Synthesis and characterization on novel fluorescent sensors for Pd²⁺/Pd⁰ with high selectivity

Abstract

Novel fluorescent sensors L1–L3 based on triaryl 1H-imidazo[4,5-b]pyrazine have been synthesized through a new and simple route from inexpensive and readily available materials in one pot. They exhibited high selectivity for palladium detection (oxidation states of 0 and +2) based on the fluorescence quenching effect.

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Introduction

With the development of industrial, agricultural, and urban modernization in the last several decades, the pollution of heavy metals (HMs), used widely in agriculture, chemical and industrial processes, is causing serious threats to living organisms.¹ As one of heavy metals (HMs), palladium plays a key role in various fields, such as electrical and electronic industries, dental crowns, automobiles, jewellery and catalysts.² In particular, palladium-catalyzed organic reactions, namely Suzuki–Miyaura, Heck, Sonogashira and Buchwald–Hartwig, are increasingly important because of their advantages in the synthesis of complex molecules.³ However, with the wide use of the palladium, a high level of residual palladium is often found in the final product.⁴ It is well known that palladium can bind to thiol-containing amino acids, proteins, DNA and other biomolecules and thereby may seriously influence our health in an adverse way.⁵ As a consequence, much attention has been paid to the development of efficient methods for detecting palladium species (mainly Pd⁰ and Pd²⁺).

Conventional methods for the selective detection of palladium include atomic absorption spectroscopy (AAS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), solid-phase micro extraction-high performance liquid chromatography (SPME-HPLC), X-ray fluorescence (XRF), etc.⁶ In spite of their rapid and extremely sensitive analysis, they require complicated sample preparation procedures, rigorous experimental conditions, expensive and sophisticated instrumentation and experienced operators.⁷ Thus, considerable attention has been paid to fluorescent methods for the detection of palladium because of their apparent advantages, such as low cost, operational simplicity, high sensitivity and selectivity, over other methods.⁸ Meanwhile, the most desirable fluorescent sensors should possess sensitive manner by dramatic change in emission color and/or intensity.

The mechanisms for detection methods are mostly based on the chemical reactions between Pd and the detector (cycle opening of rhodamine derivatives or cleavage reactions etc.) or complexation of Pd with the detector.^8–12 As a strong fluorescence quencher, Pd²⁺ ions or Pd⁰ could be detected through fluorescence quenching.^10,11 But less molecules were proved to be highly selective sensors (see ESI† for details).¹⁰ Usually, fluorescent sensing systems for Pd²⁺ species work through the coordination of sulfur or alkynyl groups to palladium.^10,12 However, other competitive ions such as Hg²⁺ and Pt²⁺ would decrease the efficiency of detection.^8p,10b Therefore, it's meaningful to discover new fluorescent sensing systems with novel structure and high anti-interference. Although imidazole skeleton has been used for the detection of series of ions,¹³ no report, to the best of our knowledge, has been covered for the detection of Pd²⁺/Pd⁰ with imidazo[4,5-b]pyrazine derivatives.

Herein, we devoted our attention to the construction of triaryl 1H-imidazo[4,5-b]pyrazine derivatives and presented novel compounds L1–L3 (L1 was determined by X-ray diffraction) as new fluorescent sensors that are capable of reporting Pd²⁺/Pd⁰ with distinct fluorescence quenching. The detailed synthetic method for the preparation of L1–L3 is shown in Scheme 1. The reaction of benzaldehydes, 2-cyanopyrimidine and ammonium acetate in the solution of DMF at 110 °C for 4 h give the sensors. (And a plausible mechanism has been provided in ESI.†)

Scheme 1 Synthesis of L1–L3 and the crystal structure of L1.

Results and discussion

The spectroscopic properties of sensor L1 were measured in different dilute solutions (10⁻⁵ M). As shown in Fig. 1, L1 had an absorption wavelength with a maximum at about 360 nm and had slight changes in different solutions. Moreover, the fluorescence spectrum shows that L1 can emit intense fluorescence at 410 nm in series of solutions and the fluorescence quantum yield (QY) of L1 in CH₃CN was calculated to be 0.23, with tryptophan as the standard.¹⁴

Fig. 1 Absorption and fluorescence spectra of L1 (10⁻⁵ M) in different solutions.

To validate the selectivity of L1 in practice, the absorption and fluorescence spectra of L1 in the presence of various metal ions have been studied. Different metal species had little effect on the absorption of sensor L1 (Fig. S1, ESI†). Fluorescence spectral data revealed that L1 selectively responded to Pd²⁺ resulting in a serious fluorescence quenching (Fig. 2A), which could be easily observed by the naked eye (Fig. 2B), and the fluorescence quantum yield (QY) decreased to 0.05. Among other metal sources examined, little emission intensifying was observed upon addition of Cu²⁺, Cd²⁺, Ni²⁺, Co²⁺, Ba²⁺, Zn²⁺, Sn²⁺, Ca²⁺, Mg²⁺, K⁺, Na⁺, Li⁺, Pb²⁺, Fe³⁺, Ru³⁺, Hg²⁺ and Pt²⁺ (Fig. 2A). Thus, according to the obvious spectroscopy changes, the designed L1 as the fluorescence sensor can detect Pd²⁺ with high selectivity.

Fig. 2 (A) The fluorescence spectra of L1 (10⁻⁵ M) in the presence of various metal ions (5 equiv.) in CH₃CN. Excitation wavelength was 360 nm. Inset: degree of fluorescence quenching of L1 upon the addition of different metal ions. (B) Photographs of L1 in the presence of difference metal ions in CH₃CN, under 365 nm UV light.

Fig. 3A displays the changes to the emission spectra of L1 in the presence of different concentrations of Pd²⁺ and it can be seen that the fluorescence intensity of the emission maximum at 410 nm decreased linearly with the increase of Pd²⁺ concentration. The Job's plot¹⁵ (Fig. 3B) revealed that L1 formed a 1 : 1 stoichiometrical complex with Pd²⁺.

Fig. 3 (A) Fluorescent spectra (L1 = 10⁻⁵ M in repeat units, Pd²⁺ = 0, 0.2, 0.4, 0.6, 0.8, 1, 2, 3, 4, 5 × 10⁻⁵ M) and (B) the Job's plot for determining the stoichiometry of L1 and Pd²⁺. Excitation wavelength was 360 nm.

As the absorption spectra of L1 displayed no obvious red-shift after the addition of Pd²⁺ ions which might indicate that the mode of linkage between the ligand and the Pd²⁺ is mostly intrachain instead of interchain.^9c,10d The binding mode of L1–Pd²⁺ is illustrated in Scheme 2. The solution of L1 displayed an intense fluorescence before the addition of Pd²⁺ ion, and owing to the charge transfer (CT) from the sensor to the metal after the complexation with Pd²⁺, a serious fluorescence quenching occurred.

Scheme 2 Proposed Pd²⁺ sensing process.

To study the selectivity of sensor L1 toward other palladium species, a series of palladium complexes with different initial oxidation states or anions such as Pd(dba)₂, PdCl₂(dppf) and Pd(OAc)₂, were introduced and examined (Fig. 4). Interestingly, fluorescence of L1 could be similarly quenched by these palladium species, indicating that the response process depends on palladium metal itself, while not the oxidation state, ligands and anions.

Fig. 4 Comparison of the fluorescence change of L1 (10⁻⁵ M) in the presence of different palladium species (5 equiv.) in CH₃CN. Excitation wavelength was 360 nm (A: L1 only; B: L1 + PdCl₂; C: L1 + Pd(dba)₂; D: L1 + PdCl₂(dppf); E: L1 + Pd(OAc)₂).

As is well known, an important property of a sensor is high selectivity toward the analyte in the presence of other competitive species. To further investigate the selectivity of sensor L1 for Pd²⁺ ion, competition experiments were performed in the presence of Pd²⁺ (3 equiv.) mixed with each of the guest cations (10 equiv.) namely Cu²⁺, Cd²⁺, Ni²⁺, Co²⁺, Ba²⁺, Zn²⁺, Sn²⁺, Ca²⁺, Mg²⁺, K⁺, Na⁺, Li⁺, Pb²⁺, Fe³⁺, Ru³⁺, Hg²⁺ and Pt²⁺, respectively. According to Fig. 2A and 5, different degree of emission intensification was observed upon addition of miscellaneous competitive metal ions while the fluorescence exhibited serious quenching in the presence of Pd²⁺ for each system. This result indicated that sensor L1 displayed a high selectivity and anti-interference toward palladium species when coexisting with other competitive metal ions.

Fig. 5 (A) Fluorescence intensities of L1 (10⁻⁵ M) in the presence of various metal ions (10 equiv.) and Pd²⁺ (3 equiv.) in CH₃CN and described by (B) the degree of fluorescence quenching. The black bars represent the fluorescence emission intensities of L1 only or L1 in the presence of different competing ions, the red bars represent the fluorescence emission intensities of L1 in the presence of different competing ions and Pd²⁺ (excitation wavelength was 360 nm). 1: No metal ion; 2: Ba²⁺; 3: Ca²⁺; 4: Cd²⁺; 5: Co²⁺; 6: Cu²⁺; 7: Fe³⁺; 8: K⁺; 9: Li⁺; 10: Mg²⁺; 11: Na⁺; 12: Ni²⁺; 13: Pb²⁺; 14: Ru³⁺; 15: Sn²⁺; 16: Zn²⁺; 17: Hg²⁺; 18: Pt²⁺.

In addition, the effect of pH on the fluorescence of sensor L1 in the absence and presence of Pd²⁺ ion was then evaluated in the pH range from 3.1 to 10.2. As shown in Fig. 6, there was little effect on the fluorescence intensity of sensor L1 under acidic or neutral conditions (pH ≤ 7.0). However, the fluorescence intensity of L1 was rapidly decreased when the conditions turned basic. To our delight, the fluorescence of L1 exhibited serious quenching in the presence of Pd²⁺ ion under different pH value conditions. The results indicate that sensor L1 could successfully allow Pd²⁺ detection in a wide pH range.

Fig. 6 The fluorescence spectra of L1 (10⁻⁵ M) and L1 in the presence of 5 equiv. of Pd²⁺ ion in H₂O under different pH conditions. Excitation wavelength was 360 nm.

As the analogs of sensor L1, L2 and L3 were also synthesized (Scheme 1) to examine the influence of molecule structure on the detection of Pd²⁺. We repeated the detecting experiments under the same conditions (Fig. 7). The wavelength of both absorption and fluorescence of L3 (376 nm/445 nm) had an obvious red-shift due to the better electron donating property of methoxyl (Fig. S2 and S3, ESI†). It's quite clear that both L2 and L3 exhibited fluorescence quenching in the presence of Pd²⁺ (Fig. 7A and B). Although the degree of quenching was not obvious for L3 compared with the other two sensors, a distinction could still be recognized relative to other competitive ions. At the same time, different initial oxidation states of palladium species were examined (Fig. 7C and D). According to the fluorescence intensity, L2 and L3 could respond to different Pd species as well. The results above revealed that both L2 and L3 are also nice fluorescence sensors for Pd species detection with well selectivity.

Fig. 7 The fluorescence (A and B) spectra of sensor L2 and L3 (10⁻⁵ M) in the presence of various metal ions (5 equiv.) in CH₃CN. Comparison of the fluorescence changes of sensors L2 (C) and L3 (D) (10⁻⁵ M) in the presence of different palladium species (5 equiv.) in CH₃CN (1: L2 or L3 only; 2: L2 or L3 + PdCl₂; 3: L2 or L3 + Pd(dba)₂; 4: L2 or L3 + Pd(OAc)₂).

Conclusions

In summary, a series of novel fluorescent sensors based on triaryl 1H-imidazo[4,5-b]pyrazine for Pd²⁺/Pd⁰ was designed and synthesized. They can be obtained through a new and simple method from inexpensive materials in one pot. The new fluorescent sensor L1 displayed a high selectivity and anti-interference toward palladium species detection in CH₃CN solution. The optical spectra had been investigated and the fluorescence of L1 exhibited serious quenching in the presence of palladium species, QY changing from 0.23 to 0.05, while different degree of emission intensifying was observed upon addition of each competitive metal ions. The 1 : 1 binding stoichiometry between the sensor and Pd²⁺ had been elucidated through the Job's plot curve. Sensors L2 and L3 were obtained and exhibited similar properties to L1 on the detection of Pd species. For the high selectivity and anti-interference toward palladium species detection, sensors L1–L3 were expected for the potential applications in human health and environmental protection.

Acknowledgements

We gratefully acknowledge the Natural Science Foundation of China (no. 21372174), the Young National Natural Science Foundation of China (no. 21202113), the Ph.D. Programs Foundation of Ministry of Education of China (2013201130004), PAPD.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1028080. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra01343g

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