Encapsulation of silica nanoparticles by redox-initiated graft polymerization from the surface of silica nanoparticles
Graphical abstract
Polymer-encapsulated silica particles by ‘grafting from’ polymerization initiated by a redox system comprising ceric ion (Ce4+) as a oxidant and an organic reductant immobilized on the surface of silica nanoparticles.
Introduction
In recent years, chemical and physical surface modifications of inorganic nanoparticles, e.g., silica, titanic dioxide and iron oxide magnetic nanoparticles have been the subject of extensive investigation because of their potential and demonstrated applications in many fields. Among permanent chemical modifications, surface grafting of polymers, in other words, chemical binding of polymers onto the inorganic nanoparticles, is of great interest for designing new functional inorganic/organic hybrid materials. These hybrid materials show useful features of inorganic materials, such as heat and chemical resistance, and those of grafted polymers, such as thermosensitivity, photosensitivity, curing ability, biocompatibility and pharmacological activity, etc. [1], [2], [3], [4].
Polymers can be grafted onto the surface of inorganic nanoparticles by either the ‘grafting onto’ or ‘grafting from’ method. The ‘grafting onto’ method involves reacting end-functionalized polymers with the particle surfaces [5], [6], [7]. One of its main disadvantages is the relatively low grafting density. The ‘grafting from’ method involves immobilizing initiators on the surface and in situ growing macromolecular chains from the surface. Conventional free radical grafting polymerization can be initiated by thermal decomposition of azo or peroxy initiators immobilized on the particle surfaces [8], [9], [10], or by UV-induced graft polymerization [11]. Furthermore, the combination of the ‘grafting from’ method with controlled/living free radical polymerization allows manipulating the structural characteristics of the polymer shell through the changes in grafting density, composition and molecular weight. For example, both hydrophobic and water soluble polymers have been grafted onto submicrometer-sized silica particles by atom transfer radical polymerization (ATRP) [12], [13], [14], [15], reverse ATRP [16], [17], reversible addition–fragmentation chain transfer (RAFT) polymerization [18], [19], and nitroxide-mediated processes (NMP) [20], [21], [22], [23]. Well-defined polymeric capsules have been fabricated by crosslinking the polymer shells and etching the silica cores by HF [24].
On the other hand, since about 50 years ago [25], [26], [27], [28], [29], ceric ion (Ce4+) initiated grafting polymerization has been widely used for the modification of both natural and synthetic polymers containing hydroxyl or amino groups, such as cellulose, starch, chitosan, poly(vinyl alcohol), etc. Different from some other metal ions used for initiating graft polymerizations, such as Co3+ [30], and Mn3+ [31], Ce4+ ions introduce active sites to reductants by a single electron-transfer process and the formation of homopolymer is prevented. Ce4+ initiated grafting polymerization has been successfully employed to prepare polymer brushes on flat substrates [32]. At the same time, hydroxyl terminated PEG and Ce4+ are demonstrated to be an effective initiator pair capable of initiating polymerization of many monomers, e.g., acrylonitrile [33], [34], methyl methacrylate (MMA) [35], acrylamide and methacrylic acid [36], leading to the formation of block copolymers comprising PMMA, PAN, PAM or PMA sections and hydrophilic PEG sections. But to our knowledge, Ce4+ initiated grafting polymerization has not been employed to modify nanometer-sized materials, such as nanoporous materials, nanofibers and nanoparticles.
Herein, we demonstrate that core–shell structured nanoparticles with polymer shells and inorganic cores can be obtained by grafting polymerization initiated by the redox reaction between Ce4+ and hydroxyl terminated poly(ethylene glycol) (PEG) immobilized on the surface of silica nanoparticles in acid aqueous solutions. Poly(methyl methacrylate) (PMMA), a common hydrophobic polymer, and poly(N-isopropylacrylamide) (PNIPAAm), a well-known thermosensitive polymer that undergoes a reversible hydrophilic–hydrophobic change in water in response to temperature changes across a lower critical solution temperature (LCST) at about 32 °C, were employed as model polymers in this study. The final silica particles encapsulated by PMMA and PNIPAAm were referred to as silica@PMMA and silica@PNIPAAm respectively.
Compared with the living free radical polymerizations, the present Ce4+/PEG redox system does not allow a precise control of the molecular weight and polydispersibility of the grafted polymers and the thickness of the shells. However, the redox methodology has some merits, e.g., it allows a facile and mild grafting polymerization at low temperatures in aqueous solutions, involves only inexpensive, commercially available reagents, and is versatile for many monomers, especially for water soluble monomers.
Section snippets
Materials
Methyl methacrylate (MMA) was washed twice with an aqueous solution of sodium hydroxide at 5 wt% and distilled water, respectively, dried over anhydrous magnesium sulfate overnight, and then distilled over calcium hydride under vacuum. The distillates were stored at −4 °C before use. N-isopropylacrylamide (NIPAAm) and poly(ethylene glycol) acrylate (PEGA) with a number average molecular weight of 375 g/mol were purchased from Sigma–Aldrich. NIPAAm was recrystallized from a mixture of toluene
Preparation of the silica@PMMA and silica@PNIPAAm nanoparticles
The synthesis procedures of polymer-encapsulated silica nanoparticles are shown in Fig. 1. The Stöber procedure allows the preparation of monodisperse silica spheres with radii ranging from 10 nm to a few micrometers in a mixture of ethanol, ammonium hydroxide and water at low temperatures [27], [37]. The silica nanoparticles were subsequently modified by APTES [38], so that amino groups are immobilized onto the particle surfaces. Then, hydroxyl terminated PEG were grafted onto the surface of
Summary
Inorganic/organic hybrid nanoparticles with silica cores tethered with a layer of polymers were elaborated by free radical graft polymerization initiated “from” the silica surface with the redox initiation system of Ce4+ and PEG immobilized on the surface of silica. Both hydrophobic and hydrophilic polymers were successfully grafted onto the surface of silica nanoparticles. On the contrary, such graft polymerization did not occur on bare silica nanoparticles. TGA results indicated that the
Acknowledgments
We appreciate financial support from the National Natural Science Foundation of China (Grant Nos.: 20574060 and 50773066).
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