IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 21 (2010) 095504 (8pp)
doi:10.1088/0957-4484/21/9/095504
DNA-decorated carbon-nanotube-based
chemical sensors on complementary metal
oxide semiconductor circuitry
Chia-Ling Chen1, Chih-Feng Yang1 , Vinay Agarwal2 ,
Taehoon Kim3 , Sameer Sonkusale2, Ahmed Busnaina3 ,
Michelle Chen4 and Mehmet R Dokmeci1,5
1
Department of Electrical and Computer Engineering, NSF Nanoscale Science and
Engineering Center for High-rate Nanomanufacturing, Northeastern University,
360 Huntington Avenue, Boston, MA 02115, USA
2
Department of Electrical and Computer Engineering, Tufts University, Medford,
MA 02155, USA
3
Department of Mechanical and Industrial Engineering, NSF Nanoscale Science and
Engineering Center for High-rate Nanomanufacturing, Northeastern University,
360 Huntington Avenue, Boston, MA 02115, USA
4
Physics Department, Simmons College, 300 The Fenway, Boston, MA 02115, USA
E-mail: chen.ch@neu.edu, yang.chi@neu.edu, vinay.agarwal84@gmail.com,
thkim@coe.neu.edu, sameer@ece.tufts.edu, busnaina@coe.neu.edu,
michelle.chen@simmons.edu and mehmetd@ece.neu.edu
Received 18 September 2009, in final form 14 January 2010
Published 8 February 2010
Online at stacks.iop.org/Nano/21/095504
Abstract
We present integration of single-stranded DNA (ss-DNA)-decorated single-walled carbon
nanotubes (SWNTs) onto complementary metal oxide semiconductor (CMOS) circuitry as
nanoscale chemical sensors. SWNTs were assembled onto CMOS circuitry via a low voltage
dielectrophoretic (DEP) process. Besides, bare SWNTs are reported to be sensitive to various
chemicals, and functionalization of SWNTs with biomolecular complexes further enhances the
sensing specificity and sensitivity. After decorating ss-DNA on SWNTs, we have found that the
sensing response of the gas sensor was enhanced (up to ∼300% and ∼250% for methanol vapor
and isopropanol alcohol vapor, respectively) compared with bare SWNTs. The SWNTs coupled
with ss-DNA and their integration on CMOS circuitry demonstrates a step towards realizing
ultra-sensitive electronic nose applications.
(Some figures in this article are in colour only in the electronic version)
pollutants is becoming increasingly important in the need to
understand the local and global trends and the complicated
side effects of air pollution. Accordingly, there are pressing
demands for miniature, low power and ultra-sensitive gas
sensing systems. Single-walled carbon nanotubes (SWNTs)
possess hollow geometry at the nanoscale with large surface
area to volume ratios (>1500 m2 g−1 ) that give rise to very
high gas absorptive capacity and electrical mobility. Hence
SWNTs are excellent materials for ultra-sensitive gas/chemical
sensors in environmental monitoring and low power lab-ona-chip systems. Bare SWNTs are found to be sensitive to
1. Introduction
Detection of gas molecules is critical in monitoring environmental pollution arising from combustion to automotive
emissions, control of chemical processes and biomedical
applications [1]. For instance, nitrogen dioxide (NO2 ), which
is one of the most prominent air pollutants and a deadly poison
arising from the combustion of fossil fuels (vehicles, electricity
generation and industrial processes), contributes to both smog
and acid precipitation. Real-time monitoring of environmental
5 Author to whom any correspondence should be addressed.
0957-4484/10/095504+08$30.00
1
© 2010 IOP Publishing Ltd Printed in the UK
Nanotechnology 21 (2010) 095504
C-L Chen et al
also amenable to CMOS technology. Previous nanotube-based
integration approaches utilized high voltage (>20 Vpp ) DEP
assembly parameters which were incompatible with CMOS
circuitry and required additional photolithography steps [36].
The heterogeneous integration of SWNTs onto CMOS
circuitry was presented previously by our group [37]. In
this work, we report the first integration of ss-DNAdecorated SWNTs onto CMOS circuitry for chemical sensing
applications. The ss-DNA was decorated on the SWNTs
to enhance the sensitivity of the gas sensor (up to ∼300%
and ∼250% enhancement was measured for methanol vapor
and isopropanol alcohol vapor, respectively). Furthermore,
the top (M3) metal layer of the foundry CMOS process is
about 1.7 μm higher than the rest of the substrate, and hence
by assembling SWNT sensors on microelectrodes using the
M3 layer enabled us to fabricate suspended nanotube sensors
which eliminated the influence of the substrate on the sensing
performance, critical for certain applications [25, 26].
numerous gases [1–5]. Moreover, functionalization of SWNTs
with polymers [6–9] and biomolecular complexes [10–12] was
shown to enhance the specificity and sensitivity of the SWNTbased sensors.
Among nucleic acid biomolecules, single-stranded DNA
(ss-DNA) is an intriguing candidate as a molecular recognition
layer, since it can bind to SWNTs through non-covalent π –π
stacking interactions [13, 14], and can be engineered to achieve
affinity to a variety of molecular targets [15, 16]. Recently,
it was reported that SWNTs decorated with a nanoscale
layer of ss-DNA display remarkable chemical sensing
capabilities, making them promising for ‘electronic nose’
applications [11]. Label-free detection of DNA hybridization
was also achieved optically [17] and electrically [18–20] on
ss-DNA-functionalized SWNTs. It has been demonstrated that
ss-DNA can be used to sort the chirality of SWNTs [21],
and the DNA-SWNT may provide a means for ultrafast DNA
sequencing [22]. Furthermore, most of the current carbon
nanotube sensors are attached to a substrate [6, 23, 24]. Sensors
based on suspended carbon nanotubes not only maximize the
active surface area for molecular functionalization, but also
eliminate nanotube–substrate interactions which are desirable
for various nanotube-based sensing devices [25, 26].
At present, many of the SWNT-based sensors require
an additional read-out circuitry or a measurement unit for
recognition of sensing response which not only limits the range
of applications, but also are prone to parasitics due to wire
bonding pads and long interconnect lines [27]. Furthermore,
in the case of high density (n > 100) sensor arrays for
detecting multiple chemistries, the area lost from wire pads
and the signal lines would be intolerable for many applications.
Another benefit of monolithic integration is the ability to
perform signal detection, amplification, buffering, storage on
a single chip and possibly wireless transmission with onchip coils. Accordingly, integration of nanomaterials onto
CMOS circuitry is essential to realize miniaturized, high
performance sensing systems. Depending on the application,
on-chip circuitry can provide high performance due to reduced
parasitics and interconnect lines as well as provide signal
conditioning and storage on the same chip. Well-known
approaches for synthesizing carbon nanotubes include laser
ablation, arc-discharge and chemical vapor deposition [28].
None of these approaches are compatible with commercial
CMOS technology since the CNT growth processes require
relatively high temperatures (>500 ◦ C). Lowering the growth
temperatures have also been pursued, yet, to date, the highest
quality nanotubes are only obtained via growth at elevated
temperatures. Several other approaches have been developed
to incorporate nanoscale materials onto microdevices such
as drop casting [29], dip coating [30], inkjet printing [31]
and field-assisted assembly [32].
One such technique,
dielectrophoretic (DEP) assembly, utilizing a nonuniform
electric field by the application of an AC voltage, is a versatile
and low temperature technique to manipulate nanoscale
materials [33–35]. During the DEP process, nanomaterials
suspended in a solution can be attracted to regions where the
intensity of the field is maximum, known as positive DEP.
This simple, low cost and high yield assembly process is
2. Experimental details
The integration of SWNTs onto functional electronic circuitry
via a post-CMOS fabrication process can be found in
our previous publication which include a double-zincation
process to remove the Al2 O3 layer prior to DEP assembly,
incorporation of SWNTs onto CMOS circuitry via DEP
assembly and a third zincation process to fix the SWNT/metal
electrode contact problem [37]. In this research, we first
demonstrate successful decoration of ss-DNA onto SWNTs
assembled on a CMOS circuitry, then demonstrate enhanced
gas sensing with ss-DNA-decorated SWNTs compared with
bare SWNTs both with passive electrodes on the CMOS
chip and also via SWNT sensors connected to active CMOS
circuitry and finally explore sensitivity enhancement versus
different ss-DNA sequences. The ss-DNA sequences chosen
for the experiments are shown here:
Seq. 1: 5 to 3 GAG TCT GTG GAG GAG GTA GTC
Seq. 2: 5 to 3 GTG TGT GTG TGT GTG TGT GTG TGT
Seq. 3: 5 to 3 CTT CTG TCT TGA TGT TTG TCA AAC.
Seq. 1 and seq. 3 were chosen based on previous
fluorescence [38] and electrical [11] measurements showing
sensitivity to various volatile compounds. Seq. 2 was chosen
based on a previous report that it can readily wrap around
SWNTs and resulting in effective sorting of the SWNTs [21].
The oligonucleotides were obtained from Invitrogen (Carlsbad,
CA) and diluted in deionized water to make a stock solution of
100 μmol. After odor responses of the bare SWNT sensors
were measured, a 2 μl drop of ss-DNA solution was applied to
them for 45 min in a humid environment and then dried with
a nitrogen stream (figure 1(a)) [11]. About 20 devices were
selected for detailed analysis here. The I –V measurements
(HP 4155A, semiconductor parameter analyzer) from the
assembled SWNTs on the CMOS chip before (34.67 k) and
after (54.44 k) ss-DNA decoration are shown in figure 1(b).
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Nanotechnology 21 (2010) 095504
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C-L Chen et al
Amplifier
(b)
Assembly area
ss-DNA decorated SWNTs
CMOS
(c)
(d)
200 nm
200nm
Figure 1. (a) The optical photograph of the CMOS chip and a schematic drawing of the DNA-decorated SWNT sensors integrated onto
CMOS circuitry, (b) I –V characterization of SWNTs on CMOS circuitry before and after ss-DNA decoration. Ss-DNA decoration was found
to increase the resistance while maintaining sufficient conduction in the devices. (c) Typical side-view SEM micrographs of the suspended
SWNTs assembled between the microelectrodes realized by the M3 layer of the CMOS chip. (d) A close-up view of ss-DNA-decorated
SWNTs on CMOS circuitry.
alcohol. The experiments were performed by measuring their
resistances under a probe station (SUSS, MicroTec, PM5)
using a multimeter (HP 34401A) with LabView control.
The chemical response of the bare SWNTs on the CMOS
chip was characterized first. The resistance of the sensors
was measured for 20 min under ambient conditions, which
showed the stability of the devices. Then the sensors were
exposed to chemical vapors for 15 min, during which their
resistance increased and reached a stable value. Finally, the
vapor source was removed and the sensors were allowed to rest
in ambient conditions for 30 min in order for their resistance
to recover the original values prior to being exposed to the
chemical vapors. Three cycles are shown in figure 2 to
demonstrate the reproducibility and stability of the sensing
response. The resistances were normalized to the value when
exposed to air ( R0 ∼ 20 k in this case). Under methanol
vapor (figure 2(a) (black data)), the increase in the measured
resistance of SWNTs was about 13.41 ± 1.03%, suggesting that
the methanol vapor near the SWNTs changed the electrostatics
in such a way that decreased the hole carrier density in the
SWNTs [39]. Next, three different sequences of ss-DNA were
decorated onto the SWNTs to enhance their chemical sensing
properties. Exposing ss-DNA-decorated SWNT sensors to
methanol vapor increased their resistance by about 18.43 ±
0.81%, 58.02 ± 3.36% and 24.7 ± 1.34% for seq. 1, seq. 2
and seq. 3, respectively, as shown in figure 2(a) (red, green and
Ss-DNA decoration on SWNTs was found to increase the
resistance of SWNTs by about 57.02%. This could be due
to weak carrier scattering by the molecular coating, as well
as some displacement of a small number of SWNTs during
the decoration of ss-DNA and/or the drying process. There
is increasing interest in the controlled assembly of suspended
nanostructures for sensing applications since they expose more
of the nanosensor surface. Selecting M3 as the integration layer
in our process allowed us to realize suspended structures since
the M3 layer was 1.7 μm above the substrate. Typical sideview SEM micrographs of suspended SWNTs integrated on
CMOS circuitry before and after ss-DNA decoration are shown
in figures 1(c) and (d), respectively.
3. Results and discussion
Chemical sensing tests were performed on both bare SWNTs
and ss-DNA-decorated SWNTs which were assembled onto
microelectrodes on the CMOS chip. These microelectrodes
were stand-alone test structures and not connected to the
on-chip amplifiers and hence were utilized for the initial
characterization of our sensors. Here, two chemical vapors,
methanol and isopropanol alcohol, were used to test the sensing
response of the SWNT sensors. The saturation vapor pressure
at 20 ◦ C is 97.48 Torr for methanol and 33 Torr for isopropanol
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DNA-decorated SWNTs were exposed to isopropanol alcohol
vapor and their responses were measured. Measurements
shown in figure 2(b) (red, green and yellow data for seq.
1, seq. 2 and seq. 3, respectively) indicated that, under
isopropanol alcohol vapor, the resistance of the same sample
increased by about 5.65 ± 0.3%, 11.25 ± 0.33% and 7.38 ±
0.49% and the measured chemical sensing response of ssDNA-functionalized SWNT sensors were enhanced by about
73.84%, 248.30% and 128.48% for seq. 1, seq. 2 and seq. 3,
respectively.
Both data show that the ss-DNA-decorated SWNT sensors
reached a stable resistance value which was within a few
minutes after exposure to the vapors of the analytes. Similarly,
after removal of the vapor source, the resistance of the sensors
dropped down to half (50%) of the highest resistance value
within a minute before taking some extra time to return to
its initial value. Comparing our sensor’s response with other
researchers’ results, we found that our response and recovery
time is of the order of a few minutes, which is longer than
the few seconds that was reported by Staii et al [11]. Possible
reasons include: first, the sensing response and the recovery
time includes the time it takes for the chemical analytes to
be delivered to the sensors. In our experimental set-up the
chemical vapors are delivered to the SWNTs through diffusion
in the chamber where Staii et al delivered the chemical analytes
to the devices through a carrier gas with a 0.1 ml s−1 flow
rate. The dimensions of our chamber are about 10 cm ×
10 cm × 2.5 cm and the chemical analyte inside a container
was placed in the center of the cell. The diffusion coefficient
is about 0.2 cm2 s−1 for methanol [40] and 0.0959 cm2 s−1 for
IPA [41], respectively. The
√ diffusion time in one dimension can
be estimated by L = 2 Dt , where L is the diffusion length,
D is the diffusion coefficient and t is the diffusion time. The
diffusion time is calculated to be around 31.2 s and 65.17 s
for methanol and IPA, respectively. From this calculation, we
noticed that the diffusion time for the chemical analytes to
reach the walls of the cell is in the range of 30–60 s for one
dimension. For a three-dimensional cell, the diffusion time
should be longer. Second, our devices consist of small bundles
of SWNTs and it is reasonable to assume that it would take a
longer time for the chemical analytes to reach the saturation
sensing response for all tubes.
The sensing mechanisms for SWNT sensors are most
commonly attributed to charge transfer or chemical gating [1],
while the exact mechanism is not well understood. Charge
transfer is unlikely to take place here because neither methanol
nor isopropanol are strong electron donors/acceptors. One
possible mechanism is that ss-DNA forms kinked structures
when adsorbed on SWNTs [14], forming binding pockets for
chemical analytes. These binding pockets in the adsorbed ssDNA may provide preferred orientations for analyte molecules,
thus allowing detection of polar molecules (e.g. methanol
and isopropanol) in an uncharged state. It is possible that
the presence of the binding pockets locally enhance the
chemical vapor concentration around the nanotubes, thus
enhancing the sensing response. Since the binding pocket
morphology depends on the DNA sequence, this could give
rise to the sequence dependence of the gas sensing response.
Methanol out
Methanol in
(b)
IPA out
IPA in
Figure 2. Change in sensor resistance upon chemical vapor
exposure. Resistances are normalized to the value when exposed to
air. (a) Bare SWNTs respond to methanol vapor (black line) with
about 13.41 ± 1.03% increase in resistance. The SWNTs decorated
with ss-DNA show enhanced response to methanol (red, green and
yellow lines for seq. 1, seq. 2 and seq. 3, respectively) with a
resistance increase of about 18.43 ± 0.81%, 58.02 ± 3.36% and
24.7 ± 1.34%, respectively. (b) Bare SWNTs respond to isopropanol
alcohol vapor (black line) with about 3.23 ± 0.50% increase in
resistance. The same SWNTs decorated with ss-DNA show
enhanced response to isopropanol alcohol (red, green and yellow
lines for seq. 1, seq. 2 and seq. 3, respectively) with a resistance
increase of about 5.65 ± 0.30%, 11.25 ± 0.33% and 7.38 ± 0.49%,
yellow data for seq. 1, seq. 2 and seq. 3, respectively). The
enhanced chemical sensing property of ss-DNA-functionalized
SWNT sensors compared with that of bare SWNTs was
measured to be about 37.43%, 332.66% and 84.19% for seq. 1,
seq. 2 and seq. 3 under methanol vapor. We also exposed bare
and ss-DNA-decorated SWNTs to isopropanol alcohol vapor.
When bare SWNTs were exposed to isopropanol alcohol vapor,
an increase in resistance of about 3.23 ± 0.5% was measured
(figure 2(b) (black data)). The change in resistance of bare
SWNTs under isopropanol alcohol vapor was less than that
when they were exposed to methanol vapor. This observation is
due to the differences in the chemical nature of the solvents, as
well as the different saturation vapor concentrations between
the two chemicals. Next, three different sequences of ss4
Nanotechnology 21 (2010) 095504
C-L Chen et al
(a)
60
12
Response (R/R0)
(c) 14
Response (R/R0)
(b) 70
50
40
30
20
10
20
40
60
80
10
8
6
4
2
100
5
Vapor Pressure (Torr)
10
15
20
25
30
35
Vapor Pressure (Torr)
Figure 3. Preliminary data regarding the sensor response versus vapor pressure of ss-DNA (seq. 2)-decorated SWNTs upon exposure to IPA
and methanol (a) Time versus response measurement. (b) Vapor pressure versus response measurement for methanol. (c) Vapor pressure
versus response measurement for IPA.
Table 1. ss-DNA decorated SWNT under vapor testing.
Odor
Methanol (97.48 Torr)
IPA (33 Torr)
R/R0 (%)
Improvement (%)
Time constant (min)
R/R0 (%)
Improvement (%)
Time constant (min)
Bare SWNTs
SWNTs + seq 1
SWNTs + seq 2
SWNTs + seq 3
13.41 ± 1.03
—
2.85
3.23 ± 0.50
—
3.02
18.43 ± 0.81
37.43
1.03
5.65 ± 0.30
73.84
2.56
58.02 ± 3.36
332.66
2.36
11.25 ± 0.33
248.30
10.03
24.7 ± 1.34
84.19
1.74
7.38 ± 0.49
128.48
2.42
results are summarized in table 1. From the time constant
calculations, we have found that the time constant for different
DNA sequences are sequence-dependent. For instance, seq.
2 (the highest response) has a higher time constant compared
with seq. 1 and seq. 3. The possible reason could be that
the binding between seq 2 DNA and gas molecules is stronger
than that between seq 1 and seq 3. Under methanol vapor, the
change in sensing response was 18.43%, 58.02% and 24.7%
for seq. 1, seq. 2 and seq. 3, respectively, and the calculated
time constant was 1.03, 2.36 and 1.74 for seq. 1, seq. 2 and seq.
3, respectively. These data suggest that the binding between
seq. 2 and gas molecules is the strongest among the three ssDNA sequences used. A similar trend was observed when the
decorated SWNT sensors were exposed to IPA vapor where the
SWNTs decorated with seq. 2 had the maximum response.
Since seq. 2 has the maximum response, preliminary
experiments regarding the sensor response versus vapor
pressure have been conducted and the measurements are shown
in figure 3. The vapor pressure was modified by adding a
The data from bare SWNTs and ss-DNA-decorated SWNTs
measured after exposure to both methanol and isopropanol
alcohol vapors were listed in table 1 for comparison. The
gas sensing response is expected to be DNA-sequencedependent, which is one of the motivating factors to use
DNA-decorated SWNTs as selective gas sensors. Notice that
from table 1, after decoration with ss-DNA sequence 2, the
sensing property of the SWNT sensors is much higher (58.02 ±
3.36% for methanol and 11.25 ± 0.33% for isopropanol
alcohol) compared with the bare SWNTs (13.41 ± 1.03%
for methanol and 3.23 ± 0.50 for isopropanol alcohol). The
enhancements in the sensitivity were calculated to be ∼300%
and ∼250% for methanol vapor and IPA vapor, respectively.
It has been reported that the GT sequence can wrap around
SWNTs better compared to other sequences [21]. To fully
understand the relationship between sequence dependence and
the enhancement in sensing response, a numerical analysis has
been conducted to determine the time constants in the decay of
the sensor response after removal of the gas vapor and these
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Nanotechnology 21 (2010) 095504
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(b)
(c)
Figure 4. Measured AC amplifier gain in response to variations in gas vapors. Corresponding to different gas vapors, the gain decreased
according to gain ∼−RSWNT /Ri . (a) For seq. 1 ss-DNA-decorated SWNT sensors, while exposed to methanol vapor, the gain of the inverting
amplifier increased by about 20.71%, while exposed to the isopropanol alcohol vapor, the gain increased by 7.75%. (b) For seq. 2
ss-DNA-decorated sensors, while exposed to methanol vapor, the gain of the inverting amplifier increased by about 55.00%, while exposed to
the isopropanol alcohol vapor, the gain increased by 13.70%. (c) For seq. 3 ss-DNA-decorated sensors, while exposed to methanol vapor, the
gain of the inverting amplifier increased by about 31.19%, while exposed to the isopropanol alcohol vapor, the gain increased by 8.25%.
Table 2. Gas sensing results measured from the amplifier.
SWNTs + seq 1
Odor
Vapor pressure
(Torr)
Air
Methanol 97.48
IPA
33
Gain G/G 0 (%) R/R0 (%)
−1.02 —
−1.23 20.71
−1.10 7.75
—
18.43
5.65
SWNTs + seq 2
Gain G/G 0 (%) R/R0 (%)
−1.00 —
−1.55 55.00
−1.14 13.70
solvent to the chemical analyte, which was dipropylene glycol,
which has low vapor pressure (0.06 mm Hg @ 25 ◦ C), and
the CNTs do not respond to it [11]. For methanol vapor
measurements under different vapor pressures, the resistance
change was 18.58%, 27.73%, 43.97% and 58.02% for vapor
pressures of 24.37 Torr, 48.74 Torr, 73.11 Torr and 97.48 Torr,
respectively. Similar results are obtained for IPA vapor
measurements at different vapor pressures; the resistance
change was 3.43%, 6.68%, 8.09% and 11.25% for vapor
pressures of 8.25 Torr, 16.5 Torr, 24.75 Torr and 33 Torr,
respectively. We also compared our results with data from [11]
and [42] with the same sequences of DNA decorated onto
SWNTs under the same chemical analyte (methanol). Our
measurements show similar values to the results obtained
in [11] and [42] and even a slightly higher sensitivity which we
think may be due to two reasons: one is that the DEP assembly
of SWNTs forms bundles of SWNTs while Staii et al have
used a single SWNT for their sensor. The other reason is that
our SWNTs are suspended from the substrate which reduced
the surface effects from the substrate and increased the active
surface area for sensing.
To demonstrate SWNT gas sensors with active circuitry,
we next assembled ss-DNA-decorated SWNTs onto microelectrodes that are connected to an operational amplifier (op-amp)
on the CMOS chip. The SWNTs were assembled onto the
feedback path ( Rref ) of a Miller-compensated single-ended opamp [43] configured as an inverting amplifier with an external
resistor ( Ri ) connected to a circuit. Measured input and
output signals from the inverting op-amp with three different
—
58.02
11.25
SWNTs + seq 3
Gain G/G 0 (%) R/R0 (%)
−1.09 —
−1.43 31.19
−1.18 8.25
—
24.7
7.38
sequences of ss-DNA-decorated SWNTs in the feedback path
are displayed in figure 4 and the gain of the inverting amplifier
was measured as −1.02, −1.00 and −1.09 for seq. 1, seq. 2
and seq. 3, respectively. We next exposed the SWNT sensors
on the CMOS chip to various gaseous vapor conditions and
characterized their gas sensing properties. We expected that,
as the sensors were exposed to vapors of different gases, their
resistance would increase, which would lead to an increase in
amplifier gain compared to the value measured under ambient
conditions. Figure 4 shows the response of the ss-DNAdecorated SWNT sensors to different gas vapors (methanol
and isopropanol alcohol) measured from the output of the
operational amplifier. Table 2 shows that the measured gain
of the inverting amplifier when exposed to methanol vapor was
measured to be −1.23, −1.55 and −1.43 for seq. 1, seq. 2 and
seq. 3, which was 20.71%, 55.00% and 31.19% higher than
the value measured under ambient conditions, respectively.
A similar trend was observed while the chip was exposed to
isopropanol alcohol vapor. Under IPA vapor, the measured
gain of the inverting amplifier was found to be −1.10, −1.14
and −1.18 for seq. 1, seq. 2 and seq. 3, which was 7.75%,
13.70% and 8.25% higher compared to the measurements
from the same sample under ambient conditions, respectively.
The measurements taken from the amplifier agreed well with
the measurements obtained from the SWNT sensors attached
to plain electrodes not connected to active amplifiers. This
data validated successful integration of the ss-DNA-decorated
SWNT sensors onto CMOS circuitry.
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4. Conclusions
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In summary, we have demonstrated successful integration of
ss-DNA-decorated SWNTs onto functional CMOS circuitry
for chemical sensing applications. Two-terminal resistances of
bare SWNTs and ss-DNA-decorated SWNTs were measured
and it was found that decorating ss-DNA onto SWNTs
increased the resistance between the SWNTs and the zinccoated electrodes by 57.02%, due to weak charge carrier
scattering, as well as some displacement of a small number of
the SWNTs during the decoration of ss-DNA and/or the drying
process. Nonetheless, ss-DNA-decorated SWNTs remained
sufficiently conductive for sensing applications. The chemical
sensing properties have been tested for both bare SWNTs and
ss-DNA-attached SWNTs on the CMOS chip with and without
amplifier connections. Bare SWNTs were found to be sensitive
to the vapors of methanol and isopropanol where an increase in
resistance of 13.41 ± 1.03% and 3.23 ± 0.5% was measured,
respectively. Ss-DNA decoration has enhanced the sensing
response of the nanotube sensors to methanol by 37.43%,
332.66% and 84.19% for seq. 1, seq. 2 and seq. 3, respectively.
With DNA decoration, the enhanced sensing response of the
nanotube sensors to isopropanol alcohol was measured as
73.84%, 248.30% and 128.48% for seq. 1, seq. 2 and seq.
3, respectively. The initial response and the recovery of the
bare and ss-DNA-decorated devices were fairly fast, within
a few minutes of vapor exposure and removal. Moreover,
ss-DNA-decorated SWNTs were successfully integrated onto
CMOS circuitry and their sensing response with and without
integration with the CMOS circuitry was in agreement with
each other. The gain of the inverting amplifier integrated
with ss-DNA-decorated SWNTs increased by about 20.71%,
55.00% and 31.19% for seq. 1, seq. 2 and seq. 3 when the
devices were exposed to methanol, respectively, and 7.75%,
13.70% and 8.25% for seq. 1, seq. 2 and seq.3 when
the devices were exposed to isopropanol vapors, respectively.
This work demonstrated the first ss-DNA-decorated SWNTs
integrated onto CMOS circuitry for sensing applications. The
methodology presented is simple and versatile with potential
applications in the realization of high sensitivity and low power
nanotube-based biological and chemical sensors.
Acknowledgments
This work was supported by the National Science Foundation
Nanoscale Science and Engineering Center (NSEC) for Highrate Nanomanufacturing (NSF grant 0425826). The authors
would like to thank Nantero Inc for supplying us with the
CMOS grade SWNT solution and Professor J Hopwood for
valuable suggestions. This research was conducted at the
George J Kostas Nanoscale Technology and Manufacturing
Research Center at Northeastern University.
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