Nanomechanical systems that are designed to trap and release molecules from pores in response to a stimulus are currently the subject of intense investigation. Such systems have potential applications for precise drug delivery. A photo-activated material, for example, could release a drug under external control at a specific time and location for phototherapy. Nanomaterials suitable for this type of operation must consist of both an appropriate container and a photoactivated moving component.Mesoporous silica made by sol-gel methodology [24][25][26] is a very useful container for molecules because the pores of both particles [24,27] and films [25] are accessible to liquids and gases. The pores in the silica materials are formed by templating agents, usually surfactants, that are removed after the formation of the structure is completed. [24,25] Diffusion of molecules driven by concentration gradients, including biomolecules, into and out of the pores has been demonstrated. [28] Methods have been developed to make tubular pores whose inner diameters can be varied between about 1 and 10 nm. [29] The pore walls and silica frameworks can readily be derivatized with a large variety of molecules. [30][31][32] [5,[33][34][35][36][37][38][39] Photoactivated moving parts based on the photoisomerization of azobenzene derivatives have been used in conjunction with mesoporous silica. [9][10][11][12][13][14] The dynamics of the motion in pores of azobenzene derivatives, with substituents ranging from hydrogen atoms to dendrimers, has been studied. [40] The decrease in the size of trans to cis isomers of azobenzene molecules attached to pore interiors has been used to regulate the transport of molecules through pores to electrodes.[13] Of most relevance to this Communication, the back and forth wagging motion has been demonstrated to act as a molecular impeller that regulates the release of molecules from the pores of silica nanoparticles under "remote control" upon photoexcitation.[9,14] Azobenzene-driven release, unlike that regulated by many other nanomachines, can occur in aqueous environments.A second category of molecular machines, called nanovalves, has been made by attaching various caps and molecular moving parts to the pore entrances, to control the entrance and egress of molecules into and from the pores. Herein we report the use of nanoimpeller-controlled mesostructured silica nanoparticles to deliver and release anticancer drugs into living cells upon external command. Using lightactivated mesostructured silica (LAMS) nanoparticles, luminescent dyes and anticancer drugs are only released inside of cancer cells that are illuminated at the specific wavelengths that activate the impellers. The quantity of molecules released is governed by the light intensity and the irradiation time. Human cancer cells (a pancreatic cancer cell line, PANC-1 and a colon cancer cell line, SW480) were exposed to suspensions of the particles and the particles were taken up by the cells. Confocal microscopy imaging of cells containing t...

Azobenzene derivatives act as both impellers and gatekeepers when they are tethered in and on mesoporous silica nanoparticles. Continuous excitation at 457 nm, a wavelength where both the cis and trans conformers absorb, produces constant isomerization reactions and results in continual dynamic wagging of the untethered terminus. The 2 nm diameter pores are loaded with luminescent probe molecules, azobenzene motion is stimulated by light, and the photoinduced expulsion of the probe from the particles that is caused by the motion is monitored by luminescence spectroscopy. The light-responsive nature of these materials enables them to be externally controlled such that the expulsion of dye molecules from the mesopores can be started and stopped at will. These results open the possibilities of trapping useful molecules such as drugs and releasing them on demand.

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