Chalcone-Based Colorimetric Chemosensor for Detecting Ni²⁺

Abstract

The first chalcone-based colorimetric chemosensor DPP (sodium (E)-2,4-dichloro-6-(3-oxo-3-(pyridine-2-yl)prop-1-en-1-yl)phenolate) was synthesized for detecting Ni²⁺ in near-perfect water. The synthesis of DPP was validated by using ¹H, ¹³C NMR and ESI-MS. DPP selectively sensed Ni²⁺ through the color variation from yellow to purple. Detection limit of DPP for Ni²⁺ was calculated to be 0.36 μM (3σ/slope), which is below the standard (1.2 μM) set by the United States Environmental Protection Agency (EPA).The binding ratio of DPP to Ni²⁺ was determined as a 1:1 by using a Job plot and ESI-mass. The association constant of DPP and Ni²⁺ was calculated as 1.06 × 10⁴ M⁻¹ by the non-linear fitting analysis. In real samples, the sensing application of DPP for Ni²⁺ was successfully performed. DPP-coated paper-supported strips could also be used for detecting Ni²⁺. The binding mechanism of DPP to Ni²⁺ was proposed by ESI-MS, Job plot, UV-vis, FT-IR spectroscopy, and DFT calculations.

1. Introduction

Nickel ion is a pivotal metal ion in biological systems, such as respiration, biosynthesis, and metabolism [1]. In addition, it is widely employed in industrial areas, such as Ni-Cd batteries, electroplating, machinery, and catalyst [2,3,4,5,6]. With its industrial usage, a large amount of nickel is released into nature as a pollutant [7], increasing the possibility of nickel exposure. Nickel is toxic, and can cause several illnesses, such as allergies, lung injuries, and respiratory disease [8,9,10,11]. Consequently, the acceptable amount of nickel in drinking water recommended by the United States Environmental Protection Agency (EPA) is limited to 1.2 μM [12]. Thus, there is a need to design methods capable of detecting nickel ions easily and quickly in the environment.

Several analytical tools are used to detect Ni²⁺, such as atomic absorption spectrometry, electrochemical methods, inductively coupled plasma mass spectrometry, and fluorescence techniques and distance-based measurement [13,14]. The methods require expensive equipment and a skilled operator [15]. In contrast, the colorimetric chemosensor has no such drawbacks [16,17,18,19]. In addition, paper-supported colorimetric sensors adsorbed on paper or thread have an additional benefit, such as semi-quantitative detection with a faster and cheaper analysis [20]. Therefore, it is useful to develop paper-supported colorimetric chemosensors for detecting Ni²⁺. Several colorimetric chemosensors that detect Ni²⁺ were studied in the past few years. Although most of the reported sensors operate in organic solvents, only a few colorimetric chemosensors with functional groups, such as naphthalimide, pyridine, coumarin, Schiff-base, quinone, and polymer with dye detect Ni²⁺ in near-perfect water [21,22,23,24,25,26]. Thus, the design of colorimetric chemosensors capable of sensing Ni²⁺ in water is of high significance.

Pyridine could provide a binding site for cations [27,28,29] and have a water-soluble hydrophilic character [30,31,32]. The chalcone structure has a conjugated $\mathsf{\pi }$-electronic system, which provides good chelating ability with metal ions [33,34,35]. In addition, the α,β-unsaturated carbonyl in the chalcone structure makes a push–pull chromophore [36,37]. In particular, the chalcone structure can be easily synthesized by using aldol condensation [38]. Thus, we predicted that a chalcone-based chemosensor with a pyridine group can detect metal ions, such as nickel, through a color change in near-perfect water and be applied for paper-supported semi-quantitative detection.

Herein, we present the first chalcone-based colorimetric chemosensor DPP for the detection of Ni²⁺ in near-perfect water. Chemosensor DPP can sense Ni²⁺ with a low detection limit by colorimetric variation from yellow to purple. In addition, DPP could apply to real water and its paper-supported strip could detect Ni²⁺ easily and quickly. The binding mechanism of DPP to Ni²⁺ was described by UV-visible titrations, ESI-mass, Job plot, FT-IR spectroscopy, and DFT calculations.

2. Materials and Methods

2.1. Materials and Equipment

Sodium hydroxide, 2-acetylpyridine, and 3,5-dichlorosalicylaldehyde were acquired commercially from Alfa, TCI, and Samchun in Korea, respectively. Methanol was acquired from Samchun in Korea. Metal cation solutions were prepared using metal nitrate or perchlorate salts. The pH buffer solutions were acquired commercially from Samchun in Korea. A Varian spectrometer was used to obtain ¹³C and ¹H NMR spectra. Absorption spectra were measured with a Perkin Elmer Lambda 365 UV-Vis. A Thermo MAX instrument was employed to collect ESI-MS spectra. FT-IR spectra were obtained by using a Thermo Fisher Scientific Fourier Transform Infrared Spectrophotometer.

2.2. Synthesis of DPP

DPP was synthesized by the aldol condensation of 2-acetylpyridine and 3,5-dichlorosalicylaldehyde. 2-Acetylpyridine (342 μL, 3.0 mmol) and 10% NaOH 5 mL were added in methanol 15 mL. The solution was stirred for 1 h. Then, 3,5-dichlorosalicylaldehyde (390 mg, 2.0 mmol) was added to the solution, which was additionally stirred at 23 °C for 1 day. The red powder precipitated was filtered, washed with ether, and dried. Yield: 392 mg (61%). ¹H NMR: δ = 8.72 (d, 1H), 8.33 (d, 1H), 8.02 (m, 3H), 7.60 (t, 1H), 7.16 (d, 1H), 7.07 (d, 1H); ¹³C NMR (175 MHz, DMSO-d₆): δ = 188.87, 167.12, 155.00, 144.42, 137.29, 129.82, 127.32, 126.94, 126.52, 123.18, 121.93, 114.13, 109.01. ESI-mass: m/z calcd. for C₁₄H₉Cl₂NO₂⁻ + 2H₂O, 328.02; found, 327.63.

2.3. UV-Vis Titrations

DPP (3.2 mg, 1 × 10⁻⁵ mol) was dissolved in DMF (1.0 mL) and 6 μL of the DPP stock (10 mM) was diluted to 2.994 mL PBS buffer (10 mM PBS, pH 7.4) to give 20 mM. Ni(NO₃)₂ (2.91 mg, 1 × 10⁻⁴ mol) was dissolved in 5.0 mL of buffer, and 3-66 μL of the Ni²⁺ stock (2 × 10⁻³ M) was added to DPP (2 × 10⁻⁵ M). UV-vis spectra were taken after 5 s.

2.4. Job Plot

Then, 3–27 μL of a DPP stock (10 mM) prepared in 1.0 mL of DMF was transferred to several quartzes. Then, 3–27 μL of the Ni²⁺ solution (1 × 10⁻² M) acquired with nitrate salt in a 1.0 mL buffer was added to diluted DPP. Each quartz cell was filled with PBS buffer to 3.0 mL. UV-vis spectra were taken after 5 s.

2.5. Interference Tolerance Test

Sensor DPP (3.2 mg, 1 × 10⁻⁵ mol) was dissolved in DMF (1 mL). An amount of 1.0 × 10⁻⁴ mol of Al(NO₃)₃, Cu(NO₃)₂, Cr(NO₃)₃, Pb(NO₃)₂, Hg(NO₃)₂, Co(NO₃)₂, Ni(NO₃)₂, Ca(NO₃)₂, Mg(NO₃)₂, Mn(NO₃)₂, In(NO₃)₃, Ga(No₃)₂, NaNO₃, AgNO₃, Fe(NO₃)₃, Fe(ClO₄)₂, Cd(NO₃)₂, and KNO₃ was dissolved in 5.0 mL buffer, respectively. An amount of 48 μL of each metal (2 × 10⁻² M) and Ni²⁺ ion (2 × 10⁻² M) was added into a 3.0 mL PBS buffer to afford 16 eq., respectively. An amount of 6 μL of the DPP stock (1 × 10⁻² M) was added to each solution. A UV-vis spectrum of each solution was taken after 5 s.

2.6. pH Effect

Then, 6 µL of the DPP stock (1 × 10⁻³ M) dissolved in DMF (1.0 mL) was diluted to 2.994 mL of each pH buffer to make 3 × 10⁻⁵ M. Ni(NO₃)₂ (2.91 mg, 1 × 10⁻⁴ mol) was dissolved in 5.0 mL buffer solution. Then, 48 µL of the Ni²⁺ stock was added to each DPP. UV-vis spectra were taken after 5 s.

2.7. Water Sample Test by the Spiking Method

The real water sample analysis was performed to determine the spiked Ni²⁺ in samples collected from drinking and tap water in our laboratory. Sensor DPP (3.2 mg, 1 × 10⁻⁵ mol) was dissolved in DMF (1.0 mL). Then, 6 µL of the DPP stock (1 × 10⁻³ M) was diluted in 2.994 mL of a sample solution containing the spiked Ni²⁺ (6 μM). UV-vis spectra were taken after 5 s.

2.8. Test Strip

The test strip assay was achieved with DPP. Filter paper cut into pieces was dipped in a DPP media at a concentration of 1 mM (1.0 mL, MeOH) and dried for 1 h. After the filter paper completely dried off, various concentrations (10, 50, and 100 μM) of Ni²⁺ solutions dissolved in buffer were employed to determine the lowest visible amount. A concentration of 50 μM of varied cation solutions (Zn²⁺, Al³⁺, Mn²⁺, K⁺, Cd²⁺, Fe²⁺, Ca²⁺, Fe³⁺, Cr³⁺, Hg⁺, Mg²⁺, Cu²⁺, Co²⁺, Pb²⁺, In³⁺, Na⁺, Ga³⁺, and Ni²⁺) was employed to analyze the selectivity of the test strip.

2.9. Calculations

The detecting mechanism of DPP to Ni²⁺ was investigated by using the Gaussian16 program [39] for theoretical calculations. They were based on B3LYP density functional methods [40,41]. The 6-31G(d,p) [42,43] and Lanl2DZ [44] basis sets were used for calculations of elements and Ni²⁺, respectively. The solvent effect of water was checked by employing IEFPCM [45]. With the optimized patterns of DPP and DPP-Ni²⁺, 20 of the lowest triplet-triplet transitions were calculated by using the TD-DFT method to investigate the transition states of the two compounds.

3. Results and Discussion

DPP was gained by the aldol condensation of 2-acetylpyridine and 3,5-dichlorosalicylaldehyde and affirmed by ¹H NMR, ¹³C NMR, and ESI-mass (Figure 1).

3.1. Spectroscopic Studies of DPP with Ni²⁺

Colorimetric sensing capability of DPP was examined with cations (Zn²⁺, Al³⁺, Mn²⁺, K⁺, Cd²⁺, Fe²⁺, Ca²⁺, Fe³⁺, Cr³⁺, Hg⁺, Mg²⁺, Cu²⁺, Co²⁺, Pb²⁺, In³⁺, Na⁺, Ga³⁺, and Ni²⁺) in buffer (pH = 7.4, Figure 2).

In adding diverse cations to DPP, only Ni²⁺ showed significant spectral change with a prominent increase of 550 nm (Figure 2a) and distinguishable color change from yellow to purple (Figure 2b). Meanwhile, other cations did not exhibit any significant spectral or visual changes, suggesting that DPP can sense exclusively Ni²⁺ with a color change. We executed the UV-vis titration to analyze the binding feature of DPP with Ni²⁺. As the Ni²⁺ was added into DPP, the absorbance of 373 nm and 550 nm was prominently increased, and that of 325 nm and 453 nm was visibly decreased. A complete isosbestic point was detected at 391 nm, suggesting that sensor DPP and Ni²⁺ would create a species (Figure 2c). In particular, DPP is the first chalcone-based sensor among chemosensors previously addressed for the sensing of Ni²⁺ in near-perfect water (

Table 1. Examples of chemosensors for detection of Ni²⁺.

Sensor	Detection Limit (μM)	Test Strip	Reference
	0.057	Yes	[21]
	0.074	No	[22]
	0.037	No	[23]
	0.0012	No	[24]
	-	No	[25]
	1.78	No	[26]
	0.36		This work

).

A Job plot experiment was achieved to determine the binding feature of DPP and Ni²⁺ (Figure 3). The result illustrated that DPP and Ni²⁺ made a 1:1 binding stoichiometry.

The 1:1 stoichiometry was assured by the ESI-MS test (Figure 4).

The peak of 385.73 (m/z) was assignable to be [(DPP + Ni²⁺ − Na⁺ + 2H₂O)]⁺ [calcd. 385.95].

According to the calibration curve with nickel ion, the association constant of DPP and Ni²⁺ was calculated as 1.06 × 10⁴ M⁻¹ by the non-linear fitting analysis (Figure 5a) [46]. Detection limit of DPP to Ni²⁺ was determined as 0.36 μM (3σ/slope, Figure 5b).

Furthermore, FT-IR analysis was performed to investigate the interaction of DPP and Ni²⁺ (Figure 6). The band at 1644 cm⁻¹ associated with the carbonyl group (C=O) of DPP moved to 1619 cm⁻¹ [47,48], signifying that the carbonyl oxygen might bind to Ni²⁺.

With the outcomes of Job plot, ESI-mass, and IR analysis, the possible feature of DPP with Ni²⁺ was proposed (Scheme 1).

The inhibition experiment was conducted to identify the exclusive selectivity of DPP for Ni²⁺ in a competitive environment (Figure 7).

When nickel and other metals of the same concentration existed together, DPP was hardly disturbed by other metals except for Cr³⁺. The detecting ability of DPP to Ni²⁺ was inspected in a pH range of 6–9 (Figure 8).

DPP showed the ability to sense Ni²⁺ at pH 7–9. Test-strip experiments were performed with filter papers coated with DPP for practical application. DPP showed a colorimetric change from yellow to purple at 50 μM Ni²⁺ (Figure 9a) and selectively detected Ni²⁺ among varied metal ions (Figure 9b). This result indicated that DPP could be applied to detecting Ni²⁺ by using a test strip.

The real water sample analysis was performed to determine the spiked Ni²⁺ in samples collected from drinking and tap water (

Table 2. Determination of Ni^{2+ a}.

Sample	Ni2+ Added (μM)	Ni2+ Found (μM)	Recovery (%)	R.S.D (n = 3) (%)
Drinking water	0.0	0.0	-	-
	6	6.09	101.48	0.37
Tap water	0.0	0.0	-	-
	6	5.98	99.68	0.24

^a Conditions: [DPP] = 20 μM in PBS buffer.

).

The acceptable recovery percentage and relative standard deviation (R.S.D.) were obtained, meaning that DPP could measure Ni²⁺ substantially in a real environment.

3.2. Theoretical Study

To understand the sensing process of DPP to Ni²⁺, theoretical calculations of DPP and DPP-Ni²⁺ were carried out. The calculations of DPP-Ni²⁺ were based on the 1:1 association of DPP and Ni²⁺, which was suggested by ESI-MS and Job plot. The energy-optimized structures of DPP and DPP-Ni²⁺ are shown in Figure 10.

The dihedral angle (6N, 1C, 11C, and 12O) of DPP is calculated as 27.06°, showing a twisted structure. DPP-Ni²⁺ complex with the dihedral angle of −3.84° forms a tetrahedral structure with 2H₂O. With the energy-optimized structures, TD-DFT calculations were performed to study the electron transitions of DPP and DPP-Ni²⁺. For DPP, excited state 1 (472.73 nm) was regarded to be the HOMO → LUMO transition, which showed an ICT character (Figure 11 and Figure 12).

Its molecular orbitals indicated the shift of electron cloud from the 2,4-dichlorophenol moiety to the pyridine one. The ICT character contributes to the yellow color of DPP. For DPP-Ni²⁺, excited state 8 (553.47nm) consists of the HOMO → LUMO (alpha), HOMO → LUMO+1 (beta), and HOMO → LUMO+2 (beta). The HOMO → LUMO (alpha) showed the ICT character from the 2,4-dichlorophenol group to the pyridine one. The HOMO → LUMO+1 (beta) and HOMO → LUMO+2 (beta) displayed both the ICT characters from the 2,4-dichlorophenol group to the pyridine one and LMCT characters from DPP to nickel (Figure 12 and Figure 13).

In addition, the calculated excitation energy of DPP-Ni²⁺ decreased compared to free DPP when the complex was formed (Figure 12). Calculated theoretical values demonstrated the redshift of the UV-vis transitions, which is consistent with experimental results. With Job plot, ESI-MS, DFT calculations, and FT-IR, we proposed the colorimetric sensing of Ni²⁺ by DPP (Scheme 1).

4. Conclusions

We developed a chalcone-based colorimetric chemosensor DPP that can efficiently detect Ni²⁺ by a colorimetric variation from yellow to purple. With Job plot and ESI-MS, the association mode of DPP to Ni²⁺ was analyzed to be a 1:1 ratio. The detection limit and binding constant of DPP to Ni²⁺ were 0.36 μM and 1.06 × 10⁴ M⁻¹, respectively. The detection limit of DPP is below the United States Environmental Protection Agency (EPA) guideline (1.2 μM) for Ni²⁺. It is noteworthy that DPP is the first chalcone-based colorimetric chemosensor to detect Ni²⁺ in near-perfect aqueous media. Practically, DPP could recognize Ni²⁺ in real water. In addition, the DPP-coated paper-supported strip showed a clear color variation from yellow to purple only in Ni²⁺. The binding mechanism of DPP to Ni²⁺ was explained by Job plot, ESI-mass, UV-vis, FT-IR, and calculations.