Introduction

Palladium is a widely used transition metal in various fields such as pharmaceutical synthesis, electrical and electronic industries, medical devices, automobiles, and catalysts [1,2,3]. A large number of palladium ions are released as they are used for various purposes, and the released palladium ions have a harmful effect on the environment and the human body [4,5,6,7]. Therefore, it is required to develop methods capable of easily and quickly detecting palladium ions [8, 9].

To detect Pd2+, there are various analytical methods like inductively coupled plasma mass spectrometry, X-ray fluorescence (XRF), solid-phase micro-extraction coupled high-performance liquid chromatography, and atomic absorption spectrometry [10,11,12]. However, these analytical methods require expensive equipment, trained professionals, and prolonged sample preparation time [13,14,15,16,17]. Due to these shortcomings, chemosensors are attracting attention as an alternative analytical method [18,19,20,21]. Chemosensors have advantages such as high sensitivity, specificity, fast response, and technical simplicity [22,23,24,25,26,27,28].

Pyridine can endow cations with a binding site through a lone pair electron of nitrogen atom [29,30,31]. Also, fluorophores including pyridine moiety are known to exhibit strong fluorescence [32]. Triphenylamine has various properties such as high fluorescence quantum yields, visible region wavelength, strong UV-vis and luminescent properties, which are useful characteristics for developing chemosensors [33,34,35,36,37,38,39]. Chalcone structure is known for optically active structure [40,41,42]. Also, a conjugate \({\uppi }\)-electronic system of this structure provides the chelating ability for metal ions [43,44,45]. Due to these properties, the chalcone structure is useful to develop chemosensors detecting metal ions [45, 46]. Pd2+ is known as a fluorescent quencher [47,48,49]. This property is useful for the development of a sensor that detects Pd2+ through quenching [50]. Therefore, we expected that the combination of the pyridine and the chalcone structure having triphenylamine might produce a sensor that detects Pd2+ with turn-off.

Herein, we present a fluorescent and colorimetric chemosensor DiPP for detecting Pd2+. DiPP was the first chalcone-based chemosensor to detect Pd2+ through both fluorescence and color change methods. Chemosensor DiPP was able to detect Pd2+ with low detection limits (0.67 µM and 0.80 µM) by fluorescence turn-off and colorimetric variation of yellow to purple. Also, the test strip absorbed with DiPP could detect Pd2+ easily and quickly through fluorescence turn-off and color change. The binding feature of DiPP to Pd2+ was addressed by UV-visible titrations, ESI-mass, 1 H NMR titration, DFT calculations and Job plot.

Experimental Section

General

Chemicals were commercially acquired from Alfa Aesar and TCI. 13 C and 1 H NMR spectra were gained with a Varian spectrometer. With Perkin Elmer spectrometers, emission and absorption data were recorded. A Thermo MAX instrument provided ESI-MS spectra.

Synthesis of Sensor DiPP ((E)-3-(4-(Diphenylamino)Phenyl)-1-(Pyridin-2-yl)prop-2-en-1-one)

DiPP was synthesized according to the literature method [51]. 2-Acetylpyridine (342 µL, 3.0 × 10− 3 mol) and 5 mL of 10% NaOH were added in 15 mL of MeOH. The solution was stirred for 50 min. 4-(Diphenylamino)benzaldehyde (558 mg, 2.0 × 10-3 mol) was added to the solution. The mixture was stirred at 20 oC for 16 h. An orange powder was washed with ether several times and dried in the oven. The dried powder was dissolved in chloroform and purified by column chromatography using chloroform. Yield: 436 mg (58%). 1 H NMR in CD3CN: 8.79–8.77 (d, 1 H), 8.13–8.08 (m, 2 H), 8.06–8.02 (t, 1 H), 7.81–7.77 (d, 1 H), 7.71–7.66 (m, 3 H), 7.40–7.36 (m, 4 H), 7.18–7.11 (m, 6 H), 6.92–6.90 (d, 2 H). 13 C NMR in deuterated DMSO:188.5 (1 C), 153.8 (1 C), 150.0 (1 C), 149.2 (1 C), 146.3 (2 C), 144.1 (1 C), 137.8 (1 C), 130.5 (2 C), 130.0 (4 C), 127.6 (1 C), 127.4 (1 C), 125.6 (4 C), 124.7 (2 C), 122.5 (1 C), 120.6 (2 C). ESI-mass: calcd for ([DiPP + H+ + 2H2O + 2THF])+ : 557.30, found 557.58.

Fluorescent and UV-vis Titrations

6 µL (1 mM) of DiPP (3.8 mg, 1 × 10− 5 mol) dissolved in 10 mL of tetrahydrofuran (THF) was diluted in 2.994 mL THF to provide 2 × 10− 6 M. 3–54 µL (2 × 10− 3 M) of Pd(NO3)2 (2.5 mg) dissolved in THF (5.0 mL) were added to DiPP (3 mL, 2 × 10− 6 M). Their fluorescence spectra were taken in 10 s. For UV-vis, 15 µL (1 mM) of DiPP (1 × 10− 5 mol, 3.8 mg) dissolved in 10 mL of THF was diluted in 2.985 mL THF to provide 5 × 10− 6 M. 3–33 µL (0.4–4.4 eq) of Pd(NO3)2 (2 mM) dissolved in THF were added to DiPP (3 mL, 5 × 10− 6 M). Their UV-visible spectra were taken in 10 s.

Competition

DiPP (1 × 10− 5 mol, 3.8 mg) was dissolved in 10 mL of THF. 0.06 mmol of KNO3, NaNO3, In(NO3)3, Cr(NO3)3, Ga(NO3)3, Fe(NO3)3, Al(NO3)3, Hg(NO3)2, Ni(NO3)2, Ca(NO3)2, Co(NO3)2, Mn(NO3)2, Cu(NO3)2, Cd(NO3)2, Pb(NO3)2, Mg(NO3)2, Zn(NO3)2, and Pd(NO3)2 was dissolved in 3,000 µL THF. 4.5 µL of each metal (2 × 10− 2 M) and Pd2+ ion (2 × 10− 2 M) was added into 2,985 µL THF to afford 15 equiv. 6 µL (1 × 10− 3 M) of the DiPP stock was added to the solutions. Their fluorescence spectra were taken in 10 s. For the UV-vis, 2.7 µL of each metal (2 × 10− 2 M) and Pd2+ ion (2 × 10− 2 M) was added into 2,980 µL THF to afford 3.6 equiv. 15 µL (1 × 10− 3 M) of the DiPP stock was added to the solutions. Their UV-visible spectra were taken in 10 s.

Quantum Yields of DiPP and DiPP-Pd2+

Standard fluorophore fluorescein (ФF = 0.79) was used for quantum yield [47].

ΦF(X) = ΦF(S)(ASFX/AXFS) (nX/nS)2

(ФF: fluorescence quantum yield, s: standard, A: absorbance, n: refractive index of the solvent, F: area of fluorescence emission curve and x: unknown)

Job Plot

A stock solution of sensor DiPP (1 mM) was prepared in 10 mL of THF. Pd2+ solution (1 × 10− 3 M) with nitrate salt was acquired in 10 mL of THF. 3–27 µL of the DiPP stock was transferred to several quartzes. 27 − 3 µL of the Pd2+ stock was added to diluted DiPP. THF was added to each quartz up to 3,000 µL. Fluorescence spectra of the solutions mixed were taken in 10 s.

1 H NMR Titration

Two NMR tube of DiPP (3.8 mg, 1 × 10− 5 mol) dissolved in CD3CN (250 µL) was prepared. In one tube, 250 µL of CD3CN was added to make a 20 mM DiPP sample. In the other tube, Pd(NO3)3 (2.3 mg, 1 × 10− 5 mol) dissolved in CD3CN (250 µL) was added to prepare a 20 mM DiPP-Pd2+ sample. 1 H NMR data were recorded in 10 s.

Calculations

To investigate the detecting mechanism of DiPP to Pd2+, the Gaussian16 program [53] was used for calculations. They were based on m06 density functional [54,55,56]. 6-31G(d,p) [57, 58] and Lanl2DZ [59] basis sets were employed for calculations of Pd2+ and elements. The solvent effect on THF was considered by employing IEFPCM [60]. With the optimized features of DiPP and DiPP-Pd2+, 20 of the lowest singlet-singlet transitions were calculated with TD-DFT to study the transition states of DiPP and DiPP-Pd2+.

Results and Discussion

Molecule DiPP was gained through the aldol condensation of 4-(diphenylamino)benzaldehyde with 2-acetylpyridine (Scheme 1). DiPP was affirmed by 1 H NMR, 13 C NMR, and ESI-MS (Figs. S1-S3).

Scheme 1
scheme 1

Synthesis of DiPP

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Fluorescent selectivity of DiPP was studied with diverse cations (K+, Ag+, Cu2+, Co2+, Zn2+, Cd2+, Ca2+, Mn2+, Mg2+, Pb2+, Ni2+, Hg2+, Cr3+, Ga3+, Na+, In3+, Fe3+, Al3+, and Pd2+) in THF. As exhibited in Fig. 1, DiPP and DiPP with most metals represented strong fluorescence at 527 nm (λex = 418 nm). By contrast, Pd2+ showed a clear quenching with DiPP at 527 nm. The quantum yields (Ф) of DiPP and DiPP-Pd2+ were given to be 0.71 and 0.088, respectively. Thus, DiPP worked as a fluorescent turn-off chemosensor for the obvious probing of Pd2+. To study the photophysical feature of DiPP to Pd2+, fluorescent titrations were checked (Fig. 2). The fluorescence of DiPP at 527 nm smoothly decreased until Pd2+ increased to 15 equiv (Fig. 2). The decrease in fluorescence intensity of DiPP with the increasing amount of Pd2+ ions was proposed as chelation enhanced quenching (CHEQ) mechanism. The developed sensor determined Pd2+ in the linear range of 0–10 µM, with a low detection limit of 0.67 µM (3\({\upsigma }\)/k) (R2 = 0.995) (Fig. 3) [61]. A competitive test was performed to know if DiPP could exclusively bind to Pd2+ with the other coexisting metals (Fig. S4). Most cations did not display the binding of DiPP to Pd2+. However, about 50% of the interference was observed from Cr3+ and more than 90% from K+ ions.

Fig. 1
figure 1

Fluorescent intensity variations of DiPP (2.0 µM) with cations (15 equiv; λex = 418 nm)

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Fig. 2
figure 2

Fluorescent variations of DiPP (2.0 µM) with varied amounts of Pd2+ (λex = 418 nm)

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Fig. 3
figure 3

Analysis of the detection limit for Pd2+ by DiPP (2 µM) based on the fluorescence intensity at 527 nm (λex = 418 nm). The standard deviations are represented by the error bar (n = 3)

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To check the colorimetric probing of DiPP to Pd2+, the UV-vis variation was studied with diverse cations in THF (Fig. 4). DiPP and DiPP with most cations showed no or little absorbance at 575 nm. However, the addition of Pd2+ caused an obvious increase in absorbance at 575 nm and a colorimetry variation of pale yellow to purple. Therefore, DiPP could also be performed as a colorimetry chemosensor for the nicely selective probing of Pd2+. Importantly, as far as we know, DiPP is the first chalcone structure-based probe among chemosensors to detect Pd2+ through both fluorescence and color change methods. (Table S1).

Fig. 4
figure 4

(a) UV-vis absorbance variations of DiPP (5.0 µM) with varied cations (3.6 equiv). (b) Color changes of DiPP (5.0 µM) with cations (3.6 equiv)

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To understand the colorimetric sensing feature of DiPP to Pd2+, UV-vis titrations were tested (Fig. 5). Absorbance of DiPP at 340 and 575 nm obviously increased, and that of 425 nm decreased until the amount of Pd2+ got to 3.6 equiv. A sound isosbestic point at 456 nm signified that the combination of DiPP with Pd2+ formed a species. The detection limit of DiPP with Pd2+ based absorbance change was calculated to be 0.80 µM (3\({\upsigma }\)/k) in the range from 0 to 14 µM (R2 = 0.995) (Fig. S5) [61].

Fig. 5
figure 5

UV-vis variations of DiPP (5.0 µM) with different concentrations of Pd2+

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A competitive test was achieved to know whether DiPP could exclusively bind to Pd2+ among the coexisting metals for colorimetric chemosensors (Fig. S6). The color change was not disturbed by most metals but was disturbed by 50% from Cr3+ and 75% from K+ ions. For the practical test, filter papers coated with DiPP were employed. The test strips could probe Pd2+ via a fluorescence turn-off and a colorimetry change from yellow to light navy blue (Fig. 6). Cu2+ and Ni2+ showed some inhibition in the fluorescent test kit. Thus, the DiPP-coated test strip can have the practical application to rapidly and readily recognize Pd2+.

Fig. 6
figure 6

Photographs of DiPP-coated test strips (1 mM). (a) DiPP-test strips immersed in Pd2+ (0 and 500 µM) under UV light. (b) DiPP-test strips were immersed in varied metal ions (500 µM) under UV light. (c) Color variation of DiPP-test strips immersed in Pd2+ (0 and 500 µM). (d) Color change of DiPP-test strips immersed in varied metal ions (500 µM)

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Detecting Mechanism of DiPP to Pd2+

To determine the reaction ratio of DiPP with Pd2+, a Job plot method was applied and showed the biggest value at a 0.5 molar fraction (Fig. S7). It meant that a Pd2+ bound to a DiPP with a 1 : 1 ratio. Positive-ion ESI-MS displayed that the peaks of 626.18 (m/z) and 756.49 (m/z) corresponded to [DiPP + Pd2+ + NO3− + 2MeOH + H2O]+ (calcd, 626.11) and [DiPP + Pd2+ + NO3− + MeOH + 2H2O + 2THF]+ (calcd, 756.21) (Fig. S8). In addition, the 1 H NMR titration was applied to illustrate how to interact DiPP with Pd2+ (Fig. 7). As the Pd2+ were added, the protons H1 and H4 showed an up-field shift, whereas H2 and H3 moved down-field. The protons H5 and H6 showed a large up-field shift, respectively. In contrast, the protons of tri-phenyl amine showed relatively small movement to the down-field, except for H7 and H7’, which were close to the binding site. The outcomes drove us to suppose that Pd2+ may bind with the nitrogen of the pyridine moiety and the oxygen of the carbonyl group. The binding constants of the DiPP-Pd2+ complex were given to be 7.0 × 104 M− 1 based on fluorescence intensity and 1.7 × 104 M− 1 based on UV-vis absorbance from the Benesi-Hildebrand equation (Figs. S9 and S10). With Job plot, ESI-MS, and 1 H NMR titration, the likely feature of DiPP-Pd2+ was supposed (Scheme 2).

Fig. 7
figure 7

1 H NMR titration of DiPP with Pd2+ (0 and 1.0 equiv)

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Scheme 2
scheme 2

Proposed structure of DiPP-Pd2+

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Calculations

To demonstrate the sensing feature of DiPP to Pd2+, theoretical calculations of DiPP and DiPP-Pd2+ were achieved. The 1:1 association of DiPP and Pd2+ was applied to calculations of DiPP-Pd2+, which was supposed by Job plot and ESI-MS. The optimized features of DiPP and DiPP-Pd2+ are demonstrated in Fig. 8. The dihedral angle (48 N, 38 C, 37 C, and 49O) of DiPP is calculated as 179.67 °, indicating that the carbonyl oxygen and the pyridine nitrogen are in the plane. In the DiPP-Pd2+ complex, DiPP as a bidentate ligand chelates Pd2+ using the pyridine nitrogen and the carbonyl oxygen, and two NO3− are bound in the vacant sites. As a result, the optimized DiPP-Pd2+ complex showed a square planar structure. With the optimized features, TD-DFT calculations were carried out for studying the electron transitions of DiPP and DiPP-Pd2+. For DiPP, the HOMO → LUMO transition of 441.47 nm was regarded as the major transition, showing an ICT character (Figs. S11 and S12). Its molecular orbitals (MOs) displayed the shift of the electron cloud from the triphenylamine moiety to the pyridine one. This ICT character leads to the yellow color of DiPP. For DiPP-Pd2+, the HOMO → LUMO related to the 558.94 nm showed a similar ICT property to free DiPP (Figs. S12 and S13). The energy gap between HOMO and LUMO was decreased when the DiPP-Pd2+ complex was formed (Fig. S12). Therefore, the color variation of yellow to purple in DiPP-Pd2+ might be due to the change of band-gap energy, resulting in a redshift. With ESI-MS, Job plot, calculations, and 1 H NMR titration, we proposed the binding feature of Pd2+ to DiPP (Scheme 2).

Fig. 8
figure 8

Energy-optimized forms of (a) DiPP and (b) DiPP-Pd2+

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Conclusion

We addressed a chemosensor DiPP based on a chalcone structure having triphenylamine that can exclusively detect Pd2+ by a fluorescent turn-off and colorimetry variation of pale yellow to purple. The association ratio of DiPP to Pd2+ was analyzed to be a 1: 1 ratio with ESI-MS and Job plot. The calculated detection limits of DiPP for Pd2+ were 0.67 µM and 0.80 µM through fluorescence and colorimetry. Specifically, it is noteworthy that DiPP could exclusively distinguish Pd2+ from in the same group Ni2+. Also, the colorimetric and fluorescent test strips coated with DiPP rapidly and easily recognized Pd2+. Interestingly, DiPP was the first chalcone-based fluorescent and colorimetric probe to detect Pd2+. The binding mechanisms of DiPP to Pd2+ could be supposed through NMR titration, Job plot, DFT calculations, fluorescent and UV-visible titrations, and ESI-mass.