Langmuir 2001, 17, 6027-6029
Monolayer Grafting of Organo-Silica Nanoparticles on Poly(ethylene naphthalate) Films
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Scheme 1. Modification of PEN Film
J. S. Wall,† B. Hu,† J. A. Siddiqui,‡ and R. M. Ottenbrite*,† Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, and Dupont Films, Bermuda Hundred, Hopewell, Virginia 23860 Received April 4, 2001. In Final Form: July 2, 2001
Introduction Monolayer assemblies of nanoparticles are receiving increasing attention due to their potential for uses in functional templates, catalysis, sensor arrays, optical materials, and photonic crystal devices.1 A variety of methods have been used to assemble particles into regular arrays for these applications, including deposition on polyelectrolyte layers,1 capillary forces,2 sedimentation,3 self-assembled monolayer templates,4 complex-fluid templation,5 and electrophoretic deposition.6 A procedure was developed, in our laboratory, to produce organo-silica hybrid nanoparticles using (aminopropyl)triethoxysilane (APS) as the catalyst and comonomer with other hydrophobic organo-silanes in a sol-gel condensation reaction.7 Particles were produced from APS with other comonomers including methyltrimethoxysilane, vinyltrimethoxysilane, allyltrimethoxysilane, and (p-chloromethyl)phenyltrimethoxy-silane. All of these organosilica nanoparticles had amine groups on the surface. Herein, we report a unique monolayer grafting of these novel organo-silica nanoparticles to the surface of poly(ethylene naphthalate) (PEN) film. The particles are covalently bonded to the surface through amide bonds that formed between the amine groups on the nanoparticles and the surface-activated PEN film. Experimental Section Poly(ethylene naphthalate) film (85 µm) was supplied by Dupont/Teijin Films, Hopewell, VA. (Aminopropyl)triethoxysilane, Tergitol NP-9, toluene, dimethylacetamide (DMAc), tris(2-aminoethyl)amine (TAEA), 1-[3-(dimethyl-amino)propyl]-3ethylcarbodiimide hydrochloride, and poly(acrylic acid) (PAA) (1 250 000 Mw) were purchased from Aldrich. Prior to use, toluene was refluxed over sodium and then distilled. Allyltrimethoxysilane (ATS) was purchased from Gelest. Spectra/Por regenerated cellulose dialysis membranes (3500 Mw cutoff) were purchased from Fisher Scientific. Dialysis membranes were soaked in deionized water and then rinsed with deionized water to remove glycerol. All other reagents were used as received. Nanoparticles were prepared similarly to a previously published procedure.7 Allyltrimethoxysilane (0.800 mL) was dispersed in a premixed 20 mL solution of deionized water and † ‡
Virginia Commonwealth University. Dupont Films.
(1) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825-7834. (2) Denkov, N. D.; Velev, O. D.; Dralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26. (3) Miguez, H.; Meseguer, F.; Lo´pez, C.; Mifsud, A.; Moya, J. S.; Va`zquez, L. Langmuir 1997, 13, 6009-6011. (4) Masuda, Y.; Seo, W. S.; Koumoto, K. Jpn. J. Appl. Phys. 2000, 39, 4596-4600. (5) Dabbs, D. M.; Aksay I. A. Annu. Rev. Phys. Chem. 2000, 51, 601622. (6) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706-709. (7) Ottenbrite, R. M.; Wall, J. S.; Siddiqui, J. A. J. Am. Ceram. Soc. 2000, 83, 3214-3215.
1.00 g Tergitol NP-9, and then 0.800 mL of (aminopropyl)triethoxysilane was added. The reaction mixture was stirred overnight, and the particle dispersion obtained was dialyzed against deionized water to remove excess surfactant and oligomeric silanes. The PEN film was modified as follows. Samples of PEN (Dupont Polyester Film, 3 cm × 10 cm × 85 µm thick) were rinsed with distilled water, methanol, and hexane and then dried at reduced pressure overnight. TAEA (3 mL) was added to a 250 mL roundbottom flask containing 100 mL of toluene and 10 PEN film samples. The reaction mixture was heated at 80-90 °C under a nitrogen atmosphere for 4 h. The film samples were removed, rinsed with toluene, methanol, and distilled water, and then dried at reduced pressure overnight to give (PEN-TAEA). In a 250 mL round-bottom flask, 1.0 g of PAA was dissolved in 100 mL of DMAc, and 10 samples of PEN-TAEA film were added. Nitrogen was slowly bubbled through the mixture while it was heated to 130 °C for 3 h. The film samples were removed, washed consecutively with 5% NaHCO3, distilled water, and methanol, and then dried at reduced pressure overnight. The PAA on the PEN surface was activated with a watersoluble carbodiimide using a procedure similar to that reported by Kishida et al.8 The film was immersed in a 1 mg/mL solution of 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride cooled to 4° C for 30 min. The film was removed from the solution, rinsed with cold water, and placed in a dispersion of nanoparticles for 2 h. The grafted film was removed from the dispersion and washed with copious amounts of water and then with THF. The stability of the nanoparticles on the surface was evaluated by (a) immersion of the grafted film in 1 N hydrochloric acid for times up to 2 h and (b) ultrasonication of the grafted film in a Bransonic 3200 ultrasonic cleaner for times up to 1 h. Scanning electron microscopy (SEM) was carried out on a Quantum DS-130S dual stage electron microscope. Atomic force microscopy (AFM) was performed on a Digital Instruments Nanoscope IIIa in tapping mode. Reflectance FT-IR spectra were obtained with a Nicolet 550 infrared spectrometer.
Results and Discussion The procedure to graft nanoparticles to the surface of the film involved three steps: (a) preparation of the organosilica nanoparticles with surface amine groups, (b) modification of the film surface with PAA, and (c) grafting the organo-silica nanoparticles to the film surface. (8) Kishida, A.; Ueno, Y.; Maruyama, I.; Akashi, M. Biomaterials 1994, 15, 1170-1174.
10.1021/la0105073 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/16/2001
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Langmuir, Vol. 17, No. 19, 2001
Notes
Figure 2. (A) AFM of 150-400 nm particles and (B) AFM of 80-100 nm particles.
Figure 1. (A) SEM of 1.25 × 106 MW PAA-modified PEN film (bar ) 1 µm). (B) SEM of 150-400 nm particles grafted to PEN film (bar ) 1 µm). (C) SEM of 80-100 nm particles grafted to PEN film (bar ) 667 nm).
Nanoparticles were prepared similarly to a previously published procedure.7 Two distinct particle sizes were prepared by adjusting the concentration of APS in the initial reaction mixture. A ratio of 1.6 mL APS/0.8 mL ATS with 1.00 g of surfactant produced particles with a size range of 150-400 nm, while a ratio of 0.8 mL APS/0.8 mL ATS produced particles with a size range of 80-100 nm. The ninhydrin test (a dark purple color) showed that there were a significant number of amine groups located on the surface of the nanoparticles available for further reaction. These surface amine groups were used to graft the nanoparticles to the surface of the activated PEN film described below. The PEN film surface was activated, before nanoparticle grafting, by the process shown in Scheme 1. The film surface was reacted with TAEA to form an amine-modified
surface through an aminolysis reaction. Subsequently, PAA, 1 250 000 Mw, was attached to the amine surface through the initial formation of ionic ammonium carboxylate bonds. After heating at 130 °C in DMAc for 3 h, these salts were converted to the corresponding amide bonds to afford a stable, highly functionalized carboxylic acid surface.9 After the PEN film was modified with TAEA and PAA, the reflectance IR showed new peaks appearing at 3450-3300 cm-1 (free N-H and hydrogen bond of N-H) and 1680 cm-1 (carbonyl of amide group). These peaks indicated that the PAA had been attached through amide bonds to the surface of PEN films. The carboxylic acid groups were then activated, to amine substitution, with a water-soluble carbodiimide using a procedure similar to that for protein immobilization on poly(methyl methacrylate) films.8 The activated PEN film was immersed, for 2 h, in a dispersion of nanoparticles prepared as described above. The film was removed from the dispersion and washed with water and then THF to remove physically adsorbed particles. The films, after air-drying, were clear to hazy depending on the size of the particles grafted to the surface. The film samples were observed by SEM (Figure 1). The 80-100 nm particles show near monolayer coverage, with approximately 230 particles/µm2, while the 150400 nm particles are much more disperse. When the same procedure was used to immobilize 2-3 µm APS-modified glass beads, no particles were observed to have adhered to the PEN film surface (data not published). It appears that the amide bonds that formed between the more (9) Suzuki, K.; Siddiqui, S.; Chappel, C.; Siddiqui, J. A.; Ottenbrite, R. M. Polym. Adv. Technol. 2000, 11, 92-97.
Notes
massive particles and the film may be inadequate to permanently secure them to the film. To establish that the monolayer of particles was indeed bonded and not just adsorbed to the surface, the film was subjected to two different rigorous environments. The first involved immersing the monolayer films in 1 N HCl for 10 min, 20 min, 40 min, 1 h, and 2 h. Films were also subjected to ultrasonication for times up to 1 h. Since both of these rigorous methods are capable of inducing bond cleavage, some erosion of the particles from the surface was expected; however, SEM analysis of these films showed very little change in the monolayer density. The density of the monolayer remained >200 particles/ µm2. Unactivated films were similarly immersed in dispersions of nanoparticles. Although some nanoparticles did adhere to the film, they were easily removed by simple washing. AFM was used to study the monolayer on the surface of the films. In Figure 2 are micrographs that show (a) the roughness of the modified film surface and (b) the grafted nanoparticles on the film. The two different particle size ranges, 80-100 and 150-400 nm, as determined by SEM, were confirmed by the AFM analysis. The AFM data
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clearly illustrate that the grafted particles are one particle in depth, forming a monolayer array. The graft density of the 80-100 nm particles appears to be slightly less than that observed by SEM. This is due to slight variations in monolayer density on the film surface and the relatively small sample area. The observation that no aggregates were seen in any of the SEM or AFM micrographs supports the fact that our procedure provides stable organo-silica nanoparticles. Conclusions A unique monolayer of organo-silica nanoparticles was chemically bonded to the surface of PEN film. Nanoparticles were prepared with (aminopropyl)triethoxysilane, which afforded amine groups on the particle surface. The surface of the PEN film was modified with poly(acrylic acid) which, after activation, was reacted with the amine groups on the nanoparticles. SEM and AFM showed a dense monolayer of 80-100 nm diameter particles on the film surface. Stable grafting was demonstrated by rigorous conditions that failed to remove the particles from the treated films. LA0105073
