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Ind. Eng. Chem. Res. 2007, 46, 464-471
Covalent Molecular Assembly in a Supercritical Medium: Formation of Nanoparticles Encapsulated in Immobilized Dendrimers Sreenivasa Reddy Puniredd and M. P. Srinivasan* Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576
This work demonstrates the feasibility of forming metallic nanoparticles within the confines of dendrimerladen ultrathin films that are immobilized on a solid surface. The functional property of the poly amido amine dendrimer (PAMAM) as a sequestering agent for Cu nanoparticles was exploited by the introduction of copper acetylacetonate (Cu(acac)2) as a precursor. The nanoparticle precursors were introduced into the matrix using supercritical carbon dioxide and liquid tetrahydrofuran (THF) as processing media. The precursors were subsequently reduced to form the nanoparticles. The resulting film assemblies were characterized via X-ray photoelectron and UV-visible absorption spectroscopies, atomic force, scanning electron, transmission electron microscopies, and ellipsometry. Higher yield, a denser distribution of nanoparticles, and greater stability toward polar solvent attack were observed when the structures were assembled in supercritical CO2 than when THF was employed. TEM images revealed that the embedded nanoparticles are fairly tightly distributed in terms of size with an average diameter of 7 nm. 1. Introduction Controlling the particle size and distribution is a critical issue for applications involving nanoparticles. For sensing, catalysis, and other applications involving electronic and optical properties,1,2 it is also necessary to immobilize the nanoparticles on a solid support. Templating techniques for immobilizing nanoparticles on to surfaces involve creation of electrostatic interactions3-5 or covalent bonds6-9 between the nanoparticle and the surface. Additionally, with the objective of forming particles with controlled size and distribution, dendrimers have been used as templates for preparing metallic, bimetallic, and semiconductor nanoparticles.10-12 Dendrimer-encapsulated metal nanoparticles are prepared typically by a two-step process: the metal precursors are sequestered within the dendrimer, and then the precursors are chemically/thermally reduced. Because the synthesis relies on dendrimeric templates, the resulting metal nanoparticles can be monodispersed in size. The preparation of a range of Cu, Pd, Pt, Ag, and Au metallic nanoparticles with dendrimers as templates has been reported.13-18 The properties of metal-dendrimer nanocomposites depend on the nature of the terminal groups of the dendrimers. Amine peripheral groups are able to induce agglomeration and precipitation of large metal clusters.19 This may be caused by the complexation of many metal precursors with the periphery of amine-terminated dendrimers, which typically results in formation of nanosized metal clusters stabilized by multiple dendrimers.20 Tripathy et al. reported the electrostatic LbL assembly of gold-dendrimer nanoclusters.21 In a similar work, Luo et al. reported the assembly of ultrathin films forming Ag-PAMAM dendrimer nanocomposites.22 Pd and Pt nanoparticles prepared within dendrimers have been immobilized onto Au surfaces via covalent amide bond formation between the unquaternized amine groups on the dendrimer periphery and anhydrideactivated self-assembled monolayers (SAMs).23 In the reports mentioned above, a solvent in the liquid state is typically used as the processing medium, especially to convey * To whom correspondence should be addressed. Tel.: +6565162171. Fax: +65-67791936. E-mail:
[email protected].
the precursor to the dendrimer-containing matrix. However, the viscosity of the liquid retards penetration into narrow spaces or pores. Even when gaseous phases under their critical temperatures are used, capillary condensation may occur in fine spaces. Such problems may be avoided and penetration of the dendrimer matrix promoted when a material in the supercritical state is used, on account of its relatively lower viscosity and higher diffusivity. Interest in using carbon dioxide in its supercritical state (SCCO2) is stimulated by the environmentally benign nature of CO2 in comparison with the organic solvents used in technological processes. It is widely applied as a carrier in impregnation24,25 and polymer synthesis.26 It has been used for formation of self-assembled monolayers of alkanethiols on gold surfaces,27,28 self-assembled structures of block copolymers,29 and synthesis of nanoparticles such as Ag, Pt, Pd, Ir, and AgPd from their organometallic precursors in SCCO2.30,31 Preparation of Pt nanoparticles in poly(4-methyl-1-pentene) and poly(tetrafluroethylene),32-34 Ag nanoparticles on the surface of polyimide film,35,36 poly(styrene-divinylbenzene),37 and poly(etherether ketone)38 and Pt-Pd nanocomposite in carbon molecular sieves(CMS),39 using SCCO2 have been reported. However, there is little information in published literature about using SCCO2 as a processing medium for dendrimers.40 While sequestration of nanoparticles in regular confined spaces such as dendrimeric systems and immobilization of nanoparticles on surfaces have been separately demonstrated, our present work focuses on immobilizing the sequestering agent (dendrimer) in thin film systems formed by covalent molecular assembly followed by formation of the nanoparticle within the immobilized dendrimer matrix. It would be of much interest, for example, in catalysis, to be able to anchor catalytically active nanoparticles in porous matrices using dendrimeric components as sequestering agents.41 It would be of interest also to know whether the use of SCCO2 for impregnating the matrix with the precursor would result in the attendant benefits expected of the supercritical medium, its advantageous transport and solubility properties, being manifested in the size and distribution of the nanoparticles formed in the matrix.
10.1021/ie060822q CCC: $37.00 © 2007 American Chemical Society Published on Web 12/09/2006
Ind. Eng. Chem. Res., Vol. 46, No. 2, 2007 465 Chart 1. Experimental Setup for the SCF Process
Recently, we showed that the SCCO2 could be used as a vehicle for building ultrathin films of oligoimide on silicon dioxide surface.42,43 The quality of the films in SCF medium was better than that for films formed in N,N-dimethylacetamide (DMAc), and the SCF-grown films were mechanically and thermally more stable. We have also investigated the formation of covalently bonded, multilayered, dendrimer-containing ultrathin films consisting of PAMAM and PMDA (pyromellitic dianhydride) using supercritical carbon dioxide as a vehicle for the deposition process.44 In the current work, we have used SCCO2 as the carrier for conveying the organometallic precursor compounds to dendrimer-containing matrices, which were formed by covalent molecular assembly in SCCO2. The functional property of the PAMAM-containing film as a sequestering agent for nanoparticles is demonstrated by the introduction of copper acetylacetonate complex (Cu(acac)2) in SCCO2 and subsequently reducing the complex to form metallic nanoparticles. We have compared the properties of the films with those obtained by introducing precursors in a liquid solvent (tetrahydrofuran (THF)). 2. Experimental Section 2.1. Materials. Pyromellitic dianhydride (PMDA), second generation of polyamidodiamine dendrimer (PAMAM), copper acetylacetonate ((Cu(acac)2) (all from Aldrich), 3-cyanopropyltrichlorosilane (CPS, Lancaster), and 4,4′-diaminodiphenylether (DDE, Fluka) were used as received. Trifluoroacetic anhydride (from Aldrich), carbon dioxide (SOXAL Code P40J purified grade with
