Introduction

Zinc is the second most abundant transition metal ion in the human body after iron. It plays an important role in various physiological and pathological processes, including DNA synthesis, gene expression, enzyme regulation structure and neuron signal transmission (Wang et al. 2020a, b, c, d; Zhang et al. 2021). The lack of Zn2+ in adults can lead to neurological disorders, Alzheimer's disease and diabetes. Lack of Zn2+ in children can lead to decreased immune function, diarrhea and even death (Vetriarasu et al. 2019; Liu et al. 2020a, b). In recent years, zinc ion is widely used in electroplating industry, which causes more and more serious environmental pollution. Therefore, it is very important for human health and environment to design a high selective zinc ion detection probe (Zhao et al. 2019; Yu et al. 2017b, a). Although many chemical sensors for zinc detection have been studied before, new fluorescent probes for selective detection of zinc ions in physiological pH conditions and environmental systems are still in great demand. Due to its d10 configuration, Zn2+ sensor is usually interfered by other d10 metal ions (such as CdII and HgII) (Anand et al. 2018; Bian et al. 2021b, a). As we all know, Schiff base fluorescent probe is a kind of metal ion probe which is relatively simple to synthesize and widely used. It has the advantages of convenient operation, simple detection method, fast detection and high sensitivity, and has attracted great attention. Because of the C = N group in its structure, its rigid structure and fluorescence are enhanced after chelating with metal ions (Patil et al. 2018; Xu et al. 2021a, b).

In this paper, we designed and synthesized a fluorescent probe SW with high selectivity and sensitivity for the detection of Zn2+ ions, which can be used for visual detection. The probe has the advantages of simple preparation and low cost (Wei et al. 2020; Pannipara et al. 2018). It has a strong practical value for the test paper detection of Zn2+ ions under 365 nm UV light and the detection of Zn2+ ions in actual water samples.

Experimental

Materials and methods

The O-benzylhydroxylamine (99%), 4-aminoacetophenone (99%), and salicylic aldehyde (98%) used in the experiment were purchased from Alfa Aesar. The remaining reagents and solvents are all analytical reagents and can be used without further purification. The water used in the experiment is distilled water. The X-4 microscopic melting point instrument produced by Beijing Tyco Instrument Limited company was used for melting point measurement, and no calibration was performed before use. German Vario EL V3.00 automatic element analyzer was used for the analysis of C, H, and N elements. 1H NMR spectra were recorded in DMSO-d6 solution using Bruker AV series DRX-500 MHz nuclear magnetic resonance instrument. Fluorescence spectra were recorded using Hitachi (Japan) F-7000 fluorescence spectrophotometer. Ultraviolet–visible absorption spectrum is measured by Hitachi UV-3900 spectrometer. The B3LYP/6-31G function is used as the basis of geometric optimization, and the Gaussian 09 software program is used for DFT calculation.

By adding various metal cations in DMSO/H2O (V/V = 9:1) medium, and the probe SW concentration was kept constant (2.0 × 10–4 mol/L). All metal cation solutions (1.0 × 10–2 mol/L) Na+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Fe3+, Mg2+, Mn2+, Zn2+, Ni2+, Pb2+, Hg2+ and Cu2+ were prepared from the nitrate salts, while the Zn2+ solutions was prepared by Zn(NO3)2. The excitation wavelength used for fluorescence spectrometry is 368 nm, the entrance slit is 5 nm and the exit slit is 5 nm.

Synthesis of the probe SW

SW was synthesized routes shown in Scheme 1. O-benzylhydroxylamine (1.18 g, 9.0 mmol) and 4-aminoacetophenone (1.22 g, 9.0 mmol) were dissolved in anhydrous ethylalcohol (15 mL), and then 3 drops of glacial acetic acid were added to the mixed solution and refluxed at 65 °C for 6 h. The mixture solution is cooled, filtered to obtain a yellowish solid, washed with anhydrous ethanol/water (V/V = 1:4) and dried under vacuum (Sun et al. 2015; Wu et al. 2021), and obtained in 2.05 g of ({4-amino}phenyl)ethanone O-benzyl oxime as crystalline solid. Yield: 89.0%. M.p.351 ~ 352 K. Anal. Calcd. for C15H16N2O(%): C, 74.97; H, 6.71; N, 11.66. Found (%): C, 74.68; H, 6.80; N, 11.52.

Scheme 1
scheme 1

Synthetic routes of SW

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({4-amino}phenyl)ethanone O-benzyloxime (1.51 g, 6 mmol) and salicylaldehyde (0.74 g, 6 mmol) were dissolved in ethanol (15 mL), and the mixed solution was stirred at 65 ℃ for 6 h. After cooling to room temperature, vacuum distillation and filtration were carried out. The precipitate was washed with ethanol/n-hexane (V/V = 1:4). After vacuum drying, 1.37 g SW dark yellow solid product was obtained (Fig S1). Yield: 61.3%. M.p. 229 ~ 230 ℃, Anal. Calcd. for C22H20N2O2 (%): C, 76.72; H, 5.85; N, 8.13. Found (%): C, 76.97; H, 5.23; N, 8.25. 1H NMR (500 MHz, DMSO-d6) δ 12.96 (s, 1H), 8.99 (s, 1H), 7.78—7.72 (m, 2H), 7.68 (dd, J = 7.7, 1.8 Hz, 1H), 7.47–7.36 (m, 7H), 7.35—7.29 (m, 1H), 7.03—6.95 (m, 2H), 5.22 (s, 2H), 2.25 (s, 3H).

Results and discussion

Fluorescence recognition of Zn2+ by probe SW

The response of probe SW to different metal cations was studied by the fluorescence method at room temperature (Li et al. 2021a, b, c). As shown in Fig. 1a, fifteen cations (Na+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Fe3+, Mg2+, Mn2+, Zn2+, Ni2+, Pb2+, Hg2+ and Cu2+) (1.0 × 10–2 mol/L) were added to the solution of probe SW (2.0 × 10–4 mol/L) (DMSO/H2O, V/V = 9:1), respectively. When the excitation wavelength is 368 nm, the probe SW has a weak fluorescence emission peak at 437 nm. After the addition of other metal ions, the intensity of emission peak has no obvious enhancement except Zn2+ ions. Furthermore, the emission peak was red-shifted from 437 to 502 nm and the intensity increased by 16 times upon addition of Zn2+ ions (Dong et al. 2017; Wang et al. 2020a). Under the UV lamp, the solution of 15 kinds of metal ions to be measured was added to DMSO/H2O solution (V/V = 9:1) of SW in turn, a strong bright green fluorescence is produced only added Zn2+ ions, and other metal cations have basically no obvious effect (Anand et al. 2017; Pan et al. 2020a, b), the fluorescence remains unchanged or quenched (Fig. 1b). It can prove that probe SW could selectively identify Zn2+ ion among other metal ions to be measured. It is helpful to study the high sensitivity of probe SW to Zn2+ ion by anti-interference experiment (Ozdemir 2016; Sun et al. 2019). In Fig. 2, after adding Zn2+ ion to probe SW solutions containing different metal ions, other metal cations had no markedly effect on the fluorescence recognition to Zn2+ ions except for the slight quenching of Cu2+ ion and Co2+ ion had a slight effect (Liu et al. 2020a). The results show that SW has good selectivity for Zn2+ ions.

Fig.1
figure 1

a Fluorescence emission spectra of probe SW with various metal ions in DMSO/H2O (v/v = 9:1) medium (λex = 368 nm); b Fluorescence changes upon different cationic added to the SW solution in DMSO/H2O (v/v = 9:1) solution under 365 nm UV lamp

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Fig. 2
figure 2

Fluorescence emission spectra (λex = 368 nm) of the probe SW in the presence of Zn2+ and various cationic in DMSO/H2O (v/v = 9:1) solution

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In order to quantitatively evaluate the fluorescence sensing behavior of SW, we carried out fluorescence titration experiment. As shown in Fig. 3, the Zn2+ solution (1.0 × 10−3 mol/L) was gradually added to the probe SW solution (2.0 × 10−4 mol/L) (Upadhyay et al. 2018; Wang et al. 2021a, b), the free SW exhibited a weak emission peak at 437 nm, a new peak emission present at 507 nm and its intensity gradually increased when the concentration of Zn2+ ion increases continuously (Wang et al. 2017; Chang et al. 2020). When the Zn2+ content reaches 0.5 equivalent, the fluorescence emission intensity reaches the maximum, indicating that the optimal binding ratio of Zn2+ to probe SW is 1:2. Bringing the results of titration experiments into the Benesi–Hildebrand equation \(\{1/(F-{F}_{0})=1/({F}_{\mathrm{max}}-{F}_{0}\)) + \({1/K}_{d}\)[C]\(\times {1/(F}_{\mathrm{max}}-{F}_{0}\))} (Sudipa et al. 2019; Xu et al. 2021a), and the binding constant is calculated as Ka = 7.7 × 104 M−1 (Fig. S2a). Where, F0, F and Fmax are the fluorescence intensity without Zn2+, the fluorescence intensity at any given Zn2+ concentration and the fluorescence intensity after titration saturation, respectively. The limit of detection LOD for Zn2+ toward SW was calculated using LOD = 3σ/slope and was found to be 2.53 × 10−8 mol/L (Fig. S2b) (Wang et al. 2020b; Purkait et al. 2019). Among them, the standard deviation σ was calculated by measuring five consecutive fluorescence intensities of probe SW, and the slope was obtained by plotting the relationship between the emission intensity of SW and the concentration of Zn2+ (Zhang et al. 2020; Bian et al. 2021b; Fan et al. 2020). The calculated results are lower than the acceptable limit (7.0 × 10−6 mol/L) of WHO drinking water (Lu et al. 2020; Kang et al. 2019b). Compared with other Zn2+ sensors reported previously, the detection limit is lower and the sensitivity is higher (Table 1).

Fig. 3
figure 3

Fluorescence spectra of the probe SW in DMSO/H2O (v/v = 9:1) solution with increasing concentration of Zn2+ (0–0.5equiv)

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Table 1 Comparison of some recently reported probes for Zn2+ detection
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In order to test and verify the rapid detection performance of probe SW for Zn2+, the Zn2+ response time experiment was carried out (Zhang et al. 2019). As shown in Fig. S3, after adding Zn2+, the response time of probe SW was 3 min (Pan et al. 2020a; Li et al. 2021a). The fluorescence reversibility studied by adding Zn2+ and ethylenediaminetetraacetic acid (EDTA, c = 1.0 × 10–3 mol/L) to SW solution. In Fig S4, when EDTA was introduced to SW and Zn2+ mixed solution, the fluorescence intensity at 502 nm was diminished. Then, Zn2+ solution was added again, and the fluorescence intensity was close to the initial fluorescence value, and the above experimental steps were repeated, and the fluorescence intensity continued to decrease and increase (Wang et al. 2021a; Mu et al. 2020). Thus, the probe SW can monitor Zn2+ reversibly.

UV–Vis spectroscopic studies of Zn2+ by probe SW

In the UV–Vis spectrum, as shown in Fig S5a, the free ligand SW exhibited a weak absorb peak at 425 nm. When Zn2+ ions were added to probe SW solution, a new absorption peak appeared at 458 nm, and the solution changed from colorless to yellow (Fig. S5b). At the same time, the addition of Co2+, Fe3+ and Cu2+ metal ions also slightly changed the color. Therefore, UV–Vis spectrum can be used as an assistant method to detect the Zn2+ by this probe SW. As shown in Fig. S6, the Zn2+ solution (1.0 × 10−3 mol/L) was gradually added to the probe SW solution (2.0 × 10−4 mol/L) for UV–visible titration experiments (Wang et al. 2020c; Liu et al. 2018). With the continuous increase of Zn2+ concentration, a new absorption peak appears at 458 nm, while the absorption peak at 352 nm gradually decreases. At the same time, an isoabsorptive point appeared at 285 nm and until the content of Zn2+ solution reached 0.5 equivalents, the peaks at 458 nm and 352 nm remained unchanged (Fig. S7b). The UV–Vis spectral characteristics support the molar ratio of metal to ligand of 1:2, and the association constant Ka = 5.6 × 104 M−1 (Fig. S7a).

The detection mechanism of the probe SW for Zn2+

1H NMR titration experiment was carried out in DMSO-d6 (Long et al. 2020). As shown in Fig. 4, with the addition of Zn2+ from 0 to 0.5 equivalent, the phenolic hydroxyl protons H1 (δ = 12.87 ppm) gradually disappeared until completely disappeared, indicating the deprotonation process of hydroxyl induced by Zn2+. With the addition of Zn2+, azomethine H2 (HC = N) shifts from 9.02 ppm to 8.65 ppm, which may be due to the coordination between imine-N and Zn2+. In the complex SW-Zn2+, the 1H NMR titration data supports 1:2 metal–ligand ratio (Xu et al. 2020; Yu et al. 2017a). Job's plot further analyzes the coordination ratio between SW and Zn2+ (Fig. S8), when the mole fraction of Zn2+ ion is 0.345, the fluorescence intensity reaches the maximum value, showing that the binding ratio of Zn2+ ion to SW is 1:2 (Liu et al. 2020b; Xue et al. 2019).

Fig. 4
figure 4

1H NMR titration of SW with different concentrations of Zn2+ in d6-DMSO

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As shown in Scheme 2, the fluorescence enhancement recognition mechanism of Zn2+ by probe SW may be mainly due to the presence of Zn2+ hindering the PET (light-induced electron transfer) effect of SW. Owing to the rotation of imine group (-CH = N) in free SW, there is a very weak fluorescence in SW solution. The addition of Zn2+ ions can coordinate with the probe SW, which inhibits the free rotation of imine group of SW molecules and produces CHEF effect (Liu et al. 2018; Zhang et al. 2018), resulting enhanced fluorescence. The mass spectrum peak of the complex appeared at 751.22 (Fig. S9), which further confirmed that the complex was consistent with the conjecture.

Scheme 2
scheme 2

Plausible mechanism for Zn2+ recognition by SW

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DFT computation

In order to further study the geometry and interaction of SW and Zn2+, density functional theory (DFT) calculation was carried out. The geometry structures of SW and SW-Zn2+ were optimized by 6-31G/LanL2DZ basic setting program using Gausans-09 software (Feng et al. 2021; Rout et al. 2019). As shown in Fig. 5, one the Zn2+ ion coordinated with two ligand SW molecules. Among, the coordination atoms are hydroxyl O atoms and the imine N atoms on C = N groups of two SW molecules, respectively. The LUMO and HOMO energies of SW were −1.551 eV and −5.659 eV, respectively. The energy gap (∆E = ELUMO-EHOMO) was 4.108 eV, and the SW molecule was delocalized on the entire conjugated skeleton except for the oxime phenyl group. After SW coordinating with Zn2+, LUMO and HOMO energies were −1.931 eV and −5.386 eV, respectively. Correspondingly, the energy gap was 3.455 eV, and the electron density was mainly delocalized on the two Schiff groups and mainly on entire salicylaldehyde and aminoacetophenone conjugated skeleton except for the oxime phenyl group. The electron density of HOMO–LUMO transition showed the fluorophore-metal charge transfer, indicating that with the PET effect, the excited electrons were readily averted to metal ions (Cui et al. 2019; Li et al. 2021b). The decrease of ∆E indicates that there is good coordination ability between Zn2+ and SW and formed a stable environment.

Fig. 5
figure 5

optimization molecular structure of SW and SW-Zn2+, molecular orbital diagrams of HOMO and LUMO

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Practical application of probe SW

In practical application, the probe SW is designed as a Zn2+ responsive strip sensor with good selectivity, and it can quickly and simply detect Zn2+ ions by changing the wavelength, which has great practical value (Kang et al. 2019a). The filter strips were immersed in DMSO/H2O solution of probe SW for 1 h, and dried at low temperature, then immersed in different concentrations of Zn2+ ions and other metal ions for 30 min, and the changes were observed under 365 nm UV lamps (Diao et al. 2018). As shown in Fig. 6a, with the increase of Zn2+ concentration, the color of the test paper changed from colorless to green. Whereas the other metal ions did not cause obvious changes in the color of test paper except Zn2+ ions (Fig. 6b), indicating that SW probe had high selectivity for Zn2+.

Fig. 6
figure 6

Fluorescent tests paper of probe SW. a The changes after adding different concentrations of Zn2+ ions; b addition of different metal ions (1.0 × 10−2 mol/L)

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In order to test the practicability of Zn2+ ions in real water samples, drinking water, tap water and Yellow River Water were collected for sensing experiments (Sarkar et al. 2020). All samples were filtered through 0.2 mm filter membrane and tested for three times. The calibration curve was obtained by measuring the Zn2+ ions concentration (Ke et al. 2020). As shown in Table 2, the detection results show that the probe SW can effectively detection Zn2+ ions and has high recovery (98%-103%), good analytical precision (RSD < 3%), which meets the detection requirements. Therefore, the probe SW can be effectively used for the detection of Zn2+ ion concentration in actual water samples and has practical value in environmental analysis.

Table 2 Determination of Zn2+ ions in actual water samples
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Conclusions

In summary, a simple Schiff base ligand SW has been designed and synthesized, and it has higher sensitivity and selectivity to Zn2+ in DMSO/H2O (v/v = 9:1) solution. And the detection limit of the SW to Zn2+ is down to 2.53 × 10−8 mol/L, which was lower than the limited value defined by WHO. By means of 1H NMR, MS analysis and theoretical calculation, we obtained the binding mode of probe SW to Zn2+ is 2:1. In addition, the probe SW can be used for the detection of Zn2+ ions in the test paper under the UV light at 365 nm and the detection of Zn2+ ions in actual water samples, which has a potential application prospect.