5168
Langmuir 2006, 22, 5168-5173
Shell Cross-Linked Au Nanoparticles Ste´phanie Koenig and Victor Chechik* Department of Chemistry, UniVersity of York, Heslington, York YO10 5DD, U.K. ReceiVed January 6, 2006. In Final Form: March 26, 2006 The organic layer of thiol-protected Au nanoparticles (ca.3 nm in diameter) was cross-linked using ring-opening metathesis polymerization or Michael addition of polyfunctional amines. The shell cross-linked nanoparticles showed increased stability toward thermal treatment and oxidative etching. The Au core of cross-linked nanoparticles was removed in an attempt to prepare hollow capsules. However, Au etching resulted in insoluble materials.
Introduction Thiol-protected Au nanoparticles exemplify an extremely flexible type of supramolecular assemblies. Their relative stability and ease of functionalization make them good candidates for the preparation of functional materials, including catalysis,1 sensing,2 etc. However, these nanoparticles suffer from limited stability against heating, prolonged storage, exposure to etchants, and other ligands. The stability can be enhanced by using polydentate ligands; for instance, bifunctional phosphines have been shown to provide better protection for the surface of CdSe nanoparticles as compared with monofunctional parent compounds.3 Even noncovalent intermolecular interactions between adjacent ligands on the nanoparticle surface (e.g., H-bonds between the amide functionalities) are known to increase nanoparticle stability.4 We were interested in improving the stability of nanoparticles by cross-linking the organic shell. This effectively converts the monodentate ligands on the surface of Au nanoparticles into a polydentate network. A similar approach was successfully used to stabilize liposomes.5 Besides, we expected that the shell crosslinked nanoparticles would be able to maintain their shape and stability even after removal of the metal core, and therefore, etching of the core would result in hollow, nanosized organic capsules. This was inspired by cross-linking of self-assembled aggregates of block copolymers6 and preparation of hollow capsules using self-assembly of oppositely charged polyelectrolytes on the surface of appropriate nanostructure template materials7 (including Au nanoparticles8). Several recent reports describe cross-linking of the organic shell of inorganic nanoparticles. For instance, Feldheim et al. polymerized pyrrole derivatives around Au nanoparticles adsorbed in the voids of porous alumina.9 In a different approach, the same group used metathesis to cross-link the organic ligand * To whom correspondence should be addressed. E-mail: vc4@ york.ac.uk. (1) Pasquato, L.; Pengo, P.; Scrimin, P. Supramol. Chem. 2005, 17, 163. (2) Haes, A. J.; Stuart, D. A.; Nie, S. M.; Van Duyne, R. P. J. Fluoresc. 2004, 14, 355. (3) Kim, S.; Bawendi, M. G. J. Am. Chem. Soc. 2003, 125, 14652. (4) Paulini, R.; Frankamp, B. L.; Rotello, V. M. Langmuir 2002, 18, 2368. Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549. (5) Jung, H. M.; Price, K. E.; McQuade, D. T. J. Am. Chem. Soc. 2003, 125, 5351. Liu, S.; O’Brien, D. F. J. Am. Chem. Soc. 2002, 124, 6037. (6) Thurmond, K. B.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1997, 119, 6656. (7) Antipov, A. A.; Sukhorukov, G. B. AdV. Colloid Interface Sci. 2004, 111, 49. (8) Gittins, D. I.; Caruso, F. AdV. Mater. 2000, 12, 1947. (9) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C.; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. Marinakos, S. M.; Anderson, M. F.; Ryan, J. A.; Martin, L. D.; Feldheim, D. L. J. Phys. Chem. B 2001, 105, 8872.
containing three terminal alkene functionalities.10 Liu et al. crosslinked the polymer brushes in the organic shell using a bifunctional reagent.11 Peng et al. used cross-linking to improve stability of semiconductor nanoparticles. Two strategies were used, alkene metathesis of the eight-legged dendron ligand with a thiol anchor group at the focal point12 and cross-linking of N-hydroxysuccinimide ester-modified dendron-coated nanoparticles (formed in situ by reacting alcohol-terminated particles with a bifunctional linker) with polyamines.13 Some of these cross-linked particles were reported to form nanocapsules upon removal of the inorganic core.10,12 Here, we report cross-linking of Au nanoparticle surface using ring-opening metathesis polymerization (ROMP) of norborneneterminated ligands. ROMP on the surface of Au nanoparticles was first reported by Mirkin et al. who used this reaction to grow polymer brushes on the nanoparticle surface.14 A similar reaction in the absence of additional monomer should lead to shell crosslinked nanoparticles. An alternative approach employing Michael addition of polyamines to the acrylate-terminated particles is also described. The shell cross-linked nanoparticles show improved stability; however, attempts to prepare hollow capsules by etching away the sacrificial metal core led to insoluble polymeric materials. Ring-Opening Polymerization at the Nanoparticle Surface. ROMP requires a cyclic alkene. We used norbornene derivative 2 as a polymerizable ligand for Au nanoparticles. Compound 2 can be conveniently prepared by Diels-Alder reaction of acrylateterminated ligand 1 (available from our previous work15) with cyclopentadiene. Encouraged by the absence of byproducts in the NMR spectra of the crude reaction mixture, we performed a similar reaction on the surface of Au nanoparticles to prepare norbornenefunctionalized particles 4 (Scheme 1). The 1H NMR spectra of the resultant dark brown material showed broad peaks at the same chemical shift as for norbornene-terminated free ligand. Spectrum assignment is shown in Figure 1. Norbornene-terminated particles 4 were also characterized by TEM (Figure 2). The mean diameter of the particles was 2.9 ( (10) Wu, M. S.; O’Neill, A.; Brousseau, L. C.; McConnell, W. P.; Shultz, D. A.; Linderman R. J.; Feldheim, D. L. Chem. Commun. 2000, 775. (11) Luo, S.; Xu, J.; Zhang, Y.; Liu, S.; Wu, C. J. Phys. Chem. B 2005, 109, 22159. (12) Guo, W. H.; Li, J. J.; Wang, Y. A.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 3901. (13) Guo, W. H.; Li, J. J.; Wang, Y. A.; Peng, X. G. Chem. Mater. 2003, 15, 3125. (14) Watson K. J., Zhu J., Nguyen S. T., Mirkin C. A. J. Am. Chem. Soc. 1999, 121, 462. (15) Koenig S., Chechik V., Langmuir 2003, 19, 9511.
10.1021/la0600604 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/29/2006
Shell Cross-Linked Au Nanoparticles
Langmuir, Vol. 22, No. 11, 2006 5169
Scheme 1. Synthesis of Norbornene-Functionalized Nanoparticles
0.7 nm, which was slightly smaller than precursor acrylateterminated particles 3 (3.7(0.7 nm). The discrepancy is most likely due to the measurement error. ROMP on the surface of Au nanoparticles coated with a mixture of a similar norbornene ligand and an alkanethiol was reported by Reinhoudt et al.16 This reaction was, however, carried out at a very high nanoparticle concentration (ca.3 mg mL-1) with a large amount of catalyst (ca. 1 mg mL-1). Under these conditions, aggregation of nanoparticles was observed by TEM, suggesting that inter- and intraparticle cross-linking has taken place. Therefore, we thought that diluting the reaction mixture and
Figure 1. 1H NMR spectra of compound 2 and norbornene-modified Au nanoparticles 4.
Figure 2. TEM image and size distribution histogram of norborneneterminated gold nanoparticles 4.
reducing the amount of catalyst would afford only intraparticle surface cross-linkage. The cross-linking of the organic shell on the nanoparticle surface was achieved using Grubbs second generation catalyst 5.17 We found that addition of >5 mol% catalyst led to the nanoparticle aggregation, even if the reaction is carried out at high dilution. Therefore, the amount of catalyst was reduced to 4 mol%. The concentration of nanoparticles was 0.07 mg mL-1. We found that, under these conditions, metathesis successfully occurs and TEM showed no aggregation; however, the solubility of the resultant nanoparticles decreases significantly. We argue that the poor solubility of the shell cross-linked nanoparticles may be due to the highly rigid structure of the polymer layer, which prevents efficient interactions with the solvent. To improve the solubility, we carried out ROMP in the presence of stoichiometric amount of unfunctionalized norbornene. We believe that copolymerization of norbornene with the nanoparticle ligand reduces the rigidity of the organic structure 6. On the other hand, the amount of norbornene added is insufficient to form of any significant amount of norbornene homopolymer. These ROMP cross-linked nanoparticles 6 showed good
solubility (e.g., in CHCl3 and CH2Cl2) and no aggregation according to TEM (Figure 3). The mean diameter of the particles was 2.3 ( 0.6 nm. The 1H NMR spectrum (Figure 4) was extremely broad, and the overall intensity very weak, consistent with the rigid structure of the polymerized norbornene layer (e.g., not all nanoparticles gave visible signals due to slow tumbling rate of the organic layer). A small amount of starting material along with product peaks can be observed. According to the NMR spectrum, 70% of the norbornene units of
Figure 3. TEM image of ROMP cross-linked nanoparticles 6.
5170 Langmuir, Vol. 22, No. 11, 2006
Koenig and Chechik
Figure 4. 1H NMR of ROMP cross-linked gold nanoparticles 6.
nanoparticles 4 have polymerized. This value is likely to be higher due to the broadness of the NMR peaks of polymerized layer; however, we were unable to determine the extent of crosslinking more accurately. Polyamine Cross-Linking of Au Nanoparticles. We earlier reported a 1H NMR study of a Michael addition of linear (nbutylamine) and branched amines (second generation diaminobutane, DAB G2) to the acrylate-modified nanoparticles 3.15 The reaction was accompanied by transesterification with the solvent (MeOH); however, we optimized reaction conditions to minimize this side process. Reaction of nanoparticles 3 with DAB G2 (eight terminal amine groups) gave cross-linked particles 7. However, the properties of these materials were not explored.
Here, we explore the stability of nanoparticles 7; in addition, we performed cross-linking of nanoparticles 3 with third generation DAB and second and fourth generation PAMAM dendrimers. The particles were purified from small molecules and any excess dendrimer by gel permeation chromatography. Stability toward Thermal Treatment. The thermal properties of thiol-protected Au nanoparticles are well documented in the literature.18 Upon heating, small nanoparticles aggregate into (16) Li, X.-M.; Huskens, J.; Reinhoudt, D. N. Nanotechnology 2003, 14, 1064. (17) Choi, T.-L.; Grubbs, R. H. Angew. Chem., Int. Ed. 2003, 42, 1743.
bigger structures. Maye and Zhong proposed that two consecutive processes are responsible for the size and shape evolution of the particles during heat treatment.19 The first involves thermally driven desorption of the ligands from the gold surface followed by coalescence of the nanoparticles cores. The second implicates the reshaping or resizing that minimizes the chemical potentials followed by competitive re-encapsulation of thiolate shells under the annealing conditions. These ripening properties of gold nanoparticles are often used to manipulate the size and shape of gold nanoparticles. Due to the growth of the particles during thermal treatment, the ripening process is accompanied by color change from dark brown to dark red or purple; eventually, a less-soluble or insoluble product is formed.20 This can be easily monitored by UV-vis spectroscopy that shows an increase of the plasmon band of the particles.21 We have examined the thermal stability of polyamine 7 and ROMP cross-linked Au nanoparticles 6 as compared to model acrylate-terminated 3 and norbornene-terminated particles 4, respectively. The temperature of the samples was increased with 5 °C steps (each step was 5 min long). The temperature at which the color of the particle solution changed was ascertained using UV-vis spectroscopy. The results are shown in Figure 5. The UV spectra of all samples were nearly constant at low temperatures. However, after a certain threshold, the UV spectra showed substantial changes, consistent with the particle growth and/or aggregation. The nanoparticle color also visibly changed to red at this temperature. The data in Figure 5 show that polyamine cross-linked nanoparticles undergo changes at temperatures ca. 10 °C higher than the parent non-cross-linked nanoparticles. This observation is consistent with the increased stability brought about by cross-linking. Similar results were obtained for ROMP cross-linked nanoparticles 6. Model norbornene-terminated particles 4 were stable to only 75 °C, while cross-linked particles 6 were stable until 110 °C. It is interesting to note that norbornene-functionalized nanoparticles are substantially less stable than their acrylate coun(18) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719. Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935. (19) Maye, M. M.; Zhong, C.-J. J. Mater. Chem. 2000, 10, 1895. (20) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C.-J. Langmuir 2000, 16, 490. (21) Love, C. S.; Chechik, V.; Smith, D. K.; Brennan, C. J. Mater. Chem. 2004, 14, 919.
Shell Cross-Linked Au Nanoparticles
Langmuir, Vol. 22, No. 11, 2006 5171
Figure 5. Normalized UV-vis spectra of polyamine cross-linked 7 (a) and parent acrylate-protected Au nanoparticles 3 (b) at different temperatures. The insets show the intensity of the plasmon peak (530 nm) as a function of temperature.
terparts, both before or after cross-linking. This difference could be due to the bulky norbornene ring which probably prevents efficient packing of the ligands around the gold core. Thus, the ligand chains have weaker intermolecular interactions and less energy is required to desorb the ligands from the gold surface.10 After cross-linking, the norbornene-terminated particles remain less stable than the acrylate ones, but the difference becomes smaller. Stability toward Etching. Nanoparticle stability toward etching was explored using methyl iodide as etchant. We noticed that prolonged treatment of nanoparticles with this reagent leads to their decomposition. We originally assumed that methyl iodide would methylate the thiolate-based protecting ligands and hence lead to their desorption. However, 1H NMR analysis of the decomposition products revealed the presence of the disulfide ligand and no sign of the expected CH3S group. It appears that the decomposition of gold nanoparticles is due to iodide radicals released via photochemical decomposition of methyl iodide. Iodide radicals then induce the oxidation of the gold cores of the particles into Au(I) or Au(III). However, the amount of iodine is insufficient to dissolve all Au, and the nanoparticles precipitate. Treatment of acrylate-coated nanoparticles 3 with 5% (v/v) CH3I resulted in complete precipitation of nanoparticles within 24 h. The cross-linked particles 7, however, proved more stable. Although some color change was detectable after treatment with CH3I overnight, complete precipitation of nanoparticles 7 can only be achieved after 5 days in the presence of 15 times higher concentration of CH3I. Similar results were obtained with the ROMP cross-linked nanoparticles 6. The sample of norbornene-coated particles 4 precipitated after overnight treatment with 50% (v/v) CH3I. The disulfide ligand was identified in the 1H NMR spectra of the supernatant. However, only some color change was observed for the cross-linked nanoparticles 6. Complete destruction of nanoparticles required 80% (v/v) CH3I and 6 weeks. To summarize, cross-linked nanoparticles showed significantly enhanced stability toward etching. The cross-linking does not prevent some nanoparticle growth and aggregation, as evidenced by the color change of both types of cross-linked particles after overnight treatment with CH3I. However, these aggregates appear quite stable over a prolonged period of time, presumably due to the strength of their stabilization by the cross-linked ligands. Figure 6 shows the TEM picture of cross-linked nanoparticles 7 after ca. 24 h treatment with CH3I. Large nanoparticle aggregates are clearly observed in the image; however, they remain completely soluble.
Figure 6. TEM image of Au aggregates in the reaction mixture of polyamine cross-linked particles 7 with CH3I.
Etching of Au Core: toward Nanocapsules. Formation of a linear polymer on the nanoparticle surface is insufficient to efficiently cross-link the organic structure, and upon removal of the Au core, the polymer may be expected to unravel. We aimed therefore to achieve a second level of cross-linking within the organic layer by forming disulfide bonds between adjacent ligands. We hoped to form disulfide bonds during oxidative etching of the Au core. Several methods have been reported in the literature for etching the metal core from gold nanoparticles. All require a mild oxidant and a good ligand. A common method uses oxygen in the presence of cyanide anions; it is frequently employed to characterize the ligands adsorbed on the gold surface.22 However, it is likely that in this case the ligand is first released as a thiolate ion, which is then slowly oxidized to a disulfide. If the thiolate has sufficient time to escape into the bulk solution prior to disulfide formation, the cross-linking would not be efficient. Besides, the strong basicity of the cyanide would likely facilitate transesterification of the ester groups in the ligand. Halogens can also oxidize gold because they can form stable gold-halide complexes. The most stable complexes are obtained with iodine. Iodine has also been used to recover ligands adsorbed on the surface of gold nanoparticles, for example, to determine the extent of a place-exchange reaction.23 Oxidative etching of Au nanoparticles with iodine is an equilibrium process; hence, a substantial excess of iodine is required to prevent the formation of an insoluble black residue of Au(0) aggregates. (22) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (23) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782.
5172 Langmuir, Vol. 22, No. 11, 2006
Koenig and Chechik
Table 1. Elemental Analysis for Second Generation PAMAM Cross-Linked Nanoparticles 7 after Etching of the Au Core
found, %
calcd for C142H288N58O28 (dendrimer) + C14H25SO2 (ligand) + KI, %
elements
ratio 1:2
ratio 1:10
ratio 1:2
ratio 1:10
C H N S O K I
31.43 5.07 6.33 3.24 -
28.35 4.79 9.02 1.48 -
31.41 4.98 6.41 3.42 6.95 11.03 35.80
28.32 4.63 9.13 1.74 6.77 11.63 37.76
Importantly, the oxidation of gold nanoparticles with iodine is thought to proceed via oxidative addition of I2 to Au(I) thiolate on the gold surface, followed by reductive elimination of disulfide from the resulting Au(II) complex.24 Thus, this method should allow the formation of disulfide bonds during gold etching, before reorganization of the structure. We previously used this method to remove the gold cores from thiolated cyclodextrin-protected particles to form disulfide bridges between the cyclodextrin units and, thus, fabricated hollow spheres.25 To further confirm the validity of this method, we have etched the core of acrylateprotected nanoparticles before cross-linking. As the solubility of I2 in polar solvents such as water is quite poor, we used iodide solutions to form soluble I3- anion that acts as the oxidant. The 1H NMR spectrum after addition of I - confirmed the presence 3 of the expected acrylate disulfide 1. Unfortunately, addition of cross-linked nanoparticle solution to excess iodine in a mixed methanol-water solution of KI resulted in immediate formation of precipitate. Both types of cross-linked nanoparticles produced materials which were completely insoluble in any solvents. Modifications to the etching procedure (e.g., using different solvents or different reagent ratios) gave the same result. Etching of the Au core from acrylate nanoparticles protected by different dendrimers (e.g., third generation DAB, second generation PAMAM) also led to an insoluble precipitate. Etching of Au from acrylate particles crosslinked with fourth generation PAMAM dendrimer initially resulted in a transparent solution. However, after solvent removal, the material became insoluble. 1H NMR analysis of the soluble polymer showed no peaks at all, which is likely due to the extremely low mobility of the organic structure (caused by high rigidity of the collapsed capsule, vide infra). To confirm the composition of the insoluble material, we carried out elemental analysis for polyamine nanoparticles prepared by cross-linking nanoparticles 3 with second generation PAMAM dendrimer using two different amine/acrylate molar ratios (Table 1). These data can be used to estimate the composition of the insoluble material (we assumed that it contains a mixture of cross-linking dendrimer, acrylate ligand, and KI26). Basically, the amount of ligand can be determined from the sulfur analysis, the amount of dendrimer from the nitrogen analysis, and carbon/ hydrogen data can be used to double check the calculation. We have used a similar approach to determine the amount of ligand, dendrimer, and KI in the mixture using multiparameter optimization. The excellent agreement between the calculated elemental composition for this ligaind/dendrimer/KI ratio and experimental data confirms that the precipitate is the dendimer (24) Jiang, T.; Wei, G.; Turmel, C.; Bruce, A. E.; Bruce, M. R. M. MetalBased Drugs 1994, 1, 419. (25) Sun, L.; Crooks, R. M.; Chechik, V. Chem. Commun. 2001, 359. (26) It was difficult to quantitatively remove KI due to difficult handling of small amount of sticky precipitate.
cross-linked ligand; it is, however, impossible to establish whether the capsular structure has been preserved or a random polymer was formed. The results showed that the acrylate/amine ratio in the precipitate was 1:1.18 when the cross-linkage was performed with a 1:2 ratio of acrylate/amine groups, and 1:3.3 when a 1:10 ratio of acrylate/amine groups was used. Hence, we conclude that intramolecular cross-linkage at the surface of the particles was somewhat favored compared to the intermolecular reaction. The insolubility of the cross-linked polymer after removal of the Au core is surprising, as the particles before etching are very stable and soluble. It is possible that the cross-linking was not very efficient and a random disulfide-linked polymer formed upon etching. However, this is unlikely. The efficiency of crosslinking is confirmed by the improved stability of the nanoparticles (vide supra). Besides, addition of the cross-linking dendrimer to the solution of disulfide ligand 1 (which was expected to give rise to a random polymer) did not produce any precipitation. This clearly shows a templating effect of Au nanoparticles The poor solubility of the organic structures is most likely due to their rigidity. With the Au nanoparticle template, the organic ligands are well spread on the nanoparticle surface and can efficiently interact with the solvent. We speculate that upon the removal of the core, the structure collapses (e.g., if the crosslinking is not efficient enough to preserve the nanocapsule shape). In the collapsed structure, the organic groups have much less flexibility to interact with the solvent, which makes the whole structure insoluble.
Conclusions We have prepared shell cross-linked Au nanoparticles by polymerizing the organic ligands with ROMP or cross-linking them with polyamine dendrimers. The efficiency of cross-linking was confirmed by NMR. The shell cross-linked particles showed higher stability toward heating and etching with methyl iodide, further confirming the efficiency of cross-linking process. However, oxidative etching of the Au core with iodine led to immediate precipitation of all organic material. It is likely that, without the Au template, the polymerized capsules collapse to form rigid and, hence, insoluble structures. We believe that solubilization of such materials would only be possible if they are modified with functional groups capable of very strong interactions with the solvents. For instance, charged cross-linking agents (e.g., sulfonates or quaternary ammonium salts) may render the capsules soluble in water. In the absence of such interactions, the capsular assemblies would remain insoluble. Experimental Section Silica column chromatography was carried out using BDH silica gel. Preparative gel permeation chromatography (GPC) was performed on Bio-Beads S-X1 (200-400 mesh) supplied by BioRad. 1H and 13C NMR spectra were recorded on a JEOL-E270 or on a JEOL ECX 400. Transmission electron microscopy was performed with a FEI TECNAI G2 instrument at 120 kV. The samples were prepared by evaporating of a drop of the nanoparticle solution onto a copper grid coated with a carbon support film. Microanalysis was carried out at the University of Manchester on a Carlo Erba EA 1108 elemental analyzer. All chemicals were used as received without further purification unless stated otherwise. Cyclopentadiene was distilled from dicyclopentadiene before use. Norbornene-Terminated Disulfide 2. 11,11′-Dithiodiundecyl dipropenoate 1 (50 mg, 9.71 × 10-5 mol) was dissolved in THF (0.5 mL) and freshly distilled cyclopentadiene (0.5 mL). The colorless solution was heated at 60 °C overnight. The solvent was then evaporated. The excess cyclopentadiene was removed by column
Shell Cross-Linked Au Nanoparticles chromatography (SiO2, hexane/CHCl3 1:10, Rf ) 0.17) to give the pure product as a colorless sticky oil (25.5 mg, 41% yield). 1H NMR (CDCl3), δ: 66.08-6.22 (m, HC endo and exo, HD endo), 5.885.95 (m, HD exo), 3.93-4.11 (m, 2H, CH2O), 3.20 (s, broad, HB exo), 3.02 (s, broad, HB endo), 2.86-2.97 (m, HE endo and exo, HA exo), 2.67 (t, 2H, J ) 7.5 Hz, CH2S), 2.18-2.25 (m, HA endo), 1.84-1.96 (m, HG (a) endo and HG (b) exo), 1.50-1.71 (m, CH2CH2S, CH2CH2O, HG (b) endo and HG (a) exo), 1.20-1.45 (m, HF (a) and HF (b), CH2). 13C NMR (CDCl3), δ: 176.28 and 174.80 (CdO), 137.97, 137.67, 135.73 and 132.32 (CC and CD), 64.53 and 64.29 (CH2O), 49.58 (CF), 46.58, 46.32 and 45.68 (CB), 43.33, 43.18, 42.49 and 41.59 (CA and CE), 39.12 (CH2S), 30.27 (CG), 29.44, 28.63, 28.48 and 25.91 (CG and CH2). MS (CI): m/z 647 [M, 16%], 581 [M - cyclopentadiene, 38%], 515 [M - 2 cyclopentadiene, 59%], 443 [M - 2 cyclopentadiene - O(CO)CHCH2, 42%], 371 [M - 2 cyclopentadiene - 2 O(CO)CHCH2, 6%], 325 [one branch of the disulfide, 67%], HRMS, calcd 647.416780, found 647.416945. Norbornene-Terminated Gold Nanoparticles 4. 11,11′-Dithiodiundecyl dipropenoate-protected particles 3 (40 mg, 4.46 × 10-5 mol of acrylates) were dissolved in THF (1 mL) and cyclopentadiene (1 mL) and heated at 60 °C overnight. The solvent was then rotary evaporated, and the product was purified by GPC (Bio-Beads S-X1, CHCl3) to afford pure particles (37.4 mg) as a dark brown sticky oil. 1H NMR (CDCl ), δ: 6.02-6.28 (m, broad, H endo and exo, 3 C HD endo), 5.90 (s, broad, HD exo), 3.81-4.21 (m, broad, 2H, CH2), 3.19 (s, broad, HB exo), 2.80-3.09 (m, broad, HB endo, HE endo and exo, HA exo), 2.33 (s, broad, COCH2CH3), 2.13-2.26 (m, broad, HA endo), 1.90 (s, broad HG (a) endo and HG (b) exo), 0.90-1.70 (m, broad, CH2, HG (b) endo and HG (a) exo, HF). ROMP at the Surface of Norbornene-Terminated Particles 4. CH2Cl2 was dried prior to use by distillation over CaH2. As the catalyst 5 could easily be deactivated by the presence of water, the
reaction was carried out in strictly anhydrous conditions (e.g., flamedried glassware) on a Schlenk line.
Langmuir, Vol. 22, No. 11, 2006 5173 Norbornene-terminated particles 4 (7 mg) were dissolved in dry CH2Cl2 (100 mL) under Ar. A solution of norbornene (0.7 mg) in dry CH2Cl2 (0.8 mL) was injected into the particle solution followed by a solution of Grubbs’ second generation catalyst (0.28 mg) in dry CH2Cl2 (1 mL). The reaction mixture was stirred at room temperature under Ar for 20 min. Acetone (20 mL) was added to the reaction mixture to quench the catalyst. The mixture was stirred for further 5 min before rotary evaporation of the solvents. The product 6 was purified by GPC. 1H NMR (CDCl3), δ: 6.14 (m, broad, starting material), 5.89 (m, broad, starting material), 5.30 (s, broad, CdC), 4.01 (s, broad, CH2O), 3.19 (s, broad, starting material), 2.90 (s, broad, starting material), 0.95-1.75 (CH2). Polyamine cross-linked nanoparticles were prepared as reported earlier.15 Thermal Stability. Acrylate-terminated gold nanoparticles 3 or shell cross-linked particles 7 (2.7 mg) were dissolved in DMF (6 mL) and heated in an oil bath from room temperature to 110 °C (temperature of the bath). After 5 min at 110 °C, an aliquot of the particle solution (0.02 mL) was diluted with DMF (3.5 mL) and analyzed by UV-vis spectroscopy. The temperature of the reaction mixture was increased by 5 °C and kept constant at 115 °C for 5 min. Another sample of the reaction mixture was analyzed by UVvis spectroscopy. This process was repeated until the particles became red and the UV-vis plasmon band of the particles was clearly visible. The same procedure was applied to norbornene-terminated particles 4 and ROMP shell cross-linked particles 6 except that they were heated from room temperature to 75 °C at the beginning of the process. Stability toward Etching with CH3I. A solution of acrylateterminated gold nanoparticles 3 (3 mg) in CHCl3 (2 mL) was mixed with CH3I (0.1 mL) and kept at room temperature. The same procedure was repeated with cross-linked particles 7 (10 mg), CHCl3 (0.5 mL), MeOH (0.5 mL), and CH3I (3 mL); norbornene-terminated gold nanoparticles 4 (7 mg), CDCl3 (ca.0.7 mL), and CH3I (0.7 mL); ROMP shell cross-linked gold nanoparticles 6 (ca. 5 mg), CDCl3 (ca.0.75 mL), and CH3I (ca.3 mL). Gold Etching from Shell Cross-Linked Gold Nanoparticles. In a typical experiment, shell cross-linked particles (5 mg) were dissolved in CHCl3 (2 mL) and added dropwise with stirring to a solution of iodine (42 mg) and KI (860 mg) in MeOH (20 mL). A pale yellow-orange precipitate formed in the reaction mixture. It was isolated by centrifugation, washed, and dried.
Acknowledgment. The authors thank Dr P. Ionita for TEM images. EPSRC is acknowledged for funding (Grant No. GR/R54675/01). LA0600604
