# A Fluorimetric Method Based on an Imidazole Compound for Cu2+ Determination in Tap Water

Authors: Zafer Ocak
Journal: Journal of Chemistry
 (2020)

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## Abstract

Ion sensor properties of 4-(1-(4-hydroxy-3-methoxybenzyl)-1H-benzo[d]imidazol-2-yl)-2-methoxyphenol (L) in acetonitrile-water (1 : 1) were evaluated by fluorescence spectrometry. Pronounced quenching in the fluorescence spectra of the ligand was only observed for the Cu2+ ion among many metal ions. Linear fluorescence responses of the ligand at 360 nm as the function of the Cu2+ concentration were used for the determination of the Cu2+ ion in spiked tap water samples. Recovery values (R%) were satisfactory, and relative standard deviation (RSD%) was below 5.00 in intraday and interday measurements. Detection and quantification limits were 0.28 and 0.84 μg/L, respectively. The assay based on external calibration only took a few minutes.


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## Body

## 1. Introduction

Benzimidazole compounds are obtained as a result of the reactions between diamine compounds and aldehyde compounds. In fact, the formation of the Schiff bases is expected from the reactions between such compounds. However, sometimes the reaction proceeds to a further step, and the cyclization of the formed diimine compound results in benzimidazole compounds. Many benzimidazole derivative compounds synthesized in this manner have been reported in the literature [1–8]. Sometimes a catalyst is used in the synthesis of benzimidazole compounds [1, 4–6]. Acidification was effective in the formation of the cyclization [2, 3].Benzimidazole compounds have generally been investigated for their biological activity and have been reported to be potent compounds with high biological activity [9–15]. The structural properties of these compounds also make them fluorescent compounds. However, there are a limited number of studies on the fluorescence properties of benzimidazole compounds [16–21]. There are studies reporting that benzimidazole derivative compounds interact selectively with metal ions [18–20]. Benzimidazole derivatives which are proposed as fluorescent sensors for iodide and phosphate ions are mentioned [19, 21]. According to our knowledge, some fluorescent sensors based on benzimidazole derivatives to detect the Cu2+ ion in water were proposed in the literature [18, 22–24]. However, there are no methods based on fluorescent benzimidazole compounds used in the determination of Cu2+ in real samples.In this work, a Cu2+ determination method based on measuring the fluorescence of the benzimidazole derivative compound is proposed. Therefore, it is desirable to draw attention to the ion sensor properties of benzimidazole derivative compounds as fluorescent compounds. The ligand used has high fluorescence property. It has been shown that it can be used as an analytical reagent in the determination of sensitive Cu2+. The synthesis of the compound is simple and economical. The usage of an inexpensive and sensitive fluorescence method is advantageous compared to existing expensive and laborious Cu2+ detection methods. In addition, the proposed method has been applied to tap water samples, and it has been desired to demonstrate practical availability.


## 2. Experimental

### 2.1. Chemicals and Apparatus

For synthesis,o-phenylenediamine, p-vanillin, and methanol were obtained from Merck and used directly. Spectrometric-grade acetonitrile as the solvent of ligand was used in fluorescence measurements and was purchased from Merck. Standard metal solutions of 1000 mg/L (Merck) were used to prepare the working solutions. The stock solution was used to prepare working solutions. The absorbance spectra of the ligand were recorded by using an Analytik Jena Specord 210 spectrophotometer. The fluorescence spectra were recorded with a PTI QM-4 spectrofluorimeter.


### 2.2. Synthesis of the Ligand

The ligand (L) (Figure1) was prepared according to the modified version of the report in the literature [25]. For this, o-phenylenediamine (1 mmol) and aldehyde (2 mmol) were refluxed in ethanol for 10 h. The progress of the reaction was monitored by thin layer chromatography (petroleum ether : ethanol : benzene; 1 : 1 : 1). After the completion of the reaction, the solution was concentrated under reduced pressure to obtain the crude product, which was recrystallized from ethanol to attain the pure product. The characterization data such as 1H-NMR, IR spectra, and the melting point of the solid compound were consistent with the literature [25, 26].Figure 1
The benzimidazole compound (L) used in this study.Mp 189–190°C;1H NMR (DMSO-d6, 200 MHz): δ 3.61 (s, 3H, OCH3), 3.69 (s, 3H, OCH3), 5.42 (s, 2H), 6.32–6.37 (m, 1H, ArH), 6.62–6.67 (m, 2H, ArH), 6.87–6.91 (d, 1H ArH), 7.14–7.24 (m, 4H), 7.42–7.48 (m, 1H, ArH), 7.61–7.67 (m, 1H, ArH), 9.00 (s, 1H, OH), and 9.57 (s, 1H, OH); 13C NMR (DMSO-d6, 50 MHz): δ 47.19 (NCH2), 55.31 (OCH3), 110.51 (ArCH), 110.76 (ArCH), 112.84 (ArCH), 115.42 (ArCH), 118.38 (ArCH), 118.71 (ArCH), 120.97 (ArCH), 121.81 (ArCH), 121.91 (ArCH), 127.73 (ArC), 135.90 (ArC), 142.48 (ArC), 145.72 (ArC), 147.47 (ArC), 147.57 (ArC), 148.08 (ArC), and 153.44 (ArC); FT-IR (KBr, cm−1): 1662.5 and 3400.4; ESI-MS (m = z): 377.07 [M+1]+. Anal. calcd. for C22H20N2O4 : C, 70.19; H, 5.37; and N, 7.44. Found: C, 70.05; H, 5.48; and N, 7.59.


### 2.3. Absorbance and Fluorescence Measurements

Absorption and emission spectra of the free ligand were measured in acetonitrile : water (1 : 1). The ligand concentrations were 2.6 × 10−5 and 2.0 × 10−7 M for absorption and fluorescence measurements, respectively. The effect of excess metal ions on the fluorescence spectra was investigated in acetonitrile : water (1 : 1). Pd2+, Sc3+, Sr2+, Ag+, Bi3+, Co2+, Cd2+, Ni2+, Cr3+, Zn2+, K+, Ca2+, Fe3+, Be2+, Li+, Mn2+, Al3+, Mg2+, Na+, and Cu2+ ions and 1 cm quartz cell were used in these measurements. The ligand was excited by 300 nm to record the fluorescence spectra.Spectrofluorimetric titrations with the Cu2+ ion were performed at constant ligand concentration and varying metal concentration. The Cu2+ concentration range was between 0 and 30 μg/L. The ligand concentration was 2.0 × 10−7 M.


### 2.4. The Proposed Method to Determine the Cu2+ Ion

Measurements were made in acetonitrile and water (1 : 1) using an equal volume of ligand solution and water sample. An external calibration graph based on the change in fluorescence intensity of the ligand with increasing metal ion concentration was used to determine the Cu2+ ion in the spiked tap water. KTU campus water (Trabzon, Turkey) was used as a tap water sample.


### 2.5. Determination of Metal-Ligand Interaction

The metal-ligand complex formation in the system was determined with the molar ratio method by means of fluorescence measurements. For this, the molar ratio graph was plotted by the data of the spectrofluorimetric titration of the Cu2+ ion with the ligand. The composition of the CuL complex was determined from the extrapolation of two curves with different slopes. The complex stability constant was calculated according to the method in the literature [27].


## 2.1. Chemicals and Apparatus

For synthesis,o-phenylenediamine, p-vanillin, and methanol were obtained from Merck and used directly. Spectrometric-grade acetonitrile as the solvent of ligand was used in fluorescence measurements and was purchased from Merck. Standard metal solutions of 1000 mg/L (Merck) were used to prepare the working solutions. The stock solution was used to prepare working solutions. The absorbance spectra of the ligand were recorded by using an Analytik Jena Specord 210 spectrophotometer. The fluorescence spectra were recorded with a PTI QM-4 spectrofluorimeter.


## 2.2. Synthesis of the Ligand

The ligand (L) (Figure1) was prepared according to the modified version of the report in the literature [25]. For this, o-phenylenediamine (1 mmol) and aldehyde (2 mmol) were refluxed in ethanol for 10 h. The progress of the reaction was monitored by thin layer chromatography (petroleum ether : ethanol : benzene; 1 : 1 : 1). After the completion of the reaction, the solution was concentrated under reduced pressure to obtain the crude product, which was recrystallized from ethanol to attain the pure product. The characterization data such as 1H-NMR, IR spectra, and the melting point of the solid compound were consistent with the literature [25, 26].Figure 1
The benzimidazole compound (L) used in this study.Mp 189–190°C;1H NMR (DMSO-d6, 200 MHz): δ 3.61 (s, 3H, OCH3), 3.69 (s, 3H, OCH3), 5.42 (s, 2H), 6.32–6.37 (m, 1H, ArH), 6.62–6.67 (m, 2H, ArH), 6.87–6.91 (d, 1H ArH), 7.14–7.24 (m, 4H), 7.42–7.48 (m, 1H, ArH), 7.61–7.67 (m, 1H, ArH), 9.00 (s, 1H, OH), and 9.57 (s, 1H, OH); 13C NMR (DMSO-d6, 50 MHz): δ 47.19 (NCH2), 55.31 (OCH3), 110.51 (ArCH), 110.76 (ArCH), 112.84 (ArCH), 115.42 (ArCH), 118.38 (ArCH), 118.71 (ArCH), 120.97 (ArCH), 121.81 (ArCH), 121.91 (ArCH), 127.73 (ArC), 135.90 (ArC), 142.48 (ArC), 145.72 (ArC), 147.47 (ArC), 147.57 (ArC), 148.08 (ArC), and 153.44 (ArC); FT-IR (KBr, cm−1): 1662.5 and 3400.4; ESI-MS (m = z): 377.07 [M+1]+. Anal. calcd. for C22H20N2O4 : C, 70.19; H, 5.37; and N, 7.44. Found: C, 70.05; H, 5.48; and N, 7.59.


## 2.3. Absorbance and Fluorescence Measurements

Absorption and emission spectra of the free ligand were measured in acetonitrile : water (1 : 1). The ligand concentrations were 2.6 × 10−5 and 2.0 × 10−7 M for absorption and fluorescence measurements, respectively. The effect of excess metal ions on the fluorescence spectra was investigated in acetonitrile : water (1 : 1). Pd2+, Sc3+, Sr2+, Ag+, Bi3+, Co2+, Cd2+, Ni2+, Cr3+, Zn2+, K+, Ca2+, Fe3+, Be2+, Li+, Mn2+, Al3+, Mg2+, Na+, and Cu2+ ions and 1 cm quartz cell were used in these measurements. The ligand was excited by 300 nm to record the fluorescence spectra.Spectrofluorimetric titrations with the Cu2+ ion were performed at constant ligand concentration and varying metal concentration. The Cu2+ concentration range was between 0 and 30 μg/L. The ligand concentration was 2.0 × 10−7 M.


## 2.4. The Proposed Method to Determine the Cu2+ Ion

Measurements were made in acetonitrile and water (1 : 1) using an equal volume of ligand solution and water sample. An external calibration graph based on the change in fluorescence intensity of the ligand with increasing metal ion concentration was used to determine the Cu2+ ion in the spiked tap water. KTU campus water (Trabzon, Turkey) was used as a tap water sample.


## 2.5. Determination of Metal-Ligand Interaction

The metal-ligand complex formation in the system was determined with the molar ratio method by means of fluorescence measurements. For this, the molar ratio graph was plotted by the data of the spectrofluorimetric titration of the Cu2+ ion with the ligand. The composition of the CuL complex was determined from the extrapolation of two curves with different slopes. The complex stability constant was calculated according to the method in the literature [27].


## 3. Results and Discussion

### 3.1. Absorption and Emission of the Ligand

The absorption and emission spectra of the ligand were determined in acetonitrile : water (1 : 1). In Figure2, the absorption and fluorescence spectrum of the ligand was shown. As seen from Figure 2, there is an absorption band resulting from the n ⟶ n∗ transition about 285 nm in the absorption spectra. When the ligand was excited with 300 nm, the maximum emission was observed at 360 nm in the fluorescence spectra (Figure 2).Figure 2
UV absorption and emission spectra of the ligand in acetonitrile-water (1 : 1). Ligand concentrations are 2.6 × 10−5 M and 2.0 × 10−7 M for absorption and emission spectra, respectively. The excitation wavelength is 300 nm.The effect of 10 equivalent excess of metal ions on the fluorescence spectra of the ligand in acetonitrile : water (1 : 1) was investigated. In Figure3, the effect of metal ions on the fluorescence spectra was shown. There were no significant changes on the fluorescence spectra of the ligand with the influence of ions such as Pd2+, Sc3+, Sr2+, Ag+, Bi3+, Co2+, Cd2+, Ni2+, Cr3+, Zn2+, K+, Ca2+, Fe3+, Be2+, Li+, Mn2+, Al3+, Mg2+, and Na+. However, the Cu2+ ion causes an effective fluorescence quenching. Figure 4 shows the fluorescence intensity at 360 nm of the free ligand and the ligand solution containing metal ions. As seen from Figure 4, the ligand shows selectivity for the Cu2+ ion over the other metal ions. This result is consistent with the outcome in the literature. It is known from the literature that benzimidazole and Schiff base compounds show selectivity for Cu2+ due to donor groups such as C=N and OH groups in their structure [22, 28].Figure 3
The effect of metal ions on fluorescence spectra ofL in acetonitrile-water (1 : 1). L: 2.0 × 10−7 M; metal ion: 2.0 × 10−6 M.Figure 4
The fluorescence intensity at 360 nm ofL with the metal ions. L: 2.0 × 10−7 M; metal ion: 2.0 × 10−6 M.


### 3.2. Complexation between Cu2+ and the Ligand

To investigate the interaction between Cu2+ and the ligand, spectrofluorimetric titrations were carried out. Figure 5 shows the change in the fluorescence spectra of the ligand with the increasing Cu2+ concentration in acetonitrile-water (1 : 1) media. As seen in Figure 5, there is regular fluorescence quenching in all spectra, while Cu2+ concentration increases. From the fluorescence quenching at 360 nm, the molar ratio graph was plotted. As seen from Figure 5 inset, the CuL complex composition is 1 : 1.Figure 5
The change in the fluorescence spectra ofL with the increasing Cu2+ ion concentration (from 0 to 30 μg/L) in acetonitrile-water (1 : 1). Inset: the molar ratio graph plotted from the change in the fluorescence at 360 nm. L: 2.0 × 10−7 M.From the spectrofluorimetric titration data, the change in I0/(I0-I) versus 1/[Cu2+] is plotted to determine the stability constant of the CuL complex (Figure 6). From the intercept divided to the slope, the complex stability constant is calculated. The log K value is found as log K = 5.51 in the mentioned conditions.Figure 6
The change in I0/(I0-I) versus 1/[Cu2+] to determine the complex stability constant.


### 3.3. The Determination Method for the Cu2+ Ion

A linear concentration range between 0 and 15.2μg/L was found for the spectrofluorimetric determination of Cu2+ ions in a standard solution with the ligand. A calibration graph in this range was used to determine Cu2+ in spiked water samples. The R% values were satisfactory in the Cu2+ determination of the samples by external calibration; namely, there is no matrix effect in copper determination with the proposed method.Figure7 shows the change in the fluorescence intensity of the ligand with increasing Cu2+ ion concentration. The calibration graph was obtained from the fluorescence quenching of the ligand at 360 nm. Figure 7 inset shows the linear range. The deviation from Beer’s law was seen after 15.2 μg/L. The correlation coefficient R2 was 0.9842.Figure 7
The change in the fluorescence intensity with the increasing Cu2+ concentration at 360 nm in acetonitrile-water (1 : 1). L: 2.0 × 10−7 M. Inset: deviation from Beer’s law after 15 μg/L of Cu2+ concentration.


### 3.4. The Method Validation

The analytical performance data of the Cu2+ determination method are given in Table 1. The limit of detection (LOD) and limit of quantification (LOQ) values were calculated as 3 s/m and LOQ (9 s/m), respectively, by the standard deviation (s) of the blank response for eleven measurements and the slope of the calibration line (m) according to the IUPAC recommendations. The LOD and LOQ values were found to be 0.28 and 0.84 μg/L, respectively. The method accuracy was showed by spiking recovery measurements. In these measurements, the Cu2+ concentration in the spiked tap water samples was 5.1 μg/L. The R% was between 96.1 and 102.0, as seen in Table 2. The method precision was established by analyzing a series of spiked tap water samples intraday and interday. The number of repeated measurements is three in the precision experiments, and the corresponding results are given in Table 2. Relative standard deviation (RSD%) was used to express the method precision. RSD% value was calculated as 3.63 and 5.00 for the intraday and interday measurements, respectively.Table 1
Analytical performance data for the Cu2+ determination method.
Wavelength (nm)
360
Limit of detection (μg/L)
0.28
Limit of quantification (μg/L)
0.84
Linear range (μg/L)
0–15.2
Ligand concentration (M)
2.0 × 10–7
Ligand volume (mL)
2.0
Total volume (mL)
4.0
Solvent
Acetonitrile : water (1 : 1)
Waiting time
1-2 min
Correlation coefficient (R2)
0.9842Table 2
Analytical performance data for the Cu2+ determination method.
Added (mg/L)
Found (mg/L) ± RSD (%)
R (%)
5.1
4.9 ± 3.1
96.1
5.1
5.2 ± 0.7
102.0
5.1
5.2 ± 0.4
102.0


### 3.5. Comparison with Other Methods

It is reported in the literature that metals are generally determined by atomic methods [29, 30]. However, the atomic methods require the usage of expensive instruments and suffer from matrix effects and extraction process before the measurements [29]. Moreover, the appropriate detection limits can be achieved by time-consuming preconcentration procedures [30]. Methods based on measurements of fluorescence intensity of a fluorophore are much cheaper and simpler. Also, detection limits comparable to the atomic methods can be obtained by these methods under optimum conditions. The LOD values of some methods proposed for the fluorimetric Cu2+ determination in tap water are summarized in Table 3 [31–47]. Most of the recommended methods, as seen in Table 3, are based on the use of nonenvironmentally friendly nanostructures containing heavy metals such as gold, copper, cadmium, and iron. The preparation and characterization of such materials are also complicated and expensive. In this study, the fluorescence of a simple imidazole compound is used to develop a fast method in order to determine Cu2+ in tap water. Moreover, the limit of detection of the proposed Cu2+ determination method is lower than most of the methods proposed in Table 3. Unlike most methods in the literature, the measurement process is only a few minutes.Table 3
Detection limits of fluorimetric Cu2+ determination methods in tap water.
Reagent
LOD
Reference
Schiff base
0.1 nM
[30]
N-CQDs
0.09μM
[31]
PVA/CdS QDs
0.12 nM
[32]
CA-CdS QDs
9.2 × 10–9 M
[33]
Coordination polymer
3.0μM
[34]
Fe3O4@SiO2-NH2-morin
7.5 nM
[35]
DPA-capped CdTeQDs
0.4 × 10–9 M
[36]
AuNCs
0.33 nM
[37]
AuNCs
0.9μM
[38]
CuNCs core/shell NPs
1.3 ppm
[39]
CuNPs
5.6μM
[40]
CdTeQDs
1.55 × 10−8 M
[41]
CdSe/ZnSQDs
0.14 nM
[42]
Fe3O4-MOFs
0.1 nM
[43]
Fe3O4@SiO2
6 nM
[44]
Porous silicon NPs
0.1µM
[45]
SrF2 nanocrystals
2 nM
[46]
Imidazole compound
0.28μg/L
This study


## 3.1. Absorption and Emission of the Ligand

The absorption and emission spectra of the ligand were determined in acetonitrile : water (1 : 1). In Figure2, the absorption and fluorescence spectrum of the ligand was shown. As seen from Figure 2, there is an absorption band resulting from the n ⟶ n∗ transition about 285 nm in the absorption spectra. When the ligand was excited with 300 nm, the maximum emission was observed at 360 nm in the fluorescence spectra (Figure 2).Figure 2
UV absorption and emission spectra of the ligand in acetonitrile-water (1 : 1). Ligand concentrations are 2.6 × 10−5 M and 2.0 × 10−7 M for absorption and emission spectra, respectively. The excitation wavelength is 300 nm.The effect of 10 equivalent excess of metal ions on the fluorescence spectra of the ligand in acetonitrile : water (1 : 1) was investigated. In Figure3, the effect of metal ions on the fluorescence spectra was shown. There were no significant changes on the fluorescence spectra of the ligand with the influence of ions such as Pd2+, Sc3+, Sr2+, Ag+, Bi3+, Co2+, Cd2+, Ni2+, Cr3+, Zn2+, K+, Ca2+, Fe3+, Be2+, Li+, Mn2+, Al3+, Mg2+, and Na+. However, the Cu2+ ion causes an effective fluorescence quenching. Figure 4 shows the fluorescence intensity at 360 nm of the free ligand and the ligand solution containing metal ions. As seen from Figure 4, the ligand shows selectivity for the Cu2+ ion over the other metal ions. This result is consistent with the outcome in the literature. It is known from the literature that benzimidazole and Schiff base compounds show selectivity for Cu2+ due to donor groups such as C=N and OH groups in their structure [22, 28].Figure 3
The effect of metal ions on fluorescence spectra ofL in acetonitrile-water (1 : 1). L: 2.0 × 10−7 M; metal ion: 2.0 × 10−6 M.Figure 4
The fluorescence intensity at 360 nm ofL with the metal ions. L: 2.0 × 10−7 M; metal ion: 2.0 × 10−6 M.


## 3.2. Complexation between Cu2+ and the Ligand

To investigate the interaction between Cu2+ and the ligand, spectrofluorimetric titrations were carried out. Figure 5 shows the change in the fluorescence spectra of the ligand with the increasing Cu2+ concentration in acetonitrile-water (1 : 1) media. As seen in Figure 5, there is regular fluorescence quenching in all spectra, while Cu2+ concentration increases. From the fluorescence quenching at 360 nm, the molar ratio graph was plotted. As seen from Figure 5 inset, the CuL complex composition is 1 : 1.Figure 5
The change in the fluorescence spectra ofL with the increasing Cu2+ ion concentration (from 0 to 30 μg/L) in acetonitrile-water (1 : 1). Inset: the molar ratio graph plotted from the change in the fluorescence at 360 nm. L: 2.0 × 10−7 M.From the spectrofluorimetric titration data, the change in I0/(I0-I) versus 1/[Cu2+] is plotted to determine the stability constant of the CuL complex (Figure 6). From the intercept divided to the slope, the complex stability constant is calculated. The log K value is found as log K = 5.51 in the mentioned conditions.Figure 6
The change in I0/(I0-I) versus 1/[Cu2+] to determine the complex stability constant.


## 3.3. The Determination Method for the Cu2+ Ion

A linear concentration range between 0 and 15.2μg/L was found for the spectrofluorimetric determination of Cu2+ ions in a standard solution with the ligand. A calibration graph in this range was used to determine Cu2+ in spiked water samples. The R% values were satisfactory in the Cu2+ determination of the samples by external calibration; namely, there is no matrix effect in copper determination with the proposed method.Figure7 shows the change in the fluorescence intensity of the ligand with increasing Cu2+ ion concentration. The calibration graph was obtained from the fluorescence quenching of the ligand at 360 nm. Figure 7 inset shows the linear range. The deviation from Beer’s law was seen after 15.2 μg/L. The correlation coefficient R2 was 0.9842.Figure 7
The change in the fluorescence intensity with the increasing Cu2+ concentration at 360 nm in acetonitrile-water (1 : 1). L: 2.0 × 10−7 M. Inset: deviation from Beer’s law after 15 μg/L of Cu2+ concentration.


## 3.4. The Method Validation

The analytical performance data of the Cu2+ determination method are given in Table 1. The limit of detection (LOD) and limit of quantification (LOQ) values were calculated as 3 s/m and LOQ (9 s/m), respectively, by the standard deviation (s) of the blank response for eleven measurements and the slope of the calibration line (m) according to the IUPAC recommendations. The LOD and LOQ values were found to be 0.28 and 0.84 μg/L, respectively. The method accuracy was showed by spiking recovery measurements. In these measurements, the Cu2+ concentration in the spiked tap water samples was 5.1 μg/L. The R% was between 96.1 and 102.0, as seen in Table 2. The method precision was established by analyzing a series of spiked tap water samples intraday and interday. The number of repeated measurements is three in the precision experiments, and the corresponding results are given in Table 2. Relative standard deviation (RSD%) was used to express the method precision. RSD% value was calculated as 3.63 and 5.00 for the intraday and interday measurements, respectively.Table 1
Analytical performance data for the Cu2+ determination method.
Wavelength (nm)
360
Limit of detection (μg/L)
0.28
Limit of quantification (μg/L)
0.84
Linear range (μg/L)
0–15.2
Ligand concentration (M)
2.0 × 10–7
Ligand volume (mL)
2.0
Total volume (mL)
4.0
Solvent
Acetonitrile : water (1 : 1)
Waiting time
1-2 min
Correlation coefficient (R2)
0.9842Table 2
Analytical performance data for the Cu2+ determination method.
Added (mg/L)
Found (mg/L) ± RSD (%)
R (%)
5.1
4.9 ± 3.1
96.1
5.1
5.2 ± 0.7
102.0
5.1
5.2 ± 0.4
102.0


## 3.5. Comparison with Other Methods

It is reported in the literature that metals are generally determined by atomic methods [29, 30]. However, the atomic methods require the usage of expensive instruments and suffer from matrix effects and extraction process before the measurements [29]. Moreover, the appropriate detection limits can be achieved by time-consuming preconcentration procedures [30]. Methods based on measurements of fluorescence intensity of a fluorophore are much cheaper and simpler. Also, detection limits comparable to the atomic methods can be obtained by these methods under optimum conditions. The LOD values of some methods proposed for the fluorimetric Cu2+ determination in tap water are summarized in Table 3 [31–47]. Most of the recommended methods, as seen in Table 3, are based on the use of nonenvironmentally friendly nanostructures containing heavy metals such as gold, copper, cadmium, and iron. The preparation and characterization of such materials are also complicated and expensive. In this study, the fluorescence of a simple imidazole compound is used to develop a fast method in order to determine Cu2+ in tap water. Moreover, the limit of detection of the proposed Cu2+ determination method is lower than most of the methods proposed in Table 3. Unlike most methods in the literature, the measurement process is only a few minutes.Table 3
Detection limits of fluorimetric Cu2+ determination methods in tap water.
Reagent
LOD
Reference
Schiff base
0.1 nM
[30]
N-CQDs
0.09μM
[31]
PVA/CdS QDs
0.12 nM
[32]
CA-CdS QDs
9.2 × 10–9 M
[33]
Coordination polymer
3.0μM
[34]
Fe3O4@SiO2-NH2-morin
7.5 nM
[35]
DPA-capped CdTeQDs
0.4 × 10–9 M
[36]
AuNCs
0.33 nM
[37]
AuNCs
0.9μM
[38]
CuNCs core/shell NPs
1.3 ppm
[39]
CuNPs
5.6μM
[40]
CdTeQDs
1.55 × 10−8 M
[41]
CdSe/ZnSQDs
0.14 nM
[42]
Fe3O4-MOFs
0.1 nM
[43]
Fe3O4@SiO2
6 nM
[44]
Porous silicon NPs
0.1µM
[45]
SrF2 nanocrystals
2 nM
[46]
Imidazole compound
0.28μg/L
This study


## 4. Conclusions

p-Vanillin derivative benzimidazole compound (L) was proposed as highly selective and sensitive fluorescence sensors for Cu2+ detection. The fluorescence quenching of L at 360 nm with increasing metal ion concentration provides Cu2+ determination in tap water. LOQ value of the proposed method is better than most of the literature and is much under the legal limits in tap water. According to EPA, the maximum contaminant level goal for copper in primary drinking water is 1.3 mg/L. Moreover, using an external calibration graph is enough to determine Cu2+ ions in tap water. The proposed method is very simple and has high sensitivity and selectivity to determine copper in tap water.



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