Sequential Electrostatic Assembly of Amine-Derivatized Gold and Carboxylic Acid-Derivatized Silver Colloidal Particles on Glass Substrates Ashavani Kumar, A. B. Mandale, and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received March 24, 2000. In Final Form: June 7, 2000 The formation of alternating layers of positively charged gold and negatively charged silver colloidal particles on glass substrates via electrostatic interaction is described. The charging of the gold and silver colloidal particles is accomplished by self-assembly of 4-aminothiophenol (4-ATP) and 4-carboxythiophenol (4-CTP) monolayers on the colloidal particles respectively and subsequent ionization of the functional groups at appropriate pH values of the colloidal solution. Glass substrates, which are negatively charged at pH > 3, are immersed first in the positively charged amine-derivatized gold solution leading to the formation of a monolayer of the gold particles and charge reversal of the glass surface. Thereafter, the gold particle covered glass surface is immersed in the negatively charged carboxylic acid-derivatized colloidal silver solution and the silver particles electrostatically self-assembled on the glass surface. This process may be continued to yield multilayer structures of the colloidal particles. The kinetics of electrostatic self-assembly of the colloidal particles on glass, the formation of the multilayer films, and their thermal stability have been followed with UV-vis spectroscopy, X-ray diffraction, ellipsometry, and X-ray photoemission spectroscopy measurements. Introduction The organization of colloidal particles as monolayers/ multilayers is a problem of topical interest and a number of different methods are currently being investigated to realize such structures. Surface-modified colloidal par- ticles, in particular, are amenable to self-assembly on different surfaces. Some of the more popular methods for the formation of monolayers of surface-modified colloidal particle films include self-assembly of hydrophobized particles during solvent evaporation, 1 covalent binding with suitable substrates, 2 and organization at the air- water interface. 3 One of the goals in this area of research is to develop protocols for the generation of crystalline arrays of nanoparticles wherein both the size and separa- tion between the nanoparticles in the array may be altered. Applications based on the collective properties of the organizates require such flexibility in controlling the nanoarchitecture of the films. 4 As far as multilayers or superlattices of colloidal gold particles are concerned, dithiols have been shown to be efficient cross-linking molecules between different layers. 5 Musick et al. 5b have shown that there is a dramatic fall in resistivity of multilayer gold particle films linked with dithiols beyond a particular film thickness (analogous to an insulator-metal transition) while Sarathy et al. 5c have recently demonstrated the interesting possibility of grow- ing heterocolloidal particle superlattices of Pt, Au, and CdS nanoparticles using the dithiol strategy. In this laboratory, we have shown that electrostatically im- mobilized colloidal particles at the air-water interface may be transferred onto suitable substrates by the Langmuir-Blodgett (LB) technique, leading to the for- mation of superlattice structures. 6 The LB technique has recently been extended to the formation of heterocolloidal particle superlattice structures as well. 6i In this paper, we extend our studies on the use of electrostatic interac- tions in the self-assembly of surface-modified colloidal particles to demonstrate the sequential assembly of positively charged gold and negatively charged silver colloidal particles on glass substrates. We remark here that such a layer-by-layer electrostatically controlled self- assembly process has been used with success in forming multilayers of polyelectrolytes, charged biopolymers such as DNA and proteins, and inorganic colloids. 7 The charging * To whom correspondence should be addressed. Phone: +91 20 5893044. 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