NANO LETTERS 2002 Vol. 2, No. 1 3-7
Letters
Preparation of Polymer Coated Gold Nanoparticles by Surface-Confined Living Radical Polymerization at Ambient Temperature Tarun K. Mandal,† Michael S. Fleming, and David R. Walt* The Max Tishler Laboratory for Organic Chemistry, Department of Chemistry, Tufts UniVersity, 62 Talbot AVenue, Medford, Massachusetts 02155 Received July 16, 2001; Revised Manuscript Received November 17, 2001
ABSTRACT Nanometer sized core−shell particles containing a gold core and poly(methyl methacrylate) (PMMA) shells were prepared by surface-confined living radical polymerization on gold nanoparticles. Gold nanoparticles were prepared in the presence of 11-mercaptoundecanol (MUD) and subsequently esterified with 2-bromoisobutyryl bromide (BIB). Atom transfer living radical polymerization was conducted on the Br-terminated gold nanoparticles using copper(I) bromide/1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (Me4Cyclam) as a room-temperature catalyst. FTIR spectroscopy confirmed the presence of polymer on the nanoparticle surfaces. The resulting gold/polymer nanocomposite particles are stable in suspension. Transmission electron microscopy and UV−vis spectroscopy were performed to characterize the gold nanoparticles (GNPs).
Introduction. Core-shell composite nanomaterials containing a metal core, such as gold, supported by a polymeric shell have been employed to prepare nanoscale building blocks for assembly into functional materials.1,2 Metal particles, particularly gold, silver, and copper have been studied in more detail because of their unique optical properties.3 Metal nanoparticles strongly absorb and scatter light and are therefore useful for preparing electronic and photonic devices as well as new analytical procedures. Metal surface modification and protection from chemical corrosion are important requirements for many applications.4 Unsta* Corresponding author. † Present Address: Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-32, India. 10.1021/nl015582c CCC: $22.00 Published on Web 01/09/2002
© 2002 American Chemical Society
bilized metal nanoparticle suspensions are susceptible to aggregation.5 Many different methods have been reported for stabilizing metal nanoparticles.6-12 One way to tailor the metal surface is by self-assembly of functionalized thiols on the surface.13 This treatment results in a self-assembled monolayer (SAM) around each nanoparticle that helps to stabilize the suspension. Polymeric stabilizers such as polyamidoamine dendrimers9 and thiol functionalized polymers11,14 have also been used to stabilize gold or silver nanoparticles. This method requires a large concentration of stabilizers, which can interfere with surface functionalization. Furthermore, polymer adsorption onto nanometer sized metal particles does not provide a compact core-shell system because of steric restrictions. Core-shell material structures,
Figure 1. Schematic representation for the preparation of initiator and polymer-coated gold nanoparticles.
consisting of an inorganic core and an organic polymer shell, can be prepared with a wide range of functionality, such as block copolymers or end functionalized polymers on gold nanoparticles,1,4,7,15,16 or ceramic micro/nanoparticles.17 To prepare core-shell materials having a dense polymer shell, a “grafting from” technique must be applied using different surface-initiated polymerization methods. Surface confined “living radical” polymerization has been used recently for the synthesis of organic-inorganic hybrid materials. Better uniformity and controlled thickness of the polymer shells are possible if living polymerization techniques are applied. Living radical,17-19 living ionic polymerization techniques,6 or ring opening metathesis polymerization7 can be used to make such core-shell materials. These hybrid materials include micro- or nanoscale coreshell materials,17,19 hybrid organic-inorganic copolymers, dispersed nanosized silicates in polymers,20 nonporous materials,21 and polymer brushes on flat silica22,23 or gold surfaces.24,25 Recently, we reported the synthesis of polymer coated silica microspheres using surface-confined living radical polymerization as well as hollow polymer microspheres by removing the silica core by chemical etching.19 The properties of the core-shell nano- or microparticles prepared by this method can be tuned by changing the composition of the particle or by attaching copolymers with different composition and functionality. Shell thickness and surface uniformity can also be controlled by this polymerization technique. This paper describes the preparation of polymer-coated gold nanoparticles using surface-confined living polymerization at room temperature. The procedure is based on our previously described approach for making polymer-coated silica microspheres.19 Self-assembled monolayers (SAMs) of bromofunctionalized thiolates on gold nanoparticles were used to initiate living radical polymerization. Since the Au-S 4
bond is not stable at high temperature, the polymer chains grown from the terminal bromine on a SAM are not stable to high temperatures (110-150 °C). To solve this problem, we have used copper(I) bromide/1,4,8,11-tetramethyl-1,4,8,11tetraazacyclotetradecane (Me4Cyclam) as a catalyst system to grow the polymer layer from the thiol assembled gold surface at room temperature. In this method, gold nanoparticles are first prepared in the presence of 11-mercapto1-undecanol. A SAM consisting of MUD then forms around individual gold nanoparticles, thereby stabilizing the suspension. The MUD functionalized gold nanoparticles are then esterified with 2-bromoisobutyryl bromide to prepare a Brterminated gold surface. Finally, Br-terminated gold nanoparticles are used as macroinitiators for living radical polymerization of methyl methacrylate from the gold nanoparticle surfaces (Figure 1). The resulting PMMA-coated gold particles were characterized by transmission electron microscopy and UV-vis spectroscopy. Materials. 11-Mercapto-1-undecanol (97%) (MUD), sodium borohydride, hydrogen tetrachloroaurate (III) hydrate, triethylamine, 2-bromoisobutyryl bromide (BIB), methyl methacrylate (MMA), and copper(I) bromide (CuBr) were from Sigma-Aldrich Chemical Co. (Milwaukee, WI). Glacial acetic acid was from Fisher Scientific Inc. (Pittsburgh, PA). 1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecane (Me4Cyclam) was purchased from Acros Organics (Pittsburgh, PA). HPLC grade solvents were used in both the reaction and washing steps. Methyl methacrylate was passed through DHR-4 column (SP2 Scientific Polymer Products, Inc., Ontarion, NY) to remove the inhibitor and was then stored under argon. Tetrahydrofuran (THF) was dried using LiAlH4 and distilled before use. Triethylamine was dried over molecular sieves and distilled. All other reagents were used without further purification. Methods. Preparation of Thiol Functionalized Gold Nano Lett., Vol. 2, No. 1, 2002
Nanoparticles. The method we have used for the production of gold nanoparticles is a modification of the Brust method.26 We substituted 11-mercapto-1-undecanol in the preparation of thiol-capped gold nanoparticles. To prepare the thiolcapped gold nanoparticles, concentrated solutions of gold chloride (HAuCl4) and 11-mercapto-1-undecanol were first prepared in ethanol. 10 mL of a solution containing 6 mM gold chloride and 12 mM 11-mercapto-1-undecanol was prepared by diluting the concentrated stock solutions in ethanol. All solutions were prepared fresh prior to reduction. Glacial acetic acid (167 µL) was then added to the solution with constant stirring. Reduction was carried out by adding 65 µL of a 1.4 M solution of sodium borohydride (NaBH4) in 5 µL portions with constant stirring. The suspension was allowed to stir slowly overnight in the dark and at room temperature in order to form stable thiol monolayers on the gold nanoparticles. After this time, the suspensions appeared grayish-pink in color. Undecanol-coated gold nanoparticles were placed into an eppendorf tube and washed three times (centrifugation at 10 000 g, for 5 min) with ethanol to remove excess alkanethiol from the suspension. The nanoparticles were then washed three times with dry tetrahydrofuran (THF) and derivatized immediately with a living radical BIB initiator. Initiator Modified Gold Nanoparticle Preparation. The scheme for initiator attachment is shown in Figure 1. Undecanol monolayer-coated gold nanoparticles were suspended in 2 mL of dry distilled THF and placed in a 4 mL glass vial. 23 µL (0.165 mmol) of dry triethylamine was added to the nanoparticle suspension. The vial was then sealed with a silicone rubber septum. 27 µL (0.22 mmol) of BIB was injected by syringe into the suspension. The mixture was stirred for 40 min at room temperature. Excess reagent was removed with three cycles of centrifugation (12 000 g), supernatant removal, and resuspending with 1 mL THF each time. Finally, BIB-modified gold nanoparticles were suspended in 0.5 mL p-xylene to perform living radical polymerization. Poly(Methyl Methacrylate)-Coated Gold Nanoparticle Preparation. A 4 mL glass vial was charged with the above gold nanoparticle suspension, 0.008 g (0.0056 mmol) of CuBr, and 0.012 g (0.047 mmol) of Me4Cyclam. The vial was then sealed with a silicone rubber septum, and argon was bubbled through the mixture for 20 min to ensure that oxygen was removed completely. Argon was also bubbled through neat MMA monomer to remove oxygen. First, the mixture was sonicated for 1 min to accelerate dissolution into p-xylene, and then 1 mL of MMA was injected into the mixture by syringe. The polymerization continued with constant stirring (magnetic stir bar/stir plate) for 2, 6, or 18 h at room temperature. After polymerization, the coated nanoparticles were separated from the suspension by centrifugation and then washed several times by centrifuging/ resuspending in THF and methanol. Characterization. FTIR (Nicolet Magna-760, Nicolet Instrument Corporation, Madison, WI) spectroscopy was used to identify the polymer on the gold nanoparticle surface and also to identify the presence of the BIB initiator on the Nano Lett., Vol. 2, No. 1, 2002
gold surface. Spectra were obtained at a resolution of 2 cm-1, and averages of 64-100 spectral/scans (for enhanced signal) were obtained in the wavenumber range 400∼4000 cm-1. Spectra of the undecanol-modified and BIB-modified gold nanoparticles were recorded from KBr pellets, prepared by mixing the nanoparticles with KBr in 1:100 (wt/wt) ratio. FTIR spectra for the PMMA-coated gold nanoparticles were obtained at room temperature by casting a THF suspension of nanoparticles on KBr pellets. UV-vis spectra were recorded on a Beckman model DU 530 spectrometer from a dispersion of PMMA or initiator attached gold nanoparticles. To determine the molecular weight of the adsorbed PMMA on the gold surface, the PMMA chains were first removed from the surface by treating the PMMA-gold nanoparticles with 5 mM I2 in CH2Cl2 solution. Gold nanoparticles were then separated by centrifugation. The PMMA was isolated from the supernatant containing residual I2 by precipitation in methanol. PMMA was further purified by dissolving and precipitating in THF and methanol, respectively. Detached and collected PMMA was identified by FTIR. Molecular weights and molecular distributions were obtained on a Waters 2690 Separation Module (Waters Corporation, Milford, MA) connected to a Waters 410 differential refractometer with CHCl3 as the carrier solvent. Molecular weights were calibrated using polystyrene standards. TEM was performed on a Phillips CM 10 electron microscope. 5 µL of a suspension of gold nanoparticles was dried onto Formvar-coated copper grids and imaged at an accelerating voltage of 80 kV. Results and Discussion. A two step process was used to prepare initiator-modified gold nanoparticles, which were then used as macroinitiators for methyl methacrylate atom transfer living radical polymerization. First, a self-assembled monolayer of undecanol was prepared on gold nanoparticles (GNPs) by reducing HAuCl4 in the presence of MUD. This process yields a stable suspension of -OH-capped GNPs, which are pink in color. The second step involves the esterification of -OH-capped GNPs with BIB to produce the anchored initiator, 2 (Figure 1). Initiator-modified GNP suspensions turn faint blue in color, which indicates that some aggregation occurs during the initiator attachment process. A UV-vis spectrum of the BIB-modified GNP suspension is red shifted compared to the spectrum of MUD-modified GNPs. The resulting surface-modified GNPs could be readily redispersed in organic solvents for the living radical polymerization step. The presence of the surface initiator groups on the GNPs was confirmed by FTIR spectroscopy. The FTIR spectra of initiator-assembled and polymer-coated GNPs will be discussed in more detail later in this section. The initiator-modified GNPs were then subjected to living radical polymerization of methyl methacrylate. As expected, polymer growth was mostly confined to the surface of the initiator assembled GNPs. We observed that a very small amount of polymer formed in solution. It is known that alkanethiol-based SAMs on gold surfaces are not completely stable in organic solvents.24,27 Some of the assembled 5
Figure 3. Transmission electron micrographs of (a) initiatorassembled, and (b) PMMA-coated gold nanoparticles.
Figure 2. UV-vis spectra of PMMA-coated GNPs (top spectrum), initiator-assembled GNPs (middle spectrum), and undecanolmodified GNPs (bottom spectrum).
initiators are expected to desorb from the gold surface during polymerization, which then results in the production of polymer in solution. PMMA-coated GNPs were washed several times with THF, with the expectation that physically adsorbed polymer would be rinsed off. FTIR spectra of PMMA-GNPs showed bands corresponding to poly(methyl methacrylate). The PMMA-GNP core-shell composite can be redispersed in a variety of organic solvents such as CH2Cl2 and THF. Suspension in these solvents resulted in a pinkishred color characteristic of unaggregated gold particles. MUD and BIB-modified GNP suspensions were unstable and precipitated very quickly from a THF suspension, but the PMMA-coated GNPs formed a very stable dispersion in THF and remained so for as long as the sample was kept (at least one week). Although we did not investigate longer term stability issues, we observed no changes in the UV-vis spectrum of the sample during this time. In addition, no precipitation was visible in the preparation. We believe any evidence of lack of stability would have been detected by either UV-vis absorption spectra or by visual inspection. Non-PMMA coated nanoparticle controls precipitated within several hours. The PMMA-GNP composite cannot be redispersed in methanol or ethanol, whereas BIB-GNPs or MUD-GNPs can be redispersed. The difference in stability and solubility between the MUD- or BIB-modified GNPs and the PMMA-GNP composite suspensions suggests that the surface has been extensively modified. As shown in Figure 2, the MUD-GNP suspension exhibited a plasmon absorption band at 542 nm, which is red-shifted when the -OH groups on the surface were esterified with the initiator. Association of nanoparticles or the change of dielectric constant28 of the surrounding medium would give rise to such a shift and might be responsible for 6
the peak broadening observed in the BIB-modified GNP spectra. Centrifugation of the BIB-modified GNPs may affect the particle size distribution, which also could affect the plasmon band position.29 PMMA-GNP core-shell composite suspensions have a sharp blue-shifted plasmon band at 552 nm when polymer is on the gold surface, compared to the BIB-modified GNPs. TEM images of uncoated and polymer-coated GNPs are shown in Figure 3. Average sizes of the GNPs are in the range of 50-70 nm estimated from TEM micrographs. According to Mie theory, the gold nanoparticle size can be correlated with the gold plasmon band.30 On this basis, the measured GNP sizes are in close agreement with the plasmon peak in the UV-vis spectra of the GNP suspension (Figure 2). We observed a thin polymer layer around the gold core, which appears very faint in TEM pictures because the polymer layer contains less electron density relative to the gold nanoparticle cores (Figure 3b). To confirm that the gold nanoparticles were coated with initiator and polymer, FTIR characterization was performed on each of the samples. The FTIR spectrum of the thiolderivatized GNPs shows bands at 2847 and 2917 cm-1, characteristic of stretching vibrational modes of the methylene groups in the assembled alkane chain of undecanol (Figure 4a). The FTIR spectrum of initiator immobilized GNPs also exhibits bands corresponding to C-H stretching along with a peak at 1735 cm-1, characteristic of the ester carbonyl group in the BIB initiator (Figure 4b). The FTIR spectrum of the PMMA-coated GNP (Figure 4c) is similar to the spectrum of neat PMMA (Figure 4d) and shows the characteristic carbonyl peak 1732 cm-1 of PMMA, confirming that nanoparticles are coated with polymer. The molecular weight and polydispersity of the PMMA coating were measured by removing PMMA from the gold surface using I2. The resulting soluble PMMA was characterized by gel permeation chromatography (GPC). For the 2 h and 6 h cleaved samples, GPC yields a Mn of 10 800 and 27 500, respectively. The polydispersity indices (PDI) of these two samples are 1.36 and 1.21, which is consistent with that expected from living polymerization (PDI < 1.5).31 There is a possibility that the detached PMMA chain might exist as disulfides. To verify that we were dealing with the free thiol, the cleaved PMMA solution was treated with NaBH4 and the GPC measurements were repeated. The same Nano Lett., Vol. 2, No. 1, 2002
Figure 4. FTIR spectra of (a) MUD-modified gold nanoparticles, (b) BIB-initiator assembled gold nanoparticles, (c) PMMA-coated gold nanoparticles, and (d) pure PMMA.
Mn and PDI were obtained as before NaBH4 reduction, indicating that no disulfides were formed. It has been reported that atom transfer radical polymerizations (ATRP) of methacrylamide using Me4Cyclam catalyst are poorly controlled, resulting in high polydispersity polymers.32 These same authors were able to prepare low polydispersity block copolymers of methacrylamide by using polyacrylates prepared by ATRP as macroinitiators with the CuBr/Me4Cyclam catalyst system.32 In conclusion, nanometer sized core-shell composite materials having a metal core and a polymer shell were prepared by surface confined living radical polymerization using Me4Cyclam at ambient temperature. This strategy should be easily extended to other inorganic nanoparticle templates. For example, there are methods for preparing CdS quantum dots from aqueous solutions with particles on the order of 1-10 nm in diameter. Preparation of polymer-coated quantum dots should be possible with the methods we describe herein. The approach should also be extendable to the preparation of core-shell materials with block copolymer shells by the sequential activation of the dormant chain in the presence of different monomers during polymerization. References (1) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746-748.
Nano Lett., Vol. 2, No. 1, 2002
(2) Simard, J.; Briggs, C.; Boal, A. K.; Rotello, V. M. J. Chem. Soc., Chem. Commun. 2000, 1943-1944. (3) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer Series in Material Science, No. 25; Springer-Verlag: Berlin, 1995. (4) Quaroni, L.; Chumanov, G. J. Am. Chem. Soc. 1999, 121, 1064210643. (5) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer Series in Material Science, No. 25; Springer-Verlag: Berlin, 1995. (6) Jordan, R.; West, N.; Ulman, A.; Chou, Y.-M.; Nuyken, O. Macromolecules 2001, 34, 1606-1611. (7) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462-463. (8) Walker, C. H.; St. John, J. V.; Wisian-Neilson, P. J. Am. Chem. Soc. 2001, 123, 3846-3847. (9) Groehn, F.; Bauer, B. J.; Akpalu, Y. A.; Jackson, C. L.; Amis, E. J. Macromolecules 2000, 33, 6042-6050. (10) Grohn, F.; Bauer, B. J.; Amis, E. J. Mater. Res. Soc. Symp. Proc. 2001, 628, CC2.7.1-CC2.7.6. (11) Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 6364-6369. (12) Hao, E.; Lian, T. Chem. Mater. 2000, 12, 3392-3396. (13) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. (14) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696-12697. (15) Song, Q.; Ai, X.; Wang, D.; Hong, X.; Wei, L.; Yang, W.; Liu, F.; Bai, Y.; Li, T.; Tang, X. J. Nanoparticle Res. 2000, 2, 381-385. (16) Marinakos, S. M.; Brousseau, L. C., III; Jones, A.; Feldheim, D. L. Chem. Mater. 1998, 10, 1214-1219. (17) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 1999, 121, 74097410. (18) Huang, X.; Wirth, M. J. Anal. Chem. 1997, 69, 4577-4580. (19) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Chem. Mater. 2000, 12, 3481-3487. (20) Weimer, M. W.; Chen, H.; Giannelis, E. P.; Sogah, D. Y. J. Am. Chem. Soc. 1999, 121, 1615-1616. (21) Volksen, W.; Miller, R. D.; Hawker, C. J.; Hedrick, J. L.; Toney, M.; Rice, P.; Magbitang, T.; Lee, V.; Huang, E.; Rodbell, K.; Lynn, K.; Petkov, M. P.; Weber, M. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 2001, 42, 163-164. (22) Husseman, M.; Malmstrom, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 14241431. (23) Huang, W.; Baker, G. L.; Bruening, M. L. Angew. Chem., Int. Ed. Engl. 2001, 40, 1510-1512. (24) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597-605. (25) Huang, W.; Skanth, G.; Baker, G. L.; Bruening, M. L. Langmuir 2001, 17, 1731-1736. (26) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655-6. (27) Bain, C. D.; Troughton, E. B.; Tao, Y.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (28) Mulvaney, P. Langmuir 1996, 12, 788-800. (29) Colloidal Gold: Principles, Methods and Applications; Hayat, M. A., Ed.; Academic Press: San Diego, CA, 1991. (30) Mie, G. Ann. Physik 1908, 25, 377. (31) Patten, T. E.; Matyjaszewski, K. Acc. Chem. Res. 1999, 32, 895903. (32) Teodorescu, M.; Matyjaszewski, K. Macromolecules 1999, 32, 4826-4831.
NL015582C
7
