Comparison Study of the Solution Phase versus Solid Phase Place

Monofunctionalized gold nanoparticles may also be prepared through a solution phase place exchange reaction. In this study, we compared the efficiency...
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Langmuir 2004, 20, 8343-8351

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Comparison Study of the Solution Phase versus Solid Phase Place Exchange Reactions in the Controlled Functionalization of Gold Nanoparticles Andrew W. Shaffer, James G. Worden, and Qun Huo* Department of Polymers and Coatings, North Dakota State University, 1735 NDSU Research Park Drive, Fargo, North Dakota 58105 Received March 16, 2004. In Final Form: June 3, 2004 Gold nanoparticles offer tremendous potential in the areas of nanoelectronics, bio- and chemosensors, and catalysis. However, before these applications are realized, the surface functionality of nanoparticles must be better controlled. Our lab has recently reported a novel synthetic approach for making monofunctionalized nanoparticles through a solid phase place exchange reaction. Monofunctionalized gold nanoparticles may also be prepared through a solution phase place exchange reaction. In this study, we compared the efficiency of these two separate approaches toward controlled functionalization of gold nanoparticles by 1H NMR, Fourier transform infrared (FT-IR), and transmission electron microscopy (TEM) analysis. We found that the solid phase place exchange approach is much more efficient at producing monofunctionalized gold nanoparticles. 1H NMR data were used to give a semiquantitative count of substituted bifunctional ligands, and FT-IR spectra supported these findings. Furthermore, we used a diamine coupling reaction of nanoparticles to show the presence of single or multiple functional groups on the nanoparticle surface by TEM analysis.

Introduction Top-down and bottom-up are two popular approaches in nanomaterial design and synthesis.1 The top-down approach starts from a bulky material followed by “removing” excess materials to obtain the desired nanomaterials with expected structures.2 In contrast, the bottom-up approach begins with the synthesis of nanoscale building blocks and reaches the development of the expected nanomaterials or nanodevices through chemical or physical assembly processes.3 Comparing these two different approaches, the top-down approach is more straightforward and offers the final product from a onestep process. A major disadvantage of the top-down approach is that the diversity of the nanomaterial structures created from such an approach is quite limited. On the other hand, the bottom-up approach is much more versatile and provides almost infinite opportunities in creating nanomaterials with novel structures and properties from the same building block materials. However, the assembling process has proven to be a significant challenge. To address this, a majority of the current work has been oriented toward using chemical interaction based self-assembling tools to organize nanoscale building blocks into the desired materials. Given the example of nanoparticles, covalent and noncovalent bonding between functionalized and nonfunctionalized nanoparticles are relied upon to assemble nanoparticles into one-, two-, and three-dimensional superstructures.4 Although extensive efforts have been invested in the self-assembling study of * To whom correspondence should be addressed. Phone: 701231-8438. Fax: 701-231-8439. E-mail: [email protected]. (1) Nalwa, H. S., Ed. Nanostructured Materials and Nanotechnology; Academic Press: London, 2002. (2) (a) Jager, E. W. H.; Smela, E.; Inganas, O. Science 2000, 290, 1540-1545. (b) Craighead, H. G. Science 2000, 290, 1532-1535. (c) Wallraff, G. M.; Hinsberg, W. D. Chem. Rev. 1999, 99, 1801-1821. (3) (a) Special report in Chem. Eng. News. from October 16, 2002. (b) Zhang, S. Mater. Today 2003, 6, 20-27. (c) Lieber, C. M. MRS Bull. 2003, 28, 486-491.

nanoscale building blocks, current existing approaches are far from ideal. These limitations include the following: (1) the types and complexity of the structures that can be created are still very limited; (2) the reproducibility of the assembled structures is often questionable; and, more importantly, (3) the assembled network structures are often unpredictable and cannot be precisely controlled. Without the ability to control the material structures, the realization of functional nanomaterials and nanodevices is inevitably hindered. The crucial problem behind this significant hurdle is our lack of control on the chemical functionality of the nano building blocks. If nanoparticles or other building blocks can have the same controlled functionality as typical organic molecules, then it could be possible to use synthetic chemistry to develop complicated nanomaterials and nanodevices with precisely controlled structures, just like the synthesis of complex organic compounds from small molecular units. With predefined chemical functionality, one can expect that supramolecular self-assembly will lead to nanomaterials with precisely controlled structures and properties. Our group recently reported a novel solid phase synthesis approach to modify gold nanoparticles with controlled numbers of chemical functional groups.5 In our approach, we used a solid phase synthesis technique to control the number of bifunctional thiol ligands attached to nanoparticles during the place exchange reaction. The bifunctional alkanethiol ligands with a carboxylic acid group are first immobilized on a solid support such as (4) (a) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (b) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549-561. (c) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293-346. (d) Hamley, I. W. Angew. Chem., Int. Ed. 2003, 42, 16921792. (e) Feldheim, D. L.; Keating, C. D. Chem. Soc. Rev. 1998, 27, 1-12. (f) Schmid, G.; Baumle, M.; Geerkens, M.; Heim, I.; Osemann, C.; Sawitowski, T. Chem. Soc. Rev. 1999, 28, 179-185. (g) Fendler, J. H. Chem. Mater. 1996, 8, 1616-1624. (h) Fendler, J. H. Chem. Mater. 2001, 13, 3196-3210. (5) Worden, J. G.; Shaffer, A. W.; Huo, Q. Chem. Commun. 2004, 518-519.

10.1021/la049308k CCC: $27.50 © 2004 American Chemical Society Published on Web 08/20/2004

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Scheme 1. Solution Phase versus Solid Phase Place Exchange Reactions between BtAu and 11-Mercaptoundecanoic Acid or 6-Mercaptohexanoic Acid in the Controlled Chemical Functionalization of Gold Nanoparticles

polymeric Wang resin. If the functional group density of the solid support is low enough, one can ensure that neighboring thiol ligands attached to the solid support are sufficiently far apart. When the resin-bound thiol ligands are allowed to undergo a place exchange reaction with gold nanoparticles, only one resin-bound thiol ligand will be attached to the nanoparticle. Upon cleavage from the resin, nanoparticles with a single carboxylic acid functional group are obtained as the desired product. The monofunctionalized gold nanoparticles could in turn be used in the bottom-up fabrication of nanoscale materials in a designed and controlled manner. For example, using an alkyldiamine, we have shown that the single carboxylic acid group modified nanoparticles can be linked together to make large numbers of nanoparticle dimers. Following our pioneer work, Jacobson et al. published an almost identical approach in the synthesis of monofunctionalized gold nanoparticles.6 Theoretically, one could also make nanoparticles with single surface functional groups through a typical solution phase place exchange reaction by strictly controlling the stoichiometric ratio of the incoming thiol ligands versus the alkylthiolate ligands attached to the nanoparticle surface. Hostetler et al. reported a systematic study on the place exchange reaction kinetics and dynamics in solution.7 By using an incoming ligand ratio of 1:20 versus the number of ligands present on the nanoparticle surface, it was found that a low number of ligands could be attached to the gold nanoparticles. Although theoretically deemed feasible, we were surprised to find that no study has actually been reported on using this method to make single functional group modified nanoparticles. Alivisatos et al. reported gold-nanoparticle-DNA conjugates with a single or discrete number of DNA molecules attached by the typical solution phase place exchange reaction.8 However, the single DNA strand modified nanoparticles were obtained by electrophoresis, largely based on the size and charge of DNA molecules rather than the nanoparticle itself. Moreover, it is difficult to use such nanoparticleDNA conjugates directly for other applications not associated with DNAs. The purpose of this study was to examine and compare the efficiency of the solution phase versus solid phase based (6) Soon, K.-M.; Mosley, D. W.; Peelle, B. R.; Zhang, S.; Jacobson, J. M. J. Am. Chem. Soc. 2004, 126, 5064-5065. (7) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782-3789. (8) (a) Zanchet, D.; Micheel, C. M.; Parak, W. J.; Gerion, D.; Alivisatos, A. P. Nano Lett. 2001, 1, 32-35. (b) Zanchet, D.; Micheel, C. M.; Parak, W. J.; Gerion, D.; William, S. C.; Alivisatos, A. P. J. Phys. Chem. B 2002, 106, 11758-11763.

place exchange reactions for the controlled chemical functionalization of nanoparticle materials, as outlined in Scheme 1. For this purpose, we synthesized gold nanoparticles with different percentages of carboxylic acid groups on the nanoparticle surface by the solution phase place exchange reaction. 1-Butanethiol-protected gold nanoparticles (BtAu) synthesized by the Brust-Schiffrin reaction, with core diameters of 1.8 nm were used as the starting material.9 In the solution phase place exchange reaction, 11-mercaptoundecanoic acid was attached to the nanoparticle surface at different percentages using incoming ligand ratios versus the existing butanethiolate ligands of 1:100, 1:50, 1:20, 1:2, and 2:1. The solid phase place exchange reaction was carried out under conditions reported previously to obtain gold nanoparticles with a single or minimum number of carboxylic acid groups using 6-mercaptohexanoic acid bifunctional ligands.5 The functionality of the nanoparticle samples was characterized by 1H nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FT-IR) spectroscopy. Furthermore, we designed a coupling reaction to examine the number and distribution of functional groups on the nanoparticle surfaces using transmission electron microscopy (TEM) analysis, as reported previously.5 Experimental Section Hydrogen tetrachloroaurate(III) hydrate (HAuCl4) was purchased from Strem Chemicals. Tetraoctylammonium bromide (TOAB), sodium borohydride (NaBH4), 1-butanethiol, 11-mercaptoundecanoic acid, 1,7-diaminoheptane, 1,2-ethylenediamine, methyl-d3 alcohol-d, chloroform-d, methyl sulfoxide-d6, lipophilic Sephadex LH-20 gel, and all solvents were purchased from SigmaAldrich. 1H NMR spectra were taken on a 300 MHz Varian mercury spectrometer. A recycle delay of 5 s and a minimum of 25 scans were used to ensure a good signal-to-noise ratio. Additionally, the spectra were acquired with a line-broadening factor of 1. The spectra were analyzed using ACD Labs NMR processor software. FT-IR spectra were taken on a Nicolet 850 Magna IR spectrometer. Thin films were drop cast onto a single salt plate and taken in transmission mode. A background of the empty IR chamber was automatically subtracted from the spectra. Baseline correction was the only manipulation done to the spectra. A JEOL 100CX transmission electron microscope operated at an accelerating voltage of 80 kV was used for imaging of the nanoparticle products. The samples were dissolved in a suitable solvent, and then, 1 µL of the sample solution was drop cast onto a Formvar coated copper grid and the solvent wicked off with filter paper. (9) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (b) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30.

Controlled Functionalization of Gold Nanoparticles 1. Synthesis of BtAu Nanoparticles. 1-Butanethiol-monolayer-protected gold nanoparticles (BtAu) were synthesized according to the Brust-Schiffrin reaction to attain gold nanoparticles with average core diameters of 1.8 nm.9 Briefly, tetraoctylammonium bromide (2.5 equiv) was stirred vigorously in toluene in a 500 mL round-bottom flask. HAuCl4‚xH2O (1 equiv) in 100 mL of deionized water was added. As the AuCl4- was transferred from the aqueous phase to the organic phase, the solution changed from a bright yellow to a red-brown color. The organic phase was separated, and the desired amount of 1-butanethiol was added to the solution. The solution was stirred for 30 min followed by the addition of NaBH4 (10 equiv) in deionized water over 10 s. The resulting solution quickly turned dark black. The solution was stirred at room temperature for 3-4 h. Following the reaction, the black organic phase was isolated and the toluene solvent removed using a rotary evaporator. The black product was suspended in a minimum amount of ethanol and filtered on a glass frit filter. The product was washed with copious amounts of ethanol and acetone and vacuum-dried. The BtAu product was then annealed by refluxing it in toluene at 140-150 °C for 6-8 h to improve the monodispersity of the sample.10 2. Solution Phase Place Exchange Reaction. The solution phase place exchange reactions were completed according to the procedure outlined by Hostetler et al. between BtAu nanoparticles and 11-mercaptoundecanoic acid.7 Five samples were prepared with the ratio of incoming 11-mercaptoundecanoic acid ligands versus the 1-butanethiolate ligands varying from 1:100 to 1:50, 1:20, 1:2, and 2:1. The average number of ligands per particle is assumed to be around 100, according to the core diameter of BtAu particles.9b In a 100 mL round-bottom flask, BtAu nanoparticles (∼50-100 mg) were combined with the proper amount of 11mercaptoundecanoic acid in dichloromethane (∼3 mg of BtAu/ mL). The reactions were protected with nitrogen to limit the amount of disulfide formation and stirred gently at room temperature for 72-96 h, until a sufficient equilibrium had established. It became necessary to add ∼25 mL of methanol to the 1:2 and 2:1 place exchange reaction flasks after 36 h of reaction because the partially reacted nanoparticles became insoluble in pure dichloromethane. After the exchange reaction was complete, the reaction mixture was concentrated using a rotary evaporator. The products were then cleaned depending on the nature of the protecting monolayer. All of the samples were initially cleaned by washing them on a glass frit filter using copious amounts of a suitable solvent. The 1:100, 1:50, and 1:20 place exchange products were washed with ethanol and acetone. The 1:2 and 2:1 place exchange products were washed using dichloromethane. The 1:100, 1:50, and 1:20 place exchange products were further purified by gel permeation chromatography using lipophilic Sephadex LH-20 gel to eliminate small organic thiol ligand molecules. For the 1:2 and 2:1 place exchange products, NMR spectral analysis indicated that these two samples were free of small ligands after washing them with a large amount of dichloromethane; therefore, no further purification was conducted on these samples. 3. Solid Phase Place Exchange Reaction. The solid phase place exchange reactions were carried out as reported previously.5 Briefly, acetyl-protected 6-mercaptohexanoic acid, synthesized according to a reported procedure,11 was coupled to Wang resin in a 1:2 molar ratio (thiol/hydroxyl group) using a standard loading procedure for attaching amino acids to Wang resin.12 After this was washed with DMF, dichloromethane, and methanol followed by drying, the acetyl group was deprotected using ammonia solution (3 M in dioxane/water, 4:1, v/v) for 12-24 h. Ellman’s agent (5,5′-dithio-bisnitrobenzoic acid, DTNB) was used to monitor the deprotection of the acetyl group visually. The deprotected resin beads turned bright yellow after mixing them with DTNB solution. After the washing/drying cycle, again, a dichloromethane solution of BtAu nanoparticles (30 mg in 6 mL (10) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C.-J. Langmuir 2000, 16, 490-497. (11) Svedhem, S.; Hollander, C. A.; Shi, J.; Konradsson, P.; Lierberg, B.; Svensson, S. C. T. J. Org. Chem. 2001, 66, 4494-4503. (12) (a) Fields, G. B.; Noble, R. L. Int. J. Pept. Protein Res. 1990, 35, 161-214. (b) Fields, G. B. Methods in Enzymology: Solid-Phase Peptide Synthesis; Academic Press: New York, 1997; Vol. 289.

Langmuir, Vol. 20, No. 19, 2004 8345 of CH2Cl2) was added and mixed with solid resin. The exchange reaction was allowed to proceed for 4 h at 45 °C followed by 12 h at room temperature with gentle shaking. During this time, the beads turned dark black. After the dark beads were filtered and washed with warm dichloromethane, they were dried under vacuum and then suspended in dichloromethane (8 mL) for 30 min. The nanoparticles were then cleaved from the resin by adding 2.0 mL of trifluoroacetic acid (equivalent to 20% TFA in CH2Cl2) and shaking the beads at room temperature for 4 h. The dark nanoparticle solution was then collected, and the nanoparticles were recovered by blowing off the solvent using a stream of N2 gas. The crude product was further purified by the following procedure. The dark solids were washed with petroleum ether 15-20 times with occasional sonication followed by centrifugation. After each cycle, the washing solution was tested for organic impurities by thin-layer chromatography (TLC). The washing cycle was repeated until UV active species no longer appeared on the TLC plates and the washing solution became almost neutral. After being dried, the nanoparticles were redissolved in a mixture solvent of 9:1 dichloromethane and methanol with a trace amount of trifluoroacetic acid and purified by size exclusion chromatography. The dark nanoparticle sample was collected in one portion. The sample was blown dry with N2 stream and washed two additional times with petroleum ether and dried. The washing solution was found to be neutral, and NMR analysis confirmed that the nanoparticle product was free of small organic molecules. The total yield of the purified BtAuCOOH nanoparticles is around 50%, referencing to the total weight of BtAu added to the reaction mixture during the solid phase place exchange reaction. 4. Nanoparticle Coupling Reaction with 1,2-Ethylenediamine or 1,7-Heptanediamine. The coupling reaction of nanoparticles with 1,2-ethylenediamine or 1,7-heptanediamine was conducted by using a diisopropylcarbodiimide in situ activation method.12 The concentration of diamine and diisopropylcarbodiimide was 0.1 M in a mixed dichloromethane and DMF solvent. The concentration of the nanoparticles was ∼0.5 mg/mL. For the 2:1 and 1:2 place exchange products, the nanoparticles all precipitated out from solution within a few hours after the coupling reaction was initiated. The precipitates were insoluble in many tested organic solvents and unsuitable for TEM analysis. For this reason, these coupling products were not analyzed by TEM. For the 1:20 place exchange product, a significant portion of nanoparticles also precipitated out from solution. The precipitates were removed by centrifugation, and only the remaining soluble solution was subjected to TEM analysis. The 1:100 and 1:50 reaction products remained in solution after the coupling reaction. For the solid phase place exchange reaction product BtAuCOOH, a very small amount of precipitates were formed after the coupling reaction and only the clear solution portion was subjected to TEM analysis. To perform the TEM analysis, ∼1 µL of the reaction mixture was diluted in ∼10 mL of dichloromethane. This solution was compared to an unreacted BtAu sample by TEM analysis.

Results and Discussion 1H

NMR Analysis. The 1H NMR spectra of free 1. ligands, 1-butanethiol, and 11-mercaptoundecanoic acid in chloroform-d are shown in parts a and b of Figure 1, respectively. For 1-butanethiol (Figure 1a), the peak assignments include 0.96 ppm (t, methyl protons at -C5), 1.25 ppm (t, thiol proton at -S1), 1.3-1.4 ppm (m, methylene protons at -C4), 1.5-1.6 ppm (m, methylene protons at -C3), and 2.56 ppm (q, methylene at -C2). For 11-mercaptoundecanoic acid (Figure 1b), the five peak assignments are 1.2-1.4 ppm (m, methylene protons at -C4′-C9′), 1.5-1.7 ppm (m, methylene protons at -C3′C10′), 2.33 ppm (t, methylene protons at -C11′), 2.50 ppm (q, methylene protons at -C2′), and 2.63 ppm (t, thiol proton at -S1′). The 1H NMR spectra of all the nanoparticle products, including BtAu, the five solution phase place exchange products, and the solid phase place exchange product are

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Figure 1. 1H NMR spectra of 1-butanethiol (a) and 11mercaptohexanoic acid (b).

Figure 2. 1H NMR spectra of BtAu (a), five solution phase place exchange products from reaction ratios of 1:100 (b), 1:50 (c), 1:20 (d), 1:2 (e), and 2:1 (f), and the solid phase place exchange product BtAuCOOH (g).

presented in Figure 2. The spectrum of unmodified BtAu, as shown in Figure 2a, resembles the spectra of alkylthiolate-protected gold nanoparticles from many previous reports.13 Three sets of methyl and methylene proton peaks can be identified from the bound thiolate ligand, which appeared at 0.96 ppm (methyl protons at -C5), 1.33 ppm (methylene protons at -C4), and 1.59 ppm (methylene protons at -C3). These three peaks are significantly broadened and overlapping. Additionally, the C2 protons closest to the -SH group, the so-called R protons, disappeared into the baseline due to a significant broadening effect and were undetectable. Line broadening of organic ligands bound to metal and metal nanoparticle (13) (a) Badia, A.; Demers, L.; Dickinson, L.; Morin, F. G.; Lennox, R. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104-11105. (b) Hasan, M.; Bethell, D.; Brust, M. J. Am. Chem. Soc. 2002, 124, 1132-1133. (c) Kohlmann, O.; Steinmetz, W. E.; Mao, X.-A.; Wuelfing, P.; Templeton, A. C.; Murray, R. W.; Johnson, C. S., Jr. J. Phys. Chem. B 2001, 105, 8801-8809. (d) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36.

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surfaces has been well documented and attributed to possible reasons such as variations in spin-spin relaxation times (T2), Knight shifts, particle size distributions, and adsorption sites on the particle surface.13 It should be mentioned that with different BtAu samples the spectra show slightly different features, such as the relative intensity and the degree of line broadening of the different peaks. These differences are more or less associated with the polydispersity of the nanoparticle cores and the ordering of the alkylthiolate monolayer. In addition, two sets of weak peaks, one small quartet at 2.45 ppm and one triplet at 2.63 ppm, were also present in the BtAu spectra. The sharpness of these two peaks indicates that they do not correspond to tightly bound thiol ligands. After stringent washing and purification by gel permeation chromatography followed by vacuumdrying, these two peaks persist in the spectra. No solvents used in the synthesis and purification steps appeared in this region as well. It is believed that the triplet at 2.63 ppm corresponds to the oxidized disulfide ligands and the quartet at 2.45 ppm corresponds to 1-butanethiol ligands. Both ligands are loosely associated, rather than tightly bound with the gold nanoparticle, leading to the observed two sets of sharp peaks. The spectra of solution phase place exchange products from reaction ratios of 1:100, 1:50, 1:20, 1:2, and 2:1 are illustrated in Figure 2b-f, respectively. From spectrum b, it can be seen that a new broad peak at 1.95 ppm has appeared. This peak is attributed to the methylene protons adjacent to the carboxylic acid group (-CH2-COOH, the C11′ proton) from the new 11-mercaptoundecanoic acid attached to the nanoparticle surface. In free solution, this peak appears at 2.45 ppm as a triplet. All the other peaks in these spectra are attributed to the methylene protons from the 11-mercaptoundecanoic acid or the remaining 1-butanethiol ligands bound to the nanoparticles. Similar spectral features were observed from all the other place exchange products. The -CH2-COOH proton peak appeared around 2.0-2.5 ppm in all the spectra. Furthermore, with an increased place exchange reaction ratio, the intensity of this peak increased gradually. The place exchange reaction product from a reaction ratio of 2:1 displays a large -CH2-COOH proton peak with very little evidence of methyl protons. We take this as an indication of complete exchange of 1-butanethiol ligands with 11mercaptoundecanoic acid. Because the -CH2-COOH proton peak can be clearly identified, we devised the following equation to calculate the place exchange level. Assuming the average number of 1-butanethiol ligands attached to the original BtAu nanoparticle is N and the place exchange percentage is p, then the integration ratio of the -CH2-COOH peak versus all the other methylene and methyl proton peaks should follow the equation

2pN 2p ) ) 7(1 - p)N + 16pN 7 + 9p Integration of -CH2-COOH protons (1) Integration of remaining methylene and methyl protons In this equation, the total number of -CH2-COOH protons is 2pN, the number of all the methyl and methylene protons from the remaining 1-butanethiolate ligands is 7(1 - p)N (protons from C3 to C5), and the number of methylene protons from the newly attached 11-mercaptoundecanoic acid is 16pN (protons from C3′ to C10′). From this equation, the place exchange percentage

Controlled Functionalization of Gold Nanoparticles

(p) can be conveniently calculated on the basis of the NMR spectral integration ratio. It should be noticed here that this calculation does not require one to know the exact number of ligands attached to the BtAu nanoparticles. On the basis of the calculation, it is estimated that the place exchange products from reaction ratios of 1:100, 1:50, 1:20, 1:2, and 2:1 contain approximately 4.9, 9.1, 37.6, 52.5, and 100% 11-mercaptoundecanoic acid ligand, respectively. The calculated percentage of 11-mercaptoundecanoic acid in the 1:100 ratio place exchange product is not as accurate as that in the higher exchange ratio products due to the extremely low intensity of the -CH2COOH peak. On the other hand, when the place exchange reaction ratio is too high, such as the case of the 1:2 and 2:1 reaction ratios, the calculation of the place exchange percentage according to the above equation is not very accurate either, due to significant overlapping of the -CH2-COOH proton peak with other methyl and methylene proton peaks. When comparing the spectra of all the solution phase place exchange products, another interesting result was observed with regards to the chemical shift of the -CH2COOH protons. This proton peak shifted to higher ppm with an increased percentage of 11-mercaptoundecanoic acid attached to the nanoparticle surface. The exact chemical shift of the five solution phase place exchange products from reaction ratios of 1:100, 1:50, 1:20, 1:2, and 2:1 appeared at 1.95, 2.0, 2.0, 2.25, and 2.25 ppm, respectively. This downfield chemical shift is clearly associated with the effect of increased hydrogen bonding between carboxylic groups with higher substitution levels. When comparing the spectrum of the solid phase place exchange product BtAuCOOH (Figure 2g) with that of the 1:100 solution phase place exchange product, a very similar weak peak was observed at 1.85 ppm, which is readily attributed to the -CH2-COOH proton from the newly attached 6-mercaptohexanoic acid ligand. This peak further shifted upfield compared to that of the 1:100 solution phase place exchange reaction product, clearly suggesting that there are few carboxylic groups attached to the nanoparticle surface. By referring to eq 1, a similar equation, eq 2, was derived to calculate the percentage of 6-mercaptohexanoic acid exchanged to nanoparticles through the solid phase place exchange reaction:

2p 2pN ) ) 7(1 - p)N + 6pN 7 - p Integration of -CH2-COOH protons (2) Integration of remaining methylene and methyl protons According to this equation, the solid phase place exchange percentage between 6-mercaptohexnoic acid and 1-butanethiolate is ∼4.5%. For a gold nanoparticle with a core diameter of 1.8 nm, there are ∼100 thiol ligands on the nanoparticle surface.9 According to this number, it can be estimated that there should be less than five bifunctional mercaptohexanoic acid ligands per nanoparticle. Again, due to the low intensity of the -CH2-COOH proton peak, the calculated result is only semiquantitative. Although NMR spectroscopic analysis can give quantitative or semiquantitative data on the percentages of functional groups attached to the nanoparticle surface, as discussed above, it does not provide elaborate information on the exact number and distribution of functional groups. Whether through solution phase synthesis or solid phase synthesis, our ultimate goal is to develop nanoparticles with a single surface functional group. A

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significant challenge involved in this study is that at very low exchange levels NMR analysis is not accurate enough to give quantitative information, due to the large integration ratio of nonfunctionalized versus functionalized ligands. This difficult has already been fully demonstrated in the two low place exchange level samples, namely, the 1:100 solution phase place exchange and solid phase place exchange reaction products. However, by combining NMR results with IR and TEM analysis, as discussed below, more substantial conclusions on the number and distribution of functionality can be made. 2. FT-IR Analysis. FT-IR has been proven to be a powerful tool in the analysis of monolayer structures assembled on 2D planar or 3D nanocluster surfaces.9,14 In our study, we used FT-IR spectroscopy to examine the functionality of the nanoparticles from both solution phase and solid phase place exchange reactions. Two regions of vibration bands are of particular interest in this study: the C-H stretching band and the carbonyl stretching band. The C-H stretching bands provide valuable information on the ordering of the self-assembled monolayer (SAM) structure, and the carbonyl stretching band is a direct indication of the carboxylic acid functional groups attached to the nanoparticle surface. In the C-H stretching region, there are five peaks of interest. These include the symmetric (r+) and antisymmetric (r-) CH3 stretches and the two symmetric (d+, d+s) and one antisymmetric (d-) CH2 stretches. The exact wavenumbers of these vibration bands are often used as a sensitive indicator of the ordering of the alkyl chains. For example, in crystalline polyethylene, d- lies at 2920 cm-1 and d+ is often found at 2850 cm-1. In solution, these values shift to 2928 and 2856 cm-1, respectively.14c,d Higher energies for the methylene stretching vibrations are often linked with a lower crystallinity of the solution based ligands or gauche defects of a monolayer film. Many previous studies on 2D SAMs have shown that the d- and d+ peaks appeared at values of 2920 and 2850 cm-1, respectively, indicating the presence of an all-trans hydrocarbon chain in the monolayer film. Allara et al. reported that these stretches occurred at 2954 cm-1 (r-), 2870 cm-1 (r+), 2924 cm-1 (d-), 2834 cm-1 (d+), and 2859 cm-1 (d+s) for alkanethiolate ligands attached to gold nanoclusters.14 The FT-IR spectra of the unmodified BtAu sample and the five solution phase place exchange products from reaction ratios of 1:100, 1:50, 1:20, 1:2, and 2:1 are presented in Figures 3 and 4. The C-H stretching region is shown in Figure 3, and the carbonyl region is shown in Figure 4. Additionally, the wavenumbers and assignments of the C-H and carbonyl stretches for each sample are summarized in Tables 1 and 2, respectively. Compared to the previously reported studies on monolayer-protected gold nanoclusters, the unmodified BtAu sample showed all five peaks of interest within the C-H stretching region (Figure 3a). The r- and r+ peaks appeared at 2953 and 2870 cm-1, respectively. The three CH2 stretches, d-, d+, and d+s, appeared at 2920, 2857, and 2839 cm-1, respectively. These values suggest that the butanethiolate ligands are between the crystalline state and the liquid state. In other words, the monolayer is not very well ordered. This result corresponds to a well-known fact that short alkyl chains tend to give a more disordered monolayer film on flat metal surfaces and nanoclusters. When (14) (a) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604-3612. (b) Nuzzo, R.G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (c) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (d) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

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Shaffer et al. Table 1. FT-IR C-H Stretching Vibration Band and Assignments of BtAu, the Solid Phase Place Exchange Product BtAuCOOH, and Five Solution Phase Place Exchange Products from Ligand Reaction Ratios of 1:100, 1:50, 1:20, 1:2, and 2:1 sample

wavenumber (cm-1)

assignment

2839.00 2857.02 2869.74 2920.16 2953.13

sym. CH2 stretch (d+s) sym. CH2 stretch (d+) sym. CH3 stretch (r+) antisym. CH2 stretch (d-) antisym. CH3 stretch (r-)

BtAu

BtAuCOOH

1:100

1:50

Figure 3. FT-IR spectra of the C-H stretching vibration region of BtAu (a) and five solution phase place exchange products from reaction ratios of 1:100 (b), 1:50 (c), 1:20 (d), 1:2 (e), and 2:1 (f).

1:20

1:2

2:1

Solid Phase Reaction absent sym. CH2 stretch (d+s) 2850.85 sym. CH2 stretch (d+) 2868.81 sym. CH3 stretch (r+) 2921.11 antisym. CH2 stretch (d-) 2950.66 antisym. CH3 stretch (r-) Solution Phase Reaction absent sym. CH2 stretch (d+s) 2857.96 sym. CH2 stretch (d+) 2870.13 sym. CH3 stretch (r+) 2923.77 antisym. CH2 stretch (d-) 2955.49 antisym. CH3 stretch (r-) absent sym. CH2 stretch (d+s) 2853.46 sym. CH2 stretch (d+) 2870.58 sym. CH3 stretch (r+) 2923.5 antisym. CH2 stretch (d-) 2956.67 antisym. CH3 stretch (r-) absent sym. CH2 stretch (d+s) 2850.85 sym. CH2 stretch (d+) 2868.21 sym. CH3 stretch (r+) 2917.92 antisym. CH2 stretch (d-) 2951.41 antisym. CH3 stretch (r-) absent sym. CH2 stretch (d+s) 2850.4 sym. CH2 stretch (d+) 2868.61 sym. CH3 stretch (r+) 2920.17 antisym. CH2 stretch (d-) 2951.12 antisym. CH3 stretch (r-) absent sym. CH2 stretch (d+s) 2850.16 sym. CH2 stretch (d+) 2868.77 sym. CH3 stretch (r+) 2920.15 antisym. CH2 stretch (d-) weak antisym. CH3 stretch (r-)

Table 2. FT-IR Carbonyl Stretching Vibration Band of BtAu, the Solid Phase Place Exchange Product BtAuCOOH, and Five Solution Phase Place Exchange Products from Ligand Reaction Ratios of 1:100, 1:50, 1:20, 1:2, and 2:1 sample

wavenumber of CdO stretch (cm-1)

BtAu

BtAuCOOH

Figure 4. FT-IR spectra of the carbonyl stretching vibration region of BtAu (a) and five solution phase place exchange products from reaction ratios of 1:100 (b), 1:50 (c), 1:20 (d), 1:2 (e), and 2:1 (f).

comparing the solution phase place exchange samples with + BtAu, the intensity of the r and r peaks decreases as the place exchange percentage increases (Figure 3b-f). The trend is emphasized by the relative lack of the r+ and rpeaks in the 2:1 place exchange product. At very low levels of place exchange reaction, as is the case for the 1:100 and 1:50 solution phase place exchange products, the monolayer appeared to be slightly disordered, judging from the wavenumber of the two -CH2 stretching bands located near 2925 and 2855 cm-1 for both samples (Figure 3b and c). We believe this slight disordering was caused by the insertion of a small amount of incoming ligands into the existing monolayer, which disturbed the monolayer ordering. With a continued increase of the

1:100 1:50 1:20 1:2 2:1

Solid Phase Reaction 1733.92 Solution Phase Reaction 1728.87 1728.06 1707.94 1706.46 1707.12

exchange level, the monolayer became ordered again, as indicated by the -CH2 stretching band at 2920 and 2850 cm-1 of the 1:20, 1:2, and 2:1 place exchange products (Figure 3d-f). This increased degree of ordering is aided by increased hydrogen bonding between carboxylic groups on the nanoparticle surface, especially for the 1:2 and 2:1 place exchange products, corresponding to what was observed from the 1H NMR spectral results. The carbonyl stretching vibration band occurring around 1700 cm-1 offers additional supportive information concerning the identity of the monolayer ligands. The unmodified BtAu sample shows no evidence of carbonyl groups (Figure 4a). The CdO stretching vibration band of the 1:100, 1:50, 1:20, 1:2, and 2:1 solution phase place

Controlled Functionalization of Gold Nanoparticles

Figure 5. FT-IR spectra of the solid phase place exchange product BtAuCOOH at the C-H stretching vibration region (a) and the carbonyl stretching vibration region (b).

exchange products appeared at 1729, 1728, 1708, 1706, and 1707 cm-1, respectively (Figure 4b-f and Table 2). The exact wavenumber of the carbonyl stretching vibration band often reveals important information concerning the hydrogen bonding effect.15 With more hydrogen bonding present, this stretching vibration band appears at lower wavenumbers. As the place exchange ratio increases from 1:100 to 2:1, more bifunctional thiol ligands are attached to the nanoparticle surface, leading to the observed decrease in the wavenumber at which the carbonyl stretching vibration band occurs. Such a phenomenon has been well documented in many previous studies on selfassembled monolayer films on planar and nanocluster substrates.14 The FT-IR spectra of the C-H stretching and carbonyl regions for the solid phase BtAuCOOH sample are shown in parts a and b of Figure 5, respectively. The antisymmetric d- and symmetric d+ bands of the CH2 stretching vibration band appeared at 2920 and 2850 cm-1, respectively, indicating the presence of a relatively ordered monolayer film on the nanoparticle surface. This is an important piece of information demonstrating that the monolayer film structure remained intact after the solid phase reactions, including the place exchange reaction and acidic cleavage conditions. Compared to that of the solution phase place exchange reaction product from the lowest ratio of 1:100, the CdO stretching vibration band of the BtAuCOOH sample occurred at an even higher wavenumber of 1734 cm-1. Such a high wavenumber strongly supports that there is no or minimal hydrogen bonding present in the BtAuCOOH nanoparticle sample. Combining this with the results from NMR analysis, it is reasonable to believe that there is only one or a few bifunctional thiol ligands attached to nanoparticles by the solid phase place exchange reaction. Although quantitative analysis of their NMR spectra suggests that both the 1:100 solution phase place exchange product and the solid phase place exchange product BtAuCOOH contain ∼5% of bifunctional ligands, FT-IR analysis demonstrated that the solid phase place exchange product BtAuCOOH contains an even less number of carboxylic groups on the nanoparticle surface. 3. TEM Analysis of Nanoparticles and their Coupling Product with Alkyldiamine. While NMR and FT-IR spectroscopy give structural information about the (15) (a) Roeges, N. P. G. A Guide to the Complete Interpretation of Infrared Spectra of Organic Structures; John Wiley & Sons: New York, 1994; pp 163-165. (b) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; John Wiley & Sons: New York, 1972; p 198.

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monolayer film attached to the nanoparticle surface, TEM analysis allows direct visualization of individual nanoparticles. In our previous study, we reported the coupling reaction of carboxylic group modified gold nanoparticles through a coupling reaction with an alkyldiamine such as 1,2-ethylenediamine or 1,7-heptanediamine.5 It is assumed that, if there is only one functional group attached to the nanoparticle surface, the coupling reaction will lead to the formation of nanoparticle dimers. In contrast, nanoparticles with more than one functional group distributed isotropically on the surface will most likely lead to the formation of higher order nanoparticle structures such as trimers, tetramers, and various large aggregates. Here, we used the same coupling reaction to examine the functionality of the solution phase place exchange product. To ensure that all the carboxylic groups were reacted with diamine, we used a large excess of diamine and DIPCDI, at a concentration of 0.1 M, to drive the reaction to completion. For the two higher place exchange level products obtained from the solution phase reaction, namely, the 1:2 and 2:1 solution phase place exchange products, when diamine and diisopropylcarbodiimide were added to the nanoparticle solution, almost all the nanoparticles precipitated out in a few minutes. This is attributed to the formation of large aggregates through the coupling of multifunctional group modified nanoparticles by diamine molecules. Because the precipitate was not soluble in most solvents tested, TEM analysis was not conducted on these samples. The 1:20 place exchange product showed a significant amount of precipitates after the coupling reaction was completed. The precipitates were discarded, and only the soluble solution was subjected to TEM analysis. The 1:50 and 1:100 place exchange products showed no visible precipitates. The TEM images of unmodified BtAu, the diamine coupling product of the solution phase place exchange products from reaction ratios of 1:20 and 1:100, and the coupling product of the solid phase place exchange product BtAuCOOH are shown in Figure 6. Control images of each exchange product are not displayed because TEM images of uncoupled place exchange nanoparticle products look similar to that of the BtAu sample, which appeared primarily as individual particles, as shown in Figure 6a. When referring to Figure 6b, one can see the presence of large aggregates from the 1:20 place exchange product, indicating that multiple functional groups were attached to the nanoparticle surface. For the 1:100 place exchange product, an even distribution of dimers, trimers, tetramers, and other small oligomers were present in the TEM image, as shown in Figure 6c. The 1:50 place exchange reaction product shows TEM images similar to the 1:100 place exchange reaction product. This result suggests that there is an even distribution of nanoparticles with one, two, three, and multiple functional groups attached across the sample. That means that, in the solution phase place exchange reaction, the incoming bifunctional ligands were randomly exchanged to different particles, at different locations of each particle. In all the cases, nonfunctionalized nanoparticles are also present in a substantial amount. Other than these features, it was also noticed that, after the diamine coupling reaction, the nanoparticle size of the two solution place exchange products appeared to be increased slightly. This is possibly due to the fusion of aggregated nanoparticles, a phenomenon that has been observed in some other instances but not fully understood

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Figure 6. TEM images of BtAu (a) and the 1,7-heptanediamine coupling reaction product of the 1:20 solution phase place exchange product (b), the 1:100 solution phase place exchange product (c), and the solid phase place exchange product BtAuCOOH (d). In parts c and d, the circles indicate dimers, the arrows indicate trimers, and the squares indicate tetramers.

yet.16 It was surmised that, with a few carboxylic groups evenly distributed on the nanoparticle surface, the diamine molecules bridged a group of nanoparticles together by electrostatic interactions with the carboxylic groups, eventually leading to the fusion of this group of nanoparticles into larger nanoparticles through electron beam irradiation during TEM analysis. In contrast to the solution phase place exchange reaction, the solid phase place exchange reaction gave nanoparticle dimers as the major product after the diamine coupling reaction, as shown in Figure 6d, although a small portion of individual nanoparticles and nanoparticle trimers and larger oligomers can also be found in the TEM images. In conjunction with the NMR and FT-IR analysis discussed above, the TEM analysis offers strong evidence to support that the nanoparticle dimers shown in these images are most likely made of nanoparticles with single carboxylic groups. Further quantitative analysis of the images revealed that the percentage of single functional group modified nanoparticles is ∼50-70% of the nanoparticle sample. Compared to the solution phase place (16) Yonezawa, T.; Onoue, S.; Kimizuka, N. Chem. Lett. 2002, 11721173.

exchange reaction, the solid phase place exchange reaction is clearly a much more efficient approach for making nanoparticles with a single functional group. In the solution phase place exchange reaction, the incoming ligands have the freedom to move around to attack the nanoparticles from any location, therefore, leading to a random distribution of an uncontrolled number of functional groups attached to the nanoparticle. This limit is clearly manifested in the 1:100 solution phase place exchange product. In contrast, in the solid phase place exchange reaction, the incoming ligands are immobilized on the polymer beads with a predefined density. Once the nanoparticle is attached to the beads, this nanoparticle will not react further with other ligands to allow the attachment of multiple surface functional groups. This is probably the most important advantage of the solid phase place exchange reaction versus the solution phase place exchange reaction in the controlled functionalization of nanoparticle materials. Although many experimental conditions and parameters involved in the solid phase place exchange reaction remain to be explored to obtain the single functional group modified nanoparticles with optimum yield, the potential of this approach has already been demonstrated in the work conducted so far.

Controlled Functionalization of Gold Nanoparticles

Conclusion In this study, we conducted a systematic comparison study of solution phase place exchange reactions versus a solid phase place exchange reaction in the preparation of gold nanoparticles with single surface functional groups. While theoretically one could use the solution phase place exchange reaction to control the number of functional groups attached to the nanoparticle surface, practically, it is not efficient for this purpose. With the advantages mentioned above, the solid phase place exchange reaction has a much better potential to be developed into a general methodology to prepare nanoparticles with single or other controlled numbers of surface functional groups. Such nanoparticles may be treated as extended “molecules” and could be further used to react with other nanoparticles or chemicals to form higher order nanoparticle oligomers, macromolecules, and assemblies. The ultimate goal of this research is the development of a “total synthesis” strategy to fabricate nanodevices such as quantum cellular au-

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tomata17 using classical chemical reactions, similar to the synthesis of more complicated organic compounds and polymers from smaller molecular units. Acknowledgment. We would like to thank Mr. Scott Payne for his assistance with the TEM micrographs and Mr. Damien Matthew for his assistance with the FT-IR analysis. This work was supported by the NDSU Faculty Start-up Fund, the NDSU Research Foundation, North Dakota EPSCoR, and the National Science Foundation CAREER award (DMR 0239424). Supporting Information Available: 1H NMR spectra and integration of the solid phase place exchange product BtAuCOOH and the solution phase place exchange products. This material is available free of charge via the Internet at http://pubs.acs.org. LA049308K (17) (a) To´th, G.; Lent, C. A. Phys. Rev. A 2001, 63, 52315-52324. (b) Snider, G. L.; Orlov, A. O.; Amlani, I.; Zuo, X.; Bernstein, G. H.; Lent, C. S.; Merz, J. L. J. Appl. Phys. 1999, 85, 4283-4285.