CoreâShellâCorona Silica Hybrid Nanoparticles Templated by

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Core-Shell-Corona Silica Hybrid Nanoparticles Templated by Spherical Polyelectrolyte Brushes: A Study by Small Angle X-Ray Scattering Haoya Han, Li Li, Weihua Wang, Yuchuan Tian, Yunwei Wang, Junyou Wang, Regine von Klitzing, and Xuhong Guo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02239 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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Core-Shell-Corona Silica Hybrid Nanoparticles Templated by Spherical Polyelectrolyte Brushes: A Study by Small Angle X-Ray Scattering

Haoya Hana,b, Li Lia*, Weihua Wangc, Yuchuan Tiana, Yunwei Wanga, Junyou Wanga, Regine von Klitzingb, Xuhong Guoa,d*

State Key Laboratory of Chemical Engineering, East China University of Science and

a

Technology, 200237 Shanghai, P.R. China Department of Physics, Technical University Darmstadt, Alarich-Weiss-Strasse 10,

b

64287 Darmstadt, Germany Sinopec Shanghai Research Institute of Petrochemical Technology, 201208 Shanghai,

c

P.R. China Engineering Research Center of Materials Chemical Engineering of Xinjiang

d

Bingtuan, Shihezi University, 832000 Xinjiang, P.R. China

*

To whom correspondence should be addressed. E-mail: [email protected] (Li

Li), [email protected] (Xuhong Guo) 1

ACS Paragon Plus Environment

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ABSTRACT: Core-shell-corona silica/polymer hybrid nanoparticles with narrow size distribution were prepared in the template of spherical polyelectrolyte brushes (SPB) which consist of a solid polystyrene (PS) core densely grafted with linear poly (acrylic acid) (PAA) chains. The microstructure of obtained hybrid nanoparticles was studied by small angle X-ray scattering (SAXS) and in combination with dynamic light scattering (DLS) and transmission electron microscope (TEM). The generation of silica shell within the brush is confirmed by the significant increase of the electron density in the shell, and the silica shell showed a unique inner-loose-outer-dense structure, whose thickness is pH sensitive while insensitive to ionic strength as revealed by fitting SAXS data. After dissolving the PS core, hollow silica nanoparticles were obtained and determined by SAXS which should be ideal carriers for pH-triggered drug delivery. SAXS is confirmed to be a powerful method to characterize the core-shell-corona silica/polymer hybrid and hollow silica nanoparticles.

KEYWORDS: Small angle X-ray scattering; Spherical polyelectrolyte brush; Silica; Nanoparticle; Core-shell-corona

INTRODUCTION In the past three decades, the fabrication of hybrid micro- and nanoparticles which are composed of either organic or inorganic cores and shells of different chemical compositions has attracted intense attentions.1-4 Due to their particular properties such as charged surface, increased stability, high surface area, magnetic or optical properties, these core-shell particles are widely applied in drug delivery, catalysis, coatings, hybrid materials, or immobilization of enzymes and proteins.1-4 Among them, core-shell 2


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polymer-silica nanoparticles with a porous morphology and tailored structures have versatile characteristics such as low toxicity, large specific area, mechanical and thermal stabilities, and surface permeability.5-6 By either calcination or decomposition upon exposure to solvents which can dissolve the template cores, hollow silica nanoparticles are obtained and used as carriers in controlled drug release because of their nontoxic nature, adjustable pore diameter, and high specific surface area with abundant Si-OH bonds on the pore surface.7-9 Using polymeric templates for the fabrication of polymer-silica hybrid particles and hollow silica has been well studied.1, 10-21 The choice of the templates, including most used polystyrene colloidal particles,10-16,18,20 polymeric surfactants22 and micelles,5-6 and the preparation methods such as sol–gel reactions13-14, 21, 23-25 and selfassembly techniques1 strongly affects the structure of thus obtained silica shell, which plays an important role in its applications such as the storage, adsorption and separation of functional small molecules, catalyst, drug carriers, and theranostic agents. Although the structure of polymer-silica hybrid particles has been well studied through different methods such as the most commonly used electron microscopy, the inner microstructural details in the wet state remain unclear due to the limitations of this method. For silica hybrid particles fabricated with core-shell-corona templates,5 the polymer corona was difficult to observe by TEM or cryo-TEM due to the dry measuring state and the low contrast of the polymer chain. Dynamic Light Scattering (DLS) was able to determine the hydrodynamic size and the size distribution of the particles but failed to observe the existence of silica layer. Therefore, an efficient method to investigate the inner microstructure of the particles, especially the formulation of the silica shell in solution is highly required.

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Here small angle X-ray scattering (SAXS) was employed to investigate the static and dynamic inner microstructure of silica hybrid particles with a core-shell-corona structure templated by spherical polyelectrolyte brushes (SPB), which consist of a polystyrene (PS) core and a shell of linear polyelectrolyte chains densely grafted onto the core.26-33 Besides the unique advantages such as reliability, economy, nondestructivity and high-efficiency, SAXS is particularly suitable for characterization of silica/polymer hybrid nanoparticles due to three reasons: i) the remarkable contrast of excess electron density between silica (325.0 nm-3) and polymers (6.4 and 44.6 nm-3 for polystyrene and poly(acrylic acid), respectively) allows us to carry out the measurements at low concentrations in order to disregard the interactions of the particles,34-36 ii) the very narrow size distribution of the particles ensures us to observe the characteristic oscillations of spheres on SAXS curves, iii) the excellent dispersibility and stability of the particles allows us to observe the structure of single particles. By building the radial distribution model of the electron density, a clear image of the mass density distribution of the silica/polymer hybrid nanoparticles, the inner microstructure of silica shell, and the structural variation upon changing pH or salt concentration can be obtained, which has not been systematically studied to the best of our knowledge. This work shows that SAXS is a powerful and efficient method to characterize the silica/polymer hybrid and hollow silica nanoparticles, which are ideal candidates for controlled drug delivery with an inner-loose-outer-dense structure and pH sensitivity.37

EXPERIMENTAL SECTION Materials 4


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Styrene, acrylic acid (AA), potassium persulfate (KPS), sodium dodecyl sulfonate (SDS), sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium chloride (NaCl), and ammonia solution (NH3·H2O, 25 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Tetraethoxysilane (TEOS), absolute ethanol (C2H5OH, 99.5%), trichloromethane (CHCl3) and N, N'-methylenebisarylamide (BIS) were purchased from J & K Chemical. Methacryloyl chloride (MC) was purchased from Tokyo Chemical Industry Co., Ltd. 2-Hydroxy-4’-hydroxyethoxy-2-methyl propiophenone (HMP) was purchased from Acros Organics. Styrene and AA were destabilized by reduced pressure distillation and stored in 4 C. KPS and SDS were recrystallized from deionized water. Water was purified using reverse osmosis and ion exchange in a Millipore Milli-Q system. All other chemicals were used as received without further purification.

Photo-initiator

(2-[p-(2-hydroxy-2-methylpropiophenone)]-ethylene

glycol-methacrylate) (HMEM) was synthesized as presented in our previous publication.38

Preparation of SPB Synthesis of SPB consisted of three main steps: thermal-emulsion polymerization of PS core, covalent attachment of photoinitiator and photo-emulsion polymerization to graft PAA chains. Typically, 0.24 g SDS as a surfactant and 0.6 g KPS as initiator were dissolved in 140 g pure water followed by the addition of 10 g styrene monomer into the system. After 30 min mechanical stirring and equilibration, the polymerization process was carried out in 80 C under a nitrogen atmosphere with continuous stirring (300 r/s) for 1 hour then cooled down to 70 C which is appropriate for the attachment of photo-initiator. Then 1 g HMEM dissolved in 8 g acetone was added in a “starved” 5


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condition (6s per drop). After another 1 hour, a thin shell of photo-initiator covering the PS surface was formed by the copolymerization of styrene and HMEM. The core latex was then purified by filtration to remove agglomerate and dialysis against vast pure water to remove excess surfactant. 100 g PS core latex (3 wt%), 3 g AA and 300 g water were added into a homemade photo-reactor. PAA chains were densely grafted from the surface of PS cores by photo-emulsion polymerization in UV radiation under a nitrogen atmosphere with vigorous stirring. After the reaction, the latex was purified by dialysis and ultrafiltration to remove excess monomer and free PAA chains.

Preparation of Silica Hybrid Nanoparticles SPB were used as templates to prepare silica hybrid nanoparticles. 10 g SPB latex (1 wt%) and 1.8 mL ammonia water were dispersed in 90 mL ethanol under vigorous stirring. 0.8 g TEOS dissolved in 5 mL ethanol was then added dropwise (2.4 ml/h) into the emulsion with the help of a homemade microscale sampling pump and kept stirring for 16 h at room temperature. The product was separated by centrifugation and washing with ethanol and water alternatively for 5 rounds to remove the solvent and free silica particles and then freeze dried.

Preparation of Hollow Silica Nanoparticles 0.2 g as-obtained powder was added into 20 mL CHCl3 under slightly stirring for 48 h to make sure the PS was entirely dissolved. The product was centrifuged and washed with ethanol, hydrochloric acid solution (1 mM) and water alternatively for 5 times and finally freeze dried.

Characterization 6


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SAXS measurements were performed at the Shanghai Synchrotron Radiation Facility (SSRF, BL16B1 beamline, Shanghai, China). The incident wavelength was 0.12 nm and the sample to detector distance was set as 5 m. With a 2D-CCD detector, the scattering information was recorded in the scattering vector q-range of 0.05-0.8 nm1

, between which sufficient information about the particles could be acquired. All

solutions were injected into cells having an optical path length of 1 mm, front and back of which were covered by two pieces of polyimide film. All SAXS data was azimuthally averaged and corrected for the background of the solvent and the empty cell. The accumulation time is 200 s for all samples, where enough intensity can be acquired with no harm to the detector. The data was further corrected for detector response, sample transmission and normalized to absolute intensity. Multiple pure water samples were measured at 25 C for absolute intensity calibration.39 Latex concentrations used in SAXS measurements were low (ca. 1 wt%) to minimize the effect of particle interaction. The ionic strength of all aqueous solutions was tuned through the addition of NaCl. The pH of solutions was adjusted with NaOH or HCl standard aqueous solutions. It should be kept in mind that the pH was adjusted while keeping the ionic strength constant.32 The particles were characterized in advance by TEM (JEOL-2100F), DLS (PSS Nicomp 380) and TGA (WRT-2P) to acquire information about their size, the morphology and the mass ratio of the component, etc.

THEORY The analysis given here follows the lines reported recently.34-35,40-46 For a suspension of monodisperse particles with spherical symmetry, the scattering intensity as function of the scattering vector q ( 𝑞 = (4𝜋/𝜆)sin⁡(𝜃/2) ), 𝜆 (wavelength of 7


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incident beam), 𝜃 (scattering angle) is given by 𝐼(𝑞) = 𝑁𝑃(𝑞)𝑆(𝑞)

(1)

where N is the number density, P(q) is the scattering intensity of an isolated particle and S(q) denotes the structure factor which takes into account the mutual interaction of the particles. Previous articles demonstrated that the structure factor is only obvious at the smallest q region for systems of spherical particles with high concentration.47-48 Since samples concentration in our analysis is low (< 1 wt%) and the measuring q values are no less than 0.03 nm-1, the effect of structure factor on scattering intensity can be safely neglected. We measured the same sample at different concentrations and observed no particle interaction at low concentrations. For example, PS latex were measured at concentrations of 0.43, 0.87, 1.74, and 3.47 wt %, where obtained SAXS curves overlap significantly after normalizing the concentration by dividing each value, indicating no observable particle interactions exists for PS sample below 3.47 wt %. We assume S(q) = 1 in our work, which means the particle innerstructure can be obtained by analyzing scattering data. The scattering intensity of an isolated silica hybrid particle can be decomposed in principle into three independent contributions:30-31, 35, 40, 42-45 𝐼0 (𝑞) = 𝐼𝑐𝑠 (𝑞) + 𝐼𝑠ℎ𝑒𝑙𝑙 (𝑞) + 𝐼𝑃𝑆 (𝑞)

(2)

Ics(q) is the contribution of the overall core-shell structure of the particle, i.e., the scattering intensity caused by a composite particle having a homogeneous core and homogeneous shells. The core and the shells are characterized by a different electron density shown schematically in Figure 1a. The shell consists of a inhomogeneous silica and polymer chains (Figure 1b) whose inhomogeneity is taken into account by the 8


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second term 𝐼𝑠ℎ𝑒𝑙𝑙 (𝑞). The final term 𝐼𝑃𝑆 (𝑞) is due to the density fluctuation of the solid PS which is neglected in our discussion.49 It should be noted that all contributions originate from statistically independent fluctuations.30,40 In the case of a spherical particle, Ics(q) is given by B2(q) with the scattering amplitude B(q) being defined as30, 48 𝐵(𝑞) = 4π ∫ 𝑏[𝜌(𝑟) − 𝜌𝑚 ]

sin 𝑞𝑟 𝑞𝑟

𝑟 2 𝑑𝑟

(3)

here b is the Thomson scattering length (b=0.282*10-14 m), [𝜌(𝑟) − 𝜌𝑚 ] denotes the radial excess electron density along the radial distance 𝑟, i.e., the contrast between the radial electron density of the particle⁡ 𝜌(𝑟) and the electron density of solvent (water) 𝜌𝑚 . Based on our former ‘five layers’ model35, here a ‘double-shell’ model was established to analyze silica hybrid nanoparticles as schematically shown in Figure1b: an inner shell with regards to the silica-PAA complex and a PAA shell in regards to the PAA chain segments out of the silica-PAA complex which results in a corona structure. The radius of the PS core is set as r0 while the silica shell is radially divided into five layers, with 𝑟𝑖 (1 ≤ 𝑖 ≤ 5) representing the outer radius of the ith silica layer. The PAA corona is divided into only two layers with 𝑟𝑗 (6≤ 𝑗 ≤ 7) representing the outer radius of jth PAA layer. The silica shell is more subtly divided due to its much higher excess electron density which results in its domination on the scattering intensity. Thus, [𝜌(𝑟𝑖 ) − 𝜌𝑚 ], [𝜌(𝑟𝑗 ) − 𝜌𝑚 ] respectively indicate the excess electron density of the ith or jth layer. (𝑟5⁡ -⁡ 𝑟0 ) and (𝑟7 ⁡ - 𝑟5 ) respectively represent the thickness of the silica shell and of the PAA shell.

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Figure 1. Double-shell fitting model of silica hybrid nanoparticles. (a) Radial excess electron density [𝜌(𝑟) − 𝜌𝑚 ] distribution of the particle. The x-axis is the radial distance r representing the distance from the center to local positions of silica hybrid nanoparticles. The y-axis refers to the excess electron density of individual layers with respect to water; (b) Corresponding schematic illustration of the double-shell model. The color yellow, dark blue, pupil and light blue refer to polystyrene core, silica shell, PAA shell, and water, respectively.

The B(q) of a silica hybrid nanoparticles is the sum of the scattering amplitude of the PS core (B0(q)) and that of the double shell (Bi(q) and Bj(q)). Thus, 𝐼𝑐𝑠 = 𝐵 2 (𝑞) = [𝐵0 (𝑞) + 𝐵𝑖 (𝑞)+𝐵𝑗 (𝑞)]2

(4)

where36, 43 𝐵0 (𝑞) = 4π𝑏[𝜌(𝑟0 ) − 𝜌𝑚 ]

sin 𝑞𝑟0 −𝑞𝑟0 cos 𝑞𝑟0 𝑞3

10


(5)

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𝐵𝑖 (𝑞) = ∑51 4π𝑏[𝜌(𝑟𝑖 ) − 𝜌𝑚 ]

(sin 𝑞𝑟𝑖 −𝑞𝑟𝑖 cos 𝑞𝑟𝑖 )− (sin 𝑞𝑟𝑖−1 −𝑞𝑟𝑖−1 cos 𝑞𝑟𝑖−1 )

𝐵𝑗 (𝑞) = ∑76 4π𝑏[𝜌(𝑟𝑗 ) − 𝜌𝑚 ]

𝑞3 (sin 𝑞𝑟𝑗 −𝑞𝑟𝑗 cos 𝑞𝑟𝑗 )− (sin 𝑞𝑟𝑗−1 −𝑞𝑟𝑗−1 cos 𝑞𝑟𝑗−1 ) 𝑞3

(6) (7)

Figure 2. Decomposition of the scattering intensity of silica hybrid nanoparticles. The circles represent experimental data. Lines denote the fitting result (solid line), the spatial inhomogeneity of the silica shell (long dashed line) and the fluctuations of the PAA shell (short dashed line), respectively. The radial excess electron density profile is shown in the inset.

The second term Ishell(q) is caused by the spatial inhomogeneity of the shell as mentioned above, which consists of static spatial inhomogeneity of the silica shell Iin(q) and thermal density fluctuation of the PAA chains Ifluct(q).40 The presented data are described in good approximation by an empirical decomposition as follows30, 40, 50-53 𝐼𝑠ℎ𝑒𝑙𝑙 (𝑞) = 𝐼𝑖𝑛 (𝑞)+𝐼𝑓𝑙𝑢𝑐𝑡 (𝑞) = 𝐼𝑖𝑛 (0) exp(−𝑟𝑔2 𝑞 2 ) + 11


𝐼𝑓𝑙𝑢𝑐𝑡 (0) 1+𝜉 2 𝑞 2

(8)

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Here Iin(0) and Ifluct(0) are treated as adjustable parameters in our analysis. rg denotes the radius of gyration of the static inhomogeneity (see Figure 2) and is found to be of the order of 10 nm whereas 𝜉 is the correlation length of the spatial correlations of the chains and is of the order of a few nanometers. The contribution of Iin(q) and Ifluct(q) are 2-5 orders of magnitude lower compared with that of Ics(q) at small scattering angles (q < 0.2 nm-1). Both Ics(q) and Iin(q) decrease rapidly with increasing q and are greatly diminished at high scattering angles (q > 0.5 nm-1), where Ifluct(q) become dominated. To describe the small but non-negligible degree of variance of both the core size and shell thickness Gaussian distributions are introduced as follows 1

𝑝(𝑟; 𝑟̅ ; 𝜎) = 𝜎√2𝜋 exp⁡(−

(𝑟−𝑟̅ )2 2𝜎2

)

(9)

where 𝑝(𝑟; 𝑟̅ ; 𝜎) is the probability density of particles with a characteristic parameter 𝑟, 𝑟̅ and 𝜎 denote the mean and the standard deviation of 𝑟. Thus, 2 𝑟0 ~𝑁(𝑟̅0 , 𝜎𝑃𝑆 ); 2 (𝑟5 − 𝑟0 )~𝑁(𝑟̅5 − 𝑟̅0 , 𝜎𝑠𝑖𝑙𝑖𝑐𝑎 ); 2 (𝑟7 − 𝑟5 )~𝑁(𝑟̅7 − 𝑟̅5 , 𝜎𝑃𝐴𝐴 )

namely the core radius 𝑟0 the silica shell thickness ( 𝑟5 − 𝑟0 ) and the PAA shell thickness ( 𝑟7 − 𝑟5 ) meet the Gaussian distribution with 𝜎𝑃𝑆 , 𝜎𝑠𝑖𝑙𝑖𝑐𝑎 and 𝜎𝑃𝐴𝐴 indicating the corresponding standard deviations, respectively.

RESULTS AND DISCUSSION Generation of Silica Shell

12


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The generation of the silica shell in SPB is schematically shown in Figure 3. SAXS curves of SPB and silica hybrid nanoparticles at the same particle number density are compared in Figure 4. A dramatic increase in the scattering intensity is observed, which is attributed to the high electron density of silica compared to the polymer template, i.e., the value (325.0 nm-3) of the former is one order of magnitude higher than that (44.6 nm-3) of the latter. Thus, the significant increase of the scattering intensity indicates the generation of silica.

Figure 3. Schematic diagram of the fabrication of silica hybrid nanoparticles and hollow silica nanoparticles.

The prominent oscillations of SAXS curves indicate that both the template SPB and the hybrid particles are in spherical shape and with very narrow size distributions. There is no obvious smear of the minimums of the SAXS curves after the generation of 13


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silica, giving us a clue that the generated silica locates within or closely surrounds the template particles evenly instead of existing as small free silica particles in the ambient solution. Silica and SPB appear as a whole, otherwise, the minimums of the SAXS curves will be smeared by the free silica nanoparticles with high electron density.

Figure 4. SAXS results of PS, SPB, and silica hybrid nanoparticles. The squares (□) and circles (○) represent the experimental scattering intensities of SPB and silica hybrid nanoparticles, respectively. The solid lines are the respective fitting results. The inset shows the radical distributions of excess electron density of SPB (red) and silica hybrid nanoparticles (black). Samples are measured at pH = 7 and [NaCl] = 10 mM.

As is well known, ammonium ions act as the catalyst for the precipitation of silica precursor to form amorphous silica in basic solutions.33 The template particles SPB are 14


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grafted with anionic PAA chains, whose carboxyl groups are extensively ionized by the addition of ammonium. Due to the Donnan effect54-55, mostly positively charged ammonium ions are absorbed as counterions around PAA chains and act as nucleation centers for the silica precursors. The precipitation should mainly take part inside the brush layer rather than in the ambient solution. As a result, silica shell is generated within the brush layer of the template SPB.

Figure 5. TEM photographs of (a) SPB; (b) silica hybrid nanoparticles; (c) enlarged silica hybrid nanoparticles; (d) hollow silica nanoparticles. The insets are corresponding schematic diagrams.

The generation of silica within SPB is confirmed by TEM as shown in Figure 5. Obviously, the size distributions of the template and the hybrid silica particles are narrow. The silica shell can be determined by comparing Figure 5a and b as silica has 15


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higher contrast than that of polymers. The PAA brush layer is hardly visible by TEM since the PAA chains were collapsed onto the PS core surface when drying samples for TEM measurement. Therefore, the size determined by TEM is actually the size of the PS core, which is around 47 nm in radius averagely (Figure 5a). The average size of silica hybrid nanoparticles is ca. 67 nm in radius (Figure 5b), which means the thickness of silica shell around 20 nm.

100

Intensity (a.u.)

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50

0 100

1000

Diameter (nm)

Figure 6. Hydrodynamic size and size distribution of PS (yellow), SPB (blue) and silica hybrid nanoparticles (red) determined by DLS at pH = 7, [NaCl] = 10 mM. The mean diameters derived from the Gaussian fits (solid lines) are 104, 303, and 230 nm, respectively.

From the excess electron density profile in the inset of Figure 4, parameters on the size and the electron density profile of the particles were obtained. The statistic average shell thickness decreased after coating silica. It is confirmed by DLS at the same pH and salt concentration in Figure 6. The sizes of all the three kinds of particles 16


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follow the Gaussian distributions. The shell thickness of silica hybrid nanoparticles (63 nm) seems much smaller than that of SPB (100 nm). However, the hydrodynamic size obtained by DLS55-57 is always larger than that determined by SAXS. SAXS focuses on the electron density distribution or mass distribution information while DLS reflects the contour of the particles.

Interestingly, the electron density of silica hybrid nanoparticles increased gradually from the core surface to the edge of silica layer in a radial direction (inset in Figure 4). Since the electron density is in direct proportion to the mass density for a certain matter, the mass density of silica is therefore the lowest in the vicinity of the PS core surface and the highest at the edge of the silica shell. The number density of PAA chains segments or carboxyl groups in SPB is the highest on the PS core surface and decreases radially outward due to the spherical geometry. As a result, the concentration of the counterions (ammonium ions) decays radially outward. The ammonium ions act as the catalyst in the sol-gel process, so the reaction rate in the inner area of the shell should be higher than that of the outer one at the early stage. As a result, the free space at inner SPB shell will be easily blocked by the loose accumulation of silica seeds after nucleation. The grating density of PAA chains on the core is in the range of 0.03-0.06 nm-2,55 which means the average distance between the neighbor chains on the core surface is 4-6 nm. Considering the chains have certain degrees of curling, the space for the silica seeds is even less. Along the radial direction the distance between chains increases due to the spherical geometry of the shell, giving more space for silica seeds to pack closely and form a network.

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Once the silica network is formed, it appears as a barrier for the reactant TEOS to diffuse through it. In other words, the outer silica layer obtains more TEOS supply than the inner one. The part near the PS core, however, grows relatively slow or even be frozen despite its high catalyst concentration, since most TEOS has no chance to penetrate the silica network and diffuse into this area. Finally, an inhomogeneous silica layer is formed, whose density increases gradually from inside to outside. The interesting electron density profile (inset in Figure 4) of the hybrid nanoparticles results from the inner-loose-outer-dense structure of silica shell. It is also worth noting that the excess electron density of silica as shown in the inset of Figure 4 is no more than 210 e/nm3, which is much lower than the value of pure silica (325.0 e/nm3), indicating the lower density of the generated silica shell compared to the common silica. This is easy to understand since the silica is generated in SPB and mixed with PAA chains which have the much lower mass density or electron density. Obviously, the obtained silica layer templated by SPB is relatively porous, while this unique inner-loose-outer-dense structure as revealed by SAXS is well suited for drug delivery since the outer dense silica layer is ideal as a protective shield while the inner loose layer has more space to load drugs.

Effect of pH Figure 7 displays the SAXS curves and the corresponding fitting results of silica/polymer hybrid nanoparticles at pH of 5, 7, and 9. At the acidic condition (pH 5), the carboxyl groups in PAA chains are only slightly or partially deionized and the corona of the hybrid particles are in a collapsed state. With the increase in pH to the neutral condition (pH 7), the carboxyl groups further deionized. The negatively charged 18


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PAA chains repel each other to form a more swelling state, resulting in the increase of thermal fluctuation and thus the scattering intensity, especially at relatively high q values. When the pH is further increased to the basic condition at pH 9, the scattering intensity increases only slightly. As shown Figure 7b, the electron density distribution changed significantly with pH. When pH increased from 5 to 7, the silica hybrid layer seems to move outward, and the thickness of the silica shell increases by ca. 2 nm, indicating the silica shell expands along with the stretch of the PAA chains. The electron density of the inner silica layers decreased, while the most compact outmost layer kept unchanged. As discussed above, the inner silica layers are relatively looser compared to the outer ones. The expansion of the silica shell mainly happens in the loose inner layers rather than the compact outer ones. When the pH is further increased to 9, the thickness of the silica shell increased to a smaller extent (ca. 1.5 nm), and the electron density of the whole silica shell decreases, even for the compact outmost silica layer which is probably due to the partial dissolution of the silica at the base condition. By comparing the fitting results as shown in Figure 7, the minimums shift obviously towards the small q values upon increasing pH, indicating an increase of the gyration radius. It is mainly due to the expansion of the silica shell with high electron density which increases the radius of gyration, which confirms that the silica shell possesses a porous structure and pH sensitivity, ensuring its potential applications in such as controlled drug delivery.

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Figure 7. (a) SAXS curves and fitting results of silica/polymer hybrid nanoparticles at different pH, the inset shows the locally enlarged SAXS curves; (b) Respective profiles of the radial excess electron density distributions. Symbols denote: (△) pH = 5, (○) pH = 7; (□) pH = 9. All measurements are carried out at the same mass fraction of 0.1 wt% and the same salt concentration ([NaCl] = 10 mM).

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Figure 8. Shell thicknesses of SPB and silica hybrid nanoparticles as a function of pH determined by SAXS and DLS with a salt concentration of 10 mM. Symbols denote: SPB by DLS (○) and SAXS (□); silica hybrid nanoparticles by DLS (△) and SAXS (▽). Dashed lines are to guide the eyes. The radius of PS core is 48 nm measured by SAXS.

The shell thicknesses of SPB and silica/polymer hybrid nanoparticles at different pH determined by SAXS and DLS are compared in Figure 8. Both methods showed the same change trend for both SPB and silica/polymer hybrid nanoparticles, i.e. the shell thickness increased with the increase of pH. The thicknesses determined by SAXS were lower than those by DLS for both SPB and silica/polymer hybrid nanoparticles because the hydrodynamic size measured by DLS is always larger than the radius determined by SAXS as discussed above.

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Upon increasing pH 3 to 7, the thickness measured by DLS seemed more sensitive and increased more obviously than that measured by SAXS for both SPB and silica/polymer hybrid nanoparticles (Figure 8). It is worth to note that samples for DLS measurements were highly diluted to avoid particle interaction while those for SAXS were in relatively high concentration (ca. 1 wt%) in order to obtain enough scattering singles. The silica/polymer hybrid nanoparticles became unstable at pH over 10.5 due to the partial reaction and corrosion of silica as mentioned above. At pH 11 precipitation occurred during SAXS measurement for silica/polymer hybrid nanoparticles. To sum up, the hybrid silica with pH-triggered swelling and dissolution should be ideal candidates for controlled drug release.

Effect of Salt Concentration The SAXS curves of silica/polymer hybrid nanoparticles at different NaCl concentrations are shown in Figure 9. Upon increasing salt concentration from 1 to 100 mM, the scattering intensity increased very slightly and the minimums shifted inconspicuously towards small q values. Considering that the carboxyl groups were not fully dissociated at pH 6.5 (see Figure 8), the added salt most probably enhanced the carboxyl dissociation.52 Meanwhile, the captured counterions in the shell led to the increase in osmotic pressure and thus the slight swelling of the hybrid silica layer, as well as the electron density of the layers.

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Figure 9. SAXS curves and fitting results of silica hybrid nanoparticles at different NaCl concentrations. Symbols denote: (△) 1 mM; (□) 10 mM; (○) 100 mM. All measurements are carried out at the same mass fraction of 0.1 wt% and pH 6.5. The respective profiles of the radial excess electron density distribution are shown in the inset.

As shown in the electron density profile in Figure 9, the thickness of the silica shell increased by less than 1 nm upon increasing the salt concentration from 1 to 100 mM, indicating that the effect of salt concentration on the thickness of the silica shell is less prominent compared to that of pH. Therefore, silica shell can be considered as resistant to the change of salt concentration, making it a promising carrier for drug delivery.

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Hollow silica nanoparticles were obtained by dissolution the PS core (Figure 5d) and characterized by SAXS (Figure 10). Comparing to the silica/polymer hybrid nanoparticles, several differences are observed with the absence of the PS cores. The hollow structure is confirmed by the zero electron density profile in the center (Figure 9b). The size of the cavity is smaller than the PS core, indicating that after removing the core the cavity is partially filled with polymer chains or extended silica shell. In Figure 10b, for hollow silica nanoparticles, the electron density between 23 to 48 nm is higher than that of the solvent water, which is probably attributed to the inward stretch of the PAA chains. With more PAA chain segments ‘released’ from the silica shell, the thermal fluctuations become more obvious as shown by the increased scattering intensity at large q values (q > 0.4 nm-1). As shown in Figure 10b, the thickness of hollow silica nanoparticles is larger than that of the silica layer before removing PS core, indicating the expansion of silica shell. The shell expands rather inwards than outwards. It is easy to understand that without hindering of the core the loose packed inner silica layers are prone to inwards expansion. This inward expansion is analogous to the shrink of silica hybrid nanoparticles, which has been reflected from the shift of the minimums of the SAXS curves to smaller q values (Figure 10a). It confirms that the obtained hollow silica nanoparticles keep the inner-loose-outer-dense structure and can be ideal carriers for controlled drug delivery.

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Figure 10. (a) SAXS curves and fitting results of hollow silica nanoparticles and silica hybrid nanoparticles. The inset shows the locally enlarged SAXS curves; (b) Respective profiles of the radial excess electron density distributions. Sample concentrations are normalized and pH adjusted to 7. 25


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CONCLUSIONS We found that SAXS is a very suitable and powerful method to observe the microstructures of silica/polymer hybrid nanoparticles and hollow silica nanoparticles. The generation of silica within the template SPB was observed by the dramatic increase of scattering intensity. A unique inner-loose-outer-dense structure of the obtained silica shell was determined by SAXS. The silica/polymer hybrid nanoparticles showed pH sensitivity, while insensitive to the change of salt concentration. After dissolving the PS core, hollow silica nanoparticles were obtained and characterized by SAXS, which should be ideal carriers for pH-triggered drug release.

ACKNOWLEDGMENT We gratefully thank the financial support from the NSFC Grants (51273063 and 21476143), the Fundamental Research Funds for the Central Universities, 111 Project Grant (B08021), Shanghai Synchrotron Radiation Facility, and the China Scholarship Council.

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