X-ray Inducible Luminescence and Singlet Oxygen Sensitization by

Article pubs.acs.org/IC

X‑ray Inducible Luminescence and Singlet Oxygen Sensitization by an Octahedral Molybdenum Cluster Compound: A New Class of Nanoscintillators Kaplan Kirakci,*,† Pavel Kubát,‡ Karla Fejfarová,$ Jiří Martinčík,$,∥ Martin Nikl,$ and Kamil Lang*,† Institute of Inorganic Chemistry of the Czech Academy of Sciences, v.v.i, Husinec-Ř ež 1001, 250 68 Ř ež, Czech Republic J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, v.v.i., Dolejškova 3, 182 23 Praha 8, Czech Republic $ Institute of Physics of the Czech Academy of Sciences, v.v.i., Cukrovarnická 10/112, 162 00 Praha 6, Czech Republic ∥ Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Břehová 7, 115 19 Praha 1, Czech Republic † ‡

S Supporting Information *

ABSTRACT: Newly synthesized octahedral molybdenum cluster compound (nBu4N)2[Mo6I8(OOC-1-adamantane)6] revealed uncharted features applicable for the development of X-ray inducible luminescent materials and sensitizers of singlet oxygen, O2(1Δg). The compound exhibits a red-NIR luminescence in the solid state and in solution (e.g., quantum yield of 0.76 in tetrahydrofuran) upon excitation by UV−vis light. The luminescence originating from the excited triplet states is quenched by molecular oxygen to produce O2(1Δg) with a high quantum yield. Irradiation of the compound by X-rays generated a radioluminescence with the same emission spectrum as that obtained by UV−vis excitation. It proves the formation of the same excited triplet states regardless of the excitation source. By virtue of the described behavior, the compound is suggested as an efficient sensitizer of O2(1Δg) upon X-ray excitation. The luminescence and radioluminescence properties were maintained upon embedding the compound in polystyrene films. In addition, polystyrene induced an enhancement of the radioluminescence intensity via energy transfer from the scintillating polymeric matrix. Sulfonated polystyrene nanofibers were used for the preparation of nanoparticles which form stable dispersions in water, while keeping intact the luminescence properties of the embedded compound over a long time period. Due to their small size and high oxygen diffusivity, these nanoparticles are suitable carriers of sensitizers of O2(1Δg). The presented results define a new class of nanoscintillators with promising properties for X-ray inducible photodynamic therapy.

■

quantum dots, and upconverting nanoparticles.3,5,6 Although these concepts improve a light penetration length of exciting light, conventional PDT is still limited to the treatment of tumors on or under the skin, or on the lining of some internal organs or cavities.2 A recent alternative approach relies on the use of scintillating nanoparticles that upon exposure to ionizing radiation, such as X-rays, emit luminescence in the visible region, which, in turn, activates a conjugated photosensitizer through Förster resonance energy transfer (FRET).6−12 Lanthanide-doped fluoride (e.g., LaF3:Ce3+, LaF3:Tb3+), Tb2O3, or SrAl2O4:Eu2+ nanoparticles, or Gd3+/Eu3+ micelles, have been used as radioluminescent sources, and these were conjugated with common photosensitizers such as porphyrins or Rose Bengal. X-ray inducible photodynamic therapy combines radiotherapy with PDT, and this approach has practically no limitation to the penetration depth in tissue that is achievable by X-rays. We initiated research on a new type of sensitizers, other than

INTRODUCTION Photodynamic therapy (PDT) is an approved medical technique for the treatment of various malignancies.1,2 This therapy is based on the destruction of tumoral tissues by reactive oxygen species, most often by the singlet oxygen, O2(1Δg). The photosensitized generation of O2(1Δg) requires oxygen, visible light, and a photosensitizer that efficiently harvests light energy and transfers it to ground-state oxygen that is, in turn, excited into its singlet state. PDT allows for an accurate targeting of tumor cells, and therefore, it is less invasive than other treatments, like surgery, radiation therapy, and chemotherapy. Moreover, photosensitizers can be useful also in diagnosis or imaging, because they can be luminescent and produce O2(1Δg) simultaneously.3,4 A limiting factor of this therapy is the poor transmission of visible light by tissues that reduces the efficiency of tumor destruction when located in deeper tissues. To overcome some of these difficulties, various PDT methodologies that utilize the tissue transparency window in the near-infrared (700−1000 nm) have been developed. Among promising strategies is the use of novel sensitizers with near-infrared absorption, two-photon absorbing sensitizers, © XXXX American Chemical Society

Received: October 4, 2015

A

DOI: 10.1021/acs.inorgchem.5b02282 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry porphyrins,13 that can be directly excited via ionizing radiation in order to increase the efficiency of the excitation process. In this respect, octahedral transition metal cluster complexes [M6Li8La6]n (M = Mo, W, Re), where Li is a face-capping halogen or chalcogen ligand and La is an apical organic or inorganic ligand, are potential candidates. The advantage over porphyrins is their high proton number enhancing the interaction probability with X-rays and relevant photophysical properties, even in the solid state or in the form of aggregates. Despite the evidence of effective absorption of X-rays by [M6Li8La6]n complexes, making them promising contrast agents for X-rays computed tomography,14 their radioluminescent properties have never been recognized. On the other hand, their photophysical properties have been extensively investigated. Upon excitation from the UV to the green spectral regions, these complexes exhibit a broad red-NIR luminescence which is efficiently quenched by oxygen leading to the formation of O2(1Δg).15 The deliberate selection of apical ligands allows for an optimization of the properties toward a projected function. For instance, the coordination of carboxylate ligands to the {Mo6I8}4+ core provides complexes with high luminescence quantum yields and high O2(1Δg) productivity in air atmosphere, and allows for additional functionalization. 16−18 The instability of most of the molybdenum cluster complexes in water at physiological pH constitutes an obstacle for their use in biological applications that can be overcome through their immobilization in inorganic or organic nanocarriers.17,19−23 From a variety of nanocarriers, polystyrene nanoparticles are fairly biocompatible, cell permeable, nontoxic, and very permeable to oxygen, and their excretion is slow.24 Thus, these nanoparticles containing suitable luminescent molecules or photosensitizers of O2(1Δg) have a good performance in imaging of oxygen inside cells or in photosensitized killing of cells. In addition, some polymers including polystyrene are used for the detection of radiation, i.e., as a base in plastic scintillators, which exhibit luminescence upon exposure to ionizing radiation, such as X-rays. The transfer of the absorbed energy from the polymer to Ir-based luminophores has already led to new radioluminescent materials for bioimaging.25−27 The above-mentioned features initiated our interest to investigate radioluminescence properties of octahedral molybdenum cluster compound, (n-Bu4N)2[Mo6I8(OOC-1-adamantane)6] (1) (Figure 1), and its applicability for the preparation of nanoscintillators. We present the synthesis of 1 and the full characterization of its structural and photophysical properties. In contrast to previously investigated nanosized systems,6−11 we suggest a concept that makes energy transfer from the nanoscintillator to the photosensitizer unnecessary since 1 acts as a nanoscintillator and sensitizer of O2(1Δg) at the same time. It eliminates energy losses occurring during energy transfer, a process that is strictly affected by the local arrangement of the interacting components, which is usually difficult to control at the nanoscale. We also discuss proof-of-concept experiments with nanoparticles prepared via nanoprecipitation of sulfonated polystyrene (PS-SO3).

■

Figure 1. Crystallographic representation of the determined molecular structure of 1 with 50% thermal ellipsoids (blue, molybdenum; magenta, iodine; red, oxygen; black, carbon; hydrogen atoms are omitted for clarity). All manipulations were performed under an inert atmosphere using a Schlenk line. Synthesis of (n-Bu4N)2[Mo6I8(OOCC10H15)6] (1). A mixture of Na2[Mo6I8(OMe)6] (200 mg, 110 μmol) and of adamantane-1carboxylic acid (119 mg, 660 μmol) in 5 mL of THF was heated at reflux for 3 days. Upon cooling, a 61 mg amount of (n-Bu4N)Cl (220 μmol) in 15 mL of tetrahydrofuran (THF) was added to the orange slurry, and the dispersion was stirred for 1 h. The solid part was removed by centrifugation (10 000 rpm/5 min), and the orange solution was reduced to 5 mL on a rotary evaporator. Then, 20 mL of diethyl ether was added, and the precipitate was separated by centrifugation (10 000 rpm/5 min). The precipitate was dried under reduced pressure to yield 222 mg of an orange powder (64% yield). ESI-MS m/z: 1333.158 [M]2−. 1H NMR (400 MHz, DMSO-d6, δ ppm): 0.94 (t, 24H, CH3), 1.31 (sextet, 16H, CH2), 1.58 (br s, 16H, CH2), 1.62 (t, 18H, CH2), 1.68 (br s, 18H, CH2), 1.90 (br s, 18H, CH), 3.17 (t, 16H, CH2). Anal. Calcd (%) for C102H162I8Mo6O12N2: C 38.29, H 5.10, N 0.88. Found: C 38.38, H 4.78, N, 0.79. UV−vis (THF): λmax/nm (ε/M−1 cm−1) = 299 (1.06 × 104), 355 (6.6 × 103), 405 sh (4.9 × 103). Crystal data for C66H90I8Mo6O12, 2(C16H36N), 2(C4H8O), C6.42: Mr = 3372.45 g mol−1, 0.344 × 0.222 × 0.145 mm3, triclinic, P1̅, a = 14.4212(2) Å, b = 14.6648(3) Å, c = 17.2369(3) Å, α = 75.211(1)°, β = 76.710(1)°, γ = 78.358(2)°, V = 2956.31(11) Å 3, Z = 1, ρcalc = 1.894 g cm−3, μ = 2.765 mm−1, λ = 0.710 73 Å, 2.84−29.29°, T = 120 K, 64 437 reflections, 14 621 independent reflections (Rint = 0.0292), 642 parameters, 29 restraints, full-matrix least-squares refinement on F2, analytical absorption correction, final R1 (I > 3σ(I)) = 0.0388, final wR2 = 0.1105, largest difference peak and hole 1.78 and −1.04 e− Å−3. The crystallographic data are summarized in Table S1 in the Supporting Information. The structure was deposited with the reference number CCDC 1024201. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Preparation of 1/PS Films. Compound 1 (100 mg, 31.7 μmol) was dissolved in chloroform (20 mL), and polystyrene (900 mg) was added to the orange solution. The suspension was stirred at 60 °C until the gain of approximately 5 mL of a homogeneous mixture. This mixture was then spread on a Teflon foil and allowed to dry overnight to provide films with a thickness of approximately 0.2 mm. Preparation of 1@PS-SO3 Nanoparticles. Compound 1 (5 mg, 1.6 μmol) was dissolved in acetone (20 mL), and partially sulfonated polystyrene nanofibers (45 mg) were added to the orange solution. The suspension was stirred for 1 h at room temperature to dissolve both components; then deionized water (50 mL) was rapidly added.

EXPERIMENTAL SECTION

Reagents and General Procedures. Starting compound Na2[Mo6I8(OMe)6] and partially sulfonated polystyrene nanofibers were prepared according to previously published procedures.28,29 All chemicals were obtained from Sigma-Aldrich and used as received. Solvents for synthesis were dried and degassed by standard methods. B

DOI: 10.1021/acs.inorgchem.5b02282 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Acetone was removed by evaporation at 60 °C under magnetic stirring. The resulting dispersion was filtrated, and its volume was adjusted to a total volume of 50 mL with deionized water ([1] = 32 μM). Luminescence Measurements. Corrected luminescence spectra were recorded on a Fluorolog 3 spectrometer (Horiba Jobin Yvon) with a cooled TBX-05-C photon detection module. The same instrument was used for luminescence lifetime experiments using an excitation at 390 nm (SpectraLED-390). The decay curves were fitted to exponential functions by the iterative reconvolution procedure of the DAS6 software (v. 6.4, Horiba Jobin Yvon, 2009). The luminescence spectra of O2(1Δg) were detected using a Hamamatsu H10330-45 photomultiplier applying excitation wavelengths between 350 and 500 nm. The luminescence quantum yields, ΦL, in oxygenfree THF solutions and the quantum yields of O2(1Δg) formation, ΦΔ, in air-saturated THF were measured by the comparative method as described in the Supporting Information. In some cases, time-resolved luminescence was measured using a LKS 20 laser kinetic spectrometer (Applied Photophysics, U.K.) equipped with R928 photomultiplier (Hamamatsu). Samples were excited by a Lambda Physik FL3002 dye laser (wavelength 425 nm, pulse width 28 ns). The oxygen solubility in air-saturated water (0.28 mM) was used for the calculation of quenching constants. Radioluminescence spectra were measured by a Horiba Jobin Yvon 5000 M spectrofluorometer. The detection part was based on a single grating monochromator with a grating blazing wavelength of 500 nm and a photon counting detector TBX-04. For excitation was used a tungsten DX-W 10 × 1-S 2400 W X-ray tube (short anode, Seifert GmbH, Germany) powered by an ISO-DEBYEFLEX 3003 high voltage supply set to 40 kV and 15 mA. Measured spectra were corrected for the spectral dependence of detection sensitivity. The emission spectrum of bismuth germinate powder (Bi4Ge3O12, abbreviated as BGO) was used as a reference of absolute emission intensity. X-ray Crystallographic Study of 1·THF. Diffraction data were collected at 120 K on a four-cycle diffractometer Gemini (Rigaku Oxford Diffraction) equipped with an Atlas CCD detector, using monochromated Mo Kα radiation (λ = 0.710 73 Å) from a sealed Xray tube monochromatized with graphite monochromator, and collimated with a fiber-optics Mo-Enhance collimator. Integration of the CCD images was performed using the CrysAlisPro software (Agilent Technologies, 2010). The same program was used for indexing of the crystal shape and absorption correction. Crystal structure was solved by direct methods with the program Sir2002.30 The obtained solution was used for the subsequent refinement based on F2 utilizing the Jana2006 software.31 All atomic types, except the hydrogen and carbon atoms in disordered butyl groups, were refined anisotropically. The hydrogen atoms were added to ideal positions with a C−H distance of 0.96 Å and refined as riding. The methyl hydrogen atoms were allowed to rotate freely about the adjacent C−C bonds. The isotropic atomic displacement parameters of hydrogen atoms were evaluated as 1.2−1.5 times Ueq of the parent atom. Tetrabutylammonium cations and THF molecules were found to be disordered over two sets of sites; their relative occupancies were refined to 0.705(4):0.295(4), and 0.782(6):0.218(6), respectively. The C−N and C−C distances in the disordered molecules were restrained to 1.48 (1) and 1.54 (1) Å, respectively. The bond angles in the N1x−C49x chain of the minor fraction of disordered tetrabutylammonium cation were restrained to 110.0(5)°. The atoms of THF and C46x−C49x atoms of Bu4N+ were refined isotropically. The severely disordered solvent molecules (cyclohexane or THF) in voids were approximated by refining occupancies of two carbon atoms (C54 and C55). This was considered as the best approximation for the disordered solvent molecules in order to avoid an alteration of the experimental data using the SQUEEZE procedure.

reported method.17 Compound 1 was obtained by the reaction of Na2[Mo6I8(OMe)6] with 6 equivalents of adamantane-1carboxylic acid in tetrahydrofuran (THF), followed by the replacement of sodium countercations for tetrabutylammonium cations by metathesis in order to increase the solubility of 1 in organic solvents and to improve its dispersibility in polymeric matrices. The formation of the product was monitored using 1 H NMR in d6-DMSO (Figure S1 in the Supporting Information). The disappearance of the 1H signals at 11.99 ppm belonging to COOH protons of starting adamantane-1carboxylic acid was in accordance with the coordination of the ligands to the {Mo6I8}4+ core through their carboxylate function. When compared with starting adamantane-1-carboxylic acid, the 1H NMR signals of the C(2,6,7)H, C(3,5,8)H, and C(4,9,10)H protons of 1 exhibited modest downfield shifts by 0.05, 0.1, and 0.05 ppm, respectively. The integrated intensities of nonoverlapping peaks were in accordance with a 6/2 molar ratio between the adamantane-1-carboxylate ligands and tetrabutylammonium cations. The HR ESI-MS spectra of 1 in THF revealed a single peak with a m/z of 1333.158 corresponding to double-charged anionic species [Mo6I8(OOCC10H15)6]2− (Figure S2 in the Supporting Information). C, H, N elemental analysis with deviations within the 0.4% margin confirmed the purity of the bulk material used for the measurement of the photophysical properties and the preparation of polymer composites. The crystal structure of 1 was studied by X-ray diffraction on a pertinent single crystal grown by slow diffusion of cyclohexane into a concentrated THF solution of 1. As shown in Figure 1, the apical adamantane ligands are coordinated through one of the carboxylate oxygen atom. Intramolecular distances ranging 2.66−2.67, 2.75−2.80, and 2.09−2.11 Å for Mo−Mo, Mo−Ii, and Mo−Oa bonds, respectively, are in accordance with those reported for [Mo6I8(OOC-R)6]2− complexes.17,18 The structure of 1·solvent can be described as an AA stacking of distorted hexagonal layers of the molecules along the c axis with tetrabutylammonium countercations located between the layers and solvents molecules in structural voids (Figure S3 in the Supporting Information). The absorption spectrum of 1 in THF had maxima at 299 and 355 nm with a shoulder at approximately 405 nm and an onset at approximately 500 nm (Figure 2). The photophysical properties are summarized in Table 1. Compound 1 exhibited a

■

RESULTS AND DISCUSSION Precursor Na2[Mo6I8(OMe)6], used for the grafting of adamantane moieties, was prepared according to a previously

Figure 2. Absorption spectrum of 1 in THF. Inset: Luminescence emission spectrum of 1 upon excitation at 440 nm in deoxygenated THF. C

DOI: 10.1021/acs.inorgchem.5b02282 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

also occur under X-ray irradiation. Indeed, these properties directly follow from the relaxation of the excited triplet states and are not affected by the way these triplet states were formed. Accordingly, a low radioluminescence intensity in air atmosphere of approximately 10%, compared with a scintillating standard, BGO powder, could be extrapolated to approximately 60% in an oxygen-free environment (Figure 3). The behavior of the excited states under ionizing radiation is schematically shown in Scheme 1.

Table 1. Photophysical Properties of 1 in THF, 1/PS Film, and 1@PS-SO3 Dispersions in Water at Room Temperaturea sample

λL/nm

1 1/PS

710 710

1@PS-SO3

690

ΦL 0.76

τL/μs

ΦΔ

225 30 (13%), 164 (87%)b 22 (32%), 57 (68%)c 7.7 (22%), 85 (78%)d 1.0 (37%), 4.9 (63%)e

0.76

b

λL is the maximum of the luminescence emission band, ΦL is the quantum yield of luminescence (oxygen-free THF, excited at 440 nm), τL is the lifetime of the triplet states (amplitude fractions in parentheses), and ΦΔ is the quantum yield of singlet oxygen formation in air-saturated THF (excited at 355 and 440 nm). bArgon atmosphere. cAir atmosphere. dDispersion saturated with argon. e Dispersion saturated with air. a

Scheme 1. Diagram Illustrating the X-ray Induced Luminescence and Production of O2(1Δg) by 1a

broad luminescence band with a maximum at 710 nm and a quantum yield, ΦL, of 0.76 in oxygen-free THF (Figure 2). The luminescence lifetime, corresponding to the lifetime of the triplet states, was 225 μs and is comparable to the lifetimes of other highly emissive [Mo6I8(OOC-R)6]2− cluster complexes.15,18,21 We found that the red luminescence is efficiently quenched by oxygen which initiated the investigation of the production of O2(1Δg) by measuring its luminescence at 1270 nm. The measured quantum yield of O2(1Δg) formation in airsaturated tetrahydrofuran, ΦΔ, of 0.76 indicated that 1 is a good O2(1Δg) photosensitizer. In general, the spectral and photophysical features of 1 resembled to those of [Mo6I8(OOCR)6]2− complexes.15,17,18,21 The clustering of heavy atoms in 1 and the high ΦL value encouraged us to investigate the properties of 1 upon absorption of X-rays generated by a tungsten tube. The proof-of-concept experiments were performed with a powdered sample in air atmosphere. X-ray excitation led to the appearance of a broad red-NIR luminescence band, identical to the luminescence spectrum of 1 obtained by excitation of the powder at 440 nm (Figure 3). The result pointed out that the radioluminescence originates from the same excited triplet states as those produced by UV−vis excitation. In this respect, the quenching of the luminescence by oxygen upon excitation of the powder of 1 at 440 nm and the subsequent production of O2(1Δg) evidenced by its characteristic luminescence band at 1274 nm (Figure S4 in the Supporting Information) should

a

Photoionization produces a free electron and an electron hole in the inner shell of heavy atoms of the complex core. The subsequent relaxation of the ionized complex by a cascade of radiative and nonradiative transitions leads to the formation of excited singlet (S1) and triplet (T1) states.

With X-rays having energy up to 100 keV, the photoeffect predominates in the interaction of X-ray photons with 1 producing free electrons and electron holes, mainly in the inner shell of heavy atoms (Mo, I) composing the complex core. The subsequent filling of inner-shell vacancies is characterized by a cascade of X-ray fluorescence photons, Auger electrons, and thermal relaxation. Finally, recombination of the ionized complexes with free electrons and the relaxation of these electrons result in the formation of excited singlet states and triplet states of the complexes and their relaxation via luminescence or O2(1Δg) production. It is worth noting that only a part of the absorbed energy from X-ray photons will be contained in the place of the primary interaction, i.e., the {Mo6I8}4+ inorganic core: primary photoelectrons and secondary particles, such as Auger electrons and X-ray fluorescence photons migrate over distances that are, in most cases, larger than the complex size and should be able to ionize or excite the neighboring complexes. The free electrons recombining with the ionized complexes very likely correspond to primary photoelectrons and Auger electrons emitted by the neighboring complexes. In general, the deposited energy in a macroscopic volume is shared between the surrounding/host material (in the case of cancer tissue it is an aqueous medium) and the active scintillation nanophase.32 For intended application reasons, compound 1 was embedded in a polystyrene (1/PS) matrix in order to investigate whether the radioluminescence properties of 1 were maintained in this polymer carrier. The 1/PS film with 10 wt % of 1 exhibited a radioluminescence band with a maximum at approximately 710 nm (Figure 4). A second band at

Figure 3. Radioluminescence signal of powder of 1 (a), compared with that of BGO powder in an absolute intensity (d), was measured in air atmosphere upon excitation with an X-ray tube (40 kV, 15 mA). The effect of air was obtained by measuring the luminescence signals in air (b) and oxygen-free atmosphere (c) under excitation at 440 nm. D

DOI: 10.1021/acs.inorgchem.5b02282 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

from the matrix with 10 wt % of 1. From the presented results it follows that the radioluminescence produced by the 1/PS film at approximately 710 nm originates from both direct excitation of 1 by X-rays and from energy transfer through the scintillating polymeric matrix. Similarly to the powder material, the radioluminescence of embedded 1 in polystyrene can be expected to be quenched by oxygen to produce O2(1Δg). Polystyrene is known as an oxygen-permeable polymer with a low diffusion barrier for oxygen (oxygen diffusion coefficient D ∼ 3 × 10−7 cm2 s−1).33 Indeed, luminescence decay curves were well-characterized by the Stern−Volmer equation, 1/τT = 1/τT0 + kq[O2], where τT and τT0 are amplitude weighted mean lifetimes of the triplet states at given concentration of oxygen [O2] and under oxygenfree conditions, respectively, and kq is a bimolecular quenching rate constant. The obtained kq of 0.80 ± 0.02 Pa−1 s−1 for the 1/PS film (Figure S5 in the Supporting Information) indicated good accessibility of excited molecules to oxygen. Efficient energy transfer within the polymer matrix, accessibility of excited molecules to oxygen, and observed formation of O2(1Δg) via its luminescence at approximately 1270 nm make the PS matrix an excellent carrier for the design of X-ray switchable materials. For comparison, we also investigated the properties of a poly(methyl methacrylate-co-methacrylic acid) film with embedded 1 (10 wt %; preparative details in the Supporting Information). This polymer has low oxygen solubility and no scintillating properties. As a result, the oxygen quenching of the luminescence was much less effective than in

Figure 4. Absolute intensity comparison of the radioluminescence of the 1/PS film (10 wt % of 1) (a) with that of a pure PS film (b). The samples were measured in air atmosphere and were excited by an Xray tube (40 kV, 15 mA).

approximately 315 nm belonged to the X-ray induced fluorescence of the PS matrix. The intensity of this band was considerably reduced when compared with that of a pure PS film. The favorable overlap of this fluorescence band with the absorption spectrum of 1 (Figure 2) suggested the occurrence of energy transfer from the excited PS matrix to embedded 1. Indeed, the transfer efficiency estimated from the fluorescence intensities of the pure PS film and of 1/PS reached approximately 85% indicating an effective energy harvesting

Figure 5. 1@PS-SO3 nanoparticles: AFM images of two nanoparticles with maximum tip heights of 38 and 34 nm (a, b); size distribution in water dispersions at room temperature measured by light scattering (c). E

DOI: 10.1021/acs.inorgchem.5b02282 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry case of 1/PS (Figure S6 in the Supporting Information), and no generation of O2(1Δg) was detected. The application of hydrophobic or water unstable compounds in biology requires their encapsulation in suitable nanocarriers. In the presented case, we selected polystyrene nanoparticles with 10 wt % of 1 (1@PS-SO3) that were prepared using a nanoprecipitation process. The partial sulfonation of PS nanofibers used for the preparation of 1@ PS-SO3 was essential for obtaining nanosized particles. The diameter of nanoparticles was 50 nm with a narrow size distribution (PDI = 0.19) as measured by dynamic light scattering (DLS) (Figure 5C). The AFM observations of separated nanoparticles (Figure 5A,B) gave a thickness and lateral sizes in accordance with the hydrodynamic diameters measured by DLS. On the contrary, the use of nonsulfonated polystyrene material led to large sedimenting particles with a diameter of approximately 400 nm. The hydrophobic character of compound 1 imposed by the adamantane moieties appears to be fundamental for the encapsulation of 1 in the polymeric nanocarriers without the need of covalent bonding. Indeed, a reported attempt to encapsulate (n-Bu4N)2[Mo6I8(NO3)6] in polystyrene particles by the precipitation method resulted in a segregation of both components due to the hydrophilic character of the nitrate apical ligands.20 As shown in Figure 6, the luminescence maximum of 1@PSSO3 nanoparticles in water was slightly blue-shifted to 690 nm

Figure 7. Luminescence decay curves of 1@PS-SO3 recorded at 700 nm in argon- (a), air- (b), or oxygen-saturated (c) water dispersions; excitation at 425 nm. Inset: Stern−Volmer plot of rate constant vs oxygen concentration (d).

1@PS-SO3 water dispersions using amplitude weighted mean lifetimes gave a kq of (9.8 ± 0.7) × 108 M−1 s−1 (Figure 7) that is close to a value typical for aqueous solutions of photosensitizers (1−2 × 109 M−1 s−1).34 The high bimolecular quenching rate constant indicates good accessibility of the excited complexes to oxygen to produce O2(1Δg). Since O2(1Δg) has a short lifetime it limits its reactivity to the proximity of a site where it was formed. In fact, O2(1Δg) produced in a matrix can be formed at the surface and/or in the bulk, and so the reactions of O2(1Δg) with a target species, commonly located at the surface or in the close surroundings, depend on the ability of O2(1Δg) to reach these locations. The lifetime of O2(1Δg) in PS is approximately 14 μs,35 during which interval it diffuses over a mean radial distances of 85 nm that is more than three times longer than the nanoparticle radius, as estimated using d = (6tD)1/2,36 where D is the diffusion coefficient for oxygen (approximately 3 × 10−7 cm2 s−1) and t is the time at which 5% of the originally produced O2(1Δg) remains (i.e., t = 3 × τΔ, where τΔ is the O2(1Δg) lifetime). Thus, the PS matrix not only contributes to the excitation of 1 by X-rays, but acts as a carrier allowing for effective concentrations of O2(1Δg) in the surroundings.

■

Figure 6. Luminescence emission spectra of 1@PS-SO3 dispersions in water saturated by air (a) and in the absence of oxygen (b). Excited at 440 nm.

CONCLUSION In summary, the octahedral molybdenum cluster compound reported here represents the first radioluminescent compound with suggested X-ray induced O2(1Δg) production. These properties are highly desirable in order to simplify the architecture of nanoscintillators for X-ray induced photodynamic therapy. In order to take a step toward this application, the compound was incorporated in PS matrices. The resulting PS films exhibited enhanced radioluminescence intensity due to energy harvesting from the scintillating PS matrix via energy transfer. Sulfonated PS nanoparticles with the embedded compound showed promising properties in aqueous media, namely, the stability of key photophysical parameters and oxygen accessibility of the embedded molecules. From a prospective point of view, this study paves the way toward the investigation of the radioluminescence properties of other octahedral cluster compounds. The variety of possible metals (e.g., W, Re with a higher Z number than in case of Mo) and ligands is propitious with regards to the tuning and optimization of designed properties. In addition, the successful application of molybdenum and rhenium octahedral cluster

when compared with the spectrum of 1 in THF. The effects of encapsulation on the stability and photophysical properties of 1 followed from the analysis of corresponding luminescence spectra over a time period (Figure S7 in the Supporting Information). The shape and luminescence intensities over the course of 12 days were unchanged indicating that the photophysical properties of embedded 1 remained intact. This protective effect can be explained by the hydrophobic environment provided by the PS matrix not enabling the hydrolysis of compound 1 that otherwise would proceed in a water environment. The luminescence of nanoparticles was efficiently quenched by oxygen that is characteristic of the O2(1Δg) production similarly to the above-described features (Figures 6 and 7). Time-resolved luminescence spectroscopy revealed that the luminescence decay can be best analyzed by a two-exponential model in both air-saturated solutions and in oxygen-free solutions (Table 1). The Stern−Volmer plot constructed for F

DOI: 10.1021/acs.inorgchem.5b02282 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(17) Kirakci, K.; Kubát, P.; Dušek, M.; Fejfarová, K.; Šícha, V.; Mosinger, J.; Lang, K. Eur. J. Inorg. Chem. 2012, 2012, 3107−3111. (18) Sokolov, M. N.; Mihailov, M. A.; Peresypkina, E. V.; Brylev, K. A.; Kitamura, N.; Fedin, V. P. Dalton Trans. 2011, 40, 6375−6377. (19) Aubert, T.; Cabello-Hurtado, F.; Esnault, M.-A.; Neaime, C.; Lebret-Chauvel, D.; Jeanne, S.; Pellen, P.; Roiland, C.; Le Polles, L.; Saito, N.; Kimoto, K.; Haneda, H.; Ohashi, N.; Grasset, F.; Cordier, S. J. Phys. Chem. C 2013, 117, 20154−20163. (20) Efremova, O. A.; Shestopalov, M. A.; Chirtsova, N. A.; Smolentsev, A. I.; Mironov, Y. V.; Kitamura, N.; Brylev, K. A.; Sutherland, A. J. Dalton Trans. 2014, 43, 6021−6025. (21) Kirakci, K.; Šícha, V.; Holub, J.; Kubát, P.; Lang, K. Inorg. Chem. 2014, 53, 13012−13018. (22) Cordier, S.; Dorson, F.; Grasset, F.; Molard, Y.; Fabre, B.; Haneda, H.; Sasaki, T.; Mortier, M.; Ababou-Girard, S.; Perrin, C. J. Cluster Sci. 2009, 20, 9−91. (23) Cordier, S.; Grasset, F.; Molard, Y.; Amela-Cortes, M.; Boukherroub, R.; Ravaine, S.; Mortier, M.; Ohashi, N.; Saito, N.; Haneda, H. J. Inorg. Organomet. Polym. Mater. 2015, 25, 189−204. (24) Wolfbeis, O. S. Chem. Soc. Rev. 2015, 44, 4743−4768. (25) Campbell, I. H.; Crone, B. K. Appl. Phys. Lett. 2007, 90, 012117. (26) Osakada, Y.; Pratx, G.; Hanson, L.; Solomon, P. E.; Xing, L.; Cui, B. Chem. Commun. 2013, 49, 4319−4321. (27) Sguerra, F.; Marion, R.; Bertrand, G. H. V.; Coulon, R.; Sauvageot, E.; Daniellou, R.; Renaud, J.-L.; Gaillard, S.; Hamel, M. J. Mater. Chem. C 2014, 2, 6125−6133. (28) Kirakci, K.; Fejfarová, K.; Kučeráková, M.; Lang, K. Eur. J. Inorg. Chem. 2014, 2014, 2331−2336. (29) Henke, P.; Kozak, H.; Artemenko, A.; Kubát, P.; Forstová, J.; Mosinger, J. ACS Appl. Mater. Interfaces 2014, 6, 13007−13014. (30) Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2003, 36, 1103. (31) Petricek, V.; Dusek, M.; Palatinus, L. Z. Kristallogr. - Cryst. Mater. 2014, 229, 345−352. (32) Bulin, A.-L.; Vasil’ev, A.; Belsky, A.; Amans, D.; Ledoux, G.; Dujardin, C. Nanoscale 2015, 7, 5744−5751. (33) Gao, Y.; Baca, A. M.; Wang, B.; Ogilby, P. R. Macromolecules 1994, 27, 7041−7048. (34) Kubát, P.; Mosinger, J. J. Photochem. Photobiol., A 1996, 96, 93− 97. (35) Jesenská, S.; Plíštil, L.; Kubát, P.; Lang, K.; Brožová, L.; Popelka, Š.; Szatmáry, L.; Mosinger, J. J. Biomed. Mater. Res., Part A 2011, 99A, 676−683. (36) Pimenta, F. M.; Jensen, R. L.; Holmegaard, L.; Esipova, T. V.; Westberg, M.; Breitenbach, T.; Ogilby, P. R. J. Phys. Chem. B 2012, 116, 10234−10246. (37) Osakada, Y.; Pratx, G.; Sun, C.; Sakamoto, M.; Ahmad, M.; Volotskova, O.; Ong, Q.; Teranishi, T.; Harada, Y.; Xing, L.; Cui, B. Chem. Commun. 2014, 50, 3549−3551. (38) Chen, H.; Colvin, D. C.; Qi, B.; Moore, T.; He, J.; Mefford, O. T.; Alexis, F.; Gore, J. C.; Anker, J. N. J. Mater. Chem. 2012, 22, 12802−12809.

compounds as luminescent probes in living cells could be extended to X-ray luminescence computed tomography, a new imaging method based on the optical detection of X-rayexcitable phosphor nanoparticles.25,37,38 Thus, these materials may offer the development of feasible methods for tumor theranostic applications.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02282. Experimental details, crystallographic data, 1H NMR spectra, mass spectrum, luminescence spectrum, and decay curves (PDF) Crystallographic data (CIF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +(420) 266 172 194. *E-mail: [email protected]. Phone: +(420) 266 172 193. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Czech Science Foundation (No. 13-05114S). We thank Prof. J. Mosinger for providing us sulfonated polystyrene nanofibers.

■

REFERENCES

(1) Singh, S.; Aggarwal, A.; Bhupathiraju, N. V. S. D. K.; Arianna, G.; Tiwari, K.; Drain, C. M. Chem. Rev. 2015, 115, 10261−10306. (2) Hu, J.; Tang, Y.; Elmenoufy, A. H.; Xu, H.; Cheng, Z.; Yang, X. Small 2015, 11, 5860−5887. (3) Wu, X.; Chen, G.; Shen, J.; Li, Z.; Zhang, Y.; Han, G. Bioconjugate Chem. 2015, 26, 166−175. (4) Lovell, J. F.; Liu, T. W. B.; Chen, J.; Zheng, G. Chem. Rev. 2010, 110, 2839−2857. (5) Cheng, L.; Wang, C.; Feng, L.; Yang, K.; Liu, Z. Chem. Rev. 2014, 114, 10869−10939. (6) Lucky, S. S.; Soo, K. C.; Zhang, Y. Chem. Rev. 2015, 115, 1990− 2042. (7) Chen, W.; Zhang, J. J. Nanosci. Nanotechnol. 2006, 6, 1159−1166. (8) Bulin, A.-L.; Truillet, C.; Chouikrat, R.; Lux, F.; Frochot, C.; Amans, D.; Ledoux, G.; Tillement, O.; Perriat, P.; Barberi-Heyob, M.; Dujardin, C. J. Phys. Chem. C 2013, 117, 21583−21589. (9) Tang, Y.; Hu, J.; Elmenoufy, A. H.; Yang, X. ACS Appl. Mater. Interfaces 2015, 7, 12261−12269. (10) Chen, H.; Wang, G. D.; Chuang, Y.-J.; Zhen, Z.; Chen, X.; Biddinger, P.; Hao, Z.; Liu, F.; Shen, B.; Pan, Z.; Xie, J. Nano Lett. 2015, 15, 2249−2256. (11) Kašcǎ ḱ ová, S.; Giuliani, A.; Lacerda, S.; Pallier, A.; Mercère, P.; Tóth, E.; Réfrégiers, M. Nano Res. 2015, 8, 2373−2379. (12) Bárta, J.; Č uba, V.; Pospíšil, M.; Jarý, V.; Nikl, M. J. Mater. Chem. 2012, 22, 16590−16597. (13) Luksiene, Z.; Juzenas, P.; Moan, J. Cancer Lett. 2006, 235, 40− 47. (14) Krasilnikova, A. A.; Shestopalov, M. A.; Brylev, K. A.; Kirilova, I. A.; Khripko, O. P.; Zubareva, K. E.; Khripko, Y. I.; Podorognaya, V. T.; Shestopalova, L. V.; Fedorov, V. E.; Mironov, Y. V. J. Inorg. Biochem. 2015, 144, 13−17. (15) Kirakci, K.; Kubát, P.; Langmaier, J.; Polívka, T.; Fuciman, M.; Fejfarová, K.; Lang, K. Dalton Trans. 2013, 42, 7224−7232. (16) Molard, Y.; Dorson, F.; Cîrcu, V.; Roisnel, T.; Artzner, F. Angew. Chem., Int. Ed. 2010, 49, 3351−3355. G

DOI: 10.1021/acs.inorgchem.5b02282 Inorg. Chem. XXXX, XXX, XXX−XXX