New Rhenium(I) Complexes: Synthesis, Photophysics, Cytotoxicity

Article pubs.acs.org/Organometallics

New Rhenium(I) Complexes: Synthesis, Photophysics, Cytotoxicity, and Functionalization of Gold Nanoparticles for Sensing of Esterase Mei-Jin Li,*,† Xing Liu,† Mei-Juan Nie,† Zhao-Zhen Wu,† Chang-Qing Yi,‡ Guo-Nan Chen,† and Vivian Wing-Wah Yam§ †

Key Laboratory of Analysis and Detection Technology for Food Safety (Ministry of Education and Fujian Province), Department of Chemistry, Fuzhou University, Fuzhou 350108, People's Republic of China ‡ School of Engineering, Sun Yat-Sen University, Guangzhou 510275, People's Republic of China § Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People's Republic of China ABSTRACT: A series of the luminescent Re(I) complexes, [Re(bpy-(CH 2 OH) 2 )(CO) 3 (L1)], [Re(bpy-(CH 2 OH) 2 )(CO)3(L2)]+, [Re(bpy-(CH2OH)2)(CO)3(L3)]+, and [Re(bpy(CH 2 OH) 2 )(CO) 3 (L4)] ((bpy-(CH 2 OH) 2 = 4,4′-bis(hydroxymethyl)-2,2′-bipyridine; L1 = 4-pyridylmethyl lipoate; L2 = [1,2]dithiolane-3-pentyl 4-pyrindylacetate; L3 = 4-pyridinylmethanol; L4 = 4-pyridinylmethyl pentanoate), were synthesized and characterized. The photophysical properties and cytotoxicity of the complexes and the sensing of esterase based on the gold nanoparticles (Au-NPs) modified with the complexes were also studied. The intense luminescence of the Re(I) complexes was quenched efficiently by Au-NPs due to the energy and electron transfer between the Re(I) fluorophores and Au-NPs. Upon addition of esterase to the mixture, the emission of the Re(I) complexes 1 and 2 system was enhanced significantly by the release of the Re(I) complexes from the surface of the Au-NPs.

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INTRODUCTION Rhenium(I) tricarbonyl diimine complexes have been extensively studied in recent years1 because of their rich photophysical properties, such as the long-lived excited states and metal-to-ligand charge-transfer (MLCT) luminescence in the visible region. Although many of the Re(I) complexes have been reported, most of them were only soluble in organic solvents,2 the water-soluble Re(I) complexes were rarely reported,3 and there were only few reports on Re(I) complexes containing pyridine or bipyridine ligands modified with a dithiolated group for the surface-enhanced Raman spectroscopy (SERS) studies on Ag or Au colloids.4 In recent years, some ratiometric fluorescent sensors based on energy transfer (ET) have been developed for protease sensing.5 Large emission spectral changes upon hydrolysis of the phosphodiester or ester group by the enzyme were observed. A new ET system involving the ruthenium(II) bathophenanthroline complex acting as the acceptor, and the carbostyril derivative acting as the donor, was synthesized by Bannwarth and co-workers.5c The donor and acceptor could be easily introduced into the peptide moieties and such a system could be employed for a protease assay. However, until quite recently, there are no reports about the Re(I) complexes as luminophores in the ET system for the sensing of enzymes. In the last decades, gold nanoparticles (Au-NPs) were widely used in the development and exploitation for their significant advantages.6 One appealing feature of Au-NPs is their high coefficients in the visible region, © 2012 American Chemical Society

which thus enables them to function as efficient quenchers for most fluorophores in the design of biological sensors and in biomedical applications.7 Because thiols and dithiols have a high affinity on the surface of Au-NPs by chemiadsorption, surface-modified Au-NPs have found application in chemical sensors8 and colorimetric sensors9 and have shown promise in biorelated areas, such as cell imaging,10 medical diagnostics,11 DNA diagnostics,12 and biomolecules.13 However, there are relatively few studies involving the application of Au-NPs modified by transition-metal complexes.14 Andrew and coworkers have exploited the synthetic strategy of emissive Re(I)Au-NPs, and the hybrid conjugates were found to be watersoluble.15 Herein, a series of novel water-soluble Re(I) complexes (Scheme 1) were designed and synthesized to modify Au-NPs as a luminescent biological sensor for esterase. The principle of the enzyme assay based on the functionalized Au-NPs for esterase is shown in Scheme 2. The emission of the Re(I) complex chromophores 1 and 2 was quenched after functionalization of the Re(I) complexes on the Au-NPs' surface due to the energy- or electron-transfer process as previously reported.16 The emission of the Au-NPs-Re(I) composites would change upon sensing of esterase due to the hydrolysis of the ester group, resulting in the release of the Re(I) chromophores 1 and 2 from the Au-NPs' surface. The Received: March 29, 2012 Published: June 5, 2012 4459

dx.doi.org/10.1021/om300256u | Organometallics 2012, 31, 4459−4466

Organometallics

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Scheme 1. Synthetic Routes for the Ligands and Re(I) Complexesa

a

(i) Dry CH2Cl2, triethylamine, N2; (ii) EDC·HCl, DMAP, 24 h, r.t.; (iii) dry THF, AgOTf, reflux, N2, 6 h. Synthesis of 4-Pyridylmethyl Lipoate (L1). Lipoic acid (114 mg, 0.55 mmol) and EDC·HCl (123 mg, 0.64 mmol) were dissolved in 20 mL of CH2Cl2; then triethylamine (TEA) (89 μL, 0.64 mmol) was added under a nitrogen atmosphere. After the mixture was stirred for 10 min, 4-pyridylcarbinol (50 mg, 0.46 mmol) and 4(dimethylamino)pyridine (DMAP) (11.2 mg, 0.092 mmol) were added, which was stirred at room temperature for overnight. The organic phase was washed with 8% NaHCO3 solution (3 × 10 mL) and water (2 × 5 mL); then the CH2Cl2 fraction was dried with Na2SO4, filtered, and evaporated to dryness. The residue was column chromatographed on silica gel using CH2Cl2 as eluent to give the product as a viscous oil. Yield: 65 mg, 47.8%. 1H NMR (CDCl3, 400 MHz, Me4Si, δ in ppm) δ: 8.63 (q, 2H, J = 3.6 Hz, pyridyl H), 7.27 (q, 2H, J = 4.8 Hz, pyridyl H), 5.15 (m, 2H, pyridyl-CH2), 1.52−3.59 (m, 13H, lipoic acid-H). ESI-MS: m/z 298.9 {M + 1}+. Synthesis of [1,2]Dithiolane-3-pentyl 4-Pyrindylacetate (L2). 1,2-Dithiolane-3-pentanol (D,L-lipolol) was synthesized according to the literature procedures.19 L2 was prepared using a procedure similar to that for L1 except 4-pyridineacetic acid hydrochloride (70 mg, 0.36 mmol) and D,L-lipolol (50 mg, 0.26 mmol) were used instead of lipoic acid and 4-pyridylcarbinol, respectively. Chromatography on silica gel with CH2Cl2−ethyl acetate (3:1, v/v) as eluent gave the product as a viscous oil. Yield: 30 mg, 36.9%. 1H NMR (CDCl3, 400 MHz, Me4Si, δ in ppm) δ: 8.56 (d, 2H, J = 3.0 Hz, pyridyl H), 7.24 (d, 2H, J = 3.0 Hz, pyridyl H), 3.62 (s, 2H, pyridyl-CH2), 1.3−4.10 (m, 15H, D,L-lipololH). ESI-MS: m/z 312.6 {M + 1}+.

effects of the different ester groups on the esterase sensor were also investigated to develop a kind of simple functional Re(I) complex-modified gold nanoprobes for esterase sensing in bioanalytical applications.

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EXPERIMENTAL SECTION

Materials and Chemicals. [Re2(CO)10] and AgOTf were purchased from J & K Chemical Reagent Co., Ltd., and 4(dimethylamino)pyridine (DMAP) was purchased from Alfa Aesar Chemical Reagent Co., Ltd. 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC·HCl), pentanoic acid, pyridin-4yl-methanol, and bovine pancreas α-chymotrypsin (BPC) were purchased from Aladdin Reagent Co. Bovine pancreas trypsin (BPT) was purchased from Sangon Biotech (Shanghai) Co., Ltd. A porcine liver esterase (PLE) suspension in (NH4)2SO4 (3.2 M, pH 8.0) was purchased from Sigma-Aldrich Co. Other chemicals were of analytical grade and used as received. All synthetic operations were performed under a dry nitrogen atmosphere using Schlenk techniques and vacuum-line systems. The stock solutions were diluted to a series of concentrations with double-distilled water (DDW) before use. The pH of the trihydroxymethyl aminomethane was adjusted with chlorhydric acid (Tris-HCl). All reagents were used as received, and solvents were purified by standard methods. The 4-pyridine ligands, L1, L2, L3, and L4, [4,4′-bis(hydroxymethyl)-2,2′-bipyridine] (bpy(CH2OH)2),17 and Re(CO)5Br18 were synthesized according to the literature procedures. 4460

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Scheme 2. Schematic Representations of Luminescence Sensing for Esterase

as a yellow solid. Yield: 16 mg, 32%. 1H NMR (CDCl3, 400 MHz, Me4Si, δ in ppm) δ: 8.95 (d, 2H, J = 5.6 Hz, bipyridyl H), 8.74 (s, 2H, bipyridyl H), 8.08 (d, 2H, J = 6.4 Hz, pyridyl H), 7.62 (d, 2H, J = 5.6 Hz, bipyridyl H), 7.26 (s, 2H, pyridyl H), 5.05 (s, 2H, pyridyl-CH2), 4.89 (s, 4H, bipyridyl-CH2), 3.54 (t, 2H, J = 8.4 Hz, CH2-OH), 1.35− 3.20 (m, 13H, lipoic acid-H). ESI-MS: m/z 784 {M − OTf−}+. Anal. Calcd for C30H31F3N3O10ReS3·H2O: C, 37.89; H, 3.50; N, 4.42. Found: C, 37.81; H, 3.50; N, 4.28. Synthesis of [Re(bpy-(CH2OH)2)(CO)3L2] (2). The procedure was similar to [Re(bpy-(CH2OH)2)(CO)3L1], except L2 was used instead of L1. Chromatography on silica gel (CH2Cl2−CH3OH, 10:1, v/v) afforded the desired complex 2 as a yellow powder (14 mg, yield 26.6%). 1H NMR (CDCl3, 400 MHz, Me4Si, δ in ppm) δ: 8.93 (t, 2H, J = 2.8 Hz, bipyridyl H), 8.69 (d, 2H, J = 8.8 Hz, bipyridyl H), 8.05 (d, 2H, J = 2.0 Hz, pyridyl H), 7.65 (d, 2H, J = 5.2 Hz, bipyridyl H), 7.28 (s, 2H, pyridyl H), 4.88 (s, 4H, bipyridyl-CH2), 4.07 (m, 2H, COCH2), 3.67 (m, 2H, CH2-OH), 3.55 (s, 2H, pyridyl-CH2), 1.26−3.39 (m, 13H, D,L-lipolol-H). ESI-MS ion cluster: m/z at 798 {M − OTf−}+. Anal. Calcd for C31H33F3N3O10ReS3·H2O: C, 38.58; H, 3.66; N, 4.35. Found: C, 38.67; H, 3.60; N, 4.21. Synthesis of [Re(bpy-(CH2OH)2)(CO)3(L3)] (3). The procedure was similar to that for [Re(bpy-(CH2OH)2)(CO)3L1], except L3 was used instead of L1. Chromatography on silica gel (CH2Cl2−CH3OH, 10:1, v/v) afforded the desired complex 3 as a yellow oil, which was dried in vacuum for several hours to give the complex 3 as a yellow solid. Yield: 16.4 mg, 52%. 1H NMR (CDCl3, 400 MHz, Me4Si, δ in ppm) δ: 9.25 (d, 2H, J = 5.6 Hz, bipyridyl H), 8.57 (s, 2H, bipyridyl H), 8.32 (d, 2H, J = 6.4 Hz, pyridyl H), 7.85 (d, 2H, J = 5.2 Hz, bipyridyl H), 7.36 (d, 2H, J = 6.0 Hz, pyridyl H), 4.77 (s, 4H,

Synthesis of 4-Pyridinylmethyl Pentanoate (L4). L4 was prepared using a procedure similar to that for L1, except pentanoic acid (120 μL, 1.10 mmol) was used instead of lipoic acid. Chromatography on silica gel with hexane−ethyl acetate (3:1, v/v) as eluent gave the product as a viscous oil. Yield: 231 mg, 65.2%. 1H NMR (CDCl3, 400 MHz, Me4Si, δ in ppm) δ: 8.60 (q, 2H, J = 6.0 Hz, pyridyl H), 7.25 (d, 2H, J = 6.0 Hz, pyridyl H), 5.13 (s, 2H, pyridylCH2), 2.42 (t, 2H, J = 7.2 Hz, CO-CH2), 0.93−1.66 (m, 7H, pentanoic acid-H). ESI-MS: m/z 194.3 {M + 1}+. Synthesis of [Re(bpy-(CH2OH)2)(CO)3Br]. [Re(bpy-(CH2OH)2)(CO)3Br] was prepared by modification of a literature method.20 Re(CO)5Br (100 mg, 0.25 mmol) was added to a solution of 4,4′bis(hydroxymethyl)-2,2′-bipyridine (bpy-(CH2OH)2) (60 mg, 0.26 mmol) in toluene (10 mL) under N2, and the mixture was heated to reflux for 4 h. The solvent was then removed in vacuum; the crude product was washed three times with diethyl ether and dried by vacuum to give the product as a bright yellow solid. Yield: 124 mg, 87.3%. Synthesis of [Re(bpy-(CH2OH)2)(CO)3(L1)] (1). AgOTf (21 mg, 0.08 mmol) was added to a solution of [Re(bpy-(CH2OH)2)(CO)3Br] (30 mg, 0.053 mmol) in THF (10 mL) under N2; then the mixture was heated to reflux for 3 h in the dark. The precipitate was filtered off, then the pyridine ligand L1 (80 mg, 0.26 mmol) was added to the filtrate, and the mixture was heated to reflux for an additional 6 h. THF was removed by rotary evaporation, and the yellow oil product was obtained. The crude product was purified by column chromatography on silica gel using CH2Cl2−CH3OH (10:1, v/v) as eluent to give the desired product as the second band. The product was dried in vacuum for several hours to afford the complex 1 4461

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bipyridyl-CH2), 4.48 (s, 2H, pyridyl-CH2), 2.51 (s, 2H, bipyridl-OH), 2.09 (s, 1H, pyridyl-OH). ESI-MS ion cluster: m/z at 596.6 {M − OTf−}+. Anal. Calcd for C22H19F3N3O9ReS·H2O: C, 34.65; H, 2.78; N, 5.51. Found: C, 34.52; H, 2.71; N, 5.38. Synthesis of [Re(bpy-(CH2OH)2)(CO)3(L4)] (4). The procedure was similar to that for [Re(bpy-(CH2OH)2)(CO)3L1], except L4 was used instead of L1. Chromatography on silica gel (CH2Cl2−CH3OH, 8:1, v/v) afforded the desired complex 4 as a yellow oil, which was dried in vacuum for several hours to give the complex 4 as a yellow solid. Yield: 19.2 mg, 40.6%. 1H NMR (CDCl3, 400 MHz, Me4Si, δ in ppm) δ: 8.91 (d, 2H, J = 5.6 Hz, bipyridyl H), 8.76 (s, 2H, pyridyl H), 8.06 (d, 2H, J = 6.4 Hz, bipyridyl H), 7.61 (d, 2H, J = 5.6 Hz, bipyridyl H), 7.26 (s, 2H, pyridyl H), 5.04 (s, 2H, pyridyl-CH2), 4.89 (s, 4H, bipyridyl-CH2), 2.36 (t, 2H, J = 7.6 Hz, CO-CH2), 1.59 (m, 2H, CH2OH), 0.89−1.34 (m, 7H, pentanoic acid). ESI-MS ion cluster: m/z at 680.1 {M − OTf−}+. Anal. Calcd for C27H27F3N3O10ReS: C, 39.13; H, 3.28; N, 5.07. Found: C, 39.12; H, 3.31; N, 5.08. Preparation of Au-NPs. Gold colloids were prepared by sodium citrate reduction of HAuCl4 as in those methods developed by Natan and co-workers.21 All glassware was thoroughly cleaned with aqua regia (3:1 HCl/HNO3). A 25 mL aqueous solution of HAuCl4 (0.50 mM) was heated to boiling, and 1 mL of trisodium citrate (2%) was added. In about 25 s, the boiling solution turns faintly blue. After approximately 70 s, the blue color suddenly changes into a dark red, indicating the formation of gold particles. The particles formed by this method were allowed to stand for 24 h before their use for further studies. The average diameter of the gold nanoparticles was about 14 nm. Cell Culture. HepG2 cells (American Type Culture Collection, Manassas, VA) were cultured in RMPI 1640 medium supplemented with 10% fetal bovine serum and 1% penicillin and streptomycin in a humidified 5% CO2 atmosphere. Cell Viability Assay. MTT assay was used to determine the viability of HepG2 cells upon treatment with rhenium complexes, as described in detail elsewhere.22 HepG2 cells were seeded in 96-well tissue culture plates at the density of 4 × 106 cells per well and incubated for 3 days. After the treatment with rhenium complexes for 24 h, the plates were washed twice with culture medium, and then MTT was added and incubated for another 4 h. Cells without treatment of rhenium complexes were used as a control. The relative cytotoxicity was expressed in percentage of [ODsample − ODblank]/ [ODcontrol − ODblank] × 100. Data were collected from three independent experiments and expressed as the mean ± standard deviation (SD). The statistical differences were analyzed by a paired Student’s t test. P values less than 0.05 were considered to indicate statistical differences. Physical Measurements and Instrumentation. 1H NMR spectra were measured on a Bruker Avance III 300 MHz spectrometer. Chemical shifts (parts per million) were reported relative to tetramethylsilane (Me4Si). Positive-ion electrospray ionization electrospray mass spectra (ESI-MS) were recorded on a Thermo Finnigan LCQ Deca XP mass spectrometer (Finnigan, USA) using acetonitrile as the mobile phase. The elemental analyses (C, H, N) were performed on a Vario MICRO instrument. UV−vis absorption spectra in Tris-HCl buffer solution were taken on a Lambda 750 spectrophotometer. Steady-state emission spectra were carried out on a Hitachi fluorescence F-4600 spectrophotometer (PMT: 700 V). Emission lifetimes were determined on an Edinburgh Analytical Instrument (F900 fluorescence spectrometer using a hydrogen lamp). TEM were carried out on a Tecnai G2 F20 S-TWIN instrument (200 kV, FEI USA).

(CH2OH)2)OTf] with L1, L2, L3, and L4 in THF solution at 67◦C for 6 h, respectively, and purified by silica gel column chromatography to give the complexes as a pale yellow solid. The hydroxyl groups in the new Re(I) complexes would enhance the solubility of the complexes in aqueous solution. The identities of the ligands and Re(I) complexes were confirmed by 1H NMR spectroscopy, ESI mass spectrometry, and elemental analysis. The Re(I) complexes were also characterized by IR spectroscopy, which showed that three strong stretches were present in the carbonyl region (1900− 2100 cm−1); these bands are typical of the facial arrangement of three carbonyls in an octahedral geometry.24 Photophysical Properties of the Re(I) Complexes. The newly synthesized Re(I) carbonyl complexes with cationic 2,2′bipyridyl ligands exhibit a good solubility in water. The electronic absorption spectra of these complexes in aqueous solution (Figure 1) were dominated by very intense high-

Figure 1. UV−visible absorption spectra of the Re(I) complexes (6.7 × 10−5 M), Au-NPs, and the complexes after addition of Au-NPs (0.36 mL) in 5 mM Tris-HCl buffer solution (pH = 8) at room temperature.

energy absorption bands at ca. 300−320 nm with molar extinction coefficients of the order of 104 dm3 mol−1 cm−1, which are attributable to the spin-allowed intraligand (IL) π → π* transitions of the bipyridine diimine moieties. One additional moderately intense MLCT [dπ(Re) → π*(N−N)] wide absorption band with molar extinction coefficients of the order of 103 dm3 mol−1 cm−1 was also observed at the lowerenergy region at ca. 345−350 nm for the four complexes, which are typical of Re(I) tricarbonyl diimine complex systems.25 The modification of the “spectator” pyridine ligand with an ester group has a very slight effect on the electronic absorption spectra of the complexes. The electronic absorption data for the Re(I) complexes are summarized in Table 1. Table 1. Photophysical Data for the New Re(I) Complexes complex

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1

RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes for the ligands and complexes are shown in Scheme 1. The new ligands L1, L2, and L4 were synthesized by modification of a literature method,23 and the obtained pyridine ligands were a pale yellow oil. The mononuclear Re(I) complexes 1, 2, 3, and 4 were prepared by reaction of the [Re(CO) 3 (bpy-

2 3 4 a

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absorption λmax/nm ε/ dm3 mol−1 cm−1 303 (17 105), 346 (7565) 302 (18 465), 345 (7610) 308 (18 720), 350 (7350) 303 (17 483), 347 (8030)

emission λem/nm (τo/μs)

ϕema (%)

315 (16 035),

542 (0.08)

9.00

314 (15 085),

542 (0.09)

10.00

319 (15 450),

544 (0.14)

16.00

317 (17 380),

546 (0.12)

11.00

Aerated aqueous solution. dx.doi.org/10.1021/om300256u | Organometallics 2012, 31, 4459−4466

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Photophysical Properties of the Re(I) Complexes after Addition of Au-NPs. Upon addition of 14 nm Au-NPs to the complex 1 in Tris-HCl buffer solution, the intense absorptions at ca. 518 nm of Au-NPs decreased while a new band appeared at a longer wavelength of 611 nm by a red shift of 93 nm, as shown in Figure 1, suggesting that gold nanoparticles were ready to bind the new rhenium(I) complexes through the dithiolated group and aggregated subsequently. TEM images (Figure 3) clearly show the aggregate formation of Re(I)-AuNPs. The similar phenomena were observed for complex 2. The results showed that the positive charge of the rhenium(I) complex units might facilitate the strong chemisorptions of the thiolated group to the citrate negatively charged gold nanoparticle surface.27 This surface charge neutralization, which accompanies the binding of a thio to gold nanoparticles, would induce aggregation and shifts the absorption to the infrared, as previous reported.28 With addition of Au-NPs to the complexes 3 and 4 in TrisHCl buffer solution, the intense absorption band at ca. 518 nm would shift to 631 nm by a large red shift of 113 nm with the color changing from red to blue (Figure 1). The results showed that the complexes 3 and 4 would result in a more severe aggregation of the Au-NPs; these changes were attributable not only to the cancelation of negative citrate charges but also to the direct adsorption of the Re(I) complexes 3 and 4 by replacing citrate ions on the gold nanoparticle surface.29 The emission of the Re(I) complexes in Tris-HCl buffer solution was quenched upon addition of Au-NPs, as shown in Figure 4. The luminescence of the Re(I) complexes was quenched by about ca. 85% and 90% for 1 and 2, respectively, after addition of 0.36 mL of Au-NPs. The emission band of Re(I) complexes also has a small red shift from 542 to 550 nm. This result could be explained by that the Re(I) complexes were readily absorbed on the Au-NPs' surface by the dithiolated group and resulted in highly efficient energy transfer between the Re(I) complexes and Au-NPs, as previously reported.16b The emission intensities of the complexes 3 and 4 were quenched by ca. 77% and 74.6% after addition of 0.36 mL of Au-NPs. One possible explanation could be that the formation of Re(I)-Au composites caused by the adsorption of the Re(I) chromophores 3 and 4 onto the Au-NPs' surface resulted in the decrease of free Re(I) chromophores 3 and 4 in solution. The emissions of the Re(I) complexes 3 and 4 adsorbed onto the Au-NPs' surface were quenched, probably due to the highly efficient electron transfer between the Re(I) complexes and AuNPs, as reported previously.29

The Re(I) complexes were found to emit strong emission maxima at ca. 542−546 nm in aqueous solution at room temperature assigned as derived from a triplet MLCT state, similar to that observed in the previous studies of related Re(I) diimine complexes.26 The emission lifetimes of the Re(I) complexes 1, 2, 3, and 4 in the aerated aqueous solution are 80, 90, 140, and 120 ns, respectively. The emission quantum yields are 9.0, 10.0, 16, and 11% for the complexes 1, 2, 3, and 4, respectively, which are higher than those for the typical compound [Re(bipy)(CO)3Br] (4%) in aerated aqueous solution. The photophysical data for the new rhenium(I) complexes are listed in Table 1. Cytotoxicity Studies. MTT assay has been employed to measure the metabolic activity of mitochondria in the cells based on the principle that living cells are capable of reducing the lightly colored tetrazolium salt into an intense colored formazan derivative. Figure 2 shows the viability of HepG2 cells

Figure 2. Viability of HepG2 cells upon treatment with the rhenium(I) complexes for 24 h.

upon treatment with the Re(I) complexes for 24 h. The results reveal that the complexes 1 and 2 exhibit a severe cytotoxicity toward HepG2 cells, where the treatment of HepG2 cells with a series of dilutions (5, 25, 50, 75, and 100 μM) of the new rhenium(I) complexes resulted in a decrease in cell viability; at the highest dosage (100 μM), complexes 1 and 2 decreased the cell viability by 34% and 42% in 24 h, respectively. Complex 3 exhibits very low cytotoxicity toward HepG2 cells, even at the highest dosage (100 μM), and complex 3 only decreased the cell viability by 3% in 24 h, probably due to the effect of the dithiols in the complexes 1 and 2 on the cells.

Figure 3. TEM images of the gold nanoparticles (left) and gold nanoparticles modified by the Re(I) complex 1 (right). 4463

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Figure 6. (a) The emission spectra changes of Re(I) complex 2-AuNPs composites (6.7 × 10−5 M Re(I) complex, 0.33 mL of Au-NPs) after addition of estrase. (b) The change in emission intensity at 542 nm as a function of time.

dithiolated group-free complex 4; the emission intensity of Re(I) complexes 3-Au-NPs and 4-Au-NPs composites showed no obvious change after addition of esterase, indicating that the emission changes of Au-NPs modified by the Re(I) complexes 1 and 2 were ascribed to the cleavage of the ester group by esterase, which resulted in leaving the dithiolated group adsorbing on the Au-NPs' surface as protecting groups. The Re(I) complex fluorophores more loosely associated on the surface, and hence, the efficiency of the energy transfer was weakened. There was no dithiolated group in the Re(I) complex 4 so that the Re(I) complex units would adsorb directly to the Au-NPs' surface by replacing the negative citrate ion. The esterase has no obvious effect on this kind of adsorption interaction, although the ester group in 4-Au-NPs composites could be cut off by esterase. The results also demonstrated that the dithiolated group was important for the modification of the rhenium(I) complexes on the Au-NPs' surface. Furthermore, trypsin and chymotrypsin were used instead of esterase, and no obvious emission spectra changes of the Re(I)-Au-NPs system was found, which showed that the two enzymes cannot recognize the ester group in the new Re(I) complexes.

Figure 4. Emission spectral changes of the Re(I) complexes (6.7 × 10−5 M) in 5 mM Tris-HCl buffer solution (pH = 8) upon addition of 14 nm Au-NPs: (a) complex 1; (b) complex 2; (c) complex 3; (d) complex 4.

Esterase Sensing Studies of the Au-NPs Modified by Re(I) Complexes. The emission intensity increased gradually within 40 min upon addition of 1 μL of porcine liver esterase (PLE) to the Re(I)-Au-NPs composite aqueous buffer solution, as shown in Figure 5. The emission intensity of the complex 1-

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Figure 5. (a) The emission spectra changes of Re(I) complex 1-AuNPs composites (6.7 × 10−5 M Re(I) complex) after addition of PLE. (b) The change in emission intensity at 545 nm as a function of time.

CONCLUSION A series of new water-soluble Re(I) complexes were successfully synthesized and were functionalized on the AuNPs' surface to sense esterase. The Au-NPs modified by complex 1 showed a higher sensitivity toward esterase than complex 2. Spectra changes were not observed in a control experiment using ester-free complex 3 and dithiolated groupfree complex 4, indicating that the ester group and dithiols were important for this kind of simple esterase sensor by the Re(I)Au-NPs composites. The results confirmed that the Re(I) complexes with disulfide ligands could be functionalized on the Au-NPs' surface to act as an esterase biosensor.

Au-NPs system was enhanced by ca. 1.7 times with no obvious shift of the emission wavelength at ca. 543 nm. This phenomenon may be ascribed to that esterase can recognize the ester bond and cut off the special bond in the ligand, resulting in removing Re(I) complexes away from the surface of Au-NPs and increasing the concentration of the free Re(I) complex fluorophores in the solution. On the other hand, in view of the fact that the energy-transfer efficiency is strongly dependent on the distance between the donor and the acceptor, hydrolysis of the ester group on the ligand of the Re(I) complexes by esterase would separate the donor and acceptor far apart and hence lead to the blocking of the energy-transfer process. Schematic representations of the luminescence sensing for esterase are shown in Scheme 2. The emission intensity of Re(I) complex 2-Au-NPs composites was enhanced about ca. 1.1 times after addition of PLE, as shown in Figure 6. The results demonstrated that the Au-NPs modified by Re(I) complex 1 showed a higher sensitivity toward esterase than complex 2. Control experiments were carried out with ester-free complex 3 and

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 591 22866135. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Scientific Foundation of China (NSFC Nos. 20801014, 21171038, 31100723), the Program for Changjiang Scholars and Innovative Research 4464

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Organometallics

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Team in University (No. IRT1116), and the funding from Fuzhou University (No. 022309).

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dx.doi.org/10.1021/om300256u | Organometallics 2012, 31, 4459−4466

Organometallics

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Article

NOTE ADDED AFTER ASAP PUBLICATION In the version of this paper that was published on June 5, 2012, an author name was left out of the author list. The version of the paper that appears as of June 7, 2012, has the full author list.

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dx.doi.org/10.1021/om300256u | Organometallics 2012, 31, 4459−4466