Controlled transport of latex beads through vertically aligned carbon nanofiber membranes

Article abstract of DOI:10.1063/1.1490142

Stripes of vertically aligned carbon nanofibers (VACNFs) have been used to form membranes for size selectively controlling the transport of latex beads. Fluidic structures were created in poly(dimethylsiloxane) (PDMS) and interfaced to the VACNF structure

Full text of DOI:10.1063/1.1490142

Controlled transport of latex beads through vertically aligned carbon nanofiber
membranes
L. Zhang, A. V. Melechko, V. I. Merkulov, M. A. Guillorn, M. L. Simpson, D. H. Lowndes, and M. J. Doktycz
Citation: Applied Physics Letters 81, 135 (2002); doi: 10.1063/1.1490142
View online: http://dx.doi.org/10.1063/1.1490142
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 1
1 JULY 2002
Controlled transport of latex beads through vertically aligned carbon
nanofiber membranes
L. Zhang,a) A. V. Melechko,b) V. I. Merkulov, M. A. Guillorn,a) and M. L. Simpsona),c)
Molecular-Scale Engineering and Nanoscale Technologies Group, Oak Ridge National Laboratory, P.O. Box
2008, MS 6006 Oak Ridge, Tennessee 37831
D. H. Lowndesd)
Thin Film and Nanostructured Materials Physics Group, Solid State Division, Oak Ridge National
Laboratory, P.O. Box 2008, MS 6056, Oak Ridge, Tennessee 37831
M. J. Doktycze)
Life Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS 6123, Oak Ridge,
Tennessee 37831
͑Received 18 January 2002; accepted for publication 2 May 2002͒
Stripes of vertically aligned carbon nanofibers ͑VACNFs͒ have been used to form membranes for
size selectively controlling the transport of latex beads. Fluidic structures were created in
poly͑dimethylsiloxane͒ ͑PDMS͒ and interfaced to the VACNF structures for characterization of the
membrane pore size. Solutions of fluorescently labeled latex beads were introduced into the PDMS
channels and characterized by fluorescence and scanning electron microscopy. Results show that the
beads size selectively pass through the nanofiber barriers and the size restriction limit correlates with
the interfiber spacing. The results suggest that altering VACNF array density can alter fractionation
properties of the membrane. Such membranes may be useful for molecular sorting and for
mimicking the properties of natural membranes. © 2002 American Institute of Physics.
͓DOI: 10.1063/1.1490142͔
Sophisticated forms of nanotechnology are likely to find
wide spread use for biomedical applications. By either mim-
icking natural structures, or by interfacing biological and
synthetic nanostructures, so-called bionanotechnology may
enable devices for therapeutic treatment, disease diagnostics,
or prosthetics. Key to the use of nanotechnology is the di-
rected assembly of devices with nanoscale features. Such
nanoscale engineering is characteristic of natural systems
and will be necessary for functional use of man-made nano-
scale materials as well. Vertically aligned carbon nanofibers
͑VACNFs͒ are nanostructures whose synthesis can be
controlled.1–3 VACNFs are prepared by a catalytically con-
trolled plasma enhanced chemical vapor deposition
͑PECVD͒ process that allows for their directed assembly.
The position, diameter, shape, height, orientation, and chemi-
cal composition can be reproducibly and reliably defined.1–3
This ‘‘bottom-up’’ approach to construction can be combined
with ‘‘top-down’’ fabrication techniques to realize complex
microscale devices with functional nanoscale features. Here,
we report the construction of dense arrays of VACNFs that
serve as membranes for controlling the transport of latex
beads through microfluidic channels. Such membrane de-
vices may be useful for analytical separations of biomol-
ecules and as functional mimics of natural membranes.
For VACNF membranes, the ‘‘pore’’ size is determined
by the interfiber spacing. Both the density and area of the
membrane should define the rate and the size limits of trans-
port. This type of membrane is analogous to the artificial gel
structures described by Austin and others.4–6 In those cases,
the artificial gels were constructed from micromachined sili-
con posts and were used for the electrophoretic separation of
DNA molecules. The VACNFs described here provide the
advantage of defining nanoscale structure within microscale
and larger features using a single top-down fabrication step.
Also, VACNF growth is an additive rather than subtractive
process, so they can be grown on a wide variety of sub-
strates, including insulating materials that are often used in
fluidic applications. Furthermore, VACNFs are likely to ex-
hibit many of the same desirable mechanical properties of
related carbon materials such as multiwall carbon nanotubes
͑MWNTs͒. However unlike MWNTs, the sidewalls of VAC-
NFs are more chemically reactive and present more options
for chemical functionalization. The exquisite control of fiber
placement and morphology should enable greater separation
capabilities, extending to molecular scale specificity.
To prepare the membrane structures, VACNFs were
grown on 3 cmϫ3 cm n-type ͑100͒-oriented Si substrates. A
10-nm-thick layer of Ni–Fe ͑1:1͒ alloy on a 10-nm-thick Ti
adhesion layer was deposited on the substrates. The Ni/Fe
layer was used as a catalyst for growth of the VACNFs. The
catalyst was patterned using contact photolithography to
form 50 ␮m wide catalyst stripes or by deposition of the
catalyst through a shadow mask consisting of a blade cut slit
in Al foil resulting in irregularly shaped catalyst lines. Acety-
a͒
Also with: the University of Tennessee Material Science and Engineering
Department.
b͒
Also with: the University of Tennessee Center for Environmental Biotech-
nology.
c͒
Also with: The University of Tennessee Electrical and Computer Engineer-
ing Department; electronic mail: simpsonml1@ornl.gov
d͒
Electronic mail: lowndesdh@ornl.gov
e͒
Author to whom correspondence should be addressed; electronic mail:
doktyczmj@ornl.gov
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0003-6951/2002/81(1)/135/3/$19.00 135 © 2002 American Institute of Physics
On: Fri, 21 Nov 2014 21:30:54

136
Appl. Phys. Lett., Vol. 81, No. 1, 1 July 2002
Zhang et al.
FIG. 3. Fluorescence microscope images of ͑a͒ 100 nm beads flowing in a
100 ␮m wide channel; ͑b͒ 500 nm beads flowing in a 100 ␮m wide channel;
and ͑c͒ 750 nm beads flowing in a 50 ␮m wide channel.
FIG. 1. Scanning electron microscopy ͑SEM͒ images of a typical VACNF
forest grown from a Ni/Fe catalyst stripe.
to the silicon base and its transparency allowed for conve-
nient viewing of the experiment.
lene (C2H2) as the carbon source and ammonia (NH3) as an
etchant, at gas flow rates of 65 and 80 sccm respectively,
were used in the PECVD process. The dc plasma discharge
was operated at 100 mA and the growth temperature was
710 °C. In all cases, the growth rate and time were selected
to produce nanofibers with a height of ϳ2.2–2.4 ␮m. An
example of the resulting ‘‘forest’’ of nanofibers, randomly
spaced within the catalyst stripe, is shown in Fig. 1. Since
the same growth and catalyst conditions were used for all
samples, the morphology and density of the fiber forests
were similar. Further details on the growth and application of
VACNFs can be found elsewhere.1–3
Fluid channel structures were prepared by casting poly-
͑dimethylsiloxane͒ ͑PDMS͒ ͑Dow Sylgard 184͒ onto a sili-
con positive relief mold.7,8 The mold was prepared using
photolithography and reactive ion etching. 50 and 100 ␮m
wide channels, 2 ␮m deep and 1.5 cm long were fabricated
by pouring a 10:1 mixture ͑elastomer:curing agent͒ of PDMS
onto the mold, followed by a 65 °C cure for 1 h. The result-
ing channel structure was peeled from the mold and overlaid
on the VACNF containing silicon substrate with the PDMS
channel oriented perpendicular to the VACNF stripe as
shown in Fig. 2. The channels were constructed to be slightly
shorter than the fiber height so that the VACNFs could pre-
sumably extend into the soft PDMS lid forming a floor to
ceiling barrier. The PDMS afforded relatively good sealing7,8
The transport of fluorescently labeled latex beads was
used to assess the VACNF membrane performance. Beads of
various diameters, ranging from 100 to 1000 nm ͑Poly-
science, Inc.͒, were diluted to 0.15% in an aqueous solution
containing 1% sodium dodecyl sulfate. These solutions were
introduced at one end of the open channels, and the transport
of the beads was monitored using a Zeiss Axiovert 135 fluo-
rescence microscope. Bead flow proceeded by a combination
of capillary action and hydrostatic pressure. A high-
resolution scanning electron microscope was used to study
the morphology of the VACNFs and the distribution of latex
beads after the fluidic experiment.
The membrane function of the VACNF barrier can be
seen in Fig. 3. In the case of the 100 ␮m wide PDMS chan-
nel, beads smaller than ϳ500 nm diffused through the fiber
barrier ͓Fig. 3͑a͔͒, while larger beads were halted at the bar-
rier ͓Fig. 3͑b͔͒. Fluorescence on both sides of the barrier
indicates that the beads traveled through the VACNFs, while
fluorescence on only the input side of the barrier indicates
that the beads were halted. Beads below 500 nm were able to
travel through the barrier in the 100 ␮m wide channel, while
the transport of larger beads was halted. In all experiments,
beads collected at the entrance to the fiber barrier, even in the
case of the smallest beads ͑100 nm͒. These observations are
consistent with measurements of the interfiber spacing of
ϳ250 ϩ/Ϫ150 nm, as determined by electron microscopy.
This implies that by controlling the density of the nanofibers,
the pore size of the membrane can be controlled.
Post experiment electron micrographs ͑Fig. 4͒ demon-
strate the robustness of the nanofibers to mechanical and flu-
idic forces. Before acquiring these images, the channels were
dried and the PDMS layer was removed. There was no ap-
parent damage to the VACNFs. For experiments where beads
were observed to traverse the CNF barrier, some beads were
observed at the downstream end of the channels and trapped
within the fiber array ͓Fig. 4͑a͔͒. For cases where the
VACNF barrier completely blocked transport, beads were
observed to accumulate only at the entrance to the VACNF
membrane ͓Fig. 4͑b͔͒.
FIG. 2. ͑a͒ A top view schematic of the PDMS channel and VACNF mem-
brane. PDMS channels were placed over the nanofiber stripes. ͑b͒ A cross-
sectional view along the axis of fluid flow.
These controlled transport experiments also highlight a
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significant practical issue concerning the construction of a
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Appl. Phys. Lett., Vol. 81, No. 1, 1 July 2002
Zhang et al.
137
modulus for PDMS of ϳ7.5ϫ105 Pa.9 Incomplete penetra-
tion of the nanofibers into the PDMS also may account for
fluid leakage perpendicular to the membrane. Apparently, the
PDMS detaches from the silicon surface in this region. This
detachment distance, Ld , is on the order of microns and
should be proportional to (t3h2)1/4 where t is the thickness of
PDMS and h is the height of nanofibers.10 Therefore, a wider
gap should be expected when using taller nanofibers and
thicker layers of PDMS. The irregularly shaped fiber arrays
apparently allowed for better contact with the silicon surface.
These problems highlight the issues related to engineering on
multiple length scales and must be addressed before practical
implementation can be realized.
In conclusion, PDMS channels containing a stripe of
VACNFs were used as initial fluidic structures to character-
ize the use of VACNFs as membrane structures. Fluores-
cently labeled latex beads size selectively traveled through
the nanofiber barriers, and various widths and shapes of fiber
array had different effects on the transport behavior of the
beads. These results strongly suggest that VACNF arrays can
serve as synthetic membranes and may be employed for con-
trolling molecular transport.
The authors would like to thank Pamela Fleming for
help with sample preparation. This research was sponsored
by the Laboratory Directed Research and Development Pro-
gram of Oak Ridge National Laboratory ͑ORNL͒, managed
by UT-Battelle, LLC for the U.S. Department of Energy un-
der Contract No. DE-AC05-00OR22725 and by the Defense
Advanced Research Projects Agency. A portion of this work
was conducted at the Cornell Nanofabrication Facility ͑a
member of the National Nanofabrication Users Network͒
which is supported by the National Science Foundation un-
der Grant No. ECS-9731293, its users, Cornell University,
and Industrial Affiliates.
FIG. 4. Postexperiment SEM images of ͑a͒ 500 nm beads trapped within the
nanofiber array and ͑b͒ 750 nm beads stopped by the nanofiber array.
sealed device containing a VACNF barrier. The PDMS
sealed well along the length of the channel. However, as can
be seen in Fig. 3, fluid leaked perpendicular to the channel at
the edge of the barrier. This leakage was more pronounced
for the fiber stripes with consistent width ͑Ni/Fe catalyst pat-
tern defined with photolithography͒ compared to the irregu-
larly shaped VACNF barriers ͑Ni/Fe catalyst pattern defined
by deposition through irregularly shaped shadow mask͒. This
is most likely related to the mechanical characteristics of
PDMS. Complete penetration of the nanofibers into the
PDMS outside the region of the channel may not have oc-
curred. This could prevent a floor-to-ceiling barrier in the
channel and allow beads to travel over the nanofibers. Wider
channel structures can allow sagging of the PDMS across the
top of the nanofibers, preventing the transport of smaller
beads through this alternative path. This sagging can be on
the order of a few hundred nanometers based on a Young’s
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