Reactive oxygen species generation in aqueous solutions containing GdVO4:Eu3+ nanoparticles and their complexes with methylene blue
Hubenko et al. Nanoscale Research Letters
Reactive oxygen species generation in 3+ aqueous solutions containing GdVO :Eu 4 nanoparticles and their complexes with methylene blue
Kateryna Hubenko 0
Svetlana Yefimova 0
Tatyana Tkacheva 0
Pavel Maksimchuk 0
Igor Borovoy 0
Vladimir Klochkov 0
Nataliya Kavok 0
Oleksander Opolonin 0
Yuri Malyukin 0
0 Institute for Scintillation Materials National Academy of Sciences of Ukraine , 60 Nauky ave., Kharkiv 61072 , Ukraine
It this letter, we report the study of free radicals and reactive oxygen species (ROS) generation in water solutions containing gadolinium orthovanadate GdVO4:Eu3+ nanoparticles (VNPs) and their complexes with methylene blue (MB) photosensitizer. The catalytic activity was studied under UV-Vis and X-ray irradiation by three methods (conjugated dienes test, OH· radical, and singlet oxygen detection). It has been shown that the VNPs-MB complexes reveal high efficiency of ROS generation under UV-Vis irradiation associated with both high efficiency of OH· radicals generation by VNPs and singlet oxygen generation by MB due to nonradiative excitation energy transfer from VNPs to MB molecules. Contrary to that under X-ray irradiation, the strong OH. radicals scavenging by VNPs has been observed.
Reactive oxygen species; Nanoparticles; Photocatalytic activity; Radical scavenging
Radiation therapy (RT) remains an important
component of cancer treatment with approximately 50% of all
cancer patients receiving RT during their course of
]. The exact mechanism of cell death due to
radiation is still an area of active investigation.
Doublestranded breaks of nuclear DNA are considered to be
the most important cellular effect of radiation leading to
irreversible loss of the reproductive integrity of the cell
and eventual cell death [
]. Such radiation damage can
be caused by (i) direct ionization and (ii) indirect
ionization via free radicals and reactive oxygen species
(ROS), chemically reactive species containing oxygen,
formed from the radiolysis of cellular water and oxygen
]. In clinical therapy, damage is
commonly indirect ionizing. In the process, water loses an
electron and becomes highly reactive. Then, through a
three-step chain reaction, water is sequentially converted
into a number of radicals and molecular products:
hydrated electrons ( ea−qÞ , hydrogen atom( H∙), hydroxyl
radical OH·, hydroperoxyl radical ( HO:2Þ , hydrogen
peroxide (H2O2), and hydrogen molecules (H2) [
Hydrated electrons and hydrogen atoms are strong
reducing agents. In contrast, hydroxyl radicals are very
strong oxidative species and immediately remove
electrons from any molecule in its path, turning that
molecule into a free radical and thus propagating a
chain reaction . When dissolved molecular oxygen is
presented in irradiated water, its reduction produces
superoxide radical ( O:2− ) and is the precursor of most
other ROS including singlet oxygen (1O2) [
Recently, it has been shown that high atomic number
(Au, Ag, Hf, Gd, Ti based) nanoparticles (NPs) [
semiconductor NPs (metal-oxide TiO2 ZnO, CuO,
CeO2, Al2O3; quantum dots ZnS, ZnS, LaF3, etc.) [
], and some inorganic NPs (carbon nanotubes)
] enhance the efficiency of RT. Theoretical
principles of X-rays–NP interaction are well described [
]. A cascade interaction of high-energy photons with
the NP’s lattice occurs through the photoelectric effect
and the Compton scattering effect mainly. Compton,
Photo-, or Auger-electrons can induce the emission of
secondary electrons, which can escape into the
environment and will be captured by an acceptor (i.e.,
water, biomolecule, oxygen, nitrogen oxides) localized
near the surface of NPs and induce biomolecular radicals
and ROS production [
8, 12, 14
]. Radiosensitizing effects
of NPs is associated with biomolecular radicals and ROS
generation as a final stage of X-ray interaction with NPs.
In semiconductor NPs, such as metal-oxide NPs, the
cytotoxic effect associated with ROS generation can be
also induced by UV irradiation [
]. The mechanism
is that when NPs are irradiated with the UV light
(energy greater than the band gap), the charge separation is
induced generating a hole (h+) in the valance band and
an electron (e−) in the conducting band. Electrons and
holes exhibit high reducing and oxidizing ability,
]. The electrons can react with molecular
oxygen to produce superoxide radical ( O2− ) through a
reductive process, whereas the holes can abstract
electrons from water and/or hydroxyl ions generating
hydroxyl radicals (OH·) through an oxidative process
]. For TiO2, CeO2, Al2O3, and ZnO nanoparticles,
the 1O2 generation via the oxidation of O2− was reported
18, 21, 22
One more approach to enhance the efficiency of
cancer therapy (photodynamic therapy, PDT) using
scintillating NPs was proposed by Chen and Zhang [
approach combines X-ray excited (scintillating) NPs and
photosensitizer (PS) molecules. Scintillating NPs serve
as an energy transducer transferring energy harvested
from X-ray irradiation to the PS that generates singlet
oxygen for tumor destruction. This approach allows
deep cancer treatment and enhances both PDT and RT.
Up to now, a variety of scintillation NPs and their
complexes with PS have been studied as X-ray inducible
photodynamic agents [
12, 14, 23–28
Recently, we have reported on the creation of
complexes on the base of scintillating gadolinium
orthovanadate NPs doped with europium ions GdVO4:Eu3+
(VNPs) and methylene blue (MB) photosensitizer and
study of electronic excitation energy transfer in the
]. It was shown that due to the effective
excitation energy conversion in the complexes, they
could be prospective as an X-ray inducible
photodynamic agent. The aim of this paper was to study the
efficiency of ROS generation in water solutions
containing VNPs and their complexes with MB under UV and
Gadolinium chloride 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. L-a-phosphatidylcholine (PC) from egg yolk,
cationic dye 3,7-bis(dimethylamino)phenazathionium
chloride (methylene blue (MB), Mw = 373.90 g/mol),
1,2-Benzopyrone (Coumarin, Mw = 146.14 g/mol) were
purchased from Sigma-Aldrich (USA) and used as
received. Antracene-9,10-dipropionic acid disodium salt
(ADPA, Mw = 366.32 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. All other chemicals were of
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 [
]. 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 was added to the mixture. Then,
obtained solution was mixed with 37.5 mL of disodium
EDTA solution (0.01 M). Then, 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 colorless 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. For
this purpose, the obtained solution was purred in a
dialysis sac (Cellu Sep T2, membrane with a molecular
weight cut-off of 12 KDa, pore size ~ 2.5 nm) and placed
in a 2 L glass with distilled water. After each 6 h, the
water was refreshed.
Instrumentation and characterization
Synthesized VNPs were characterized using transmission
electron microscopy (TEM-125 K 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). Fluorescence and
fluorescence excitation spectra were taken with a
spectrofluorimeter Lumina (Thermo Scientific, USA).
Preparation of VNPs–MB complexes
Solutions for investigations were prepared as follows.
First, stock solutions of MB in water (1 mmol/L) were
prepared. To obtain VNPs–MB aqueous solutions,
required amount of the dye stock solution and VNPs
aqueous solution were 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 was added in a
flask and gently shaken during 1 h for VNPs–MB
complex formation. The concentration of MB in the
obtained solution was 10 μmol/L. The concentrations of
nanoparticles were 0.1, 1, or 10 mg/mL.
Active oxygen and free radical species detection
The formation of ROS under the UV/X-ray irradiation
of aqueous solutions containing VNPs, MB, or
VNPs-MB complexes was detected spectroscopically
using several methods.
Conjugated dienes formation test
Lipid oxidation under UV irradiation was measured
using PC liposomes suspension. Unilamellar PC lipid
vesicles were prepared by the extrusion method [
Briefly, appropriate amount of PC (25 mg/ml) in
chloroform was placed in a flask and dried until complete
chloroform evaporation using a rotary evaporator
(Rotavapor R-3, Buchi). The thin lipid-dyes film was
then hydrated with 10.8 ml of double distilled water.
The obtained lipid suspension was finally extruded
through 100 nm pore size polycarbonate filter using a
mini-extruder (Avanti Polar Lipids, Inc., USA). The
concentration of PC was 1.2 mmol/L. For conjugated dienes
formation test, 1 mL of the PC liposome suspension was
mixed with 1 mL of VNPs water solution (MB water
solution or VNPs–MB water solution). The final MB
concentration was 10 μmol/L and VNPs, 1 g/L. PC
concentration in the solutions was 0.6 mmol/L. The
obtained aqueous solutions were placed in quartz cuvettes
(10 × 10 mm) and irradiated with 250 W mercury lamp
(band pass l = 310–400 nm, light flux was 43 W/cm2)
for 30 min. Then, the absorbance of the suspensions was
recorded at 234 nm (conjugated dienes maximum) using
a Specord 200 spectrophotometer (Analytik Jena,
Germany). The concentration of conjugated dienes formed
in water without any additives (NPs, MB, or VNPs–MB
complexes) was taken as a control. Each experimental
point was the mean value of at least three independent
tests. Statistical processing was carried out using the
software package Statistika v. 5.0 (StatSoft, USA).
OH· radical detection
To detect the hydroxyl radical formation in the solution
under UV irradiation, coumarin was used as a probe
molecule. Coumarin reacts with OH· radicals producing
highly fluorescent 7-hydroxycoumarin [
Experimental procedure was as follows. Coumarin aqueous
solution (0.1 mmol/L) was mixed with MB (10 μmol/L),
VNPs (0.1, 1 or 10 g/L), or VNPs–MB aqueous
solutions. The obtained aqueous solutions were placed
in quartz cuvettes (10 × 10 mm) and irradiated with
He-Cd laser λexc = 325 nm for 1 h. In case of X-ray
irradiation, the cuvette was irradiated from above (from
the open part) by an X-ray using ISOVOLT 160 Titan E
apparatus with a tungsten cathode for 30 min. The
voltage on the tube was 30 kV (20 mA). The distance from
the X-ray tube to the irradiated samples was 25 cm. The
fluorescence spectra (excited at 325 nm) of the solutions
were recorded with a spectrofluorimeter Lumina
(Thermo Scientific, USA). The relative intensity of
7-hydroxycoumarin fluorescence was analyzed.
Singlet oxygen detection
1O2 production in the solutions containing VNPs, MB,
or VNPs–MB complexes was analyzed on the evaluation
of ADPA fluorescence spectra [
measurements were carried out in quartz cuvettes (10 ×
10 mm). ADPA aqueous solution (10 μmol/L) was
mixed with MB (10 μmol/L), NPs (1 g/L), or VNPs–MB
aqueous solutions in a cuvettes. The solutions were
irradiated at 457 nm using High Stability Blue Solid
State Laser MBL-457, 50 mW (Changchun New
Industries Optoelectronics Tech. Co., Ltd.). The fluorescence
emission of ADPA excited at 378 nm was collected at
different time scales (0, 10, 20, 30, 40, and 60 min) using
a spectrofluorimeter Lumina (Thermo Scientific, USA).
Results and discussions
Characteristic of synthesized VNPs
Figure 1 a and Additional file 1: Figure S1 show the
TEM images of synthesized VNPs with a side
distribution histogram and an XRD pattern, which support the
GdVO4:Eu3+ NPs crystalline structure. Synthesized
GdVO4:Eu3+ NPs are of spindle-like form with a 8 ×
25 nm ± 5 nm size and tetragonal phase structure of
zircon type. The negative charge of the NPs surface
(ζ-potential is − 18.75 ± 0.15 mV, pH = 7.8) is due to
carboxylate groups of disodium EDTA stabilizer used
during the synthesis. The overage hydrodynamic
diameter of GdVO4:Eu3+ nanoparticles is 44.0 ± 0.3 nm. The
absorption spectrum of GdVO4:Eu3+ NPs represents of
the intense wide band in the 250–350 nm spectral range
that corresponds to a charge transfer from oxygen
ligands to the vanadium atom in VO3− group (Fig. 1b)
]. Doping GdVO4 NPs with Eu3+ ions imparts strong
fluorescence to VNPs in the red spectral range, which is
governed by the transition within the f–electron
configuration of the europium ions [
] (will not be
discussed in this paper).
It is known that the size of NPs affects the optical
energy gap in semiconductor materials. The band gap
energy, Eg, can be estimated from the absorption edge
wavelength of the inter-band transition according to the
Tauc’s relationship [
ðahvÞð1=nÞ ¼ A
where a is absorption coefficient, hv is the incident
photon energy, A is the energy independent constant (the
band tailing parameter), and n is the constant (power
factor of the transition mode), which depends on the
material nature (crystalline or amorphous). The value of
n denotes the nature of the transition, n = 1/2 for direct
allowed transitions, n = 3/2 for direct forbidden
transitions, n = 2 for indirect allowed transitions, and n = 3 for
indirect forbidden transition [
]. GdVO4 is a direct gap
semiconductor, for which n = 1/2 [
] Thus, Eq. (1) can
be rewritten as:
ðαhvÞ2 ¼ A
Absorption coefficient (a) is calculated from
absorbance as a = 2.303D/l, where D is absorbance and l is the
Figure 1c represents the energy dependence of (ahv)2
for synthesized GdVO4:Eu3+ nanoparticles. The band
gap value Eg was determined by extrapolation of the
linear portion of the (ahv)2 curve versus the photon
energy hv to zero. The obtained value Eg = 4.13 eV is
higher than that reported for GdVO4:Eu3+ powders with
crystallite size ranging from 14.4 to 43 nm (3.56–3.
72 eV) [
]. We suppose it could be due to the
difference in used synthesize methods that in our case
gives smaller NPs with narrow and blue-shifted
absorption band as compared to that obtained by the
hydrothermal or Pechini’s methods.
Photo-induced free radicals generation (conjugated dienes test)
It is commonly accepted that tree types of ROS ( OH ;
O2− , and 1O2 ) generating in NPs systems under UV
irradiation contribute to the major oxidative stress in
biological systems [
]. Although photocatalytic
activity of such metal-oxide NPs as TiO2, ZnO, CuO,
CeO2, Al2O3, and Fe2O3 is well-described [
research has studied the photocatalytic activity of ReVO4
]. It was shown that ReVO4 NPs are effective
in photocatalytic destruction of organic pollutants.
However, no research has studied the types of ROS generated
by ReVO4 NPs under UV irradiation.
To mimic biological environment, we used PC
liposome suspension and detected free radicals generation
under UV irradiation in the suspensions containing MB,
VNPs, or VNPs–MB complexes on lipid oxidation
(conjugated dienes formation test) [
]. Lipid oxidation by
molecular oxygen via radical chain reactions can be
initiated by ionizing radiation when ROS and free radicals
appear in the system [
]. Radical chain reactions
involving polyunsaturated fatty acids cause a rearrangement
of the double bonds leading to conjugated dienes. The
resulting conjugated dienes exhibit an absorption band at
234 nm that could be detected photometrically. Figure 2
shows relative concentrations of conjugated dienes formed
in lipid suspensions containing MB, VNPs, or VNPs–MB
complexes. It could be seen that in all solutions, the
concentration of conjugated dienes increases as compared to
the pure PC liposome suspension. However, the efficiency
of this process differs. Methylene blue is one of the
conventional photosensitizer molecules with the main
absorption maxima λmax = 665 nm and a less intense absorption
band in the UV spectral range (Additional file 1: Figure
S2). Under UV irradiation of MB, the two major
photochemical processes may take place [
]. MB excited by
UV light undergoes intersystem crossing process (Qp=0.54
]) to the long-lived triplet state (3MB*) and reacts with
oxygen molecules (3O2) forming singlet oxygen (1O2):
MBþ þ hv→3MBþ
3MBþ þ 3O2→MBþ þ 1O2
The second photochemical process may take place at
high MB concentrations. The ground state MB
molecules can work as reducing agents donating an
electron to the MB triplet and forming the semi-reduced
radical (MB·) and semi-reduced radical, respectively
3MBþ þ MBþ→MB
The oxidation of MB· by molecular oxygen returning
the ground state dye and leading to superoxide radical
MB þ 3O2 →MBþ þ O2−
Singlet oxygen and superoxide radicals, as well as MB
radicals formed in Reactions (4)–(6) can affect the lipid
oxidation process. In diluted solution where no MB dimer
formation is observed ([MB] < 20 μM), Reactions (3) and
(4) will dominate [
]. However, in VNPs–MB complexes
due to increased MB concentration within VNPs surface
], the second photochemical process can take place.
Thus, the increase of the conjugated diene formation in
the lipid suspension containing MB can be explained by
MB action as 1O2 photogenerator under UV irradiation. It
should be noted that the efficiency of this process is much
smaller than that under MB excitation within
longwavelength absorption maximum.
In the suspension containing GdVO4:Eu3+ nanoparticles,
lipid oxidation is more effective. This effect could be
ascribed to the photocatalytic behavior of VNPs under UV
irradiation. Conducting band electrons (e−) and valence
band holes (h+) formed under UV irradiation (E > Eg)
can interact with molecular oxygen and water
molecules adsorbed on the NPs surface by following
18, 20, 47
H2O þ hþ→OH
OH− þ hþ→OH
2 þ hþ→1O2
Hydroxyl ions formed during water photolysis and
adsorbed on NPs surface can also interact with holes to
produce hydroxyl radicals:
Moreover, the oxidation of O2− produces singlet
18, 21, 22
2O2− þ 2Hþ→H2O2
Its reaction with hydrogen ions leads to hydrogen
as a result of its interaction with electrons hydroxyl
radicals and hydroxyl ions can be formed:
H2O2 þ e−→OH
Thus, the increase in efficiency of conjugated dienes
concentration in a suspension containing VNPs (Fig. 2,
column 3) can be ascribed to the products generating
via Reactions (7)–(12) and facilitating lipid oxidation.
In the lipid suspension containing complexes VNPs–
MB, the highest conjugated dienes concentrations can
be explained by products generated both via Reactions
(3)–(6) and Reaction (7)–(12) (Fig. 2, column 4).
Moreover, in VNPs–MB complexes in Reaction (3) and (4),
singlet oxygen generation could take place both due to
direct MB excitation and via nonradiative excitation
energy transfer from VNPs to MB that is rather effective in
this composition [
Hydroxyl radical detection
The next step was to examine more exactly the
efficiency of OH· and 1O2 generation in the solutions under
UV/X-ray irradiation. Coumarin was used as a probe
molecule to validate the appearance of hydroxyl radicals
in the solutions under consideration. It is known that
OH· radicals are one of the main products of water
photolysis/radiolysis under UV/X-ray irradiation [
In water solution, OH· radicals interact with coumarin
molecules to form highly fluorescent product
7-hydroxycoumarin (see scheme in Fig. 3) that could be
detected spectroscopically by the appearance of a new
band (λmax ~ 460 nm) shifted toward the
longwavelength spectral region with respect to the coumarin
fluorescence band (λmax ~ 400 nm), Fig. 3 [
higher the concentration of OH· radicals in the solution
is, the more effective coumarin oxidation and,
consequently, the more intense the long-wavelength band are.
Thus, analysis of the relative intensity of the
longwavelength band could provide the information about
the concentration of OH· radicals in the solution under
effect of various factors.
The fluorescence emission spectra of the coumarin
water solution containing MB, VNPs, or VNPs–MB
complexes measured after 1 h of UV illumination is presented
in Fig. 3. It is shown that UV irradiation of coumarin
water solution without any additives (control) provokes a
formation of a new long-wavelength fluorescence band
that indicates OH· radicals generation and coumarin
oxidation (Fig. 3). In the presence of MB molecules in the
solution, the relative intensity of this band does not change
that indicates no additional effects of MB on OH· radicals
generation (Fig. 3). In the solution containing VNPs, the
intensity of the 7-hydroxycoumarin band increases
remarkably (Fig. 3) due to photocatalytic activity of VNPs
under UV irradiation, Reactions (8), (9) and (12). Let us
note that the sharp peaks around 535–540 nm belong to
the europium ion fluorescence in GdVO4:Eu3+
nanoparticles (intraconfiguration transitions). In the
solution containing VNPs–MB complexes, the relative
intensity of the 7-hydroxycoumarin band was about twice
as smaller as compared to that in the solution containing
VNPs that points to the less effective OH· radicals
production (Fig. 3). That can be explained by the fact that
the MB dye adsorption within the VNPs surface can
prevent water molecules and hydroxyl ions adsorption
and, consequently, reduces VNPs photocatalitic activity
concerning OH· radicals generation via Reactions (8) and
(9). Moreover, in VNPs–MB complexes, a part of
adsorbed energy is transferred nonradiatively to MB
] that also decreases the efficiency of
electron-hole pairs production and, consequently, VNPs
capability for OH· radicals generation in such complexes.
Unexpected results were observed under X-ray
irradiation of the solutions containing VNPs (Fig. 4). Contrary
to the case of UV irradiation, we observe that the
relative intensity of 7-hydroxycoumarin band decreases as
compared to the coumarin water solution without
nanoparticles that indicates the scavenging of OH· radicals
formed in the solutions as a result of water radiolysis.
The observed effect is strongly depended on the VNP
concentrations (Fig. 4). It should be noted that the main
discussion concerning nanoparticles’ ability to serve as a
ROS scavenger is focused mainly on CeO2 nanocrystals
]. The main features that forces
nanoceria to act as ROS scavenger are generally attributed to
high content of oxygen vacancies and Ce3+ ions in
nanoceria and its switching between 3+ and 4+ oxidation
states. However, the critical dependence of nanoceria
biological activity on its size and self-regeneration
mechanism is still under discussion [
]. We note also that
protective effects of GdVO4:Eu3+ and CeO2 NPs against
X-ray-induced damages were observed in our group
earlier in vivo experiments [
]. To the best of our knowledge,
the ability of GdVO4:Eu3+ nanoparticle to sweep OH·
radicals generated in the water solution under X-ray
irradiation has been observed for the first time and requires
further more in-depth research.
Singlet oxygen generation
To evaluate the efficiency of VNPs–MB complexes of
1O2 generation in water, we use the method-based
ADPA oxidation by singlet oxygen with a formation of
non-fluorescent endoperoxide ADPAO2 (Fig. 5). Thus,
in the presence of singlet oxygen, the ADPA
fluorescence is quenched irreversibly. We should note that
under UV irradiation, ADPA molecules undergo strong
photobleaching that complicates the identification of
MB, VNPs, or VNPs–MB complexes impacts associated
with the 1O2 generation. To overcome this drawback, we
apply laser irradiation at 457 nm, which matches one of
the excitation peaks of Eu3+ ions doped in GdVO4
nanocrystals (Additional file 1: Figure S3). Figure 5
shows that, the ADPA molecules undergo no
photochemical reactions at the irradiation of 457 nm
light. In the solution containing MB, a slight decrease of
ADPA intensity in time could be observed (Fig. 5) that is
associated with MB slight excitation at this wavelength
and action as photosensitizer according to Reaction (3)
and (4). The same effect is observed for the solution
containing VNPs (Fig. 5) and could be ascribed to the
formation of O2− radicals on the surface of VNPs
(Reaction (7)) followed by its oxidation according to
Reaction (10) with singlet oxygen generation. The
stronger ADPA fluorescence quenching is observed in
VNPs–MB complexes. The efficiency of this process is
twice as higher as in the solution with MB or VNPs. The
higher efficiency of singlet oxygen generation in the
solution containing VNPs–MB complexes is associated
with the energy transfer from VNPs to the MB in the
complexes, in which VNPs serve as energy transducer
for MB photosensitizer.
Unfortunately, due to ADPA sensor instability, we
were not successful to measure the efficiency of the 1O2
generation in water solution under X-ray excitation.
The efficiency of ROS generation in water solutions
containing GdVO4:Eu3+ nanoparticles and their complexes
with MB have been analyzed under UV-Vis and X-ray
irradiation by three methods (conjugated dienes test, OH·
radical, and singlet oxygen detection). Complexes
VNPs–MB reveal high efficiency of ROS generation
under UV-Vis irradiation associated with both high
efficiency of OH· radicals generation by VNPs and 1O2
generation by MB due to nonradiative excitation energy
transfer from VNPs to MB molecules. For the first time,
the strong OH· radicals scavenging by VNPs has been
observed under X-ray irradiation. Our observation
indicates that VNPs–MB complexes can be potentially used
to activate photodynamic therapy.
Additional file 1: Supplementary materials. (DOCX 939 kb)
MB: Methylene blue; PS: Photosensitizer; ROS: Reactive oxygen species;
VNPs: Gadolinium orthovanadate GdVO4:Eu3+ nanoparticles
The authors strongly appreciated to Mrs. Olga Sedyh for her assistance with
nanoparticles synthesis and Dr. O. Sorokin for fruitful discussion of the results
This work was supported by National Academy of Sciences of Ukraine
(Project № 0116U002612).
Availability of data and materials
The datasets generated during and/or analyzed during the current study are
available from the corresponding authors on reasonable request.
The idea of the research was developed by SY with the assistance of YM.
Nanocrystals were synthesized by VK. ADPA was synthesized by IB. Conjugated
dienes test was carried out by NK. Spectroscopic investigations and interpretation
of spectral bands were done by KH, TT, and PM. X-ray irradiation experiments
were carried out by OO. KH and SY drafted the manuscript text. All authors have
read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published
maps and institutional affiliations.
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