Highly Sensitive Determination of Ammonia Nitrogen by Nanocubic Copper Prepared by Direct Electrochemical Deposition

Abstract

The precise and rapid detection of ammonia nitrogen is of paramount importance in safeguarding water environments. In this study, we introduce a novel approach for electrochemical ammonia sensing using nanocubic copper electrodes, fabricated through a straightforward electrodeposition technique. A comprehensive characterization of the copper electrode sheds light on the pivotal role of deposition in shaping the morphology of copper particles, subsequently impacting the ammonia sensing capabilities. Through an in-depth investigation of the electrochemical behavior of nanocubic copper electrodes, we unveil how ammonia enhances electron transfer during copper oxidation by forming robust coordination with Cu(II) and simultaneously disrupting the oxide layer on the copper surface. This synergistic effect process has been effectively harnessed for the rapid electrochemical quantification of ammonia nitrogen. Linear scan voltammetry results reveal a direct relationship between peak currents and ammonia nitrogen concentrations spanning the broad range of 0.1–100 ppm. Notably, the nanocubic copper electrode exhibits a low detection limit, exceptional resistance to interference and impressive repeatability. Moreover, our practical testing in real water samples show the nanocubic copper electrode’s superior performance over the spectrometric method in determining ammonia nitrogen concentrations. These findings underscore the potential of the nanocubic copper electrode for high-performance ammonia sensing applications.

1. Introduction

Ammonia-nitrogen is a prevalent water pollutant commonly found in industrial wastewater.^1,2 It consists of un-ionized ammonia and ammonium, the concentrations of which are pH- and temperature-dependent. Excessive ammonia entering aquatic environments will cause water eutrophication and disrupt the equilibrium of the water ecosystem.³ Furthermore, ammonia poses a toxic threat to humans, fish, and crustaceans, particularly affecting aquatic organisms in their early stages of development.^4,5 Therefore, an accurate detection and regular monitoring of ammonia nitrogen concentrations in aquatic environments are of paramount importance.

Currently, spectroscopic methods have been widely employed for the detection of ammonia nitrogen in water systems. These methods include Nessler’s reagent the sodium reagent colorimetric method,⁶ salicylic acid spectrophotometry,⁷ and gas-phase molecular absorption spectroscopy.⁸ However, spectroscopic approaches often suffer from complicated procedures, slow detection rates, and the use of significant quantities of toxic and harmful reagents. In contrast, electrochemical methods, especially the modified electrode methods,^9,10 offer the inherent advantages, such as rapid response, ease of operation, and high sensitivity, which are more in line with the requirements for fast, accurate detection and online real-time monitoring of ammonia nitrogen.

Gas-sensitive electrodes¹¹ and ion-selective electrodes¹² have been successfully developed for ammonia nitrogen detection. However, they have limitations in terms of narrow detection ranges, limited sensitivity, and susceptibility to factors like temperature and pH,¹³ thereby limiting their practical applications. In contrast, voltammetric methods based on nanoelectrode materials have attracted great interest for electrochemical ammonia nitrogen detection due to their specific selectivity and ultra-high sensitivity.^14–16

As the pivotal active material of a modified electrode, introducing the specific properties of nanomaterials, such as high specific area, excellent adsorption ability and catalytic performance, could dramatically enhance the reaction rate, selectivity and sensitivity of electrodes.^17–20 Various nanomaterials have been explored to construct electrochemical sensors for the determination of ammonia nitrogen in water.^1,21 Baciu et al.¹⁴ fabricated a silver-electrodecorated carbon nanotubes-epoxy composite electrode, which displayed excellent electrocatalytic activity towards the direct oxidation of ammonium. The composite electrode displayed good detection sensitivity of 0.613 mA/mmol L⁻¹(mM) and a low detection limit of 1 µM for the electrochemical sensing of ammonia. Massafera et al. utilized polypyrrole nanowires, directly electrodeposited onto Au substrates, as an electrochemical transducer for the oxidation of ammonia, which was successfully employed for ammonia sensing in solution.²² To boost the catalytic activity, Pt nanoparticles were further decorated on polypyrrole by Zhang et al.¹⁶ The results showed that the composite could be utilized as an electrochemical sensor for ammonia in solution. Alternatively, the sensing performance for ammonia would be improved when the morphology of Pt was controlled to be nanosheet.²³ Recently, it has been demonstrated that copper is an attractive candidate for ammonia sensing. Valentini et al. found that MWCNT/Cu nanocomposite paste electrodes were sufficiently sensitive for ammonia detection at a low potential and a higher current response.¹⁵ Subsequently, Yang et al. adopted a one-step hydrothermal synthetic procedure to construct stereo rosette-like copper particles on carbon cloth. The rosette-like copper electrode exhibited high sensitivity to ammonia with a low detection limit.²⁴ The morphology and structure of electrode materials significantly influence the electrochemical behavior of electrodes, thereby enhancing the response performance of electrochemical sensors. Electrochemical deposition offers an effective means to control the morphology and size of nanometal particles, featuring advantages like simplicity of operation and excellent reproducibility. In this study, a straightforward constant potential deposition method was employed to deposit nanocubic copper metal particles onto a platinum substrate. Subsequently, the electrochemical behavior of these particles in ammonia-containing solutions was investigated, with the aim of utilizing them to construct an electrochemical sensor for ammonia nitrogen detection.

2. Experimental

2.1 Materials

Sulfuric acid, Copper (II) sulfate pentahydrate (CuSO₄·5H₂O), Sodium carbonate, Sodium bicarbonate, Ammonium sulfate were purchased form Sinopharm Group Chemical Reagent Co., Ltd. Pt sheets (10 × 10 × 0.1 mm, 2.5 × 4 × 0.1 mm) were provided by Shanghai Lerton Industry Co., Ltd. Ultrapure water (18.2 MΩ/cm, Sichuan Youpu Ultrapure Technology Co., Ltd.) was used throughout the experiments.

2.2 Instrumentation

The microstructure and composition distribution features were assessed using scanning electron microscopy (SEM, SU8010, Hitachi, Tokyo, Japan) and energy-dispersive X-ray spectroscopy (EDX, X-MaxN, Oxford Instruments, UK). X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha, USA) was employed to analyze the elemental states and surface properties of the prepared Cu electrode. Electrochemical experiments were conducted using a Zahner Zennium electrochemical workstation (Zahner, Germany) within a conventional three-electrode system comprising a saturated calomel electrode (SCE) as the reference electrode, and Pt sheets serving as both the counter electrode and working electrode.

2.3 Fabrication of nanocubic copper electrode

The nanocubic copper electrode was prepared through constant potential deposition onto Pt sheets. Typically, a mixed solution of 0.9 M H₂SO₄ and 0.075 M CuSO₄ was employed as the deposition solution. The electrochemical deposition was carried out using an electrochemical workstation within a three-electrode system. Here, the Pt sheets served as both the working and counter electrodes, while a saturated calomel electrode was used as the reference electrode. Deposition potentials ranged from −0.2 to −0.4 V, and the potential was maintained for a duration of 60 s.

2.4 Electrochemical tests

All electrochemical tests were performed on the electrochemical workstation using 50 mM carbonate buffer solution (CBS) as the supporting electrolyte¹⁵ and ammonium sulfate as the source of ammonia nitrogen. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted in the potential range of −0.8 V–0.1 V with varying sweep rates.

2.5 Determination of ammonia by the spectrophotometric method

The spectrophotometric determination of ammonia nitrogen with salicylic acid²⁵ involves the following process: In an alkaline medium (pH = 11.6), in the presence of nitrosoferricyanide sodium [Na₂(Fe(CN)₆)NO]·2H₂O, ammonia and ammonium ions in water react with salicylate and hypochlorite ions to form a blue compound. This compound absorbs at a wavelength of 697 nm, and the absorbance is measured with a spectrophotometer.

3. Results and Discussion

3.1 Characterization of nanocubic copper electrode

We employed constant potential deposition technique to fabricate a nanocubic copper electrode by depositing copper onto a platinum (Pt) surface. Cyclic voltammetry (CV) analysis of the pristine Pt electrode in a CuSO₄ solution (Fig. S1) revealed a reduction peak at −0.3 V, indicating the conversion of Cu²⁺ to Cu⁰. Consequently, we selected a deposition potential of −0.3 V for generating nanocubic copper. The morphology of the nanocubic copper deposits on the Pt electrode surface was analyzed using scanning electron microscopy (SEM). Figures 1a and 1b show SEM images of the nanocubic copper electrode obtained after a 60 s deposition at −0.3 V. Evidently, the Pt surface was completely covered by nanocubic copper particles with a predominantly cubic shape and an average diameter of approximately 40 nm. Elemental mapping analysis confirmed the substantial deposition of copper particles across the Pt surface. SEM analysis at deposition potentials of −0.2 V and −0.4 V (Fig. S2) revealed that higher deposition potentials lead to fewer but larger microparticles on Pt surface. Decreasing the deposition potential to −0.4 V caused most particles to merge together, resulting in a loss of the cubic shape. Therefore, −0.3 V was determined to be the suitable deposition potential to maintain the desired cubic morphology and a large surface area. Energy-dispersive X-ray spectroscopy (EDS) confirmed that the electrode primarily consisted of metal copper nanoparticles, with a small amount of oxygen attributed to adsorbed oxygen on copper particles and Pt sheets, along with some oxidized copper.

Figure 1.

(a, b) SEM imagines of nanocubic Cu electrode deposited at −0.3 V. (c–e) Element mapping of (c) Cu and O, (d) Cu and (e) O. (f) EDS spectrum.

X-ray photoelectron spectroscopy (XPS) was employed to gain insight into the elemental states and surface properties of the electrodeposited nanoscale Cu electrode. In the sum spectrum (Fig. 2a) of nanoscale Cu deposited on Pt sheets at −0.3 V for 60 s, C 1s, O 1s and Cu 2p peaks were observed. The high-resolution XPS measurement of Cu 2p (Fig. 2b) displayed two major peaks at 932.8 eV and 952.6 eV, corresponding to Cu 2p_3/2 and Cu 2p_1/2,²⁶ respectively. These peaks could be fitted with two doublets assigned to CuO and Cu₂O (and/or Cu) indicating the presence of Cu⁺ (or Cu⁰) rather Cu²⁺ as Cu²⁺ peaks typically appear at higher binding energies. The presence of a small satellite peak at 944.5 eV suggested the existence of Cu²⁺ on the surface, as it is not typically found for Cu₂O.²⁷ The nanoscale copper electrode also showed an O 1s peak at ∼531 eV (Fig. 2c), which could be deconvoluted into three peaks attributed to lattice oxygen of CuO, Cu₂O, chemisorbed oxygen species (e.g., O⁻, O₂⁻, O²⁻) and OH, respectively.²⁶ This suggested some surface oxidation of the copper nanocubes, consistent with the EDX results.

Figure 2.

(a) XPS survey spectra of the nanocubic copper electrode deposited at −0.3 V and the corresponding fine spectra of (b) Cu 2p and (c) O 1s.

3.2 Electrochemical behavior of nanocubic copper in ammonia

The electrochemical behavior of the nanocubic copper electrode in the presence ammonia was investigated using cyclic voltammetry (CV) in a potential range of −0.8 V to 0.1 V. In a 50 mM carbonate buffer (pH = 10), both with and without ammonia, the CV curves (Fig. 3a) of the nanocubic copper electrode displayed three oxidative peaks at −0.42 V, −0.26 V, and −0.03 V, along with two reductive peaks at −0.54 V and −0.18 V. These peaks corresponded to oxygen adsorption, the oxidation of Cu(0) to Cu(I), and the oxidation of Cu(I) and/or Cu(0) to Cu(II) and the reduction of Cu(II) to Cu(I) and Cu(I) to Cu(0),²⁸ respectively. The presence of 10 ppm NH₃ in the solution resulted in similar profiles, with slight shifts in peak potential. Notably, oxidative peaks II (−0.26 V), III (0.03 V), and reductive peak V (−0.18 V) showed pronounced changes in peak currents, indicating their sensitivity to ammonia. Pristine Pt electrodes immersed in ammonia-containing solution (inset of Fig. 3a) remained essentially unchanged, confirming that the observed changes were specific to the nanocubic copper electrode. These changes were attributed to the formation of a stable copper-ammonia complex due to the interplay between ammonia and copper facilitating copper oxidation.²⁹

Figure 3.

(a) Cyclic voltammograms of nanocubic Cu electrodes and bare Pt electrode (insert) in 50 mM carbonate buffer solution (pH = 10) with the presence and absence of 10 ppm ammonia. Scan rate = 50 mV s⁻¹. (b) Cyclic voltammograms of nanocubic Cu electrode in the presence of different concentrations of ammonia ranged from 1 to 20 ppm. The insets are the plots of peak current versus ammonia concentration. (c) Linear scan voltammograms of nanocubic copper in carbonate buffer (pH = 10) with the presence of 10 ppm NH₃ at different scan rates. Inset shows the liner fitting plot of the logarithm of the peak current (i_p) at −0.03 V versus the logarithm of the scan rate (ν). (d–f) Linear scan voltammograms of nanocubic copper electrodes without (d) and with polarization at −0.26 V (e) and −0.03 V (f) in different concentrations of ammonia.

To establish the relationship between the redox peak current of the nanocubic copper electrode and ammonia concentration, the CV was used to detect the response of nanocubic electrode with NH₃ concentration from 1 to 20 ppm in 50 mM CBS (pH = 10) (Fig. 3b). The oxidation peak current (i_p(III)) at −0.03 V exhibited a linear relationship with ammonia concentration, as described by the equation i_p(III) = 0.00134 c + 0.00159, R² = 0.99. Similarly, the reduction peak around −0.6 V showed an increase in peak current with ammonia concentration, described by i_p(V) = −0.00171 c − 0.02409, R² = 0.947. Therefore, the oxidation peak near −0.03 V was selected for quantitative ammonia nitrogen analysis.

The resulted current through the electrode surface includes Faraday and capacitive currents.³⁰ Specifically, when the peak current predominantly consists of Faraday current, the electrode surface process can be categorized into two types. Specifically, when the peak current predominantly consists of Faraday current, the electrode surface process can be categorized into two types. When peak current is linear with square root of scan rate (ν^1/2), the electrode reaction is a diffusion control process.³¹ When peak current is linear to the scan rate (ν), the electrode reaction is a surface-controlled process.³² In the research, the faradic peak current (i_p) is obtained by subtracting the background current from the actual peak current in the LSV curves. Figure 3a shown the influence of the scan rate on the oxidation peak obtained from LSV plots in 50 mM CBS (pH = 10). The oxidation peak currents (−0.03 V) increased with rising scan rates across the range of 5–200 mV s⁻¹. The inset in Fig. 3a depicts linear relationships between the peak currents, excluding background (capacitive) currents, and the scan rate within the 5–200 mV s⁻¹ range. The regression equation is i_p = 9.66 × 10⁻⁴ ν + 0.01247 (R² = 0.996). Without ammonia, another linear relationship exists between the peak currents and the scan rate within the range of 5–200 mV s⁻¹. The regression equation is i_p = 6.48 × 10⁻⁴ ν + 0.00387 (R² = 0.998) (as depicted in Fig. S3), indicating a surface-controlled process on the electrode surface. This signifies that ammonia does not change the faradic process on nanocubic Cu and just accelerate the electrochemical oxidation of copper through a complex.

Electrochemical polarization treatments were applied to copper electrodes deposited at −0.3 V for 60 s to alter their surface states with various copper species.³³ Figure 3d depicts the LSV plots for the as-prepared copper electrode within the range of −0.15 to 0.1 V in 50 mM CBS (pH = 10). As illustrated, in the blank solution, two peaks with distinct peak current emerge at 0 V and 0.07 V, respectively. Upon introduction of ammonia to the solution, the current of both peaks increased. Notably, the addition of ammonia results in the shift of the oxidation peak at higher potential and nearly coalescing with the peak at lower potential.

Previous investigations of the voltammetric behavior of nanocubic copper electrodes demonstrated the polarization of the prepared nanocubic copper electrodes at −0.26 V would lead to an increased concentration of Cu(I) on the nanocubic copper particles. As shown in Fig. 3e, after polarizing at −0.26 V for 5 min in 50 mM CBS (pH = 10), the oxidation peak at low potential becomes pronounced in blank solution, leading to the speculation that the peak at low potential corresponds to the Cu(I)/Cu(II) conversion, while the peak at high potential corresponds to the Cu(0)/Cu(II) conversion, through the following reaction:¹⁵

  

\begin{equation} \text{Cu$_{2}$O} + \text{H$_{2}$O} + \text{CO$_{3}^{2-}$}-\text{2 e$^{-}$} \to \text{Cu$_{2}$(OH)$_{2}$CO$_{3}$} \end{equation} (1)

  

\begin{equation} \text{2 Cu} + \text{2 OH$^{-}$} + \text{CO$_{3}^{2-}$}-\text{4 e$^{-}$} \to \text{Cu$_{2}$(OH)$_{2}$CO$_{3}$} \end{equation} (2)

Notably, the Figs. 3d and 3e illustrate that with the gradually increasing NH₃ concentration, the peak current at low potential undergoes significant augmentation, whereas the peak current at high potential increases at a more modest pace. This trend suggests that ammonia facilitated the oxidation of Cu(I) and Cu(0), particularly the Cu(I)/Cu(II) conversion, probably through the following reactions:

  

\begin{equation} \text{Cu$_{2}$O} + \text{8 NH$_{3}$} + \text{H$_{2}$O}-\text{2 e$^{-}$} \to \text{2 Cu(NH$_{3}$)$_{4}{}^{2+}$} + \text{2 OH$^{-}$} \end{equation} (3)

  

\begin{equation} \text{Cu} + \text{4 NH$_{3}$}-\text{2 e$^{-}$} \to \text{Cu(NH$_{3}$)$_{4}{}^{2+}$} \end{equation} (4)

Polarization at −0.03 V in CBS (pH = 10), facilitates the transformation of these electrodes into the Cu(II) state. As depicted in Fig. 3f, the LSV curve appears relatively flat in the blank solution, signifying a weak electrode oxidation process during this stage, which is attributed to the development of a stable copper oxide layer¹⁵ on the surface of the nanocubic copper electrode during the polarization. This oxide layer hampers internal copper oxidation due to its inadequate conductivity. Upon the addition of ammonia to the solution, two obvious peaks emerge, indicating that ammonia could significantly boosts the oxidation of both Cu(I) and Cu(0). This effect could arise from the creation of a stable copper-ammonia complex with Cu(II), which disrupts the stable oxide layer on the surface of the nanocubic copper electrode, as described in the following reaction:

  

\begin{equation} \text{Cu$_{2}$(OH)$_{2}$CO$_{3}$} + \text{8 NH$_{3}$} \to \text{2 Cu(NH$_{3}$)$_{4}{}^{2+}$} + \text{2 OH$^{-}$} + \text{CO$_{3}{}^{2-}$} \end{equation} (5)

These results reveal that the robust coordination between ammonia and Cu(II) expedites the oxidation process of copper near −0.03 V by enhancing electron transfer and disrupting the non-conductive oxide layer on copper surface. The extent of this promotion correlated directly with ammonia concentration.

To investigate the impact of deposition potential on the electrochemical sensitivity of nanocubic copper electrodes to ammonia nitrogen, firstly we maintained a fixed deposition time of 60 s, varied deposition potential. Figure 4a shows the plots of the oxidation peak current versus ammonia concentration around −0.03 V. At −0.2 V and −0.3 V deposition potentials, the peak current at the potential of −0.03 V exhibited a linear increase with rising ammonia nitrogen concentrations, displaying a robust linear correlation. However, as the deposition potential decreased to −0.4 V, the peak current deviated from the proportional relationship with ammonia concentration, suggesting an initial increase followed by a decline. Ultimately, −0.3 V was determined as the optimal deposition potential. Furthermore, we investigated the influence of deposition time on the electrochemical response of nanocubic copper electrodes to ammonia nitrogen. The deposition potential is fixed at −0.3 V and the deposition time is varied. Figure 4b shows different deposition duration exhibited a good linear relationship with ammonia concentration, with the response current to the same ammonia concentration being relatively consistent. This indicated that a deposition time of 30 s was sufficient to cover the platinum surface with nanocubic copper. Considering both the response magnitude and the strength of the linear correlation, a deposition time of 60 s was chosen as optimal. Finally, as shown in Fig. 4a, the influence of solution pH on the sensing ability of nanocubic copper electrode for ammonia was investigated via LSV in 50 mM CBS with (0–2 ppm) NH₃. At pH 9.2, the peak current increased with ammonia concentration. But the linear correlation was weaker at ammonia concentration below 1 ppm. At pH 10, the peak current consistently increased linearly with rising ammonia levels, offering the best correlation. At pH 11, the peak current remained nearly unchanged with ammonia concentration. Ammonia’s state in solutions is pH-dependent and it mainly exists as NH₃ when pH exceeds 9.75.³⁴ At pH 10, ammonia mostly takes on the NH₃ form and prevents the precipitation of Cu²⁺ with OH⁻ and CO₃²⁻. Thus, a pH of 10 was selected for the subsequent ammonia determination.

Figure 4.

The plots of the peak current of LSV near −0.03 V of nanocubic copper electrodes towards to different ammonia concentration. (a) Nanocubic copper electrodes deposited at different potential for 60 s. (b) Nanocubic copper electrodes deposited at −0.3 V for varied duration. (c) The influence of solution pH on the peak current responses.

The electrochemical response of the nanocubic copper electrode was investigated using the LSV in 50 mM CBS (pH = 10). As shown in Fig. 5a, while the concentration of NH₃ increased from 0.1 ppm to 100 ppm, the oxidation peak current at the potential of about −0.03 V also increased proportionally. Two linear regions were observed in Fig. 5b, when NH₃ concentration falls between 0.1 to 2 ppm, the linear equation is one is i_p = 0.00242 c + 0.000193 (R² = 0.996) for ammonia concentration between 0.1 to 2 ppm and the other is i_p = 0.00118 c + 0.0047 (R² = 0.999) for NH₃ concentrations within the range of 2 to 100 ppm. Compared to other materials, the nanocubic copper has a wider detection range, as shown in Table 1. Meanwhile, the limit of detection (LOD) was determined to 0.08 ppm (S/N = 3). Notably, this linear range and low LOD is superior to some ammonia sensors reported previously.

Figure 5.

(a) LSV plots of nanocubic copper electrode in 50 mM carbonate buffer solution (pH = 10) to the successive addition of ammonia from 0.1 ppm to 100 ppm. (b) Calibration curves for ammonia.

Table 1. Comparison of the performance of the detection ammonia with other electrochemical sensors.

Material	Method	Linear Range	LOD	year	Ref
Ag-CNT	DPV	0.2–1 mM	1 µM	2017	14
Cu/MWCNT	DPV	3–100 µM	3.74 µM	2018	15
Pt-PPy/Ni foam	DPV	500 nM–400 µM	12.36 nM	2020	16
Pt-PANI-CC	DPV	0.5–550 µM	77.2 nM	2020	23
PtIr3	I-V	1–11 mM	4.88 µM	2019	35
PANi-Nafion-Cu	I-t	1–1000 µM	0.5 µM	2016	36
Pt-Ni(OH)2-NF	DPV	5–500 µM	2.74 µM	2022	37
CuO codoped ZnO	I-V	77 µM–0.7 M	8.9 µM	2011	38
Pt-PPy-CC	DPV	1–450 µM	0.24 µM	2021	39
Nanocubic Cu	LSV	5.88 µM–5.88 mM	4.7 µM	—	This work

The nanocubic copper electrode demonstrated good repeatability for ammonia determination with a standard deviation of 2.3 % for seven successive detections of 2 ppm ammonia (Fig. 6a). The effect of interference on ammonia detection has also been studied. Interference from various species, such as Cl⁻, NO₃⁻, HPO₄²⁻ and glucose was minimal (<10 %) with the addition of 20 ppm interference, indicating the electrode’s excellent anti-interference capability. Reproducibility is examined by conducting independent experiments with seven different of nanocubic copper electrodes for the voltametric detection of 2 ppm ammonia. A relative standard deviation of 4.39 % indicates a satisfactory reproducibility of the nanocubic copper electrode for ammonia sensing.

Figure 6.

(a) The repeatability, (b) anti-interference and (c) reproducibility of nanocubic copper electrodes. Ammonia concentration for all experiments is 2 ppm.

The practical applicability of nanocubic copper electrodes for the ammonia nitrogen determination in real water sample from sewage treatment plants was demonstrated. After the filtration, these water samples were analyzed using nanocubic copper electrodes and the ammonia concentration was subsequently determined based on the established working curve. For comparison, the ammonia concentration in these sample was determined by spectrophotometric method. As shown in Table 2, the results were consistent with those obtained through spectrophotometric analysis, confirming the reliability of the nanocubic copper sensor for real water samples.

Table 2. Detection results of ammonia nitrogen content in river and lake water using the nanocubic Cu electrode-based electrochemical method and salicylic acid spectrophotometric method.

Method	Sample 1/ppm	Sample 2/ppm	Sample 3/ppm
Nanocubic copper sensor	7.6	6.6	1.7
Spectrophotometry	7.7	6.5	1.1

4. Conclusions

In conclusion, we have successfully synthesized nanocubic copper electrodes using a straightforward constant potential deposition technique. The morphology of these copper particles was found to be strongly dependent on the deposition potential, influencing the sensing capabilities of the resulting electrodes. Through a comprehensive investigation using cyclic voltammetry and linear sweep voltammetry, we gained valuable insights into the electrochemical behavior of these nanocubic copper electrodes. The results demonstrate that the electrochemical oxidation of Cu(0)/Cu(I) within these electrodes exhibits a progressively increased peak current with rising ammonia concentration. This highlights the tremendous potential of nanocubic copper electrodes for the sensitive detection of aqueous ammonia. Furthermore, we observed that the presence of ammonia leads to a gradual transition of copper oxidation from a surface reaction-dominated mechanism to a diffusion-controlled one, which is crucial in enhancing the electrooxidation of nanocubic copper. The remarkable performance of our nanocubic copper-based sensor was highlighted by its high sensitivity and impressively low detection limit of 0.08 ppm. Additionally, the sensor exhibited excellent selectivity in ammonia detection, making it a promising candidate for environmental monitoring applications. The successful application of our sensor in real water samples from sewage treatment plants further underscores its potential utility.

Acknowledgments

This work was supported by grants the Hubei Province Technology Innovation Project (2018AAA056), the Natural Science Foundation of Hubei Province of China (No. 2023AFB731).

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.24791466.

• 1) The precise and rapid detection of ammonia nitrogen is of paramount importance in safeguarding water environments. In this study, we introduce a novel approach for electrochemical ammonia sensing using nanocubic copper electrodes, fabricated through a straightforward electrodeposition technique. A comprehensive characterization of the copper electrode sheds light on the pivotal role of deposition in shaping the morphology of copper particles, subsequently impacting the ammonia sensing capabilities. Through an in-depth investigation of the electrochemical behavior of nanocubic copper electrodes, we unveil how ammonia enhances electron transfer during copper oxidation by forming robust coordination with Cu(II) and simultaneously disrupting the oxide layer on the copper surface. This synergistic effect process has been effectively harnessed for the rapid electrochemical quantification of ammonia nitrogen. Linear scan voltammetry results reveal a direct relationship between peak currents and ammonia nitrogen concentrations spanning the broad range of 0.1–100 ppm. Notably, the nanocubic copper electrode exhibits a low detection limit, exceptional resistance to interference and impressive repeatability. Moreover, our practical testing in real water samples show the nanocubic copper electrode’s superior performance over the spectrometric method in determining ammonia nitrogen concentrations. These findings underscore the potential of the nanocubic copper electrode for high-performance ammonia sensing applications.

CRediT Authorship Contribution Statement

Qianhui Luo: Data curation (Lead), Investigation (Lead), Methodology (Equal), Writing – original draft (Lead)

Yulin Zheng: Investigation (Equal)

Qing Chang: Conceptualization (Equal), Formal analysis (Lead)

Yuqun Xie: Investigation (Equal), Methodology (Equal)

Guodong Jiang: Conceptualization (Lead), Investigation (Lead), Methodology (Lead), Supervision (Lead), Writing – review & editing (Lead)

Conflict of Interests

The authors declare no competing interests.

Funding

Hubei Province Technology Innovation Project : 2018AAA056

the Natural Science Foundation of Hubei Province of China: No. 2023AFB731

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