Triphenylphosphine-assisted highly sensitive fluorescent chemosensor for ratiometric detection of palladium in solution and living cells

Abstract

Accurate and sensitive detection of palladium (Pd) is essential for both environmental and human health applications. This paper describes a triphenylphosphine (PPh₃)-assisted highly sensitive fluorescent chemosensor for ratiometric detection of Pd²⁺ concentrations up to 300 nM with an LOD of 1 nM. During the detection, PPh₃ plays a crucial role in improving the sensitivity of this chemosensor. The chemosensor also has suitable water-solubility which allows detection of Pd²⁺ in solution. Our studies show that it exhibits excellent selectivity toward Pd²⁺ and undergoes a color change from colorless to yellow to allow visual detection of Pd²⁺. Besides detection in solution, this chemosensor also shows great potential as an imaging reagent to detect Pd²⁺ as low as 0.03 ppm in diverse cells.

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1. Introduction

Palladium (Pd) detection has received great attention in chemical and biological studies due to the fact that it is a critical material in a variety of industrial applications; however, it brings potential harm to the environment and human health. In industry, Pd-derived compounds form efficient universal catalysts,^1–3 which are used on a large scale to promote numerous chemical conversions in the production of fine chemicals, synthetic drugs and polymers.^4,5 Pd metal is an indispensable material in many areas such as automobile catalytic converters, electronics technology, hydrogen storage, dentistry, jewelry and photography.^6,7 However, Pd-containing substances released from industry can cause serious problems for environment and human health.⁸ Current studies indicate Pd can be absorbed by biological materials, and accumulated in the food chain. Once human eats food containing Pd, the heavy metal enters the human body and binds to thiol-containing amino acids, proteins, DNA or other macromolecules.^9,10 The binding to these biological moieties can disturb various physiological processes inside the human body. According to the governmental restrictions, the dietary Pd intake should not exceed than 1.5–15 μg per day per person and the Pd threshold in drugs is 5–10 ppm.¹¹ In order to have systems help detecting and monitoring the Pd levels, sensitive and accurate method for Pd detection is in great need.

Known for its high sensitivity and specificity, operational simplicity and non-invasive detection, fluorescent chemosensor is particularly preferable for the quantitative or quantitative analysis for environmental and biological samples.^12–15 Using Pd's specific coordination or catalysis reactions as recognition mechanisms,¹⁶ a large number of fluorescent chemosensors have been reported to detect Pd²⁺,^17–29 which is the most abundant existing form of the palladium metal under physiological conditions or in environment. Some of these chemosensors are suitable for measuring Pd²⁺ in biological system.^30–34 However, most of these chemosensors for quantitatively measuring Pd²⁺ concentration are based on fluorescent fluctuation in a single emission channel. The drawback for utilizing single emission channel is that it can bring in uncertainties in detection due to different factors such as variability in chemosensor distribution, instability in the excitation source and effects of the microenvironment.^35–37 In contrast, ratiometric chemosensors can help eliminate some of these uncertainties by introducing the self-calibration mechanism using two emission channels. Though a small number of ratiometric fluorescent chemosensors have been developed, some of them have insufficient soluble in an aqueous medium.^38–41 Additionally, we also noticed among these studies, a high concentration (≥20 μM) of Pd²⁺ are usually used to be incubated with living cells to test the performance of these chemosensor in cellular environment.^{30,32–34,42,43} However, the results drawn from these experiments may not directly prove the chemosensor performance in the detection of biological samples containing low levels (e.g. nanomolar level) of Pd²⁺.

To address these issues, we previously designed a 4-hydroxynaphthalimide-derived ratiometric fluorescent chemodosimeter for imaging palladium in living cells,⁴³ shown in Scheme 1. Based on the excellent internal charge transfer (ICT) structure of 4-hydroxynaphthalimide fluorophore,^44,45 the fluorescence wavelength of the chemodosimeter presented a 73 nm redshift after Pd-catalyzed cleavage of propargyl ethers owing to the stronger electron-donating ability of oxygen anion.^46–48 The chemosensor displayed an appropriate solubility in PBS aqueous solution and high selectivity for Pd detection via measuring fluorescence intensity ratios between the two emission channels (500 nm/558 nm). However, even though the chemodosimeter with a detection limit of 70 nM presented excellent sensitivity for Pd detection in vitro, a high level of Pd²⁺ (40 μM) was still needed to be incubated with cells to perform intracellular imaging experiment. A number of the ratiometric chemosensors that has been reported recently also suffered from the similar problem in high level of Pd²⁺ for incubation.^{30,32–34,42,43} These studies indicated that intracellular complex testing conditions might affect the chemosensor sensitivity for Pd²⁺ detection. As a continuation of our previous work, we are dedicated to improve the chemosensor sensitivity for intracellular Pd²⁺ detection. In this work, we developed a high sensitive fluorescent chemosensors 1 for intracellular Pd²⁺ imaging based on triphenylphosphine (PPh₃)-supported Pd-catalyzed deallylation reaction (Scheme 1). For the similar chemical structures, chemosensor 1 still maintains the advantages of our previous chemodosimeter including proper aqueous solubility and dual-channel fluorescence emission. However, the detection limit in vitro decreased from 70 nM to 1 nM and the Pd²⁺ concentration used in cell incubation was reduced from 40 μM to 300 nM. The analytical performance of the two chemosensors was compared in Table 1 in ESI.† To account for these difference, we found that PPh₃ play a crucial role in improving the sensitivity of chemosensor 1 for Pd²⁺ detection in solution and living cells.

Scheme 1 Proposed recognition mechanisms of chemosensor 1 and previous chemosensor toward Pd²⁺.

2. Materials and methods

2.1 Materials

The N-butyl-4-chloro-1,8-naphthalimide was synthesized according to our previous work.⁴⁹ All other materials were purchased from commercial suppliers and used without further purification. Silica gel (200–300 mesh, Qingdao Haiyang Chemical Co.) was used for column chromatography. ¹H NMR spectra were acquired on a Bruker AMX400 spectrometer. Chemical shifts (δ) were reported in ppm relative to a Me₄Si standard. Electrospray ionization (ESI) mass spectra were measured with an LC-MS 2010A (Shimadzu) instrument. Fluorescence measurements were obtained on a Hitachi F-2500 fluorescence spectrometer with a 10 mm quartz cuvette. The pH measurements were made with a Sartorius basic pH-meter PB-10. Confocal imaging was performed in Olympus FV1000-LX81. HPLC was performed on a TechMate C18-ST (5 μM, 4.6 × 250 mm) column (TechMate Technology CO., LTD.) using an HPLC system composed of two pumps (LC-6AD, SHIMADZU) and a detector (SPD-20AV, SHIMADZU).

Scheme 2 The synthesis of chemosensor 1 and its Pd⁰-catalyzed deallylation product.

2.2 Synthetic procedures

Chemosensor 1. To a mixture of N-butyl-4-chloro-1,8-naphthalimide (863.2 mg, 3 mmol), K₂CO₃ (1.3 g, 10.0 mmol) and 18-crown-6 (30.0 mg, 0.1 mmol) in 10 mL DMF was added allyl alcohol (900.0 mg, 15 mmol). The resulting solution was heated to 100 °C for 12 h. After cooling to room temperature, the reaction mixture was dropped into 200 mL doubly distilled water. The product was exacted from water using CHCl₃ as solvent. After removal of CHCl₃, the residues were purified by silica gel column chromatography using CHCl₃/petroleum ether (v/v, 1 : 10) as eluent to afford 1 as a pale yellow solid (431 mg, 46%) ​(Scheme 2). ¹H-NMR (400 MHz, CDCl₃-d₆) δ (*10⁻⁶): 0.98 (t, J = 7.3 Hz, 3H), 1.42–1.48 (m, 2H), 1.68–1.75 (m, 2H), 4.17 (t, J = 7.5 Hz, 2H), 4.85 (d, J = 5.0 Hz, 2H), 5.42 (d, J = 10.5 Hz, 1H), 5.55 (d, J = 17.2 Hz, 1H), 6.14–6.21 (m, 1H), 7.03 (d, J = 8.2 Hz, 1H), 7.26 (s, 1H), 7.70 (t, J = 7.8 Hz, 1H), 8.53 (d, J = 8.2 Hz, 2H), 8.60 (d, J = 8.0 Hz, 2H). ¹³C-NMR (100 MHz, CDCl₃-d₆): δ (*10⁻⁶): 13.99, 20.56, 30.44, 40.25, 69.73, 106.39, 115.47, 118.85, 122.71, 123.81, 126.08, 128.77, 129.64, 131.68, 132.05, 133.40, 159.80, 164.09, 164.68. ESI-MS calcd for C₁₉H₁₉NO₃ [M + Na]⁺ 332.1, found 332.1.

The reaction product of chemosensor 1 with Pd(PPh₃)₄. To a solution of chemosensor 1 (620 mg, 2.00 mmol) in THF (20 mL) was added morpholine (195 mg, 2.20 mmol), sodium borohydride (90.8 mg, 2.40 mmol) and Pd(PPh₃)₄ (11.6 mg, 0.01 mmol) at 24 °C. After reaction for 4 h, 3 N HCl (4 mL) was added very slowly to quench the reaction. Then, the solution was extracted with EtOAc, and the organic phase was washed with brine, dried over Na₂SO₄, then filtered and the solvent was evaporated. The residue was purified by silica gel flash chromatography (CHCl₃/petroleum ether, v/v, 1 : 50) to afford compound 2 as a yellow solid (500 mg, 90%). ¹H-NMR (400 MHz, DMSO-d₆) δ (*10⁻⁶): 0.85 (t, J = 6.8 Hz, 3H), 1.23–1.39 (m, 2H), 1.56–1.64 (m, 2H), 4.03 (t, J = 7.4 Hz, 2H), 7.16 (d, J = 8.0 Hz, 1H), 7.75–7.79 (m, 1H), 8.35 (d, J = 1.2 Hz, 1H), 8.37 (d, J = 1.2 Hz, 1H), 8.48 (d, J = 7.6 Hz, 1H), 8.54 (d, J = 8.4 Hz, 1H), 11.85 (s, 1H). ¹³C-NMR (100 MHz, DMSO-d₆): δ (*10⁻⁶): 14.12, 20.25, 30.16, 110.31, 113.00, 122.16, 122.73, 125.87, 129.19, 129.51, 131.40, 133.83, 160.60, 163.35, 164.01. HRMS (ESI positive) calcd for C₁₆H₁₅NO₃ [M + H]⁺ 270.11247, found 270.11240.

2.3 Cell culture and imaging

HepG2 cells and RAW 264.7 macrophage cells (gifted from the center of cells, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences) were cultured in culture media (DMEM/F12) supplemented with 10% FBS, 1.5 g L⁻¹ sodium bicarbonate, 1% antibiotics (penicillin/streptomycin, 100 U mL⁻¹) at 37 °C under a humidified atmosphere containing 5% CO₂. Before use, HepG2 cells were seeded in a 6-well plate at a density of 10⁴ cells per well in culture media. For detecting Pd²⁺, the cells were incubated with the chemosensor 1 (5 μM) and PPh₃ (4 μM) for 15 min, washed twice with culture media, and observed under confocal fluorescence microscope. Then, the cells were incubated with Pd²⁺ (final concentration: 300 nM) for 15 min and were measured the fluorescence changes. RAW 264.7 cells were treated with the same procedures as HepG2 cells.

3. Results and discussion

3.1 Spectral response of chemosensor 1 to Pd²⁺ in PPh₃-reducing environment

First, we assessed the catalyzed deallylation reaction between chemosensor 1 and Pd²⁺ in PPh₃-reducing environment. Referring to the previous literatures,^50,51 Pd²⁺ can be reduced to Pd⁰(PPh₃)n (n = 1–4) in situ by using PPh₃ as a reducing agent and ligand, shown in Scheme 1. The generated Pd⁰(PPh₃)n catalyzes the allylic oxidative insertion to cleave the allylic C–O bond in chemosensor structures, accompanied by changes in fluorescence signal. Under simulated physiological conditions (PBS solution, 10 mM, pH 7.4), the solution containing chemosensor 1 (5 μM) and PPh₃ (4 μM) exhibited absorption and emission band centered at 375 nm and 460 nm, respectively (Fig. 1). Upon an addition of Pd²⁺ (0.5 μM), the maximum absorption peak showed a 75 nm redshift to 450 nm, with the solution color changed from colorless to yellow (Fig. 1A). Meanwhile, the maximum emission peak underwent a 100 nm redshift to 560 nm (Fig. 1B). These results confirmed that chemosensor 1 could serve as a naked-eye and ratiometric indicator for Pd²⁺ detection in solution.

Fig. 1 (A) Absorption spectra and (B) fluorescence spectra of chemosensor 1 (5 μM, containing 4 μM PPh₃) before and after addition of Pd²⁺ (0.5 μM), measured in PBS buffer (10 mM, pH 7.4). The blank line is for chemosensor 1 and the red line is for chemosensor 1 + Pd²⁺. Inserts show the changes of the color and fluorescence of the solution before and after addition of Pd²⁺. Each spectrum was acquired after Pd²⁺ addition for 1 h at room temperature. λ_ex = 410 nm.

To confirm the mechanism of chemosensor 1 in sensing Pd²⁺, the reaction process was monitored by high phase liquid chromatography (HPLC). As shown in Fig. S1,† in PBS solution (pH = 7.4), chemosensor 1 can converted into 4-hydroxy-naphthalimide in the presence of PdCl₂ and PPh₃ (Fig. S1C†). However, in THF medium (see synthetic procedures), the deallylation reaction can be hardly performed in the presence of Pd(PPh₃)₄ (Fig. S1A†). To enhance turnover, NaBH₄ was chosen as a reducing agent because of its known ability to efficiently reduce Pd²⁺ to Pd⁰.⁵⁰ The result showed that NaBH₄ significantly enhanced the Pd-catalyzed deallylation of chemosensor 1 in THF medium (Fig. S1B†).

3.2 The effect of PPh₃ on the recognition reaction

Next, we examined the effect of PPh₃ on the recognition reaction. As seen in Fig. 2, the fluorescence spectral of chemosensor 1 (5 μM) presented limited change in the presence of Pd²⁺ (0.5 μM), indicating the deallylation reaction is hardly performed under Pd²⁺ catalysis. Chemosensor 1 showed almost the same fluorescence spectra in the absence and presence of PPh₃ (4 μM), confirming that PPh₃ itself does not affect the chemosensor emission spectra. Additionally, PPh₃ and Pd(PPh₃)₄ show no fluorescence emission in detection wavelength range. The fluorescence spectral only underwent a large redshift in both presence of PPh₃ and Pd²⁺. These results revealed that PPh₃ plays a crucial role in recognition reaction. According to the previous study by Koide's group,^50,51 Pd²⁺ can be converted to Pd⁰(PPh₃)n in PPh₃-reducing environment. As a result, we achieved the same conversion in the chemosensor spectral when Pd(PPh₃)₄ was used in lieu of PdCl₂ and PPh₃ (Fig. 2). Moreover, in PPh₃-reducing environment, an optimal time for the reaction between chemosensor 1 and Pd²⁺ was between 60 to 90 min (Fig. S2†).

Fig. 2 Fluorescence response of chemosensor 1 (5 μM) toward PPh₃ (4 μM), Pd²⁺ (0.5 μM), Pd(PPh₃)₄ (0.5 μM), PPh₃ (4 μM) + Pd²⁺ (0.5 μM), respectively, and the fluorescence spectral of PPh₃ (4 μM) and Pd(PPh₃)₄ (0.5 μM) in wavelength range of 430–700 nm. All experiments were performed in PBS buffer (10 mM, pH 7.4). λ_ex = 410 nm.

3.3 Sensitivity of chemosensor 1 toward Pd²⁺

Chemosensor 1 showed high sensitivity for ratiometric detection of Pd²⁺ in PBS solution (10 mM, pH 7.4). As seen in Fig. 3, upon an increasing addition of Pd²⁺ (final concentration: 0–250 nM), the solution containing chemosensor 1 (5 μM) and PPh₃ (4 μM) presented a gradual decrease in fluorescent intensity at 460 nm, with a corresponding increase at 560 nm. In addition, a well-defined isoemission point at around 517 nm was also observed, indicating that a new species was formed. The logarithm of the fluorescence intensity ratio at two emission peaks, log(F₅₆₀/F₄₆₀), changed from −1.1 to 0.31 after addition of Pd²⁺ (250 nM). There was a good linearity (R² = 0.990) between log(F₅₆₀/F₄₆₀) and Pd²⁺ concentrations in the range of 20–250 nM (Fig. 2). The detection limit of chemosensor 1 is 1 nM (see ESI†), to our knowledge, this value is the lowest in reported ratiometric chemosensors for measuring Pd²⁺.

Fig. 3 Fluorescence responses of chemosensor 1 (5 μM, containing 4 μM PPh₃) toward Pd²⁺ (final concentration: 0, 4, 8, 10, 15, 20, 25, 30, 40, 50, 60, 80, 100, 150, 200, 250 nM). Insert: the linear relationship between log(F₅₆₀/F₄₆₀) and Pd²⁺ concentrations. All experiments were performed after reacting with metal ions for 1 h in PBS solution (10 mM, pH = 7.4). λ_ex = 410 nm.

3.4 Selectivity of chemosensor 1 toward Pd²⁺

The selectivity of chemosensor 1 toward various metal ions was also investigated. Under the same conditions (10 mM PBS, pH 7.4), chemosensor 1 exhibited no apparent fluorescence change in the presence of other metal ions, even other π-philic metals such as Ag, Ni, Au, Co, Hg (Fig. 4A). Moreover, the interference of the above-mentioned metal ions on monitoring Pd were also studied (Fig. 4B). These results showed chemosensor 1 possesses high selectivity toward Pd²⁺ even in the presence of other metal ions.

Fig. 4 (A) Fluorescence responses of chemosensor 1 (5 μM, containing 4 μM PPh₃) toward various metal ions (0.5 μM for Pd²⁺, 3 μM for other ions). (B) Fluorescence responses of chemosensor 1 (5 μM, containing 4 μM PPh₃) toward Pd²⁺ (0.5 μM) in the presence of various metal ions (3 μM for other ions). All experiments were performed after reacting with metal ions for 1 h in PBS solution (10 mM, pH = 7.4). λ_ex = 410 nm.

3.5 Ratiometric imaging of Pd²⁺ in living cells

To further demonstrate the practical application of chemosensor 1 in living systems, the experiments were carried out in HepG2 and RAW 264.7 macrophage cells. As seen in Fig. 5, the cells incubated with chemosensor 1 (5 μM) and PPh₃ (4 μM) for 15 min showed an intense intracellular fluorescence, indicating that chemosensor 1 could penetrate cell membranes. Then, upon addition of Pd²⁺ (300 nM, equivalent to 0.03 ppm), the fluorescence intensity ratios at two emission channels increased immediately (Fig. 5c). These results demonstrate that the cellular uptake of Pd²⁺ as low as 0.03 ppm, which is substantially lower than the specified threshold in drugs (5–10 ppm), could be determined by using chemosensor 1 as an imaging reagent. Moreover, the RAW 264.7 cells showed a similar phenomenon in dual emission channels under stimulation of Pd²⁺ (Fig. 6), indicated that chemosensor 1 can be used for Pd²⁺ detection in diverse cells.

Fig. 5 Ratiometric imaging of Pd²⁺ in living HepG2 cells by using confocal fluorescence microscope. The cells were incubated with chemosensor 1 (5 μM) and PPh₃ (4 μM) for 15 min: (a) the bright-field transmission image; (b) the ratio images generated from green channel at 515 ± 30 nm and blue channel at 450 ± 35 nm. Then, the cells were incubated with Pd²⁺ (300 nM) for 15 min: (c) the ratio images generated from green channel at 515 ± 30 nm and blue channel at 450 ± 35 nm. Incubation was performed at 37 °C under a humidified atmosphere containing 5% CO₂. Scale bar = 10 μm.

Fig. 6 Fluorescence imaging of Pd²⁺ in RAW 264.7 macrophage cells. The cells were incubated with chemosensor 1 (5 μM) and PPh₃ (4 μM) for 15 min: (a) bright-field image, (b) blue channel at 450 ± 35 nm, (c) green channel at 515 ± 30 nm, and (d) mixed image generated from (c) and (b). Then, the cells were incubated with Pd²⁺ (300 nM) for 15 min: (e) bright-field image, (f) blue channel at 450 ± 35 nm, (g) green channel at 515 ± 30 nm, and (h) mixed image generated from (f) and (g). Incubation was performed at 37 °C under a humidified atmosphere containing 5% CO₂.

To estimate the cytotoxicity of chemosensor 1, we performed 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays in RAW 264.7 cells with 5, 10 and 20 μM chemosensor 1 (containing 4 μM PPh₃) for 24 h, respectively. The experimental results in Fig. S3,† showed that our probe exhibited very low toxicity to cultured cells at a concentration of 5 μM in the experimental conditions.

4. Conclusions

In conclusion, we have developed a highly sensitive 4-hydroxynaphthalimide-derived fluorescent chemosensor 1 for ratiometric detection of Pd²⁺ in PPh₃-reducing environment. Chemosensor 1 can detect Pd²⁺ quantitatively by ratiometric fluorescence method with a 100 nm red-shifted emission wavelength in the concentration up to 300 nM with a detection limit of 1 nM. During the detection, PPh₃ play a crucial role in improving the chemosensor sensitivity. The chemosensor also has suitable water-solubility, exhibits excellent selectivity toward Pd²⁺, and undergoes a color change from colorless to yellow for “naked eye” detection of Pd²⁺. Finally, we demonstrated that chemosensor 1 can be used as an imaging reagent to detect Pd²⁺ as low as 0.03 ppm in diverse cells.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (No. 21275018, 21575015 and 21203008), Beijing National Laboratory for Molecular Sciences (BNLMS) (20140121), Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No. 20121101110049) and the 111 Project (B07012) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17819c

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