Polymer-Stabilized Gold Nanoparticles with High Grafting Densities

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Langmuir 2004, 20, 2867-2873

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Polymer-Stabilized Gold Nanoparticles with High Grafting Densities Muriel K. Corbierre, Neil S. Cameron,† and R. Bruce Lennox* Department of Chemistry, McGill University, 801 Sherbrooke St. West, Montreal, Quebec H3A 2K6, Canada Received August 25, 2003. In Final Form: December 8, 2003 A series of polymer-coated Au nanoparticles have been prepared using the “grafting-to” approach. Thiolterminated polystyrene and poly(ethylene oxide) ligands are found to form dense brushes on the faceted gold nanoparticle surfaces. Depending on the polymer, the ligand grafting densities on the gold nanoparticles are 1.2- to 23.5-fold greater than those available via self-assembled monolayer formation of the corresponding two-dimensional gold surfaces.

Introduction Functionalized metal nanoparticles have been the subject of intense activity for the past decade, particularly since Brust, Schiffrin, and co-workers introduced new synthetic procedures which provide access to fairly monodisperse, thiol-stabilized gold nanoparticles.1,2 Murray and co-workers extended the solubility possibilities of these materials by using place-exchange reactions of one thiol for another.3,4 Gold nanoparticles are of special interest due to their potential applications in biomedical, electronic, and optical materials.5-11 To date, few linear polymer-stabilized metal nanoparticles with a single point of attachment have been reported in the literature.12-19 Using a “grafting-to” * To whom correspondence may be addressed. E-mail: [email protected]. † Current address: IMI-NRC, 75 de Mortagne Blvd., Boucherville, QC J4B 6Y4, Canada. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (2) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (3) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (4) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782-3789. (5) Vossmeyer, T.; Guse, B.; Besnard, I.; Bauer, R. E.; Mullen, K.; Yasuda, A. Adv. Mater. 2002, 14, 238-242. (6) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959-1964. (7) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856-2859. (8) Demaille, C.; Brust, M.; Tsionsky, M.; Bard, A. J. Anal. Chem. 1997, 69, 2323-2328. (9) Al-Rawashdeh, N. A. F.; Sandrock, M. L.; Seugling, C. J.; Foss, C. A. J. J. Phys. Chem. B 1998, 102, 361-371. (10) Evans, S. D.; Johnson, S. R.; Cheng, Y. L.; Shen, T. J. Mater. Chem. 2000, 10, 183-188. (11) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. Adv. Mater. 2001, 13, 1501-1505. (12) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696-12697. (13) Teranishi, T.; Hosoe, M.; Miyake, M. Adv. Mater. 1997, 9, 6567. (14) Teranishi, T.; Kiyokawa, I.; Miyake, M. Adv. Mater. 1998, 10, 596-599. (15) Jordan, R.; West, N.; Ulman, A.; Chou, Y.-M.; Nuyken, O. Macromolecules 2001, 34, 1606-1611. (16) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ru¨hm, A.; Lennox, R. B. J. Am. Chem. Soc. 2001, 123, 10411-10412. (17) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Nano Lett. 2002, 2, 3-7. (18) Lowe, A. B.; Sumerlin, B. S.; Donovan, M. S.; McCormick, C. L. J. Am. Chem. Soc. 2002, 124, 11562-11563.

approach, Murray and co-workers reported the synthesis of poly(ethylene glycol)-functionalized gold nanoparticles (PEO-Au).12 Corbierre et al. reported polystyrene-functionalized gold nanoparticles (PS-Au) by the covalent attachment of a thiol-terminated polystyrene prepared by anionic polymerization.16 Alternatively, a “graftingfrom” method allows poly(methyl methacrylate) to be grown at the surface of gold nanoparticles using a surfaceconfined living radical polymerization process.17 Poly(nbutyl acrylate) can also be grown at the surface of gold nanoparticles by atom transfer radical polymerization (ATRP).19 In the “grafting-from” technique, an alkylthiolcapped gold nanoparticle is usually used as a precursor to the polymer capped material. Chart 1 shows a graphic representation of a gold nanoparticle composed of a truncated octahedral gold core with several polymer chains grafted to the core facets. Exploring the many interesting electronic, electrical and spectroscopic properties of dispersed metal nanoparticles6-8,20,21 is of course dependent upon having access to the nanocomposite of interest. The process of blending or dispersing nanoparticles in a polymer matrix is, however, problematic, as it is necessary that the nanoparticles and the host matrix be chemically compatible (e.g., a favorable χ parameter22). In a previous study, tetradecanethiol-stabilized gold nanoparticles (C14-Au) were incorporated into several polymer matrices composed of polystyrene (PS) and poly(dimethylsiloxane) (PDMS). However this leads systematically to aggregates, which are made of up to hundreds of particles. Reasoning that nanoparticles decorated with a ligand whose chemical nature matches the matrix would be less prone to aggregation (i.e., χ ) 0), we have shown that PS125-Au nanoparticles can be individually dispersed in high molecular weight PS matrices.16 However, in addition to χ parameter considerations, the blending of polymer brushes into polymer matrices is also sensitive to both the brush and matrix molecular weights, as well as the (19) Nuss, S.; Bo¨ttcher, H.; Wurm, H.; Hallensleben, M. L. Angew. Chem., Int. Ed. 2001, 40, 4016-4018. (20) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; O’Horo, M. P.; Ganguly, B. N.; Mehrotra, V.; Russell, M. W.; Huffman, D. R. Science 1992, 257, 219-223. (21) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302-1317. (22) The Flory-Huggins parameter (χ) describes the energy of interaction of a given polymer segment with other polymers or solvent molecules. When the interactions are favourable (i.e., miscibility is achieved), χ is negative or zero, and when the interactions are unfavourable, χ is positive.

10.1021/la0355702 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/25/2004

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Corbierre et al.

Tetradecanethiol (C14-SH), R-methylstyrene, sec-butyllithium (1.4 M in cyclohexane), styrene, propylene sulfide, HAuCl4.3H2O, and Superhydride were bought from Aldrich and used as received unless otherwise indicated. Tetrahydrofuran, ethanol, methanol, and chloroform were bought from Fisher Scientific. Deuterated chloroform (99.8%) was obtained from Cambridge Isotope Laboratories Inc. Thiol-Terminated Polymer Synthesis. Two families of thiol-terminated polymer were used in this study: thiolated polystyrene (PS-SH) and thiolated poly(ethylene oxide) (PEOSH). A PEO45-SH sample (Mn 2100 g/mol, PI 1.05, i.e., ∼45 EO units) was obtained from Polymer Source Inc., Dorval, QC, and was used as received. Subsequent characterization of the sample by solution 1H NMR and matrix-assisted laser desorption/ ionization time-of-flight (MALDI-TOF) spectroscopy showed that the sample contains uncharacterized impurities and disulfide of the parent thiol. The thiolated polystyrenes were synthesized in our laboratories by anionic polymerization based on a method reported by Stouffer and McCarthy.23 All glassware was flamedried, and the reactions were carried out in an ultrapure nitrogen atmosphere. Transfer of solvents and reagents was performed by cannula and septa. THF, freshly distilled from sodium/ benzophenone, was transferred into a round-bottom flask, and a few drops of R-methylstyrene (RMS) were added. The reaction vessel was titrated dropwise with the initiator, sec-butyllithium (sec-BuLi), until a red color persisted. Styrene monomer, freshly cryo-distilled from fluorenyllithium, was then added. Because reaction conditions are optimal at 243 K, the flask was cooled in a dry ice/acetone bath prior to the initiator addition (10% excess). The red color typical of RMS anions was temporarily replaced by the orange/yellow color associated with the living polystyryl chain ends until all the styrene was consumed and the red color was regenerated as the chains were end-capped with the remaining RMS. The population of polystyrene anions was titrated with one unit of propylene sulfide in order to generate the thiol endgroup. An aliquot of the first block was withdrawn prior to the addition of propylene sulfide. Both the homopolymer and the thiol end-capped polymer were quenched in HCl/methanol. The volume of the reaction mixture was then reduced by rotary evaporation, and the concentrated polymer mixture was precipitated by dropwise addition to a large volume of reagent grade

methanol. The polymers were recovered by vacuum filtration and rinsed several times with MilliQ H2O and methanol. The samples were dried in a vacuum oven for 2 days at 40 °C. 1H and 13C solution NMR of the PS-SH samples was used to determine the purity and the presence of the thiol end group, respectively. The molecular weights were determined both by GPC (gel permeation chromatography) and MALDI-TOF spectroscopy. Two PS-SH samples were synthesized: PS125-SH (Mn 13300 g/mol, PI 1.7, i.e., 125 PS units) and PS19-SH (Mn 2000 g/mol, PI 1.2, i.e., 19 units). Gold Nanoparticle Synthesis. Gold nanoparticles, functionalized with C14-SH, PS-SH, or PEO-SH, were synthesized following the method reported by Yee et al.24 All glassware was rinsed with aqua regia and then several times with MilliQ water, and was oven-dried overnight at 160 °C. Prior to use, the components of the reaction vessel were flame-dried under vacuum and then kept under an ultrapure nitrogen atmosphere. In a typical synthesis, 5 mL of freshly distilled THF was used to dissolve 200 mg of HAuCl4‚3H2O, which was then introduced to the reaction vessel. The color of the HAuCl4 in THF solution was pale yellow. Five milliliters more of THF was used to solubilize 0.125 mL of C14-SH, which was added to the flask. The solution became slightly orange, with greenish undertones for the first few minutes. The orange solution was stirred over a magnetic stir plate overnight in the dark. The reduction started with the addition of 1 mL of Superhydride (lithium triethylborohydride, Aldrich, 0.1 M in THF) at once, at which point the solution became very dark with a deep red Bordeaux wine color. Subsequently, Superhydride was added by increments of 0.5 mL over a couple of hours until no more gas evolution was observed. The solution was stirred overnight in the dark. A few milliliters of dry ethanol were then added to the reaction mixture to selectively precipitate the nanoparticles. The solution was poured into two glass centrifugation tubes and centrifuged down until all the nanoparticles precipitated. The supernatant was discarded, the nanoparticles were redissolved in a small quantity of THF, a few milliliters of dry ethanol were added, and the centrifugation process was repeated until there were no free thiols and salts left in the supernatant, as indicated by thin-layer chromatography (TLC) and 1H NMR. The preparation procedure of PS-Au and PEO-Au nanoparticles was similar. However, given the high molecular weights of the polymer-thiol samples, with the exception of PS19-SH, a smaller initial molar ratio of polymer-thiol/HAuCl4 was used (1:3.5 for PS125-SH, 1:1.1 for PS19-SH, 1:12 and 1:6 for PEO45SH, yielding PEO45-Au A and PEO45-Au B, respectively) as compared to the ratio used in the C14-SH/HAuCl4 (1:1.1) system. The stirring times of the gold salt and the polymer-thiol, prior to the reduction process, were extended to 24 h. The reduction was then carried out as above. In solution, the PS-Au nanoparticles appeared red-purple in color, whereas the PEO-Au nanoparticles were pink. The purification process for PS-Au was similar to that used for C14-Au. The purification process for PEO-Au involved the use of a stirred high-pressure dialysis cell with a regenerated cellulose membrane (MW cutoff 30 kDa, 60 PSI, from Millipore). Gold Nanoparticle Characterization. The gold nanoparticles were characterized by solution transmission electron microscopy (TEM), 1H NMR, and UV-visible (UV-vis) spectroscopy of dilute solutions. For PEO-Au, a drop of dilute aqueous solution was deposited on a TEM copper grid (400 mesh) first covered with a thin film of Formvar-supported carbon. Excess solution was wicked away with the corner of a Kimwipe tissue, and the grid was left to dry at ambient pressure and temperature, and then under vacuum. For C14-Au, a few drops of a dilute solution in CHCl3 deposited at the surface of water in a small dish allows for the formation of a thin film of nanoparticles, mainly in a monolayer state. After the solvent was allowed to evaporate, a copper grid (400 mesh) covered with a Formvar film was dipped in the monolayer and allowed to dry at ambient pressure and temperature and then under vacuum. Alternatively, a drop of dilute CHCl3 solution of PS-Au was deposited directly onto a 400 mesh copper grid covered with a thin carbon film

(23) Stouffer, J. M.; McCarthy, T. J. Macromolecules 1988, 21, 12041208.

(24) Yee, C. K.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3486-3491.

a

A truncated octahedral shape represents the gold core.

polymer brush grafting density. We demonstrate here how the “grafting-to” approach, combined with the variation of the characteristics of the thiol-terminated polymers used and the synthetic reaction conditions, can be used to control both the grafting density and the molecular weight of the polymer brush on the Au nanoparticle core. Matching of the grafting density and matrix leads to a tuning of the dispersibility of the nanoparticles. Experimental Section

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Figure 1. Transmission electron micrographs of the nanoparticles studied. The average diameters and standard deviations are 3.9(0.5) nm for C14-Au, 7.7(1.8) nm for PS125-Au, 4.4(1.2) nm for PS19-Au, 4.4(1.6) nm for PEO45-Au A, 9.8(3.9) nm for PEO45-Au A (aged), and 3.6(2.0) nm for PEO45-Au B. Table 1. Some Physical Properties of the Gold Nanoparticles Studied nanoparticles

Mn of ligand (g/mol)

dcore (nm)a

dnanoparticle (nm)b

C14-Au PS125-Au PS19-Au PEO45-Au B PEO45-Au A PEO45-Au A (aged)e

230.45 13300 2000 2100 2100 2100

3.9(0.5) 6.2(1.7) 4.4(1.2) 3.6(2.0) 3.8(1.9) 9.2(3.8)

n.a. 40 14 30 n.a. n.a.

% organic/ % Auc 14.4/85.6 56.5/43.5 50.0/50.0 29.3/70.7 n.a. 24.8/75.2

λmax (nm)d (solvent) 509 (CHCl3) 525 (CHCl3) 517 (CHCl3) 515 (H2O) 520 (H2O) 519 (H2O)

a TEM, apparent diameter of the circumsphere of the truncated octahedral gold cores, with the standard deviations in parentheses; only symmetrical nanoparticles were taken into account. b Hydrodynamic diameter of the core plus polymer chains, from DLS. c Weight percent, from TGA. d Absorbance maximum of the surface plasmon resonance measured by UV-vis spectroscopy. e Sample was 1 year old when measured.

(Electron Microscopy Sciences), and the solvent was allowed to evaporate at ambient pressure and temperature. For routine analysis, a JEOL 2000FX transmission electron microscope was used at a 80 kV setting. High-resolution TEM on a JEOL JEM 2011 microscope at 200 kV acceleration voltage was also performed in order to study the shape of the gold cores. Both negatives and digital pictures using a Gatan Bioscan CCD camera (model 792) were obtained. The CCD camera was interfaced with a PC running Digital Micrograph software. The negatives were scanned using an Epson 1200 Photo scanner with a negative adapter at a resolution of 300 dpi. The TEM images were analyzed using SigmaScan Pro 4.0. Calibrated areas of approximately 300 nanoparticles per image were measured, and using the reasonable assumption that the cores of the nanoparticles are truncated octahedra, the diameters (of their circumsphere) were estimated. Histograms of the populations of diameters were constructed using SigmaPlot 5.0. TEM images of the particles studied are shown in Figure 1. 1H NMR of the nanoparticles dissolved in CDCl was carried 3 out on a 400 MHz Varian Mercury spectrometer in order to assess the purity of the nanoparticles. A broadening of the peaks, caused by the binding of the ligands on the nanoparticles, was also observed in all cases. UV-vis spectra were obtained on a Varian Cary 50 spectrophotometer, between 300 and 800 nm wavelength. Dilute nanoparticle solutions (in chloroform or water) were measured in quartz cuvettes, using pure solvent as a reference. The plasmon bands measured (Table 1) for the various types of samples are typical of small, dispersed gold nanoparticles in solution. The small spectral differences observed between samples are related to the different gold core sizes, as well as the different dielectric

constants of both the capping layer and the solvents used in the measurement.25 To determine the Au/thiol ratios (in combination with TEM results), thermogravimetric analysis (TGA) of the Au nanoparticles was carried out on a TA Q500 instrument, with a Pt pan at a heating rate of 10 °C/min. The runs were performed under N2 gas, and the gas was switched to air for 10 min at the end of the run at 700 °C to ensure complete combustion of the organic material. N2 and air gases were used at a rate of 50 mL/min. A Brookhaven BI9000 dynamic light scattering (DLS) instrument was used to measure the hydrodynamic diameters of the polymer-coated Au nanoparticles. The incident light source was a frequency doubled YAG laser at a wavelength of 532 nm (150 mW). The C14-Au nanoparticles were too small (dcore ∼ 3 nm) for DLS measurements, at the lower detection limit of the instrument. However, the polymer-stabilized nanoparticles were large enough to obtain meaningful results, using NNLS and CONTIN analysis of the autocorrelation functions. Table 1 summarizes some of the physical characteristics of the Au nanoparticles studied. Preparation of Self-Assembled Monolayers on TwoDimensional (2D) Gold Substrates. Of particular interest is the comparison of the grafting densities of the polymer monolayers on the 3D nanoparticles and the grafting densities of the identical polymers packed in monolayers on 2D gold. For this purpose, polymer-thiol and C14-SH monolayers were prepared by incubating Au slides in 1 mM THF solutions of thiol for 3 days. The polycrystalline gold (reported below as 2D gold surface) (25) Kreibig, U.; Vo¨llmer, M. Optical Properties of Metal Clusters, 2nd ed.; Kreibig, U., Ed.; Springer-Verlag: New York, 1995.

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Table 2. Dry Thickness of a Monolayer of Thiol-ligands on 2D Gold Slides Adsorbed from a THF Solution, and Their Respective Calculated Grafting Densities and Area per Molecule

ligand

dry thicknessa (Å)

grafting density δ (chains/nm2)

area/ligand molecule (Å2/chain)

C14-SH PS19-SH PS125-SH PEO45-SH

15.3(0.2) 18(1) 9.1(0.4) 25.3(0.9)

3.3b 0.6 0.04 0.9

30c 150 2300 110

a The standard errors for the thickness are shown in parentheses and are based on six measurements. Issues related to accuracy are described in the Experimental Section. b Using 0.846 g/cm3 as the tetradecanethiol bulk density (Aldrich). c The area per molecule obtained here is comparable (somewhat larger) to that reported from single-crystal gold diffraction studies.28

had been freshly evaporated (80 nm thick) on an adhesive layer of titanium (3 nm thick) on silicon (110) wafers. After incubation with the thiols, the slides were rinsed with copious amounts of THF, then MilliQ water, and anhydrous ethanol and were dried under a stream of pure nitrogen prior to ellipsometry measurements. The refractive index of the fresh bare gold substrate was measured by ellipsometry (Ns, 0.158; Ks, -3.572). The refractive indices used for PS, PEO, and C14-SH were 1.59, 1.45, and 1.459, respectively. An Optrel Multiskop Null ellipsometer equipped with a He/Ne laser beam oriented at a 70° incident angle was used for the ellipsometry measurements. Six measurements were performed on each gold slide and averaged. The standard errors on the thickness are reported in Table 2. The dry brush thickness measured on each sample and the resulting calculated areas per molecule and grafting densities of the ligands adsorbed on 2D surfaces are also presented in Table 2. For the grafting density and area per molecule calculations, bulk density polymer values were assumed. Bulk density values of 1.05 g/cm3 for PS and 1.21 g/cm3 for PEO were used.26 Whereas the absolute errors in thickness measurements are not known, we refer to the measurement of the tetradecanethiol self-assembled monolayer as an internal standard. The thickness obtained using literature refractive indices yields a thickness within 2 Å of literature values.27 The accuracy of the ellipsometric thickness obtained for the polymer-thiols is therefore in this range.

Results and Discussion Of the samples studied, the C14-Au nanoparticles are both the smallest and most monodisperse, with a core diameter of 3.9 ( 0.5 nm (Figure 1). Similar particles have been characterized as having cores in the shape of truncated octahedra.28,29 Indeed, high-resolution TEM confirms that the cores are not spherical but instead are faceted. In fact, several gold core shapes observed correspond to different geometrical projections of a truncated octahedron (Figure 2). Most of the gold cores that appear spherical could in fact correspond to a truncated octahedral projection, if the hexagonal (and largest) face is in contact with the substrate (Figure 2). We therefore assume in our calculations that each of the gold cores in these nanoparticle samples is in fact a truncated octahedron. The grafting density of C14-S- on the gold core is high, with 4.35 chains/nm2 (23 Å2/chain, Table 3). This grafting density is consistent with the small size of the ligand. The area per molecule of C14-S- on the Au nanoparticle core is comparable to the footprint of docosyl mercaptan (C22(26) Polymer Handbook, 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; John Wiley and Sons: Toronto, 1999. (27) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (28) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546-558. (29) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428-433.

Figure 2. Identification of some of the geometrical core representations on a high-magnification TEM image of the gold nanoparticle samples. Inset: Some geometrical views of the truncated octahedral gold core.

SH) bound to planar Au(111) surfaces (21.4 Å2/chain).28 This corresponds to a crystalline, tight packing of the alkanethiol chains at an angle ca. 30° from the surface normal. The thickness measured of the 2D C14-SH SAM (on polycrystalline gold) found by ellipsometry measurements (15 Å, Table 2) is also consistent with this type of packing. The polydispersity in C14-Au core diameter (standard deviation 0.5 nm) is similar to that of alkanethiol-stabilized gold nanoparticles reported in the literature by the Brust1 or the Ulman24 techniques. The C14-Au nanoparticles synthesized by the Ulman technique (diameter ca. 4 nm) are significantly larger than the ones produced by the Brust method (ca. 2.0-2.5 nm). The differences in the two methods arise from the choice of reducing agent and solvent, and the presence of a phase transfer catalyst in the Brust technique.1 However, how each factor influences the size of the nanoparticles is not yet well understood. Polymer-stabilized gold nanoparticles have larger diameters and polydispersities compared to small moleculestabilized nanoparticles (Table 1). Most of the nanoparticles appear to be roughly spherically symmetric, although several of the larger nanoparticles appear to be rather irregular in TEM images. Facets can be distinguished on a large number of particles by high-resolution TEM (Figure 2). The steric bulk of the polymer chains probably influences the growth rate of the gold core significantly. Effective capping of the gold core is probably lessened by a bulky ligand, leading to more polydisperse nanoparticles than in the n-alkanethiol-Au case. The PS19-Au nanoparticles are smaller and more monodisperse (d ) 4.4 ( 1.2 nm) than the PS125-Au nanoparticles (d ) 7.7 ( 1.8 nm).30 We also note that the composition of the two types of nanoparticles, determined from combined TGA and TEM data,31 is quite different (Table 3). The grafting (30) We note that the PS125-Au nanoparticles have a tendency to aggregate when stored at 4 °C. Indeed, after a year, the majority of the sample deposits as dimer and trimer aggregates as observed by TEM (see Figure 1). Gentle heating of these nanoparticles in a toluene solution for 10 min breaks the aggregates apart. However, no change in the UV-vis spectra was noted after heating for this sample.

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Table 3. Calculated Characteristics of the Polymeric Ligands on Gold Nanoparticles polymeric ligands on truncated octahedral coresb

polymeric ligands on spherical coresd

nanoparticle

compositiona,b x/yc

grafting density (chains/nm2)

area/molecule (Å2/chain)

grafting density (chains/nm2)

area/molecule (Å2/chain)

C14-Au PS19-Au PS125-Au PEO45-Au B PEO45-Au A

180/1250 177/1800 97/5000 40/1000 38/1200

4.35 3.45 0.94 1.15 0.98

23 29 106 87 102

5.5 4.2 1.2 2.0 1.2

18 24 83 50 83

a Average number of thiols and Au atoms per nanoparticle, calculated from TGA and TEM data, assuming regular truncated octahedral gold cores. b Only symmetrical nanoparticles were taken into account, since the assumption of regular truncated octahedral gold cores is needed for the determination of these results. c x ) number of thiol molecules per nanoparticle; y ) number of gold atoms per nanoparticle. d Calculated with the assumption of a spherical gold core for comparison purposes.

density of the PS ligand is 0.94 chains/nm2 on the PS125Au nanoparticles (yielding a 106 Å2/chain area per molecule) and is 3.45 chains/nm2 on PS19-Au (yielding a 29 Å2/chain area per molecule). The greater thiol coverage in the PS19-Au case arises, at least in part, from the fact that the hydrodynamic radius of a PS19-SH tethered chain is substantially less than that of a PS125-SH tethered chain. The area per molecule on nanoparticles of PS125-SH is 1.06 nm2, whereas for PS19-SH it is 0.29 nm2.32 The theoretical Flory radius of a tethered chain can be calculated: Rf ) aN3/5, where a is a monomer size and N is the number of monomers per chain.33 In practice, Rf is dependent on the solvent (among other conditions) and thus is not accurately determined. However, taking the ratio of the Flory radii of tethered polymer chains of the same chemical nature (same a) but with different molecular weight (different N) allows one to account for the effect of only molecular weight on the tethered coil dimensions. The Rf ratio (of P125 over P19) is 3.1, whereas the area per molecule ratio is 3.5 for the PS125 and PS19 chains. It is also of interest to compare the PS-SH area per molecule on the nanoparticles (3D) and on planar gold (2D). While the area per molecule of C14-SH is quite similar on a 2D surface and the 3D particle, the PS-SH ligands have dramatically different areas per molecule on the two surfaces. The PS19-SH chain has an area per molecule of 155 Å2/chain on 2D surfaces but only 29 Å2/chain on Au nanoparticles. The PS125-SH chain has an area per molecule of 2300 Å2/chain on a 2D surface yet 106 Å2/ chain on a 3D nanoparticle. It is probable that during the particle synthesis, the enthalpy corresponding to the formation of the Au-S bonds effectively counteracts the entropic cost associated with the stretching of the polymer chains. The presence of void volumes, caused by the faceting of the gold cores, relaxes the local packing constraints, thus allowing the chain to “expand” into a greater area (Chart 2). The larger area per molecule of PS19 compared to PS125 thus correlates with the greater void volume fraction arising at the facet edges of a small truncated octahedral core (i.e., PS19-Au) compared to a larger one (i.e., PS125-Au) (Figure 3). In the case of the adsorption to 2D surfaces, the entropic cost is probably greater than that on the faceted surface. The grafting density here is 0.04 chains/nm2 for the adsorption of PS125SH onto 2D gold slides. This is consistent with a report (31) TEM provides the average diameter of the circumsphere of the nanoparticle core. TGA provides the weight fraction of organic/Au for the nanoparticles. The combination of data allows for the simple calculation of the nanoparticle average composition. (32) The surprisingly small area per molecule of PS19-SH on the nanoparticles (29 Å2/chain) can be explained by a combination of the nature of the thiol end group which has the same conformation as an alkanethiol and of the presence of the voids at the facet edges which provides extra volume for the dense PS chains. (33) de Gennes, P. G. Macromolecules 1980, 13, 1069-1075.

Figure 3. Graph of the calculated percentage of void volume compared to the volume of the void plus the volume of the brush, as a function of the core radius (nm), for various brush thickness. Chart 2. Representation of a Void Volume (Red) between Two Polymer Brushes (Blue) Adsorbed on a Gold Core (Black)

of a grafting density of 0.02 chains/nm2 for the adsorption of a longer PS400-SH in toluene onto 2D gold slides.34 Due to the relatively high polydispersity (1.7) of our PS125-SH sample, it is reasonable to assume that the 2D gold surface will be preferentially covered by the lower molecular weight fraction of the polymer sample.35,36 If the entire range of chains actually binds, then the grafting density values calculated for the 2D surfaces will be lower. The thiol/gold synthesis ratio influences the extent of thiol coverage of the nanoparticles, to a greater degree (34) Koutsos, V.; van der Vegte, E. W.; Hadziioannou, G. Macromolecules 1999, 32, 1233-1236. (35) Dan, N. Macromolecules 1994, 27, 2310-2312. (36) Dijt, J. C.; Cohen Stuart, M. A.; G. J. Fleer. Macromolecules 1994, 27, 3219-3228.

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than it influences the size and polydispersity of the gold core. The PEO45-Au A and the PEO45-Au B nanoparticles were prepared with an initial thiol/gold mole ratio of 1:12 and 1:6, respectively. Although their initial diameter size and polydispersities are similar (see Table 1), it is interesting to note that the sizes and polydispersities of the PEO45-Au A nanoparticles increased considerably over a year of storage at 4 °C (3.8-9.2 nm). This change is attributable to their lower thiol coverage, as the more densely coated PEO45-Au B sample remains stable over the same time period. The PEO-S chain density on the Au core is 0.98 chains/nm2 (102 Å2/chain) and 1.15 chains/ nm2 (87 Å2/chain) for PEO45-Au A and PEO45-Au B, respectively, in their initial states. Therefore a lesser molar thiol/Au synthesis ratio (1:12) leads to a lower grafting density. Because of the bulkiness of the PEO45 chains, the effect of the initial ligand concentration on the final grafting density or nanoparticle size is less than that in the case of small ligands.12 Although the grafting densities of PEO45-Au A and PS125-Au are similar (0.9 chains/nm2), the ligand bulkiness is significantly different. The lesser bulkiness of PEO might also contribute to the lesser stability of the PEO45-Au A nanoparticles. Finally it should be noted that the size of the gold core of the PEO45-Au A sample differs significantly from the PEO-Au nanoparticles prepared by Murray and co-workers (diameter 2.8 ( 1 nm, grafting density 2.85 chains/nm2).12 Although the same thiol/gold mole synthesis ratio (1:12) was used in these two studies, the preparation method differed (Yee24 vs Brust 2-phase1). The dimensions of the resulting polymer brushes on the Au cores in a given solvent were determined using DLS measurements. The hydrodynamic diameters of PS125-Au and PS19-Au in THF, and of PEO45-Au B in H2O, were determined. Combined with TEM and TGA data, this allows for the determination of the PS-SH area per molecule at the thiol-solvent interface.37 Beginning with the PS19 system, its area per molecule at the THF interface is 2.4 nm2. An area per molecule at the solvent interface which is this large stems from the high apparent curvature of the Au core. Clearly, the constraints of a terminal tether and high grafting density force the PS attached to the Au core into a highly extended conformation. Surface-attached PS19 chains swollen in THF stretch to a DLS-derived average length of 4.8 nm, corresponding to the end-to-end distance of the fully extended PS19-SH chain (4.8 nm). For the case of the chain adopting a coil conformation, the calculated mean end-to-end distance is 1.2 nm. In the case of a free PS chain in a good solvent, a coil is the most probable conformation. It is noteworthy that PS19 also extends significantly from the surface in the dry state (determined from TEM images of the nanoparticles where the average edge-to-edge distance between PS19-Au nanoparticles is 5.4 ( 1.7 nm (Figure 4). Half of this distance (2.7 nm) is less than the extended polymer end-to-end distance (4.7 nm) but greater than the unperturbed coil (1.2 nm). Although in this dry state the PS19-SH chains are collapsed, they clearly remain highly extended given the physical constraints imposed on them by lateral chain interactions. The PS19-SH ellipsometric thickness is consistent with the less densely grafted chains (0.6 chains/ nm2 on 2D surfaces) also having a lesser dry thickness (1.9 nm). This result is comparable to the dry thickness/ grafting density obtained by Cohen Stuart et al. for a vinylterminated PS20 passively grafted from the melt onto hydrogen-terminated silicon surfaces (dry thickness: 3.0 nm, 0.95 chains/nm2),38 given that polymer adsorption (37) Assuming a spherical thiol-solvent interface.

Corbierre et al.

Figure 4. Edge-to-edge distances obtained from TEM images for (A) PS125-Au nanoparticles and (B) PS19-Au nanoparticles.

from the melt generally leads to denser brushes as opposed to adsorption from a solution. The DLS results compare favorably with those obtained by Schmidt et al., whose PS40-grafted chains on microgel spheres extend up to 6 nm in a toluene solution.39 However, the grafting density of PS19 on gold nanoparticles in this study (3.45 chains/ nm2) is considerably greater than the PS40/10 nm microgel core (0.2 chains/nm2) system. Although this is consistent with the PS chains being shorter, the different techniques used to graft the polymers and to measure the grafted chain number are probably important variables. In the case of the PS125 system, the DLS-derived dimension is 17 nm, whereas its calculated end-to-end distance is 31.4 nm, and the square root of the average mean square of the unperturbed dimensions is 7.7 nm. The surface-attached polymer chains thus are neither highly extended nor do they adopt a coil conformation, due to the restrictions imposed by the neighboring chains in the brush. The brush thickness in THF corresponds to a polymer area per molecule of 33.5 nm2 at the THF interface. This large value is in part attributable to the high apparent curvature of the gold core. Finally, the TEMderived thickness of the nanoparticle dry brush (7 nm) is considerably greater than the ellipsometric thickness of the dry 2D brush (0.9 nm). (38) Maas, J. H.; Fleer, G. J.; Leermakers, F. A. M.; Cohen Stuart, M. A. Langmuir 2002, 18, 8871-8880. (39) Lindenblatt, G.; Scha¨rtl, W.; Pakula, T.; Schmidt, M. Macromolecules 2000, 33, 9340-9347.

Polymer-Stabilized Gold Nanoparticles

DLS of the PEO45-Au B sample shows two populations, one with a 30 nm average diameter (corresponding to the single nanoparticles) and the other with a 120 nm average diameter. The 120 nm population is probably due to interparticle PEO interactions leading to the formation of small aggregates composed of three or four nanoparticles. The maximum aggregate diameter measured by DLS is 250 nm. Precedence for “soft” associative behavior of PEO chains has previously been reported by van de Ven et al.40 The 30 nm average diameter for the PEO45-Au B nanoparticles in water corresponds to a solvated brush thickness of 13 nm and a contact area at the polymer/ water interface of 35 nm2/chain. This contact area is 40fold larger than the area per molecule at the gold core interface (0.87 nm2/chain), due to the apparent curvature of the core and the conformational properties of the ligand. The solvated brush thickness (13 nm) is smaller than the extended end-to-end distance of the PEO45 chain (17.7 nm). However, the conformation of the PEO chains at the surface of the gold core is more difficult to assess. Grunze et al.41 observed highly extended alkanethiol-terminated PEO45 chains (ellipsometric dry thickness: 120 Å) grafted on 2D gold surfaces and concluded that they adopted a helical conformation. In the present study, the dry thickness of the PEO45 chains grafted on 2D surfaces is 25 Å, similar to the early stage thickness ( 10 min). The alkanethiol spacers might facilitate the ordering of the polymer chains in the Grunze study. Unlike Grunze et al.,41 who observe an eventual 120 Å thick PEO45 layer, we do not observe an increase in the dry thickness of PEO films on 2D slides (even after 2 days of incubation) past (40) Polverari, M.; van de Ven, T. G. M. J. Phys. Chem. 1996, 100, 13687-13695. (41) Tokumitsu, S.; Liebich, A.; Herrwerth, S.; Eck, W.; Himmelhaus, M.; Grunze, M. Langmuir 2002, 18, 8862-8870.

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25 Å. The solvent-wetted, densely grafted PEO chains on nanoparticles described here are extended in a comparable fashion to early stage, but not late stage, adsorption onto a 2D surface. This implies that these PEO chains are in a restrained coil but not helical conformation when they are adsorbed to the nanoparticle surface. Summary Gold nanoparticles stabilized with polymer ligands varying in terms of chemical structure and molecular weight were prepared. The control of the molecular weight and the synthetic parameters provides access to different grafting densities. Overall, higher grafting densities were obtained on 3D nanoparticles compared to polymer-thiol adsorption on 2D gold slides. Voids created by the juxtaposed facets provide additional volume to the polymeric ligands. There are several advantages to grafting polymer brushes onto Au nanoparticles, compared to growing polymers on flat surfaces or on organic nanoparticles. First, their characterization is facile, since the composition is derived from a combination of TEM/TGA analysis. Furthermore, DLS assists in studying the conformation in solution of the polymer chains grafted onto the nanoparticles. Finally, the synthesis of the polymer prior to grafting (the “grafting-to” approach) allows for full characterization of the polymer ligand, which is more difficult in the “grafting-from” cases. Acknowledgment. We thank Dr. C. Bartels for performing some of the DLS measurements and O. M. Tanchak for his help with the ellipsometry measurements. NSERC, FQRNT, and Merck-Frosst provided the funds for this research. Supporting Information Available: Histograms representing the nanoparticle core size distributions and UV-vis spectra of the nanoparticles in solution. This material is available free of charge via the Internet at http://pubs.acs.org. LA0355702