Introduction

Metal ions play important roles in various important biological and chemical processes, such as electron transfer, oxygen transport, and drug metabolism [1,2,3]. The misregulation, deficiency, or excess of metal ions is a known contributing factor in some diseases [1]. Thus, the development of novel fluorophores for highly sensitive and selective detection of ions are crucial. Among the heavy metal ions in the human body, Cu2+ plays an essential role in the progression of several neurogenerative diseases, such as Parkinson’s, Alzheimer’s, and Huntington’s disease [4,5,6,7,8,9]. Compared with existing methods for detecting copper ions [10,11,12,13,14,15,16], such as inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence microscopy, and nano-secondary ion mass spectrometry (Nano-SIMS), the fluorescence detection method is superior in terms of high optical selectivity and sensitivity, fast response times, and in situ detection. It is noteworthy that the World Health Organization reported a safe upper limit of 31.47 μM of Cu2+ ions in drinking water and 100–150 g/dL in blood [17,18,19]. Hence, designing a fluorescent probe with a detection limit below these upper limits with adequate specificity for copper ion recognition is worthy of in-depth consideration.

Among many fluorophores developed for ion detection, 4,4-difluoro-4-bora3a,4a-diaza-s-indacene (BODIPY) fluorescent dyes have received considerable interest because of their appropriate photophysical properties, such as high photochemical stability, molar absorption coefficient, and fluorescent quantum yield [20,21,22,23,24,25,26,27,28,29,30]. BODIPY dyes have been widely used in diverse organic functional materials, such as labeling reagents, chemosensors, and laser dyes, and have been well investigated for applications such as probes for cations and anions, photosensitizers in photodynamic therapy, and dye-sensitized solar cells [20,21,22,23,24,25,26,27,28,29,30]. However, relatively few fluorescent probes based on BODIPY have been developed for Cu2+ detection [31,32,33,34,35,36,37,38], and most of them suffer from interference by zinc ions, or the formed BODIPY-Cu2+ complexes are easily affected by sulfide ions. Therefore, the development of BODIPY-based fluorescent probes that can recognize Cu2+ ions in mixed media with high selectivity and sensitivity is still necessary.

Considering this background and following our previous work [39, 40], probe 4 based on nonsymmetric benzo[a]-fused and thieno[3,2-b]thiophene[b]-fused BODIPY dyes was newly prepared (Scheme 1) in this study. This sensor can detect Cu2+ in polar mixed solvents with a low detection limit and high sensitivity. In addition, based on the results of high-resolution mass spectrometry (HR-MS), the binding ratio of the sensor and Cu2+ was determined to be 1:1. The HOMO and LUMO orbital distributions of the BODIPY–Cu2+ complex and dipicolylethylenediamine (DPEN) unit in the chemical sensor were obtained by density functional theory (DFT) calculations for a comprehensive understanding of the recognition mechanism of fluorescence quenching. The probe was used to effectively visualize Cu2+ in live cells.

Scheme 1
scheme 1

Designed structures of probe based on benzo[a]fused and thieno[3,2-b] thiophene-fused BODIPY for detection of Cu2+

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Materials and Methods

Instruments and Reagents

All reagents and solvents were purchased from Energy Chemicals (China) and used without further purification. All reactions were carried out under a dry argon atmosphere by using Schlenk line technique. 1H NMR, 13C NMR and 19F NMR spectra were recorded on a Bruker DRX 500 spectrometer and referenced to the residual proton signals of the solvent. HR-MS were recorded on a Bruker Daltonics microTOF-Q II spectrometer. UV–Vis spectra were obtained by a Shimadzu UV-1800 spectrophotometer. Fluorescence spectra of the samples were recorded on a Horiba JobinYvon Fluorolog-3 spectrofluorimeter. All the solvents employed for the spectroscopic measurements were of spectroscopic grade.

Synthesis

The precursors 1–3 were synthesized and provided in supporting information.

Synthesis of Probe 4

Dye 3 (38.9 mg, 0.1 mmol), DPEN (24.2 mg, 0.1 mmol), and K2CO3 (110 mg, 0.8 mmol) were dissolved in dichloromethane (25 mL) under argon, the mixture was stirred at 50 ℃ for 1 h. Then, the solvent of the mixture was removed in vacuo and the residue purified by column chromatography over silica gel (petroleum ether/dichloromethane = 4:1) to give a deep red powder 4 (52.3 mg, 88%). 1H NMR (500 MHz, CDCl3) δ/ppm 8.54 (d, J = 4.0 Hz, 2 H), 7.85 (m, 4 H), 7.78 (t, J = 4.0 Hz, 2 H), 7.64 (d, J = 8.0 Hz, 2 H), 7.42 (t, J = 8.0 Hz, 2 H), 7.30 (d, J = 8 Hz, 1 H), 7.16 (t, J = 8.0 Hz, 2 H), 6,98 (s, 1 H), 6.71 (s, 1 H), 4.06 (dd, J = 4.0 Hz, 2 H), 3.97 (s, 4 H), 3.08 (t, J = 4.0 Hz, 2H). 13C NMR (126 MHz, CDCl3) δ/ppm 158.3, 158.1, 158.0, 148.9, 142.1, 138.1, 137.7, 135.0, 132.2, 131.2, 129.8, 127.5, 126.0, 125.5, 125.4, 125.1, 124.1, 122.7, 121.3, 120.6, 107.5, 107.3, 60.6, 51.5, 41.6. 19F NMR (376 MHz, CDCl3) δ/ppm − 145.12 (q, BF2). HR-MS (ESI) calcd C31H25BF2N6S2Na:[M]+  = 617.1541, found [M]+  = 617.1551.

Preparation of Solutions and Spectral Measurements

A stock solution of probe 4 (1.0 mM) was prepared in CH3CN. The tested sample solutions of 4 (1 μM) for analysis were prepared by adding 10 μL of the stock solution into the 2 mL of CH3CN/Water (90:10, v/v), during the titration experiments, a 2 mL solution of 4 was poured into a quartz optical cell with a 1 cm optical path length and the concentrated ion and blank solutions were added using a micro-pipette. The series of solutions of ions (1 mM for Al3+, Fe2+, Fe3+, Hg2+, Ca2+, Co2+, Cd2+, Mg2+, Ba2+, Mn2+, Ni2+, Zn2+, Na+, K+, Li+, Cu+, and 1 mM, 10 mM for Cu2+) and a blank solution were prepared in deionized water. Spectral data were collected immediately after each addition. For all measurements, the excitation and emission slit widths were set at 5 nm.

Cell Culture and Confocal Imaging

The human NSCLC cell line, PC9 (purchased from the American Type Culture Collection, Manassas, VA, USA) was cultured in DMEM medium supplemented with 10% fetal bovine serum (FBS), 100 μg/mL streptomycin, 100 U/mL penicillin and was maintained at 37 ℃ in a humidified atmosphere containing 5% CO2. For the fluorescence imaging study, 1 × 105 PC9 cells were seeded on a round cover slides in 24-well plates and cultured for 24 h. The cells were incubated with 5 μM probe 4 (in PBS) for 30 min at 37 ℃, and then washed with PBS for 3 times. The cells were subsequently incubated with 10 μM CuCl2 for 8 min, followed by PBS washing for 3 times again. Fluorescence quenching by the intracellular CuCl2 was studied using a confocal fluorescence microscopy (LSM 710 NLO; Carl ZeissMeditec, Dublin, CA, USA).

Computational Details

The ground state and excited state structures of compounds 4 and 4-Cu were optimized using the density functional theory (DFT) method with B3LYP functional and 6-31G(d, p) basis set (LanL2DZ basis for Cu atoms). The LanL2DZ basis set was assigned to the elements of Cu atoms, which guarantees a reasonable balance of the computational cost and the reliability of the results. All the calculations were performed with the Gaussian16 program package [41].

Results and Discussion

Scheme 2 presents the synthesis route of probe 4. Starting material 1 was prepared in 75% yield according to reported literature [39]. Precursor 2 was synthesized by a reduction reaction performed by refluxing 1 with ethylene glycol in the presence of potassium hydroxide for 2 h. Subsequently, 2 was used for condensation reactions with 3-halogeno-1-formylisoindoles in the presence of POCl3 as a catalyst, followed by treatment with BF3•OEt2 under basic conditions to produce 3 in 40% yield. Target product 4 was obtained by the nucleophilic substitution reactions of 3 and DPEN moieties. We further optimized the reaction conditions. Increasing the reaction temperature further increased the reaction yield of 4 to as high as 88%. The reaction time was longer at this temperature and the yield did not increase, which may be attributed to the fact that basic potassium carbonate affects the stability of the product. The structure of 4 was well characterized by 1H NMR, 13C NMR, 19F NMR, and HR-MS.

Scheme 2
scheme 2

Synthesis and structure of probe 4. (i) KOH, (CH2OH)2, reflux, 2 h; (ii) 3-chloro-2H-isoindole-1-carbaldehyde, phosphorus oxychloride, 0 ℃, 1 h, triethylamine (10 min) and boron trifluoride etherate, room temperature, 2 h; (iii) DPEN, potassium carbonate, and dichloromethane, 36 ℃

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To assess the selectivity of probe 4, its absorption and fluorescence behavior were well investigated upon the addition of various metal ions in CH3CN/water (90/10, v/v). The main absorption and emission band of probe 4 can be observed at 517 and 585 nm, respectively, in the CH3CN/water (90/10, v/v) solution (Fig. 1). No distinct changes can be observed in the absorption maximum in the presence of common anions such as Al3+, Fe2+, Fe3+, Hg2+, Ca2+, Co2+, Cd2+, Mg2+, Ba2+, Zn2+, Mn2+, Ni2+, Na+, K+, Li+, Cu+; however, the maximum band of the absorption and fluorescence spectra decrease upon the addition of Cu2+. The absorption peak at 517 nm decreases gradually, along with the emergence of a new absorption peak at 573 nm, with increasing Cu2+ content (Fig. 1c). These changes were accompanied by a change in the color of the solution of 4 from orange to light pink. In addition, the fluorescence emission of probe 4 was quenched completely by Cu2+, along with a color change from yellow to black. The detection limit of Cu2+ was calculated to be 8.49 μM based on 3σ/k (Figs. 1d and S1 in ESI) [42, 43]. Therefore, probe 4 is a highly selective and sensitive sensor for Cu2+ in the CH3CN/water (90/10, v/v) solution.

Fig. 1
figure 1

The maximum absorption spectra (a) and emission spectra (b) of 4 (10−4 M) in CH3CN/water (90/10) after the addition of 200 equiv. of various anions. (c) Absorption spectra of 4 (10−5 M) in solution (CH3CN/water: 90/10) after the addition of Cu2+ (1 mM). Inset: photo pictures of 4 (10−5 M) before and after adding Cu2+ solution. (d) Fluorescence spectra of 4 (10−5 M) in solution (CH3CN/water: 90/10) after the addition of Cu2+ (1 mM) nm with excitation wavelength at 500 nm. Inset: Plot of emission intensity versus Cu2+ concentration (R2 = 0.9923)

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We attempted to fabricate single crystals to better explain the structure–activity relationship between the probe and ions; however, unfortunately, we did not succeed. HR-MS tests were performed on the separated complexes. As shown in Fig. 2, the mass spectrum of 4 in the presence of Cu2+ has one peak at m/z = 656.0895, which is assigned as [4 − H+ + Cu2+]+ (m/z = 656.0866). These results further confirm that the ratio of probe 4 to Cu2+ in the complex was 1:1. To determine the stability of the complex, a solution of 4–Cu2+ was added dropwise to an aqueous solution of sodium sulfide or EDTA (Ethylenediaminetetraacetic acid); no significant change was observed in its absorption spectrum (Fig. S2), indicating that the complexes were very stable. In addition, we tested the effects of light and different pH levels on the probe stability (Fig. S3). Compared to that exhibited by commercial dye rhodamine 6G, probe 4 exhibited better photostability under continuous irradiation with a 525 nm laser for 30 min. No distinct change was observed in the fluorescence emission of the probes at different pH levels. The experimental results show that probe 4 is stable under light illumination and acid/alkali environments, which confirms its excellent application range.

Fig. 2
figure 2

The HR-MS spectra of 4-Cu2+

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The dark toxicity of probe 4 was first investigated using an MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) assay (Fig. S4). When the probe concentration reached 32 μM, the cell viability was still above 80%, which indicates that probe 4 had little dark toxicity. To demonstrate the practicality of probe 4 for biological applications, we examined the potential applications of 4 for imaging intracellular Cu2+ in PC9 cells based on confocal fluorescence imaging experiments. DAPI (4',6-diamidino-2-phenylindole) with blue fluorescence was used to stain the cell nuclei. After staining with probe 4 (5 μM) for 30 min, intracellular red fluorescence was observed (top images in Fig. 3). However, incubation with Cu2+ significantly reduced the intracellular red fluorescence intensity, demonstrating that complex 4–Cu2+ was attained in a cellular environment, as shown in Fig. 3 (lower images). These findings indicate that probe 4 can be used for the detection of intracellular Cu2+ in living cells.

Fig. 3
figure 3

The confocal fluorescence images of PC9 cells after incubation with probe 4 for 30 min in the absence(top) and (bottom) the presence of Cu2+. Scale bars: 20 μm

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For an improved understanding of the conformation of BODIPY–Cu2+ complexes, the minima structures of 4 and 4-Cu2+ were optimized by DFT calculations using B3LYP in combination with the 6-31G(d, p) basis set for S, B, H, C, N, and F atoms and the LANL2DZ basis set for Cu atoms (Gaussian 16) [41]. As shown in Fig. 4, the Cu atom was coordinated in a tetrahedral geometry by four nitrogen atoms from the DPEN unit with atomic distances of 2.10, 2.22, 2.07, and 2.06 Å. The lowest-energy excitation of 4 was predicted to occur at 507 nm (f = 0.6) and to arise primarily from the HOMO → LUMO transitions. However, the lowest-energy excitations of 4–Cu2+ are produced by the HOMO-1 → LUMO transitions with an oscillator strength of 0.13. The HOMO of 4–Cu2+ was almost delocalized in the DPEN–Cu2+ unit. Thus, the LUMO is in the BODIPY core containing one nitrogen atom of DPEN. Accordingly, distinct intramolecular charge transfer was observed, which fits well with the experimentally observed “turn-off” fluorescence phenomenon in the emission of 4 before and after complexing with the Cu2+ ion.

Fig. 4
figure 4

The optimized molecular structures of probe 4 and 4-Cu2+ complex were calculated by DFT using the B3LYP in combination with 6-31G (d, p) basis set for S, B, H, C, N and F atoms and LANL2DZ basis set for Cu atom

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Conclusions

A probe based on nonsymmetric benzo[a]-fused and thieno[3,2-b]thiophene[b]-fused BODIPY was developed and fully characterized by NMR and HR-MS. Results indicated that probe 4 can be used to detect Cu2+ ions with high sensitivity and selectivity, without any interference from other competitive metal ions. HR-MS spectra confirmed the stoichiometry binding of 4 with Cu2+ forming a 1:1 complex in the acetonitrile/water (90/10) solution. Imaging experiments confirmed that probe 4 can be applied in the detection of intracellular Cu2+ in living cells.