Smart Optical Composite Materials: Dispersions of MetalâOrganic

Smart Optical Composite Materials: Dispersions of Metal−Organic Framework@Superparamagnetic Microrods for Switchable Isotropic−Anisotropic Optical Properties Karl Mandel,*,†,‡ Tim Granath,‡ Tobias Wehner,§ Marcel Rey,⊥,∥ Werner Stracke,† Nicolas Vogel,⊥,∥ Gerhard Sextl,†,‡ and Klaus Müller-Buschbaum*,§ †

Fraunhofer Institute for Silicate Research, ISC, Neunerplatz 2, D97082 Würzburg, Germany Chair of Chemical Technology of Materials Synthesis, Julius-Maximilians-University Würzburg, Röntgenring 11, D97070 Würzburg, Germany § Institute of Inorganic Chemistry, Julius-Maximilians-University Würzburg, Am Hubland, 97074 Würzburg, Germany ⊥ Institute of Particle Technology and ∥Interdisciplinary Center for Functional Particle Systems, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 4, 91058 Erlangen, Germany ‡

S Supporting Information *

ABSTRACT: A smart optical composite material with dynamic isotropic and anisotropic optical properties by combination of luminescence and high reflectivity was developed. This combination enables switching between luminescence and angle-dependent reflectivity by changing the applied wavelength of light. The composite is formed as anisotropic core/ shell particles by coating superparamagnetic iron oxide−silica microrods with a layer of the luminescent metal−organic framework (MOF) 3∞[Eu2(BDC)3]·2DMF·2H2O (BDC2− = 1,4-benzenedicarboxylate). The composite particles can be rotated by an external magnet. Their anisotropic shape causes changes in the reflectivity and diffraction of light depending on the orientation of the composite particle. These rotation-dependent optical properties are complemented by an isotropic luminescence resulting from the MOF shell. If illuminated by UV light, the particles exhibit isotropic luminescence while the same sample shows anisotropic optical properties when illuminated with visible light. In addition to direct switching, the optical properties can be tailored continuously between isotropic red emission and anisotropic reflection of light if the illuminating light is tuned through fractions of both UV and visible light. The integration and control of light emission modes within a homogeneous particle dispersion marks a smart optical material, addressing fundamental directions for research on switchable multifunctional materials. The material can function as an optic compass or could be used as an optic shutter that can be switched by a magnetic field, e.g., for an intensity control for waveguides in the visible range. KEYWORDS: smart materials, metal organic frameworks, composites, optical properties, switching, isotropy, anisotropy, superparamagnetism, magnetic actuation temperature changes,14,15 solvents with switchable polarity,11 or thin films that may change transparency16 or hydrophilicity.6 It has not been possible to date to unify isotropy and anisotropy of one class of properties in one material. This is a challenge because materials intrinsically show either one or the

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mart materials, i.e., materials with switchable, tunable, and self-adapting properties and functionalities, are of broad scientific and technological interest1−3 and promise to enable interactive, multifunctional properties.4−7 Such smart materials include, e.g., photonic crystals sensitive to thermal, optical, or magnetic changes,8−10 surfactants with tunable stabilization properties,5,11 membranes with switchable, selective transport properties,12,13 actuators which respond to © 2016 American Chemical Society

Received: October 25, 2016 Accepted: December 12, 2016 Published: December 12, 2016 779

DOI: 10.1021/acsnano.6b07189 ACS Nano 2017, 11, 779−787

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ACS Nano other property, meaning that physical properties of materials, such as magnetization, index of refraction, conductivity, emission, etc., are either equal in all directions (isotropic) or direction dependent (anisotropic). A material system combining both properties must therefore be a composite with the possibility of selectively addressing individual components and thus the desired macroscopic property via an external stimulus. Recently, luminescent inorganic−organic complexes and metal−organic frameworks (MOFs), respectively, have been shown to enable core/shell formation with magnetic particles in dispersions.17,18 Maji et al. demonstrated the construction of bifunctional inorganic−organic hybrid nanocomposites with a Prussian blue magnetic core and a luminescent lanthanide probe.18 We also showed the combination of iron oxide nanoparticles in a silica matrix with lanthanide-based MOFs as hybrid composite particles.17 These architectures demonstrate that luminescence and magnetism can be maintained simultaneously. MOFs are remarkable luminescent materials19,20 that are of current interest for lighting,21 optical devices,22 and stimuli responsive sensing. 23,24 Due to the hybrid character, luminescence phenomena can derive from ligand fluorescence and phosphorescence,25 charge and energy transfer,26 metalbased luminescence,27 and host−guest interactions.28 Moreover, linkers and metal ions can interact perfectly by sensitization of, e.g., the characteristic 4f-emission of Ln3+ ions by suitable ligands.29 The possibility of manipulating optical properties of magnetic colloidal particles has been recently reported. These particles where assembled into a photonic crystal, and the interparticle distance was shown to be adjustable via an applied magnetic field.9,30,31 As a result, the transmission of light through the photonic crystal was controlled via particle alignment in the external magnetic field.32,33 Ultimately, this magnetic control leads to adjustable superstructures for which defined colors can be turned off or on by varying between horizontal and vertical magnetic fields. Here, we set out to design a material system capable of switching from isotropic to anisotropic optical properties in a fully tunable and reversible way. The key requirement to incorporate both isotropic and anisotropic properties is the combination of MOFs with superparamagnetic and anisotropic particle structures. We combine these individual elements by coating an anisotropic, rod-shaped superparamagnetic iron oxide/silica core with a thin, uniform MOF shell (Figure 1). The anisotropic nature of the core induces anisotropic light reflection properties, which can be controlled via a magnetic field. The core is further coated with a luminescent Eu3+-based MOF34,35 as shell material on a silica matrix, which emits isotropic, visible light upon UV irradiation. In combination, the optical properties of the system can be switched between anisotropic and isotropic properties via the choice of excitation wavelength while being rotated in a magnetic field.

Figure 1. Proposed constitution of a smart composite capable of continuous switching between isotropic and anisotropic properties upon external control: A magnetic anisotropic core enables the alignment of the particles by a magnetic field and, thus, switchable, anisotropic reflection properties (yellow arrow represents incoming and reflected light). This core is coated by a fluorescent shell material that produces isotropic light emission (red arrows) when excited by UV irradiation.

itation was induced via addition of a cyclohexane-acetone mixture acting as an antisolvent (see the Materials and Methods for details). During this process, the iron oxide nanoparticles are subsequently linked via silica while anisotropy is induced by the magnetic field. Ultimately, silica-coated iron oxide nanoparticle chains fuse together into solid Fe3O4·SiO2 microrod superstructures. Figure 2 depicts the synthesis scheme and scanning electron microscopy (SEM) images of the flat rods that are obtained as a function of the applied magnetic field. The magnetic field in the sample depends on the distance of a magnet which is placed in the proximity of the reaction vial. The magnetic field decreases (as a x−3 function) with increasing distance x of the magnet to the vial (see Figure 2a). The aspect ratio, i.e., the degree of anisotropy, increases with increasing field. In fact, perfectly rod-shaped structures (Figure 2b) are obtained at a magnetic field strength >120 mT (prevailing in the center of the reaction vial; measured in situ with a Hall probe). Still, a decent rodlike character prevails for structures which form at a magnetic field strength >30 mT. Exposed to weaker fields, the structures more and more lose their rodlike character, and less defined, irregular structures result (Figure 2f). The rods obtained at the highest magnetic fields were used in the further MOF composite formation process. The iron−oxide nanoparticles induced superparamagnetic properties to the hierarchical anisotropic superstructures (Figure 2g). Contrary to the intuitive assumption that the brown-colored dispersion of microrods behaves like a blackbody in terms of interaction with light, it was observed that a strong reflection of visible light with pronounced dynamic oscillations between higher and lower reflectivity occurs when the dispersed particles are magnetically rotated (Supplementary Video 1). Figure 3 and Supplementary Videos 1−3 illustrate these dynamic optical properties. In the presence of a magnet, the rods align along the magnetic field lines and transiently assemble into larger, anisotropic superstructures. This produces a strong macroscopic orientation that can be externally directed by variation of the direction of a magnetic field (Figure 3a and Supplementary Video 2). Removing the magnet leads to disassembly of the superstructures into individual, nonaligned microrods (Supplementary Figure S1). The macroscopic orientation enables control of the transmission and reflection of visible light. We investigate the dependence of the light transmission properties on the orientation of the microrods by illuminating a cuvette containing the particle dispersion with red laser light (λ = 650

RESULTS AND DISCUSSION In order to create the anisotropic particle core as proposed in Figure 1, superparamagnetic microrods were synthesized by precipitating SiO2 onto iron oxide (Fe3O4) nanoparticles in the presence of a magnetic field. To this end, tetraethyl orthosilicate as the silica source was added to a sol of superparamagnetic iron oxide nanoparticles.36 Silica precip780


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Figure 2. Rods are formed from iron oxide nanoparticles by exposing a suspension of these superparamagnetic particles to a magnetic field while adding silica and an antisolvent. The magnetic field induces an alignment of the nanoparticles, which become interconnected by silica that is precipitated in traces onto the nanoparticle chains (a). The rodlike character (single structure: SEM inset image c) of the solid product depends on the magnetic field and distance to the sample: With increasing distance of the magnet to the vial, the magnetic field decays and the rodlike character of the resulting solids gradually gets lost (SEM images b−f). The rods possess superparamagnetic properties (g).

nm) and monitoring the angular distribution of light on a white screen placed around the sample (Figure 3b and Supplementary Video 3). A schematic representation of the experimental configuration and the involved angles is shown in Figure 3d. First, we rotate the rods in the xy-plane of the experiment. This alignment corresponds to a variation of the inplane angle φ without changing the azimuthal angle θ (Figure 3d). The alignment of the rods in top view is shown in the microscope images of Figure 3a. When the rods are aligned parallel to the incoming light beam (defined as φ = 180°), only little light reflection is observed and most of the light passes the suspension, leading to a high intensity spot on the screen directly behind the cuvette. When the particles are rotated with respect to the incident light beam (φ = 135°), the transmitted light intensity is decreased. The most pronounced changes occur at perpendicular alignment of the microrods with respect to the incident light beam (φ = 90°): high light intensity is observed within the cuvette, indicating an increase in light reflectivity. Similarly, a higher concentration of microrods in the dispersion shows strongly anisotropic reflectivity depending on the orientation of the rods in a magnetic field (Figure 3c and Supplementary Video 1): Rods that are oriented perpendicular to the light source (90°) caused strong reflectivity, while an alignment parallel to the light source (180°) significantly

decreased the reflection of light. To exclude potential wavelength dependence, the same experiments were repeated with a blue laser, resulting in the same anisotropic properties (Supplementary Figure S2). To quantify the anisotropic reflection properties, we determined the back-reflected light intensity for the same microrod orientations as in the experiments before, i.e., when the microrods are oriented in plane (θ = 0) but with different orientations with respect to the incident light beam (variation of φ) (Figure 3d−f). For each orientation, the sample was illuminated with visible light of λ = 500 nm, and the absolute reflection intensity of the sample was determined close to the conditions of total reflectance (5° offset with respect to the incident light beam). From this quantitative experiment, a clear difference in reflection with changes of a factor of about 3.6 in intensity as a function of the orientation of the rods is observed, which is in agreement with the visual impression shown in the previous experiments. These results clearly demonstrate the anisotropic, magnetically controllable reflection properties of the composite particles. Furthermore, we note a strong anisotropy in the intensity of transmitted light in the z-direction (Figure 3g). If the rods are aligned within the xy-plane and perpendicular to the incident light beam (θ = 0°; φ = 90°), the intensity of transmitted light 781


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Figure 3. Optical properties of the superparamagnetic microrod suspension aligned by a magnetic field. All images are taken from Supplementary Videos 1−3. The orientation of the microrods is shown in the top column and specified by the in-plane angle φ and the azimuthal angle θ as schematically shown in d). (a) Light microscope image of the assembly and orientation of the microrods into larger rodlike structures oriented along the magnetic field; scale bar: 50 μm. (b) Angular distribution of transmitted light (λ = 650 nm) depending on the in-plane orientation of the microrods; the sample is placed in the center with a white screen placed around to visualize the angular distribution of light intensity upon interaction with the particle dispersion; illumination occurs from the bottom of the photographs onto microrods oriented with respect to the incident light beam as shown in (a). (c) Macroscopic change in observed reflectivity of a sample with different positions of a magnet illuminated under ambient conditions. (d) Schematic illustration of the setup and orientation of the microrods with respect to the incident light beam described by the in-plane angle φ and the azimuthal angle θ. (e) Absolute intensities of the reflected light (λ = 500 nm) for different in-plane orientation angles φ of the microrods with respect to the incident light beam. (f) Quantification of the dependence of the intensity of reflected light for a dispersion with different in-plane orientation of the microrods. (g) Angular distribution of the light intensity (λ = 650 nm) depending on the azimuthal orientation of the microrods, characterized by the angle θ as described in (d). 782


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Figure 4. (a) Luminescent composite material 3∞[Eu2(BDC)3]·2DMF ·2H2O@Fe3O4·SiO2. (b) Photoluminescence spectra of the composite in comparison to the pure MOF. (c) Schematic depiction of the composite system (a detailed crystal structure of the MOF can be found in Supplementary Figure S3). (d) SEM-EDX of the modified particles. (e) PXRD analysis of the composite in comparison to a powder pattern simulated from X-ray single-crystal data of the MOF. (f) Photograph of the system in the presence of a magnetic field, illustrating magnetic separation.

photoluminescence in layers thin enough to maintain the anisotropic particle shape. Contrary to organic chromophores, the rigidity of a framework structure increases stability. Therefore, we chose a MOF with significant chemical and mechanical stability. This is also beneficial to prevent a rub-off of the composite shell. Both of these points can be found simultaneously for the MOF used as a framework compound. The MOF shell was generated by reacting 3∞[Eu2(BDC)3]· 2DMF·2H2O with the Fe3O4·SiO2 microrod cores in DMF. The reaction product was subsequently washed with DMF three times and magnetically separated after each washing cycle in order to ensure that only magnetic composite particles were collected. The shape of the microrods is maintained upon modification as can be seen in a high resolution SEM image (Figure 4d). Via energy dispersive X-ray analyses (EDX) of the rods, it was also possible to detect Eu (Figure 4d, inset). The presence of the MOF 3∞[Eu2(BDC)3]·2DMF·2H2O shell on the magnetically separated particles was further confirmed by powder X-ray diffraction (Figure 4e, crystal structure shown in Supplementary Figure S3). We consider the formation of bonds as a result of this treatment. Two principle possibilities can apply: (a) binding of Eu-ions to the oxo and hydroxo groups of the silica matrix due to the high oxophilicity of the lanthanides and (b) ligand contact of the carboxylate to core atoms, e.g., as

shows a pronounced anisotropy with high intensities perpendicular to the rod orientation (i.e., the intensity profile is stretched along the z-plane) (Figure 3g, first image). If we rotate the microrods along the azimuthal angle θ without changing the in-plane orientation with respect to the light source (i.e., φ = 90°), the anisotropic intensity profile on the screen rotates as well: the region of high intensity remains perpendicular to the rod orientation (Figure 3g). We tentatively attribute this property to diffraction of light passing through the oriented rods. Similar to a multislit experiment, diffraction occurs preferentially perpendicular to the orientation of the slits. Since the spacing of the rods (or the slits in between the oriented rods) is not regular, we do not observe defined maxima and minima as expected from multislit interference but a broad distribution of light intensity perpendicular to the long axis of the rods. The superparamagnetic microrods were subsequently coated with a luminescent metal−organic framework (MOF, Figure 4). The carboxylate MOF 3∞[Eu2(BDC)3]·2DMF·2H2O was chosen as a MOF component,34 as it exhibits a pronounced red luminescence and is stable under atmospheric conditions and in many solvents. In general, coordination compounds can benefit from essential ligand sensitization of emissive metal states, as shown in this example, to provide a strong 783


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Figure 5. (a) Light reflection properties of the MOF-functionalized nanocomposite rods. Absolute reflection intensities of the beam (λ = 500 nm) for different angles between magnet and beam. (b) Differences in the reflection intensities for different angles. (c) Visualization of the angle dependence of the magnetic field on the light reflection. (d) Emission properties of the MOF-functionalized nanocomposite rods for different angles between magnet and beam (λ = 300 nm). (e) Emission intensities showing no difference for different directions of the magnetic field. (f) Visualization of the angle-independent isotropic luminescence properties (λExc = 302 nm) of the MOF-functionalized microrods.

nature and independent of an external magnetic field and its orientation (Figure 5d−f). Thus, the particle system can be switched between purely isotropic and anisotropic behavior. This is done by changing the wavelength of an illuminating light source between the UV excitation for luminescence and visible light for reflection, while the particles are rotated by an external magnetic field. Moreover, by mixing UV and visible light and tailoring the respective fractions, the system can also be continuously changed from purely isotropic red emission to purely anisotropic, dynamic reflection of visible light. Parts c and f of Figure 5 show photographs highlighting the dual optical properties of the composite particles in the presence of a rotating magnetic field. Visible white light induces a clear anisotropy with orientation-dependent light intensity (Figure 5c), while isotropic, red luminescence is not orientation dependent (Figure 5f). Thus, the rotating magnetic field induced time-dependent oscillations of the intensity of reflected white light as the microrod orientation changes in response to the magnetic field. In contrast, the luminescence properties of the system behave isotropically and do not show a change in intensity in the same rotating magnetic field. The dynamic nature of this remarkable effect is best seen in motion in a video. Supplementary Video 4 shows the MOF@superparamagnetic microrods under the influence of a rotating magnetic field, while the intensity of visible light is reduced and intensity of UV light increased. A continuous transition from anisotropic reflection to isotropic emission is observed. Supplementary Video 5 shows the optical properties of dispersions of pure magnetic microrods (left), MOF@superparamagnetic microrods (middle), and pure MOF (right)

hydrogen bonds to the oxo and hydroxo groups of the silica matrix. In corroboration, during our investigations, no rub-off of the shell was observed. However, a direct analytical proof of the binding is difficult due to the thin atomic layer of this contact. Figure 4a shows the optical properties of the composite particles. The dispersion shows pronounced luminescence in the red region of the visible spectrum upon excitation with UV light. The magnetic properties remain unaffected by the coating, and the dispersion can be actuated in the presence of a magnetic field (Figure 4f). The photoluminescence spectra of the pure MOF and the composite microrods are depicted in Figure 4b. The emission is almost exclusively based on intra-4ftransitions of the Eu3+ cations 5D0 → 7FJ, J = 0−6.37 This metal-based emission is the result of an efficient energy transfer from the BDC2− ligand, which provides complete energy uptake for excitation without participating in the emission.38 Importantly, this luminescence property is isotropic, as the emission is spherical. Thus, the MOF shell adds the isotropic optical emission component to the system. The combination of anisotropic light reflection with the isotropic emission gives rise to dynamic, switchable optical properties of the composite particle dispersion. Figure 5 shows the angle-dependent optical properties and demonstrates this dynamic, switchable nature of the interaction with light. Similar to the data shown in Figure 3, when illuminated with visible light (λ = 500 nm), the composite MOF@superparamagnetic microrod particles show anisotropic light reflection behavior that depends on the direction of the magnetic field lines (Figure 5a−c). The light emission, arising from the luminescent MOF shell excited via UV irradiation, however, is isotropic in 784


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with 10 mL of cyclohexane, yielding a two-phase system of “oil” on top of “water”. After that, 125 mL of acetone was quickly poured (within 1 s) into the mixture, while the whole system was exposed to a magnetic field. The magnetic field was obtained from an NdFeB hand-held magnet. To obtain highly anisotropic rods, the magnetic field strength in the reaction vial should be ∼120 mT. The field strength in our experiments was controlled with a Hall probe. The strength was varied by changing the distance of the magnet to the vial. The quick addition of the antisolvent acetone caused a turbulent mixing of all reactants, ultimately yielding TEOS precipitation. During the whole synthesis, no additional stirring was performed. The setup is depicted in Figure 2, where for clarity reasons, the setup (the vial) is not drawn to scale; moreover, in the real experiment, the magnet is typically placed under the beaker. The finally obtained precipitate was magnetically separated (solid liquid separation typically occurred within 30 s), decanted, and washed initially with 2.5 mL of NaOH (0.01 M) in 150 mL of acetone, two times with 150 mL of an acetone−water mixture (2:1), and finally, two times with 20 mL of DMF. Redispersion was performed within 100 mL of DMF. All synthesis steps were performed at room temperature. Synthesis of 3∞[Eu2(BDC)3]·2DMF·2H2O. Prior to the MOF synthesis, Eu(NO3)3·5H2O was synthesized according to ref 40 by dissolving Eu2O3 in an aqueous solution of 6.0 M HNO3 and evaporating at 100 °C. Na2BDC was synthesized from NaOH and H 2 BDC (terephthalic acid) in H 2 O according to ref 41. 3 ∞[Eu2(BDC)3]·2DMF·2H2O was synthesized according to ref 34 by reaction of Na2BDC with Eu(NO3)3·5H2O in a mixture of DMF/ H2O (1:1). Synthesis of 3∞[Eu2(BDC)3]·2DMF·2H2O@Fe3O4·SiO2 Microrods. For microparticle modification with a luminescent MOF, 3 ∞[Eu2(BDC)3]·2DMF·2H2O (2.35 mg, 2.40 μmol) was placed with a suspension of Fe3O4·SiO2 microrods (24.0 μmol) in 1 mL of DMF in a Duran glass ampule. The ampule was repeatedly frozen in liquid nitrogen and degassed after evacuation and sealing of the ampule, and the microparticles were dispersed in an ultrasonic bath at 60 °C for approximately 6 h. For purification, the MOF@microparticle system was separated magnetically and washed three times with 1 mL of DMF, respectively. Powder X-ray Diffraction (PXRD). Powder analysis was carried out on a Bruker D8 Discover diffractometer with Da Vinci design, focusing Göbel mirror, and linear LynxEye detector. Powder samples were prepared by grinding with a mortar and placed on low background silicon wafer. The samples were measured in parallel beam geometry in reflection mode using CuKα radiation (λ = 1.54056 Å) Magnetic Measurements. Magnetic properties were studied with a vibrating sample magnetometer (VSM, VersaLabTM 3 T cryogenfree vibrating sample magnetometer) by cycling the applied field from −30 to +30 kOe two times with a step rate of 100 Oe s−1. The temperature was set to 20 °C. Optical Properties. The alignment of the superparamagnetic microrods was using an Ergolux optical microscope in bright field mode with a 16× objective. Videos were taken with a B/W CMOS camera (DCC1545M, Thorlabs). The transmission and scattering was analyzed by illuminating 0.5 mL of the 0.01 wt % microrod dispersion in a cuvette (Brand, ISO 9001 14001) with a red laser light (λ = 650 nm, 0.5 mW). A white screen was placed around the sample to visualize the angular distribution of scattered and reflected light. Videos were taken with a camera (Panasonic DMC-FZ72) mounted on the top of the setup (Supplementary Figure S2). The alignment of the magnetic microrods was changed by moving a magnet along the screen, approximately 10 cm away from the sample. The change in reflection properties was visualized by illuminating a glass vial containing 1.5 mL of the 0.1 wt % microrod suspension with polarized white light (Schott, 150 W). The camera (Panasonic DMCFZ72) as well as the light source was placed in front of the vial. Similarly, the alignment of the microrods was changed with a magnet placed approximately 10 cm away from the sample.

under the same illumination conditions. The reference samples show either anisotropic (pure rods) or isotropic (pure MOFs) characteristics. The dual optical properties are only observed in the composite architecture.

CONCLUSION The combination of anisotropic superparamagnetic silicacovered Fe3O4 microrods with a luminescent MOF shell of 3 ∞[Eu2(BDC)3]·2DMF·2H2O resulted in the generation of a smart optical composite material with switchable properties based on the presence of isotropic and anisotropic optical properties. The MOF shell contributes red Eu3+-based luminescence as the isotropic optical component, while the silica/Fe3O4 core gives rise to an anisotropic reflection of visible light that is dependent on the orientation of an applied magnetic field. The combination of angle-dependent reflectivity and transmittance with isotropic luminescence creates a particle system with switchable optical properties. Furthermore, the optical properties can be continuously tuned between isotropic and anisotropic behavior depending on the energy and fraction of a light source without changing the wavelength of the emitted light. The fundamental challenge that exists for homogeneous materials, namely that properties of one material class are either isotropic or anisotropic is overcome successfully with this type of composite. Potential functions of this combinatory material cover e.g. an optic compass or an optic shutter for waveguides switched by a magnetic field as well as to control the intensity of waveguides in the visible range by continuous tailoring of anisotropic and isotropic properties. Further sophistication is given by the relation of reflection and emission, which can be used to sense this tailoring ratiometrically. Altogether, this marks a smart optical material that enables the precise tuning of optical properties, which may be suitable for the design of switchable stimuli-responsive optical materials. MATERIALS AND METHODS Materials. Iron(III) chloride hexahydrate (FeCl3·6H2O, 99%), iron(II) chloride tetrahydrate (FeCl2·4H2O, 99%), ammonia solution (aqueous NH3, 28−30 wt %), N,N-dimethylformamide (DMF, 99%), and sodium hydroxide (NaOH, 98%) were obtained from SigmaAldrich, Germany. Cyclohexane (100%) was obtained from VWR International, Germany. Tetraethyl orthosilicate (TEOS, 99%) and terephthalic acid (C8H6O4, 98%) were obtained from abcr GmbH, Germany. Nitric acid (HNO3, 65% solution) was obtained from Otto Fischar GmbH & Co. KG, Germany. Eu2O3 (99.9%) was obtained from research chemicals. All reagents were used without further purification. Synthesis of a Ferrofluid Containing Superparamagnetic Iron Oxide Nanoparticles. Superparamagnetic iron oxide nanoparticles were synthesized by the coprecipitation method,39 and stabilization was performed as described in a former publication.36 In brief, 2.16 g (8 mmol) of FeCl3·6H2O and 795 mg (4 mmol) of FeCl2· 4H2O were dissolved in 100 mL of deionized water. Then 5 mL of an aqueous ammonia solution (NH3 (aq), 28−30%) was quickly added under stirring. The black precipitate was magnetically separated and washed until neutral pH was reached. Then 10 mL of 1 M nitric acid (HNO3) was added and the solution subsequently diluted with 100 mL of water. Synthesis of Fe3O4·SiO2 Microrods. To obtain stable magnetically manipulable microrods, 1 mL of TEOS was dropped (as 10 individual droplets with a 1 mL Eppendorf pipet, added within 10 s) into 20 mL of the as-prepared ferrofluid. The reaction vial was a beaker glass, which was, when filled with the ferrofluid, covered at a height of about 2 cm with the fluid. Subsequently, the fluid was slowly overlaid 785


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ACS Nano Photoluminescence Spectroscopy. Excitation and emission spectra were recorded with a Horiba Jobin Yvon Fluorolog 3 photoluminescence spectrometer equipped with a 450 W Xe lamp, an integration sphere, Czerny−Turner double grating (1200 grooves per mm), excitation and emission monochromators, and FL-1073 PMT detector. To block the first and second harmonic oscillation of the light source, an edge filter (400 nm) was used. Excitation spectra were recorded from 250 to 500 nm and corrected for the spectral distribution of the lamp intensity with a photodiode reference detector. Emission spectra were recorded from 400 to 800 nm and corrected for the spherical response of the monochromators and the detector using correction spectra provided by the manufacturer. For determination of the reflection properties, a magnet was placed beside the sample in the sample chamber and moved circularly around the sample in 45° steps. Scanning Electron Microscopy. Scanning electron microscopy was carried out with a Zeiss Auriga 60 FE-SEM at 1 kV acceleration voltage for image acquisition and at 5 kV for EDX analysis using a SDD EDS detector from Ametek/EDAX.

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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/acsnano.6b07189. Figure S1: Microscopy images of magnetic microrods in the presence and absence of a magnetic field. Figure S2: Schematic crystal structure of the MOF 3∞[Eu2(BDC)3]· 2DMF·2H2O. Figure S3: Experimental setup of the laser experiments (PDF) Demonstration of the dynamic reflective behavior of the microrods when rotated via a magnet in visible light at ambient conditions (MPG) Actuation of the rods with a magnet and their alignment into anisotropic, highly directional superstructures (MPG) Controlling the transmission and reflection of visible light in the red part of the spectrum (λ = 650 nm) by recording the angular distribution of light on a white screen at different positions of a magnet (MPG) MOF@superparamagnetic microrods under the influence of a rotating magnetic field while the intensity of visible light is reduced and that of the UV light increased (MPG) Same experiment as in video 4 that compares luminescence of dispersions of blank microrods, MOF@superparamagnetic microrods, and pure MOF (MPG)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Karl Mandel: 0000-0002-1445-0702 Notes

The authors declare no competing financial interest.

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