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Simultaneous detection of ammonia and nitrate using a modified electrode with two regions
Microchemical Journal 154 (2020) 104649
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Simultaneous detection of ammonia and nitrate using a modified electrode with two regions
T
Jingyi Wanga,b, , Peng Diaoa, ⁎
a b
⁎
Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G1H9, Canada
ARTICLE INFO
ABSTRACT
Keywords: Electrochemical detection Ammonia Nitrate Modified electrode
Simultaneous detection of ammonia and nitrate has significance in the application areas of environment and industry. Conventional methods include additional steps to separate ammonia and nitrate before detection. A more convenient method is required to detect ammonia and nitrate directly in their mixture. In this work, a modified electrode with electrodeposited Pt region and Ag region was prepared to simultaneously detect ammonia and nitrate for the first time. The Pt region and Ag region were designed to selectively determine ammonia and nitrate, respectively, free of interference with each other. A nonlinear correlation between ammonia oxidation current and ammonia bulk concentration and a linear correlation between nitrate reduction current and nitrate bulk concentration were found. The simultaneous detection limits of ammonia and nitrate were 3.946 μM and 0.134 mM, respectively. The modified electrode showed good long-term stability for the simultaneous detection of ammonia and nitrate with relative standard deviations of 7.10% and 3.93%, respectively. The modified electrode exhibited good anti-interference ability toward Na+, K+, Cl− and SO42−. This work demonstrated a simple and fast approach for detecting two components in solution.
1. Introduction Ammonia and nitrate are two of the important species in inorganic nitrogen cycle. Determination of ammonia and nitrate are explored in water [1–4], soil [5] and nitrogen oxidation/reduction product [6] samples in the application fields of environment and industry. The reported methods for the detection of ammonia and nitrate mixture are colorimetric [1], potentiometric [2], fluorometric [3], chemiluminescence [4], and spectrophotometric [5] combined with flow system to detect ammonia and nitrate in different flow channels [1–5], or with extraction steps [6] to detect ammonia and nitrate separately. Simultaneous dual- or multi-component chemical analysis, determining more than one components at the same time without additional component separation steps, improves the efficiency of detection [7–10]. In electrochemical sensing systems, the application of electrode arrays can enhance the power of electrochemical multi-component detection, in which each electrode is modified with materials of different physical and chemical properties, and can thus add new dimensions of information from analytes [11]. In addition, the modification carried out on different regions of a single substrate facilitates the integration of sensing elements [12–16]. We have applied the partitional modification
method [17] to fabricate two regions on a substrate used as a sensor for simultaneous determination of glucose and nitrite by correcting the influence of glucose on nitrite detection [18]. There are two approaches to further improve the efficiency of electrochemical multi-component detection on a sensor with multi-regions: (1) adding more modified regions with low selectivity [19,20], which puts the work load on data collection and analysis; (2) fabricating highly selective sensing elements, which is much more efficient and of cause a great challenge. Obviously, the second option is ideal. Each of the regions has a sensing element of high selectivity and only responds to one component. The multi-component detection process can be even faster by using a single current channel for data collection. In this case, the sensing elements should respond their corresponding analytes at different electrochemical potentials to avoid interference current [21,22]. Herein, an indium tin oxide (ITO) coated glass substrate was modified into two regions, a Pt region and an Ag region, which have high selectivity toward ammonia and nitrate, respectively. The designed and fabricated electrode was used to simultaneously detect ammonia and nitrate through a signal current channel.
⁎ Corresponding author at: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G1H9, Canada; Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China. E-mail addresses: [email protected] (J. Wang), [email protected] (P. Diao).
https://doi.org/10.1016/j.microc.2020.104649 Received 6 November 2019; Received in revised form 10 January 2020; Accepted 16 January 2020 Available online 17 January 2020 0026-265X/ © 2020 Elsevier B.V. All rights reserved.
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
2. Materials and experimental methods
2.4. Surface morphology characterization
2.1. Materials
The morphology of modified electrode was characterized using scanning electron microscopy (SEM, Philips FEI XL30 SFEG) with an accelerating potential of 10 kV.
Hydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6⋅6H2O), silver nitrate (AgNO3), and all other chemicals were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. All the chemicals were of analytical grade and used directly without further purification. The aqueous solutions were prepared using deionized (DI) water. The stock solutions of ammonia and nitrate were prepared at various concentrations, i.e., 1 mM, 10 mM, 0.1 M, and 1 M for ammonia, and 2 mM, 20 mM, 0.2 M, and 2 M for nitrate. The ITO coated glass slides were obtained from CSG Holding Co. possessing a square resistance of 15 Ω.
3. Results and discussion 3.1. Electrochemical characterization of modified electrode Typical SEM images of Pt/ITO and Ag/ITO (Fig. S1) show crossed and standing Pt flakes of nanoscale, as well as anisotropic Ag particles with sizes ranging from nanometers to micrometers, respectively. The electrochemical active surface area (AEC) of Pt and Ag particles can be determined using electrochemical approaches as described in the Supplementary Material. Typical values for AEC of Pt particles and Ag particles are 0.461 cm2 and 0.0267 cm2, respectively. It is obvious that the AEC of Pt particles is more than ten times larger than that of Ag particles. This result can be reasonably explained by the surface morphologies of Pt and Ag particles as the Pt particles possessing protruding flakes with high surface roughness but the Ag particles exhibiting relatively smooth surface with low roughness. The obtained Pt/ ITO and Ag/ITO were used for detecting ammonia and nitrate respectively and simultaneously in 0.5 M KOH saturated with N2. As shown in Fig. 1a, in the absence of ammonia (black, dashed line), typical CV features for platinum electrode in alkaline solution [23,24] are observed on Pt/ITO. The reduction peak originates from ca. −0.10 V is attributed to the reduction of surface platinum oxides formed in the positive-going sweep. The peaks of hydrogen adsorption and desorption on Pt appear below −0.55 V in the negative- and positive-going sweeps, respectively. In the presence of ammonia (red, solid line), the ammonia oxidation is observed above −0.50 V with a peak current at ca. −0.30 V in the positive-going sweep, and meanwhile the reduction peak of surface platinum oxides is diminished. In addition, the new peaks appearing below −0.55 V are different from those measured in the absence of ammonia. The new peaks arise from NHx,ads desorption and surface nitrogen oxides reduction [24,25]. The CV features of Ag/ ITO in 0.5 M KOH with (red, solid line) and without nitrate (black, dashed line) are shown in Fig. 1b. In the absence of nitrate, the reduction peak starts from −0.30 V in the negative-going sweep reflects OH− desorption from Ag [26]. While in the presence of nitrate, another reduction process could be observed in the negative-going sweep initiating from −0.9 V with a peak at −1.3 V, which is the electro-reduction of nitrate [26,27]. It should be noted that the current values are negative in the potential range of −1.0 to −0.4 V even in the positive potential sweep in the CV curves of Pt/ITO and Ag/ITO electrodes in 0.5 M KOH. This is attributed to the nature of ITO substrate. In the above potential range, the reversible reduction of Sn(IV) to Sn(II) takes place [28] resulting in reduction currents in both positive and negative potential sweeps (Fig. S3). However, this potential range is still in the inert potential range of ITO substrate [29], which allows us to use it as a substrate electrode in the operation condition. When electrically connecting the Pt region and Ag region together, the electrochemical characteristics of (Pt//Ag)/ITO can be recorded. The CV curve of (Pt// Ag)/ITO in 0.5 M KOH is shown as the black dotted line in Fig. 1c, which presents a combination of the electrochemical responses from Pt/ ITO and Ag/ITO. In the presence of ammonia (red dashed line), the ammonia oxidation peak appears in the positive-going sweep; while in the presence of nitrate (solid blue line), the reduction peak appears in the negative-going sweep. The potential sweep window (from −1.0 to 0.0 V) was carefully selected to avoid any interference to the simultaneous detection of ammonia and nitrate at (Pt//Ag)/ITO. The negative potential limit was chosen to prevent the interfering current, i.e., high hydrogen evolution current at E<−1.0 V on the Pt region, and to prevent the formation NH3 at lower electrode potentials, because the produced NH3 could
2.2. Electrochemical experiments Electrochemical experiments were conducted on a CHI750C electrochemical workstation (CH Instrument Co.) at room temperature. A conventional three-electrode cell with a working electrode, a saturated calomel (SCE) as reference electrode, and a Pt foil as counter electrode, was used for the electrodeposition and the cyclic voltammetric (CV) experiments. The single component detection and simultaneous detection of ammonia and nitrate were conducted in 12 mL 0.5 M KOH saturated with high pure N2. Different concentrations of ammonia and nitrate stock solutions were added into the KOH solution to obtain the desired concentrations of analytes in the solution for detection. The added volumes of ammonia and nitrate stock solutions were less than 4% of the total volume of KOH solution. The CV curves were recorded in the presence of various concentrations of ammonia and nitrate, and the response currents originated from ammonia and nitrate were collected. All potentials are reported with respect to SCE. 2.3. Preparation of dual-region modified electrode The ITO coated glass slide was pre-fabricated with the ITO film divided into two closely-spaced regions as illustrated in Scheme 1. As a result, the two ITO film regions were electrically insulated from each other. Electrodeposition was performed on each of the regions to conduct surface modification. Prior to use, the ITO coated glass slide was cleaned by sonication in 0.5 M KOH and acetone for 5 min and 15 min, respectively. The surface was rinsed with DI water after each cleaning step, and then dried with high pure N2. Platinum particles and silver particles were electrodeposited onto the two ITO regions respectively. A similar procedure has been reported in our previous work [8]. The detailed deposition processes are as follows: (1) The cleaned ITO coated glass slide was immersed in 0.1 M HCl containing 1 mM H2PtCl6. One of the two ITO regions was electrically connected to the electrochemical workstation as a working electrode. A potential of 1.4 V was applied to the ITO region for 120 s followed by CVs from −0.3 V to 1.6 V at 0.1 V s−1 for 5 cycles. Then a deposition potential of −0.3 V was applied for 600 s to obtain the Pt-modified region (Pt/ITO). (2) After electrodeposition of Pt, the ITO coated slide was rinsed with DI water and transferred into 0.5 M KNO3 containing 5 mM AgNO3. The other ITO region was electrically connected to the electrochemical workstation, and −0.8 V was applied for 20 ms for 4 times to deposit Ag seeds on the surface. Then a deposition potential of 0.35 V was applied for 1800 s to obtain Ag-modified region (Ag/ITO). The final electrode was a dualregion modified ITO electrode denoted as (Pt//Ag)/ITO with the Pt region and Ag region electrically connected with each other. Therefore, the (Pt//Ag)/ITO performed as a single working electrode in its electrochemical characterization and simultaneous detection of ammonia and nitrate. Before use, the Pt region was activated by conducting CVs in 0.5 M H2SO4 from −0.28 to 1.4 V at 0.5 V s−1 until the CV feature did not change with cycle number. 2
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
Fig. 1. CV curves of (a) Pt/ITO in 0.5 M KOH with (red, solid line) and without (black, dashed line) 50 mM NH3 (b) Ag/ITO in 0.5 M KOH with (red, solid line) and without (black, dashed line) 5.52 mM NO3− (c) (Pt//Ag)/ITO in 0.5 M KOH (black dotted line), 0.5 M KOH with 0.11 mM NH3 (red, dashed line) and 0.5 M KOH with 10.9 mM NO3− (blue, solid line). Potential sweep rate: 0.1 V s−1.
CNH3 (mol L−1) can be obtained from the CV curves, which is presented in the inset of Fig. 2a. A nonlinear correlation between Ip,NH3 and CNH3 is observed in the investigated CNH3 range, which suggests a surface adsorption involved rate-determining mechanism. The rate-determining step and reversibility of the reaction were evaluated by recording the CV curves at different potential sweep rates (v, V s−1) as shown in Fig. S4. The linear dependence of Ip,NH3 on v, and of peak potential Ep,NH3 on lnv suggest an irreversible adsorption-controlled electrode reaction [30]. At 25 °C, Ip,NH3and Ep,NH3 can be expressed as [30]:
subsequently mislead the detection of bulk NH3. The positive potential limit was chosen to be lower than Ag anodic dissolution potentials, and to prevent the formation of nitrogen oxides at higher electrode potentials, because the produced nitrogen oxides could raise a reduction current in the potential range of nitrate reduction on the Ag region, and thus subsequently influence the detection of nitrate. In the selected potential window −1.0 to 0.0 V, there are no interference current from the addition of nitrate at the ammonia oxidation potentials, and no interference current from the addition of ammonia at the nitrate reduction potentials, which are the basic requirements for simultaneous determination of the two components on (Pt//Ag)/ITO.
Ip,NH3 = 3.2. Single component detection on (Pt//Ag)/ITO
n F 2AEC v
Ep,NH3 = E 0 +
Fig. 2a and b show the CV curves of (Pt//Ag)/ITO in 0.5 M KOH with various concentrations of ammonia and nitrate, respectively. In Fig. 2a, the ammonia oxidation peak current increases as the increasing of ammonia concentration in solution. Meanwhile the current at −1.0 V does not vary with the ammonia concentration, which means the presence of ammonia would not interfere the current response at −1.0 V for nitrate detection. Similarly, in Fig. 2b, the increase of nitrate concentration does not give rise to the current response at ammonia detection region of −0.5 to 0 V in the positive-going sweep, indicating no interference from nitrate to ammonia detection. The variation of the peak current Ip,NH3 as a function of the bulk concentration of ammonia
NH3
2.718RT RT RT k 0 ln F F v
(1)
(2)
where n is the number of electron transferred in reaction, α the transfer coefficient of the reaction, F the Faraday constant in C, AEC the electrochemical active surface area in cm2, ΓΝΗ3 the amount of ammonia adsorbed on the electrode per unit area in mol cm−2, R the gas constant in J mol−1 K−1, T the absolute temperature in K, E0’ the formal potential of electrode in V, and k0 the standard rate constant in cm s−1. To derive ΓΝΗ3 the Langmuir adsorption isotherm can be applied in the ammonia adsorption on Pt during the electro-oxidation by assuming that the adsorbed species do not interact with each other [31], and is 3
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
Fig. 2. (a) (Pt//Ag)/ITO in 0.5 M KOH with various NH3 concentrations (b) (Pt//Ag)/ITO in 0.5 M KOH with various NO3− concentrations, potential sweep rate: 0.1 V s−1. Insets in (a) and (b) show the nonlinear dependence of Ip,NH3 on CNH3 and the linear dependence of INO3- (−1.0 V) on CNO3- respectively.
NH3
G0 RT
= exp
(5) −1
Τhe ΔG° for physisorption is between −20 and 0 kJ mol , the physisorption together with chemisorption is in the range of −80 to −20 kJ mol−1 and chemisorption is in the range of −400 to −80 kJ mol−1 [32]. The ΔG° for ammonia adsorption in this work is calculated to be −20.7 ± 1.1 kJ mol−1, which suggests a possible physisorption or a combination of physisorption and chemisorption. The nitrate reduction current has a positive linear correlation with the nitrate concentration in solution. The rate-determining step of the nitrate electro-reduction process was also evaluated as demonstrated in Fig. S5. The results indicate an irreversible diffusion-controlled electrode process [27] by knowing that the peak current is proportional to the square root of v and shifts negatively with elevated v. At 25 °C, Ip,NO3- can be expressed as [30]:
Ip,NO3 = 299n -
NH3 CNH3
3.3. Simultaneous detection of ammonia and nitrate on (Pt//Ag)/ITO
(3)
Fig. 3 shows the CV curves of (Pt//Ag)/ITO in 0.5 M KOH in the presence of both ammonia and nitrate at various concentrations. The potential sweep rate was set to be 0.02 V s−1 to obtain a better repeatability of detection results. The ammonia oxidation current and the nitrate reduction current can be recognized in the positive-going sweep and negative-going sweep respectively. The insets are the calibration curves, where the nonlinear current response to ammonia concentration and linear current response to nitrate concentration are presented and are well fitted by Eqs. (4) and (7). The detection limits are 3.946 μM for ammonia, and 0.134 mM for nitrate with a signal to noise ratio of 3. The detection limit for ammonia is calculated by assuming that at extreme low concentrations, Ip,NH3 has a linear correlation with CNH3 as can be deduced from Eq. (4), and thus the slope of the linear regime of Ip,NH3 vs CNH3 is used for the calculation. The detection limit for ammonia is roughly thirty times lower than that for nitrate, which can be
where ΓNH3,S is the amount of ammonia adsorbed on the electrode per unit area at saturation in mol cm−2, and βNH3 the equilibrium parameter for ammonia. Introducing Eq. (3) into Eq. (1) gives
Ip,NH3 =
n F 2AEC v NH3,s 2.718RT 1 +
(7)
where B is a constant.
NH3,s NH3 CNH3
1+
−1
INO3 ( 1.0V) = Bv1/2CNO3
expressed as [30]:
=
2
where DNO3 is the diffusion coefficient of nitrate in cm s , and CNO3the bulk concentration of nitrate in mol L−1. The variation of nitrate reduction current at −1.0 V (INO3- (−1.0 V)) as a function of CNO3- is depicted in Fig. 2b, and INO3- (−1.0 V) is proportional to CNO3- (Fig. 2b inset). Since INO3- at any point on the wave varies with v1/2 and CNO3[30], a similar equation as Eq. (6) gives the current as:
Fig. 3. CV curves of (Pt//Ag)/ITO in 0.5 M KOH with (a) 4.98 μM NH3 and 0.266 mM NO3− (b) 9.92 μM NH3 and 0.595 mM NO3− (c) 26.4 μM NH3 and 1.09 mM NO3− (d) 59.0 μM NH3 and 1.74 mM NO3− (e) 0.108 mM NH3 and 2.54 mM NO3− (f) 0.269 mM NH3 and 3.50 mM NO3− (g) 0.589 mM NH3 and 4.44 mM NO3−, potential sweep rate: 0.02 V s−1. Insets are the nonlinear dependence of Ip,NH3 on CNH3 and the linear dependence of INO3- (−1.0 V) on CNO3-.
NH3
(6)
1/2A D 1/2v 1/2C EC NO3 NO3
NH3 CNH3 NH3 CNH3
(4)
which could successfully explain the nonlinear correlation between Ip,NH3 and CNH3. The fitting of experimental data with the theoretical description has a coefficient of determination R2=0.997 as seen in the solid line of Fig. 2a inset. The fitting results directly give the value of βNH3, and can be used to calculate the standard free energy of adsorption ΔG0 using Eq. (5): 4
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
Table 1 Simultaneous determination of ammonia and nitrate concentrations of simulated sample.
Ammonia Nitrate
Cadded (mM)
Idetected (μA)
Ccalculated (mM)
Relative Deviation (%)
0.206 1.05
10.4 44.5
0.219 1.12
6.31% 6.67%
diagrams [34]. The Ca(OH)2 and Mg(OH)2 colloids and precipitates would attach the electrode surface and cover the active sites for detection. Therefore, Ca2+ and Mg2+ needed to be removed from solution before detection. 4. Conclusion Fig. 4. Reproducibility of (Pt//Ag)/ITO electrode. 26.4 μM NH3 and 0.264 mM NO3−.
In conclusion, a modified electrode with a Pt region and an Ag region was fabricated to simultaneously detect ammonia and nitrate. The Pt region and Ag region possessed high selectivity toward ammonia and the nitrate, respectively. In the range of detection concentrations, the presence of ammonia did not interfere the detection of nitrate, and vice versa, which facilitated the simultaneous detection of both components. The ammonia oxidation on the electrodeposited Pt was an adsorptioncontrolled electrochemical reaction. By introducing Langmuir isotherm, the theoretical model describing the nonlinear correlation between the ammonia oxidation current and ammonia concentration was developed. The standard free energy of adsorption ΔG° for ammonia adsorption was −20.7 ± 1.1 kJ mol−1, and a possible physisorption or a combination of physisorption and chemisorption was suggested by the result. The linear correlation between the nitrate reduction current and nitrate concentration indicated that the diffusion of nitrate ions to the Ag particle surface was the slowest step in the whole electrochemical reduction process. The modified electrode showed a reliable performance for the simultaneous detection of ammonia and nitrate with detection limits of 3.946 μM and 0.134 mM, respectively. Moreover, the response currents of simultaneous detection on the modified electrode after stored in air every six days slightly changed with relative standard deviations of 7.10% and 3.93% for ammonia and nitrate, respectively, suggesting good stability of the modified electrode for long-term use. The simultaneous detection of simulated sample showed relative
correlated to the higher AEC of Pt/ITO than that of Ag/ITO. The reproducibility of (Pt//Ag)/ITO for simultaneous determination of ammonia and nitrate is shown in Fig. 4. The tests were performed every six days, and after each test, the electrode was rinsed with DI water, dried with high pure N2, and preserved in air at room temperature. The relative standard deviations of the response currents of ammonia and nitrate are 7.10% and 3.93%, respectively, indicating a good reproducibility of the performance of (Pt//Ag)/ITO. The simulated water sample containing ammonia and nitrate concentrations the same as those in the groundwater [33] was tested. The results indicate a good performance of the modified electrode for the simultaneous determination of ammonia and nitrate concentrations with relative deviations lower than 10% (see Table 1). The interference of the commonly coexisted ions in water/wastewater, e.g., Na+, K+, Cl− and SO42− in the simultaneous detection of ammonia and nitrate on (Pt// Ag)/ITO was tested by adding the above ions at the same concentration as ammonia or nitrate in solution. The obtained CV curves did not show an obvious change after adding Na+, K+, Cl− and SO42− suggesting good anti-interference ability of (Pt//Ag)/ITO. While divalent cations, such as Ca2+ and Mg2+ formed Ca(OH)2 and Mg(OH)2 colloids and precipitates in 0.5 M KOH according to their species distribution vs pH
Scheme 1. Preparation of modified electrode with a Pt region and an Ag region. 5
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
deviations of detected concentrations from the actual concentrations of ammonia and nitrate were less than 10%. The modified electrode exhibited good anti-interference ability toward Na+, K+, Cl− and SO42−. This work demonstrates the simultaneous detection of ammonia and nitrate through an electrochemical approach for the first time. It is expected that the dual- and multi-region modification of electrode provide a simple and fast way for simultaneous detection.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC 20973020 and 20773007); Program for New Excellent Talents in University (NCET-08–0034); Program for Changjiang Scholars and Innovative Research Team in University (IRT 0805); and Innovation Foundation of BUAA for PhD Graduates. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.microc.2020.104649. References [1] E. Canelli, Simultaneous automated determination of chloride, nitrite, nitrate, and ammonia in water and wastewater, water, air, Soil Poll. 5 (1976) 339–348. [2] M. Trojanowicz, R. Lewandowski, Multiple potentiometric system for continuous determination of chloride, fluoride, nitrate and ammonia in natural waters, Fresen. J. Anal. Chem. 308 (1981) 7–10. [3] T. Aoki, S. Uemura, M. Munemori, Continuous flow method for simultaneous determination of nitrate and ammonia in water, Environ. Sci. Tech. 20 (1986) 515–517. [4] T. Aoki, S. Fukuda, Y. Hosoi, H. Mukai, Rapid flow injection analysis method for successive determination of ammonia, nitrite, and nitrate in water by gas-phase chemiluminescence, Anal. Chim. Acta 349 (1997) 11–16. [5] C.E. López Pasquali, P. Fernández Hernando, J.S. Durand Alegría, Spectrophotometric simultaneous determination of nitrite, nitrate and ammonium in soils by flow injection analysis, Anal. Chim. Acta 600 (2007) 177–182. [6] G.T. Richardson, J.A. Davies, J.G. Edwards, Separation and determination of micromolar ammonia, nitrite, and nitrate from a single small sample, Fresen. J. Anal. Chem. 343 (1992) 473–474. [7] H. Karimi-Maleh, C.T. Fakude, N. Mabuba, G.M. Peleyeju, O.A. Arotiba, The determination of 2-phenylphenol in the presence of 4-chlorophenol using nanoFe3O4/ionic liquid paste electrode as an electrochemical sensor, J. Colloid Interf. Sci. 554 (2019) 603–610. [8] Z. Shamsadin-Azad, M.A. Taher, S. Cheraghi, H. Karimi-Maleh, A nanostructure voltammetric platform amplified with ionic liquid for determination of tert-butylhydroxyanisole in the presence kojic acid, J. Food. Meas. Charact. 13 (2019) 1781–1787. [9] F. Tahernejad-Javazmi, M. Shabani-Nooshabadi, H. Karimi-Maleh, 3D reduced graphene oxide/FeNi3-ionic liquid nanocomposite modified sensor; an electrical synergic effect for development of tert-butylhydroquinone and folic acid sensor, Compos. Part B-Eng. 172 (2019) 666–670. [10] H. Karimi-Maleh, O.A. Arotiba, Simultaneous determination of cholesterol, ascorbic acid and uric acid as three essential biological compounds at a carbon paste electrode modified with copper oxide decorated reduced graphene oxide nanocomposite and ionic liquid, J. Colloid Interf. Sci. 560 (2020) 208–212.
6
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Simultaneous detection of ammonia and nitrate using a modified electrode with two regions
T
Jingyi Wanga,b, , Peng Diaoa, ⁎
a b
⁎
Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G1H9, Canada
ARTICLE INFO
ABSTRACT
Keywords: Electrochemical detection Ammonia Nitrate Modified electrode
Simultaneous detection of ammonia and nitrate has significance in the application areas of environment and industry. Conventional methods include additional steps to separate ammonia and nitrate before detection. A more convenient method is required to detect ammonia and nitrate directly in their mixture. In this work, a modified electrode with electrodeposited Pt region and Ag region was prepared to simultaneously detect ammonia and nitrate for the first time. The Pt region and Ag region were designed to selectively determine ammonia and nitrate, respectively, free of interference with each other. A nonlinear correlation between ammonia oxidation current and ammonia bulk concentration and a linear correlation between nitrate reduction current and nitrate bulk concentration were found. The simultaneous detection limits of ammonia and nitrate were 3.946 μM and 0.134 mM, respectively. The modified electrode showed good long-term stability for the simultaneous detection of ammonia and nitrate with relative standard deviations of 7.10% and 3.93%, respectively. The modified electrode exhibited good anti-interference ability toward Na+, K+, Cl− and SO42−. This work demonstrated a simple and fast approach for detecting two components in solution.
1. Introduction Ammonia and nitrate are two of the important species in inorganic nitrogen cycle. Determination of ammonia and nitrate are explored in water [1–4], soil [5] and nitrogen oxidation/reduction product [6] samples in the application fields of environment and industry. The reported methods for the detection of ammonia and nitrate mixture are colorimetric [1], potentiometric [2], fluorometric [3], chemiluminescence [4], and spectrophotometric [5] combined with flow system to detect ammonia and nitrate in different flow channels [1–5], or with extraction steps [6] to detect ammonia and nitrate separately. Simultaneous dual- or multi-component chemical analysis, determining more than one components at the same time without additional component separation steps, improves the efficiency of detection [7–10]. In electrochemical sensing systems, the application of electrode arrays can enhance the power of electrochemical multi-component detection, in which each electrode is modified with materials of different physical and chemical properties, and can thus add new dimensions of information from analytes [11]. In addition, the modification carried out on different regions of a single substrate facilitates the integration of sensing elements [12–16]. We have applied the partitional modification
method [17] to fabricate two regions on a substrate used as a sensor for simultaneous determination of glucose and nitrite by correcting the influence of glucose on nitrite detection [18]. There are two approaches to further improve the efficiency of electrochemical multi-component detection on a sensor with multi-regions: (1) adding more modified regions with low selectivity [19,20], which puts the work load on data collection and analysis; (2) fabricating highly selective sensing elements, which is much more efficient and of cause a great challenge. Obviously, the second option is ideal. Each of the regions has a sensing element of high selectivity and only responds to one component. The multi-component detection process can be even faster by using a single current channel for data collection. In this case, the sensing elements should respond their corresponding analytes at different electrochemical potentials to avoid interference current [21,22]. Herein, an indium tin oxide (ITO) coated glass substrate was modified into two regions, a Pt region and an Ag region, which have high selectivity toward ammonia and nitrate, respectively. The designed and fabricated electrode was used to simultaneously detect ammonia and nitrate through a signal current channel.
⁎ Corresponding author at: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G1H9, Canada; Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China. E-mail addresses: [email protected] (J. Wang), [email protected] (P. Diao).
https://doi.org/10.1016/j.microc.2020.104649 Received 6 November 2019; Received in revised form 10 January 2020; Accepted 16 January 2020 Available online 17 January 2020 0026-265X/ © 2020 Elsevier B.V. All rights reserved.
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
2. Materials and experimental methods
2.4. Surface morphology characterization
2.1. Materials
The morphology of modified electrode was characterized using scanning electron microscopy (SEM, Philips FEI XL30 SFEG) with an accelerating potential of 10 kV.
Hydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6⋅6H2O), silver nitrate (AgNO3), and all other chemicals were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. All the chemicals were of analytical grade and used directly without further purification. The aqueous solutions were prepared using deionized (DI) water. The stock solutions of ammonia and nitrate were prepared at various concentrations, i.e., 1 mM, 10 mM, 0.1 M, and 1 M for ammonia, and 2 mM, 20 mM, 0.2 M, and 2 M for nitrate. The ITO coated glass slides were obtained from CSG Holding Co. possessing a square resistance of 15 Ω.
3. Results and discussion 3.1. Electrochemical characterization of modified electrode Typical SEM images of Pt/ITO and Ag/ITO (Fig. S1) show crossed and standing Pt flakes of nanoscale, as well as anisotropic Ag particles with sizes ranging from nanometers to micrometers, respectively. The electrochemical active surface area (AEC) of Pt and Ag particles can be determined using electrochemical approaches as described in the Supplementary Material. Typical values for AEC of Pt particles and Ag particles are 0.461 cm2 and 0.0267 cm2, respectively. It is obvious that the AEC of Pt particles is more than ten times larger than that of Ag particles. This result can be reasonably explained by the surface morphologies of Pt and Ag particles as the Pt particles possessing protruding flakes with high surface roughness but the Ag particles exhibiting relatively smooth surface with low roughness. The obtained Pt/ ITO and Ag/ITO were used for detecting ammonia and nitrate respectively and simultaneously in 0.5 M KOH saturated with N2. As shown in Fig. 1a, in the absence of ammonia (black, dashed line), typical CV features for platinum electrode in alkaline solution [23,24] are observed on Pt/ITO. The reduction peak originates from ca. −0.10 V is attributed to the reduction of surface platinum oxides formed in the positive-going sweep. The peaks of hydrogen adsorption and desorption on Pt appear below −0.55 V in the negative- and positive-going sweeps, respectively. In the presence of ammonia (red, solid line), the ammonia oxidation is observed above −0.50 V with a peak current at ca. −0.30 V in the positive-going sweep, and meanwhile the reduction peak of surface platinum oxides is diminished. In addition, the new peaks appearing below −0.55 V are different from those measured in the absence of ammonia. The new peaks arise from NHx,ads desorption and surface nitrogen oxides reduction [24,25]. The CV features of Ag/ ITO in 0.5 M KOH with (red, solid line) and without nitrate (black, dashed line) are shown in Fig. 1b. In the absence of nitrate, the reduction peak starts from −0.30 V in the negative-going sweep reflects OH− desorption from Ag [26]. While in the presence of nitrate, another reduction process could be observed in the negative-going sweep initiating from −0.9 V with a peak at −1.3 V, which is the electro-reduction of nitrate [26,27]. It should be noted that the current values are negative in the potential range of −1.0 to −0.4 V even in the positive potential sweep in the CV curves of Pt/ITO and Ag/ITO electrodes in 0.5 M KOH. This is attributed to the nature of ITO substrate. In the above potential range, the reversible reduction of Sn(IV) to Sn(II) takes place [28] resulting in reduction currents in both positive and negative potential sweeps (Fig. S3). However, this potential range is still in the inert potential range of ITO substrate [29], which allows us to use it as a substrate electrode in the operation condition. When electrically connecting the Pt region and Ag region together, the electrochemical characteristics of (Pt//Ag)/ITO can be recorded. The CV curve of (Pt// Ag)/ITO in 0.5 M KOH is shown as the black dotted line in Fig. 1c, which presents a combination of the electrochemical responses from Pt/ ITO and Ag/ITO. In the presence of ammonia (red dashed line), the ammonia oxidation peak appears in the positive-going sweep; while in the presence of nitrate (solid blue line), the reduction peak appears in the negative-going sweep. The potential sweep window (from −1.0 to 0.0 V) was carefully selected to avoid any interference to the simultaneous detection of ammonia and nitrate at (Pt//Ag)/ITO. The negative potential limit was chosen to prevent the interfering current, i.e., high hydrogen evolution current at E<−1.0 V on the Pt region, and to prevent the formation NH3 at lower electrode potentials, because the produced NH3 could
2.2. Electrochemical experiments Electrochemical experiments were conducted on a CHI750C electrochemical workstation (CH Instrument Co.) at room temperature. A conventional three-electrode cell with a working electrode, a saturated calomel (SCE) as reference electrode, and a Pt foil as counter electrode, was used for the electrodeposition and the cyclic voltammetric (CV) experiments. The single component detection and simultaneous detection of ammonia and nitrate were conducted in 12 mL 0.5 M KOH saturated with high pure N2. Different concentrations of ammonia and nitrate stock solutions were added into the KOH solution to obtain the desired concentrations of analytes in the solution for detection. The added volumes of ammonia and nitrate stock solutions were less than 4% of the total volume of KOH solution. The CV curves were recorded in the presence of various concentrations of ammonia and nitrate, and the response currents originated from ammonia and nitrate were collected. All potentials are reported with respect to SCE. 2.3. Preparation of dual-region modified electrode The ITO coated glass slide was pre-fabricated with the ITO film divided into two closely-spaced regions as illustrated in Scheme 1. As a result, the two ITO film regions were electrically insulated from each other. Electrodeposition was performed on each of the regions to conduct surface modification. Prior to use, the ITO coated glass slide was cleaned by sonication in 0.5 M KOH and acetone for 5 min and 15 min, respectively. The surface was rinsed with DI water after each cleaning step, and then dried with high pure N2. Platinum particles and silver particles were electrodeposited onto the two ITO regions respectively. A similar procedure has been reported in our previous work [8]. The detailed deposition processes are as follows: (1) The cleaned ITO coated glass slide was immersed in 0.1 M HCl containing 1 mM H2PtCl6. One of the two ITO regions was electrically connected to the electrochemical workstation as a working electrode. A potential of 1.4 V was applied to the ITO region for 120 s followed by CVs from −0.3 V to 1.6 V at 0.1 V s−1 for 5 cycles. Then a deposition potential of −0.3 V was applied for 600 s to obtain the Pt-modified region (Pt/ITO). (2) After electrodeposition of Pt, the ITO coated slide was rinsed with DI water and transferred into 0.5 M KNO3 containing 5 mM AgNO3. The other ITO region was electrically connected to the electrochemical workstation, and −0.8 V was applied for 20 ms for 4 times to deposit Ag seeds on the surface. Then a deposition potential of 0.35 V was applied for 1800 s to obtain Ag-modified region (Ag/ITO). The final electrode was a dualregion modified ITO electrode denoted as (Pt//Ag)/ITO with the Pt region and Ag region electrically connected with each other. Therefore, the (Pt//Ag)/ITO performed as a single working electrode in its electrochemical characterization and simultaneous detection of ammonia and nitrate. Before use, the Pt region was activated by conducting CVs in 0.5 M H2SO4 from −0.28 to 1.4 V at 0.5 V s−1 until the CV feature did not change with cycle number. 2
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
Fig. 1. CV curves of (a) Pt/ITO in 0.5 M KOH with (red, solid line) and without (black, dashed line) 50 mM NH3 (b) Ag/ITO in 0.5 M KOH with (red, solid line) and without (black, dashed line) 5.52 mM NO3− (c) (Pt//Ag)/ITO in 0.5 M KOH (black dotted line), 0.5 M KOH with 0.11 mM NH3 (red, dashed line) and 0.5 M KOH with 10.9 mM NO3− (blue, solid line). Potential sweep rate: 0.1 V s−1.
CNH3 (mol L−1) can be obtained from the CV curves, which is presented in the inset of Fig. 2a. A nonlinear correlation between Ip,NH3 and CNH3 is observed in the investigated CNH3 range, which suggests a surface adsorption involved rate-determining mechanism. The rate-determining step and reversibility of the reaction were evaluated by recording the CV curves at different potential sweep rates (v, V s−1) as shown in Fig. S4. The linear dependence of Ip,NH3 on v, and of peak potential Ep,NH3 on lnv suggest an irreversible adsorption-controlled electrode reaction [30]. At 25 °C, Ip,NH3and Ep,NH3 can be expressed as [30]:
subsequently mislead the detection of bulk NH3. The positive potential limit was chosen to be lower than Ag anodic dissolution potentials, and to prevent the formation of nitrogen oxides at higher electrode potentials, because the produced nitrogen oxides could raise a reduction current in the potential range of nitrate reduction on the Ag region, and thus subsequently influence the detection of nitrate. In the selected potential window −1.0 to 0.0 V, there are no interference current from the addition of nitrate at the ammonia oxidation potentials, and no interference current from the addition of ammonia at the nitrate reduction potentials, which are the basic requirements for simultaneous determination of the two components on (Pt//Ag)/ITO.
Ip,NH3 = 3.2. Single component detection on (Pt//Ag)/ITO
n F 2AEC v
Ep,NH3 = E 0 +
Fig. 2a and b show the CV curves of (Pt//Ag)/ITO in 0.5 M KOH with various concentrations of ammonia and nitrate, respectively. In Fig. 2a, the ammonia oxidation peak current increases as the increasing of ammonia concentration in solution. Meanwhile the current at −1.0 V does not vary with the ammonia concentration, which means the presence of ammonia would not interfere the current response at −1.0 V for nitrate detection. Similarly, in Fig. 2b, the increase of nitrate concentration does not give rise to the current response at ammonia detection region of −0.5 to 0 V in the positive-going sweep, indicating no interference from nitrate to ammonia detection. The variation of the peak current Ip,NH3 as a function of the bulk concentration of ammonia
NH3
2.718RT RT RT k 0 ln F F v
(1)
(2)
where n is the number of electron transferred in reaction, α the transfer coefficient of the reaction, F the Faraday constant in C, AEC the electrochemical active surface area in cm2, ΓΝΗ3 the amount of ammonia adsorbed on the electrode per unit area in mol cm−2, R the gas constant in J mol−1 K−1, T the absolute temperature in K, E0’ the formal potential of electrode in V, and k0 the standard rate constant in cm s−1. To derive ΓΝΗ3 the Langmuir adsorption isotherm can be applied in the ammonia adsorption on Pt during the electro-oxidation by assuming that the adsorbed species do not interact with each other [31], and is 3
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
Fig. 2. (a) (Pt//Ag)/ITO in 0.5 M KOH with various NH3 concentrations (b) (Pt//Ag)/ITO in 0.5 M KOH with various NO3− concentrations, potential sweep rate: 0.1 V s−1. Insets in (a) and (b) show the nonlinear dependence of Ip,NH3 on CNH3 and the linear dependence of INO3- (−1.0 V) on CNO3- respectively.
NH3
G0 RT
= exp
(5) −1
Τhe ΔG° for physisorption is between −20 and 0 kJ mol , the physisorption together with chemisorption is in the range of −80 to −20 kJ mol−1 and chemisorption is in the range of −400 to −80 kJ mol−1 [32]. The ΔG° for ammonia adsorption in this work is calculated to be −20.7 ± 1.1 kJ mol−1, which suggests a possible physisorption or a combination of physisorption and chemisorption. The nitrate reduction current has a positive linear correlation with the nitrate concentration in solution. The rate-determining step of the nitrate electro-reduction process was also evaluated as demonstrated in Fig. S5. The results indicate an irreversible diffusion-controlled electrode process [27] by knowing that the peak current is proportional to the square root of v and shifts negatively with elevated v. At 25 °C, Ip,NO3- can be expressed as [30]:
Ip,NO3 = 299n -
NH3 CNH3
3.3. Simultaneous detection of ammonia and nitrate on (Pt//Ag)/ITO
(3)
Fig. 3 shows the CV curves of (Pt//Ag)/ITO in 0.5 M KOH in the presence of both ammonia and nitrate at various concentrations. The potential sweep rate was set to be 0.02 V s−1 to obtain a better repeatability of detection results. The ammonia oxidation current and the nitrate reduction current can be recognized in the positive-going sweep and negative-going sweep respectively. The insets are the calibration curves, where the nonlinear current response to ammonia concentration and linear current response to nitrate concentration are presented and are well fitted by Eqs. (4) and (7). The detection limits are 3.946 μM for ammonia, and 0.134 mM for nitrate with a signal to noise ratio of 3. The detection limit for ammonia is calculated by assuming that at extreme low concentrations, Ip,NH3 has a linear correlation with CNH3 as can be deduced from Eq. (4), and thus the slope of the linear regime of Ip,NH3 vs CNH3 is used for the calculation. The detection limit for ammonia is roughly thirty times lower than that for nitrate, which can be
where ΓNH3,S is the amount of ammonia adsorbed on the electrode per unit area at saturation in mol cm−2, and βNH3 the equilibrium parameter for ammonia. Introducing Eq. (3) into Eq. (1) gives
Ip,NH3 =
n F 2AEC v NH3,s 2.718RT 1 +
(7)
where B is a constant.
NH3,s NH3 CNH3
1+
−1
INO3 ( 1.0V) = Bv1/2CNO3
expressed as [30]:
=
2
where DNO3 is the diffusion coefficient of nitrate in cm s , and CNO3the bulk concentration of nitrate in mol L−1. The variation of nitrate reduction current at −1.0 V (INO3- (−1.0 V)) as a function of CNO3- is depicted in Fig. 2b, and INO3- (−1.0 V) is proportional to CNO3- (Fig. 2b inset). Since INO3- at any point on the wave varies with v1/2 and CNO3[30], a similar equation as Eq. (6) gives the current as:
Fig. 3. CV curves of (Pt//Ag)/ITO in 0.5 M KOH with (a) 4.98 μM NH3 and 0.266 mM NO3− (b) 9.92 μM NH3 and 0.595 mM NO3− (c) 26.4 μM NH3 and 1.09 mM NO3− (d) 59.0 μM NH3 and 1.74 mM NO3− (e) 0.108 mM NH3 and 2.54 mM NO3− (f) 0.269 mM NH3 and 3.50 mM NO3− (g) 0.589 mM NH3 and 4.44 mM NO3−, potential sweep rate: 0.02 V s−1. Insets are the nonlinear dependence of Ip,NH3 on CNH3 and the linear dependence of INO3- (−1.0 V) on CNO3-.
NH3
(6)
1/2A D 1/2v 1/2C EC NO3 NO3
NH3 CNH3 NH3 CNH3
(4)
which could successfully explain the nonlinear correlation between Ip,NH3 and CNH3. The fitting of experimental data with the theoretical description has a coefficient of determination R2=0.997 as seen in the solid line of Fig. 2a inset. The fitting results directly give the value of βNH3, and can be used to calculate the standard free energy of adsorption ΔG0 using Eq. (5): 4
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
Table 1 Simultaneous determination of ammonia and nitrate concentrations of simulated sample.
Ammonia Nitrate
Cadded (mM)
Idetected (μA)
Ccalculated (mM)
Relative Deviation (%)
0.206 1.05
10.4 44.5
0.219 1.12
6.31% 6.67%
diagrams [34]. The Ca(OH)2 and Mg(OH)2 colloids and precipitates would attach the electrode surface and cover the active sites for detection. Therefore, Ca2+ and Mg2+ needed to be removed from solution before detection. 4. Conclusion Fig. 4. Reproducibility of (Pt//Ag)/ITO electrode. 26.4 μM NH3 and 0.264 mM NO3−.
In conclusion, a modified electrode with a Pt region and an Ag region was fabricated to simultaneously detect ammonia and nitrate. The Pt region and Ag region possessed high selectivity toward ammonia and the nitrate, respectively. In the range of detection concentrations, the presence of ammonia did not interfere the detection of nitrate, and vice versa, which facilitated the simultaneous detection of both components. The ammonia oxidation on the electrodeposited Pt was an adsorptioncontrolled electrochemical reaction. By introducing Langmuir isotherm, the theoretical model describing the nonlinear correlation between the ammonia oxidation current and ammonia concentration was developed. The standard free energy of adsorption ΔG° for ammonia adsorption was −20.7 ± 1.1 kJ mol−1, and a possible physisorption or a combination of physisorption and chemisorption was suggested by the result. The linear correlation between the nitrate reduction current and nitrate concentration indicated that the diffusion of nitrate ions to the Ag particle surface was the slowest step in the whole electrochemical reduction process. The modified electrode showed a reliable performance for the simultaneous detection of ammonia and nitrate with detection limits of 3.946 μM and 0.134 mM, respectively. Moreover, the response currents of simultaneous detection on the modified electrode after stored in air every six days slightly changed with relative standard deviations of 7.10% and 3.93% for ammonia and nitrate, respectively, suggesting good stability of the modified electrode for long-term use. The simultaneous detection of simulated sample showed relative
correlated to the higher AEC of Pt/ITO than that of Ag/ITO. The reproducibility of (Pt//Ag)/ITO for simultaneous determination of ammonia and nitrate is shown in Fig. 4. The tests were performed every six days, and after each test, the electrode was rinsed with DI water, dried with high pure N2, and preserved in air at room temperature. The relative standard deviations of the response currents of ammonia and nitrate are 7.10% and 3.93%, respectively, indicating a good reproducibility of the performance of (Pt//Ag)/ITO. The simulated water sample containing ammonia and nitrate concentrations the same as those in the groundwater [33] was tested. The results indicate a good performance of the modified electrode for the simultaneous determination of ammonia and nitrate concentrations with relative deviations lower than 10% (see Table 1). The interference of the commonly coexisted ions in water/wastewater, e.g., Na+, K+, Cl− and SO42− in the simultaneous detection of ammonia and nitrate on (Pt// Ag)/ITO was tested by adding the above ions at the same concentration as ammonia or nitrate in solution. The obtained CV curves did not show an obvious change after adding Na+, K+, Cl− and SO42− suggesting good anti-interference ability of (Pt//Ag)/ITO. While divalent cations, such as Ca2+ and Mg2+ formed Ca(OH)2 and Mg(OH)2 colloids and precipitates in 0.5 M KOH according to their species distribution vs pH
Scheme 1. Preparation of modified electrode with a Pt region and an Ag region. 5
Microchemical Journal 154 (2020) 104649
J. Wang and P. Diao
deviations of detected concentrations from the actual concentrations of ammonia and nitrate were less than 10%. The modified electrode exhibited good anti-interference ability toward Na+, K+, Cl− and SO42−. This work demonstrates the simultaneous detection of ammonia and nitrate through an electrochemical approach for the first time. It is expected that the dual- and multi-region modification of electrode provide a simple and fast way for simultaneous detection.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC 20973020 and 20773007); Program for New Excellent Talents in University (NCET-08–0034); Program for Changjiang Scholars and Innovative Research Team in University (IRT 0805); and Innovation Foundation of BUAA for PhD Graduates. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.microc.2020.104649. References [1] E. Canelli, Simultaneous automated determination of chloride, nitrite, nitrate, and ammonia in water and wastewater, water, air, Soil Poll. 5 (1976) 339–348. [2] M. Trojanowicz, R. Lewandowski, Multiple potentiometric system for continuous determination of chloride, fluoride, nitrate and ammonia in natural waters, Fresen. J. Anal. Chem. 308 (1981) 7–10. [3] T. Aoki, S. Uemura, M. Munemori, Continuous flow method for simultaneous determination of nitrate and ammonia in water, Environ. Sci. Tech. 20 (1986) 515–517. [4] T. Aoki, S. Fukuda, Y. Hosoi, H. Mukai, Rapid flow injection analysis method for successive determination of ammonia, nitrite, and nitrate in water by gas-phase chemiluminescence, Anal. Chim. Acta 349 (1997) 11–16. [5] C.E. López Pasquali, P. Fernández Hernando, J.S. Durand Alegría, Spectrophotometric simultaneous determination of nitrite, nitrate and ammonium in soils by flow injection analysis, Anal. Chim. Acta 600 (2007) 177–182. [6] G.T. Richardson, J.A. Davies, J.G. Edwards, Separation and determination of micromolar ammonia, nitrite, and nitrate from a single small sample, Fresen. J. Anal. Chem. 343 (1992) 473–474. [7] H. Karimi-Maleh, C.T. Fakude, N. Mabuba, G.M. Peleyeju, O.A. Arotiba, The determination of 2-phenylphenol in the presence of 4-chlorophenol using nanoFe3O4/ionic liquid paste electrode as an electrochemical sensor, J. Colloid Interf. Sci. 554 (2019) 603–610. [8] Z. Shamsadin-Azad, M.A. Taher, S. Cheraghi, H. Karimi-Maleh, A nanostructure voltammetric platform amplified with ionic liquid for determination of tert-butylhydroxyanisole in the presence kojic acid, J. Food. Meas. Charact. 13 (2019) 1781–1787. [9] F. Tahernejad-Javazmi, M. Shabani-Nooshabadi, H. Karimi-Maleh, 3D reduced graphene oxide/FeNi3-ionic liquid nanocomposite modified sensor; an electrical synergic effect for development of tert-butylhydroquinone and folic acid sensor, Compos. Part B-Eng. 172 (2019) 666–670. [10] H. Karimi-Maleh, O.A. Arotiba, Simultaneous determination of cholesterol, ascorbic acid and uric acid as three essential biological compounds at a carbon paste electrode modified with copper oxide decorated reduced graphene oxide nanocomposite and ionic liquid, J. Colloid Interf. Sci. 560 (2020) 208–212.
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