Preparation of Polymer Brush Grafted Anionic or Cationic Silica

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Preparation of Polymer Brush Grafted Anionic or Cationic Silica Nanoparticles: Systematic Variation of the Polymer Shell Marek Sokolowski,*,† Christoph Bartsch,† Vivian J. Spiering,† Sylvain Prev́ ost,‡ Marie-Sousai Appavou,§ Ralf Schweins,‡ and Michael Gradzielski*,† †

Stranski Laboratorium für Physikalische Chemie, Technische Universität Berlin, Straße des 17 Juni 124, 10623 Berlin, Germany Institut Laue - Langevin, DS/LSS, 71 Avenue des Martyrs, Cedex 9 38042 Grenoble, France § Jülich Center for Neutron Scattering JCNS at Heinz Maier-Leibnitz-Zentrum (MLZ), Forschungszentrum Jülich GmbH, Lichtenbergstraße 1, 85748 Garching, Germany Downloaded via KAOHSIUNG MEDICAL UNIV on August 30, 2018 at 03:54:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Polymer brush grafted anionic SiO2@PMAA (poly(methacrylic acid)) and cationic SiO2@PDMAEMA (poly(2-(dimethylamino)ethyl methacrylate)) inorganic/polymer hybrid nanoparticles with different core radii (dNP = 50− 140 nm) and different amounts of attached polymer were synthesized via surface-initiated atomic transfer radical polymerization (ATRP). To avoid irreversible aggregation, a three-step surface modification had to be employed, thereby keeping the nanoparticles always dispersed. For SiO2@PMAA the shell thickness changes with the monomer concentration, while for SiO2@PDMAEMA the grafting density was changed by monomer concentration and the shell thickness remained constant. We assume that the control over the grafting density relies on the nature of the complexation potential of the PDMAEMA. The structural characterization of the polymer grafted SiO2-NPs was done in detail by different scattering methods combined with thermogravimetric analysis, and details of the brush characteristics are obtained by small-angle neutron scattering (SANS). With this approach we were able to produce silica nanoparticles with anionic and cationic polymer shells, where the softness of the NPs can be controlled by the amount of polymer, which are pH-responsive and colloidally stable over a large pH range.

1. INTRODUCTION Nanoparticles are typically of rather hard nature (in particular with respect to their steric interaction), such as Au, Ag, silica nanoparticles, or polymer lattices. Nanoparticles of different sorts have found widespread application over the past years, and they have been the topic of many fundamental investigations. One main concern in recent time regarding nanoparticles has been their potential toxicity (“nanotoxicity”), especially for nanoparticles considered for potential medical or biological applications.1,2 In that respect, many types of different studies have been done,3−6 and such experiments showed for instance for neutral quantum dots the importance of the charge distribution, where nanoparticles with cationic charges in the outer layer accumulate in cells while the ones with anionic charges in the outer shell showed minimal nonspecific adsorption.7 One particular aspect here is the direct interaction of nanoparticles with phospholipid membranes.8 This can for instance be studied by mixing nanoparticles with phospholipid vesicles or supported lipid bilayers, which for the case of silica nanoparticles has been shown to lead to nanoparticle loaded vesicles,9 which are formed through an invagination process that is controlled by the ratio of nanoparticles size, adhesion © XXXX American Chemical Society

energy between nanoparticle and membrane, and the bending elasticity of the membrane.10 Accordingly, a key parameter here is the interaction between the nanoparticles and the membrane. As mentioned above, so far one has mainly concentrated on rather “hard” nanoparticles, but certainly it is equally interesting to investigate “soft” nanoparticles, not only with respect to their interaction with membranes but also for introducing the particle softness as an additional tuning parameter. Such particles can be achieved by having a polymer shell around the nanoparticles, where silica nanoparticles here are particularly attractive due to the well-developed surface chemistry of silica, which allows different ways of attaching a polymer shell to a silica surface.11 By doing so, one can introduce a soft, steric interaction potential due to the presence of the polymer on the nanoparticle surface, and second one also has the possibility to have modified electrostatic interactions, if a charged polymer chain is employed. For instance, such polymer modified silica nanoparticles have been prepared for the case of poly(butyl acrylate) (PBA) Received: May 14, 2018 Revised: August 11, 2018

A

DOI: 10.1021/acs.macromol.8b01019 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), phosphorus pentoxide, tert-butyl methacrylate (tBuMA), tetraethyl orthosilicate (TEOS), toluene, trifluoroacetic acid (TFA), and sodium hydroxide were purchased from Sigma-Aldrich and used without further purification. Anhydrous ethyl acetate was prepared by mixing “wet solvent“ with phosphorus pentoxide in a mass ratio of 5:1 and evaporating the ethyl acetate after 15 min of exposure time. This anhydrous ethyl acetate was stored with a molecular sieve (5 Å). Millipore water was taken from a Millipore system having a resistivity higher than 18 MΩ cm. D2O for the SANS measurements was obtained from Eurisotop (Gif-sur-Yvette, France) in 99.9% isotopic purity. 2.2. Methods. ζ-Potential measurements were performed with a nano zetaziser (Malvern). The electrophoretic mobility was measured at room temperature in Millipore water and transferred into ζpotentials by applying the Smoluchowski approximation. Transmission measurements were done at λ = 632 nm using a UV− vis Cary instrument. Densities of PDMAEMA solution (same reaction conditions except they were not surface initiated, but the polymerization was done in solution) were measured at several concentrations with a densimeter (DMA 4500 from Anton Paar). From these values we deduced the specific density of the polymer chains in solution. Titrations were performed with a commercial Titrando instrument (Metrohm) by measuring the pH as a function of the added volume of 0.1 M HCl or NaOH stock solutions. The polymer concentration was adjusted to 1 wt %, and the starting pH was set to pH = 2 for PDMAEMA and pH = 12 for PMAA. The degree of ionization β was calculated via

by means of nitroxide-mediated polymerization (NMP) after surface initializing Stoeber silica nanoparticles appropriately.12,13 Core−shell silica/PBA has also been employed to produce films with interesting optical properties by dipcoating.14 Polystyrene-modified silica nanoparticles can also be obtained by surface-initiated anionic polymerization and have been studied in some detail in THF solution.15 A rather versatile way for different polymer modifications is given by the reversible addition−fragmentation chain transfer polymerization (RAFT) method, which has been shown to allow to attach a large variety of different polymers,16 which then may also allow to attach polymers that are water-soluble, as aqueous dispersions are particularly interesting for many applications. One may also employ the technique of surface-initiated atom transfer radical polymerization (ATRP) for attaching pHresponsive polymer brushes as done for poly(2(diethylamino)ethyl methacrylate) (PDEA) and multiresponsive poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA).17−19 In a similar fashion by this method poly(2-(2methoxyethoxy)ethyl methacrylate) (PMDM) has been attached, thereby yielding nanoparticles possessing a lower critical solution temperature (LCST) in aqueous solution,20 and this had similarly been reported previously for the case of poly(N-isopropylacrylamide (PNIPAM).21 In a different approach via vapor deposition polymerization PDMAEMAcoated SiO2-NPs have been produced that showed antibacterial properties.22 In contrast, only very little work has been reported on the topic of silica NPs modified with an anionic polymer shell. Of course, such stimuli-responsive polymer-coated silica nanoparticles are particularly interesting systems as they do not only combine the hardness of the silica core with the softness of the polymer shell, but in addition the polymer properties can be tuned by one or more external parameters; i.e., one has functional hybrid colloids. In general, it might be noticed that not so many investigations have been done on polymer-modified silica nanoparticles soluble in water where the extent of the polymer shell was systematically varied, and this applies even more so with respect to their structure and stability in aqueous solution. Accordingly, we were now interested in that point and prepared silica nanoparticles that contained shells of poly(dimethylaminoethyl methacrylate) (PDMAEMA) of different grafting densities, which, depending on pH, will be more or less positively charged (inverting the negative charge that silica nanoparticles normally possess). In addition, we synthesized negatively charged core−shell nanoparticles of poly(methacrylic acid) (PMAA) of different shell thickness depending on the degree of polymerization. The colloidal structure of these hybrid core−shell nanoparticles was studied by scattering methods (light, SAXS, and SANS) as a function of the degree of surface polymerization and of pH, with the aim of gaining a comprehensive structural understanding of these particles in aqueous solution, with an emphasis on the structure of the polymer shell. Light scattering here can give an overall picture of the particles, while SANS, due to its contrast options and wavelength, can deliver a more refined structural picture.

β=

VPH − VEP,2 VEP,1 − VEP,2

(1)

with added volume of stock solution at the measured pH VpH, the volume at the first equivalence point VEP,1 (neutralization of excess acid or base), and the volume at the second equivalence point VEP,2 (neutralization of polymer). Light scattering (DLS and SLS) was measured on an ALV/CGS-3 instrument (ALV, Langen, Germany) at 25 °C. Intensity fluctuations were recorded by an ALV500/E multiple-τ correlator of measurement angles of 40°−120° in 5° steps. The decay rate Γ was calculated by fitting the correlation functions with the cumulant method.23 The diffusion coefficient was calculated from the slope of the decay rate Γ versus q2 and then converted into a hydrodynamic radius RH with the Stokes−Einstein equation. For SLS the scattering intensity at a certain q-vector was calculated from the count rate, and the forward scattering I0 and RG were calculated using the Guinier approximation, which is valid up to qRG ≤ 1.0.24−26 From the forward scattering I0 the molecular weight was calculated via app MW =

I0 Kcg

(2)

with the knowledge of the mass concentration [g/mL] and optical constant K = [2π(dn/dcg)nsol]2/(λ4NA) including the refractive index increment dn/dcg for the specific solvent with nsol and NA the Avogadro constant. dn/dcg was calculated from refractive index measurements at different concentrations for NP solution and has a value for SiO2-NP dn/dcg = 0.0755 mL/g, neglecting the relatively small differences for different concentrations and polymer types. Thermogravimetric analysis (TGA) was performed using a TA Instruments Q50 V6.7 Build 203 instrument. In a typical measurement previously freeze-dried samples were heated in a synthetic air environment over a temperature range of 20−700 °C and a heating rate of 10 °C/min. SAXS measurements were performed at ESRF in Grenoble on the ID02-beamline with a sample-to-detector distance (SDD) of 10 m and an X-ray wavelength of λX‑ray = 0.0996 nm, resulting in a q-range of 0.0067−1 nm−1. The scattering length densities were calculated using the NIST calculator.27 The molecular weight of the silica core was calculated from the forward scattering I0:

2. EXPERIMENTAL SECTION 2.1. Materials. Acetic acid, 3-aminopropyltriethoxysilane (APTS), 2-bromo-2-methylpropionyl bromide (BIBB), ethyl acetate, copper(I) bromide (99.99%), dichloromethane (DCM), L-lysine, hydrochloric acid, N,N-dimethylaminoethyl methacrylate (DMAEMA), B


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Macromolecules Scheme 1. Surface Modification and Polymerization of PDMAEMA or PMAA onto SiO2-NPs

app MW =

NAI0ρSiO

0.2 for “full contrast conditions”. A detailed sample preparation is given in the Supporting Information. SLD values were calculated using the NIST calculator with a silica density of ρSiO2 = 2.2 g/cm3 and a polymer density of ρpolymer = 1.19 g/cm3.32,33 Details are given in the Supporting Information. 2.3. Synthesis Details for the Preparation of PolymerModified Silica Nanoparticles. 2.3.1. SiO2 Synthesis. In a 250 mL round-bottom flask L-lysine was dissolved in 100 g of water, and the mixture was heated to the desired temperature. 15 g of TEOS was added dropwise within 10 min to the solution, and the mixture was stirred for a given time at given specific stirrer speed (Table S1). For purification the dispersion was dialyzed six times against 5 L of Millipore water. 2.3.2. SiO2@NH2 Synthesis: Amino Functionalization. To a SiO2 dispersion (d = 50−140 nm) a mixture of APTS and acetic acid was added dropwise at 70 °C within 10 min, and then the mixture was stirred at 70 °C for 16 h (Table S2). For purification the aqueous SiO2@NH2 dispersions were dialyzed five times against 5 L of Millipore water (pH 4 preadjusted with acetic acid). 2.3.3. SiO2@Br Synthesis: Introducing the Surface Initiator. For introducing the ATRP initiator onto the NP surface, SiO2@NH2-NPs were transferred into toluene using azeotropic distillation of water. In a typical distillation procedure a round-bottom flask with 70 mL of aqueous SiO2@NH2-NP dispersion was mixed with 30 mL of toluene and 100 mL of anhydrous ethyl acetate, condensing the ethyl acetate over the steam pressure (T ∼ 40 °C) of toluene and water until all of the water had disappeared. During the distillation process the sample was continuously sonicated in a sonication bath. A portion of 100 mL of anhydrous ethyl acetate (in total ∼1 L) was added into the roundbottom flask; after evaporating all of the water and ethyl acetate the desired NP concentration in toluene was adjusted, and the sample was treated 20 min in a sonication bath and with sonication tip finger (pulse time 2 s, pause time 5 s). In a protective atmosphere (N2) a solution of 0.5 mL of BIBB in 1 mL of toluene was added dropwise to a mixture of 60 mL of toluene, 15 mL of triethylamine, and SiO2@NH2 which was cooled in a water−ice bath, and the mixture was vigorously stirred (rpm = 400 with a big stirrer) for 16 h. After addition of BIBB, the temperature of the water−ice bath was slowly adjusted to RT. For purification the brown SiO2@Br mixture was diluted with MeOH (V = 30 mL) and gently centrifuged (rpm = 2000), and the supernatant brown solvent was removed. The brown SiO2@Br-NPs were washed with MeOH followed by gentle centrifugation. This step was repeated several times (∼3×). Finally, the particles were dispersed in water by sonicating 20 min in a bath and with a sonication tip finger (pulse time 3 s, pulse break 5 s), keeping the temperature below 25 °C. The concentration was adjusted to c = 3.0 g/L by adding MeOH to the dispersion,

2

2

Δη2cg

(3)

with the density of silica ρSiO2 = 2.2 g/cm and the scattering length density contrast Δη of silica to water. Small-angle neutron scattering (SANS) was measured at MLZ in Munich on the Instrument KWS-1 and at ILL in Grenoble on the instrument D11 with a wavelength spread of 10% and 9%, respectively. Three configurations were used with wavelength λ0 = 5.0 Å for SDD = 1.5 and 8 m, λ0 = 10 Å with SDD of 20 m for KWS1,28 and λ0 = 6.0 Å for SDD = 1.5, 8, and 34 m for D1129 to cover a q range of 0.015−1 nm−1. Transmissions were measured for both wavelengths at a SDD of 8 m. In all scattering data the intensities were divided by transmission and sample thickness (1 mm) corrected for the scattering of the empty cell and normalized to the scattering of water, thereby yielding absolute intensity values. The scattering intensity was approximated using 3

I(q) = 1NP(q)S(q)

(4)

1

with N being the number density of the NPs, P(q) the form factor, and S(q) the structure factor.29 For P(q) the scattering of the bare NPs was described by a sphere form factor while data for the polymer grafted NPs were fitted with a core−shell form factor with spherical symmetry, where the scattering length density of the shell ηshell is given by the volume fraction of solvent contained in the shell, and ηshell decays exponentially to the scattering length density of the solvent ηsolv (see Figure 4). Swelling of the polymer shell at the core radius RC for ηshell by the solvent is described by ηshell, R = XCηsolv + (1 − XC)ηpolymer C

(5)

where XC is the volume fraction of the solvent at the core radius and ηpolymer of the polymer in dried state. Data analysis was done using the software package of SasFit.30 For samples where aggregation took place we further employed a fractal model introduced by Teixeira et al. to describe such agglomerates:31

Df Γ(Df − 1) sin([Df − 1] arctan(qζf )) ÅÄÅ ÑÉ(Df − 1)/2 1 Ñ (qr0)Df ÅÅÅÅ1 + 2 2 ÑÑÑÑ q ζf Ñ ÅÇ Ö (6) where Df is the fractal dimension, ζf is a cutoff length for the fractal size, r0 is the radius of the single particles, and Γ is the gamma function. Samples prepared for SANS were prepared with a VD2O/ VH2O ratio 0.58/042 to match the contrast of the silica core later, i.e., at contrast match conditions (CM), and with a VD2O/VH2O ratio 0.8/ Sexp(q , ζ , Df , r0) = 1 +

C


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NPs in water impossible, and to avoid an −Br to −OH exchange, the SiO2@Br-NPs had to be transferred into MeOH for the subsequent polymerization, as keeping the particles in a stable colloidal state is only possible in a very polar solvent. Nonetheless, SiO2@Br-NPs were coagulating over time as they exhibit low surface potentials (ζ ∼ −20 mV measured in methanol) and always have to be sonicated shortly before polymerization. We analyzed our NPs with different scattering techniques and ζ-potential measurements to obtain the surface charge, which is a confirmation of successful surface modification. The success of the amine functionalization was confirmed by the charge inversion from an anionic to a cationic surface and a reversal to an anionic surface charge once the bromine functionalization had taken place (Table S4). 3.2. Polymerization. For polymerization, we used surfaceinitiated ATRP with Cu(I)Br as catalyst and PMDETA as ligand in a methanol−water mixture (VMeOH/VH2O = 1:1 for DMAEMA or VMeOH/VH2O = 4:1 for tBuMA), keeping the SiO2@Br-NP concentration low to avoid fusion of polymer chains from different particles. Polymerizations were performed with high monomer and Cu to initiator molar ratio, and the monomer concentration was varied from 2.4 to 122 mM per reaction batch, while keeping temperature, catalyst, and SiO2@Br-NPs weight concentration constant. Cationic PDMAEMA was directly synthesized using the monomer DMAEMA, thereby yielding SiO2-NPs with a grafted PDMAEMA shell (SiO2@PDMAEMA). In contrast, anionic polyelectrolyte brushes were achieved by polymerization of tBuMA and subsequent hydrolysis to yield poly(methacrylic acid) brushes (SiO2@PMAA). After the reaction, polymer-coated SiO2-NPs were transferred into water by dialysis, where polymers are charged due to the adjusted pH. The polymer charging is a necessary step in the purification procedure due to electrostatically enhanced colloidal stability, where dispersion was achieved via sonication and filtration. PMAA can be deprotonated (pKa ∼ 4.5),37 and typically the pH of the solutions was 9. In contrast, the dimethylamino moiety of PDMAEMAH+ (pKa ∼ 7.6)38 can be protonated, leading to a cationic polymer at pH values lower than 7, where we always used acetic acid for the pH adjustment. It should be noted that we tried in a similar fashion to polymerize neutral acrylamide onto SiO2-NPs, but this always led to substantially larger aggregates. Apparently charging is a necessary step for keeping the NPs colloidally stable. Thus, it is intrinsically challenging to obtain such core− shell NPs with uncharged polymer shells that do not aggregate. Besides, we also tried to polymerize polymers on small commercial Ludox-NP (diameter of 17 nm), but this always led to formation of large aggregates. Polymer grafted NPs were studied by several techniques including scattering (light, SAXS, and SANS), ζ-potential measurements, TGA, and TEM analysis to gain detailed information regarding their structure. 3.3. Characterization of the Polymer-Modified SiO2NPs. 3.3.1. Structure of the (Functionalized) SiO2-NPs. As a first step we characterized the initially prepared SiO2-NPs, where SAXS shows nicely the marked monodispersity of the particles as well as their systematic size variation (Figure 1). The SAXS curves were fitted with a model of polydisperse spheres (for details see Figure S6). In our case average diameters of 48.6, 64.8, and 140.6 nm and polydispersity

resulting in a yellow SiO2@Br-MeOH dispersion. The whole procedure was done in a similar fashion for the different NPs of 50, 70, and 140 nm diameter and is depicted in Scheme 1. 2.3.4. SiO2@PDMAEMA. Water, MeOH, and the SiO2@Br dispersion were bubbled with N2 for 20 min to remove all dissolved oxygen. In a 100 mL Schlenk flask under a protecting atmosphere (N2) 55 mg of Cu(I)Br (0.38 mmol) was added quickly. Immediately 20 mL of water, 13 mL of MeOH, and 7 mL of SiO2@Br dispersion were added, and the green/yellow dispersion was bubbled with N2 for 10 min. The final H2O/MeOH ratio was VH2O/VMeOH = 1/1. Now the desired amount of the monomer DMAEMA and 73 μL (0.38 mmol) of PMDETA (ligand) were added simultaneously into the Schlenk flask, and the green mixture was stirred for 24 h at RT. Depending on the added monomer amount the color changed from green to blue, where more added monomer resulted in a slower color change. For purification the mixture was centrifuged gently (rpm = 2000), and the excess blue solvent was removed. The orange SiO2@PDMAEMA-NPs solution containing Cu−monomer complexes was diluted with 40 mL of water and dialyzed against 5 L of Millipore water at pH 4 adjusted with acetic acid. Then SiO2@PDMAEMA-NPs were dispersed using a sonication bath (20 min) and a sonication tip finger (5 min, pulse time 2 s, pulse break 5 s), keeping the temperature below T = 15 °C. Finally, the dispersion was filtered with a 5 μm cellulose acetate syringe filter and subsequently sonicated with a tip finger at same conditions for 1 min (see the Supporting Information). 2.3.5. SiO2@PMAA. The SiO2@Br dispersion with the Cu(I)Br was prepared as before, but with a final volume ratio of VH2O/VMeOH = 1/ 4, due to the improved solubility of tBuMA. Now the desired amount of the monomer tBuMA and 73 μL (0.38 mmol) of PMDETA were added simultaneously into the Schlenk flask, and the green mixture was stirred for 24 h. Depending on the amount of added monomer, the color changed from green to blue; more added monomer resulted in a slower color change. The NP mixture was gently centrifuged (rpm = 2000), washed once with MeOH, centrifuged, and then dispersed in 20 mL of DCM. To the NP−DCM mixture TFA was added quickly to achieve a ratio of VDCM/VTFA = 1/1, and the mixture was stirred 1 h at 300 rpm. For purification, the NPs were gently centrifuged and dispersed in DCM again (3×) and then washed and dispersed in water. The aqueous NP dispersion was dialyzed eight times against Millipore water preadjusted to pH 9 using NaOH. Then SiO2@PMAA-NPs were dispersed in a sonication bath (20 min) and using a sonication tip finger (5 min, pulse time 2 s, pulse break 5 s), keeping it below T = 15 °C. The final dispersion was filtered with a 5 μm cellulose acetate syringe filter and subsequently sonicated with a tip finger for 1 min (see the Supporting Information).

3. RESULTS AND DISCUSSION 3.1. Preparation of Polymer-Modified Silica Nanoparticles. Silica nanoparticles (SiO2-NPs) were synthesized with a modified Stoeber method with TEOS as precursor but using L-lysine as catalyst, where the size and polydispersity can be well controlled and tuned by the reaction time, temperature, and stirring speed.34,35 Here we synthesized three NPs with diameters of 50, 70, and 140 nm. Successful amino functionalization was achieved with APTS in water.36 The amino group was then reacted with BIBB to place the initiator group on the surface. For this reaction the NPs had to be transferred into toluene due to the need of an unpolar and aprotic solvent for preventing hydrolysis of the bromide acid moiety. Drying of the NPs had to be avoided as otherwise irreversible agglomeration of SiO2-NPs could take place. Thus, SiO2@NH2-NPs had to be transferred from water into toluene via azeotropic distillation with anhydrous ethyl acetate. However, the number of aggregates increases particularly for the smallest NPs as sonication cannot separate all aggregates into single NPs after toluene transfer. The −Br moiety is a good leaving group, which renders storage of the SiO2@BrD


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20−700 °C, where the final mass value was read at 700 °C. As expected, the measured mass loss Δm for SiO2@polymer-NPs is much smaller than if 100% of the monomer would have been converted (theoretical Δm losses from 70−98%, depending on the monomer to NP ratio; Table S5), which implies that only 10−40% of added monomer polymerize onto the surface, a condition as normally seen for surface polymerizations by means of ATRP,39 where polymerization efficiency is expected to be low according to heterogeneous surface liquid reaction (see the Supporting Information). An interesting observation is that the amount of grafted polymer increases with increasing particle size for SiO2@PDMAEMA (while it is rather constant for SiO2@PMAA). This result is unexpected due to a decrease of total surface area with increasing NPs size one would expect a reduced polymer amount for the same grafting density and polymer length. In general, the amount of grafted polymer is smaller for SiO2@PMAA than for SiO2@PDMAEMA-NPs, potentially as result of the harsh reaction conditions in the hydrolysis reaction when transforming hydrophobic SiO2@ PtBUMA to hydrophilic SiO2@PMAA or lower monomer reactivity. During that process, some of the polymer chains might be cleaved from the core. Several approaches can be used to calculate the molecular weight of NPs and polymer grafted NPs using LS, SANS, or SAXS. In our case the most accurate way was a combination of SAXS and TGA, where TGA gives the total amount of bound polymer. The MW of SiO2-NP core was calculated from the forward scattering intensity I0 (eq 3) extrapolated by a Guinier analysis. The sample concentration was cNP ∼ 0.2 wt %, and the precise concentration of the NP solution was determined gravimetrically afterward by evaporating the water. The MW of

Figure 1. SAXS data (dots) of pure silica nanoparticles with corresponding sphere form factor fit (dark line).

indices for the diameters of 0.086, 0.074, and 0.070 were deducted, respectively. The resulting values for Mw (eq 3) are given in Figure 1 and Table 1. Very similar fit results were obtained for further modifications steps, but the reduced surface potential and the lower polarity of MeOH compared to those of water tend to led to more agglomeration. 3.3.2. Polymer-Modified SiO2-NPs: Variation of Polymer Type. Because of the temperature inertness of SiO2, TGA is a powerful technique to determine the amount of grafted polymer on the NPs. The studied temperature range was

Table 1. Mass Loss Δm from TGA (Which Gives the Amount of Grafted Polymer of the Nanoparticles), Molecular Weight MW from SAXS in Combination with TGA Results from SLS, Hydrodynamic Radii RH, Shell Thicknesses tshell, and ζ-Potential Values for SiO2@PDMAEMA and SiO2@PMAA Nanoparticlesa RNP (nm)

Δm (wt %)

24.3 24.3 24.3 24.3 24.3 24.3 24.3 32.4 32.4 32.4 32.4 32.4 32.4 32.4 70.3 70.3 70.3 70.3 70.3 70.3 70.3

0.0 3.0 4.5 5.5 7.0 11.5

8.69 8.99 9.14 9.25 9.41 9.94

× × × × × ×

SiO2@PDMAEMA (cationic) 107 2.79 × 108 27 7 10 1.96 × 108 39 107 1.8 × 109 42 107 2.16 × 108 43 107 2.65 × 108 41 107 2.83 × 108 40

0.0 17.5 20.5 23.5 27.5 28.5

2.06 2.54 2.65 2.76 2.93 2.98

× × × × × ×

108 108 108 108 108 108

1.64 2.36 3.23 2.55 2.93 4.04

× × × × × ×

108 108 108 108 108 108

0.0 27.5 28.0 28.1 31.5 33.5

2.12 3.02 3.87 3.89 4.11 4.36

× × × × × ×

109 109 109 109 109 109

2.26 2.91 8.27 1.05 1.10 1.27

× × × × × ×

109 108 108 109 109 109

MW (SAXS +TGA) (g/mol)

MW (SLS) (g/mol)

tshell (nm)

ζ (pH 4) (mV)

Δm (wt %)

0 13 16 17 16 17

+49.0 +50.1 +44.9 +51.0 +52.8 +51.1

35 47 49 47 48 49

0 14 16 15 15 16

+45.7 +32.3 +43.2 +41.8 +42.4 +42.2

72 78 79 80 78 82

0 7 9 10 8 12

+53.2 +35.8 +36.8 +44.4 +47.4 +48.3

0.0 2.5 3.0 7.5 10.5 13.0 14.5 0.0 4.5 5.5 7.0 7.8 8.0 13.5 0.0 1.5 4.5 5.5 6.5 7.0 4.0

RH (nm)

MW (SAXS +TGA) (g/mol) 8.69 8.94 9.19 9.73 1.05 1.12 1.16 2.06 2.17 2.29 2.35 2.41 2.43 2.60 2.12 2.16 2.26 2.35 2.40 2.44 2.37

× × × × × × × × × × × × × × × × × × × × ×

107 107 107 107 108 108 108 108 108 108 108 108 108 108 109 109 109 109 109 109 109

MW (SLS) (g/mol)

RH (nm)

SiO2@PMAA (anionic) 2.79 × 108 26 1.91 × 108 48 1.77 × 108 60 2.96 × 109 89 2.27 × 109 88 2.33 × 109 111 9.67 × 108 118 1.64 × 108 36 3.35 × 108 53 4.62 × 108 71 3.47 × 108 78 5.72 × 108 78 5.20 × 108 97 5.11 × 108 94 2.26 × 109 72 8.87 × 108 75 1.96 × 109 78 3.23 × 109 82 1.20 × 1010 100 2.09 × 109 103 2.31 × 109 83

tshell (nm)

ζ (pH 9) (mV)

0 24 35 65 63 86 93 0 20 39 45 45 64 61 0 5 8 12 30 33 13

−54.3 −32.8 −39.1 −38.2 −36.9 −38.2 −39.8 −46.9 −39.9 −48.1 −41.6 −47.9 −40.7 −49.9 −55.4 −48.6 −35.8 −44.0 −42.5 −42.3 −44.2

a For Δm loss the value 0.0 represents none grafted SiO2@NH2 and SiO2-NP as reference for cationic or anionic NPs. Rcore obtained from fitting results using a spherical core form factor fit from SAXS for pure SiO2-NPs. Light scattering result done in high charge conditions (SiO2@NH2 and SiO2@PDMAEMA at pH 4; SiO2 and SiO2@PMAA at pH 9). tshell calculated by subtracting RH with Rcore.

E


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Figure 2. (right) Hydrodynamic radii RH of polymer grafted NPs and (left) shell thicknesses for different core sizes: (▲) d = 50 nm, (●) d = 70 nm, and (■) d = 140 nm (full symbols: SiO2@PDMAEMA; open symbols: SiO2@PMAA).

polymer grafted NPs was calculated by multiplying the MW of the core (obtained from SAXS via eq 3) with the polymer percentage obtained from TGA (see Figure S8, where also details of the calculation are given). In addition, the MW of polymer grafted NPs was measured by SLS. The refractive index increment dn/dcg of polymer grafted NPs is similar to that of pure SiO2-NPs as that is by far the largest fraction of the hybrid NPs. MW values calculated from SLS are tendentially in good agreement with the results of SAXS+TGA data. However, data exhibit for the smallest NPs a higher Mw from SLS compared to SAXS+TGA data, indicating that the smallest NPs have a higher tendency for agglomeration. In contrast, for the larger NPs MW from SLS is smaller compared to values obtained SAXS+TGA data but usually in the same magnitude. Here we assume that at low concentration a repulsive structure factor is still present which reduces the real scattering intensity of the NPs at low q values. Thus, tendentially smaller approximated I0 values result in smaller calculated MW values. DLS data were measured at fully charged conditions for both polymers, i.e., at pH 4 for SiO2@PDMAEMA and at pH 9 for SiO2@PMAA (see Figure 2). From it the shell thickness tshell could be estimated by subtracting the core radius from the hydrodynamic radius RH of polymer grafted NPs: tshell = RH − RNP

monomer concentration, while the shell thickness always remains constant at tshell ∼ 16 nm. We assume that this unexpected result has its origin in the complexation potential of the polymer and monomer. Once a critical degree of polymerization is reached the chains may tend to chelate the copper by replacing PMDETA with PDAEMA, and further monomer propagation is no longer possible.41,42 A higher monomer concentration increases the probability to grow new chains on different initiator moieties once remaining chains are already trapped. Those complexes can break down by charging the dimethylamino moiety of the polymer. The charging is necessary because it separates the intra- and interparticle bridged polymer chains and enhances the colloidal stability due to electrostatic repulsion. For a more detailed discussion the reader is referred to the Supporting Information Chapter 6. 3.4. pH-Responsive Properties. Both polymers are pHresponsive (PMAA, pKa ∼ 4.3,37 and PDMAEMAH+, pKa ∼ 7.638). In Figures 3a and 3c the hydrodynamic radius RH of the PDMAEMA and PMAA modified NPs is shown as a function of pH. It remains constant for the SiO2@PMAA but increases somewhat with increasing pH for the SiO2@PDMAEMA, which might be attributed to increasing attractive interactions (as their charge becomes reduced at higher pH). Indicated in Figures 3a and 3c is also the point at which the samples are no longer stable and start phase separating (2Φ), which is especially a problem for SiO2@PDMEAMA upon increasing pH. For colloidal stability polymer charging is important because of the electrostatic repulsion between the particles. In addition, there will be steric repulsion resulting from the polymer chains. For the case that the polymer is uncharged attractive vdW forces (they should be pronounced between the SiO2 surfaces and between them and the polymer chains) may dominate the steric repulsion force as we always see NP aggregation and phase separation. However, the stability of these polymermodified NPs is largely controlled by the charge density which is related to the ζ-potential, which is shown in Figures 3b and 3d for the two particle types as a function of pH. As expected, it decreases systematically with increasing pH, having a general tendency for positive values for SiO2@PDMEAMA (Figure 3b) and negative values for SiO2@PMAA (Figure 3d).

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The results for both polymers differ from each other. An expected shell thickness increase for PMAA can be observed resulting from an increase of degree of polymerization of grafted chains as a function of the monomer concentration. For anionic polymer grafted NPs one can control tshell simply by the employed tBuMA concentration. In contrast, SiO2@ PDMAEMA shows the interesting and unexpected result that tshell does not increase with increasing amount of grafted polymer. The only reasonable interpretation is an increase of the grafting density for higher polymer content. That observation strongly differs from results of groups using surface-initiated ATRP, where the grafting density for PDMAEMA on a flat substrate was controlled by the initiator density.40 On the basis of our results, we can control the grafting density of the PDMAEMA brush polymer just by the F


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Figure 3. pH-dependent values for hydrodynamic radii RH and ζ-potentials for the smallest NPs (for larger NPs see the Supporting Information; the wt % in the legend refers to the amount of grafted polymer): (a) RH of SiO2@PDMAEMA (2Φ indicating the onset of precipitation), (b) ζpotentials of SiO2@PDMAEMA, (c) RH of SiO2@PMAA (2Φ indicating the onset of precipitation), (d) ζ-potentials of SiO2@PMAA, (e) turbidity for several precipitation/dispersion cycles for SiO2@PDAMEMA with different polymer content, and (f) turbidity measurements of SiO2@ PDMAEMA after one charging/discharging cycle and subsequently applied sonication (US).

The minimum needed ζ-potential for colloidal stabilization (marked as a red dot-dashed line in Figure 3b) is normally assumed to be around |ζmin| ∼ 25 mV, which is the potential where the electrostatic energy of an elementary charge is equal to the thermal energy kT. Pure SiO2-NPs have always a negative potential in this pH range due to deprotonatable OH moieties on the surface. However, at pH 4 and 5 the ζpotential potential is above −25 mV, but light scattering shows no aggregation. Anionic SiO2@PMAA-NPs have always a negative surface potential, which quickly becomes more negative from pH 4 to 6 and beyond the potential is weakly changing or almost constant for all amounts of grafted polymer (Figure 3d), indicating that charge saturation is reached at pH 6 and not a function of the polymer content.37 In contrast, the ζ-potential of cationic SiO2@NH2 and SiO2@PDMAEMA is negative only in strongly basic media due to the deprotonation of still free Si−OH moieties. For cationic NPs if the ζ-potential drops below +25 mV, NPs begin to precipitate (Figure 3a,b). Independent of the amount of grafted polymer the ζ-potential values are close to each other at pH 4 within experimental

error, indicating that here charge saturation of the polymer is achieved. Furthermore, and different than SiO2@PMAA, a higher polymer content results in higher values at higher pH values, which may explain the delayed pH-dependent aggregation with increasing grafting density of cationic PDMAEMA. For the most highly grafted SiO2@PDMAEMA at pH 8 light scattering shows colloidally stable solutions (Figure 3a). However, they show slow agglomeration over time (several weeks), which is in good agreement with the DLVO theory where weak electrostatic repulsion stabilizes NPs kinetically only for short time periods. In contrast, at pH 7 SiO2@ PDMAEMA with higher degree of grafting remains stable a long time as colloidal solution. NPs are pH-responsive and can be precipitated and dispersed simply by the variation of the pH. We followed the forced precipitation and dispersion cycle with turbidity measurements. Once precipitated by changing pH NPs cannot disperse completely back by adjusting the pH back to 4 with subsequent intense mixing and a certain amount of aggregates remain in the solution. With each precipitation and dispersion cycle the concentration of aggregates increases G


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Macromolecules (Figure 3e), but one can achieve the initial particle concentration again using a sonication tip finger (Figure 3f). It is important to mention that electrostatic long-range stabilization reduces with each charging/discharging step due to concomitant increase of total salt concentration.

Also, the higher the polymer density of SiO2@PDMAEMA, the less sensitive are the NPs to the added electrolytes (NaCl), which is in good agreement with theoretical predictions from Pincus for polyelectrolyte brushes anchored at surfaces.43 Light scattering data exhibit (Figure S10) a mean hydrodynamic radius gain (aggregation) with increasing salt concentration, but SiO2@PDMAEMA with the largest amount of grafted polymer do not aggregate until a salt concentration of cNaCl = 50 mM. 3.5. Small-Angle Neutron Scattering (SANS) at pH 4 and 9. SANS measurements allowed to obtain detailed structural information about the polymer shell. Samples were measured in bulk contrast (using 80% D2O, as the NPs cannot be employed in a dry or very concentrated state) and a H2O/ D2O mixture that matches the silica core, and therefore one sees only the polymer shell. We measured cationic and anionic brush grafted NPs with the smallest core size dNP = 50 nm at constant pH 4 or 9, respectively. The obtained SANS data are displayed in Figure 5 for SiO2@PDMAEMA with 5.5 wt % grafted polymer and in Figure 6 as well as Figures S24 and S25 for the other SiO2@PDMAEMA and SiO2@PMAA systems, respectively (for the larger nanoparticles of 70 and 140 nm the SANS curves are given in Figures S26−S28). Looking at the data displayed in Figure 5, one finds that in bulk contrast (Figure 5a) one sees very well the form factor oscillation that indicates the high monodispersity of the NPs, while in the

Figure 4. Scattering length density (SLD) profiles of the core−shell form factor with spherical symmetry, with a constant shell contrast as well as a linear and an exponential decay. The purple lines describe ηshell in full contrast conditions, and the dark lines describe the contrast match conditions.

Figure 5. Comparison of the different scattering length density profiles for the example SANS data of SiO2@PDMAEMA with 5.5 wt % of grafted polymer: (a) SANS data (dots) with different models of the shell profiles (lines) and (b) corresponding residuals in in CM conditions; (c) SANS data (dots) with different models (lines) and (d) corresponding residuals in full contrast conditions. H


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Figure 6. (a, b) SANS data of SiO2@PMAA with corresponding core−shell form factor fits: (a) contrast match (b) full contrast. (c, d) SANS data of SiO2@PDMAEMA with corresponding core−shell form factor fit: (c) contrast match; (d) full contrast. (e, f) SANS intensity of the polymermodified SiO2−NPs (the wt % of grafted polymer is given in the inset) after subtracting the scattering intensity of the SiO2-NPs. For better legibility data were cumulatively shifted with a factor of 10 for each scattering curve. Data measured at KWS-1.

in the polymer shell and therefore about the polymer density profile. In the literature, a sigmoidal decaying volume fraction profile toward the solvent has been described for polymer brushes in good solvent conditions for flat surfaces.44 However, if the surface is curved, the polymer density is expected to decay exponentially toward the continuum.15,45−47 For our

polymer contrast (contrast match, Figure 5c) one sees a monotonous decrease of the scattering intensity that is steeper than a q−2 law as would be expected for a simple shell structure. However, especially the polymer contrast is very suited for gaining information regarding the scattering length distribution I


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Table 2. Values from the SANS Analysis for SiO2@PDMAEMA and SiO2@PMAA-NPs at Both Contrast Conditionsa Xpolymer (wt %)

cg (g/L)

N (cm−1 nm−2)

1

3.0 5.5 7.0 11.5

3.4 4.0 3.4 4.2

0.219 0.252 0.212 0.252

3.0 5.5 7.0 11.3

1.3 2.4 2.4 3.2

0.090 0.149 0.150 0.195

SiO2@PMAA at pH 9 2.5 7.5 10.5 13.0 2.5 7.5 10.5 13.0

2.0 2.6 2.1 2.9 1.1 1.4 1.5 1.0

tshell (nm) ηcore = 9.6 14.2 14.2 15.5 ηcore < 9.5 13.5 13.8 15.4 ηcore = 13.3 19.8 23.0 29.2 ηcore < 13.6 19.8 24.0 29.8

SiO2@PDMAEMA at pH 4

0.133 0.160 0.127 0.172 0.074 0.085 0.091 0.062

XC

α

Rind (nm)

Rf (nm)

Df

0.54 0.49 0.42 0.37

4.01 3.80 3.99 2.35

32 36 37 40

108 128 120 141

2.95 2.73 2.89 2.43

0.55 0.50 0.46 0.37

4.78 5.40 5.46 4.74

35 40

106 128

2.27 2.50

39

110

2.80

0.45 0.48 0.45 0.47

2.31 6.65 8.85 12.96

40 43

121 110

2.70 2.63

34

130

2.88

0.45 0.48 0.45 0.47

5.49 8.85 11.78 16.10

38 44 40 50

120 150 122 194

2.60 2.30 2.89 2.87

ηsolvent

ηsolvent

ηsolvent

ηsolvent

a

For the form factor: polymer graft content Xpolymer, weight concentration cg, particle number density 1N, shell thickness tshell, polymer volume fraction at the core radius XC, exponential decay rate α. For the structure factor: radius of individual scatterer Rind, cutoff size Rf and fractal dimension Df. Fit parameters are tshell, XC, and α. For all core−shell structures fixed values of Rcore = 24.3 nm and PDI = 0.11 were employed; details about particle number density calculation and SLD values are summarized in the Supporting Information.

Figure 7. Fit parameters from SANS modeling for SiO2@PDMAEMA and SiO2@PMAA core−shell structures (d = 50 nm) in contrast match ηcore = ηsolvent (▲, SiO2@PDMAEMA; ●, SiO2@PMAA) and full contrast conditions ηcore < ηsolvent (△, SiO2@PDMAEMA; ○, SiO2@PMAA); Plotted are the (a) radius of the core−shell NPs R, (b) volume fraction at the core XC, and (c) the exponential diffuse parameter α versus the amount of grafted polymer. Note that at 0 wt % is the core radius of pure silica nanoparticles.

a spherical form factor given by RNP = 24.3, 32.6, and 70.3 nm and PDI = 0.11, 0.11, and 0.12, which were kept constant for further modeling (see the Supporting Information). To test the validity of an exponential decaying scattering length density within the shell, the data were fitted by keeping the intensity at low q constant for the three different shell types: constant (α = −∞), linear (α = 0), and exponential (α > 0) ηshell decay toward the solvent (Figure 5). The volume fraction of water at the core radius XC increases with constant shell > linear shell > exponential shell. Thus, we have two free parameter with α describing the SLD profile and XC for adjusting the scattering intensity to the intensity of the data in the low q-regime, and both are coupled as otherwise we would increase the amount of the polymer in the shell with constant shell > linear shell > exponential shell. All other parameters were kept constant. In the model some variation in the shell

system we used a spherical core−shell form factor where the size, polydispersity, and scattering length density of the core ηcore were fixed by the values obtained for the pure SiO2-NPs by using a spherical form factor model. For the scattering length density profile of the shell for a comparison we employed as different functions a constant shell contrast, a linear function, or an exponential relation. (see Figure 4 for a comparison of the different profiles). The decay of the scattering length density of the shell ηshell was described by the parameter α while the volume fraction of the water (D2O) at the core radius XC and shell thickness Xshell influences ηshell,Rc and ηshell,Rc+tshell; i.e., the latter is set to 1 assuming fully decaying ηshell to ηsolvent.30 For all SANS data for modeling the core−shell structures, the radius of the silica core and its polydispersity were taken from the SANS data of the pure SiO2-NPs in full contrast using J


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Macromolecules thickness tshell was allowed (limits were set from 0 to RH). Differences between the model and the experimental data can be observed in the high q regime (q = 0.2−1 nm−1), especially after subtracting the background. For contrast match (CM) and full contrast conditions the exponential shell approximates most accurately the measured data as seen from the residuals which become smallest for the exponential approximation (Figure 5b,d). However, it might be noted that there is not an enormous difference in the quality of the fits, which indicates that the core−shell model is appropriate, but it is difficult to gain much more detailed insights about the corona profile from the given data. The other SANS data were fitted with the same model, and the outcome is shown in Figure 6 for the 50 nm SiO2-NPs (in Figures S24−S28 for the other samples). The curves in full contrast (Figure 6a,c) show the form factor minimum, which becomes less pronounced with increasing polymer content. This is not a sign of increasing polydispersity of the particles but mainly of the changing overall contrast conditions, as the scattering of the polymer shell becomes more and more important. In the polymer contrast (contrast match of the SiO2 core) one sees no marked features of the form factor. Only for the SiO2@PDMAEMA (Figure 6c) one sees a weak oscillation around q ∼ 0.6−0.7 nm−1, indicating a shell thickness of ∼9− 10 nm. For all polymer grafted samples α typically has positive values of 3−10 and describes an exponential ηshell decay (see Table 2 and Table S7). The scattering curves look similar, but there is evidence for aggregation seen in low q regime, where the intensity increases further. To account for that, we employed a fractal model for the structure factor introduced by Teixeira et al.31 When comparing the obtained fit parameters (Figure 7), one finds for both types of polymer-modified NPs an increase of the effective particle radius with increasing polymer content. However, this increase is much less pronounced for the SiO2@PDMAEMA. Another interesting point is that for SiO2@PMAA the volume fraction at the core surface XC is remarkably constant with increasing polymer grafting, but α increases (Table 2). In contrast and different than the anionic NPs for SiO2@ PDMAEMA XC decreases with increasing amount of grafted polymer, thus confirming an increase of the grafting density (in agreement with the light scattering results depicted in Figure 2 which showed a constant size of the polymer shell with increasing polymer content). Comparing the fit results from both contrast the absolute values differ somewhat but show the same trend for increasing polymer grafting (Figure 7). Similar results are obtained from the data for larger nanoparticles (d = 70 and 140 nm) (see the Supporting Information). For the larger NPs the decrease in the form factor oscillation by increasing polymer content is somewhat less marked than for the smaller ones, simply due to the fact that the relative amount of polymer is less. For completeness we also described the SANS data in a different way and more carefully with respect to the polymer contribution, especially in the high q regime. For that the data were modeled with a homogeneous spherical core with polymer chains with Gaussian statistics attached to its surface, as proposed by Pedersen and Gerstenberg48 for the case of diblock copolymer micelles. Furthermore, the aggregation was taken into account with the fractal structure factor and in addition with short-range excluded volume effect from cores sticking together in the aggregates. This was modeled via a

polydisperse hard sphere structure factor implemented via the decoupling approximation.49 The approximated data are in good agreement with the experimental, but the model is overpredicting the oscillations minimas (Figure S29). Note that in reality the form factor is certainly much more complex. Details about the model can be found in the Supporting Information in section 7.9. In parallel to modeling the core−shell structures we did some model free analysis. In full contrast conditions we subtracted the scattering intensity of the SiO2-NPs from that of the SiO2@polymer data to gain a pseudo-shell-form factor (Figure 6e,f; this is neglecting all scattering cross-terms and therefore can only be considered to be a very rough approximation of the really scattering of the polymer shell). Directly observable for anionic samples the minima are shifted toward lower q values with increasing polymer grafting, which is in good agreement with the LS data and indicates an increasingly thicker polymer shell. In comparison for cationic NPs the first minimum remains remarkably constant at q = 0.126 nm−1. Thus, this is direct confirmation for the increase of shell thickness for the anionic versus cationic NPs, where the shell thickness is almost constant. The average radius of the SiO2@PDMAEMA shell can be estimated as R = π/qmin = 24.92 nm (using a thin shell form factor), indicating that the mass average of the shell is rather close to the core radius. In summary, the SANS curves allow to deduce information regarding the density profile of the polymer shell and confirm the marked differences in the structure of the anionic (PMAA) and cationic (PDMAEMA) shell, where once one sees an increase in thickness and in the other case of the grafting density, with increasing amount of monomer employed.

4. CONCLUSION We successfully grafted anionic and cationic polyelectrolyte brushes onto silica nanoparticles of different sizes in the radii range 24−70 nm, where the polymerization was done by a surface-initiated atom transfer radical polymerization (ATRP). As cationic polymer we employed PDMAEMA and for the anionic polymer PMAA, where the latter was obtained after hydrolysis of the initially formed PtBuMA. The amount of attached polymer was controlled by the amount of added monomer. Charging of the NPs is necessary to have colloidal stability in water and to inhibit agglomeration in solution. This means that upon reducing the charge density, which means increasing pH for PDMAEMA and decreasing pH for PMAA, one observes agglomeration and precipitation in the samples. In addition, it was observed that obtaining well-defined and colloidally stable NPs became increasingly more difficult with decreasing NP size. The obtained hybrid NPs were characterized in detail by means of thermogravimetric analysis (TGA), static and dynamic light scattering (SLS and DLS), and small-angle neutron scattering (SANS). For anionic brushes (PMAA) the shell thickness is simply controlled by the monomer concentration as typical observed for living radical polymerization. In contrast for the cationic NPs (PDMAEMA) the shell thickness is independent of the amount of added monomer and apparently the grafting density is determined via the monomer concentration. We assume that the unexpected result is due to the complexation potential of PDMAEMA, which may chelate copper at a certain degree of polymerization, and further monomer addition on that chain is no longer possible. K


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thylaminoethyl methacrylate brush grafted silica nanoparticles; SiO2@PMAA, poly(methacrylic acid) brush grafted silica nanoparticles; ζ-potential, zeta potential.

SANS shows that the polymer profile of the shell is best described by an exponentially decaying polymer density. As expected, the shell is largely swollen by solvent, but the modeling also clearly confirms the constant thickness of the PDMAEMA shell seen before by light scattering, and the shell simply becomes denser with increasing polymer content. In summary, we have obtained well-defined silica− polyelectrolyte hybrid nanoparticles, whose core size and polymer shell can be varied systematically in polymer content, thereby controlling the softness of the shell via the thickness of the shell are the grafting density. They are available as anionic and cationic NPs and are colloidally stable over a large pH range, and as these hybrid NPs are based on weak polybases or polyacids they show pH-responsive behavior.

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(1) Leroueil, P. R.; Hong, S. Y.; Mecke, A.; Baker, J. R.; Orr, B. G.; Holl, M. M. B. Nanoparticle interaction with biological membranes: Does nanotechnology present a janus face? Acc. Chem. Res. 2007, 40, 335−342. (2) Karakoti, A. S.; Hench, L. L.; Seal, S. The potential toxicity of nanomaterials - the role of surfaces. JOM 2006, 58, 77−82. (3) Gil, P. R.; Oberdörster, G.; Elder, A.; Puntes, V.; Parak, W. J. Correlating Physico-Chemical with Toxicological Properties of Nanoparticles: The Present and the Future. ACS Nano 2010, 4, 5527−5531. (4) Winnik, F. M.; Maysinger, D. Quantum Dot Cytotoxicity and Ways To Reduce It. Acc. Chem. Res. 2013, 46, 672−680. (5) Hsiao, I.-L.; Gramatke, A. M.; Joksimovic, R.; Sokolowski, M.; Luch, A.; Gradzielski, M.; Haase, A. Size and Cell Type Dependent Uptake of Silica Nanoparticles. J. Nanomed. Nanotechnol. 2014, 5, 248−258. (6) Wang, Y.; Santos, A.; Evdokiou, A.; Losic, D. An overview of nanotoxicity and nanomedicine research: principles, progress and implications for cancer therapy. J. Mater. Chem. B 2015, 3, 7153− 7172. (7) Han, H.-S.; Martin, J. D.; Lee, J.; Harris, D. K.; Fukumura, D.; Jain, R. K.; Bawendi, M. Spatial Charge Configuration Regulates Nanoparticle Transport and Binding Behavior In Vivo. Angew. Chem. 2013, 125, 1454−1459. (8) Michel, R.; Gradzielski, M. Experimental Aspects of Colloidal Interactions in Mixed Systems of Liposome and Inorganic Nanoparticle and Their Applications. Int. J. Mol. Sci. 2012, 13, 11610− 11642. (9) Michel, R.; Kesselman, E.; Plostica, T.; Danino, D.; Gradzielski, M. Internalization of Silica Nanoparticles into Fluid Liposomes: Formation of Interesting Hybrid Colloids. Angew. Chem., Int. Ed. 2014, 53, 12441−12445. (10) Dobereiner, H. G.; Lipowsky, R. Vesicles in contact with nanoparticles and colloids. Europhys. Lett. 1998, 43, 219−225. (11) Vansant, E. F. in Characterization and Chemical Modification of the Silica Surface, 1st ed.;Van Der Voort, P., Vrancken, K. C., Eds.; Elsevier: Amsterdam, 1995; Vol. 93. (12) Inoubli, R.; Dagréou, S.; Khoukh, A.; Roby, F.; Peyrelasse, J.; Billon, L. ‘Graft from’ polymerization on colloidal silica particles: elaboration of alkoxyamine grafted surface by in situ trapping of carbon radicals. Polymer 2005, 46, 2486−2496. (13) Inoubli, R.; Dagréou, S.; Delville, M.-H.; Lapp, A.; Peyrelasse, J.; Billon, L. In situ thermo-dependent trapping of carbon radicals: a versatile route to well-defined polymer-grafted silica nanoparticles. Soft Matter 2007, 3, 1014−1024. (14) Deleuze, C.; Delville, M. H.; Pellerin, V.; Derail, C.; Billon, L. Hybrid Core@Soft Shell Particles as Adhesive Elementary Building Blocks for Colloidal Crystals. Macromolecules 2009, 42, 5303−5309. (15) Kim, C. J.; Sondergeld, K.; Mazurowski, M.; Gallei, M.; Rehahn, M.; Spehr, T.; Frielinghaus, H.; Stühn, B. Synthesis and characterization of polystyrene chains on the surface of silica nanoparticles: comparison of SANS, SAXS, and DLS results. Colloid Polym. Sci. 2013, 291, 2087−2099. (16) Moraes, J.; Ohno, K.; Maschmeyer, T.; Perrier, S. Synthesis of silica-polymer core-shell nanoparticles by reversible addition-fragmentation chain transfer polymerization. Chem. Commun. 2013, 49, 9077−9088. (17) Perruchot, C.; Khan, M. A.; Kamitsi, A.; Armes, S. P.; von Werne, T.; Patten, T. E. Synthesis of Well-Defined, Polymer-Grafted Silica Particles by Aqueous ATRP. Langmuir 2001, 17, 4479−4481. (18) Chen, X.; Randall, D. P.; Perruchot, C.; Watts, J. F.; Patten, T. E.; von Werne, T.; Armes, S. P. Synthesis and aqueous solution properties of polyelectrolyte-grafted silica particles prepared by

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01019. Details of synthesis, thermogravimetric analysis spectra, ζ-potential measurement, scattering length densities estimation, details about the SAXS and SANS data, TEM pictures (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(M.G.) E-mail [email protected], Ph +49 30 314 24934, Fax +49 30 314 26602. *(M.S.) E-mail [email protected], Ph +49 30 314 24790, Fax +49 30 314 26602. ORCID

Marek Sokolowski: 0000-0002-6989-9996 Michael Gradzielski: 0000-0002-7262-7115 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the DFG via the International Research Training Group 1524 “SelfAssembled Soft Matter Nano-Structures at Interfaces”. We thank Christina Eichenhauer, Sören Selve, and Ulrich Gernert for providing TGA data and TEM and SEM pictures of polymer grafted NPs, respectively. A. Laschewsky (Universität Potsdam, Germany) is thanked for enlightening discussions about surface polymerization. We also thank the European Synchrotron Facility (ESRF) and Theyencheri Narayanan for the provision of in-house beam time and the JCNS and ILL for funding the travel.

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ABBREVIATIONS APTS, 3-aminopropyltriethoxysilane; BIBB, 2-bromo-2-methylpropionyl bromide; CM, contrast match; DCM, dichloromethane; DMAEMA, N,N-dimethylaminoethyl methacrylate; tBuMA, tert-butyl methacrylate; PDI, polydispersity index; PMDETA, N,N,N′,N″,N″-pentamethyldiethylenetriamine; TEOS, tetraorthoethoxy silicate; TFA, trifluoroacetic acid; SDD, sample to detector distance; SLD, scattering length density; SiO2-NPs, silica nanoparticles; SiO2@NH2-NPs, amino-functionalized silica nanoparticles; SiO2@Br, initiatorfunctionalized silica nanoparticles; SiO2@PDMAEMA, dimeL


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