Fabrication and Phase Behavior of Thermo- and/or pH-Responsive Polymer-Grafted SiO\({}_{2}\) Nanoparticles  
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1 School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710062, China; [EMAIL_ADDRESS] (X.W.); [EMAIL_ADDRESS] (J.P.)  
[PERSON]  
1 School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710062, China; [EMAIL_ADDRESS] (X.W.); [EMAIL_ADDRESS] (J.P.)  
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2 School of Chemical Engineering and Technology, Hainan University, Haikou 570228, China; [PERSON] [EMAIL_ADDRESS] (J.P.)  
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3 Shanxi Key Laboratory of Chemical Additives for Industry, College of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China 33333333333333333333333333333333333333333  
thermo-responsive polymers display a lower critical solution temperature (LCST), providing potential biomedical applications, for instance, drug delivery and biosensors [24; 25; 26]. As known, poly(ethylene glycol) (PEG) is a neutrally charged, water-soluble, non-toxic polymer, and thus, it has been widely applied in biomedical applications. Further, the nonlinear PEG analogues, for example, macromolecules prepared from oligo(ethylene glycol) methacrylate (OEGMA), are as biocompatible as their linear counterparts [27; 28]. It has been found that polymers of oligo(ethylene glycol) methacrylate (POEGMA) are thermo-responsive, and the LCST of the POEGMA polymers increases with increasing the side-chain length of the oligo(ethylene glycol) unit [29]. Importantly, the copolymerization of two oligo (ethylene glycol) methacrylate (OEGMA) of different chain-lengths can form thermo-responsive copolymers that their LCST can be tuned by changing the ratio of the two OEGMA monomers [29; 30; 31]. Thus, POEGMA based thermo-responsive polymers can be considered smart biocompatible materials for various applications in biotechnology.  
In this study, silica nanoparticles were firstly synthesized by the Stober method and followed with modification by 3-aminopropyltrimethoxysilane (APTMS) and 2-bromoisobutyryl bromide (BiBB) to synthesize spherical particle initiator (SiO\({}_{2}\)-Br). The SI-ATRP method was then used to graft oligo(ethylene glycol) methacrylate (OEGMA, \(M_{n}=300\) g/mol) and/or tert-butyl acrylate (tBA) monomers onto the surface of silica nanoparticles. Through this synthesis procedure, we prepared SiO\({}_{2}\)-\(g\)-POEGMA, SiO\({}_{2}\)-\(g\)-PtBA and SiO\({}_{2}\)-\(g\)-P(OEGMA-_stat_-tBA) nanoparticles. Next, SiO\({}_{2}\)-\(g\)-PtBA and SiO\({}_{2}\)-\(g\)-P(OEGMA-_stat_-tBA) were then hydrolyzed to obtain SiO\({}_{2}\)-\(g\)-PAA and SiO\({}_{2}\)-\(g\)-P(OEGMA-_stat_-AA), respectively. Fourier transform infrared spectra (FT-IR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), gel permeation chromatography (GPC), and proton nuclear magnetic resonance (\({}^{1}\)H NMR) were used to characterize the prepared polymer-grafted nanoparticles. Further, thermo- and/or pH-responsive behavior of SiO\({}_{2}\)-\(g\)-POEGMA, SiO\({}_{2}\)-\(g\)-PAA, and SiO\({}_{2}\)-\(g\)-P(OEGMA-_stat_-AA) in an aqueous solution was investigated by dynamic light scattering (DLS) under different temperature and at different pH.