Hydrophilic Silica/Copolymer Nanoparticles and Protein-Resistance Coatings ()
1. Introduction
Hydrophiphilic block copolymers are mostly used in biological sciences [1] [2] and advance material [3]. In the recent research, the significant hydrophiphilic block copolymers are mainly used poly (poly (ethylene glycol) methyl ether methacrylate, P (PEGMA), or poly (poly (ethylene glycol) methacrylate, P(PEG), as the hydrophilic block [4]-[7]. Actually, both P(PEGMA) and P(PEG) consist of a linear methacrylate backbone reactive functional group with a side chain of poly(ethylene glycol) (PEG), which could generate a wealth of new polymeric materials by employing the recently developed living radical polymerization techniques of reversible addition fragmentation chain transfer (RAFT) [8] [9] and atom transfer radical polymerization (ATRP) [10]-[12]. In order to achieve the polymer films with adjusted properties by varying the monomer composition based on PEGMA monomer, well-defined block copolymers are synthesized, such as P(PEGMA)-b-poly(2,5-dibromo 3-vinylthiophene) by RAFT for uniform cross-linked nanoparticles [8], poly (glycidyl methacrylate-co-poly (ethylene glycol) methyl ether methacrylate) nanoparticles by ATRP [12]. For investing the effect of concentration and temperature on the micelle formation by P(PEGMA) in aqueous solution, the P(PEGMA) homopolymersare synthesized by aqueous ATRP [13]. Furthermore, The PEG-based materials are commonly used as biofouling-resistant materials. Thus, many hydrophobic surfaces modified with PEG-based materials are greatly used to reduce protein adsorption [14]-[16]. Actually, the distribution density and the chain length of PEG grafted on the surfaces are two key parameters in determining protein repelling behaviour [17]. Therefore, it is hoped that a hydrophilic block copolymers as antibiofouling coating could be designed with the combination of two different hydrophilic segments.
This paper reports the synthesis, the tem-responsive and surface properties as protein-resistance coatings of the hydrophiphilic silica graftedblock copolymers of SiO2-g-P(PEGMA)-b-P(PEG) nanoparticles, which are synthesized via SI-ATRP approach by poly [poly (ethylene glycol) methyl ether methacrylate)] and (P(PEGMA)) poly (poly (ethylene glycol) methacrylate, P(PEG)) in three molar ratios of SiO2-Br/PEGMA/12FMA = 1/42.46/19.44, 1/42.46/38.88 and 1/42.46/77.76. Their chemical structures are characterized by nuclear magnetic resonance (1H-NMR) spectroscopy, Fourier transform infrared spectroscopy (FI-IR) and thermo gravimetric analysis (TGA). The nanoparticles in water are characterized by transmission electron microscopy (TEM). The transition points and phase separation processes for the lower critical solution temperature (LCST) and pH-Re- sponse are examined by dynamic light scattering (DLS). The surface properties of films are investigated by atomic force microscope (AFM) and the quartz crystal microbalance with dissipation (QCM-D). The obtained results provide a prospected insight on the application of SiO2-g-P(PEGMA)-b-P(12FMA) nanoparticles as protein-resistance coatings.
2. Experimental
2.1. Materials
SiO2-initiator (SiO2-Br) was prepared by silanization of hydrosilylated undec-10-enyl, 2-bromo-2-methyl propionate (UBMP) and silica nanoparticle using the previous method [18]. Poly (ethylene glycol) methyl ether methacrylate (PEGMA, ~475 g・mol−1) and poly (ethylene glycol) methacrylate (PEG, ~500 g∙mol−1) was supplied Aldrich. CuCl and CuCl2 were purified. N, N, N', N,' N''-pentamethyldiethylenetriamine (PMDETA), ethanol, tetrahydrofuran (THF) and other solvents were used as analytical purity. Bovine serum albumin (BSA) was prepared in distilled water and phosphate buffer solution (PBS) buffer solution.
2.2. Synthesis of SiO2-g-P(PEGMA)-b-P(PEG) by SI-ATRP
The SiO2-g-P(PEGMA)-b-P(PEG) are prepared by silica surface-initiating atom transfer radical polymerization (SI-ATRP) of poly (ethylene glycol) methyl ether methacrylate (PEGMA) and poly(ethylene glycol) methacrylate (PEG). The procedure of SiO2-g-P(PEGMA)-b-P(PEG) polymerization was similar in previous work [19]. The conversion of PEGMA was 82% and the conversion of PEG was 80% after 12 h reaction. In this paper, three molar ratios of SiO2-Br/PEGMA/PEG=1/42.46/19.44, 1/42.46/38.88 and 1/42.46/77.76 for Sample S1, S2 and S3 were used to obtain SiO2-g-P(PEGMA)-b-P(PEG) hybrid particles. The results from 1H-NMR, FI-IR and TGA were able to confirm that the synthesis of SiO2-g-P(PEGMA)-b-P(PEG)was proceed as expected by SI- ATRP approach.
3. Characterization
The chemical structure, morphology of nanoparticles, transition points and phase separation processes for the lower critical solution temperature (LCST), surface properties of films and protein-resistance behavior was invested and the detail information was showed in previous work [19].
4. Results and discussion
4.1. The Morphology of Hydrophiphilic Nanoparticles
Because the nanoparticles formed by hydrophilic block copolymer are strongly depended on the composition of blocks, the influence of different molar ratios of SiO2-Br/P(PEGMA)/P(PEG) in Sample S1 (1/42.46/19.44), S2 (1/42.46/38.88) and S3 (1/42.46/77.76) on the morphology of SiO2-g-P(PEGMA)-b-P(PEG) nanoparticles in water is explored by TEM (Figure 1), based on the good solubility of hydrophilic block of P(PEGMA) and P(PEG) in water. In Figures 1(a)-(c), the spherical particles composed of ~220 nm silica core and different thickness of P(PEGMA) shell are observed for SiO2-g-P(PEGMA)-b-P(PEG) of Sample S1-S3. All the spherical particles have a good dispersion in water. While, the thickness of -P(PEGMA)-b-P(PEG) shell is increased with increasing the PEGMA concentration from Sample S1 to Sample S3, but much content of P(PEGMA) segment in Sample S3 leads to the overlapping cross of copolymer (Figure 1(c)).
4.2. The Tem-Responsive of LCST in Water Solution
The lower critical solutiontemperature (LCST) and the transition points and phase separation behavior of SiO2- P(PEGMA)-b-P(PEG) nanoparticles in dilute aqueous solutionis determinedby DLS measurementsas the concentration of 1.0 mg∙ml−1. The LSCT is taken as the initial turn point in the hydrodynamic diameter (Dh) versus temperature.The variation of Dh in Figure 2(a) exhibits an abrupt increase at the transition point for SiO2- P(PEGMA)-b-P(PEG) with only one transition point during heating. It is noticed that the LCSTfor Sample S1 and Sample S2of SiO2-P(PEGMA)-b-P(PEG)is at 60˚C and 67˚C, which is much lower than the LCST of the SiO2-P(PEGMA) (80˚C) [19]. This is because the grafting P(PEG) segment onto SiO2-P(PEGMA)enhances the intermolecular forceand the intermolecular force is enhancedwith the increasing content of P(PEG). the enhanced intermolecular forcecoulddrops the LCST of SiO2-P(PEGMA). Sample S3 has the less content of P(PEG) segment to reduce the effect of intermolecular force to obtain a high LCST at 77˚C. This indicates that the effect of intermolecular force is gradually enhanced with the increasing of hydrophilic block of P(PEG). Thus, the interaction of SiO2-g-P(PEGMA)-b-P(PEG) nanoparticles is increased and the interaction between water and -P(PEGMA)-b-P(PEG) is reduced from Sample S1 to S3, which makes the LCST of SiO2-g- P(PEGMA)-b-P(PEG)reduce gradually.
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(a) (b) (c)
Figure 1. TEM images of SiO2-g-P(PEGMA)-b-P(PEG) in water for S1 (a), S2 (b) and S3 (c).
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Figure 2. DLS measurements of LCST for Sample S1-S3 in water (a) and Dependence of hydrodynamic diameter (Dh) of SiO2-g-P(PEGMA)-b-P(PEG)on pH (b) at the concentration of 0.2 mg∙ml−1.
Actually, the pH-Responsive self-assembly behavior of SiO2-g-P(PEGMA)-b-P(PEG) was also analyzed by DLS. The Dh of SiO2-g-P(PEGMA)-b-P(PEG) in water is decreased with the ascending of pH in Figure 2(b). The hydration is weakened and the intermolecular forces is reinforced due to the deprotonation degree of P(PEG) segment rose with increasing acidity of aqueous solution. This result leads the pH is decreased with the ascending of pH increased. When pH is above 5.0, the Dh remains essentially unchanged with the ascending of pH. This is due to the hydroxyl of P(PEG) segment combines with the hydroxyl of aqueous solution and this hindered the intermolecular aggregates. So the SiO2-g-P(PEGMA)-b-P(PEG) particles have a good dispersion when pH is above 5.0.
4.3. The Chemical Composition, Morphology and Water Adsorption of Films
The chemical compositions of SiO2-g-P(PEGMA)-b-P(PEG) particles are analyzed by XPS in Figure 3. The particles for S1 (Figure 3(a)), S2 (Figure 3(b)) and S3 (Figure 3(c)) are mainly composed of O, C and Si elements and the electron binding energy of O1s, C1s and Si2p at 530.1 eV, 283.0 eV and 101.2 eV, respectively. With the increasing amount of P(PEG) segment, the single of Si is gradually weakened and the single of C is increased. This has been proved by the chemical composition in Figures 3(a)-(c).
The morphology of film surface for SiO2-g-P(PEGMA)-b-P(PEG) nanoparticles casted from water solution is investigated by AFM (Figures 3(d)-(f)). For three SiO2-g-P(PEGMA)-b-P(PEG) films, due to the decreasing of silica content and the increasing of the P(PEG) segment, the root mean square roughness (Ra) for Sample S1, S2 and S3 is slightly reduced as Ra = 29.7 nm, 28.1nm and 26.8 nm for (Figures 3(d)-(f)), respectively. They are all distributed with particle raised agglomerates and the particle raised agglomerates are reduced from Sample S1 to S3. Therefore, with the increase of P(PEG) segment, the surface roughness of films is decreased in water.
Because the chemical composition and surface roughness contribute much to the surface water adsorption of SiO2-g-P(PEGMA)-P(PEG) films monitored by QCM-D in Figure 4(a). The Δf in the adsorption curves is used to indicate the adsorbed amounts of probe liquids, the ΔD is used to indicate the viscoelasticity of the film (the higher value of ΔD indicating the higher viscoelasticity of the film). In Figure 4(a), the similar absorption curves for water-casted films by SiO2-g-P(PEGMA)-b-P(PEG) (Sample S1, S2 and S3)indicate that Δf and ΔD
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Figure 4. QCM-D curves on the surface of Sample S1, S2 and S3water-casted films (a) and Protein- resistance behavior on the surface of SiO2-g-P(PEGMA)-b-P(PEG) films (Sample S1, S2 and S3) (b).
quickly reach the adsorption equilibrium when water is absorbed on the surface of films, indicating that the P (PEGMA) and P(PEG) chains are distributed on the surface in a relatively ordered structure. The adsorbed amount of water is increased from S1 to S3 (Δf = −447.67, −615.48 and −836.11 Hz, respectively) due to the increasing of the P(PEG) content. At the same time, the viscoelasticity of films is correspondingly increased (ΔD = 96.51, 106.23 and 121.12 × 10−6). These indicates the increasing P(PEG) chains in water results the increase of the water adsorption amount and decrease of the surface roughness (Figures 3(d)-(f)).
4.4. The Application to Protein-Resistance
It is well known that PEG-modified surfaces are normally resisted to protein adsorption [20], and therefore are used to demonstrate the ability of antibiofouling. The less adsorption of BSA demonstrates the better antibiofouling of film. To correlate the relationship between the wettability and the anti-bacteria of obtainedSiO2-g-P(PEGMA)-b-P(PEG), the BSA protein-resistance of the copolymer films is investigated by using the method of QCM-D (Figure 4(b)). The S1-S3 films show just a slightly increase in △f (−7.25 Hz, −7.15 Hz and −6.96 Hz, respectively) after the adsorption of BSA (Figure 4(b)), compared with PBS adsorption. When the PBS solution is continued flowing on the surface about 120 min, the △f almost have no changes to suggest a very little adsorbed amount of BSA protein on the film surfaces. This indicates that Samples S1-S3 have good protein resistance due to their hydrophilic character and sterical effects of PEG chains. The introduction of P(PEG) segments could slight increase the protein-repelling adsorption of SiO2-g-P(PEGMA)-b-P(PEG) films (Δf = −6.96 Hz ~ −7.25 Hz) compared with SiO2-g-P(PEGMA) films (Δf = −9.5 Hz) [19]. This is attributed to the well-distributed hydrophilic PEG chains on the film surfaces to give a repellent property to protein adsorption.
Acknowledgements
This work has been financially supported by the National Natural Science Foundation of China (NSFC Grants No. 51373133, 51573145 and 51073126), by the National Basic Research Program of China (973 Program, No.2012CB720904), by the International Cooperation Project of Shaanxi Province (No.2014KW11) and the State Administration of Cultural Heritage (20110128). The authors also wish to express their gratitude for the MOE Key Laboratory for Non-equilibrium Condensed Matter and Quantum Engineering of Xi’an Jiaotong University.
NOTES
*Corresponding author.