Detection of Ammonium Ion by An Electrochemical Sensor Based on Cu-BTC

There is a growing interest in utilizing metal-organic frameworks (MOFs) for the development of electrochemical sensors with superior performance. In this work, a study on the detection of ammonium ion (NH4 +) on metal-organic frameworks (MOFs) Cu-BTC surface has been conducted by using both experimental and computational methods. By using DFT calculation, the adsorption energy of ammonium ion (NH4 +) on the MOF surfaces was determined. The calculation result showed that NH4 + molecules can be adsorbed on the surface of Cu-BTC with an adsorption energy value of -1.51 eV. Additionally, we performed the synthesis of Cu-BTC and, using CV (Cyclic Voltammetry), we obtained a working area of around -0.113 V. Furthermore, chronoamperometry tests revealed that the addition of ammonium at concentrations of 0.05, 0.1, and 0.15 mM resulted in changes in the current. The sensor also showed good stability and an increase in peak current at each tested concentration. This confirms that the MOFs tested can be utilized as ammonium ion sensors.


Introduction
Monitoring ammonium ions is crucial for addressing environmental and health concerns due to their prevalence in wastewater, fertilizers, and industrial effluents [1] [2].Ammonium ions are commonly found in wastewater, soil, and natural water bodies.The NH 4 + concentration becomes one of the indicators that determine the quality of a water environment.For instance, when the NH 4 + level is excessively high, that is, more than 2 mg per L of water, it leads to eutrophication (rapid overgrowth of aquatic plants due to excess nutrients leaching from fertilizers) and poor water quality [2] [3].
Not only in the environmental aspect, but the measurement of ammonium ions also plays a crucial role in the field of health.For instance, the ammonium levels in urine can monitor the clinical treatment of kidney diseases [4].Therefore, tracking ammonium ions levels is essential for sustainable practices, pollution control, and even health care improvement.Electrochemical-based sensors have gained significant attention for their high sensitivity, selectivity, rapid response, and portability, enabling real-time monitoring in various scenarios [1] [2] [5] [6] [7].However, to enhance the efficiency of these sensors for ammonium ion detection, a thorough understanding of their detection mechanism is vital.
Metal organic frameworks (MOFs) are advanced crystalline materials created by combining metal ions or clusters with organic linkers [5].MOFs possess a high surface area, flexible structure, and adjustable pore size, making them highly versatile in gas adsorption and storage, separation processes, and catalysis applications.Because of their unique properties, MOFs show great potential in electrochemical sensing [8].Previous studies have reported the capability of copper (II)-based MOF connected by benzene-1,3,5tricarboxylate (Cu-BTC) to detect ammonia in gas form [9]. Inspired by that finding, we try to investigate the ability of the Cu-BTC to detect the ammonia in its ionic form (NH 4 + ).
In this study, Cu based nanosheet MOFs or HKUST-1 were synthesized using a solvent method at room temperature.MOFs morphology, structure, and composition were characterized by some methods like scanning electron microscopy (SEM) and X-Ray Diffraction (XRD) [9].Next, chronoamperometry testing was conducted to determine whether there is a response (in the form of current changes) due to the variation in NH 4 + concentration around Cu-BTC.The adsorption mechanism of NH 4 + on the active site of Cu-BTC was studied using density functional theory (DFT) calculation.

Material Preparations
In this work, the method was used to prepare Cu-BTC.First, copper solution was prepared by dissolving 0.05 M of copper nitrate trihydrate (Cu(NO 3 ) 2 •3H 2 O) in water and trimesic acid solution containing 40 wt% of TEOA was also prepared in a different glass.Next, both solutions were mixed with a ratio of 3:2 to obtain blue precipitation.The precipitate was then washed with ethanol and water several times to remove the excess of the precursor and dried at 60 °C overnight.The product of Cu-BTC nanoparticles was labeled as Cu-BTC NPS.

Electrochemical Characterizations
Electrochemical testing is also an important part of Cu-BTC characterization.Electrochemical testing on Cu-BTC was first performed using cyclic voltammetry (CV) test.This CV test is commonly used to investigate reduction and oxidation processes of Cu-BTC.This testing also can elucidate electron transfer processes and the presence of intermediate [10].This initial test was performed using a screenprinted carbon electrode (SPCE).This sensor has three main electrodes, they are reference, working, and counter electrode, all of which are present in a single unit.The CV testing was conducted using the method in previous research for dengue virus immunosensor [11].Prior to the testing, SPCE needs to be activated using 1 M H 2 SO 4 .The SPCE is treated with H 2 SO 4 until all three electrodes are fully covered.Then the SPCE is connected to PalmSens CH Instrument.The activation process begins with a scan rate of 100 mV/s and a total of 50 scans.After the drying process, the SPCE can be considered active and ready for CV testing.The dropping process of Cu-BTC onto SPCE starts by mixing 4 mg of Cu-BTC with 950 μL of double distilled water.Then it was placed on the working electrode of the SPCE and allowed to dry for about 3 to 4 hours.During this time, the CV testing was performed by connecting to the CH instrument.The electrode will receive a specific potential and currents are generated from the electron transfer process, resulting in a CV graph.
The chronoamperometry (CA) test was then conducted after identifying the peak on CV graph.This test was used to determine the current response as a function of time when a square step signal was applied to the working electrode.This CA test was initiated by drop-casting water onto electrode's surface.Subsequently, following the CV test result, the potential value is recorded over time with a run time of 10000.Then, the electrode is drop-casted with an NH 4 + solution with varying concentrations, starting from 0.05 mM, 0.1 mM, and 0.15 mM.This will yield varied results and the fluctuation in current indicates that the Cu-BTC functions as a sensor that is sensitive to ammonia concentration changes.

Computational Methods
DFT calculations have been employed to investigate the adsorption trends of ammonium ion on Cu-BTC MOFs.All calculations were performed using the B3LYP [12][13] functional in Gaussian 09 program [14].For all optimization calculations, the basis set employed consisted of 6-31++G(d,p) for H, C, N, and O atoms, and LANL2DZ for Cu atoms.The Cu-BTC active site structure selected for the calculations is the paddle-wheel structure with open metal sites (OMS), as depicted in Figure 1.The self-consistent field was adjusted to achieve quadratic convergence, with an additional iteration in cases where the initial first order SCF did not converge (XQC) for all calculations.

Morphology and Structure Characterization
Results obtained from SEM method showed that Cu-BTC MOFs had a morphology of a double-sided pyramidal diamond shape material.Whereas XRD characterization method showed that synthesis of Cu-BTC MOFs had a similar diffraction pattern from those in the previous research [5].From both methods, it can be concluded that Cu-BTC MOFs were successfully synthesized.XRD and SEM results are shown in Figures 2-4.XRD measurements were conducted using a diffractometer with Cu K α (154060 Å) that ranged from 2θ = 5.000 o up to 2θ = 70.002o .The measurements resulted in significant peak values at 6.7 o , 9.5 o , 11.6 o , 13.3 o , 18.9 o , 26 o that does not differ significantly from the reference [5].
Furthermore, from the obtained dataset, percentage of crystallinity and amorphousness of Cu-BTC MOFs were 61.6% and 38.4%, respectively.This indicates that the crystalline form of Cu-BTC is more dominant than the amorphous form.As a result, the SEM characterization is expected to reveal a more dominant crystalline shape.From Figure 3, the results of SEM characterization aligned with the expectations, based on XRD observations.Although there were several differences between experimentbased results from reported reference, the results were sufficiently accurate as the differences were not too significant.The variations in the results were attributed to several factors, some of which were different accuracy levels of instrument used, different parameters and variability between sample used with reported reference, and data processing factors.

Behaviour
The graph resulting from CV and CA testing is used to describe the electrochemical behaviour of Cu-BTC.The first graph is cyclic voltammograms, which is shown in Figure 4.It is a graph illustrating the process of applying a voltage within a specific range, usually from negative to positive potential, along the x-axis and the resulting current response along the y-axis.In the testing for Cu-BTC, a forward scan is conducted, where the voltage initially increased until the current is reaching a maximum point and then stopped for once in potential limit called switch potential.Then a subsequent voltage decrease occurs until it meets the lowest point and come back to the initial state.From this graph, the position of the anodic peak, cathodic peak, and the half-cell potential value can be determined, which is useful for defining the redox reaction within.
The first thing to notice is the shape of the CV graph.It appears to be asymmetry.This suggest that electron transfer within Cu-BTC is unlikely to be naturally reversible.It is also evident from the shape difference between cathodic and anodic peak.Using the IUPAC convention, the anodic peak is in the upper, while the cathodic peak is the lower one.However, there is only anodic peak in the CV graph.
When potential scan was performed form positive to negative potential, there is no cathodic peak.It indicates that the reduction process of electrolyte species is not occurring.The absence of reduction is caused by the fact that the electrolyte contains only ammonium ions.On the other hand, knowing that the anodic peak is located at -0.113 V and the maximum density current is 1.2 A/cm 2 , it can be said that the Cu-BTC has decent electrochemical properties.After determining the anodic peak location, a current response measurement is performed in response to a square signal input.This square signal input is intended to assess whether the Cu-BTC is sufficiently responsive to sudden change in ammonium ion concentrations under the maximum electrochemical condition.The result shown in Figure 5 depicts significant and distinct changes in current.At the 145 seconds of the experiment, chronoamperometry test is carried out with a dosage of 0.05 mM ammonia.A decrease in current occurs at the beginning, followed by fluctuating increase in current after the adding.The initial increase is acknowledged to be less reliable due to inconsistencies which may result in inaccurate measurements of ammonia levels.The fluctuating values are possibly caused by impurities within the electrode surfaces and the lack of calibration.Nonetheless, there is still an overall increase in current by 0.32 A.
Subsequently, at 470 seconds, the ammonium ion concentration is changed to 0.1 mM.This change here is more stable and demonstrates the usual responses to the square signal input.There is an increase in current by 0.19 A with steady-state time of approximately 70 s.Then, at the 650 seconds of the experiment, ammonium ion concentration is raised again to 0.15 mM and the observed change in current remains stable as before.The increase in current is slightly larger, at 0.21 A but with a faster steadystate time of 50 s.This indicates that as the total ammonium ion concentration increases, the response of Cu-BTC sensor becomes faster and provides a larger response.However, the magnitude of current changes is still relatively small, indicating that this sensor needs to be paired with an instrumental amplifier to increase the sensor's sensitivity.

DFT Calculations
The adsorption energy of NH 4 + on the Cu-BTC paddle-wheel active site is calculated using the following equation: The calculated adsorption energy (E_adsorption) for the ammonium ion is approximately -1.5 eV.This demonstrates that NH 4 + adsorption on the Cu-BTC paddle wheel active site is stable.Given that the detection of NH 4 + ions in an electrochemical sensor necessitates both the adsorption of NH 4 + and its subsequent reduction, the stable NH 4 + adsorption on the Cu-BTC active site strongly indicates that this material can effectively function as a suitable NH 4 + sensor.

Conclusion
To summarize, this study involved the synthesis of a typical type of nanosheet-structured MOFs (HKUST-1) nanomaterials based on Cu (copper), which was then utilized as the foundation for a novel electrochemical sensor designed for detecting ammonium ion (NH 4 + ).The porous structure of HKUST-1 was found to facilitate substantial electron transfer, leading to effective electrocatalytic activity towards ammonium ion.The synthesized Cu-BTC is a success based on the tests conducted.Firstly, from the morphology and structure characterization, the XRD showed the material has a dominant crystallinity up to 61.6% and that makes the SEM result is close to the reference.Secondly, from the electrochemical behaviour aspect, cyclic voltammetry (CV) and chronoamperometry (CA) test were conducted.The CV test showed that this material has a decent electrochemical property to be an ammonium ion sensor based on the captured clear anodic peak.Additionally, the CA test showed an inconsistency due to the impurities of electrode surfaces and lack of calibration, yet the material is stable enough to be a sensor provided that an amplifier is applied to.Lastly, the DFT calculations showed that adsorption energy calculated is -1.5 eV, indicating the interaction between ammonium ion and Cu-BTC perfectly enhances the stability of system.Overall, the Cu-BTC sensor displayed decent sensitivity and favourable stability.Importantly, successful application in real sample testing underscored its potential practical significance.

Cu 4 +Figure 1 .
Figure 1.This image represents the results of geometry optimization for Cu-BTC MOF, Ammonium ion, and Cu-BTC MOF interacting with Ammonium ion.