Actuatable Membranes Based on Polypyrrole-Coated Vertically Aligned Carbon Nanofibers
L. Fletcher1,2 S. T. Retterer3,5 T. E. McKnight4 A.
V. Melechko1,5 J. D. Fowlkes2,5
M. L. Simpson1,2,5 and M. J. Doktycz3,5
1Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.;
2Materials Science and Engineering Department, University of Tennessee, 434 Dougherty Hall, Knoxville, Tenn.;
3Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.;
4Measurement Science & Systems Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.;
5Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tenn.
The fabrication and characterization of synthetic, actuatable, nanoporous membranes has been achieved. Nanoscale features that emerge during synthesis of vertically aligned carbon nanofibers (VACNF) were tailored further by controlled polymerization of a polypyrrole (pPy) surface layer. Volume changes in the resulting hybrid nanostructures can be modulated by application of an electric field. An electric charge on the membrane causes ionic flow and subsequent swelling of the material. When a vertically aligned array of these coated nanofibers was integrated with fluidic and electronic elements, nano-actuators were created that were capable of reversibly controlling membrane pore size. Examination of planar pPy films by atomic force microscopy during the application of cyclic voltages revealed that pPy volume is stable for prolonged periods (400 s) and reproducible over multiple actuations. Coating the VACNF membrane components with pPy enabled modulation of membrane permeability by physically reducing the interfiber spacing and the resultant, nanometer-scale pore size. Dynamic switching between open and restricted flow states is facilitated by external electrical control. Application of -1.0 V bias to the underlying CNF electrode leads to swelling of the pPy coating and reduced permeability of the membrane. Removal of the bias restores membrane permeability. Altered transport of 50 to 200 nm fluorescent-labeled latex beads, streptavidin, and fluorescein isothiocynate was demonstrated by actuation of the nanoscale pores. The reversible swelling of the channels could be used to gate or induce flow in microfluidic channels.
Nanoporous membranes are applicable to a variety of research fields due to their ability to selectively separate molecules with high efficiency. Of particular interest are methods for controlling membrane selectivity through externally applied stimuli and integrating such membrane structures within multiscale systems. Membranes comprised of deterministically grown, VACNF are compatible with these needs. VACNF membranes can regulate molecular transport by physically selecting species as they pass between the fibers. Defined interfiber spacing allows nanoscale control of membrane pore structure and resultant size selectivity. Subsequent physical or chemical modification of VACNF structures enables the tuning of physical pore size and chemical specificity allowing further control of membrane permeability. The membranes are relatively thin, as compared to pores in oxide or polymer films, yet are highly robust. Because of the short transport distances, species can diffuse rapidly through the membrane. Carbon nanofiber, actuatable polymer hybrid membranes represent the first steps toward the creation of dynamic membrane structures, capable of reversibly controlling transport on the molecular scale. Reversible control of nanoscale volume structures may find multiple uses in applications requiring fluidic flow control, molecular separations, chemical dosing, or nanoscale actuation.
B. L. Fletcher, S.T. Retterer, T. E. McKnight, A. V. Melechko, J. D. Fowlkes, M. L. Simpson, M. J. Doktycz, “Actuatable Membranes Based on Polypyrrole-Coated Vertically Aligned Carbon Nanofibers,” ACS Nano 2, 247-254 (2008).
This research was supported by NIH Grant EB000657. A portion of this research was conducted at the Center for Nanophase Materials Sciences (User Project CNMS2006-064), which is sponsored by the Division of Scientific User Facilities, Department of Energy. A.V.M. and M.L.S. acknowledge support from the Division of Materials Sciences and Engineering, Department of Energy.