Phase Transfer of Surface-Modified Gold Nanoparticles by

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Langmuir 2003, 19, 6987-6993

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Phase Transfer of Surface-Modified Gold Nanoparticles by Hydrophobization with Alkylamines K. Subramanya Mayya† and Frank Caruso*,‡ Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany, and Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia Received January 6, 2003. In Final Form: March 25, 2003 The complete transfer from an aqueous to an organic solution of negatively charged carboxylate- and sulfonate-modified gold nanoparticles in the presence of primary amines is demonstrated. The transfer of carboxylate-modified gold nanoparticles to chloroform was found to be dependent on the pH of the aqueous phase. The sulfonate-modified gold nanoparticles readily transferred to chloroform in the presence of sodium chloride. Fourier transform infrared measurements showed that the transfer of particles occurred via acid-base pair formation between the carboxylate or sulfonate moieties on the nanoparticles and the protonated amine groups of the alkylamines. The transfer of the gold nanoparticles to the organic phase occurred under similar conditions for alkylamines with C12 and C18 chain lengths. Transmission electron microscopy studies showed that the phase-transferred gold nanoparticles arranged into ordered arrays upon evaporation of the organic solvent.

Introduction The size-dependent optoelectronic properties of metal and semiconductor nanoparticles have led to intense investigations of these particles both as isolated entities and in the form of ordered assemblies.1 Recent advances in the understanding of this intermediate state of matter has largely been possible because of the number of wet chemical synthetic routes available to synthesize a diverse range of nanoparticles with varied properties. A widely used synthetic approach to prepare nanoparticles is based on the Brust et al. method,2 wherein colloidally stable alkanethiol-capped nanoparticles (also known as monolayer protected clusters, MPCs) of various sizes with relatively high monodispersity are obtained.3 These nanoparticles can be isolated as solids and are readily dispersible in a variety of organic media. MPCs are generally stabilized with thiol-based ligands that are known to covalently bind to metals, with their properties primarily determined by the terminal functional groups.3b A wide range of MPCs, including metals and alloy clusters, have been synthesized in high yield.3 Metal and semiconductor nanoparticles synthesized in the organic phase are known to exhibit size-dependent dispersability in various solvents, making them convenient systems for the utilization of solvent-precipitation techniques to obtain highly monodisperse nanoparticles.4 Such * To whom correspondence should be addressed. Fax +61 3 8344 4153. E-mail: [email protected]. † Max Planck Institute of Colloids and Interfaces. ‡ The University of Melbourne. (1) (a) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (b) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545. (c) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. Rev. Mater. Sci. 1998, 49, 371. (2) Brust, M.; Walker, M.; Bethel, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (3) (a) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (b) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27 and references therein. (4) Wilson, W. L.; Szajowski, P. F.; Brus, L. E. Science 1993, 262, 1242.

monodisperse nanoparticles can be self-assembled into well-ordered, close-packed structures upon evaporation of the solvent.1c,5 The formation of such nanoparticle structures has been the main impetus for a number of recent reports on the transfer of nanoparticles from aqueous to organic solutions6 because there has been limited success in forming ordered structures from nanoparticles in aqueous solutions by using other noncovalent interactions (for example, electrostatic interactions).7 A wide variety of nanoparticles, including gold, silver, CdTe, and CdS, have been transferred from aqueous to organic solutions via hydrophobization by using various methods, a majority of which are based on the direct coordination of alkanethiols or alkylamines to the nanoparticle surface.6 An alternative strategy to phase-transfer nanoparticles is to exploit electrostatic interactions between the transfer agent (or molecule) and the nanoparticle surface (which may be premodified). Recent interest has focused on the transfer of nanoparticles from aqueous to organic phases based on electrostatic interactions via the formation of acid-base pairs between surface-modified nanoparticles and surfactants. Investigations have resulted in only a partial transfer of nanoparticles to the organic phase when single-long-chain surfactants were used.6b,9 In a study on the transfer of carboxylic acidderivatized gold nanoparticles into the organic phase, Kimura and co-workers reported that the nanoparticles transferred mainly to the aqueous-organic interface and (5) (a) Wang, Z. L. Adv. Mater. 1998, 10, 13. (b) Kiely, C. J.; Fink, J.; Zheng, J. G.; Brust, M.; Bethell, D.; Schiffrin, D. J. Adv. Mater. 2000, 12, 640 and references therein. (c) Brown, L. O.; Hutchison, J. E. J. Phys. Chem. B 2001, 105, 8911. (6) (a) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (b) Chen, S.; Yao, H.; Kimura, K. Langmuir 2001, 17, 733. (c) Liz-Marzan, L. M.; Lado-Tourino, I. Langmuir 1996, 12, 3585. (d) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (e) VijayaSarathy, K.; Kulkarni, G. U.; Rao, C. N. R. J. Chem. Soc., Chem. Commun. 1997, 537. (f) Wang, W.; Efrima, S.; Regev, O. Langmuir 1998, 14, 602. (g) Sastry, M.; Kumar, A.; Muhkerjee, P. Colloids Surf., A 2001, 181, 255. (h) Lala, N.; Lalbegi, S. P.; Adyanthaya, S. D.; Sastry, M. Langmuir 2001, 17, 3766. (i) Kurth, D. G.; Lehmann, P.; Lesser, C. J. Chem. Soc., Chem. Commun. 2000, 949. (j) Gopanik, N.; Talapin, D. V.; Rogach, A. L.; Eychmuller, A.; Weller, H. Nano Lett. 2002, 2, 803. (7) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18 and references therein.

10.1021/la034018+ CCC: $25.00 © 2003 American Chemical Society Published on Web 06/21/2003

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not completely to the organic phase when single-longchain surfactants were employed.6b Similarly, the transfer of carboxylate-modified nanoparticles by alkylamines to the aqueous-organic interface (and not to the organic phase) was also recently reported.9 In this work, we demonstrate that the complete phase transfer of carboxylateand sulfonate-derivatized gold nanoparticles to the organic phase can be achieved with single-chain primary amines by utilizing electrostatic interactions. Although primary amines are known to bind directly to the surfaces of gold nanoparticles, and even though they have been previously employed to transfer gold nanoparticles to organic phases,6f,8b,10 the binding of amines to other metal nanoparticle surfaces is not well-understood. Hence, the significance of the present study lies in the fact that it provides a general phase transfer protocol to transfer metal (as well as, for example, semiconductor) nanoparticles by exploiting electrostatic interactions. Here, we employ surface-modified gold nanoparticles as a model system to effect the complete phase transfer of the nanoparticles. Further, we study the influence of the functional groups used to derivatize the nanoparticles upon the transfer across the aqueous-organic interface using primary amines. The transfer of the gold nanoparticles of both 6- and 12-nm diameter from the aqueous to the organic phase was performed in the presence of excess alkylamine (C12 or C18), and the influence of different capping agents upon the transfer of the nanoparticles has been studied. The main advantage of employing this system for the formation of ordered nanostructures is that the particle separation can be tuned externally (for example, by varying the surface pressure of a Langmuir monolayer of such hydrophobized nanoparticles)8 and also by employing alkylamines with appropriate hydrocarbon chain lengths. Additionally, other nanoparticles can also be surface-functionalized with suitable surfactants, making them amenable to the method used here. Experimental Section Materials. Hydrogen tetrachloroaurate(III) (Alfa Aesar, Karlsruhe), sodium borohydride (Aldrich), tetraoctylammonium bromide (TOAB; Fluka), mercaptoundecanoic acid (MUA; Aldrich), 4-carboxythiophenol (4-CTP; Aldrich), triphenylphosphine3,3′,3′′-trisulfonic acid trisodium salt (TPP; Fluka), R,R,R-tris(hydroxymethyl)methylamine (TRIS; Aldrich), diisobutylphenoxyethoxyethyl-dimethylbenzylammonium chloride (hyamine; Fluka), 4-dimethylaminopyridine (DMAP; Fluka), octadecylamine (ODA; Aldrich), and dodecylamine (DDA; Aldrich) were all used as received without further purification. Instrumentation. Samples requiring sedimentation were centrifuged using a Sigma 3K30 laboratory centrifuge in 2-mL disposable Eppendorf tubes. UV-visible samples were placed in quartz cuvettes (Suprasil, Hellma; path length of 1 cm) and analyzed using a double-beam spectrophotometer (Cary 4E, Varian) with a resolution of 0.1 nm. A solvent spectrum was subtracted from all the measured spectra. Transmission electron microscopy (TEM) measurements were performed on a Philips CM12 microscope using an accelerating voltage of 120 kV. Samples for TEM were prepared by drying a 5-µL drop of the same sample on carbon-coated copper grids (Plano). Analytical ultracentrifugation (AUC) measurements were performed on a Beckman-Coulter Optima XL-I ultracentrifuge. Particle size distributions were estimated from sedimentation velocity experiments at 20 °C and 1500 rpm. Twelve-millimeter self-made (8) (a) Kim, B.; Tripp, S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955. (b) Chen, X. Y.; Li, J. R.; Jiang, L. Nanotechnology 2000, 11, 108. (c) Sastry, M.; Gole, A.; Patil, V. Thin Solid Films 2001, 384, 125. (d) Burghard, M.; Philipp, G.; Roth, S.; Von Klitzing, K.; Pugin, R.; Schmid, G. Adv. Mater. 1998, 10, 11. (9) Mayya, K. S.; Sastry, M. Langmuir 1999, 15, 1902. (10) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723.

Mayya and Caruso double-sector titanium centerpieces were used. The instrumentation and calculation of the size distribution of the nanoparticles from the sedimentation coefficient are described elsewhere.11 Electrophoretic mobilities of the gold nanoparticles were measured with a Malvern Zetasizer 4 by taking the average of five measurements at the stationary level, as described elsewhere.12 Fourier transform infrared (FTIR) measurements were performed on an IMPACT400 (Nicolet, Madison, WI) instrument. Colloid particles dispersed in the organic phase were repeatedly washed and concentrated (by centrifugation followed by redispersion) using pure organic solvent, mixed with finely powdered 200-mg KBr, and dried. Measurements were performed on a 12-mm pellet of dried KBr containing the sample [prepared at a load of 9 tons/ mm2 using a PIN 15.001 hydraulic press (Graseby/Specac, U.K.)]. Background subtraction was performed for all spectra using the spectrum measured on the pure KBr pellet. Synthesis of Carboxylate-Derivatized Gold Nanoparticles. Gold nanoparticles were prepared using the method described previously.13a Briefly, 300 mg of HAuCl4 was dissolved in 30 mL of deionized water and added to a separating flask containing 80 mL of 25 mM tetraalkylammonium bromide dissolved in toluene. The mixture was mixed well to phasetransfer the gold salt from the aqueous to the organic phase. The aqueous phase was then separated, and the TOAB-gold chloride complex dissolved in the toluene phase was washed three times with water, following which the toluene solution was transferred to the round-bottomed flask containing a magnetic stirrer bar. To the stirring solution of the TOAB-gold chloride complex was added dropwise an aqueous solution of sodium borohydride (300 mg in 15 mL). The orange-colored toluene phase of the TOABgold complex turned red, indicating the formation of the gold nanoparticles. The stirring was continued for an additional 30 min, after which the solution was allowed to stand without stirring for 3 h. The gold colloidal dispersion was then transferred to the separating flask, and the aqueous phase was separated, followed by washing with excess deionized water. The gold nanoparticles in the toluene phase were then filtered and maintained at 65 °C for 60 min. The as-prepared gold nanoparticles were then capped with MUA and 4-CTP, as follows. To the hot solution of gold nanoparticles in toluene was added 10 mL of a heated solution (65 °C) of the capping molecule in toluene (30 times molar excess to that of gold), and it was allowed to stand overnight. The nanoparticles were separated from the toluene phase, and the precipitate was washed with an excess of warm toluene (65 °C), after which the precipitate was dispersed in 40 mL of 0.1 M TRIS and used as a stock solution for further hydrophobization using alkylamines. In the case of the 4-CTPcapped gold nanoparticles, 4-CTP was dissolved in tetrahydrofuran because it is insoluble in toluene. Synthesis of Sulfonate-Derivatized Gold Nanoparticles. Method A. Gold nanoparticles capped with DMAP (∼6-nm mean diameter) were prepared using the method described previously.13b Sulfonate-derivatized gold nanoparticles were synthesized by the addition of a 30 M excess aqueous solution of TPP to the gold sol. Upon addition of a TPP solution (1 mM), the nanoparticle dispersion destabilized, changing color from red to black via blue, as a result of the increase in the ionic strength of the nanoparticle dispersion. The flocculates were separated by centrifugation (1000 g) and redispersed in deionized water. The precipitate was found to readily disperse in deionized water, leading to the formation of a red-colored sol, which was washed several times with deionized water (by centrifugation) to remove excess counterions. The obtained sulfonate-derivatized gold nanoparticles were negatively charged (ζ potential ) -58 mV) and were stable for several months. Method B. Gold nanoparticles were synthesized using the standard citrate reduction method.14 A total of 1 mL of the gold chloride solution (1%) was added to 100 mL of boiling water (11) Lessard, R. A.; Franke, H. Materials Characterization and Optical Probe Techniques. Crit. Rev. Opt. Sci. Technol. 1997, CR69. (12) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317. (13) (a) Gittins, D. I.; Caruso, F. ChemPhysChem 2002, 1, 110. (b) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001. (14) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 55.

Surface-Modified Gold Nanoparticles (under reflux to maintain a constant volume). To this solution was added 2.5 mL of trisodium citrate (1%). After 10 min, the reaction vessel was removed from the heating element and allowed to cool to room temperature, during which time the solution had changed color from pale yellow to gray blue, purple, and then wine red. Spectrophotometric analysis showed a plasmon absorption band (λmax) at 517 nm. TEM analysis showed that the particles were relatively monodisperse, near-spherical, and of a mean diameter of ∼12 nm. Surface modification of the obtained gold nanoparticles was performed by the addition of varying amounts of 0.1 M TPP. Above TPP concentrations of 10 mM, the gold nanoparticles were found to destabilize as a result of the increase in the ionic strength of the solution. The destabilized nanoparticles were then centrifuged, redispersed in deionized water, and characterized by using UV-visible spectrophotometry, microelectrophoresis, and TEM. Phase Transfer of Surface-Modified Gold Nanoparticles. Phase transfer of the surface-modified gold nanoparticles from the aqueous to the organic phase was performed as follows. A total of 1 mL of the stock solution of the surface-modified gold nanoparticles was dispersed in 10 mL of deionized water and placed in a separating flask, followed by the addition of 10 mL of the organic solvent (toluene/chloroform) containing appropriate amounts of alkylamine. The concentrations of the surfacemodified gold nanoparticles were estimated by using the absorbance at the peak plasmon absorption. A typical concentration in a 10-times-diluted solution of the as-prepared gold sol corresponds to an absorbance at the peak plasmon absorption (525 nm) of 0.19. The biphasic mixture in the separating flask was vigorously mixed for 5 min and allowed to stand so that the two phases could separate. Chloroform was used to disperse the hydrophobized, MUA-capped nanoparticles because the concentration of ODA required to effect the complete phase transfer of the nanoparticles used was above the solubility limit of ODA in toluene when acid was not employed. The sulfonate-derivatized gold nanoparticles were transferred to chloroform containing primary amines in the presence of sodium chloride, as follows. A chloroform solution of primary amine was added to the separating flask containing the gold nanoparticle dispersion. The biphasic mixture was vigorously mixed and allowed to stand so that the phase separation could take place. Aliquots of 5 µL of 2 M NaCl were added, and the biphasic mixture was vigorously mixed, following which the solution was allowed to stand (so that the phase separation could take place). This process was continued until the aqueous phase turned colorless. The two phases were then separated for further characterization.

Results and Discussion A. Hydrophobization of Carboxylate-Modified Gold Nanoparticles. The phase transfer of carboxylatemodified gold nanoparticles from the aqueous to the organic phase was effected by vigorously mixing a biphasic mixture of water and toluene containing carboxylatemodified gold nanoparticles and primary amines, respectively. The carboxylic acid derivatization was performed by using two different bifunctional molecules, one with a long hydrocarbon chain (MUA) and the other with a benzene ring (4-CTP). The nanoparticles stabilized with either of these molecules, dispersed in 0.1 M TRIS (pH ) 9), were negatively charged (ζ potential ) -47 mV) and were stable for several months. We found that the 4-CTPcapped gold nanoparticles spontaneously transferred to the organic phase containing primary amines (1 mM) at pH ) 9. Figure 1a shows the UV-visible spectra of the 4-CTPcapped gold nanoparticles in water (dashed curve) and toluene (solid curve). The peak plasmon absorption for the 4-CTP-capped gold nanoparticles shifted from 522 to 529 nm upon transfer from water to the toluene phase, which is due to the change in refractive index of the medium upon the transfer of the nanoparticles from water (∼1.33) to toluene (∼1.47).6a,15 Figure 1b shows the AUC data for the 4-CTP-capped gold nanoparticles in water

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Figure 1. UV-visible spectra (a) and AUC curves (b) of the 4-CTP-capped gold nanoparticles in water (dashed curves) and toluene (solid curves). The nanoparticles were phase-transferred using 0.0025 M ODA. The concentrations of the gold nanoparticles used for recording the spectra are different in the water and toluene dispersions.

(dashed curve) and toluene (solid curve). As can be seen, there is no change in the size distribution (within experimental error) of the gold nanoparticles upon the transfer from the aqueous to the organic phase. The MUA-capped gold nanoparticles were similarly transferred to the organic phase; however, the concentration of amine (0.025 M) required to transfer the nanoparticles was 1 order of magnitude higher than that required to transfer the same number of 4-CTP-capped gold nanoparticles at pH ) 9 (see Supporting Information). The concentration of ODA required to effect the phase transfer was higher than the solubility limit of ODA in toluene, and, therefore, the MUA-capped gold nanoparticles were phase-transferred to chloroform. The transfer of the MUA-capped gold nanoparticles was dependent on the concentration of ODA in the organic phase. To investigate this concentration-dependent phase-transfer behavior, 0.1 mL of the stock solution of the MUA-capped gold nanoparticles (corresponding to an absorbance at the peak plasmon absorption (525 nm) ) 0.187 (dashed spectrum, Figure 2a) was dispersed in 1 mL of TRIS in a 2-mL Eppendorf tube. The nanoparticles were then transferred to 0.5 mL of chloroform containing various concentrations of ODA (see Experimental Section). A total of 0.2 mL of the gold nanoparticles dispersed in chloroform was then diluted to 3 mL and characterized by using UVvisible spectrophotometry (Figure 2). The peak plasmon absorption of the MUA-capped gold particles shifted from 525 nm (dashed spectrum, Figure 2a) to 529 nm upon the transfer from the aqueous to the organic phase in the presence of ODA. The amount of gold nanoparticles transferred to chloroform increased with an increasing ODA concentration in the chloroform phase (Figure 2a,b). At lower ODA concentrations, smaller amounts of the gold nanoparticles transferred to the organic phase while a fraction of the nanoparticles remained at the aqueousorganic interface (by visual inspection). As the concentration of ODA was increased, the amount of the nanoparticles transferred from water to chloroform increased (solid spectra, Figure 2a), which was accompanied by an increase in the particle concentration at the aqueous-organic interface (observed by visual inspection). At a concentration of 0.05 M ODA, the aqueous phase turned colorless, with the partitioning of the nanoparticles occurring between the aqueous-organic interface and chloroform. (15) Mulvaney, P. Langmuir 1996, 12, 788.

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Figure 2. UV-visible ODA-concentration-dependent phase transfer of the MUA-capped gold nanoparticles from water to chloroform at pH ) 9. (a) Spectra of the ODA-modified MUAcapped gold nanoparticles in chloroform with an increasing ODA concentration (0.01, 0.02, 0.04, 0.06, 0.08, and 0.1 M) and MUA-capped gold nanoparticles dispersed in 0.1 M TRIS (dashed spectrum). (b) Absorbance at 529 nm (left scale) versus the ODA concentration and percentage transfer (right scale) as a function of the ODA concentration. The percentage of transfer was calculated by taking the absorbance of the 0.1 M ODAcapped nanoparticle system as 100%.

At a concentration of 0.1 M ODA, the complete transfer of the gold nanoparticles to chloroform occurred with negligible amounts of the nanoparticles at the aqueousorganic interface. From a comparison of the UV-visible spectra of a similar number of nanoparticles dispersed in water and chloroform, it can be inferred that the absorbance of the nanoparticle dispersion at λmax increased by 1-2 factors upon the transfer of the nanoparticles from water to chloroform. An increase in the absorbance due to the increase in the refractive index (in the present case from 1.33 to 1.45) of the medium has been previously observed for gold nanoparticles stabilized with polymeric comb stabilizers and gold nanoparticle-polyelectrolyte complexes.6a,16 The twofold increase in the absorbance upon the transfer from water to chloroform is somewhat higher than was expected purely on the basis of the solvent refractive index changes,6a but it is likely that the presence of the additional layer of amine affects the “actual refractive index” experienced by the nanoparticles. The transfer yields at various amine concentrations were calculated assuming the absorbance at the peak plasmon absorption obtained in the chloroform phase for 0.1 M ODA corresponds to 100% transfer. Figure 2b shows the increase in absorbance at λmax of the gold nanoparticles dispersed in chloroform as a function of increasing ODA concentration. Langmuir-type adsorption behavior has been previously reported for the transfer of carboxylate-modified gold nanoparticles from an aqueous to an organic solution.6b However, such an adsorption mechanism was not arrived at in this work because the fatty amines employed here are known to be sparingly soluble in water, compared to the phase-transfer agent used previously, which is insoluble in water. This is evident from the red shifts of λmax (UV-visible spectra not shown) of the gold nanoparticles left behind in the aqueous phase at intermediate ODA concentrations. Such a stabilization of the surface-modified nanoparticles via the formation of interdigitated bilayers using fatty lipids has been previously observed.17 (16) Schmitt, J.; Ma¨chtle, P.; Ech, D.; Mo¨hwald, H.; Helm, C. A. Langmuir 1999, 15, 3256. (17) Patil, V.; Mayya, K. S.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281.

Mayya and Caruso

The difference between the concentrations of ODA required for the complete phase transfer of the MUA- and 4-CTP-capped gold nanoparticles to the organic phase could be due to the combined effect of MUA and 4-CTP having different degrees of deprotonated carboxylic acid groups and the differences in the surface coverage of the capping molecules on the nanoparticle surface. The difference in the surface coverage of the capping molecules is likely due to the difference in the solubilities of MUA and 4-CTP in toluene (4-CTP is insoluble in toluene and, therefore, was dissolved in THF; see Experimental Section). The difference in the surface coverage of the carboxylate groups on the nanoparticle surface capable of binding to ODA was estimated using the so-called antagonistic approach (see Supporting Information), where small aliquots of a water-soluble, dilute cationic surfactant solution are added to the nanoparticle dispersion. The effective surface charge on the gold nanoparticles is expected to reduce upon complexation with the cationic surfactant, leading to flocculation of the nanoparticles. Hyamine, a cationic surfactant capable of binding strongly to the carboxylic acid groups, was used for this purpose. From the titration experiments, it is estimated that the ratio of carboxylic acid moieties available for complexation on the 4-CTP-capped nanoparticles to those on the MUAcapped nanoparticles is ∼1:5 (see Supporting Information). This is in qualitative agreement with the observed ratio of alkylamine required to transfer the nanoparticles from the aqueous to the organic phase, namely, 1:10 (the calculation was performed assuming that all of the ODA took part in the transfer process; see Supporting Information). However, for a given number of MUA-capped nanoparticles,18 the transfer of the nanoparticles from the aqueous to the organic phase occurred at lower amine concentrations when the pH of the solution was lowered to pH ) 7. At this pH, the complete transfer of the MUAcapped nanoparticles to the organic phase containing 2.5 × 10-3 M ODA occurred. The lower amount of ODA required to transfer the nanoparticles at the reduced pH is likely due to the increase in the number of ionized amine groups (NH3+; pKa ) 9), suggesting enhanced electrostatic interaction between the amine and the carboxylic acid groups. pH-dependent electrostatic interactions for these systems have been previously reported.19 This pH dependency clearly demonstrates that ODA facilitates the transfer of the nanoparticles from the aqueous to the organic phase. The transfer of the 4-CTP-capped gold nanoparticles at a lower pH was not attempted because the complete transfer occurred at pH ) 9 and lower amine concentrations. The above data are in agreement with that of previous work on the transfer of carboxylate-derivatized gold nanoparticles using primary amines,9 where the maximum transfer of the gold nanoparticles to the aqueous-organic interface (not the organic phase) was found to occur at an intermediate pH as a result of the maximization of the electrostatic interaction between the amine and the carboxylic acid groups. In the current work, it is likely that the transfer of the nanoparticles to the organic phase occurred because of the higher coverage of carboxylate groups on the nanoparticle surface in comparison to the previous report.9 In the present method of synthesis, the formation of gold-thiolate covalent linkages are favored (18) Estimation of the number of gold nanoparticles was made by using the absorbance value at the peak plasmon absorption of a dilute nanoparticle dispersion. (19) (a) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575. (b) Sastry, M.; Rao, M.; Ganesh, K. N. Acc. Chem. Res. 2002, 35, 847 and references therein.

Surface-Modified Gold Nanoparticles

Figure 3. TEM images of ODA-stabilized, surface-modified gold nanoparticles: (a) 4-CTP-capped gold nanoparticles, (b) high-magnification image of part a, (c) MUA-capped gold nanoparticles, and (d) high-magnification image of part c.

in the organic phase and, further, the ratio of gold salt to the surfactant (see Experimental Section) is higher than that employed earlier.9 One of the primary advantages of nanoparticles in organic solvents (for example, MPCs) is that they can be stored as solids, which can be subsequently readily dispersed in a variety of organic solvents. The gold nanoparticles transferred to the organic phase reported here were similarly isolated as a solid (upon the evaporation of the solvent). The solid was readily dispersible in a variety of organic solvents and was stable for months with no signs of aggregation. Figure 3 shows TEM images of the (parts a and b) 4-CTP- and (parts c and d) MUAcapped gold nanoparticles hydrophobized and transferred to the organic phase using ODA. The TEM images showed the nanoparticles to be well-separated, with the nanoparticles tending to organize upon evaporation of the organic solvent. As was mentioned earlier, monodisperse nanoparticles dispersed in organic solvents are known to form well-ordered superlattice structures upon evaporation of the solvent.1b,c,5 Whereas the self-assembly of surfactant-stabilized, hydrophobic metallic nanoparticles to form ordered superlattices is known to take place as a result of the inherent property of the metal (defined by the Hamaker constant),20 the long-range order is generally dependent on the chain length of the surfactant.1c Therefore, the nanoparticles hydrophobized with differentchain-length primary amines would be desirable for the formation of superlattices with long-range order. We found that both the 4-CTP- and the MUA-capped gold nanoparticles transferred to the organic phase by DDA takes place under similar conditions as those used for ODA. B. Hydrophobization of Sulfonate-Modified Gold Nanoparticles. Sulfonate-modified nanoparticles of two different sizes (6 and 12 nm) were employed. The sulfonatederivatized gold nanoparticles were transferred to the organic phase by using an excess (0.5 M) of ODA in the organic phase. The pH of the nanoparticle dispersion was (20) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466.

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6. At this pH, the amine groups of ODA are positively charged (NH3+), and the transfer of the TPP-capped gold nanoparticles is expected to occur spontaneously from water to chloroform containing ODA or DDA. Interestingly, the transfer of the TPP-capped gold nanoparticles occurred only in the presence of sodium chloride. The concentration of sodium chloride required to transfer the nanoparticles was found to be proportional to the TPP concentration used to derivatize the gold nanoparticles. A control experiment aimed at transferring the sulfonatederivatized gold nanoparticles to the organic phase in the presence of salt and in the absence of a primary amine did not result in phase transfer. Alternatively, when the phase transfer of TPP-capped nanoparticles was attempted in the absence of salt but in the presence of a primary amine, a change in color from red to purple of the aqueous sol was observed, while the chloroform phase remained colorless. The change in color of the aqueous sol is probably due to the flocculation caused upon the coordination of the nanoparticles with the primary amine present in the aqueous phase, which transferred across the aqueous organic interface because of the partitioning into both phases. The transfer of the metal nanoparticles from the aqueous to the organic phase in the presence of relatively large amounts of salt has been previously observed for sodium oleate-capped metal colloids, with less salt needed to effect the transfer for doubly charged ions.21 However, the exact mechanism of the phase transfer of the nanoparticles is not well-understood. In the present study, however, the transfer of the gold nanoparticles to the organic phase occurs via the acid-base interactions between the negatively charged sulfonate groups on the nanoparticles and the positively charged amine groups of the alkylamine in the presence of NaCl. The presence of counterions is likely to assist hydrophobization of the sulfonate-derivatized gold nanoparticles, thus effecting the transfer of the gold nanoparticles to chloroform. The transfer of the gold nanoparticles to the organic phase via the direct coordination of the primary amine to the nanoparticle surface, as was observed previously, can be ruled out because the particles are capped with TPP.6g,10,22 Ligand exchange of TPP with ODA, as was observed by Brown and Hutchison, can be ruled out because a change in the size of the nanoparticles coupled with ligand exchange was not observed.22 Also, the phase transfer upon the hydrophobization of the TPP-capped nanoparticles was spontaneous, as opposed to the slow process characteristic of ligand exchange. The gold nanoparticles prepared using method B showed a peak plasmon absorption at 517 nm (dashed spectrum, Figure 4). The peak plasmon absorption shifted from 517 to 519 nm upon capping the nanoparticles with TPP (2.5 mM; spectrum a, Figure 4). The TPP-capped nanoparticles were then transferred to chloroform in the presence of 1 mM NaCl, following which the peak plasmon absorption further shifted to 529 nm (spectrum b, Figure 4). The 10-nm shift in the peak plasmon absorption of the TPPcapped gold nanoparticles is due to the change in the refractive index of the medium upon the transfer from water (∼1.33) to chloroform (∼1.44).6a,15 The gold nanoparticles (prepared by using both methods A and B) transferred to the organic phase in this manner were washed three times with chloroform to remove any uncoordinated amine, and the nanoparticles could be easily (21) (a) Hirai, H.; Aizawa, H.; Shiozaki, H. Chem. Lett. 1992, 96, 5908. (b) Hirai, H.; Aizawa, H. J. Colloid Interface Sci. 1993, 161, 471. (22) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 882.

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Figure 4. UV-visible spectra of citrate-reduced gold nanoparticles (dashed spectrum), followed by capping with TPP (spectrum a), transfer to chloroform in the presence of excess ODA, and washing with pure chloroform (spectrum b).

Mayya and Caruso

Figure 6. FTIR spectra in the fingerprint region of the gold nanoparticles dispersed in the organic phase: (a) 4-CTPmodified gold nanoparticles, (b) MUA-modified gold nanoparticles (transferred at pH ) 9), (c) MUA-modified gold nanoparticles (transferred at pH ) 7), and (d) TPP-modified gold nanoparticles (dashed spectrum). Table 1. FTIR Spectral Features of Primary-Amine-Hydrophobized, Surface-Modified Gold Nanoparticles6b,23,24 surface modifier MUA 4-CTP MUA (at pH ) 7)

Figure 5. TEM images of ODA-stabilized, TPP-capped gold nanoparticles prepared using method B (∼12 nm; part a) and method A (∼6 nm; part b).

separated and redispersed in a variety of nonpolar solvents. The sulfonate-derivatized gold nanoparticles transferred to chloroform were characterized by TEM. Parts a and b of Figure 5 are representative TEM images of 12- and 6-nm TPP-capped nanoparticles, respectively, transferred to chloroform containing ODA. The particles are well-separated and further support the UV-visible spectrophotometry data. The interparticle distances were measured by drawing parallel lines passing through the centers of the nanoparticles from the area of the image where the particles are arranged in ordered structures (Figure 5b). The separation between the lines was measured to be 5.8 nm. A thickness per bilayer of TPP was previously reported to be about 1.2 nm.23 The separation between the particles corresponding to the primary amines is, therefore, ∼4.6 nm. This value corresponds approximately to a bilayer of ODA, indicating that the TPP-capped gold nanoparticles are covered with a layer of ODA. Calculation of the interparticle distance (in ordered areas) for the 12-nm-sized particles (Figure 5a) yields similar values within experimental error. C. Fourier Transform Infrared (FTIR) Characterization. The surface-modified gold nanoparticles hydrophobized by using primary amines were characterized by FTIR spectroscopy. The FTIR spectra for all the samples showed bands corresponding to methylene symmetric and asymmetric stretches at 2850 and 2920 cm-1, corresponding to the hydrocarbon chain of ODA or DDA, indicating that in the solid state the hydrocarbon chains surrounding (23) Schmid, G. Chem. Rev. 1992, 92, 1709.

1010 1078 1082 1174 1377 1394 1406 1469 1465 1487 1488 1583 1566 1647 1637 1740 2852 2850 2922 2920

1080 1377 1406 1467 1490 1566 1602 1647 1735 2852 2920

peak assignment in-plane aromatic CsH bending vibrations Ν(NsCsO) NCO stretch in-plane aromatic C-H bending vibrations νs(CsNsC) symmetric stretch νs(COO-) and δ(R-CH2) δ(CH2) methylene scissors deformation (CdO) in carboxylic acid salt (asymmetric) (CdO) amide carbonyl stretch free protonated carboxylic acid groups symmetric methylene stretch asymmetric methylene stretch

the nanoparticles are highly ordered.24 The FTIR spectra in the fingerprint region for the hydrophobized gold nanoparticles are shown in Figure 6, and the various band positions are given in Table 1. The most prominent feature of the 4-CTP-modified gold nanoparticles (spectrum a, Figure 6) is the carboxylate (COO-) band at 1394 cm-1, indicating that most of the 4-CTP molecules on the gold nanoparticles are in the deprotonated form, consistent with the formation of COO-NH3+ ion pairs. Similar bands corresponding to the formation of ion pairs are observed for the MUA-capped gold nanoparticles transferred at both pH ) 7 and 9. The presence of the bands at 1400 and 1600 cm-1 on formation of COO-NH3+ has been previously reported for self-assembled monolayers of MUA on planar gold surfaces upon the adsorption of polylysine.25 The weak band at 1740 cm-1 (spectrum a) corresponds to the free carboxylic acid groups (COOH), indicating the presence of a small fraction of unassociated carboxylic acid groups of 4-CTP on the nanoparticles.26 Such a peak was not observed for the MUA-capped gold nanoparticles transferred at pH ) 9 (spectrum b, Figure 6). As was mentioned (24) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (25) Jordan, C. E.; Frey, B. L.; Kornguth, S.; Corn, R. M. Langmuir 1994, 10, 3642. (26) (a) Frey, B. L.; Corn, R. M. Anal. Chem. 1996, 68, 3187. (b) Creager, S. E.; Steiger, C. M. Langmuir 1995, 11, 1852.

Surface-Modified Gold Nanoparticles

earlier, the amount of amine required to transfer the MUAcapped nanoparticles was lower at pH ) 7 compared with that required at pH ) 9. When the MUA-capped gold nanoparticles were transferred at pH ) 7, the carboxylic acid group band (spectrum c, Figure 6) at 1735 cm-1 reappeared, indicating that fewer carboxylic acid groups take part in the phase transfer via the formation of acidbase pairs with primary amines compared with the phase transfer at pH ) 9. The lowering of the pH of the aqueous sol followed by the protonation of carboxylate groups contributes to the hydrophobization of the MUA-capped gold nanoparticles. This is in agreement with our observation that the amount of ODA required to transfer the 4-CTP-capped gold nanoparticles was lower than that required to transfer the MUA-capped gold nanoparticles because the number of deprotonated carboxylic acids on the 4-CTP-capped gold nanoparticles at pH ) 9 is lower than that on the MUA-capped gold nanoparticles. Additionally, the IR data supports the results obtained from the titration experiments (see Supporting Information) in that the number of deprotonated carboxylic acid groups on the MUA-capped gold nanoparticles was higher than that in the case for the 4-CTP-capped gold nanoparticles at pH ) 9. The additional peaks observed for the 4-CTPand MUA-capped nanoparticles are assigned to the amide bond of ODA upon complexation with carboxylic acid groups. The TPP-capped gold nanoparticles transferred to the organic phase by using primary amines in the presence of sodium ions were also characterized by FTIR spectroscopy. The spectra showed bands corresponding to CH2 symmetric and asymmetric stretches (2850 and 2920 cm-1; spectra not shown) and a CH2 scissors band (1467 cm-1) due to the presence of ODA or DDA. As was mentioned previously, the position of the methylene stretch indicates that the hydrocarbon chains on the nanoparticles in the solid state are well-ordered. Also, two bands corresponding to sulfonate symmetric and asymmetric peaks at 1040 and 1139 cm-1, respectively, were observed (spectrum d, Figure 6). This indicates that the sulfonate groups are deprotonated and interact with the amine groups via the formation of SO3-NH3+ ion pairs. Conclusions The complete phase transfer of negatively charged, surface-modified gold nanoparticles from the aqueous to the organic phase using primary amines was achieved.

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Surface modification of gold nanoparticles was conducted by using three different capping molecules, which render a negative surface charge to the nanoparticles. The phase transfer of the surface-modified nanoparticles to the organic phase is based on the attractive electrostatic interaction between the negatively charged surface groups of the nanoparticles (namely, COO- and SO3-) and the positively charged NH3+ groups of the primary amines. The carboxylate-modified gold nanoparticles were found to transfer to the organic phase under alkaline conditions. FTIR measurements of the phase-transferred MUAcapped gold nanoparticles indicated that the hydrophobization of nanoparticles was promoted at a lower pH due to the protonation of carboxylate groups on the nanoparticles. The ODA-concentration-dependent transfer of the MUA-capped gold nanoparticles to chloroform showed that at intermediate concentrations a significant fraction of the nanoparticles remained at the water-chloroform interface, while the complete transfer of the nanoparticles to chloroform occurred at higher concentrations. The phase transfer of the sulfonate-derivatized gold nanoparticles was found to occur only in the presence of salt. The ionic strength required to transfer the sulfonate-derivatized gold nanoparticles was proportional to the concentration of the capping molecule (TPP). The method described here represents a viable method for the phase transfer of various kinds of nanoparticles with appropriate surface modification. Such transferred nanoparticles could find applications in the fields of magnetics, optics, and microelectronics. Acknowledgment. This work was funded by the German Federal Ministry of Education, Science, Research and Technology (BMBF). We are grateful to H. Co¨lfen and K. Tauer for helpful discussions. U. Blo¨ck, C. Pilz, and A. Vo¨lkel are thanked for the high-resolution TEM analysis, microelectrophoresis measurements, and AUC measurements, respectively. H. Mo¨hwald is thanked for the support of this work within the Max Planck Institute. Supporting Information Available: Details of the antagonistic titration experiments followed using UV-visible spectrophotometry and the calculation of the ratios of carboxylate groups available for coordination with primary amines. This material is available free of charge via the Internet at http://pubs.acs.org. LA034018+