- Email: [email protected]
GdVO4:Eu3+ nanoparticles – Methylene Blue complexes for PDT: Electronic excitation energy transfer study
Journal of Luminescence 192 (2017) 975–981
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
GdVO4:Eu3+ nanoparticles – Methylene Blue complexes for PDT: Electronic excitation energy transfer study
MARK
⁎
S.L. Yefimova , T.N. Tkacheva, P.O. Maksimchuk, I.I. Bespalova, K.O. Hubenko, V.K. Klochkov, A.V. Sorokin, Yu.V. Malyukin Institute for Scintillation Materials National Academy of Sciences of Ukraine, 60 Nauky ave., 61072 Kharkiv, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: Gadolinium orthovanadate nanoparticles Photosensitizer Methylene Blue Electronic excitation energy transfer
Europium-doped gadolinium orthovanadate nanoparticles (GdVO4:Eu3+) were used as a donor in fluorescence resonance energy transfer (FRET) experiments in water solutions. Using Methylene Blue as an excitation energy acceptor, we have observed highly efficient FRET between GdVO4:Eu3+ nanoparticles and MB. Despite the strong decrease of both the donor GdVO4:Eu3+ luminescence and lifetime, the FRET-induced emission of the acceptor MB has not been observed. This feature has been explained by small fluorescence quantum yield of MB, high efficiency of the intersystem crossing and triplet state formation for MB and small amount of MB molecules bonded to the GdVO4:Eu3+ nanoparticles. The long lifetime, large Stokes shift, narrow emission, X-ray excited luminescence and high FRET efficiency (∼ 90%) make GdVO4:Eu3+ nanoparticles prospective as an excitation source for the photosensitizer MB activation in such applications as photodynamic therapy.
1. Introduction Despite the better understanding of tumor biology and improved diagnostic devices observed in last years, cancer remains one of the world's most devastating diseases being the second leading cause of death [1,2]. Current cancer treatments include surgery, radiation, chemotherapy, hormone therapy and immune therapy [1,2]. Unfortunately, conventional anti-cancer treatments have many disadvantages including high toxicity, effects on healthy cells, tissues and organs and variety of side effects. Photodynamic therapy (PDT) is an alternative approach to conventional and developed cancer treatment methods and was clinically used for treatment of certain kinds of cancers, pre-cancer lesions and other diseases [3–6]. PDT involves the use of light-sensitive dye called photosensitizer (PS) combined with visible light of the appropriate wavelength to match the absorption spectrum of the PS. The third principal component is oxygen molecules. Absorbed photons transform PS from its ground state (singlet state) into the excited singlet state (see Scheme 1). The excited molecule can fall back to its ground state with concomitant emission of fluorescence. All photosensitizers are usually poorly fluorescent molecules due to high quantum yield of an electron spin conversion to long-lived triplet state (intersystem crossing process). This triplet PS can interact with molecular oxygen (3O2) and neighbor molecules in two pathways: an electron or hydrogen atom transfer (type I reaction) or energy transfer (type II reaction) (see Scheme 1) leading to the formation of reactive oxygen species (ROS) and singlet oxygen (1O2), respectively [4–10]. It is ⁎
believed that 1O2 produced through type II reaction is the primarily ROS that causes damage to nearby biomolecules. Despite the numerous advantages of the PDT approach, this method has the main drawback, i.e. the low penetration depth of exciting light into human tissue (about 6 mm) even in the near infrared spectral range (700–1100 nm), where most tissue chromophores absorb weakly [11,12]. That requires direct light delivery for PDT activation and restricts PDT use to superficial tumor [4–6]. One of the possible ways to overcome this drawback is using X-ray luminescent (scintillating) nanoparticles as an excitation or “light” source for photosensitizer molecules to activate PDT, which has become a hot topic as a new method for cancer treatment [13–15]. Within this concept, X-ray excited PS is composed of scintillating and photosensitizing parts with an effective energy transfer between them. In order to use nanoparticles to activate PS molecules, both parts could be close enough spatially. A typical mechanism for energy transfer is fluorescence excitation energy transfer (FRET). In above-mentioned case, FRET refers to the transfer of energy the initially excited donor (scintillating NPs) to an acceptor (PS molecule). Efficient energy transfer requires also that the emission spectrum of the donor overlap the absorption band of the acceptor. Since the first announcement by W. Chen and J. Zhang in 2006 [13] this idea has been exploited in many research groups using various scintillation nanoparticles, e.g. CeF3, LaF3:Ce3+, _LaF3:Tb3+, LuF3:Ce3+, CaF2:Mn2+, CaF2:Eu2+, BaFBr:Eu2+, BaFBr:Mn2+, CaPO4:Mn2+, SrAl2O4:Eu2+, rare-earth elements oxides, ZnO, ZnS, ZnS:Cu, CO, TiO2 and different ways of their binding with PS molecules [16–24].
Corresponding author. E-mail address: [email protected] (S.L. Yefimova).
http://dx.doi.org/10.1016/j.jlumin.2017.08.044 Received 31 January 2017; Received in revised form 18 August 2017; Accepted 21 August 2017 Available online 23 August 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
37.5 mL of Na3VO4 (0.01 M) was flowed drop wise (рН = 10.5). The mixture was intensively stirred by using a magnetic stirrer and heated on a water bath under a reflux condenser for 24 h at 100 °С. Obtained colourless transparent solution scatters light under the side illumination (the Tyndall cone). Then the solution was cooled and dialyzed against water for 24 h to remove the excess of ions. A dialysis membrane with a molecular weight cut-off of 12 kDa was used. 2.3. Instrumentation and characterization Synthesized VNPs were characterized using Transmission Electron Microscopy (TEM-125K electron microscope, Selmi, Ukraine) and Dynamic Light Scattering method (ZetaPALS analyzer, Brookhaven Instruments Corp., USA). Absorption spectra were measured using a Specord 200 spectrometer (Analytik Jena, USA). Luminescence spectra were taken with a spectrofluorimeter Lumina (Thermo Scientific, USA). Luminescence decay curves were registered using a computer-controlled setup based on a grating monochromator MDR-23 operating in Time-Correlated Single Photon Counting (TCSPC) mode (Picoquant TimeHarp 260, Germany). An excitation of samples was induced by a YAG:Nd3+ pulsed laser NL202 (forth harmonic, λexc ~ 266 nm, EKSPLA, Lithuania). The Instrumental Response Function (IRF) was about 7 ns. The obtained luminescence decay curves were analyzed using FluoFit software for data analysis (PicoQuant, Germany). The X-ray luminescence was excited by an X-ray generator «РЕЙС» (U = 25 kV, I = 37 µA) and registered using the same computed controlled setup combined with a Hamamatsu R9110 PMT. Absolute quantum yield of GdVO4:Eu3+ photoluminescence was measured using a homemade integrating sphere (diameter of 100 mm), which provides reflectance > 99% over the 400–1000 nm range. As an excitation source, we use a xenon lamp, the necessary excitation wavelength (λexc = 280 nm) were selected using a grating monochromator MDR-23. The luminescence was collected using a microspectrometer USB4000 (OceanOptics, USA) connected with an integrated sphere. The absolute quantum yield was calculated using the method described in [31]. The experimental setup was adjusted and tested on a standard dye (rhodamine 6 G in ethanol).
Scheme 1. Schematic representation of photosensitizer (PS) energetic levels and photodynamic reactions with molecular oxygen.
In the present paper, we discuss the possibility for gadolinium orthovanadate nanoparticles doped with europium ions GdVO4:Eu3+ (VNPs) to be used as an excitation source for Methylene Blue (MB) photosensitizer. VNPs exhibit strong absorption in the UV spectral range and luminescence excited both within the UV band and under Xray excitation with luminescence bands overlapping the MB absorption band. One more advantage of lanthanide NPs as an energy donor is very long lifetime (in the millisecond range). In our previous papers, we reported on biological activity of rare-earth orthovanadate nanoparticles (pro-/antioxidant properties, effects on bioenergy processes in cell mitochondria, selective accumulation in cell nuclei) and ordered dye adsorption near the surface of nanoparticles [25–27]. Thus, we consider VNPs to be prospective candidates for the PDT application. MB is a well-known photosensitizer used in variety applications including PDT [28,29]. At the first stage, we analyze the efficiency of VNPs as an electronic excitation energy donor in fluorescence resonance energy transfer experiments in aqueous solutions. Our study indicates high potential of GdVO4:Eu3+ nanoparticles as an excitation source for MB activation. 2. Experimental 2.1. Chemicals
2.4. Preparation of VNPs – dyes aqueous solutions
Gadolinium chlorides GdCl3·6H2O (99.9%), europium chloride EuCl3·6H2O (99.9%), disodium EDTA·2Na (99.8%) and anhydrous sodium metavanadate NaVO3 (96%) were obtained from Acros organic (USA) and all used without further purification. Sodium hydroxide NaOH (99%) was purchased from Macrohim (Ukraine). Sodium orthovanadate Na3VO4 solution was obtained by adding a 1 M solution of NaOH in aqueous solution NaVO3 to pH = 13. Cationic dyes 3,7-bis(dimethylamino)phenazathionium chloride (Methylene Blue (MB), Mw = 373,90 g/mol), 3,3-diethyloxacarbocyanine iodide (DiOC2, Mw = 460.31 g/mol) were purchases from Sigma-Aldrich (USA) and used as received. Cationic dye 1,1′-dimethyl-3,3,3′,3′-tetramethylindodicarbocyanine tetrafluoroborate (DiDC1, Mw = 484.12 g/mol) was obtained from the dye collection of Dr. Igor Borovoy (Institute for Scintillation Materials, NAS of Ukraine) with the purity controlled by thin layer chromatography (Fig. 1). All other chemicals were of analytical grade.
Solutions for investigations were prepared as following. First, stock solutions of the dyes (MB, DiDC1 or DiOC2) in chloroform (1 mM) were prepared. To obtain VNPs – dyes aqueous solutions with different concentrations of dyes, required amount of the dye stock solution was added in a flask and carefully stirred using a rotary evaporator (Rotavapor R-3, Buchi) during 1 h to a complete evaporation of chloroform. Then 1 mL of a VNPs aqueous solution (1 g/L) was added and a flask was gently shaken during 1 h for dye – VNPs complex formation. Thus, the concentration of VNPs in solutions was kept constant (1 g/l), while the dye concentration is varied from 1·10−6 M to 1·10−4 M. We took no action to remove unbounded dye molecules, a large fraction of dye molecules in a solution is in unbound form. 3. Results and discussion Following a colloidal route, we obtained well-crystallized spindle-like GdVO4:Eu3+ nanoparticles of 10×50 nm ± 5 nm size, as verified by TEM image (Fig. 2a). To stabilize GdVO4:Eu3+ nanoparticles in aqueous solutions, disodium EDTA was used, which carboxylate groups impart a negative charge to the nanoparticle surface. The overage hydrodynamic diameter of GdVO4:Eu3+ nanoparticles is 56.4 ± 1.5 nm, zeta-potential is – 20.03 ± 2.2 mV (pH = 7.8). Photoluminescence properties of ReVO4:Eu3+ nanoparticles are well-studied [32–36]. The luminescence excitation and emission spectra of synthesized GdVO4:Eu3+ nanoparticles are presented in Fig. 2b,c. The excitation spectrum corresponding to the
2.2. Synthesis of GdVO4:Eu3+ colloidal solutions Aqueous colloidal solutions of gadolinium orthovanadate nanoparticles doped with europium ions Gd0.9Eu0.1VO4 (GdVO4:Eu3+) were synthesized according to the method reported earlier [30]. First, 0.4 mL of aqueous solution of gadolinium chloride (1 M) was mixed with 0.05 mL of europium chloride (1 M), then 49.55 mL of doubly distilled water were added to the mixture. Then obtained solution was mixed with 37.5 mL of disodium EDTA solution (0.01 M). Then, 976
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
Fig. 1. Chemical structures of used organic dye molecules.
Eu3+ emission of VNPs (Fig. 2b) consists of the intense wide band in the 250–350 nm spectral range and several weaker bands in the longer-wavelength region (see inset in Fig. 2b). The wide band corresponds charge transfer from oxygen anions to vanadium ions, whereas the weak bands
belong to the f→f transitions of europium ions within the 4f6 electron configuration [32]. The luminescence spectrum of GdVO4:Eu3+ nanoparticles (Fig. 2c) results from transition within the f–electron configuration of the europium ions. The main contribution is the 5D0–7F2,4 forced
Fig. 2. Transmission electronic microscopy (TEM) image (a), absorption (b), luminescence, λexc = 280 nm (c) and X-ray induced luminescence (d) spectra of synthesized GdVO4:Eu3+ nanoparticles.
977
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
Fig. 3. Luminescence spectrum of GdVO4:Eu3+ nanoparticles, λexc = 280 nm and absorption and luminescence spectra of MB, λexc = 650 nm (a) and DiDC1, λexc = 630 nm (b). Normalized data.
acceptor molecules can be attached to the VNPs surface thus increasing the FRET efficiency [26,27]. Formation VNPs-dye complexes was confirmed by increasing VNPs hydrodynamic diameter (91.1 nm) of particles and decreasing zeta-potential (–10.5 mV) in solution containing VNPs and MB molecules (see Supplementary data, Fig. 1S). As shown in Fig. 4 in the presence of MB in a solution, both VNPs luminescence intensity and lifetime decrease. It should be noted that the quenching of VNPs luminescence could be governed by several mechanisms: (i) static quenching (formation of ground state non-luminescent VNPs-dye complexes due to dye adsorption on the surface of NPs) and (ii) dynamic quenching related to the processes in the VNPs excited state and leading to non-radiative deactivation of excitation energy (e.g., energy or electron transfer) [38,39]. It is well known that both mechanisms provoke the decrease of luminescence intensity, whereas lifetime decrease is associated with dynamic quenching only [38,39]. Therefore, simple dye molecule adsorption within the surface of VNPs cannot affect the lifetime without additional processes in the excited state. Electron transfer between Eu3+ and MB is unlikely to appear. MB cation can be easily reduced via electron accepting with the formation of colourless so-called leukoform of the MB dye [40], but Eu3+ ion cannot give an electron. Moreover, we did not observe solution decolouration upon VNPs–MB complexation. Thus, we consider VNPs luminescence intensity and lifetime decrease in the presence of MB to be due to fluorescence resonance energy transfer. The changes in the VNPs luminescence parameters depend on the MB concentration (Fig. 4). Luminescence decay curves both in presence and in absence of MB are found to be two-exponential (Fig. 4b). In case of a solution without MB (Fig. 4b, curve 1), the decay curve can be fitted to the equation y=10% exp ( − t /0.324 ms ) +90% exp ( − t /0.899 ms ) . Two-exponential luminescence decay was observed for Eu3+ ions in Y0.6Eu04VO4 nanocrystals and was explained by the existence of two types of Eu3+ located close to the surface of the nanocrystal and in the volume, respectively [41]. Eu3+ ions located near the surface are well known to be more affected by the non-radiative decay process [34,41]. Thus the shorter component in the luminescence decay is associated with the luminescence of Eu3+ ions located near the surface of VNPs. The average luminescence lifetime was calculated as < τav > = f1 τ1 + f2 τ2 , where fi is the fractional contribution of i-th component to the luminescence decay (0.1 and 0.9 for τ1 and τ2 , respectively) [38,39] and is 0.842 ms. The increase in the MB concentrations in the solutions leads to the gradual decrease of both the short and long lifetime components (Table 1). At highest MB concentration, the average lifetime reduces in
electric-dipole transitions [34]. Other components of weaker importance are the 5D0–7F1,3 magnetic dipole transitions [34]. For europium content of 10%, quantum yield QD = 0.16 , for excitation in vanadate matrix (λexc = 280 nm) is obtained that correlates with the data presented in [37]. GdVO4:Eu3+ nanoparticles exhibit also X-ray excited luminescence (Fig. 2d). The photoluminescence and X-ray excited luminescence spectra of VNPs perfectly match the absorption spectrum of MB (Fig. 3a). Thus, VNPs could be used as a source to excite MB at the expense of fluorescence resonance energy transfer between VNPs (excitation energy donor) and MB (excitation energy acceptor) [38,39]. Efficient energy transfer requires the emission band of the donor to be overlapped with the absorption band of the acceptor and that the donor and the acceptor are close enough spatially to permit the energy transfer (10–100 Å) [38,39]. This energy transfer results in a decrease of donor emission, and an increase of acceptor emission, as well as a decrease of donor lifetime [38,39]. We calculated the normalized spectral overlap between the donor emission and the acceptor absorption, J (λ ),(M−1cm3) [38]:
J (λ ) =
∞ ∫0 FD (λ )⋅εA (λ )⋅λ4dλ , ∞ ∫0 FD (λ ) dλ
(1)
where FD (λ ) is the donor luminescence spectrum, ε (λ ) is the extinction coefficient of the acceptor at λ, (M−1 cm−1). To calculate J(λ) we took into account only 5D0–7F2 and 5D0–7F4 electric-dipole transitions centered at 618 and 700 nm, respectively (Fig. 2c). The 5D0–7F1,3 magnetic dipole transitions do not contribute to FRET. Using the average index of refraction for VNPs n = 2.0704 [40], an orientation parameter k 2 = 2/3 assuming random orientation of the donor and acceptor transition dipoles, calculated quantum yield of the VNPs QD = 0.16, and obtained J(λ) = 4.807·10–13 M−1 cm3, we calculated the Förster distance between the donor and acceptor R0, Å [38]:
R 0 = 9, 78⋅103 (k 2⋅n−4QD J (λ ))1/6
(2)
For the VNPs – MB pair, R0 = 36.5 Å that could ensue the effective FRET [38,39]. In water solutions, VNPs are stabilized with disodium EDTA, which form a shell around the nanoparticles. We exploited the fact that MB cations can be adsorbed within this shell near the surface of VNPs due to coulomb interactions between the negatively charged carboxylate groups of EDTA shell and MB cations that will ensure the necessary distance between the donor and acceptor required for FRET. Due to the size of VNPs (hydrodynamic diameter of 56 nm), several 978
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
Fig. 4. Luminescence spectra, λexc = 280 nm (a) and luminescence decay curves, λreg = 618 nm (b) of GdVO4:Eu3+ nanoparticles in the solutions with different MB concentrations: 1 – without MB; 2 – 1·10−6 M; 3 – 5·10−6 M; 4 – 1·10−5 M; 5 – 5·10−5 M; 6 – 1·10−4 M. The inset shows the luminescence spectrum of MB at λexc = 650 nm.
Table 1 GdVO4:Eu3+ VNPs luminescence decay parameters changes depending on MB concentration. [MB], M
% f1
τ1, ms
% f2
τ2, ms
〈τav〉, ms
0 1 5 1 5 1
10 10 13 10 36 32
0.324 0.310 0.314 0.203 0.171 0.096
90 90 87 90 64 68
0.899 0.862 0.680 0.579 0343 0.236
0.842 0.807 0.592 0.541 0.281 0.191
× × × × ×
10−6 10−6 10−5 10−5 10−4
E=
IDA , ID
(3)
E=
τDA , τD
(4)
where IDA (τDA ) and ID (τD ) are the donor luminescence intensity (lifetime) in the presence and absence of the acceptor, respectively. At highest MB concentration in the solution (1·10−4 M), the FRET efficiencies calculated using Eqs. (3) and (4) are 91% and 77%, respectively. It should be noted that the use of lifetime in Eq. (4) could be a source of confusion, because Eq. (4) assumes that the decay of the donor is single exponential in the absence and presence of the acceptor. In case of multi-exponential decay, one should use the average lifetimes, but the value obtained could be not correct [38]. On the other hand, Eq. (3) takes into account the emission intensities arisen both from Eu3+ ions located near the surface and closer to the center of VNPs and gives a more accurate determination of the FRET efficiency [38,41]. Despite the effective decrease of the donor GdVO4:Eu3+ VNPs
4.4 times and becomes 0.191 ms. At the same time, we have observed some increase in the short component fractional contribution up to 32–36% at high MB concentrations that confirms the stronger quenching of Eu3+ ions located near the surface of VNPs. The efficiency of FRET can be calculated using luminescence intensity or decay parameters using Eqs. (3) and (4) [38]:
Fig. 5. Luminescence spectra, λexc = 280 nm (a) and luminescence decay curves, λreg = 618 nm (b) of GdVO4:Eu3+ nanoparticles in the solutions with different DiDC1 concentrations: 1 – without DiDC1; 2 – 1·10−6 M; 3 – 5·10−6 M; 4 – 1·10−5 M. The inset shows the luminescence spectrum of the solution with [DiDC1] = 5·10−6 M.
979
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
Fig. 6. Luminescence spectra, λexc = 280 nm (a) and luminescence decay curves, λreg = 618 nm of GdVO4:Eu3+ nanoparticles in the solutions with different DiOC2 concentrations: 1 – without DiOC2; 2 – 1·10−6 M; 3 – 5·10−6 M; 4 – 1·10−5 M; 5 – 5·10−5 M.
4. Conclusion
luminescence, we did not observe the FRET-induced emission of the acceptor MB (Fig. 4a), while at the excitation within the acceptor absorption band, the intrinsic MB emission is observed (see inset in Fig. 4a). We did not observe any MB aggregation on the surface of VNPs even at high dye concentrations in the solution (see Supplementary data, Fig. 2S) that could be the reason of MB emission quenching. We explain this phenomenon as follows. MB is non-fluorescent dye, which shows residual emission (fluorescence quantum yield Qf = 0.04 in ethanol) [28,42]. Moreover, it is known that MB possesses high intersystem crossing quantum yield (Qp= 0.54), i.e. excited MB molecule undergoes a conversion to its long-lived triplet state (see Scheme 1) [28]. The excited triplet state of organic molecules in aqueous solutions at room temperature is deactivated in the non-radiative way. For PS molecules, this way includes the interaction with molecular oxygen or electron-transfer reactions as shown in Scheme 1. We also should take into consideration that in aqueous solutions containing GdVO4:Eu3+ VNPs and MB, only a small part of the dye molecules will be adsorbed on the surface of VNPs, and half of them will undergo a non-radiative deactivation via triplet state formation. To confirm this arguments, we use other dye molecule (DiDC1, see Fig. 1) as an excitation energy acceptor, which does not belongs to the photosensitizers family. Fig. 3b shows the absorption and emission spectra of DiDC1 dye. The calculated using Eqs. (1) and (2) overlap integral J (λ ) = 1.06·10–12 M−1 cm3 and Förster distance R0 = 41.7 Å indicate that DiDC1 is also a good candidate as an excitation energy acceptor for VNPs in FRET experiments [38]. Fig. 5a shows that the addition of DiDC1 into the VNPs solution provokes a sharp decrease of the donor VNPs luminescence and an appearance of the sensitized acceptor luminescence with the maximum centered at 665 nm. The average lifetime of donor luminescence is also reduced depending on the acceptor concentration (Fig. 5b). At the DiDC1 concentration of 1·10−5 M, the VNPs luminescence lifetime is 5.5 times shorter < τav > = 151μs than that in the solution without any acceptor molecules indicating the efficient FRET with E~100% calculated using Eq. (1). We also used cationic dye DiOC2 (Fig. 1), which absorption spectrum does not overlap the emission spectrum of GdVO4:Eu3+ VNPs and, consequently, it cannot be the excitation energy acceptor for VNPs. Indeed, the addition of DiOC2 dye does not cause any changes in both the GdVO4:Eu3+ VNPs luminescence intensity and decay lifetimes (Fig. 6).
Thus, the efficiency of electronic excitation energy transfer between GdVO4:Eu3+ nanoparticles and cationic organic dye MB in water solutions under UV excitation has been studied. We have shown that the addition into the water solution even a small amount of MB provokes strong decrease of both GdVO4:Eu3+ luminescence intensity and lifetime. At optimal MB concentration, the FRET efficiency of about 90% has been obtained. The average luminescence lifetime of GdVO4:Eu3+ is more than 4 times reduced. Despite the strong quenching of the donor VNPs luminescence, we did not observe FRET-induced emission of the acceptor MB that could be explained by (i) high intersystem crossing quantum yield for MB (Qp= 0.54) with a non-radiative deactivation of the triplet level, (ii) small fluorescence quantum yield (Qf = 0.04 in ethanol) and (iii) a small amount of MB molecules in the solution bonded to the GdVO4:Eu3+ nanoparticles (about 10%). The long lifetime, large Stokes shift, narrow emission and X-ray excited luminescence make GdVO4:Eu3+ nanoparticles prospective as an excitation source for photosensitizer MB in PDT applications. Acknowledgments This work was supported by National Academy of Sciences of Ukraine (Project № 0116U002612). Authors thanks to Dr. I.A. Borovoy (Institute for Scintillation Materials of NAS of Ukraine) for DiDC1, dye making available. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.08.044. References [1] American Cancer Society, Cancer Facts & Figures 2016, American Cancer Society, Atlanta, 2016. [2] D. Peer, J.M. Karp, S. Hong, O.C. Farokhzad, R. Margalit, R. Langer, Nat. Nanotechnol. 2 (2007) 751–760. [3] I. Macdonald, T. Dougherty, J. Porphyr. Phthalocyanines 5 (2001) 105–129. [4] H.I. Pass, J. Natl. Cancer Inst. 85 (1993) 443–456. [5] D. Dolmans, D. Fukumura, R.K. Jain, Nat. Rev. Cancer 3 (2003) 380–387.
980
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
[25] N.S. Kavok, K.A. Averchenko, V.K. Klochkov, S.L. Yefimova, Yu.V. Malyukin, Eur. Phys. J. E: Soft Matter Biol. Phys. 37 (2014) 127. [26] T.N. Tkacheva, S.L. Yefimova, V.K. Klochkov, I.A. Borovoy, Yu.V. Malyukin, J. Mol. Liq. 199 (2014) 244–250. [27] Ganna Grygorova, Vladimir Klochkov, Svetlana Yefimova, Yuri Malyukin, Chem. Phys. Lett. 621 (2015) 46–51. [28] L.M. Moreira, J.P. Lyon, A.P. Romani, D. Severino, M.R. Rodriguesand, H.P.M. de Oliveira, Adv. Asp. Spectrosc. 14 (2012) 393–422. [29] V.V. Chagovets, M.V. Kosevich, S.G. Stepanian, O.A. Boryak, V.S. Shelkovsky, V.V. Orlov, V.S. Leontiev, V.A. Pokrovskiy, L. Adamowicz, V.A. Karachevtsev, J. Phys. Chem. C 116 (2012) 20579–20590. [30] V.K. Klochkov, A.I. Malyshenko, O.O. Sedyh, Yu.V. Malyukin, Funct. Mater. 1 (2011) 111–115. [31] D.O. Faulkner, J.J. McDowell, A.J. Price, D.D. Perovic, N.P. Kherani, G.A. Ozin, Laser Photonics Rev. 6 (2012) 802–806. [32] C. Hsu, R.C. Powell, J. Lumin. 10 (1975) 273–293. [33] J.H. Kang, W.B. Im, D.C. Lee, J.Y. Kim, D.Y. Jeon, Y.C. Kang, K.Y. Jung, Solid State Commun. 133 (2005) 651–656. [34] A. Huignard, V. Buissette, A.C. Franville, T. Gacoin, J.P. Boilot, J. Phys. Chem. B 107 (2003) 6754–6759. [35] S. Georgescu, Е Cotoi, АМ Voiculescu, О Toma, Rom. Rep. Phys. 60 (2008) 947–955. [36] K. Riwotzki, M. Haase, J. Phys. Chem. B 105 (2001) 12709–12713. [37] A. Huignard, V. Buissette, G. Laurent, T. Gaconu, J.-P. Boilot, Chem. Matter 14 (2002) 2264–2269. [38] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Publishers, New York, Boston, Dordrecht, London, Moscow, 1999. [39] B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley, Weinheim, 2002. [40] V.S. Shelkovsky, Biophys. Bull. 33 (1) (2015) 5–21. [41] D. Casanova, D. Giaume, Th Gacoin, J.-P. Boilot, A. Alexandrou, J. Phys. Chem. B 110 (2006) 19264–19270. [42] I.I.I. John Olmsted, J. Phys. Chem. 83 (1979) 2581–2584.
[6] T. Dai, B.B. Fuchs, J.J. Coleman, R.A. Prates, C. Astrakas, T.G.St Denis, M.S. Ribeiro, E. Mylonakis, M.R. Hamblin, G.P. Tegos, Front. Microbiol. 3 (2012) 120–136. [7] B.W. Henderson, T.J. Dougherty, Photochem. Photobiol. 55 (1992) 145–157. [8] M.B. Vrouenraets, G.W. Visser, G.B. Snow, G.A. van Dongen, Anticancer Res. 23 (2003) 505–522. [9] C.S. Foote, Photochem. Photobiol. 54 (1991) 659–672. [10] P. Mroz, A. Yaroslavsky, G.B. Kharkwal, M.R. Hamblin, Cancers 3 (2011) 2516–2539. [11] A.-L. Bulin, C. Truillet, R. Chouikrat, F. Lux, C. Frochot, D. Amans, G. Ledoux, O. Tillement, P. Perriat, M. Barberi-Heyob, C. Dujardin, J. Phys. Chem. C 117 (2013) 21583–21589. [12] V. Ntziachristos, C. Bremer, R. Weissleder, Eur. Radiol. 13 (2003) 195–208. [13] Wei Chen, Jun Zhang, J. Nanosci. Nanotechnol. 6 (2006) 1159–1166. [14] P. Retif, S. Pinel, M. Toussaint, C. Frochot, R. Chouikrat, Th Bastogne, M. BarberiHeyob, Theranostics 5 (2015) 1030–1044. [15] J. Xu, J. Gao, Q. Wie, J. Nanomater. (2016), http://dx.doi.org/10.1155/2016/ 8507924. [16] S. Clement, W. Deng, E. Camilleri, B.C. Wilson, E.M. Goldys, Sci. Rep. 6 (2016), http://dx.doi.org/10.1038/srep19954. [17] L. Ma, X. Zou, B. Bui, W. Chen, K.H. Song, T. Solberg, Appl. Phys. Lett. 105 (2014) 013702. [18] X. Zou, M. Yao, L. Ma, M. Hossu, X. Han, P. Juzenas, W. Chen, Nanomedicine 9 (2014) 2339–2351. [19] S. Kašcáková, A. Giuliani, S. Lacerda, A. Pallier, P. Mercère, É. Tóth, M. Réfrégiers, Nano Res. 8 (2015) 2373–2379. [20] H. Zhang, Y. Shan, L. Dong, J. Biomed. Nanotechnol. 10 (2014) 1450–1457. [21] A.-L. Bulin, A. Vasil`ev, A. Belsky, D. Amans, G. Ledoux, Ch Dujardi, Nanoscale 7 (2015) 5744–5751. [22] M.Yu Losytskyy, L.O. Vretik, O.A. Nikolaev, A.I. Marynin, N.F. Gamaley, V.M. Yashchuk, Mol. Cryst. Liq. Cryst. 639 (2016) 169–176. [23] D.K. Chatterjee, Li.S. Fong, Y. Zhang, Adv. Drug Deliv. Rev. 60 (2008) 1627–1637. [24] H. Chen, G.D. Wang, Y.-J. Chuang, Z. Zhen, X. Chen, P. Biddinger, Z. Hao, F. Liu, B. Shen, Z. Pan, Nano Lett. 15 (2015) 2249–2256.
981
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
GdVO4:Eu3+ nanoparticles – Methylene Blue complexes for PDT: Electronic excitation energy transfer study
MARK
⁎
S.L. Yefimova , T.N. Tkacheva, P.O. Maksimchuk, I.I. Bespalova, K.O. Hubenko, V.K. Klochkov, A.V. Sorokin, Yu.V. Malyukin Institute for Scintillation Materials National Academy of Sciences of Ukraine, 60 Nauky ave., 61072 Kharkiv, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: Gadolinium orthovanadate nanoparticles Photosensitizer Methylene Blue Electronic excitation energy transfer
Europium-doped gadolinium orthovanadate nanoparticles (GdVO4:Eu3+) were used as a donor in fluorescence resonance energy transfer (FRET) experiments in water solutions. Using Methylene Blue as an excitation energy acceptor, we have observed highly efficient FRET between GdVO4:Eu3+ nanoparticles and MB. Despite the strong decrease of both the donor GdVO4:Eu3+ luminescence and lifetime, the FRET-induced emission of the acceptor MB has not been observed. This feature has been explained by small fluorescence quantum yield of MB, high efficiency of the intersystem crossing and triplet state formation for MB and small amount of MB molecules bonded to the GdVO4:Eu3+ nanoparticles. The long lifetime, large Stokes shift, narrow emission, X-ray excited luminescence and high FRET efficiency (∼ 90%) make GdVO4:Eu3+ nanoparticles prospective as an excitation source for the photosensitizer MB activation in such applications as photodynamic therapy.
1. Introduction Despite the better understanding of tumor biology and improved diagnostic devices observed in last years, cancer remains one of the world's most devastating diseases being the second leading cause of death [1,2]. Current cancer treatments include surgery, radiation, chemotherapy, hormone therapy and immune therapy [1,2]. Unfortunately, conventional anti-cancer treatments have many disadvantages including high toxicity, effects on healthy cells, tissues and organs and variety of side effects. Photodynamic therapy (PDT) is an alternative approach to conventional and developed cancer treatment methods and was clinically used for treatment of certain kinds of cancers, pre-cancer lesions and other diseases [3–6]. PDT involves the use of light-sensitive dye called photosensitizer (PS) combined with visible light of the appropriate wavelength to match the absorption spectrum of the PS. The third principal component is oxygen molecules. Absorbed photons transform PS from its ground state (singlet state) into the excited singlet state (see Scheme 1). The excited molecule can fall back to its ground state with concomitant emission of fluorescence. All photosensitizers are usually poorly fluorescent molecules due to high quantum yield of an electron spin conversion to long-lived triplet state (intersystem crossing process). This triplet PS can interact with molecular oxygen (3O2) and neighbor molecules in two pathways: an electron or hydrogen atom transfer (type I reaction) or energy transfer (type II reaction) (see Scheme 1) leading to the formation of reactive oxygen species (ROS) and singlet oxygen (1O2), respectively [4–10]. It is ⁎
believed that 1O2 produced through type II reaction is the primarily ROS that causes damage to nearby biomolecules. Despite the numerous advantages of the PDT approach, this method has the main drawback, i.e. the low penetration depth of exciting light into human tissue (about 6 mm) even in the near infrared spectral range (700–1100 nm), where most tissue chromophores absorb weakly [11,12]. That requires direct light delivery for PDT activation and restricts PDT use to superficial tumor [4–6]. One of the possible ways to overcome this drawback is using X-ray luminescent (scintillating) nanoparticles as an excitation or “light” source for photosensitizer molecules to activate PDT, which has become a hot topic as a new method for cancer treatment [13–15]. Within this concept, X-ray excited PS is composed of scintillating and photosensitizing parts with an effective energy transfer between them. In order to use nanoparticles to activate PS molecules, both parts could be close enough spatially. A typical mechanism for energy transfer is fluorescence excitation energy transfer (FRET). In above-mentioned case, FRET refers to the transfer of energy the initially excited donor (scintillating NPs) to an acceptor (PS molecule). Efficient energy transfer requires also that the emission spectrum of the donor overlap the absorption band of the acceptor. Since the first announcement by W. Chen and J. Zhang in 2006 [13] this idea has been exploited in many research groups using various scintillation nanoparticles, e.g. CeF3, LaF3:Ce3+, _LaF3:Tb3+, LuF3:Ce3+, CaF2:Mn2+, CaF2:Eu2+, BaFBr:Eu2+, BaFBr:Mn2+, CaPO4:Mn2+, SrAl2O4:Eu2+, rare-earth elements oxides, ZnO, ZnS, ZnS:Cu, CO, TiO2 and different ways of their binding with PS molecules [16–24].
Corresponding author. E-mail address: [email protected] (S.L. Yefimova).
http://dx.doi.org/10.1016/j.jlumin.2017.08.044 Received 31 January 2017; Received in revised form 18 August 2017; Accepted 21 August 2017 Available online 23 August 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
37.5 mL of Na3VO4 (0.01 M) was flowed drop wise (рН = 10.5). The mixture was intensively stirred by using a magnetic stirrer and heated on a water bath under a reflux condenser for 24 h at 100 °С. Obtained colourless transparent solution scatters light under the side illumination (the Tyndall cone). Then the solution was cooled and dialyzed against water for 24 h to remove the excess of ions. A dialysis membrane with a molecular weight cut-off of 12 kDa was used. 2.3. Instrumentation and characterization Synthesized VNPs were characterized using Transmission Electron Microscopy (TEM-125K electron microscope, Selmi, Ukraine) and Dynamic Light Scattering method (ZetaPALS analyzer, Brookhaven Instruments Corp., USA). Absorption spectra were measured using a Specord 200 spectrometer (Analytik Jena, USA). Luminescence spectra were taken with a spectrofluorimeter Lumina (Thermo Scientific, USA). Luminescence decay curves were registered using a computer-controlled setup based on a grating monochromator MDR-23 operating in Time-Correlated Single Photon Counting (TCSPC) mode (Picoquant TimeHarp 260, Germany). An excitation of samples was induced by a YAG:Nd3+ pulsed laser NL202 (forth harmonic, λexc ~ 266 nm, EKSPLA, Lithuania). The Instrumental Response Function (IRF) was about 7 ns. The obtained luminescence decay curves were analyzed using FluoFit software for data analysis (PicoQuant, Germany). The X-ray luminescence was excited by an X-ray generator «РЕЙС» (U = 25 kV, I = 37 µA) and registered using the same computed controlled setup combined with a Hamamatsu R9110 PMT. Absolute quantum yield of GdVO4:Eu3+ photoluminescence was measured using a homemade integrating sphere (diameter of 100 mm), which provides reflectance > 99% over the 400–1000 nm range. As an excitation source, we use a xenon lamp, the necessary excitation wavelength (λexc = 280 nm) were selected using a grating monochromator MDR-23. The luminescence was collected using a microspectrometer USB4000 (OceanOptics, USA) connected with an integrated sphere. The absolute quantum yield was calculated using the method described in [31]. The experimental setup was adjusted and tested on a standard dye (rhodamine 6 G in ethanol).
Scheme 1. Schematic representation of photosensitizer (PS) energetic levels and photodynamic reactions with molecular oxygen.
In the present paper, we discuss the possibility for gadolinium orthovanadate nanoparticles doped with europium ions GdVO4:Eu3+ (VNPs) to be used as an excitation source for Methylene Blue (MB) photosensitizer. VNPs exhibit strong absorption in the UV spectral range and luminescence excited both within the UV band and under Xray excitation with luminescence bands overlapping the MB absorption band. One more advantage of lanthanide NPs as an energy donor is very long lifetime (in the millisecond range). In our previous papers, we reported on biological activity of rare-earth orthovanadate nanoparticles (pro-/antioxidant properties, effects on bioenergy processes in cell mitochondria, selective accumulation in cell nuclei) and ordered dye adsorption near the surface of nanoparticles [25–27]. Thus, we consider VNPs to be prospective candidates for the PDT application. MB is a well-known photosensitizer used in variety applications including PDT [28,29]. At the first stage, we analyze the efficiency of VNPs as an electronic excitation energy donor in fluorescence resonance energy transfer experiments in aqueous solutions. Our study indicates high potential of GdVO4:Eu3+ nanoparticles as an excitation source for MB activation. 2. Experimental 2.1. Chemicals
2.4. Preparation of VNPs – dyes aqueous solutions
Gadolinium chlorides GdCl3·6H2O (99.9%), europium chloride EuCl3·6H2O (99.9%), disodium EDTA·2Na (99.8%) and anhydrous sodium metavanadate NaVO3 (96%) were obtained from Acros organic (USA) and all used without further purification. Sodium hydroxide NaOH (99%) was purchased from Macrohim (Ukraine). Sodium orthovanadate Na3VO4 solution was obtained by adding a 1 M solution of NaOH in aqueous solution NaVO3 to pH = 13. Cationic dyes 3,7-bis(dimethylamino)phenazathionium chloride (Methylene Blue (MB), Mw = 373,90 g/mol), 3,3-diethyloxacarbocyanine iodide (DiOC2, Mw = 460.31 g/mol) were purchases from Sigma-Aldrich (USA) and used as received. Cationic dye 1,1′-dimethyl-3,3,3′,3′-tetramethylindodicarbocyanine tetrafluoroborate (DiDC1, Mw = 484.12 g/mol) was obtained from the dye collection of Dr. Igor Borovoy (Institute for Scintillation Materials, NAS of Ukraine) with the purity controlled by thin layer chromatography (Fig. 1). All other chemicals were of analytical grade.
Solutions for investigations were prepared as following. First, stock solutions of the dyes (MB, DiDC1 or DiOC2) in chloroform (1 mM) were prepared. To obtain VNPs – dyes aqueous solutions with different concentrations of dyes, required amount of the dye stock solution was added in a flask and carefully stirred using a rotary evaporator (Rotavapor R-3, Buchi) during 1 h to a complete evaporation of chloroform. Then 1 mL of a VNPs aqueous solution (1 g/L) was added and a flask was gently shaken during 1 h for dye – VNPs complex formation. Thus, the concentration of VNPs in solutions was kept constant (1 g/l), while the dye concentration is varied from 1·10−6 M to 1·10−4 M. We took no action to remove unbounded dye molecules, a large fraction of dye molecules in a solution is in unbound form. 3. Results and discussion Following a colloidal route, we obtained well-crystallized spindle-like GdVO4:Eu3+ nanoparticles of 10×50 nm ± 5 nm size, as verified by TEM image (Fig. 2a). To stabilize GdVO4:Eu3+ nanoparticles in aqueous solutions, disodium EDTA was used, which carboxylate groups impart a negative charge to the nanoparticle surface. The overage hydrodynamic diameter of GdVO4:Eu3+ nanoparticles is 56.4 ± 1.5 nm, zeta-potential is – 20.03 ± 2.2 mV (pH = 7.8). Photoluminescence properties of ReVO4:Eu3+ nanoparticles are well-studied [32–36]. The luminescence excitation and emission spectra of synthesized GdVO4:Eu3+ nanoparticles are presented in Fig. 2b,c. The excitation spectrum corresponding to the
2.2. Synthesis of GdVO4:Eu3+ colloidal solutions Aqueous colloidal solutions of gadolinium orthovanadate nanoparticles doped with europium ions Gd0.9Eu0.1VO4 (GdVO4:Eu3+) were synthesized according to the method reported earlier [30]. First, 0.4 mL of aqueous solution of gadolinium chloride (1 M) was mixed with 0.05 mL of europium chloride (1 M), then 49.55 mL of doubly distilled water were added to the mixture. Then obtained solution was mixed with 37.5 mL of disodium EDTA solution (0.01 M). Then, 976
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
Fig. 1. Chemical structures of used organic dye molecules.
Eu3+ emission of VNPs (Fig. 2b) consists of the intense wide band in the 250–350 nm spectral range and several weaker bands in the longer-wavelength region (see inset in Fig. 2b). The wide band corresponds charge transfer from oxygen anions to vanadium ions, whereas the weak bands
belong to the f→f transitions of europium ions within the 4f6 electron configuration [32]. The luminescence spectrum of GdVO4:Eu3+ nanoparticles (Fig. 2c) results from transition within the f–electron configuration of the europium ions. The main contribution is the 5D0–7F2,4 forced
Fig. 2. Transmission electronic microscopy (TEM) image (a), absorption (b), luminescence, λexc = 280 nm (c) and X-ray induced luminescence (d) spectra of synthesized GdVO4:Eu3+ nanoparticles.
977
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
Fig. 3. Luminescence spectrum of GdVO4:Eu3+ nanoparticles, λexc = 280 nm and absorption and luminescence spectra of MB, λexc = 650 nm (a) and DiDC1, λexc = 630 nm (b). Normalized data.
acceptor molecules can be attached to the VNPs surface thus increasing the FRET efficiency [26,27]. Formation VNPs-dye complexes was confirmed by increasing VNPs hydrodynamic diameter (91.1 nm) of particles and decreasing zeta-potential (–10.5 mV) in solution containing VNPs and MB molecules (see Supplementary data, Fig. 1S). As shown in Fig. 4 in the presence of MB in a solution, both VNPs luminescence intensity and lifetime decrease. It should be noted that the quenching of VNPs luminescence could be governed by several mechanisms: (i) static quenching (formation of ground state non-luminescent VNPs-dye complexes due to dye adsorption on the surface of NPs) and (ii) dynamic quenching related to the processes in the VNPs excited state and leading to non-radiative deactivation of excitation energy (e.g., energy or electron transfer) [38,39]. It is well known that both mechanisms provoke the decrease of luminescence intensity, whereas lifetime decrease is associated with dynamic quenching only [38,39]. Therefore, simple dye molecule adsorption within the surface of VNPs cannot affect the lifetime without additional processes in the excited state. Electron transfer between Eu3+ and MB is unlikely to appear. MB cation can be easily reduced via electron accepting with the formation of colourless so-called leukoform of the MB dye [40], but Eu3+ ion cannot give an electron. Moreover, we did not observe solution decolouration upon VNPs–MB complexation. Thus, we consider VNPs luminescence intensity and lifetime decrease in the presence of MB to be due to fluorescence resonance energy transfer. The changes in the VNPs luminescence parameters depend on the MB concentration (Fig. 4). Luminescence decay curves both in presence and in absence of MB are found to be two-exponential (Fig. 4b). In case of a solution without MB (Fig. 4b, curve 1), the decay curve can be fitted to the equation y=10% exp ( − t /0.324 ms ) +90% exp ( − t /0.899 ms ) . Two-exponential luminescence decay was observed for Eu3+ ions in Y0.6Eu04VO4 nanocrystals and was explained by the existence of two types of Eu3+ located close to the surface of the nanocrystal and in the volume, respectively [41]. Eu3+ ions located near the surface are well known to be more affected by the non-radiative decay process [34,41]. Thus the shorter component in the luminescence decay is associated with the luminescence of Eu3+ ions located near the surface of VNPs. The average luminescence lifetime was calculated as < τav > = f1 τ1 + f2 τ2 , where fi is the fractional contribution of i-th component to the luminescence decay (0.1 and 0.9 for τ1 and τ2 , respectively) [38,39] and is 0.842 ms. The increase in the MB concentrations in the solutions leads to the gradual decrease of both the short and long lifetime components (Table 1). At highest MB concentration, the average lifetime reduces in
electric-dipole transitions [34]. Other components of weaker importance are the 5D0–7F1,3 magnetic dipole transitions [34]. For europium content of 10%, quantum yield QD = 0.16 , for excitation in vanadate matrix (λexc = 280 nm) is obtained that correlates with the data presented in [37]. GdVO4:Eu3+ nanoparticles exhibit also X-ray excited luminescence (Fig. 2d). The photoluminescence and X-ray excited luminescence spectra of VNPs perfectly match the absorption spectrum of MB (Fig. 3a). Thus, VNPs could be used as a source to excite MB at the expense of fluorescence resonance energy transfer between VNPs (excitation energy donor) and MB (excitation energy acceptor) [38,39]. Efficient energy transfer requires the emission band of the donor to be overlapped with the absorption band of the acceptor and that the donor and the acceptor are close enough spatially to permit the energy transfer (10–100 Å) [38,39]. This energy transfer results in a decrease of donor emission, and an increase of acceptor emission, as well as a decrease of donor lifetime [38,39]. We calculated the normalized spectral overlap between the donor emission and the acceptor absorption, J (λ ),(M−1cm3) [38]:
J (λ ) =
∞ ∫0 FD (λ )⋅εA (λ )⋅λ4dλ , ∞ ∫0 FD (λ ) dλ
(1)
where FD (λ ) is the donor luminescence spectrum, ε (λ ) is the extinction coefficient of the acceptor at λ, (M−1 cm−1). To calculate J(λ) we took into account only 5D0–7F2 and 5D0–7F4 electric-dipole transitions centered at 618 and 700 nm, respectively (Fig. 2c). The 5D0–7F1,3 magnetic dipole transitions do not contribute to FRET. Using the average index of refraction for VNPs n = 2.0704 [40], an orientation parameter k 2 = 2/3 assuming random orientation of the donor and acceptor transition dipoles, calculated quantum yield of the VNPs QD = 0.16, and obtained J(λ) = 4.807·10–13 M−1 cm3, we calculated the Förster distance between the donor and acceptor R0, Å [38]:
R 0 = 9, 78⋅103 (k 2⋅n−4QD J (λ ))1/6
(2)
For the VNPs – MB pair, R0 = 36.5 Å that could ensue the effective FRET [38,39]. In water solutions, VNPs are stabilized with disodium EDTA, which form a shell around the nanoparticles. We exploited the fact that MB cations can be adsorbed within this shell near the surface of VNPs due to coulomb interactions between the negatively charged carboxylate groups of EDTA shell and MB cations that will ensure the necessary distance between the donor and acceptor required for FRET. Due to the size of VNPs (hydrodynamic diameter of 56 nm), several 978
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
Fig. 4. Luminescence spectra, λexc = 280 nm (a) and luminescence decay curves, λreg = 618 nm (b) of GdVO4:Eu3+ nanoparticles in the solutions with different MB concentrations: 1 – without MB; 2 – 1·10−6 M; 3 – 5·10−6 M; 4 – 1·10−5 M; 5 – 5·10−5 M; 6 – 1·10−4 M. The inset shows the luminescence spectrum of MB at λexc = 650 nm.
Table 1 GdVO4:Eu3+ VNPs luminescence decay parameters changes depending on MB concentration. [MB], M
% f1
τ1, ms
% f2
τ2, ms
〈τav〉, ms
0 1 5 1 5 1
10 10 13 10 36 32
0.324 0.310 0.314 0.203 0.171 0.096
90 90 87 90 64 68
0.899 0.862 0.680 0.579 0343 0.236
0.842 0.807 0.592 0.541 0.281 0.191
× × × × ×
10−6 10−6 10−5 10−5 10−4
E=
IDA , ID
(3)
E=
τDA , τD
(4)
where IDA (τDA ) and ID (τD ) are the donor luminescence intensity (lifetime) in the presence and absence of the acceptor, respectively. At highest MB concentration in the solution (1·10−4 M), the FRET efficiencies calculated using Eqs. (3) and (4) are 91% and 77%, respectively. It should be noted that the use of lifetime in Eq. (4) could be a source of confusion, because Eq. (4) assumes that the decay of the donor is single exponential in the absence and presence of the acceptor. In case of multi-exponential decay, one should use the average lifetimes, but the value obtained could be not correct [38]. On the other hand, Eq. (3) takes into account the emission intensities arisen both from Eu3+ ions located near the surface and closer to the center of VNPs and gives a more accurate determination of the FRET efficiency [38,41]. Despite the effective decrease of the donor GdVO4:Eu3+ VNPs
4.4 times and becomes 0.191 ms. At the same time, we have observed some increase in the short component fractional contribution up to 32–36% at high MB concentrations that confirms the stronger quenching of Eu3+ ions located near the surface of VNPs. The efficiency of FRET can be calculated using luminescence intensity or decay parameters using Eqs. (3) and (4) [38]:
Fig. 5. Luminescence spectra, λexc = 280 nm (a) and luminescence decay curves, λreg = 618 nm (b) of GdVO4:Eu3+ nanoparticles in the solutions with different DiDC1 concentrations: 1 – without DiDC1; 2 – 1·10−6 M; 3 – 5·10−6 M; 4 – 1·10−5 M. The inset shows the luminescence spectrum of the solution with [DiDC1] = 5·10−6 M.
979
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
Fig. 6. Luminescence spectra, λexc = 280 nm (a) and luminescence decay curves, λreg = 618 nm of GdVO4:Eu3+ nanoparticles in the solutions with different DiOC2 concentrations: 1 – without DiOC2; 2 – 1·10−6 M; 3 – 5·10−6 M; 4 – 1·10−5 M; 5 – 5·10−5 M.
4. Conclusion
luminescence, we did not observe the FRET-induced emission of the acceptor MB (Fig. 4a), while at the excitation within the acceptor absorption band, the intrinsic MB emission is observed (see inset in Fig. 4a). We did not observe any MB aggregation on the surface of VNPs even at high dye concentrations in the solution (see Supplementary data, Fig. 2S) that could be the reason of MB emission quenching. We explain this phenomenon as follows. MB is non-fluorescent dye, which shows residual emission (fluorescence quantum yield Qf = 0.04 in ethanol) [28,42]. Moreover, it is known that MB possesses high intersystem crossing quantum yield (Qp= 0.54), i.e. excited MB molecule undergoes a conversion to its long-lived triplet state (see Scheme 1) [28]. The excited triplet state of organic molecules in aqueous solutions at room temperature is deactivated in the non-radiative way. For PS molecules, this way includes the interaction with molecular oxygen or electron-transfer reactions as shown in Scheme 1. We also should take into consideration that in aqueous solutions containing GdVO4:Eu3+ VNPs and MB, only a small part of the dye molecules will be adsorbed on the surface of VNPs, and half of them will undergo a non-radiative deactivation via triplet state formation. To confirm this arguments, we use other dye molecule (DiDC1, see Fig. 1) as an excitation energy acceptor, which does not belongs to the photosensitizers family. Fig. 3b shows the absorption and emission spectra of DiDC1 dye. The calculated using Eqs. (1) and (2) overlap integral J (λ ) = 1.06·10–12 M−1 cm3 and Förster distance R0 = 41.7 Å indicate that DiDC1 is also a good candidate as an excitation energy acceptor for VNPs in FRET experiments [38]. Fig. 5a shows that the addition of DiDC1 into the VNPs solution provokes a sharp decrease of the donor VNPs luminescence and an appearance of the sensitized acceptor luminescence with the maximum centered at 665 nm. The average lifetime of donor luminescence is also reduced depending on the acceptor concentration (Fig. 5b). At the DiDC1 concentration of 1·10−5 M, the VNPs luminescence lifetime is 5.5 times shorter < τav > = 151μs than that in the solution without any acceptor molecules indicating the efficient FRET with E~100% calculated using Eq. (1). We also used cationic dye DiOC2 (Fig. 1), which absorption spectrum does not overlap the emission spectrum of GdVO4:Eu3+ VNPs and, consequently, it cannot be the excitation energy acceptor for VNPs. Indeed, the addition of DiOC2 dye does not cause any changes in both the GdVO4:Eu3+ VNPs luminescence intensity and decay lifetimes (Fig. 6).
Thus, the efficiency of electronic excitation energy transfer between GdVO4:Eu3+ nanoparticles and cationic organic dye MB in water solutions under UV excitation has been studied. We have shown that the addition into the water solution even a small amount of MB provokes strong decrease of both GdVO4:Eu3+ luminescence intensity and lifetime. At optimal MB concentration, the FRET efficiency of about 90% has been obtained. The average luminescence lifetime of GdVO4:Eu3+ is more than 4 times reduced. Despite the strong quenching of the donor VNPs luminescence, we did not observe FRET-induced emission of the acceptor MB that could be explained by (i) high intersystem crossing quantum yield for MB (Qp= 0.54) with a non-radiative deactivation of the triplet level, (ii) small fluorescence quantum yield (Qf = 0.04 in ethanol) and (iii) a small amount of MB molecules in the solution bonded to the GdVO4:Eu3+ nanoparticles (about 10%). The long lifetime, large Stokes shift, narrow emission and X-ray excited luminescence make GdVO4:Eu3+ nanoparticles prospective as an excitation source for photosensitizer MB in PDT applications. Acknowledgments This work was supported by National Academy of Sciences of Ukraine (Project № 0116U002612). Authors thanks to Dr. I.A. Borovoy (Institute for Scintillation Materials of NAS of Ukraine) for DiDC1, dye making available. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.08.044. References [1] American Cancer Society, Cancer Facts & Figures 2016, American Cancer Society, Atlanta, 2016. [2] D. Peer, J.M. Karp, S. Hong, O.C. Farokhzad, R. Margalit, R. Langer, Nat. Nanotechnol. 2 (2007) 751–760. [3] I. Macdonald, T. Dougherty, J. Porphyr. Phthalocyanines 5 (2001) 105–129. [4] H.I. Pass, J. Natl. Cancer Inst. 85 (1993) 443–456. [5] D. Dolmans, D. Fukumura, R.K. Jain, Nat. Rev. Cancer 3 (2003) 380–387.
980
Journal of Luminescence 192 (2017) 975–981
S.L. Yefimova et al.
[25] N.S. Kavok, K.A. Averchenko, V.K. Klochkov, S.L. Yefimova, Yu.V. Malyukin, Eur. Phys. J. E: Soft Matter Biol. Phys. 37 (2014) 127. [26] T.N. Tkacheva, S.L. Yefimova, V.K. Klochkov, I.A. Borovoy, Yu.V. Malyukin, J. Mol. Liq. 199 (2014) 244–250. [27] Ganna Grygorova, Vladimir Klochkov, Svetlana Yefimova, Yuri Malyukin, Chem. Phys. Lett. 621 (2015) 46–51. [28] L.M. Moreira, J.P. Lyon, A.P. Romani, D. Severino, M.R. Rodriguesand, H.P.M. de Oliveira, Adv. Asp. Spectrosc. 14 (2012) 393–422. [29] V.V. Chagovets, M.V. Kosevich, S.G. Stepanian, O.A. Boryak, V.S. Shelkovsky, V.V. Orlov, V.S. Leontiev, V.A. Pokrovskiy, L. Adamowicz, V.A. Karachevtsev, J. Phys. Chem. C 116 (2012) 20579–20590. [30] V.K. Klochkov, A.I. Malyshenko, O.O. Sedyh, Yu.V. Malyukin, Funct. Mater. 1 (2011) 111–115. [31] D.O. Faulkner, J.J. McDowell, A.J. Price, D.D. Perovic, N.P. Kherani, G.A. Ozin, Laser Photonics Rev. 6 (2012) 802–806. [32] C. Hsu, R.C. Powell, J. Lumin. 10 (1975) 273–293. [33] J.H. Kang, W.B. Im, D.C. Lee, J.Y. Kim, D.Y. Jeon, Y.C. Kang, K.Y. Jung, Solid State Commun. 133 (2005) 651–656. [34] A. Huignard, V. Buissette, A.C. Franville, T. Gacoin, J.P. Boilot, J. Phys. Chem. B 107 (2003) 6754–6759. [35] S. Georgescu, Е Cotoi, АМ Voiculescu, О Toma, Rom. Rep. Phys. 60 (2008) 947–955. [36] K. Riwotzki, M. Haase, J. Phys. Chem. B 105 (2001) 12709–12713. [37] A. Huignard, V. Buissette, G. Laurent, T. Gaconu, J.-P. Boilot, Chem. Matter 14 (2002) 2264–2269. [38] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Publishers, New York, Boston, Dordrecht, London, Moscow, 1999. [39] B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley, Weinheim, 2002. [40] V.S. Shelkovsky, Biophys. Bull. 33 (1) (2015) 5–21. [41] D. Casanova, D. Giaume, Th Gacoin, J.-P. Boilot, A. Alexandrou, J. Phys. Chem. B 110 (2006) 19264–19270. [42] I.I.I. John Olmsted, J. Phys. Chem. 83 (1979) 2581–2584.
[6] T. Dai, B.B. Fuchs, J.J. Coleman, R.A. Prates, C. Astrakas, T.G.St Denis, M.S. Ribeiro, E. Mylonakis, M.R. Hamblin, G.P. Tegos, Front. Microbiol. 3 (2012) 120–136. [7] B.W. Henderson, T.J. Dougherty, Photochem. Photobiol. 55 (1992) 145–157. [8] M.B. Vrouenraets, G.W. Visser, G.B. Snow, G.A. van Dongen, Anticancer Res. 23 (2003) 505–522. [9] C.S. Foote, Photochem. Photobiol. 54 (1991) 659–672. [10] P. Mroz, A. Yaroslavsky, G.B. Kharkwal, M.R. Hamblin, Cancers 3 (2011) 2516–2539. [11] A.-L. Bulin, C. Truillet, R. Chouikrat, F. Lux, C. Frochot, D. Amans, G. Ledoux, O. Tillement, P. Perriat, M. Barberi-Heyob, C. Dujardin, J. Phys. Chem. C 117 (2013) 21583–21589. [12] V. Ntziachristos, C. Bremer, R. Weissleder, Eur. Radiol. 13 (2003) 195–208. [13] Wei Chen, Jun Zhang, J. Nanosci. Nanotechnol. 6 (2006) 1159–1166. [14] P. Retif, S. Pinel, M. Toussaint, C. Frochot, R. Chouikrat, Th Bastogne, M. BarberiHeyob, Theranostics 5 (2015) 1030–1044. [15] J. Xu, J. Gao, Q. Wie, J. Nanomater. (2016), http://dx.doi.org/10.1155/2016/ 8507924. [16] S. Clement, W. Deng, E. Camilleri, B.C. Wilson, E.M. Goldys, Sci. Rep. 6 (2016), http://dx.doi.org/10.1038/srep19954. [17] L. Ma, X. Zou, B. Bui, W. Chen, K.H. Song, T. Solberg, Appl. Phys. Lett. 105 (2014) 013702. [18] X. Zou, M. Yao, L. Ma, M. Hossu, X. Han, P. Juzenas, W. Chen, Nanomedicine 9 (2014) 2339–2351. [19] S. Kašcáková, A. Giuliani, S. Lacerda, A. Pallier, P. Mercère, É. Tóth, M. Réfrégiers, Nano Res. 8 (2015) 2373–2379. [20] H. Zhang, Y. Shan, L. Dong, J. Biomed. Nanotechnol. 10 (2014) 1450–1457. [21] A.-L. Bulin, A. Vasil`ev, A. Belsky, D. Amans, G. Ledoux, Ch Dujardi, Nanoscale 7 (2015) 5744–5751. [22] M.Yu Losytskyy, L.O. Vretik, O.A. Nikolaev, A.I. Marynin, N.F. Gamaley, V.M. Yashchuk, Mol. Cryst. Liq. Cryst. 639 (2016) 169–176. [23] D.K. Chatterjee, Li.S. Fong, Y. Zhang, Adv. Drug Deliv. Rev. 60 (2008) 1627–1637. [24] H. Chen, G.D. Wang, Y.-J. Chuang, Z. Zhen, X. Chen, P. Biddinger, Z. Hao, F. Liu, B. Shen, Z. Pan, Nano Lett. 15 (2015) 2249–2256.
981