Ultrahigh Magnetically Responsive Microplatelets with TunableFluorescence EmissionRafael Libanori,† Frieder B. Reusch,† Randall M. Erb,‡ and Andre R. Studart*,†

†Complex Materials, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland‡Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts 02115, United States

*S Supporting Information

ABSTRACT: Tuning the optical properties of suspensions by controlling the orientation and spatial distribution of suspendedparticles with magnetic fields is an interesting approach to creating magnetically controlled displays, microrheology sensors, andmaterials with tunable light emission. However, the relatively high concentration of magnetic material required to manipulatethese particles very often reduces the optical transmittance of the system. In this study, we describe a simple method ofgenerating particles with magnetically tunable optical properties via sol−gel deposition and functionalization of a continuouslayer of silica on ultrahigh magnetically responsive (UHMR) alumina microplatelets. UHMR microplatelets with tunablemagnetic response in the range of 15−36 G are obtained by the electrostatic adsorption of 2 to 13% of superparamagnetic ironoxide nanoparticles (SPIONs) on the alumina surface. The magnetized platelets are coated with a 20−50 nm layer of SiO2through the controlled hydrolysis and condensation reactions of tetraethylorthosilicate (TEOS) in an NH3/ethanol mixture.Finally, the silica surface is covalently modified with an organic fluorescent dye by conventional silane chemistry. Because of theanisotropic shape of the particles, control of their orientation and distribution using magnetic fields and field gradients enableseasy tuning of the optical properties of the suspension. This strategy allows us to gain both spatial and temporal control over thefluorescence emission from the particle surface, making the multifunctional platelets interesting building blocks for themanipulation of light in colloid-based smart optical devices and sensors.

■ INTRODUCTION

Magnetically responsive particles have been explored in manyapplications ranging from mechanical sensing1,2 to micro-robotics3−5 to cancer treatment.6,7 By tailoring the surfacechemistry of such particles, multifunctional systems thatcombine a magnetic response with other mechanical, chemical,or optical properties have been proposed in a variety ofdifferent fields. This has led, for example, to the development ofmagnetic systems to transport and release drugs in specific sitesin the human body,8,9 to catalyze chemical reactions with aminimum loss of catalysts,10,11 to trace fluid flow inextracorporeal blood purification,12 and to monitor chemical

concentrations and temperatures of fluids on the microscaleoptically.1,13

Although magnetic techniques enable remote control andhigh penetration depths, the high field strengths and highconcentrations of magnetic material often required for particlemanipulation remain limiting factors in several applications.14

The use of anisotropic microparticles partially coated withsuperparamagnetic iron oxide nanoparticles (SPIONs) wasrecently shown to be an effective strategy for circumventing

Received: July 18, 2013Revised: October 26, 2013Published: October 31, 2013

Article

pubs.acs.org/Langmuir

© 2013 American Chemical Society 14674 dx.doi.org/10.1021/la4027305 | Langmuir 2013, 29, 14674−14680

these limiting issues by enhancing the magnetic response andminimizing the undesired effects of gravity and thermal motionon the position and orientation of the magnetically responsiveparticles.15,16 In this approach, coating the nonmagneticmicroparticles with SPIONs can be easily performed byregulating the pH of the aqueous solution relative to themicroparticle’s isoelectric point to create a surface charge that isopposite to that of the charged ligands stabilizing the SPIONsin solution. The resulting SPION-labeled microparticles werefound to respond to magnetic fields as low as 10 G (1 mT) at asurface coverage of less than 15%. Such an ultrahigh magneticresponse (UHMR) allows for a major reduction in the amountof magnetic material required for remote manipulation. Forinstance, UHMR alumina platelets can be made responsive to amagnetic field of only 300 G (30 mT) using SPIONconcentrations that are as low as 0.01 vol % relative to thevolume of platelets. Such SPION content is low enough tomaintain the white color of the Al2O3 platelets, enabling thepotential utilization of such a system in optical applications inwhich the light absorption of the SPIONs has so far been amajor limitation.Alumina platelets exhibiting an ultrahigh magnetic response

have been used for the assembly of advanced composites withdeliberately reinforced microstructures15 as well as for thecreation of swellable bilayered composites with programmableshape-changing effects.17 However, the combination of opticalanisotropy and magnetic control offered by such microparticlesis yet to be explored for the development of colloidal systemswith switchable optical properties.Previous work has shown that magnetic fields provide an

effective means to control the optical reflectivity of suspensionscontaining aluminum microplatelets dispersed in a ferrofluid.18

In this case, aligning the highly reflective metallic plateletsperpendicular to the direction of light through negativemagnetophoresis significantly hampers the optical transmissionof an otherwise translucent suspension. However, the relativelyhigh concentration of SPIONs present in the ferrofluid reducesthe overall optical transmittance of the system even in thetranslucent state.Here, we report a simple and upscalable procedure for

obtaining multifunctional alumina platelets exhibiting both amagnetic response and fluorescence emission. Magneticnanoparticles adsorbed on the alumina surface are protectedby a continuous silica shell deposited through a modifiedStober method,19 which can be further functionalized with afluorescent dye to emit light, when illuminated at a specificwavelength range.20 The manipulation of such multifunctionalplatelets with low magnetic fields allows for both spatial andtemporal control of the fluorescence emission from the particlesurface, offering a straightforward mechanism for controllinglight emission in smart optical devices and probing rheologicalproperties of fluids on the microscale.

■ EXPERIMENTAL METHODSMaterials. The following chemicals were used in the study:

tetraethylorthosilicate (TEOS, Merck), ethanol absolute (EtOH,Scharlau, 99.9%), ammonium hydroxide (NH4OH, Merck, 25%),N,N-dimethylformamide (DMF, Honeywell, 99.8%), anionic ferrofluid(EMG 705, Ferrotec), poly(vinylpyrrolidone) (PVP, Sigma-Aldrich,360 kg/mol), rhodamine B isothiocyanate (RBITC, Sigma-Aldrich),and 3-aminopropyltriethoxysilane (APTES, Acros, 99%). All chemicalsare of technical grade unless otherwise stated. The aluminum oxideplatelets (Al Pearl) with average diameter of 7.9 μm and thickness of330 nm (Figure S1 in the Supporting Information) were kindly

supplied by Antaria, Australia. Deionized water was used in allexperiments.

Magnetization of Alumina Platelets. Two different suspensionswere initially prepared by suspending 10 g of alumina platelets in 300mL of water at pH 7 and 0.375 mL of ferrofluid EMG 705 in 200 mLof water at pH 7. The latter was then slowly added to the plateletsuspension, and the resulting mixture was stirred at room temperatureuntil the supernatant was clear. The ultrahigh magnetically responsive(UHMR) platelets were then removed by filtration, washed with 1500mL of water, and dried at 60 °C for 4h.

Silica Coating of Alumina Platelets. The surface modification ofUHMR alumina platelets with silica was performed by a modifiedStober method described elsewhere.19 The complete surfacemodification process is schematically shown in Figure 1. In summary,

UHMR platelets are first functionalized with nonionic amphiphilicpolymer poly(vinylpyrrolidone) (PVP, Mw = 360 kg mol−1) in waterand subsequently transferred to 140 mL of an ammonia/ethanolsolution containing 0.56 mol/L of ammonia. Next, the silica coating isformed by slowly adding tetraethyl orthosilicate (TEOS) with asyringe pump at specific flow rates to control the extent of secondaryprecipitation of SiO2 particles. In a typical synthesis, 1.5 g of theUHMR platelets obtained in the previous step is dispersed into 200mL of an aqueous solution containing 0.35 g·L−1 PVP, which is keptstirring overnight. The platelets are then filtered and washed with 100mL of ethanol. After drying at room temperature for approximately 1h, the platelets are transferred to 140 mL of an ammonia/ethanolsolution containing 0.56 mol·L−1 of ammonia and stirred for 30 min at600 rpm. For a nominal SiO2 thickness of 20 nm, 20 mL of a 2.65 vol% TEOS solution in absolute ethanol was added with a syringe pumpoperated at a flow rate of 800 μL·h−1. Finally, the silica-coated platelets(SiO2@UHMR-Al2O3) are filtered, washed with approximately 1 L ofethanol, and dried at 70 °C for 4 h.

Figure 1. (a) Modified Stober method used to coat magnetizedalumina platelets with silica. (b) Syringe pump utilized to adjust theinjection rate of TEOS solution and thus control the extent ofsecondary precipitation of silica nanoparticles.

Langmuir Article

dx.doi.org/10.1021/la4027305 | Langmuir 2013, 29, 14674−1468014675

Because the relatively low concentration of TEOS in the solutioninjected into the reaction vessel limited the production of silica-coatedplatelets to about 1.6 g, an alternative similar procedure was alsodeveloped to scale up the production of SiO2@UHMR-Al2O3 platelets.For the PVP surface modification of such a large batch, 10 g of UHMRalumina platelets was suspended in a 1 L solution of water containing4.375 g·L−1 PVP. Next, platelets were filtered and suspended in 800mL of an ammonia/ethanol solution with an ammonia concentrationof 0.56 mol·L−1. The silica coating step was then performed throughthe controlled addition of 40 mL of a 8.3 vol % TEOS solution inEtOH at a flow rate of 800 μL·h−1. The concentration of TEOS wasincreased to carry out the reaction in less than 50 h. However, plateletswere more prone to the formation of small agglomerates whenprocessed following this scaled-up route.Functionalization of Silica-Coated Platelets. Fluorescent silica-

coated platelets were prepared by adding a previously synthesizedfluorescent silane during the silica coating process described above.20

The synthesis of the fluorescent silane was carried out by adding0.0855 mL of APTES to 9.2 mL of an EtOH solution containing 7.6mg·mL−1 RBITC dye. The mixture was allowed to react at roomtemperature and in the dark for 12 h under continuous magneticstirring. Six milliliters of the resulting solution was directly added to asuspension consisting of an ammonia/ethanol solution (0.56 mol/Lammonia) and alumina platelets freshly coated with a silica layer asdescribed above for the small-batch synthesis procedure. Thesuspension was stirred for 48 h at room temperature, before theplatelets were finally filtered and washed with 500 mL of deionizedwater.Structural, Chemical, and Magnetic Characterization. Infra-

red spectra were obtained in diffuse reflectance mode between 4000and 370 cm−1 with 64 scans at a resolution of 4 cm−1 (DRIFT, System2000 FT-IR, PerkinElmer). A mixture of 5 wt % platelets in KBrpowder was initially ground in an agata mortar and subsequentlymilled in a vibration mill for 1 min to generate FT-IR samples afterpowder compaction. A spectrum obtained for pure KBr was used toset the background reflectance.A FEI Quanta 200 FEG electron microscope was used to acquire

qualitative energy dispersive X-ray (EDX) spectra in low-vacuummode. Local EDX data was collected for 100 s using an accelerationvoltage of 5 kV. SEM images were acquired in a Zeiss LEO 1530electron microscope using an operating voltage of 5 kV.The magnetic response of SiO2@UHMR-Al2O3 platelets was

evaluated by first sonicating a suspension of 0.01 g of modifiedplatelets in 10 mL of a 5 wt % PVP aqueous solution to break downsmall agglomerates that might have formed during the silica coatingprocedure. To enable observation in an optical microscope, a dropletof such a suspension was added to a microscope glass slide coveredwith a transparent Teflon foil. The glass slide was placed inside an air-core solenoid (Ward’s Natural Science, Rochester, NY), which wasdirectly set up on the optical microscope. The amplitude of the staticmagnetic field was adjusted by controlling the electrical currentrunning through the solenoid. The minimum magnetic field requiredto align the platelets was defined as the amplitude at which at least50% of the platelets were aligned parallel to the applied field. For theexperiments on the alignment dynamics, the response of the UHMRplatelets to a static magnetic field of 50 G was observed with aninverted light microscope (DMI3000B, Leica, Germany) and recordedwith a high-speed camera (Phantom v9.0, Vision Research, USA). Toshow our ability to control the fluorescence emission from the plateletsover time, we also performed experiments under a rotating magneticfield. Fluorescence emission from the platelet surface was measuredwith a fluorescence microscope (DM6000B, Leica, Germany)equipped with a red protein cutoff filter. In this case, a smallneodymium magnet disc (2.5 cm × 0.5 cm) attached to an electricalmotor was used to apply a rotating magnetic field of 30 Hz, which wasdeliberately switched on and off over time to control the orientationand emission of the fluorescent platelets.

■ RESULTS AND DISCUSSION

Chemical and structural analyses of the surface of the aluminaplatelets after each modification step allowed us to identify themost important parameters involved in the controlled synthesisof multifunctional platelets combining fluorescence andmagnetic response. The formation of a silica layer on theplatelet surface is particularly critical because it requires tightcontrol over the nucleation process of hydrolyzed TEOSmolecules. Therefore, such a reaction was first investigated onbare alumina surfaces. The uniformity and the roughness of thesilica layer deposited on the surface of alumina platelets werefound to be affected greatly by the saturation level of the silicaprecursor solution and by the colloidal stability of the aluminamicroplatelets in the ammonia/ethanol mixture.Stabilization of the platelets in the reaction medium is crucial

to reducing agglomeration and ensuring complete exposure ofthe particle surface during the coating procedure. In agreementwith earlier studies,19 we observed that the as-received aluminaplatelets could be effectively suspended in water using thenonionic amphiphilic polymer poly(vinylpyrrolidone) (PVP).Because the as-received platelets were found to be slightlyhydrophobic, the amphiphilic PVP molecules likely adsorbthrough hydrophobic interactions with the alumina surface.This enables the anchoring of the macromolecules on theparticle surface while allowing its more hydrophilic moieties toextend toward the aqueous phase and thus provide the stericlayer needed for effective particle stabilization.Coating PVP-stabilized platelets with silica resulted in either

uniform and smooth or nonuniform and rough layersdepending on the saturation level of the initial TEOS solution.Control over the silica saturation level was achieved byadjusting the rate of TEOS injection into the reactionmedium.21 To obtain a more uniform and smooth coating,the concentration of TEOS (C) must be sufficiently low tosuppress homogeneous nucleation in the bulk liquid but highenough to enable the deposition of a continuous silica filmthrough heterogeneous nucleation on the surface of the seedingplatelets. For the micrometer-sized alumina platelets used here,we found that an addition rate of 800 μL/h for a 2.65 vol %solution of TEOS in EtOH yields a homogeneous, smoothsilica coating and drastically reduces the agglomeration ofplatelets (Figure 2a). This flow rate corresponds to the additionof about 21 molecules·nm−2·h−1. Our results indicate that suchan addition rate lies within the optimum window in which theconcentration of hydrolyzed silica precursor falls between thecritical values needed to induce surface heterogeneous and bulkhomogeneous nucleation (Chetero < C < Chomo). Despite themicroscopically smooth surface of the coated platelets, closerobservation shows that the coating exhibits a granular texturewith numerous asperities approaching a few tens of nanometersin size (Figure 2b). Such asperities increase the effective surfacearea of the platelet, providing more room for the laterattachment of functional molecules.In contrast, nonuniform and rougher coatings are obtained

when TEOS reaches concentrations beyond the critical valueneeded for homogeneous nucleation (C > Chomo, Figure 2c,d).The rougher surface probably results from the deposition ofsilica nanoparticles that initially formed via homogeneousnucleation in the bulk solution and only afterward adsorbed onthe platelet surface. Such conditions were met by adding 40 mLof a 2.65 vol % solution of TEOS in EtOH in the form ofdiscrete 1 mL aliquots per hour using a pipet. The rougher

Langmuir Article

dx.doi.org/10.1021/la4027305 | Langmuir 2013, 29, 14674−1468014676

platelets were also more prone to agglomeration, as shown inFigure 2c. Spherical silica particles with a diameter as large as400 nm were observed on the surface of the alumina plateletscoated by adding the TEOS precursor at a higher rate (Figure2d).The controlled precipitation procedure using a low TEOS

addition rate is also an effective means of obtaining amonolayer of silica on the surface of alumina platelets initiallyprelabeled with magnetic nanoparticles (UHMR-Al2O3). Four-ier transform infrared spectroscopy (FT-IR, DRIFT mode) andenergy dispersive X-ray spectroscopy (EDX) confirmed thepresence of the continuous silica layer on the surface of treatedUHMR-Al2O3 platelets. The spectrum of SiO2@UHMR-Al2O3platelets exhibits typical infrared absorption bands of SiO2, suchas the stretching of free (3660 cm−1) and associated −O−Hgroups (2800−3500 cm−1) and the asymmetric (1186 cm−1)and symmetric (1067 cm−1) bending of Si−O−Si groups, asdisplayed in Figure 3a. The absorption band at 1655 cm−1 isassigned to the stretching of carboxyl groups of the pyrrolidonemoiety,22 suggesting that PVP remains adsorbed on plateletsurface after the coating step. SEM investigation employing aback-scattered electron detector reveals the presence of ironoxide nanoparticles adsorbed on the platelet surface, shown asbrighter spots in Figure 3b. Semiquantitative analysis usingEDX on selected regions of the UHMR-Al2O3 surface indicatesthat SiO2 is present both on the platelet surface itself (red curvein Figure 3b) and at the brighter spots corresponding to theiron oxide particles (blue curve in Figure 3b). These resultssuggest that the continuous silica coating was deposited on topof the initially adsorbed magnetic nanoparticles, providing anew hydroxylated surface for further functionalization withsilane coupling agents.The ultrahigh magnetic response and the alignment

dynamics of the SiO2@UHMR-Al2O3 platelets are shown inFigure 4. By using different amounts of SPIONs within themagnetic layer, it is possible to tune the magnetic response ofthe silica-coated alumina platelets. For example, increasing theSPION coverage from 2.0 to 13.0% decreases the magneticfield required for alignment from 36 to 15 G (Figure 4a,b). Themagnetic response is defined here as the magnetic field requiredto align at least 50% of the platelets in the direction of theexternal field. The surface coverage with SPIONs was estimated

by image analysis using scanning electron micrographs similarto those shown in Figure 2. According to our previous work,the surface coverage with SPIONs increases linearly with theamount of ferrofluid used during the coating procedure.According to this linear relationship, 3.4 μL of ferrofluid pergram of platelets is needed to obtain 1% surface coverage withSPIONs.15

The dependence of the alignment field on the surfacecoverage of SiO2@UHMR-Al2O3 platelets with SPIONs is ingood agreement with earlier experimental data and theoreticalpredictions for UHMR-Al2O3 platelets without an outer silicacoating (Figure 4c).15,16 The theoretical curve depicted inFigure 4c is obtained by estimating the external magnetic fieldat which the magnetic energy, Um, gained through plateletalignment with the field dominates the gravitational energy, Ug,gained via sedimentation of the platelet in the plane of thesubstrate (more details in Figure S2 in the SupportingInformation).15 Oblate ellipsoids with an experimentallydetermined diameter and thickness of 7.9 and 0.33 μm,respectively, were assumed in these calculations (Figure S1 inthe Supporting Information). Our experimental data indicatethat the additional silica layer deposited on the surface of theUHMR platelets does not significantly increase the amplitudeof the external field required for alignment. This can beattributed to the small contribution of the thin silica coating tothe total mass of the platelet (7−17%) and the very weakdiamagnetic response of silica.In addition to the equilibrium states evaluated above, the

dynamics of platelet alignment was also studied by quantifyingthe change in the orientation angle of a single platelet over time

Figure 2. Silica-coated platelets obtained by adding TEOS using (a, b)a syringe pump or (c, d) a regular pipet to achieve slow and fastprecursor addition rates, respectively.

Figure 3. (a) FTIR spectra of bare (black) and silica-coated (red)platelets. (b) EDX analysis on the surface of the silica-coated plateletsindicating the presence of silica alone (red square) and silica combinedwith iron oxide (blue square).

Langmuir Article

dx.doi.org/10.1021/la4027305 | Langmuir 2013, 29, 14674−1468014677

while an external magnetic field was switched on and off. Theorientation angle was geometrically calculated from the opticalmicroscopy images by following the change in the projectedlength of the platelet as a function of time (Figure 4d). Anglesgreater than 80° could not be measured precisely because of thelimited resolution of our optical experimental setup. Inagreement with previous studies,23 our results reveal thatplatelets initially suspended in deionized water align within timescales as short as 140 ms when subjected to an out-of-planemagnetic field. Upon removal of the magnetic field, the platelettakes 1370 ms to return to its resting configuration driven bygravitational forces (Figure 4d). Overall, these resultsdemonstrate the fast response time of these multifunctionalplatelets, which is crucial to their potential use as micro-rheological sensors or magnetically controlled displays.To explore the potential of the SiO2@UHMR-Al2O3 system

further, we covalently attached an organic dye to the silicacoating to provide a light-emitting molecule on the plateletsurface. In fact, the attachment of chromophore groups to silicasurfaces rather than directly on magnetic particles has beenshown to reduce significantly the quenching of the dye forsufficiently thick silica shells.24,25 To produce fluorescentplatelets, a solution of rhodamine B thioisocyanate (RBTIC)coupled with 3-aminopropyltriethoxysilane (APTES) was

added to a suspension of freshly coated alumina platelets in aammonia/ethanol mixture. Silanol groups (Si−OH) of hydro-lyzed RBTIC-APTES condense with the hydroxyl groupspresent on the surface of the silica coating, as schematicallyshown in Figure 5a. Infrared spectroscopy confirmed theformation of the thiourea group (shown in blue in Figure 5a)expected from the reaction between RBTIC and APTES, asindicated by the absorption band at 1390 cm−1 in Figure 5b.26

The covalent coupling of the fluorescent silane on the silicasurface through the formation of Si−O−Si bridges (greenbonds in Figure 5a) was also confirmed by absorption bands at1208 and 1155 cm−1. Although not exploited here, other silanecoupling agents carrying different functional groups can also becovalently attached to the SiO2@UHMR-Al2O3 platelets usingsimilar procedures. Another possible surface modification wasdemonstrated in previous work in which the surface of the samesilica-coated platelets was modified with a hydrophobic silaneto enable the reinforcement of a polydimethylsiloxane (PDMS)matrix through efficient stress transfer at the platelet−matrixinterface.27

The multifunctional nature of the resulting platelets allows usto gain both spatial and temporal control over the fluorescentemission from the modified particle. As an example of spatiallylocalized fluorescence, Figure 5c shows the alignment of the

Figure 4. Optical microscope images showing the alignment response of SiO2@UHMR-Al2O3 platelets with SPION surface coverages of (a) 2.0%and (b) 13.0% under low static magnetic fields (H). (c) Dependence of the magnetic alignment field on the SPION surface coverage for silica-coatedand noncoated UHMR platelets. (d) Alignment dynamics of the SiO2@UHMR-Al2O3 platelets showing the evolution of the platelet angle over timewhen an out-of-plane static magnetic field is switched on and off. Note that the scales before and after the break in the time axis are different.

Langmuir Article

dx.doi.org/10.1021/la4027305 | Langmuir 2013, 29, 14674−1468014678

platelets in the local field of magnetic domains uniformlypatterned on a low-coercivity magnetic tape. Besides such astatic configuration, the emitted fluorescence can also bedynamically controlled to exhibit any arbitrary temporal patternby exposing the multifunctional platelets to changing magneticfields. To illustrate our ability to control the emission intensitydynamically using magnetic fields, we performed plateletalignment tests by first applying a rotating magnetic field at afrequency of 30 Hz under an optical microscope (Figure 5d).At such high frequency, each platelet cannot track the rotatingfield and will experience phase ejection, aligning its long axisparallel to the plane of the rotating magnetic field.16,23 Thisconfiguration exposes a lower surface area to the detector,resulting in the reduced fluorescence emission shown in Figure5d (configuration 1). By switching off the magnetic field, theplatelet response becomes dominated by gravitational and dragforces and slowly returns to the horizontal configuration(configuration 3 in Figure 5d). This configuration exposes ahigher surface area and maximizes the fluorescence intensityreaching the detector. As observed in previous studies,16,23

platelet orientation along the plane of the rotating magneticfield occurs much faster (configuration 4 in Figure 5d) than itsgravity-driven return to the resting position on the glasssubstrate (configuration 2 in Figure 5d). In applications wherefaster response times are required, such as in light-emittingdisplays, the transition from an out-of-plane to an in-planeconfiguration can be greatly accelerated by using a competingfield applied orthogonally to the direction of the initial rotatingmagnetic field.18 Alternatively, in microrheology lower

frequencies can be applied to enable fast rolling of the plateletsat the same speed of the rotating magnetic fields.

■ CONCLUSIONSSilica-coated platelets exhibiting an ultrahigh magnetic response(SiO2@UHMR-Al2O3) were successfully synthesized byelectrostatically adsorbing SPIONs on the platelets followedby the controlled precipitation of silica on their surfaces. Themagnetic nanoparticles were completely incorporated into thesilica coating, and the introduced Si−OH groups allowed forthe addition of new functionalities to the platelets throughstandard silica modification methods. The magnetic response ofthe SiO2@UHMR-Al2O3 platelets was not affected by thepresence of the silica coating and can be tailored by changingthe surface coverage with SPIONs. The time scale required forthe alignment of modified platelets using a static magnetic fieldof 50 G was found to be on the order of 100 ms. Furthermore,fluorescent dyes were covalently attached to the silica surface toyield fluorescent platelets that can be manipulated usingmagnetic fields as low as 10−20 G. The orientation andposition of the magnetically responsive fluorescent micro-platelets could be deliberately tuned to gain spatial andtemporal control over the fluorescence emission from theparticles, making them potential building blocks for the creationof smart light-emitting displays and sensors.

■ ASSOCIATED CONTENT*S Supporting InformationSize distribution of the micrometer-sized alumina platelets usedin this study and calculations of the minimum magnetic field

Figure 5. (a) Scheme showing the covalent attachment of a fluorescent dye molecule to the surface of a silica-coated platelet. (b) FTIR spectra of theSiO2@UHMR-Al2O3 (black line) and fluorescent RBTIC-SiO2@UHMR-Al2O3 (red line) platelets. (c) Spatial control of the magnetic fluorescentplatelets on the surface of a low-coercivity magnetic tape. (d) Temporal control of the fluorescent emission of RBTIC-SiO2@UHMR-Al2O3 plateletsin a fluid under a rotating magnetic field.

Langmuir Article

dx.doi.org/10.1021/la4027305 | Langmuir 2013, 29, 14674−1468014679

required for platelet alignment. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: andre.studart@mat.ethz.ch.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Prof. Markus Niederberger and Prof. LudwigGauckler for allowing access to their equipment and JonathanSander for experimental assistance with the fluorescencemicroscope. We gratefully acknowledge Antaria (Bentley,Australia) for providing the alumina platelets for theseexperiments. We are also grateful for the support of theElectron Microscopy Center of ETH Zurich (EMEZ),especially Dr. Karsten Kunze.

■ REFERENCES(1) McNaughton, B. H.; Kehbein, K. A.; Anker, J. N.; Kopelman, R.Sudden Breakdown in Linear Response of a Rotationally DrivenMagnetic Microparticle and Application to Physical and ChemicalMicrosensing. J. Phys. Chem. B 2006, 110, 18958−18964.(2) Wilhelm, C. Out-of-Equilibrium Microrheology inside LivingCells. Phys. Rev. Lett. 2008, 101, 028101.(3) Ghosh, A.; Fischer, P. Controlled Propulsion of ArtificialMagnetic Nanostructured Propellers. Nano Lett. 2009, 9 (), 2243−2245.(4) Nelson, B. J.; Kaliakatsos, I. K.; Abbott, J. J. Microrobots forMinimally Invasive Medicine. Annu. Rev. Biomed. Eng. 2010, 12, 55−85.(5) Nishimura, K.; Uchida, H.; Inoue, M.; Sendoh, M.; Ishiyama, K.;Arai, K. I. Magnetic Micromachines Prepared by Ferrite PlatingTechnique. J. Appl. Phys. 2003, 93, 6712−6714.(6) Jordan, A.; Scholz, R.; Wust, P.; Fahling, H.; Roland, F. MagneticFluid Hyperthermia (MFH): Cancer Treatment with AC MagneticField Induced Excitation of Biocompatible Superparamagnetic Nano-particles. J. Magn. Magn. Mater. 1999, 201, 413−419.(7) Ito, A.; Shinkai, M.; Honda, H.; Kobayashi, T. MedicalApplication of Functionalized Magnetic Nanoparticles. J. Biosci. Bioeng.2005, 100 (), 1−11.(8) Gupta, A. K.; Gupta, M. Synthesis and Surface Engineering ofIron Oxide Nanoparticles for Biomedical Applications. Biomaterials2005, 26, 3995−4021.(9) Berry, C. C.; Curtis, A. S. G. Functionalisation of MagneticNanoparticles for Applications in Biomedicine. J. Phys. D: Appl. Phys.2003, 36, R198−R206.(10) Shokouhimehr, M.; Piao, Y. Z.; Kim, J.; Jang, Y. J.; Hyeon, T. AMagnetically Recyclable Nanocomposite Catalyst for Olefin Epox-idation. Angew. Chem., Int. Ed. 2007, 46 (), 7039−7043.(11) Gijs, M. A. M.; Lacharme, F.; Lehmann, U. MicrofluidicApplications of Magnetic Particles for Biological Analysis andCatalysis. Chem. Rev. 2010, 110, 1518−1563.(12) Ettenauer, M.; Posnicek, T.; Brandl, M.; Weber, V.;Falkenhagen, D. Magnetic Fluorescent Microparticles as Markers forParticle Transfer in Extracorporeal Blood Purification. Biomacromole-cules 2013, 8, 3693−3696.(13) Anker, J. N.; Behrend, C. J.; Huang, H.; Kopelman, R.Magnetically-Modulated Optical Nanoprobes (MagMOONs) andSystems. Proc. Fifth Int. Conf. Sci. Clin. Appl. Magn. Carriers 2005,293, 655−662.(14) van der Beek, D.; Petukhov, A. V.; Davidson, P.; Ferre, J.; Jamet,J. P.; Wensink, H. H.; Vroege, G. J.; Bras, W.; Lekkerkerker, H. N. W.Magnetic-Field-Induced Orientational Order in the Isotropic Phase ofHard Colloidal Platelets. Phys. Rev. E 2006, 73, 041402.

(15) Erb, R. M.; Libanori, R.; Rothfuchs, N.; Studart, A. R.Composites Reinforced in Three Dimensions by Using Low MagneticFields. Science 2012, 335, 199−204.(16) Erb, R. M.; Segmehl, J.; Charilaou, M.; Loffler, J. F.; Studart, A.R. Non-Linear Alignment Dynamics in Suspensions of Platelets underRotating Magnetic Fields. Soft Matter 2012, 8, 7604−7609.(17) Erb, R. M.; Sander, J. S.; Grisch, R.; Studart, A. R. Self-ShapingComposites with Programmable Bioinspired Microstructures. Nat.Commun. 2013, 4, 1712.(18) Bubenhofer, S. B.; Athanassiou, E. K.; Grass, R. N.; Koehler, F.M.; Rossier, M.; Stark, W. J. Magnetic Switching of Optical Reflectivityin Nanomagnet/Micromirror Suspensions: Colloid Displays As aPotential Alternative to Liquid Crystal Displays. Nanotechnology 2009,20, 485302.(19) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. AGeneral Method To Coat Colloidal Particles with Silica. Langmuir2003, 19, 6693−6700.(20) Van Blaaderen, A.; Vrij, A. Synthesis and Characterization ofColloidal Dispersions of Fluorescent, Monodisperse Silica Spheres.Langmuir 1992, 8, 2921−2931.(21) Stober, W.; Fink, A.; Bohn, E. Controlled Growth ofMonodisperse Silica Spheres in the Micron Size Range. J. ColloidInterface Sci. 1968, 26, 62−69.(22) Behen, J. J.; Dwyer, R. F.; Bierl, B. A. Determination ofPolyvinylpyrrolidone by an Infrared Absorption Method. Anal.Biochem. 1964, 9, 127−132.(23) Erb, R. M.; Segmehl, J.; Schaffner, M.; Studart, A. R. TemporalResponse of Magnetically Labeled Platelets under Dynamic MagneticFields. Soft Matter 2013, 9, 498−505.(24) Corr, S. A.; Rakovich, Y. P.; Gun’ko, Y. K. MultifunctionalMagnetic-Fluorescent Nanocomposites for Biomedical Applications.Nanoscale Res. Lett. 2008, 3, 87−104.(25) Lu, Y.; Yin, Y.; Mayers, B. T.; Xia, Y. Modifying the SurfaceProperties of Superparamagnetic Iron Oxide Nanoparticles through ASol−Gel Approach. Nano Lett. 2002, 2, 183−186.(26) Ritchie, R. K.; Spedding, H.; Steele, D. A Spectroscopic Study ofThiourea Derivatives(III): Infrared and Raman Spectra of NN′-Disubstituted Thioureas. Spectrochim. Acta, Part A 1971, 27, 1597−1608.(27) Erb, R. M.; Cherenack, K. H.; Stahel, R. E.; Libanori, R.;Kinkeldei, T.; Munzenrieder, N.; Troster, G.; Studart, A. R. LocallyReinforced Polymer-Based Composites for Elastic Electronics. ACSAppl. Mater. Interfaces 2012, 4, 2860−2864.

Langmuir Article

dx.doi.org/10.1021/la4027305 | Langmuir 2013, 29, 14674−1468014680