Langmuir 1998, 14, 5921-5928
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Facile Surface Modification of Colloidal Particles Using Bilayer Surfactant Assemblies: A New Strategy for Electrostatic Complexation in Langmuir-Blodgett Films Murali Sastry,* K. S. Mayya, and Vijaya Patil Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received June 16, 1998. In Final Form: July 28, 1998 Preliminary investigations have recently indicated that interdigitated bilayer assemblies of fatty acid molecules form spontaneously on colloidal silver particle surfaces while such bilayer structures are not observed on planar silver films. In this paper, this problem is probed further through contact angle and quartz crystal microgravimetry measurements of monolayer formation of lauric acid molecules on welldefined hydrophobic monolayers (formed from octadecanethiol chemisorbed on gold films) as a function of solution pH. The repulsive interaction between the ionized carboxylic acid groups in the lauric acid molecules prevents the formation of bilayer assemblies on planar surfaces. However, nanoscale surface curvature of colloidal particles permits interdigitation of the hydrocarbon chains in the bilayers, thereby maximizing the hydrophobic interaction as well as considerably reducing the electrostatic repulsive interactions of the headgroups, leading to stable bilayer assemblies. The strategy based on bilayer formation on colloidal particles is flexible and is used to derivatize colloidal silver particles with carboxylic acid and amine functional groups and thereafter electrostatically immobilize them at the air-hydrosol interface using the conjugate Langmuir monolayer. Good quality multilayer colloidal particle films can be deposited by the Langmuir-Blodgett technique, indicating that the bilayer assemblies on the colloidal particles are quite robust. This novel approach considerably extends the scope for the generation of nanoscale architectures using self-assembly of surface-modified colloidal particles.
Introduction Self-assembled monolayers (SAMs)1 provide a versatile route for surface modification important in a multitude of applications ranging from nonlinear optical materials,2 substrates for cell growth,3 and to high-density memory devices4 as well as in the formation of nanoparticle monolayer films via covalent attachment.5 The above list is by no means exhaustive and newer applications continue to emerge as our understanding of the electronic and chemical properties of SAMs evolves. Following the work of Brust et al.6 in which self-assembly of dodecanethiol on * To whom correspondence should be addressed. Present address: Department of Materials and Nuclear Engineering, Building 090, University of Maryland, College Park, MD 20742-2115. Phone: 301405-6566. Fax: 301-314-7136. E-mail:
[email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C. H.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. (3) Tidwell, C. D.; Ertel, S. I.; Ratner, B. D.; Tarasevich, B. J.; Atre, S.; Allara, D. L. Langmuir 1997, 13, 3404. (4) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Segi, T.; Swuzki, Y.; Ichimura, K. Langmuir 1992, 8, 2601. (5) (a) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (b) Bandyopadhyay, K.; Patil, V.; Vijayamohanan, K.; Sastry, M. Langmuir 1997, 13, 5244. (6) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (7) (a) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (b) Terrill, R. H.; Postlewaithe, 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. (c) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (d) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (e) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (f) Leff, D. V.; Ohara, P. C.; Heath, J. C.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (g) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (h) Wang, Z. L.; Harfenist, S. A.; Whetten, R. L.; Bentley, J.; Evans, N. D. J. Phys. Chem. B 1998, 102, 3068.
colloidal gold particles was demonstrated for the first time, there have been a number of studies on the self-assembly of alkane thiols,7 aromatic thiols,8 and primary amines on colloidal gold.9 The question that naturally follows is whether SAMs of suitable molecules on colloidal metal particles (which are curved on a nanoscale, hereafter referred to as 3-D SAMs) and on planar metal surfaces such as thin films (henceforth referred to as 2-D SAMs) exhibit any fundamental differences. While it is generally agreed that the colloidal particle surface curvature leads to the accommodation of more surfactant molecules per metal atom than on planar surfaces7f and to an increase in the volume of terminal groups in the chemisorbed molecules (and hence, more conformational disorder in the distal groups),7b-e the conclusion is that 2-D and 3-D SAMs are in most respects similar.7b-e,8c We have recently provided evidence that this geometric effect does, however, lead to significant differences and enables the selfassembly of interdigitated bilayers of fatty acid molecules on colloidal silver particles while such a bilayer structure has not been reported to form on 2-D silver films.10 In this paper, we seek a more comprehensive understanding of why bilayers of fatty lipid molecules form spontaneously on nanoscale curved surfaces and not on planar surfaces. To study bilayer formation on planar surfaces, an important requirement is the presence of a well-defined close-packed hydrophobic “primary” monolayer (monolayer in contact with the metal surface, graphic B, inset of Figure 1). We therefore deviate marginally from the approach adopted in our earlier work where fatty acid molecules, lauric acid, themselves constituted the (8) (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. (9) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (10) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281.
S0743-7463(98)00707-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/03/1998
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nanoscale architectures using self-assembly of surfacemodified colloidal particles. Experimental Details
Figure 1. Contact angles of a sessile water drop on lauric acid secondary monolayers self-assembled from aqueous solutions at different pH (lauric acid concentration, 10-4 M). Graphic A shows the lauric acid secondary monolayer formed on the primary SAM while graphic B shows the primary ODT SAM on gold formed from a 10-3 M ethanolic solution of ODT.
primary monolayer.10 While lauric acid does chemisorb on silver surfaces, we have chosen to use SAMs of octadecanethiol (ODT) on gold to provide the hydrophobic primary monolayer. We have been guided by the approach of Bain and co-workers who have studied monolayer formation (the “secondary” monolayer, graphic A, inset of Figure 1) from an aqueous solution of various ionic, nonionic, and zwitterionic surfactants on SAMs of ODT on gold.11 For uniformity, we have used the same methodology for investigating bilayer formation on colloidal particles as well (i.e., capping the particles with ODT to form the primary monolayer followed by incorporation of the fatty lipid secondary monolayer). Contact angle and quartz crystal microbalance (QCM) measurements of lauric acid monolayers formed on ODT SAMs as a function of the solution pH show that lauric acid monolayers do not form at pH values greater than 5. At pH values lower than 5 where progressively less ionization of the carboxylic acid groups occurs, sparsely covered monolayers are observed to form (∼25% surface coverage). It is interesting that secondary monolayers of lauric acid do form on ODT-capped silver colloidal particles even at a pH of 9 where the carboxylic acid groups are fully charged. The energetics of this main difference between self-assembly in 2-D and 3-D SAMs is briefly discussed. The colloidal particle surface modification due to the secondary monolayer is checked using electrostatic immobilization of the colloidal particles at the air-water interface using the complementary Langmuir monolayer as demonstrated by us for carboxylic acid-derivatized colloidal silver,12 gold,13a,b and CdS Q-state particles.13c We show that derivatization of nanoscale curved surfaces with bilayers is general and can be used for other fatty lipids as well (e.g., fatty amines) and thus provides a versatile route for surface modification of colloidal particles without the use of bifunctional molecules. This ability considerably enhances the range and scope of creating (11) (a) Ward, R. N.; P. B. Davies, P. B.; Bain, C. D. J. Phys. Chem. 1993, 97, 7141. (b) Bain, C. D.; Davies, P. B.; Ward, R. N. Langmuir 1994, 10, 2060. (c) Ward, R. N.; Duffy, D. C.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1994, 98, 8536. (12) (a) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S. G. J. Phys. Chem. B 1997, 101, 4954. (b) Mayya, K. S.; Sastry, M. J. Phys. Chem. B 1997, 101, 9790. (c) Mayya, K. S.; Sastry, M. Langmuir 1998, 14, 74. (13) (a) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575. (b) Mayya, K. S.; Patil, V.; Sastry, M. J. Chem. Soc., Faraday Trans. 1997, 93, 3377. (c) Mayya, K. S.; Patil, V.; Madhu Kumar, P.; Sastry, M. Thin Solid Films 1998, 312, 308.
SAM formation of ODT on gold films was investigated using QCM by immersion of a gold-coated 10-MHz AT-cut quartz crystal in a 10-3 M ethanolic solution of ODT. The changes in the resonance frequency as a function of time of immersion of the quartz crystal in the monomer solution was measured ex-situ using an Elchema frequency counter after careful washing of the crystal with ethanol and drying in flowing nitrogen. The stability and resolution of the quartz crystal frequency counter was (1 Hz which translates into a mass resolution of 4.3 ng/cm2 . The frequency change was converted to mass uptake using the Sauerbrey formula.14 After a close-packed SAM of ODT had formed, adsorption of lauric acid molecules on the hydrophobic SAM surface was monitored using ex-situ QCM in a similar fashion by immersion of the SAM-covered quartz crystal in 10-4 M solution of the fatty acid in deionized water maintained at pH values in the range 2-9. The pH of the lauric acid solution was adjusted using ammonia and dilute H2SO4. Prior to ex-situ measurement of the mass uptake, the films were washed carefully by slow immersion in a beaker containing 50 mL of deionized water and then dried in flowing nitrogen.15 Please note that, in this study, the formation of the lauric acid secondary monolayer on hydrophobic surfaces was done from an aqueous solution rather than an ethanolic solution as performed in the earlier work.10 The modified procedure was adopted to enable comparison with the secondary monolayer formation on the colloidal particle surface where the incorporation was performed in an aqueous phase.10 Contact angles of a sessile water drop (1 µL) on the bare ODT SAM on gold and the secondary lauric acid monolayers formed at different pH values were recorded after monolayer equilibration had occurred using a Rame Hart 100 goniometer. After each contact angle measurement, the integrity of the bilayer structures was checked by measurement of the quartz crystal resonance frequency. No frequency changes were observed after each contact angle measurement. Silver colloidal particles were prepared in an aqueous medium as described elsewhere.12 This procedure yields silver particles of 70 ( 12 Å in diameter with the colloidal solution pH being close to 9. The colloidal particles were then capped with ODT molecules by mixing to 9 mL of the hydrosol 1 mL of an ethanolic solution of ODT whose concentration was adjusted to 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.10 We note that the quantity of ethanol used for ODT capping in the water/ ethanol mixture is important in obtaining a stable sol. It was observed that significant aggregation of the colloidal particles occurred within 30 min when less than 0.5 mL of ethanol (concentration of ODT suitably increased) was used for thiol capping of the silver sol. Excess ethanol apparently stabilizes the thiol-capped solid in water over a long time scale (typically 48 h) by solvating the alkyl tails of the thiol molecules in the primary monolayer. After formation of the alkanethiol primary monolayer, incorporation of the lauric acid and octadecylamine secondary monolayers was achieved in a similar manner using ethanolic solutions of the respective fatty lipids (10-5 M concentration). 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. Before additional studies, the bilayercapped sols were heated mildly (to ca. 40 °C) for 1 h to get rid of the excess ethanol in the sol. The surface modification resulting from the formation of the secondary monolayer was studied by complexation of the colloidal (14) Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206. (15) Careful washing of the QCM crystals during the secondary monolayer (lauric acid) formation was done with water since rinsing with ethanol (as adopted for ODT monolayer formation) resulted in loss of the secondary monolayer. The secondary monolayer formed was robust and resistant to water cleaning as evidenced by QCM measurements which showed no frequency changes between repeated cycles of immersion in pure water.
Facile Surface Modification of Colloidal Particles particles with Langmuir monolayers at the air-colloidal solution interface. More specifically, we have used fatty amine (fatty acid) molecules in the Langmuir monolayer for electrostatic immobilization of carboxylic acid- (amine-) derivatized colloidal particles. Such a strategy has been used successfully in this lab for the organization of derivatized colloidal particles at the airhydrosol interface and has been shown to provide a fair degree of flexibility in controlling the colloidal particle density in builtup Langmuir Blodgett (LB) films.12,13 Langmuir monolayers of octadecylamine and arachidic acid were spread using chloroform on the carboxylic acid and amine-derivatized hydrosols, respectively, and the π-A isotherms measured at room temperature as a function of time on a Nima 611 Langmuir trough equipped with a Wilhelmy plate for pressure sensing. The compression rate was 50 cm2 /min for all the isotherms. Control π-A isotherms were recorded with time for arachidic acid Langmuir monolayers on the carboxylic acid-derivatized sol and octadecylamine Langmuir monolayers on the amine-terminated silver sol as well as for uncharged octadecanol monolayers on both the hydrosols as an additional test of the nature of colloidal surface derivatization. On equilibration of the monolayer, the silver cluster films were transferred to gold-coated quartz crystals and hydrophobized quartz substrates by the LB technique16 for QCM and optical absorption spectroscopy measurements, respectively. The quartz substrates were hydrophobized with one monolayer of lead arachidate since it has been observed that the transfer ratio improves significantly if a hydrophobic substrate is used.12a QCM and optical absorption measurements were carried out for films of different thickness to check the integrity of the colloidal particle monolayers transferred.
Results and Discussion 2-D versus 3-D Bilayer Formation. We first address the problem of monolayer formation of lauric acid on hydrophobic surfaces provided by SAMs of ODT on gold (2-D SAM formation). As briefly mentioned in the Experimental Section, secondary monolayer formation with lauric acid (10-4 M concentration) was studied as a function of pH of the aqueous fatty acid solution after a stable hydrophobic SAM of ODT on gold had formed. As will be shown subsequently, QCM measurements show that close-to-unity coverage for ODT occurs within 3-4 h of immersion of the gold film in the ethanolic solution of the monomer. Figure 1 shows a plot of the contact angles with sessile water drops of the lauric acid on ODT bilayer assemblies formed at different lauric acid aqueous solution pH values. Each measurement indicates a completely new film formed on the hydrophobic ODT surface. These measurements were made after 24 h of immersion of the ODT SAM in aqueous lauric acid solutions and careful washing and drying of the films as described in the Experimental Section and ref 15. The time of immersion was determined from QCM studies of the secondary monolayer formation to be discussed below. It can be seen that, at high pH values (>5), the contact angle for the expected bilayer assembly is close to 110° which corresponds to the value for the hydrophobic ODT surface. This indicates that the lauric acid secondary monolayer does not form at these pH values (graphic B, Figure 1). At solution pH values lower than 5, however, a systematic fall in the contact angle with reducing pH is observed, indicating a concomitant increased adsorption of the lauric acid molecules (exposing the carboxylic acid groups at the surface). The contact angle stabilizes at approximately 73° below pH 4 but is still much higher than the contact angles (
