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  • Open Access

Energy Transfer in Ce0.85Tb0.15F3 Nanoparticles-CTAB Shell-Chlorin e6 System

  • 1Email author,
  • 1,
  • 2,
  • ^3,
  • 4 and
  • 1
^Deceased
Nanoscale Research Letters201712:294

https://doi.org/10.1186/s11671-017-2077-x

  • Received: 1 January 2017
  • Accepted: 13 April 2017
  • Published:

Abstract

Formation and electronic excitation energy transfer process in the nanosystem consisting of Ce0.85Tb0.15F3 nanoparticles, cetrimonium bromide (CTAB) surfactant, and chlorin e6 photosensitizer were studied. It was shown that chlorin e6 molecules bind to Ce0.85Tb0.15F3 NP in the presence of CTAB forming thus Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem. We consider that binding occurs via chlorin e6 embedding in the shell of CTAB molecules, formed around NP. In the Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem, electronic excitation energy transfer from Ce3+ to chlorin e6 takes place both directly (with the 0.33 efficiency for 2 μM chlorin e6) and via Tb3+.

Keywords

  • Ce0.85Tb0.15F3 nanoparticles
  • Chlorin e6
  • Electronic excitation energy transfer
  • Photodynamic therapy

Background

Photodynamic therapy (PDT) is the method for the treatment of cancer, where the photosensitizer excitation by the light leads to the generation of singlet oxygen that is toxic for the tumor tissue [1]. But despite of several advantages, the drawback of this method is the very small depth of the light penetration into the tissue [2]. Thus, the idea of X-ray-inducible PDT based on X-ray-excited sensitizers composed of scintillating and photosensitizing parts with the electronic excitation energy transfer (EEET) from the first to the last one seems attractive [3]. Recently, a number of nanosystems based on this concept were described with various materials used as scintillators, different photosensitizer molecules, and several ways of binding them into a system [2, 49]; X-rays induced singlet oxygen generation [2, 6, 7, 9], cell destruction [4, 5, 7], and tumor destruction in mice [7] were demonstrated.

One of the options to choose scintillator for the above-described nanosystems is using lanthanide fluoride nanoparticles (NPs) [5, 9, 10]. Since f-f transitions of the majority of lanthanides are strongly forbidden, while in the case of Ce3+ its d-f transitions are allowed, generally, the lanthanide ions except Ce3+ cannot be efficiently excited by light in the UV-visible spectral region [11]. Thus, for the study of photophysics properties, CeF3-based [8, 12, 13] or Ce3+-doped [8, 10, 11] nanoparticles are used; excitation of Ce3+ results either in its own emission or in that of another lanthanide ion dopant (e.g., Tb3+) due to EEET [12, 13]. Excitation energy transfer from lanthanide nanoparticles to electrostatically bound [8, 9] or covalently attached [8, 10] photosensitizers was demonstrated.

Chlorin e6 is a known compound with photosensitizing properties used in PDT of cancer [14, 15]; its combination with scintillating lanthanide fluoride NP could be promising for the X-ray-inducible PDT. Nanosystems containing conjugates of La0.9Ce0.1F3 to chlorin e6 were studied in [10], though Tb-doped NPs were not investigated with chlorin e6 because of poor spectral overlap [10]. At the same time, EEET from Tb-doped NP to protoporphyrin IX was demonstrated in [16]. Thus, the possible role of Tb3+ doping agent in EEET pathways taking place in nanosystems containing lanthanide fluoride NP and chlorin e6 photosensitizer should be studied. X-ray-induced scintillation emission spectra of CeF3 NP (both undoped and Tb3+-doped) were similar to their photoluminescence spectra excited in ultraviolet spectral region [9, 1618]. EEET processes in Tb3+-doped cerium fluoride NP are also expected to be similar for the cases of X-rays and ultraviolet light excitation. Therefore, studies of energy transfer between the sensitizer and Tb3+-doped CeF3 NP can be carried out using UV excitation as a model. Here, we study formation and EEET process in the nanosystem consisting of Ce0.85Tb0.15F3 nanoparticles, cetrimonium bromide (CTAB) surfactant, and chlorin e6 photosensitizer.

Methods

Materials

Hydrofluoric acid, isopropyl alcohol, cerium(III) chloride heptahydrate, and terbium(III) chloride heptahydrate were acquired at Sigma-Aldrich Co. and used without further purification. Chlorin e6 (Frontier Scientific Inc.) was kindly provided by T.Y. Ohulchanskyy (Institute for Lasers, Photonics and Biophotonics at the State University of New York at Buffalo). Fifty millimolar TRIS-HCl buffer (pH 7.2) was used as solvent.

Synthesis and Characterization of Nanoparticles

Ce0.85Tb0.15F3 NPs (0.07 M water solution) were synthesized as described in [19]. Briefly, a mixture of 1.58 g of cerium(III) chloride heptahydrate (4.25 mmol, i.e., 85%) and 0.293 g of terbium(III) chloride heptahydrate (0.75 mmol, i.e., 15%) was dissolved in 15 mL of water and added to 150 mL of isopropyl alcohol. Hydrofluoric acid (20 mmol), dissolved in 50 mL of isopropyl alcohol, was added drop-wise to a cerium and terbium salt solution under vigorous stirring. The resulting white sediment was filtered and washed carefully by pure isopropyl alcohol several times. Then, the suspension was slightly dried to form a paste-like substance and dispersed in 110 ml of distilled water using an ultrasonic bath. The resulting transparent colloid solution was boiled for 5 min to remove residual alcohol.

Particle size distribution was studied by the dynamic light scattering (DLS) technique using ZetasizerNano ZS (Malvern Instruments) apparatus (Fig. 1). For the obtained Ce0.85Tb0.15F3 NP, intensity distribution of the hydrodynamic diameter gave main maximum at 62 ± 36 nm (about 97% of intensity) with negligibly small addition of larger fractions. The Z potential of the synthesized NPs was determined as +41 ± 14 mV.
Fig. 1
Fig. 1

Distribution of DLS intensity for Ce0.85Tb0.15F3 nanoparticles, averaged for 5 measurements

A representative transmission electron microscopy (TEM) image of Ce0.85Tb0.15F3 nanoparticles obtained in the above-described reaction is provided in Fig. 2. TEM was performed using a Leo 912 AB Omega electron microscope operating at 100 kV. Before the analysis, sols were brought onto the copper grids using micropipette without any specific pretreatment and dried in ambient air. Comparison of Figs. 1 and 2 shows that the size of the nanoparticles obtained by TEM is smaller than the hydrodynamic diameter at the maximum of DLS intensity distribution. We believe this is connected with the peculiar property of the DLS method that the DLS intensity by a particle is proportional to the sixth power of its radius; thus, larger particle makes higher contribution to the DLS intensity as compared to smaller one. Recalculation of the obtained intensity distribution to nanoparticle number distribution according to the mentioned sixth power relation resulted in a maximum at 24 ± 7 nm that is in agreement with the TEM results.
Fig. 2
Fig. 2

TEM image of Ce0.85Tb0.15F3 nanoparticles

Preparation of Samples

Concentrated solution of chlorin e6 (10 mM) was prepared in DMF. To prepare the solution of the studied nanosystems, an aliquot (20 μL per 1 mL) of 0.07 M water solution of Ce0.85Tb0.15F3 nanoparticles was added to the CTAB solution (0.05 mg/mL CTAB concentration was found to be optimal) in 50 mM TRIS-HCl buffer (pH 7.2). An aliquot of chlorin e6 concentrated solution was then added; in order to minimize reabsorption, 2 μM concentration of chlorin e6 was used; at this concentration, chlorin e6 has negligible absorption at the maximum wavelength of Ce3+ emission, while at the wavelength of the Soret band maximum (near 400 nm; optical density about 0.3 for the used concentration of chlorin e6), Ce3+ emission is already weak. Besides, 5 and 10 μM concentrations of chlorin e6 were additionally used in Tb3+ luminescence decay measurements. Solution of chlorin e6 (2 μM) in the presence of concentrated micelles-forming CTAB (1 mg/mL) was used for the comparison.

Spectral Measurements

Absorption spectra were measured using Specord M40 spectrophotometer (Carl Zeiss, Germany). Luminescence excitation, emission, and anisotropy spectra as well as the curves of luminescence decay in millisecond timescale were registered with the help of the Cary Eclipse fluorescent spectrophotometer (Varian, Australia). Absorption and fluorescence measurements were performed in 1 cm × 1 cm quartz cell at room temperature. Quantitative estimation of the efficiency of Ce3+ to chlorin e6 EEET (ECe−Ce6) was performed as described in [20] by comparison of chlorin e6 fluorescence intensities upon excitation of Ce3+ (\( {I}_{{\mathrm{emCe}}_6}^{{\mathrm{exCe}}^{3+}} \), contribution to this intensity of the own excitation of chlorin e6 at this wavelength was subtracted) and chlorin e6 itself (\( {I}_{{\mathrm{emCe}}_6}^{{\mathrm{exCe}}_6} \); optical densities of Ce3+ and chlorin e6 at the used excitation wavelengths were equal) according to:
$$ {E}_{Ce- Ce6}=\frac{I_{{\mathrm{emCe}}_6}^{{\mathrm{exCe}}^{3+}}}{I_{{\mathrm{emCe}}_6}^{{\mathrm{exCe}}_6}} $$
(1)
The value of E Ce−Ce6 could be also estimated by comparison of the integral intensities of Ce3+ emission in the presence (IntNPCe6) and in the absence (IntNP) of chlorin e6, given that the reabsorption could be neglected, as:
$$ {E}_{\mathrm{Ce}-\mathrm{Ce}6}=1-\frac{{\mathrm{Int}}_{\mathrm{NP}\;{\mathrm{Ce}}_6}}{{\mathrm{Int}}_{\mathrm{NP}}} $$
(2)

When performing estimation of the efficiency of Ce3+-to-chlorin e6 EEET by (1) and (2), spectral sensitivity of the fluorescent spectrophotometer on the excitation and emission wavelength was taken into account.

Decay curves of Tb3+ luminescence were fitted by three exponents; the relative intensities B 1, B 2, and B 3 are calculated as B i  = A i  × τ i A i × τi (i = 1, 2, 3; τ 1, τ 2, and τ 3 are the decay times; A 1, A 2, and A 3 are amplitudes of corresponding exponents). Quantitative estimation of the efficiency of Tb3+-to-chlorin e6 EEET for each of the three decay components (E Tb−Ce6(τ i )) was performed by comparison of the decay times of corresponding components of Tb3+ luminescence at 543 nm in the absence of chlorin e6 (τ i Tb) and in its presence (τ i TbCe6) as:
$$ {E}_{\mathrm{Tb}-\mathrm{Ce}6}\left({\tau}_i\right)=1-\frac{\tau_i^{{\mathrm{Tb}\mathrm{Ce}}_6}}{\tau_i^{\mathrm{Tb}}} $$
(3)

Amplitudes A 1, A 2, and A 3 are affected (to different extent for different components) by addition of chlorin e6 due to decreased EEET from Ce3+ to Tb3+. Thus, the expression (3) cannot be applied for calculation of total Tb3+-to-chlorin e6 EEET efficiency using average decay time values.

Three-exponential fit of the Tb3+ decay curves was also used to estimate real decrease of Tb3+ luminescence intensity upon addition of chlorin e6. Cary Eclipse fluorescent spectrophotometer uses pulsed xenon lamp (80 Hz; 2 μs pulse width at half peak height) for luminescence excitation, setting the intensity measured before each pulse (thus, in about 12.5 ms after previous pulse) as zero for the background correction. Hence, the intensity of Tb3+ luminescence that has decay times in ms range is artificially reduced; but addition of chlorin e6 leads to the decay time decrease and thus less significant artificial reduction of the intensity. So, the Tb3+ luminescence quenching apparent from the spectra is less strong than the real one. Thus, we estimated the real intensities of Tb3+ luminescence (and thus its real decrease upon chlorin e6 addition) using the results of the three-exponential fit of the luminescence decay curve as A 1 × τ 1 + A 2 × τ 2 + A 3 × τ 3.

Results and Discussion

EEET in NP-CTAB-Chlorin e6 Nanosystem

First of all, it should be mentioned that it is not possible to prepare the nanosystem with EEET consisting of the synthesized Ce0.85Tb0.15F3 NP and the monomer form of chlorine e6 based only on electrostatic interactions without any linking group or some binding substance. While in the distilled water, chlorin e6 molecules form aggregates on Ce0.85Tb0.15F3 NP that leads to complete quenching of the chlorin e6 fluorescence; in 50 mM TRIS-HCl buffer (pH 7.2), no manifestation of interaction was observed in chlorin e6 absorption and emission spectra. Such dependence on medium should be connected with the change in chlorin e6 molecule taking place at pH value about 6.1 [14].

In order to form model nanosystem containing Ce0.85Tb0.15F3 NP and chlorin e6, surfactant CTAB was used that was reported as a stabilizer for lanthanide fluoride NPs in [11]. Thus, to the solution of NPs in the presence of 0.05 mg/mL CTAB, 2 μM of chlorin e6 was added. Absorption spectrum of the obtained solution in comparison with these of chlorin e6 free and in the presence of CTAB micelles (i.e., CTAB of the concentration 1 mg/mL) is presented in Fig. 3. Absorption spectra of chlorin e6 in the presence of CTAB micelles and CTAB-NP nanosystems are similar but significantly different from chlorin e6 spectrum in buffer. Thus, we could suppose that in both cases, chlorin e6 molecules are built in a shell formed by CTAB molecules; supposed arrangement of the components in the NP-CTAB-chlorin e6 nanosystem is schematically presented in Fig. 4.
Fig. 3
Fig. 3

Absorption spectra of 2 μM chlorin e6 free in solution (black solid line) and in the presence of CTAB micelles (1 mg/mL CTAB; green dash-dotted line) and Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem (2 μM chlorin e6; 0.05 mg/mL CTAB; red dashed line)

Fig. 4
Fig. 4

Schematic representation of the supposed arrangement of the components in the NP-CTAB-chlorin e6 nanosystem. In the symbol representing chlorin e6 molecule, the minus placed in the circle stands for the net negative charge of the molecule that could be due to some or all of its three carboxylic groups. Bromide counter ions of the CTAB molecules are not shown for simplicity

Fluorescence emission spectra of Ce0.85Tb0.15F3 NP in the presence of CTAB (Fig. 5) demonstrate broad band corresponding to Ce3+ emission (320 nm) and narrow bands of Tb3+ ions (490, 543, 584, and 621 nm) as described in the literature [13]. Addition of chlorin e6 (Fig. 5) results in decrease of the intensity of Ce0.85Tb0.15F3 NP emission bands as well as in appearance of the band corresponding to chlorin e6 fluorescence (670 nm). This could be explained as EEET of Ce0.85Tb0.15F3 NP excitations to the chlorin e6 molecules bound to the CTAB shell of Ce0.85Tb0.15F3 NP. This conclusion could be also supported by fluorescence excitation measurements (Fig. 6). In the normalized fluorescence excitation spectra of chlorin e6 (emission at 680 nm) besides its own Soret and Q-bands, we observe the band at 250 nm, which coincides with the one in the excitation spectrum of Ce0.85Tb0.15F3 NP (emission at 320 nm); this is consistent with EEET from Ce3+ ions to chlorin e6. The increase in the fluorescence anisotropy of chlorin e6 in the presence of NP-CTAB (data not presented) is one more proof of the formation of NP-CTAB-chlorin e6 nanosystem.
Fig. 5
Fig. 5

Fluorescence emission spectra of Ce0.85Tb0.15F3 NP in the presence of CTAB (0.05 mg/mL CTAB; black solid line), Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem (2 μM chlorin e6; 0.05 mg/mL CTAB; red dashed line), and difference of Ce3+ bands in these spectra (blue dash-dotted line). Excitation wavelength 250 nm

Fig. 6
Fig. 6

Normalized fluorescence excitation spectra of Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem (2 μM chlorin e6; 0.05 mg/mL CTAB; black solid line (emission of chlorin e6 at 680 nm) and blue short dashed line (emission of Ce3+ at 320 nm)) and chlorin e6 in the presence of CTAB micelles (1 mg/mL CTAB; emission at 680 nm; red dashed line)

It is seen from Fig. 5 that the addition of chlorin e6 leads to the narrowing and short-wavelength shift of the Ce3+ emission band. Difference of the unquenched and quenched Ce3+ bands gives the broad band with the maximum near 355 nm (Fig. 5) that most possibly corresponds to the emission of the perturbed Ce3+ states; these states were supposed to be the traps for the non-perturbed Ce3+ excitations transferring these excitations to either Tb3+ dopant or to the attached photosensitizer [8]. It should be added that the mentioned difference spectrum does not contain any significant component similar to that of Soret band of chlorin e6; thus, at these concentrations, the impact of reabsorption on the Ce3+ emission quenching by chlorin e6 could be considered as negligible. Based on Ce3+ emission spectra in the presence and in the absence of chlorin e6, efficiency of EEET could be estimated by comparing integral intensities of Ce3+ emission according to (2); the value of EEET efficiency equal to 0.33 was obtained for the 2 μM concentration of chlorin e6. Increasing the chlorin e6 concentration results in more significant EEET efficiency values, but these values also contain higher reabsorption contribution.

Efficiency of EEET from Ce3+ to chlorin e6 could be also estimated from the fluorescence excitation spectra by comparison of chlorin e6 fluorescence intensities upon excitation of Ce3+ (at 271 nm; contribution to this intensity of the own excitation of chlorin e6 at this wavelength was subtracted using the normalized excitation spectrum of chlorin e6 in CTAB micelles (Fig. 6)) and chlorin e6 itself (at 406 nm; optical densities of Ce3+ at 271 nm and chlorin e6 at 406 nm are equal) according to (1). Surprisingly, the value of EEET efficiency of about 0.06 was obtained that is much less than the value of 0.33 obtained according to (2). We could suppose that the EEET efficiency calculation based on (1) cannot be applied in our case. Perhaps Ce3+-to-chlorin e6 EEET brings chlorin e6 molecule to the vibronic levels with higher ability to further intersystem conversion as compared to the photoexcitation at 406 nm; this would cause decreased fluorescence quantum yield of chlorin e6 leading to lower values of  apparent EEET efficiency than calculated by (1).

It should be mentioned that the close proximity of Ce0.85Tb0.15F3 NP causes the strong decrease in the fluorescence intensity of chlorin e6; the same effect was noticed in [10]. The possible reason for this could be the heavy atom effect, i.e., more intensive transition of the excitations to the triplet state due to the close proximity of Ce and Tb atoms causing spin-orbit interactions in chlorin e6 molecule. One more possible explanation could be EEET between chlorin e6 molecules in the case where they are bound to NP-CTAB nanosystem at the mutual distances that are close enough for chlorin e6-chlorin e6 energy transfer.

EEET Pathways in NP-CTAB-Chlorin e6 Nanosystem

It is interesting to study in more details the pathway of EEET from NP to chlorin e6. It is known that EEET from Ce3+ to Tb3+ ions takes place inside Ce0.85Tb0.15F3 nanoparticles [12, 13]. When adding chlorin e6 to the nanosystem, the following additional processes could occur besides the mentioned Ce3+-to-Tb3+ EEET: (i) EEET from Ce3+ perturbed states directly to chlorin e6 and (ii) EEET from excited Tb3+ ions to chlorin e6 molecules.

First of all, since the excited state lifetime of Tb3+ ions is extremely long as compared to that of Ce3+ [8, 13], decrease in the Ce3+ emission intensity upon addition of chlorin e6 means the direct EEET from Ce3+ to chlorin e6 (with the 0.33 efficiency for 2 μM chlorin e6). Further, it is seen from Fig. 5 that together with the decrease in Ce3+ emission, this of Tb3+ diminishes as well. The apparent decrease in the Tb3+ emission band intensity upon addition of chlorin e6 is about 20–23%, but the total intensity values of the millisecond Tb3+ emission are biased by the spectrofluorometer (see "Spectral measurements" subsection in the “Methods” section); real intensity decrease was estimated using Tb3+ luminescence decay curves as 56% (543-nm band for 2 μM chlorin e6) exceeding that for Ce3+ (33% for 2 μM chlorin e6). Thus, the Tb3+ emission quenching could be due both to (i) Tb3+-to-chlorin e6 EEET and to (ii) decreased Ce3+-to-Tb3+ EEET (and thus reduced population of excited levels of Tb3+) caused by competition with the direct Ce3+-to-chlorin e6 EEET. The observed decrease in the Tb3+ emission decay time upon the addition of chlorin e6 (Fig. 7) points to the existing of EEET from excited Tb3+ ions to chlorin e6 molecules; such transfer was also reported for protoporphyrin IX in [16]. It should be mentioned that while the spectral overlap of Tb3+ emission with chlorin e6 absorption could be poor, extremely high values of the donor (i.e., Tb3+) excited state lifetime could still lead to efficient EEET at significant distances.
Fig. 7
Fig. 7

Luminescence decay curve (broad red line) and its three-exponential fit (thin black line) of Tb3+ band at 543 nm of Ce0.85Tb0.15F3 NP in the presence of 0.05 mg/mL CTAB (a) and Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem for 2 μM (b), 5 μM (c), and 10 μM (d) concentration of chlorin e6; CTAB concentration 0.05 mg/mL. Excitation wavelength 250 nm

To analyze in more details the quenching of Tb3+ emission, let us look at the components of the three-exponential fit of the decay curve for the most intensive Tb3+ luminescence band at 543 nm (Table 1). It is seen that the decay times of all three components decrease upon the addition of chlorin e6 pointing to EEET from Tb3+ to chlorin e6 for all of them. EEET efficiency calculated according to (3) for all three components turns out to be the highest (0.42 for 2 μM of chlorin e6) for the shortest component τ 1 and the lowest (but still as high as 0.15 for 2 μM of chlorin e6) for the longest component τ 3. At the same time, while the amplitude A 1 of the shortest component stays about the same at different concentrations of chlorin e6, that of the medium one A 2 does not change at 2 μM of chlorin e6 and decreases almost twice at its highest concentration of 10 μM. At the same time, the amplitude A 3 of the longest component decreases the most strongly (more than twice at 2 μM and more than 10 times at 10 μM of chlorin e6). We could thus suppose that the luminescence intensity corresponding to the shortest component τ 1 (and the medium one τ 2 at the low concentrations of chlorin e6) decreases mainly due to EEET from Tb3+ to chlorin e6. At the same time, the intensity of the longest component τ 3 diminishes due to both (i) EEET from Tb3+ to chlorin e6 (that reduces τ3) and (ii) decreased population of Tb3+ excited states due to competition of EEET from Ce3+ to Tb3+ with that from Ce3+ to chlorin e6 (that reduces A 3). We could further speculate that the short-time emitting Tb3+ ions receive excitations without competition with Ce3+-to-chlorin e6 EEET. At the same time, these short-time emitting Tb3+ ions surprisingly demonstrate the highest Tb3+-to-chlorin e6 EEET efficiency. The possible explanation could be as follows (Fig. 8). We could suppose that Ce3+ perturbed states (that demonstrate luminescence near 355 nm (Fig. 5)) are mostly connected with Ce3+ ions situated close to the surface of the NP. In this case, Tb3+ ions with shorter decay times are supposed to be situated near the NP surface as well and close to perturbed Ce3+ ions (Fig. 8, A). This results in (i) high Ce3+-to-Tb3+ EEET rate (due to short distance) leaving no place for competition by Ce3+-to-chlorin e6 EEET, and (ii) high rate of subsequent Tb3+-to-chlorin e6 EEET. At the same time, Tb3+ ions with longer luminescence decay times are supposed to be situated further from the NP surface (generally, higher distance from surface means lower impact of various quenchers that is consistent with higher luminescence decay times); they receive excitations from the perturbed Ce3+ ions which do not neighbor short-time emitting surface Tb3+ ions in close proximity (Fig. 8, B). This results in (i) lower Ce3+-to-Tb3+ EEET rate that permits partial redirection of Ce3+ excitation flow to the Ce3+-to-chlorin e6 EEET pathway and (ii) lower rate of subsequent Tb3+-to-chlorin e6 EEET.
Table 1

Parameters of the three-exponential fit of the decay curve of Tb3+ luminescence band at 543 nm for Ce0.85Tb0.15F3 NP in the presence of 0.05 mg/mL CTAB and Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem (2, 5, or 10 μM of chlorin e6; 0.05 mg/mL CTAB)

Chlorin e6 concentration

τ 1, ms

A 1, a.u.

B 1

E Tb−Ce6 1 )

τ 2, ms

A 2, a.u.

B 2

E Tb−Ce6 2 )

τ 3, ms

A 3, a.u.

B 3

E Tb−Ce6 3 )

0 μM

0.69

34

0.02

2.35

81

0.17

6.13

147

0.81

2 μM

0.40

48

0.04

0.42

1.71

84

0.29

0.27

5.18

64

0.67

0.15

5 μM

0.31

47

0.06

0.54

1.32

63

0.36

0.44

4.51

29

0.58

0.27

10 μM

0.27

44

0.10

0.61

1.06

46

0.42

0.55

3.98

14

0.48

0.35

τ 1, τ 2, and τ 3 are the decay times; A 1, A 2, and A 3 are the amplitudes; B 1, B 2, and B 3 are the relative intensities (calculated as B i  = A i  × τ i A i  × τ i , i = 1, 2, 3) of the three-exponential fit; E Tb−Ce6(τ i ) is the EEET efficiency calculated for the corresponding decay component according to the expression (3)

Fig. 8
Fig. 8

Schematic representation of the supposed EEET pathways (arrows) between perturbed Ce3+ ions, Tb3+ ions, and chlorin e6 molecules bound to Ce0.85Tb0.15F3 NP (CTAB shell is not presented for simplicity)

Basing on the above observations, photophysics processes in the Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem could be as follows. Suggesting that perturbed Ce3+ ions are mainly localized near to the NP surface, for the part of these ions situated close to Tb3+ ones EEET only to Tb3+ takes place. For the other Ce3+ ions, competition between EEET to Tb3+ (localized at higher distance from the NP surface) and to chlorin e6 exists. For both cases, excitations of Tb3+ are further transferred to chlorin e6.

Conclusions

  1. 1.

    Chlorin e6 molecules bind to Ce0.85Tb0.15F3 NP in the presence of CTAB forming thus the Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem. We consider that binding occurs via chlorin e6 embedding in the shell formed around NP by CTAB molecules.

     
  2. 2.

    In the Ce0.85Tb0.15F3 NP-CTAB-chlorin e6 nanosystem, electronic excitation energy transfer from Ce 3+ to chlorin e6 takes place both directly (with the 0.33 efficiency for 2 μM chlorin e6) and via Tb3+.

     

Notes

Abbreviations

CTAB: 

Cetrimonium bromide

DLS: 

Dynamic light scattering

EEET: 

Electronic excitation energy transfer

NP: 

Nanoparticles

PDT: 

Photodynamic therapy

TEM: 

Transmission electron microscopy

Declarations

Authors’ Contributions

ML and LK carried out the spectral measurements and calculations and wrote the article. OS performed the synthesis of nanoparticles and participated in the discussion of the nanosystem model. AM performed the DLS characterization of the nanoparticles. NG and VY participated in the design of the study, discussion of the results, and coordination. All authors read and approved the final manuscript.

Competing Interests

The authors declare that they have no competing interests.

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Authors’ Affiliations

(1)
Taras Shevchenko National University of Kyiv, Volodymyrska Str., 64/13, Kyiv, 01601, Ukraine
(2)
Zabolotny Institute of Microbiology and Virology, NAS of Ukraine, 154 Zabolotnogo Str., Kyiv, 03680, Ukraine
(3)
R.E. Kavetsky Institute for Experimental Pathology, Oncology and Radiobiology, NAS of Ukraine, 45 Vasylkivska Str., Kyiv, 03022, Ukraine
(4)
National University of Food Technologies, Volodymyrska Str. 68, Kyiv, 01601, Ukraine

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