Carbon nanotubes have three key attributes that make them of great interest for novel membrane applications 1) atomically flat graphite surface allows for ideal fluid slip boundary conditions and extremely fast flow rates 2) the cutting process to open CNTs inherently places functional chemistry at CNT core entrance for chemical selectivity and 3) CNT are electrically conductive allowing for electrochemical reactions and application of electric fields gradients at CNT tips. The CNT membrane, with tips functionalized with charged molecules, is a nearly ideal platform to induce electro-osmotic flow with high charge density at pore entrance and a nearly frictionless surface for the propagation of plug flow. Through diazonium electrochemical modification we have successfully bound anionic surface charge to CNT tips and along CNT cores. Both cationic and neutral fluxes through the CNT membrane are observed as a function of bias. Ionic mobilities are enhanced by a factor of ~5-8 with rectification seen for large anion/cation mixtures proving conducting through CNTs. High electro-osmotic flows of 0.05 cm/s at negative 300mV bias are seen. The analysis of charged dye molecules (Ru(bpy)3^2+) showed a factor of 50 enhancement in on state (-300mV) and of state (+300mV) primarily due to electrophoretic effects. Neutral molecules (caffeine) within ionic solutions showed a factor of 10 enhancement in on state (-300mV) and of state (+300mV). The enhanced transport of neutral species under bias is the primary proof of the existence of an electro-osmotic processes. The optimization of this process occurred after 2 steps; 1) use of CNTs with diameter under 2nm to avoid the effects of screening (Debye length ~1nm) and 2) use of quad-valent charged dye tethered to CNT entrance and core. Use of the electro-osmotic phenomenon for responsive/programmed transdermal drug delivery devices is discussed with the voltage gated delivery of clonidine and nicotine across CNT membrane at therapeutically useful fluxes.
- Carbon Nanotube Membranes,