Embed Size (px)
Citation preview
• Electrochemical detection of nutrients without any addition of liquid reagents and no interferences
• Methods validated with natural samples• Calibrationless method in progress for low
concentration of silicate
• Development of electronic cards for silicate• Fabrication of first in situ phosphate prototype• Adaptation of calibrationless method to phosphate• Adaptation of silicate and phosphate sensors for in
situ measurements in the ocean• Decrease the detection limit of nitrate
[1] Lacombe et al., Silicate electrochemical measurements in seawater: Chemical and analytical aspects towards a reagentless sensor; Talanta 77 (2008) 744-750[2] Giraud/Lesvenet al., Reagentless and calibrationless silicates measurement in oceanic water; Talanta 97 (2012) 157-162[3] Joncaet al., Phosphate determination in sea water: Toward a reagentless electrochemical method; Talanta, 87 (2011) 161-167.[4] Joncaet al., Electrochemical behaviour of isopoly- and heteropolyoxomolybdates formed during anodic oxidation of molybdenum in seawater; Int. J. of Electrochem. Sci. 7 (2012) 7325-7348[5] Joncaet al., Reagentless and silicate interference free electrochemical phosphate detection in seawater; Electrochim. Acta 88 (2013) 165-169[6] Fajerwerget al., An original nitrate sensor based on silver nanoparticles electrodeposited on a gold electrode; Electrochem. Comm. 12 (2010) 1439-1441
ANESIS: Autonomous Nutrient Electrochemical Sensor In SituC. Barus1, J. Jońca1, W. Giraud1, N. Striebig2, M. Armengaud2, K. Fajerwerg3, M. Comtat4, V. Garçon1
1 - Laboratoire d’É tudes en Géophysique et Océanographie Spatiales, Toulouse / 2 - Observatoire Midi-Pyrénées, Toulouse3 - Laboratoire de Chimie de Coordination, Toulouse / 4 - Laboratoire de Génie Chimique, Toulouse, FRANCE
The use of 2 working electrodes with different sizes allows to determine silicate concentration directly, without any calibration step [2].
Macro Electrode (ME) (Ø~2 mm):
Ultra-Micro Electrode (UME)(Ø~15 µm):
� 2 equations, 2 unknowns:simultaneous determination of C and DI: intensity (A), n: electron number, S: surface (cm2), C: concentration (mol.cm-3), F: Faraday constant, D: diffusion coefficient (cm2.s-1), r: radius (cm), ν: scan rate (V.s-1)
2/12/122/35 ......10.69,2 vCDrnI MEME π=
UMEUME rCDFnI .....4=
The detection of nitrate occurs on gold working electrode covered by silver nanoparticles. Two simultaneous catalytic processes are observed increasing the obtained electrochemical signal [6].
NO3- + 2H+ + 1e- = NO2
• + H2O
2 NO2• + H2O = NO3
- + 2H+ + NO2-
O2 + 2H+ + 2e- = H2O2
H2O2 + NO2- = NO3
- + H2O
Calibrationlessmethod
- Nutrients limit oceanic primary production and serve as water mass tracers- Long term monitoring and real time transmission of nutrients data will allow us to better understand biogeochemical cycles
- Monitoring of chemicals in the ocean requires an in situminiaturized autonomous instrumentation - Electrochemistry provides: miniaturisation, reagentless and calibrationless methods, reduction of energy requirement
Conclusions and perspectives
NitratePhosphateSilicate
References
As silicate is not electroactive species a complexation at acidic medium (pH ≤ 1.5) with molybdates is required [1].
Si(OH)4 + 12 MoO42- + 24 H+ ���� H4Si(Mo12O40) + 12 H2O
The reagents needed are formed in situby a simple oxidation of molybdenum metal. The acidic pH is reached thanks to a non-proton exchanged membrane which avoids the reduction of protons on the counter-electrode.
Mo + 4 H2O ���� MoO42- + 8 H+ + 6 e-
1st sensor prototype:
Pump50 µL
Molybdenum cellDetection cell
Cyclic voltammograms of silicate (140 µM) obtained with this prototype (≠ flow rates) compared with agitated solution
Scientific rationale
Counter-electrode
Workingelectrode (Au)
MoReference
Non proton exchangemembrane
[email protected] – Collaborative on Oceanographic Chemical Analysis – COCA Workshop, 26-29 March 2013
Electrochemical method is in excellent agreement with the colorimetric analysis. The average deviation obtained for all phosphate concentrations is 4.9 % (natural seawater samples off Peru).
Amperometry at rotating gold electrode (0.29 V –2000 rpm): Standard addition of phosphate 0.5-3.5
µM and silicate 8.5 and 154.5 µM in sea water.
y = 0.9819x + 0.0962R2 = 0.985
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Phosphate concentration by electrochemistry, µµµµM
Co
nce
ntr
atio
n p
ho
sph
ates
sp
ectr
op
ho
tom
etry
,µµ µµ
M
Electrochemical detection of phosphate is possible in acidic medium (pH ≃ 1) in the presence of molybdates [3].
PO43- + 12 MoO4
2- + 27 H+ ���� H3PMo12O40 + 12 H2O
MoO42- and H+ formed via anodic oxidation of molybdenum [3,4].
Mo + 4 H2O ���� MoO42- + 8 H+ + 6 e-
Silicate interferenceissue: Use of anappropriate ratio H+/MoO4
2- of 70 and an electrochemical cell with a specialisedmembrane technology adaptation [5].
C
Au MoRef
Non proton exchangemembrane
H+MoO42-
H+
Proton exchangemembrane
-1.0
-0.8
-0.6
-0.4
-0.2
0 50 100 150 200 250 300
Time (s) - Phosphate
J (µ
A.c
m-2)
0 100 200 300 400 500 600
Time (s) - Silicate
Silicate
Phosphate
H+/MoO42- = 70, pH 1
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.1 0.2 0.3 0.4 0.5
E (V/Ref)
I (µ
A)
agitated solutionsensor with flow rate = 200 µL.s-1
sensor with flow rate = 3.33 µL.s-1
Au
Ag NPs Ag NPs
O2 H2O2
NO2- + NO3
-
NO3-
NO2•
NO3-
NO2•
[NO3-]= 9mM
500 mV/s – aerated artificial seawaterLOD = 10 µM
E (V/ECS)
j (m
A.c
m-2)
Au
Ag
Au+AgNPs
