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Supplemental Material
Three micron gap gold microelectrodes were fabricated on SiO2/Si substrate using
standard photolithographic technique and electron beam deposition.
Prior to the
synthesis of polyaniline nanowires the electrode patterns were cleaned by piranha
solution (70 vol. % H2SO4/30% vol. H2O2) for 2 minutes, rinsed with water and then
dried under a stream of nitrogen.
A pair of gold microelectrodes was connected to conductive copper tape for
connection to a potentiostat and to the measurement systems. An EG & G Galvanostat /
Potentiostat (Princeton Applied Research Model 273A) was used for electrodepostion of
polyaniline nanowires. The electrochemical synthesis of polyaniline nanowires was
carried out using an aqueous solution containing 0.5 M aniline and 1.0 M HClO4 in
double distilled water. A standard three-electrode electrochemical cell was used wherein
both gold microelectrodes were shorted and used as a working electrode, a Pt foil served
as a counter electrode, and a standard Ag/AgCl functioned as a reference electrode. The
polyaniline nanowires were synthesized (to bridge the gap between two gold
microelectrodes) using two-step electrodepostion method wherein applied current density
was varied in stepwise manner to facilitate nanowire formation. The schematic of
polyaniline nanowires based chemiresistive sensor is as shown in figure S1.
For the functionalization of polyaniline nanowires network by gold nanoparticles,
the gold microelectrodes with polyaniline nanowires were inserted into an aqueous
solution containing 0.1 M KCl and 0.5 mM HAuCl4 and then subjected to 5 cycles using
cyclic voltammetry between + 0.2 V to -0.5 V vs Ag/AgCl reference electrode along with
Pt counter electrode in a three electrode electrochemical cell. The scan rate was fixed at
2
50 mV/s. Optical microscopy (Hirox HI-Scope Advanced KH-3000, River Edge, NJ,
USA) was used for visual verification of polyaniline nanowires bridging two gold
electrodes. Polyaniline nanowires network before and after functionalization with gold
nanoparticles was also characterized by scanning electron microscope and Energy
Dispersive X-ray analysis (EDAX) (SEM, Leo SUPRA 55, Model 1550).
For gas sensing studies, the electrodes were wire-bonded to a chip holder and
each electrode was connected in series with a load resistance of comparable value to the
resistance of the sensor in order to optimize resolution1. The circuit was subjected to 0.5
V DC potential and the current was continuously monitored with sample rate of five
samples per second. Electrical resistance of the sensor was determined by continuous
monitoring the voltage over the load resistor and applying Ohm’s Law. A 1.3 cm3 glass
chamber with inlet and outlet ports was positioned over the microfabricated chip with a
sandwiched O-ring and sealed by using a clamp. Gas at 200 std. cm3 min-1 flowed
through the glass chamber. Dry air (purity: 99.998%) and H2S (purity: 99.99%) were
used as the carrier and analyte gases, respectively (Air gas Inc., Riverside, CA, USA).
The carrier gas was diluted with the analyte gas to obtain different H2S concentrations.
Analyte and carrier gas flow rates were regulated by mass flow controllers (Alicat
Scientific Incorporated, Tucson, AZ). A custom LabView computer program was
developed to control and monitor the voltage of the nanowire circuit using a Field Point
analog module from National Instruments (Austin, TX, USA)1.
References
1
C.M.Hangarter, M.A.Bangar, S.C. Hernandez., W.Chen, M. A. Deshusses, A.
Mulchandani, and N. V. Myung, Appl. Phy. Lett. 92, Art. 073104 (2008).
3
Figure S1. Schematic of polyaniline nanowire based chemiresistive sensor.