Porous MgNiO2 Chrysanthemum Flower Nanostructure Electrode for Toxic Hg2+ Ion Monitoring in Aquatic Media

A simple hydrothermal synthesis approach was used to synthesize porous MgNiO2 Chrysanthemum Flowers (CFs) nanostructures and applied as a sensing electrode for quick detection of hazardous mercury (Hg2+ ions). The morphological, structural, and electrochemical properties of MgNiO2 CFs were investigated. The morphological characteristic of MgNiO2 CFs, with a specific surface area of 45.618 m2/g, demonstrated strong electrochemical characteristics, including cations in different oxidation states of Ni3+/Ni2+. Using a three-electrode system for electrochemical detection, the MgNiO2 CFs based electrode revealed a good correlation coefficient (R2) of ~0.9721, a limit of detection (LOD) of ~11.7 μM, a quick response time (10 s), and a sensitivity of 8.22 μA∙μM−1∙cm−2 for Hg2+ ions over a broad linear range of 10–100 μM. Moreover, the selectivity for Hg2+ ions in tap water and drinking water was determined, and a promising stability of 25 days by MgNiO2 CFs electrode was exhibited. The obtained results indicate that the developed MgNiO2 CFs are a promising electrode for detecting hazardous Hg2+ ions in water and have the potential to be commercialized in the future.


Introduction
With the expansion of industry, environmental contamination has become a social concern as a result of mining for materials.Mine development seriously pollutes the aquatic environment along with soil.Additionally, industries such as chemical fertilizers, cosmetics, and gold and aluminum mining contribute significantly to heavy metal pollution in water [1].Because heavy metal pollution is not biodegradable and may accumulate in the bodies of living beings, it is regarded as a major source of harmful environmental contamination.It causes several physical and mental health issues for humans and animals, including terrestrial and aquatic creatures [2].Metallic elements are necessary for the human body in trace amounts, but their concentration range has a significant influence on human health.When the concentration range of a metal is below its toxic range, it is deemed safe, but when it exceeds the permissible limit, it causes a variety of cytological and physiological effects [3].
Hg 2+ ions are the most dangerous and dominant toxicant in the environment, and ingestion of Hg 2+ ions might damage reproductive organs, bones, brain function, kidneys, and liver, resulting in damaging the nervous system, hair, vision, and hearing loss [4,5].Human chromosomes and genetic abnormalities are both caused by the intake of Hg 2+ ions [6].Hg 2+ is a highly toxic heavy metal that might cause serious problems in aquatic environments.Mercury is a major source of environmental concern due to its stability at polluted sites and complex biological toxicity processes [7][8][9][10][11][12].The accumulation of Sensors 2023, 23, 7910 2 of 14 these metals in the body constitutes a serious hazard to human health, and therefore, the development of highly sensitive technologies for measuring trace levels of heavy metal ions has garnered considerable attention [13][14][15][16].
Many approaches for measuring heavy metals have been developed over the last few decades.Inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence spectroscopy (XRF), and atomic absorption spectrometry (AAS) are common techniques for assessing these metal ions [17].However, the time-consuming method, hefty maintenance expenses, and pricey complex instruments severely limit their practical applicability.In modern society, the sensitive and selective identification of dangerous heavy metals using cost-effective and acceptable methodologies is critical [18].Electrochemical detection techniques have recently garnered a lot of interest for heavy metal ion detection because of their capacity to detect ions with a fast analysis time, low power cost, and high sensitivity [17].In an electrochemical approach, heavy metal ions generate changes in current, potential, electrochemical impedance, capacitance, or electrochemical luminescence that can be utilized to detect them [19].
Over the past few decades, nano-metal oxides have been widely explored in the field of electrochemical detection.Nano-metal oxides are synthesized to achieve varying sizes, stability, and morphologies.Because of these variances, these materials exhibit a wide range of electrical and photochemical characteristics, making them valuable for a wide range of applications [20].Metal oxides, primarily transition metal oxides, have been utilized to alter electrodes for the detection of a variety of analytes [21], and just a few have been exploited for heavy metal detection [22].Recently, secondary transition metal oxide or binary metal oxide-based materials have presented remarkable catalytic properties [23].Apart from other metal oxides, blending MgO and NiO to form MgNiO 2 materials has received immense attention because this combination significantly elevates the active sites for excellent catalytic reaction.In addition, various synthetic techniques can yield different catalytic impacts on electrochemical and photoelectrochemical systems.MgNiO 2 is commonly produced using solid-state reactions or chemical approaches, such as coprecipitation, sol-gel, or hydrothermal synthesis [3].In most reports, a solid state approach has been used to produce MgNiO 2 materials in which high-purity magnesium oxide (MgO) and nickel oxide (NiO) powders are thoroughly mixed in the desired stoichiometric ratio at high temperature ranges from 800 • C to 1000 • C [24].However, the hydrothermal technique involves utilizing a closed physical system and a chemical process that occurs in an aqueous solution at temperatures exceeding 100 • C to synthesize diverse chemical compounds and materials.Usually, hydrothermal synthesis offers an improved approach for obtaining small, porous, uniformly sized nanomaterials.Recently, there has been a surge in research focused on hydrothermal synthesis for secondary metal oxide materials, including MgFe 2 O 4 , MgNiO 2 , LiFePO 4 etc. [25].
In this study, MgNiO 2 CFs were synthesized using a simple hydrothermal approach and utilized for the three-electrode electrochemical system for the detection of toxic Hg 2+ ions.Morphological, structural, optical, and electrochemical investigations are performed for as-synthesized MgNiO 2 CFs, and the sensor performances in terms of sensitivity, stability, selectivity, repeatability, and detection limit for Hg 2+ ions are thoroughly investigated.The sensing behavior of MgNiO 2 CFs based electrode is examined by measuring the cyclicvoltametry (CV) and linear sweep voltammetry (LSV) in 0.1 M PBS (pH = 7) by varying the concentration of Hg 2+ ions.MgNiO 2 CFs based electrode shows a broad linear range of 1 µM -1 mM and a limit of detection (LOD) of ~373.9 nM with a good sensitivity of 9.008 µA•µM −1 •cm −2 for Hg 2+ ions.

Synthesis of MgNiO 2 CFs
MgNiO 2 was synthesized by a simple hydrothermal method.A total of 0.214 g of magnesium acetate tetrahydrate (CH 3 COO) 2 Mg•4H  USA) and 0.6 g of urea (CO(NH 2 ) 2 , Sigma-Aldrich, Missouri, United States) were dissolved in 40 mL of deionized (DI) water [26].After that, the solution was magnetically stirred at room temperature for 1 h.The mixed solution was subjected to Teflon-lined stainless steel for 10 h at 120 • C.After completion of the reaction, the product was washed with DI water and ethanol and centrifuged for 15 min at ~3000 rpm to obtain a white solid product.The product was then dried overnight at 60 • C in an oven and calcined at 650 • C for 6 h.

Characterization of MgNiO 2 CFs
Field emission scanning electron microscopy (FESEM, Hitachi S-4700, Tokyo, Japan) and a transmission electron microscope (TEM, H-7650, Hitachi, Tokyo, Japan) were used to identify the morphological properties.Energy dispersive X-ray spectroscopy (EDS) was used to determine the elemental composition.To explain the crystal characteristics, X-ray diffraction (XRD, Rigaku, Woodlands, TA, USA, Cu K, = 1.54178Å) in the Bragg angle range of 20 • to 80 • was used.A UV-visible spectrophotometer (JASCO, V-670) was used to measure the absorption characteristics.The structural characteristics were determined by Fourier transform infrared (FTIR, IR300, Nicolet, QC, Canada,) spectroscopy in the 400-4000 cm −1 region and Raman (Renishaw, Wotton-under-Edge, Old Town, UK) spectroscopy in the 200-1400 cm −1 ranges, respectively [26].Surface characteristics of MgNiO 2 CFs, such as specific surface area and pore size distribution, were investigated using the Brunauer-Emmett-Teller (BET) method with a Micromeritics Tristar 3000.The X-rays Photoelectron Spectroscopy (XPS; KRATOS AXIS-Nova, Manchester, UK) evaluated the surface composition and element states with a 0-1400 eV energy range.

Electrochemical Sensing of Hg 2+ Ions Using MgNiO 2 CFs Electrode
For the observation of Hg 2+ ions in the solution medium, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were used.The electrolyte was prepared using 0.1 M phosphate buffered saline (PBS) at concentrations ranging from 1-100 µM Hg 2+ ions.For electrode preparation, a cleaned 3 mm diameter screen printed electrode (SPE) was used, and a paste of 0.05 wt% of MgNiO 2 powder with nafion solution was deposited on the SPE surface by the doctor blade method [27][28][29][30].Thereafter, the nafion binder was subsequently removed by annealing SPE for 15 min at 80 • C in an oven.Three electrodes with SPE as working electrode, Ag/AgCl reference electrode, and a gold wire counter electrode were used to detect Hg 2+ ions.For cyclic voltammetry (CV) measurements, the scan rate was fixed at 10 mV/s using different concentration of Hg 2+ ions.The sensitivity of the electrochemical sensor is estimated by dividing the slope of the calibrated plot by the active area of the sensing electrode.The limit of detection (LOD) was calculated from the following equation: where SD = standard deviation

Morphological Properties and Element Analysis of MgNiO 2 CFs
FESEM was employed to investigate the morphology of MgNiO 2 nanostructures, as shown in Figure 1a-c.At low magnification, the synthesized MgNiO 2 exhibited the morphology of Chrysanthemum Flowers (CFs) with an average diameter of ~6.34 µm, as shown in Figure 1a,b.Figure 1c exhibits the MgNiO 2 CFs at high magnification with thin sheet aggregation and an average thickness of ~25 nm.The existence of several pores in the sheet was apparent, suggesting that the synthesized MgNiO 2 CFs are porous and have a high specific surface area.Additionally, SEM-EDS was used to verify the purity of the synthesized MgNiO 2 CFs.The EDS spectra of MgNiO 2 CFs are shown in Figure 1d.Herein, no peaks other than Mg, Ni, and O were seen, suggesting that the synthesized MgNiO 2 CFs are pristine.The traces of Pt peaks can be seen in an EDS image, which might be due to the surface coating of Pt during FESEM.Furthermore, the atomic percentages (at %) of Mg, Ni, and O were 17.9%, 19.2%, and 62.9%, respectively, with a comparatively high proportion of oxygen perhaps owing to oxygen or moisture in the air sticking to the MgNiO 2 CFs.The FESEM results confirmed that the synthesized MgNiO 2 was pure with a unique Chrysanthemum Flower structure.
synthesized MgNiO2 CFs.The EDS spectra of MgNiO2 CFs are shown in Figure 1d.Herein, no peaks other than Mg, Ni, and O were seen, suggesting that the synthesized MgNiO2 CFs are pristine.The traces of Pt peaks can be seen in an EDS image, which might be due to the surface coating of Pt during FESEM.Furthermore, the atomic percentages (at %) of Mg, Ni, and O were 17.9%, 19.2%, and 62.9%, respectively, with a comparatively high proportion of oxygen perhaps owing to oxygen or moisture in the air sticking to the MgNiO2 CFs.The FESEM results confirmed that the synthesized MgNiO2 was pure with a unique Chrysanthemum Flower structure.TEM and HRTEM were used to confirm the morphology of MgNiO2 CFs.The TEM image in Figure 2a corresponds with FESEM results, exhibiting the size of MgNiO2 CFs as ~5-7 µm.Figure 2b,c is a high magnification TEM image that reveals a clear lattice structure, showing that the synthesized MgNiO2 CFs have an excellent crystalline phase.The HRTEM image, as shown in Figure 2d, exhibited a lattice distance of ~0.217 nm, which corresponds to the (200) crystal plane [31,32].

Structural and Surface Properties of MgNiO 2 CFs
The crystalline characteristics of MgNiO 2 CFs were identified by X-ray diffraction (XRD), as shown in Figure 3a.The observed diffraction peaks at ~36.40  200) plane.The Scherrer equation [33] was used for calculating the size of the crystals of MgNiO 2 CFs: where τ is the crystal size, β is the full width at half maximum (FWHM), θ is the Bragg angle, k is the Scherrer constant (crystal shape = 0.94), and λ is the X-ray wavelength  [34,35].Furthermore, quite weak bands were observed at ~2359 cm −1 and ~1476 cm −1 , corresponding to atmospheric CO 2 adsorption [36].The Raman spectroscopy observations of MgNiO 2 CFs are presented in Figure 3c.The peak at ~496 cm −1 was attributed to a single phonon (1P) TO mode, and the peak at ~1076 cm −1 was assumed to be the result of LO modes.The existence of NiO was the primary cause for the observed Raman peaks [37].According to the Raman spectra, the synthesized MgNiO 2 CFs showed high phase purity.

Structural and Surface Properties of MgNiO2 CFs
The crystalline characteristics of MgNiO2 CFs were identified by X-ray diffraction (XRD), as shown in Figure 3a.The observed diffraction peaks at ~36.40° (111), ~42.40° (200), ~61.98° (220), ~74.58° (311), and 78.52° (222) correspond to typical MgNiO2 (JCPDS 24-0712).A prominent diffraction peak appeared at ~42.40°, indicating that MgNiO2 CFs of MgNiO2 CFs are presented in Figure 3c.The peak at ~496 cm −1 was attributed to a single phonon (1P) TO mode, and the peak at ~1076 cm −1 was assumed to be the result of LO modes.The existence of NiO was the primary cause for the observed Raman peaks [37].According to the Raman spectra, the synthesized MgNiO2 CFs showed high phase purity.4a, the overall specific surface area of MgNiO2 CFs was ~45.618 m 2 /g, with voids providing the total specific surface area at ~41.041 m 2 /g.Herein, because of the high porosity of MgNiO2 CFs, the specific surface area increased significantly.Figure 4b shows the average diameter of the pores as ~61.871 nm.The results showed a match with the morphological properties of FESEM results, implying that the increased surface area of MgNiO2 CFs might enhance the active sites for the detection of target heavy metals, resulting in enhanced sensitivity.4a, the overall specific surface area of MgNiO 2 CFs was ~45.618 m 2 /g, with voids providing the total specific surface area at ~41.041 m 2 /g.Herein, because of the high porosity of MgNiO 2 CFs, the specific surface area increased significantly.Figure 4b shows the average diameter of the pores as ~61.871 nm.The results showed a match with the morphological properties of FESEM results, implying that the increased surface area of MgNiO 2 CFs might enhance the active sites for the detection of target heavy metals, resulting in enhanced sensitivity.
of MgNiO2 CFs are presented in Figure 3c.The peak at ~496 cm −1 was attributed to a single phonon (1P) TO mode, and the peak at ~1076 cm −1 was assumed to be the result of LO modes.The existence of NiO was the primary cause for the observed Raman peaks [37] According to the Raman spectra, the synthesized MgNiO2 CFs showed high phase purity   4a, the overall specific surface area of MgNiO2 CFs was ~45.618 m 2 /g, with voids providing the total specific surface area at ~41.041 m 2 /g.Herein, because of the high porosity of MgNiO2 CFs, the specific surface area increased significantly.Figure 4b shows the average diameter of the pores as ~61.871 nm.The results showed a match with the morphological properties of FESEM results implying that the increased surface area of MgNiO2 CFs might enhance the active sites for the detection of target heavy metals, resulting in enhanced sensitivity.X-ray photoelectron (XPS) analysis was utilized to determine the surface binding energies of MgNiO 2 CFs. Figure 5a shows the survey profile, which exhibits distinct peaks of O 1s, Ni 2p, and Mg 1s, indicating the existence of Ni, Mg, and O in MgNiO 2 CFs. Figure 5b depicts a high-resolution O 1s spectra with peaks at ~528.17 eV, ~529.90 eV, and ~530.98 eV resulting from oxygen bonding with Ni 2+ , Mg 2+ , and Ni 3+ ions, respectively [38,39].It suggests that the MgNiO 2 CFs bonding is composed of three ions: Ni 2+ , Mg 2+ , and Ni 3+ .In Figure 5c, the high-resolution spectrum of Ni 2p depicts the peaks at ~853.14 eV and 855.19 eV, which might be due to Ni 2p 3/2 of Ni 2+ and Ni 3+ , while the peaks at ~870.72 eV and ~872.19 eV are due to Ni 2p 1/2 of Ni 2+ and Ni 3+ .The MgNiO 2 CFs were assumed to be in the Ni 3+ and Ni 2+ states [40].The peaks at ~859.59 eV and ~861.13 eV were Ni 2p 3/2 satellites, whereas the peaks at ~877.27 eV, ~878.75 eV, and ~880.47 eV were Ni 2p 1/2 satellites.Furthermore, the difference between the Ni 2p 3/2 and Ni 2p 1/2 double peaks was ~17.58 eV, indicating the existence of various oxidized Ni 3+ and Ni 2+ ions [41,42].Figure 5d shows the high-resolution Mg 1s spectra, which has binding energies of ~1302.12eV for Mg and ~1303.24eV for Mg-O [23].Thus, the existence of Ni 3+ , Ni 2+ , and Mg 2+ ions was explained by the XPS spectra of MgNiO 2 CFs.
Sensors 2023, 23, x FOR PEER REVIEW 7 of 1 X-ray photoelectron (XPS) analysis was utilized to determine the surface binding energies of MgNiO2 CFs. Figure 5a shows the survey profile, which exhibits distinct peak of O 1s, Ni 2p, and Mg 1s, indicating the existence of Ni, Mg, and O in MgNiO2 CFs.Figur 5b depicts a high-resolution O 1s spectra with peaks at ~528.17 eV, ~529.90 eV, and ~530.98 eV resulting from oxygen bonding with Ni 2+ , Mg 2+ , and Ni 3+ ions, respectively [38,39].I suggests that the MgNiO2 CFs bonding is composed of three ions: Ni 2+ , Mg 2+ , and Ni 3+ .In Figure 5c, the high-resolution spectrum of Ni 2p depicts the peaks at ~853.14 eV and 855.19 eV, which might be due to Ni 2p3/2 of Ni 2+ and Ni 3+ , while the peaks at ~870.72 eV and ~872.19 eV are due to Ni 2p1/2 of Ni 2+ and Ni 3+ .The MgNiO2 CFs were assumed to be in th Ni 3+ and Ni 2+ states [40].The peaks at ~859.59 eV and ~861.13 eV were Ni 2p3/2 satellites whereas the peaks at ~877.27 eV, ~878.75 eV, and ~880.47 eV were Ni 2p1/2 satellites Furthermore, the difference between the Ni 2p3/2 and Ni 2p1/2 double peaks was ~17.58 eV indicating the existence of various oxidized Ni 3+ and Ni 2+ ions [41,42].Figure 5d shows th high-resolution Mg 1s spectra, which has binding energies of ~1302.12eV for Mg and ~1303.24eV for Mg-O [23].Thus, the existence of Ni 3+ , Ni 2+ , and Mg 2+ ions was explained by the XPS spectra of MgNiO2 CFs.

Optical Properties of MgNiO2 CFs
UV-vis in the range of 300-800 nm was applied to investigate the optica characteristics of the MgNiO2 CFs.The absorption peak of MgNiO2 CFs at ~303 nm i shown in Figure 6a.The optical band gap of MgNiO2 CFs was calculated using the Tau Equation ( 2) based on the UV-vis graph [43].
where α is the absorption coefficient, hν is photon energy, A is absorbance, Eg is optica band gap, and n is a number (n = 2) describing the transition process.The optical band gap of ~3.41 eV was determined by (αhν) 2 versus hν energy, as shown in Figure 6b.Thi value was lower than the optical bandgap of conventional NiO (~3.6 eV), implying tha

Optical Properties of MgNiO 2 CFs
UV-vis in the range of 300-800 nm was applied to investigate the optical characteristics of the MgNiO 2 CFs.The absorption peak of MgNiO 2 CFs at ~303 nm is shown in Figure 6a.The optical band gap of MgNiO 2 CFs was calculated using the Tau Equation ( 2) based on the UV-vis graph [43]. (αhν where α is the absorption coefficient, hν is photon energy, A is absorbance, E g is optical band gap, and n is a number (n = 2) describing the transition process.The optical band gap of ~3.41 eV was determined by (αhν) 2 versus hν energy, as shown in Figure 6b.This value was lower than the optical bandgap of conventional NiO (~3.6 eV), implying that MgNiO 2 CFs can be excited more easily than typical NiO [44].The photoluminescence emission spectra of MgNiO 2 CFs were measured in the 400-800 nm region at room temperature.Three emission peaks appear at ~507 nm, ~566 nm, and ~651 nm.Herein, the peaks at ~507 nm and ~566 nm were related to NiO, and the PL peaks were upshifted by a smaller particle size and appeared as a double peak at ~20 nm [45].These findings matched well to the crystal size determined by XRD (~25 nm).MgO is shown by the peak at ~651 nm [46].Herein, the photoluminescence peak was caused by an electronic shift involving 3d 8 electrons of Ni 2+ ions [47].Direct recombination between electrons in the conduction band and holes in the valence band induced the PL spectrum to consist of strong and wide peaks [48].
OR PEER REVIEW 8 of 14 MgNiO2 CFs can be excited more easily than typical NiO [44].The photoluminescence emission spectra of MgNiO2 CFs were measured in the 400-800 nm region at room temperature.Three emission peaks appear at ~507 nm, ~566 nm, and ~651 nm.Herein, the peaks at ~507 nm and ~566 nm were related to NiO, and the PL peaks were upshifted by a smaller particle size and appeared as a double peak at ~20 nm [45].These findings matched well to the crystal size determined by XRD (~25 nm).MgO is shown by the peak at ~651 nm [46].Herein, the photoluminescence peak was caused by an electronic shift involving 3d8 electrons of Ni 2+ ions [47].Direct recombination between electrons in the conduction band and holes in the valence band induced the PL spectrum to consist of strong and wide peaks [48].

Sensing Performance, Selectivity, and Real Sample Performance of MgNiO2 CFs
The electrochemical characteristics of the MgNiO2 CFs modified electrode towards the detection of Hg 2+ ions were investigated using cyclicvoltammetry (CV).A threeelectrode system was utilized with MgNiO2 CFs as the working electrode, Ag/AgCl as the counter electrode, and Pt as the reference electrode [49].The target electrolyte was prepared by dissolving different concentration of Hg 2+ ions (1-100 µM) in phosphate buffer solution (PBS, pH = 7.0), and the CV plots were measured at a scan rate of 50 mV/s, as shown in Figure 7a.In our work, as the concentration of Hg 2+ ion increased, the oxidation or reduction current increased correspondingly.The maximum oxidation current peak of ~5.20 µA was observed for 100 µM of Hg 2+ ions, which was 3 times higher than the oxidation current for 1 µM.Notably, a higher oxidation peak suggested a quicker electron transfer process in the electrochemical system and a stronger electrocatalytic behavior of the electrode [31].The excellent and rapid sensing response of the MgNiO2 2+ 3+ 2+

Sensing Performance, Selectivity, and Real Sample Performance of MgNiO 2 CFs
The electrochemical characteristics of the MgNiO 2 CFs modified electrode towards the detection of Hg 2+ ions were investigated using cyclicvoltammetry (CV).A three-electrode system was utilized with MgNiO 2 CFs as the working electrode, Ag/AgCl as the counter electrode, and Pt as the reference electrode [49].The target electrolyte was prepared by dissolving different concentration of Hg 2+ ions (1-100 µM) in phosphate buffer solution (PBS, pH = 7.0), and the CV plots were measured at a scan rate of 50 mV/s, as shown in Figure 7a.In our work, as the concentration of Hg 2+ ion increased, the oxidation or reduction current increased correspondingly.The maximum oxidation current peak of ~5.20 µA was observed for 100 µM of Hg 2+ ions, which was 3 times higher than the oxidation current for 1 µM.Notably, a higher oxidation peak suggested a quicker electron transfer process in the electrochemical system and a stronger electrocatalytic behavior of the Sensors 2023, 23, 7910 9 of 14 electrode [31].The excellent and rapid sensing response of the MgNiO 2 CFs electrode for Hg 2+ ions might be related to the conductive character of Ni 3+ /Ni 2+ in the charge transfer process [50].Due to excellent electrocatalytic efficiency towards Hg 2+ ions, the MgNiO 2 CFs electrode exhibited an increase in anodic current.In order to calculate the sensitivity of the MgNiO 2 CFs electrode, the oxidation current versus Hg 2+ ions concentration is displayed in Figure 7b.The fabricated electrode showed a correlation coefficient (R 2 ) of ~0.9721, a limit of detection (LOD) of ~11.7 µM, a constant sensitivity of ~8.22 µA•µM −1 •cm −2 , and high linearity in the 10-100 µM wide range.The existence of significantly promising sensitivity might be due to the large surface area of MgNiO 2 CFs, which allows considerable analytic adsorption on the electrode surface [21].The detection of Hg 2+ ion as reported by other workers is discussed in Table 1.In comparison, MgNiO 2 CFs based electrode displayed a low LOD and a high sensitivity across a large linear range towards the detection of the Hg 2+ ion.2023, 23, x FOR PEER REVIEW 9 the sensitivity of the MgNiO2 CFs electrode, the oxidation current versus Hg 2+ concentration is displayed in Figure 7b.The fabricated electrode showed a correl coefficient (R 2 ) of ~0.9721, a limit of detection (LOD) of ~11.7 µM, a constant sensitiv ~8.22 µA•µM −1 •cm −2 , and high linearity in the 10-100 µM wide range.The existen significantly promising sensitivity might be due to the large surface area of MgNiO2 which allows considerable analytic adsorption on the electrode surface [21].The dete of Hg 2+ ion as reported by other workers is discussed in Table 1.In comparison, Mg CFs based electrode displayed a low LOD and a high sensitivity across a large linear r towards the detection of the Hg 2+ ion.To investigate the electrode selectivity, the MgNiO 2 CFs electrode were tested by electrochemical method for the detection of other heavy metal ions, such as Cr 3+ and Cu 2+ .The PBS electrolytes of Cr 3+ and Cu 2+ ions were prepared in the same way as the Hg 2+ ions.The MgNiO 2 CFs electrode was measured by CV investigation at a constant scan rate of 50 mV/s.Figure 8a depicts that the oxidation current increases with increasing Cr 3+ ion concentration.The result was comparable to Hg 2+ , but the oxidation peak emerged at a lower voltage range, implying that Cr 3+ might be evaluated independently of Hg 2+ .The CV curve of Cu 2+ in Figure 8b  To investigate the electrode selectivity, the MgNiO2 CFs electrode were tested by electrochemical method for the detection of other heavy metal ions, such as Cr 3+ and Cu 2+  The PBS electrolytes of Cr 3+ and Cu 2+ ions were prepared in the same way as the Hg 2+ ions The MgNiO2 CFs electrode was measured by CV investigation at a constant scan rate of 50 mV/s.Figure 8a depicts that the oxidation current increases with increasing Cr 3+ ion concentration.The result was comparable to Hg 2+ , but the oxidation peak emerged at a lower voltage range, implying that Cr 3+ might be evaluated independently of Hg 2+ .The CV curve of Cu 2+ in Figure 8b revealed erratic results regardless of concentration rise indicating that the MgNiO2 CFs electrode has no selectivity for Cu 2+ .The Hg 2+ ions sensing behavior in drinking water was tested to determine the actua usage of the MgNiO2 CFs electrode.The CV plots of Hg 2+ ion in drinking water are shown in Figure 9a, exhibiting increasing concentrations of Hg 2+ ion, and higher voltage values This confirms the actual usability of the MgNiO2 CFs electrode.Moreover, the shapes of CV curves differed from almost similar oxidation peaks (as observed in Figure 7a), which might be related to direct testing in drinking water.To evaluate the stability of the MgNiO CFs electrode, as shown in Figure 9c, the sensitivity performances for Hg 2+ ions were measured at regular intervals over a period of 25 days.The I-V properties of the MgNiO CFs electrode were examined after every 5-days, and the I-V current values for the detection of Hg 2+ ions remained at roughly ~86% of the initial value without exhibiting a significant drop in the performance.This demonstrates an excellent stability of the MgNiO2 CFs electrode toward the detection of Hg 2+ ions.The obtained stability results clearly showed the long-term viability of the MgNiO2 CFs electrode towards electrochemical sensing for the detection of harmful Hg 2+ ions.The Hg 2+ ions sensing behavior in drinking water was tested to determine the actual usage of the MgNiO 2 CFs electrode.The CV plots of Hg 2+ ion in drinking water are shown in Figure 9a, exhibiting increasing concentrations of Hg 2+ ion, and higher voltage values.This confirms the actual usability of the MgNiO 2 CFs electrode.Moreover, the shapes of CV curves differed from almost similar oxidation peaks (as observed in Figure 7a), which might be related to direct testing in drinking water.To evaluate the stability of the MgNiO 2 CFs electrode, as shown in Figure 9c, the sensitivity performances for Hg 2+ ions were measured at regular intervals over a period of 25 days.The I-V properties of the MgNiO 2 CFs electrode were examined after every 5-days, and the I-V current values for the detection of Hg 2+ ions remained at roughly ~86% of the initial value without exhibiting a significant drop in the performance.This demonstrates an excellent stability of the MgNiO 2 CFs electrode toward the detection of Hg 2+ ions.The obtained stability results clearly showed the long-term viability of the MgNiO 2 CFs electrode towards electrochemical sensing for the detection of harmful Hg 2+ ions.The selectivity of the MgNiO2 CFs electrode toward Hg 2+ ions was further examined by measuring the current response in PBS electrolytes with Hg 2+ ions (10 µM) and mixtures with other metal ions.As shown in Figure 9c, the current responses were considerably decreased when Fe 3+ (10 µM) and Cr 3+ (10 µM) ions were mixed with Hg 2+ ions.In other words, interfering species such as Fe 3+ (10 µM) and Cr 3+ (10 µM) ions and Hg 2+ ions in PBS electrolytes resulted in lowering the current response compared to a high current response observed only by Hg 2+ ions.Figure 9d displays the percentage response of interfering metal ions by the MgNiO2 CFs electrode.With the MgNiO2 CFs electrode, ~100% sensitivity of Hg 2+ ions was achieved, whereas the sensing responses were lowered towards Cr 3+ + Fe 3+ ions (~62%) and Cr 3+ + Fe 3+ + Hg 2+ ions (~44%).Thus, the MgNiO2 CFs electrode expressed good selectivity for Hg 2+ ions compared to other heavy metal ions.

Conclusions
A simple hydrothermal technique was used to synthesize MgNiO2 nanomaterials in the form of Chrysanthemum Flowers (CFs).The synthesized MgNiO2 CFs showed significant size pores of ~61.871 nm with a high specific surface area of ~45.618 m 2 /g, which created an extensive sensing active site.The Ni ions in the MgNiO2 CFs showed multiple Ni 3+ /Ni 2+ oxidation states, which supported good conductive characteristics and promising electronic behavior.The MgNiO2 CFs electrode had a high correlation coefficient (R 2 ) of ~0.9721, a low limit of detection (LOD) of ~11.7 µM, a fast reaction time (10 s), and a sensitivity of ~8.22 µA•µM −1 •cm −2 towards the detection of Hg 2+ ions over a wide linear range of 10-100 µM.Furthermore, the MgNiO2 CFs electrode was tested to detect other heavy metal ions, namely Cr 3+ and Cr 2+ , and the obtained results confirmed The selectivity of the MgNiO 2 CFs electrode toward Hg 2+ ions was further examined by measuring the current response in PBS electrolytes with Hg 2+ ions (10 µM) and mixtures with other metal ions.As shown in Figure 9c, the current responses were considerably decreased when Fe 3+ (10 µM) and Cr 3+ (10 µM) ions were mixed with Hg 2+ ions.In other words, interfering species such as Fe 3+ (10 µM) and Cr 3+ (10 µM) ions and Hg 2+ ions in PBS electrolytes resulted in lowering the current response compared to a high current response observed only by Hg 2+ ions.Figure 9d displays the percentage response of interfering metal ions by the MgNiO 2 CFs electrode.With the MgNiO 2 CFs electrode, ~100% sensitivity of Hg 2+ ions was achieved, whereas the sensing responses were lowered towards Cr 3+ + Fe 3+ ions (~62%) and Cr 3+ + Fe 3+ + Hg 2+ ions (~44%).Thus, the MgNiO 2 CFs electrode expressed good selectivity for Hg 2+ ions compared to other heavy metal ions.

Conclusions
A simple hydrothermal technique was used to synthesize MgNiO 2 nanomaterials in the form of Chrysanthemum Flowers (CFs).The synthesized MgNiO 2 CFs showed significant size pores of ~61.871 nm with a high specific surface area of ~45.618 m 2 /g, which created an extensive sensing active site.The Ni ions in the MgNiO 2 CFs showed multiple Ni 3+ /Ni 2+ oxidation states, which supported good conductive characteristics and promising electronic behavior.The MgNiO 2 CFs electrode had a high correlation coefficient (R 2 ) of ~0.9721, a low limit of detection (LOD) of ~11.7 µM, a fast reaction time (10 s), and a sensitivity of ~8.22 µA•µM −1 •cm −2 towards the detection of Hg 2+ ions over a wide linear range of 10-100 µM.Furthermore, the MgNiO 2 CFs electrode was tested to detect other heavy metal ions, namely Cr 3+ and Cr 2+ , and the obtained results confirmed that the

Figure 1 .
Figure 1.FESEM images at low magnification (a,b), high magnification (c), and EDX spectrum (d) of MgNiO 2 CFs.TEM and HRTEM were used to confirm the morphology of MgNiO 2 CFs.The TEM image in Figure 2a corresponds with FESEM results, exhibiting the size of MgNiO 2 CFs as ~5-7 µm.Figure 2b,c is a high magnification TEM image that reveals a clear lattice structure, showing that the synthesized MgNiO 2 CFs have an excellent crystalline phase.The HRTEM image, as shown in Figure2d, exhibited a lattice distance of ~0.217 nm, which corresponds to the (200) crystal plane[31,32].

Sensors 2023, 23 , 7910 5 of 14 (
Cu = 1.54178Å).Herein, the strongest lattice plane of ~42.4 • (200) is chosen, and an FWHM (β) value of 0.3983 • and a crystal size (τ) of ~22.35 nm is obtained.This is similar to the thickness of a single sheet, as shown in FESEM, indicating that the MgNiO 2 sheets are agglomerated into a single layer crystal structure.In addition, MgNiO 2 CFs showed no additional peaks, confirming the purity of the synthesized nanostructures.FTIR analysis was employed to characterize the structural properties of the MgNiO 2 CFs, as depicted in Figure3b.The O-H stretching vibration of water molecules was adsorbed on the surface causes the appearance of a common wide peak at ~3413 cm −1 .The Mg-O vibration coupling frequency was involved in the existence of a peak at ~571 cm −1 , whereas the Ni-O vibration coupling frequency was confirmed by the peak at ~407 cm −1 .The observed peaks in MgNiO 2 CFs matched well with the reported values in the published literature

Figure 4
Figure4exhibits the adsorption/desorption of N2 gas via Brunauer-Emmett-Teller (BET) analysis of MgNiO2 CFs.As shown in Figure4a, the overall specific surface area of MgNiO2 CFs was ~45.618 m 2 /g, with voids providing the total specific surface area at ~41.041 m 2 /g.Herein, because of the high porosity of MgNiO2 CFs, the specific surface area increased significantly.Figure4bshows the average diameter of the pores as ~61.871 nm.The results showed a match with the morphological properties of FESEM results, implying that the increased surface area of MgNiO2 CFs might enhance the active sites for the detection of target heavy metals, resulting in enhanced sensitivity.

Figure 4
Figure 4 exhibits the adsorption/desorption of N 2 gas via Brunauer-Emmett-Teller (BET) analysis of MgNiO 2 CFs.As shown in Figure4a, the overall specific surface area of MgNiO 2 CFs was ~45.618 m 2 /g, with voids providing the total specific surface area at ~41.041 m 2 /g.Herein, because of the high porosity of MgNiO 2 CFs, the specific surface area increased significantly.Figure4bshows the average diameter of the pores as ~61.871 nm.The results showed a match with the morphological properties of FESEM results, implying that the increased surface area of MgNiO 2 CFs might enhance the active sites for the detection of target heavy metals, resulting in enhanced sensitivity.

Figure 4
Figure4exhibits the adsorption/desorption of N2 gas via Brunauer-Emmett-Teller (BET) analysis of MgNiO2 CFs.As shown in Figure4a, the overall specific surface area of MgNiO2 CFs was ~45.618 m 2 /g, with voids providing the total specific surface area at ~41.041 m 2 /g.Herein, because of the high porosity of MgNiO2 CFs, the specific surface area increased significantly.Figure4bshows the average diameter of the pores as ~61.871 nm.The results showed a match with the morphological properties of FESEM results implying that the increased surface area of MgNiO2 CFs might enhance the active sites for the detection of target heavy metals, resulting in enhanced sensitivity.

Figure 7 .
Figure 7. CV plot (a), calibrated oxidation current versus concentration of Hg 2+ ions (b), an curves with varying Hg 2+ ion concentrations from 1 µM~100 µM concentration of the Hg 2+ ion M PBS and (c) CV plots of bare SPE and MgNiO2 CFs modified electrode in PBS with Hg 2+ (µM

Figure 7 .
Figure 7. CV plot (a), calibrated oxidation current versus concentration of Hg 2+ ions (b), and I-V curves with varying Hg 2+ ion concentrations from 1 µM~100 µM concentration of the Hg 2+ ion in 0.1 M PBS and (c) CV plots of bare SPE and MgNiO 2 CFs modified electrode in PBS with Hg 2+ (µM).To investigate the effect of MgNiO 2 on sensing behavior, CV measurements of Hg 2+ ions in 0.1 M PBS electrolyte on bare SPE and MgNiO 2 CFs modified SPE were performed.According to Figure 7c, the bare SPE posed the least current response to 1 µM concentration of Hg 2+ ions in PBS electrolyte, whereas a prominent current response to 1 µM concentration of Hg 2+ ions was recorded by MgNiO 2 CF-modified SPE.This noted change in current response indicates the sensing behavior of MgNiO 2 CFs toward Hg 2+ ions at very low revealed erratic results regardless of concentration rise, indicating that the MgNiO 2 CFs electrode has no selectivity for Cu 2+ .ensors 2023, 23, x FOR PEER REVIEW 10 of 14 µM concentration of Hg 2+ ions was recorded by MgNiO2 CF-modified SPE.This noted change in current response indicates the sensing behavior of MgNiO2 CFs toward Hg 2 ions at very low concentrations.Therefore, the obtained result showed strong electrocatalytic characteristics of MgNiO2 CFs electrode to sense Hg 2+ ions at low traces.

Figure 9 .
Figure 9. (a) CV plots for real samples sensing in drinking water, and (b) stability test of MgNiO2 CFs modified electrode.(c) I-V curves and (d) current response (%) of MgNiO2 CFs modified electrode with interfering metal ions in PBS electrolyte.