Supplementary information
Highly sensitive capacitive gas sensing at ionic liquid–electrode interfaces
Zhe Wang †, Min Guo†, Xiaoyi Mu‡, Soumyo Sen§, Thomas Insley§ , Andrew J. Mason‡ , Petr
Král §,∥,⊥, , Xiangqun Zeng † *
†
Department of Chemistry, Oakland University, Rochester, MI, USA
‡
Department of Electrical & Computer Engineering, Michigan State University, East Lansing,
MI, USA
§
Department of Chemistry, ∥,Physics, and ⊥Biopharmaceutical Sciences, University of Illinois
at Chicago, IL, USA
*
Corresponding Author ([email protected])
Content
1. Supplementary Note: Methods and experimental
(1) Figure
S1.
Ionic
liquid
1-butyl-1-methylpyrrolidinium
structure,
electrode
and
sensor
bis(trifluoromethylsulfonyl)imide
setup
(abbreviated
(a)
as
[C4mpy][NTf2] in this manuscript. It was also abbreviated as [Bmpy][NTf2] in the
literature) was selected as the electrolyte. (b) Microfabricated Au electrode fabrication
steps; (c) Geometry of the fabricated flexible electrode; (d) Cross-section view of
experimental fixture with different gas feeding pathways.
(2) Figure S2. Capacitance response with two different SO2 gas feeding pathways. DC bias
at -0.5V, AC frequency is 3Hz.
(3) Figure S3. Bode Plot of [C4mpy][NTf2]/Au system with different SO2 concentrations.
Phase vs. frequency curves at different SO2 concentrations in frequency range of 100
MHz to 0.1 Hz. DC bias: -0.5V.
(3) Table S1. Measured kinetic diameter (KD) values of selected gas molecules.
2. Supplementary Note: SO2 detection and Langmuir adsorption model.
(1) Figure S4. Long term stability measurements of SO2 sensing. DC bias: -0.5V and AC
frequency : 3Hz.
3. Supplementary Note: Definition of selectivity coefficients
4. Supplementary Note: Atomistic molecular dynamics (MD) simulations of the ionic
liquid-Au electrode interface with or without solvated SO2
(1) Figure S5. Atomistic molecular dynamics (MD) simulations of the ionic liquid-Au
electrode interface with solvated SO2.
(2) Figure S6. Distribution of cations and anions of [C4mpy][NTf2] with the distance from
the gold surface; Distribution of cations (A) in the absence and (B) in the presence of
SO2 gas molecules;
Distribution of anion (C) in the absence and (D) in the presence
of SO2 gas molecules.
Supplementary Note: Methods and Experimental
Figure S1. Ionic liquid structure, electrode and sensor setup. (a) 1-butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide
([C4mpy][NTf2])
was
selected
as
the
electrolyte.
(b)
Microfabricated Au electrode fabrication steps; (c) Geometry of the fabricated flexible electrode; (d)
Cross-section view of experimental fixture with different gas feeding pathways.
As shown in figure S1(b) a thin gold film was deposited on a porous PTFE film with a
interdigitated pattern. Physical vapor deposition (PVD) of gold was performed using an Edward 360
thermal evaporator. In order to pattern electrode, a stainless steel sheet hard mask was prepared by
electric discharge machining (EDM), tightly mounted against PTFE on PVD sample stage, and
removed after deposition, shown as STEP 1 and STEP 2 in Figure S1 (b). The flexible
microfabricated electrode is shown in Figure S1 (c). A working electrode (WE) and a counter
electrode (CE) are configured as interdigitated electrodes. A reference electrode is placed along the
working electrode. The finger width of and the gap between the CE and WE are both 200 µm. The
thickness of gold was 500 nm, ensured good continuity of the thin film without blocking the pores.
The gold electrodes were coated with 200 µm [C4mpy][NTf2] using a droplet process, as shown in
STEP 3 of Figure S1(b).
The fabricated electrode was placed in a gas chamber, sealed with an O-ring, as illustrated in
Figure S1 (c). Both the upper chamber and lower chamber has gas inlet and outlet for introducing
analyte gas and diluting gas (nitrogen or air) either from the top or bottom of the sensor. The total gas
flow was maintained at 200 ± 0.05 sccm (standard cubic centimeters per minute) by digital mass-flow
controllers (MKS Instruments Inc). The mass flow controllers were used to adjust the ratio of the
analyte gas and nitrogen flow rates. The total gas flow was set 200 sccm and it was maintain as a
constant at atmospheric pressure. EIS testing was carried out with a VersasStat MC potentiostat
(Princeton Applied Research, Oak ridge, TN, USA). A DC bias plus a sinusoidal AC signal with 10
mV peak-to-peak amplitude was applied for sensing SO2.
Figure S2. Capacitance response with different gas feeding pathways
Impedance response measured at 3 Hz with different gas pathways at 2 ppm SO2 under DC
bias=-0.5V. The capacitance changes were measured when SO2 was introduced either from the top
ionic liquid layer or directly from the ionic liquid-electrode interface (the gas permeable membrane
side in figure S1(d). The SO2 introduced directly to the IL/electrode interfacial region at 20 th second
gives a faster response time ((T90=10 seconds) in comparison to SO2 that was introduced via the IL
bulk (T90=55 seconds). The mass transport of SO2 in the bulk ionic liquid contributes to the slow
response due to the high viscosity of the ionic liquid. The maximum capacitances measured in both
cases are identical confirming that the sensing signal is due to the capacitance change at the interface
when exposed to the analyte SO2.
Figure S3. Bode Plot of [C4mpy][NTf2]/Au system with different SO2 concentration. Phase vs.
frequency curves at different SO2 concentrations in frequency range of 100 MHz to 0.1 Hz.
Table S1. Measured Kinetic diameter values of selected gas molecules from literatures1
Molecule
Diameter
(nm)
CH4
CO2
O2
NO2
NO
SO2
H2
CO
0.38
0.33
0.346
0.33
0.317
0.41
0.29
0.376
Supplementary Note: Langmuir adsorption model for SO2 detection
The gas absorption process at IL-electrode interface can be explained by Equation (S1)

K1C SO2 , IL
1 K1C SO2 , IL
(S1)
where θ is surface coverage of the working electrode, CSO2,IL is the concentration of SO2 in IL, K1 is the
ratio of the direct and inverse rate constants in absorption equilibrium equation. Since CSO2,IL is
proportional to SO2 concentration (CSO2) in gas according to the Henry law, kH was defined as the Henry
constant:
kH 
C SO2 , IL
C SO2
(S2)
If K = kH ×K1 , Equation (S2) can be rewritten as:

KC SO2
1  KC SO2
(S3)
Define Cdl0 as double-layer capacitance value when the surface coverage of SO2 is zero (θ=0) and Cdl1 as
double-layer capacitance value when the surface coverage of SO2 is 100% (θ=100%). The double-layer
capacitance Cdl can be expressed in terms of θ as follows
Cdl  Cdl0  (1   )  Cdl1 
(S4)
Derived from Equation (S3) and (S4), Cdl can be rewritten as
Cdl  Cdl0  (Cdl1  Cdl0 ) 
KC SO2
1  KC SO2
(S5)
Equation (S5) reveals the relationship between double-layer capacitance and SO2 concentration.
Picking the data in the Figure 4 (a) at frequency of 3Hz, Cre response as a function of CSO2 is plotted as
blue dots in Figure 4 (b). Using Equation (S5) to fit the test data, the fitting curve is plotted as red line
with R2=0.9990. The relationship between Cre and CSO2 is
Cre  43.34F  30.00F 
CSO2
CSO2  3.06 ppm
(S6)
Equation (S6) presented as equation 2 in the manuscript.
Figure S4. Long stability measurement using kinetic size-spectra measurement. The capacitance
at 3 Hz with 2 ppm SO2 is plotted over the measurement period of 90 days. The signals are
normalized on the first day of sensor signal. These values were calculated by taking the mean of at
least three measurements. During the rest of the measurements the whole system was blanked with
nitrogen. Figure S4 demonstrates IL double layer capacitor stability over 90 days during which a
capacitance change of ± 1 % is observed without any significant loss in the capacitance signal,
revealing influence of SO2 to IL EDL was reproducible for SO2 detection.
Supplementary Note: Definition of selectivity coefficients
We compared the signal of interfering gases to the signal of SO2 at 2 ppm (cSO2(2ppm)). The total signal
will be given by modified version of calibration sensitivity equation (S8) which is accepted by the
international Union of Pure and Applied Chemistry (IUPAC). That is,
S= Sbl+mSO2,cSO2 + mici (S7)
where S is the measured signal, ci is the concentration of interference species i, mi is the
corresponding calibration sensitivity, and Sbl is the signal from the blank. A selectivity coefficient, ki ,
expressing the sensitivity of the sensor toward species i with respect to SO2 is given by
ki = mi/mSO2
(S8)
Supplementary Note: Atomistic molecular dynamics (MD) simulations of the ionic liquid-Au
electrode interface with or without solvated SO2
The computational modeling was
done by preparing the ionic liquid [C4mpy][NTf2]
(1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide) in a box (42 x 42 x 84 Å3). We
simulated the system in the NVT ensemble, using the Langevin dynamics with a damping constant of
γLang = 0.1 ps-1 and a time step of 2 fs. These systems with differently charged gold electrodes
(electric fields) were equilibrated with NAMD [7] for 100 ns. The ionic liquid was simulated for 5 ns
with or without randomly distributed (120) SO2 gas molecules (1512.28 mmol/lt) and then placed
above a gold (111) surface (40x40x40 Å3), prepared by VESTA [1]. We considered differently charged
surface gold atoms to show the effect of electric field on the distribution of cations, anions and SO 2
molecules [8]. For ionic liquid, we used parameters from the forcefield developed by Borodin and
Smith [2], while SO2 parameters were taken from a CHARMM general forcefield [3,4]. The van der
Waals (vdW) interactions associated with all the molecules were also calculated using the CHARMM
general forcefield, while the vdW parameters of gold atoms on the (111) surface were taken from Iori
et. al.’s [5]. The systems were neutralized by changing the number of cations and anions. Nonbonding
interactions were calculated using a cut-off distance of d=10 Å and long range electrostatic
interactions were calculated by the PME method [6] in the presence of the periodic boundary
conditions. We analyzed our results for last 30 ns of the simulations.
Figure S5. Atomistic molecular dynamics (MD) simulations of the ionic liquid-Au electrode
interface with solvated SO2
(A) Distribution of SO2 above the gold surface obtained for its
different charging. (B) The number of SO2 molecules within the first peak, separated 4 Å away from
the surface, obtained at different surface charging.
Figure S6. Distribution of cations and anions of the ionic liquid with the distance from the gold
surface. Distribution of cations (A) in the absence and (B) in the presence of SO 2 gas molecules.
(C-D) The same for anions.
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