A reversible molecular valve

Abstract

In everyday life, a macroscopic valve is a device with a movable control element that regulates the flow of gases or liquids by blocking and opening passageways. Construction of such a device on the nanoscale level requires (i) suitably proportioned movable control elements, (ii) a method for operating them on demand, and (iii) appropriately sized passageways. These three conditions can be fulfilled by attaching organic, mechanically interlocked, linear motor molecules that can be operated under chemical, electrical, or optical stimuli to stable inorganic porous frameworks (i.e., by self-assembling organic machinery on top of an inorganic chassis). In this article, we demonstrate a reversibly operating nanovalve that can be turned on and off by redox chemistry. It traps and releases molecules from a maze of nanoscopic passageways in silica by controlling the operation of redox-activated bistable [2]rotaxane molecules tethered to the openings of nanopores leading out of a nanoscale reservoir.controlled release ͉ nanomachine ͉ nanovalve P rogress in designing and synthesizing nanovalves that control access to and from the pores in inorganic materials is proceeding apace. Our previous work (1), involving the fabrication of pseudorotaxane-derivatized, nanostructured thin films, demonstrated a reusable but irreversible nanovalve that acts like a cork in a bottle, opening the orifices of a two-dimensional hexagonal array of cylindrical pores and allowing the contents to spill out. Other examples, using more traditional control mechanisms, have been described in the literature. Based on photodriven motions involving cis-trans isomerizations of NAN double bonds, azobenzenes tethered to the walls of the nanoporous membranes function (2) as a regulatory mechanism for mass transport through the channels of nanoporous materials. Using the intermolecular dimerization of tethered coumarins, dimers act (3, 4) as a net to control the access to and from the pores of derivatized MCM-41 upon photoactivation and subsequent dedimerization. Chemically linked CdS nanoparticles on derivatized mesoporous silica nanospheres function (5) as caps to control the release of chemicals from the pores. Although, in the latter two examples, covalent bonds are broken to control the release of the pore's contents, the first instance is one in which a change in a molecule's configuration, and hence shape, does the trick. Based on pH-sensitive intermolecular hydrogen bonds between poly-(ethyloxazoline) and poly(methacrylic acid), a polymer gel can be eroded electrically, leading to the release of a trapped insulin load (6). A heat-responsive polymer, poly(N-isopropylacrylamide) (PNIPAAm), has also been used (7) as the source of hindrance at the pores' orifices to modulate the transport of solute, albeit in an irreversible fashion. By using monolithic copolymers to define pores with tunable sizes ranging from tens to thousands of nanometers, PNIPAAm has been shown (8) to behave as a reversible valve in microfluidic chips. The electrochemical corrosio...

Autonomous artificial nanomotor powered by sunlight

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Efficient production of [ n ]rotaxanes by using template-directed clipping reactions

“…These molecular compounds are usually synthesized by a template-directed approach (22) that depends on molecular recognition and self-assembly processes. Recently, their potential applications as molecular switches for nanoelectronics (23, 24) and molecular actuators for constructing artificial muscles (25), for fabricating smart surface materials (26), and for controlling the nanoscale release of molecules trapped in mesoporous silica (27)(28)(29) were demonstrated. Polyrotaxanes and well defined, homogeneous oligorotaxanes, in which the recognition sites on a dumbbell (an axle terminated by bulky stoppers) are encircled by large rings or macrocycles (wheels) by dint of molecular recognition, have become (30-36) one of the most intensively investigated subjects in mechanical chemistry.…”

Drug delivery systems based on nucleic acid nanostructures

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