A simple and effective fluorescent probe based on rhodamine B for determining Pd²⁺ ions in aqueous solution

Abstract

A new simple-structure fluorescent probe RL based on the special affinity of Pd²⁺ to double bond ligands has been designed and synthesized. RL shows high selectivity and sensitivity for Pd²⁺ with a detection limit of 21.3 nM (2.3 ppb) in the presence of other competing metal ions in aqueous media. Furthermore, RL has potential applications in Pd²⁺ analysis in natural water and in a reactor involving a Pd²⁺catalytic reaction.

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Introduction

Recently, palladium has been widely used in various materials owing to its specific physical and chemical properties. For example, it is widely used as a catalyst in synthesis because of its excellent catalytic ability.¹ Pd-catalyzed reactions are efficient and are used in pharmacy^2–5 and various materials. However, the use of palladium in the pharmaceutical industry could result in high levels of residual palladium in the final organic phase following the syntheses.⁴ Palladium can be transported into biological materials and enriched by the food chain to cause a potential health hazard. For example, it can coordinate with DNA, thiol-containing amino acids, proteins, and vitamin B6 and disturb several cellular processes.⁶ Governmental restrictions on the levels of residual heavy metals in end products are very strict, its threshold for palladium in drugs is 5–10 ppm.⁷ Thus, great attention has been focused on the residual palladium in the final product⁸ and effective methods to detect trace palladium are necessary.

Traditional analytical methods (atomic absorption spectrometry, plasma emission spectroscopy, solid phase micro extraction-high-performance liquid chromatography, and X-ray fluorescence)^9–11 for the quantification of Pd usually suffer from the high cost of the instrumentation and their complex operation. Thus, fluorescent methods garner the attention of several researchers¹² because of the easy preparation and convenient operation.

As a fluorescence quencher, Pd²⁺ could be detected by probes through fluorescence quenching.¹³ However, a fluorescence enhanced probe would be more efficient for Pd²⁺ detection. Recently, Holdt et al. designed a few fluorescence enhanced probes for Pd²⁺ by using a new concept of photoinduced electron transfer (PET) fluoroionophores in fluorophore- spacer-receptor systems.¹⁴ Koide et al. reported a turn-on fluorescent sensing system for palladium based on the catalytic Tsuji–Trost allylic oxidative insertion reaction of fluorescein derivatives.¹⁵ Nearly all published ratiometric fluorescence probes for palladium are based on the Pd-catalyzed cleavage reactions and can recognize palladium with high sensitivity and excellent selectivity.¹⁶ Unfortunately, the reaction conditions are relatively strict and have a long response time.^16b,e The well-known fluorophore rhodamine dye is unique for the design of turn-on fluorescence probe. Rhodamine dyes has been utilized to construct chemosensors for metal ions.¹⁷ Peng and co-workers developed a novel rhodamine chemosensor for palladium species by taking advantage of both the π-affinity of Pd to allyl groups and the well-known ring-opening process of the spirolactam of rhodamine B.¹⁸ Herein, we designed and synthesized a simple structure palladium specific probe RL, which can show excellent selectivity and high sensitivity for Pd²⁺ over other metal ions and common anions. The response of the probe to palladium was based on a coordination reaction. A 110-fold increase of fluorescent intensity was recorded after addition of Pd²⁺. Promisingly, RL could be a potential fluorescent probe for quantitative determination of palladium in environmental water samples and in reactors.

Results and discussion

RL was synthesized via a three-step procedure (Scheme 1). Intermediate compounds 1, and 2 were synthesized from rhodamine B according to the literature.¹⁹ Reaction of 2 with sodium hydride at 0 °C for 30 min and then at room temperature for 24 h afforded probe RL in 53% yield. RL was characterized by IR, ¹H NMR, ¹³C NMR and HRMS (ESI†).

Scheme 1 The synthesis of fluorescent probe RL.

At first, we assessed the spectroscopic properties of probe RL. The spectroscopic properties of RL were measured in EtOH/PBS solution (3 : 7,v/v, pH = 7.2) at a micromolar concentration. The absorption spectrum of probe RL (10 μM) showed an absorption maximum at 560 nm (Fig. S1†). The addition of only Pd²⁺ (0–5 equiv.) under the above conditions resulted in the enhancement of absorbance centered at 560 nm. This is ascribed to the interaction of Pd²⁺ with the probe.

Fig. 1 shows the fluorescence spectra of probe RL upon titration with PdCl₂. The fluorescence spectrum of probe RL (10 μM) in the absence of palladium in EtOH/PBS solution (3 : 7, v/v, pH = 7.2) exhibits very weak fluorescence intensity at 585 nm. With continuous addition of PdCl₂, the fluorescence intensity at 585 nm increased smoothly and gradually peaked, which indicated the formation of a new complex between RL and Pd²⁺. Over 110-fold fluorescence enhancement was observed when the concentration of Pd²⁺ ions was 35 μM. The fluorescence spectra exhibited no significant changes above 35 μM. The enhancement of the fluorescence intensity at 585 nm followed the sigmoidal curves and the association constant calculated for the metal complexation was 9.13 × 10⁴ M⁻¹ (Fig. S2†). The fluorescence intensities are linearly proportional to the amount of Pd²⁺ in the range of 0–8 μM (Fig. 1, inset). The detection limit is 21.3 nM (2.3 ppb), which is more sensitive compared with those in the literature (Table S1†). It also means that RL can be used for Pd-polluted water analysis.

Fig. 1 Fluorescence titration of RL (10 μM) upon addition of Pd²⁺ (from bottom to top: 0, 2, 4, 6, 8, 10, 15, 20, 25, 30, 35, 40, 45, 50 μM) in EtOH/PBS (3 : 7, v/v, pH = 7.2) at room temperature. Inset: the linear relationship between RL and Pd²⁺; λ_ex = 555 nm.

To better understand the coordination details between RL and Pd²⁺ ions, the coordination reaction between RL and Pd²⁺ ions was further explored by the titration of the complex with sulfide (S²⁻) (Fig. S3†). The addition of Na₂S to the mixture of RL and Pd²⁺ resulted in diminution of the fluorescence intensity at 585 nm, which indicated the discoordination of Pd²⁺ and the regeneration of free probe RL.

The Job plot shows 1 : 1 stoichiometry between the probe and Pd²⁺, which indicates the formation of a 1 : 1 complex (Fig. 2). Another evidence of the binding mode comes from HRMS (Fig. S8†)of the complex of RL and Pd²⁺.

Fig. 2 Job plot of probe RL responding Pd²⁺. The total concentration of RL and Pd²⁺ is 20 μM.

We think the vinyl-amine portion might be the key factor in the Pd²⁺-sensing process. RL displayed a good response to Pd²⁺ which might be due to the interaction between the vinyl group and Pd²⁺, because vinyl group is similar to allyl group which is well-known group to bind Pd²⁺.^18a,b Bearing these in mind, the proposed mechanism (Scheme 2) involved the opening of the spirolactam ring by Pd²⁺ complexation.

Scheme 2 Proposed mechanism for RL–Pd²⁺ complexation.

The time course of changes in the fluorescence intensity of the complexation between RL and Pd²⁺ ion was evaluated. From the plot of the time-dependent fluorescence spectra, we observed that fluorescence intensity reached the relative saturation point in 60 min (Fig. S4†). Therefore, other fluorescent measurements were recorded after a 2 h incubation period.

The high selectivity towards the analyte over the other competitive species is an important feature of the probe. The common cations were used to conduct selectivity experiment of RL (10 μM) in EtOH/PBS solution. Only Pd²⁺ ions induced a prominent fluorescence enhancement, other transition and alkali metal ions had no or little effect on the emission of probe RL (Fig. 3).

Fig. 3 Fluorescence responses of RL (10 μM) toward common metal ions (20 μM) in EtOH/PBS (3 : 7, v/v, pH = 7.2) at room temperature. Bars represent the fluorescence intensity at 585 nm. 1: K⁺, 2: Ca²⁺, 3: Na⁺, 4: Mg²⁺, 5: Al³⁺, 6: Zn²⁺, 7: Fe³⁺, 8: Pb²⁺, 9: Co²⁺, 10: Ni²⁺, 11: Cu²⁺, 12: Hg²⁺, 13: Ag⁺, 14: Au³⁺, 15: Ru²⁺, 16: Rh²⁺, 17: Pd²⁺; λ_ex = 555 nm.

To further explore the selectivity of RL for Pd²⁺, interference experiments involving the effects of other metal ions and anions (Fig. 4) in the identification of Pd²⁺ ions were also performed. Compared with the intensity of RL response toward Pd²⁺ in the absence of any interference ions, the metal ions caused only tiny variations in the fluorescence intensity except Hg²⁺, and none of the anions caused any significant quenching of the fluorescence. Thus, these free cations and anions would have very little influence towards RL and do not hamper the fluorogenic detection of Pd²⁺.

Fig. 4 Fluorescence intensity of RL (10 μM) after addition of Pd²⁺ (20 μM) in the presence of other metal ions (20 μM) and common anions (20 μM) in EtOH/PBS (3 : 7, v/v, pH = 7.2). (a) 1: Pd²⁺, 2: Zn²⁺, 3: Na⁺, 4: Mg²⁺, 5: Pb²⁺, 6: Ni²⁺, 7: K⁺, 8: Hg²⁺, 9: Cu²⁺, 10: Co²⁺, 11: Fe³⁺, 12: Cr³⁺, 13: Cd²⁺, 14: Ca²⁺, 15: Ba²⁺, 16: Al³⁺, 17: Ag⁺, 18: Au³⁺, 19: Rh²⁺, 20: Ru²⁺; (b) 1: Pd²⁺, 2: Cl⁻, 3: Br⁻, 4: CH₃COO⁻, 5: HPO₄²⁻, 6: HSO₄⁻, 7: SO₄²⁻, 8: CO₃²⁻.

The spirolactam ring of the rhodamine moiety in RL is susceptible to the changes in pH. It always opens in acidic media and indicates the fluorescence of rhodamine. Therefore, checking the fluorescence properties of RL in solutions with different pH values is very important. The acid–base titration experiments were carried out by adjusting the pH with an aqueous solution of NaOH and HCl. It can be clearly seen that the fluorescence intensity of probe RL did not emit any characteristic fluorescence in the pH range 7–12 (Fig. 5). RL–Pd²⁺ complex can emit strong fluorescence at near neutral pH. Thus, we chose the near-neutral pH range (pH = 7.2) as a test system in the other assays.

Fig. 5 pH-dependent variation in fluorescence intensity at 585 nm of RL (10 μM) and RL/Pd²⁺ (20 μM) in EtOH/PBS (3 : 7, v/v, pH = 7.2), λ_ex = 555 nm.

According to the reported test procedures for water contamination^18c and residual Pd detection in a reactor,^18a we also performed proof-of-concept experiments to demonstrate the potential applications of this probe for Pd analysis in environmental water and residual Pd in a reactor. Firstly, experiments were performed on Yellow River and tap water for Pd²⁺ detection. The water samples were collected and filtered, prepared as EtOH–H₂O solution (EtOH : H₂O = 3 : 7), and then spiked with different amounts of PdCl₂ (0–12 μM). After the addition of probe RL (10 μM), PdCl₂ could be readily detected with a large fluorescence enhancement which was almost linearly dependent on the concentrations of palladium ion (Fig. 6). The results demonstrated that our detection system could detect trace amounts of Pd²⁺ ions in environmental samples.

Fig. 6 Proof-of-concept experiment with probe RL (10 μM) for Pd²⁺ detection in EtOH – tap water (■) and EtOH – Yellow river water (●) solution at μM levels. λ_ex = 555 nm.

Then, we carried out an experiment to monitor Pd in a reactor using probe RL. A THF solution of PdCl₂ was stirred in four flasks for 1 h at room temperature. After the solution was poured out, the four flasks were treated with different washing procedures, respectively (no wash; brushing with detergent only; brushing with detergent and washing with water; brushing with detergent, washing with water and acetone). RL solutions were added to the ‘cleaned’ flasks and stirred overnight. As shown in Fig. 7, the different fluorescent responses can also be detected, which demonstrated that the RL sensing method can be used for the Pd analysis in a reactor.

Fig. 7 Probe RL solutions exposed to PdCl₂ with different wash procedures (no wash (1); brushing with detergent only (2); brushing with detergent and washing with water (3); brushing with detergent, washing with water and acetone (4)).

Experimental

Materials and reagents

All reagents and solvents were purchased from commercial sources and used without further purification. The solutions of metal ions were prepared from nitrate salts which were dissolved in distilled water. Distilled water was used throughout the process of absorption and fluorescence determination. All samples were prepared at room temperature, shaken for 10 s and stood for 2 h before measurement. Dilute hydrochloric acid or sodium hydroxide was used for tuning pH values. All solvents used in spectroscopic analysis are spectroscopic grade.

Apparatus

Thin-layer chromatography (TLC) was conducted on silica gel 60F₂₅₄ plates (Merck KGaA). Melting points were determined on an XD-4 digital micro melting point apparatus. ¹H and ¹³C NMRspectra were recorded on a Bruker Advance 300 spectrometer, using d₆-DMSO as solvent and tetramethylsilane (TMS) as internal standard. IR spectra were recorded with an IR spectrophotometer VERTEX70 FT-IR (Bruker Optics). High-resolution mass spectrometry (HRMS) spectra were recorded on a Q-TOF6510 spectrograph (Agilent). Fluorescent measurements were recorded on a Perkin Elmer LS-55 luminescence spectrophotometer and UV-vis spectra were recorded on a U-4100 UV-Vis-NIR Spectrometer (Hitachi). The pH measurements were performed on aPHS-3C digital pH-meter (YouKe, Shanghai, China).

Preparation of the probe (RL)

Intermediate compounds 1, 2 were synthesized from rhodamine B according to the literature. Sodium hydride (125 mg of 60% suspension in mineral oil, 3.00 mmol) was dispersed in 10 mL freshly dry tetrahydrofuran. To this slurry, 2 (203 mg, 0.37 mmol, dissolved in 5 mL tetrahydrofuran) was added dropwise at 0 °C over a period of 30 min. The reaction mixture was stirred at room temperature for 24 h. After removal of the solvent in a rotary evaporator under reduced pressure, the residue was purified by flash column chromatography on silica gel (5 : 1 petroleum–ethyl acetate) to afford a white solid (92 mg, 53%). m.p. 196–198 °C; FT-IR (KBr): 3488, 2965, 2925, 2900, 1697, 1614, 1516, 1328, 1218, 1120 cm⁻¹.¹H NMR (300 MHz, d₆-DMSO), δ: 1.08 (t, J = 6.9 Hz, 12H), 3.31 (q, J = 6.9 Hz 8H), 4.21 (d, J = 9.9 Hz, 1H), 4.25 (d, J = 13.5 Hz, 1H), 6.33 (b, 4H), 6.38 (s, 2H), 6.81 (dd, J = 9.9, 13.5 Hz 1H), 7.01 (dd, J = 6.0 Hz 1H), 7.52 (m, 2H),7.85 (q, J = 6.6, Hz, 1H); ¹³C NMR (75 MHz, d₆-DMSO), δ: 12.88 (4C), 44.10 (4C), 63.80, 97.37, 97.67 (2C), 105.72 (2C), 108.63 (2C),123.36, 124.07, 126.43, 128.06 (2C), 128.38, 128.98, 134.27, 148.89 (2C), 152.50 (2C), 154.66, 166.35 ppm; TOF-HRMS: C₃₀H₃₃N₃O₂ [RL + H]⁺: 468.2651, found 468.2585.

Preparation of the stock solution

The stock solution of RL was prepared in EtOH. The stock solution of PdCl₂ was prepared in 3 : 1 (v/v) MeOH/brine. Parent stock solutions (0.01 M) of the nitrate salts of K⁺, Ca²⁺, Na⁺, Mg²⁺, Al³⁺, Zn²⁺, Fe³⁺, Pb²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Cr³⁺, Cd²⁺ and Ba²⁺ were prepared in distilled water. The stock solution of HgCl₂ (0.01 M) was prepared in distilled water. The stock solution of AuCl₃, dirhodiumtetraacetate, and dichloro(p-cymene)ruthenium(II) dimer (0.001 M) were prepared in 1 : 1 (v/v) MeOH/H₂O. Test solutions were prepared by placing 100 μL of the RL stock solution to 10 mL with 30% EtOH in PBS (10 mM, pH = 7.2) solution.

Conclusions

In summary, we have rationally developed a simple-structure and sensitive fluorescent probe for the palladium species based on the coordination mechanism under mild conditions. The probe shows high selectivity and sensitivity for Pd²⁺ with a detection limit of 21.3 nM (2.3 ppb) in the presence of other competing metal ions in aqueous media. Proof-of-concept experiments have also displayed its potential application for trace-Pd analysis in water. Additionally, it provides a convenient method for residual palladium detection in reactors.

Acknowledgements

This study was supported by 973 Program (2010CB933504).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Supplementary figures and characterization of the compound. See DOI: 10.1039/c4ra12649a

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