A facile strategy to fabricate covalently linked raspberry-like nanocomposites with pH and thermo tunable structures

Abstract

The preparations of covalently linked raspberry-like composite particles often suffer from uncontrolled particle shape and surface morphology, tedious reactions to introduce surface reactive groups, inefficient inter-particle reactions, and rigorous requirements for the formation of hierarchical structure. In this study, we developed a facile strategy to fabricate a kind of size-controlled, positively charged, alkoxysilanes-functionalized nanoparticles (Tsi-PDMAEMA-PSt NPs) via a combination of the ability of RAFT polymerization to design macromolecular architectures and the process of polymer self-assembly to produce well-defined NPs. Tsi-PDMAEMA-PSt NPs can effectively deposit on the outer surface of negatively charged silica microspheres and then form stable silica@polymer particles by the reaction between alkoxysilanes and surface silanols. The surface morphology, particle size, ζ-potential, structure stability as well as pH and thermo-responsiveness of the prepared composite particles were investigated. The results indicated that the prepared silica@polymer particles possessed unique raspberry-like surface structures with high stability and controllability. Moreover, the surface morphology and dispersion state of silica@polymer particles in water can respond to the change of pH and temperature. Consequently, considering the high simplicity and controllability, the design herein provided a promising route to prepare the long-stable raspberry-like composite microspheres with unique surface morphologies and stimuli-responsive properties for a wide range of possible applications.

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Introduction

In the past several decades, the organic–inorganic nanocomposites with hierarchical structure and specific surface have gained a great amount of attention, as this kind of materials not only possess the physicochemical properties of both the organic and inorganic phase, but also surmount the individual shortcomings and give rise to synergistic functions or unique morphologies that are absent in the individual components.^1–5 Among various nanocomposites, the raspberry-like core–shell composite nanoparticles (NPs), consisted of a central sphere surrounded by smaller particles, have attracted tremendous interest due to their unique morphologies, uneven surfaces with high roughness, large specific surface area and high light-scattering ability.^6–10 These raspberry-like NPs have witnessed their widespread applications in a verity of fields, such as heterogeneous catalysis,^6,7,11–13 optical, magnetic and electrical materials,^14–16 delivery and controlled release of therapeutic agents,^17,18 anti-biofouling or anti-fogging coating,¹⁹ especially for the fabrication of superhydrophobic or superhydrophilic surfaces.^9,20–26

A number of approaches have been developed for the preparation of the raspberry-like composite particles. Generally speaking, these preparation approaches can be classified into three types according to the formation process of raspberry-like colloidal particles.^27,28 (1) The big central particles are generated in advance, and then the smaller corona particles are in situ formed on the surface of the core particles by various emulsion polymerization technologies or silica nucleation via a sol–gel process. This strategy involves a tedious production process and often suffers from uncontrolled particle structure and size.^28,29 (2) The small corona particles are present in advance, subsequently the large central spheres are in situ formed by Pickering emulsion polymerization. This route is obviously limited due to the unavoidable presence of coalescence and flocculation, the requirement of a long equilibration process, and the difficulty of getting a desired size.^28,30 (3) Both the small corona particles and large core particles are prepared in advance, and then the smaller corona particles are anchored onto the larger core particles by a self-assembled heterocoagulation process based on the interactions between the binary particles,^27,28 such as hydrogen bonding interactions, electrostatic interactions and hydrophobic interactions. Undoubtedly, the self-assembled heterocoagulation of large and small particles should be the most simple and widely used route for the synthesis of raspberry-like composites in large scale because of its versatility, simplicity and controllability.^31–35 However, the weak interactions between the associated particles always result in the irreversible loss of the surface structures and functionalities in routine use.^33–38 As a result, some heterocoagulated chemical reactions between the core and corona particles, such as amine–epoxy coupling reaction,^25,39–41 amine–aldehyde reaction,⁴² and pyridinium reaction,⁴³ are developed to construct stable raspberry-like particles after hetero-assembly of ready-made particles. Nevertheless, these procedures generally involve surface modifications to introduce reactive groups on the surface of both the core and corona particles. The surface modifications often require drastic reaction conditions, multistep reactions and tedious purification. In addition, the rigorous requirements for the formation and maintenance of hierarchical structure, and especially the very low efficiency of inter-particle reactions also impeded their applications in practice.^40,42,43 Therefore, challenges still remain to develop simple and efficient methods for the preparation of the hierarchical particles with strong mechanical stability and durability.

A variety of polymeric particles and inorganic nanomaterials have been used to construct raspberry-like particles. Among the various nanomaterials, silica-based particles are of greatest significance, largely due to their excellent biocompatibility, chemical inertia, high thermal and mechanical stability, facile preparation and easy functionalization.^44–46 For example, the in situ silica mineralization from different precursors allows for the deposition of silica onto the shell and formation of corona nodules to produce raspberry-like morphology.^{9,10,20,47–50} However, silica self-nucleation often causes uncontrolled morphology and size.^48–51 In addition, the silica particles dispersed in aqueous solution have a negative surface charge, as the isoelectric point of silica is in the range pH 2 to 3.^51–54 Apparently, the electronegative silica particles as corona particles can deposit onto the outer surface of positively charged microspheres.^9,24,55 They also can serve as core particles to adsorb the electropositive corona particles.^17,39,56,57 As a result, silica particles are the most popular candidate to form hierarchical nanospheres by the self-assembled heterocoagulation. More importantly, the presence of silanol (Si–OH) groups on the surface of silica particles can not only help to immobilize substances through hydrogen bonds (≡Si–O–H⋯OR),^46,58 but also provide anchoring sites for the covalent modification of the silica surface. Especially, the surface Si–OH groups can effectively react with hydrolyzed organotrialkoxysilanes to form stable siloxane bonds (Si–O–Si) under a mild condition in the absence of other reactants, which have been widely used for surface chemical modification of silica-based substrates.^59–61 Consequently, considering the aforementioned advantages, the combination of silica particles and organotrialkoxysilanes-functionalized particles would provide a facile and efficient approach to covalently heterocoagulate hierarchical particles for the construction of stable and durable raspberry-like particles with well-defined morphologies and functions. However, quite surprisingly, to the best of our knowledge, there appears to be no study that has been conducted on the preparation of stable raspberry-like particles through the combination of heterocoagulation and condensation reaction between the silanol groups and organotrialkoxysilanes. One of the main reasons for this phenomenon may be the extremely low efficiency of inter-particle reactions.^40,42,43

Herein we developed a facile strategy to covalently anchor polymer NPs onto the surface of silica for the fabrication of long-stable raspberry-like composite particles. Firstly, the trimethoxysilane-end-capped poly(dimethylaminoethyl methacrylate)-block-polystyrene (Tsi-PDMAEMA-PSt) were readily prepared via a RAFT polymerization in the presence of a trialkoxysilanes-functionalized RAFT reagent (benzyl (3-trimethoxylsilylpropyl)trithiocarbonate, BTPT). The amphiphilic Tsi-PDMAEMA-PSt can self-assemble in water to obtain the positively charged trialkoxysilanes-functionalized polymer NPs, which are able to effectively deposit on the surface of negatively charged silica microspheres due to electrostatic heterocoagulation and then form the covalent bonds between alkoxysilanes and surface silanols. As shown in Scheme 1, the whole process required neither surface modification for silica particles and polymer NPs nor additional stabilizer or surfactant. The trialkoxysilanes-functionalized polymer NPs play multiple roles. The cationic hydrophilic PDMAEMA chains, on the one hand, have a high adsorption affinity on core silica particles because of the electrostatic interactions and hydrogen bonds.^46,58 On the other hand, PDMAEMA as a typical kind of pH and thermoresponsive polyelectrolyte will render this dual-responsive property to the resultant raspberry-like particles, allowing for conveniently tuning particle size and surface functions. More importantly, the trialkoxysilanes groups locate at the end of the flexible PDMAEMA chains that are adherently attached on the outer surface of silica particles, thereby facilitating contact between the trialkoxysilanes and surface silanols. And thus the inter-particle condensation reaction will possibly occur according to the intermolecular mode, allowing for the efficient formation of the covalent linkages. As a result, the prepared raspberry-like silica@polymer core–shell particles not only possessed excellent stability without loss of the polymer NPs coatings even after ultrasonication, but also exhibited pH and thermal responsive behavior. Consequently, the design herein provided a promising route to prepare the long-stable raspberry-like composite microspheres with unique surface morphologies and stimuli-responsive properties for a wide range of possible applications.

Scheme 1 Schematic illustrations of the preparation process of stable raspberry-like core–shell composite particles and their pH and thermal responsive behaviour.

Materials and methods

Materials

Tetraethylorthosilicate (TEOS, ≥99%), anhydrous ethanol, ammonium hydroxide aqueous solution (28% by weight in water), and other reagents, unless otherwise specified, were purchased from Jiangtian Chemical Reagents Co., Ltd. (Tianjin, China) and used as received without further purification. The trimethoxysilyl-end-functionalized RAFT agent benzyl (3-trimethoxylsilylpropyl)trithiocarbonate (BTPT) was synthesized according to a previous study.⁶² The detailed information and characterization of BTPT were given in Scheme S1, Fig. S1 and S2 in the ESI.† Styrene (St) was washed with an aqueous solution of NaOH (5%) and fractionally distilled under vacuum to remove inhibitor and polymer. N,N-Dimethylaminoethyl methacrylate (DMAEMA) was purified prior to polymerization by passing through the basic alumina column to remove inhibitors. 2,2′-Azobis(isobutyronitrile) (AIBN) was obtained from Aladdin Industrial Corporation (Shanghai, China) and purified by recrystallization. HPLC grade tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were purchased from Merck (Darmstadt, Germany). The uniform-size silica nanoparticles with mean diameter of about 500 nm were prepared using a modified method originally described by Stöber et al.^63,64 In a typical synthesis, 50 mL TEOS solution (4.5 mL TEOS in 50 mL ethanol, about 0.4 mol L⁻¹) was quickly injected into a mixture of deionized water (25 mL), NH₃·H₂O (7.5 mL), and ethanol (17.5 mL) at room temperature under magnetic stirring. After reacting for 6 h, the microspheres were collected by centrifugation and cleaned with ethanol and water. The silica spheres were redispersed in water or dried into powders for size, ζ-potential and morphology analysis.

Synthesis of trimethoxysilyl-end-capped amphiphilic copolymer

The well-defined trimethoxysilyl-end-capped poly(N,N-dimethylaminoethyl methacrylate)-block-polystyrene (Tsi-PDMAEMA-PSt) was prepared by a two-step sequential RAFT polymerization in the presence of BTPT as RAFT agent and AIBN as initiator in THF at 60 °C, as shown in Scheme S2 in the ESI.† As the first block, the polymer PSt with pre-ordained polymerization degree was prepared and then it was used as macro-RAFT agent in the subsequent polymerization to prepare well-defined Tsi-PDMAEMA-PSt. The molecular weight of PSt was determined by GPC analysis and ¹H-NMR data via comparing the intensity of characteristic resonance of the trimethoxyl proton resonances (about 3.6 ppm) of BTPT to that of methylene proton resonances at 1.18–1.72 ppm of PSt. The PSt (M_n = 7.85 × 10³ Da calculated from GPC data, the number-average degrees of polymerization (DP_n) ≈ 72) as macro-RAFT agent was chain extended by DMAEMA to obtain Tsi-PDMAEMA-PSt under a condition similar to that for PSt. The composition and structure of Tsi-PDMAEMA-PSt were determined by GPC, FTIR and ¹H-NMR, and the content of PDMAEMA block was calculated by comparing the peak area of aromatic proton resonances (6.45–7.30 ppm) of PSt to that of methylene proton resonances (OCH₂–, about 4.08 ppm) of the PDMAEMA side chain.

Preparation of Tsi-PDMAEMA-PSt nanoparticles

The prepared Tsi-PDMAEMA-PSt copolymers are amphiphilic and thus can self-assemble into NPs in water. The Tsi-PDMAEMA-PSt NPs were prepared by a nanoprecipitation method. Typically, the copolymer Tsi-PDMAEMA-PSt (5.0 mg) was dissolved in 1.0 mL THF. And then the solution was added drop-wise into 10 mL distilled water under moderate stirring. The solution was then swept by nitrogen airflow under stirring to allow complete evaporation of THF at room temperature. The solution with final concentration of 0.5 mg mL⁻¹ was filtered through a 0.1 μm filter before use. The size, ζ-potential and morphology of Tsi-PDMAEMA-PSt NPs were measured by laser particle size analyzer and electron microscopy.

Fabrication of raspberry-like composite microspheres

Raspberry-like composite microspheres were prepared by an electrostatic interaction-induced self-assembly of heterocoagulation between positively charged Tsi-PDMAEMA-PSt NPs and the negative silica microspheres, and a subsequent hydrolytic condensation reaction between trimethoxysilyl groups on the PDMAEMA chains and silanol groups on the surface of silica microspheres. Typically, 2.5 mg of silica microspheres (particle size, about 500 nm by TEM) were added into 50 mL of Tsi-PDMAEMA-PSt NPs water dispersion solution (0.5 mg mL⁻¹). The pH was adjusted to approximately 6.0–6.5 by adding drop-wise hydrochloric acid (1 mol L⁻¹ solution). The mixture solution was left for 12 h under gentle magnetic stirring at room temperature to form stable heterocoagulated particles. The coverage of the corona NPs on the surfaces of core silica particles can be controlled through the adjustment of the mass ratio between the core and corona particles. The heterocoagulated composite particles were collected by centrifugation and washing with water to remove the non-adsorbed polymer NPs, and then the resulting self-assembled hybrid particles consisting of a large silica core surrounded with a large number of smaller polymer NPs were redispersed in a mixture of anhydrous ethanol and triethylamine (ethanol/triethylamine = 95/5, v/v). After incubation for another 6 h at 60 °C under slow stirring, the final raspberry-like hybrid microspheres with stable covalent bond linkages between core and corona particles were obtained by repeated centrifugation and washing with ethanol.

The stability of the prepared raspberry-like composite microspheres in water was investigated by comparison of the morphology and size before and after ultrasonic treatment in a water bath of 25 °C according to previous studies.^13,31,36

Due to the characteristic of PDMAEMA chains, the prepared raspberry-like composite microspheres can exhibit pH and thermal responsive behaviors. The effect of pH and thermal on the morphology and particles size was performed. Typically, the prepared raspberry-like composite microspheres were redispersed again in water in a in a glass bottle. The pH of solution was adjusted by dropwise adding 1 mol L⁻¹ HCl or NaOH aqueous solution and monitored with a pH meter (PH-3C, INESA Scientific Instrument Co., Ltd). The thermal effect on the morphology and particles size of the prepared hybrid NPs was investigated in the temperature range of 20–60 °C at the heating rate of 1 °C min⁻¹.

Characterization

¹H-NMR spectra of the products were recorded on a Varian Inova-500M instrument (Varian Inc., Palo Alto, USA) with CDCl₃ as a solvent and tetramethylsilane (TMS) as the internal standard. Fourier transform infrared spectroscopy (FT-IR) was carried out using KBr disks in the region of 4000–500 cm⁻¹ by using BIO-RAD FT-IR 3000 (BIO-RAD Company, Hercules, USA). The molecular weights and polydispersities index (PDI) of the polymers were determined with an Agilent 1100 gel permeation chromatographer (GPC) equipped with a refractive index detector, using Shodex GPC KF-803L column with molecular weight range 500–42 000 calibrated with PSt standard samples. DMF was used as eluent at a flow rate of 1.0 mL min⁻¹ at 30 °C. The structures and morphologies of polymer NPs, silica microspheres and the resulting composite microspheres were observed under a Japan JEM-2100F transmission electron microscopy (TEM) system at an operated voltage of 200 kV. For TEM measurement, the sample was prepared by adding a drop of NPs solution onto the copper grid, and then the sample was air-dried and measured at room temperature. After a gold sputter coating by a sputter-coating apparatus (Q150R S, Quorum), their general surface morphologies were also observed by the Hitachi S-4800 field emission scanning electron microscope (SEM) equipped with Energy Dispersive X-ray (EDX) microanalysis, operated at a voltage of 30 kV, and EDX spectrum was also carried out with it. The samples for SEM observation were prepared by direct deposition of the NPs or microspheres onto an aluminum foil. Dynamic light scattering (DLS) and ζ-potential of the samples were measured by using a Zetasizer Nano ZS90 (Malvern Instruments, Worcestershire, UK) and the mean diameter of the particles as well as ζ-potential were obtained from the instrument's Zetasizer software.

Results and discussion

Preparation and characterization of Tsi-PDMAEMA-PSt and polymer NPs

The trimethoxysilane-functionalized positively charged polymer NPs are generated from the self-assembly of the trimethoxysilyl-end-capped amphiphilic copolymer (Tsi-PDMAEMA-PSt) in water. RAFT polymerization has been proved to be as a powerful, flexible, and easy-to-use platform to synthesize well-defined amphiphilic copolymers with functionalized end-groups, as one of the typical features of the RAFT process is the retention of the RAFT agent (Z–C(=S)–S–R) at the polymer chain ends, leading to the presence of the R and Z group at their α- and ω-ends.⁶⁵ As a result, the Tsi-PDMAEMA-PSt with precisely controlled architecture, predetermined chain composition and specific terminal group can be very readily prepared via a RAFT polymerization in the presence of a trialkoxysilanes-functionalized RAFT reagent. The chain structure and chemical composition of the macro-RAFT agent Tsi-PSt and Tsi-PDMAEMA-PSt copolymer were characterized by ¹H-NMR, FTIR and GPC, as shown in Fig. 1.

Fig. 1 ¹H-NMR spectra (A), FTIR spectra (B), GPC curves (C) of Tsi-PSt and Tsi-PDMAEMA-PSt.

As shown in Fig. 1(A), the ¹H-NMR spectra of Tsi-PSt and Tsi-PDMAEMA-PSt exhibited the resonance signals of methine and methylene protons in backbone (1.20–2.00 ppm) and phenyl protons (6.20–7.75 ppm). In addition, Tsi-PDMAEMA-PSt showed the characteristic signal of methyl and methylene protons of PDMAEMA at 0.82–1.10 ppm (e), 2.30 ppm (h), and at about 2.58 and 4.05 ppm (f and g), respectively, suggesting the formation of Tsi-PDMAEMA-PSt. Typical signals of the aromatic ring, methyl and methylene appeared at 3030 cm⁻¹, 2920 cm⁻¹, 2830 cm⁻¹, 2770 cm⁻¹, 1605 cm⁻¹, 1250–1450 cm⁻¹ and 695–760 cm⁻¹ in FTIR spectra of Tsi-PSt and Tsi-PDMAEMA-PSt. The spectrum of Tsi-PDMAEMA-PSt also exhibited the characteristic peaks of carbonyl (C=O) and carbon–oxygen bond in ester bonds of PDMAEMA at about 1730 cm⁻¹ and 1145 cm⁻¹, respectively. On inspection of Fig. 1(B), besides the characteristic peaks of PSt and PDMAEAM, the characteristic bands at 805 cm⁻¹ and 1035 cm⁻¹ that were associated with the stretching of Si–O– and Si–O–C groups in spectra of Tsi-PSt and Tsi-PDMAEMA-PSt were observed. These results showed that trimethoxysilyl-end-capped polymers Tsi-PSt and Tsi-PDMAEMA-PSt were obtained. The GPC curves of Tsi-PSt and Tsi-PDMAEMA-PSt were shown in Fig. 1(C). It can be found that both polymers showed symmetrical and unimodal GPC curves with narrow molecular weight distributions (PDI < 1.40). The molecular weights can be calculated from the ¹H-NMR data and obtained by GPC analysis. Typically, the well-defined Tsi-PDMAEMA-PSt with PSt block DP_n of about 72 and PDMAEMA block DP_n of about 86 was obtained and selected as a representative sample. In addition, the results indicated that the molecular weights calculated from ¹H-NMR were close to those obtained by GPC. The results above indicated that the BTPT-mediated RAFT polymerization has very good controllability and can be used to prepare organotrialkoxysilanes-functionalized polymers with controlled chain architecture and predetermined molecular weight.

The trimethoxysilane-end-capped Tsi-PDMAEMA-PSt, as an amphiphilic copolymer, could self-assemble into NPs in water. As shown in Fig. 2(A) and (B), TEM and SEM images demonstrated the well-defined spherical morphology and uniform size of Tsi-PDMAEMA-PSt NPs. The sphere-like Tsi-PDMAEMA-PSt NPs in a dry state in TEM and SEM images possessed an average diameter of approximately 54 ± 5 nm, which was slightly smaller than the wet NPs determined by DLS (about 68 ± 7 nm). Compared with the size determined by DLS, the smaller sizes observed by TEM and SEM observations can be ascribed to the shrinkage of hydrophilic shells upon drying samples.⁶⁶ Tsi-PDMAEMA-PSt NPs were generated from the self-assembly in water and thus the trimethoxysilane-functionalized Tsi-PDMAEMA as hydrophilic segments will be present on the outer surface of NPs, which will provide a direct route to incorporate reactive trimethoxysilane groups onto polymer particles for subsequent covalent bonding with silica particles. In order to determine the presence of trimethoxysilane groups on the surface of Tsi-PDMAEMA-PSt NPs, EDX was used to analyse the surface composition of as-prepared NPs. As can be seen in Fig. 2(D), the EDX spectrum of Tsi-PDMAEMA-PSt NPs showed the distinctive peaks attributed to the C, N, O and Si, which confirmed the formation of trimethoxysilane-functionalized polymer NPs. Due to protonation of the tertiary amine side groups of PDMAEMA chains (pK_a about 7.5), Tsi-PDMAEMA-PSt NPs were positively charged at relatively low pH values. Fig. 2(E) shows the ζ-potential of Tsi-PDMAEMA-PSt NPs in water as a function of the pH value. The surface charge of Tsi-PDMAEMA-PSt NPs was about 30 mV and decreased with the increase of pH when the pH value was above about 6.5. These results clearly demonstrated that trimethoxysilane-functionalized polymer NPs with positive surface charges were successfully obtained by a very simple strategy, which are favorable for the formation of stable raspberry-like particles by self-assembled heterocoagulation with negatively charged silica particles.

Fig. 2 Typical TEM image (A), SEM image (B), size distribution in aqueous solution (C); EDX analysis under SEM (D) and ζ-potential at different pH values (E) of Tsi-PDMAEMA-PSt NPs.

Preparation and characterization of raspberry-like silica@polymer particles

Silica nanoparticles are one of the most widespread nanomaterials in use, which can be easily prepared through hydrolysis–condensation reaction according to a method originally described by Stöber et al.⁶³ Fig. 3(A) and (B) show the SEM and TEM images of the morphological structure of the original silica particles. The results revealed the formation of highly monodispersed spherical silica particles with smooth surface morphology and an average particle size of about 500 nm. The ζ-potentials of the original silica spheres in water as a function of the pH value were measured and are shown in Fig. S3 in ESI.† The silica particles dispersed in water possessed a negative surface charge when the pH value was above about 3.5. Moreover, their surface charge became more and more negative with the increase of pH values, due to the dissociation of surface silanol groups. These results are in agreement with the previous reports.^51–54

Fig. 3 Characterization of silica particles and raspberry-like silica@polymer particles. Typical SEM image (A) and TEM image (B) of silica particles; typical SEM image (C) and TEM image (D) of silica@polymer particles; DLS measurements in aqueous solution (E) of silica particles and silica@polymer particles; ζ-potential at pH 6.5 (F) of silica particles and silica@polymer particles.

Apparently, Tsi-PDMAEMA-PSt NPs and silica particles are oppositely charged in the range of about pH 3.5 to 9.5, and thus they have a sufficiently high charge density to form stable complexes by electrostatic interaction-induced assembly. Moreover, it was worth pointing out that PDMAEMA chains containing multiple carbonyl groups can form hydrogen bonds (C=O⋯H–O–Si≡) with the silanol groups on the surface of silica particles, which will also facilitate the self-assembled heterocoagulation between Tsi-PDMAEMA-PSt NPs and silica particles.⁴⁶ As demonstrated above, the surface charge of Tsi-PDMAEMA-PSt NPs would decrease with the increase of pH when the pH value was above about 6.5. In addition, the ester groups and trimethoxysilane groups in the PDMAEMA segments have a high tendency to hydrolyze at a very low pH value. As a result, the electrostatic heterocoagulation of Tsi-PDMAEMA-PSt NPs and silica particles was conducted at a relatively high pH (pH 6.0–6.5). On the other hand, it has been pointed out that the total charge ratio of the positive groups on corona particles to the negative groups on core particles has a significant impact on the formation and stabilization of electrostatic complex particles.^56,67 The higher positive/negative charge ratio tended to have sufficient amounts of Tsi-PDMAEMA-PSt NPs to be adsorbed on the surfaces of negative silica particles. Therefore, the raspberry-like silica@polymer particles with high and full coverage of corona NPs can be prepared by dispersing the large core silica particles into the excessive Tsi-PDMAEMA-PSt NPs solution at pH 6.5 (mass ratio of silica particles/Tsi-PDMAEMA-PSt NPs = 1 : 10).

After a self-assembled heterocoagulation of Tsi-PDMAEMA-PSt NPs on silica particles and a subsequent condensation reaction between surface silanol groups with trimethoxysilyl groups, the stable raspberry-like hybrid microspheres consisting of a larger silica core surrounded by smaller polymer NPs were obtained. SEM and TEM were employed to investigate the surface morphologies of the prepared nanostructures. As shown in Fig. 3(C) and (D), it can be obviously observed that all composite particles with uniform size and shape have well-organized raspberry-like surface morphologies. The hierarchical structures with unique rough surface are very distinctive from the smooth surfaces of bare silica spheres prepared by the Stöber method as shown in Fig. 3(A) and (B). These results also demonstrated that the small Tsi-PDMAEMA-PSt NPs were homogeneously deposited on the surface of the large silica particles in the absence of any other stabilizer and coagulum. In addition, the average diameter of spherical raspberry-like composite particles measured from the SEM and TEM images was about 600 nm, which was significantly larger than that of bares silica particles. The size and size distribution of silica particles and raspberry-like silica@polymer particles in aqueous solution were further determined by DLS technique, as shown in Fig. 3(E). DLS results showed that the average diameter of bare silica particles was about 503 nm, which was fairly consistent with the observations of electron microscopy. However, the DLS measurement indicated that the average diameter of silica@polymer particles was about 618 nm, which was also much bigger than that of bare silica spheres. Fig. 3(F) shows the ζ-potentials of the silica particles and raspberry-like silica@polymer particles in water at pH value of 6.5. The ζ-potential changed dramatically from −35.6 ± 3.2 mV of silica particles to 22.4 ± 3.7 mV of silica@polymer particles as a consequence of the specific attachment of the positive-charged Tsi-PDMAEMA-PSt NPs on the surface of silica particles. The significant changes in the surface morphology, particle size and ζ-potential suggested that Tsi-PDMAEMA-PSt NPs were successfully coated on the surfaces of silica particles to form stable raspberry-like particles with a full coverage.

The FTIR spectra of silica particles and silica@polymer particles were shown in Fig. 4, which provided more evidence for the formation of the covalent linkages between core and corona particles. As revealed in Fig. 4, it can be observed the characteristic peaks of silica particles at about 1100 cm⁻¹ and 475 cm⁻¹, which can be attributed to the stretching and bending vibrations of –Si–O–. We can also observe the bending vibration of –Si–O–Si– at 805 cm⁻¹ and the vibrations of –Si–OH in the range of around 3200 to 3600 cm⁻¹, respectively. Besides the characteristic peaks of –Si–O–, –Si–O–Si– and –Si–OH, the FTIR spectrum of silica@polymer particles showed the characteristic peaks of Tsi-PDMAEMA-PSt, such as –CH₂– and –CH₃ stretching and bending vibrations at about 1450 cm⁻¹, 2830 cm⁻¹ and 2920 cm⁻¹, N–CH₃ antisymmetric stretching at about 2770 cm⁻¹, and –C=O stretching vibration at about 1730 cm⁻¹. In particularly, it can be found in the FTIR spectrum of silica@polymer particles that the stretching vibrations of –Si–OH in the range of around 3200 to 3600 cm⁻¹ almost disappeared, which verified the occurrence of the reaction between Si–OH groups and hydrolyzed organotrialkoxysilanes and proved the formation of the covalent linkages between core and corona particles.

Fig. 4 FTIR spectra of silica particles and silica@polymer particles.

It was worth pointing out that, due to the covalent linkages between core and corona particles, the obtained raspberry-like composite particles were very stable, which are expected to be able to retain their corona NPs and surface functionalities in routine use and storage for a long period of time. The structure stability of the raspberry-like silica@polymer particles was investigated by applying ultrasonication. The SEM and TEM images of the silica@polymer particles after ultrasonic treatment for 0.5 h were shown as in Fig. S4 in ESI.† It can be found that the coverage of silica core particles with the Tsi-PDMAEMA-PSt corona NPs kept nearly constant even after ultrasonication, suggesting the very strong stability of surface structures.^13,31,36 These results further confirmed that the linkages between core and corona particles are covalent bonds. In fact, the extremely low efficiency of inter-particle reactions often resulted in the failure of the formation of covalent bonds between two particles.^40,42,43 The highly efficient covalent binding between Tsi-PDMAEMA-PSt corona NPs and silica core particles may be contributed to (1) the intimate contact between core and corona particles due to the synergistic interactions of hydrogen bonding and electrostatic nature, (2) the spatial extension of trimethoxysilane-end-capped PDMAEMA chains in water, (3) the high reactive activity between silanols and hydrolyzed organotrialkoxysilanes.^59–61 As a result, the inter-particle condensation reaction will possibly occur in similar fashion as the intermolecular mode, leading to the efficient formation of the covalent linkages.

The hierarchical morphology of silica@polymer particles was prepared based on a heterocoagulated assembly with the aid of inter-particle electrostatic and hydrogen-bonding interactions. Besides the simplicity and versatility, one of the more significant advantages of this technique is the high controllability on surface coverage and roughness though simply altering the size of the corona and core particles or facilely adjusting the mass ratio between the core and corona particles. As demonstrated in Fig. 5, the smaller corona Tsi-PDMAEMA-PSt NPs can be efficiently deposited onto the larger core silica spheres. Moreover, the surface coverage and roughness of the raspberry-like silica@polymer particles can be well controlled through the adjustment of the mass ratio of silica particles/Tsi-PDMAEMA-PSt NPs. As shown in Fig. 5(A), when the mass ratio of silica particles to Tsi-PDMAEMA-PSt NPs was 20 : 1, the surface of the large silica cores was almost barren with a low coverage of Tsi-PDMAEMA-PSt corona. The surface coverage of the smaller NPs on the larger core particles was significantly increased with the decrease of mass ratio of core and corona particles. When the ratio between larger particles and smaller NPs was decreased to 1 : 1, the naked surface of silica cores can be hardly observed, as shown in Fig. 5(C). With further decreasing the mass ratio, it is clearly observed from Fig. 5(D) that the silica@polymer particles with almost full coverage of corona NPs can be obtained. Undoubtedly, the high simplicity and controllability of this strategy provide an efficient and promising solution to fabricate raspberry-like particles with controllable surface morphologies and tailored surface functions.

Fig. 5 Typical SEM images of raspberry-like silica@polymer particles from different mass ratio between the core and corona particles. (A) Mass ratio of silica particles/Tsi-PDMAEMA-PSt NPs = 20 : 1; (B) mass ratio of silica particles/Tsi-PDMAEMA-PSt NPs = 5 : 1; (C) mass ratio of silica particles/Tsi-PDMAEMA-PSt NPs = 1 : 1; (D) mass ratio of silica particles/Tsi-PDMAEMA-PSt NPs = 1 : 10.

pH and thermal dual-responsiveness of silica@polymer particles

Stimuli-responsive materials can respond to their surrounding environments, such as pH, temperature, light, magnetic or electric field, which have found fascinating interest for various applications. It was well known that PDMAEMA is a typical kind of pH and thermo-responsive polymer. Polymeric hydrogels or nanomaterials consisting of pH and thermo-responsive PDMAEMA segments can undergo reversible and significant changes in shape, structure, and/or property after being exposed to external pH and thermo-stimuli. Tsi-PDMAEMA-PSt NPs with a typical core–shell structure are assembled from the block copolymer composing of hydrophilic PDMAEMA and hydrophobic PSt. PDMAEMA forming the hydrophilic corona will impart Tsi-PDMAEMA-PSt NPs and the resultant silica@polymer particles with pH and thermal dual-responsiveness. As shown in Fig. 6(A) and S5 in ESI,† the silica@polymer particles can be redispersed into water without the occurrence of aggregation or sedimentation, and the dispersion solution can keep transparent and clear at a relatively low pH value (pH 6.5) and temperature (25 °C), indicating a very stable dispersion state. The good stability may arise from the combination of steric stabilization by hydrophilic PDMAEMA chains and the electrostatic stabilization because of the adequate surface charge of Tsi-PDMAEMA-PSt NPs. On the one hand, the hydrophilic and flexible PDMAEMA chains may create a hydrophilic protective shield around the silica@polymer particles, which can prevent the aggregation of particles through inter-particle steric repulsion. In addition, as demonstrated in Fig. 3(F), the silica@polymer particles are positively charged at a relatively low pH value due to the protonation of the tertiary amine side groups of PDMAEMA chains, and thus the repulsive electrostatic force among silica@polymer particles will markedly impede the occurrence of flocculation or coagulation to maintain the stability. However, the steric and electrostatic stabilization will decrease with the increase of the pH or temperature, and thus the macroscopic phase separation would occur. As shown in Fig. 6(A) and S5,† the dispersion solution of silica@polymer particles changed into completely turbid at about pH 9.86 and at around 48 °C, respectively, indicating the appearance of abundant aggregation of particles. It can also be seen that the turbid solution gradually became clear and turned into completely transparent again when the pH or temperature was decreased and returned to the original value. These phenomena confirmed the pH and thermoresponsive property of silica@polymer particles. The results also suggested that the reversible aggregation–disaggregation transitions of silica@polymer particles could be highly regulated by the change of pH or temperature, which can be mainly ascribed to the reversible hydrophilic–hydrophobic transitions of PDMAEMA chains triggered by pH or temperature, as shown in Fig. 6(B). The pH environment varies from acidic to alkaline, leading to a significant decrease in protonation degree of the tertiary amine side groups of PDMAEMA chains. The deprotonation of the tertiary amine side groups can not only result in the hydrophobic transitions of PDMAEMA chains, but also cause the nearly disappearance of electrostatic stabilization due to the extremely low surface charge of Tsi-PDMAEMA-PSt NPs under alkaline condition as shown in Fig. 2(E). Consequently, silica@polymer particles will tend to aggregate with the increase of pH value. Similarly, with the increase of temperature, PDMAEMA chains will turn from hydrophilic into hydrophobic due to the abrupt conformational changes of PDMAEMA segments upon heating, leading to the loss of steric stabilization and a significant decrease in particles stability and thus the aggregation of silica@polymer particles. Because the protonation–deprotonation conversions and hydrophilic–hydrophobic transitions of PDMAEMA chains are reversible, the aggregated silica@polymer particles will disintegrate with the decrease of pH or temperature. SEM was used to further illustrate the macroscopic phase separation and aggregation of silica@polymer particles. As shown in Fig. 6(C) and S6,† the appearance of a great number of micron-scale aggregates in SEM images confirmed the pH or thermo-triggered aggregation of silica@polymer particles. Moreover, upon closer inspection of SEM images in Fig. 6(C) and S6,† the composite particles and their aggregates seem to lose their raspberry-like surface morphologies. The surface roughness of silica@polymer particles and their aggregates was significantly lower when compared with the particles shown in Fig. 3(C). This phenomenon should be also due to the pH or thermo-induced deprotonation and hydrophobic transitions of PDMAEMA chains. On the one hand, the PDMAEMA shells of Tsi-PDMAEMA-PSt NPs will change from highly stretched structure to shrinkage due to the hydrophobic transitions of PDMAEMA chains, leading to the deformation of Tsi-PDMAEMA-PSt NPs. On the other hand, besides the hydrophobic interactions, the loss of steric and electrostatic stabilization will result in the inter-particle aggregation of Tsi-PDMAEMA-PSt NPs on the outer surface of core silica particles. Undoubtedly, the collapse of PDMAEMA shells and the aggregation of Tsi-PDMAEMA-PSt NPs will markedly change the surface structures and morphologies of silica@polymer particles. However, it was worth pointing out that the surface roughness of silica@polymer particles and their aggregates at a relatively high pH value or temperature was much higher than that of original silica particles shown in Fig. 3(A), suggesting without loss of the polymers NPs coating after pH or thermo-triggered aggregation–disaggregation transitions. These phenomena also indicated that Tsi-PDMAEMA-PSt polymers were covalently bonded to the surface of silica particles, further confirming the robust stability of surface structure of silica@polymer particles. There results demonstrated that the robust and stable raspberry-like silica@polymer particles with pH and thermal dual-responsiveness have been successfully obtained, which may have a great potential for applications in many fields, such as controlled delivery systems in pharmaceutical area, tunable superhydrophobic surfaces in engineering domain, and functional flocculating agents for the removal of toxic metals and other pollutants in environmental regulation.

Fig. 6 pH and thermal dual-responsiveness of silica@polymer particles. (A) Photographs of the pH-triggered aggregation–disaggregation transitions of silica@polymer particles at 25 °C; (B) schematic illustration of the stimulus-triggered aggregation–disaggregation transitions of silica@polymer particles (C) typical SEM image of silica@polymer particles collected at pH 9.86.

Conclusions

In this study, a facile approach was developed to prepare stable raspberry-like core–shell composite particles by the heterogeneous self-assembly and a subsequent reaction between silanol groups of silica particles and trimethoxysilane groups of alkoxysilanes-functionalized polymer NPs. The alkoxysilanes-functionalized polymer NPs were assembled from amphiphilic Tsi-PDMAEMA-PSt with well-defined chain architecture and composition. Tsi-PDMAEMA-PSt NPs with positive surface charges can effectively deposit on the surface of negatively charged silica microspheres and then form the covalent bonds between alkoxysilanes and surface silanols to produce stable silica@polymer particles. The surface morphology, particle size, ζ-potential, structure stability as well as pH and thermo-responsiveness of the prepared composite particles were investigated. The results indicated that the prepared silica@polymer particles possessed unique raspberry-like surface structures with high stability. Moreover, their surface coverage and morphology can be well controlled by the adjustment of the mass ratio of silica particles/Tsi-PDMAEMA-PSt NPs. In addition, the surface morphology, dispersion sate and stability of silica@polymer particles in water can be governed by controlling pH and temperature, indicating a kind of typical pH and thermal dual-responsive behavior. Consequently, considering the high simplicity and controllability, the design herein provided a promising route to prepare the long-stable raspberry-like composite microspheres with unique surface morphologies and stimuli-responsive properties for a wide range of possible applications.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 31470925 and 31470963) and Tianjin Research Program of Application Foundation and Advanced Technology (No. 15JCQNJC03000).

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Footnote

† Electronic supplementary information (ESI) available: Schematic illustration of synthesis and structure characterization of BTPT; schematic illustration of synthesis of Tsi-PDMAEMA-PSt; ζ-potential of silica particles in water at different pH values; typical SEM and TEM images of silica@polymer composite particles after ultrasonication treatment; photographs of the thermo-triggered aggregation–disaggregation transitions of silica@polymer particles; typical SEM image of silica@polymer particles collected at 48 °C. See DOI: 10.1039/c6ra03965k

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