Importance of Controlling Nanotube
Density for Highly Sensitive and Reliable
Biosensors Functional in Physiological
Fumiaki N. Ishikawa,†Marco Curreli,‡C. Anders Olson,‡Hsiang-I Liao,?Ren Sun,?Richard W. Roberts,‡
Richard J. Cote,‡Mark E. Thompson,§and Chongwu Zhou†,*
chanical stiffness,1,2high carrier mobility,3
and thermal conductivity.4,5Due to these
properties, numerous efforts have been de-
voted to commercialize applications that in-
corporate CNTs. These applications include
the next generation of
mechanical composites,16?18and transpar-
ent electronics.19?25Chemical and biologi-
cal sensing is one of the applications where
CNTs, especially single-walled CNTs, are
considered to be the ultimate type of sen-
sors. For example, single-walled CNTs have
the smallest diameters among various one-
dimensional structured materials, where ev-
ery atom in the CNTs is in contact with the
In the view of commercialization, re-
searchers have traditionally preferred mul-
tiple nanotube channels in a field-effect
transistor (FET) configuration (Figure 1a)
over single nanotube transistors because
the former offers several advantages, in-
cluding higher uniformity, lower noise, and
higher reproducibility.36While the use of
such networked nanotubes as FET chan-
nels was discussed in a number of previous
reports,27,29,31?36,38,40?42there is unfortu-
nately minimal investigation correlating
the role of the nanotube density to the bio-
sensor performance. Several theoretical
and experimental studies have proved that
the density of nanotubes in the FET channel
plays an important role in transistor
performance.45?48This correlation strongly
suggests that the density of nanotubes will
ingle-walled carbon nanotubes (CNTs)
possess a number of unique and
promising properties, such as me-
also affect the performance of biosensors
based on nanotube networks since it is
likely that the sensitivity to gate modula-
tion (FET performance) reflects the sensitiv-
ity to gating by charged captured analytes
(biosensor performance), as we have shown
for In2O3nanowire biosensors.49Under-
standing the role of the nanotube density
will lead to better designs of nanotube bio-
sensors and more reliable fabrication proce-
dures, both of which are extremely
In this context, we report our studies on
the role played by the nanotube density in
FET biosensor performance and demon-
strate that the control of nanotube density
is critical in achieving high/reliable perfor-
mance (e.g., high sensitivity, uniformity, re-
producibility, etc.). As a first step in the
*Address correspondence to
Received for review May 28, 2010
and accepted October 05, 2010.
© XXXX American Chemical Society
optimized nanotube biosensors to detect the nucleocapsid (N) protein of the SARS virus and demonstrated
KEYWORDS: nanotube density · percolation theory · biosensing · nanotube
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Published online Xxxxxxxxx 00, 0000
study, we fabricated devices with different nanotube
densities and then compared the biosensor perfor-
mance of those devices. We found that the low den-
sity nanotube devices offer the best performance in
terms of the magnitude of response and detection limit.
Transistor measurements revealed the semiconductor-
like behavior of the low density network and the quasi-
metallic behavior of the high density network. Two as-
pects of the semiconductor-like nature were attributed
to the enhanced sensitivity at low nanotube densities.
First, the off-current is smaller or negligible. Second, the
threshold voltage shift is enhanced due to a smaller ca-
pacitance. The latter was confirmed experimentally
where we observed larger Vtshift for lower density
samples. We note that this is the first observation of
density-dependent Vtshift, which has never been previ-
ously discussed. Lastly, using these density-optimized
devices, we detected the nucleocapsid (N) protein, a
biomarker associated with the SARS coronavirus, un-
der physiological conditions.
RESULTS AND DISCUSSION
In order to investigate the effect of the density of
nanotube on the biosensor performance, we fabri-
cated devices with different nanotube densities. The
density was controlled by varying the time (10, 20, and
60 min) of incubation in the ferritin solution, which de-
termines the density of iron-based catalyst nanoparti-
cles on the substrate. Fabrication and experimental pro-
cedures are described in detail in the Methods section
and Supporting Information. The density of nanotubes
can be classified as follows: low (10 min), medium (20
min), and high (60 min) density. Typical SEM images of
each density are shown in Figure 1b?d. While we have
carefully maintained identical nanotube growth condi-
tions for the three substrates, the nanotubes in the low
density sample appear to be slightly shorter than the
nanotubes in the medium density substrate. This varia-
tion in the length may also affect the percolation level
and thus influence the electrical characteristics of the
resulting FETs. We are confident that the observed dif-
ferences in device performance (see below) are mainly
due to different nanotube densities in the FET channel,
rather than the presence of shorter nanotubes.
We first characterized the devices as transistors be-
cause the density of nanotubes has been shown to af-
fect the transistor performance significantly,45?49and it
is likely to reflect the sensitivity of the devices as a bio-
sensor, as well.48Shown in Figure 2a?c are the typical
plots of family of source?drain current (Ids) versus
source?drain voltage (Vds) under different drain-back
gate voltage (Vg) for each nanotube density. The step of
Vgis 3 V. The device with low nanotube density (Fig-
ure 2a) exhibits clear separation along each curve, indi-
otube devices. As the density increases from low to
medium to high (Figure 2b,c), the curve separation be-
comes less clear, indicating more metallic behavior. The
linear behavior of the Ids?Vdscurves under small nega-
tive bias shows that the transport is diffusive, and the
contact/junction resistance contributes little to the
overall device resistance. This low contact resistance
can be attributed to our devices’ long channel length
(?200 ?m). Figure 2d shows, in log scale, the typical
plots of Idsversus Vgfor low (blue), medium (green), and
high (red) density nanotube devices at Vds? 1 V. The
low density device exhibited high on/off ratios (?104),
while the medium density device exhibited moderate
on/off ratios (101?2) and high density device exhibited
low on/off ratios (?10). That correlation can be ex-
plained by the conventional percolation theory ap-
plied to carbon nanotube networks.26,50Figure 2e
shows the same Ids?Vgplots in linear scale. The dashed
lines in Figure 2e are the fitting curves to extract
transconductance. Figure 2f shows on-current and
transconductance extracted from the Ids?Vgcurves.
The saturation of transconductance was observed,
while the on-current monotonically increased as the
density increased. This is consistent with previous theo-
retical simulations and experimental observations that
the capacitance of an array of aligned nanotubes satu-
rates as the density increases because nanotubes
screen each other.10,51,52To confirm the reproducibility
of the process, we measured several devices for each
density, and the distribution of on/off ratios for each
density is shown in Figure 2g?i. Devices with different
densities clearly exhibit different ranges of on/off ratio,
confirming the reproducibility of the fabrication.
We then investigated the sensing performance of
those devices using streptavidin (S-Av) as a model ana-
lyte. The sensing was carried out in 1? phosphate buff-
ered saline (PBS). Vdsand drain-liquid gate voltage (Vlg)
were 0.2 and 0 V, respectively. Constant air flow was ap-
plied to the buffer to promote mixing and minimize
the mechanical perturbation caused by adding aliquots
Figure 1. (a) Schematic diagram of the device structure. Typical
SEM images of (b) low density, (c) medium density, and (d) high
density nanotube samples.
VOL. XXX ▪ NO. XX ▪ ISHIKAWA ET AL.www.acsnano.org
of buffer. The sensing experimental setup is shown in
Figure S1 (Supporting Information). We note that the
potential of the Pt electrode, used as a gate electrode,
was stable upon exposure to streptavidin in 1? PBS, in-
dicating little possibility for false signals as previously
proposed.53The stability of the baseline under these
conditions is discussed in the Supporting Information
(Figure S2). Figure 3a shows the plot of normalized cur-
rent versus time for a low density nanotube device.
The device showed a ?2% decrease in conductance af-
ter exposure to 1 pM streptavidin. We attributed amine
groups in streptavidin to the source of the device char-
strong affinity to nanotubes with electron-donating
properties and positive charges (when protonated) that
are consistent with the observed trend of the changes
(decrease in conductance).26,29The device showed fur-
ther sequential decreases as the streptavidin concentra-
tion increased to 100 nM. On the other hand, when a
to 1 and 10 pM streptavidin, there was only a negli-
gible response (?0.5%). The device showed a change
larger than 1% when the device was exposed to 100 pM
streptavidin. Furthermore, the device with the high
density nanotube only showed responses when ex-
posed to 1 nM streptavidin. While the low density de-
vice clearly displayed the highest sensitivity, this device
was also affected by a slightly higher noise level. We
are aware that noise level can be further reduced by op-
timizing the device geometry (using a wider or shorter
channel). For this study, we deliberately kept the device
dimension constant for the three different types of de-
vices so the nanotube density was the only variable. To
summarize, Figure 3d shows plots of responses for
each device (2 high density (HD) devices, 1 medium
density (MD) device, 2 low density (LD) devices) versus
Figure 2. Idsversus Vdscurves under different gate voltage ranging from ?15 V (red) to 15 V (black) with a step of 3 V for (a)
low density, (b) medium density, and (c) high density nanotube devices. (d) Idsversus Vgcurves at Vds? 1 V in log scale for de-
vices with each density of nanotube. (e) Curves shown in (d) plotted in linear scale. (f) On-current (left axis) and transconduc-
tance (right axis) extracted from the Idsversus Vgcurves for each density. Distribution of on/off ratio for (g) low density, (h)
medium density, and (i) high density nanotube devices.
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streptavidin concentrations. The device with the low
density nanotube clearly offers the highest magnitude
of response to every concentration, while the device
with the high density nanotube offers the lowest mag-
nitude of response. The device with the medium den-
sity nanotube provided responses between those two
extremes. The inset shows the plot of the response ver-
sus log of the concentration of streptavidin. In this in-
sponses with a conductance change higher than 0.5%.
Changes above this line are considered true responses
(signal) because the addition of buffer sometimes
causes false response (noise) of ?0.2%. We did not
take the thermal noise into consideration because we
expect that we can reduce the thermal noise to negli-
gible levels compared to the noise caused by the addi-
tion of the buffer by using, for example, larger channel
width and lower temperature. According to this defini-
tion, the limit of detection (LOD) for high density de-
vices falls into the 100 pM to 1 nM range, while medium
density devices have an LOD of 10?100 pM. Low den-
sity devices even show an LOD of 1 pM to 10 pM, sug-
gesting the higher sensitivity of lower density devices.
These experiments clearly demonstrate the impor-
tance of the nanotube density on the biosensor
This enhanced sensitivity can be partially explained
by the elimination of direct metallic nanotube path-
ways at low density, as was confirmed in the transistor
measurements (Figure 2) since conduction through me-
tallic nanotubes is expected to be unaffected by the ad-
sorption of proteins. However, the elimination of metal-
lic pathways may not be the only source for this
observed enhancement. In fact, the magnitude of re-
sponse improved by 1 order of magnitude, while the
elimination of metallic nanotube pathways is expected
to improve the sensitivity only by a factor of 2?3 (from
the ratio of semiconductive and metallic nanotubes).
In order to investigate the source of the sensitivity en-
hancement, we measured the gate dependence using
liquid gate before/after the sensing experiments. We
exposed devices with each density to 111 nM streptavi-
din solution in 1? PBS, and the shift of the threshold
voltage by the exposure was measured using the liq-
uid gate. Figure 4a?c shows Ids?Vlgcurves before/af-
ter the exposure to streptavidin for devices with each
Figure 3. Plots of current versus time while devices were exposed to different concentrations of streptavidin for (a) low
density, (b) medium density, and (c) high density nanotube devices. The experiments were carried out in 1? PBS. In these
measurements, Vdswas 0.2 V and Vlgwas 0 V. (d) Plots of the response signal (showed as normalized conductance) versus log
of the concentration of streptavidin for each device with different nanotube density. The acronyms LD, MD, and HD repre-
sent low density, medium density, and high density, respectively. The inset is the same plot with the response plotted in log
scale. The dashed line represents the response level of 0.5%.
VOL. XXX ▪ NO. XX ▪ ISHIKAWA ET AL. www.acsnano.org
density. Figure 4d shows the plots of the Vtshift for
each device. Surprisingly, the devices with lower nano-
to higher density devices, which can also contribute to
the enhanced sensitivity. This is in sharp contrast to our
previous observations about In2O3nanowire biosen-
sors where devices with different transistor perfor-
mance showed the same amount of Vtshift after expo-
sure to the analyte.49The change of the Ids?Vlgcurves
trostatic interaction or charge transfer because there is
little change in the transconductance. We note that in
previous reports it has been proposed that the contact
resistance modulation is the dominant sensing mecha-
nism for carbon nanotube biosensors. However, this
discrepancy is likely to come from the difference in the
device geometry. In previous reports, a short channel
length (?4 ?m) was used,54but our devices have a long
channel length (200 ?m), making the channel resis-
tance dominant in the overall device resistance. Indeed,
the transistor curves show linear Ids?Vds, proving negli-
gible contact resistance. Furthermore, a study per-
formed by Iddo et al. showed that carbon nanotube bi-
osensors can be operated by a bulk modulation
mechanism with a contact passivation.55
We have further performed AFM imaging to esti-
mate the number of streptavidin molecules on low
and high density nanotube samples and proved that
there is no significant difference in the number of
streptavidin, as shown in Figure 4e,f. This suggests
that the enhanced Vtshift for low density nanotubes
arises from stronger interaction between nanotubes
and one streptavidin molecule.
We tentatively attributed the semiconductor behav-
ior of the low density nanotube network to the pro-
nounced Vtshift for low density devices. When model-
ing the interaction between a charged protein and a
nanotube as a capacitive coupling, the following rela-
tionship can be expressed:
where ?Q represents the amount of charges brought
Figure 4. Idsversus Vlgcurves before (red) and after (blue) exposure to streptavidin at the concentration of 111 nM for (a)
low density, (b) medium density, and (c) high density nanotube devices. (d) Plots of the shift of Vtfor devices with each den-
sity. (e,f) AFM images of low and high density carbon nanotubes after exposure to streptavidin solution. These AFM im-
ages are obtained upon scanning an area of 1 ?m ? 1 ?m.
∆Q ) CTotal× ∆V
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by the proteins, CTotalis the total capacitance of the car-
bon nanotube?double layer-liquid gate capacitor, and
?V is the potential created by the charges on the pro-
teins. Thus, the potential created by the charges is ex-
We note that these numbers are per unit nanotube
area. Total capacitance CTotalcan be written as
where CDLis the capacitance of the double layer and
CCNTis the capacitance of the carbon nanotube (quan-
tum capacitance).56,57Since CCNTis smaller for semicon-
to the lower density of states near the Fermi level,57
the equivalent potential ?V becomes larger for semi-
conductive nanotubes. Since the characteristics of the
low density nanotube network are dominated by the
semiconductive nanotubes, it is likely that the low den-
sity network experiences more electrostatic interaction
for a given amount of charges, that is, given amount
(density) of adsorbed proteins, than the high density
network which is less semiconductive.
Another possible mechanism that contributes to
the density-dependent Vtshift might be the strong
tube?tube screening for high density nanotube FET
channels. Such screening can only be meaningful when
the distance over which a charge can induce nontrivial
electrostatic potential is comparable to the distance be-
tween the tube and another tube. This means the De-
bye length determines the area over which the
tube?tube screening can play a major role. Although
the Debye length of 1? PBS buffer is about 0.7 nm
when calculated assuming a simple thermal equilib-
rium, in reality it might be significantly larger than that.
Or the picture of Debye screening might not even ap-
ply to the current system, as is reported in the work by
Liu et al.58According to this report, there is a shift in bal-
ance between ion diffusion and ion screening (electro-
static accumulation) to the diffusion side, due to ion
flows such as electrodiffusion induced by source?drain
or source?gate bias. This shift in balance leads to quali-
tatively different behaviors in the screening, and as a re-
sult, there is increased distance (?10 times) over which
a charge can induce a nontrivial, electrostatic potential
change (this might be considered as elongation of the
Debye length). Under such conditions, tube?tube
screening may affect the performance of nanotube bio-
sensors by screening the charges inside an area of 10
? 10 nm2around tube?tube junctions. In our experi-
ment, there might have been an external flow of ions
due to the SD, SG, and DG biases. In addition, we have
used external air flow to enhance the mixing of the so-
lution. This might have significantly increased the
screening length of our system. We are currently inves-
tigating the effect of such external flow on biosensing.
Finally, using devices with an optimized nanotube
density (low density), we successfully demonstrated
the detection of the SARS biomarker protein (N pro-
tein) at physiological conditions (1? PBS) with a detec-
tion limit of 5 nM. The device preparation is shown in
the Supporting Information (Figure S3). We would like
to emphasize that these detection measurements took
Figure 5. (a) Schematic diagram of the device structure used
for N protein sensing. (b,c) AFM images of a carbon nanotube
before and after the functionalization with Fn, respectively.
The insets show a schematic illustration of the nanotubes. The
white solid lines represent the lines along which the height
profiles of the nanotube were examined. These AFM images
have been resized for clarity but were originally captured
upon scanning an area of 1 ?m ? 1 ?m. (d) Plot of Idsversus
time shows the device response when sequentially exposed
to 4 ?M BSA and N protein at the concentrations of 5, 10, and
50 nM. The inset shows the plots of response versus concen-
tration of N protein. Black marks represent the data points,
and the red solid line represents a fitting using the Langmuir
VOL. XXX ▪ NO. XX ▪ ISHIKAWA ET AL.www.acsnano.org
place in a high electrolyte medium, such as 1? PBS, be-
cause we used a unique capture probe anchored to
the nanotube surface. This capture probe is an engi-
neered antibody mimic protein (AMP) based on the hu-
man fibronectin (Fn) scaffold. We have previously de-
scribed the use of this AMP for nanowires biosensors.59
Beside the high binding affinity for the target protein,
this AMP is tiny in size (on the order of 3?4 nm). The
captured analyte is thus held in closer contact with the
nanotubes than it would be if conventional antibodies
were used. Bovine serum albumin (BSA) was used to
cover the empty regions of source?drain electrodes
and nanotubes to prevent nonspecific binding59(Fig-
ure S4). The schematic diagram of the device ready for
sensing is shown in Figure 5a. The AFM images of a car-
bon nanotube before and after functionalization with
Fn are shown in Figure 5b,c, respectively. The insets in
these figures show an illustration of the nanotube
The selectivity of the device was first confirmed
against BSA. For a density-optimized device, as shown
in Figure 5d, an addition of 4 ?M BSA at t ? 400 s did
not cause any stationary change in the normalized Ids.
On the other hand, exposing the device to N protein (5
nM) rapidly led to decreased conductance by ?2%. Fur-
ther increases in the N protein concentration led to fur-
ther decreases in the normalized current, confirming
the selective detection of N protein. The inset of Fig-
ure 5d shows the plot of normalized response versus
muir isotherm model to estimate the dissociation con-
stant (the solid red line in the inset). The fitting yielded
Kdof 33 nM, while Kdestimated from surface plasmon
resonance was 3 nM.59The discrepancy might arise
from structural disorder caused by absorption of the
branches of Fn onto the nanotubes. While Fn is an-
chored onto the nanotubes at the C-terminus to main-
tain the active orientation, part of Fn might be attracted
to the nanotube surface due to hydrophobic interac-
tion, thus causing a structural disorder. Clearly, the
density-optimized device can be used as a detection
tool to find the N protein under conditions similar to
physiological fluids. This ability to operate in high elec-
trolyte concentrations is a critical characteristic for prac-
tical applications of this technology.
In conclusion, our study has revealed the impor-
tance of considering the nanotube density when de-
signing carbon nanotube biosensors. We have pro-
posed that percolation and tube?tube screening play
important roles in the enhanced sensitivity of low den-
sity devices. We have also shown how our newly devel-
oped biosensors can be used as a quick diagnostic
tool for high-profile diseases, such as SARS. The detec-
tion of the SARS biomarker in a condition closer to the
physiological conditions is a critical step toward the
practical application of such nanobiosensors.
Nanotube FET biosensors were fabricated following a previ-
ously reported method with a few modifications.44Briefly, nano-
tubes were grown by chemical vapor deposition (CVD), a
method that consistently results in high-quality, low-defect,
single-walled nanotubes with an average diameter of 1?2 nm
and average length of 5?30 ?m. The nanotube density in the
FET channel was determined by controlling the deposition time
of ferritin (catalyst nanoparticle precursor) on the Si/SiO2sub-
strate. An aqueous solution of ferritin was obtained by diluting
1:100 the stock solution of ferritin with DI water. The Si/SiO2sub-
strate was soaked in this ferritin solution for 10, 20, or 60 min.
The ferritin-coated substrate was then annealed at 700 °C for 10
for the CVD growth of carbon nanotubes. Nanotubes were
grown for 10 min at 900 °C using 2500 sccm of methane, 10
sccm of ethylene, and 600 sccm of hydrogen. Metal electrodes
(10 nm Cr and 40 nm Au) were deposited through a shadow
mask, which yielded devices with 200 ?m channel length and 5
mm channel width, as indicated in Figure 1a. This wide area of
the nanotube network makes different batches of devices have
similar electrical characteristics, decreasing the device to device
variation and increasing the reproducibility of results. Unwanted
nanotubes located outside the channel were etched by oxygen
plasma, while the channel was covered with a protecting film of
poly(methyl methacrylate). The entire fabrication process leaves
nanotubes intact and clean,44an ideal condition for the funda-
mental studies conducted here.
Acknowledgment. The authors acknowledge financial sup-
port from the L.K. Whittier Foundation, the National Institutes
of Health, and the National Science Foundation (CCF-0726815,
CCF-0702204, AI085583, and EB008275).
Supporting Information Available: Materials, experimental
setup for biosensing, stability of the Pt gate potential upon ex-
posure to streptavidin, surface functionalization of carbon nano-
tubes for N protein sensing, and surface saturation with BSA.
This material is available free of charge via the Internet at http://
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