pH-Operated Nanopistons on the Surfaces of Mesoporous Silica Nanoparticles Yan-Li Zhao, †,‡ Zongxi Li, † Sanaz Kabehie, † Youssry Y. Botros, ‡,§,| J. Fraser Stoddart,* ,‡ and Jeffrey I. Zink* ,† California NanoSystems Institute and Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, 607 Charles E. Young DriVe East, Los Angeles, California 90095, Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, Intel Labs, Building RNB-6-61, 2200 Mission College BouleVard, Santa Clara, California 95054, and National Center for Nano Technology Research, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia Received June 18, 2010; E-mail: stoddart@northwestern.edu; zink@chem.ucla.edu Abstract: The development of drug delivery systems for the targeted and on-demand release of pharmaceutical products has risen rapidly to become a contemporary challenge in the field of nanobio- technology. Biocompatible mechanized phosphonate-clothed silica nanoparticles have been designed and fabricated in which the supramolecular machinery, which covers the surfaces of the nanoparticles, behaves like nanopistons, releasing encapsulated guest molecules in a controlled fashion under acidic conditions. The mechanized nanoparticles consist of a monolayer of -cyclodextrin (-CD) rings positioned selectively around the orifices of the nanopores of the mesoporous nanoparticles. A rhodamine B/benzidine conjugate was prepared for use as the nanopistons for movement in and out of the cylindrical cavities provided by the -CD rings on the surfaces of the nanoparticles. Luminescence experiments indicated that the mechanized nanoparticles were able to store small cargo molecules (e.g., 2,6-naphthalenedisulfonic acid disodium) within their nanopores at neutral pH and then release them by passage through the cavities of the -CD rings as soon as the pH was lowered to ∼5. In further investigations, the phosphonate-covered silica nanoparticles were functionalized selectively with the -CD rings, but on this occasion, the seven linkers attaching the rings to the orifices surrounding the nanopores contained cleavable imine double bonds. The -CD rings on the surface of the nanoparticles served as gates for the storage of large cargo molecules (e.g., rhodamine B) inside the nanopores of the nanoparticles under neutral conditions. Since imine bonds can be hydrolyzed under acidic conditions, the -CD rings could be severed from the surface of the nanoparticles when the pH was decreased to 6, releasing the large cargo molecules. The results described here present a significant step toward the development of pH-responsive nanoparticle-based dual drug delivery vehicles that are potentially capable of being interfaced with biological systems. Introduction The goal of drug delivery is to administer medicinally active molecules with high specificity to diseased cells in a targeted and controlled manner. 1,2 Recently, mesoporous silica nano- particles 3 have emerged as an appealing class of drug delivery vehicles for the treatment of diseases 2,4 on account of their sophisticated design and mode of action. There is increasing evidence 5 that the nanoparticles are noncytotoxic and that when they are on the order of 100-200 nm in diameter, they can undergo cellular uptake into acidic lysosomes by means of endocytosis. 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