basic Science Institute, Gwangju Branch, Korea. For TEM analysis, a powdered sample was dispersed into deionized water (sonication for 5 min), and one drop was placed on a 200-square mesh copper grid with a carbon film and then vacuum-dried. The TEM and EDS-mapping images were obtained by a 200 kV accelerated electron beam. The solid-state UV-vis absorbance spectra of the prepared samples were obtained with a UV-vis spectrometer (Thermo EVOLUTION 220). The spectra were collected in the range 200–800 nm. The local symmetry and chemical environment of the Co2+ ions were verified by XAS with the 8C Nanoprobe XAFS beam-line at the Pohang Accelerator Laboratory, Pohang, Korea. The XAS results were calibrated with a thin Co metal foil and collected using a thin powdered layer obtained by placing the powdered sample between the transparent adhesive tape. The XAS spectra were measured in the fluorescent mode.
Conclusions We have successfully substituted Mg2+ in a MgAl-LDH lattice with Co2+ utilizing a low-concentration CoCl2 solution (0.02 M) under hydrothermal conditions. Considering the potential application toward drug delivery systems, the particle size of pristine MgAl-LDH was homogeneously controlled to ∼250 nm. Incorporation of Co2+ proved to be closely related to the dissolution of Mg2+ from the LDH lattice and reaches saturation after ∼24 h. We could not observe any significant impurity phase resulting from surface precipitation, or other unexpected reactions, during the substitution reaction. Both the solid-state UV-vis and X-ray absorption spectroscopies provided evidence that the substituted Co2+ ions were well stabilized in the octahedral sites of an LDH that were formerly occupied by Mg2+ ions. It was notable that the crystallinity, particle size, and morphology of pristine LDH were well preserved during the substitution reaction. Therefore, we suggest a soft chemical hydrothermal reaction as a potential method to incorporate divalent ions into an LDH lattice through isomorphous substitution, thus preserving the physical properties of layered double hydroxides.
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Acknowledgements
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This paper was financially supported by the National Research Foundation of Korea (NRF) with grants funded by the Korean government (MSIP) (2005-0049412 and 2010-0024370).
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24 J.-M. Oh, S.-J. Choi, G.-E. Lee, J.-E. Kim and J.-H. Choy, Chem.–Asian J., 2009, 4, 67–73. 25 Z. P. Xu, M. Niebert, K. Porazik, T. L. Walker, H. M. Cooper, A. P. J. Middelberg, P. P. Gray, P. F. Bartlett and G. Q. Lu, J. Controlled Release, 2008, 130, 86–94. 26 K.-M. Kim, J.-H. Kang, A. Vinu, J.-H. Choy and J.-M. Oh, Curr. Top. Med. Chem., 2013, 13, 488–503. 27 J.-H. Choy, S.-Y. Kwak, J.-S. Park and Y.-J. Jeong, J. Mater. Chem., 2001, 11, 1671–1674. 28 S.-Y. Kim, J.-M. Oh, J. S. Lee, T. J. Kim and J.-H. Choy, J. Nanosci. Nanotechnol., 2008, 8, 5181–5184. 29 J.-M. Oh, C.-B. Park and J.-H. Choy, J. Nanosci. Nanotechnol., 2011, 11, 1632–1635. 30 Z. P. Xu, N. D. Kurniawan, P. F. Bartlett and G. Q. Lu, Chem.–Eur. J., 2007, 13, 2824–2830. 31 A. W. Musumeci, T. L. Schiller, Z. P. Xu, R. F. Minchin, D. J. Martin and S. V. Smith, J. Phys. Chem. C, 2009, 114, 734–740. 32 B. I. Lee, J. S. Bae, E. S. Lee and S. H. Byeon, Bull. Korean Chem. Soc., 2012, 33, 601–607. 33 S. Komarneni, N. Kozai and R. Roy, J. Mater. Chem., 1998, 8, 1329–1331. 34 J. R. Conner, in Chemical fixation and solidification of hazardous wastes, Van Nostrand Reinhold, New York, 1990, pp. 32–46. 35 T. Suzuki, K. Ishigaki and M. Miyake, J. Chem. Soc., Faraday Trans. 1, 1984, 80, 3157–3165. 36 M. C. Richardson and P. S. Braterman, J. Mater. Chem., 2009, 19, 7965–7975. 37 M. Park, C. L. Choi, Y. J. Seo, S. K. Yeo, J. Choi, S. Komarneni and J. H. Lee, Appl. Clay Sci., 2007, 37, 143–148. 38 Z. Liu, R. Ma, M. Osada, K. Takada and T. Sasaki, J. Am. Chem. Soc., 2005, 127, 13869–13874. 39 Y. Zhang, X. Xia, J. Kang and J. Tu, Chin. Sci. Bull., 2012, 57, 4215–4219. 40 W. Liu, X. Li, D. Jiang, J. Qian, Q. Liu, X. Yang and K. Wang, CrystEngComm, 2014, 16, 2395–2403. 41 J.-M. Oh, S.-H. Hwang and J.-H. Choy, Solid State Ionics, 2002, 151, 285–291. 42 A. I. Y. Tok, F. Y. C. Boey, Z. Dong and X. L. Sun, J. Mater. Process. Technol., 2007, 190, 217–222. 43 F. M. Labajos, V. Rives and M. A. Ulibarri, J. Mater. Sci., 1992, 27, 1546–1552. 44 J.-M. Oh, D.-H. Park, S.-J. Choi and J.-H. Choy, Recent Pat. Nanotechnol., 2012, 6, 200–217. 45 I. I. Volchenskova and K. B. Yatsimirskii, Theor. Exp. Chem., 1971, 4, 415–420. 46 M. Mendolia, H. Cai and G. C. Farrington, The study of polyether solvation mechanisms using UV-vis spectroscopy, Technical report no. 1992-26, Office of Naval Research, Arlington, 1992. 47 A. A. Verberckmoes, M. G. Uytterhoeven and R. A. Schoonheydt, Zeolites, 1997, 19, 180–189. 48 J. Taghavimoghaddam, G. Knowles and A. Chaffee, Top. Catal., 2012, 55, 571–579.
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51 V. B. Sapre and C. Mande, J. Phys. C: Solid State Phys., 1972, 5, 793. 52 S.-M. Paek and Y.-I. Kim, J. Alloys Compd., 2014, 587, 251–254. 53 F. Leroux, E. M. Moujahid, C. Taviot-Guého and J.-P. Besse, Solid State Sci., 2001, 3, 81–92.
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49 L. J. Zhao, L. F. Duan, Y. Q. Wang and Q. Jiang, J. Phys. Chem. C, 2010, 114, 10691–10696. 50 C. J. Wang, Y. A. Wu, R. M. J. Jacobs, J. H. Warner, G. R. Williams and D. O’Hare, Chem. Mater., 2010, 23, 171–180.
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Dalton Trans., 2014, 43, 10430–10437 | 10437
As featured in: Volume 43 Number 27 21 July 2014 Pages 10261–10652
Dalton Transactions An international journal of inorganic chemistry www.rsc.org/dalton
Isomorphous substitution of divalent metal ions in layered double hydroxides through a soft chemical hydrothermal reaction We proposed a soft chemical reaction to isomorphously substitute aqueous Co(II) ions for Mg(II) in the lattice of layered double hydroxides (LDHs). This method was proven to introduce external metal ions into the crystal lattice without changing crystallinity, size and morphology of LDH nanomaterials.
Themed issue: Layered Inorganic Solids ISSN 1477-9226
PAPER B. Marler et al. Topotactic condensation of layer silicates with ferrierite-type layers forming porous tectosilicates
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Isomorphous substitution of divalent metal ions in layered double hydroxides through a soft chemical hydrothermal reaction† Tae-Hyun Kim,a Won-Jae Lee,b Ji-Yeong Lee,a Seung-Min Paek*b and Jae-Min Oh*a We have successfully incorporated Co2+ ions into layered double hydroxides (LDHs) comprising Mg and Al hydroxides via isomorphous substitution utilizing a soft chemical hydrothermal reaction. The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis showed that the incorporation of Co2+ into an LDH was highly dependent on the dissolution of Mg2+. The X-ray diffraction (XRD) patterns showed that the crystalline phase, as well as the crystallinity of pristine LDH, was well preserved without the evolution of impurities during the substitution reaction. It was notable that the size (∼250 nm) and hexagonal plate-like morphology of LDHs did not change significantly upon Co2+ substitution. Trans-
Received 5th February 2014, Accepted 4th March 2014
mission electron microscopy-energy dispersive spectroscopy (TEM-EDS) exhibited homogeneous distribution of Co2+ in the LDH particles obtained by this substitution reaction. Solid-state UV-vis and X-ray
DOI: 10.1039/c4dt00373j
absorption spectroscopy (XAS) verified that the incorporated Co2+ ions were well stabilized in the
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octahedral sites of an LDH, which were formerly occupied by Mg2+ ions.
Introduction Layered materials comprising various chemical compositions of metal oxides or metal hydroxides have swelling properties, high specific surface areas, exchangeable interlayer substances, and modifiable external surfaces resulting from their characteristic 2D nanosheet-stacking structure.1,2 Therefore, they have been considered as versatile materials with high applicability in various fields, including catalyst, catalytic support, biomedical, and environmental fields.3–7 Among the various classes of layered materials, layered double hydroxides (LDHs), also known as anionic clays, along with layered hydroxy salts and hydroxy double salts, are unique materials that possess positively charged layers to accommodate anionic chemical species between the layers. The general chemical formula of an LDH is M(II)1−xM(III)x(OH)2(An−)x/n· mH2O (0 < x < 1; m and n are integers), where M(II) and M(III) stand for divalent and trivalent metal cations, respectively, and a Department of Chemistry and Medical Chemistry, College of Science and Technology, Yonsei University, Wonju, Gangwondo 220-710, Korea. E-mail: [email protected]; Fax: +82-33-760-2182; Tel: +82-33-760-2368 b Department of Chemistry, Kyungpook National University, Taegu 702-701, Korea. E-mail: [email protected]; Fax: +82-53-950-6330; Tel: +82-53-950-5335 † Electronic supplementary information (ESI) available: ICP-AES results of Co2+ substitution under room temperature, calculated lattice parameters of Co2+-sub2+ stituted LDHs, XRD patterns of Co -substituted LDHs with a high concentration 2+ of Co in solution, and SEM-EDS mapping results. See DOI: 10.1039/c4dt00373j
10430 | Dalton Trans., 2014, 43, 10430–10437
An− represents the anionic species.8 The structure of an LDH is based on that of Mg(OH)2 (brucite), where Mg(OH)6 octahedra are infinitely propagated in the direction of the xy-plane by sharing their edges.9 Partial substitution of Mg2+ with trivalent cations such as Al3+ produces permanently positively charged layers; this substitution is further compensated by interlayerexchangeable anions.10,11 It has been known that selected LDHs such as MgAl-LDH and ZnAl-LDH have low biological toxicity,12 and can be dissolved in ionic species under weakly acidic physiological conditions.13,14 Thus, LDHs have been attracting tremendous interest, especially in the field of biomedical applications. Furthermore, LDHs have found application as cellular delivery carriers for anionic drug molecules,15–18 because the plasma membranes are usually negatively charged.19 It has been reported that LDHs can effectively deliver not only anionic anticancer agents such as methotrexate and 5-fluorouracil but also therapeutic genes to enhance the medicinal efficacy.18,20–22 The main reason for the effective cellular delivery by LDHs is clathrin-mediated endocytosis.18 It has been demonstrated that the size and morphology of an LDH play key roles in cellular uptake and intracellular trafficking.23–25 Especially, the particle size of an LDH is essential in cellular delivery with a maximized cellular uptake efficiency of ∼200 nm particles.24 Although LDHs exhibit high cellular-delivery efficacy, it is difficult to label them with a traceable moiety for detection in biological systems. Thus, they cannot be, as yet, utilized in a
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practical drug delivery system. Recent studies on nanomedicine pursue theranosis, a simultaneous diagnosis and therapy strategy.26 Tracing of LDHs in a biological system would increase the possibility of LDHs serving as practical drug delivery systems for nanomedicine. There have been several reports on the labelling of LDHs with tracers. Fluorescent dyes, magnetic resonance contrasting Gd complexes, and radioisotopes have been incorporated into LDHs via intercalation or surface modification.24,27–30 However, the introduction of these tracers into the lattice of LDHs is required for robust labelling. Recently, Musumeci et al. have directly incorporated radioactive 57Co2+ and 67Ga3+ into the LDH lattice to develop radiolabelled LDHs.31 Byeon et al. have developed a new type of LDHs that consists of only rare earth metals having photoluminescence properties.32 To optimise the application of labelled-LDHs to nanomedicine, it is of utmost importance to obtain LDHs with wellcontrolled size and morphology. MgAl-LDH is one of the most extensively studied LDHs in terms of size and morphology control. It is possible to fabricate MgAl-LDHs with particle sizes ∼200 nm and plate-like shapes with relatively low aspect ratios (diameter/thickness), which are favourable properties for cellular uptake.24 In this context, we aimed to synthesise MgAl-LDHs with controlled size and morphology whilst incorporating Co2+ into the LDH lattice via isomorphous substitution under soft chemical reaction conditions. 57Co is a radioisotope detectable by positron emission tomography (PET) and can thus be employed as a radiolabel. In this study, we utilized non-radioisotopic Co2+ for incorporation into LDHs as a proof-of-concept experiment. The possibility that metal ions in solid LDHs can be isomorphously substituted by the ions in solution was first suggested by Komarneni et al.,33 possibly via diadochy, a wellknown cation-exchange process in calcite or hydroxyapatite.34,35 Richardson et al. observed a similar phenomenon by suspending LDHs in metal-ion-containing solutions for a very long period of 5 days.36 Infrared (IR) and diffuse reflectance visible-to-near infrared spectroscopies were utilized to confirm the isomorphous substitution in LDHs. However, considerable structural reorganization in the LDH lattice was also reported. Park et al. studied the reaction between LDHs and metal ions, Cu2+ and Pb2+, and observed the evolution of metal hydroxide precipitates on the surface of LDHs.37 In this study, our overarching goal was to incorporate Co2+ into MgAl-LDHs that possess a well-controlled size and morphology via isomorphous substitution under soft chemical hydrothermal treatment. To accomplish this, we desired to: (1) preserve the crystallinity, size, and shape of pristine LDH, (2) introduce Co2+ homogeneously into the lattice, (3) minimize the formation of surface precipitation impurities, and (4) carry out the substitution in a short time under simple reaction conditions. In this paper, we demonstrate the successful substitution of Mg2+ with Co2+ in MgAl-LDH utilizing both structural characterisation (X-ray diffraction (XRD), solid state UV-vis spectroscopy, and X-ray absorption spectroscopy (XAS)) and microscopic techniques (scanning electron microscopy (SEM)
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and transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS)).
Results and discussion The chemical formulae of pristine LDH and Co2+-substituted LDH were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Table 1). Co2+ was found to gradually incorporate over time into the LDH lattice upon exposure to a 0.02 M Co2+ solution under 150 °C hydrothermal conditions. The graph in Fig. 1 displays time-dependent changes in the Co2+ and Mg2+ molar ratio with regard to the net composition of divalent ions (Mg2+ + Co2+) in the LDH lattice. We could clearly observe the time-dependent increase in Co2+ and the corresponding decrease in Mg2+ through an S-type curve. The S-type curve of Co2+ substitution was different from the L-type Co2+ uptake reported by Komarneni et al.,33 suggesting that hydrothermal conditions required a minimum time of 3 h for effective metal substitution.
Table 1 Chemical formula, particle diameter, and thickness of pristine and Co2+-substituted LDHs ( particle diameter and thickness of LDH particles were calculated as the average of 50 randomly selected particles from SEM images). Samples are named in the format LDH-Co-n, where n stands for reaction time (in hours) of the LDH sample in 0.02 M Co2+ solution under 150 °C hydrothermal conditions
Sample
Chemical formula
Pristine LDH LDH-Co-1
Mg2.54Al1(OH)7.08(CO3)0.5· mH2O Mg2.02Co0.008Al1 (OH)6.06(CO3)0.5·mH2O Mg2.05Co0.025Al1 (OH)6.15(CO3)0.5·mH2O Mg1.65Co0.59Al1 (OH)6.48(CO3)0.5·mH2O Mg1.31Co0.97Al1 (OH)6.56(CO3)0.5·mH2O Mg1.25Co1.04Al1 (OH)6.58(CO3)0.5·mH2O
LDH-Co-3 LDH-Co-6 LDH-Co-12 LDH-Co-24
Diameter (nm)
Thickness (nm)
250 ± 20
80 ± 8
250 ± 22
80 ± 10
250 ± 20
80 ± 10
250 ± 20
90 ± 10
280 ± 27
110 ± 21
290 ± 30
140 ± 19
Fig. 1 Time-dependent change of Mg2+ (open circle, right-side y-axis) and Co2+ (solid circle, left-side y-axis) content in the LDH lattice. The y-axis is the molar ratio between Mg2+ or Co2+ and net divalent metal (Mg2+ + Co2+).
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Both curves for Co2+ increase and Mg2+ decrease were topto-bottom symmetrical, suggesting that Co2+ incorporation into the LDH lattice was closely related to Mg2+ dissolution into the aqueous environment, thus implying that the substitution of Mg2+ with Co2+ is isomorphous. Co2+ substitution reaches a plateau at ∼24 h (Fig. 1) at which time the Mg2+/Co2+ molar ratio was almost 1 (Table 1). At room temperature, substitution of Co2+ into MgAl-LDH does not occur to a significant extent, even at high Co2+ concentrations (0.8 M) after 24 h of reaction time (Table S1†). Whilst Co2+ could be incorporated into the LDH lattice upon dissolution of Mg2+, there still remained concern pertaining to the isomorphous nature of substitution between Mg2+ and Co2+. The aqueous Co2+ might not exist in the previously occupied Mg2+ site in LDHs, forming cobalt hydroxide precipitates similarly to the report by Park et al.37 Various cobalt hydroxide precipitates such as α-Co(OH)2 (JCPDS no. 741057),38 β-Co(OH)2 (JCPDS no. 30-0443)39 and Co2(OH)3Cl (JCPDS no. 73-2134)40 could form through hydrolysis of Co2+ under basic conditions. The powder XRD patterns for pristine and Co2+-substituted LDHs are shown in Fig. 2. Pristine
MgAl-LDH showed a well-crystallised and single-phased hydrotalcite-like (JPCDS no. 14-0191) structure (Fig. 2(a)). Sharp peaks corresponding to (003) and (006) revealed that the pristine LDH possessed well-organised 2D layer stacking. Other reflection peaks attributed to the lattice, such as (013), (015), (016), (018), (019), (110), and (113), were observed at 35.9°, 38.4°, 41.5°, 45.5°, 49.9°, 60.7°, and 62.0°, respectively.41 We found a slight shift in the peak position of (110) according to reaction time. The calculated lattice parameter a decreased until 3 h and increased afterwards within a small range of ±0.01 Å (Table S2†). This was attributed to the formation of stable crystals during hydrothermal treatment as reported by Tok et al.42 The XRD patterns for Co2+-substituted LDHs were nearly identical to those of pristine LDHs in terms of peak position and intensities; no significant impurity phase (corresponding to α-Co(OH)2, β-Co(OH)2 or Co2(OH)3Cl) was found. This result confirms that there was neither evolution of an impurity phase nor structural deformation during the substitution reaction. However, an impurity phase corresponding to Co2(OH)3Cl (JCPDS no. 73-2134) was found when Co2+ solutions of higher concentration (≥0.4 M) were treated with LDHs under hydrothermal conditions (Fig. S1†). Therefore, we concluded that 0.02 M Co2+ concentration was appropriate for isomorphous substitution at 150 °C. The overall particle size and morphology of MgAl-LDH, the most important factors in drug delivery application, were wellpreserved after the Co2+-substitution reaction, as proved by the SEM images of pristine and Co2+-substituted LDHs (Fig. 3). Pristine LDH (Fig. 3(a)) showed the typical shape of hexagonal hydrotalcite as prepared by hydrothermal methods.41,43 The average particle size and standard deviation of pristine LDH (determined by the examination of 50 randomly selected particles) were between ∼250 and ∼20 nm, demonstrating a narrow particle size distribution suitable for cellular uptake.24 Upon hydrothermal treatment in Co2+ solutions, no significant changes in the overall particle shape were observed and the hexagonal plate-like morphology of pristine LDH was preserved (Fig. 3(b)–(f ), Table 1).
Fig. 2 Powder X-ray diffraction patterns of (a) pristine and Co2+-substi2+ tuted LDHs in 0.02 M Co solution under 150 °C hydrothermal conditions: (b) LDH-Co-1, (c) LDH-Co-3, (d) LDH-Co-6, (e) LDH-Co-12, and (f ) LDH-Co-24.
Fig. 3 Scanning electron microscopic images of (a) pristine and Co2+-sub2+ stituted LDHs in 0.02 M Co solution under 150 °C hydrothermal conditions: (b) LDH-Co-1, (c) LDH-Co-3, (d) LDH-Co-6, (e) LDH-Co-12, and (f) LDH-Co-24.
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According to the Student’s t-test, LDH-Co-1, LDH-Co-3, and LDH-Co-6 were determined to have statistically the same particle diameter as that of pristine LDH, while LDH-Co-12 and LDH-Co-24 showed slightly increased particle sizes. It is believed that the LDH particles grew by an aging effect whilst exchanging with Co2+ (in solution) under hydrothermal conditions. However, this does not imply the evolution of a solid impurity phase through surface precipitation. No significant signals were observed in XRD patterns, thus confirming the absence of surface precipitation (Fig. 2). The distribution of Co2+ ions in the LDH lattice upon substitution with 0.02 M Co2+ solution (under hydrothermal conditions) was visualized with energy dispersive spectroscopy (EDS) mapping (Fig. 4). In order to observe the distribution of metal ions (Mg2+, Al3+, and Co2+) in a single particle, we magnified the images to contain a few particles and carried out EDS mapping experiments. Yellow, cyan, and magenta colours indicate the locations of Mg2+, Al3+, and Co2+, respectively. We could clearly observe that Co2+ was homogeneously distributed throughout the entire particle in both LDH-Co-1 (Fig. 4(b′)) and LDH-Co-24 (Fig. 4(c′)), while no cobalt was detected in pristine LDH (Fig. 4(a′)). Furthermore, the content of cobalt increased significantly with increasing reaction times, which was in good agreement with the ICP-AES results (Table 1). The elemental distribution mapping, at low magnification, was also carried out with SEM-EDS mapping and likewise showed a homogeneous distribution of the three different metal ions in the LDH-Co-24 sample (Fig. S2†). If the Co2+ substitution only occurred at the edge or surface of LDH particles or if cobalt hydroxide precipitates covered the LDH particles, the Co2+ distribution would be localized to only the outer parts of LDH particles. We therefore conclude that the successful replacement of Mg2+ with Co2+ occurs through isomorphous substitution as corroborated by the EDS mapping results. We next carried out solid-state UV-vis spectroscopic studies to verify that the substituted Co2+ ions were stabilized in the
Fig. 4 Transmission electron microscopic images (a–c) and corresponding energy dispersive spectroscopy (EDS) mapping images (a’–c’) of (a, a’) pristine LDH and Co2+-substituted LDHs in 0.02 M Co2+ solution under 150 °C hydrothermal conditions: (b, b’) LDH-Co-1 and (c, c’) LDH-Co-24. (Legend: yellow = Mg2+, cyan = Al3+, magenta = Co2+.)
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Fig. 5 Solid state UV-vis absorption spectra of Co2+-substituted LDHs: (a) LDH-Co-1, (b) LDH-Co-3, (c) LDH-Co-6, (d) LDH-Co-12, (e) LDH-Co-24, and those of conventionally prepared Co2+-containing LDHs: (f) MgCoAlLDH and (g) CoAl-LDH (solid line: observed peak, dotted line: fitted peaks, open circle: summation of fitted peaks).
octahedral sites of an LDH that were formerly occupied by Mg2+ ions (Fig. 5). For the comparative study, we prepared MgCoAl-LDH (Mg1.18Co1.12Al(OH)6.6·mH2O) and CoAl-LDH (Co2.23Al-(OH)6.46(CO3)0.5·mH2O) through conventional coprecipitation10,44 as reference samples. There are three possible spin-allowed d–d transitions in the octahedral symmetry of Co2+ ions: ν1 (4T1g(F) → 4T2g(F)), ν2 (4T1g(F) → 4A2g(F)) and ν3 (4T1g(F) → 4T1g(P)).45 The UV-vis spectra of all samples showed broad peaks ranging from 400 to 650 nm. To analyse the peak positions and relative intensities in detail, we separated the peaks by utilizing the Gaussian peak function in OrginPro® 8.0. As the ν1 transition is generally observed in the near-IR region and the ν2 and ν3 transitions are known to occur in the visible light region,46 we assigned the two peaks at ∼600 and 530 nm as ν2 and ν3, respectively. It is also known that the ν2 transition, a 2-electron process with very low probability, is rarely observed,46 which is in good agreement with our results showing reduced absorbance for ν2. The small peak observed around 450 nm was also attributed to the absorption of the
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Co(OH)6 octahedron.47 The strongest peak was attributed to ν3, indicating the existence of Co2+ ions in octahedral sites. The characteristic pink colour of Co(OH)6-containing compounds was clearly observed in all samples. We found that the ν3 transition shifted bathochromically upon increasing the cobalt content from 519.3 nm for LDH-Co-1 to 528.4 nm for LDH-Co24 (Table 2). This kind of red shift in cobalt transition may be attributed to the increasing Co2+ content according to substitution time.48,49 The Racah parameter (B) and octahedral splitting energy (Δo) were calculated from the UV-vis spectra and the Tanabe–Sugano diagram with a d7 configuration. As summarised in Table 2, the calculated B and Δo values of Co2+-substituted LDH lay in the regions 830–840 cm−1 and 9250–9350 cm−1, respectively. Although the values showed slight decreases in energy in correlation with increasing reaction time and Co2+ content, these values were fairly comparable with those of [Co(OH)6]4− (Δo = 9200 cm−1 and B = 825 cm−1), as previously reported.50 The UV-vis spectroscopic results suggested that all of the Co2+ ions in the LDH lattice were well stabilized in the octahedral sites formerly occupied by Mg2+ ions. The local structure and chemical environment around Co2+ in Co2+-substituted LDHs were also confirmed by XAS. As XAS is a very sensitive spectroscopic tool that reflects the subtle changes in symmetry and bond lengths in coordinate compounds, it would elucidate the precise chemical environment around the substituted Co2+. The Co K-edge X-ray absorption near-edge structure (XANES) of Co2+-substituted LDHs and reference compounds (MgCoAl-LDH and CoAl-LDH) showed intense white lines attributed to the 1s → 4p transition at ∼7720 eV (Fig. 6(A)). The main edge positions of the Co2+-substituted LDHs and reference compounds were nearly identical. Furthermore, we could not find any significant preedges that would correspond to Co in tetrahedral symmetry, corroborating that the Co2+ ions existed in octahedral sites. It was notable that the threshold energy of Co-containing LDHs (dotted line in Fig. 6(A)) was ∼3.02 eV higher than that of β-Co(OH)2 (dashed line in Fig. 6(A)). This chemical shift was attributed to the different effective nuclear charge of Co in the LDH phase as compared with that of Co in β-Co (OH)2. According to Sapre’s report,51 the chemical shift becomes larger with increasing effective nuclear charge. Mean-
Table 2 Peak positions of ν2 and ν3 transitions, calculated Racah parameters (B), and octahedral splitting energy (Δo) values for Co2+-substituted LDH compounds. (The Racah parameter and octahedral ligand field split7 ting energy were calculated on the basis of the d Tanabe–Sugano diagram.)
Sample
ν2 (Oh) (nm)
ν3 (Oh) (nm)
B (cm−1)
Δo (cm−1)
LDH-Co-1 LDH-Co-3 LDH-Co-6 LDH-Co-12 LDH-Co-24 MgCoAl-LDH CoAl-LDH
591.8 587.1 592.8 593.6 593.6 596.0 601.8
519.3 520.6 523.6 526.8 528.4 527.8 534.7
840 840 840 830 830 830 820
9350 9370 9300 9270 9250 9240 9140
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Fig. 6 (A) XANES spectra of (a) LDH-Co-1, (b) LDH-Co-3, (c) LDH-Co-6, (d) LDH-Co-12, (e) LDH-Co-24, (f ) MgCoAl-LDH, (g) CoAl-LDH, and (h) β-Co(OH)2. (B) Fourier transformed EXAFS spectra for (a) LDH-Co-1, (b) LDH-Co-3, (c) LDH-Co-6, (d) LDH-Co-12, (e) LDH-Co-24, (f ) MgCoAl-LDH, and (g) CoAl-LDH.
while, the shorter bond distances between neighbouring ions can trigger an inductive effect, which results in the larger ionicity and higher chemical shifts.52 Thus, Co2+ in an LDH (lattice parameter a = 0.306 nm) contains shorter Co–O bond lengths than with β-Co(OH)2 (a = 0.312 nm) and exhibits greater chemical shifts. Similar changes in the chemical shift between β-Co(OH)2 and Co2+-containing LDH in Co K-edge have been reported by Leroux et al.53 Therefore, it was concluded that all the Co2+ ions in the substituted LDHs were stabilized in octahedral sites of pure LDH structure, and cobalt hydroxide impurities did not develop. To compare the local structure around the Co2+ ion in substituted LDH with that of conventionally prepared reference LDHs (MgCoAl-LDH and CoAl-LDH), we analysed the Fourier-transforms (FTs) of the extended X-ray absorption fine structure (EXAFS) in Co K-edge by utilizing the UWXAFS code (Fig. 6(B)). The peaks near 1.7 Å (dashed line in Fig. 6(B)) and 2.7 Å (dotted line in Fig. 6(B)) in FTs (non-phase-shift-corrected) could be ascribed to the contribution of Co–O and Co– (Mg, Co, Al), respectively. The first shells for LDH-Co-1 and LDH-Co-3 were very broad, suggesting that LDH-Co-1 and LDH-Co-3 had poorly crystalline characteristics surrounding the Co2+ ions. Such results were in good agreement with the chemical formulae of LDH-Co-1 and LDH-Co-3 obtained from ICP-AES (Table 1), in which a very small amount of Co2+ was found in both LDH-Co-1 and LDH-Co-3. However, as the reaction time increased, the FTs for all the samples gradually resembled that of MgCoAl-LDH, in which the chemical formula was closer to LDH-Co-24. Furthermore, the spectral features of LDH-Co-12 and LDH-Co-24 were virtually identical to that of MgCoAl-LDH, clarifying that Co2+ ions were successfully incorporated into the LDH framework under proper hydrothermal treatment. The second shell intensities of the samples were relatively small owing to the nanocrystalline nature of Co2+-substituted LDHs.
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Experimental
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Materials Mg(NO3)2·6H2O, Al(NO3)3·9H2O, and NaHCO3 were purchased from Sigma-Aldrich Co., Ltd (USA); NaOH pellets and CoCl2 were obtained from Daejung Chemicals & Metals Co., Ltd (Korea). All chemicals were used without further purification. Synthesis of pristine LDH, MgCoAl-LDH, and CoAl-LDH For the preparation of pristine MgAl-LDH having controlled particle size, morphology, and homogeneous distribution, a mixed metal solution (0.1875 M of Mg2+ and 0.0973 M of Al3+) was titrated by an alkaline solution (0.75 M of NaOH and NaHCO3) until pH ∼9.5, followed by the hydrothermal treatment at 150 °C for 48 h.41 The final products were washed several times with deionized water. The pristine LDH was stored in a slurry state after centrifugation at 10 000 rpm. For the comparative study, we prepared MgCoAl-LDH and CoAl-LDH through a conventional coprecipitation method. A mixed metal solution (0.04685 M of Mg2+, Co2+, and Al3+ for MgCoAl-LDH; and 0.0937 M of Co2+ and 0.04685 M of Al3+ for CoAl-LDH, respectively) was titrated with an alkaline solution (0.375 M of NaOH and NaHCO3) until the pH reached ∼9.5. The precipitates were aged for 24 h at room temperature. The resulting precipitate was collected by centrifugation and washed several times with deionized water and then dried with a lyophilizer. Substitution of Co2+ under hydrothermal condition For the substitution of Co2+, an LDH was mixed with a CoCl2 solution (0.02 M) and directly subjected to hydrothermal treatment. First, pristine LDH was suspended in 5 mg mL−1 of deionized water and 80 mL of the suspension was mixed with 100 mL of a 0.02 M Co2+ solution. After vigorous stirring for ∼3 min, the mixtures were transferred to autoclaves and hydrothermally treated at 150 °C. At each time point (1, 3, 6, 12, and 24 h), the reaction was quenched and Co2+-substituted LDHs were collected by centrifugation, and washed with deionized water and lyophilized. The Co2+-substituted LDHs are denoted LDH-Co-n, where n stands for reaction time in hours. Characterisation The powder XRD patterns were obtained with a Bruker AXS D2 phaser (LYNXEYE™ detector) by utilizing Ni-filtered Cu-Kα radiation (λ = 1.5406 Å) with a 1 mm air-scattering slit and a 0.1 mm equatorial slit. Data were collected from 3° to 70° (2θ) with time step increments of 0.02° and 0.5 s per step, respectively. ICP-AES (Perkin Elmer Optima-7300DV) was utilized to determine the chemical composition of pristine LDH, Co2+-substituted LDH (LDH-Co-1–LDH-Co-24), and Co2+-containing LDHs (MgCoAl-LDH, CoAl-LDH). The powdered sample was completely dissolved in hydrochloric acid and diluted with ultrapure deionized water to obtain a metal concentration within 10–100 ppm. The ICP-AES measurement was repeated three times and the average values were utilized.
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The particle size and morphology of pristine and Co2+-substituted LDHs were measured with SEM on Quanta 250 FEG. For the SEM measurements, the powdered samples were sus−1 pended in deionized water (∼0.4 mg mL ) and sonicated for 3 min to disperse the particles. One drop of the suspension was placed on a piranha-solution-treated silicon wafer. After vacuum drying at 40 °C, the sample was coated with Pt/Pd plasma for 50 s and the images were collected by a 30 kV accelerated electron beam. To obtain the average particle size and sample thickness, 50 particles from at least five different sites were randomly selected for measurement. To visualize the distribution of metal ions in Co2+-substituted LDHs, TEM and EDS mappings were carried out with a Tecnai F20 instrument at the Korean Basic Science Institute, Gwangju Branch, Korea. For TEM analysis, a powdered sample was dispersed into deionized water (sonication for 5 min), and one drop was placed on a 200-square mesh copper grid with a carbon film and then vacuum-dried. The TEM and EDS-mapping images were obtained by a 200 kV accelerated electron beam. The solid-state UV-vis absorbance spectra of the prepared samples were obtained with a UV-vis spectrometer (Thermo EVOLUTION 220). The spectra were collected in the range 200–800 nm. The local symmetry and chemical environment of the Co2+ ions were verified by XAS with the 8C Nanoprobe XAFS beam-line at the Pohang Accelerator Laboratory, Pohang, Korea. The XAS results were calibrated with a thin Co metal foil and collected using a thin powdered layer obtained by placing the powdered sample between the transparent adhesive tape. The XAS spectra were measured in the fluorescent mode.
Conclusions We have successfully substituted Mg2+ in a MgAl-LDH lattice with Co2+ utilizing a low-concentration CoCl2 solution (0.02 M) under hydrothermal conditions. Considering the potential application toward drug delivery systems, the particle size of pristine MgAl-LDH was homogeneously controlled to ∼250 nm. Incorporation of Co2+ proved to be closely related to the dissolution of Mg2+ from the LDH lattice and reaches saturation after ∼24 h. We could not observe any significant impurity phase resulting from surface precipitation, or other unexpected reactions, during the substitution reaction. Both the solid-state UV-vis and X-ray absorption spectroscopies provided evidence that the substituted Co2+ ions were well stabilized in the octahedral sites of an LDH that were formerly occupied by Mg2+ ions. It was notable that the crystallinity, particle size, and morphology of pristine LDH were well preserved during the substitution reaction. Therefore, we suggest a soft chemical hydrothermal reaction as a potential method to incorporate divalent ions into an LDH lattice through isomorphous substitution, thus preserving the physical properties of layered double hydroxides.
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Acknowledgements
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This paper was financially supported by the National Research Foundation of Korea (NRF) with grants funded by the Korean government (MSIP) (2005-0049412 and 2010-0024370).
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