Langmuir 1999, 15, 2745-2748
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Photodeposition of Molecular Layers on Nanoparticle Substrates Daniel B. Wolfe,†,‡,§ Steven J. Oldenburg,‡ Sarah L. Westcott,‡ Joseph B. Jackson,‡ Mark S. Paley,§ and Naomi J. Halas*,‡ Department of Chemistry, Department of Electrical and Computer Engineering, The Rice Quantum Institute, and the Center for Nanotechnology Science and Technology, Rice University, 6100 South Main, Houston, Texas 77005, and Universities Space Research Association, National Aeronautics and Space Administration, Space Sciences Laboratory-ES 76, Marshall Space Flight Center, Huntsville, Alabama 35812 Received November 13, 1998 We report the photodeposition of polymeric layers of nanometer scale thickness onto silica nanoparticle substrates. This was accomplished by ultraviolet irradiation of a solution of functionalized diacetylene monomers in which the silica nanoparticles were suspended. Following photodeposition, the coated nanoparticles were analyzed using transmission electron microscopy and UV-visible spectroscopy. Highly regular polydiacetylene films with thicknesses from 2.5 to 25 nm were produced. The thickness measurements were facilitated by the attachment of small gold nanoparticles onto the surface of the silica nanoparticle substrates prior to photodeposition, to provide contrast in the final TEM image. UV-visible spectroscopy of the deposited films indicates that approximately 40% less conjugation is present relative to that of macroscopic polydiacetylene thin films grown with the same approach. This process yields a unique “nanolaminate” coating which may be useful in the modification of the physical, chemical, or optical properties of nanoparticles.
Introduction There is currently a great deal of interest in the control of nanoparticle properties via modification of their surfaces. Characteristics such as solubility,1 resonant optical properties,2 electrical and mechanical aspects of nanoparticle-constituent materials,3,4 and even the agglomeration of accompanying colloidal particles in solution5 are all influenced by chemical functionalization or film growth on nanoparticle surfaces. There has also been interest in the coating of nanoparticles with polymer layers, given the wide range of adjustable physical and chemical properties of these materials. The recent preparation and characterization of polystyrene nanoparticles coated with either polypyrrole or polyaniline by oxidative catalytic polymerization of the monomer in solution with polystyrene nanoparticle substrates has been reported.6-8 These nanoparticles were characterized with X-ray photoelectron spectroscopy * To whom correspondence should be addressed. E-mail: halas@ faraday.rice.edu. † Department of Chemistry, Rice University. ‡ Department of Electrical and Computer Engineering, The Rice Quantum Institute, and the Center for Nanotechnology Science and Technology, Rice University. § National Aeronautics and Space Administration. (1) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (2) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (3) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (4) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (5) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396-5401. (6) Perruchot, C.; Chelimi, M. M.; Delamar, M.; Lascelles, S. F.; Armes, S. P. Langmuir 1996, 12, 3245. (7) Barthet, C.; Armes, S. P.; Chehimi, M. M.; Bilem, C.; Omastova, M. Langmuir 1998, 14 (4), 5032. (8) Barthet, C.; Armes, S. P.; Lascelles, S. F.; Luk, S. Y.; Stanley, H. M. E. Langmuir 1998, 14 (4), 2032.
(XPS),6,7 an established technique for the characterization of macroscopic polymer surfaces.9-12 This method provides useful information regarding the chemical nature of the polymer coating but yields little structural information, since the typical sampling depth of XPS is 2-5 nm. Pellet conductivity measurements have also been utilized as an indirect method for determining the thickness of the polymer layer.7 Transmission electron microscopy (TEM) can provide a direct and quantitative method for determining the thickness of a nanoparticle polymer coating; in principle, however, low mass contrast between the nanoparticle core and coating materials prevents distinguishing between the nanoparticle and its coating in the TEM image. The photodeposition of polydiacetylenes has proven to be a robust and versatile process for depositing polymer thin films onto a variety of macroscopic substrates.13 This technique involves ultraviolet (UV) irradiation of a monomer solution through a UV-transparent substrate which is in contact with the solution. In regions where the substrate-monomer surface is irradiated, a polydiacetylene film is deposited. Masking studies have clearly shown that this process is a surface reaction and does not involve the transport of polymer particles from solution to the substrate.13 The macroscopic films obtained by this method are of high optical quality, and the process is very reproducible. The photodeposition process provides the (9) Briggs, D.; Seah, M. P. Practical Surface Anaylsis, Auger and X-ray Photoelectron Spectroscopy, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, U.K., 1990; Vol. 1. (10) Deslandes, Y.; Mitchel, D. F.; Paine, A. J. Langmuir 1993, 9, 1468. (11) Maeda, S.; Gill, M.; Armes, S. P.; Fletcher, I. W. Langmuir 1995, 11, 1899. (12) Beadle, P. M.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 1996, 12, 1784. (13) Paley, M. S.; Frazier, D. O.; Abdeldeyem, H.; Armstrong, S.; McManus, S. P. J. Am. Chem. Soc. 1995, 117, 4775.
10.1021/la981593i CCC: $18.00 © 1999 American Chemical Society Published on Web 03/25/1999
2746 Langmuir, Vol. 15, No. 8, 1999 Scheme 1. Synthetic Scheme for the Preparation of DAPNA.
flexibility to polymerize with a UV laser and create patterned films. Total film thickness is limited to 2 µm due to the UV absorption depth, however, which limits the usefulness of this technique for the fabrication of optical devices. In this paper we show that highly controlled polydiacetylene coatings can be successfully grown onto silica nanoparticle surfaces via a straightforward adaptation of this photodeposition method. The nanoparticle substrates are suspended in solution in the presence of the monomer and irradiated. The resulting nanoparticles are characterized using UV-visible spectroscopy and TEM. The use of gold colloid-decorated silica nanoparticle substrates for polymer photodeposition greatly facilitates TEM evaluation of the homogeneity and thickness of the resultant nanoparticle polymer films. These new coated particles may present a novel constituent for new organic-inorganic composite materials and may exhibit interesting and useful optical or nonlinear optical properties. Experimental Section The diacetylene monomer used for this study (3) is shown in Scheme 1. This compound is able to undergo photodeposition on macroscopic substrates such as quartz and mica.13 The major difference between this molecule and the one reported previously is the presence of the tertiary aniline instead of the secondary aniline. The tertiary aniline is sterically hindered enough to rule out the attachment of the molecule to the surface of the nanoparticle through an amine linkage. Furthermore, we would not expect any side reactions to occur at this site. The compound is prepared as shown in Scheme 1 and is described below. All reagents listed were purchased from Aldrich Chemical Co. and used without further purification unless otherwise noted. All solvents used were reagent grade or better and used straight from the container. 1H NMR studies were performed on a 250 MHz Bruker spectrometer at Rice University. Transmission electron microscope images were obtained using lacey carbon grids purchased from Electron Microscopy Sciences. The images were taken at Rice University on a JEOL JEM-2010 transmission electron microscope. Synthetic Methods. N-Methyl-N-propargyl-4-nitroaniline (1).14 4-Fluoro-1-nitrobenzene (10.20 g, 72.3 mmol) is placed in a round bottom flask with N-methylpropargylamine (5 g, 72.3 mmol), potassium carbonate (11.22 g, 81 mmol), triethylamine (1.7 mL), and DMSO (4 mL). The mixture is stirred at 50 °C for 2 days and then poured into cold water. The crude precipitate (14) Garito, F.; Horner, C. J.; Kalyanaraman, P. S.; Desai, K. N. Makromol. Chem. 1980, 181, 1605.
Wolfe et al. is isolated by vacuum filtration and washed repeatedly with water. The product is then purified by recrystallization from hot toluene twice to yield pure 1. Yield: 10.18 g (74%). 1H NMR (d6-acetone): δ 2.79 (t, 1H), 3.19 (s, 3H), 4.33 (d, 2H), 6.96 (d, 2H), 8.13 (d, 2H). N-Methyl-N-(3-bromopropargyl)-4-nitroaniline (2).15,16 A solution of 1 (5 g, 26.2 mmol) in acetone (50 mL) is prepared in a round bottom flask equipped with a stir bar. Silver nitrate (436 mg) is added, and the mixture is allowed to stir for 15 min at room temperature until the mixture appears cloudy. Solid N-bromosuccinimide (5.45 g, 30.4 mmol) is added slowly, and the mixture is allowed to stir at room temperature for another 20 min. The mixture is then poured over ice water, and a yellow precipitate appears. This crude precipitate is isolated by vacuum filtration. The product is purified by dissolving the precipitate in ethyl acetate and washing with water. The organic layer is then separated and dried over MgSO4, and the solvent is removed in a vacuum to yield pure 2 in the form of yellow crystals. This product must be stored in the absence of light and at 0 °C. Yield: 5.57 g (79%). 1H NMR (d6-acetone): δ 3.19 (s, 3H), 4.40 (s, 2H), 6.95 (d, 2H), 8.13 (d, 2H). N-Methyl-N-(2,4-hexadiyn-6-ol)-4-nitroaniline (DAPNA) (3).17 A solution of propargyl alcohol (0.250 mL, 4.3 mmol), ethylamine (70% aqueous solution, 5 mL), and tetrahydrofuran (5 mL) is prepared in a three-neck round bottom flask equipped with an addition funnel and a nitrogen inlet. The contents of the flask are kept under nitrogen at all times. The addition funnel is charged with 2 (1.12 g, 4.3 mmol) in tetrahydrofuran (5 mL) under N2. CuCl (0.08 g) is added to the solution in the flask and immediately turns the solution blue. The contents of the addition funnel are added dropwise over 20 min, causing a color change to green. Once the reaction is complete, a few crystals of hydroxylamine hydrochloride are added, turning the solution yellow. This solution is allowed to stir under N2 for 4 h at room temperature covered with aluminum foil to block out light. Pouring the solution into cold water and isolating the precipitate by vacuum filtration terminates the reaction. The crude product is then purified by column chromatography (1% acetone/99% methylene chloride) and the solvent is removed in a vacuum to yield pure (3) as yellow crystals. Yield: 550 mg (52%). TLC (1% acetone/99% methylene chloride): Rf ) 0.3. 1H NMR (d6acetone): δ 3.19 (s, 3H), 4.24 (d, 2H), 4.40 (t, 1H), 4.50 (s, 2H), 6.95 (d, 2H), 8.13 (d, 2H). The substrate material is prepared through established procedures resulting in silica nanoparticles of 100-150 nm in diameter.18 These particles are prepared in a water/ethanol mixture. Previous work has shown that ethanol is not an ideal solvent for photodeposition.19 Therefore, the particles are centrifuged and redispersed in 1,2-dichloroethane twice. This solvent has yielded the highest quality thin films of DAPNA through photodeposition onto planar substrates.13 Initial experiments showed there was insufficient mass contrast in the TEM images between the polydiacetylene coating and the silica nanoparticle to obtain quantitative information on coating thickness. To overcome this difficulty, we proceeded to prepare small gold nanoparticles (5 nm diameter) stabilized with sodium citrate20 and attached these to silica nanoparticles coated with 5 wt % (aminopropyl)trimethoxysilane/95 wt % propyltrimethoxysilane.2-5 The gold particles in solution (20 mL) were added to a small amount of functionalized silica nanoparticles in solution (5 mL), and the resulting mixture was agitated for 1 min and allowed to react at room temperature for 1 h. After centrifuging and redispersing the mixture, we obtained silica particles with immobilized gold nanoparticles attached at a low surface coverage (
