Langmuir 1998, 14, 2707-2711
2707
Surface Derivatization of Colloidal Silver Particles Using Interdigitated Bilayers: A Novel Strategy for Electrostatic Immobilization of Colloidal Particles in Thermally Evaporated Fatty Acid/Fatty Amine Films Vijaya Patil and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received October 27, 1997. In Final Form: March 9, 1998 We have recently demonstrated that interdigitated bilayers of fatty acid molecules self-assemble on nanoscale curved surfaces, thereby highlighting an important difference between self-assembly on planar surfaces where such structures have not been reported to form. On a more practical level, this approach leads to a new strategy for derivatization of colloidal particle surfaces without the use of terminally functionalized molecules. In this paper, we use this new strategy to derivatize colloidal silver particle surfaces with carboxylic acid and amine functional groups and, thereafter, to incorporate the colloidal particles in thermally evaporated conjugate fatty lipid films via electrostatically controlled diffusion from the sol. The diffusion of the colloidal particles in the thermally evaporated organic films has been followed using quartz crystal microgravimetry and modeled on the basis of a 1-D diffusion process and the cluster diffusivities determined. It is observed that both the charge on the clusters and the length of the alkyl tail of the molecules in the secondary monolayer (via a hydrophobic interaction contribution) influence the cluster diffusivity in the thermally evaporated films. The cluster-incorporated films have been further characterized using optical absorption spectroscopy and X-ray photoemission spectroscopy.
Introduction The formation of thin films and packaging of nanoparticles in different matrices constitutes an important technological challenge en route to commercial exploitation of the exciting properties of nanoscale matter. Among the many approaches currently being investigated for the formation of thin films of nanoparticles,1-6,19 we have focused primarily on two methods wherein carboxylic acid derivatized colloidal particles are immobilized at the airwater interface using fatty amine Langmuir monolayers7-9 and via diffusion in thermally evaporated fatty amine films.10-12 The colloidal particle immobilization at the air-water interface and diffusion into the evaporated films was controlled via attractive electrostatic interaction between the oppositely charged amine and carboxylic acid * Author for communication. Telephone: 0091-212-337044. Fax: 0091-212-337044/330233. E-mail:
[email protected]. Current address: Materials Science & Engineering Center, University of Maryland at College Park, College Park, MD 20742. E-mail:
[email protected]. (1) Stroscio, J. A.; Eigler, D. M. Science 1991, 254, 1319. (2) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (3) Patil, V.; Mayya, K. S.; Sastry, M. J. Mater. Sci. Lett. 1997, 16, 899. (4) Yi, K. C.; Horvolgyi, Z.; Fendler, J. H. J. Phys. Chem. 1994, 98, 3872. Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (5) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Chem. Soc., Faraday Trans. 1994, 90, 673. (6) Urquhart, R. S.; Furlong, D. N.; Gegenbach, T.; Geddes, N. J.; Grieser, F. Langmuir 1995, 11, 1127. Pan, Z.; Liu, J.; Peng,.; Li, T.; Wu, Z.; Zhu, Z. Langmuir 1996, 12, 851. (7) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S. G. J. Phys. Chem. B. 1997, 101, 4954. (8) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575. (9) Mayya, K..S.; Sastry, M. J. Phys. Chem. B. 1997, 101, 9790. (10) Sastry, M.; Patil, V.; Mayya, K. S. Langmuir 1997, 13, 4490. (11) Sastry, M.; Patil, V.; Sainkar, S. R. J, Phys. Chem. B 1998, 102, 1404. (12) Patil, V.; Sastry, M. Langmuir 1997, 13, 5511.
groups, and furthermore, the cluster density in both cases could be easily controlled by varying the colloidal solution pH.7,8,10 Interesting cluster size dependent fractionation effects have also been reported for films formed using the Langmuir Blodgett technique9 and for films formed via diffusion.12 We have recently demonstrated that nanoscale curvature enables the formation of interdigitated bilayers of fatty acids on colloidal silver particle surfaces, such bilayers not having been reported for planar surfaces.13 From an application perspective, formation of interdigitated bilayers affords a flexible strategy for derivatization of colloidal particle surfaces without using terminally functionalized (bifunctional) capping molecules. In this paper, we show that this approach is general and that colloidal silver particles can be derivatized using amine and carboxylic acid functional groups. Thereafter, we investigate with quartz crystal microgravimetry (QCM) the electrostatically controlled diffusion of surface modified colloidal particles into thermally evaporated fatty acid and fatty amine films (depending on the nature of cluster derivatization) by immersion of the films in colloidal solution. The kinetics of cluster incorporation has been analyzed in terms of a 1-D diffusion model11 and cluster diffusivities in the organic matrix determined. After equilibration of the cluster density in the films, they were further investigated with optical absorption spectroscopy, and a chemical analysis was performed using X-ray photoemission spectroscopy (XPS). Experimental Details Silver colloidal particles were prepared in an aqueous medium as described elsewhere.7 This procedure yields a clear yellow sol at a pH close to 9 containing silver particles of 70 ( 12 Å diameter. The colloidal particles were capped with octadecanethiol molecules by mixing 9 mL of the hydrosol with 1 mL of an ethanolic solution of octadecanethiol whose concentration was adjusted to (13) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281.
S0743-7463(97)01159-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/17/1998
2708 Langmuir, Vol. 14, No. 10, 1998
Patil and Sastry
Scheme 1. Cartoon Showing the Primary and Secondary Monolayers Formed on Colloidal Silver Particles and Their Diffusion into the Conjugate Fatty Lipid Film: (A) Primary Monolayer Formed by Octadecanethiol/Lauric Acid Followed by Capping with a Lauric Acid Secondary Monolayer and (B) Primary Monolayer Formed by Octadecanethiol Followed by Capping with Octadecylamine
yield a surfactant concentration of 10-5 M. At this concentration, complete coverage of the silver colloid surface is expected to be achieved with the formation of a primary monolayer (Scheme 1).13 As in our previous work,13 we shall refer to the chemisorbed alkanethiol monolayer as the “primary” monolayer whereas the interdigitated monolayer leading to the colloidal particle derivatization is termed the “secondary” monolayer (Scheme 1). After formation of the alkanethiol primary layer, the secondary monolayer incorporation with lauric acid (sol 1, Scheme 1A) and octadecylamine (sol 2, Scheme 1B) was achieved in a similar manner using ethanolic solutions of the respective fatty lipids. The silver sol was also capped with bilayers of lauric acid as described elsewhere (sol 3, Scheme 1A).13 Attempts to form bilayers of octadecylamine on silver colloidal particles failed, possibly due to poor binding of the amine group with the silver surface. The capping of the silver particles was monitored at each stage using optical absorption spectroscopy carried out on a Hewlett-Packard 8452 diode array spectrophotometer operated at a resolution of 2 nm. The amine (sol 2, Scheme 1B) and carboxylic acid derivatized sols (sols 1 and 3, Scheme A) were extremely stable with negligible changes in the optical absorption spectra over weeks. Thin films of octadecylamine and arachidic acid of 500 Å thickness were deposited on Si (111) wafers, quartz substrates, and gold-coated 6 MHz AT cut quartz crystals by thermal evaporation in an Edwards E 306A coating system equipped with a liquid nitrogen trap. The pressure during film deposition was better than 1 × 10-7 Torr, and the deposition rate was 1 Å/min. The film thickness/deposition rate was monitored in situ using a water-cooled quartz crystal microbalance (QCM). After deposition of the fatty lipid films, the kinetics of cluster incorporation was followed by immersing the fatty amine/fatty acid coated quartz crystals in the silver hydrosols 1, 3, 2, respectively, and measuring the frequency change ex situ after thorough washing and drying of the crystals. Sols 1-3 were used without further treatment. At close to the as-prepared pH (pH ) 9), we have observed maximum electrostatic interaction between the charged amine and carboxylic acid groups.7 Control experiments were also performed wherein the fatty amine and acid films were immersed in similarly derivatized silver sols (e.g., an octadecylamine film immersed in sol 2). The change in the quartz crystal resonance frequency was measured using an Edwards FTM5 QCM that had a frequency resolution and stability of (1 Hz. For the 6 MHz crystal used in this study, this yields a mass resolution of 12 ng/cm2. Measured frequency changes were converted to a mass loading using the Sauerbrey
Figure 1. (A) Optical absorption spectra of the as-prepared silver sol (curve 1); silver sol capped with 10-5 M octadecanethiol (curve 2), octadecanethiol capped silver sol on further adsorption of 10-5 M lauric acid measured 15 min after adsorption (curve 3), and the bilayer capped sol (curve 3) after 24 h of capping with lauric acid (curve 4, dotted line). The surface plasmon resonance wavelengths are given next to the respective spectra. (B) Optical absorption spectra of the as-prepared silver sol (curve 1), silver sol capped with 10-5 M octadecanethiol (curve 2), octadecanethiol capped silver sol on further adsorption of 10-5 M octadecylamine measured 15 min after adsorption (curve 3), and the bilayer capped sol (curve 3) after 24 h of capping with octadecylamine (curve 4, dotted line). The surface plasmon resonance wavelengths are given next to the respective spectra. formula.14 A parallel investigation of the kinetics of cluster incorporation in the fatty amine/acid films on quartz substrates was carried out using optical absorption spectroscopy. The chemical nature of the cluster-incorporated films was studied using X-ray photoemission spectroscopy (XPS). After complete cluster incorporation in the fatty amine/acid films was ascertained using QCM measurements, the films were mounted in a VG Scientific ESCA 3 MK II spectrometer operated at a pressure better than 1 × 10-9 Torr. The excitation source was Mg KR X-rays (1253.6 eV energy), and the measurement was done in the constant analyzer energy (CAE) mode at a pass energy of 50 eV leading to an overall spectral resolution of ∼1 eV. The Ag 3d, C 1s, N 1s, S 2p, and O 1s core level spectra were recorded at an electron takeoff angle (ETOA, angle made by the electron emission direction measured from the surface plane) of 70°. General scans from all the films revealed no measurable sign of impurities. The measured spectra were treated using a Shirley background subtraction procedure15 and decomposed into chemically distinct components using a nonlinear least-squares procedure. Since the overall film structure after cluster incorporation is quite disordered, we shall not attempt a quantitative analysis of the XPS data. For brevity, we show only the core level spectra recorded from a fatty amine film in which clusters from sol 1 had been incorporated. The core level binding energies (BEs) obtained from the spectra of the other films were nearly identical.
Results and Discussion The formation of the primary and secondary monolayers (Scheme 1) on the silver colloidal particle surface in sols 1 and 2 was followed stepwise using optical absorption spectroscopy and is shown in Figure 1. Silver exhibits a strong and sharp surface plasmon resonance, which is extremely sensitive to surface modification16 and is (14) Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206. (15) Shirley, D. A. Phys. Rev. B. 1972, 5, 4709. The background subtraction of the core level spectra was done with an application written by MS using Mathcad. Mathcad is a commercial mathematical package for the PC available from Mathsoft Inc., Cambridge, MA 02142, and the application SHIRL.MCD is available from the Mathsoft public domain on the Internet. (16) Henglein, A. J. Phys. Chem. 1993, 97, 5457. Mulvaney, P. Langmuir 1996, 12, 788.
Colloidal Silver Particles
therefore a good system for following the adsorption of surfactants using optical absorption spectroscopy. Figure 1A shows the optical absorption spectra of the as prepared uncapped silver hydrosol (curve 1), the silver sol 15 min after capping with 10-5 M octadecanethiol (curve 2), and the hydrosol capped with the alkanethiol (curve 2) after further capping with 10-5 M lauric acid measured 15 min after lauric acid capping (curve 3) and 24 h after capping with lauric acid (curve 4, dotted line). It can be seen that the plasmon resonance intensity is reduced at each stage of the primary and secondary monolayer formation and is accompanied by a red shift of the absorption maximum. This indicates the sequential formation of a primary alkanethiol and secondary fatty acid monolayer on the silver particle surface. The plasmon resonance wavelengths are given next to the curves in the figure. We note here that the bilayer capped hydrosol is extremely stable as seen by the almost exact superposition of the spectra recorded 15 min and 24 h after capping with lauric acid (Figure 1A, curves 3 and 4). Figure 1B shows similar optical absorption spectra for the as-prepared silver hydrosol (curve 1), sol shown as curve 1 15 min after capping with octadecanethiol (curve 2), sol shown as curve 2 15 min after capping with octadecylamine (curve 3), and finally sol shown as curve 3 recorded 24 h after capping with the long chain amine secondary monolayer (curve 4, dotted line). The aminederivatized sol was also extremely stable with no evidence of flocculation of the particles even after weeks of storage. This result, together with the similar exceptional stability of the lauric acid capped sol, indicates electrostatic stabilization of the sol and supports the structure illustrated in Scheme 1. Large shifts in the surface plasmon resonance are observed at each stage of monolayer and bilayer formation with the fatty amine molecules (values indicated in the figure). It is difficult to rationalize the large shifts in the plasmon resonance on bilayer formation purely in terms of changes in the cluster electronic properties. Flocculation of the colloidal particles could lead to similar changes in the optical properties of the sol, but given the long-term stability of the bilayer-capped sols, this does not appear likely. Further work is required to clarify this interesting point, which will be addressed in a future paper. In an earlier work, we have provided evidence for the formation of an interdigitated bilayer structure13 on the silver sol, and this aspect will not be pursued further herein. We emphasize that the spirit of this investigation is to demonstrate that facile surface modification of colloidal particles can be done using interdigitated bilayers (obviating the need for bifunctional capping molecules) and, thereafter, demonstrate the formation of organic film/colloidal particle composite films via an electrostatically controlled diffusion process. The kinetics of cluster incorporation in 500 Å thick fatty amine/fatty acid films during immersion in the appropriate sols was followed by ex-situ QCM measurements and is shown in Figure 2. The data points indicated by solid circles and triangles in Figure 2 correspond to mass uptake kinetics of the thermally evaporated octadecylamine film during immersion in sols 1 and 3 respectively while the solid squares pertain to the QCM kinetics of the thermally evaporated arachidic acid film on immersion in sol 2. The solid lines are fits to the data based on a 1-D diffusion model.11 The following observations may be made from the figure. Even though the film thickness in the three cases was nearly identical (to within (25 Å), the equilibrium mass loadings are quite different. More specifically, the cluster density in the films transferred from the different sols is in the order sol 1 > sol 2 > sol 3. The
Langmuir, Vol. 14, No. 10, 1998 2709
Figure 2. QCM mass uptake kinetics for a 500 Å thick octadecylamine film immersed in sol 1 (filled circles) and sol 3 (filled triangles) while the filled squares refer to the mass uptake data for a 500 Å thick arachidic acid film immersed in sol 2. The solid lines are based on fits to the data using a 1-D diffusion model. Table 1. Parameters Obtained from Fits to the QCM Mass Uptake Data for 500 Å Thick Octadecylamine/ Arachidic Acid Films on Immersion in Sols 1-3 (See Text for Details) Based on a 1-D Diffusion Model for Cluster Incorporation experiment octadecylamine in sol 1 arachidic acid in sol 2 octadecylamine in sol 3
mass/cluster, Co D mo (ng) (×1017/cm3) (×104Å2/h) L (Å) 2.62 × 10-9 2.71 × 10-9 2.48 × 10-9
5.87 4.29 1.61
5.04 8.00 1.28
1200 1000 700
nature of the time variation of the mass uptake is also quite different with clusters incorporated from sol 2 reaching an equilibrium concentration faster than in the other cases. Control experiments wherein the fatty acid/ fatty amine films were immersed in carboxylic acid/amine derivatized silver sols respectively did not show any complexation of the colloidal particles even after prolonged exposures, thereby underlining the role played by electrostatic interactions in the cluster diffusion process. The above results taken together with the long-term stability of the sols show quite clearly that surface derivatization with the amine and carboxylic acid groups occurs as shown in Scheme 1. To quantify the kinetics of cluster incorporation, we have analyzed the QCM data shown in Figure 2 in terms of a 1-D diffusion model. The basic details of the diffusion model and its application to the incorporation of silver colloidal particles derivatized with a bifunctional molecule, 4-carboxythiophenol, in thermally evaporated amine films have been described elsewhere and will not be repeated here.11 This model leads to estimation of the cluster diffusivity, D, as well as the cluster concentration at the film/sol interface, Co. To fit the QCM mass uptake data shown in Figure 2 to the 1-D diffusion model, the mass of silver colloidal particles in sols 1, 2, and 3 was taken to be 2.62 10-9, 2.71 × 10-9, and 2.48 × 10-9 ng respectively (Table 1). The choice of the film thickness, which represents an important boundary condition for the QCM data fitting procedure, is more critical. It was observed in our previous studies that the film thickness increases considerably after cluster incorporation and that close to 60% of the equilibrium cluster density is reached within 10 h of immersion.11 Such a behavior is seen in the films of this study as well and proceeding in the spirit of the earlier study, we have taken the film thickness for the fits
2710 Langmuir, Vol. 14, No. 10, 1998
to be the values measured at the end of the immersion cycle. The final film thickness (“L”) after equilibrium cluster densities had been achieved was determined from optical interferometry to be 1200, 1000, and 700 Å for the 500 Å octadecylamine/arachidic acid films immersed in sols 1, 2, and 3, respectively. The values of Co and cluster diffusivities determined from fits to the QCM data shown in Figure 2 are listed in Table 1. From the parameters listed in Table 1, the following observations may be made. The cluster densities at the film/solution interface are nearly 4-5 orders of magnitude greater than the silver colloidal particle density in the bulk of the solution (∼1.1 × 1013/cm3). Such large “counterion” concentration enhancements have been observed at charged Langmuir monolayer surfaces and can be explained in terms of a Poisson-Boltzmann-Stern formalism.17 It is also observed that the colloidal particle density at the film-colloidal solution interface and diffusivity is least for the silver particles capped with bilayers of lauric acid (sol 3). The diffusivity is highest for clusters incorporated from sol 2 (octadecanethiol primary monolayer and octadecylamine secondary monolayer) while Co is nearly the same for sols 1 and 2. From purely electrostatics considerations, one expects nearly similar parameters for 500 Å amine films immersed in carboxylic acid terminated sols (sols 1 and 3). However, it appears that the attractive electrostatic interaction is considerably weaker for the clusters capped with lauric acid bilayers. Since the density of positive charges (from the -NH3+ groups) in the film in both cases is expected to be the same (i.e., same film thickness), this indicates that the effective negative charge on the silver clusters capped with lauric acid bilayers is less than that for clusters capped first with octadecanethiol followed by lauric acid. The degree of packing of the molecules in the primary monolayer will determine to a large extent the quality of the secondary lauric acid monolayer. The strong thiolate linkage which is known to form for octadecanethiol molecules with silver surfaces18 is expected to lead to a octadecanethiol primary monolayer with better packing of the alkyl chains than for clusters capped with lauric acid. This may be responsible for a smaller surface charge on the lauric acid bilayer capped clusters and thus lead to a smaller diffusivity and cluster concentration at the film/solution interface and consequently to smaller equilibrium cluster density in the film. However, application of a similar argument based on electrostatics alone appears to fail when applied to the diffusion of clusters from sols 1 and 2. At the moment, we can only offer a tentative explanation for the fact that the cluster diffusivity is higher for the amine capped sol while simultaneously, the equilibrium cluster density is smaller than for films immersed in sol 2 (lauric acid capped sol). The secondary monolayer is expected to be partially solvated by water, especially toward the terminal groups due to the large volume available to the groups. During diffusion into the film, the secondary monolayer could expel water by accommodating alkyl chains from the host (the fatty amine/acid film) via an interdigitation process, thus maximizing the hydrophobic interaction. This contribution to the energetics of the diffusion process would be higher for secondary monolayers with longer alkyl chains. This argument would satisfactorily explain the larger diffu(17) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1985; Chapter 12. Borukhov, I.; Andelman, D.; Orland, H. Phys. Rev. Lett. 1997, 79, 435. (18) (a) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (b) Sellers, H.; Ulman, A.; Schnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.
Patil and Sastry
Figure 3. Optical absorption spectra recorded after silver particle equilibration in 500 Å thick octadecylamine and arachidic acid films immersed in sols 1-3. The experimental film/sol combination is indicated next to the respective curves.
sivity observed for the octadecylamine terminated clusters over lauric acid terminated clusters. The concomitant larger cluster volume would also reduce the equilibrium cluster density in the film, as is observed (Table 1, sols 1 and 3). Further work is required taking secondary monolayer molecules of different chain lengths for the derivatization procedure as well as thermally evaporated molecules with different alkyl chains before a statement can be made on this aspect. The silver colloidal particle diffusion into the thermally evaporated fatty lipid films was followed independently using optical absorption spectroscopy as well. The form of the plasmon resonance intensity increase with time curve was similar to that observed in the QCM mass uptake data. In Figure 3, the optical absorption spectra recorded for 500 Å thick octadecylamine/arachidic acid films immersed in sols 1, 3, 2, respectively, after cluster density equilibration in the films had been achieved, are shown. The film/sol experimental combinations are indicated next to the respective curves. The values of the surface plasmon resonance intensity are in complete agreement with the trends observed with QCM (Figure 2), i.e., cluster density decreasing in the order sol 1 > sol 2 > sol 3. The surface plasmon resonance wavelength increases progressively from 460 to 465 and finally to 480 nm as the cluster density in the films increases (sols 3, 2, and 1 respectively). This indicates that interactions between the colloidal particles (which increases with increasing cluster density) alters the optical properties of the films significantly. The C 1s, Ag 3d, S 2p, and O 1s core level spectra recorded from a 500 Å thick octadecylamine film immersed in sol 1 after equilibration of the silver particle density are shown in Figure 4a-d, respectively. The solid lines are nonlinear least-squares fits to the data assuming individual spectral components are Gaussians. The intensity of the chemically distinct species, their binding energies (BEs) and the full width at half-maximum (fwhm) were left free in the fitting routine. The parameters obtained from the fits are listed in Table 2. The C 1s spectrum (Figure 4a) shows the presence of two components. The component at 285 eV is assigned to carbons in the alkyl tails of the capping molecules/molecules in the fatty lipid matrix. The component at 286.9 eV may be attributed to carbons coordinated to the thiol groups in the octadecanethiol molecules19 or to carbons coordi-
Colloidal Silver Particles
Langmuir, Vol. 14, No. 10, 1998 2711
Figure 4. Core level spectra recorded from a 500 Å thick octadecylamine film immersed in sol 1 after cluster equilibration together with the individual spectral components (solid lines): (a) C 1s; (b) Ag 3d; (c) S 2p; and (d) O 1s core levels. Table 2. Parameters Obtained from Fits to the Core Level Spectra Recorded from a 500 Å Thick Octadecylamine Film Immersed in Sol 1 core level
BE1 (eV)
BE2 (eV)
C 1s Ag 3d S 2p O 1s
285.0 368.2 162.3 532.1
286.9 374.3 163.4
intensity 1 (counts/s)
intensity 2 (counts/s)
2331 1292 254 310
91 868 136
nated to the amine groups in octadecylamine. The contribution from the carboxylate carbon, which is expected at ∼289 eV,20 is too weak to be satisfactorily stripped from the envelope. This may be due to the attenuation of the carboxylate carbon signal by inelastic scattering from the film matrix. The Ag 3d spectrum (Figure 4b) shows the presence of only the spin-orbit components separated by 6.1 eV with the individual peak intensities in the expected ratio of 1.5. There is no evidence for a chemically distinct thiolate silver species. This result is in agreement with our work on the covalent attachment of silver clusters onto thiolterminated SAMs19 and also agrees with the work of Brust et al. where XPS studies did not show the presence of a chemically distinct species in alkanethiol capped gold colloidal particles.21 The S 2p spectrum shown in Figure 4c showed the presence of only the spin-orbit components (19) Bandyopadhyay, K.; Patil, V.; Vijayamohanan, K.; Sastry, M. Langmuir 1997, 13, 5244. (20) Burns, F. C.; Swalen, J. D. J. Phys. Chem. 1982, 86, 5123. Davies, M. C.; Lynn, R. A. P.; Davis, S. S.; Hearn, J.; Watts, J. F.; Vickerman, J. C.; Johnson, D. Langmuir 1994, 10, 1399.
separated by 1.14 eV in the expected ratio of 2.18a The BE of the S 2p3/2 component at 162.3 eV indicates the formation of a thiolate species with no uncoordinated thiol groups (expected BE ) 164 eV).18a,19 This result implies that all the octadecanethiol molecules are chemisorbed on the cluster surface with no thiol molecules in the secondary monolayer. The O 1s spectrum (Figure 4d) could satisfactorily be accounted for by a single component at 532 eV. This BE is in excellent agreement with the O 1s BE determined from yttrium arachidate Langmuir-Blodgett films22 and is therefore assigned to the oxygen in the carboxylic acid groups of lauric acid molecules. The expected intensity of the carboxylate carbon accounting for differences in ionization cross sections of the C 1s and O 1s electrons as well as the 2:1 oxygen:carboxylate carbon ratio is ∼50 counts/s. This value is close to the noise level in the C 1s spectrum (Figure 4a) and explains why the carboxylic acid carbon component could not be distinguished. As mentioned earlier, we have not attempted a quantitative analysis of the film due to uncertainty in the surface structure. In conclusion, it has been shown that colloidal silver particles can be derivatized with amine and carboxylic acid groups using a new strategy based on interdigitated bilayers. The formation of the interdigitated bilayer structures which leads to amine/carboxylic acid derivatization of the colloidal particle surface was shown through electrostatically controlled diffusion into thermally evaporated fatty lipid films on immersion in the appropriate sol. The cluster incorporation process has been modeled in terms of a 1-D diffusion model and the cluster diffusivites determined. It is observed that the surface charge on the colloidal particles as well as the hydrophobic component arising from the packing of the alkyl chains from the secondary monolayer and host matrix contributes significantly to the energetics (and kinetics) of the diffusion process. A chemical analysis of a representative film showed no evidence for a thiolate silver species with all the octadecanethiol molecules forming thiolate linkages with the cluster surface. This new strategy for surface modification of colloidal particles obviates the need for bifunctional molecules and shows scope for development. Acknowledgment. V.P. wishes to thank the Council of Scientific and Industrial Research, Government of India for a research fellowship. LA9711591 (21) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (22) Ganguly, P.; Paranjape, D. V.; Sastry, M.; Chaudhary, S. K.; Patil, K. R. Langmuir 1993, 9, 487.
