Formation of Water-Dispersible Gold Nanoparticles Using a

Drying of the colored aqueous phase results in the formation of a highly stable reddish powder of gold nanoparticles that may be readily redispersed i...
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Langmuir 2003, 19, 1168-1172

Formation of Water-Dispersible Gold Nanoparticles Using a Technique Based on Surface-Bound Interdigitated Bilayers Anita Swami, Ashavani Kumar, and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune - 411 008, India Received September 9, 2002. In Final Form: November 21, 2002 We demonstrate the phase transfer of dodecylamine (DDA)-capped colloidal gold particles dispersed in an organic solvent into water containing the cationic surfactant, cetyltrimethylammonium bromide (CTAB). Vigorous shaking of the biphasic mixture results in the rapid phase transfer of DDA-capped gold nanoparticles from the organic to aqueous phase, the aqueous phase acquiring a pink, foamlike appearance. Drying of the colored aqueous phase results in the formation of a highly stable reddish powder of gold nanoparticles that may be readily redispersed in water. The water-dispersible gold nanoparticles have been investigated by UV-vis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). These studies indicate the presence of interdigitated bilayers consisting of a DDA primary monolayer directly coordinated to the gold particle surface and a secondary monolayer of CTAB, this secondary monolayer providing sufficient hydrophilicity to facilitate gold nanoparticle phase transfer and render them water-dispersible. The CTAB-DDA stabilized particles may be dispersed in water at very high nanoparticle concentrations with stability even in the presence of high amounts of electrolyte and over a wide range of solution pH.

Introduction Metal nanoparticles have tremendous application in the areas of catalysis,1 opto-electronics,2 film growth seeding,3 and so forth because of their size-dependent optical and electronic properties. Consequently, several protocols have been developed for synthesis of metal nanoparticles in water/polar solvents4 as well as in nonpolar organic solvents,5-7 each method possessing characteristic advantages and disadvantages. An important drawback of methods for the synthesis of nanoparticles in an aqueous environment is that ionic interactions limit the concentration of nanoparticles in the aqueous phase to very dilute levels.8a Development of protocols for achieving high nanoparticle concentration in water, particularly that of gold, would be extremely useful for biological applications of nanoparticles such as biosensors9,10 and DNA sequence determination11 and therefore, development of alternative approaches to tackle this problem are desirable. * To whom correspondence should be addressed. Ph: +91 20 5893044. Fax: +91 20 5893952/ 5893044. E-mail: sastry@ ems.ncl.res.in. (1) Hoffman, A. J.; Mills, G.; Yee, H.; Hoffman, M. R. J. Phys. Chem. 1992, 96, 5546. (2) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (3) Heath, J. R.; Gates, S. M.; Chess, C. A. Appl. Phys. Lett. 1994, 64, 3569. (4) Handley, D. A. Colloidal Gold: Principles, Methods and Applications; Hayat, M. A., Ed.; Academic Press: San Diego, 1989; Vol. 1, Chapter 2. (5) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (6) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (7) Selvakannan, P. R.; Mandal, S.; Pasricha, R.; Adyanthaya, S. D.; Sastry, M. Chem. Commun. 2002, 1334. (8) (a) Gittins, D. J.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001. (b) Gittins, D. J.; Caruso, F. ChemPhysChem 2002, 3, 110. (9) Schneider, B. H.; Dickinson, E. L.; Vach, M. D.; Hoijer, J. V.; Howard, L. V. Biosens. Bioelectron. 2000, 15, 13. (10) Weller, H. Angew. Chem., Int. Engl. 1993, 32, 41. (11) (a) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640. (b) Cao, Y.-W. C.; Jin, R.; Mirkin, C. A. Science 2002, 297, 1536.

One possible method could be based on the phase transfer of metal nanoparticles synthesized in an organic medium (where ionic effects are expected to be unimportant) to water. While many different procedures have been developed for the transfer of aqueous gold nanoparticles into organic solvents,12-18 reports on the phase transfer of gold nanoparticles in the reverse direction are less frequent.8,19 Previous research into the phase transfer of nanoparticulate material from organic to aqueous solutions by Rotello and co-workers19 involved a place exchange mechanism to functionalize alkanethiol-capped organically soluble gold nanoparticles with carboxylic acid groups. However, this process permanently changes the chemistry of the particle surface and results in only a small proportion of transferred material.19 Recently, Gittins and Caruso8a have reported a facile and rapid onestep method for the direct and complete transfer of gold and palladium nanoparticles synthesized in toluene and stabilized by tetraalkylammonium salts across the phase boundary (organic to aqueous).8a This was accomplished by addition of an aqueous 0.1 M 4-dimethlyaminopyridine (DMAP) solution to aliquots of the gold/platinum nanoparticles in toluene. The DMAP molecules replace the tetraalkylammonium salts and form a labile donoracceptor complex with the gold atoms on the surface of the nanoparticles through the endocyclic nitrogen atoms.8a These authors have shown that the aqueous gold and palladium solutions are extremely stable with no sign of degradation even after storage for several months. Gittins (12) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (13) Liz-Marzan, L. M.; Lado-Tourino, I. Langmuir 1996, 12, 3585. (14) Sarathy, K. V.; Kulkarni, G. U.; Rao, C. N. R. Chem. Commun. 1997, 537. (15) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (16) Wang, W.; Efrima, S.; Regev, O. Langmuir 1998, 14, 602. (17) Sastry, M.; Kumar, A.; Mukherjee, P. Colloids Surf., A 2001, 181, 255. (18) Lala, N.; Lalbegi, S. P.; Adyanthaya, S. D.; Sastry, M. Langmuir 2001, 17, 3766. (19) Simard, J.; Briggs, C.; Boal, A. K.; Rotello, V. M. Chem. Commun. 2000, 1943.

10.1021/la026523x CCC: $25.00 © 2003 American Chemical Society Published on Web 01/17/2003

Formation of Water-Dispersible Gold Nanoparticles

and Caruso8b have also demonstrated the phase transfer of silver, gold, platinum, and palladium nanoparticles using several exchanging ligands such as mercaptoundecanioc acid (MUA), mecaptosuccinic acid, and so forth. An important aspect of the work was the nonspecific bioconjugation of the protein, bovine serum albumin (BSA), with MUA-functionalized gold nanoparticles, possibly through electrostatic and hydrogen bonding interactions between the protein and the ionized carboxylate ions on the nanoparticle surface.8b In a different approach, we demonstrate in this paper that dodecylamine-capped gold nanoparticles (DDA-Au) dispersed in chloroform can be quantitatively transferred into water containing cetyltrimethylammonium bromide (CTAB). Vigorous shaking of the biphasic mixture (DDAAu-in-chloroform/CTAB-in-water) results in the swift transfer of the hydrophobized nanoparticles (DDA-Au) into the aqueous phase. These nanoparticles are exceptionally stable in the aqueous phase at high concentration and can be stored as a reddish powder, which can be readily redispersed in water. Analysis of the phase-transferred gold nanoparticle solution and the purified powder of nanoparticles was done by UV-vis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared (FTIR) spectroscopy. While interdigitated bilayers have been used to stabilize Ag,20 magnetite nanoparticles,21 and gold nanorods22 in water, we are not aware of reports on their use in achieving nanoparticle phase-transfer and waterdispersibility. Presented below are details of the investigation. Experimental Details DDA-capped gold nanoparticles in chloroform were synthesized as described in a previous report.17 To 100 mL of the gold colloidal solution prepared by borohydride reduction of chloroauric acid, 100 mL of a 2 × 10-4 M solution of dodecylamine (DDA) in chloroform was added to yield immiscible layers of the colorless organic solution at the bottom of the red-colored gold hydrosol. Vigorous shaking of the biphasic mixture resulted in extremely rapid transfer (within 30 s) of the gold colloidal particles into the organic phase. The organic phase was separated out and the powder of DDA-capped gold nanoparticles was obtained by evaporating the solvent by rotovapping. The powder was purified by ethanol washing and redispersed in chloroform. In a typical experiment, 25 mL of DDA-Au nanoparticles in chloroform was added to 25 mL of 10-3 M CTAB solution in water. The concentration of gold in chloroform was estimated to be 2 × 10-4 M by UV-vis spectroscopy. Vigorous shaking of the biphasic mixture resulted in phase transfer of the gold nanoparticles from chloroform to water, giving a pink, foamlike appearance to aqueous layer. This layer was then separated from the organic layer, dried, and the resulting dry powder redispersed in 25 mL of double-distilled water. The redispersed solution was centrifuged three times at 10 000 rpm and 25 °C for 20 min to remove uncoordinated CTAB molecules from solution. Further removal of water from the solution by rotovapping and drying under vacuum in N2 atmosphere gave a reddish, dry powder of surface-modified gold nanoparticles. UV-vis spectra of the asprepared gold nanoparticles, hydrophobized gold nanoparticles, and redispersed nanoparticles after phase transfer from organic phase to aqueous phase by CTAB were recorded on a Shimadzu dual-beam spectrophotometer (model UV-1601 PC) operated at a resolution of 0.5 nm. FTIR spectroscopic measurements of a drop-coated film of the phase-transferred gold nanoparticles on Si(111) substrate were done on a Shimadzu FTIR-8201 PC instrument operated at 2 cm-1 resolution. Calorimetric measurements of the purified powder of phase-transferred Au (20) Patil, V.; Mayya, S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281. (21) Shen, L.; Laibinis, P.; Hatton, T. A. Langmuir 1999, 15, 447. (22) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368.

Langmuir, Vol. 19, No. 4, 2003 1169 nanoparticles were done using DSC-7 Perkin-Elmer unit from 25 to 325 °C at a heating rate of 10 °C/min. under nitrogen environment. Thermogravimetric analysis of the phase-transferred gold nanoparticle powder was done on TGA-7 PerkinElmer instrument from 0 to 800 °C at a scanning rate of 10 °C/min. TEM measurements were performed on a JEOL Model 1200EX instrument operated at an accelerating voltage of 120 kV. Samples for TEM were prepared by placing a drop of the purified phase-transferred gold nanoparticle solution on a carboncoated TEM copper grid. The mixtures were allowed to dry for 1 min and the extra solution was removed using a blotting paper.

Results and Discussion Figure 1A shows pictures of the gold nanoparticle solution at different stages of surface modification. Tube 2 contains DDA-capped nanoparticles in chloroform by a method of phase transfer described in an earlier report.17 As mentioned in the Experimental Section, vigorous shaking of the biphasic mixture (DDA-Au-in -chloroform/ CTAB-in-water) resulted in the swift transfer of the hydrophobized nanoparticles (DDA-Au, tube 2) into the aqueous phase. Tube 3 in Figure 1A contains gold nanoparticles in water phase transferred from chloroform. For comparison, the gold hydrosol prepared by borohydride reduction of chloroauric acid (10-4 M) is also shown (tube 1). The ruby red color of gold nanoparticle solutions is due to excitation of surface plasmon vibrations in the nanoparticles and occurs at ca. 520 nm.17,23 Figure 1B shows the UV-vis spectra of the gold solutions, curves 1, 2, and 3 corresponding to tubes 1, 2, and 3, respectively, in Figure 1A. Curve 4 corresponds to the spectrum of the chloroform layer after phase transfer of DDA-capped gold nanoparticles into water. The disappearance of the absorption at 520 nm in curve 4 clearly indicates that almost complete transfer of gold nanoparticles (DDA-Au) from chloroform to water had been achieved. The surface plasmon wavelength of gold nanoparticles phase transferred into water was 521 nm and no time-dependent change in the UVvis spectra of this solution was observed. This clearly shows that the gold nanoparticles in water are quite stable with little evidence for aggregation. An important observation is that although the difference in refractive index of the two phases is rather large (refractive indices: chloroform ) 1.446; water ) 1.33), the shift in the surface plasmon resonance wavelength consequent to phase transfer is only 1 nm. It is well known that the plasmon resonance of gold nanoparticles shifts to the red as the refractive index of the medium increases.23 While the exact reason for the insensitivity of the bilayer-capped gold nanoparticle plasmon resonance to solution refractive index is not fully understood, we speculate that this may be due to the presence of the interdigitated bilayer on the nanoparticle surface which prevents direct interaction of the water with the nanoparticle surface. To understand how CTAB molecules interact with DDAcapped gold nanoparticles and facilitate nanoparticle phase transfer, TGA and DSC measurements of the purified powder of the phase-transferred Au nanoparticles were done. Figure 2A and 2B shows the DSC thermograms and TGA data obtained for the surface-modified Au nanoparticles, respectively. In both figures, curve 2 corresponds to data recorded from the phase-transferred powder and curve 1 is that recorded from pure CTAB. Figure 2A shows three endothermic peaks at 65, 100, and 255 °C for purified CTAB-DDA-Au nanoparticles (curve 1). Figure 2B shows weight losses of ca. 68% and 100% at 253 °C for CTAB-DDA-Au nano and pure CTAB powder, respectively. From the comparison of Figure 2A and B, it (23) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427.

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Figure 1. (A) Picture showing various stages of modification of gold nanoparticles: as-prepared borohydride reduced gold hydrosol (tube 1), DDA-modified gold nanoparticle solution in chloroform (tube 2), DDA-modified nanoparticles after phase transfer into an aqueous solution of CTAB (tube 3). (B) UV-vis spectra corresponding to the three gold nanoparticle solutions shown in A.; curves 1, 2, 3 correspond to solutions from tubes 1, 2, 3, respectively. Curve 4 corresponds to the spectrum recorded from the organic layer after phase transfer.

Figure 2. (A) DSC data recorded from pure CTAB (curve 1) and surface-modified gold nanoparticles with interdigitated bilayer of DDA and CTAB (curve 2). (B) TGA data recorded for pure CTAB (curve 1) and surface-modified gold nanoparticles with interdigitated bilayer of DDA and CTAB (curve 2).

is observed that the broad endothermic peak at 65 °C (Figure 2A, curve 2) is not accompanied by a weight loss and that pure CTAB does not show any signature in this region (Figure 2A, curve 1). We believe that the endothermic peak at 65 °C is due to the melting of ordered regions of the hydrocarbon chains arising from interdigitated segments of the primary DDA and CTAB secondary monolayers (boxed regions, Scheme 1). In a previous study, some of us have shown that silver nanoparticles capped with interdigitated bilayer of lauric acid show similar no weight loss endothermic features at ca. 50 °C.20 An interdigitated structure is expected to be energetically favorable in an aqueous environment because of the maximization of hydrophobic interactions between the interdigitated hydrocarbon chains.24 On the basis of this result, we propose that the phase transfer of the DDAcapped gold nanoparticles and their redispersibility in

Figure 3. FTIR spectra of surface-modified gold nanoparticles with interdigitated bilayer of DDA and CTAB (curve 1) and pure CTAB (curve 2) in different spectral windows.

water arise because of formation of an interdigitated structure as shown in Scheme 1. Further analysis of the DSC and TGA data from pure CTAB and CTAB-DDA-Au nanoparticle powder yields another endothermic process at 253 °C accompanied by weight losses in both cases (Figure 2A and B). This feature may be attributed to desorption of free CTAB molecules as well as a fraction of the surface-bound CTAB-DDA bilayers. This desorption temperature is in good agreement with the reported temperature by Leff et al.6 for DDAmodified gold nanoparticles. The TGA data show complete weight loss at 253 °C in the pure CTAB (Figure 2B, curve 1) while a 68% weight loss at same temperature is observed from the CTAB-DDA-Au powder (Figure 2B, curve 2). This (24) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1985; p 102.

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Figure 4. (A) Transmission electron micrograph (TEM) of a film of gold nanoparticles phase transferred into water (CTABDDA-Au). (B) Histogram showing the particles size distribution measured from Figure 4A. Scheme 1. Diagram Showing the Formation of an Interdigitated Secondary Monolayer of CTAB Molecules on the Surface of DDA-Modified Gold Particles (CTAB-DDA-Au)

The box encloses ordered, interdigitated regions of the bilayers (see text for details).

is followed by negligible weight loss in the temperature range 312-800 °C. The weight loss at 253 °C (curve 2) is attributed to desorption of free, uncomplexed CTAB molecules as well as a fraction of the bilayers of CTAB and DDA on the gold nanoparticles. The percentage weight contribution of the surface-bound DDA and CTAB molecules is higher than a theoretical estimation of ca. 25% (by assuming the area occupied by DDA molecule as 25 Å2 on the surface of gold nanoparticles with diameter 35 Å and ratio of DDA and CTAB as 1:1). It is clear from the TGA data that the higher weight loss observed in this study is due to the presence of uncoordinated CTAB molecules in the powder. The feature seen at ca. 100° in both samples (Figure 2A) is due to the moisture entrapped in CTAB and the CTAB-DDA-Au powder. A drop-coated film of aqueous solution of purified powder of CTAB-DDA-Au was prepared on a Si(111) substrate and analyzed by FTIR spectroscopy. The spectra obtained from a CTAB-DDA-Au nanoparticle film (curve 1) as well as a solution-cast pure CTAB film (curve 2) in the spectral range 1200-1800 cm-1 and 2700-3600 cm-1 are shown in Figure 3. Since CTAB forms interdigitated bilayer structure with primary monolayer of DDA and is not directly bonded to the gold nanoparticle surface, the FTIR spectrum of surface-modified gold nanoparticles (CTAB-

DDA-Au) looks similar to that of pure CTAB. Both curves show resonance at 1480 cm-1, which can be assigned to methylene scissoring vibrations.25 The methylene antisymmetric and symmetric vibrations are observed at 2930 and 2857 cm-1, respectively, in both cases.25 A drop-coated film of aqueous solution of purified CTABDDA-Au powder was formed on carbon-coated copper grid by solvent evaporation and analyzed by transmission electron microscopy (TEM). Figure 4A shows a representative TEM image of CTAB-capped gold nanoparticles and Figure 4B is the particle size distribution (PSD) measured from Figure 4A. It is clear from the picture that the particles are fairly polydisperse. An analysis of the PSD histogram yielded gold nanoparticles of size 16 ( 2 nm. The average size of the nanoparticles is roughly four times that of the as-prepared gold nanoparticles formed by borohydride reduction of chloroauric acid.26 At this moment, it is not clear whether the increase in particle size consequent to phase transfer is due to the aggregation of the gold nanoparticles during phase transfer or due to the self-assembly of CTAB-capped gold nanoparticles during solvent evaporation for sample preparation. (25) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (26) Patil, V.; Malvankar, R. B.; Sastry, M. Langmuir 1999, 15, 8197.

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The electrolyte-induced precipitation of colloidal gold nanoparticles in aqueous phase is well known in the literature. Strouse et al. have shown that the colloidal gold nanoparticles can be precipitated by addition of electrolyte and furthermore, that under certain conditions, they can be redispersed in water.27 The stability of CTABDDA-Au nanoparticle solution of this study was checked as a function of electrolyte (NaCl) concentration. Curve 1 in Figure 5A is the UV-vis spectrum of the as-prepared CTAB-DDA-Au nanoparticles in water and curves 2, 3, 4, 5, and 6 correspond to spectra of the CTAB-DDA-Au gold nanoparticles solution after addition of 0.1, 0.5, 1, 2, and 3 M NaCl, respectively. The UV-vis spectra after addition of different amounts of NaCl are almost identical indicating that even the large concentration of salt does not destabilize the colloidal solution. Since gold nanoparticle solution of CTAB-DDA-Au was very stable at high ionic concentrations, it would be interesting to test the stability of gold solution as a function of solution pH. Figure 5B shows the UV-vis spectra recorded from CTAB-DDA-Au nanoparticle solution under different solution pH conditions (pH listed next to the corresponding spectra). As in the case of the spectra recorded from the gold nanoparticle solution at different ionic strengths, the spectra recorded as a function of solution pH are almost identical with little evidence for aggregation of the particles. The exceptional stability of the bilayer-capped gold colloidal solution as a function of ionic strength and pH is a salient feature of this work. In summary, the formation of water-dispersible gold nanoparticles by phase transfer of DDA-capped gold nanoparticles from chloroform to water bearing CTAB molecules has been demonstrated. The phase transfer is achieved by interdigitation of hydrocarbon tails of CTAB (27) Cumberiand, S. L.; Strouse, G. F. Langmuir 2002, 18, 269.

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Figure 5. (A) UV-vis spectra of phase-transferred gold nanoparticles from organic to aqueous phase (CTAB-DDA-Au) as a function of NaCl concentration. Curve 1, before addition of NaCl; curve 2, after addition of 0.1 M NaCl; curve 3, at 0.5 M NaCl; curve 4, at 1 M NaCl; curve 5, at 2 M NaCl; and curve 6, at 3 M NaCl. (B) UV-vis spectra of gold nanoparticles transferred to aqueous phase via interdigitated bilayers (CTABDDA-Au) as a function of solution pH (pH of the gold nanoparticle solution is indicated next to the respective curves).

molecules with that of hydrocarbon sheath formed by primary monolayer of DDA on gold nanoparticle surface. Acknowledgment. A. S. and A. K. would like to thank the Council for Scientific and Industrial Research (CSIR), Govt. of India, for research fellowships. This work was partially funded by a grant from the Department of Science and Technology (DST), Govt. of India, and is gratefully acknowledged. LA026523X