Tetradecanuclear and Octadecanuclear Gold(I) Sulfido Clusters

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Tetradecanuclear and Octadecanuclear Gold(I) Sulfido Clusters: Synthesis, Structures, and Luminescent Selective Tracking of Lysosomes in Living Cells Chun-Yu Liu,†,‡ Xue-Rui Wei,§ Yuan Chen,† Hui-Fang Wang,† Jian-Feng Ge,*,† Yu-Jie Xu,§ Zhi-Gang Ren,*,† Pierre Braunstein,∥ and Jian-Ping Lang*,†,‡

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†

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People’s Republic of China ‡ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, People’s Republic of China § Technology School of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X), Medical College of Soochow University, No.199, RenAi Road, Suzhou 215123, Jiangsu, People’s Republic of China ∥ Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 rue Blaise Pascal-CS 90032, 67081 Strasbourg, France S Supporting Information *

ABSTRACT: Reactions of the phosphanyl-g old(I) precursor [(AuCl)2(bdppmapy)] (1; bdppmapy = N,N-bis(diphenylphosphanylmethyl)-2-aminopyridine) with Na2S in a 1:1 or 1:2 molar ratio gave rise to one tetradecanuclear and one octanuclear Au(I) sulfido cluster, [Au14S6(bdppmapy)5]Cl2 (2) and [Au18S8(bdppmapy)6]Cl2 (3), respectively. The former displays a new structural framework in gold cluster chemistry. Compounds 2 and 3 showed strong green luminescence and were employed as excellent imaging probes to selectively light up the lysosomes of living cells. Their long-term tracking of lysosomes can be achieved for up to 36 h, while tracking with commercial Lyso-Tracker Red under the same conditions was limited to 3 h. Our work demonstrated the possibility of constructing novel gold(I) sulfido clusters supported by special P−N hybrid ligands and the potential application of these clusters as long-term selective trackers of lysosomes in bioimaging.

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selectivity.20,21 Therefore, novel probes are in urgent demand to address these challenges. In the past few decades, the assembly of highly luminescent polynuclear AuI clusters has attracted much attention.22−26 Polynuclear sulfido AuI clusters with phosphine ligands are among the most stable systems.27−29 In comparison with the other luminescent biomarkers mentioned above, AuI clusters with specific structural frameworks may exhibit higher quantum yield, better solubility, and lower cytotoxicity.30 However, to our knowledge, gold clusters have not yet been employed as lysosome trackers for live cell imaging. Furthermore, the longterm tracking ability of probes is an important element in cell imaging systems. Most commercial small-molecule trackers, including the Lyso-Tracker DND series and neutral red, showed high pH dependence, which limits their performance in longterm tracking.31 Other probes such as quantum dots32,33 and nanoparticles34,35 could extend the imaging time, but unfortunately they also increased cytotoxicity.36,37 Therefore, it is

INTRODUCTION

To visualize the cellular molecular recognition events in live cells, organelle-specific markers have attracted much attention for decades.1−3 As one of the most important types of organelles, lysosomes play an essential role in many cellular physiological processes, including plasma membrane repair, immune response, protein degradation, and cell homeostasis.4,5 To understand the role of lysosomes in cellular processes and reveal their functions, great efforts have been recently devoted to develop specific and sensitive lysosomal trackers.6−8 Typical luminescent dyes such as fluorene9 and rhodamine fluorophores10,11 have been developed as potential biomarkers for lysosome imaging and marking. However, these probes suffer from limitations, including poor photostability, low selectivity, and high cytotoxicity.12 Many papers have reported the use of metal complexes13−15 or gold nanoparticles16,17 as molecular imaging probes. Several distinct advantages, such as biodegradability associated with the presence of relatively labile metal− ligand bonds, allow them to perform as biomolecular trackers for cell imaging.18,19 However, these probes still suffer from limitations such as poor solubility, high cytotoxicity, and low © XXXX American Chemical Society

Received: November 27, 2018

A

DOI: 10.1021/acs.inorgchem.8b03298 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

P{1H} NMR (400 MHz, CD2Cl2, ppm): δ 20.23 (s, bdppmapy). Quantum yield in the solid state: 20.7%. Synthesis of [Au18S8(bdppmapy)6]Cl2 (3). A mixture containing 1 (0.0955 g, 0.1 mmol) and Na2S·9H2O (0.0480 g, 0.2 mmol) in CH2Cl2 (10 mL) was stirred for 2 h at ambient temperature to form a yellow-green solution and a gray precipitate. After filtration, slow diffusion of diethyl ether into the solution afforded yellow block crystals of 3 after 5 days, which were collected by filtration, washed with diethyl ether, and dried in air. Yield: 0.0656 g (86.6% based on Au). Anal. Calcd for C93H84Au9ClN6P6S4: C, 32.76; H, 2.47; N, 2.47. Found: C, 32.56; H, 2.52; N, 2.61. IR (KBr disk): 3435 (s), 3048 (w), 2963 (m), 1628 (m), 1592 (s), 1473 (s), 1435 (vs), 1384 (s), 1262 (s), 1223 (w), 1099 (vs), 1026 (s), 860 (m), 803 (s), 741 (w), 692 (m), 509 (w), 482 (w) cm−1. 1H NMR (400 MHz, CD2Cl2, ppm): δ 8.37 (s, 3H, py), 8.16− 8.06 (m, 3H, py), 7.99−7.87 (m, 4H, phenyl), 7.86−7.77 (m, 4H, phenyl), 7.68 (dd, J = 12.5, 7.4, 7H, phenyl), 7.63−7.36 (m, 20H, phenyl), 7.36−7.17 (m, 12H, phenyl), 7.17−7.08 (m, 3H, phenyl), 7.04 (dd, J = 11.2, 4.4, 1H, phenyl), 6.99−6.91 (m, 3H, phenyl), 6.86 (dd, J = 13.2, 6.6, 3H, phenyl), 6.72 (d, J = 8.6, 1H, py), 6.53 (ddd, J = 11.3, 7.8, 4.6, 2H, py), 6.48−6.43 (m, 1H, py), 6.33 (ddd, J = 24.0, 15.3, 8.0, 5H, py), 5.91 (d, J = 8.6, 1H, methylene), 5.81−5.65 (m, 2H, methylene), 5.59 (d, J = 9.7, 3H, methylene), 5.36 (m, 7.5H, CD2Cl2 and methylene), 4.79 (s, 2H, methylene), 4.65 (dd, J = 14.5, 9.4, 1H, methylene). 31P{1H} NMR (400 MHz, CD2Cl2, ppm): δ 27.48 (s, bdppmapy). Quantum yield in the solid state: 26.6%. Single-Crystal X-ray Crystallography. Single crystals of 1−3 coated with Paratone oil on a Cryoloop pin were mounted on a Rigaku Mercury (1) or a Bruker APEX-II single-crystal X-ray diffractometer employing graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at 293 K (1) or 100 K (2 and 3). CrystalClear (Rigaku and MSc, Ver. 1.3, for 1) and Bruker SAINT (2 and 3) were employed for the refinement of cell parameters and the reduction of collected data, whereas absorption corrections (multi-scan) were applied. The structures were solved by direct methods and refined on F2 using SHELXS-2016 programs.42 Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were generated geometrically. The SQUEEZE tool of PLATON was applied to 2 and 3 for the existence of large solvent-accessible voids.43 A summary of the pertinent crystal data and structural refinement parameters of 1−3 is given in Table S1. DFT Calculations. Density functional theory (DFT) calculations were performed with the Gaussian 09 program44 using the Cartesian coordinates at their solid-state structures. Calculations were performed with the functional of B3LYP-D3.45,46 The 6-31G(d) basis set was used for P, C, N, S, and H and the LanL2DZ relativistic effective core potential for Au. Cell Culture. HeLa cells were used in this study, which were cultured in Roswell Park Memorial Institute culture medium (RPMI1640) supplemented with penicillin (100 U mL−1), 10% calf serum, Lglutamine (2.5 × 10−4 M), and streptomycin (100 μg mL−1) at 37 °C in a humidified incubator with 5% CO2; the cells were loaded onto a glassbottom coverslip and cultured for 48 h before use. Luminescence Imaging. The cells were washed with PBS and then incubated solely with 10 μM of compounds 2 or 3 in DMSO/ culture medium (1/99, v/v) for 6 h at 37 °C. Cell imaging was then carried out after washing the cells with phosphate buffer saline (PBS) solution. Cells incubated with complex 2 or 3 were excited at 458 nm with a semiconductor laser, and the emission was collected at 480−560 nm in the green channel. All of the images were obtained using the same settings; the overlap between the biomarkers and the compounds 2 or 3 could be obtained by considering the luminescent intensities in the green and red channels within selected ROIs (regions of interest). For colocalization experiments of 2 or 3, the cells were first incubated with 2 or 3 for 6 h and then incubated with Lyso-Tracker Red for another 10 min before imaging (concentration 10 μM for 2 or 3, 50 nM for LysoTracker Red; λex 458 nm for 2 or 3, 561 nm for Lyso-Tracker Red; λem 520 ± 40 nm for 2 or 3, 600 ± 20 nm for Lyso-Tracker Red). 31

necessary to develop efficient low-toxicity gold clusters as longterm trackers in bioimaging. In this work, we deliberately chose a P−N hybrid ligand, N,Nbis(diphenylphosphanylmethyl)-2-aminopyridine (bdppmapy), because it can be used as a versatile linker in the construction of high-nuclearity Au(I) clusters38−40 and its pyridyl groups are anticipated to prolong the imaging period. The preformed phosphanyl-gold(I) complex [(AuCl)2(bdppmapy)] (1) was treated with different amounts of Na2S to produce one tetradecanuclear and one octanuclear Au(I) sulfido cluster, [Au14S6(bdppmapy)5]Cl2 (2) and [Au18S8(bdppmapy)6]Cl2 (3), respectively. Both clusters were structurally determined and confirmed to be excellent luminescent long-term selective probes for lysosomes in live cells.

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EXPERIMENTAL SECTION

Materials and Methods. All reagents were commercially available and used as received. The phosphine ligand N,N-bis(diphenylphosphanylmethyl)-2-aminopyridine (bdppmapy) was prepared according to the literature procedure.40 The complex [Au(tht)Cl] (tht = tetrahydrothiophene) was synthesized according to the literature procedure.41 The analytical instruments engaged in this work are the same as those used in our previous works.38 MALDI-TOF mass spectra were recorded on a Bruker Autoflex II mass spectrometer in positive reflection mode. Electrospray ionization mass spectrometry (ESI-MS) spectra were performed on an Agilent 1200/6200 mass spectrometer using MeCN or DMSO/H2O (1/99, v/v) as the mobile phase. Photoluminescence spectra and quantum yields were obtained on a HORIBA PTI QuantaMaster40 spectrofluorometer. The UV−vis absorption spectra of 2 and 3 in CH2Cl2 were measured on a Varian Cary-50 UV−vis spectrophotometer. Luminescence imaging was performed with a Leica TCS SP5 II confocal laser scanning microscope. Synthesis of [(AuCl)2(bdppmapy)] (1). A mixture containing [Au(tht)Cl] (0.1605 g, 0.5 mmol) and bdppmapy (0.1225 g, 0.25 mmol) in CH2Cl2 (10 mL) was stirred for 2 h at ambient temperature to form a clear colorless solution. Slow diffusion of hexane into the solution afforded colorless block crystals of 1 after 2 days, which were collected by filtration, washed with hexane, and dried in air. Yield: 0.2223 g (93.1% based on Au). Anal. Calcd for C31H28Au2Cl2N2P2: C, 38.95; H, 2.93; N, 2.93. Found: C, 38.79; H, 3.03; N, 2.87. IR (KBr disk): 3416 (m), 3051 (w), 1593 (s), 1566 (w), 1476 (s), 1435 (vs), 1384 (w), 1318 (w), 1219 (w), 1161 (m), 1102 (s), 998 (w), 862 (w), 744 (s), 692 (s), 514 (m) cm−1. 1H NMR (400 MHz, CD2Cl2, ppm): δ 8.06−7.97 (m, 2H, py), 7.73−7.60 (m, 8H, phenyl), 7.53 (dd, J = 7.9, 6.0, 4H, phenyl), 7.49−7.39 (m, 8H, phenyl), 6.98−6.90 (m, 2H, py), 4.79 (s, 4H, methylene). 31P{1H} NMR (400 MHz, CD2Cl2, ppm): δ −16.59 (s, bdppmapy). Synthesis of [Au14S6(bdppmapy)5]Cl2 (2). A mixture containing 1 (0.0955 g, 0.1 mmol) and Na2S·9H2O (0.0240 g, 0.1 mmol) in 10 mL of CH2Cl2/EtOH (1/1, v/v) was stirred for 2 h at ambient temperature to form a yellow solution and a gray precipitate. The resulting mixture was filtered, and the solution was layered with diethyl ether. Yellow block crystals of 2 were obtained after 5 days, which were then collected by filtration, washed with diethyl ether, and dried in air. Yield: 0.0710 g (91.0% based on Au). Anal. Calcd for C155H136Au14Cl2N10P10S6: C, 34.01; H, 2.49; N, 2.56. Found: C, 33.97; H, 2.33; N, 2.67. IR (KBr disk): 3442 (s), 1628 (m), 1592 (s), 1474 (s), 1435 (s), 1400 (m), 1384 (m), 1314 (w), 1218 (w), 1158 (w), 1099 (s), 1084 (s), 998 (m), 859 (s), 741 (m), 691 (m), 508 (m), 481 (w) cm−1. 1H NMR (400 MHz, CD2Cl2, ppm): δ 8.36 (s, 5H, py), 8.19−8.07 (m, 5H, py), 7.91 (d, J = 3.4, 8H, phenyl), 7.82 (d, J = 3.0, 4H, phenyl), 7.68 (dd, J = 12.3, 7.3, 13H, phenyl), 7.62−7.55 (m, 6H, phenyl), 7.55−7.36 (m, 25H, phenyl), 7.26 (dd, J = 21.2, 13.1, 22H, phenyl), 7.17−7.10 (m, 5H, phenyl), 7.09−7.01 (m, 2H, phenyl), 6.95 (t, J = 6.9, 4H, phenyl), 6.87 (s, 6H, phenyl), 6.60−6.52 (m, 3H, py), 6.49−6.43 (m, 2H, py), 6.42− 6.25 (m, 10H, py), 5.92 (d, J = 8.3, 2H, methylene), 5.80−5.68 (m, 3H, methylene), 5.59 (dd, J = 14.7, 9.3, 5H, methylene), 5.36 (m, 17.2H, CD2Cl2 and methylene), 4.67 (dd, J = 14.2, 8.8, 5H, methylene). B

DOI: 10.1021/acs.inorgchem.8b03298 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Reaction of Phosphanyl-Gold(I) Precursor [(AuCl)2bdppmapy] (1) with Na2S·9H2Oa

Counteranions (Cl−) in the clusters have been omitted.

a

Figure 1. Crystal structures of a side view (a) and top view (b) of the [Au14(μ3-S)6(μ-bdppmapy)5]2+ dication of 2. Color legend: yellow, Au; red, S; orange, P; gray, C; blue, N. Phenyl rings and hydrogen atoms have been omitted.

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m/z 2699.1 (attributed to [Au14S6(bdppmapy)5]2+) and 1799.7 (attributed to [Au14S6(bdppmapy)5 + H]3+) for 2 and m/z 2248.4 for 3 (attributed to [Au18S8(bdppmapy)6 + H]3+) in their ESI mass spectra using MeCN or DMSO/H2O (1/99, v/v) as the mobile phase (Figures S12−S17). Crystal Structure of 2. Compound 2 crystallizes in the triclinic P1̅ space group, and the [Au14S6(bdppmapy)5]2+ dication of 2 consists of an Au6S2(bdppmapy) unit and an Au8S4(bdppmapy)4 crown connected to each other through Au−S and aurophilic interactions (Figure 1). Each face of the distorted Au6S2 cubane consists of one S and three Au atoms. The S atoms are positioned across the body diagonal of the cubane. The Au atoms of the cubane are linked to one P atom from the bottom bdppmapy (Au4 and Au5) or capped with an Au2S triangle (Au1−Au3, Au6) (Figures S18 and S19). Each S atom of the cluster assumes a triply bridging coordination mode that links three Au atoms from the Au6S2 cubane (for S1 and S2)

RESULTS AND DISCUSSION Syntheses and Characterization. The phosphanyl-gold(I) precursor [(AuCl)2(bdppmapy)] (1) was prepared in 93.1% yield from the reaction of [Au(tht)Cl] with bdppmapy (molar ratio 2:1) in CH2Cl2 followed by a standard workup. Then, 1 was utilized to assemble the luminescent polynuclear μ3-sulfido clusters 2 and 3, respectively (Scheme 1). Compound 2 was isolated in 91.0% yield (based on Au) from the reaction of 1 with Na2S·9H2O in a 1:1 molar ratio in CH2Cl2/EtOH (v/v 1/1) followed by a standard workup. Changing this molar ratio to 1:2 and the solvent to CH2Cl2 afforded 3 in 86.6% yield (based on Au). Complexes 1−3 were characterized by single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), infrared spectra (IR), elemental analysis, and thermogravimetric analysis (TGA) as well as 1H and 31P{1H} NMR spectroscopy (Figures S1−S11). The structures of 2 and 3 were stable in solution, as confirmed by the molecular ion peaks observed at C

DOI: 10.1021/acs.inorgchem.8b03298 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Crystal structures of a side view (a) and top view (b) of the [Au18(μ3-S)9(μ-bdppmapy)6]2+ dication of 3. Color legend: yellow, Au; red, S; orange, P; gray, C; blue, N. Phenyl rings and hydrogen atoms have been omitted.

Figure 3. UV−vis absorption spectra (left) of bdppmapy and 1−3 in CH2Cl2 and emission spectra (right) of 2 and 3 in the solid state. Inset: photograph of the samples of 2 and 3 in the solid state under irradiation at 365 nm.

quantum yields of 20.7% and 26.6% at room temperature, respectively. Interestingly, they were also emissive (λem 536 nm for 2; λem 516 nm for 3) in DMSO/PBS buffer (1/99 v/v; pH 7) (Figures S22 and S23). Their UV−vis absorption spectra showed absorption peaks at ca. 330 nm for bdppmapy, ca. 320 nm for 1, ca. 320 and 365 nm for 2, and ca. 320, 395, and 440 nm for 3 (Figure 3), respectively. The absorption of 2 in CH2Cl2 fell in the range of UV, which indicated that the solution should be colorless. However, its solution was yellow, which was probably due to its luminescence excited by visible light.30 A significant part of visible light (400−450 nm) was involved in its excitation spectrum. To further understand the electronic structures of the emissions of 2 and 3, density functional theory (DFT) calculations were performed. The major frontier orbitals of 2 and 3 are illustrated in Figure S24. The HOMO states of 2 or 3 were mainly delocalized around the central fragment and result from the gold and sulfur atoms. The LUMO state of 2 and the LUMO, LUMO+1, and LUMO+2 states of 3 were largely delocalized over the surrounding units, with the largest contributions from the pyridyl and phenyl groups of the ligands. This result suggests that the excited state of the emissions for 2 and 3 can be assignable to a metal to ligand charge transfer (MLCT). Then, time-dependent UV−vis absorption and timedependent emission spectra were employed to examine the

or one Au atom from the Au6S2 cubane and two Au atoms from the capping Au2S triangles (for S3−S6). The Au···Au separations fall within the range of 2.937(8)−3.361(8) Å, implying the presence of aurophilic interactions (Table S2).47 Numerous polynuclear Au(I) clusters with nuclearities of 6, 10, and 12 have been reported, but the coexistence of Au14 and Au18 clusters appears to be unique, and no Au14 cluster analogous to 2 has been reported before.48 Crystal Structure of 3. Compound 3 crystallizes in the trigonal R3̅ space group, and the 18 gold(I) atoms in the [Au18S8(bdppmapy)6]2+ dication of 3 are linked by eight triply bridging S atoms and surrounded by six bdppmapy ligands. Similarly to the related [Au18S8(trans-dppee)6]2+ (trans-dppee = trans-1,2-bis(diphenylphosphino)ethylene) and [Au18S8(dppe)6]2+ (dppe = 1,2-bis(diphenylphosphino)ethane)49,50 the [Au18S8(bdppmapy)6]2+ dication of 3 consists of two Au9S4(bdppmapy)3 units connected with each other through Au−S and aurophilic interactions (Figure 2). The cluster core could also be regarded as an Au6S2 cubane surrounded by two Au6S3(bdppmapy)3 crowns (Figures S20 and S21). The Au···Au contacts vary from 2.985(7) to 3.367(7) Å and are similar to those in 2 (Table S3). Photoluminescent Properties of 2 and 3. Compounds 2 and 3 were stable in air and emitted bright yellow-green light (λem 540 nm for 2; λem 518 nm for 3) in the solid state, with D

DOI: 10.1021/acs.inorgchem.8b03298 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Colocalization confocal luminescence images of HeLa cells incubated with probes 2 and 3 and their ROI (regions of interest) analysis: (A, F) bright-field transmission images; (B, G) confocal images (green channel) of cells with 2 or 3; (C, H) confocal images (red channel) of cells with LysoTracker Red; (D, I) merged images of red and green channels; (E, J) fluorescence intensities of the ROIs across the cells. Red channel emission was collected in the range of 580−620 nm upon excitation at 561 nm, while green channel emission was collected in the range of 480−560 nm upon excitation at 458 nm.

stability of 2 and 3 (Figures S25−S27). The absorbance intensity and emission intensity were retained up to 36 h without obvious attenuation, suggesting a high stability of 2 and 3 in solution. Cell Imaging Properties. Confocal luminescence imaging of HeLa cells was used to evaluate the potential applications of 2 and 3. The commercial lysosome biomarker probe Lyso-Tracker Red DND-99 efficiently targets lysosomes and was chosen to costain the cells, since 2 and 3 gave green emissions. The concentration of Lyso-Tracker Red was kept at 50 nM, while those of 2 and 3 were kept at 10 μM in the medium in order to obtain intracellular images with a satisfactory signal to noise ratio. In order to demonstrate the potential utility of 2 and 3 for cellular imaging, the cytotoxicities of HeLa cells and WS-1 normal human fibroblast cells were assessed. The viability of unprocessed HeLa cells or WS-1 cells was assumed to be 100%. The CCK8 assay results indicated that 2 and 3 had low cytotoxicity (HeLa cells, ∼100% viability for 2 and ∼85% viability for 3; WS-1 cells, ∼100% viability for 2 and ∼100% viability for 3) over a concentration range from 0 to 50 μM, appropriate for cell imaging (Figures S28 and S29). In comparison with some current AuNP or metal complex biomarkers, the lower cytotoxicity of 2 and 3 is a favorable

characteristic for their use as lysosome trackers in living cells.51,52 HeLa cells were incubated with 10 μM of 2 or 3 in DMSO/ culture medium (1/99 v/v) at 37 °C for 6 h, and intense intracellular luminescence was monitored in lysosomes (Figure 4). To confirm the labeling specificity, colocalization experiments were performed by costaining HeLa cells with both commercial Lyso-Tracker Red and 2 or 3. The luminescence of 2 or 3 in HeLa cells showed nearly complete colocalization with the Lyso-Tracker Red signals (Figure 4D,I), thus indicating that 2 and 3 can specifically localize in lysosomes of living cells. Long-term tracking of lysosome experiments with 2 and 3 was studied through real-time monitoring. HeLa cells with 2 or 3 were incubated for up to 36 h, and the luminescence intensity in lysosomes gradually increased with the incubation time (Figures 5a, A−H). After ca. 12 h, the luminescence intensity reached a maximum, which remained for up to 36 h without smudging any other organelles. In contrast, the luminescence of Lyso-Tracker Red decreased rapidly (Figure 5a, I−L), and the residual intensity was estimated to be 20% within 1.5 h. Figure 5b illustrates the quantified luminescence intensities of HeLa cells labeled with 2, 3, and Lyso-Tracker Red at different incubation times. The increase of luminescence intensity could be attributed to a continuous cell uptake of 2 or 3, while LysoE

DOI: 10.1021/acs.inorgchem.8b03298 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

h but decreased rapidly after 24 h and the residual intensity was about 37% at 36 h (Figure S32 and Figure 5b). This result indicated that the pyridyl groups in 2 and 3 play an important role in the long-term tracking of lysosome. The photostability of 2 and 3 was explored by photobleaching experiments. The signal loss of 2 or 3 was relatively small, whereas the luminescence intensity of Lyso-Tracker Red decreased rapidly (Figure 6). Compounds 2 and 3 retained

Figure 5. (a) Luminescence images of HeLa cells incubated with 10 μM of 2 (A−D), 3 (E−H), and Lyso-Tracker Red (I−L) at different incubation times. (b) Quantified luminescence intensities of 2, 3, LysoTracker Red, and [Au18S8(dppe)6]Cl2 after different incubation times.

Figure 6. (a) Photobleaching assay of 2 or 3 (green) and Lyso-Tracker Red (red) in Hela cells under continuous excitation for 10 min. (b) Normalized emission intensity loss of 2, 3, and Lyso-Tracker Red with increasing bleaching time.

Tracker Red was readily expelled from cells. Obviously, the stable labeling properties of 2 and 3 make them promising candidates for potential long-term lysosome trackers. To verify the importance of pyridyl groups of 2 and 3 in the long-term tracking of lysosomes, a similar long-term tracking experiment with [Au18S8(dppe)6]Cl2, an Au18 cluster with a core structure similar to that of 2 but with the different phosphine ligand dppe,49 was studied for comparison. [Au18S8(dppe)6]Cl2 gave orange emissions (λem 579 nm) in DMSO/PBS buffer (1/ 99 v/v; pH 7) (Figure S30), and commercial lysosome biomarker probe Lyso-Tracker Green DND-26 was chosen to costain the cells. The colocalization confocal luminescence images of HeLa cells incubated with 10 μM of [Au18S8(dppe)6]Cl2 in DMSO/culture medium (1/99 v/v) showed its good selectivity of lysosomes in living cells (Figure S31). HeLa cells with [Au18S8(dppe)6]Cl2 were then incubated for up to 36 h, and the luminescence intensity in lysosomes gradually increased with the incubation time, which reached a maximum after ca. 12

more than 75% intensity in 10 min, but Lyso-Tracker Red lost almost 80% of its luminescence signal within the same period. This photobleaching assay revealed that 2 and 3 exhibit superior photostability in comparison to Lyso-Tracker Red.

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CONCLUSION In conclusion, we have prepared one tetradecanuclear and one octadecanuclear Au(I) sulfido cluster with a bridging P−N hybrid ligand (bdppmapy), [Au14S6(bdppmapy)5]Cl2 (2) and [Au18S8(bdppmapy)6]Cl2 (3). Compound 2 displays a new cluster skeleton within the gold cluster family. 2 and 3 show strong luminescence and specifically image lysosomes in living cells. The long-term tracking of lysosomes can be achieved by 2 and 3 for up to 36 h without any evident luminescence intensity decay or smudging any other organelles. This work provides a F

DOI: 10.1021/acs.inorgchem.8b03298 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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facile access to the high-nuclearity gold(I)/sulfido/P−N hybrid ligand clusters with strong luminescence, which could be employed as efficient luminescent biomarkers for bioimaging.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03298. Additional information regarding PXRD, MALDI-TOF mass, ESI-MS, cytotoxicity, and selected bond distances and angles of 2 and 3 (PDF) Accession Codes

CCDC 1855648−1855650 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*J.-F.G.: e-mail, [email protected]. *Z.-G.R.: e-mail, [email protected]. *J.-P.L.: e-mail, [email protected]; tel, +86-512-65882865; fax, +86-512-65880328. ORCID

Jian-Feng Ge: 0000-0003-2479-7600 Pierre Braunstein: 0000-0002-4377-604X Jian-Ping Lang: 0000-0003-2942-7385 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 21531006, 21671144, 21773163), the State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry (Grant No. 2018kf-05), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Project of Scientific and Technologic Infrastructure of Suzhou (Grant No. SZS201708) for Postgraduates in Universities of Jiangsu Province. The authors highly appreciate Mr. Cheng-Rong Lu at Soochow University for his DFT calculations on different UV−vis absorption wavelengths of 2 and 3. We are grateful to the useful comments of the editor and the reviewers.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03298 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03298 Inorg. Chem. XXXX, XXX, XXX−XXX