| The
Self Assembly of Starbrust Dendrimers
Starburst dendrimers are mono-disperse, highly branched organic macromolecules, roughly spherical in shape, with characteristic sizes in the range 10-100 Angstroms. They exhibit container properties, i.e. they show the ability to encapsulate functional organic or inorganic "cargo" (metal and metal-oxide clusters, bio-active species, chromophores). Their end-group chemistry can be adjusted to tune long-ranged electrostatic and short-ranged adhesive interactions between dendrimers in solution, and between the dendrimers and a substrate. We want to understand how to balance these interactions to promote the self organization of dendrimers into structured thin layers. In combination with their container properties, there is the potential for spontaneously organized functional thin films. We are addressing the self-assembly of starburst dendrimers by examining the adsorption of poly(amidoamine) (PAMAM) dendrimers from dilute solution onto cleaned gold in-situ by a quartz crystal microbalance technique. All experiments are carried out at 21 degrees C, with either anhydrous ethanol or deionized water as solvent. In ethanol, the equilibrium absorbances correspond to about a monolayer and increase weakly (roughly linearly) with dendrimer generation, G. The adsorption is likely driven by the weak short-ranged adhesive interaction between the primary amine end-groups on PAMAM and gold. In aqueous media, the results are completely different. Under the conditions employed (low ionic strength and pH 6.5-7) the PAMAM amine end-groups tend to protonate in aqueous solution, so that the macromolecules bear positive charge. The equilibrium absorbance grows exponentially with generation G up to G = 6. Estimates indicate that multi-layers form in the aqueous system with the number of layers increasing exponentially. Multi-layers can be explained by a favorable electrostatic image-charge interaction between the charged dendrimers in solution and the gold substrate. For G = 7 a drastic drop in absorbance is seen which may be the result of surface crowding discussed in connection with the ''dense-shell'' transition. Similar conclusions are drawn using atomic force microscopy (AFM). For
more details contact Chris
Durning. Others involved in this research are Lajos Balogh,
George Flynn, Donald Tomalia, and Nicholas Turro. |
