Langmuir 2007, 23, 12799-12801
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Aggregation-Resistant Water-Soluble Gold Nanoparticles Layal L. Rouhana, Jad A. Jaber, and Joseph B. Schlenoff* Department of Chemistry and Biochemistry, Center for Materials Research and Technology (MARTECH), The Florida State UniVersity, Tallahassee, Florida 32306 ReceiVed July 18, 2007. In Final Form: September 30, 2007 Stable, water-soluble gold nanoparticles, Au NPs, having an average diameter of ca. 4 nm, were prepared using place exchange reactions. The nanoparticles, capped with novel zwitterionic disulfide ligands, showed remarkable stability in saline media with salt concentrations as high as 3.0 M. Similarly, the Au NPs did not precipitate out of solution when charged polyelectrolytes or biopolymers were added, indicating the absence of nonspecific interactions. The stability and degree of association of Au NPs were characterized using UV-vis absorption spectroscopy, quasielastic light scattering, and surface-enhanced Raman scattering.
In the past few decades, gold colloids have been the subject of great interest, due to their remarkable physical and chemical properties.1 Their uniformity and stability, as well as size-related electronic, magnetic, and optical characteristics,1,2 make them promising in the fields of catalysis,3 nanophotonics,4 biosensors, and drug delivery,5,6 among others. Many of these applications, especially those in biomedicine and biotechnology, require aggregation-resistant, water-soluble nanoparticles which are capable of surviving the complex in vivo environment.7-10 Historically, this has been accomplished by encapsulating the particles with a stabilizing shell whose chemical nature, amount, and valency determine stability.2,11 In the case of thiol-passivated Au NPs, also known as thiolate gold monolayer-protected clusters (MPCs), the Brust-Schiffrin biphasic synthesis offers a straightforward way to prepare stabilized Au NPs in an organic phase in the presence of thiols.12 Thiols form self-assembled monolayers (SAMs) on gold, passivating and stabilizing the surface.13 To enhance the water compatibility of the Au NPs, Brust-like syntheses use watersoluble functional thiols, such as tiopronin,14glutathione,15 and poly(ethylene glycol).16-18 *
[email protected]. (1) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (2) Kim, T.; Lee, K.; Gong, M. S.; Joo, S. W. Langmuir 2005, 21, 9524-9528. (3) Zidki, T.; Cohen, H.; Meyerstein, D. Phys. Chem. Chem. Phys. 2006, 8, 3552-3556. (4) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212-4217. (5) Prosperi, D.; Morasso, C.; Mantegazza, F.; Buscaglia, M.; Hough, L.; Bellini, T. Small 2006, 2, 1060-1067. (6) Paciotti, G. F.; Myer, L.; Weinreich, D.; Goia, D.; Pavel, N.; McLaughlin, R. E.; Tamarkin, L. Drug DeliVery 2004, 11, 169-183. (7) Loo, C.; Lowery, A.; Halas, N.; West, J.; Drezek, R. Nano Lett. 2005, 5, 709-711. (8) El-Sayed, I. H.; Huang, X.; El-Sayed, M. A. Nano Lett. 2005, 5, 829-834. (9) Liu, Y.; Shipton, M. K.; Ryan, J.; Kaufman, E. D.; Franzen, S.; Feldheim, D. L. Anal. Chem. 2007, 79, 2221-2229. (10) Noh, S. M.; Kim, W.-K.; Kim, S. J.; Kim, J. M.; Baek, K.-H.; Oh, Y.-K. Biochim. Biophys. Acta 2007, 1770, 747-752. (11) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 67826786. (12) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802 (13) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103-1169. (14) Templeton, A. C.; Chen, S. W.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66-76. (15) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643-10646. (16) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696-12697. (17) Zheng, M.; Davidson, F.; Huang, X. Y. J. Am. Chem. Soc. 2003, 125, 7790-7791. (18) Latham, A. H.; Williams, M. E. Langmuir 2006, 22, 4319-4326.
Other techniques to prepare aqueous and stable nanoparticles involve the transfer of the Au NPs from the organic medium in which they are synthesized to an aqueous medium using surfactants.1 However, this process is hindered by the surfactant’s tendency to bind irreversibly to the nanoparticle surface, modifying its chemistry, in addition to an incomplete transfer of nanoparticles from one phase to another.19 Gittins and Caruso effected complete transfer of Au NPs prepared in toluene into an aqueous phase using 4-dimethylaminopyridine as a phase transfer agent. These NPs were produced at high concentrations and described as “indefinitely” stable.20 Direct syntheses of water-soluble Au MPCs are also found in the literature, such as a route using a fully cationic quaternary ammonium thiolate as the stabilizing shell.21 These Au NPs protected with polar thiolates are resistant to core aggregation upon drying and redissolve spontaneously, conserving their original characteristics. Tatumi and Fujihara described an imidazolium sulfonate-terminated thiol as the capping agent during synthesis, leading to nanoparticles that were not soluble in pure water, but were soluble and stable in aqueous solutions at high salt concentrations.22 Among all of the direct reduction techniques that produce aqueous Au NPs, the citrate reduction route remains the most popular.1 Citrate functions as a reducing agent and/or an electrostatic capping ligand in the presence of other reducing agents (i.e., NaBH4).11,23 Stabilization arises from electrostatic repulsion between neighboring Au NPs due to the negative surface charge created by the citrate layer. Low concentrations of salt destabilize these systems. We describe a new surface modification procedure involving ligand exchange of citrate-stabilized Au NPs with a zwitterion disulfide. The sulfur functionality easily displaces more weakly adsorbed citrate, but because stabilization relies less on electrostatics, NP stability in salt or macromolecule solutions is enhanced. Materials and Methods All reagents were used as received from Sigma-Aldrich. All aqueous solutions were prepared using deionized water (Barnstead, E-pure, Milli-Q). (19) Schmid, G.; Klein, N.; Korste, L.; Kreibig, U.; Schonauer, D. Polyhedron 1988, 7, 605-608. (20) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001-3004. (21) Cliffel, D. E.; Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2000, 16, 9699-9702. (22) Tatumi, R.; Fujihara, H. J. Chem. Soc., Chem. Commun. 2005, 83-85. (23) Turkevitch, J. Discuss. Faraday Soc. 1951, 11, 55-75.
10.1021/la702151q CCC: $37.00 © 2007 American Chemical Society Published on Web 11/16/2007
12800 Langmuir, Vol. 23, No. 26, 2007
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Scheme 1. Zwitterionic Ligands Attached to a Spherical Au NP (not to scale)
A zwitterion disulfide was synthesized as described in detail in the Supporting Information. This disulfide is expected to behave the same way as the corresponding thiol, since it is well-established that the chemisorption of a n-alkyl disulfide on a planar,24 or NP,25 gold surface occurs in a similar manner to that of the corresponding alkanethiol, leading to the formation of indistinguishable SAMs. Citrate-capped Au NPs were prepared following a standard route (see Supporting Information).26 Transmission electron microscopy (TEM) showed the NPs to have an average diameter of 3.4 ( 0.6 nm (see Supporting Information). Exchange reactions of the citrate capping ligand with the zwitterion disulfide were effected as follows: To a vigorously stirred solution of citrate-capped Au NPs, different volumes of an aqueous solution of the disulfide (0.01 M) were added and mixed for 12 h. The various reaction mixtures had the following molar ratios of zwitterion:citrate in solution: (1:1), (1:2), (1:4), (1:10), (1:20), (1:25), (1:50), (1:100), (1:133), (1:200), (1:400). Scheme 1 represents a full shell of zwitterion capping ligands on the gold surface. Surface plasmon resonance spectra of gold solutions were recorded using a Cary UV-vis spectrophotometer, at different ratios of zwitterion to citrate, in the presence of different NaCl concentrations, positively and negatively charged polyelectrolytes, and in the presence of proteins. Quasi-elastic light scattering (QELS) was used to measure the hydrodynamic radius, Rh (nm), of Au NPs capped with citrate and those capped with a mixed shell of citrate/zwitterion, with a molar ratio of (20:1), at various concentrations of NaCl. Thermal gravimetric analysis (TGA) was used to assess the density of ligands, and surface-enhanced Raman spectroscopy (SERS) was employed to characterize ligands on NPs.
Results and Discussion The stability of the Au NPs solutions was followed using UV-vis absorption measurements. When Au NPs precipitate, their absorbance maximum red-shifts and broadens.2 Figure 1A shows the behavior of citrate-capped NPs with and without NaCl. The red shift and peak broadening in the UV-vis spectrum show that these particles aggregate when the ionic strength reaches 0.1 M. Au NPs capped with zwitterionic ligands, from a molar ratio of 20:1 in solution, exhibited a plasmon resonance peak at 505 nm, in the absence of NaCl, in contrast with the citrate-capped NPs which absorb at 512 nm. This blue shift is consistent with chemisorption of the disulfide ligand.1 A lack of peak broadening (24) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (25) Porter, L. A.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378-7386. (26) Tom, R. T.; Suryanarayanan, V.; Reddy, P. G.; Baskaran, S.; Pradeep, T. Langmuir 2004, 20, 1909-1914.
Figure 1. (A) UV-vis absorption spectra of citrate-Au in 0 M NaCl (upper curve), and citrate-Au in 0.1 M NaCl (lower curve), showing the aggregation of as-made citrate-capped nanoparticles. (B) UV-vis spectra of zwitterion-Au (from 20:1 mixture of citrate/ zwitterion) in 0, 0.1, 0.5, 1, 2, and 3 M NaCl (lower to upper), showing the stability of the zwitterion Au NPs.
in the presence of NaCl concentrations up to 3 M indicates resistance to aggregation (Figure 1B). Note that the peaks become sharper with increasing salt concentration, probably due to the change in refractive index of the medium.27 These particles were stable for at least 12 months in 3 M NaCl. TGA yielded a weight loss of about 12%, consistent with a zwitterion thiol area of 0.25 nm2 on the 1.7-nm-radius Au NPs. It is known that ligand displacement/exchange reactions on Au NPs28 are much more rapid and complete than those on planar Au.29 A kinetics study was performed, where the absorbance as a function of time was monitored for two solutions containing citrate-capped and zwitterion-capped NPs after an aliquot of NaCl solution was added to step the concentration to 0.05 M salt (at t ) 0). The zwitterion-capped NPs showed stable absorbance upon addition of salt (Figure 2), whereas the absorbance of the citrate NPs increased after a few minutes as the nanoparticles agglomerated and precipitated on the cuvette walls. Rh was followed in situ with quasi-elastic light scattering. No significant differences in hydrodynamic radii were observed between particles capped with zwitterion (20:1) in pure water and those in different salt concentrations, indicating a lack of agglomeration (see Supporting Information). Note that there was a slight increase in the Rh value between the citrate-capped Au NPs (2.8 ( 0.1 nm) and the zwitterion Au NPs (3.2 ( 0.1 nm) owing probably to the fact that the zwitterionic ligand has a longer chain than that of citrate, although the difference in the Rh values (12%) is close to the instrumental error (6%). (27) Ghosh, S. K.; Nath, S.; Kundu, S.; Esumi, K.; Pal, T. J. Phys. Chem. B 2004, 108, 13963-13971. (28) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782-3789. (29) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 1252812536.
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Langmuir, Vol. 23, No. 26, 2007 12801
Figure 2. Absorbance vs time spectra of Au-citrate (upper) and Au-zwitterion (20:1) (lower) in the presence of 0.05 M NaCl. Table 1. Au NP Stability in the Presence of Proteins and Polyelectrolytes capping ligand solution composition PSSa PAAa PDADMACa PAHa PVBTMACa BSAb lysozymeb
citrate
zwitterion
stable stable aggregated aggregated aggregated stable aggregated
stable stable stable stable stable stable stable
a 10 mM concentration, in the presence of 0.15 M NaCl. b 150 µg/mL, in the presence of 0.15 M NaCl.
The same Au NPs, monitored by UV-vis absorbance measurements, did not precipitate out of solution upon addition of positively charged polyelectrolytes, such as poly(diallyldimethylammonium chloride) PDADMAC, poly(allylamine hydrochloride) PAH, and poly(vinylbenzyltrimethyl ammonium chloride) PVBTMAC, or negatively charged polyelectrolytes, such as poly(styrene sulfonic acid) PSS and poly(acrylic acid) PAA. Likewise, the addition of negatively charged biopolymers, such as bovine serum albumin BSA, or a positively charged protein, such as lysozyme, did not lead to the aggregation of the colloidal particles, indicating the absence of nonspecific interactions, as shown in Table 1. For a molar ratio of citrate/zwitterion higher than 20:1 (i.e., 25:1; 50:1; 100:1; 133:1; 200:1; 300:1), behavior similar to that of citrate-capped nanoparticles was observed, and the Au NPs precipitated out of solution when the ionic strength exceeded 0.1 M. The ligands on the surface of the nanoparticles were probed by surface-enhanced Raman spectroscopy. This technique provides an enhancement of the Raman scattering signal from analytes adsorbed on roughened or nanoparticulate metallic
Figure 3. Upper to lower, surface-enhanced Raman spectrum of Au-citrate, Raman spectrum of sodium citrate, and surface-enhanced Raman spectrum of Au zwitterion NPs. The spectra are offset. Excitation source: 785 nm; 25 mW; 20 s integration.
surfaces using the appropriate wavelength of light.30 In this study, nanoparticles were deposited by evaporating the aqueous phase, providing a film which was expected to be SERS-active. The SERS spectrum of Au capped with citrate showed clear signals in the region expected for citrate (Figure 3). Despite several attempts with several films, no SERS signals could be obtained from zwitterion-capped nanoparticles (Figure 3). We interpret these results as indicating that spacing is maintained between zwitterion-capped NPs, preventing the modest aggregation that favors SERS response. In conclusion, we were able to modify the surface of Au NPs, produced by a popular technique, using a place exchange reaction with a novel disulfide zwitterion. Although used at a small concentration (20 times less than citrate), the disulfide adsorbed onto the surface, dramatically improving colloid stability against aggregation by salt, negatively and positively charged polyelectrolytes, and negative and positive proteins. These stable colloids have the potential uses in catalysis and biological applications. Acknowledgment. This work was supported by the FSU Center for Materials Research and Technology, MARTECH. The authors thank Dr. Jin Gyu Park for helping with the SERS measurements. Supporting Information Available: Description of disulfide zwitterion and Au NPs synthesis, quasi-elastic light scattering data, TEM, TGA, and surface-enhanced Raman spectroscopy experimental setup. This material is available free of charge via the Internet at http:// pubs.acs.org. LA702151Q (30) Kho, K. W.; Shen, Z. X.; Zeng, H. C.; Soo, K. C.; Olivo, M. Anal. Chem. 2005, 77, 7462-7471.
