Upload
zhiqiang-chen
View
222
Download
0
Embed Size (px)
Citation preview
8/6/2019 Highly efficient electroosmotic flow through functionalized carbon nanotube
http://slidepdf.com/reader/full/highly-efcient-electroosmotic-ow-through-functionalized-carbon-nanotube 1/8
Highly efficient electroosmotic flow through functionalized carbon nanotube
membranesJi Wu, Karen Gerstandt, Mainak Majumder,† Xin Zhan and Bruce J. Hinds*
Received 22nd March 2011, Accepted 16th May 2011
DOI: 10.1039/c1nr10303b
Carbon nanotube membranes with inner diameter ranging from 1.5–7 nm were examined for enhanced
electroosmotic flow. After functionalization via electrochemical diazonium grafting and carbodiimide
coupling reaction, it was found that neutral caffeine molecules can be efficiently pumped via
electroosmosis. An electroosmotic velocity as high as 0.16 cm sÀ1 VÀ1 has been observed. Power
efficiencies were 25–110 fold improved compared to related nanoporous materials, which has important
applications in chemical separations and compact medical devices. Nearly ideal electroosmotic flow
was seen in the case where the mobile cation diameter nearly matched the inner diameter of the single-
walled carbon nanotube resulting in a condition of using one ion is to pump one neutral molecule at
equivalent concentrations.
Introduction
Electro-osmotic flow (EOF) can be the primary method to
control fluid flux for applications including lab-on-chip diag-
nostics, medical implants, drug delivery and chemical separa-
tions.1–6 The key advantage of EOF compared to pressure driven
pumps is the ability for compact integration of numerous flow
paths that can be electrically addressed. Unfortunately EOF is
a relatively inefficient process based on interfacial flow that hasshown limited but promising success in device applications.7–9
EOF relies on a high density of fixed surface charge (usually
anionic) that allows only the counterion (cation) to flow in the
direction of electric field pumping neutral solvent and solutes in
the flow without the interference of free anions moving in the
opposite direction. This is a surface phenomenon seen only
within the Debye screening length ($2 nm) of the surface, and
hence requires a nanoporous membrane or channel. The ideal
conditions for efficient EOF are to have high surface charge
density, small diameter and a slippery interface to efficiently
transmit flow through the membrane.
Currently, significant scientific attention has been focused on
carbon nanotube membranes due to their potential applicationsin chemical separations, water purification, chemical sensors and
drug delivery.10–16 Carbon nanotubes (CNTs) have three key
attributes (1) atomically flat graphitic planes allowing fast fluid
flow (2) ability to covalently functionalize entrances of CNT
pores with charged groups and (3) they are electrically conduc-
tive allowing for the concentration of electric field at the CNT
tip. Molecular Dynamic (MD) simulations predicted a 10 000
fold fluid flow enhancement over the slippery graphitic inter-
face17 and the ability to support ionic flow.18,19 Both predictions
were experimentally confirmed11–13 and are the basis for exam-
ining EOF efficiency within CNT membranes. By placing anionic
charged groups at the entrance to the CNT to exclude mobile
anions, cations alone will be accelerated down the tube and
generate EOF. The primary hypothesis of this report is that the
slippery graphite core, that supports fast pressure driven flow,
will yield high efficiency EOF. Power efficiency is a particularlyimportant parameter for compact medical devices such as
programmable transdermal drug delivery or in large scale
chemical separations.
CNT membranes can be fabricated via several
approaches11,12,20–23 and the electroosmotic flow had been
initially examined in non-graphitic CNTs. Sun and Crooks
embedded a single carbon nanotube into an epoxy matrix and
then microtomed CNT membranes.22 However no enhanced
fluid flux or electroosmosis compared to classical materials was
observed since the diameter of their CNTs was large (500 nm)
and they were not highly ordered graphitic tubes.22 A chemical
vapor deposition (CVD) method was used to coat a layer of
amorphous carbon (a-C) on the wall of the Anodized AluminaOxide (AAO) membrane template to obtain a-CNT membranes.
This electro-osmotic investigation showed significant improve-
ment in the electro-osmotic velocity however this was signifi-
cantly less than the electrophoretic flow of charged ions21
presumably because the tubes were much larger (120 nm i.d.)
than the Debye screening length ($1.5 nm) of the pore surface.
Using ordered graphitic MWCNTs Majumder et al. found that
the conformational change of gate-keeper molecules can be
utilized to effectively separate chemical species of different sizes
but did not directly study the phenomenon of electroosmosis.14
Recently functionalized DWCNT membranes have been used for
Department of Chemical and Materials Engineering, University of Kentucky, Kentucky, 40506, USA. E-mail: [email protected]
† Current address: Dept. Mech. Engr. Monash University, Australia.
This journal is ª The Royal Society of Chemistry 2011 Nanoscale
Dynamic Article LinksC<Nanoscale
Cite this: DOI: 10.1039/c1nr10303b
www.rsc.org/nanoscale PAPER
View Online
8/6/2019 Highly efficient electroosmotic flow through functionalized carbon nanotube
http://slidepdf.com/reader/full/highly-efcient-electroosmotic-ow-through-functionalized-carbon-nanotube 2/8
very power efficient electrophoretic/osmotic pumping of nicotine
through human skin at therapeutically useful doses6 and
extremely high Li+ and K+ mobilities are reported in SWCNT
channels.24 Needed is an electroosmotic study on ordered
graphitic CNTs to examine whether there are large gains in EOF
power efficiency due to the slippery CNT interface.
We report herein efficient electro-osmotic flow (EOF) across
CNT membranes that were fabricated by microtoming multi-
walled CNTs (7 nm i.d.) and single walled CNT (1.5 nm i.d.)epoxy composites. The EOF was studied by measuring the fluxes
of both charged and neutral probe molecules through the
membranes under external electric field. An electrochemical
diazonium grafting method was employed to enhance the
surface anionic charge density of CNTs, which was further
enhanced via a carbodiimide coupling reaction with the dye
molecule containing four negatively charged SO3À groups.
Improvements in EOF power efficiencies of 25–40 folds are seen
in the system.
Experimental section
Fabrication of carbon nanotube (CNT) membranes
CNT membranes were fabricated using an approach similar to
a prior report for single CNT membrane flow22 and modified for
a high CNT loading. To describe it briefly, multi-walled CNTs
with an average core diameter of $7 nm and length of 150 mm
were prepared via a chemical vapor deposition (CVD) approach
using ferrocene/xylene as the feeding gas.25 SWCNTs were
purchased from CheapTubes.com. Next, 5 wt% CNTs were
mixed with Epon 862 epoxy resin (Miller Stephenson Chem.
Co.), hardener methylhexahydrophthalic anhydride (MHHPA,
Broadview Tech. Inc.) and 0.1 g surfactant Triton-X 100
(Sigma) using a ThinkyÔ centrifugal shear mixer. As-prepared
CNTs–epoxy composite was cured at 85 C according to the
commercial epoxy procedure before being cut into CNT
membranes using a microtome equipped with a glass blade.
The typical thickness of the as-cut CNT membrane is about
5 microns. Modest pullout of only a few percent of the
membrane thickness is seen on regions of the sample after
microtoming. Finally, the residual epoxy on the tips of CNTs
was removed by H2O plasma oxidation.
Synthesis of 4-carboxy phenyl diazonium tetrafluoroborate
The diazonium compound was synthesized following a method
reported by D’Amour and Belanger.26 2.74 g (0.02 mol) of
p-aminobenzoic acid (Aldrich) was dissolved in 20 ml of water,
which was heated at about 50 C until it was dissolved. 0.044 molof concentrated HCl was added dropwise to the solution, fol-
lowed by cooling of the solution to À3 C. 0.022 mol of NaNO2
(Sigma) in10 mlof water at 0 C was added slowly to the solution
in $30 minutes followed by 1 h reaction time. The solution was
filtered, and then 0.022 mol of NaBF4 (Aldrich) solution was
added to the filtrate at À3 C. A light yellow precipitate was
formed. The precipitate was filtered and washed with ice water
and cold ether. The product was dried in vacuum and preserved
in a desiccator at 4 C. 1H NMR (400 MHz, CDCl3, d): (4-car-
boxyphenyl)-diazonium tetrafluoroborate, two doublets at
8.87–8.83 and 8.46–8.42 ppm.
Functionalization of CNT membranes
As-prepared MWCNT membranes were grafted with benzoic
acid by electrochemically reducing 5 mM 4-carboxy phenyl
diazonium tetrafluoroborate in 0.1 M HCl and 0.1 M KCl
electrolyte at À0.6 V for 4 minutes. The MWCNT membrane
was grafted using a static approach without internal fluid flow,
and named S.G. CNT.14 After the grafting reaction, the
membrane was thoroughly rinsed using de-ionized (DI) water,0.1 (M) KCl, and IPA to dissolve any unwanted byproducts. In
the next step, Direct Blue 71 dye was coupled to benzoic acid via
one step carbodiimide chemistry method: 10 mg of EDC and
5 mg Sulfo-NHS were dissolved into 4 ml of 50 mM Direct Blue
71 (dye, Aldrich) in 0.1 (M) MES buffer for 12 h at ambient
temperature, after which the membrane was washed with 0.1 M
MES buffer, 0.1 M KCl solution and DI water to remove the
excess reagents. These functionalized membranes are referred to
as S.G. MWCNTs–dye, as diagrammed in Fig. 1d. For SWCNT
membranes, functionalization was achieved by H2O plasma
oxidation to produce carboxylate groups at CNT tips.11,27
Functionalization of AAO membranes
Anodic aluminium oxide (AAO) membrane with an average pore
diameter of 20 nm and a thickness of 60 mm was purchased
from Whatman Company. AAO membranes were immersed in
20 ml toluene containing 2 ml 3-(triethoxysilyl)propylamine
(C6H17NO3Si, Sigma) that was refluxed overnight with argon gas
protection. The amine functional group was further coupled to
sulfoacetic acid (HO3SCH2CO2H, Sigma Aldrich) using the
same one step carbodiimide chemistry as described above to
make the AAO membrane negatively charged.
Characterization of CNT membranes
Electrochemical impedance spectroscopy28
measurements wereemployed to characterize the surface chemistry of CNT
membranes, which were performed in the frequency range of 100
kHz–0.2 Hz with a sinusoidal amplitude modulation of 10 mV
Fig. 1 SEM images of MWCNT membrane (a) cross-section view; (b)
top view; (c) TEM image of microtome-cut MWCNTs with open tips; (d)
the molecular structure on the functionalized CNT membrane (S.G.
CNTs–dye, grey: C; red: O; blue: N; yellow: S).
Nanoscale This journal is ª The Royal Society of Chemistry 2011
View Online
8/6/2019 Highly efficient electroosmotic flow through functionalized carbon nanotube
http://slidepdf.com/reader/full/highly-efcient-electroosmotic-ow-through-functionalized-carbon-nanotube 3/8
using a Model 263A Potentiostat and FRD 100 Frequency
Response Analyzer from Princeton Applied Research. Faradaic
EIS measurements were carried out at 230 mV using an elec-
trolyte consisting of 5 mM K3Fe(CN)6 and K4Fe(CN)6, 0.1 M
KCl, and 10 mM K2CO3 (pH 10.8). A Ag/AgCl reference cell
(BASI Corp.) was used with Pt wire counter electrodes. An
S-4300 HITACHI Scanning Electron Microscope (SEM) and
a JEOL 2010F Transmission Electron Microscope (TEM) were
used to examine the microstructure of the CNT membranes. Forthe preparation of TEM samples several pieces of CNT
membranes were dissolved in 1 ml 98% concentrated sulfuric acid
to remove the epoxy matrix. The acidic solution was then added
to 100 ml deionized water and centrifuged. A drop of centrifuged
solution containing CNTs was applied to a TEM grid coated
with lacey carbon.
Permeation measurements
A U-shape tube installed with three electrodes was employed for
all the permeation studies as shown in Fig. 3c. CNT membrane,
platinum wire and Ag/AgCl in saturated KCl were employed as
working electrode, counter electrode and reference electrode,respectively. Constant potential was provided using a Model
263A Potentiostat. A 50 nm thick Au/Pd film was sputtered onto
the edge of the CNT membrane area as a contact for applying
biases. The distance between counter and working electrodes is
about 10 cm and all membrane sample areas were 0.07 cm2. Two
model molecules, Ru(bpy)32+ and caffeine were used to investi-
gate the electrophoresis and electro-osmosis properties of CNT
membranes. Typically, the donor solution is composed of 5 mM
Ru(bpy)32+ or 5 mM caffeine in 0.01 M KCl aqueous solution.
Permeate solution is 0.01 M KCl in DI water, which is used to
balance any potential osmosis pressure from the donor solution.
The concentration of the molecules was measured using an
Ocean Optics UV-Vis spectrometer (USB4000, Ocean OpticsInc.). 286 nm and 272 nm peaks were used to calculate Ru(bpy)3
2+
and caffeine concentrations, respectively. Concentrations of
metallic ions, including potassium and calcium were quantified
using inductively coupled plasma-atomic emission spectrometry
(Varian Vista-PRO CCD Simultaneous ICP-AES). Potassium
and calcium standards were purchased from ULTRA Scientific.
Results and discussion
CNT membranes used for all the studies were prepared using
a microtome-cut method22 modified for high CNT loadings
(5–10%) of multi-walled (MWCNTs) or single-walled
(SWCNTs) to give more porosity.6 Triton X-100 surfactant wasadded to disperse CNTs more uniformly in the polymeric epoxy
matrix. The surfactant containing phenyl functional group has
a strong interaction with the graphitic plane via p – p stacking so
that CNTs can be well dispersed.29,30 The as-prepared CNT
membranes were characterized using a Scanning Electron
Microscope (SEM) and a Transmission Electron Microscope
(TEM) as shown in Fig. 1. To obtain TEM images of CNTs, the
polymeric epoxy was removed by dissolution in concentrated
sulfuric acid. SEM cross-section view of CNT membranes
(Fig. 1a) clearly shows that the space between CNTs is
completely filled with the epoxy resin, which indicates that the
membrane is defect-free. Typically, the CNT membrane has
a thickness of $5 mm as shown in Fig. 1a. Regions of CNTs can
extend out of the membrane surface as shown in Fig. 1b. TEM
imaging (Fig. 1c) confirms that CNTs were cut open by the
microtome glass knife, allowing mass transport through CNT
cores. It is notable that frequent change of glass knives is
required since the blade will become blunt after $20 cuttings.
H2O plasma oxidation was carried out to remove polymeric
residuals from the cutting process. Fig. 1d shows the schematicstructure of the CNT membrane with chemical functionalization.
The integrity of CNT membranes from defects was tested by
the Au colloid permeation experiment.11,13 It was found there is
no permeation of 10 nm Au colloids (520 nm optical absorption
peak) through the CNT membrane with an average pore size of
7 nm. The porosity of the CNT membrane was evaluated from
the steady-state Ru(bpy)32+ or K+ flux through MWCNT and
SWCNT membranes using the following equation:13
Ap ¼ (J Dx)/(DDC ), 3 ¼ 100 (Ap/Am) (1)
where Ap (cm2) is the available pore area, J (mol sÀ1) is the
experimental steady-state flux of Ru(bpy)32+, Dx (cm) is thethickness of the membrane measured by scanning electron
microscopy, DC (mol cmÀ3) is the concentration difference
between the feed and the permeate, 3 is the porosity of the
membrane, and Am (cm2) is the membrane area exposed to the
solution. It is assumed that Ru(bpy)32+ can diffuse with a bulk
diffusivity (5.16Â 10À6 cm2 sÀ1) inside the CNTs with$7 nmcore
diameter.13,14 Porosities of 0.003% are typically seen for
MWCNT and 0.009% for SWCNT membranes.
As-prepared MWCNT membranes were functionalized with
benzoic acid via a static electrochemical diazonium grafting
method.14,28 Electro-chemical grafting of aryl diazonium salts is
an efficient method to modify inert conductive materials such as
graphite and glassy carbon with covalently bonded organicmolecules of high density.31,32 The pK a of benzoic acid is near 4.2,
thus the surface of CNTs is negatively charged at the pH value
used (pH ¼ 7).33 It should be pointed out that a high charge
density is critical to obtaining an efficient electro-osmosis
pumping.1,21 The charge density can be further quadrupled via
a carbodiimide coupling reaction with Direct Blue 71 dye.14
Schematically the overall composite is shown in Fig. 1d.
Surface chemistry of functionalized CNT membranes was
characterized using Faradaic Electrochemical Impedance Spec-
troscopy28 and it is an effective method demonstrating surface
modifications of electrodes.34 Fig. 2 shows the Faradaic EIS
Nyquist plots of the Fe(II/III)(CN)6 redox couple using as-
prepared and functionalized CNT membranes as the workingelectrode surface. The semicircle portion (Fig. 2a), observed at
higher frequencies, corresponds to the electron transfer-limited
process, whereas the linear part at the lower frequencies repre-
sents the electrochemical process limited by diffusion.35 When
electron transfer processes are very fast, such as in the case of
Fig. 2b, a linear diffusion tail can be clearly seen. However, a very
slow electron-transfer step results in a large semicircle region that
is not accompanied by a straight line, such as shown in Fig. 2a
after diazonium grafting. The Nyquist plots have a single semi-
circle, the diameter of which is corresponding to the electron
transfer resistance (Ret). The intercept of the semicircle with the
This journal is ª The Royal Society of Chemistry 2011 Nanoscale
View Online
8/6/2019 Highly efficient electroosmotic flow through functionalized carbon nanotube
http://slidepdf.com/reader/full/highly-efcient-electroosmotic-ow-through-functionalized-carbon-nanotube 4/8
Zreal axis at high frequencies is equal to the sum of solution and
contact resistances.28,35 EIS Nyquist plots (Fig. 2a) show that the
charge transfer resistance (Ret) has increased from 200 to
$300 000 ohms after electrochemical diazonium grafting. The
grafted molecule, benzoic acid, is anionic at a pH value of 10.8
(pK a ¼ 4.2), and repels the ferro/ferri cyanide anions resulting in
an increased Ret. After coupling anionic dye molecules, the
density of negative charge is quadrupled and further steric bulk
leads to a further Ret increase as shown in Fig. 2a.The experimental setup for permeation, electrophoretic and
electro-osmotic studies is shown in Fig. 3c, and the area of the
CNT membrane is 0.07 cm2. Two model molecules, charged Ru
(bpy)32+ and electrically neutral caffeine were used for the inves-
tigation (Fig. 3b). Fig. 3a shows the flux of Ru(bpy)32+ and
caffeine through a S.G. MWCNT–dye membrane applying biases
ranging fromÀ300 to +300 mV. The voltage dependenttransport
of neutral caffeine molecules is direct evidence of the EOFprocess.
Under an external electric field, the general trend is that fluxes
of both cationic and neutral molecules were enhanced under
negative biases, whereas the fluxes were decreased applying
positive biases (Fig. 3a), consistent with both electrophoresis and
EOF. Compared to the diffusional flux at 0 mV, the flux of Ru
(bpy)32+ can be enhanced by more than 8 times applying a À300
mV using an electrolyte solution containing 0.01 M KCl. Using
a feed solution of stronger ionic strength (0.1 M), however, the
flux of Ru(bpy)32+ was reduced by four fold consistent with
increased screening of surface charge. Without the influence of
external electric field, the diffusional fluxes of both molecules are
similar (<20% variation) in the electrolyte solutions of differentionic strengths (0.1 M and 0.01 M). The steady state flux of Ru
(bpy)32+ through the CNT membrane can be calculated using the
Nernst–Planck equation:
J (x) ¼ ÀDdC (x)/d(x) À zDFC (x)/RT df(x)/dx + C yeo (2)
where D, C and z are diffusion coefficient, concentration, charge
of the permeate molecule, respectively. F is Faraday constant and
yeo is the electro-osmosis velocity. dC (x)/d(x) and df(x)/dx are
trans-membrane concentration and potential gradients, respec-
tively. In this case the flux J (x) is through open CNT pore area,
not the total area of the sample. The porosity of the MWCNT
membrane used for Fig. 3 is 0.0027% and sample area is 0.07 cm2
.The three terms of eqn (2) are passive diffusional, electropho-
retic, and electro-osmotic transport processes, respectively.
According to the Nernst–Planck equation, electrophoresis does
not vary with the ionic strength of electrolyte solutions, given
that the trans-membrane concentration and potential gradients
remain fixed. Thus electroosmotic convective flow is the only
remaining mechanism for the enhanced flow of Ru(bpy)32+ using
a feed solution of low ionic strength.
However a more direct measure of electroosmotic flow is the
flux of neutral molecule, caffeine.36 Eqn (2) can be simplified as:
J (x) ¼ ÀDdC (x)/d(x) + C yeo ¼ J diff. + J eo (3)
Subtracting diffusion (J diff.) from total flux, J (x), gives electro-
osmotic flow (J eo). Electroosmotic velocity yeo can be calculated
from eqn (4):
yeo ¼ J eo/C (4)
ApplyingÀ300 mV bias, the observed yeo is as highas 0.036 cm
sÀ1 (0.12 cm sÀ1 VÀ1) for a S.G. MWCNT–dye membrane as was
calculated using the data shown in Fig. 3 and Table 1. This
compares favorably to Takamura et al. who reported a very high
electroosmotic flow velocity of up to 0.035 cm sÀ1 VÀ1 in SiO2
microfluidic channel with a depth of 400 nm, which was fabri-
cated using photolithography.37 The increase in EOF was nearly
proportional to the negative applied bias while for the positivebias there was a slight decrease in flux consistent with a small
EOF in the opposite direction due to a low concentration of
caffeine in the permeate solution. Notably, this asymmetric EOF
was also observed in carbonaceous AAO membranes.21
Diazonium salts of benzoic acid can be electrochemically
grafted onto CNTs14,31,32 to increase surface charge density and
electroosmotic flow. Fig. 4 shows the plots of caffeine fluxes vs.
the applied trans-membrane voltage for as-fabricated CNTs and
S.G. CNT membranes. After diazonium grafting, the caffeine
electroosmotic flow has been enhanced by 90% consistent
with surface functionalization. It should be pointed out that
Fig. 2 (a) Nyquist plotsof Faradaic Electrochemical Impedance Spectra
of bare, static diazonium grafted and S.G. CNT–dye membranes; solu-tion used is 5 mM K3Fe(CN)6 and K4Fe(CN)6 in 0.1 M KCl and 10 mM
K2CO3 aqueous solution (pH 10.8), and applied bias is +230 mV; (b)
amplified Fig. 3a.
Nanoscale This journal is ª The Royal Society of Chemistry 2011
View Online
8/6/2019 Highly efficient electroosmotic flow through functionalized carbon nanotube
http://slidepdf.com/reader/full/highly-efcient-electroosmotic-ow-through-functionalized-carbon-nanotube 5/8
z potential of unmodified CNT membranes is negligible and little
EOF is seen.
The effective electrophoretic mobilities of K+ and Ca2+ ions
through MWCNTs and SWCNTs were also calculated and
compared with their bulk values.38,39 The effective electropho-
retic mobility (m) is measured by the ion flux as a functionof bias:
m ¼ J pore  Dx/eCV (5)
where J pore is ionic flux through the open pore area, Dx
membrane thickness, C concentration of ion, e elemental charge,V applied bias across the membrane. Table 2 shows mobilities of
5 Â 10À8 m2 VÀ1 sÀ1 in the SWCNT cores which is very close to
the bulk values, supporting the hypothesis that CNTs are
a relatively non-interacting CNT surface. It should be noted that
the MWCNTs have sterically bulky dye molecules at pore
entrances while SWCNTs have only carboxylate functionality
from plasma oxidation. The increased steric hindrance reduces
the mobility in MWCNTs compared to the bulk case, while in the
unhindered SWCNT case is close to bulk mobility. Recently, the
mobilities of ions within CNTs have been indirectly measured,
through lifetimes of current pulse events, to be dramatically
Fig. 3 (a) Flux rates of caffeine and Ru(bpy)32+ through the S.G. CNT-dye membrane (0.07 cm2) with a porosity of 0.0027% under external electric
field; the solution used is 5 mM caffeine or Ru(bpy)32+ in 0.01 M or 0.1 M KCl aqueous solution. (b) Molecular structures of caffeine (1,3,7-trime-
thylxanthine) and Ru(bpy)32+ permeates; (c) diagram of the experimental setup for the permeation studies. Note: C.E, R.E and W.E represent counter,
reference and working electrodes, respectively.
Table 1 Effect of applied voltage on the flux, enhancement factor (E ),and electro-osmotic velocity of caffeine through the S.G. MWCNT–dyemembrane
J appa/mA cmÀ2
Fluxa/nanomolesper cm2 per h E V /mV yeo/cm sÀ1
0 6.8 1.0 0 01.8 9.3 1.4 À100 0.0183.7 11.7 1.7 À200 0.0235.3 18.2 2.7 À300 0.036
a The area of the MWCNT membrane used is 0.07 cm2 and the porosity is0.0027%, the same as Fig. 3a.
This journal is ª The Royal Society of Chemistry 2011 Nanoscale
View Online
8/6/2019 Highly efficient electroosmotic flow through functionalized carbon nanotube
http://slidepdf.com/reader/full/highly-efcient-electroosmotic-ow-through-functionalized-carbon-nanotube 6/8
enhanced by 2 orders of magnitude.24 This dramatic mobility
enhancement is not seen under our conditions and may reflect
a mass transport limitation of ions to CNT pore entrance in the
large area membrane geometry. However dramatic EOF veloci-
ties are not seen in this study, suggesting that mobilities within
CNTs are closer to bulk value. Mobilities of a variety of ions
through CNT membranes are an object of ongoing research.
It is important to compare EOF performance in CNT
membranes to other nanoporous systems to see if there is an
advantage of the fast CNT cores. This would become apparent in
the achieved electroosmotic velocity (normalized to voltage),
power efficiency, and ion pumping efficiency. Table 3 summa-
rizes the comparison of EOF performance for CNT membranes,
AAO/a-CNT membranes, AAO without functionalization, and
AAO with anionic surface functionalization (SO3À). Electroos-
motic velocity was 1–3 orders of magnitude faster for the CNT
samples supporting the primary hypothesis of enhanced EOF.
Pumping power efficiency is a critical parameter for application
of EOF, whether in compact medical devices or industrialseparations. The power efficiency here is defined as power, that is
current times voltage drop, divided by moles transported.
Improvements in power consumption range from 25–112 fold.
For the high performance template-prepared a-CNT
membranes21 presumably the large diameter (120 nm), that is
well beyond the 3 nm Debye screening length, was the primary
limiting factor and could be improved with reduced diameter. It
is important to note here that in that study, the voltage drop
across the membrane could not be reported due to it being
a constant current experiment. However this paper reported the
formation of bubbles on the counter electrode thus the bias was
above 1.2 V and the power efficiencies shown here are the highest
estimate. Because our membranes have low porosities, we usedcaffeine instead of phenol21 for its higher UV-Vis absorption
coefficient in the permeate concentration assay. The caffeine
should give comparable measures of EOF as phenol since both
are a measure of neutral solvent volume flux.
To increase the surface charge density AAO membranes were
treated with strong acids to increase the functional density of
hydroxyl groups and EOF has been systematically studied.40,41 It
is difficult to directly compare the results of those papers to the
CNT membranes since the applied biases ranged from 4 to 40
volts, well above the voltage to split water. This process generates
bubbles and changes the pH which acts as electrophoretic ions.
To make a more appropriate comparison, we functionalized the
Fig. 4 Caffeine flux through the as-prepared and diazonium grafted
MWCNT membrane with a porosity of 0.0025% as a function of bias;
donor solution used is 5 mM caffeine in 0.01 M KCl aqueous solution. A
much lower electro-osmosis effect is observed in the case of bare CNTmembrane due to the low surface charge density.
Table 2 Electrophoretic mobility of K+ in MWCNT membrane and Ca2+ in SWCNT membranea
Types of membrane Types of ionsAppliedvoltage/mV
Electrophoreticmobility/m2 VÀ1 sÀ1
Bulk electrophoreticmobility/m2 VÀ1 sÀ138,39
MWCNTs K+À0.3 6.4 Â 10À9 7.6 Â 10À8
SWCNTs Ca2+À0.3 5.0 Â 10À8 6.2 Â 10À8
a Note: porosities of MWCNT and SWCNT membranes are 0.015 and 0.0085%, respectively.
Table 3 Comparison of electro-osmosis power consumption of MWCNT, SWCNT, a-CNT/AAO and AAO membranes
Diameter/nm
V eo/cm sÀ1 VÀ1
Power consumption /W h per nanomole
Powerconsumption ratio
Ratio of cationsto caffeine
MWCNTs 7 1.6 Â 10À1 2.5 Â 10À8 1 18SWCNTs 1.5 1.8 Â 10À1 3.3 Â 10À8 1.3 1a
AAO/a-CNT21 120 2.2 Â 10À3 9.9 Â 10À7 40 — b
AAO membrane 20 1.1 Â 10À4 2.8 Â 10À6 112 172c
AAO membrane functionalized (SO3À) 20 3.7 Â 10À4 6.2 Â 10À7 25 38c
a Larger hydrated Ca2+ used as cation for SWCNT, while all others use K+. b Due to water hydrolysis above 1.2 V, direct comparison to ref. 21 is notpossible. c Ratios of cations to caffeine for AAO and functionalized AAO membranes were calculated using current data: flux of K+
¼ (It/(eN ))/T ; I iscurrent, t is time in second, e is elementary charge, T is time in hour and N is Avogadro constant. Note: porosities of MW and SWCNT membranes usedare 0.015 and 0.0085%, respectively.
Nanoscale This journal is ª The Royal Society of Chemistry 2011
View Online
8/6/2019 Highly efficient electroosmotic flow through functionalized carbon nanotube
http://slidepdf.com/reader/full/highly-efcient-electroosmotic-ow-through-functionalized-carbon-nanotube 7/8
AAO membrane with a high density of anionic sulfonate groups
(SO3À) via a combination of silanol and carbodiimide chemistry.
The applied voltages (0.3 to 0.6 V) are below the splitting waterand similar to our CNT membrane EOF transport conditions. It
should be pointed out that electrochemical diazonium grafting
cannot be applied to the AAO membrane since it is an insulator.
Asshownin Table3, theMWCNT membraneconsumes 112times
less energy to pump the same amount of electrically neutral
caffeine compared to the unmodified AAO membrane, which is
attributed to the lack of high density charged functional groups.41
After surface functionalization, its electroosmotic velocity has
been enhanced due to the presence of a high density of SO 3À
functional groups on its inner wall; however it is still 25 times less
energy efficient compared to the MWCNT membrane, due to the
non-ideal surface properties of the AAO pore structure.
Though power efficiency is important, another form of effi-ciency is the number of ions required to pump neutral molecules.
As shown in Fig. 5a, using the same concentration of ions and
neutral molecules, a much higher EOF efficiency could be ach-
ieved when the size of pumping ions (cations) fits the diameter of
CNTs. Consequently, fewer ions are needed to pump the same
amount of neutral molecules, which is energetically more
favorable. Fig. 5c shows a 3-dimensional model of Ru(bpy)32+
moving in a (12,12) SWCNT channel. For the smallest diameter
SWCNTs (nominal $1.5 nm i.d.) using large diameter Ca2+ (0.8
nm hydrated diameter) or Ru(bpy)32+ (1.1 nm diameter), a 1 to 1
ratio of ion to neutral was achieved which is a 40 fold
Fig. 5 (a) Schematic of highly efficient electro-osmotic pumping of
caffeine using various cations, such as Ru(bpy)32+ (dia.z 1 nm), Ca2+ or
K+ in SW or MWCNTs functionalized with negatively charged carbox-
ylate groups: MWCNTs have $7 nm inner diameter; SWCNTs have
inner diameters ranging from 0.8–2 nm; CAF: caffeine ($0.5 nm in
diameter); (b) TEM image of SWCNTs with$2 nm inner diameter; (c) 3-
dimensional model of Ru(bpy)32+ moving in a (12,12) SWCNT.
Fig. 6 (a) Flux of caffeine and Ru(bpy)32+ through the water plasma
etched SWCNT membrane; (b) flux of Ca2+ and caffeine through a water
plasma etched SWCNT membrane with a pore diameter 1.4Æ 0.5 nm; (c)
flux of K+ and caffeine through a water plasma etched MWCNT
membrane with an average pore diameter of 7 nm. Solutions used are 5
mM Ru(bpy)32+ and 5 mMcaffeinefor (a),5 mMCa2+ and 5 mM caffeine
for (b), 10 mM K+ and 5 mM caffeine for (c). Porosities of CNT
membranes used for (a), (b) and (c) are 0.0023, 0.0085 and 0.015%,
respectively, and the membrane area is 0.07 cm2.
This journal is ª The Royal Society of Chemistry 2011 Nanoscale
View Online
8/6/2019 Highly efficient electroosmotic flow through functionalized carbon nanotube
http://slidepdf.com/reader/full/highly-efcient-electroosmotic-ow-through-functionalized-carbon-nanotube 8/8
improvement over other systems. This approaches the ideal case
of a single ion pushing a column of solvent through the CNT
cores. Fig. 6a and b show the efficient pumping of neutral
caffeine compared to Ca2+ and Ru(bpy)32+ with EOF velocity,
power consumption and comparative ratios summarized in
Table 3. It is expected that even higher EOF efficiency can be
obtained through improving the size distribution of SWCNTs
(CheapTubes.com) diameters since a significant fraction (50%) is
>1.5 nm i.d. with several SWCNT diameters as large as 5 nmbeing seen in our TEM analysis (Fig. 5b). In the case of
MWCNTs, with larger inner diameter (7 nm), more solvent and
neutral molecules are able to move around the cation, reducing
the cation pumping efficiency (Fig. 6c). Using a 10 mM KCl and
5 mM caffeine donor solution, as many as 18 K+ ions (with
a hydrated ionic diameter of 0.66 nm) are required to pump one
neutral caffeine molecule through the MWCNT membrane. In
the case of unmodified AAO membrane, as many as 178 K+ ions
are required to pump one caffeine molecule. As expected, the
unmodified AAO membrane has inefficient electroosmotic
pumping due to the lack of a high density of charged functional
groups.41 After grafting with sulfonate functional groups only 38
potassium ions are needed per caffeine, which is significantlyhigher than that needed for SWCNTs (Table 3).
Conclusion
A facile microtome-cutting method has been developed to fabri-
cate MWCNT and SWCNT membranes that show enhanced
electroosmotic flow rates and efficiency. Direct observation of
neutral molecular transport under bias demonstrated the
phenomena of electroosmotic pumping with velocities as high as
0.16 cm sÀ1 VÀ1, 82 fold higher than related nanoporous AAO/
a-CNT materials. Changes in electroosmotic flow as a function of
ionic strength and surface charge functionality were also consis-
tent with the electroosmotic flow phenomena. Appreciable elec-troosmotic pumping was observed at voltages (0.3–0.6 V) that are
far below voltages of water splitting. Importantly power effi-
ciencies were 25–110 fold improved to comparable nanoporous
materials which has important application in separations and
portable medical devices. High electroosmotic efficiency was seen
in terms of the ratio of ions needed to pump neutral molecules.
1 : 1 ratios are seen in small diameter SWCNTs that is
approaching the condition for ideal electroosmosis where a single
ion pushes a column of solvent and neutral molecules.
Acknowledgements
We would like to thank Dali Qian and Rodney Andrews from theCenter for Applied Energy, University of Kentucky, for
supplying MWCNTs. Facility support was provided by the
Center for Nanoscale Science and Engineering and Electron
Microscopy Center at the University of Kentucky. Financial
support from NIH NIDA (R01DA018822), NSF CAREER
(0348544), and DARPA (W911NF-09-1-0267).
References
1 Y. Chen, Z. Ni, G. Wang, D. Xu and D. Li, Nano Lett., 2007, 8, 42– 48.
2 Z. Guo, T. S. Zhao and Y. Shi, J. Chem. Phys., 2005, 122, 144907.
3 R. Qiao and N. R. Aluru, Nano Lett., 2003, 3, 1013–1017.4 D. R. Reyes, D. Iossifidis, P.-A. Auroux and A. Manz, Anal. Chem.,
2002, 74, 2623–2636.5 H. A. Stone, A. D. Stroock and A. Ajdari, Annu. Rev. Fluid Mech.,
2004, 36, 381–411.6 J. Wu, K. S. Paudel, C. Strasinger, D. Hammell, A. L. Stinchcomb
and B. J. Hinds, Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 11698– 11702.
7 I. Vlassiouk, S. Smirnov and Z. Siwy, Nano Lett., 2008, 8, 1978–1985.8 A. v. d. Berg, H. G. Craighead and P. Yang, Chem. Soc. Rev., 2010,
39, 899–900.9 H. Daiguji, P. Yang and A. Majumdar, Nano Lett., 2003, 4, 137–142.
10 F. Fornasiero, H. G. Park, J. K. Holt, M. Stadermann,C. P. Grigoropoulos, A. Noy and O. Bakajin, Proc. Natl. Acad. Sci.U. S. A., 2008, 105, 17250–17255.
11 B. J. Hinds, N. Chopra, T. Rantell, R. Andrews, V. Gavalas andL. G. Bachas, Science, 2004, 303, 62–65.
12 J. K. Holt, H. G. Park, Y. Wang, M. Stadermann, A. B. Artyukhin,C. P. Grigoropoulos, A. Noy and O. Bakajin, Science, 2006, 312,1034–1037.
13 M. Majumder, N. Chopra, R. Andrews and B. J. Hinds, Nature, 2005,438, 44.
14 M. Majumder, X. Zhan, R. Andrews and B. J. Hinds, Langmuir,2007, 23, 8624–8631.
15 P. Nednoor, N. Chopra, V. Gavalas, L. G. Bachas and B. J. Hinds,Chem. Mater., 2005, 17, 3595–3599.
16 C. L. Strasinger, N. N. Scheff, J. Wu, B. J. Hinds and
A. L. Stinchcomb, Subst. Abuse: Res. Treat., 2009, 3, 31.17 G. Hummer, J. C. Rasaiah and J. P. Noworyta, Nature, 2001, 414,
188–190.18 S. Joseph and N. R. Aluru, Nano Lett., 2008, 8, 452–458.19 S. Joseph, R. J. Mashl, E. Jakobsson and N. R. Aluru, Nano Lett.,
2003, 3, 1399–1403.20 S. Kim, J. R. Jinschek, H. Chen, D. S. Sholl and E. Marand, Nano
Lett., 2007, 7, 2806–2811.21 S. A. Miller, V. Y. Young and C. R. Martin, J. Am. Chem. Soc., 2001,
123, 12335–12342.22 L. Sun and R. M. Crooks, J. Am. Chem. Soc., 2000, 122, 12340–
12345.23 M. Yu, H. H. Funke, J. L. Falconer and R. D. Noble, Nano Lett.,
2008, 9, 225–229.24 C. Y. Lee, W. Choi, J.-H. Han and M. S. Strano, Science, 2010, 329,
1320–1324.
25 R. Andrews, D. Jacques, A. M. Rao, F. Derbyshire, D. Qian, X. Fan,E. C. Dickey and J. Chen, Chem. Phys. Lett., 1999, 303, 467–474.26 M. D’Amour and D. Belanger, J. Phys. Chem. B , 2003, 107, 4811–
4817.27 M. Majumder, N. Chopra and B. J. Hinds, J. Am. Chem. Soc., 2005,
127, 9062–9070.28 M. Majumder, K. Keis, X. Zhan, C. Meadows, J. Cole and
B. J. Hinds, J. Membr. Sci., 2008, 316, 89–96.29 Q.B. Zheng,Q. Z.Xue,K. O.Yan,L. Z.Hao, Q.Li and X.L. Gao, J.
Phys. Chem. C , 2007, 111, 4628–4635.30 Z. Liang, J. Gou, C. Zhang, B. Wang and L. Kramer, Mater. Sci.
Eng., A, 2004, 365, 228–234.31 J. L. Bahr,J. Yang, D. V. Kosynkin, M. J. Bronikowski,R. E. Smalley
and J. M. Tour, J. Am. Chem. Soc., 2001, 123, 6536–6542.32 J. Pinson and F. Podvorica, ChemInform, 2005, 36, 429.33 Determination of Organic Structures by Physical Methods, ed. F. C.
Nachod and J. J. Zuckerinan, Academic Press, New York, London,
1971.34 Electrochemical Methods: Fundamentals and Applications, ed. A. J.
Bard and L. R. Faulkner, Wiley, New York, 1980.35 E. Katz and I. Willner, Electroanalysis, 2003, 15, 913–947.36 Handbook of Chemistry and Physics, ed. C. D. Hodgman, Chemical
Rubber Publishing Company, Cleveland, 1951.37 Y. Takamura, H. Onoda, H. Inokuchi, S. Adachi, A. Oki and
Y. Horiike, Electrophoresis, 2003, 24, 185–192.38 J. L. Beckers, J. Chromatogr., A, 1997, 764, 111–126.39 D. R. Crow, Principles and Applications of Electrochemistry,
Chapman & Hall, UK, 4th edn, 1994.40 W. Chen, J.-H. Yuan and X.-H. Xia, Anal. Chem., 2005, 77, 8102–
8108.41 J. Y. Miao, Z. L. Xu, X. Y. Zhang, N. Wang, Z. Y. Yang and
P. Sheng, Adv. Mater., 2007, 19, 4234–4237.
Nanoscale This journal is ª The Royal Society of Chemistry 2011
View Online