Article pubs.acs.org/Langmuir
Magnetic Mesoporous Photonic Cellulose Films Michael Giese,† Lina K. Blusch,‡ Maik Schlesinger,‡ Georg R. Meseck,‡ Wadood Y. Hamad,§ Mohammad Arjmand,∥ Uttandaraman Sundararaj,*,∥ and Mark J. MacLachlan*,‡ †
Institut für Organische Chemie, Universität Duisburg-Essen, Universitätsstrasse 7, 45141 Essen, Germany Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada § FPInnovations 2665 East Mall, Vancouver, BCBritish Columbia V6T 1Z4, Canada ∥ Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada ‡
S Supporting Information *
ABSTRACT: Novel hybrid materials of cellulose and magnetic nanoparticles (NPs) were synthesized and characterized. The materials combine the chiral nematic structural features of mesoporous photonic cellulose (MPC) with the magnetic properties of cobalt ferrite (CoFe2O4). The photonic, magnetic, and dielectric properties of the hybrid materials were investigated during the dynamic swelling and deswelling of the MPC films. It was observed that the dielectric properties of the generated MPC films increased tremendously following swelling in water, endorsing efficient swelling ability of the generated mesoporous films. The high magnetic permeability of the developed MPC films in conjunction with their superior dielectric properties, predominantly in the swollen state, makes them interesting for electromagnetic interference shielding applications.
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and explored its functionality in sensing.15 In this approach, CNCs were mixed with a solution of urea-formaldehyde (UF) to form a UF/CNC composite with the CNCs organized in a chiral nematic order. After removal of the UF with alkali and drying the films with supercritical CO2, the remaining pure cellulose material is mesoporous and has a high surface area. The alkaline treatment of the films to remove the UF leads to simultaneous desulfation of the CNCs, preventing subsequent redispersion of these films in water. Additionally, a reduction of crystallinity is observed, making MPC films more flexible and swellable than usual CNC-based films. The chiral nematic structure of MPC results in a one-dimensional photonic material with iridescent coloration.16 Recently, we have been able to show that MPC hybrids with gold NPs show chiroptical effects associated with their surface plasmon resonances, making these materials interesting for sensing applications.17 In contrast to previous studies, where cellulose was used as a simple substrate for magnetic NPs, we report here a hybrid material combining the magnetic properties of CoFe2O4−NPs with the reversible swelling and photonic behavior of chiral nematic MPC. The capability of absorbing polar solvents (water) was validated with dielectric spectroscopy over the Xband (8.2−12.4 GHz). The high dielectric properties of the developed MPC films, particularly in the swelled state, coupled
INTRODUCTION The preparation of hybrid materials has gained tremendous attention due to the potential synergy of the combined organic/ inorganic materials. Inspired by nature, supramolecular chemists follow the bottom-up approach in order to obtain novel composite materials with extraordinary properties. In this context, cellulose decorated with magnetic nanoparticles (NPs) provides magnetic nanocomposites for purification/filtration membranes, magneto-responsive actuators, anticounterfeiting papers, radiofrequency materials, and flexible data storage.1−7 However, most reports in this field are found on cellulose fibrils loaded with various ferrites.8,9 Lumen-loading10 of nanofibrils with presynthesized NPs leaves the external surface of the fibrils free of magnetic NPs. Thus, the NPs are “protected” within the lumen and do not interfere with interfiber bonding. In contrast, synthesis of NPs inside a mesoporous network allows complexation of the metal ion precursors by surface carboxylate and sulfate groups, and supports stepwise conversion to metal hydroxides and then to ferrites.11 The nanofibrils subsequently act as NP stabilizers, prevent aggregation, and support the synthesis of small NPs with narrow polydispersity. Unconventional methods such as impregnation of papers with a suspension of magnetic NPs in an acrylic monomer and subsequent wet polymerization have also been used.12 Besides cellulose nanofibrils, cellulose nanocrystals (CNCs) have also been employed to make magnetic NP composites.13,14 We recently reported the preparation of mesoporous photonic cellulose (MPC) with a chiral nematic nanostructure © XXXX American Chemical Society
Received: August 9, 2016 Revised: August 22, 2016
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DOI: 10.1021/acs.langmuir.6b02974 Langmuir XXXX, XXX, XXX−XXX
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termination. Two measurements were carried out: one with the backing alone, and one with the samples and backing together. The network analyzer working with one port sent a signal down the waveguide incident to the backing/sample, and then the S-parameter of each sample was recorded. The S-parameter was converted to the dielectric properties using the Reflection-Only Epsilon ArbitraryBacked Model. The measurements were performed for both dry samples and wet samples (soaked in water for 6 h). Synthesis of Mesoporous Photonic Cellulose (MPC). A mixture of urea (2.0 g, 67 mmol) and formaldehyde (10 g of a 37% solution in H2O stabilized with 10−15% MeOH) was stirred until everything dissolved. One drop of an HCl solution (37% in H2O) was added, and the opaque solution was heated to 100 °C for 30 min until it became clear. Next, 1 mL of the UF precursor solution was added to 5 mL of a CNC suspension (CNC-Na, pH = 6.9, 3 wt %) and stirred for 10 min at room temperature. The mixture was transferred to a cellulose acetate surface (5 cm diameter) and allowed to dry for 48 h under ambient conditions. To terminate the polymerization, these films were cured at 120 °C for 16 h in an oven to give MPC. In order to remove UF from the composites, the film was heated to 70 °C in an aqueous solution of KOH (15%) for 16 h. These films were then washed with H2O and EtOH and finally dried under ambient conditions. Synthesis of Magnetic MPC Loaded with CoFe2O4 Nanoparticles (NPs). Into a solution of CoCl2·6H2O (0.998 g, 4.19 mmol, 1 equiv) and FeSO4·7H2O (2.296 g, 8.26 mmol, 2 equiv) in H2O (20 mL), a MPC film was dipped and soaked for 1.5 h at r.t. Films were carefully taken out and wiped with a paper sheet before being transferred into an aqueous solution (20 mL) of KNO3 (1.838 g, 18.2 mmol) and NaOH (0.25 g, 6.25 mmol) for 2 h at 80 °C. In order to clean films, they were washed with water and soaked overnight. Films were soaked in ethanol prior to supercritical drying to prevent collapse of the structure. Mesoporous CoFe2O4@MPC films were obtained by drying ethanol-soaked films with supercritical CO2. High Loaded Films (CoFe2O4@MPC_H). 0.998 g (4.19 mmol) of CoCl2·6H2O, 2.296 g (8.26 mmol) of FeSO4·7H2O; 1.838 g (18.2 mmol) of KNO3. CHN Analysis: C, 39.43; N, 0.85; H, 5.88. Low Loaded Films (CoFe2O4@MPC_L). 0.227 g (0.954 mmol) of CoCl2·6H2O, 0.574 g (2.06 mmol) of FeSO4·7H2O; 0.460 g (4.55 mmol) of KNO3. CHN Analysis: C, 40.12; N, 1.02; H, 6.23.
with their magnetic properties suggest that these materials may be suitable candidates for electromagnetic interference (EMI) shielding.
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EXPERIMENTAL SECTION
Materials and Methods. All compounds were used as received from suppliers without further purification. Aqueous suspensions of cellulose nanocrystals (CNCs) were supplied from FPInnovations (CNC-Na: 3.0 wt %, pH = 6.9; conductivity: 377 μS/cm, particle size: ∼100−300 nm long; ∼ 5−20 nm wide), obtained by a procedure described previously.18 Polarized Optical Microscopy. Images were acquired on an Olympus BX41 microscope with crossed polarizers. UV−Visible/Near-IR Spectroscopy. Spectra were obtained on a Cary 5000 UV−vis/NIR spectrophotometer using the films on glass microscope slides perpendicular to the beam path. Circular Dichroism (CD) Spectroscopy. CD spectra were recorded on a JASCO J-710 spectropolarimeter by using the same procedure as for the UV−vis data. Films were soaked overnight in the mentioned solvent and small pieces that did not completely cover the aperture were used in order to prevent saturating the detector. Nitrogen Adsorption. Nitrogen adsorption isotherms were measured at 77 K using a Micromeritics ASAP 2000 analyzer using sample weights of ∼100 mg. Pore-size distributions were obtained from the adsorption branches using the Barrett−Joyner−Halenda (BJH) method. Scanning Electron Microscopy (SEM). Images were taken on a Hitachi S4700 electron microscope with sputter-coated samples (2.5 nm of gold/palladium 60/40). Transmission Electron Microscopy (TEM). Millimeter-sized triangles of the material were embedded in Spurr’s resin. To improve penetration of the resin into the nanostructure, the resin was incorporated by consecutive microwave-assisted soaking steps in which the resin to acetone ratio was increased from 1:3 in the first step to pure resin in the final step. The embedded samples were microtomed, and images of the cross sections were recorded on a Hitachi H7600 electron microscope. Particle size distributions represent the Feret’s diameter that was semiautomatically obtained using the particle analysis feature of FIJI (https://fiji.sc/). Powder X-ray diffraction (PXRD). Data were obtained on a Bruker D8 Advance instrument using Cu Kα irradiation and a NaI scintillation detector. The deconvolution of the peaks of the diffraction patterns was done by the use of DIFFRACplus TOPAS software (Bruker-AXS) on the basis of the Rietveld refinements.19 The degree of crystallinity Xc (the fraction in weight occupied by the crystallites) was calculated using Ruland’s theoretical approach.20 Supercritical Drying. Critical point drying using supercritical CO2 was performed on an Autosamdri-815 critical point dryer (Tousimis Research Corporation, Rockville, Maryland, USA). Magnetic Properties. The magnetic properties were investigated by using a Quantum Design MPMS-XL7 SQUID magnetometer operating between 1.8 and 300 K for dc fields ranging between −7 and 7 T. The data was corrected for the diamagnetic contribution of the sample holder. Dielectric Properties. Dielectric properties measurements over the X-band frequency range (8.2−12.4 GHz) were performed using an E5071C network analyzer (ENA series 300 kHz−20 GHz). The samples under the test were backed by an arbitrary but repeatable
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RESULTS AND DISCUSSION In our study, we employ mesoporous MPC as a template for the formation of CoFe2O4−NPs inside the pores of the films (Scheme 1).21,22 MPC films were synthesized according to our previous report15 and then soaked in an aqueous solution of CoCl2 and FeSO4, and subsequently transferred into a basic developer solution containing KNO3 to oxidize ferrous to ferric ions. Scheme 1 illustrates the sequence of steps used to make the new materials. Two sets of samples were synthesized with different concentrations of precursors to modify the loading; they are denoted as CoFe2O4@MPC_L (for low concentration) and CoFe2O4@MPC_H (high concentration). The hybrid materials were characterized by UV/vis and circular dichroism (CD) spectroscopy, electron microscopy, energy dispersive X-ray spectroscopy (EDX), and nitrogen adsorption techniques. The magnetic properties of the hybrid B
DOI: 10.1021/acs.langmuir.6b02974 Langmuir XXXX, XXX, XXX−XXX
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materials, surface carboxylate and/or sulfate groups on the cellulose served to stabilize the NPs.11 Such functional groups have also been found important to complex the ferrous ions and guarantee a high loading.7 The MPC used in this work presents mainly surface hydroxyl groups, since desulfation accompanies the treatment of the composite films in base.15 To determine the size of the NPs, cross sections of the films were prepared by embedding small pieces of the materials in resin followed by microtoming. Despite efforts to improve the penetration of the resin into the samples, the films partly delaminated during microtoming, which made image acquisition using transmission electron microscopy (TEM) challenging. However, in the obtained images, particles appeared to be well dispersed throughout the films (Figure 1C, 1D, and S6). Interestingly, the particles follow the layered structure within the films, which indicates higher mass transport within the layer planes. In line with the reduced loading, the particle density was found to be smaller in CoFe2O4@MPC_L than in CoFe2O4@ MPC_H. The latter sample also showed a slightly decreasing particle density toward the center of the material, which can be expected given that the diffusion of both precursors and developer solution decreases with increasing material thickness. For both materials a broad particle size distribution was found with a mean of 108 nm for CoFe2O4@MPC_L and 150 nm for CoFe2O4@MPC_H, respectively. The irregular shape of the particles suggests that they are aggregates composed of smaller particles, but it was not possible to obtain images of sufficiently high resolution to further support this assumption. Nitrogen adsorption measurements were performed to determine the influence of the NP loading on the porosity of the MPC. After supercritical drying, the pristine MPC films used in our experiments show a characteristic type IV isotherm with a Barrett−Joyner−Halenda (BJH) pore size distribution of 7 nm, average pore volume of 0.4 cm3 g−1, and Brunauer− Emmett−Teller (BET) surface area of 188 m2 g−1. The mesoporosity is preserved in the films after loading with magnetic NPs and drying from ethanol with supercritical CO2. The loading reduces the BET surface area of the films (79 and 91 m2 g−1 for CoFe2O4@MPC_H and CoFe2O4@MPC_L, respectively, and average pore volumes of 0.2 and 0.3 cm3 g−1, respectively) (Figure S4). The BJH pore-sizes are calculated to
materials were investigated by superconducting quantum interference device (SQUID) magnetometry, and the dielectric properties were measured over the X-band frequency range (8.2−12.4 GHz). Successful loading of the cellulose samples with CoFe2O4 (CoFe2O4@MPC) is obvious to the naked eye (Figure 1A, 1B).
Figure 1. (A) Photographs of as-prepared CoFe2O4@MPC with high (left) and low loading (right), (B) after drying with supercritical CO2 from EtOH. TEM images of a cross section of CoFe2O4@MPC_L (C) and CoFe2O4@MPC_H (D), showing the distribution and size of the NPs embedded in MPC.
The formerly transparent samples appear brown with an angledependent iridescence, due to the preserved chiral nematic structure of the MPC. In addition, the samples respond to magnetic fields, as proven by a simple neodymium magnet. PXRD data confirm the formation of crystalline cobalt ferrite (Figure S1). The homogeneous loading of the NPs within the films was confirmed by EDX (Figure S2, S3). The EDX maps show homogeneously distributed Co and Fe for both samples, CoFe2O4@MPC_L and CoFe2O4@MPC_H. The NP size critically depends on chosen reaction conditions, such as concentration, temperature, stabilizing agent, and host material. In most published magnetic cellulosic
Figure 2. SEM micrograph showing the pristine MPC (A) as well as the CoFe2O4@MPC_H (B). The pictures clearly demonstrate the preservation of the twisted layer structure of the MPC after modification and additional aggregates of CoFe2O4 on the surface (see arrow). (C) CD spectra of films of CoFe2O4@MPC_L and CoFe2O4@MPC_H when soaked in EtOH. C
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with the NPs size and the degree of crystallinity, giving higher values for bigger NPs and high crystallinity.28,29 The magnetic hysteresis of the MPC hybrids reveals a magnetization (Ms) of 3.9 emu/g for CoFe2O4@MPC_H at 300 K, while a magnetization of only 0.2 emu/g was detected for CoFe2O4@MPC_L (Figure 3). Coercivity (HC = 217−644
be about 12 nm for both loadings. The significant reduction of the surface area of the hybrid materials compared to that of the pristine MPC is attributed to the additional basic treatment during the NP synthesis. Scanning electron microscopy (SEM) images of samples with both high and low loadings of NPs show the characteristic twisted-layer structure of the MPC, confirming the preservation of the chiral nematic structure (Figure 2A,B and Figure S5). Thus, the loading procedure does not significantly change the structural features of MPC. However, the micrographs of CoFe2O4@MPC_H prove the assumption that the ferrite nanoparticles tend to aggregate in the composite materials (Figure 2B, see arrow). Additional proof for the preservation of the chiral nematic structure was provided by the photonic properties of the samples. The maximum reflection wavelength (λmax) for a chiral nematic liquid crystal is governed by the following equation:23,24
Figure 3. (A) Magnetization saturation curves for CoFe2O4@MPC_H (red) and CoFe2O4@MPC_L (blue). (B) Zoomed version of the hysteresis curves showing the coercivity and remanence values.
λmax = navg ·P·sin(θ )
where P is the helical pitch, θ is the angle of incident light, and navg is the average refractive index. Thus, the reflection and color of the chiral nematic structures are dependent on the viewing angle, the average refractive index of the material, and the pitch of the chiral nematic assembly. We analyzed the NP/ CNC composite films by both UV−visible and CD spectroscopies; analytical results are summarized in Table S1. In general, the films are opaque and they are only transparent enough for CD or UV−vis analysis when swollen in either EtOH or water (Figures S7 and S8). The intense absorption of the magnetic NPs dominates the UV−vis spectra. Films with low loading of NPs soaked in EtOH show an intense absorption from 350 to 700 nm, and highly loaded samples show complete absorption in the range 300−500 nm. However, CD spectroscopy offers a significant opportunity to probe the chiral nematic structure of the composites. The CD spectra of the composite films soaked in EtOH show reflectance peak wavelengths of 540 nm for CoFe2O4@MPC_L and 630 nm for CoFe2O4@MPC_H. Knowing the relative swellability of MPC in polar liquids,15 we compared the swelling of the samples in EtOH with the swelling in water. The enhanced swelling of the composite films in water leads to stretching of the chiral nematic structure, leading to a longer pitch and thus causing a red-shift of the reflectance peak wavelengths to 560 and 655 nm, respectively. These data are consistent with previous observations for pristine MPC.15 However, the NP-loaded materials swell less in polar liquids than do pristine MPC films, suggesting that the magnetic NPs participate in cross-linking of the MPC, which inhibits swelling. Indeed, surface OH groups on the cellulose are likely complexed to the NPs or participate in hydrogen bonding to water on the NP surface. Nevertheless, the dynamic swelling of these hybrid materials allows for external control over the interparticle distance. Since the interparticle distance affects the photonic and magnetic properties of the hybrid materials, we investigated the magnetic properties of the hybrid materials. As mentioned, PXRD results confirmed the formation of crystalline cobalt ferrite (CoFe2O4@MPC), which is ferromagnetic.25 Below their Curie temperature, these NPs are magnetic. Cobalt ferrites are hard ferrites with a high coercivity (Hc) and high remanence (MR) after magnetization, but below a threshold size (10−20 nm),26 they become superparamagnetic.7,27 Overall, the magnetization (MS) strongly correlates
Oe) and remanence (MR = 0.04−1.58 emu/g) indicate ferromagnetic contributions and do not show superparamagnetic behavior. These results are in agreement with the previously reported effects of the NP size on the magnetization.30,31 Owing to the extensive utilization of electronic devices, EMI has been discussed as a new category of environmental pollution.32 Therefore, the field of EMI shielding has gained considerable attention within the past decade. Generally, an EM wave is composed of two components, i.e. electric field and magnetic field. These two components interchangeably transform into each other depending on the intrinsic impedance of the medium where the EM wave propagates.33 Thus, an effective shield must be able to attenuate both electric and magnetic fields efficiently. In other words, a shield must possess sufficient amount of nomadic charges and/or electric/magnetic dipoles to interact with an incident EM wave.34 Essentially, EMI shielding arises from Ohmic loss, polarization loss, and magnetic loss. Ohmic loss comes from the dissipation of energy by nomadic charges through conduction, hopping, and tunneling mechanisms; polarization loss originates from the energy required overcoming the momentum to reorient electric dipoles in each half cycle of the alternating field, and magnetic loss derives from the interaction of the EM wave with magnetic dipoles.35,36 Under an applied EM wave, the level of energy dissipation by Ohmic loss and polarization loss is represented by imaginary permittivity and real permittivity, respectively, whereas the amount of magnetic loss is estimated by measuring magnetic permeability.35 Our data showed that the generated MPCs had magnetic properties due to the existence of the magnetic NPs in their structure, making the obtained MPC hybrids appealing for application in magnetic shielding. However, in order to further verify the shielding performance of the generated MPCs, their dielectric properties in both dry and wet states were investigated over the X-band frequency range. The dielectric properties for the films with low and high loading of magnetic NPs showed very similar results; therefore, herein we just present the dielectric properties for the film with low loading (Figure 4; see Figure S9 for all details). D
DOI: 10.1021/acs.langmuir.6b02974 Langmuir XXXX, XXX, XXX−XXX
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Figure 4. (A) Real permittivity and (B) imaginary permittivity for the magnetic NP-loaded films with low loading over the X-band frequency range. The results are presented for both dry and wet films.
Notes
The real permittivity for CoFe2O4@MPC_L in the dry state was found to be around 6 (Figure 4A), which is higher than that of common polymers (∼2) over the X-band.37 This can be attributed to dipolar polarization originating from the motion of the hydroxyl groups within the amorphous regions of cellulose.38 It is worth noting that the high conductivity of magnetic NPs also contributed to the electronic polarization of the cellulose matrix, contributing to real permittivity.34,39 Interestingly, it was observed that swelling the MPC films in water led to a significant increase in the real permittivity (∼38). This increase can be ascribed to the dipolar polarization of the absorbed water, and also the positive impact of absorbed water on the ease of rotation and polarizability of the side groups and other flexible portions of cellulose.40 It should be noted that the presence of hydroxyl groups in the molecular structure of cellulose intensifies the water-absorption process.41 Similar results were found for the imaginary permittivity of CoFe2O4@MPC_L. While the dry samples yielded imaginary permittivities around 5, the imaginary permittivity significantly increased for the water-swollen samples (∼20, Figure 4B). As the cellulose structure is inherently insulating, the obtained imaginary permittivity for the dry samples is due to the presence of the conductive magnetic NPs in the MPC structure.40 The high imaginary permittivity in the waterswollen samples can be related to the presence of water as a conducting medium, facilitating the transference of nomadic ions/charges across the samples. In summary, we prepared novel composite materials combining the structural and optical features of mesoporous photonic cellulose (MPC) with the magnetic properties of CoFe2O4 NPs. The high magnetic permeability and superior dielectric properties, particularly in the swollen state, of the developed MPC films make them appealing for EMI shielding applications.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from ArboraNano, NSERC, the German Academic Exchange Service (DAAD) (fellowships to M.G. and M.S.), the Humboldt Foundation (fellowship to L.K.B.), and the Swiss National Science foundation (fellowship to G.R.M.) is gratefully acknowledged. We appreciate the in-kind contribution of CNCs from FPInnovations and CelluForce Inc. Special thanks to Didier Savant and Daniel Leznoff for support with the magnetic measurements. Derrick Horne from UBC Bioimaging Facilities is acknowledged for help with TEM image acquisition.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02974. PXRD patterns, electron micrographs, EDX data, nitrogen sorption, and CD/UV−vis measurements of the films (PDF)
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REFERENCES
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DOI: 10.1021/acs.langmuir.6b02974 Langmuir XXXX, XXX, XXX−XXX
