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Nitrate amperometric sensor in neutral pH based on Pd nanoparticles on epoxy-copper electrodes
Accepted Manuscript Title: Nitrate amperometric sensor in neutral pH based on Pd nanoparticles on epoxy-copper electrodes Author: Albert Gut´es Carlo Carraro Roya Maboudian PII: DOI: Reference:
S0013-4686(13)00669-5 http://dx.doi.org/doi:10.1016/j.electacta.2013.03.199 EA 20334
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
31-1-2013 27-3-2013 29-3-2013
Please cite this article as: A. Gut´es, C. Carraro, R. Maboudian, Nitrate amperometric sensor in neutral pH based on Pd nanoparticles on epoxy-copper electrodes, Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.03.199 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nitrate amperometric sensor in neutral pH based on Pd nanoparticles on epoxycopper electrodes Albert Gutés, Carlo Carraro, Roya Maboudian*
Roya Maboudian 201 Gilman Hall
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Department of Chemical and Biomolecular Engineering
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Author for correspondence:
University of California
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Berkeley, CA, 94720
Fax: +1 510 642 4778 Email: [email protected]
http://cheme.berkeley.edu/faculty/maboudian/
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Website
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Tel: +1 510 643 7957
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Nitrate amperometric sensor in neutral pH based on Pd nanoparticles on epoxycopper electrodes Albert Gutés, Carlo Carraro, Roya Maboudian* Department of Chemical and Biomolecular Engineering, University of California,
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Berkeley, CA, 94720, USA
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Abstract
Amperometric nitrate sensing at neutral pH using an epoxy-copper electrode
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modified with palladium nanoparticles is presented. After epoxy-copper is hardened and polished, electroless deposition is employed for the deposition of
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Pd nanoparticles. Scanning electron microscopy and energy-dispersive X-ray spectroscopy reveal the morphology and composition of the Cu/Pd surface. The effect of Pd deposition time towards nitrate electroreduction is investigated,
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highlighting the importance of the bimetallic catalyst. The optimized electrode shows a linear response at pH=7 in the range from 2 to 35 ppm of nitrate. The
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simplicity and cost effectiveness of the fabrication process makes this Cu/Pd electrode a good candidate for distributed nitrate monitoring and determination the
Nitrate sensor, electrochemistry, epoxy-copper, palladium
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Keywords
field.
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nanoparticles, bimetallic catalyst 1. Introduction
Nitrate ground water pollution has become a growing concern in the last few decades due to its toxicity at ppm concentration levels [1-2]. Nitrate in groundwater can be divided into four categories depending on its source: (i) waste materials, (ii) natural sources, (iii) irrigation in agriculture, and (iv) row crop agriculture [3-5]. Overuse of nitrogen-based fertilizers and subsequent leakage of nitrates into waters have caused a tremendous increase and spread of this water pollutant worldwide [6]. Moreover, NOx increase in the atmosphere 1 Page 2 of 19
has been traced to overfertilization [7]. For these reasons, nitrate detection and monitoring is of great importance. Optical UV detection of nitrates at 220 nm can be employed [8], but is limited to the screening of clear unpolluted water samples with low organic contents. Other methodologies for nitrate detection include ion chromatography and the standard cadmium reduction method [9]. The Cd reduction method is a lengthy process and generates cadmium waste
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with the associated disposal issues; thus the final cost of analysis is high.
A promising method for the detection of nitrate is based on its electroreduction One of the most
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in the presence of metallic or bimetallic catalyst [10-14].
explored bimetallic catalysts with strong electrocatalytic reduction properties
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towards nitrate electroreduction is Cu-Pd. Much of the literature on the use of Cu-Pd or other metallic catalysts has been limited to nitrate electroreduction in
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acidic [15-21] and basic [22-27] media but since the majority of ground and drinking waters are close to neutral pH, the previously developed sensing materials cannot be directly used without sample pre-treatment. The majority of
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studies have focused on the determination of the nitrate electroreduction mechanism or on the catalytic and selective effect of the Cu-Pd electrode, De Vooys et al. presented a
ed
showing very high conversion rates to N2.
thorough study on the use of Cu-Pd electrodes for the electroreduction of nitrate, [28] proposing mechanisms in acidic and basic media, noting the
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differences as pH values change, as well as the influence of the Cu to Pd ratio and its effects towards selectivity and activity of the electrode. In addition, a
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recent review by Rosca et al. [29] presents an exhaustive compilation of the various electrocatalytical paths for nitrogen compounds, including nitrate, but again acidic or basic media were used in all nitrate electroreduction measurements. A few reports on nitrate detection by electroreduction in neutral media can be found in the literature [30], but in all cases, the detected nitrate ranges of concentrations exceed the legislation limits [31]. Here we present a simple and robust fabrication method for nitrate sensors with a linear response range of 2 to 35 ppm measured at pH=7.0. To achieve the required sensitivity, the Pd to Cu ratio had to be carefully optimized which we have achieved via electroless deposition schemes. To the best of our knowledge, this is the first time that electrochemical reduction of nitrate at neutral pH has been achieved in this concentration range. 2 Page 3 of 19
2. Experimental 2.1 Electrode construction Figure 1 shows schematically the fabrication of the epoxy-Cu electrodes.
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Epoxy-copper (Epoxy Technologies Epotek 430) is prepared as directed by the
manufacturer. PVC tubing of 3 mm inner diameter is cut in 2 cm long pieces. On
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one end, a 3 mm in diameter electrical cable is introduced 1 cm into the tubing.
Epoxy-Cu is applied to the other end until filling the remaining 1 cm cavity in the
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tube. After hardening, the epoxy-Cu is polished with sandpaper of decreasing grain size with the final polishing performed with alumina pastes of 1-, 0.3- and
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0.05-m size (CHI polishing kit CHI120) until mirror-shine is achieved. Extensive rinsing with deionized (DI) water (18 MΩ, Barnsted Nanopure Infinity) is performed prior to drying with nitrogen flow.
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Deposition of the Pd nanoparticles is performed, right after polishing, as follows. The deposition solution consists of 1 mM K2PdCl4 (Aldrich, 99.99% purity) and
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20 mM KCl (Aldrich, 99% purity) and is used for all Pd depositions. Excess of chloride is necessary to stabilize the PdCl42- anion in solution, avoiding its precipitation as PdCl2. Polished epoxy-Cu electrodes are immersed in the
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aforementioned solution for varied time duration, ranging from 10 to 300 s, then rinsed in DI water and dried in a nitrogen flow. As explained in the results
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section, the optimal deposition time towards nitrate reduction corresponds to 2min deposition time.
2.2 Characterization instruments Electrochemical measurements (cyclic voltammetry and coulometry) are performed using a CH660D potentiostat-galvanostat (CH Instruments, USA). A standard three-electrode configuration is used with a Ag/AgCl reference electrode, a Pt wire auxiliary electrode and the modified epoxy-Cu/Pd nanoparticle working electrode. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses are obtained using a JEOL JSM 6340F field emission scanning electron microscope. 3 Page 4 of 19
2.3 Amperometric detection All measurements are performed in a 0.1 M phosphate buffer solution (PBS, Fluka 99.4%) with pH adjusted to 7.0. Potassium nitrate (Fluka 99.8%)standard solution containing 1000 ppm nitrate in PBS pH = 7.0 is prepared daily. Cyclic
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voltammetry is performed between +0.2 and -1.2V at a scan rate of 50 mV/s
without stirring, starting at +0.2V. 10 cycles are performed in order to reach
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reproducible voltammograms. The measurements are performed in open glass beakers without any oxygen scavenging or removal. 3-step voltammetry is
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performed by applying potentials (vs. Ag/AgCl) as follows, measuring each 20 ms: +0.2V for 2 s (Cu-Pd cleaning / oxidation step) followed by 2 s at -0.2V (Cu-
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Pd regeneration step) and a final measuring step at -1V for 3 s. All the 3-step measurements are performed without stirring. The average of the final 10recorded currents is used as analytical data. The first two steps (cleaning and
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regeneration) are found to be crucial in order to avoid surface poisoning reported previously for nitrate electroreduction [21,30] and are selected by
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direct observation on the obtained cyclic voltammograms, where maximum currents are achieved on the positive scan at around -0.1 V due to Cu/Pd oxidation and maximum currents are achieved on the negative scan at around -
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0.2 V during Cu/Pd reduction.
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3. Results and discussion
3.1 Characterization of the epoxy-Cu/Pd nanoparticle substrate Figure 2 shows the SEM image of the polished epoxy-copper prior to palladium nanoparticles deposition (a) and after 2 min of palladium deposition (b). As can be observed, the electrode surface morphology changes substantially upon Pd deposition, with the formation of Pd nanostructures and films. Figure 2c shows the EDX spectrum of the same sample as in Figure 2b. The spectrum confirms the Pd deposition, with Pd/Cu ratio of 1:4.6. The Al signal is attributed to the presence of Al in the epoxy-Cu formulation as-received.
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3.2 Catalytic reduction of nitrate by cyclic voltammetry Figure 3a shows the typical cyclic voltammograms of the bare Cu electrode in a 0.1M PBS solution at pH=7.0 and in the presence of 20 ppm of nitrate. Figure 3b shows the response obtained with the epoxy-Cu/Pd electrode (2 min of palladium deposition) in the same PBS and nitrate dissolutions. Scan rate is 50
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mV/s in both cases. As can be observed, a reduction tail is obtained in the negative scan starting at -0.8V, corresponding to nitrate reduction on the Cu/Pd
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surface. In order to minimize possible interferences of hydrogen evolution, a described next.
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3.3 Three-step nitrate amperometric sensing
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less extreme potential of -1V is selected for the third step in the nitrate detection
To characterize fully the electrodes’ responses, three-step voltammetry
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measurements without solution stirring are performed, following a similar protocol as described in the literature [32] as described in section 2.3. Figure 4a
ed
shows the typical current responses obtained for the three-step voltammetry on the bare epoxy-Cu electrode while figure 4b shows the same response using an epoxy-Cu electrode after a 2 min Pd deposition time. In both cases PBS pH=7.0
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baseline (black line) and after nitrate is added to a final concentration of 20 ppm (red dashed line) is shown. It can be observed that no significant difference is
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obtained when no Pd is present on the surface of the electrode (Figure 4a) while an increase in the current is obtained when the Cu/Pd electrode is used (Figure 4b). The current differences in the first two steps when using the Cu/Pd electrode are due to the liberation and regeneration of adsorbed nitrogenated compounds on the electrode surface that otherwise would cause its poisoning as reported earlier [21,30]. The currents recorded during the last second of the three-step voltammetry measurements are averaged and used as experimental data and plotted against the different nitrate concentrations in the electrochemical cell obtained by nitrate addition. Stirring of the solution is performed after each addition at 600 rpm for 5 s in order to homogenize the nitrate concentration in the electrochemical cell. Before each three-step measurement, the stirring is 5 Page 6 of 19
stopped. Figure 5a shows the responses obtained from the electrodes with various Pd deposition times. As can be observed, nitrate reduction sensitivity increases with the amount of deposited Pd showing an optimal response at a deposition time of 120 s. After this Pd deposition time, the response signal decreases. The observed behavior may be understood by the analysis put forth by Gao and Namely, for nitrate
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Li [33] for PdCu bimetallic catalyst for nitrate reduction.
reduction, both Cu and Pd must be available during the electron transfer
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process. The need for Pd in the catalyst mixture is demonstrated in Figure 4a, where an electrode with no deposited Pd is used and signal remained
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unchanged in the presence of nitrate. The need of copper is demonstrated by using an epoxy-silver electrode instead of an epoxy-copper electrode with a 2-
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min Pd deposition following the same procedures as before [32].
Figure 5b shows the calibration plot obtained when using this electrode. As can be observed, the data obtained in the absence of Cu in the catalyst only
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presents noise. We conjecture that for Pd deposition times longer than 2 min, Cu starts being covered by Pd to such an extent that it is no longer available to
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nitrate in solution. Figure 6 shows the calibration curve for the optimal 2 min Pd deposition time electrode on a log-log plot. The linear range for nitrate reduction at pH=7.0 is 2 to 35 ppm, with excellent linear correlation. The achieved
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detection range is well in the legislation limits of 10 ppm [31]. Reproducible response of the electrodes is obtained over periods of months. This is attributed
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to the regenerative nature of the 3-step measurements described in section 2.3.Table 1 compares the performance of the here presented Cu-Pd electrode to those reported previously. The table clearly shows that the Cu-Pd electrode provides an excellent response towards nitrate reduction when compared to previous reports in the literature with the added novelty of sensing in neutral pH. The electrochemical nitrate detection scheme presented here should not be influenced by many ions that are usually present in natural waters (such as HCO3-, SO42- Ca2+, and Mg2+) since none of these cations or anions can be either reduced or oxidized in the potential range of our measurements. The possibility of nitrite interference was studied using the optimal 2 min Pd electrode by adding 1000 ppm NaNO2 (Fluka 99.8%) to 25 ml PBS buffer as performed for nitrate measurements, with no response observed over the 6 Page 7 of 19
examined range of ?? to 50 ppm. Previous studies on the electroreduction of nitrite by copper complexes [34] suggest that nitrite reduction follows a different reaction pathway than nitrate reduction that might not be possible in the presence of Pd.
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4. Conclusions In conclusion a facile fabrication method for a new electrochemical nitrate
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sensor based on epoxy Cu and electrolessly deposited Pd is presented. The
main advantage of the presented electrode is the possibility of nitrate
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determination in the legislation-relevant range at neutral pH. Nitrate response is achieved by a three-step amperometry consisting of a surface cleaning and
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regeneration followed by a nitrate electroreduction. Stable current is used as analytical signal for the calibration of the electrodes. Optimization of the amount of deposited Pd has been performed and a final condition of 1mM Pd
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dissolution with a 2-min deposition time is found to provide the highest sensitivity towards nitrate reduction. The linear range provided by the here
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presented electrode is 2-35 ppm, a suitable range for its use in the detection of nitrates in drinking water and crop soils among others. The simplicity of the fabrication process, the low cost of the starting components, the high rate of
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precursor utilization expected in the electroless deposition and the possibility of using inexpensive counter electrodes, such as graphite, open potential for mass
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production of the described sensor. Acknowledgements
The authors are grateful to Mr. Peter Lobaccaro for his help with SEM and EDX imaging. This work was supported by National Science Foundation under Grant# EEC-0832819 (Center of Integrated Nanomechanical Systems). References [1] E.H. Burden, Toxicology of nitrates and nitrites with particular reference to potability of water supplies, Analyst 102 (1961) 429. 7 Page 8 of 19
[2] M.H. Ward, T.M. deKok, P. Levallois, J. Brender, G. Gulis, B.T. Nolan, J. Van Derslice, Workgroup report: Drinking-water nitrate and health-recent findings and research needs, Environ. Health Perspect. 113 (2005) 1607. [3] L.W. Canter, Nitrates in Groundwater, CRC Press, Boca Raton, FL, 1996. [4] B.A. Stewart, F.G. Viets, G.L. Hutchins, Agricultures effect on nitrate pollution of groundwater, J. Soil Water Conserv. 23 (1968) 13.
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[5] M.N. Almasri, J.J. Kaluarachchi, Assessment and management of long-term
nitrate pollution of ground water in agriculture-dominated watersheds, J. Hydrol.
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295 (2004) 225.
[6] J.M. Beman, K.R. Arrigo, P.A. Matson, Agricultural runoff fuels large
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phytoplankton blooms in vulnerable areas of the ocean, Nature 434 (2005) 211. [7] S. Park, P. Croteau, K.A. Boering, D.M. Etheridge, D. Ferretti, P.J. Fraser,
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K.R. Kim, P.B. Krummel, R.L. Langenfelds, T.D. van Ommen, L.P. Steele, C.M. Trudinger, Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940, Nature Geosci. 5 (2012) 261.
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[8] T. Kamiura, M. Tanaka, Determination of nitrate in suspended particulate matter by high-performance liquid-chromatography with UV detection, Anal.
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Chim. Acta 110 (1979) 117.
[9] APHA. Standard methods for the examination of water and wastewater. 18th ed. American Public Health Association, Washington, DC, 1992.
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[10] K. Fajerwerg, V. Ynam, B. Chaudret, V. Garçon, D. Thouron, M. Comtat, An original nitrate sensor based on silver nanoparticles electrodeposited on a
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gold electrode, Electrochem. Comm. 12 (2010) 1439. [11] F. Gauthard, F. Epron, J. Barbier, Palladium and platinum-based catalysts in the catalytic reduction of nitrate in water: effect of copper, silver, or gold addition, J. Catal. 220 (2003) 182. [12] G.E. Dima, A.C.A. de Vooys, M.T.M. Koper, Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions, J. Electranal. Chem. 554-555 (2003) 15. [13] L.A. Estudillo-Wong, E.M. Arce-Estrada, N. Alonso-Vante, A. ManzoRobledo, Electro-reduction of nitrate species on Pt-based nanoparticles: Surface area effects, Catal. Today 166 (2011) 201. [14] M. J. Moorcroft, J. Davis, R. G. Compton, Detection and determination of nitrate and nitrite: a review, Talanta 54 (2001) 785. 8 Page 9 of 19
[15] N. G. Carpenter, D. Pletcher, Amperometric method for the determination of nitrate in water, Anal. Chim. Acta 317 (1995) 287. [16] A. G. Fogg, S. P. Scullion, T. E. Edmonds, Assessment of online nitration reactions as a means of determining nitrate by reverse flow-injection with reductive amperometric detection at a glassy-carbon electrode, Analyst 114 (1989) 579.
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[17] A. G. Fogg, S. P. Scullion, T. E. Edmonds, B. J. Birch, Direct reductive amperometric determination of nitrate at a copper electrode formed insitu in a
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capillary-fill sensor device, Analyst 116 (1991) 573.
[18] M. J. Moorcroft, L. Nei, J. Davis, R. G. Compton, Enhanced electrochemical
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detection of nitrite and nitrate at a Cu-30Ni alloy electrode, Analytical Letters 33 (2000) 3127.
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[19] Z. Zhao, X. Cai, Determination of trace nitrite by catalytic polarography in ferrous iron thiocyanate medium, J. Electroanal. Chem. 252 (1988) 361. [20] J. Davis, M. J. Moorcroft, S. J. Wilkins, R. G. Compton, M. F. Cardosi,
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Electrochemical detection of nitrate and nitrite at a copper modified electrode, Analyst 125 (2000) 737.
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[21] A. O. Solak, P. Gulser, E. Gokmese, F. Gokmese, A new differential pulse voltammetric method for the determination of nitrate at a copper plated glassy carbon electrode, Mikrochim. Acta 134 (2000) 77.
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[22] S. Cattarin, Electrochemical reduction of nitrogen oxyanions in 1-M sodiumhyrdoxide solutions at silver, copper and CuInSe2 electrodes, J. Appl.
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Electrochem. 22 (1992) 1077.
[23] H.L. Li, J.Q. Chambers, D.T. Hobbs, Electroreduction of nitrate ions in concentrated
sodium-hydroxide
solutions
at
lead,
zinc,
nickel
and
phtalocyaninie-modified electrodes, J. Appl. Electrochem. 18 (1988) 454. [24] J.O. Bockris, J. Kim, Electrochemical reductions of Hg(II), rutheniumnitrosyl complex, chromate, and nitrate in a strong alkaline solution, J. Electrochem. Soc. 143 (1996) 3801. [25] J.D. Genders, D. Hartsough, D.T. Hobbs, Electrochemical reduction of nitrates and nitrites in alkaline nuclear waste solutions, J. Appl. Electrochem. 26 (1996) 1.
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[26] M. Fedurco, P. Kedzierzawski, J. Augustynski, Effect of multivalent cations upon reduction of nitrate ions at the Ag electrode, J. Electrochem. Soc. 146 (1999) 2569. [27] R. Tenne, K. Patel, K. Hashimoto, A. Fujishima, Efficient electrochemical reduction of nitrate to ammonia using conductive diamond film electrodes, J. Electroanal. Chem. 347 (1993) 409.
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[28] A.C.A. de Vooys, R.A. van Santen, J.A.R. van Veen, Electrocatalytic reduction of NO3- on palladium/copper electrodes, J. Mol. Catal. A: Chem. 154
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(2000) 203. Electrocatalysis, Chem. Rev. 109 (2009) 2209.
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[29] V. Rosca, M. Duca, M.T. de Groot, M.T.M. Koper, Nitrogen Cycle [30] O. Ghodbane, M. Sarrazin, L. Roue, D. Belanger, Electrochemical of
nitrate
on
pyrolytic
graphite-supported
Cu
and
Pd-Cu
an
reduction
electrocatalysts, J. Electrochem. Soc. 155 (2008) F117.
http://water.epa.gov/drink/contaminants/basicinformation/nitrate.cfm,
consulted on October 31st 2012.
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[31]
[32] A. Gutes, C. Carraro, R. Maboudian, Nonenzymatic glucose sensing based Acta 56 (2011) 5855.
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on deposited palladium nanoparticles on epoxy-silver electrodes, Electrochim. [33] W. Gao, F. Li, Catalytic Hydrogenation of Nitrate Ions over Pd-Cu/ZSM-5
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Catalyst, Advanced Materials Research 197-198 (2011) 967. [34] J.G. Woollard-Shore, J.P. Holland, M.W. Jones, J.R. Dilworth, Nitrite
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reduction by copper complexes, Dalton Trans. 39, (2010) 1576.
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Table 1. Comparison of analytical performance of some electrochemical nitrate sensors Electrode material
Solution
nitrate concentration (M)
Ref.
AgNP on Au
Synthetic seawater
10-5 – 10-2
[10]
Polycrystalline metal
0.5M H2SO4 or
0.1
[12]
1M NaCl
1
[30]
Pt-based NP
0.5M NaOH
0.001 – 1
Vitreous carbon
0.1 M Na2SO4 at
10-4 - 10-3
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Cu/graphite
pH = 2.9
5 ·10-4 – 5 ·10-3
Concentrated H2SO4
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Glassy carbon
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Cu/graphite and Pd–
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HClO4
0.5 - 2M H2SO4
Epoxy-Cu-Pd-NP
PBS buffer pH=7.0
[15] [16]
10-4 - 10-3
[17]
3.2·10-5 – 5.6·10-4
This work
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Electrodeposited Cu
[13]
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Figure Captions: Figure 1: Top: electrode construction diagram. A 3mm inner diameter PVC tube (a) is cut into 2cm long pieces. Half of the tube is filled with a 3mm electrical cable (b) and the rest is filled with epoxy-Cu and hardened (c). After hardening, the electrode surface is polished with sandpaper and alumina slurry until a
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electrode (left) and after the 2 min Pd deposition (right).
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mirror-shine surface is obtained (d). Bottom: picture of the unmodified Cu-epoxy
Figure 2: SEM images of (a) polished epoxy Cu and (b) after a 2 min Pd
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deposition; (c) EDX spectrum of pristine epoxy-Cu and (d) EDX spectrum of
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sample shown in (b).
Figure 3: (a) Cyclic voltammograms for the bare Cu electrode in PBS (black line) and in a PBS solution containing 20 ppm nitrate (red dashed line). (b)
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Cyclic voltammograms obtained at pH=7.0 on the epoxy-Cu/Pd electrode (2 min Pd deposition) in a 0.1M PBS solution (black line) and in a PBS solution at +0.2V
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containing 20 ppm nitrate (red dashed line). Scan rate 50 mV/s. Scans started
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Figure 4: (a) Three-step voltammetry on the epoxy-Cu in PBS pH=7.0 baseline (black line) and in the presence of 20 ppm nitrate (red dashed line). (b) Same
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measurements as in (a) when Pd has been deposited on the epoxy-Cu electrode surface for 2 min. Blue dashed line shows the applied voltage along the 7 second measurement. Figure 5: (a) calibration responses for the studied set of Pd deposition times. Plotted currents correspond to the average of the currents recorded during the last second in the three-step measurements after nitrate additions. (b) Calibration response of an epoxy-Ag/Pd electrode with Pd deposited for 2 min. Figure 6: Linear response of the optimal 2 min Pd deposition in the range of 2 35 ppm of nitrate.
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S0013-4686(13)00669-5 http://dx.doi.org/doi:10.1016/j.electacta.2013.03.199 EA 20334
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
31-1-2013 27-3-2013 29-3-2013
Please cite this article as: A. Gut´es, C. Carraro, R. Maboudian, Nitrate amperometric sensor in neutral pH based on Pd nanoparticles on epoxy-copper electrodes, Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.03.199 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nitrate amperometric sensor in neutral pH based on Pd nanoparticles on epoxycopper electrodes Albert Gutés, Carlo Carraro, Roya Maboudian*
Roya Maboudian 201 Gilman Hall
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Department of Chemical and Biomolecular Engineering
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Author for correspondence:
University of California
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Berkeley, CA, 94720
Fax: +1 510 642 4778 Email: [email protected]
http://cheme.berkeley.edu/faculty/maboudian/
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ed
Website
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Tel: +1 510 643 7957
Page 1 of 19
Nitrate amperometric sensor in neutral pH based on Pd nanoparticles on epoxycopper electrodes Albert Gutés, Carlo Carraro, Roya Maboudian* Department of Chemical and Biomolecular Engineering, University of California,
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Berkeley, CA, 94720, USA
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Abstract
Amperometric nitrate sensing at neutral pH using an epoxy-copper electrode
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modified with palladium nanoparticles is presented. After epoxy-copper is hardened and polished, electroless deposition is employed for the deposition of
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Pd nanoparticles. Scanning electron microscopy and energy-dispersive X-ray spectroscopy reveal the morphology and composition of the Cu/Pd surface. The effect of Pd deposition time towards nitrate electroreduction is investigated,
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highlighting the importance of the bimetallic catalyst. The optimized electrode shows a linear response at pH=7 in the range from 2 to 35 ppm of nitrate. The
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simplicity and cost effectiveness of the fabrication process makes this Cu/Pd electrode a good candidate for distributed nitrate monitoring and determination the
Nitrate sensor, electrochemistry, epoxy-copper, palladium
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Keywords
field.
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in
nanoparticles, bimetallic catalyst 1. Introduction
Nitrate ground water pollution has become a growing concern in the last few decades due to its toxicity at ppm concentration levels [1-2]. Nitrate in groundwater can be divided into four categories depending on its source: (i) waste materials, (ii) natural sources, (iii) irrigation in agriculture, and (iv) row crop agriculture [3-5]. Overuse of nitrogen-based fertilizers and subsequent leakage of nitrates into waters have caused a tremendous increase and spread of this water pollutant worldwide [6]. Moreover, NOx increase in the atmosphere 1 Page 2 of 19
has been traced to overfertilization [7]. For these reasons, nitrate detection and monitoring is of great importance. Optical UV detection of nitrates at 220 nm can be employed [8], but is limited to the screening of clear unpolluted water samples with low organic contents. Other methodologies for nitrate detection include ion chromatography and the standard cadmium reduction method [9]. The Cd reduction method is a lengthy process and generates cadmium waste
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with the associated disposal issues; thus the final cost of analysis is high.
A promising method for the detection of nitrate is based on its electroreduction One of the most
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in the presence of metallic or bimetallic catalyst [10-14].
explored bimetallic catalysts with strong electrocatalytic reduction properties
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towards nitrate electroreduction is Cu-Pd. Much of the literature on the use of Cu-Pd or other metallic catalysts has been limited to nitrate electroreduction in
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acidic [15-21] and basic [22-27] media but since the majority of ground and drinking waters are close to neutral pH, the previously developed sensing materials cannot be directly used without sample pre-treatment. The majority of
M
studies have focused on the determination of the nitrate electroreduction mechanism or on the catalytic and selective effect of the Cu-Pd electrode, De Vooys et al. presented a
ed
showing very high conversion rates to N2.
thorough study on the use of Cu-Pd electrodes for the electroreduction of nitrate, [28] proposing mechanisms in acidic and basic media, noting the
pt
differences as pH values change, as well as the influence of the Cu to Pd ratio and its effects towards selectivity and activity of the electrode. In addition, a
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recent review by Rosca et al. [29] presents an exhaustive compilation of the various electrocatalytical paths for nitrogen compounds, including nitrate, but again acidic or basic media were used in all nitrate electroreduction measurements. A few reports on nitrate detection by electroreduction in neutral media can be found in the literature [30], but in all cases, the detected nitrate ranges of concentrations exceed the legislation limits [31]. Here we present a simple and robust fabrication method for nitrate sensors with a linear response range of 2 to 35 ppm measured at pH=7.0. To achieve the required sensitivity, the Pd to Cu ratio had to be carefully optimized which we have achieved via electroless deposition schemes. To the best of our knowledge, this is the first time that electrochemical reduction of nitrate at neutral pH has been achieved in this concentration range. 2 Page 3 of 19
2. Experimental 2.1 Electrode construction Figure 1 shows schematically the fabrication of the epoxy-Cu electrodes.
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Epoxy-copper (Epoxy Technologies Epotek 430) is prepared as directed by the
manufacturer. PVC tubing of 3 mm inner diameter is cut in 2 cm long pieces. On
cr
one end, a 3 mm in diameter electrical cable is introduced 1 cm into the tubing.
Epoxy-Cu is applied to the other end until filling the remaining 1 cm cavity in the
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tube. After hardening, the epoxy-Cu is polished with sandpaper of decreasing grain size with the final polishing performed with alumina pastes of 1-, 0.3- and
an
0.05-m size (CHI polishing kit CHI120) until mirror-shine is achieved. Extensive rinsing with deionized (DI) water (18 MΩ, Barnsted Nanopure Infinity) is performed prior to drying with nitrogen flow.
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Deposition of the Pd nanoparticles is performed, right after polishing, as follows. The deposition solution consists of 1 mM K2PdCl4 (Aldrich, 99.99% purity) and
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20 mM KCl (Aldrich, 99% purity) and is used for all Pd depositions. Excess of chloride is necessary to stabilize the PdCl42- anion in solution, avoiding its precipitation as PdCl2. Polished epoxy-Cu electrodes are immersed in the
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aforementioned solution for varied time duration, ranging from 10 to 300 s, then rinsed in DI water and dried in a nitrogen flow. As explained in the results
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section, the optimal deposition time towards nitrate reduction corresponds to 2min deposition time.
2.2 Characterization instruments Electrochemical measurements (cyclic voltammetry and coulometry) are performed using a CH660D potentiostat-galvanostat (CH Instruments, USA). A standard three-electrode configuration is used with a Ag/AgCl reference electrode, a Pt wire auxiliary electrode and the modified epoxy-Cu/Pd nanoparticle working electrode. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses are obtained using a JEOL JSM 6340F field emission scanning electron microscope. 3 Page 4 of 19
2.3 Amperometric detection All measurements are performed in a 0.1 M phosphate buffer solution (PBS, Fluka 99.4%) with pH adjusted to 7.0. Potassium nitrate (Fluka 99.8%)standard solution containing 1000 ppm nitrate in PBS pH = 7.0 is prepared daily. Cyclic
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voltammetry is performed between +0.2 and -1.2V at a scan rate of 50 mV/s
without stirring, starting at +0.2V. 10 cycles are performed in order to reach
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reproducible voltammograms. The measurements are performed in open glass beakers without any oxygen scavenging or removal. 3-step voltammetry is
us
performed by applying potentials (vs. Ag/AgCl) as follows, measuring each 20 ms: +0.2V for 2 s (Cu-Pd cleaning / oxidation step) followed by 2 s at -0.2V (Cu-
an
Pd regeneration step) and a final measuring step at -1V for 3 s. All the 3-step measurements are performed without stirring. The average of the final 10recorded currents is used as analytical data. The first two steps (cleaning and
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regeneration) are found to be crucial in order to avoid surface poisoning reported previously for nitrate electroreduction [21,30] and are selected by
ed
direct observation on the obtained cyclic voltammograms, where maximum currents are achieved on the positive scan at around -0.1 V due to Cu/Pd oxidation and maximum currents are achieved on the negative scan at around -
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0.2 V during Cu/Pd reduction.
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3. Results and discussion
3.1 Characterization of the epoxy-Cu/Pd nanoparticle substrate Figure 2 shows the SEM image of the polished epoxy-copper prior to palladium nanoparticles deposition (a) and after 2 min of palladium deposition (b). As can be observed, the electrode surface morphology changes substantially upon Pd deposition, with the formation of Pd nanostructures and films. Figure 2c shows the EDX spectrum of the same sample as in Figure 2b. The spectrum confirms the Pd deposition, with Pd/Cu ratio of 1:4.6. The Al signal is attributed to the presence of Al in the epoxy-Cu formulation as-received.
4 Page 5 of 19
3.2 Catalytic reduction of nitrate by cyclic voltammetry Figure 3a shows the typical cyclic voltammograms of the bare Cu electrode in a 0.1M PBS solution at pH=7.0 and in the presence of 20 ppm of nitrate. Figure 3b shows the response obtained with the epoxy-Cu/Pd electrode (2 min of palladium deposition) in the same PBS and nitrate dissolutions. Scan rate is 50
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mV/s in both cases. As can be observed, a reduction tail is obtained in the negative scan starting at -0.8V, corresponding to nitrate reduction on the Cu/Pd
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surface. In order to minimize possible interferences of hydrogen evolution, a described next.
an
3.3 Three-step nitrate amperometric sensing
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less extreme potential of -1V is selected for the third step in the nitrate detection
To characterize fully the electrodes’ responses, three-step voltammetry
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measurements without solution stirring are performed, following a similar protocol as described in the literature [32] as described in section 2.3. Figure 4a
ed
shows the typical current responses obtained for the three-step voltammetry on the bare epoxy-Cu electrode while figure 4b shows the same response using an epoxy-Cu electrode after a 2 min Pd deposition time. In both cases PBS pH=7.0
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baseline (black line) and after nitrate is added to a final concentration of 20 ppm (red dashed line) is shown. It can be observed that no significant difference is
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obtained when no Pd is present on the surface of the electrode (Figure 4a) while an increase in the current is obtained when the Cu/Pd electrode is used (Figure 4b). The current differences in the first two steps when using the Cu/Pd electrode are due to the liberation and regeneration of adsorbed nitrogenated compounds on the electrode surface that otherwise would cause its poisoning as reported earlier [21,30]. The currents recorded during the last second of the three-step voltammetry measurements are averaged and used as experimental data and plotted against the different nitrate concentrations in the electrochemical cell obtained by nitrate addition. Stirring of the solution is performed after each addition at 600 rpm for 5 s in order to homogenize the nitrate concentration in the electrochemical cell. Before each three-step measurement, the stirring is 5 Page 6 of 19
stopped. Figure 5a shows the responses obtained from the electrodes with various Pd deposition times. As can be observed, nitrate reduction sensitivity increases with the amount of deposited Pd showing an optimal response at a deposition time of 120 s. After this Pd deposition time, the response signal decreases. The observed behavior may be understood by the analysis put forth by Gao and Namely, for nitrate
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Li [33] for PdCu bimetallic catalyst for nitrate reduction.
reduction, both Cu and Pd must be available during the electron transfer
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process. The need for Pd in the catalyst mixture is demonstrated in Figure 4a, where an electrode with no deposited Pd is used and signal remained
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unchanged in the presence of nitrate. The need of copper is demonstrated by using an epoxy-silver electrode instead of an epoxy-copper electrode with a 2-
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min Pd deposition following the same procedures as before [32].
Figure 5b shows the calibration plot obtained when using this electrode. As can be observed, the data obtained in the absence of Cu in the catalyst only
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presents noise. We conjecture that for Pd deposition times longer than 2 min, Cu starts being covered by Pd to such an extent that it is no longer available to
ed
nitrate in solution. Figure 6 shows the calibration curve for the optimal 2 min Pd deposition time electrode on a log-log plot. The linear range for nitrate reduction at pH=7.0 is 2 to 35 ppm, with excellent linear correlation. The achieved
pt
detection range is well in the legislation limits of 10 ppm [31]. Reproducible response of the electrodes is obtained over periods of months. This is attributed
Ac ce
to the regenerative nature of the 3-step measurements described in section 2.3.Table 1 compares the performance of the here presented Cu-Pd electrode to those reported previously. The table clearly shows that the Cu-Pd electrode provides an excellent response towards nitrate reduction when compared to previous reports in the literature with the added novelty of sensing in neutral pH. The electrochemical nitrate detection scheme presented here should not be influenced by many ions that are usually present in natural waters (such as HCO3-, SO42- Ca2+, and Mg2+) since none of these cations or anions can be either reduced or oxidized in the potential range of our measurements. The possibility of nitrite interference was studied using the optimal 2 min Pd electrode by adding 1000 ppm NaNO2 (Fluka 99.8%) to 25 ml PBS buffer as performed for nitrate measurements, with no response observed over the 6 Page 7 of 19
examined range of ?? to 50 ppm. Previous studies on the electroreduction of nitrite by copper complexes [34] suggest that nitrite reduction follows a different reaction pathway than nitrate reduction that might not be possible in the presence of Pd.
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4. Conclusions In conclusion a facile fabrication method for a new electrochemical nitrate
cr
sensor based on epoxy Cu and electrolessly deposited Pd is presented. The
main advantage of the presented electrode is the possibility of nitrate
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determination in the legislation-relevant range at neutral pH. Nitrate response is achieved by a three-step amperometry consisting of a surface cleaning and
an
regeneration followed by a nitrate electroreduction. Stable current is used as analytical signal for the calibration of the electrodes. Optimization of the amount of deposited Pd has been performed and a final condition of 1mM Pd
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dissolution with a 2-min deposition time is found to provide the highest sensitivity towards nitrate reduction. The linear range provided by the here
ed
presented electrode is 2-35 ppm, a suitable range for its use in the detection of nitrates in drinking water and crop soils among others. The simplicity of the fabrication process, the low cost of the starting components, the high rate of
pt
precursor utilization expected in the electroless deposition and the possibility of using inexpensive counter electrodes, such as graphite, open potential for mass
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production of the described sensor. Acknowledgements
The authors are grateful to Mr. Peter Lobaccaro for his help with SEM and EDX imaging. This work was supported by National Science Foundation under Grant# EEC-0832819 (Center of Integrated Nanomechanical Systems). References [1] E.H. Burden, Toxicology of nitrates and nitrites with particular reference to potability of water supplies, Analyst 102 (1961) 429. 7 Page 8 of 19
[2] M.H. Ward, T.M. deKok, P. Levallois, J. Brender, G. Gulis, B.T. Nolan, J. Van Derslice, Workgroup report: Drinking-water nitrate and health-recent findings and research needs, Environ. Health Perspect. 113 (2005) 1607. [3] L.W. Canter, Nitrates in Groundwater, CRC Press, Boca Raton, FL, 1996. [4] B.A. Stewart, F.G. Viets, G.L. Hutchins, Agricultures effect on nitrate pollution of groundwater, J. Soil Water Conserv. 23 (1968) 13.
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[5] M.N. Almasri, J.J. Kaluarachchi, Assessment and management of long-term
nitrate pollution of ground water in agriculture-dominated watersheds, J. Hydrol.
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295 (2004) 225.
[6] J.M. Beman, K.R. Arrigo, P.A. Matson, Agricultural runoff fuels large
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phytoplankton blooms in vulnerable areas of the ocean, Nature 434 (2005) 211. [7] S. Park, P. Croteau, K.A. Boering, D.M. Etheridge, D. Ferretti, P.J. Fraser,
an
K.R. Kim, P.B. Krummel, R.L. Langenfelds, T.D. van Ommen, L.P. Steele, C.M. Trudinger, Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940, Nature Geosci. 5 (2012) 261.
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[8] T. Kamiura, M. Tanaka, Determination of nitrate in suspended particulate matter by high-performance liquid-chromatography with UV detection, Anal.
ed
Chim. Acta 110 (1979) 117.
[9] APHA. Standard methods for the examination of water and wastewater. 18th ed. American Public Health Association, Washington, DC, 1992.
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[10] K. Fajerwerg, V. Ynam, B. Chaudret, V. Garçon, D. Thouron, M. Comtat, An original nitrate sensor based on silver nanoparticles electrodeposited on a
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gold electrode, Electrochem. Comm. 12 (2010) 1439. [11] F. Gauthard, F. Epron, J. Barbier, Palladium and platinum-based catalysts in the catalytic reduction of nitrate in water: effect of copper, silver, or gold addition, J. Catal. 220 (2003) 182. [12] G.E. Dima, A.C.A. de Vooys, M.T.M. Koper, Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions, J. Electranal. Chem. 554-555 (2003) 15. [13] L.A. Estudillo-Wong, E.M. Arce-Estrada, N. Alonso-Vante, A. ManzoRobledo, Electro-reduction of nitrate species on Pt-based nanoparticles: Surface area effects, Catal. Today 166 (2011) 201. [14] M. J. Moorcroft, J. Davis, R. G. Compton, Detection and determination of nitrate and nitrite: a review, Talanta 54 (2001) 785. 8 Page 9 of 19
[15] N. G. Carpenter, D. Pletcher, Amperometric method for the determination of nitrate in water, Anal. Chim. Acta 317 (1995) 287. [16] A. G. Fogg, S. P. Scullion, T. E. Edmonds, Assessment of online nitration reactions as a means of determining nitrate by reverse flow-injection with reductive amperometric detection at a glassy-carbon electrode, Analyst 114 (1989) 579.
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[17] A. G. Fogg, S. P. Scullion, T. E. Edmonds, B. J. Birch, Direct reductive amperometric determination of nitrate at a copper electrode formed insitu in a
cr
capillary-fill sensor device, Analyst 116 (1991) 573.
[18] M. J. Moorcroft, L. Nei, J. Davis, R. G. Compton, Enhanced electrochemical
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detection of nitrite and nitrate at a Cu-30Ni alloy electrode, Analytical Letters 33 (2000) 3127.
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[19] Z. Zhao, X. Cai, Determination of trace nitrite by catalytic polarography in ferrous iron thiocyanate medium, J. Electroanal. Chem. 252 (1988) 361. [20] J. Davis, M. J. Moorcroft, S. J. Wilkins, R. G. Compton, M. F. Cardosi,
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Electrochemical detection of nitrate and nitrite at a copper modified electrode, Analyst 125 (2000) 737.
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[21] A. O. Solak, P. Gulser, E. Gokmese, F. Gokmese, A new differential pulse voltammetric method for the determination of nitrate at a copper plated glassy carbon electrode, Mikrochim. Acta 134 (2000) 77.
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[22] S. Cattarin, Electrochemical reduction of nitrogen oxyanions in 1-M sodiumhyrdoxide solutions at silver, copper and CuInSe2 electrodes, J. Appl.
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Electrochem. 22 (1992) 1077.
[23] H.L. Li, J.Q. Chambers, D.T. Hobbs, Electroreduction of nitrate ions in concentrated
sodium-hydroxide
solutions
at
lead,
zinc,
nickel
and
phtalocyaninie-modified electrodes, J. Appl. Electrochem. 18 (1988) 454. [24] J.O. Bockris, J. Kim, Electrochemical reductions of Hg(II), rutheniumnitrosyl complex, chromate, and nitrate in a strong alkaline solution, J. Electrochem. Soc. 143 (1996) 3801. [25] J.D. Genders, D. Hartsough, D.T. Hobbs, Electrochemical reduction of nitrates and nitrites in alkaline nuclear waste solutions, J. Appl. Electrochem. 26 (1996) 1.
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[26] M. Fedurco, P. Kedzierzawski, J. Augustynski, Effect of multivalent cations upon reduction of nitrate ions at the Ag electrode, J. Electrochem. Soc. 146 (1999) 2569. [27] R. Tenne, K. Patel, K. Hashimoto, A. Fujishima, Efficient electrochemical reduction of nitrate to ammonia using conductive diamond film electrodes, J. Electroanal. Chem. 347 (1993) 409.
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[28] A.C.A. de Vooys, R.A. van Santen, J.A.R. van Veen, Electrocatalytic reduction of NO3- on palladium/copper electrodes, J. Mol. Catal. A: Chem. 154
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(2000) 203. Electrocatalysis, Chem. Rev. 109 (2009) 2209.
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[29] V. Rosca, M. Duca, M.T. de Groot, M.T.M. Koper, Nitrogen Cycle [30] O. Ghodbane, M. Sarrazin, L. Roue, D. Belanger, Electrochemical of
nitrate
on
pyrolytic
graphite-supported
Cu
and
Pd-Cu
an
reduction
electrocatalysts, J. Electrochem. Soc. 155 (2008) F117.
http://water.epa.gov/drink/contaminants/basicinformation/nitrate.cfm,
consulted on October 31st 2012.
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[31]
[32] A. Gutes, C. Carraro, R. Maboudian, Nonenzymatic glucose sensing based Acta 56 (2011) 5855.
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on deposited palladium nanoparticles on epoxy-silver electrodes, Electrochim. [33] W. Gao, F. Li, Catalytic Hydrogenation of Nitrate Ions over Pd-Cu/ZSM-5
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Catalyst, Advanced Materials Research 197-198 (2011) 967. [34] J.G. Woollard-Shore, J.P. Holland, M.W. Jones, J.R. Dilworth, Nitrite
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reduction by copper complexes, Dalton Trans. 39, (2010) 1576.
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Table 1. Comparison of analytical performance of some electrochemical nitrate sensors Electrode material
Solution
nitrate concentration (M)
Ref.
AgNP on Au
Synthetic seawater
10-5 – 10-2
[10]
Polycrystalline metal
0.5M H2SO4 or
0.1
[12]
1M NaCl
1
[30]
Pt-based NP
0.5M NaOH
0.001 – 1
Vitreous carbon
0.1 M Na2SO4 at
10-4 - 10-3
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Cu/graphite
pH = 2.9
5 ·10-4 – 5 ·10-3
Concentrated H2SO4
an
Glassy carbon
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Cu/graphite and Pd–
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HClO4
0.5 - 2M H2SO4
Epoxy-Cu-Pd-NP
PBS buffer pH=7.0
[15] [16]
10-4 - 10-3
[17]
3.2·10-5 – 5.6·10-4
This work
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Electrodeposited Cu
[13]
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Figure Captions: Figure 1: Top: electrode construction diagram. A 3mm inner diameter PVC tube (a) is cut into 2cm long pieces. Half of the tube is filled with a 3mm electrical cable (b) and the rest is filled with epoxy-Cu and hardened (c). After hardening, the electrode surface is polished with sandpaper and alumina slurry until a
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electrode (left) and after the 2 min Pd deposition (right).
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mirror-shine surface is obtained (d). Bottom: picture of the unmodified Cu-epoxy
Figure 2: SEM images of (a) polished epoxy Cu and (b) after a 2 min Pd
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deposition; (c) EDX spectrum of pristine epoxy-Cu and (d) EDX spectrum of
an
sample shown in (b).
Figure 3: (a) Cyclic voltammograms for the bare Cu electrode in PBS (black line) and in a PBS solution containing 20 ppm nitrate (red dashed line). (b)
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Cyclic voltammograms obtained at pH=7.0 on the epoxy-Cu/Pd electrode (2 min Pd deposition) in a 0.1M PBS solution (black line) and in a PBS solution at +0.2V
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containing 20 ppm nitrate (red dashed line). Scan rate 50 mV/s. Scans started
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Figure 4: (a) Three-step voltammetry on the epoxy-Cu in PBS pH=7.0 baseline (black line) and in the presence of 20 ppm nitrate (red dashed line). (b) Same
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measurements as in (a) when Pd has been deposited on the epoxy-Cu electrode surface for 2 min. Blue dashed line shows the applied voltage along the 7 second measurement. Figure 5: (a) calibration responses for the studied set of Pd deposition times. Plotted currents correspond to the average of the currents recorded during the last second in the three-step measurements after nitrate additions. (b) Calibration response of an epoxy-Ag/Pd electrode with Pd deposited for 2 min. Figure 6: Linear response of the optimal 2 min Pd deposition in the range of 2 35 ppm of nitrate.
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Figure 1
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Figure 4
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Figure 5
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