A reversible molecular valve
Proceedings of the National Academy of Sciences of the United States of America
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
... 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 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 corrosion of gold into soluble gold chloride is the mechanism exploited for the use of a gold membrane in microelectromechanical systems to cover etched silicon reservoirs filled with drugs. Upon application of a positive potential, electrochemical corrosion of the gold membranes releases the drugs (9-11). Porous silica has also been used for uncontrolled release of drugs (12). Molecular machines (13-17) are attractive systems for deployment in the development of nanovalves on account of their abilities to be customized and optimized for a range of functions at the nanoscale. The attachment of supramolecular and molecular machines to nanoparticles has been achieved in a number (18-21) of different classes of systems. In our most recent research, described in this report, the movable control element that has been used to control the flow of molecules in a nanovalve is a bistable, redox-controllable rotaxane R 4ϩ , shown in Fig. 1 . Mechanically interlocked molecules of this kind (22, 23) have been studied of late for their ability to be switched either chemically or electrochemically in solution (24) , as a Langmuir-Blodgett monolayer on a silica surface (25), in a solid-state polymer matrix (26) , as a self-assembled monolayer on gold (27) , and as a monolayer sandwiched between two electrodes in crossbar memory devices (28, 29) . In R 4ϩ , the movable part of the molecule is the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT 4ϩ ) component that can be induced to move between two different recognition sites or stations on the dumbbell component. In its ground state, the CBPQT 4ϩ ring prefers to encircle the tetrathiafulvalene (TTF) station, rather than the dioxynaphthalene (DNP) one, which is separated from the TTF station by an oligoethyleneglycol chain incorporating a rigid terphenylene spacer. Because the stabilization energy between CBPQT 4ϩ and TTF is Ͼ2 kcal͞mol more than that between CBPQT 4ϩ and DNP, it follows that, in Ͼ95% of the molecules, the CBPQT 4ϩ ring encircles the TTF station. Two-electron oxidation of the TTF station with Fe(ClO 4 ) 3 to give the TTF 2ϩ dication destabilizes its interaction (coulombic repulsion) with the CBPQT 4ϩ ring, which moves to the DNP station in its electromechanical excited state. The preference for the CBPQT 4ϩ ring to encircle the DNP station instead of the TTF 2ϩ dication is even greater on account of the large difference (Ϸ8 kcal͞mol, at least) in their stabilization energies. Reduction of the TTF 2ϩ dication back to the neutral TTF unit by ascorbic acid heralds the return of the CBPQT 4ϩ ring to the TTF station in a thermally activated process (24). Experimental Procedures General Methods. Chemicals were purchased from Aldrich and used as received. 3-isocyanatopropyltriethoxysilane (3-ICPES) was purchased from Gelest (Morrisville, PA) and used after distillation. Rhodamine B was purchased from Lambda Physik (Acton, MA) and used as received. Tris(2,2Ј-phenylpyridyl)iridium(III) [Ir(ppy) 3 ]was a gift from Mark Thompson (University of Southern California, Los Angeles). The bromide 1 (30), 4-hydroxy-3,5diisopropyl-benzaldehyde (5) (31), Grignard reagent 8 (30), tosylate 10 (30), and a,aЈ-[1,4-phenylenebis(methylene)]bis(4,4Ј-bipyridium) bis(hexafluorophosphate) (12⅐2PF 6 ) (32) were all prepared according to procedures described in the literature. Solvents were dried following methods described in ref. All reactions were Freely available online through the PNAS open access option.