Dendrimer-Directed Synthesis of Shell Cross-Linked Nanocages with Amine Interior Walls

Young-Woong Suh1, Christopher Downing2, Jeong Kyu Lee3, Mayfair C. Kung3, and Harold H. Kung3. (1) Clean Technology Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul, 136-791, South Korea, (2) Chemistry, Northwestern University, 2145 Sheridan Road, E136, Evanston, IL 60208-3120, (3) Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, E136, Evanston, IL 60208-3120

We have recently reported a method for the preparation of a 2 nm diameter siloxane nanocage, derived from shell cross-linked micelle (SCM) templates, which possesses amine functional groups tethered to the interior surface of the shells [1]. Such a nanocage has several potential applications, for example, as a nanoreactor for controlled chemical reaction or drug delivery, a chemical sensor, etc. The reported method, however, does not permit easy control of the size or attaining a high degree of size uniformity, although it is possible to control the size by tuning the length of hydrocarbon chain. Here we describe a new, facile preparation method derived from carbamate-incorporated dendrimers for the purpose of synthesizing a molecular-size nanocage with cross-linked shells and amine interior walls. This new method can control the nanocage size in the range from 4 nm to 10 nm by the number of dendrimer generation and the size of core molecule. The preparation is based on the protection/deprotection chemistry of Fmoc (9-fluorenylmethoxycarbonyl) and TBDMS (t-butyldimethylsilyl) groups in order to produce carbamate groups in the dendrimer backbones. The Fmoc group is converted to primary amine by the base, piperidine, while the TBDMS group to hydroxyl group by the acid, HF. By properly combining these groups, dendrons of different generations could be prepared that possess triethoxysilyl groups at one end (shell) and amine groups for linkage to the core molecule at the other end. All intermediate products in the synthesis can be characterized with 1H, 13C and 29Si NMR, and MALDI-TOF mass spectrometry. Finally, the dendrimer, obtained by the reaction between the core molecule and dendrons, would be shell cross-linked by hydrolysis of the ethoxysilyl groups and subsequent condensation of the resulting silanols, followed by cleavage of carbamate bonds to produce uniform size nanocages. The characterization of this procedure and the properties of nanocages for binding of metal ions will be presented.

References [1] Y.-W. Suh, M.C. Kung, Y. Wang, and H.H. Kung, J. Am. Chem. Soc. 2006, 128, 2776.