Investigation into the Interaction between Surface-Bound Alkylamines

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Investigation into the Interaction between Surface-Bound Alkylamines and Gold Nanoparticles Ashavani Kumar, Saikat Mandal, P.R. Selvakannan, Renu Pasricha, A. B. Mandale, and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received February 7, 2003 In addition to alkanethiols and phosphine derivatives, alkylamines have been investigated as capping agents in the synthesis of organically dispersible gold nanoparticles. However, reports pertaining to gold nanoparticle derivatization with alkylamines are relatively scarce and their interaction with the underlying gold support is poorly understood. In this paper, we attempt a more detailed examination of this problem and present results on the Fourier transform infrared spectroscopy, thermogravimetry, nuclear magnetic resonance, and X-ray photoemission (XPS) characterization of gold nanoparticles capped with the alkylamines laurylamine (LAM) and octadecylamine (ODA). The capping of the gold nanoparticles with the alkylamines was accomplished during phase transfer of aqueous gold nanoparticles to chloroform containing fatty amine molecules. Thermogravimetry and XPS analysis of purified powders of the amine-capped gold nanoparticles indicated the presence of two different modes of binding of the alkylamines with the gold surface. The weakly bound component is attributed to the formation of an electrostatic complex between protonated amine molecules and surface-bound AuCl4-/AuCl2- ions, while the more strongly bound species is tentatively assigned to a complex of the form [AuCl(NH2R)]. The alkylamine monolayer on the gold nanoparticle surface may be place exchanged with other amine derivatives present in solution.

Introduction Metal nanoparticles have important applications in the area of catalysis,1 optoelectronics,2 electron microscopy markers,3 DNA detection,4 etc. due to their size- and shapedependent optical, electrical, and electronic properties. The nanoparticles are in general unstable due to their high surface energy and need to be stabilized against aggregation by suitable surface modification. Insofar as the synthesis of gold nanoparticles in a nonpolar organic environment is concerned, attempts have been made to stabilize them by capping with alkanethiols,5 ω-functionalized alkanethiols,6 aromatic thiols,7 and other thiol derivatives.8 The ability to modify the surface of gold nanoparticles with terminally functionalized thiol molecules is important in the use of the colloidal particles as scaffolds for novel chemical reactions,9 in gold sol based immunoassays,10 and in the self-assembly of the particles * To whom correspondence should be addressed. Telephone: +91 20 5893044. Fax: +91 20 5893952/5893044. E-mail: sastry@ ems.ncl.res.in. (1) (a) Hoffman, A. J.; Mills, G.; Yee, H.; Hoffman, M. R. J. Phys. Chem. 1992, 96, 5546. (b) Mukherjee, P.; Patra, C. R.; Ghosh, A.; Kumar, R.; Sastry, M. Chem. Mater. 2002, 14, 1678. (2) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (3) Baschong, W.; Wrigley, N. G. J. Electron. Microsc. Tech. 1990, 14, 313. (4) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (5) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Porter, L. A.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378. (6) (a) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (b) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (7) (a) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (b) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir 1997, 13, 51. (c) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 3944. (8) (a) Maye, M. M.; Chun, S. C.; Han, L.; Rabinovich, D.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 4958. (b) Tan, Y.; Li, Y.; Zhu, D. Langmuir 2002, 18, 3392. (c) Liu, J.; Mendoza, S.; Roman, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304.

in thin film form.11 Gold nanoparticles stabilized with alkanethiols in volatile organic solvents are also attractive from the point of view that they self-assemble into closepacked structures on evaporation of the solvent leading to uniform monolayers of nanoparticles.12 It is clear from the above that thiol chemistry as a means of surface derivatization of gold nanoparticles has received considerable attention and is the method of choice in most studies. Indeed, the nature of the gold nanoparticlethiolate bond13 and the mechanism of place exchange involving replacement of one gold nanoparticle surfacebound thiolate species with another is fairly well understood.14 Recently, there have been sporadic reports on the synthesis of organic solutions of gold nanoparticles derivatized with alkylamine15a-f and amine derivatives.15g As in the case of alkanethiols, the self-assembly of alkylamine molecules was first investigated on thin films of gold.16 Xu et al. showed that, under certain conditions, vapor phase deposition of octadecylamine (ODA) molecules on gold thin films resulted in the formation of ordered (9) Templeton, A. C.; Hostetler, M. J.; Warmouth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am.Chem. Soc. 1998, 120, 4845 and references therein. (10) (a) Storhoff, J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959. (b) Sastry, M.; Lala, N.; Patil, V.; Chavan, S. P.; Chittiboyina, A. G. Langmuir 1998, 14, 4138. (11) (a) Sastry, M.; Rao, M.; Ganesh, K. N. Acc. Chem. Res. 2002, 35, 847. (b) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (12) Wang, Z. L. Adv. Mater. 1998, 10, 13 and references therein. (13) Hasan, M.; Bethell, D.; Brust, M. J. Am. Chem. Soc. 2002, 124, 1132. (14) Song, Y.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 7096. (15) (a) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (b) Green, M.; O’Brien, P. Chem. Commun. 2000, 183. (c) Chen, X. Y.; Li, J. R.; Jiang, L. Nanotechnology 2000, 11, 108. (d) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 882. (e) Brown, L. O.; Hutchison, J. E. J. Phys. Chem. B 2001, 105, 8911. (f) Sastry, M.; Kumar, A.; Mukherjee, P. Colloids Surf., A 2001, 181, 255. (g) Selvakannan, P. R.; Mandal, S.; Pasricha, R.; Adyantaya, S. D.; Sastry, M. Chem. Commun. 2002, 1334. (16) Xu, C.; Sun, L.; Keplay, L. J.; Crooks, R. M. Anal. Chem. 1993, 65, 2102.

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monolayers of the alkylamines. However, attempts to selfassemble ODA from solution on gold thin films was not very successful due to competition of the Au0-amine interaction with the amine-solvent interaction.16 In an important shift to nanoscale gold surfaces, Heath and coworkers showed that primary amines form partially covalent bonds with gold nanoparticles and that the stability of amine-capped gold nanoparticles is a finitesize effect which is largely kinetic, rather than thermodynamic in origin.15a While most of the reports on surface derivatization of gold nanoparticles with alkylamine and other amine derivatives agree that the interaction of the amines with gold is strong and similar in this respect to thiolate bonds, a detailed investigation into the chemistry of the interaction has been lacking. We attempt to address this issue in this paper. From a detailed temperaturedependent analysis of alkylamine-capped gold nanoparticles prepared by a phase-transfer protocol reported in an earlier study from this group,15f we have identified two different modes of bonding of the amine molecules with the gold nanoparticle surface. One is a weakly bound electrostatic complex involving the protonated amine molecules and gold nanoparticle surface-bound chloroaurate ions while the strongly bound species is argued to be due to [AuCl(NH2R)] complexes. Furthermore, we show that gold nanoparticle surface-bound alkanethiols may be replaced by alkylamines by a place-exchange mechanism while the converse process, i.e., exchange of surfacebound alkylamines with alkanethiols, does not occur. Presented below are details of the investigation. Experimental Details In a typical experiment, colloidal gold was synthesized by borohydride reduction of aqueous HAuCl4 solution (1.5 × 10-3 g of HAuCl4 in 100 mL of water) as described in detail elsewhere.17 This results in a clear ruby red colloidal gold solution at pH 9 with the size of the gold nanoparticles being 53 ( 8 Å. To two separate 100 mL batches of the gold colloidal solution, 100 mL of a 2 × 10-4 M solution of laurylamine (LAM) and 100 mL of a 2 × 10-4 M solution of ODA in chloroform were added to yield immiscible layers of the red colored gold hydrosol on top of the colorless organic solution. Vigorous shaking of the test tubes resulted in the extremely rapid transfer (within 30 s) of the gold colloidal particles into the organic phase as evidenced by the red coloration of the organic phase (and a corresponding loss of color from the aqueous phase) when the two layers separated out.15f We would like to point out that in order to achieve near complete transfer of the gold nanoparticles to the organic phase, excess ODA/LAM (excess in relation to the chloroaurate ion concentration in water by at least a factor of 2) in chloroform had to be taken. Apparently, the surface coverage of the ODA/LAM molecules bound to the gold nanoparticle surface determines a certain minimum hydrophobicity required for complete phase transfer below which the nanoparticles were observed to be immobilized at the water-chloroform interface. UV-vis spectroscopy measurements of the different ODA-Au and LAM-Au organic solutions were performed on a Hewlett-Packard HP 8542A diode array spectrophotometer operated at a resolution of 2 nm. After phase transfer of the gold nanoparticles into chloroform had been accomplished, the ODA-Au and LAM-Au solutions were rotavapped, leading to a dark brownish powder. This powder was washed repeatedly with ethanol to get rid of uncoordinated ODA/LAM molecules in the powder. Known quantities (10 mg/ mL) of the purified alkylamine-capped gold nanoparticle powders were then dissolved in CDCl3, and the solutions were analyzed by nuclear magnetic resonance spectroscopy (NMR). Proton NMR spectra of the gold nanoparticle solutions were recorded on a Bruker AC 200 MHz instrument and scanned in the range 0-15 ppm. For comparison, the proton NMR spectrum of pure laurylamine in CDCl3 was also recorded. (17) Patil, V.; Malvankar, R. B.; Sastry, M. Langmuir 1999, 15, 8197.

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Figure 1. (A) UV-vis spectra of LAM- and ODA-capped gold nanoparticles after redispersion in different organic solvents. Curves 1 and 3: LAM-capped gold nanoparticles in chloroform and toluene, respectively. Curves 2 and 4: ODA-capped gold nanoparticles in chloroform and toluene, respectively. (B) UVvis spectra of LAM-capped gold nanoparticles in chloroform before (curve 1) and after addition of 10-1 M (curve 2), 10-2 M (curve 3), and 10-3 M (curve 4) ethylenediamine to the solution. The purified alkylamine-capped gold nanoparticle powder was readily redispersible in different organic solvents such as toluene, benzene, and chloroform. Thin films of the gold nanoparticles were formed on Si(111) substrates by immersing the substrate in amine-capped gold nanoparticles in CHCl3 and allowing the solvent to evaporate. The films thus formed were subjected to Fourier transform infrared spectroscopy (FTIR) and X-ray photoemission spectroscopy (XPS) measurements. FTIR measurements were carried out in the diffuse reflectance mode on a Shimadzu-8201 PC instrument at a resolution of 4 cm-1. XPS measurements on the gold nanoparticle 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 3f, Cl 2p, and N 1s 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 was ∼1 eV for the XPS measurements. The core level spectra were background corrected using the Shirley algorithm18 and the chemically distinct species resolved using a nonlinear leastsquares fitting procedure. The core level binding energies (BE) were aligned with the adventitious carbon binding energy of 285 eV. Samples for transmission electron microscopic (TEM) analysis were prepared by placing drops of the ODA- and LAM-capped gold colloidal solutions on carbon-coated copper TEM grids. The films on the TEM grids were allowed to stand for 2 min, following which the extra solution was removed using a blotting paper and the grid was allowed to dry prior to measurement. TEM measurements were performed on a JEOL Model 1200EX instrument operated at an accelerating voltage of 120 kV. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) profiles of carefully weighed quantities of purified powders of ODA- and LAM-capped gold nanoparticles were recorded on a Seiko Instruments Model TG/DTA 32 instrument at a heating rate of 10 °C/min.

Results and Discussion As mentioned briefly in the experimental section, purified powders of LAM- and ODA-capped gold nanoparticles can be readily redispersed in different organic solvents. Curves 1 and 2 in Figure 1 correspond to UVvis spectra recorded from Au-LAM and Au-ODA nanoparticle solutions in chloroform, respectively. A strong resonance at ca. 520 nm is seen in both spectra, with this (18) Shirley, D. A. Phys. Rev. B 1972, 5, 4709.

Interaction of Alkylamines and Gold Nanoparticles

Figure 2. 1H NMR spectra of pure laurylamine (curve 1) and purified powders of Au-ODA (curve 2) and Au-LAM (curve 3) dispersed in CDCl3.

absorption band arising due to excitation of surface plasmon vibrations in the gold nanoparticles. The UVvis spectra of these two nanoparticle solutions remained unchanged with time, indicating the particle size distribution in the organic phase was extremely stable. Curves 3 and 4 in Figure 1 correspond to UV-vis spectra recorded from Au-LAM and Au-ODA solutions in toluene respectively, and as in the case of the nanoparticles in chloroform, a strong plasmon absorption band at 520 nm appears in both solutions. The UV-vis studies thus demonstrate that the alkylamine-gold nanoparticles can be stored as a powder and dispersed in toluene/chloroform with little indication of aggregation of the particles. Purified powders of Au-LAM and Au-ODA were dispersed in CDCl3 and characterized by 1H NMR spectroscopy. Curves 1, 2, and 3 in Figure 2 correspond to the 1H NMR spectra recorded from the pure laurylamine, ODA-capped gold nanoparticles dispersed in CDCl3, and LAM-capped gold nanoparticles dispersed in CDCl3, respectively. The NMR spectrum of pure amine (curve 1) shows prominent resonances at 2.7, 1.72, 1.26, and 0.85 ppm which correspond to the protons from the R-CH2, β-CH2, methylene, and methyl groups from the hydrocarbon chain, respectively and are identified in the schematic given in the inset of Figure 2. The spectra of Au-ODA (curve 2) and Au-LAM (curve 3) are essentially similar and show broad multiplets at 1.55, 1.26, and 0.89 ppm which correspond to β-CH2, methylene, and methyl groups from the hydrocarbon chains of the surface-bound amine molecules. It is clear from a comparison of the spectrum of the pure amine and amine-modified gold nanoparticles that the resonance at 2.7 ppm, typical of R-CH2 protons in a pure amine (curve 1) is not observed in the amine-modified gold nanoparticle sample (curves 2 and 3). This resonance appears to have broadened considerably and shifted from 2.7 ppm in pure laurylamine (curve 1) to 2.0-0.5 ppm in the gold-amine complexes (curves 2 and 3) due to the coordination of the alkylamine molecules to gold nanoparticles through nitrogen atoms. Such a large chemical shift in the R-CH2 protons is due to the presence of the metal core, which can create large inhomogeneities in the magnetic field about local chemical environments. Similar observations were reported by Heath and co-workers,15a where they attributed similar broadening and unusual chemical shifts in proton NMR spectra of alkylamine-capped gold nanoparticles to ligation of the alkylamines with the gold surface. Earlier, 13C NMR studies of alkanethiol monolayers bound to the surface of gold nanoparticles also showed similar broadening of the

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Figure 3. (A) Representative TEM image of laurylaminecapped gold nanoparticles solution-cast onto a carbon-coated grid. (B) Histogram of the edge-to-edge distances measured from the laurylamine-capped gold nanoparticle film shown in (A).

Figure 4. (A) Representative TEM image of octadecylaminecapped gold nanoparticles solution-cast onto a carbon-coated grid. (B) Histogram of the edge-to-edge distances measured from the octadecylamine-capped gold nanoparticle film shown in (A).

resonance arising from carbons next to the sulfur atoms19,20 (in thiol headgroups), indicating strong interaction of sulfur atoms with the gold surface. The complete absence of the resonance at 2.7 ppm in Au-ODA (curve 2) and Au-LAM (curve 3) proton NMR spectra indicates that the purification process to remove uncoordinated fatty amine molecules in the powders is highly successful. As mentioned briefly in the Introduction, one of the advantages of the synthesis of hydrophobized gold nanoparticles in volatile organic solvents is that the nanoparticles spontaneously assemble into hexagonally ordered close-packed structures on solvent evaporation. Figures 3A and 4A show representative TEM images recorded from solution-cast films of LAM- and ODA-capped gold nanoparticles, respectively. The TEM micrographs show that the particles of size 53 ( 8 Å assemble into reasonably ordered two-dimensional structures that are in many regions hexagonally close-packed. The fairly large polydispersity of the gold nanoparticles conspires against extension of the hexagonal ordering to larger length scales. Even though the gold nanoparticles in this study are not monodisperse, it is observed from the TEM micrographs that the particles are well separated from each other with an apparently uniform interparticle separation. Figures 3B and 4B are plots of the histograms of the gold (19) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia; Reven, L. Langmuir 1996, 12, 1262. (20) Terrill, R. H.; Postelwaithe, T. A.; Chen, C.; Poon, C.; Terzis, A.; Chen, A.; Hutchinson, 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.

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Figure 5. TGA/DTA data recorded from purified powders of LAM- and ODA-capped gold nanoparticles. Curves 1 and 2 correspond to TGA data from LAM- and ODA-capped nanoparticles, respectively, and are associated with the axis on the left. Curves 3 and 4 correspond to the DTA data from LAM- and ODA-capped gold nanoparticles, respectively, and are associated with the axis on the right.

nanoparticle edge-to-edge distances measured from the images shown in Figures 3A and 4A, respectively, from 150 gold nanoparticle pairs. The histograms are strongly peaked at 20 Å (Figure 3B) and 25 Å (Figure 4B) for the Au-LAM and Au-ODA films, respectively. The welldefined spacing of the amine-capped gold nanoparticles is due to steric effects arising from the protective alkylamine sheath surrounding the particles that prevents physical contact with the metal cores. The increase in interparticle spacing in the case of the Au-ODA nanoparticle film relative to the Au-LAM film is due to the increased chain length of the ODA molecules surrounding the metal core. The interparticle separation in both cases is smaller than twice the length of the LAM and ODA molecules, indicating that the hydrocarbon chains from contiguous gold nanoparticles are interdigitated.19,20 From the UV-vis and TEM studies, it is clear that the gold nanoparticles are hydrophobic and, therefore, covered with a monolayer of the alkylamine molecules. It would be important to study the nature of the interaction between the surface-bound alkylamine molecules and the gold nanocore in terms of the strength of the bond and its chemical nature. To do so, TGA/DTA measurements on carefully weighed amounts of purified powders of AuLAM and Au-ODA were carried out from room temperature to 1000 °C, and the data obtained are shown in Figure 5 (left axis, TGA data: curve 1, Au-LAM, and curve 2, Au-ODA; right axis, DTA data: curve 3, AuLAM, and curve 4, Au-ODA). Two prominent weight losses are observed at 255 and 520 °C for the Au-LAM powder and amount to ca. 7% and 1%, respectively (curve 1). The corresponding characteristic temperatures for the Au-ODA complex are 260 and 500 °C and are accompanied by weight losses of ca. 6% and 5%, respectively (curve 2). The presence of two distinct temperatures at which weight loss occurs indicates the possibility of two different modes of binding of the alkylamine molecules with the gold nanoparticle surface. While an interpretation of the individual weight losses at the different temperatures would require a chemical analysis (to be presented later), the overall weight loss in the two samples is clearly due to decomposition of the alkylamine molecules bound to the gold nanoparticle surface. The total weight loss during heating to 1000 °C in the Au-LAM and Au-ODA powders is thus ca. 8% and 11%, respectively. The temperature at which maximal rate of mass loss occurs agrees well with TGA data reported in the literature on the desorption of alkanethiols/alkylamines covalently

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Figure 6. FTIR spectra of drop-coated films of pure laurylamine (curve 1) and LAM-capped gold nanoparticles on Si(111) wafer at room temperature (curve 2), after heating the AuLAM film at 260 °C for 1 h (curve 3), and after heating the Au-LAM film at 540 °C for 1 h (curve 4).

bound to colloidal gold.15a,20 The percentage weight contribution of the LAM and ODA molecules present as a close-packed monolayer on the gold nanoparticle surface (diameter 53 Å) estimated theoretically by assuming the area occupied by LAM/ODA molecule to be 25 Å2 (8% and 12% for LAM and ODA, respectively) agrees well with that observed experimentally. The broad exothermic feature at 400 °C observed in the LAM-Au powder (curve 3) accompanies the second and more prominent weight loss in this material (ca. 8%). This feature is attributed to sintering of the gold nanoparticles consequent to complete decomposition of the protective LAM monolayer. A similar exothermic peak is observed in the case of the Au-ODA powder (curve 4) albeit at a higher temperature (ca. 480 °C), indicating that the ODA monolayer is more stable than the LAM monolayer. It is clear from the TGA/DTA data that two important decompositon/desorption processes occur in the alkylamine-capped gold nanoparticles in the vicinity of 260 and 540 °C, suggesting two different binding modes for the alkylamines. To chemically differentiate between the two different alkylamine species on the surface of gold nanoparticles, temperature-dependent FTIR and XPS measurements were carried out on Au-LAM particles. The behavior of Au-ODA nanoparticles in these experiments was quite similar and has been omitted for brevity. Figure 6 shows the FTIR spectra recorded from solutioncast films of pure laurylamine (curve 1) and LAM-capped gold nanoparticles on a Si(111) substrate at room temperature (curve 2) and after heating at 260 and 540 °C for 1 h (curves 3 and 4, respectively). A comparison of the spectra in Figure 6 reveals the presence of three prominent features at ca. 2850, 2920, and 3418 cm-1 in the bare LAM film (curve 1) while the 3418 cm-1 feature is missing in the Au-LAM film on Si (curves 2 and 3). The 3418 cm-1 band in the bare laurylamine film corresponds to N-H stretch vibrations from LAM molecules, which disappears on their coordination with the surface of gold nanoparticles. The methylene antisymmetric and symmetric vibrations from the hydrocarbon chains of laurylamine are observed at 2920 and 2850 cm-1, respectively, in both the native amine film and the LAM-stabilized gold particle film (curves 1 and 2, respectively, Figure 6) and indicate that the hydrocarbon chains in the LAM monolayer surrounding the gold particles are in a close-packed,

Interaction of Alkylamines and Gold Nanoparticles

Figure 7. (A) Au 4f core level spectra recorded from a LAMstabilized gold nanoparticle film grown on a Si(111) substrate as a function of temperature. Curve 1: film at room temperature. Curves 2 and 3: after heating the Au-LAM film at 260 and 540 °C for 1 h, respectively. The spin-orbit components are shown in the figure. (B) N 1s core level spectra recorded from a LAMstabilized gold nanoparticle film grown on a Si(111) substrate as a function of temperature. Curve 1: film at room temperature. Curves 2 and 3: after heating the Au-LAM film at 260 and 540 °C for 1 h, respectively. The spectrum in curve 1 has been decomposed into two components by a nonlinear least-squares procedure.

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lack of an additional Au 4f component arising from AuCl4-/ AuCl2- ions may be due to the very small surface concentration of the ions that is below the detection limits of the XPS instrument. The binding of the laurylamine and octadecylamine molecules to the gold nanoparticles is through the amine functionality (proton NMR data, Figure 2), and therefore, it should be possible to resolve (if any) chemically distinct binding modes suggested by the TGA data (Figure 5) through the N 1s core level spectrum. The N 1s spectrum was recorded from the Au-LAM film after different heat treatments and is shown in Figure 7B. Curve 1 represents the N 1s spectrum before heat treatment, while curves 2 and 3 correspond to spectra recorded after 1 h heating of the films at 260 and 540 °C, respectively. The N 1s spectrum from the as-deposited Au-LAM film could be resolved into two chemically distinct species centered at 399.3 and 401.2 eV. The higher BE component is assigned to electron emission from nitrogens in the protonated amine groups electrostatically complexed with AuCl4- ions present on the surface of the gold nanoparticles (eq 1).

[Au0n(AuCl4-)m](aq) + mRNH3+(org) f [Au0n(AuCl4-)m](RNH3+)m(org) (1)

crystalline state.21 Another important observation is that the intensity of the methylene symmetric and antisymmetric vibration modes decreases on heating the film at 260 °C (curve 3), indicating that a fraction of the laurylamine molecules desorb/decompose at this temperature. Further heat treatment of the Au-LAM film at 560 °C resulted in complete loss of the methylene symmetric and antisymmetric vibrations (Figure 6, curve 4), indicating almost total decomposition/desorption of the LAM molecules. These observation are in good agreement with the TGA data where the Au-LAM sample exhibited mass losses at ca. 260 and 520 °C (Figure 5, curve 1). A solution-cast film of LAM-capped gold nanoparticles was formed on Si(111) substrates and analyzed by XPS as a function of temperature.The general scan spectrum of the film at room temperature showed the presence of C1s, N 1s, Cl 2p, and Au 4f core levels with no evidence of impurities. The film was sufficiently thick; therefore, no signal was measured from the substrate (Si 2p core level). Figure 7A shows the Au 4f spectra recorded from the Au-LAM film deposited at room temperature (curve 1) and the spectra recorded after heating at 260 and 540 °C for 1 h (curves 2 and 3, respectively). The Au 4f spectrum in all cases could be resolved into a single spin-orbit pair (splitting ∼ 3.7 eV) with a 4f7/2 binding energy (BE) of 83.8 eV (Figure 7, curve 1). There was no evidence for additional components in the Au 4f spectrum, and the result is in agreement with the XPS findings of Leff, Brandt, and Heath for alkylamine-capped gold nanoparticles.15a In addition to the Au 4f signal, a strong Cl 2p signal was also observed in the film, suggesting the presence of AuCl4-/ AuCl2- ions on the surface of the gold nanoparticles (Supporting Information, S1). The Cl 2p core level could be satisfactorily fit to two spin-orbit pairs with BEs of the 2 p3/2 peaks centered at 197.8 and 199.3 eV. The presence of two chemically distinct Cl 2p components strongly supports the possibility of both AuCl4- and AuCl2ions existing on the surface of the gold nanoparticles. The

We believe that the phase transfer of the gold nanoparticles to the organic phase by complexation with the alkylamine molecules occurs through this electrostatic process. Control experiments on the phase transfer of gold nanoparticles with the aqueous phase held at pH 12 (under conditions where the amine groups would be unprotonated) did not lead to efficient phase transfer, and the nanoparticles were found to form a film at the liquidliquid interface. We would like to mention here that Schiffrin and co-workers have observed the presence of a complex of the form R4N+Br- on the surface of gold nanoparticles when this molecule (R4N+Br-) was used as a phase-transfer reagent.22 On heating the film at 260 °C for 1 h, the form of the N 1s signal changes dramatically and the asymmetry arising from the presence of the high BE component at 401.3 eV is reduced significantly (compare curves 1 and 2, Figure 7). This indicates the almost complete loss of the electrostatically bound laurylamine component, thus identifying the chemical nature of the species decomposing/ desorbing at this temperature in the TGA data (Figure 5, curve 1). The lower BE component remains unchanged in both intensity and BE (399.3 eV, curve 2) after heating at 260 °C, suggesting that it is the more strongly bound laurylamine component and does not undergo any chemical change under these conditions. The N 1s spectrum recorded from this film after further heating at 540 °C for 1 h is shown as curve 3 in Figure 7. It is seen that the intensity of this signal is considerably reduced, indicating that the more strongly bound laurylamine component desorbs almost completely at this temperature. This result also agrees with the TGA data for Au-LAM (curve 1, Figure 5), where almost complete loss of the laurylamine layer was indicated at this temperature. The strongly bound low BE component in the N 1s spectrum of Au-LAM films needs to be identified. The presence of the Cl 2p core level unequivocally establishes the presence of either AuCl4- or AuCl2- ions on the surface of the gold nanoparticles. It is well-known that, during reduction of AuCl4- ions, Au3+ (in the AuCl4- ions) is

(21) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604.

(22) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922.

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rapidly reduced to Au1+ (in AuCl2- ions) and finally to Au0.23 The AuCl2- ions are fairly long-lived23 and could be bound to the surface of the gold nanoparticles along with AuCl4- ions, resulting in their stability in the aqueous environment. It is known that gold in the +1 oxidation state has a propensity to form complexes with different ligands such as RNC, piperidine, cyclohexylamine, and 3-bromopyridine24a-c as well as interesting and stable complexes of the form AuCl(NH2R) with alkylamines.24 We believe that during complexation of the alkylamine molecules with the gold nanoparticles prior to phase transfer, such complexes are formed on the surface of gold nanoparticles (eq 2).

[Au0n(AuCl2-)m](aq) + mRNH3+(org) f Au0n[AuCl(RNH2)]m(org) + HCl (2) These complexes are considerably more stable than the electrostatically bound complexes (eq 1, 260 °C TGA weight loss and high BE N 1s XPS feature) and may be identified with the 399.3 eV N 1s component in the Au-LAM film and the 540 °C weight loss in the TGA data (Figure 5, curve 1). It is well-known that alkanethiol molecules bound to the surface of gold nanoparticles may be place-exchanged with other thiol molecules.14 It would be of interest to see whether alylamine molecules on the surface of gold nanoparticles can also be place-exchanged. As a simple test, we studied the replacement of laurylamine molecules by another amine, ethylenediamine (EDA). The choice of a diamine was mediated by the fact that should place exchange occur, this would lead to the possibility of crosslinking of the gold nanoparticles and a visible change in (23) Henglein, A. Langmuir 1999, 15, 6738. (24) (a) Bachman, R. E.; Fioritto, M. S.; Fetics, S. K.; Cocker, T. M. J. Am. Chem. Soc. 2001, 123, 5376. (b) Ahrens, B.; Jones, P. G.; Fischer, A. K. Eur. J. Inorg. Chem. 1999, 1103. (c) Jones, P. G.; Freytag, M. Chem. Commun. 2000, 277. (d) Guy, J. J.; Jones, P. G.; Mays, M. J.; Sheldrick, G. M. J. Chem. Soc., Dalton Trans. 1977, 8. (25) (a) Gomez, S.; Philippot, K.; Colliere, V.; Chaudret, B.; Senocq, F.; Lecante, P. Chem. Commun. 2000, 1945. (b) Ahrens, B.; Friedrichs, S.; Herbst-Irmer, R.; Jones, P. G. Eur. J. Inorg. Chem. 2000, 2017.

Kumar et al.

the optical properties. Figure 1B shows UV-vis spectra of laurylamine-capped gold nanoparticles in chloroform as a function of varying EDA concentration in the solution. Curves 1, 2, 3, and 4 correspond to UV-vis spectra recorded from the as-prepared LAM-capped gold nanoparticle solution in chloroform and 0.1, 0.01, and 0.001 M EDA in the gold nanoparticle solutions, respectively. It is clear from the graph that there is considerable broadening of surface plasmon resonance and that the broadening increases as the concentration of diamine increases. The broadening of the surface plasmon band is a clear indication of aggregation of the gold nanoparticles in solution which can happen only by an exchange of the laurylamine molecules with EDA. Thus, place exchange between molecules bearing amine functionality may be accomplished. To conclude, it has been demonstrated that gold nanoparticles may be modified by alkylamine molecules yielding stable powders of gold nanoparticles that may be readily redispersed in different solvents. The particles assemble into close-packed structures on solvent evaporation with well-defined edge-to-edge distances. A salient feature of the work is the identification of two modes of binding of alkylamines with the gold nanoparticle surface and the chemical nature of the complexes. The surfacebound alkylamine monolayer may be place-exchanged with other amine derivatives in a manner similar to that observed for alkanethiols. Acknowledgment. A.K. and S.M. thank the Council for Scientific and Industrial Research (CSIR) and University Grants Commission (UGC), Government of India, for research fellowships. This work was partially supported by a grant from the Department of Science and Technology (DST), Government of India, and is gratefully acknowledged. Supporting Information Available: XPS Cl 2p core level spectrum recorded from the Au-LAM film on Si(111) substrate. This material is available free of charge via the Internet at http://pubs.acs.org. LA034209C