Highly efﬁcient electroosmotic ﬂow through functionalized carbon nanotube

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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.

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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).

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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


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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.


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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 Â 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.


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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.


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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.


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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.


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