Morphology-Controlled Coating of Colloidal Particles with Silica

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Morphology-Controlled Coating of Colloidal Particles with Silica: Influence of Particle Surface Functionalization Xu Dong,† Pan Wu,† Goetz P. Hellmann,‡ Changchun Wang,† and Christian G. Schaf̈ er*,†,‡ †

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, and Laboratory of Advanced Materials, Fudan University, 220 Handan Road, Shanghai 200433, China ‡ German Institute for Polymers (DKI), Schlossgartenstrasse 6, D-64289 Darmstadt, Germany S Supporting Information *

ABSTRACT: We present a general, convenient, and efficient synthetic concept for the coating of colloidal particles with a silica (SiO2) shell of well-defined and precisely controlled morphology and porosity. Monodisperse submicroscopic polystyrene (PS) particles were synthesized via two-stage emulsifier-free emulsion polymerization and subsequent swelling polymerization, enabling selective particle surface modification by the incorporation of ionic (methacrylic acid, MAA) or nonionic (hydroxyethyl methacrylate, HEMA or methacrylamide, MAAm) comonomers, which could be proven by zeta potential measurements as well as by determining the three-phase contact angle of the colloidal particles adsorbed at the air−water and n-decane−water interface. The functionalized particles could be directly coated with silica shells of variable thickness, porosity, and controlled surface roughness in a seeded sol−gel process from tetraethoxysilane (TEOS), leading to hybrid PS@silica particles with morphologies ranging from core−shell (CS) to raspberry-type architectures. The experimental results demonstrated that the silica coating could be precisely tailored by the type of surface functionalization, which strongly influences the surface properties of the colloidal particles and thus the morphology of the final silica shell. Furthermore, the PS cores could be easily removed by thermal treatment, yielding extremely uniform hollow silica particles, while maintaining their initial shell architecture. These particles are highly stable against irreversible aggregation and could be readily dried, purified, and redispersed in various solvents. Herein we show a first example of coating semiconducting CdSe/ZnS nanocrystals with smooth and spherical silica shells by applying the presented method that are expected to be suitable systems for applications as markers in biology and life science by using fluorescence microscopy methods, which are also briefly discussed. (emulsion, miniemulsion, suspension, dispersion).8 Although, these simple silica or polymer particles have demonstrated a high versatility for many applications, the synthesis of highly functional inorganic nanoparticles went into the focus of many research areas for the last decades, whereby a new class of hightech colloids with applications in catalysis,9 electronics and optoelectronics,10 photonics,11 biological labeling and imaging,12 sensing,13 drug targeting, and immobilization has been realized.14,15 However, most of the applications require these functional nanoparticles to be coated with a shell of a different material to modify their surface,16−18 chemical,19 reactive,20 catalytic,21 electrical,22 optical,23 or magnetic properties.24 Especially silica-coated colloidal particles are a class of materials that have attracted a great deal of attention in recent years and are nowadays widely used for numerous applications in fields of drug-delivery,25,26 sensing,27,28 imaging,29,30 and catalysis.31−33 The advantages of silica as a coating material

1. INTRODUCTION Micro- and submicrometer-sized particles, which are also referred to as colloidal particles,1 play a fundamental role in the design of novel exciting materials due to their special electronic,2 optical,3 chemical,4 and magnetic properties.5 For this reason, these materials are frequently used in basic research as well as for a huge variety of practical applications, e.g., in catalysis or sensor technology, in paint and coating industry, in the production of high-performance plastics and ceramics, rheological fluids and photographic films, in the food and cosmetics industries, as well as in pharmacy.6 The properties of these colloidal particles often depend strongly on their size, composition, and morphology and can therefore be diversely tailored for the respective applications. Nowadays, monodisperse colloidal particles can be synthesized from a huge variety of different materials. They can consist of silica (SiO2) particles, which are accessible via sol− gel processes (Stöber process),7 but also of polymer particles such as polystyrene (PS) or poly(methyl methacrylate) (PMMA), which can be synthesized with monodisperse particle size distribution by methods of heterophase polymerization © XXXX American Chemical Society

Received: November 10, 2016 Revised: January 25, 2017 Published: February 13, 2017 A

DOI: 10.1021/acs.langmuir.6b04069 Langmuir XXXX, XXX, XXX−XXX

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aqueous sol−gel solution. Shen et al.53 presented a novel method for the preparation of polymer−silica colloidal nanocomposites through emulsion polymerization of styrene using 4-vinylpyridine (4VP) as a functional comonomer. Composite particles with smooth or rough core−shell morphology were obtained through subsequent sol−gel nanocoating process. However, most of these coating methods are complicated and time-consuming processes, which are poorly controllable. Consequently, most of the methods reported are not well reproducible and do not allow the control over structure and morphology of the silica shell, as a result of ignoring the great possibilities of a selective pretreatment of the substrate particles before coating. Therefore, they are often limited to the formation of core−shell (CS) particles owing to a closed and completely smooth silica shell. In general, the precise tuning of the surface properties of the polymer particles by a selective pretreatment seems to be crucial for the morphology-controlled coating with silica, since the first seeding layer produced in the sol−gel process, which is adsorbed to the surface before a dense shell is formed, is expected to define the final shell morphology. Moreover, a precise control of the dielectric surface chemistry and therefore the silica−particle interactions as well as the shell growth conditions via optimization of the silica deposition rate in the sol−gel process should allow one to affect the morphology and structure of the final silica shells. Yet, the effect of surface properties on the silica coating is seldom discussed, and only few works have been directed to systematic investigation of the formation of well-defined silica shell morphologies in a sol−gel approach.59 In the present paper, with the aim to give a morphological control of the silica coating, we present a convenient, simple and fast synthetic concept for the controlled coating of colloidal particles of different materials and different sizes with silica. This method is based on the hydrophilization of the substrate particles by selective particle surface modification. Therefore, this work focuses on three main objects including: first, investigating the effect on the surface properties of submicroscopic PS particles by the incorporation of hydrophilic ionic (methacrylic acid, MAA) or nonionic (hydroxyethyl methacrylate, HEMA or methacrylamide, MAAm) comonomers; second, examining how homogeneous silica coatings of welldefined and precisely controlled morphology and porosity can be grown onto the hydrophilized PS particles by the addition of tetraethoxysilane (TEOS) in a seeded sol−gel process; finally, interpreting the results and application of the presented method on coating semiconducting CdSe/ZnS nanocrystals with silica, in order to demonstrate the broader applicability of this method. Further, we show a first example of using the asprepared silica-embedded nanocrystals for fluorescence microscopy.

relies on the fact that it is a low-cost material and accessible on tunable length scales accompanied by different morphologies, which basically allows one to tailor surface properties of the colloidal particles.34 Additionally, this coating endows the cores with several beneficial properties: first, the outer silica coating provides an inert and stable matrix to avoid particle aggregation and chemical transformations of the embedded cores, which makes it possible to disperse those particles in a wide range of solvents. Second, the silica shell can be easily activated, which offers new possibilities for chemical and biofunctionalization. Finally, the most important is that the silica coating process can be easily regulated, and both the porosity as well as the morphology can be precisely controlled, enabling the specific design of rough surfaces with well-defined gating and wetting properties. The coating of colloidal particles in the size range of 1 nm to 1 μm such as metal,35,36 semiconductor,37−40 magnetic,41−43 ceramic,44,45 and polymer particles has been intensively studied,46,47 and innumerable literature reports are available that prove the success of silica coating. Excellent reviews in the field of silica-containing colloidal nanocomposite particles are given by, e.g., Shen et al.,48 Armes et al.,49 and Lin et al.50 However, fundamental investigations of the coating process of colloidal particles with regard to a precisely controlled and welldefined structure and morphology of the final silica shell are rather scarce. Several feasible coating protocols, especially for polymer-based core particles, have been established, which rely on combinations of heterophase polymerization techniques with sol−gel processes. However, in most studies dealing with the coating of polymer particles with silica by using sol−gel synthesis, the surfaces of the polymer particles could be coated with the help of additives such as polymeric stabilizers,51 functional comonomers,52,53 silane coupling agents,54,55 the adsorption of polyelectrolytes56 or by treating the particles in oxygen plasma.57,58 In order to increase the chemical affinity of the polymer particles with the silica shell, two general synthetic strategies are followed: (i) Postfunctionalization of preformed polymer particles. For example, van Blaaderen et al.51 adsorbed poly(vinylpyrrolidone) (PVP) on positively or negatively charged PS particles and were able to coat them with smooth and homogeneous silica shells of variable thickness by the addition of tetraethoxysilane (TEOS) in a sol−gel process. Chiu et al.56 demonstrated the ability to grow silica shells of controllable thickness over a wide dynamic range directly on negatively charged PS particles after layer-by-layer attachment of polyelectrolyte multilayers. Kim et al.57 developed a facile technique to introduce hydroxyl groups on the surfaces of PS particles by using oxygen plasma and used these surfacemodified PS spheres as sacrificial template to fabricate silicacoated PS composites by co-condensation between hydroxyl groups with TEOS in a sol−gel process. (ii) Functionalization of polymer particles through copolymerization with functional comonomers. For example, Bourgeat-Lami et al.55 synthesized PS particles containing silanol groups in emulsion polymerization using 3-(trimethoxysilyl)propyl methacrylate (MPM) as a functional comonomer. The silanol groups were then converted into silica coatings by co-condensation with TEOS in a sol−gel process. Chen et al.52 prepared microsized, monodisperse, positively charged PS particles by dispersion polymerization using cationic 2-(methacryloyl)-ethyltrimethylammonium chloride (MTC) as the comonomer, which ensured the generation of silica coatings from the hydrolysis and condensation of TEOS via electrostatic interaction in an

2. EXPERIMENTAL SECTION 2.1. Chemicals. Styrene (S) and Disponil FES 27A were purchased from BASF SE and methacrylic acid (MAA), methacrylamide (MAAm) and hydroxyethyl methacrylate (HEMA) from Evonik Röhm GmbH. Prior to use in emulsion polymerization S was distilled under reduced pressure to remove the stabilizer 4-tert-butylcatechol from the monomer. Aqueous ammonia solution (28−30%), n-decane (98%), ethanol (99.5%), aqueous dimethylamine solution (40%) and sodium nitrite (97%) were obtained from VWR, azobis(isobutyronitrile) (98%, AIBN)), sodium peroxodisulfate (98%, SPS), were obtained from Sigma-Aldrich and teraethoxysilane (TEOS) and PDMS sylgard 184 elastomer from ABCR. The gellan B


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Langmuir gum (Kelcogel CG-LA) was a gift from CPKelco (USA). All chemicals were used as received unless otherwise mentioned. 2.2. Synthesis of Hydrophilic PS Particles. Monodisperse PS spheres were synthesized by emulsifier-free emulsion polymerization according to the literature.60 The synthesis yielded monodisperse PS seed particles with an average diameter of 590 and 620 nm, respectively, as determined by TEM measurements. The resulting PS seed particles were enlarged in a subsequent swelling polymerization step similar to a process described in the literature.61 Under argon, a 250 mL flask equipped with stirrer and reflux condenser was filled with 125 g of seed particles and stirred at 200 rpm. After 30 min, a monomer emulsion consisting of 56 g water, 0.17 g Disponil FES 27A, 15.5 g monomer (S + 4.3 mmol comonomer: MAA, HEMA, or MAAm, respectively), 0.26 g NaNO2 and 0.43 g AIBN was added and stirred for swelling procedure at 200 rpm under argon for 24 h. The mixture was then heated to 80 °C. After stirring for 24 h at 80 °C, the product was cooled to room temperature. The synthesis yielded monodisperse particles with an average diameter of 830 nm (PS), 780 nm (PSMAA), 790 nm (PSHEMA) and 790 nm (PSMAAm), respectively, as determined by TEM measurements. 2.3. Silica-Coating of Hydrophilic PS Particles. The functionalized PS particles were coated with silica by a modified Stöber method from TEOS in ethanol.62 In the standard procedure, 1.66 g of the particles were dispersed in a solution of 1.5 mL aqueous ammonia and 1 mL TEOS in 32.1 mL ethanol and stirred at room temperature overnight. In the standard procedure, the concentration of TEOS cTEOS = 0.12 M, the concentration of water cH2O = 3.6 M and the concentration of ammonia cNH3 = 0.60 M. The synthetic procedure was varied in the range 0.03 M ≤ cTEOS ≤ 0.12 M, 3.6 M ≤ cH2O ≤ 15.0 and 0.15 M ≤ cNH3 ≤ 0.6M. Free silica particles formed during the coating process were separated by several centrifugation and redispersion cycles in EtOH (3500 rpm, 30 min; 2500 rpm, 30 min; 1500 rpm, 30 min, and 1000 rpm, 30 min), before the products were dried at 60 °C in a vacuum. 2.4. Synthesis, Functionalization, and Silica-Coating of CdSe/ZnS Nanocrystals. Monodisperse water-dispersible CdSe/ ZnS nanocrystals capped with mercaptopropionic acid (MPA) were synthesized according to the literature.63 For the coating of the nanocrystals with silica, 185 mL ethanol, 50.6 mL of dimethylamine solution (40% in water), 36.5 mL of water, 55.5 μL of TEOS dissolved in 1.5 mL of ethanol, and 0.3 mL of the nanocrystal dispersion were placed in a 250 mL Erlenmeyer flask and stirred overnight. For the further growth of the silica shell, 85.0 μL of TEOS dissolved in 5 mL of ethanol was added, and the reaction mixture was again stirred overnight. 2.5. Characterization. Scanning electron microscopy (SEM) was performed on a Philips XL30 FEG at an operating voltage of 20 kV. Dried powders of hollow silica particles and square pieces (3 × 3 mm) of the PS particles embedded in PDMS were investigated. Prior to SEM investigations, the samples were sputter-coated with a conductive gold layer (20 nm). Transmission electron microscopy (TEM) was performed on a Zeiss EM10 with an operating voltage of 60 kV. For investigation of the single particles, the diluted dispersions were drop-casted on carbon-coated copper grids (Plano GmbH, Germany) and dried at room temperature. For the preparation of ultrathin sections, dried powders of the core−shell (CS) particles were embedded in an epoxy resin (UHU endfest) and cut into slices of 100 nm using an ultramicrotome Ultracut UTC (Leica) equipped with a diamond knife. TEM images were recorded with a slow-scan CCD camera TRS (Troendle). Dynamic light scattering (DLS) and zeta potential measurements of the particles were performed on a Zetasizer Nano ZS (Malvern) using a He−Ne laser at a wavelength of 632.8 nm. The experiments on diluted dispersions of the particles were carried out at an angle of 90° at 20 °C. For the measurements of the zeta potential, the purified dispersions were diluted either with Millipore water (pH = 7.4) or with the Stöber alkosol without TEOS (pH = 12.3), filled into a disposable measuring cell of the type Zetasizer Nanocells (Malvern) and measured at constant pH value.

The determination of the three-phase contact angle of the PS particles absorbed at the air−water or n-decane−water interface was carried out according to the literature using the gel trapping technique introduced by Paunov.64 A 250 mL beaker was filled with 98.0 g of Millipore water and heated to to 95 °C by using a water bath. Afterward, 2.0 g of Kelcogel CG-LA were added slowly under vigorous stirring by using a propeller stirrer until a clear solution was obtained. The hot solution was homogeneously distributed in thermostated Petri dishes at 50 °C. From the particles, dispersions in water/2-propanol (50:50) with a solid content of 2 wt % were prepared. Ten microliters of the particle dispersion was spread either directly onto the surface (air−water interface) or into the n-decane−water interface of the Kelcogel CG-LA solution (layered with n-decane) at 50 °C, through fast injection by using a syringe. The Petri dish was cooled to room temperature and the Kelcogel CG-LA solution gelled after 30 min. After completed gelation, the air respectively n-decane phase was exchanged to liquid Sylgard 184 PDMS elastomer (ratio of PDMS to curing agent of 10:1) and cured at room temperature for 48 h. The cured elastomer could be easily removed from the gel surface and could be directly used for SEM analysis. UV−vis absorption spectrum of the silica-coated CdSe/ZnS nanocrystals were measured from dilute aqueous dispersion in 1.00 cm quartz cell by using a Lambda 40 UV−vis spectrophotometer (PerkinElmer). Fluorescence emission spectra from the same sample were recorded using an USB 2000 Vis-NIR fiber spectrophotometer (Ocean Optics). For recording the fluorescence emission spectra, the samples were excited by an UV lamp NU-6 W (Benda) at a wavelength of 366 nm. For confocal laser scanning microscopy (CLSM) measurements of the silica-coated CdSe/ZnS nanocrystals, the particles were diluted in H2O, spin coated on glass cover slides (50 rps, 60 s) and embedded in poly(methyl methacrylate). Illumination of the particles was provided by using an intensity-controlled solid-state laser (GL532T-500, Shanghai Laser & Optics Century Co., Shanghai, China) operating typically at 532 nm. The laser beam was widened and coupled into a conventional, commercially available wide-field microscope (DM IRB, Leica Microsystems Wetzlar, Germany). The excitation light was focused into the back focal plane of the objective lens (HCX PL APO CS 100x/1.40−0.70 Oil 0.17/D, Leica Microsystems) to ensure a uniform illumination of the field of view in the sample. The microscope slide was placed on a homemade sample-holder mounted directly onto the objective. CdSe/ZnS nanocrystal luminescence was collected by the same objective lens and directed out of the microscope stand. In order to separate the fluorescence from the excitation beam, fluorescence was filtered by an appropriate long pass filter (BLP01-532R-25, Semrock, Rochester, USA). The fluorescent light emitted from the particles was detected in the detection window of 545−585 nm and imaged by the camera chip of a high-sensitivity electron-multiplying charge-coupled device (EMCCD, Ixon plus DU897, Andor Technology, Belfast, Northern Ireland).

3. RESULTS AND DISCUSSION 3.1. Synthesis of Hydrophilic PS Particles. Submicroscopic PS spheres were accessible by emulsifier-free emulsion polymerization. The synthesis yielded monodisperse particles that were subsequently enlarged in a second step of swelling by the addition of a monomer emulsion of styrene in water containing the monomer soluble AIBN as radical initiator and water-soluble NaNO2 as inhibitor. AIBN conducted the polymerization only in the particles, and the nitrite caught the radicals in the water phase to effectively prevent the start of polymer chains in water. After complete monomer absorption, the swollen particles were polymerized at elevated temperature. As can be seen from Figure 1a, the synthesis yielded uniform and monodisperse PS particles with an average diameter of 830 nm. Hydrophilic PS particles having surface-anchored carboxyl, hydroxy, and amide groups were also successfully synthesized by the same swelling method. For this purpose, a portion of the C


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influence of the different functional groups on the surface properties of the particles will be discussed. Unfortunately, the functional groups of the various PS particles neither were accessible to titration, nor could they be detected by spectroscopic methods due to the very low quantity of the functional groups inside the particles of only 2.3 mol % (IR spectra are given in Supporting Information Figure S2). For comparison, the different particle types were therefore characterized by zeta potential measurements. For this purpose, the purified particle dispersions were first diluted with Millipore water and measured at a constant pH value (pH = 7.4). However, since the zeta potential of colloid particles is dependent on the respective pH value, it was also measured in an ethanolic Stöber alkosol with ammonia (pH = 12.3). The results of the measurements are summarized in Table 2 (zeta potential measurements are given in Supporting Information Figure S3). Table 2. Comparison of Zeta Potential of Functionalized Colloidal Particles at Different pH Value

Figure 1. TEM images of the corresponding colloidal particles: (a) PS, (b) PSMAA, (c) PSHEMA, and (d) PSMAAm. The scale bars correspond to 1000 nm.

styrene in the swelling emulsion (4.2 mol %) was replaced by the comonomers MAA, HEMA, or MAAm. The controlled buildup of the hydrophilic PS particles was also verified by transmission electron microscopy (TEM). In Figure 1, TEM images of the corresponding particles after the swelling polymerization step are given, demonstrating that monodisperse PSMAA (Figure 1b), PSHEMA (Figure 1c) and PSMAAm (Figure 1d) are obtained by the applied synthetic procedure. From Figure 1, it can be concluded that the synthesis yielded monodisperse submicroscopic PSMAA, PSHEMA, and PSMAAm particles with an average diameter of 780, 790, and 790 nm, respectively. In addition to TEM investigations, the hydrodynamic diameters and size distributions of the particles were characterized by using dynamic light scattering (DLS). The average diameter of the polymer particles obtained by TEM (DTEM) and hydrodynamic diameters (DDLS) obtained by DLS measurements are compiled in Table 1 (DLS measurements are

comonomer

459 463 470 472

MAA HEMA MAAm

DTEM/nm 830 780 790 790

± ± ± ±

18 20 19 18

DDLS/nm

PDI

± ± ± ±

0.097 0.035 0.060 0.031

875 835 833 840

37 38 42 42

comonomer

ζpH7.4/mV

ζpH12.3/mV

459 463 470 472

MAA HEMA MAAm

−52.0 −48.6 −50.3 −53.5

−61.7 −78.4 −66.9 −68.2

As can be summarized from Table 2, a negative zeta potential was measured for all particles. The pure PS particles were also negatively charged, because of the sulfate groups incorporated by the initiator and the emulsifier. At pH = 7.4, all zeta potentials were of the same order of magnitude. The change to the alkaline environment of the Stöber alkosol led to a slight increase of the negative potential by 18% in the unfunctionalized PS particles. For the nonionically modified PSHEMA and PSMAAm particles, the negative potential increased by 30%, whereas for the ionically modified PSMAA particles the potential tremendously increased by 60%. From the zeta-potential measurements, it can be concluded that under the conditions of the Stöber process (pH = 12.3), a clear gradation between unfunctionalized, nonionically, and ionically modified PS particles could be observed. In order to accurately examine how the copolymerization of styrene with hydrophilic monomers affects the surface properties of the particles, their hydrophilicity was determined by measuring the three-phase contact angle of particles adsorbed in the air−water and n-decane−water interface by using the gel trapping technique. To measure the contact angle, the particles of the purified dispersions were spread in the respective interface. Then, the lower aqueous phase was solidified by a nonadsorbing polysaccharide, so that a monolayer of the particles was embedded and fixed in the aqueous phase. Subsequently, the upper phase (n-decane or air) was replaced by polydimethylsiloxane (PDMS). After the PDMS has been cured, the particle monolayer could be removed from the aqueous gel together with the PDMS layer. The particles embedded in the PDMS surface were imaged with a highresolution scanning electron microscope (HR-SEM) and the three-phase contact angle was determined. Figure 2 shows typical SEM images of the four types of particles, which were adsorbed as described in the air−water and n-decane−water interface. The results of the contact angle measurements are summarized in Table 3.

Table 1. Comparison of Average Particle Diameters and Standard Deviations σ Determined by TEM (DTEM) and DLS (DDLS) Measurements batch

batch

given in Supporting Information Figure S1), and were in excellent agreement with expectations, furnishing proof for the successful incorporation of the hydrophilic comonomers into the PS particles. The as-prepared particles were used as model systems to prove the general applicability of hydrophilic PS particles and to study the coating with silica shells. Therefore, it was important to characterize their surface properties. In the following, the D


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expectations. However, in the n-decane−water interface, the exact opposite behavior was apparent. Here, the pure PS particles showed the smallest contact angle with θ = 74°, which increased from HEMA with θ = 80° to MAA with θ = 87°. Only the particles with MAAm with a contact angle θ = 104° showed very different characteristics and fell out of the correlation. These contact angle measurements thus permitted the differentiation of the surface properties of the different types of particles prepared. The disadvantage, however, is that these properties could not be measured under the conditions of the Stö ber process, since the gelation induced by the polysaccharide can only take place in water. Therefore, no absolute statement could be made about the interaction of the particles with the Stöber alkosol. It can be concluded that, by means of zeta potential measurements and measurements of the three-phase contact angle, clear gradations in the surface properties of the pure and functionalized PS particles could be detected, which indicated that the particles were successfully covered with the diverse hydrophilic functionalities. 3.2. Coating of PS Particles with Silica. In order to investigate the influences of the surface properties of the hydrophilic PS particles onto the coating with silica, the wellestablished Stöber process was used. For this purpose, the particles were transferred from their aqueous dispersion to ethanol containing tetraethoxysilane (TEOS), ammonia and water, in which the TEOS precursor is hydrolyzed and condensed to form amorphous silica. After complete condensation of TEOS, the resulting particle morphology was characterized by using TEM measurements of the pristine as well as of ultrathin sample sections of the particles in order to be able to image the silica shells. Furthermore, the PS cores were additionally burnt from the corresponding hybrid particles at 550 °C, whereby hollow particles were formed and the silica shells could be imaged even better in the TEM investigations. TEM images of the PSMAA, PSHEMA, and PSMAAm particles after silica coating are shown in Figure 3. It should be noted that it was not possible to coat unfunctionalized PS particles. Furthermore, it is worth mentioning that during the coating process of all particles described herein, a small amount of free silica particles was formed, which could be easily removed through controlled centrifugation (SEM images of a representative dispersion before and after separation of free silica particles are given in Supporting Information Figure S4). The corresponding TEM images (Figure 3) proved the success of the silica coating for all three types of particles. Due to the cutting of the sample with a diamond knife the structure was slightly distorted and the corresponding particles appeared deformed in TEM images (Figure 3, middle). However, after removing of the core particles, the resulting hollow silica particles retained their initial shell structure (Figure 3, right). However, clear differences could be observed for the different surface functionalities: perfectly even and smooth, 65 nm thick silica shells grew on the PSMAA particles (Figure 3a). Also, the PSHEMA and PSMAAm particles could be coated with a silica shell, but not with a closed shell; it grew rather a porous and rough shell onto these types of particles. In particular, the silica-coated PSHEMA particles are no longer CS particles, but should be classified as raspberry-type particles. From Figure 3 it can be concluded that hydrophilic PS particles are suitable for the coating with silica in a Stöber-type process. In accordance with the zeta potential and contact angle measurements, a clear gradation between PSMAA, PSHEMA, and PSMAAm particles could be observed, which additionally

Figure 2. HR-SEM images of monolayers of monodisperse colloidal particles on a PDMS surface, which were deposited by the gel-trapping technique in the air−water and n-decane-water interface: (a,b) PS, (c,d) PSHEMA), (e,f) PSMAAm, and (g,h) PSMAA. The scale bars correspond to 500 nm.

Table 3. Comparison of the Contact Angle at the Air−Water θA/W and n-Decane−Water θD/W Interface batch

comonomer

θA/W/°

θD/W/°

459 470 472 463

HEMA MAAm MAA

108 103 99 94

74 80 104 87

The contact angle measurements (Table 3) clearly point out the influence of the hydrophilic functionalities onto the surface properties of the PS particles. As in the case of zeta potential measurements (Table 2), clear differences were also evident (Table 3, Figure 2). Moreover, the adsorption of the particles in the air−water and n-decane−water interfaces differed significantly. In the air−water interface, the pure PS particles provided a contact angle of θ = 108°, which is reduced to θ = 94° from HEMA through MAAm to MAA functionalities. This means that the pure PS particles adsorbed in the air−water interface were most likely in the aqueous phase, contrary to all E


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Figure 3. TEM images of deposited (left) and of ultrathin sections of the colloidal particles (middle) after coating the functionalized PS particles with silica in Stöber process and TEM images of the hollow particles after removing the PS cores at 550 °C (right): (a) PSMAA, (b) PSMAAm, and (c) PSHEMA. The scale bars correspond to 500 nm.

were formed with less NH3, but at all concentrations tested, the shells remained quite smooth. Already at a concentration of 0.15 M, the silica shell was almost closed, but still very thin and slightly rough (Figure 5c), while at a concentration of 0.3 M, an extremely smooth and completely closed shell was formed. At a concentration of 0.6 M, the particles could be coated with a thick, smooth and completely closed silica shell. Thus, it can be summarized that particularly thin shells are available with smaller amounts of NH3. Finally, the concentration of water was varied. As could be verified by the TEM images in Figure 6, increasingly thicker and rougher silica shells were formed with increasing amounts of water. At a water concentration of 3.6 M, the particles could be coated with an extremely smooth and closed silica shell (Figure 6a), while at a concentration of 7.0 M the silica shell formed was much thicker and had a rough surface (Figure 6b). At a concentration of 15.0 M, only the formation of a raspberrytype shell, which consists of spherical silica particles, was formed (Figure 6c). As can be concluded, the coating of PSMAA particles with silica in the Stöber-type process enabled the controlled buildup of uniform PSMAA@silica CS particles with exceptional homogeneous and smooth silica shells, while the thickness, porosity, and surface roughness of the silica shells could precisely be controlled through altering the conditions of the

indicated that the particles were covered with the diverse functional groups. The results demonstrated that the morphology of the final silica shell could be tailored through selective particle surface functionalization. Moreover, it could be confirmed that silica attracts particularly well on PSMAA particles, leading to homogeneous and smooth silica shells. In the next step, the concentrations of TEOS, NH3 and water were varied in order to study the impact on the morphology of the silica shell. First, the concentration of TEOS was reduced. The TEM images of the particles in Figure 4 clearly show that with less TEOS thinner silica shells were formed and the shells became less regular and porous. This can be explained as follows: since the concentrations of NH3 and water were kept constant, it can be assumed that unclosed shells were formed by the adsorption of a seeding layer, which grew together by the addition of further TEOS, thus forming a thicker and more closed shell. At a concentration of 0.03 M, the incomplete structure of the silica shell can be clearly seen (Figure 4c), since the TEOS was not sufficient to complete the shell formation. With a concentration of 0.06 M, a still rough, but almost closed shell was formed (Figure 4b), while a concentration of 0.12 M was sufficient to form a smooth and completely closed shell (Figure 4a). Next, the concentration of NH3 was varied. As could be proven by the TEM images in Figure 5, thinner silica shells F


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Figure 4. TEM images of deposited (left) and of ultrathin sections of the colloidal particles (middle) after coating the PSMAA particles with silica in the Stöber process and TEM images of the hollow particles after removing the PS cores at 550 °C (right) by using (a) 0.12 M TEOS, (b) 0.06 M TEOS, and (c) 0.03 M TEOS. The scale bars correspond to 500 nm.

photograph in Figure 7c, the particles could be homogeneously dispersed in the different solvents, whereby redispersion took place instantaneously. However, the silica particles did not fill with the dispersion medium during redispersion. After 1 h they separated from the dispersion and floated onto the surface of the medium (Figure 7d), which indicated that the silica shells are impermeable for these media. However, this process of dispersing and floating was completely reversible and could be repeated as often as desired. From Figure 7 it can be concluded that the hollow silica particles are highly stable against irreversible aggregation, dispersible in various media and could be readily dried, purified, and redispersed. 3.3. Coating of CdSe/ZnS Nanocrystals with Silica. As a proof of concept and to demonstrate the broader applicability, we applied the presented method based on the selective particle surface modification with hydrophilic groups to highly functional fluorescent CdSe/ZnS nanocrystals, which usually cannot be directly coated with silica. Generally, two different strategies have already been developed to embed single semiconductor nanocrystals in monodisperse spherical silica particles in the size range of 10−100 nm. Several reports can be found in the literature dealing with the encapsulation of single semiconductor nanocrystals in silica spheres by means of waterin-oil65−67 and oil-in-water microemulsions.68 However, the formation of a silica shell by using microemulsion process

sol−gel process. Moreover, all silica shells were stable enough that the particles retained their initial shell architecture after calcination, yielding extremely uniform hollow silica particles. Figure 7 shows a photograph and SEM image of a powder of silica hollow particles after calcination of the PSMAA@silica particles at 550 °C (TGA is given in Supporting Information Figure S5). After thermal decomposition of the PS cores, a white powder of the hollow silica spheres was obtained (Figure 7a), which contained no colored residues of degradation products of PS. As could be proven by the SEM image in Figure 7b, the silica shells remained completely intact and the particles appear translucent, whereby the very homogeneous shell thickness is clearly depicted by the width of the bright rings. Since these hollow particles are extremely uniform and are highly stable, they have great advantages compared to simple particles for a large number of different applications and can be used, e.g., as microcontainers for the immobilization of biochemicals, catalytically active substances and for drug delivery. Practical applications of these materials are currently under investigation. In the next step, it was verified whether the hollow silica spheres can be dispersed in different solvents at different pH values. Therefore, dispersions from the powder of hollow silica particles (Figure 7a) in H2O, aqueous NH3, aqueous HCl, THF, and toluene were prepared. As can be seen from the G


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Figure 5. TEM images of deposited (left) and of ultrathin sections of the colloidal particles (middle) after coating the PSMAA particles with silica in Stöber process and TEM images of the hollow particles after removing the PS cores at 550 °C (right) by using (a) 0.6 M NH3, (b) 0.3 M NH3, and (c) 0.15 M NH3. The scale bars correspond to 500 nm.

transferred to the ethanolic Stöber alkosol. In order to impose the dispersibility in ethanol and to functionalize the nanocrystals with hydrophilic functional groups, the hydrophobic ligands on the ZnS surface were replaced with mercaptopropionic acid (MPA). The use of MPA as hydrophilic stabilizers, rather than silane coupling agents, has several advantages: Apart from preventing irreversible particle aggregation due to chemical processes of surface attached silanes, carboxylate groups on the particle surface lead to a high degree of dispersibility and stability in ethanol and water. The MPA functionalized CdSe/ ZnS nanocrystals could therefore be stored in the dried state, could readily be dispersed in water at pH = 8 for subsequent silica coating, and moreover, the dispersions were stable over long time periods (months). Figure 8a shows a TEM image of the MPA-capped nanocrystals, proving the existence of nonaggregated and separated nanocrystals, which had a spherical shape and uniform sizes. To coat the MPA-capped CdSe/ZnS nanocrystals with silica, they were transferred from their aqueous dispersion to ethanol containing TEOS, dimethylamine, and water. The TEM images in Figure 8b revealed that after the coating process single CdSe/ZnS core were embedded in the silica particles. Moreover, the final products were perfectly spherical in shape and no free silica particles could be observed. It can be concluded that separated silica growth was effectively prevented

requires the introduction of immeasurable large amounts of surfactants as well as organic solvents as the oil phase. Moreover, the size of the silica coatings is restricted to the dispersed domain size in the microemulsion, usually varying from 20 to 80 nm. Alternatively, modified Stö ber-type processes can be used to incorporate semiconductor nanocrystals into silica spheres, which enables an accurate control over the thickness of the silica coating. For this purpose, thiolor amino-functionalized coupling agents, such as 3-mercaptopropyl trimethoxysilane (MPS),39,40,69 11-mercapto-1-undecanol (MUD),67 or 3-aminopropyl trimethoxysilane (APS),70 need to be introduced via ligand exchange to provide the surface with reactive silanol or hydroxy groups for the controlled growth of a homogeneous silica coating. However, besides the additional functionalization step, this method usually either requires a two-step coating process consisting of a slow deposition of a thin silica layer from a silicate solution in water followed by transfer into ethanol to grow thicker shells,69 or is restricted to the formation of very thin functional silica layers based on subsequent silanization with functional silanes, such as MPM,67 MPS,39 or APS.70 In our approach, CdSe/ZnS nanocrystals were prepared by a solvothermal process in organic solvent under the assistance of hydrophobic ligands. The resulting nanocrystals, however, were only dispersible in nonpolar solvents and could not be H


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Figure 6. TEM images of deposited (left) and of ultrathin sections of the colloidal particles (middle) after coating the PSMAA particles with silica in the Stöber process and TEM images of the hollow particles after removing the PS cores at 550 °C (right) by using (a) 3.6 M H2O, (b) 7.0 M H2O, and (c) 15.0 M H2O. The scale bars correspond to 500 nm.

Figure 8. TEM images of CdSe/ZnS nanocrystals (a) before and (b) after coating with silica in the Stöber process. The scale bars correspond to 50 nm.

embedded in the center of the silica shells. The average diameter of the CdSe/ZnS@silica particles was measured to be 80 nm as estimated from the TEM measurements. This method used herein is straightforward and does not need reactive coupling agents, precoating with sodium silicate or additional silanization. The major advantage of the herein presented technique is mainly that the coating of single nanocrystals with thick silica layers can be directly performed in ethanol without the addition of surfactants and other organic solvents, so it is cleaner and does not require subsequent purification steps,

Figure 7. (a) Photograph and (b) SEM image of a powder of silica hollow particles (scale bar corresponds to 2000 nm) after calcination at 550 °C. Photos of dispersions of Silica hollow particles in water, aqueous NH3, aqueous HCl, tetrahydrofuran, and toluene (from left to right): (c) after shaking and (d) after 1 h after leaving untouched.

during the synthesis and a uniform CdSe/ZnS@silica CS structure was formed in which single nanocrystal are precisely I


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embedded nanocrystals. Moreover, the absorption and emission spectra of the CS particles remained essentially unaffected, even after storing the particles in the Stöber alkosol for months under ambient conditions, which is a further proof that the nanocrystals are well protected by the silica shell. The potential use of the prepared CdSe/ZnS@silica particles as fluorescent probe for bioimaging was assessed by confocal laser scanning microscopy (CLSM). A typical CLSM image of the particles is shown in Figure 9b. As can be concluded from Figure 9b, the CdSe/ZnS@silica particles showed very bright fluorescence, although the nanocrystals are coated by a thick silica shell. Nevertheless, the CLSM measurements also revealed that the CdSe/ZnS@silica particles are highly stable against photobleaching, giving rise that herein investigated silica-coated nanocrystals are suitable systems for biolabeling and fluorescence imaging.

however, for the application of herein described approach to CdSe/ZnS nanocrystals, also an additional functionalization step with MPA had to be introduced to provide the particles with hydrophilic surface functionalities. In the next step, the optical properties of the final CdSe/ ZnS@silica particles were investigated by UV−vis absorption and fluorescence emission spectroscopy. Figure 9a shows the

4. CONCLUSIONS In conclusion, we have presented a convenient and efficient synthetic concept for the morphology-controlled coating of colloidal particles of different sizes and compositions with silica. Means of hydrophilic submicroscopic PS particles were synthesized by emulsifier-free emulsion polymerization and a subsequent swelling process, which enabled the selective particle surface modification by the incorporation of hydrophilic comonomers such as MAA, HEMA, and MAAm. Clear gradations in the surface properties of the particles were evidenced both by zeta potential measurements as well as by determining the three-phase contact angle. It could be proven that all types of hydrophilic PS particles are suitable for the coating with silica in a sol−gel process, however, respective surface properties of the colloidal particles strongly influenced the surface properties of the colloidal particles and thus the morphology of the final silica shells. Silica shells of variable thickness, porosity, and controlled surface roughness could be prepared through altering the conditions of the sol−gel process, leading to hybrid PS@silica particles with morphologies ranging from core−shell (CS) to raspberry-type architectures. Moreover, all silica shells formed were stable enough that the initial polymer particles could be removed by thermal treatment, whereby extremely uniform hollow silica spheres of various morphologies could be obtained. It has been found that these particles are highly stable and dispersible in various solvents (H2O, aqueous NH3, aqueous HCl, tetrahydrofuran, and toluene) and could be readily dried, purified, and redispersed, giving rise that herein prepared silica hollow spheres are expected to be promising candidates for application in biology and life science. Furthermore, the herein presented method, based on selective particle surface modification, was successfully applied to highly functional fluorescent CdSe/ZnS nanocrystals. MPAcapped CdSe/ZnS nanocrystals could be directly coated with smooth and spherical silica shells and the corresponding particles revealed a universal CS structure with a single nanocrystal embedded in each silica particle. Moreover, the CdSe/ZnS@silica particles prepared showed excellent fluorescence properties in CLSM measurements, which additionally verified that these particles are highly stable against photo bleaching. We envisage that herein presented method is highly reproducible and applicable for the morphology-controlled silica coating of a broad variety of different nano- and micro sized colloidal particles. Additionally, silica coating based on particle surface hydrophilization is currently being successfully

Figure 9. (a) UV−vis absorption and fluorescence emission spectra and (b) CLSM image (fluorescence mode, excitation wavelength 532 nm, detection window 545−585 nm) of CdSe/ZnS nanocrystals embedded in silica particles (80 nm). The scale bar corresponds to 2000 nm.

UV−vis absorption and fluorescence emission spectra of an ethanolic dispersion of the CdSe/ZnS@Silica particles. In the spectra, the characteristic band gap absorption and emission of the CdSe/ZnS cores at 549 and 598 nm, respectively, could be observed, suggesting that the silica shells did not affect the optical properties of the nanocrystals. The emission spectrum revealed a single and narrow emission peak with a full width at half-maximum smaller than 50 nm, indicating the uniformity of the size and shape as well as the chemical composition of the J


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used for silica encapsulation of metal particles and a fuller account about the general applicability of the presented method will be presented elsewhere.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b04069. DLS measurements of all PS particles prepared (Figure S1), ATR-FT-IR spectra of all PS particles prepared (Figure S2), zeta potential measurements of all PS particles prepared (Figure S3), SEM images of a representative dispersion before and after separation of free silica particles (Figure S4), and TGA of PSMAA@ silica particles (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Changchun Wang: 0000-0003-3183-2160 Christian G. Schäfer: 0000-0003-1250-2721 Author Contributions

The manuscript was written through contributions of all authors and all authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the State Key Project of Research and Development (Grant No. 2016YFC1100300), the National Science Foundation of China (Grant No. 21474017, 51633001) and the Natural Science Foundation of Shanghai (Grant No. 17ZR1440200). C.G.S. thanks Ulrike Kunz (Department of Materials Science, TU Darmstadt) for performing the HR-SEM measurements. The authors thank Dr. Jasmin T. Zahn and P. Hoyer (German Cancer Research Center, Heidelberg) for performing the CLSM measurements.

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ABBREVIATIONS AIBN, azobis(isobutyronitrile); APS, 3-aminopropyl trimethoxysilane (APS); CLSM, confocal laser scanning microscope; CS, core−shell; DLS, dynamic light scattering; HEMA, hydroxyethyl methacrylate; HR-SEM, high-resolution scanning electron microscope; MAA, methacrylic acid; MAAm, methacrylamide; MPA, mercaptopropionic acid; MPM, 3(trimethoxysilyl)propyl methacrylate; MPS, 3-mercaptopropyl trimethoxysilane (MPS); MUD, 11-mercapto-1-undecanol; PMMA, poly(methyl methacrylate); PS, polystyrene; SPS, sodium peroxodisulfate; TEOS, tetraethoxysilane; UV−vis, ultraviolet−visible spectroscopy; ζ, zeta potential

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REFERENCES

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Morphology-Controlled Coating of Colloidal Particles with Silica

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