J. Phys. Chem. B 2010, 114, 11117–11126
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Confinement Effects on Monosaccharide Transport in Nanochannels A. Ziemys,† A. Grattoni,† D. Fine,† F. Hussain,‡ and M. Ferrari*,†,§,| Department of Nanomedicine and Biomedical Engineering, The UniVersity of Texas Health Science Center at Houston, Houston, Texas 77030, UniVersity of Houston, Houston, Texas 77204, Department of Experimental Therapeutics, The UniVersity of Texas M. D. Anderson Cancer Center, Houston, Texas 77030, and Department of Bioengineering, Rice UniVersity, Houston, Texas 77030 ReceiVed: April 19, 2010; ReVised Manuscript ReceiVed: June 23, 2010
Transport theories based on the continuum hypothesis may not be appropriate at the nanoscale in view of surface effects. We employed molecular dynamics simulations to study the effects of confinement and concentration on diffusive transport of glucose in silica nanochannels (10 nm or smaller). We found that glucose modifies the electrical properties of nanochannels and that, below 5 nm in channel height, glucose adsorption and diffusivity are significantly reduced. With increasing concentration, the diffusivity is reduced linearly in the bulk, while it is reduced nonlinearly at the interface. The effective diffusivity reduction is related to the interface thickness, which can be 2-4 nm depending on concentration, and has an unexpected reduction at low concentrations. Results suggest that nanochannels present a one-dimensional cage environment that affects diffusivity in a fashion similar to cage-breaking diffusion. Our simulation results, consistent with the experimental observations presented here, suggest that nanoconfinement is the essential cause of the observed altered fluid diffusive transport, not accounted for by classical theories, because of coupling of confinement and concentration effects. Introduction The first silicon nanochannels, integrated in chips and membranes, were developed more than a decade ago.1,2 They prepared the way for nanofluidics, which soon merged into nanoelectromechanical systems (NEMS) after with applications for therapeutics and diagnostics.3,4 Silicon nanochannels offered new methods of controlled drug delivery that relied exclusively on transport in nanoconfined liquid.2,5 The characteristic confinement length found in NEMS ranges from 100 nm down to a few nanometers. The fluid structure at the interface is affected by the presence of both geometrical and electrical features.6,7 If a nanochannel height is comparable to the Debye length of an electrolyte solution, the electrical double layer (EDL) can affect a substantial portion of the nanoconfined fluid volume. In the case of simple symmetrical monovalent salts, the Debye length is approximately 10 nm at concentrations of 10-3 M and 1 nm at 10-1 M, and at very low charge concentrations the Debye length may be up to 100 nm. The EDL is a consequence of surface charges on the solid; modeling results suggested that even electrically neutral surfaces affect the distribution of ions and electroosmotic flows in nanochannels.8 Therefore, the layers of fluid associated with surface interfaces can be significant as compared to the rest of the nanoconfined fluid, and in fact may occupy the entire nanochannel interior. Devices with nanochannels rely on convective (active) or diffusive (passive) transport. Convective transport is driven by gradients of mechanical or electrical forces, such as in electrophoresis or electroosmosis.9 However, at the nanoscale, the * To whom correspondence should be addressed. Address: 1825 Pressler, Suite 537, Houston, TX 77030; tel: (713) 500-2470; fax: (713) 500-2462; e-mail:
[email protected]. † The University of Texas Health Science Center at Houston. ‡ University of Houston. § The University of Texas M. D. Anderson Cancer Center. | Rice University.
Reynolds numbers are low (typically
