ARTICLE pubs.acs.org/Langmuir
Structure Formation in Metal Complex/Polymer Hybrid Nanomaterials Prepared by Miniemulsion Christoph P. Hauser, Nicole Jagielski, Jeannine Heller, Dariush Hinderberger, Hans W. Spiess, Ingo Lieberwirth, Clemens K. Weiss,* and Katharina Landfester Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
bS Supporting Information ABSTRACT: Polymer/complex hybrid nanostructures were prepared using a variety of hydrophobic metal β-diketonato complexes. The mechanism of structure formation was investigated by electron paramagnetic resonance (EPR) spectroscopy and small-angle X-ray scattering (SAXS) in the liquid phase. Structure formation is attributed to an interaction between free coordination sites of metal β-diketonato complexes and coordinating anionic surfactants. Lamellar structures are already present in the miniemulsion. By subsequent polymerization the lamellae can be embedded in a great variety of different polymeric matrices. The morphology of the lamellar structures, as elucidated by transmission electron microscopy (TEM), can be controlled by the choice of anionic surfactant. Using sodium alkylsulfates and sodium dodecylphosphate, “nano-onions” are formed, while sodium carboxylates lead to “kebab-like” structures. The composition of the hybrid nanostructures can be described as bilayer lamellae, embedded in a polymeric matrix. The metal complexes are separated by surfactant molecules which are arranged tail-to-tail; by increasing the carbon chain length of the surfactant the layer distance of the structured nanomaterial can be adjusted between 2 and 5 nm.
’ INTRODUCTION Preparation of structured nanomaterials is an emerging field in nanotechnology. A variety of structured (hybrid) nanomaterials were synthesized and used for diverse applications such as biomedical and photothermal imaging, data storage, and optoelectronic devices.13 Due to their unique optical, magnetic, and catalytical properties, rare-earth elements are promising candidates for structured nanomaterials.4 Pinna et al. described rareearth hybrid nanomaterials with lamellar order, prepared via the benzyl alcohol route.5 By doping these lamellar nanohybrids consisting of luminescent rare-earth oxide (Tb3+, Eu3+, Nd3+) crystalline layers separated from each other by benzoate molecules, an emission efficiency comparable or even higher than that of commercial phosphors can be observed due to the large absorption cross-section of benzoate molecules located between the layers.6 An approach to reduce the self-quenching rate of lanthanide ions by templating was reported by Kawa and Frechet. They investigated the self-assembly of carboxylate-functionalized dendrimers around trivalent lanthanide ions.7 Nanowires/nanorods including rare-earth fluoride, hydroxide, or oxide synthesized via a hydrothermal route are very suitable host lattices for down- and up-conversion luminescence of lanthanide ions.8,9 However, for the synthesis of hybrid materials containing polymers, high-temperature methods cannot be used. For this purpose, milder techniques like that of the miniemulsion polymerization technique have already been successfully applied.10,11 r 2011 American Chemical Society
Encapsulation of organically capped EuS nanocrystals in polystyrene (PS) via miniemulsion polymerization to form raspberrylike nanocomposite particles and their magnetic properties was investigated by Pereira et al.12 Neutral, inert inner-shell lanthanide (Ln) β-diketonate complexes such as Ln(tmhd)3 (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedione) in combination with ester-containing monomers such as butyl acrylate were used by Ramírez et al. to synthesize highly organized onion-like layered nanocomposite particles via miniemulsion polymerization.13 Lanthanide-containing polymer nanoparticles without an inner structure were synthesized by encapsulation of lanthanide tris(4,4,4-trifluoro-1-(2-naphthyl-1,3-butanedione)) complexes in PS nanoparticles via miniemulsion polymerization. Their cell adhesive properties were investigated by inductively coupled plasma mass spectrometry (ICP-MS).14 Further lanthanide or transition metal complexes were encapsulated in polymeric nanoparticles by heterophase polymerization techniques for preparation of particle monolayers and luminescent films.15,16 An inductively coupled plasma optical emission spectrometry (ICP-OES) based method was established to quantify the metal content in the latexes.17 Furthermore, mesoporous CeO2 nanoparticles synthesized by an inverse miniemulsion process are able Received: July 21, 2011 Revised: September 9, 2011 Published: October 06, 2011 12859
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Langmuir to significantly reduce the onset temperature of catalytic methane oxidation.18 In this paper, we will considerably extend the work of Ramírez et al.13 and present a detailed study on the generation of metalcontaining structured soft nanomaterials. Ramírez et al.13 recognized the spontaneous formation of highly organized layered nanocomposite particles when neutral, lanthanide (Ln) β-diketonate complexes were introduced in a miniemulsion polymerization process with methacrylate monomers. TEM investigations showed that the hybrid nanomaterials have an onion-like internal structure, which was only generated when trigonal-prismatic lanthanide tris(2,2,6,6-tetramethyl-3,5-heptanedionate) (Ln(tmhd)3) complexes were used. As these complexes possess accessible coordination sites, the authors concluded that coordinative interaction with monomer or surfactant might be the reason for structure formation. However, the data did not allow giving a conclusive statement. Here, the structure formation process is further elucidated. A variety of hydrophobic metal (Me) β-diketonato complexes were subjected to the miniemulsion process, and the structure formation was studied across the relevant parameters. Structure formation can be described as bilayer formation of adducts which are generated by an interaction between free coordination sites of the Me(tmhd)n complex and a coordinating surfactant (e.g., alkylsulfate, carboxylate, or alkylphosphate). Lamellar structures are generated with layer spacings dependent on the carbon chain length of the surfactant. The morphology of the lamellar structure is determined by the type of ionic group of the surfactant. Moreover, it is demonstrated that structure formation is also induced by tmhd complexes of transition metals and is not limited to lanthanide complexes. Furthermore, we show that the structure is already generated in the nonpolymerized miniemulsion. By subsequent polymerization of the monomer (acrylates, methacrylates, styrene) the metal-containing nanostructures can be embedded in and therefore stabilized by a polymeric matrix.
’ EXPERIMENTAL SECTION Materials. The tmhd complexes of Gd, Eu, Tb, La, Yb, Co, Cr, Al, Mn, In, Bi, Ga, Cu, and Ag were purchased in a purity of 99% from Strem Chemicals. Hexadecane (HD, Merck, 99%), ethylbenzene (Merck, 99%), sodium methyl sulfate (C1S, TCI Europe, 98%), sodium hexylsulfate (C6S, Acros, 98%), sodium heptylsulfate (C7S, Alfa Aesar, 99%), sodium dodecyl sulfate (C12S, Merck, 99%), sodium tridecyl sulfate (C13S, Alfa Aesar, 99%) sodium n-octadecyl sulfate (C18S, Alfa Aesar, 99%), sodium formiate (C1C, Fluka, 99%), sodium hexanoate (C6C, TCI Europe, 99%), sodium heptanoate (C7C, TCI Europe, 97%), sodium octanoate (C8C, Fluka, 99%), sodium nonanoate (C9C, TCI Europe, 98%), sodium decanoate (C10C, TCI Europe, 99%), sodium laurate (C12C, TCI Europe, 97%), sodium stearate (C18C TCI Europe, 95%), sodium dodecylphosphate (C12P, TCI Europe), cetyltrimethylammonium bromide (CTAB, Fluka, 99%), Lutensol AT50 (C16/C18-O(CH2CH2O)50H, BASF, see structure in Supporting Information, Figure SI1), V59 (2,20 -azo(2-methylbutyronitrile), Wako), and 5-DOXYL-stearic acid (5-DSA, Sigma-Aldrich) were used as received. Methylacrylate (MA, Fluka, 99%), hexylacrylate (HA, Sigma-Aldrich, 98%), laurylacrylate (LA, Sigma-Aldrich, 90%), methylmethacrylate (MMA, Merck, 99%), hexylmethacrylate (HMA, Sigma-Aldrich, 98%), laurylmethacrylate (LMA, Fluka, 95%), benzylmethacrylate (BzMA, Aldrich, 96%), and styrene (Merck, 99%) were purified using a column packed with neutral aluminum oxide (Merck) before use. Demineralized water was used for all experiments.
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Synthesis of Me-Containing Nanomaterials. Typically, the miniemulsions were prepared following the procedure of Ramírez et al.13 A mixture of 42 mg of Gd(tmhd)3 and 300 mg of methylmethacrylate was heated to 72 °C for 30 min. After adding 16 mg of hexadecane, the organic phase was added to the surfactant solution, which was prepared from 34 mg of C12S, 51 mg of Lutensol AT50, and 2.7 g of water. The emulsion was stirred for 1 h, and then 24 mg of the oil-soluble initiator V59 was added. The miniemulsion was prepared by ultrasonication for 2 min (5 s pulse, 10 s break) at 70% amplitude with a Branson sonifier W450 digital (1/8” tip) in an ice bath. Finally, the miniemulsion was polymerized for 12 h at 72 °C. The ratios of the components for the preparation of the other dispersions were chosen accordingly. The complex was applied in a molar ratio of 1:50 with respect to monomer. Sodium alkylsulfates and sodium dodecylphosphate were used in a molar ratio of Me(tmhd)n to surfactant of 1:2. Sodium carboxylates were applied in equimolar ratios respective to Me(tmhd)n. The nonionic surfactant Lutensol AT50 was added to all surfactant solutions at 17 wt % with respect to the dispersed phase. Dialysis was performed by diluting the dispersions to a solid content of 1 wt % with demineralized water. A 30 mL amount of these diluted dispersions were dialyzed in a dialysis tube VISKING type 20/32 (Carl Roth GmbH & Co.) with 2 L of demineralized water over 2 h while changing the water after 1 h and taking samples at 15 min intervals. Characterization of Me-Containing Nanomaterials. Transmission Electron Microscopic (TEM) Analysis. The hybrid polymer dispersions were diluted 1:1000 with demineralized water. A 4 μL amount of this dispersion was deposited on a 400 mesh carbon-coated copper grid and air dried. The morphology of the Me-containing nanomaterials was analyzed by a Philips TEM 400 with an acceleration voltage of 80 kV. Element mapping was conducted on a FEI Tecnai F20 with a postcolumn EEL spectrometer (Gatan). Scanning electron micrographs were obtained using a Gemini 1530 microscope (Carl Zeiss AG, Oberkochen, Germany) Electron Paramagnetic Resonance (EPR) Analysis. CW EPR spectra were measured on a Miniscope 200 spectrometer (Magnettech GmbH) working at X-band frequencies (∼9.4 GHz). The temperature was controlled with a TC HO2 temperature controller (Magnettech). All measurements were performed at 25 °C. Samples were loaded into 1.5 mm outer diameter capillaries. A typical microwave power of 10 μW was applied during these measurements and did not lead to saturation broadening. The modulation amplitude was set to 0.02 mT with a width of the central line of the nitroxide spectrum around 1 mT. The sweep width was 10 mT. Depending on the signal-to-noise (S/N) ratio 1099 scans were averaged, with 4096 data points and a scan time of 60 s each. The presented CW EPR spectra were background corrected using a home-written MATLAB program by subtracting a first-order polynomial fitted to the first and last 15% of the spectral data points and further normalized by their maximum. Small-Angle X-ray Scattering (SAXS) Analysis. Liquid State. Miniemulsions prepared with the procedure mentioned above (see Synthesis of Me-Containing Nanomaterials) were analyzed before and after polymerization. Dried State. The Me-containing polymer dispersions were analyzed after freeze drying. Analysis of both kinds of samples was performed using a Bruker Nano-STAR with a HI-STAR-detector (2-dimensional, Bruker AXS, Cu Kα radiation: λ = 0.154 nm) at room temperature. Inductively Coupled Plasma Optical Emission Spectrometry (ICPOES) Analysis. Dispersions were diluted 1:10 with an aqueous Triton X100 (Sigma-Aldrich) solution (5 wt %). The amount of sulfur and gadolinium was determined with an ICP-OES (Activa M, Horiba Jobin Yvon) operating at 1250 W plasma power, 12 L 3 min1 plasma gas flow, 0.89 L 3 min1 nebulizer Ar flow, and 2.90 bar Ar pressure. For calibration of the ICP-OES single-element standard solutions were used. These standards were prepared from 1000 mg 3 L1 pure single-element standard solutions (Gd standard from Spex Ceritprep, S-standard from 12860
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Figure 1. 5-DSA CW EPR spectra of miniemulsions containing C18C and MMA with (red) or without (black) Eu(tmhd)3. Chem-Lab NV) by successive dilution with Milli-Q (Elix/Gradient, Millipore) water. Experimental error between different detection wavelengths and measurements is 5%.
’ RESULTS AND DISCUSSION As mentioned in the Introduction, Ramírez found indications that coordinative interactions between the complex and other components of the formulation are responsible for formation of the lamellar structure when incorporating Ln(tmhd)3 in polymeric nanoparticles using the miniemulsion technique. However, the data did not allow giving a conclusive statement. In order to investigate the parameters, interacting components, and the mechanism of structure formation, a detailed investigation of the unpolymerized (“liquid state”) miniemulsion, the dispersion, and the dried powders was conducted. Formulations were based on the data published by Ramírez, and the relevant parameters were adjusted. Investigation in the Liquid State. Electron Paramagnetic Resonance (EPR) Spectroscopy. To obtain a detailed molecular
view of the mechanism of structure formation, the nitroxidelabeled fatty acid 5-doxyl-stearic acid (5-DSA) was added to the miniemulsion (ME). Nitroxides are used for EPR analysis to obtain information about structural and dynamic changes in the investigated systems.1922 A 5 mol % amount of the surfactant sodium stearate (C18C) was substituted by 5-DSA. Methylmethacrylate (MMA) with or without europium tris(2,2,6,6tetramethyl-3,5-heptanedionate) (Eu(tmhd)3) was used as dispersed phase. As the electronic structure of Gd3+, used in Ramírez’s work, interferes with EPR measurements, e.g., by reducing the nitroxide relaxation times, the Eu complex was instead chosen. These miniemulsions were investigated with CW EPR. The resulting spectra of 5-DSA in miniemulsion with (red) and without (black) Eu(tmhd)3 are presented in Figure 1. The spectrum of the miniemulsion with Eu(tmhd)3 reveals two components of significantly different rotational mobility: a fast rotating component (1) and a very slow rotating component (2). Such an immobilized fraction of 5-DSA is not visible in the corresponding spectrum of the miniemulsion without Eu complex. The presence of the immobilized 5-DSA with its limited rotational mobility can be explained by an interaction with the Eu complex, e.g., by coordinative adduct formation. Small-Angle X-ray Scattering (SAXS). In addition to CW EPR spectroscopy, SAXS was used to investigate structure formation of the Eu-containing miniemulsion prepared with MMA and
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C18C. The corresponding SAXS diffractogram in the liquid phase is presented in Figure 2a. The peaks in the SAXS diffractogram at 2θ = 1.8°, 3.6°, and 5.4° represent first-, second-, and third-order reflections of a lamellar structure and indicate that this structure is already generated in the monomer droplets of the miniemulsion. To investigate which combination of the components induces structure formation, MMA miniemulsions with sodium dodecylsulfate (C12S), Lutensol AT50 and complex, and C12S and complex alone were analyzed with SAXS (Figure 2b). Only for the miniemulsion prepared with Eu complex and C12S a peak at 2θ = 2.5° can be observed. The diffractograms of neither the miniemulsion prepared with C12S but without complex nor the miniemulsion prepared with the nonionic surfactants show any signal. These findings indicate that the structure is only generated in the presence of both surfactant and complex and may be attributed to their mutual interaction observed with EPR spectroscopy. As MMA possesses electron-donating groups (carboxylate O), there is the possibility that monomer is participating in structure formation.13 Hence, styrene, a monomer without donating groups, was used for comparison. In the SAXS diffractogram of the miniemulsion prepared with styrene (Figure 2c) a peak at 2θ = 2.5° is also visible, indicating the presence of lamellar structure. This shows that structure formation is not limited to estercontaining monomers like MMA. Thus, it can be concluded that structure formation seems mainly independent of the monomer type. Consequently, the key factor for the structure formation process is an interaction between complex and anionic surfactant, which was also shown with CW EPR spectroscopy. SAXS investigations on a nonpolymerizable miniemulsion with ethylbenzene and complex as dispersed phase showed the same peaks in the diffractogram (see Supporting Information, Figure SI2). Lamellar structures in an onion-like arrangement are visible on transmission electron micrographs (see Supporting Information, Figure SI3), similar to those observed by Ramírez et al.13 and those obtained after polymerization. Furthermore, SAXS investigations on polymerized dispersions revealed a weak but observable peak in the range of 2θ = 2.5° (see Supporting Information, Figure SI4). These results substantiate that the structures are already generated in the monomer droplets during the miniemulsification process and are not the result of phase separation processes during drying or polymerization. Investigation in Dispersion. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Despite the insights gained from the experiments described above, no information about the stoichiometry of the adduct of complex and surfactant can be deduced from the CW EPR and SAXS experiments. In order to describe the type of coordination (monodentate or bidentate) in a more detailed way, it is necessary to determine the molar ratio between metal complex and surfactant. A Gdcontaining polymer dispersion and a pure polymer dispersion both prepared with MMA and C12S under the same conditions were dialyzed for surfactant removal. Assuming that the surfactant involved in structure formation is located in the particles, only surfactant on the surface, responsible for colloidal stabilization and free surfactant, can be removed. The amount of C12S remaining in the dialyzed sample just prior to destabilization was determined via ICP-OES by measuring the sulfur content along with the Gd content in order to calculate the molar ratio between Gd(tmhd)3 and C12S. 12861
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Figure 2. SAXS diffractograms of miniemulsions prepared with (a) Eu complex, MMA, and C18C, (b) MMA with complex and C12S in comparison to MMA with C12S and MMA with complex and Lutensol AT50, and (c) Eu complex, C12S, and different monomers.
Figure 3. (a) TEM micrograph of Gd-containing structured nanoparticles prepared with PBzMA and sodium laurate (C12C). (b) TEM micrograph with element mapping of Gd(tmhd)3/PBzMA/C12C nanostructures: (red) Gd and (blue) C.
During dialysis the Gd-containing polymer dispersion quickly coagulated. In contrast, the pure polymer dispersion remained stable even after extensive dialysis. As both dispersions were prepared with the same amount of surfactant, the results show that in the Gd-containing system a great portion of the surfactant does not act as a stabilizing agent but is indeed located in the interior of the particles. For the dialyzed Gd-containing polymer dispersion, a sulfur content of 85% of theory was measured just prior to coagulation. In the pure polymer dispersion, a sulfur content of 84% of theory was detected after the same number of dialysis steps. However, a sulfur content of roughly 10% of theory can be measured in a pure polymer dispersion extensively dialyzed for 6 days. These findings indicate that only between 10% and 15% of the used C12S is necessary for stabilization of the pure polymer particles and between 85% and 90% can be removed by dialysis without inducing destabilization. In the case of the Gd-containing polymer dispersion only 15% of the applied C12S can be removed by dialysis before coagulation occurs. It can be deduced that these 15% of C12S molecules is responsible for particle stabilization. The remaining 85% is not accessible for dialysis and thus is involved in structure formation. With the sulfur content of the dialyzed Gd-containing polymer dispersion a molar ratio between Gd(tmhd)3 and C12S of 1:3 can be calculated. On the basis of this result, C12S seems to occupy more than 2 free coordination sites of the lanthanide complex. These findings support the results of the CW EPR and SAXS experiments that show that there is an interaction between the metal complex and the anionic surfactant.
Investigation of the Lamellar Structures. Investigation in the Dried State. SAXS investigations have shown the same results for dried
and liquid samples. As the investigation on dried samples is less time consuming, measurements were performed on dried samples only. The particle sizes and solids contents of the dispersions correspond well to the theoretical solids contents and to the corresponding values of the dispersions without complex. This indicates that the Me complex does not significantly interfere with the miniemulsion polymerization process. As shown by SAXS, structure formation is not limited to estercontaining monomers. Thus, a great variety of monomers (acrylates, methacrylates, and styrene) was used for preparation of Gd-containing structured soft nanomaterials in the presence of C12S. Although the polymeric matrix is not involved in the structure formation process, the Tg of the polymer determines whether the hybrid particles form a film or retain their particulate structure. In any case, the lamellae are incorporated in the matrix (Figure 3a). Elemental Composition of the Layer Structure. The microstructural elementary composition of the structures was investigated by element mapping via electron energy loss spectroscopy (EELS). Figure 3b shows a TEM micrograph with an element map of Gd(tmhd)3/poly(benzylmethacrylate) (PBzMA)/sodium laurate (C12C) nanostructures. Normal contrast shows the lamellar structure, which will be later shown to be typical for application of carboxylate surfactants (Figure 3a and see text below). The element map shows (Figure 3b, inset) that the lamellae consist of alternating layers of Gd and carbon, indicating that the complex is 12862
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Figure 4. TEM images of Gd-containing nanomaterials prepared with different anionic surfactants C12S (A), C12P (B), and C12C (C).
separated by a layer of surfactant, polymer, or both. This supports the results of the SAXS investigations (Figure 2 b) showing that the complex is an essential part for structure formation. Surfactant Type and Chain Length. As already observed by SAXS in the liquid state, anionic surfactants, as long chain carboxylates or sulfates, induce formation of lamellar phases in the monomer droplets in miniemulsion. In contrast, the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the nonionic surfactant Lutensol AT50 did not induce structure formation. In this part, we present the investigation of the morphology of the nanostructures obtained from miniemulsions prepared with anionic surfactants with carboxylate, phosphate, and sulfate headgroups.
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As shown before, the interaction between surfactant and complex plays an important role for structure formation; thus, the molecular structure of the complexes is briefly introduced. Ln(tmhd)3 complexes consist of three chelating tmhd ligands which occupy six coordination sites of the Ln ion. The coordination sphere can be described as trigonal prism. These complexes can be regarded as hard Lewis acids with an unsaturated coordination sphere.23 Consequently, they react preferentially with Lewis bases, which occupy the vacant coordination sites by electron donation and increase the coordination number to eight.24 Several reports which deal with the coordination between β-diketonates and substrate molecules can be found in the literature, in particular this interaction is the basis for the use of Ln β-diketonates as lanthanide shift reagents for NMR. Evans et al. described the solvation numbers for Ln β-diketonates by using different substrates. For example, dimethyl sulfoxide (DMSO) and hexamethylphosphoramide (HMP) are coordinated to Eu(fod)3 (fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dione) with a molar ratio of 1:2 (complex: substrate).25 Formation of a 1:2 adduct of Eu(fod)3 with N,Ndimethylformamide (DMF) was determined by emission titration studies.26 The coordination of dimethoxy-ethane (DME) with Gd(tmhd)3 yields a 1:1 adduct in which DME acts as a bidentate ligand. 27 In this study, the ratio of surfactant to complex was chosen accordingly to these considerations. Sodium alkylsulfates and sodium dodecylphosphate (C12P) were used with a molar ratio of complex to surfactant of 1:2. In the case of sodium carboxylates, an equimolar ratio was chosen because bidentate coordination is usually observed. 28 The micrographs in Figure 4 show Gd-containing hybrid nanomaterials prepared with sulfate, phosphate, and carboxylate surfactants, all with identical alkyl chain lengths. By using C12S (A) and C12P (B), particulate structures with a diameter of 200 nm and an onion-like morphology can be observed. Dispersions prepared with sodium laurate (C12C) (C) contain kebablike structures with 100 nm in width and 400 nm in length. These structures consist of several strands of lamellae with an individual width of approximately 1520 nm. Although all of the surfactants lead to lamellar structures, the arrangement of the lamellae is different. Since the surfactants possess an equal number of C atoms, the differences in the morphology can be attributed to the headgroups of the surfactant and their interaction with the complex.29 The coordination mode influences the resulting aggregate. The ICP measurements suggest more than one monodentately coordinated sulfate ligand. The bidentately coordinating carboxylate ligand will lead to brick-like adducts, which tend to assemble to lamellar structures with low curvatures (kebab-like), while the sulfate-containing adducts will have a wedge-like shape. These are known to assemble into flexible, more curved bilayers (onions).30 In other words, the morphology of the nanocomposites can be controlled by the choice of the surfactant. The layer distances of the Gd(tmhd)3-containing structured soft nanomaterials prepared with various anionic surfactants are presented in Table 1. The layer distances d, calculated from the peak maxima of the SAXS diffractograms with the Bragg equation (nλ = 2d sin(θ) with n = order of diffraction, λ = wavelength, d = layer distance, θ = angle of the peak) are around 4 nm for the C12 anionic surfactants, which is in agreement with a C12surfactant bilayer of C12P adsorbed on an aluminum oxide surface.31 12863
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Table 1. Layer Distances (determined with SAXS) of Gd-Containing Structured Nanomaterials with Different Sodium Alkylsulfates, Sodium Carboxylates, and Sodium Dodecylphosphatea
a
chain length lc [nm]
layer distance d (Bragg) [nm]
sodium methylsulfate
0.3
n.o.
sodium hexylsulfate sodium heptylsulfate
0.9 1.0
1.9 2.2
GdC12S
sodium dodecylsulfate
1.7
3.9
GdC13S
sodium tridecylsulfate
1.8
4.3
GdC18S
sodium octadecylsulfate
2.5
5.3
GdC1C
sodium formiate
0.3
n.o.
GdC6C
sodium hexanoate
0.9
n.o.
GdC7C
sodium heptanoate
1.0
n.o.
GdC8C GdC9C
sodium octanoate sodium nonanoate
1.2 1.3
n.o. 2.8
GdC10C
sodium decanoate
1.4
2.9
GdC12C
sodium laurate
1.7
3.7
GdC18C
sodium stearate
2.5
5.2
GdC12P
sodium dodecylphosphate
1.7
3.7
sample
anionic surfactant
GdC1S GdC6S GdC7S
n.o. = none observed.
Figure 5. TEM images of Gd-containing nanostructures prepared with different anionic surfactants: (a) C13S, (b) C18S, (c) C9C, and (d) C10C. The scale bars are 100 nm.
As the surfactants have an equal number of C atoms in the chain, the layer spacing seems to be connected to the
length of the hydrophobic part of the surfactants. In order to test whether this assumption is correct, alkyl sulfates and 12864
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Figure 6. Layer thickness of Gd-containing nanostructures depending on the carbon chain length of various anionic surfactants.
carboxylates with different chain lengths were used for the preparation. Sodium alkylsulfates as well as sodium carboxylates with short chains (up to C8) are not suitable to stabilize the miniemulsion droplets and the resulting polymer dispersion alone, as their solubility in the aqueous phase is too high. Hence, 17 wt % of Lutensol AT50 was added to the dispersed phase for colloidal stabilization. As shown above, Lutensol AT50 does not participate in structure formation and is thus exclusively used for particle stabilization. The characteristics of Gd-containing dispersions are summarized in Table 1. The length of the carbon chain lc of the sodium alkylsulfates and sodium carboxylates was calculated by32 lc ¼ ðNc 3 0:127 þ 0:154Þ
ð1Þ
with Nc number of C atoms. Short chain carboxylates (C < 8) as well as methylsulfate do not induce formation of lamellar structures, which can be observed with SAXS or TEM. The morphology of the observed structures is comparable to the above-mentioned systems with C12 surfactants. Structures with particulate, onion-like structures are observed for the sulfate systems, while kebab-like structures are seen for carboxylates (Figure 5). Regarding the sulfates, the layer spacing increases from 1.9 (hexyl sulfate) to 5.3 nm (octadecyl sulfate) and from 2.8 (nonanoate) to 5.2 nm (stearate) when carboxylates are used. Irrespective of the headgroup of the surfactants, an almost linear increase of the layer spacing can be observed (Figure 6). The layer spacing induced by the carboxylates is always slightly below (12 nm) that induced by the sulfates. Assuming that the surfactants participate in the layer between the Ln-containing lamellae, e.g., as a bilayer, the layer spacing should indeed increase with increasing carbon chain length and should coincide with twice the length of the surfactants’ carbon chains (eq 1). Although the calculated values are slightly below the measured values, they are in good agreement (Figure 6, Table 1). With these results, the findings of the EPR experiments, and the well-known fact that carboxylates and sulfates can coordinate to lanthanide ions, we can construct a model for the lamellae. Primary building blocks consisting of the tmhd complex with additionally coordinated surfactant assemble into bilayer lamellae (Figure 7). As the complex and the headgroup of
Figure 7. Schematic representation of the generated bilayer structure consisting of adducts of trigonal prismatic complex (green) and surfactant.
the surfactant adds to the layer spacing, the difference between the calculated and the measured values can readily be explained. These results correspond well to former reports. Liu et al. reported formation and self-assembly of lanthanide(III) oleates as an lanthanide oleate bilayer.33 The lanthanide ions are separated by a double layer of oleate groups which are arranged tail-to-tail. The same finding was observed for lanthanide(III) dodecanoates.34 The influence of longer or bulkier components on the interlamellar distance in the case of yttria-based crystalline and lamellar nanostructures was described by Pinna et al. By using the bulkier 4-tert-butyl benzyl alcohol instead of benzylalcohol, an increase in the interlamellar distance can be observed and they deduced that the organic species are localized between the yttrium oxide layers.5 There are two prominent reasons for the absence of structures in the systems prepared with short chain carboxylates or sulfates. Either the water solubility of these compounds is so high that the interaction with the Ln(tmhd)3 complex is not favored or the interaction is present but the adducts do not possess the geometry and aspect ratio required for formation of lamellar structures. This is supported by the findings of Yada et al. They recognized that aggregation of surfactant micelles with aluminum species to form a mesophase is promoted by a longer alkyl chain of the surfactant, because the aggregation energy increases with an increase in the surfactant chain length.35 To summarize, the resulting nanostructures are best described as alternating complex/surfactant bilayers, which are embedded in a polymeric matrix. Taking a closer look at the individual particles, the structure is not present all over the colloid. In contrast, just part of the particle contains the structures. Representatively, this is visualized in Figure 3a, where the lamellar strands are bent along the rim of the particles, in Figure 4b, where parts of the particle do not show the typical onion-like structure, and also in Figure 5a, where only the outer parts of the particles show the lamellae. This is, however, expected as an excess of polymer is used for preparation. 12865
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Figure 8. PMMA particles prepared with Gd(tmhd)3 and C12S: (A) low magnification showing that most of the particles show the crumpled morphology; (B) visualized in detail.
Table 2. Characteristics of Ln-Containing Structured Nanomaterialsa ionic radii
a
layer distance
sample
lanthanide ion
[pm]
CN
d [nm]
LaC12S
La(tmhd)3
130
8
4.0
EuC12S GdC12S
Eu(tmhd)3 Gd(tmhd)3
121 119
8 8
4.0 3.9
TbC12S
Tb(tmhd)3
118
8
3.8
YbC12S
Yb(tmhd)3
113
8
4.0
InC12S
In(tmhd)3
94
6
3.8
GaC12S
Ga(tmhd)3
76
6
3.9
CrC12S
Cr(tmhd)3
76
6
4.0
CoC12S
Co(tmhd)3
69
6
4.0
CuC12S BiC12S
Cu(tmhd)2 Bi(tmhd)3
71 117
4 6
3.9 n.o.
MnC12S
Mn(tmhd)3
72
6
n.o.
AlC12S
Al(tmhd)3
68
6
n.o.
AgC12S
Ag(tmhd)
116
4
n.o.
n.o. = none observed.
In contrast to colloids prepared without the complex, which are perfectly spherical, PMMA/Gd(tmhd)3 hybrid particles exhibit a crumpled morphology. This is visualized in the micrographs (Figure 8) where a large fraction of the hybrid particles show this crumpled morphology, indicating that they all contain the layered structure. Influence of the Central Ion. Lanthanides. Lanthanides are chemically very similar, only differing in their ionic radii,36 i.e., decreasing from La3+ (130 pm) to Yb3+ (113 pm) for coordination number (CN) eight, and their physical properties such as magnetism and luminescence. From their chemical similarity, we can expect that the tmhd complexes of all of the lanthanides interact in a similar manner with the surfactant, i.e., as an additional ligand to generate building blocks for the lamellar structure. In order to evaluate the influence of the ionic radii on structure formation, different Ln(tmhd)3 complexes (Ln = La3+, Eu3+, Tb3+, Yb3+) were used for preparation. La, as the first element in the lanthanide series, has the largest ionic radius, while Eu and Tb are located in the middle, and Yb, being the last element of the series, has the smallest radius (see Table 2). In this series of experiments, C12S was used as coordinating surfactant. Structure formation was
investigated by SAXS, and the diffractograms for Ln-containing nanomaterials with various lanthanides are presented in Figure SI5 (see Supporting Information). All SAXS diffractograms exhibit a peak at 2θ = 2.2°, which indicates that all of the investigated lanthanide complexes are able to induce structure formation. As the lanthanides represent the whole range of ionic radii, we assume that structure formation can be observed for all Ln(tmhd)3 complexes. Even though there is a decrease in the size from La3+ to Yb3+, the layer thicknesses of the Ln-containing materials with different lanthanides are around 4 nm (see Table 2). Transmission electron micrographs (not shown) confirm the onion-like arrangement of the lamellar structure, underlining that C12S, and not the complex or the radius of the central ion, is responsible for this morphology. This is consistent with the observation of Binnemans et al., who reported that the layer spacing of lanthanide(III) dodecanoates is not altered using various trivalent lanthanide ions.34 Transition and Main Group Metals. The free coordination sites provided by the unique trigonal prismatic geometry of Ln(tmhd)3 seems to play a crucial role for interaction with the anionic surfactants and thus for structure formation. Ramírez et al. showed that no structures were observed when the Ln complex was substituted by the corresponding Al(tmhd)3. This octahedral complex provides no free coordination site and hence is not suitable for structure formation. Recent reports, however, show that a number of transition metal β-diketonates can interact with water as an additional ligand.37 Thus, the necessity of free and accessible coordination sites for structure formation was evaluated with further tmhd complexes of transition or main group metals. In these experiments Co(tmhd)3, Cr(tmhd)3, Mn(tmhd)3, Al(tmhd)3, Ga(tmhd)3, In(tmhd)3, and Bi(tmhd)3 were used. These tmhd complexes have a trivalent central ion coordinated with three tmhd ligands and thus are comparable to the Ln(tmhd)3 complexes. Consequently, they were applied in order to investigate the influence of the central ion. Furthermore, the influence of the coordination geometry was investigated using Bi3+ since its radius is similar to trivalent lanthanide ions but the coordination geometry is different. For example, in the comparable complex bismuth(III) tris(1-methoxy-2-methyl-2-propanolato), Bi(OCMe2CH2OMe)3, the Bi is hexacoordinated by three chelating [OCMe2CH2OMe] ligands which form a distorted octahedron.38 The effect of the ionic radius was elucidated using the main group metal complexes Ga(tmhd)3 and In(tmhd)3 12866
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Langmuir in addition to Al(tmhd)3. Furthermore, the influence of the charge of the central ion and the number of coordinated tmhd ligands was investigated with the application of biscoordinated Cu(tmhd)2 and monocoordinated Ag(tmhd). The characteristics of the Me-containing nanomaterials prepared with SDS as surfactant are listed in Table 2. The diffractograms presented in Figure SI6 (see Supporting Information) exhibit a peak at 2θ = 2.2° for Co-, Cr-, In-, Ga-, and Cu-containing nanomaterials. From peak maxima, layer distances of around 4 nm can be calculated for the Co-, Cr-, In-, Ga-, and Cu-containing nanomaterials. This layer thickness is in good agreement with the C12S bilayer, which was also observed for Ln-containing nanomaterials. However, there are some deviations concerning structure formation depending on the choice of metal complex. For example, an influence of the central ion can be observed. A diffraction peak can only be observed for the transition metal complexes of Co and Cr. This difference for the transition metals can be explained by the coordination sphere of the metal complex. Chromium(III) and cobalt(III) β-diketonato complexes possess five hollows on the peripheral surface of the three ligands. Solvent molecules can be accommodated in the hollows and interact with the metal complex.39 Even though the ionic radius of the Bi3+ ion is similar to the lanthanide ions and therefore a flexible coordination sphere is expected, no peak in the SAXS diffractogram of the Bi-containing nanomaterial can be observed. Here, it is not possible to give a concluding statement for Bi(tmhd)3 because the Bi-containing miniemulsion showed a rheopex behavior after ultrasonication. This along with the structure of the complex could be responsible for the absence of structure formation. Using the main group metal complexes Ga(tmhd)3 and In(tmhd)3, a peak in the SAXS diffractogram can be observed, in contrast to Al(tmhd)3, even though Al is in the same main group. The metals Ga and In have larger radii and thus a more flexible coordination sphere. By using Cu(tmhd)2 and Ag(tmhd), the influence of the charge of the central ion and the number of coordinated ligands was elucidated. The SAXS diffractogram of Cu-containing nanoparticles exhibits a peak at 2θ = 2.2°, implying a layer distance of 4 nm. Accordingly, there is an interaction between Cu(tmhd)2 and C12S which can probably be described as coordination of C12S in an opposite axial position to the copper(II) ion. Such 4 + 2 coordination with four coordination sites in a square plane (2 tmhd) and two sites with a longer distance at the vertices of a tetragonally distorted octahedron can be expected due to the JahnTeller effect, thus favoring such geometries. Even though there are several examples in the literature for complexes of Ag(tmhd) with phosphine ligands40 in which the silver ion is triscoordinated, no peak in the SAXS diffractogram of Ag-containing nanoparticles can be observed. To conclude, formation of Me-containing nanostructures is largely independent of the size and charge of the central ion. Furthermore, the number of coordinated tmhd ligands does not influence structure formation. In fact, the ability of a complex to induce structure formation in structured hybrid materials depends on the central ion and its ability to interact with anionic surfactants. Thus, only central ions with free coordination sites or hollows which enable interaction with C12S are suitable. To exclude the fact that structure formation is only limited to Me complexes with tmhd ligands, structure formation was also investigated using differently sized ligands. Gd(dbm)3, which contains a bulkier dbm ligand (dbm =1,3-diphenyl-1,3-propandione), was compared to a smaller and sterically less demanding acac ligand (acac =2,4-pentandione) in the form of Gd(acac)3.
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C12S was used as the coordinating surfactant. Results showed that structure formation is also observed in the systems with dbm and acac ligands. This means that as long as free coordination sites are available and accessible, the steric demand of the ligands is not the decisive factor. Unfortunately however, no detailed image of the molecular structure of the adducts of complex and coordinating surfactant could be deduced from these results.
’ CONCLUSION Successful preparation of metal-containing structured soft nanomaterials via the miniemulsion technique is presented. A variety of hydrophobic Me(tmhd)n complexes were subjected to the miniemulsion process. Structure formation was already induced in the monomer droplets of the miniemulsion. By subsequent polymerization, the nanostructures were embedded in a polymeric matrix and metal-based structured soft nanomaterials were generated. The mechanism of structure formation was studied in a detailed way and can be attributed to an interaction between free coordination sites or hollows of the metal complex and coordinating anionic surfactants. Coordination leads to an alternating self-assembly of metal complex and surfactant bilayer. The metal complexes are separated by two surfactant molecules, which are arranged tail-to-tail. The layer distance of these bilayer lamellae can be adjusted between 2 and 5 nm depending on the carbon chain length of the surfactant. The longer the chain length, the greater the layer distance. The morphology of the nanostructures is influenced by the type of anionic surfactant. Using sodium alkylsulfates and sodium dodecylphosphate, “nano-onions” are formed. Using sodium carboxylates, kebab-like structures are generated. The differences in the arrangement of the lamellae is most likely a result of different coordination modes and the resulting geometries of the adducts. Cationic and nonionic surfactants are not able to induce structure formation. Furthermore, the size of the central ion has no influence on the layer distance and appears only to depend on the chain length of the anionic surfactant. ’ ASSOCIATED CONTENT
bS
Supporting Information. Chemical structure of Lutensol AT50, additional diffractograms, one micrograph, and additional literature on the complex geometries. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Deutsche Forschungsgemeinschaft (DFG) Schwerpunktprogramm “Lanthanoidspezifische Funktionalit€aten in Molek€ul und Material” (SPP 1166). We thank Johannes Fickert for his assistance during nanomaterials synthesis, Jochen Gutmann, Michael Bach, and Daniela Mannes for SAXS measurements, and Gunnar Glasser for acquiring the scanning electron micrographs. 12867
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