SEPARATION OF
MACROMOLECULES USING ULTRATHIN
SILICON MEMBRANES
By Mary Coan
Chemical Engineering Ph.D.
OUTLINE

Ultra-filtration (UF) Membranes

Nanofabricated Membranes

Ultrathin Porous Nanocrystalline Silicon (pnc-Si) Membranes

Fabrication

Physical Properties

Tunability

Molecular Separation

Proposed Future Work

Conclusion
ULTRA-FILTRATION (UF) MEMBRANES

Pressure driven membrane separation process (Image 1)

Separates particulate matter from soluble components in
the carrier fluid

Water

PEG

Blood

Pore sizes typically range from 0.01 - 0.10 µm (Image 2)

Usedfor
Forbacteria
water Reclamation
High removal capability
and most viruses, and
colloids

Smaller pore sizes result in higher removal capabilities
http://www.dow.com/liquidseps/prod/uf_index.htm
Image #1: http://www.fumatech.com/EN/Membrane-technology/Membrane-processes/Ultrafiltration/
Image #2: http://www3.ntu.edu.sg/home/DDSun/research.html
ULTRA-FILTRATION (UF) MEMBRANES

Membrane used
Most materials that are used infor
UF
are polymeric and
bacteria
are naturally hydrophobic


Polysulfone (PS)

Polyethersulfone (PES)

Polypropylene (PP)

Polyvinylidenefluoride (PVDF)
removal
Materials are blended with hydrophilic agents to
decrease hydrophoicity (Image 1)

Potentially reduces the membranes ability to be cleaned
with high strength disinfectants

Impacts removal of bacterial growth
http://www.dow.com/liquidseps/prod/uf_index.htm
Image #1: http://www.mymedicalsuppliers.com/dialysis-equipment-and-supplies/
ULTRA-FILTRATION (UF) MEMBRANES

Four types of UF membrane modules

plate-and-frame (Image1), spiral-wound (Image2), tubular
(Image3) and hollow fiber (Image3) configurations

Suited for one or more specific applications


For high purity water


Many applications can use more than one configuration
spiral-wound and hollow fiber configurations
For more concentrated solutions

plate-and-frame and tubular configurations
http://www.appliedmembranes.com/about_ultrafiltration.htm ,
Image #1-4: http://www.hydrotech.cn/English/mofenli.asp
ULTRA-FILTRATION (UF) MEMBRANES

The selection of the proper configuration depends on the
type and concentration of colloidal material or emulsion

It must take into account the flow velocity, pressure
drop, temperature, power consumption, membrane
fouling and module cost
http://www.appliedmembranes.com/about_ultrafiltration.htm
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRA-FILTRATION (UF) MEMBRANES

Limitations of typical UF membranes:

broad pore size distributions

< 1,000 times thicker than the molecules they are
designed to separate

Results in poor size cutoff properties, filtrate loss
within the membranes, and low transport rates
1. Tong, H. D. et al. Silicon nitride nanosieve membrane. Nano Lett. 4, 283–287 (2004). 2. Kuiper, S., van Rijn, C. J. M.,
Nijdam, W. & Elwenspoek, M. C. Development and applications of very high flux microfiltration membranes. J. Membr. Sci.
150, 1–8 (1998)
ULTRA-FILTRATION (UF) MEMBRANES

Nanofabricated membranes offer more precise structural
control, yet transport is also limited by μm-scale thicknesses

New class of ultrathin nanostructured membranes (Image1)

Membrane thickness ≈ the size of the molecules being separated
(10 nm)

Membrane fragility, complex and expensive fabrication
processes have prevented the use of ultrathin membranes
for molecular separations in commercial use
1. Yamaguchi, A. et al. Self-assembly of a silica-surfactant nanocomposite in a porous alumina membrane. Nature Mater. 3,
337–341 (2004). 2. Lee, S. B. & Martin, C. R. Electromodulated molecular transport in goldnanotubule membranes. J. Am.
Chem. Soc. 124, 11850–11851 (2002). 3. Tong, H. D. et al. Silicon nitride nanosieve membrane. Nano Lett. 4, 283–287. 4.
Martin, F. et al. Tailoring width of microfabricated nanochannels to solute size can be used to control diffusion kinetics. J.
Control. Release 102, 123–133 (2005).. 5. http://www.kochmembrane.com/mww_purification.html
OUTLINE

Ultra-filtration (UF) Membranes

Nanofabricated Membranes

Ultrathin Porous Nanocrystalline Silicon (pnc-Si) Membranes

Fabrication

Physical Properties

Tunability

Molecular Separation

Proposed Future Work

Conclusion
NANOFABRICATED MEMBRANES

Part of the Ultrafilitration Membranes

Fabricated using typical microelectronic
techniques

Lithography

Focused Ion Beam

Reactive Ion Etching

Sputtering

Chemical Vapor Deposition
http://www.homecents.com/h2o/ro/index.html
NANOFABRICATED MEMBRANES

Silicon Nitride Nanoseive Membrane

Nanopores, 25 nm in diameter, were directly drilled by FIB
in a 10-nm SiN membrane (110 Kx, scale bar: 50 nm).
NANOFABRICATED MEMBRANES

Perspective SEM of a filter

Square holes in the top layer are
the entrance ports

Hexagonal outline on the surface
is the result of structurally
reinforcing trenches defined in
the first phase of fabrication

Channels revealed in the cross
section are formed by the removal
of silicon dioxide grown between
the layers of polysilicon.
NANOFABRICATED MEMBRANES

Molecule-Nanofilter Interaction at the Micro(Macro)-Nano-Micro
junction

Various factors are in play to affect the transport of biomolecules (with
various shapes and sizes) through a nanopore or a nanofluidic filter
OUTLINE

Ultra-filtration (UF) Membranes

Nanofabricated Membranes

Ultrathin Porous Nanocrystalline Silicon (pnc-Si)
Membranes

Fabrication

Physical Properties

Tunability

Molecular Separation

Proposed Future Work

Conclusion
ULTRATHIN POROUS NANOCRYSTALLINE
SILICON (PNC-SI) MEMBRANES

An UF Nanofabricated Membrane

Ultrathin: 15 nm thick

Prepared using typical silicon
fabrication techniques


Lithography

Etching
Left Image: TEM image of the
porous nanostructure of a 15-nmthick membrane

Pores appear as bright spots

Nanocrystalline silicon is in grey
or black contrast.
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
FABRICATION

Silicon fabrication techniques provide control over
average pore sizes from 5nm to 25 nm, are fully
understood and readily available

Uses precision silicon deposition and etching techniques
to create the ultrathin membrane (next slide, animation)

Instead of directly patterning pores, voids are formed
spontaneously as nanocrystals nucleate and grow in a 15nm-thick amorphous silicon (a-Si) film during a rapid
thermal annealing step
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
FABRICATION PROCESS
Pnc-Si Membrane
a-Si
Oxide
a-Si
Oxide
a-Si a-Si
500 a-Si
nm thermal
oxide
Oxide
a-Si
a-Si
Oxide
~ 500 μm
(100) Silicon Wafer
500 nm thermal oxide
Step 1: Grow 500nm thick Thermal Oxide
Step 2: Pattern Backside
Step 3: Remove front oxide and deposit a 3-layer oxide/a-Si/oxide film stack
Step 4: Rapid Thermal Anneal
Step 5: Anisptropic Etching of (100) Si Wafer using EDP
Step 6: Remove Oxide Masks
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
FABRICATION

Voids span the molecularly thin membrane to create
pores

The resulting membranes cover openings several
hundred μm across in a rigid crystalline silicon frame

Can be easily handled and used
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
PHYSICAL PROPERTIES

Several characterization techniques
were used to confirm/determine the
properties of the pnc-Si membranes

Transmission Electron Microscopy
(TEM)

Refractive Index

Atomic Force Microscopy (AFM)

Mechanical Stability using a customized
holder and Optical Microscope

Refractive Index (Right Image)

For a 15-nm-thick silicon film after
deposition (a-Si) and after
crystallization (pnc-Si)
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
PHYSICAL PROPERTIES

Sputtered a-Si:
1)
High optical density, comparable to
2) Shift in optical
Properties
microelectronic quality a-Si deposited
with chemical vapor deposition (CVD)
2)
Exhibits a clear shift in optical
properties after crystallization
3)
Resonance peaks similar to crystalline
silicon after crystallization

Results are indicative of high purity
silicon films with smooth interfaces

3) Similar Peaks
TEM images of the as-deposited a-Si
show no distinguishable voids or
crystalline features
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
PHYSICAL PROPERTIES

Membranes were transferred onto
polished quartz

Atomic Force Microscopy (AFM)

confirm the accuracy of the Refractive
Index data

Measured the step height of the
membrane edge

Confirmed the 15nm thickness of a
sample membrane

Showed highly smooth surface
morphology
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
PHYSICAL PROPERTIES

Important characteristic of pnc-Si membranes is their
remarkable mechanical stability

Mechanically Stability:

Used a customized holder to apply pressure to one side of
the membrane while an optical microscope was used to
monitor deformation

Right Top and Bottom Images are optical micrographs of a
200 μm x 200 μm x 15nm membrane

no applied pressure (Top)

more than 1 atm of differential applied pressure across it for ~ 5
minutes (Bottom)

With no differential pressure, the membrane is
extremely flat (Top), and at maximum pressure (Bottom)
the membrane elastically deforms but maintains its
structural integrity throughout the duration of test.
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
PHYSICAL PROPERTIES

pnc-Si membranes exhibit no plastic deformation

Immediately return to their flat state when the
pressure is removed

Pressurization tests were cycled three times with no
observable membrane degradation

Due to their smooth surfaces and random nanocrystal
orientation

inhibit the formation and propagation of cracks
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
TUNABILITY

Pore size distributions in pnc-Si membranes are controlled by
the Rapid Thermal Annealing Process (RTP)

Nanocrystal nucleation and growth are Arrhenius-like processes
that exhibit strong temperature dependence above a threshold
crystallization temperature of approximately 700ºC in a-Si

Existing crystallization models fail to predict void formation, and
must be extended to account for how volume contraction and
material strain lead to pore formation in ultrathin membranes
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
TUNABILITY

Pore size tunability:

3 wafers with 15-nm-thick pnc-Si
membranes were processed
identically, except for the annealing
temperature
a)
Annealed at 715ºC resulted in an
average pore size of 7.3 nm
b)
Annealed at 729ºC resulted in an
average pore size of 13.9nm
c)
Annealed at 753ºC resulted in an
average pore size of 21.3 nm

Pore size and density increase
monotonically with temperature
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
TUNABILITY

Another sample annealed at 700ºC exhibited no
crystalline structure and resulted in no voids

strong morphological dependence on temperature
near the onset of crystallization

With the ability to “tune” the average pore size
pnc-Si Membranes are well suited for:

size-selective separation of large biomolecules

Examples: proteins and DNA
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION

Two common blood proteins of different molecular
weight (MW) and hydrodynamic diameter (D) were
used to test the molecular capabilities of the pnc-Si
Membrane

Bovine serum albumin, BSA (MW=67,000 (67K), D=6.8
nm), fluorescently labelled with Alexa 488

Immunoglobulin-c, IgG (MW=150 K, D=14 nm),
fluorescently labelled with Alexa 546

Free Alexa 546 dye was used as an additional low
molecular weight (MW=1 K, D < 1 nm) species
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION
Well
50 mm bead spacer
thin diffusion chamber
Fluorescent Mixture
15nm thick
membrane
PBS
Glass Coverslip
Step 1: Fill the Diffusion Chamber with 50ml Clean Buffer solution (PBS)
Step 2: Fill the Well with 3 ml of a fluorescent mixture containing BSA and Free
Alexa 546 dye
Taking a closer look at the membrane interface as time passes
one can see the Alexa 546 dye (Species 1) flows through the
pnc-Si Membrane into the diffusion chamber while the larger
Protein (BSA, Species 2) remains in the well
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION
Images of the membrane edge were taken every 30s
 Spreading of the fluorescence signal from the
membrane edge to the diffusion chamber during
separation, is illustrated in the two false-color images
below

False
Color
images
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION
Alexa Dye vs BSA


Results from the separation of free Alexa
546 dye and BSA using membrane A
Dye passes freely through the membrane
while BSA is almost completely blocked.
BSA vs IgG


Results from the separation of IgG and BSA
through membrane B at 1 mM
concentration
BSA diffuses through the membrane 0.4
times more rapidly than IgG
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION

BSA diffuses through the membrane more rapidly than IgG

The diffusion coefficients for these molecules are within 25% of each
other

The measured rate difference indicates that pnc-Si membranes hinder
IgG diffusion relative to BSA diffusion

The increased cut-off size of membrane B allows for a increase in
BSA diffusion by 15x compared to membrane A

BSA and IgG were retained behind membranes with maximal pore
sizes 2x as large as their reported hydrodynamic diameters

electrostatic interactions and protein adsorption might create an
effective pore size smaller than that measured by TEM
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION

Negatively charged Alexa 488 dye in
the presence and absence of high salt
concentrations during separation

diffusion of the Alexa dye drops by a
factor of 10 when experiments are
conducted in deionized water

electrostatic repulsion between the dye and
a negatively charged native oxide layer on
the surface of the pnc-Si membranes

High salt concentrations increase
throughput by screening surface and
solute charges
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION

Charge effects

Modified membranes to carry abundant negative and positive surface
charges (Image 1)


In low ionic strength solutions

Positively charged membranes blocked only positively charged dyes

Negatively charged membranes blocked only negatively charged dyes
In high ionic strength phosphate buffered saline solutions

Stronger electrostatic interactions that reduce the effective pore size were
expected

Results in pnc-Si membranes that can be functionalized to
separate similarly sized molecules on the basis of their charge
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION

Factors affecting the Effective Pore Size of the pnc-Si
Membrane

Protein adsorption to the pore walls will reduce the effective
pore size

BSA adsorption shrinks, but does not occlude, the largest membrane
pores by as much as 7nm

Charge Effects

Uncertain relationship between a protein’s physical size and
hydrodynamic dimensions may reduce effective pore size

Behavior of water (hydrogen bonding) in nanoscale pores may
reduce pore size
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION

Given the long passage-times of molecules through thick
membranes, it is significant that filtrate molecules
appear downstream of pnc-Si filters within minutes

Quantified the transport through pnc-Si membranes

fluorescence microscopy experiments with bench-top
experiments

Easily remove and assay the Alexa 546 dye that diffused across
membrane A from a 100 mM starting concentration using a similar
unstirred geometry
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION

Dye diffuses over 9x faster through pncSi membrane A than dialysis
membranes

pnc-Si membrane A exhibits an initial
transport rate of 156 nmol cm-2h-1 that
slows as the 3 ml source volume depletes

Due to the lowering of the concentration
gradient across the barrier

For membrane C an increase of 10% in
dye transport was measured relative to
membrane A, despite porosities differing
by 29x (0.2% versus 5.7%)
Dye diffusion through pnc-Si membranes
compared to diffusion through standard
regenerated cellulose dialysis membranes
(Spectra/Por 7 dialysis membrane,
molecularweight cut-off550K)
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
ULTRATHIN PNC-SI MEMBRANES:
MOLECULAR SEPARATION

Dye or small molecule transport is essentially
unhindered by pnc-Si membranes

as porosities far lower than that of membrane A
should theoretically allow greater than half-maximal
diffusion through an infinitely thin porous barrier

Diffusion through the commercial membrane is
the rate-limiting transport process

Due to the observed increase in the diffusion rate
over conventional dialysis membranes

Diffusion through the bulk solution is ratelimiting for the pnc-Si membrane experiment

Enhancement of the transport rate is expected in
systems that implement active mixing, or forced
flow
Christopher C. Striemer, Thomas R. Gaborski, James L. McGrath & Philippe M. Fauchet “Charge- and size-based separation
of macromolecules using ultrathin silicon membranes” Nature, Vol 445| 15 February 2007| doi:10.1038/nature05532
OUTLINE

Ultra-filtration (UF) Membranes

Nanofabricated Membranes

Ultrathin Porous Nanocrystalline Silicon (pnc-Si) Membranes

Fabrication

Physical Properties

Tunability

Molecular Separation

Proposed Future Work

Conclusion
PROPOSED FUTURE WORK


More robust study of separation

Not limited to only two proteins at one time

Test using proteins commonly found in blood
Determine the effects of different concentrations of
proteins

Increase concentrations to those similar in Blood and
beyond

Integration into microfluidic devices

Silicon-based platform opens several avenues for
future developments

surface functionalization using well-established
chemistries

modify surface charge

reduce protein adsorption

protect the silicon from chemical attack in harsh
environments.
PROPOSED FUTURE WORK

Effects of large scale production on
the physical properties of the device



Determine low-cost feasibility
Environmental effects

Separation properties of the membrane

Physical properties of the membrane
Determine methods to “clean” the
membranes

if high-cost production
Image: http://www.rikenresearch.riken.jp/eng/frontline/4950
OUTLINE

Ultra-filtration (UF) Membranes

Nanofabricated Membranes

Ultrathin Porous Nanocrystalline Silicon (pnc-Si) Membranes

Fabrication

Physical Properties

Tunability

Molecular Separation

Proposed Future Work

Conclusion
CONCLUSION

First use of ultrathin nanomembranes for size-based molecular
separations

Separation of BSA and IgG suggests that pnc-Si can be used for
membrane-based protein fractionation

Are too close in size to be efficiently separated using conventional
membrane processes

Standard membranes cause a lot of the filtrate species to be
lost


Due to the high surface area and tortuous porosity
pnc-Si membranes should allow for recovery of both the
retentate and filtrate fractions to enable membrane-based
chromatography
CONCLUSION

pnc-Si membranes are expected to be highly efficient for
separation processes

Due to the thickness and minimal filter surface area

Diffusion transport rate of 156 nmol cm-2 h-1 for Alexa 546 dye

More than 10x faster than thick nanofabricated membranes

0.9 x faster than the authors measurements through dialysis
membranes

pnc-Si membranes with fixed charges

Can be used to separate similarly sized molecules with
different charges

adds another dimension of control for highly efficient molecular
separations