Electronic Supplementary Material (ESI) for Journal of Environmental Monitoring
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Supplementary Information
Micro Electronic Wireless Nitrate Sensor Network for Environmental
Water Monitoring
Manas Ranjan Gartia, Björn Braunschweig, Te-Wei Chang, Parya Moinzadeh, Barbara S. Minsker,
Gul Agha, Andrzej Wieckowski, Laura L. Keefer and Gang Logan Liu
Photoresist
Glass
Photolithography
Metal deposition Au (200 nm) on Ti (20 nm)
Photolithography
Ag (200 nm) on Ti (20 nm)
R
W
C
R: Reference; W: Working; C: Counter
Figure S1: Fabrication steps for the micro sensor.
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Potentiostat Circuit Board
D2A
TI 8565
(Triangle Signal)
(Bias voltage)
+
_
Amplifier A
R
+
_
SHM-DAQ
Counter
Amplifier B
Reference
Imote2 wireless
node
MAX764
±5V
Power supply
Working
Figure S2: Schematic of circuit design for miniaturized potentiostat.
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-3
2.0x10
-3
-2.0x10
Ep / V vs Ag/AgCl, (3.0 KCl)
j/ A cm
-2
0.0
-3
-4.0x10
-3
-1
-6.0x10
10 mV s
-3
-8.0x10
-1
1350 mV s
-0.86
-0.88
-0.90
-0.92
-0.94
-0.96
-0.98
0.0
0.2
0.4
0.6
0.8
-1
1.0
1.2
1.4
ν/Vs
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
0.2
E/ V vs. Ag/AgCl, (3 M KCl)
Figure S3: Cyclic voltammograms using a silver macro electrode in 10 mM NO3− and 0.1
M NaOH, with varying the scan rate from 10 to 1350 mV s-1. Inset shows the variation of
peak potential vs. scan rate.
3
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-3
-3
1.2x10
-4
8.0x10
1/2
(ip/ν ) / A V
s
-1/2 1/2
1.6x10
-4
4.0x10
0.0
0.2
0.4
0.6
0.8
-1
1.0
1.2
1.4
ν/Vs
Figure S4: Variation of i p / v1 2 vs. scan rate for nitrate reduction on silver macro
electrode in 0.1 M NaOH electrolyte.
-4
1.4x10
1/2
(ip/ν ) / A V
s
-1/2 1/2
-4
1.3x10
-4
1.2x10
-4
1.1x10
-4
1.0x10
-5
9.0x10
-5
8.0x10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
-1
ν/Vs
Figure S5: Variation of i p / v1 2 vs. scan rate for nitrate reduction using micro sensor in
0.1 M NaOH electrolyte.
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-4
1.0x10
0.0
-4
i/ A
-1.0x10
-1
4 mM (50 mV s )
Ar purged
-1
4 mM (50 mV s )
without Ar purged
-1
4 mM (100 mV s )
Ar purged
-1
4 mM (100 mV s )
without Ar purged
-4
-2.0x10
-4
-3.0x10
-4
-4.0x10
-4
-5.0x10
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2
0.0
0.2
E/ V vs. Ag/AgCl, (3 M KCl)
Figure S6: Cyclic voltammograms (50 and 100 mV s-1) for the reduction of 4 mM nitrate
in 0.1 M NaOH electrolyte with and without purging oxygen by argon.
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a
0.0
Blank
400 pM
4 nM
40 nM
400 nM
4 uM
40 uM
200 uM
400 uM
5 mM
25 mM
50 mM
-5
i/ A
-5.0x10
-4
-1.0x10
-4
-1.5x10
-4
-2.0x10
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
+
E/ V vs. Ag/Ag
b
0.0
-6
-4.0x10
i/ A
-6
Blank
0.4 pM
4 pM
40 pM
400 pM
4 nM
40 nM
400 nM
1.6 uM
-8.0x10
-5
-1.2x10
-5
-1.6x10
-5
-2.0x10
4 uM
8 uM
16 uM
40 uM
160 uM
400 uM
800 uM
1.6 mM
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
+
0.2
E/ V vs. Ag/Ag
Figure S7: Cyclic voltammograms (50 mV s-1) of nitrate with different concentration
using micro sensor in a microfluidics. (a) Higher nitrate concentration (b) lower nitrate
concentration.
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a
-6
1.7x10
-6
1.6x10
-6
1.5x10
i/A
-6
1.4x10
-6
1.3x10
-6
1.2x10
-6
1.1x10
0
1
2
3
4
5
Concentration/ μM
-6
5.0x10
b
-6
4.5x10
-6
i/A
4.0x10
-6
3.5x10
-6
3.0x10
-6
2.5x10
-6
2.0x10
-6
1.5x10
0
100
200
300
400
500
Concentration/ μM
-5
8.0x10
c
-5
7.0x10
-5
6.0x10
-5
i/A
5.0x10
-5
4.0x10
-5
3.0x10
-5
2.0x10
-5
1.0x10
0.0
0
10
20
30
40
50
60
Concentration/ mM
Figure S8: Calibration curves for nitrate using micro sensor in a microfluidics.
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Figure S9: Location from which water samples collected are shown on google map
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5.0x10
Bored well
Pond
Crystal lake
Michigan lake
Lake of the woods
-6
i/ A
0.0
-5.0x10
-6
-1.0x10
-5
-1.5x10
-5
-2.0x10
-5
-2.5x10
-5
-3.0x10
-5
-3.5x10
-5
-4.0x10
-5
Chicago (tap water)
Urbana (tap water)
San Diego (tap water)
Los Angeles (tap water)
Bottled water
Stream water
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
E/ V vs. Ag/Ag+
Figure S10: Electrochemical measurements of water samples using iNits.
-6
1.6x10
-7
8.0x10
i/ A
0.0
-7
-8.0x10
-6
-1.6x10
Blank
Sample-1
Sample-2
Sample-3
Sample-4
-0.794
-6
-0.90
-2.4x10
-0.857
-0.812
-6
-3.2x10
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
0.2
+
E/ V vs. Ag/Ag
Figure S11: Cyclic Voltammetric determination of nitrate ions in water samples using
micro sensor and microfluidics.
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0.3
0.2
0.1
0.0
400
450 500 550 600
Wavelength (nm)
0.4
0.3
0.2
0.1
0.0
400
650
450 500 550 600
Wavelength (nm)
650
Standard (5 ppm)
(Tap water)
Los Angeles
San Diego
Chicago
Urbana (IL)
0.8
Absorbance (a.u.)
Absorbance (a.u.)
0.4
Standard (5 ppm)
Stream water
Crystal lake
Lake of the woods
(outside)
0.5
Absorbance (a.u.)
Standard (1 ppm)
Bottled water
Bored well
Michigan lake (Chicago)
Lake of the woods (inside)
0.6
0.4
0.2
0.0
400
450 500 550 600
Wavelength (nm)
650
Figure S12: Nitrate reductase (NaR) method to determine the nitrate concentration. The
absorbance peak at 543 nm is used for determining the nitrate-nitrogen concentration in
water samples.
3
2
1
∗
π
∗
π
Absorbance (a.u.)
Absorbance (a.u.)
4
0
3
1 μM
5 μM
10 μM
25 μM
50 μM
100 μM
200 μM
1 mM
10 mM
100 mM
n
1
0
π
200
2
Bottled water
Bored well
Stream water
Crystal Lake
Lake of the woods (inside-pond)
Lake of the woods (outside)
Michigan Lake
Tap water (Urbana)
Tap water (Chicago)
Tap water (Los Angeles)
Tap water (San Diego)
250
300
Wavelength (nm)
350
190 200 210 220 230 240 250
Wavelength (nm)
(a) Standard nitrate samples
(b) Water samples
Figure S13: Ultraviolet screening method to determine the nitrate concentration. The
absorbance peak at 200 nm is used for determining the nitrate-nitrogen concentration in
water samples.
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Absorbance at 200 nm
2.4
2.0
Regression slope = 0.00878
y-intercept = 0.55192
2
R = 0.98634
1.6
1.2
0.8
0.4
0
50
100
150
200
Nitrate concentration (μM)
Figure S14: Calibration curve for the known concentration of nitrate standards.
0.6
100 μM
0.4
0.2
10 μM
Absorbance /Arbitr. Units
Absorbance /Arbitr. Units
0.8
0.4
Water sample-1
Water sample-2
Water sample-3
Water sample-4
0.3
0.2
0.1
0.0
200
250
300
350
Wavelength / nm
400
0.0
-0.2
200
250
300
350
400
Wavelength / nm
Figure S15: UV absorption spectra of nitrate with different concentration from 10 to 100
μM in DI water. The inset shows the UV absorption spectra of collected unknown water
samples.
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1 μM
5 μM
10 μM
25 μM
50 μM
100 μM
200 μM
0.0025
0.0000
0.003
220
230
240
Wavelength (nm)
Lake of the woods (pond)
Lake of the woods (outside)
Stream water
0.02
Tap water (Chicago)
Tap water (Los Angeles)
Tap water (San Diego)
Bored well
Tap water (Urbana)
Bottled water
Crystal Lake
Michigan Lake
0.01
0.002
0.00
0.001
220
230
240
-0.01
210
220
230
240
Wavelength (nm)
250
-5
-0.01
0.000
210
0.00
250
0.006
Second derivative
-0.0025
210
Second derivative
0.01
Second derivative
0.0050
Michigan Lake
Second derivative
Second derivative
0.0075
Regression slope = 2.6733 x 10
-4
y-intercept = 9.49123 x 10
2
R = 0.99976
0.004
0.002
250
0
Wavelength (nm)
50
100
150
200
Nitrate concentration (μM)
Figure S16: Second derivative Ultraviolet screening method to determine the nitrate
concentration. The second derivative peak at 226 nm is used for determining the nitratenitrogen concentration in water samples.
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Table S1: Water quality data of the water samples tested.
Samples
Location
Tap water
Chicago
Urbana
Los
Angeles
(LA)
San Diego
(SD)
Lake of the
woods,
Champaign
Crystal
lake,
Champaign
Michigan
lake,
Chicago
Nestle pure
life
Champaign,
IL
Lake of the
woods,
Champaign
Champaign,
IL
Sample-1,
South
campus,
UIUC
Sample-2,
South
campus,
UIUC
Sample-3,
South
campus,
UIUC
Sample-4,
South
campus,
UIUC
Lake
Bottled
water
Bored
well
Pond
Stream
water
Total
hardness
(ppm)
100
100
100
Total
Chlorine
(ppm)
0
1
0
Total
Bromine
(ppm)
0
2
0
Free
Chlorine
(ppm)
0
1
0
pH
7
8
7.8
Total
Alkalinity
(ppm)
80
150
180
Cyanuric
acid
(ppm)
40
40
30
250
0
0
0
8.4
180
40
250
0.5
1
0
8.4
180
150
250
0.5
1
0
8.4
240
100
100
0.5
1
0
8.4
180
30
100
0
0
0
7.2
120
15
500
0
0
0
8.4
180
40
250
0
0
0
8.4
120
40
500
0
0
0
8.4
240
40
400
0
0
0
8.0
200
40
500
0
0
0
8.2
120
40
400
0
0
0
8.4
200
40
450
0
0
0
8.0
240
40
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Chemicals and apparatus
A 0.1 M nitrate stock solution was purchased from Thermo scientific. A silver-bulk
electrode was prepared by sealing a 0.5 mm diameter 99.99% silver wire (Alfa Aesar,
Ward Hill, MA, USA) in a glass tube using epoxy adhesive. The electrode surface was
mechanically polished using commercial silver cleaner followed by 0.2 μm alumina
powder. Ag/AgCl, NaCl (3.0 M), and a Pt wire were used as reference and counter
electrodes, respectively. Cyclic voltammetry (CV) measurements on silver electrodes
(bulk) and micro-sensor were carried out with an Autolab PGSTAT 100 potentiostat at
room temperature. All measurements with the bulk electrode were performed under an
argon atmosphere in an electrochemical cell with a 0.1 M NaOH (electrolyte) and 0.1 M
H2SO4 liquid-liquid junction. The Ag electrode was electrochemically activated by
cycling the potential from -1.3 to 1.0 V vs. a commercial Ag/AgCl reference with a scan
rate of 1 V s-1 for 20 cycles.
Methods
Ultraviolet absorption spectroscopy
Ultraviolet (UV) absorption spectroscopy is also used to confirm and evaluate the nitrate
concentration. In our experiment, a quartz cuvette containing the liquid sample was tested
in an Evolution 60 UV-Visible Spectrophotometer for absorption measurement. For
calibration of instrumental error, another cuvette with de-ionized water was used for
reference. The scanning wavelength range is from 190 nm to 400nm, the scanning step is
1 nm and the integration time for each step is 1.5 seconds. We analyzed the collected
water samples, and solution with different nitrate concentrations, shown in Figure S12,
S13 and inset (of Figure S13). The measured UV absorption spectra of nitrate solution
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correspond to that in the literature 1, 2. The absorption of known nitrate solution increases
with the concentration while the shape of the absorption spectra remains the same. The
absorption spectrum for nitrate solution showed two distinct peaks at the wavelength of
200-210 nm and 300 nm (this peak appears at concentrations higher than 1 mM only).
We found that the absorption spectra of collected spring water samples are very similar to
that of nitrate solution with distinct absorption peaks appearing at 200-220 nm. The
nitrate concentration was determined from the calibration curve obtained using known
concentration of nitrate standards.
Analysis of cyclic voltammetric (CV) data and comparison with bond dissociation
free energy
The potential dependent free energy of activation for the nitrate reduction reaction can be
written as a quadratic function of electrode potential as follows from Marcus’s theory 3-7:

ΔG 0 
ΔG = ΔG 1 +
† 
 4ΔG0 
†
2
†
0
(5)
where ΔG 0 = F ( E − E 0 ) , F is Faraday’s constant, E 0 is the formal potential of the
overall reaction and has a value of 0.01 V 8 and ΔG0† is the intrinsic energy barrier for the
reduction reaction9. The free energy of activation at the peak potentials corresponding to
the cyclic voltammograms can be obtained by 10:
ΔG † =


RT  
RT
ln  Z
 − 0.78
F  
F ( β × n)vD 

(6)
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where R is the gas constant, T is the temperature in K, Z =
RT
in cm s-1, M is the
2π M
molecular mass of nitrate and D is the diffusion coefficient in cm2 s-1. β × n is calculated
as11:

1.86 RT
 F Ep − Ep 2

β ×n = 




(7)
where E p is the peak potential and E p 2 is the potential when the current is half of the
peak value. The resulting quadratic equation was solved for ΔG0† using a MATLAB
program. Finally, the bond dissociation free energy (BDFE) was calculated from the free
energy of activation using the equation 7
ΔG0† =
R0 + Ri + BDFE
4
(8)
where the solvent reorganization energy, R0 , is calculated using Marcus equation as
R0 =
 1 1
 − 
8πε 0 a0  η 2 ε s 
e2
(9)
Here, η and ε s are the refractive index and static dielectric constants of the solvent,
respectively and a0 is the effective radius of the analyte calculated by the equation 12:
 2a − aB 
a0 = aB  AB

 a AB

(10)
where a AB = aNO− = 2.64 A o and aB = aO− = 1.76 A o . Neglecting internal reorganizational
3
energy, Ri , (due to its small value) and substituting the earlier calculated value of ΔG0† (
intrinsic energy barrier for the reduction reaction) from Table S1, we can calculate the
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bond dissociation energy. For a nitrate concentration of 10 mM, scan rate of 0.01 Vs-1
and using an average diffusion coefficient of 1.9 ×10−5 cm 2s -1 9, we have calculated the
BDFE to be 47.2 kcal mol-1, which matches well with the value of 47.5 kcal mol-1,
reported using photodissociation measurements 13 and theoretical studies 14.
Table S1 Analysis of cyclic voltammetric (CV) data for nitrate ion reduction (10 mM)
using Micro sensor in microfluidics in 0.1 M NaOH
Scan rate/V s-1
105Ip/A
Ep/V
(Ep-Ep/2)/V
ΔG0† /kcal mol-1
0.01
0.779
-0.874
0.065
22.71
0.02
0.972
-0.879
0.069
21.78
0.04
1.229
-0.879
0.072
20.89
0.08
1.601
-0.894
0.080
19.86
0.1
1.812
-0.903
0.082
19.44
0.2
2.334
-0.913
0.084
18.35
0.3
2.712
-0.923
0.087
17.67
0.4
3.121
-0.923
0.088
17.28
0.5
3.521
-0.933
0.090
16.83
1.0
4.835
-0.952
0.092
15.53
The calculated value of ΔG0† is much greater than that expected for a step wise reduction
of nitrate forming NO3•2− . Hence, in the current system, nitrate reduction is taking place
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in a concerted manner with NO2− and O•− as intermediate products
9, 15
. The expected
reduction reaction can then be written as 9:
NO3− + e− → NO2− + O•− (rate determining step)
O•− + e− + H 2O → 2OH −
References
1. A. Ianoul, T. Coleman and S.A. Asher, Anal. Chem., 2002, 74, 1458-1461.
2. M.A. Ferree and R.D. Shannon, Water Res., 2001, 35, 327-332.
3. Marcus, R.A. On the Theory of Oxidation-Reduction Reactions Involving Electron
Transfer .1. J. Chem. Phys. 1956, 24, 966-978.
4. Marcus, R.A. Electrostatic Free Energy and Other Properties of States having
Nonequilibrium Polarization .1. J. Chem. Phys. 1956, 24, 979-989.
5. Marcus, R.A. Chemical + Electrochemical Electron-Transfer Theory. Annu. Rev. Phys.
Chem. 1964, 15, 155-&.
6. Marcus, R.A. Exchange reactions and electron transfer reactions including isotopic
exchange. Theory of oxidation-reduction reactions involving electron transfer. Part 4.—A
statistical-mechanical basis for treating contributions from solvent, ligands, and inert salt.
Discuss. Faraday Soc. 1960, 29, 21-31.
7. Saveant, J.M. Dissociative Electron Transfer, In Advances in Electron Transfer
Chemistry, Mariano, P.S., Ed.; JAI Press: Greenwich, 1994; Vol.4 pp. 53-116.
8. Handbook of Electrochemistry. Elsevier: Amsterdam, 2007;
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Electronic Supplementary Material (ESI) for Journal of Environmental Monitoring
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9. Bhat, M.A.; Ingole, P.P.; Chaudhari, V.R.; Haram, S.K. Mechanistic aspects of nitrate
ion reduction on silver electrode: estimation of O-NO(2)(-) bond dissociation energy
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polar solvent. An example of a dissociative electron transfer with significant attractive
interaction between the caged product fragments. J. Am. Chem. Soc. 2000, 122, 98299835.
11. Bard, A.J. and Faulkner, L.R. Electrochemical Methods: Fundamentals and
Applications. Wiley: New York, 2001;
12. Handbook of Chemistry and Physics. CRC Press: New York, 1999;
13. Kim, B. NO3, the Study of Molecular Properties and Photodissociation by Ab Initio
Method, Spectroscopy, and Translational Spectroscopy. 1991,
14. Zeng, X.; Chen, W.; Liu, J.; Kan, J. A theoretical study of five nitrates: Electronic
structure and bond dissociation energies. Journal of Molecular Structure-Theochem
2007, 810, 47-51.
15. Broder, T.L.; Silvester, D.S.; Aldous, L.; Hardacre, C.; Crossley, A.; Compton, R.G.
The electrochemical oxidation and reduction of nitrate ions in the room temperature ionic
liquid [C(2)mim][NTf2]; the latter behaves as a 'melt' rather than an 'organic solvent'.
New Journal of Chemistry 2007, 31, 966-972.
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