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SUPPORTING INFORMATION
Label-free chemiresistive biosensor for mercury (II) detection based on
single-walled carbon nanotubes and structure-switching DNA
Ji-Lai Gonga, c, Tapan Sarkara, Sushmee Badhulikab and Ashok Mulchandania*
a) Department of Chemical and Environmental Engineering, University of California,
Riverside, CA 92521, USA.
b) Department of Electrical Engineering, University of California, Riverside, CA
92521, USA.
c) College of Environmental Science and Engineering, Hunan University, Changsha,
410082, China.
1. Fabrication of label-free SWNTs chemiresistive biosensor
SWNTs suspension was prepared by dispersing 0.2 mg of carboxylated SWNTs
from Carbon Solution Inc., Riverside, CA, USA [P3 SWNT-COOH 80 ~ 90% purity
and semiconducting: metallic = 2:1] in 20 ml of N,N-dimethylformamide (DMF)
(Sigma Aldrich, Spectral grade) by ultrasonication for 90 minutes followed by
centrifugation at 31,000xg for 90 minutes to separate soluble fraction from the
aggregates.
Gold microelectrodes were fabricated on highly doped p-type silicon substrate by
standard lithographic patterning. First, approximately 300 nm SiO2 dielectric layer
was deposited on the substrate by low-pressure chemical vapor deposition (LPCVD).
Electrodes (200 m x 100 m) separated by a 3 m gap were written on the substrate
by photolithography, followed by deposition of 20 nm Cr layer and 180 nm of Au
layer by e-beam evaporation. Finally electrodes were defined by using standard
lift-off technique.
To bridge the gap between the gold electrodes, SWNTs were aligned across the
electrode using dielectrophoresis. A 0.1 l of SWNT suspension was dispensed on
the top of the electrode gap while applying 3 VP-P at 4 MHz frequency for a
predetermined time. After alignment, the device was washed with nanopure water to
remove the extra SWNTs and dried by gently blowing dry nitrogen gas. Desirable
resistance of the device could be achieved by varying the alignment time. The
electrode was then annealed at 300 OC for 90 minutes in reduced environment (5% H2
in N2) to improve the contact between the gold electrode and SWNTs by removing
any DMF residues between electrode and SWNTs.
1.1. Non-covalent functionalization of SWNTs with PolyT (Scheme I)
Aligned SWNTs network chemiresistor was incubated with 100 nM of
oligonucleotide polyT (5’–TTT TTT TTT TTT TTT-3’) [Integrated DNA
Technologies Inc., San Diego, CA, USA] in 10 mM pH 7.2 phosphate buffer (PB) for
2 hours at room temperature followed by thorough washing with nanopure water to
remove unbound oligos. Further, the gold pads of the electrode were blocked by
6-mercapto-1-hexanol (MCH). MCH blocking was done by incubating the device
with 6 mM of MCH solution for 1 hour at room temperature. A monolayer of MCH
was formed on exposed gold pads of the electrode due to the interaction between thiol
group and gold, which restricts further interaction/accumulation of Hg2+ on the gold
surface.
1.2. Covalent functionalization of SWNTs with amino-labeled PolyT followed by
hybridization with polyA (Scheme II)
First, SWNTs network chemiresistor was modified with 1-Pyrenebutanoic acid
succinimidyl ester (PBASE) by incubating with 6 mM of PBASE in
dimethylformamide (DMF) for 1 hour at room temperature followed by washing with
DMF three times to remove residual ester1. The capture oligonucliotide was then
covalently attached to the PBASE-modified SWNTs by incubating overnight at 4 OC
with 100 nM amino-labeled polyT (5’-/5AmMC6/TTT TTT TTT TTT TTT -3’)
[Intergrated DNA Technologies Inc., San Diago, CA, USA] in 10 mM pH 7.2 PB
through the amide bond between the amine at its 5’ end and N-hydrosuccinimide ester
of PBASE washed three time to remove excess oligo. The device was further treated
with 0.1 mM ethanolamine (EA) for 30 minutes at room temperature to block
excessive reactive groups and incubated with 0.1 % (V/V) Tween 20 in 10 mM pH 7.2
PB for 30 minutes at room temperature to prevent nonspecific binding to SWNTs. The
device gold pads were blocked by MCH blocking. Finally, the captured oligo (polyT)
was hybridized to polyA (5’ – AAA AAA AAA AAA AAA -3’) [Intergrated DNA
Technologies Inc., San Diago, CA, USA] by incubating with 100 nM PolyA in 10 nM
pH 7.2 PB solutions for 2 hour at room temperature.
2. Sensing measurements:
The sensing protocol consisted of 1) monitoring the initial resistance (RO) of the
biosensors fabricated using different scheme by measuring the source-drain current (I)
as a function of source-drain voltage (V) from -0.5 to +0.5 V using a CHI 1202 (CH
Instruments, Austin, TX, USA) electrochemical analyzer and taking the inverse of the
slope of the I-V curve from -0.1 V to +0.1 V, 2) incubation for 30 minutes at room
temperature with different concentration of Hg2+ ion sample in PB, washing three
times with PB and water, drying with nitrogen and recording the new resistance.
Figure S1 Schematic illustration of SWNTs chemiresistive label-free biosensor
fabrication steps through noncovalent functionalization of SWNTs with polyT
(Scheme I)
Figure S2 Current versus voltage (I-V) curves of SWNTs chemiresistive label-free
biosensor (fabricated via Scheme I) at various stages of fabrication and upon
exposure of 1  M of Hg2+

Figure S3 Responses of SWNTs chemiresistive label-free biosensor (fabricated via
Scheme I) to various metal ions. The concentrations of all metal ions were fixed at
1  M. Data for each metal ion was obtained from four different sensors at different
time and error bars represent ±1 standard deviation.

References
(1) R. J. Chen, Y. Zhang, D. Wang and H. Dai, J. Am. Chem. Soc., 2001, 123,
3838.