Design Concept and Potential Applications:
The present design (see figure 1) consists of a silicon block and cantilever, which is functionalized on its top surface to provide a layer where electrostatic repulsion can take place. We initially chose to functionalize the cantilever with a covalently bonded organic monolayer made of acrylic acid, although other possibilities are discussed below (such as the electrostatic deflection of charged metallic mono- or bilayers on the surface of the cantilever).
The organic monolayer can be assembled on the surface through hydrogenation of the cantilever, followed by thermal reaction of the hydrogenated surface with the olefin of the acid. The silicon block can be etched to its final dimensions, and then its lower part can be perforated so that a SWCNT can be inserted through the perforation. The SWCNT is used as the fluid transport conduit, which may be connected to other devices as part of a larger system. The choice of a covalently bonded monolayer ensures that the structure of the device will be stable and will not be subject to undesirable changes by the solution environment.
Figure 1: Side view of the nanomechanical valve, showing the silicon block construction, the acrylic acid monolayer on the top surface and a 17,17 SWCNT inserted through the silicon block. The model shown contains approximately 76,500 atoms and its main body (silicon block) is 35 nm long.
Depending on whether the discharge end of the valve is fixed or movable, two designs are considered here (see figure 2): (a) an "in-line" design, where the discharge end of the SWCNT is connected to another device and therefore it is not allowed to move. In this case, it is required that there be a surface on which the SWCNT rests, so that the action of the cantilever "crimps" the SWCNT against that surface in order to interrupt the flow through it, and (b) a "free-end" design, where the position of the discharge end of the SWCNT is not constrained, and the action of the cantilever deflects the SWCNT past the point of buckling, thus interrupting the flow.
Figure 2: Two types of valve design according to the mobility of the discharge end of the nanotube: "in-line" and "free-end" designs.
The device shown in figure 1 is designed to work in an aqueous environment (although this is not the case for systems relying on the deflection of charged metallic layers). If the pH of the surrounding environment is low, most of the molecules on the monolayer will be protonated and as a result there will not be a significant net charge on the monolayer. There may be a slight stress on the surface due to the van der Waals interactions between monolayer molecules, which may very slightly deflect the cantilever up or down, but this will have only a negligible effect on the flow through the SWCNT. If the pH of the surrounding environment is high, a certain amount of molecules on the monolayer will deprotonate, resulting in a net negative charge on the surface of the cantilever. The excess charge will cause a compressive stress that will deflect the cantilever downward , thus exerting pressure on the SWCNT and interrupting the fluid flow. Since changes in pH are reversible (by addition of acids and bases) and the device structure is inert to the environment, the process is repeatable.
It is also worth noting that the section of the monolayer assembled behind the cantilever, on the non-deflecting part of the valve (main silicon block), is a necessary design element in order to force the cantilever to deflect downwards. If the monolayer stopped behind the base, the cantilever would be relatively free to curl in an inverted "U" shape rendering inadequate to interrupt the flow through the SWCNT. Basically, the section behind the cantilever overcomes most of the upward torque around the base, which comes from the force exerted on its tip by the SWCNT.
Short term, our valve design has multiple potential uses in nano-fluidics, including fields such as medicine, biology, environmental engineering, micro- or nano-engines, ink-jet printing, and any other application where fluid transport is desired at the nanoscale level. In addition, this device can be set up for self-regulated dosing of compounds (see figure 3). This can be achieved by connecting the device to a reservoir containing the reactant, so that after flowing out through the SWCNT, the reactant changes the pH of the surrounding environment, thus causing the valve to close until the pH has returned to the level that allows it to open again. The process could continue until the reservoir is completely empty.
Figure 3: Self-regulated dosing of acid and basic compounds (e.g. drugs)
Long-term we envisioned the control valve presented here as a key element at the interface between micro and nano-scale devices. For example, fluid transport between micro-storage tanks, for reactants and products, and position controlled nanosynthesis devices will require control mechanisms such as the valve designed here to regulate the throughputs of fluid species (see Figure 4).
Figure 4: Conceptual drawing of a molecular manufacturing system showing the controlled delivery of reactants from a microstorage unit to the nanoassembly unit through a SWCNT. It is assumed that the reactant is in the fluid state. The flow is controlled by the device described in this paper. The size of this device (~22.5 – 45 nm) makes it a suitable interface between micro and nanosystems. The drawing , not drawn to scale, also shows the order of magnitude of the dimensions of each component.
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