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Benzene- and Anthracene-Mediated Assembly of Gold Nanoparticles at the Liquid-Liquid Interface Ashavani Kumar,† Saikat Mandal,† Suju P. Mathew,‡ P. R. Selvakannan,† A. B. Mandale,† Raghunath V. Chaudhari,*,‡ and Murali Sastry*,† Materials Chemistry and Homogeneous Catalysis Divisions, National Chemical Laboratory, Pune 411 008, India
Langmuir 2002.18:6478-6483. Downloaded from pubs.acs.org by AUSTRALIAN NATL UNIV on 08/14/18. For personal use only.
Received April 12, 2002 The organization of gold nanoparticles at the liquid-liquid interface between the gold hydrosol and benzene as well as anthracene in chloroform is described. Vigorous stirring of the biphasic mixture results in almost complete transfer of the gold nanoparticles from the aqueous to the benzene phase and the subsequent assembly of the gold nanoparticles at the liquid-liquid interface. In the case of anthracene in chloroform, the gold nanoparticles assembled directly at the interface forming an extremely flexible membrane. The gold nanoparticle films formed at the interface in both cases could be transferred onto different solid supports and were analyzed by a host of techniques. The films show reasonable two-dimensional ordering of the gold nanoparticles over large length scales. It was observed that the benzene and anthracene molecules are strongly bound to the gold particle surface, presumably through cation-π interactions between the aromatic molecules and nanoparticle surface-bound Au+ ions, thus opening up a hitherto unexplored avenue for the assembly of gold nanoparticles.
Introduction The controlled assembly of nanoparticles both in solution and on suitable surfaces to yield “crystals of nanocrystals” 1 is a problem of current interest with direct relevance to commercial application of nanoscale matter. Nanoparticles have been (self)-assembled on various surfaces such as those provided by Langmuir monolayers,2 terminally functionalized self-assembled monolayers (SAMs),3 surfacemodified polymers,4 and DNA-functionalized surfaces.5 Strategies for the assembly of nanoparticles in solution have relied on binding suitable bifunctional molecules to the particle surface and effecting nanoparticle crosslinking via modulation of electrostatic interaction,6 complementary DNA binding,7 and antibody-antigen interactions.8 Recently, amino acid-derivatized gold nanoparticles have been electrostatically assembled in solution onto DNA molecules9 while π-π interactions between surfacemodified gold10 and magnetic Fe3O4 nanoparticles11 have * Authors for communication. E-mail:
[email protected],
[email protected]. † Materials Chemistry Division. ‡ Homogeneous Catalysis Division. (1) Alivisatos, A. P. Science 1996, 271, 933. (2) (a) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 5, 607. (b) Sastry, M. Curr. Sci. 2000, 78, 1089. (c) Sastry, M.; Mayya, K. S.; Patil, V.; Hegde, S. G. J. Phys. Chem. B 1997, 101, 4954. (3) (a) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (b) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234. (4) (a) Freeman R. G.; et al. Science 1995, 267, 1629. (b) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem. 2000, 1, 18. (5) (a) Niemeyer, C. M.; Burger, W.; Peplies, J. Angew. Chem., Int. Ed. Engl. 1998, 37, 2265. (b) Nbindya, J. K. N.; Reiss, B. D.; Martin, B. R; Keating, C. D.; Natan, M. J.; Mallouk, T. E. Adv. Mater, 2001, 13, 249. (c) Mada, Y.; Tabata, H.; Kawai, T. Appl. Phys. Lett. 2001, 79, 1181. (6) (a) Shipway, A. N.; Lahav, M.; Gabai, R.; Willner, I. Langmuir 2000, 16, 8789. (b) El-Sayed, M. Acc. Chem. Res. 2001, 34, 257. (c) Galow, T. H.; Boal, A. K.; Rotello, V. M. Adv. Mater. 2000, 12, 576. (d) Mandal, S.; Gole, A.; Lala, N.; Sastry, M. Langmuir 2001, 17, 6262. (7) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (8) (a) Sastry, M.; Lala, N. Patil, V.; Chavan, S. P.; Chittiboyina, A. G. Langmuir 1998, 14, 4138. (b) Park, S. J.; Lazarides, A.; Mirkin, C. A.; Letsinger, R. L. Angew. Chem., Intl. Ed. 2001, 40, 2909. (9) Kumar, A.; Pattarkine, M. Bhadbade, M.; Datar, S.; Dharmadhikari, C. V.; Ganesh, K. N.; Sastry, M. Adv. Mater. 2001, 13, 341.
been used to successfully organize superstructures of the particles in solution. The liquid-liquid interface has also served as a fertile medium for nanoparticle assembly.12 Efrima and co-workers first demonstrated that metal liquidlike films (MELLFs) of silver could, under stringent conditions, be synthesized at the interface between an organic solvent such as dichloromethane and water.12a,b The MELLFs were shown to consist of silver particles of nanodimensions and were nonconducting.12a Subsequent reports have developed on this approach and the organization of negatively charged colloidal gold particles via electrostatic interactions with cationic surfactant molecules,12c and the assembly of gold nanoparticles into one-dimensional superstructures12d at the liquid-liquid interface have been shown. As part of our ongoing studies into the assembly of nanoparticles at the interface between an organic solvent and the hydrosol,12c we have observed that aromatic molecules such as benzene and anthracene present in the organic phase bind strongly with aqueous gold nanoparticles. This process leads to the immobilization of the gold nanoparticles in the form of a highly localized film at the interface. In the case of anthracene, the film at the interface exists as a highly elastic membrane. In this paper, we present details of our investigation into immobilizing gold nanoparticles using aromatic molecules in a single experiment that combines assembly in solution and immobilization at an interface. While it is well-known that nitrogen-containing aromatic molecules such as pyridine bind to aqueous gold nanoparticles through the nitrogen lone-pair electrons (and indeed, induce their aggregation),13 to the best of our knowledge there are no reports in the literature on the binding of aromatic (10) Teranishi, T.; Haga, M.; Shiozawa, Y.; Miyake, M. J. Am. Chem. Soc. 2000, 122, 4237. (11) Jin, J.; Iyoda, T.; Cao, C.; Song, Y.; Jiang, L.; Li, T. J.; Zhu, D. B. Angew. Chem., Int. Ed. 2001, 40, 2135. (12) (a) Yogev, D.; Efrima, S. J. Phys. Chem. 1988, 92, 5754. (b) Schwartz, H.; Harel, Y.; Efrima, S. Langmuir 2001, 17, 3884. (c) Mayya, K. S.; Sastry, M. Langmuir 1999, 15, 1902. (d) Wyrwa, D.; Beyer, N.; Schmid, G. Nano Lett. 2002, 2, 419. (13) (a) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435. (b) Galletto, P.; Brevet, P. F.; Girault, H. H.; Antoine, R.; Broyer, M. J. Phys. Chem. B 1999, 103, 8706.
10.1021/la025827g CCC: $22.00 © 2002 American Chemical Society Published on Web 07/25/2002
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Figure 1. (A) UV-vis spectra recorded from (1) the gold nanoparticle-benzene film on quartz (curve 1), (2) the as-prepared colloidal gold solution (curve 2), and (3) the colloidal gold solution shown as curve 2 to which a small quantity of benzene was added (curve 3, see text for details). The inset shows test tubes before (test tube on the left) and after (test tube on the right) complexation of aqueous colloidal gold particles with benzene (text for details). (B) UV-vis spectra in the range 190-400 nm recorded from the gold nanoparticle-benzene film on quartz (curve 1) and the pure benzene solution (curve 2).
hydrocarbons such as benzene and anthracene to gold nanoparticles. We believe the binding of these molecules to the gold nanoparticles leading to their cross-linking at the interface occurs through cation-π interactions involving Au+ ions present in AuCl2- complexes bound to the gold nanoparticle surface. This is a salient feature of the work and creates the exciting possibility of surface modification of gold nanoparticles without the use of the currently popular alkanethiols. The films of the twodimensional assembly of gold nanoparticles may be easily transferred onto solid supports after evaporation of the organic layer. Experimental Details The gold hydrosol was prepared by borohydride reduction of aqueous HAuCl4 solution as described elsewhere14 resulting in 35 ( 7 Å diameter gold particles in a ruby-red solution at a pH close to 9.14,15 Experiment 1. Ten milliliters of the freshly prepared gold hydrosol was taken in a test tube along with 10 mL of benzene resulting in a biphasic mixture with the colorless organic part on top and colored hydrosol below (inset of Figure 1A, test tube to the left). Vigorous shaking of the biphasic mixture for 60 min resulted in a turbid, emulsion-like solution that rapidly phase separated on cessation of stirring. The gold hydrosol was now colorless, and the organic phase was bluish as shown in the inset of Figure 1A (test tube to the right) clearly indicating transfer of the gold nanoparticles to the organic phase. The appearance of a blue color in the organic phase as opposed to the original ruby-red color of the gold hydrosol is a clear indicator of aggregation of the gold nanoparticles.7,8 Closer examination of the organic phase revealed that the blue color originated in a fairly uniformly dispersed viscous phase in benzene that, with time (typically 30 min), settled at the liquid-liquid interface. The biphasic mixture was poured into a Petri dish and the benzene phase evaporated to leave a uniform thin film of the gold nanoparticle-benzene complex on the surface of water. This film was lifted onto quartz substrates, Si(111) wafers and carboncoated transmission electron microscopy (TEM) grids for further analysis. Experiment 2. One hundred milliliters of the gold colloidal solution was taken in a separating funnel along with 100 mL of a 0.1 M solution of anthracene in hexane resulting in a biphasic mixture with the colorless organic part on top and the colored hydrosol below. Vigorous shaking of the biphasic mixture for 60 (14) Patil, V.; Malvankar, R. B.; Sastry, M. Langmuir 1999, 15, 8197. (15) The pink-purple color of gold hydrosols is due to excitation of surface plasmon vibrations in the particles: Mulvaney, P. Langmuir 1996, 12, 788.
min resulted in a turbid, emulsion-like solution that rapidly phase separated on cessation of stirring. The gold hydrosol was now colorless and a beautiful bluish membrane formed at liquidliquid interface as shown in the inset of Figure 2A. As in the previous experiment, the gold nano-anthracene film could be separated out and transferred onto quartz substrates, Si(111) wafers, and holey TEM grids for further analysis. UV-vis spectroscopy measurements of the gold nanofilms were performed on a Hewlett-Packard HP 8542A diode array spectrophotometer operated at a resolution of 2 nm while FTIR measurements were carried out in the diffuse reflectance mode on a Shimadzu-8201 PC instrument at a resolution of 4 cm-1. TEM measurements were carried out on a JEOL model 1200EX instrument operated at an accelerating voltage of 120 kV. X-ray photoelectronic spectroscopy (XPS) measurements on a gold nanobenzene film were carried out on a VG MicroTech ESCA 3000 instrument at a pressure of better than 1 × 10-9 Torr. The general scan and C 1s, Au 4f, and Cl 2p core level spectra were recorded with un-monochromatized Mg KR radiation (photon energy ) 1253.6 eV) at a pass energy of 50 eV and electron takeoff angle (angle between electron emission direction and surface plane) of 60°. The overall resolution of measurement is thus ∼1 eV for the XPS measurements. The core level spectra were background corrected using the Shirley algorithm and the chemically distinct species resolved using a nonlinear least squares procedure. The core level binding energies (BEs) were aligned with the adventitious carbon binding energy of 285 eV. Carefully washed powders of the gold nanoparticles complexed with benzene and anthracene were subjected to thermogravimetric analysis (TGA). The TGA measurements were performed on a Seiko Instruments model TG/DTA 32 instrument at a heating rate of 10 °C/min.
Result and Discussion The UV-vis spectrum recorded from the Au nanobenzene film is shown in Figure 1A (curve 1). Three welldefined absorption maxima at 520, 590, and 735 nm are observed in the spectrum. The 520 nm peak is the transverse surface plasmon component while the two peaks at larger wavelengths are the longitudinal components that indicate considerable aggregation of the gold nanoparticles.7-9 The presence of two longitudinal components is unusual and is possibly due to the presence of aggregates of two well-defined sizes. UV-vis spectra recorded from the as-prepared aqueous gold solution (curve 2) and the gold solution to which a small quantity of
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Figure 2. (A) UV-vis spectra recorded from (1) the anthracene film drop-coated on quartz (curve 1) and the gold nanoparticleanthracene membrane film formed at the liquid-liquid interface (curve 2, see text for deails). The inset shows a separation funnel with the gold nanoparticle-anthracene membrane at the liquid-liquid interface with air bubbles lifting up the membrane. (B) UV-vis spectra in the range 300-450 nm recorded from the gold nanoparticle-anthracene membrane (curve 1) and pure anthracene film (curve 2) on quartz.
benzene dissolved in ethanol was added (curve 3)16 are also shown in Figure 1A for comparison. Addition of benzene to the gold hydrosol results in a strong reduction in the plasmon resonance intensity accompanied by a broadening, which is characteristic of aggregation of the gold particles. We would like to add here that addition of just ethanol to the colloidal gold solution did not result in a detectable change in the optical properties of the solution, thus clearly indicating that benzene is responsible for the changes observed. Pyridine has been frequently used in earlier studies to induce the aggregation of gold nanoparticles in solution.13 The binding of pyridine to gold occurs through the nitrogen lone-pair electrons, this process leading to the displacement of surface-bound ions and, consequently, reduction in the repulsive interaction between the gold particles and, ultimately, aggregation.13 In the case of benzene and anthracene, it is clear that an alternative mechanism is operative and this will be dealt with subsequently. Figure 2A shows the UV-vis spectra recorded from an anthracene film solution-cast onto quartz (curve 1) and a gold nano-anthracene thin film formed at the liquidliquid interface in experiment 2 after transfer to a quartz substrate (curve 2). It can be seen that a strong and extremely broad plasmon resonance peak occurs at ca. 650 nm (curve 2) in the case of the gold nano-anthracene film while the pure anthracene film is essentially featureless in this spectral window (curve 1). This result clearly indicates a highly aggregated gold nanoparticle assembly in the gold nano-anthracene membrane. Comparison of the UV-vis spectra recorded from the gold nano-benzene (Figure 1A, curve 1) and the gold nano-anthracene films (Figure 2A, curve 2) reveals the almost complete disappearance of the transverse plasmon component and a very much red-shifted and prominent longitudinal component in the gold nano-anthracene film relative to the benzene film. This indicates that the state of aggregation of the gold nanoparticles in experiment 2 is much higher than that in experiment 1. It was observed that the gold nano(16) In this experiment, 10 mL of a solution of benzene in ethanol (from 1 mL of benzene + 10 mL of ethanol) was added to 10 mL of the gold hydrosol.
anthracene film behaved more like a flexible membrane. This is clearly shown in the experiment wherein air bubbles were introduced into the separating funnel in experiment 2. The bubbles traveled up to and beyond the liquid-liquid interface thereby lifting the blue-colored gold nano-anthracene membrane. This process is captured in the picture shown in the inset of Figure 2B. It is clear that in the case of anthracene, there is extremely large-scale assembly of the gold nanoparticles over macroscopic length scales. Such membranes are expected to have potential applications in separation processes and are being pursued in this laboratory. In the case of benzene, the film was much more tenuous and no such flexibility of the film was observed. It is well-known that aromatics such as benzene and anthracene possess strong signatures in the UV region of the electromagnetic spectrum. Figures 1B and 2B show UV-vis spectra in the UV spectral region recorded from gold nano-benzene and gold nano-anthracene films on quartz respectively (curves 1 in both cases) along with the corresponding spectra for benzene solvent and a pure solution-cast anthracene film (curves 2 in both cases). The prominent fingerlike features present in the pure anthracene film (Figure 2B, curve 2) are almost identically reproduced in the gold nano-anthracene film as well (Figure 2B, curve 1) clearly attesting to the presence of anthracene in the gold nanoparticle membrane. The three characteristic peaks at 213, 242, and 270 nm in pure benzene (Figure 1B, curve 2) are shifted to longer wavelengths and broadened in the case of benzene complexed with gold nanoparticles (Figure 1B, curve 1) indicating not only the presence of benzene in the gold nanoparticle film but also some degree of interaction with the gold nanoparticle surface as well. The gold nanoparticle-benzene/anthracene films were transferred onto Si(111) wafers as briefly described earlier and subjected to Fourier transform infrared (FTIR) analysis. Prior to this measurement, the film was heated at 60 °C for 1 h. Curve 1 in Figure 3A shows the FTIR spectrum recorded from the gold nano-benzene film. The aromatic C-H stretch vibration at ca. 3055 cm-1 is prominent in the as-prepared Au-nano film clearly at-
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Figure 3. (A) FTIR spectra recorded before (curve 1) and after (curve 2) heating the gold nanoparticle-benzene film deposited on a Si(111) wafer at 200 °C for 1 h. (B) TGA data recorded from powders of the gold nanoparticle-benzene film (curve 1) and gold nanoparticle-anthracene membrane formed at the liquid-liquid interface (curve 2).
Figure 4. (A) TEM picture of the gold nanoparticle-benzene film formed at the interface between borohydride reduced gold colloidal solution and benzene. (B) Edge-edge interparticle distance histogram of the TEM image shown in A.
testing to the presence of benzene in the film. The presence of benzene in the aggregated gold nanoparticle film under standard conditions of temperature and pressure indicates that the binding of these molecules to the nanoparticle surface must be sufficiently strong. Similar features were observed in the case of the gold nano-anthracene membrane as well and have not been shown for brevity. The strength of the interaction between aromatic hydrocarbons and gold nanoparticles was probed by TGA of the film collected at the interface as described above (Figure 3B). Curves 1 and 2 correspond to TGA data recorded from the gold nano-benzene and gold nanoanthracene films, respectively. As in the case of FTIR measurements, precaution was taken to thoroughly dry the powder by heating at 60 °C for 1 h prior to measurement. In the case of the benzene film, there is a nearly 50% weight loss at ca. 100 °C and this is attributed to desorption of surface-bound benzene molecules (curve 1). The large desorption temperature of this highly volatile molecule clearly shows the strong binding of benzene with gold nanoparticles and is a salient result of this study. The gold nano-anthracene film (curve 2) shows a 20% weight loss at a significantly larger temperature (∼160 °C). This indicates that the gold nano-anthracene complex is more stable than the benzene counterpart and also that the anthracene molecules do not desorb completely upon heating at this temperature. The FTIR spectrum of the gold nanoparticle-benzene film after heating at 200 °C for 1 h is represented as curve 2 in Figure 2A. The aromatic C-H stretch vibration at ca. 3055 cm-1 which is prominent
Figure 5. TEM picture of the gold nanoparticle-anthracene membrane formed at the liquid-liquid interface and lifted onto a carbon-coated TEM grid.
in the as-prepared Au nano-benzene film (curve 1) almost completely disappears following the heat treatment, thus supporting the TGA results presented above that the thermal treatment leads to desorption of the benzene molecules from the nanoparticle assembly. A representative transmission electron microscopy image recorded from the gold nanoparticle-benzene film is shown in Figure 4A. At low magnification (data not shown), it was observed that the holey grid was uniformly covered with extremely large domains of the gold nanoparticles without any apparent disruption in the close-
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Figure 6. (A) Au 4f core level spectrum recorded from the Au-nano-benzene film deposited on a Si(111) wafer. The inset shows the Cl 2p core level recorded from the same film. (B) C 1s core level spectrum from the Au-nano-benzene film transferred onto a Si(111) wafer.
packed assembly of the nanoparticles. The higher magnification image reveals the very regular in-plane packing of the gold nanoparticles (Figure 4A). While some degree of sintering of the particles appears to have occurred, the particles in the 2-D assembly are to a large extent well separated from one another. Figure 4B is a plot of the histogram of the interparticle separation (edge-edge) measured from the image shown in Figure 4A for 150 gold nanoparticle pairs. It is seen from Figure 4B that the edgeedge separations in the 2-D assembly are strongly peaked at 2.5 nm. Many earlier studies have demonstrated the very regular, hexagonal arrangement of silica-coated gold nanoparticles17 as well as CdSe quantum dots.19 Even though we observe a very regular edge-edge separation of the gold nanoparticles in the benzene film over large distances, there is clearly lack of periodicity in the ordering of the particles observed in earlier studies.17,18 While the exact reason for this difference is not known, we speculate that it could be due to the fact that the gold nanoparticles of this study are not very monodisperse (ca. 20% standard deviation)14 and it is known that this level of polydispersity does not lead to good two-dimensional ordering. Figure 5 shows the TEM picture recorded from one region of the gold nano-anthracene membrane. One large domain with very regular tear structures is observed in this picture. Darker regions where membrane folding had occurred are also seen in the picture. These features are consistent with the flexible membrane observed at the liquid-liquid interface (inset of Figure 2A) and also explains the difference in optical properties of the gold nano-benzene (Figure 1A) and gold nano-anthracene (Figure 2A) films mentioned earlier. The gold nanoparticles in the anthracene membrane are clearly more densely packed than those in the benzene film and display the same lack of translational order. A chemical analysis of the Au nano-benzene film transferred onto a Si(111) substrate was carried out by XPS. Figure 6A shows the Au 4f core level spectrum where it is clear that a single spin-orbit pair exists. The Au 4f7/2 binding energy (BE) of 84 eV is consistent with metallic gold and is in agreement with the observation of others.19 The C 1s spectrum is shown in Figure 6B and is rather complex. Two prominent peaks are observed at 285 and 283.2 eV. While the higher BE component may be assigned (17) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (18) Murray, C. D.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (19) (a) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (b) Brown, L. O.; Hutchinson, J. E. J. Am. Chem. Soc. 1999, 121, 882.
to adventitious carbon, the strong signal at 283.2 eV clearly indicates the presence of benzene in the film. While aromatic molecules such as benzene have been reported to have BEs close to 284.9 eV,20 we believe the shift to lower BEs is due to interaction with the gold nanoparticle surface and has been observed in the gold nano-anthracene film as well (data not shown). We do not currently understand the presence of the other lower BE C 1s peak at ca. 279 eV. The most surprising observation from the XPS measurements is the presence of fairly strong Cl 2p signal for the nanoparticle film (inset, Figure 6A, BE ca. 199 eV). The presence of Cl in the Au nano-benzene film clearly indicates the presence of gold chloride complexes on the surface of the gold nanoparticles and provides a clue as to the nature of complexation of the gold nanoparticles with the benzene molecules. This is discussed below. From the above experimental details it is clear that benzene and anthracene molecules bind to the surface of gold nanoparticles leading to the assembly of the nanoparticles in a 2-D assemblies at the liquid-liquid interface. We offer the following tentative explanation for the results obtained. Henglein has reported that aqueous reduction of Au3+ in AuCl4- anionic complexes results in the generation of Au+ ions that exhibit a characteristic absorption at ca. 203 nm.21 In the case of the gold nanoparticle-benzene film on quartz, the absorption band is observed at 202 nm, which we attribute to Au+ in nanoparticle surface-bound AuCl2- ions. This observation is strengthened by the XPS finding on the presence of Cl in the Au nano-benzene film (Figure 6A, inset). The exceptional stability of the gold colloidal solution further attests to the presence of ions on the nanoparticle surface and this was cross-checked with isoelectric focusing studies of the as-prepared colloidal solution that indicated a negative charge on the gold nanoparticle surface. It is well-known that Au+ binds to aromatic molecules through cation-π interactions22 and this is the likely mode of binding of both benzene and anthracene to the gold nanoparticles of this study. That the π-electron cloud is responsible for coordination with the gold nanoparticles is supported by our finding that reduction in the π-electron cloud density by attaching electron-withdrawing functional groups to benzene, such as would occur in benzoic acid, resulted in negligible binding of the benzoic acid molecules with the gold nanoparticles. (20) Clark, D. T.; Kilcast, D. J. Chem. Soc. B 1971, 2243. (21) Henglein, A. Langmuir 1999, 20, 6738. (22) Gimeno, M. C.; Jones, P. G.; Laguna, A.; Sarroca, C.; Calhorda, M. J.; Veiros, L. F. Chem. Eur. J. 1998, 4, 2308.
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In summary, the assembly of gold nanoparticles into two-dimensional superstructures by a simple one-step process involving binding of aromatic molecules such as benzene/anthracene and immobilization of the assembly at the liquid-liquid interface has been described. The use of a nonthiolated cross-linking agent, π-electron interactions with gold nanoparticles, and retention of crosslinked structure within the organic phase are novel aspects of this protocol that set it apart from other existing methods for solution assembly of gold nanoparticles. This approach based on binding of aromatic molecules with colloidal gold
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shows promise for the controlled assembly of the nanoparticles at the liquid-liquid interface as well as a new strategy for surface modification of gold nanoparticles. Acknowledgment. A.K. and S.M. thank the Council for Scientific and Industrial Research and University Grants Commission, Government of India, for research fellowships. LA025827G
