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Superparamagnetic Luminescent MOF@Fe3O4/SiO2 Composite Particles for Signal Augmentation by Magnetic Harvesting as Potential Water Detectors Tobias Wehner,† Karl Mandel,*,§,‡ Michael Schneider,§,‡ Gerhard Sextl,§,‡ and Klaus Müller-Buschbaum*,† †

Institute of Inorganic Chemistry, Julius-Maximilians-University Würzburg, Am Hubland, D97074 Würzburg, Germany 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 ‡

S Supporting Information *

ABSTRACT: Herein, we present the generation of a novel complex particle system consisting of superparamagnetic Fe3O4/SiO2 composite microparticle cores, coated with luminescent metal−organic frameworks (MOFs) of the constitution 2∞[Ln2Cl6(bipy)3]·2bipy (bipy = 4,4′-bipyridine) that was achieved by intriguing reaction conditions including mechanochemistry. The novel composites combine the properties of both constituents: superparamagnetism and luminescence. The magnetic properties can be exploited to magnetically collect the particles from dispersions in fluids and, by gathering them at one spot, to augment the luminescence originating from the MOF modification on the particles. The luminescence can be influenced by chemical compounds, e.g., by quenching observed for low concentrations of water. Thus, the new composite systems present an innovative concept of property combination that can be potentially used for the detection of water traces in organic solvents as a magnetically augmentable, luminescent water detector. KEYWORDS: superparamagnetism, composites, metal organic frameworks (MOFs), water detectors, magnetic particle detectors, luminescence

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and metal center. 30−32 For example, for the MOF ligand based light absorption, followed by a ligand-to-metal energy transfer, results in the specific 4f−4f emission of Eu3+ and Tb3+, which can be detected as sharp bands in the photoluminescence spectra.33 Water detection in solvents is a challenge of high application relevance. The state of the art analysis to detect water traces is the Karl Fischer titration. However, the method requires the use of chemicals, sophisticated equipment, trained lab-staff and it is typically a nonmobile laboratory method.34,35 Besides, also optical humidity sensors, which take advantage of the luminescence properties of light emitting compounds, are used for the reversible detection of water in solvents. Such sensors are, e.g., Ru(II) complexes with dppz (dipyrido[3,2a:2,3-c]phenazine) ligands36,37 or Ru−diimine complexes.38 Additionally, palladium and platinum based sensors like platinum octaethylporphyrin films39 or Pd/Pt porphyrins40 can be used for H2O sensing. Other suitable water sensors are rhodamine 6G based compounds using an optode membrane and a gelatin matrix to immobilize the luminophore41 or

INTRODUCTION Metal organic frameworks (MOFs) have gained tremendous attention.1−3 The main reason is found in a variety of interesting properties resulting from a combination of high surface area4,5 and tailorable pores6−10 with key properties such as distinct host−guest interaction leading to gas storage,6,9,10 drug-delivery systems,11,12 catalysts13−15 or luminescence16−20 and opening new ways for sensors.21,22 Although dominated by reactions in solution, mechanochemical syntheses are a particularly remarkable development.23−25 To widen the field of potential applications, MOFs have also been successfully combined to functional composite core/shell particles. Thereby, it was shown that it is possible to maintain the functions of both components in one composite material.26 A combination of luminescence with superparamagnetism has inspired composite synthesis. Luminomagnetic systems were, e.g., created by spray pyrolysis of Fe2O3 with Eu-doped Gd2O3,27 or modification of Fe3O4 with phosphors such as Eu( D B M ) 3 2 H 2 O ( D B M = d i b e n z o y l m e t h an a t e) o r YVO4:Eu3+.28,29 To the best of our knowledge, no MOF system has been utilized for this property combination, so far. In lanthanide containing MOFs, the intrinsic luminescence of the metal centers can coincide with ligand fluorescence, phosphorescence and energy transfer processes between linker © XXXX American Chemical Society

2 ∞[Ln2Cl6(bipy)3]·2bipy,

Received: December 8, 2015 Accepted: February 10, 2016

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DOI: 10.1021/acsami.5b11965 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Principle of the proposed water detector system using luminescent MOF@superparamagnetic particles: (a) dispersion of the microparticles in the H2O-contaminated solvent, (b) interaction of water with the particle shell, (c) partial loss of the luminescence function, and (d) magnetic harvesting and detection of the microparticles.

different Ln3+ based substances.42 Furthermore, the amount of water in a solvent can be determined indirectly by sensing the dissolved oxygen content in water. This can, e.g., be done by using the fluorescence method, in which the light from a fluorescent material excited via a blue emitting LED is quenched by oxygen passing through a dissolved oxygen permeable layer. The fluorescence after O2-contact is detected by a light-receiving diode.43 In addition, luminescent metal−organic frameworks have been utilized as sensors for small molecules including water, recently.18,19,42,44 Because of its vibration nodes, water is a typical quencher for luminescence, allowing to monitor its concentration by an intensity decrease of the luminescence. It is worth mentioning that many lanthanide containing MOFs show a certain sensitivity toward humidity45 (mostly due to the pronounced oxophilicity of the lanthanides) that can lead to degradation of the MOF. This degradation is ultimately observed by decrease of the luminescence intensity.46 Herein, we utilize this luminescence degradation in a composite of the MOFs 2∞[Ln2Cl6(bipy)3]·2bipy and Fe3O4/SiO2 microparticles (Ln = Eu, Tb, bipy = 4,4′-bipyridine), thereby consisting of a superparamagnetic core and a luminescent shell. The novel composite can be used for potential sensor applications for humidity/water detection. Furthermore, our composite detector is mobile, in situ applicable and does not require titration, has the simplicity of a luminescence sensor, and at the same time yielding a detection limit that is as good as Karl Fischer systems.34,35

generation. Both, mechanochemical and solvothermal approaches were developed successfully. It is intriguing that both properties superparamagnetism and luminescence can be combined in one particle system. The composite succeeds here, albeit several expected problems: Lanthanide containing MOFs are typically just weak paramagnets “diluting” the superparamagnetic effect, and the dark brown color of the Fe3O4 core is a potential problem for any emitter, as it is close to a blackbody. To the best of our knowledge, this is the first time this property combination could be achieved. Moreover, the novel composite particles can be potentially useful as an innovative system to detect chemical species such as water in liquids such as organic solvents. The composite can be dispersed in the selected fluid. Thereby, a fast and quantitative reaction even with low concentrations of water is possible. Depending on the content of water, a directly related amount of MOF gets hydrolyzed, accompanied by a reduction of the luminescence intensity according to the amount of H2O. Thereby, the detector composites are in principle reusable, until the complete MOF shell is used up. To read out the luminescence signal, the superparamagnetic nanocomposite Fe3O4/SiO2 core functions as carrier vehicle for the MOFs that allows magnetic harvesting and thereby an augmentation of the luminescence signal despite the initial distribution. This means that a low detector concentration can be used that does not require the complete solvent volume to be illuminated but can be magnetically separated and investigated, subsequently. Superparamagnetism is a magnetic nanoeffect: the particles are strong magnets when exposed to a magnetic field but completely lose their magnetic properties when the external magnetic field is removed, i.e., they are switchable magnets.47−49 These particles can be added to fluids without initial agglomeration, as there is no remanent magnetization. Combined with a chemically modified surface (in this work: the MOF system), the particle system can interact with target substances in the fluid. By “switching-on” in an external magnetic field, the particles are magnetically collectable. As individual magnetic nanoparticles are too small to be efficiently

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RESULTS AN DISCUSSION Characterization and Synthesis. To generate a composite combining the two properties (luminescence of a MOF with sensor properties plus superparamagnetism of Fe3O4), two strategies were applied: The magnetic particles were either directly contacted with the already synthesized MOFs 2 ∞[Ln2Cl6(bipy)3]·2bipy (Ln = Eu, Tb) or by formation of the respective MOFs starting from the precursor [LnCl3(py)4]· 0.5py. Therefore, superparamagnetic nanoparticles of Fe3O4 were imbedded into SiO2 microparticles prior to composite B


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Figure 2. Photoluminescence spectroscopic investigations on the MOF@magnetic particle composite systems 2∞[Ln2Cl6(bipy)3]·2bipy@Fe3O4/SiO2 (Ln = Eu top left, Tb bottom left) and photographs under daylight and under UV-light indicating the option of magnetic separation (mid). Fluorescence microscopic investigations of the Eu-MOF@magnetic particles (bottom right) in comparison with a transmission image of the composite particles indicating that the MOF is generating the shell covering all of the magnetic particles (top right).

extracted from a fluid with standard magnets, several of them are agglomerated within a matrix to form larger particles that still possess the switchable magnetic properties but are large enough to be quickly separable.48,49 The principle of using a MOF@superparamagnetic particle system for luminescent signal augmentation is depicted in Figure 1. As magnetic particles, we used a Fe3O4/SiO2 system that we recently developed. It consists of multicores of 10 nm sized magnetite nanoparticles in a silica matrix, forming micron sized particles (average size, 20 μm; BET surface area, ∼75 m2/g) that possess superparamagnetic properties (saturation magnetization 30 emu/g).48,50 The matrix of the microparticles that hosts the iron oxide nanoparticles is silica, thus bonding of surface modifying MOFs can be achieved via Ln−O bonds. As luminescent MOFs for the microparticle functionalization, we used the two-dimensional MOF system 2∞[Ln2Cl6(bipy)3]· 2bipy (Ln = Eu, Tb). For the lanthanide ions Eu3+ or Tb3+, intensive luminescence properties are observed, which are characteristic for the particular rare earth ion.33 Upon excitation with UV-light, the ligand 4,4′-bipyridine acts as a sensitizer for the lanthanide centers, followed by energy transfer to the Ln3+ ions, from where light is emitted in the visible spectral region. The emission is constituted by sharp emission bands in the photoluminescence spectra, which can be assigned to the transitions 5D0→7F0−4 for Eu3+ and 5D4→7F6−0 for Tb3+.33 For the synthesis of modified superparamagnetic nanocomposite microparticles, the magnetic carriers for the luminescent MOF on average are 20 μm in size and respond to an external magnet with a saturation magnetization of 30

emu/g. The Fe3O4/SiO2 particles, therefore, provide a sufficiently large size and saturation magnetization to ensure fast magnetic separation in fluids as well as a suitable surface areas to provide a sufficient interface for a surface functionalization. The MOF shell provides additional surface functionalization for the interaction with target substances in fluids. Further details on the pure magnetic particle system including electron micrographs have been previously reported.48,50 The modification of superparamagnetic Fe3O4/SiO2-microparticles with the MOF is achieved directly by reaction with the synthesized MOF 2∞[Ln2Cl6(bipy)3]·2bipy either (a) via a mechanochemical treatment by ball milling or (b) under solvothermal conditions in hexane. Both pathways lead to a strong connection of MOF and microparticles in a core/shell composite product. Alternatively, an in situ formation of the MOFs on the microparticle surface can be carried out. Although the usual formation reaction of the MOF system from LnCl3 and 4,4′-bipy does not work in the presence of the microparticles leading to neither coating nor to MOF formation, an alternative reaction with the precursor complex [LnCl3(py)4]·0.5py (Ln = Eu, Tb) together with 4,4′bipyridine proved successful and yields the same MOFs on the surface of Fe3O4/SiO2. PXRD analysis confirms the formation of the complex 2 particle system ∞ [Ln2Cl6(bipy)3]·2bipy@Fe3O4/SiO2 (see Figure S1). All reaction products show an intensive luminescence that can be assigned to the typical luminescence processes observed for both MOFs.33,51 Excitation spectra show C


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Figure 3. SEM images of individual composite particles (a,d) and detailed insets (b,e) for Eu (a,b) respectively Tb (d,e) MOF 2∞[Ln2Cl6(bipy)3]· 2bipy @Fe3O4/SiO2. EDX (c) for the Eu system and (f) for the Tb system on the particles reveals the MOF coating (regions marked with 1) on most of the particle. A few areas (regions marked with 2) of the particles are noncoated, which is optically visible and confirmed by EDX.

The MOFs do not need to be activated in order to function as detector system. Via distribution in a solvent, the pore content is exchanged with the solvent in an equilibrium. BET analysis of the composite systems 2∞[Eu2Cl6(bipy)3]·2bipy@ Fe3O4/SiO2 results in a surface of sBET = 50 m2/g. Thus, the specific surface area of the composite system origins from the original Fe3O4/SiO2 particle surface (70 m2/g) and not the MOF (660 m2/g40). Given that the microparticle functionalization was performed with the nonactivated MOF, too, it is reasonable that the MOF@microparticle composite does not exhibit an increased surface area or porosity and thereby has the advantage not to require primary activation. Application of the MOF@superparamagnetic Particles System as Water Detector. Because of the high oxophilicity of the lanthanide centers, the MOF part 2∞[Eu2Cl6(bipy)3]· 2bipy of the composite MOF@magnetic particle systems hydrolyzes upon contact to water or humidity, which results in gradual reduction of the luminescence intensity. Although the MOF@microparticle systems exhibit intensive luminescence properties of the MOF part, the composite materials do not show luminescence after hydrolysis, but still keep their magnetic properties. Therefore, an application of the Ln3+containing composites as water detector is possible for solvents that do not contain competing functional groups such −OH, as the latter can lead to erroneous detection. To test the MOF@magnetic particle system’s water detection ability, the influence of water on the luminescence was investigated including magnetic harvesting and signal 2 [Eu2Cl6(bipy)3]·2bipy@Fe3O4/SiO2 augmentation: 2 mg ∞ were placed in 0.5 mL of hexane as well as toluene with a respective defined content of H2O (none, 0.1%, 0.7%, 1.4%, 2.9%, 10% related to the amount of substance, mass %) under inert gas atmosphere. After the system was sealed in an inert ampule, the particles were dispersed in an ultrasonic bath, collected with a harvest magnet and separated from the H2O-

the excitation of 4,4′-bipyridine and, thus, indicate an energy transfer via antenna effect, as expected for the MOFs (see Figure 2). The formation of one particle system instead of two products that are not interconnected is verifiable as all modified composite particles combine luminescence clearly visible with the naked eye (green for Tb3+, red for Eu3+) with superparamagnetic properties (Figure 2). If the MOFs 2 ∞[Ln2Cl6(bipy)3]·2bipy were not attached to the microparticles by attractive interactions, the contact with a magnetic field would result in separation of the product into superparamagnetic particles and the nonmagnetic MOF. Product analysis via fluorescence microscopy shows that the MOF@ magnetic particle system yields luminescence properties for the complete substance with no such separation (Figure 2). In addition, the absence of nonluminescent areas is a proof for the successful formation of a MOF@magnetic particle system, because unfunctionalized microparticles would not exhibit luminescence properties. Moreover, the presence of the luminescence MOFs on the outside of the composite particles indicates formation of a core/shell system. The proposed core− shell character of the MOF@magnetic particles system is also confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), as can be seen in Figure 3: the Fe3O4/SiO2 microparticle core is almost completely coated with MOF, except a few regions where the uncoated Fe3O4/SiO2 parts are left blank. Finally, the superparamagnetic properties are also retained for the particle system (see Figure S2). Interestingly, the overlap of superparamagnetic and paramagnetic properties are well-pronounced in the magnetization curves for the Tb system and less prominent for the Eu system at high external field (approximately visible in the region above 0.8 T). This observation fits well to the fact that Tb has a susceptibility that is 10 times higher than that of Eu. D


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Figure 4. Exemplary detection of water with the composite particles 2∞[Eu2Cl6(bipy)3]·2bipy@Fe3O4/SiO2: From magnetic harvesting of the red luminescent particles (top left) to the on-the-fly “turn-off” detection of the luminescence after contact with hexane (water content of 1.4% (top mid) and 2.9% (top right)). Referring excitation and emission spectra of the modified composite particles after contact with defined amounts of H2O in hexane (mid) and toluene (bottom) and the quantitative determination of the reduction in excitation and emission intensity starting from 0.1% H2O (insets depict the absolute course of intensity for increasing water concentrations).

change including the same quantitative trend. The emission/ excitation intensity decrease occurs at the same water concentrations but is more prominent for toluene. From the concentrations investigated, calculation of an unknown water content is possible for the region of intensity change (see Figure 4 and insets). Analysis of the isolated MOF@magnetic particles after detection can be used for a quantitative determination of the water content: For example, for hexane and 0.1% water content, the characteristic Eu3+ luminescence shows an intensity reduction of 27% for the 5D0→7F4 transition (λmax = 700 nm). A water amount of 0.7% results in 87% reduction of the original luminescence intensity, whereas 1.4%

containing solvent for analysis. Both, absence of water and humidity was thereby ensured during the photoluminescence investigations. The isolated MOF@magnetic particle system was subsequently examined via photoluminescence spectroscopy. Measurable reduction of the luminescence intensity is obvious for 0.1% water, already, followed by ongoing decrease in luminescence intensity for 0.7% and so on. It is hardly visible by the eye for 1.4% water content. Further increase to a water content of 2.9% results in the weak blue luminescence originating from 4,4′-bipyridine only; thus, the quenching of the Ln3+-emission is complete (see Figure 4). The use of toluene and hexane as solvent results in a similar luminescence E


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a frequency of 15 Hz. The composite product was magnetically separated from any nonmagnetic byproduct/excess MOF. Solvothermal Modification of Fe3O4 /SiO2 with the MOF 2 ∞[Ln2Cl6(bipy)3]·2bipy. For a solvothermal approach, Fe3O4·SiO2 microparticles (18.9 mg) were placed together with 0.8 mL of hexane 2 [Tb2Cl6(bipy)3]·2bipy (64.8 μmol, 85.0 mg) or and either ∞ 2 ∞[Eu2Cl6(bipy)3]·2bipy (64.8 μmol, 84.0 mg), in a Duran glass ampule. The ampule was repeatedly frozen in liquid nitrogen and degassed. After evacuation and sealing of the ampule, the microparticles were dispersed by an ultrasonic bath for 10 min. Subsequently, the reaction mixture was heated to 90 °C with 20 °C/h; the temperature was maintained for 36 h and the reaction mixture was then cooled to room temperature with 20 °C/h. The solvent was removed under vacuum after the reaction. In Situ Formation of the MOFs on the Surface of Fe3O4/SiO2 Microparticles. For an in situ functionalization of the microparticles by formation of 2∞[Ln2Cl6(bipy)3]·2bipy, [TbCl3(py)4]·0.5py (64.8 μmol, 40.3 mg) or [EuCl3(py)4]·0.5py (64.8 μmol, 39.8 mg) was placed in a Duran glass ampule together with 4,4′-bipyridine (194 μmol, 30.3 mg), Fe3O4/SiO2microparticles (18.9 mg) and 0.8 mL of hexane. The reagents were then treated as described above for the solvothermal modification. The solvent was removed under vacuum after the reaction and the product again magnetically separated from possible nonmagnetic impurities. Water Detection Analysis. Highly dried solvents were brought to a certain water content by deliberate addition of defined amounts of water. The volume of water was set with an Eppendorf pipette (standard deviation: ±8% for 0.1% water content, ±2.5% for 0.7% water content, ±1.5% for 1.4% and 2.9% water content, and ±1% for 10% water content) followed by constant stirring to avoid phase separation. The solvent/detector system was contacted inside the photoluminescence spectrometer. 2 mg of 2∞[Eu2Cl6(bipy)3]·2bipy@ Fe3O4/SiO2 was placed in 0.5 mL of solvent (hexane, toluene) with a respective defined content of H2O (none, 0.1%, 0.7%, 1.4%, 2.9%, 10% related to the amount of substance, mass %) under inert gas atmosphere. The amount of substance of the MOF@microparticle system used for detection was set to 1.25 μmol. The system was sealed in an inert ampule, the particles were dispersed in an ultrasonic bath, collected with a harvest magnet and separated from the H2Ocontaining solvent for analysis. The photoluminescence spectroscopy was carried out on a HORIBA Jobin Yvon Spex Fluorolog 3 spectrometer equipped with a 450 W Xe lamp, double grated excitation and emission monochromators and a photomultiplier tube (R928P). All samples were investigated as solids in spectroscopically pure quartz cuvettes in front face mode at room temperature recording excitation and emission spectra time-dependent and subsequently. Thereby, the detection was monitored by the luminescence intensity. To provide absence of humidity during the PL investigation, special sealed cuvettes were applied. To observe potential chemical changes, complete spectra were recorded for identical spectrometer conditions. For the estimation of the detection limit, the intensity of transition into the 7F4-state was used.

water reduces the intensity by >99%. The intensity decrease follows an exponential function. Higher water concentrations further diminish the remaining intensity, until the Eu3+transitions cannot even be detected with a sensitive spectrometer. The detection limit of the MOF@magnetic particle system was estimated by calculation via a fit of the emission intensity change with decreasing water content (see Figure S3). Thus, a theoretical detection limit of 0.01% (9 μg) water content for toluene and of 0.03% (20 μg) water content for hexane could be calculated. Thereby, the presented MOF@ particle composite system reaches the sensitivity of the Karl Fischer titration (which is 10 μg of water traces in solvents). Even for an “on-the-fly” analysis outside a laboratory without spectrometer and only using the eye, the detector is still sensitive as “turn-off” sensor. Considering 98% intensity loss a complete turn-off by the eye, the detection limits are still only 0.8% H2O for toluene and 1.4% for hexane (see also Supporting Information).

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CONCLUSION

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EXPERIMENTAL SECTION

Altogether, we present a novel composite microparticle system that enables a combination of superparamagnetism and luminescence that, to the best of our knowledge, is thereby reported for the first time for a MOF. It is constituted of Fe3O4/SiO2 superparamagnetic core particles with a luminescent MOF shell consisting of 2∞[Ln2Cl6(bipy)3]·2bipy. This multifunctional composite system is suitable for an application as novel sensitive water detector based on irreversible quenching of the luminescence. This is combined with signal augmentation via magnetic harvesting of the particles in fluids that allows to use very low detector amounts, which would not give a visible luminescence in larger fluid volumes without harvesting. Thereby, the novel particle system exceeds MOFsensors without harvesting option. Depending on the concentration of water, the composite particle system can be used as a fast on-the-fly detector that can be read out by the eye as a “turn-off” detector or quantitatively with a photoluminescence spectrometer. Furthermore, the detector is mobile and does not require laboratory titration.

Reagents. Iron(III) chloride hexahydrate (FeCl3·6H2O, 99%+), and iron(II)chloride tetrahydrate (FeCl2·4H2O, 99%+, Sigma-Aldrich) were used without further purification. Ammonium hydroxide solution (aqueous NH3, 25 wt %) in water, nitric acid (HNO3, 1 M, diluted from a 53 wt % solution), sodium silicate (water glass) solution (36 wt %, molar ratio of SiO2:Na2O = 3:1, Na2Si3O7) were obtained from Fischer Chemicals and used without further purification. Tb4O7 (ChemPur, 99.9%), Eu2O3 (Research Chemicals, 99%) and NH4Cl (Grüssing GmbH, 99.5%) were used without further purification. 4,4′Bipyridine (Sigma-Aldrich, 98%) was purified by sublimation under high vacuum (150 °C, 10−5 mbar). The syntheses of 2∞[Ln2Cl6(bipy)3]·2bipy33,51 and [LnCl3(py)4]· 0.5py52 were performed from anhydrous halides,53 as described in the literature. Additional details are given in the Supporting Information. Synthesis of Composite Particles 2∞[Ln2Cl6(bipy)3]·2bipy @ Fe3O4/SiO2. For microparticle functionalization with the MOFs, three principle approaches have been successfully carried out. Mechanochemical Modification of Fe 3 O 4 /SiO 2 with 2 ∞[Ln2Cl6(bipy)3]·2bipy. For the mechanochemical modification, either 2 2 ∞[Tb2Cl6(bipy)3]·2bipy (64.8 μmol, 85.0 mg) or ∞[Eu2Cl6(bipy)3]· 2bipy (64.8 μmol, 84.0 mg) was placed together with Fe3O4/SiO2 microparticles (18.9 mg) and four 5 mm-steel balls in a ball milling jar (horizontal vibration principle). The reagents were milled for 3 min at

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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/acsami.5b11965. All analytical investigation methods and instrumentation that were applied (photoluminescence spectroscopy, fluorescence microscopy, PXRD, magnetic measurements, SEM, SEM-EDX and BET) as well as details on the respective investigations (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*K. Müller-Buschbaum. E-mail: [email protected]. F


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ACS Applied Materials & Interfaces *K Mandel. E-mail: [email protected].

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T. Wehner and K. Müller-Buschbaum gratefully acknowledge support of the Deutsche Forschungsgemeinschaft for the project MU-1562/7-1 “Mechanochemical Synthesis of Hybrid Materials”. We thank Werner Stracke (Fraunhofer ISC) for the SEM-EDX investigation.

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