A colorimetric and fluorometric NBD-based chemosensor for highly selective recognition of palladium(II) cations

Abstract

A chemosensor NBD-PMA with colorimetric and fluorometric responses for Pd²⁺ cations has been described. NBD-PMA selectively coordinated Pd²⁺ cations with a colour change from bright yellow to pink, which enabled detection of Pd²⁺ cations in environmental water samples with the naked eye. NBD-PMA also exhibited high selectivity and sensitivity for Pd²⁺ cations in fluorometry even in the presence of other metal cations, which makes it practically applicable for detecting Pd²⁺ cations in environmental samples and in living cells.

---

Introduction

Fluorometric and colorimetric chemosensors for the determination of metal ions have attracted active research interests in environmental and biological chemistry,^1–3 because they play important roles in living systems or have toxic impacts on our environment.^4–6 Palladium is one of the important elements used in various materials.^7–9 Extensive use and waste produced in the steel-making process have raised the pollution emissions to water sources and soils, and have even had severe effects on the growth of plants.^10–12 Therefore, the palladium content is strictly restricted in water sources, food, crops and medicaments in laws and regulations. In order to improve the sensitivity and testing on the spot without the use of special test instruments, a rapid test strip is a very important tool for seeing a colour change with the naked eye.

The NBDA (4-amino-7-nitro-2,1,3-benzoxadiazole) derivative is a typical intramolecular charge transfer (ICT) fluorophore widely used in the design of chemosensors.^13–18 Particularly, some NBD-type chemosensors containing 2-picolylamine are developed for the selective recognition of Cu²⁺, Hg²⁺ and Zn²⁺ cations.^19–26 Qian et al. reported that NBD-PMA (Scheme 1) shows no responsiveness for Zn²⁺ cations and the NBD-TPEA (Scheme 1) chemosensor shows a visible light excited fluorescence property for Zn²⁺ cations.¹⁹ These results suggest that the substituted groups on 2-picolylamine can tune the ICT capacity. Interestingly, Akerman et al.²⁷ and Dayan et al.²⁸ reported that 2-picolylamine can coordinate to Pd²⁺ cations to form stable five-membered rings.

Scheme 1 The chemical structures of NBD-PMA and NBD-TPEA.

In this paper, we have further explored the photophysical properties and selectivity of NBD-PMA for different cations and anions in detail. The results surprisingly suggest that NBD-PMA can selectively chelate to Pd²⁺ cations to result in a colour change and fluorescence quenching.

Results and discussion

The chemosensor NBD-PMA was synthesized according to the literature except for the replacement of THF with CH₃CN to afford NBD-PMA in a 65% yield as a brown crystal.¹⁹

The UV-vis spectrum of NBD-PMA was determined in aqueous acetonitrile solution (CH₃CN : H₂O = 4 : 1, v/v) and showed two typical NBD absorptions at 460 nm (ε = 1.5 × 10⁴ M⁻¹ cm⁻¹) and 331 nm (ε = 6.0 × 10³ M⁻¹ cm⁻¹) and one pyridyl absorption at 260 nm (ε = 4.7 × 10³ M⁻¹ cm⁻¹). Upon the addition of PdCl₂ solution to the NBD-PMA solution, the absorptions at 460 and 331 nm gradually disappeared, while two new absorptions at 379 nm (ε = 7.8 × 10³ M⁻¹ cm⁻¹) and 526 nm (ε = 1.8 × 10⁴ M⁻¹ cm⁻¹) were formed with three clear isosbestic points at 345, 406 and 480 nm, respectively (Fig. 1). The absorption at 460 nm showed a remarkable red-shift to 527 nm (Δλ = +67 nm), which resulted in the colour change from bright yellow to pink. In order to detect Pd²⁺ cations with the naked eye, rapid test strips at the 0–50 ppm level had been prepared (Fig. S1, ESI†). It was obvious that the 10 ppm test strip showed a different colour from that of the 0 ppm one, which is the threshold for palladium in drugs.²⁹ It was indicated that NBD-PMA could be used as a potential candidate for a colorimetric chemosensor in the determination of Pd²⁺ cations.

Fig. 1 Absorption spectra of NBD-PMA (10 μM) upon the addition of different Pd²⁺ cation concentrations. (Inset: the colour change before and after the addition of Pd²⁺ cations).

The interactions of NBD-PMA with metal cations or anions were studied in aqueous acetonitrile solution. The surveyed metal cations were representative alkali (Na⁺ and K⁺), alkaline earth (Ca²⁺, Mg²⁺ and Ba²⁺) and transition metal (Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Hg²⁺, Ag⁺, Fe³⁺, Mn²⁺, Pt²⁺ and Pd²⁺) cations. Furthermore, the anions were also studied (Fig. 2a) for detecting Pd²⁺ cations. Only the Pd²⁺ cation caused an outstanding decrease in the fluorescence of NBD-PMA at a wavelength of 530 nm. Although Cu²⁺ cations could also quench the fluorescence of NBD-PMA, the decrease of the fluorescence intensity (18%) was small compared with that of the Pd²⁺ cation (85%). It suggested that NBD-PMA showed a high selectivity for Pd²⁺ cations over other metal cations such as Hg²⁺, Zn²⁺, etc. To check further the practical applicability of NBD-PMA, the competition experiments were performed by treating NBD-PMA with 1 equimolecular amount of Pd²⁺ cations in the presence of 1 equimolecular amount of competing metal cations and the results showed that the other cations were not interference for the determination of Pd²⁺ cations (Fig. 2b).

Fig. 2 (a) Fluorescence spectra of NBD-PMA (10 μM) upon adding different cations and anions with excitation at 465 nm in aqueous acetonitrile (v/v = 1 : 4) solutions; (b) bar graph of NBD-PMA with fluorescence intensities at 530 nm in the presence of an equimolecular amount of Pd²⁺ and other metal cations (10 μM).

On titrating with Pd²⁺ cations, the intensity of the emission band progressively decreases down to a constant after the addition of more than 1 equimolecular amount of Pd²⁺ cations (Fig. S2, ESI†). The detection limit of NBD-PMA for Pd²⁺ cations was found to be 0.34 ppm²² (Fig. S3, ESI†). When S²⁻ anions were added to the coloured solution of NBD-PMA–Pd, the pink solution turned to yellow and the fluorescence increased due to the dissociation of Pd²⁺ cations (Fig. S4a, ESI†). The cyclic experiments show that the sensors can be recycled at least four times (Fig. S4b, ESI†).

To examine the binding mode of the sensor to Pd²⁺ cations, mass spectrum and NMR analyses were further performed. A peak at m/z = 447.3602 was obviously found, which is calculated as being for [NBD-PMA + PdCl₂ + H]⁺ (Fig. S5, ESI†). Furthermore, in the ¹H NMR spectra (Fig. 3), the chemical shifts of H_d and H_h in NBD-PMA shifted downfield from 4.81 and 8.55 ppm to 5.51 and 8.89 ppm upon adding Pd²⁺ cations, respectively. The chemical shift of H_f riding on the pyridyl ring shifted downfield from 7.79 to 8.06 ppm, and those of H_e and H_g riding on pyridyl ring were shifted together downfield from 7.43 and 7.32 ppm to around 7.68 ppm. However, the chemical shift of H_c riding on nitrogen shifted downfield from 9.91 to 10.05 ppm. The chemical shift of H_b shifted upfield from 6.35 to 6.16 ppm. These results indicated that NBD-PMA did participate in the complexation with Pd²⁺ cations. The Job’s plot for the complexation between NBD-PMA and PdCl₂ exhibited a 1 : 1 stoichiometry (Fig. S6, ESI†), which further confirmed the formation of the 1 : 1 complex NBD-PMA–Pd as shown in Fig. 3 and in previous reports.^27,28 The association constant (Kα) of NBD-PMA with Pd²⁺ cations was determined to be 8.5 × 10⁴ L mol⁻¹.

Fig. 3 ¹H NMR spectra of NBD-PMA in d₆-DMSO with Pd²⁺ cations.

The applicability of the chemosensor was examined for fluorescence imaging of Pd²⁺ cations in living HeLa cells. As shown in Fig. 4, the sensor-loaded cells displayed strong yellow fluorescence (Fig. 4a–d), but the fluorescence intensity decreased enormously upon incubating with Pd²⁺ cations for 30 min (Fig. 4e–h). These results displayed that this probe NBD-PMA could enter living HeLa cells to react with Pd²⁺ cations and could be used to image Pd²⁺ cations in living cells.

Fig. 4 CLSM images in living HeLa cells. (a–d) The cells were incubated with NBD-PMA (10 μM) for 30 min; (e–h) the cells were incubated with Pd²⁺ cations (10 μM) for another 30 min after incubating for 30 min with NBD-PMA (10 μM).

In order to examine the applicability of the proposed method in practical samples, the chemosensor was used to determine Pd²⁺ cations in three water samples: washing water from washing the Tsuji–Trost reaction bottle, river water from the Pearl River, and tap water from our laboratory.

All these samples were filtered through a 0.45 μm membrane and adjusted to pH 7.40. Then the resulting solutions were spiked with standard Pd²⁺ cation solutions and measured with the chemosensor NBD-PMA with a UV-2450 UV-vis spectrophotometer. The recovery results are presented in Table 1 and are satisfied. Moreover, we also use the test strip to identify the different concentration of Pd²⁺ cations in different water samples. The results showed that the test strips exhibited a remarkable colour response to 10 ppm Pd²⁺ cations compared with the blank strip (Fig. S2, ESI†).

Table 1 Determination of Pd²⁺ cations in samples (n = 5)

Sample	Pd^2+ added (μM)		Pd^2+ found (μM)	Recovery (%)	RSD (%)
Washing water		10.00	10.02	100.2	1.42
River water		10.00	9.60	96.0	1.04
Tap water		10.00	9.80	98.0	1.30

---

Conclusions

We have developed a colorimetric and fluorescent chemosensor NBD-PMA for selective and sensitive Pd²⁺ cation detection. The chemosensor NBD-PMA selectively detected Pd²⁺ cations through a colour change from bright yellow to pink visible with the naked eye. NBD-PMA can detect trace amounts of Pd²⁺ cations with excellent selectivity in the presence of potential competitors such as Hg²⁺, Co²⁺, Cu²⁺, Ag⁺ and Pt²⁺ cations. Therefore, this chemosensor can be applied to detect Pd²⁺ cations in real water samples and in living cells, which makes it a real useful tool for Pd²⁺ cation detection.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (21502056) and innovation research program for undergraduate in South China University of Technology (X. L. W.).

Notes and references

1. J. S. Kim and D. T. Quang, Chem. Rev., 2007, 107, 3780–3799.
2. S. E. Angell, C. W. Rogers, Y. Zhang, M. O. Wolf and W. E. Jones, Coord. Chem. Rev., 2006, 250, 1829–1841.
3. C. W. Rogers and M. O. Wolf, Coord. Chem. Rev., 2002, 233–234, 341–350.
4. M. F. Yardim, T. Budinova, E. Ekinci, N. Petrov, M. Razvigorova and V. Minkova, Chemosphere, 2003, 52, 835–841.
5. D. W. Boening, Chemosphere, 2000, 40, 1335–1351.
6. G. McKay and M. J. Bino, Environ. Pollut., 1990, 66, 33–53.
7. C. Locatelli, D. Melucci and G. Torsi, Anal. Bioanal. Chem., 2005, 382, 1567–1573.
8. L. Canovese, G. Chessa, F. Visentin and P. Uguagliati, Coord. Chem. Rev., 2004, 248, 945–954.
9. B. Pitteri, L. Canovese, G. Chessa, G. Marangoni and P. Uguagliati, Polyhedron, 1992, 11, 2363–2373.
10. C. D. Spicer, T. Triemer and B. G. Davis, J. Am. Chem. Soc., 2011, 134, 800–803.
11. D. G. Cho and J. L. Sessler, Chem. Soc. Rev., 2009, 38, 1647–1662.
12. T. Z. Liu, S. D. Lee and R. S. Bhatnagar, Toxicol. Lett., 1979, 4, 469–473.
13. Z. Y. Xu, J. Li, S. Guan, L. Zhang and C. Z. Dong, Spectrochim. Acta, Part A, 2015, 148, 7–11.
14. C. Y. Zhang, L. Wei, C. Wei, J. Zhang, R. Y. Wang, Z. Xi and L. Yi, Chem. Commun., 2015, 51, 7505–7508.
15. D. Lee, G. Kim, J. Yin and J. Yoon, Chem. Commun., 2015, 51, 6518–6520.
16. Y. Fu, L. Mu, X. Zeng, J. L. Zhao, C. Redshaw, X. L. Ni and T. Yamato, Dalton Trans., 2013, 42, 3552–3560.
17. W. Jiang, Q. Q. Fu, H. Y. Fan, J. Ho and W. Wang, Angew. Chem., Int. Ed., 2007, 46, 8445–8448.
18. G. Ambrosi, S. Ciattini, M. Formica, V. Fusi, L. Giorgi, E. Macedi, M. Micheloni, P. Paoli, P. Rossi and G. Zappia, Chem. Commun., 2009, 7039–7041.
19. F. Qian, C. L. Zhang, Y. M. Zhang, W. J. He, X. Gao, P. Hu and Z. J. Guo, J. Am. Chem. Soc., 2009, 131, 1460–1468.
20. S. Banthia and A. Samanta, New J. Chem., 2005, 29, 1007–1010.
21. W. Jiang, Q. Q. Fu, H. Y. Fan and W. Wang, Chem. Commun., 2008, 259–261.
22. J. H. Kim, J. Y. Noh, I. H. Hwang, J. J. Lee and C. Kim, Tetrahedron Lett., 2013, 54, 4001–4005.
23. Q. Jiang, Z. J. Guo, Y. Zhao, F. Y. Wang and L. Q. Mao, Analyst, 2015, 140, 197–203.
24. M. Vasilescu, A. Latus, R. D. Baratoiu, M. Bem and T. Constantinescu, Rev. Roum. Chim., 2011, 56, 239–245.
25. Z. C. Xu, G. H. Kim, S. J. Han, M. J. Jou, C. M. Lee, I. Shin and J. Yoon, Tetrahedron, 2009, 65, 2307–2312.
26. M. E. Jung, T. A. Dong and X. L. Cai, Tetrahedron Lett., 2011, 52, 2533–2535.
27. K. J. Akerman, C. Venter, L. A. Hunter and M. P. Akerman, J. Mol. Struct., 2015, 1091, 74–80.
28. S. Dayan, A. Çetin, N. B. Arslan, N. K. Özpozan, N. Özdemir and O. Dayan, Polyhedron, 2015, 85, 748–753.
29. C. E. Garrett and K. Prasad, Adv. Synth. Catal., 2004, 346, 889–900.

---

Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of NBD-PMA, experimental procedures, and supplementary spectra and graphs. See DOI: 10.1039/c6ra06226a

---