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One Pot, Spontaneous and Simultaneous Synthesis of Gold Nanoparticles in Aqueous and Nonpolar Organic Solvents Using a Diamine-Containing Oxyethylene Linkage PR. Selvakannan,† P. Senthil Kumar,† Arvind S. More,‡ Rahul D. Shingte,‡ Prakash P. Wadgaonkar,‡ and Murali Sastry*,† Materials and Polymer Chemistry Divisions, National Chemical Laboratory, Pune 411 008, India Received June 12, 2003. In Final Form: December 4, 2003 The simultaneous synthesis of gold nanoparticles in both water and chloroform by the spontaneous reduction of aqueous chloroaurate ions by a diamine-containing oxyethylene linkage that partitions in both phases is demonstrated. The oxidation of the diamine consequent to reduction of the gold ions results in the polymerization of the diamine and formation of a polymeric coating around the gold nanoparticles. The gold nanoparticles are uniform in size ranging between 3 and 6 nm. The nanoparticles capped with the polymer may be stored as a powder and readily redispersed in nonpolar organic solvents and water, indicating that the surface of the polymer capping the nanoparticles is significantly different in both cases.
Interest in the synthesis of gold nanoparticles dates back to the pioneering work of Faraday.1 Today, a number of recipes exist for the synthesis of gold nanoparticles of different sizes and shapes in both aqueous1,2 and nonpolar organic solutions.3 Indeed, gold nanoparticles synthesized in water may be transferred to nonpolar organic solvents4 and vice versa5 by simple surface modification strategies at the phase boundary. Current interest in this area is fuelled by potential application of gold nanoparticles in biodiagnostics6 and therapeutics.7 We have recently shown that gold nanoparticles may be synthesized in nonpolar organic solvents in a single step using the multifunctional molecule 4-hexadecylaniline (4-HDA).3b 4-HDA complexes with aqueous AuCl4ions, accomplishes their phase transfer into the organic solvent, reduces the gold ions to yield gold nanoparticles, and stabilizes them in the organic phase.3b During the course of our investigations into the use of aniline derivatives in gold nanoparticle synthesis, we have discovered that the diamine compound, viz., bis(2-(4aminophenoxy)ethyl)ether (DAEE), when taken in chloroform and stirred vigorously with aqueous chloroauric acid solution results in the spontaneous and simultaneous formation of gold nanoparticles in both chloroform and water. The gold nanoparticles are quite uniform in size (3-6 nm in diameter) and are capped with the polymer formed by the oxidation of the diamine compound. This * To whom correspondence should be addressed. Ph: +91 20 5893044. Fax: +91 20 58939522/5893044. E-mail: sastry@ ems.ncl.res.in. † Materials Chemistry Division. ‡ Polymer Chemistry Division. (1) Faraday, M. Philos. Trans. R. Soc. London 1857, 147, 145. (2) Handley, D. A. Colloidal Gold: Principles, Methods and Applications; Hayat, M. A., Ed.: Academic Press: San Diego, 1989; Vol. 1, Chapter 2. (3) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Selvakannan, PR.; Mandal, S.; Pasricha, R.; Adyanthaya, S. D.; Sastry, M. Chem. Commun. 2002, 1334. (4) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (5) Gittins, D. J.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001. (6) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (7) Sandhu, K. K.; McIntosh, C. M.; Simard, C. M.; Smith, S. W.; Rotello, V. M. Bioconjugate Chem. 2002, 13, 3.
results in the formation of a network of gold nanoparticles interconnected by the polymer. Partitioning of the diamine in the two phases may be easily controlled by varying the pH of the aqueous chloroauric acid solution thereby enabling control over the gold nanoparticle concentration in the two phases. In a typical experiment, 50 mL of 10-3 M HAuCl4 in water (pH 2.5) was taken with the same volume of 10-3 M DAEE (the procedure for the synthesis of DAEE given in the Supporting Information) in chloroform. The moment aqueous chloroauric acid was added to the diamine solution in chloroform, the appearance of a purple color in the aqueous phase could be discerned. Stirring the biphasic mixture continuously for 12 h in the dark resulted in both the organic and the aqueous phases turning into ruby red and purple colors, respectively. The appearance of purple color in the aqueous phase immediately after addition of DAEE indicates the rapid formation of gold nanoparticles, the color arising due to excitation of surface plasmons in the nanoparticles. This clearly occurs by a rapid process of phase transfer of DAEE molecules originally present in chloroform into the aqueous phase and reduction of the aqueous chloroaurate ions. On the other hand, the appearance of color in the organic phase required much more time, indicating that the phase transfer of aqueous chloroaurate ions into the organic phase by complexation with DAEE molecules at the liquid-liquid interface followed by their reduction was much slower than the parallel process in the aqueous phase. UV-vis spectra8 recorded from the chloroform and aqueous phases after 12 h of reaction are represented by curves 1 in panels a and b of Figure 1, respectively. Sharp, intense absorption bands centered at 500 nm for chloroform and 510 nm for water are observed and arise due to excitation of surface plasmons in the gold nanoparticles formed in the two phases. We note that the plasmon excitation wavelength (8) The UV-vis spectra of the Au nanoparticles in water and chloroform were measured on a Hewlett-Packard HP 8452 diode array spectrophotometer at a resolution of 2 nm. TEM measurements on the DAEE-capped Au nanoparticle films cast onto carbon-coated TEM grids were carried out on a JEOL model 1200EX instrument at an accelerating voltage of 120 kV. The proton (1H) NMR spectra were recorded on a Bruker 200 MHz instrument and scanned in the range of 0-15 ppm.
10.1021/la0350352 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/19/2003
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Figure 1. UV-vis spectra recorded from the chloroform (a) and aqueous phases (b) respectively after reaction of 10-3 M diamine in chloroform with 10-3 M aqueous chloroauric acid solution at pH 2.5 (curves 1), 4.5 (curves 2), and 6 (curves 3). Curve 4 in panel b corresponds to the spectrum recorded from gold nanoparticles synthesized in water at pH 2.5 after purification and redispersion.
in both of the phases is slightly lower than that normally observed for gold nanoparticles in solution (520-540 nm).3a,4,6 Reaction of DAEE in chloroform with aqueous HAuCl4 solutions at pH 4.5 and 6 was carried out, and the UV-vis spectra recorded from the two phases are shown as curves 2 and 3, respectively, in Figure 1a,b. As the pH of the aqueous phase is increased above 3, there is a progressive decrease in gold nanoparticle concentration in both phases, this effect being more pronounced in the aqueous phase. As the pH is increased, the degree of protonation of the amine groups decreases, resulting in less transfer of the DAEE molecules into water and a consequent reduction in gold nanoparticle concentration. In the organic phase, reduction in density of protonated amine groups translates into a smaller number of AuCl4- ions transferred into chloroform and thereby a smaller concentration of gold nanoparticles there as well. At pH values lower than 3, complete phase transfer of DAEE molecules into water was observed leading to formation of gold nanoparticles only in the aqueous phase (data not shown). An interesting feature of the spectra recorded from the gold nanoparticle solutions at different pHs is the presence of an absorption band at ca. 395 nm that accompanies the gold surface plasmon band at 500-510 nm (Figure 1a,b). Polymerization of aniline into polyaniline results in the appearance of an absorption band centered at ca. 380 nm that is attributed to π-π* transitions in the benzoid rings of the polymer.9 We believe that the 390 nm band observed in this study (Figure 1) arises from the polymer formed by oxidation of DAEE during reduction of the gold ions to form gold nanoparticles. This is most likely given that the terminal segments of the diamine used were aniline-like. Further evidence for the formation of the polymer is provided below. The gold nanoparticles in the two phases at all pHs were extremely stable in time and could be separated out as dry powders by rotavapping and readily redispersed in the respective solvents. Curve 4 in Figure 1b corresponds to the UV-vis spectrum of DAEE-capped gold nanoparticles synthesized in water at pH 3 followed by drying and redispersion in water. While gold nanoparticles synthesized in organic media are usually stable after evaporation (9) (a) McManus, P. M.; Yang, S. C.; Cushman, R. J. J. Chem. Soc., Chem. Commun. 1985, 1556. (b) Huang, K.; Qiu, H.; Wan, M. Macromolecules 2002, 35, 8653.
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Figure 2. (A) UV-vis absorption spectra of 10-3 M diamine in chloroform before (curve 1) and after reaction with water at pH 6 (curve 2), pH 4.5 (curve 3), and pH 3 (curve 4). (B) UV-vis absorption spectra of the aqueous-phase gold nanoparticles prepared by the reaction of 10-3 M diamine in chloroform with 10-3 M aqueous chloroauric acid solution at pH 2.5 before (curve 1) and after iodine treatment, centrifugation, and redispersion in water (curve 2; see the text for details).
Figure 3. Proton NMR spectra recorded from (curve 1) pure DAEE in CDCl3, (curve 2) DAEE-capped gold nanoparticles in CDCl3, and (curve 3) DAEE-capped gold nanoparticles synthesized at pH 3 and redispersed in D2O. The gold nanoparticles were prepared by reaction of 10-3 M DAEE in chloroform with 10-3 M aqueous HAuCl4 solution at pH 2.5.
of the solvent and readily redispersible in a range of nonpolar solvents,3a,b there are relatively fewer reports on the synthesis of water-dispersible nanoparticles.10,11 The fact that the polymeric DAEE oxidation product is capable of rendering the gold nanoparticles soluble in both nonpolar organic and polar solvents such as water indicates that the nature of the gold nanoparticle polymer capping agent is different in both cases. We are currently working out what conformational changes in the DAEE polymeric material would render the surface either hydrophilic or hydrophobic. To delineate partitioning of the DAEE molecule in the two phases, UV-vis spectra were recorded from aliquots of a 10-3 M solution of DAEE in chloroform after reaction with pure water at different pH values (Figure 2a). As the pH of the water is decreased, the intensity of spectral features characteristic of DAEE in the organic phase decreases, indicating that the partitioning of this molecule is pH-dependent and is maximum at pH 3. This result is in agreement with the UV-vis absorption results pertaining to gold nanoparticle formation at different pHs presented in Figure 1a,b. Figure 3 shows the proton NMR spectrum8 of pure DAEE (in CDCl3, curve 1), DAEE-capped gold nanopar(10) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (11) Selvakannan, P. R.; Mandal, S.; Phadtare, S.; Pasricha, R.; Sastry, M. Langmuir 2003, 19, 3545.
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ticles synthesized in chloroform, purified and redispersed in CDCl3 (curve 2), and DAEE-capped gold nanoparticles synthesized in water, purified and redispersed in D2O. These gold nanoparticle samples were prepared in the reaction of 10-3 M DAEE in chloroform and 10-3 M HAuCl4 in water at pH 2.5. The aromatic protons of DAEE occur at 6.66 and 6.78 ppm (protons b and c in the structure and curve 1, Figure 3), while the aromatic protons in the DAEEpolymer bound to gold particles in D2O are shifted to 7.02 and 7.27 ppm (curve 3). In the case of DAEE-polymer bound to gold in CDCl3, two chemically shifted aromatic proton pairs at 6.57, 6.71 and 7.0, 7.2 ppm are observed. The latter protons are associated with the aromatic group proximal to the polymerized amine group9b in the DAEEpolymer bound to the nanoparticle surface, while the 6.57, 6.71 ppm pair is associated with the unoxidized amine segment. Polymerization of both amine groups in DAEE would involve significant loss in intensity of the 6.57, 6.71 ppm pair, while incomplete polymerization of DAEE would be indicated by the presence of the two chemically shifted aromatic proton pairs at 6.57, 6.71 and 7.0, 7.2 ppm. This result indicates that the DAEE-polymer capping the gold nanoparticles in water is almost fully oxidized with a negligible percentage of unoxidized -NH2 groups while in the gold nanoparticles formed in the chloroform phase, the DAEE-polymer contains a large fraction of unoxidized amine groups. We do not understand at this stage how the degree of oxidative polymerization of DAEE could influence the hydrophilic/hydrophobic character of the polymer capping the gold nanoparticle surface. However, that the nature of interaction of the DAEE-polymer surface with the underlying gold nanoparticle surface is significantly different in both solvents is indicated by the differences in the chemical shifts in the methylene protons in both cases (curves 2 and 3, 3-4 ppm range). The oxyethylene linkages clearly play an important role in binding of the polymer to the gold nanoparticles. Representative transmission electron microscopy (TEM)8 images recorded from aqueous gold nanoparticles synthesized by the reaction of 10-2 and 10-3 M DAEE in chloroform with 10-3 M aqueous HAuCl4 solution at pH 2.5 are shown in panels A and B of Figure 4, respectively. In both cases, spherical structures are observed that are connected to each other by flat tapelike interconnects. The overall morphology of the nanogold-DAEE polymer structures is more clearly seen in the higher magnification TEM image shown in Figure 4B. The contrast in the structures, both in the spherical regions and in the interconnects, is uniform, making it difficult to identify the gold nanoparticles in the polymeric superstructure. Similar structures were observed by Dai, Tan, and Xu during the reduction of chloroaurate ions by o-anisidine wherein poly(o-anisidine) coated the gold nanoparticles leading to a networked superstructure.12 In the chloroform phase (Figure 4C,D), the nanostructures show a significantly larger number of well-defined spherical regions that are in physical contact with each other. The flat interconnecting regions seen in the water-phase structures are virtually absent in the chloroform phase. If the interconnected nanostructures seen in the TEM images of the aqueous- and chloroform-phase particles are due to gold, one would observe significant broadening of the gold surface plasmon bands or the appearance of a red-shifted plasmon band due to coupling of the nanoparticles in the aggregated structures. However, the plasmon bands in the UV-vis spectra of the gold nanoparticles formed in both water and chloroform (Figure 1) (12) Dai, X.; Tan, Y.; Xu, J. Langmuir 2002, 18, 9010.
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Figure 4. (A,B) TEM micrographs of gold nanoparticles synthesized in CHCl3 by the reaction of 10-2 and 10-3 M DAEE in chloroform with 10-3 M aqueous HAuCl4 solution at pH 2.5. Panels C and D are the corresponding TEM images of the gold nanoparticles synthesized in water under the above experimental conditions.
Figure 5. (A,B) TEM micrographs indicating clearly the binding of spherical nanoclusters of Au nanoparticles in the polyaniline network as prepared in the aqueous phase using 10-3 M chloroauric acid and DAEE. Panels C and D are the corresponding TEM micrographs after the gold nanoparticles are selectively leached out by iodine treatment (see the text for details).
are quite sharp with no evidence of another red-shifted component. Clearly, the interconnected structures seen in the TEM images of Figure 4 are not due to gold nanoparticles but due to the polymer formed by the oxidation of DAEE during reduction of the gold ions. The polymer coating would satisfactorily explain the 395 nm band observed in the UV-vis spectra of the gold nanoparticles in water and chloroform (Figure 1) as well as the similarity of the structures with that of poly(o-anisidine)nanogold observed by other groups.12 To conclusively establish the presence of the polymer consisting of oxidized DAEE molecules in the nanostructures and to effectively decouple the gold nanoparticles from the composite, a simple iodination experiment was performed. In this experiment, the gold nanoparticles synthesized in water by the reaction of 10-3 M DAEE in chloroform with 10-3 M aqueous HAuCl4 solution were treated with iodine in KI solution for 6 h. Iodine treatment leads to the oxidation of the gold nanoparticles to Au3+ that separates them from the polymeric network as a water-soluble component. This solution was then centrifuged, and the resultant pellet
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and supernatant were separated out. The pellet dissolves easily in N-methyl pyrrolidone (NMP), a common solvent for dissolving polyaniline. Figure 5 shows TEM images recorded from drop-cast films of the nanostructures in water obtained by reaction of 10-3 M DAEE in chloroform with 10-3 M aqueous HAuCl4 solution before (A,B) and after iodine treatment and centrifugation (C,D). The TEM images A and B now show the polymeric network together with almost spherical gold nanoparticles of 3-6 nm size distribution. The gold nanoparticles are clearly missing in the polymeric network obtained after iodine treatment (Figure 5C,D). Comparison of images before (A,B) and after iodine treatment (C,D) shows a striking retention in structure of the polymer even after removal of the gold nanoparticles. The TEM images thus clearly establish the presence of a polymeric protective network encapsulating the gold nanoparticles that is formed by the oxidation of DAEE. As shown above, the process of iodine treatment and dissolution in NMP has resulted in successful extraction of the quasi-pure polymer minus the nanoparticles. This resultant quasi-pure polymer was then redispersed in NMP, and the UV-vis absorption spectrum was recorded (Figure 2B, curve 2). The UV-vis spectrum of the asprepared gold nanoparticle-polymer solution in water is shown for comparison (Figure 2B, curve 1). The band arising due to π-π* transitions in the benzoid rings of the polymer can clearly be seen in the gold-nanoparticle-free polymeric solution at ca. 365 nm (curve 2). We attribute this apparent shift in the π-π* transition band relative to that observed in the solution of the gold nanoparticles capped with the polymer (Figure 1, ca. 395 nm) to interaction of the aromatic segments in the polymer with the gold surface. Aromatic molecules such as anthracene and benzene can bind to gold nanoparticles, presumably through cation-π interactions involving surface-bound unreduced AuCl4- ions.13 The small blue shift in the surface plasmon band of the gold nanoparticles encap-
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sulated in the polymeric network may be also be attributed to such an interaction. In addition to this band, another fairly intense and broad absorption band centered at 490 nm is observed in the UV-vis spectrum of the pure polymer (Figure 2B, curve 2). This band is attributed to excitonic absorption in quinoid rings in the polyanilinelike polymeric structures formed by the oxidation of DAEE.9 In conclusion, the one pot and simultaneous synthesis of gold nanoparticles capped with a polymeric membrane in both water and nonpolar organic solvents has been described. This is accomplished by the reaction of the diamine, bis(2-(4-aminophenoxy)ethyl)ether, present in chloroform with aqueous chloroaurate ions. The gold nanoparticles are formed in both phases by a process involving fractionation of the diamine in both phases, transfer of aqueous gold ions from water into chloroform, and reduction of gold ions by the diamine. The reduction of gold ions results in the oxidative polymerization of the diamine and formation of a polymeric capping agent for the nanoparticles. The partitioning of the diamine is pHdependent and enables variation in the concentration of gold nanoparticles in the two phases. Acknowledgment. P.R.S., S.K., A.S.M., and R.D.S. thank the Council of Scientific and Industrial Research and University Grants Commission, Government of India, for financial support. The authors gratefully acknowledge the editor and the referees for their critical and insightful reading of the manuscript. Supporting Information Available: The procedure for preparation of the DAEE molecule. This material is available free of charge via the Internet at http://pubs.acs.org. LA0350352 (13) Kumar, A.; Mandal, S.; Mathew, S. P.; Selvakannan, P. R.; Mandale, A. B.; Chaudhari, R. V.; Sastry, M. Langmuir 2002, 18, 6478.
