Controlled Crystal Growth of Two-Dimensional Layered

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Controlled crystal growth of 2-dimensional layered nanomaterials in hydrogel via modified electrical double migration method Gyeong-Hyeon Gwak, Na Kyung Kwon, Won-Jae Lee, Seung-Min Paek, So Youn Kim, and Jae-Min Oh Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01252 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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Crystal Growth & Design

Controlled crystal growth of 2-dimensional layered nanomaterials in hydrogel via modified electrical double migration method Gyeong-Hyeon Gwak,†, §Na Kyung Kwon,∥ Won-Jae Lee, ‡ Seung-Min Paek,*,‡ So Youn Kim,*,∥ and Jae-Min Oh*,† †

Department of Chemistry and Medical Chemistry, College of Science and Technology, Yonsei

University, Wonju 26493, Republic of Korea §

Beamline Research Division, Pohang Accelerator Laboratory, Pohang University of Science

and Technology, Pohang 37673, Republic of Korea Address here.

∥ School

of Energy and Chemical Engineering, Ulsan National Institute of Science and

Technology (UNIST), Ulsan 44919, Republic of Korea ‡

Department of Chemistry, Kyungpook National University, Taegu 41566, Republic of Korea

KEYWORDS. Crystal growth in gel, Layered double hydroxide, Agarose, Electrical double migration method, Homogeneous distribution.

ACS Paragon Plus Environment

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ABSTRACT. Layered double hydroxide (LDH) nanomaterials of uniform size and homogeneous distribution are successfully developed in agarose hydrogel through the electrical double migration method. To grow LDH crystals in agarose matrix, both cationic (Ni2+, Ga3+) and anionic precursors (OH- and CO32-) of LDH are simultaneously transported from solution to agarose hydrogel through electric potential, resulting in the in situ crystal growth of LDH. X-ray diffraction patterns, X-ray absorption spectra, and high-resolution transmission electron microscopic images confirm that the developed LDH is ~15 nm in size and had well-defined crystal structure. Scanning electron microscopy and small angle X-ray scattering spectroscopy showed homogeneous arrays of LDH nanocrystals along agarose sheets with homogeneous distribution. On the basis of the characterization results, we suggest that crystal nuclei are first developed on the agarose chain, and then LDHs homogeneously grow along sheets of agarose matrix.

Introduction During the past few decades, biomimetic mineralization of inorganic materials such as calcium carbonate and hydroxyapatite in hydrogel have been studied to comprehend biomineral growth which is essential in the development of functional biomaterials.1-3 When inorganic materials grow in a biopolymer network, as in biomimetic mineralization, the nucleation and growth rates can be regulated by controlling the concentrations of solutions and diffusion rate.2 Compared with conventional crystal growth in solution medium, mineralization in a biopolymer network protects crystallites from exterior perturbations such as convection current or solution turbulence, resulting in defect reudction.3 The basic method for crystal growth in hydrogel is a single diffusion system in which a hydrogel loaded with metal cations (e.g., Ca2+) is immersed into


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solution with counterions.4, 5 This method facilitates a continuous supply of ionic sources and prevents the hydrogel network from drying out. There have been improvements in diffusion systems to enhance their preparation efficiency and to control physicochemical properties: “single diffusion with vapor phase reactant method (solution medium of the ionic source was replaced with a gas phase)” 6 and “double diffusion method (two kinds of ionic source solutions were simultaneously diffused into hydrogel)”.7 The recently suggested double migration method which is electric potential aided double diffusion, achieves rapid and effective ion penetration through the hydrogel to align the concentration gradient of ionic precursors.8 Heinemann et al. introduced hydroxyapatite and calcite in gelatin through this method under various operation time and precursor concentration.8, 9 Watanabe et al. reported hydroxyapatite crystal growth in agarose gel using similar method to increase crystal growth rate, which was 15 times higher than that of simple diffusion.10 The biomimetic materials obtained by double migrations method are versatile in bone substitution or tissue engineering11, 12; however, to the best of our knowledge, the method has been only utilized in calcium containing materials like calcite or hydroxyapatite. For the first time, we tried to apply the electrical double migration method to 2-dimensional layered inorganic materials, which are emerging candidates as drug delivery,13 diagnostics,14 tissue engineering,15 environmental16 and energy applications.17 Layered double hydroxide (LDHs; [M2+(1x)M

3+

nx(OH)2][A ]x/n·mH2O,

M: metal cation, A: anionic species) is a family of 2-dimensional

inorganic material with various compositions and functionalities.18 It has attracted especial interests in biomedical fields including drug/gene reservoir,19 controlled release systems,20 cellular drug/gene delivery carriers,21, 22 and nanodiagnostics.23, 24 The LDHs are conventionally prepared by coprecipitation, in which the pH of metal salt solution is modified by alkaline


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solution to introduce supersaturation18. There have been several approaches to synthesize LDH with electrosynthesis for desired functionality. For instance, Indira et al. prepared LDHs having nickel and trivalent metals (aluminium, chromium and manganese) through electrosynthesis for Ni/Cd battery application.25 Furthermore, Li et al. recently prepared Fe-containing LDHs with hierarchical arrays through electrochemical synthesis for electrocatalyst.26 We have recently proposed that electrical double migration method could be utilized to grow LDH nanoparticles inside hydrogel polymer networks; LDHs having various metal compositions like ZnAl-LDH and MgAl-LDH were successfully prepared through this method.27-29 Especially, hydrozincite (Zn5(OH)8(CO3)), a layered zinc hydroxide, grown in agarose through this electrical double migration provided both drug reservoir and release properties to agarose.27 In this study, we develop NiGa-LDH nanomaterials through electrical double migration and suggest a crystal growth mechanism various analyses. It was verified that LDH nanomaterials can be effectively prepared in situ in agarose hydrogel with superior dispersity through electrical double migration method. First, we confirmed the existence of 2-dimensional LDH structure via high resolution-transmission electron microscopy (HR-TEM), ultraviolet-visible spectroscopy (UV-vis), X-ray diffraction (XRD), diffuse reflectance UV spectroscopy, and X-ray absorption spectroscopy (XAS). Then, the distribution and arrangement of LDH nanoparticles was investigated with scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) mapping, and small-angle X-ray scattering (SAXS). Experimental Materials


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Agarose (~120 KDa) was obtained from Bio Basic Inc., Canada. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), gallium nitrate hydrate (Ga(NO3)3·xH2O), tris(hydroxymethyl)aminomethane (NH2C(CH2OH)3, Tris), and sodium bicarbonate (NaHCO3) were purchased from Sigma-Aldrich LLC, USA. Ammonium hydroxide (NH4OH) and sodium hydroxide (NaOH) were purchased from Duksan Pure Chemicals Company, Korea. LDH nanoparticles in Agarose through Electric double migration (L-E@A) Electrical double migration method for LDH nanoparticles was carried out based on the previous report.27 First, 1 wt/wt% agarose hydrogel (4×1×3.5 cm (width × thickness × height)) was prepared in Tris-HCl buffer solution (pH 7.4) and was placed in the center of an acrylic container (4×15×4 cm). Cationic metal solution (0.16 M Ni2+ and 0.08 M Ga3+) and alkaline solution (0.08 M NaHCO3 and 1 mL NH4OH) were introduced at each side of the container (Scheme 1). Platinum electrodes were plunged into each solution, and 25 V of electric potential was applied for 20 min. Upon application of the electric potential, the color of the agarose gel gradually changed from white to green, implying development of nickel containing particles inside the agarose hydrogel. The obtained L-E@A was thoroughly washed with deionized water and then dehydrated under vacuum or lyophilized for further characterization. LDH nanoparticles through conventional Coprecipitation (L-C) For comparative study, LDH nanoparticles were prepared by a conventional coprecipitation method.30 Cationic metal solution (0.024 M Ni2+ and 0.012 M Ga3+) was titrated with basic solution containing NaOH (0.12 M) and NaHCO3 to pH ~8.5. After 24 h, the cyan-colored precipitates were washed with deionized water, centrifuged, and then lyophilized. Mixture of Agarose and Conventional LDH through melt-blending (L-C@A)


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As a reference material for L-E@A, conventionally synthesized LDH powder (L-C) was introduced in agarose hydrogel by melt-blending. First, agarose powder was clearly melted in Tris-HCl buffer (pH 7.4) to a 1wt/wt% concentration at 80°C. An equivalent mass of lyophilized L-C powder compared with the L-A@E was dispersed in hot agarose solution by vigorous stirring for 2 h. The obtained cyan-colored suspension was then placed in a cuboid mold (4×1×3.5 cm) and cooled at room temperature. Finally, the agarose hydrogel containing L-C (LC@A) was washed and dried similarly to L-E@A. Characterization An HR-TEM was operated by FEI Titan G2 ChemiSTEM Cs Probe at 200 kV. Dehydrated LE@A film was thinly sliced and then immediately placed directly on 200 mesh carbon-coated copper grids (Electron Microscopy Sciences) for observation. The transparency of three dehydrated films (agarose, L-E@A, and L-C@A) was measured with an UV-vis spectrophotometer, Shimadzu UV-1800, in transmittance mode. The XRD patterns were obtained with a Bruker D2 phaser with Ni-filtered Cu-Kα radiation (λ = 1.5406 Å) in the 2θ range from 5° to 70°. Electronic absorption spectra for Ni2+ (d8) of L-C, L-E@A, and L-C@A were obtained with a diffuse reflectance UV-vis spectrometer (Thermo EVOLUTION 220). Spectra were analyzed with a d8 Tanabe-Sugano diagram to obtain the octahedral crystal field splitting energy ∆o and Racah parameter B for Ni(OH)6 octahedron. The local symmetry and chemical environment around nickel and gallium ions were analyzed by XAS at the 8C XAFS beam line, Pohang Accelerator Laboratory, Korea. The amplitude reduction factors for the Ni Kedge were obtained from the reference α-Ni(OH)2 (trigonal crystal structure, space group: P3m1). In order to precisely determine the morphology and distribution of LDH nanoparticles in L-E@A, SEM images were obtained with FEI QUANTA 250 FEG. Energy dispersive


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spectrometry mapping (Ametek Apollo x) was used to determine the distribution of LDH nanoparticles. The particle size of L-C nanoparticles dispersed in deionized water was analyzed by dynamic light scattering (DLS; Otsuka Electronics ELSZ-1000). Particle distributions in dehydrated films were confirmed with SAXS experiments, which were conducted in transmission mode at the 4C SAXS II beamline, Pohang Accelerator Laboratory, Korea; a radiation wavelength, λ, of 1.17 Å (10.6 keV, of beam energy), sample-to-detector distance of 5.0 m, and Mar charge-coupled device area-detector were employed. The one-dimensional scattered intensity profiles, I(q), obtained by azimuthally averaged two-dimensional scattering data were plotted as a function of the scattering vector, q (q = (4π·sin(θ/2))/λ), where θ is the scattering angle. Results and discussion In order to confirm the growth of LDH in agarose biopolymer network, XRD pattern of LE@A were obtained compared with L-C and agarose film itself. Dehydrated agarose film exhibited an amorphous pattern with a broad peak in the range of 2θ 15-25°, which is a typical pattern for an amorphous polymer (Fig. 1A(a)).31 The X-ray diffraction pattern of L-C nanoparticles (Fig 1A(b)) showed well developed (003) and (006) reflections with asymmetric (0kl) peaks, which are characteristics of a hydrotalcite (JCPDS 14-0191) type LDH structure.32 Similar to L-C, L-E@A also exhibited (003), (006), (012), and (110) diffraction peaks originating from the LDH crystal. The (00l) peaks of the L-E@A in Fig. 1A(c) shifted to the lower angle region by 2θ ~ 2o compared to L-C nanoparticles, which can be rationalized by the slight lattice expansion of the LDH structure along the crystallographic c-axis due to partial intercalation of saccharides in agarose. Subtle broadening of the peaks in L-E@A compared with


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L-C was attributed to the small crystallite size of LDH nanoparticles formed in the agarose matrix. Figure 1B shows diffuse reflectance UV-vis spectra of L-C and L-E@A. The Ni2+ (d8) of Ni(OH)6 in the LDH structure was expected to have three spin-allowed transitions, ν1: 3A2g(F) →3T2g(F), ν2: 3A2g(F) →3T1g(F), and ν3: 3A2g(F) →3T1g(P). The latter two transitions were observed at approximately 390 and 700 nm, respectively, while the ν1 transition lay beyond the spectral window. The ν2 (3A2g(F) →3T1g(F)) of the two samples exhibited a humped pattern due to spin-orbit coupling.33 Light absorption in the UV region < 300 nm was attributed to ligand to metal charge transfer (LMCT: O2-→Ni2+). The crystal field splitting energy (∆o) and Racah parameter (B) of the Ni(OH)6 octahedron in the three samples calculated according to the d8 Tanabe-Sugano diagram were ~8400 and ~935 cm-1, respectively, which were in good agreement with literatural values for β-Ni(OH)2 (∆o=8600 cm-1 and B=820-925 cm-1).34 From the XRD and UV-vis spectroscopy results, the crystal phase of particles in L-E@A was clearly determined to be same as that of conventionally prepared LDH, L-C. In order to cross-confirm that the local structure around metals in L-E@A was same to that in L-C, we carried out XAS at the K-edges of Ni and Ga. The spectra were divided into X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) for detailed data interpretation. The XANES and EXAF analyses provided qualitative information (symmetry and oxidation state) and quantitative parameters (bond distance and Debye-Waller factor), respectively. The Ni K-edge XANES spectra of both L-C and L-E@A showed intense white lines at the same energy (8346.3 eV), which was attributed to the dipole allowed 1s → 4p transition (Fig. 2A(a)). The XANES spectra of Ga K-edge of L-C and L-E@A showed a main edge at 10370.3 and 10369.4 eV, respectively, which also originated from the


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dipole allowed 1s → 4p transition (Fig. 2B(a)). Both Ni and Ga K-edge XANES spectra of LE@A were similar to those of L-C in terms of shape, sharpness, and intensity of the white line, as well as the main edge position, suggesting that the local chemical environment around Ni and Ga in the L-E@A was identical to that of L-C nanoparticles. Bond length and crystal disorder around Ni and Ga in L-E@A were compared with those of LC, utilizing Fourier transforms (FTs) of EXAFS spectra which was analyzed utilizing the UWXAFS software package (Figs. 2A(b) and B(b)). FTs of the L-C (solid line) and L-E@A (open circles) were almost identical, implying that the particles in L-E@A have same bonding nature with L-C. FT peaks corresponding to the first coordination shell appeared around 1.66 and 1.57 Å (non-phase-shift-corrected) for Ni and Ga K-edge, respectively. EXAFS curve-fitting analysis of L-C revealed that the bond lengths of (Ni-O) and (Ga-O) were 2.060 and 1.979 Å, respectively (Table 1). The slight difference in bond length between (Ni-O) and (Ga-O) could be explained by the difference in ionic radii (0.83 Å for Ni2+ and 0.76 Å for Ga3+). FT peaks around 2.80 and 2.77 Å (Figs. 2A(b) and B(b)) correspond to the second coordination shell of Ni-(Ni, Ga) and Ga-(Ni, Ga), respectively. The FT amplitude of the L-E@A was similar to that of L-C in both FTs at the Ni and Ga K-edge, suggesting that the samples have same local structure around metal ions. The relatively large Debye-Waller factor (σ2) of L-E@A compared with L-C suggested the smaller crystallite size of LDH in [email protected] From XAS results, we again confirmed that NiGa-LDH nanoparticles were successfully obtained through electrical double migration route. The photograph of as-prepared L-E@A showed a homogeneous green color (Figure 3A(a)), whereas L-C@A (LDH nanoparticle containing agarose prepared by melt-blending) exhibited segregation of colored part and agarose moiety (Figure 3A(b)). After the dehydration, L-E@A


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was green and highly transparent (Figure 3B(a)), while significant opaqueness attributed to L-C aggregates were found in L-C@A (Figure 3B(b)). This suggests the existence of smaller and better distributed LDH particles in L-E@A than in L-C@A, since smaller particles are known to scatter less light than larger ones.36 It could be therefore concluded that L-E@A has NiGa-LDH nanoparticles with uniform dispersion without serious aggregation. The transparency of L-E@A was quantitatively compared with agarose or L-C@A with UV-vis transmittance spectroscopy experiments (Figure 3C), showing >60%, ~80% and

Controlled Crystal Growth of Two-Dimensional Layered

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