Distribution of CoFe2O4 Nanoparticles Inside PNIPAM-Based

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Distribution of CoFeO Nanoparticles Inside PNIPAM Based Microgels of Different Crosslinker Distributions Marcus U. Witt, Stephan Hinrichs, Nadir Möller, Sebastian Backes, Birgit Fischer, and Regine von Klitzing J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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The Journal of Physical Chemistry

Distribution of CoF e2O4 Nanoparticles Inside PNIPAM Based Microgels of Different Crosslinker Distributions Marcus U. Witt,† Stephan Hinrichs,‡ Nadir M¨oller,† Sebastian Backes,¶ Birgit Fischer,‡ and Regine von Klitzing∗,† †Technical University Darmstadt, Department of Physics, Soft Matter at Interfaces, Alarich-Weiss-Strae 10, 64287 Darmstadt,Germany ‡University Hamburg, Institute of Physical Chemistry, Grindelallee 117, 20146 Hamburg,Germany ¶Technical University Berlin, Stranski-Laboratories of Physical and Theoretical Chemistry, Strae des 17. Juni 124, 10623 Berlin,Germany E-mail: [email protected]

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Abstract The aim of this study is to tailor the inner structure of positively charged poly-(NIsopropylacrylamid-co-allylamine) (P(NIPAM-co-AA)) microgels for a better control of the distribution of negatively charged magnetic cobaltferrite (CoF e2 O4 @CA) nanoparticles (MNP) within the microgels. Therefore, two different strategies are followed for the microgel synthesis: the (one pot) batch method which leads to a higher crosslinker density in the microgel core and the feeding method which compensates different reaction kinetics of the crosslinker and the monomers. The latter one is expected to result in a homogeneous crosslinker distribution. Information about the crosslinker distribution is indirectly gained by measuring the elastic modulus via indentation experiments with an atomic force microscope. While the batch method results in a higher elastic modulus in the center of the microgel indicating a core/shell structure the feeding method leads to a constant elastic modulus over the whole microgel. The loading with MNP and their distribution is studied with transmission electron microscopy (TEM). The TEM images show a large difference in the MNP distribution which is correlated to the crosslinker distribution of both types of microgels. The batch method microgel has a low MNP concentration in the core. The feeding method microgel shows a much more homogeneous distribution of MNP across the microgel. The latter one shows also a stronger charge reversal which is a hint for a higher loading of the feeding method microgel. Dynamic light scattering (DLS) and electrophoretic mobility (EM) measurements demonstrate that for both types of microgels the temperature sensitivity is preserved after loading with MNP.

Introduction Linear poly-N -Isopropylacrylamide (PNIPAM) exhibits a lower critical solution temperature(LCST) of around 32 ◦ C in water, 1 which was found to be of interest for numerous applications. By introducing geometrical constraints via crosslinkers into the system, a hydrogel is obtained. This 3-dimensional network shows a temperature induced volume phase 2


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transition (VPTT) with a VPTT close to the LCST of the linear polymer. Heating PNIPAM gels above the volume phase transition temperature (VPTT) water becomes a poor solvent and the gel shrinks. 2–6 In comparison to macrogels, microgels respond faster to external stimuli due to the shorter equilibration time and the absence of a significant temperature gradient inside the system. 2 Furthermore, coating surfaces with microgel particles is easier than coating them with macrogels. These benefites are making microgels promising candidates for small scale applications such as microchip sensors or medical applications. Microgel coatings are used for humidity sensors 7 or glucose sensors. 8 PNIPAM based microgels are also used as microreactors 9 or as drug delivery systems. 10,11 Furthermore, microgels present a suitable model system for concentrated colloidal systems. 3 Tanaka et al. and Pelton et al. 12–15 reported the synthesis of PNIPAM based microgels by radical precipitation polymerization. Additional applications become possible with the incorporation of co-monomers, metallic nanoparticles or functional moieties. For example co-monomers can be used to trigger the volume phase transition by pH. 15–20 The combination of metallic nanoparticles with PNIPAM based microgels is interesting for nanosensors or as additional optothermic triggers for shrinking and swelling of the microgels. It was already demonstrated that AuNP are capable to trigger the shrinking of microgels when irradiated with light of the plasmon resonance frequency. 21–26 The loading with magnetic nanoparticles (MNP) is of special interest for applications in directed motion, sensors and actuators. Combining the properties of microgels and magnetic NPs leads to hybrid materials with new stimuli responses. For example the torque and counter torque acting on the MNP in an external magnetic field. 27 Embedding metallic nanoparticles into polymer networks was realized in different ways. Magnetic nanoparticles (MNP) were embedded into acrylamide 28 or N-vinylcaprolactam based microgels 29,30 or covered with a polymer shell to form core/shell structures, consisting of a single core of magnetic material or agglomerates of multiple MNP. 31–35 The same has

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been shown for gold nanoparticles covered with polymer. 23,36 Furthermore, MNPs can be absorbed on the microgel surface. 37 The hybrid materials show properties of both components. The shrinking and swelling of the microgel as well as the response to external fields of the nanoparticles (NP). For example gold nanoparticles (AuNP)can be used as hot spots to trigger the shrinking by excite plasmons with laser ligth. 26 Magnetic microgels can be deformed by applying an external magnetic field. This was simulated by Zubarev et al. and Weeber et al. 38,39 This was also measured experimentally by Bonini et al. and Backes et al. 28,40 The reported elongation of magnetic microgels in field direction is rather small compared to the theoretical predictions. 39,40 It is expected that a large volume of the magnetic microgel is not responding to the external magnetic field which might be caused by a heterogeneous distribution of MNP. In several studies a core/shell arrangement was found, 23,26,41 leaving the core free of AuNP. The empty core is unresponsive to external fields (magnetic or light) and therefore the response of the microgel was smaller as predicted in simulations. An obvious strategy to increase the response to external fields, is to increase the amount of NPs inside the microgel. The number of MNP embedded within the microgels depends on the microgel composition, which was already demonstrated by Backes et al. 40 In a former study 40 it was shown that the amount of embedded MNP within oppositely charged microgels does not increase systematically with increasing charge of the microgel. Citrate stabilized AuNP could even be incorporated into slightly negatively charged microgels. These results indicate that the charge is not the only important parameter for loading microgels with metal nanoparticles. 23 Besides the charge the steric hindrance of the nanoparticles inside the microgel is assumed to be a key parameter. The steric hindrance is directly correlated to the mesh size of the microgel network and the MNP size (including stabilizing coatings). Gawlitza et al. 23 changed the crosslinker density of the microgels and found an increasing distribution gradient of AuNP with increasing crosslinker density. This finding was related to the established opinion that the core is higher crosslinked than the shell. For

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high crosslinker densities the core of the microgel was almost particle free. In addition, the response of the MMG to an external magnetic field was smaller as predicted in literature. This also seems to be originated in an unloaded microgel core. This inactive volume is not contributing to the response to an external magnetic field. To increase the response to external fields further, the distribution of NP can be changed. A homogeneous distribution of NP results a highly responsive microgel, because the core volume is also loaded. In none of the mentioned works the influence of the internal microgel structure on the MNP distribution was studied. In former studies microgels have been synthesized with the batch method which produces heterogeneously crosslinked microgels with a dense core and a fluffy shell. 12,42 This results in a smaller mesh size in the particle center and an increasing mesh size towards the microgel surface. The heterogeneous co-polymerization of the crosslinker into the polymer network is due to the faster reaction kinetics of the crosslinker in comparison to the one of the monomer. Therefore, the MNP distribution within the microgels is heterogeneous, with a higher concentration within the fluffy shell. The present paper addresses the tailoring of the inner microgel structure in order to control the MNP distribution within the microgel. Two synthesis strategies are pursued: the commonly used batch method which results in a heterogeneous crosslinker distribution and the feeding method 42 which compensates the faster consumption of crosslinker. Acciaro et al. 42 showed in their study the homogeneous crosslinker distribution by measuring the swelling ratio in dependence of the particle size during the polymerization. This method did not give consistent results for our microgels. Therefore in the present work the crosslinker distribution will be studied by scanning the local elastic modulus of the microgels with the atomic force microscopy based indentation method. The present paper contributes to two interesting topics related to PNIPAM based microgels. First, to clarify the different structures and the distribution of crosslinker in the microgels synthesized by the batch and feeding method. Second, the influence of mesh size

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of the microgel network on the distribution of MNP.

Experimental Section Materials For the microgel synthesis the following chemicals were used: N -Isopropylacrylamide (NIPAM), N,N ’-methylene- bis(acrylamide) (BIS), Allylamine (AA),2,2’-Azobis(2-amidinopropane) dihydrochloride (AAPH) and were purchased from Sigma-Aldrich. For the MNP synthesis the following chemicals were used: Iron(II)chloride (water free) from Merck, Cobald(II)chloride hexahydrat and Ironnitrate nonahydrat from Sigma Aldrich, nitric acid and citric acid and sodium hydroxide from G¨ ussing, Disodiumcitrate from Honeywell. All chemicals were used as received, no further purification was performed. The water was purified with a Millipore Milli-Q device. The purified water had a resistance of 18.1 M Ω(@25 ◦ C). The gold coated silicon wafers are purchased at Sigma Aldrich and are cleaned with acetone.

Reaction Vessel The synthesis of the PNIPAM microgel is temperature-sensitive. Therefore, a double wall reactor was used to maintain a constant temperature. The reactor is of cylindrical shape with a maximum reaction volume of 300 ml. The temperature was controlled by a thermostat which allowed a temperature control with an accuracy of ± 0.1 K. The thermostat circulates water through the jacket of the reactor. The reactor is equipped with four ground joints for: feeding syringe, degasing with nitrogen, for a reflux condenser and a glass stirring rod.

Synthesis of Magnetic Nanoparticles (MNP) The magnetic nanoparticles (MNP) are cobalt-ferrite (CoF e2 O4 ) particles. The MNP are stabilized with citric acid. They were synthesized by a precipitation process in a boiling solution of 1 M sodium hydroxide. Cobalt (II)-chloride hexahydrate and iron(III)-chloride 6


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were added to the boiling solution. The MNP were washed with water. A solution of 2 M nitric acid and 0.35 M iron(III)-nitrate was added. The mixture was stirred at 80 ◦ C for 20 minutes. The precipitate was dialyzed for one week against sodium citrate and citric acid. 43 The MNP are negatively charged with an electrophoretic mobility of −4.03 µm × cmV −1 s−1 and a zeta potential of 56.9 mV at a pH of 8. The size of the MNP is determined with transmission electron microscopy. The average size was found to be 12.2 ± 3.2 nm.

Synthesis of Poly-NIPAM Microgel Particles (MG) The PNIPAM microgel particles were synthesized by a surfactant free precipitation polymerization introduced by Pelton and Chibante. 12 Since the MNP were negatively charged a positively charged microgel was synthesized. Therefore a positively charged initiator (AAPH) was used and a positively charged co-monomer AA was polymerized into the microgel. Two different methods were used to synthesize the microgels. The first synthesis method is the so called ”batch” method. In the batch method all the reactants were preloaded into the reactor. The polymerization was started by adding the initiator AAPH to the reaction solution. The second synthesis method is called ”feeding” method. There the reactants are added over time into the reaction. The crosslinker BIS and the monomer NIPAM have different reaction times. The BIS reacts faster and is faster consumed. This faster consumption is compensated in the feeding reaction due to a constant level of reactants. The amount of monomer available for the polymerization was for both synthesis methods the same. Batch method : All the reactants are dissolved in 140 ml MilliQ water and the solution was degased for 60 min with nitrogen. During the degasing, the solution was heated to 80 ◦ C. AA was added. 1 ml of the initiator AAPH was injected into the reactor to start the polymerization. The concentration of AAPH was 67.5

mg . ml

The reaction is stopped after

10 min by rapid cooling. Feeding method : This method is related to the synthesis used by Acciaro et al. 42 Acciaro et al. reported a feeding method with a small amount of reactant preloaded into the reactor.

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The preloading was neglected due to the small influence on the resulting microgels. 120 ml of MilliQ water was degased in the reactor for 60 min with nitrogen and heated to 80 ◦ C. NIPAM and BIS were dissolved in 20 ml MilliQ water and degased separately. After degasing the AA was added to the reactants. 1 ml polymerization initiator AAPH was injected into the reactor. The concentration of the AAPH was the same as for the batch method. 20 ml ml into the reactor with a of the monomer solution (NIPAM, BIS, AA) were pumped with 2 min

syringe. The first monomers were starting the polymerization. The reactants were fed into the reactor for 10 min. The polymerization was stopped by rapid cooling. The resulting microgels were dialyzed against MilliQ water for 7 days. The water was changed every day. The purified microgels were freeze-dried at −85 ◦ C for 72 h. The concentration of both reactants (crosslinker and co-monomer) was for both methods (batch and feeding) 2.5 mol% with respect to the total amount of all reactants (except: initiator). Overall the used amount of reactants was 20 mmol. MG1 refers to the microgels, synthesized by the batch method and MG2 to the one, synthesized by the feeding method.

Preparation of Magnetic Microgels (MMG) For the analysis the two microgels (batch method: MG1, feeding method: MG2) were loaded with MNP. The hybrid microgel contains MNP is called magnetic microgel (MMG) in the following: MMG1 (batch method) and MMG2 (feeding method). The freeze dried microgels were dispersed in MilliQ water for 24 h before usage. The concentration of the stock dispersion of microgels was 0.5 wt% at a pH of 7 (no salt/base/acid was added). The pH was chosen to be 7 as a compromise between highly charged microgel and MNP stabilization. The MNP are stabilized with citric acid at a pH of 8. The stock dispersion of the MNP was 5.8 wt%. The MNP dispersion and the microgel were placed separately in a ultrasonic bath for 10 min. 0.25 mg of microgel and MNP are used for the magnetic microgels. Therefore 50 µl of the microgel stock dispersion was mixed with 946 µl water. Afterwards 4.3 µl of MNP stock dispersion was added. The concentration of microgel and

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MNP in the samples was 0.0248 wt%. The dispersion of MNP and microgels was mixed for 30 min at 1500 rpm in a vortexer. To remove free MNP the dispersion is centrifuged at 1000 rpm (67 g) for 30 min. The supernatant contains the free MNP and the precipitant the MMG. 800 µl of the supernatant were removed and the remaining precipitant is diluted with 800 µl MilliQ water. This cleaning procedure was repeated three times until the supernatant was clear. The dispersion of MMG had a pH of 7.

Methods Dynamic Light Scattering To determine the hydrodynamic radius of the microgel and the magnetic microgel dynamic light scattering (DLS) was used. The used DLS system is a multi angle setup from LSInstruments. The measurement was performed with a solid state laser of λ = 660 nm with a maximum power of 100 mW . The scattering signal was detected with two avalanche photodiodes. The signals were analyzed with a pseudo-cross correlation in the LS-Instruments hardware correlator. Up to 19 angles were measured between 30◦ and 120◦ . The measurement time was set to 30 s at each angle. The correlation data were fitted with a self written script. The fit is based on the cumulant fit procedure. The sample was placed in an index matching bath of decahydronaphthalene. The bath is temperature controlled within a precision of ∆T = ±0.1 K. The samples were measured at various temperatures above and below the volume phase transition temperature (VPTT). The samples were diluted sufficiently. Sufficient dilution means here: negligible particle particle interaction and no multiple scattering events in the DLS setup. From the hydrodynamic radii at different temperatures (20 ◦ C and 50 ◦ C) the swelling ratio was calculated. The swelling ratio is given as α =

r(@20 r(@50

◦ C)3 ◦ C)3

.

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Electrophoretic Mobility (EM) In order to have a measure for the microgel charge the electrophoretic mobility was measured. The electrophoretic mobility (EM) was measured with the Malvern Nano-ZS (λ = 433 nm, 4 mW ). At an angle of 173◦ (back scattering) the movement of the particles is monitored in an alternating electric field.

Transmission Electron Microscopy (TEM) The magnetic microgels were imaged with a transmission electron microscope (TEM). The samples were characterized with a TEM FEI CM20 microscope (FEI, Eindhoven, The Netherlands) equipped with a LaB6 cathode. The acceleration voltage was 200 kV . A Gatan double tilt holder was used. The samples were prepared on a copper grid covered with a carbon film (300mesh, Sience Service, Munich, Germany). The samples were prepared as discribed in the literature. 40 The analyzed images have a resolution of 2.5 nm per pixel.

Atomic Force Microscopy (AFM) Atomic force microscopy was used to measure the elasticity of the particles under geometrical confinement. The microgel particles are spin coated onto a gold coated Si-waver. The waver was cleaned with toluene and ethanol. The microgel dispersion was spin coated at 1000 rpm for 120 s. The spin coating resulted in the adsorption of single microgels at the surface. All measurements were carried out with a MFP-3D AFM (Asylum Research, Oxford Instruments). The elasticity measurements were done with a cantilever of HQ:CSC38/NO Al tips. They are uncoated and have a tip cone angle of 40 0.05

N . m

◦

with a spring constant of

The cantilever was measured against a hard surface to calibrate the deflection and

to determine the exact spring constant. The elasticity of the microgels was measured in a water droplet. Force maps were measured around the particles. The approach curves were fitted by the Hertz model. For the fit 9 % (from 1 % to 10 %) of the approach curve was 10


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used. The elastic modulus was averaged over several particles and the variance calculated.

Results Microgels (MG) Hydrodynamic Radius: Fig.1 (empty symbols) shows the hydrodynamic radii of the two types of microgels versus temperature . The microgels show the well known shrinking and swelling behavior in dependence on the temperature. The process of shrinking and swelling is reversible. Tab.1 shows the hydrodynamic radii of the microgels as well as the swelling ratio. The radius at T = 20 ◦ C for the MG2 (feeding method) is slightly larger than for MG1 (batch method). At T = 50 ◦ C MG1 and MG2 have a similar size. The swelling ratio for both microgels is similar. The full swelling and shrinking curves,i.e. the change in hydrodynamic radius over the temperature change, is shown in detail in the supporting information (SI) Fig.S1. The VPTT is close to the expected 32 ◦ C. Electrophoretic Mobility (EM): Fig.2 (empty symbols) shows the EM for microgel MG1 (batch method) and MG2 (feeding method). Tab.1 shows the EM(µ). Both MG have a positive electrophoretic mobility at T = 20 ◦ C. With increasing temperature the EM increases. The increase in EM is reversible for several cycles. At 20 ◦ C the EM is similar for both gels. At T = 50 ◦ C the EM is lower for MG2 compared to MG1. The difference in EM is constant for the different heating/cooling cycles. Table 1: Characteristic measurements of the microgels, mean hydrodynamic radii Rh and the swelling ratio α and mean electrophoretic mobility µ (@20 ◦ C and @50 ◦ C) . ◦

◦

Sample Rh20 C /nm Rh50 C /nm α µ20 M G1 452 257 5.44 M G2 495 266 6.44

◦C

/µmcmV −1 s−1 0.93 1.21

Nanomechanical Properties :

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µ50

◦C

/µmcmV −1 s−1 7.32 5.78


Figure 1: Hydrodynamic radius of the microgels and magnetic microgels, MG1 (batch method) and MG2 (feeding method) and MMG1 and MMG2 respectively in dependence on the temperature for four heating/cooling cycles.

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Figure 2: Electrophoretic mobility of the synthesized microgel and magnetic microgels, MG1 (batch method) and MG2 (feeding method) and MMG1 and MMG2 respectively in dependence on the temperature for four heating/cooling cycles.

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Fig.3 shows the elastic modulus averaged over several particles. The range of the x-values reflects the lateral expansion of the microgels after adsorption. After adsorption onto the gold surface the difference in diameter is larger than before adsorption. The diameter of MG1 is around 500 nm and of MG2 700 nm. This is a hint that the microgels are flattened on the surface. The elastic modulus is calculated from the forcemaps. For the MG1 (batch method) the elastic modulus is higher in the particle center compared to the modulus at the circumference of the particles. In contrast the elastic modulus of the MG2 (feeding method) is more or less constant over the whole particle diameter. This leads to the conclusion, that the crosslinker distribution inside the microgels (MG1 and MG2) is different. The overall elastic modulus of MG1 is smaller as for MG2.

Figure 3: Elastic modulus for MG1 (batch method) and MG2 (feeding method) measured with SFM indentation method in water at 20 ◦ C. The values are averaged over 10 microgels.

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Transmission Electron Microscopy: Fig.S2 shows the TEM images of both MG. MG1 (batch method) shows a decreasing gray value from the particle center to the particle circumference. The gray value for MG2 (feeding method) is constant through the whole particle.

Magnetic Microgels (MMG) Based on these results the influence of the different crosslinker distributions on the MNP distribution can be investigated. Hydrodynamic Radius: Fig.1 (filled symbols) shows the hydrodynamic radii for the magnetic microgels (MMG1 and MMG2). The MMG1 is smaller compared to MMG2 at all measured temperatures. Again the process of shrinking and swelling is reversible over several heating/cooling cycles. Tab.2 shows the hydrodynamic radii for the two magnetic microgels. The table shows as well the swelling ratio which is similar to the one before MNP incorporation (Tab.1). The MMG are larger compared to the unloaded MG. Electrophoretic Mobility: Fig.2 (filled symbols) shows the EM for the magnetic microgels for several heating/cooling cycles, and Tab.2 shows the mean EM at 20 ◦ C and 50 ◦ C. MMG1 has a positive EM for temperatures below the VPTT. Above the VPTT the EM is negative. MMG2 has a negative EM for all temperatures. This means that MG2(MMG2) exhibits a charge reversible by loading the microgel with MNP. MMG2 is negatively charged for all measured temperatures. For MG1(MMG1) a charge reversal due to loading with MNP can only be observed for the shrunken state (50 ◦ C). The change of the EM over the heating/cooling cycles is reversible and constant. Table 2: Characteristic measurements of the magnetic microgels, mean hydrodynamic radii and mean electrophoretic mobility (@20 ◦ C and @50 ◦ C) and the swelling ratio. ◦

◦

Sample Rh20 C /nm Rh50 C /nm α µ20 M M G1 554 301 6.23 M M G2 705 406 5.23

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◦C

/µmcmV −1 s−1 0.41 −0.62


µ50

◦C

/µmcmV −1 s−1 −3. −5.78


Transmission Electron Microscopy: Fig.4 shows the TEM images of both MMG. MNP have a much higher contrast compared to the microgels and they dominate the TEM images. The polymer network of MMG1 with its dangling ends becomes visible due to the adsorbed MNP. MMG2 has a more constant contrast for the whole microgel and the surface appears smoother with much less dangling polymer ends compared to MMG1. The number of MNP inside each microgel is so high that in the two dimensional image the particles overlap and vary strongly in contrast. A conventional analysis with ImageJ 15.k and the available features, such as Particle Analysis and Watershed, do not lead to a sufficient representation of the amount of MNP inside the microgels. To determine the MNP distribution a self developed analysis with ImageJ was used.The TEM images are treated as follows: First, the background of the image was calculated using the rolling ball algorithm. Second, the calculated background is subtracted from the image. Third, the image is converted into a binary map. A box of constant length and width is extracted for each MMG. The box is slightly longer than the microgel to cover all MNP even for a more polydisperse microgel sample. The width of the box is chosen to be as big as possible without getting any significant curvature effect. The treatment is depicted in Fig.S3-S7 for both microgel systems. The black values along the width are summed up for 10 to 20 MMG and averaged. The resulting values are called black pixel frequency. The black pixel frequency is a representation of the number of MNP over the radius of the MMG. Fig.5 shows the black pixel frequency for the two MMG. The histogram for MMG1 (batch method) shows a more constant level for the black pixel frequency. With peaks of the black pixel frequency at both ends of the histogram. The histogram for MMG2 (feeding method) shows a parabolic distribution of the black pixels, with the highest counts in the center.

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Figure 4: TEM images of PNIPAM microgels after loading with MNP. a) MMG1 (batch method, MG1,) b) MMG2 (feeding method, MG2).

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Figure 5: Black pixel frequency over the particle size for the magnetic microgels. MMG1 (batch method) on the left and MMG2 (feeding method) on the right.

Discussion Microgels (MG) In the presented study two different synthesis strategies were applied in order to tailor the inner structure of PNIPAM microgels. The PNIPAM microgels were synthesized with identical amounts of reactants via the batch method (MG1) 12 and the feeding method (MG2). 42 Acciaro et al. 42 used the swelling ratio over the particle size during the polymerization to verify the homogeneously crosslinked microgel. In the present study we use mainly the elastic modulus measured with AFM to determine the distribution of crosslinker inside the microgel. The measured elastic modulus is supported by TEM imaging. The elastic modulus of the batch method microgel MG1 show a well pronounced maximum in the particle center which has been already observed by former studies. 44,45 It is explained by a dense core and a fluffy shell due to the faster consumption of the crosslinker in the polymerization compared to the monomer. The elastic modulus for the feeding method microgel MG2 is higher and constant over the whole diameter. This constant elastic modulus indicates a homogeneous distribution of crosslinker over the whole microgel. The TEM images support this findings of heterogeneously (MG1)and homogeneously (MG2) crosslinked

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microgels. The TEM images of the pure microgels MG1 (batch method) and MG2 (feeding method) (see Fig.S3 ) show a decreasing contrast towards the particle circumference for MG1 and a constant contrast for MG2. This is the result of the different distribution of crosslinker in the two microgels systems with heterogeneously crosslinked microgel MG1 and homogeneously crosslinked microgel MG2. This is further supported by the TEM images of the MMG. MMG1 shows dangling ends of polymers loaded with MNP. This supports the dense core/fluffy shell structure of the heterogeneously crosslinked MG1. The exact distribution of allylamine inside the microgel is unknown but can be estimated to be close to the crosslinker distribution. Two extreme cases are possible: First, the consumption rate of AA is faster than the consumption rate of the crosslinker. Than the distribution of AA in the batch synthesis would give a core/shell structure with a high amount of AA in the core and less in the shell. The distribution caused by feeding method would be homogeneous due to the constant feeding of AA during synthesis. In the second case the consumption rate of AA is slower as for the crosslinker. Then, the AA would be built in within the microgel with higher concentrations at the surface than in the bulk for both synthesis methods. This would hinder the loading of the microgel core due to the lack of charges in the core, since it is known that MNP cannot be embedded in pure PNIPAM microgels. 40 In contrast, the present work shows clearly a homogeneous MNP distribution within homogeneously crosslinked microgels. This is a strong hint that the AA reaction kinetics is either similar or faster than the one of the crosslinker. Therefore the AA distribution is predicted to be similar to the crosslinker concentration in both synthesis methods. At a pH of 7 the microgels are slightly positive charge. To decrease the pH and therefore increase the charge of the microgels by protonating the AA is not desired due to the destabilization of the MNP at lower pH. The MNP are stabilized with citric acid and stable at pH 8. A pH below 7 would result in an aggregation of the MNP before they can be embedded into the microgel. The swelling and shrinking of the microgels in aqueous dispersion was studied with DLS.

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Both types of microgels shrink and swell reversibly. The size of the two microgels is nearly identical in the shrunken state, which is a strong hint that the same amount of monomer is consumed during the polymerization. This is important for the comparison of the two systems (batch and feeding). The shrinking and swelling influences the surface charge density of the microgels. This influence is shown in the temperature dependent EM. The EM confirms the desired positive charge of the microgels. Also the EM studies support the reversible shrinking and swelling during heating and cooling. In the shrunken state the EM is higher than in the swollen state which is explained by rearrangment of the charges towards the surface of the microgels. 46 Comparing PNIPAM based microgels co-polymerized with AA and pure PNIPAM base microgels the PNIPAM-co-Allylamine microgels tend to have a larger hydrodynamic radius due to the internal repulsive force of the positively charged AA. This is already reported in literature. 16,47 While DLS and EM measurements give similar results for both types of microgels the mechanical properties differ a lot. Magnetic Microgel (MMG) The magnetic microgels are larger compared to the unloaded microgels. This is already known from literature for microgels 40 and brushes. 48 The increase in size (stretching of the microgel matrix) is explained by: First, the osmotic pressure, due to the increase of ion concentration in the sample. The ions drag water into the microgel. Second, the repulsive interaction between the MNP and last, the volume of the MNP itself. The shrinking/swelling behavior of the microgels is preserved after loading with MNP. The shrinking/swelling is as well reversible for several heating/cooling cycles. After loading the microgels with MNP the swelling ratio remains more or less the same compared to the pure microgel. Loading the positively charged microgels with negatively charged MNP leads to a charge reversal at 50 ◦ C. This charge reversal is more pronounced for the homogeneously crosslinked microgel (MMG2) and also detected at 20 ◦ C for MMG2. The identical EM for the MG and the difference in EM for the MMG is a strong hint that more MNP are embedded into the homogeneously crosslinked microgel(MMG2). This is supported by the increase in hydrodynamic radii which

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is much more pronounced for the homogeneously crosslinked magnetic microgel (MMG2). The higher loading with MNP leads to the conclusion that MNP penetrate deeper into the microgel since the core is less dense crosslinked than for the heterogeneously crosslinked MG1. This is further supported by TEM images. The TEM images show that the MNP distribution inside the two types of microgels (batch and feeding) is different. The MNP distribution for MMG1 (batch method) shows a more constant black pixel frequency compared to the MMG2 (feeding method). The rather flat 2D projection of the MNP distribution for MMG1 is a result of a core/shell distribution of the MNP. The electron beam passes the same amount of MNP at all places, which means that there are less particles per probed volume in the core of the microgel than in the shell. For MMG1 the MNP are distributed in the shell of the core/shell structured microgel. Furthermore the higher number of black pixels at the ends of the histogram support the core/shell structure. This was already shown for gold nanoparticle by Gawlitza et al. 23 and Gelissen et al. 41 Gelissen et al. also showed the ideal core/shell assembly and the corresponding nanoparticle distribution in such a histogram. The distribution of MNP in MMG2 is homogeneous. The parabolic distribution of the 2D projection of the MNP distribution inside the microgels is the direct consequence of the homogeneously distributed MNP. The black pixel frequency for ideal homogeneously distributed MNP would be also parabolic. The observed MNP distribution is therefore close to ideal. The observed core/shell arrangement of the MNP inside the microgel network supports the different crosslinker distributions inside the microgels. The highly crosslinked core of the batch microgel (MG1) has a small mesh size and therefore sterically hinders the MNP to penetrate deeper into the microgel network. For the feeding microgel (MG2), this is not the case and the constant crosslinker density results in a constant mesh size. Thus allowing the MNP to penetrate through the whole microgel particle. The pixel resolution of 2.5 nm per pixel is suitable to determine the MNP distribution in the microgel. The resolution of the TEM image is 5 times the average MNP size. Some drawbacks of the 2D images from 3D soft polymer microgels in high vacuum are

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to be kept in mind. Due to the high vacuum, the microgels are in a dried state adsorbed onto the surface and therefore flattened. This effect becomes more pronounced with softer microgel (less crosslinker). The used TEM techniques are not suitable to resolve or analyze particles that are laying on top of each other and limits the analysis for highly loaded hybrid systems. The number of MNP embedded into the microgel is significantly higher compared to the study by Backes et al. 40 In The presented study the MNP are stabilized with citric acid and have a core diameter of around 12 nm while Backes et al. 40 used Polyacrylic Acid (PAA) for the MNP stabilization and had a particle diameter of 15 nm. The bare magnetic nanoparticles are compadable as in the present study, but the PAA shell is much larger compared to the citrate shell. Therefore PAA@MNP are more sterically hindered.

Conclusion The present study shows the influence of the inner microgel (MG) structure on the MNP distribution for microgels and oppositely charged magnetic nanoparticles (MNP). Therefore two synthesis strategies (batch and feeding) are realized. The batch method leads to a heterogeneously crosslinked structure of the microgel and the feeding method leads to homogeneously crosslinked structure. This was proved by AFM indentation experiments and supported by TEM images. The microgels synthesized with the batch method show a decreasing elastic modulus from the microgel center towards the outer shell, while the elastic modulus is constant across the whole microgel synthesized with the feeding method. The feeding method compensates successfully the different reactions kinetics of the used monomers, crosslinker and co-monomer. The difference in internal structure leads to different distribution of the MNP. They are rather located in the outer shell of heterogeneous microgels and can enter more deeply into the core of homogeneously crosslinked microgels. The total amount of MNP is higher in homogeneously crosslinked microgels leading to a stronger increase in size

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of the microgel than for the heterogeneous one. Both species of positively charged microgels exhibit a charge reversal during loading with negatively charged MNP. A high loading with the MNP was achieved while preserving the microgel properties such as the reversible swelling and shrinking due to temperature cycling. A multi-responsive microgel that exhibit the VPT was designed. It could further be shown that the mesh size is a key parameter for embedding MNP into the microgel. Homogeneously crosslinked microgels are interesting for future applications due to the higher loading with nanoparticles and the homogeneous distribution of the particles leading to a bigger ”active” volume. This opens up future applications in drug delivery systems where the magnetic microgels act as a carrier systems and can be guided by external magnetic fields. Furthermore the magnetic microgel can be adsorbed at surfaces and work as haptic sensors in external magnetic fields.

Supporting Information Detailed change of hydrodynamic radius over temperature measured by dynamic light scattering Fig.S1 ; transmission electron microscopy image of neat microgels Fig.S2 ; untreated TEM images of MMG Fig.S3 ; background of TEM images of MMG Fig.S4 ; TEM image of MMG with subtracted background Fig.S5 ; binary map of MMG Fig.S6 ; binary map with highlighted area and extracted are of MMG Fig.S7

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Distribution of CoFe2O4 Nanoparticles Inside PNIPAM-Based

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