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Charge- and size-based separation of macromolecules using ultrathin silicon membranes

Abstract

Commercial ultrafiltration and dialysis membranes have broad pore size distributions and are over 1,000 times thicker than the molecules they are designed to separate, leading to poor size cut-off properties, filtrate loss within the membranes, and low transport rates1,2. Nanofabricated membranes have great potential in molecular separation applications by offering more precise structural control3,4, yet transport is also limited by micrometre-scale thicknesses5. This limitation can be addressed by a new class of ultrathin nanostructured membranes where the membrane is roughly as thick (10 nm) as the molecules being separated, but membrane fragility and complex fabrication have prevented the use of ultrathin membranes for molecular separations1. Here we report the development of an ultrathin porous nanocrystalline silicon (pnc-Si) membrane using straightforward silicon fabrication techniques that provide control over average pore sizes from approximately 5 nm to 25 nm. Our pnc-Si membranes can retain proteins while permitting the transport of small molecules at rates an order of magnitude faster than existing materials, separate differently sized proteins under physiological conditions, and separate similarly sized molecules carrying different charges. Despite being only 15 nm thick, pnc-Si membranes that are free-standing over 40,000 μm2 can support a full atmosphere of differential pressure without plastic deformation or fracture. By providing efficient, low-loss macromolecule separations, pnc-Si membranes are expected to enable a variety of new devices, including membrane-based chromatography systems and both analytical and preparative microfluidic systems that require highly efficient separations.

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Figure 1: Fabrication and pore morphology of pnc-Si membrane.
Figure 2: Physical properties of pnc-Si membranes.
Figure 3: Tunability of pnc-Si membrane pore size for molecular separation experiments.
Figure 4: Molecular separation and transport rates through pnc-Si membranes.

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Acknowledgements

We thank R. Krishnan for assistance with ellipsometry and AFM, and for useful discussions; J. Snyder for assistance in preparing protein-treated membrane samples for TEM; B. McIntyre for assistance with microscopy; P. Osborne for help with equipment fabrication; and the Laboratory for Laser Energetics at the University of Rochester for providing access to their spectroscopic ellipsometer. Silicon microprocessing was conducted at the Hopeman Microfabrication Facility at the University of Rochester and the Semiconductor and Microsystems Fabrication Laboratory (SMFL) at the Rochester Institute of Technology. This work was partially supported by a Johnson & Johnson Award to the University of Rochester Medical School’s Discovery Concept Fund.

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Correspondence to Christopher C. Striemer.

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Supplementary information

Supplementary Information

This file contains Supplementary Figures 1-2 and Legends and Legends for Supplementary Movies 1 and 2. Supplementary Figure 1 presents the relative transport rates of a positive and negative dye through pnc-Si membranes functionalized with positive and negative surface chemistries. Supplementary Figure 2 presents evidence that the effective pore size of untreated pnc-Si may be reduced by a monolayer of adsorbed BSA. (PDF 1005 kb)

Supplementary Movie 1

This file contains Supplementary Movie 1, which shows the simultaneous diffusion of Alexa 546 dye, and BSA through a pnc-Si membrane, illustrating the relative transport rates. (MOV 42 kb)

Supplementary Movie 2

This file contains Supplementary Movie 2, which shows the simultaneous diffusion of BSA and IgG through a pnc-Si membrane, illustrating the relative transport rates. (MOV 37 kb)

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Striemer, C., Gaborski, T., McGrath, J. et al. Charge- and size-based separation of macromolecules using ultrathin silicon membranes. Nature 445, 749–753 (2007). https://doi.org/10.1038/nature05532

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