Post on 17-Nov-2021
1
Multifunctional Rare-Earth Vanadate Nanoparticles:
Luminescent Labels, Oxidant Sensors, and MRI
Contrast Agents
Mouna Abdesselem‡1, Markus Schoeffel‡
1, Isabelle Maurin
2, Rivo Ramodiharilafy
1, Gwennhael
Autret3, Olivier Clément
3, Pierre-Louis Tharaux,
3 Jean-Pierre Boilot
2, Thierry Gacoin
2, Cedric
Bouzigues1, and Antigoni Alexandrou
1,*
1Laboratoire d'Optique et Biosciences, Ecole Polytechnique, CNRS UMR 7645 - Inserm U 696,
91128 Palaiseau Cedex, France. 2Laboratoire de Physique de la Matière Condensée, Ecole
Polytechnique, CNRS UMR 7643, 91128 Palaiseau Cedex, France. 3
Paris Centre de Recherche
Cardiovasculaire (PARCC), INSERM U970 56 rue Leblanc, 75015 Paris, France
E-mail : antigoni.alexandrou@polytechique.edu
Supplementary information
Dynamic light scattering and zeta potential. Dynamic light scattering and zeta potential of
rare-earth vanadate nanoparticles were measured with a Malvern Nano SZ Zetasizer and are
reported in Table S1.
Composition d <n> (nm) PdI Zeta Potential (mV)
GdVO4 32 0.21 48
GdVO4cit
9 0.14 -13
Gd0.6Eu0.4VO4 54 0.12 8.6
Gd0.6Eu0.4VO4/SiO2 69 0.13 -31
2
Gd0.6Eu0.4VO4/Dextran 72 0.11 -31
Gd0.6Eu0.4VO4cit
23 0.20 -34
Gd0.6Eu0.4VO4cit
/SiO2 29 0.17 -20
Gd0.6Eu0.4VO4cit
/Dextran 34 0.16 -25
Table S1: Size (number average) and zeta potential of rare-earths vanadate nanoparticles. PdI
is the polydispersity index of the size distribution. Note that dynamic light scattering
overestimates the size of polydisperse nanoparticle solutions, as can be seen from the TEM
analysis discussed below and in the main text. The superscript cit denotes nanoparticles formed
via the citrate route.
Microstructural analysis of normal-route nanoparticles from X-ray powder diffraction. At
first, we fitted the Bragg peaks individually to Pseudo-Voigt functions using the FIT subroutine
of the FullProf suite of programs.1 Only non-overlapping and non-degenerate reflections were
used, considering both Kα1 and Kα2 contributions, to obtain the peak position and their integral
breadth as well as their deconvolution in Lorentzian and Gaussian components βL and βG,
respectively. Depending on the peak shape function used, we obtain the integral breadth due to
finite size effects as iLLsL ,, βββ −= for a pure Lorentzian (index L) peak shape and as
222
,, iGGsGβββ −= for a pure Gaussian (index G) peak shape.
2 The index s designates the pure
sample effect and i stands for the contribution of the instrument resolution obtained from the
analysis of the peak breadths of a bulk GdVO4 reference sample. The pure sample contribution
βL,s is then used to determine the coherence length Lc according to the Scherrer equation:
θβ
λ
coss
c
KL = ,
3 where K is the Scherrer constant. K may vary between about 0.5 and 2 depending
on the crystallite shape.4 In the present approximation, we used a value of K = 1. The Lc values
3
obtained range between 15 nm and 30 nm similar to the major and minor axis values derived
from TEM images.
For Williamson-Hall analysis, we considered only the Lorentzian contribution (after correction
for the instrument resolution), which is generally responsible for the crystallite size broadening.5
The strain parameter ε and the coherence length L associated with the apparent crystallite size
can be estimated respectively from the slope and the intercept of the Williamson-Hall plot
according to:
λ
θε
λ
θβ )sin(2.
1)cos(.+=
L (1)
β.cos(θ)/λ is plotted against 2sin(θ)/λ in Fig. S1B. The strain parameter extracted from the linear
fit is small, 0.4 ± 0.2 %, and the apparent crystallite size is 24±0.5 nm. As the model assumes
spherical crystallites, this value is in good agreement with the average value of the long and short
axis found from the statistical analysis of the TEM images (Fig.1C). The data show a rather large
scatter around the linear fit, at least partly due to a direction dependence of the crystallite
dimensions.
4
Figure S1. (A) Microstructural analysis from the refinement of the X-ray diffraction pattern
assuming an arbitrary shape modeled by spherical harmonics. (B) Williamson-Hall plot
derived from powder X-ray diffraction for the GdVO4 particles. The red line is a linear fit to
the data.
Microstructural analysis was also performed using the Rietveld method by fitting the entire
diffraction profile according to the FullProf implementation of the procedure described in Ref. 6.
This model accounts for direction-dependent crystallite sizes. The instrument resolution function
was determined from bulk GdVO4. We modeled an anisotropic crystallite shape by a sum of
spherical harmonics which does not require any assumption on the actual shape. Only the
coefficients with even parity, 0
0P , 0
2P , 0
4P , and 0
6P , were refined according to the crystal
symmetry (tetragonal space group, I41/amd n°141). The refined diffraction pattern obtained by
this method shows very good agreement between the measured and calculated profile (see Figure
S1A) and yields a crystallite form that may be described as a compressed prolate spheroid with a
thickness of 16 nm and a length of 25 nm, in very good agreement with the statistical analysis of
the TEM observations (Fig. 1). This confirms our conclusion based on the TEM images that a
preferential crystallographic orientation is present across the whole nanoparticles. The TEM
contrast variations would thus be representative of internal porosity, of surface roughness or of a
polygranular structure with primary crystallites aggregated in an ordered way in all three
dimensions.
5
Microstructural analysis of citrate-route nanoparticles from TEM and X-ray powder
diffraction. The citrate route synthesis described below (Methods section) results in smaller
sized nanoparticles. We analyzed transmission electron microscopy (TEM) images of over 100
Gd0.6Eu0.4VO4(cit)
particles and found a spherical shape. The distribution follows a log-normal
law centered at 5±0.2 nm (the error bar is that for the log-normal fit and corresponds to the
standard error on the mean; see Fig. S2A and B). We also performed a microstructural analysis
of these particles as discussed above for the normal-route nanoparticles. The X-ray diffraction
pattern and the corresponding refinement are shown in Fig. S2C. We found an average apparent
crystallite size of 4 nm, with coherence length values ranging between 3 and 6 nm depending on
the Bragg reflections, in good agreement with the size obtained from the TEM image analysis
(Fig. S2B).
Figure S2. (A) TEM image of citrate-route Gd0.6Eu0.4VO4 nanoparticles, scale bar: 20 nm. (B)
Size distribution obtained from 104 Gd0.6Eu0.4VO4cit
particles. The solid line is a log-normal fit
of the size distribution. (C) Microstructural analysis of GdVO4cit
nanoparticles from the
refinement of the X-ray diffraction pattern assuming an arbitrary shape modeled by spherical
harmonics.
6
Surface coatings. Surface modifications with silica or dextran enhance the normal-route particle
stability by increasing their surface charge and hence the electrostatic repulsions in the colloid.
Moreover, a dextran coating confers furtivity to the nanoparticles in the blood circulation. The
efficient coating of particles is noticeable in the size increase (see Tab. S1). We further
characterized the surface modifications with Fourier-Transform Infrared (FT-IR) spectroscopy
(Fig. S3). The intense peak at 800 cm-1
corresponds to the asymmetric vibration of the V-O-V
bond. The asymmetric stretching mode of the Si-O-Si bond appears as an intense peak at 1100
cm-1
for the silica-coated nanoparticles (Fig. S3 A). Dextran-coated nanoparticles display the
characteristic peaks of dextran, namely the C-H stretching vibration around 2930 cm-1
and the
valent vibrations in C-O bonds at 1153 cm-1
and in C-C bonds at 1030 cm-1
.
Figure S3. FT-IR spectra of pristine Gd0.6Eu0.4VO4 nanoparticles, silica-coated (A) and dextran-
coated nanoparticles together with a reference dextran spectrum (B). Arrows indicate
characteristic peaks of the V-O-V (800 cm-1
), Si-O-Si (1100 cm-1
), C-C (1030 cm-1
), C-O (1153
cm-1
) and C-H (2930 cm-1
) vibrational modes.
7
Dependence of the emission spectrum on the excitation wavelength. We measured the
emission spectra of Gd0.6Eu0.4VO4 nanoparticles with excitation wavelengths of 396 nm and 466
nm. We found emission spectra similar to those obtained with 280 nm UV excitation (Fig. 2B
and S5B) with a peaks at 617 and 701 nm.
Figure S4. Emission spectra of normal-route Gd0.6Eu0.4VO4 nanoparticles with excitation
wavelengths of 396 nm and 466 nm. The peak positions are indicated. The colloid concentration
in vanadate ions was ~100 mM.
Luminescence properties of citrate-route particles. We measured the excitation and emission
spectra of a suspension of citrate-route Gd0.6Eu0.4VO4 particles (Fig. S5). These showed the same
narrow absorption and emission peaks as their normal-route counterparts with a slight peak
broadening (Fig. 2 A) as expected due to their smaller size.
8
Figure S5. Luminescence properties of citrate-route Gd0.6Eu0.4VO4 nanoparticles. (A) Excitation
spectrum (λem = 615 nm). (B) Emission spectrum (λex = 280 nm). The peak positions as well as
the corresponding transitions are indicated.
Hydrogen peroxide detection. Single 30-nm sized Gd0.6Eu0.4VO4 nanoparticles show similar
behavior both in terms of photo-reduction and recovery in response to hydrogen peroxide
addition (Fig. S6A). More than ten nanoparticles were photoreduced under strong illumination at
466 nm and their luminescence decreased by 30% with an interparticle variation of 5% (Fig.
S6A). After photoreduction and addition of 100 µM hydrogen peroxide, the luminescence of 14
single nanoparticles was measured under weak illumination yielding an average signal increase
of 20% with a response variation of 3%.
9
Figure S6. (A) Luminescence of 11 single Gd0.6Eu0.4VO4 nanoparticles photoreduced at 466 nm.
Laser intensity: 1.6 kW/cm²; acquisition time: 1s. The luminescence signal is normalized to the
initial value before photoreduction (t=0). The red line represents the average luminescence
signal. (B) Luminescence of 14 single Gd0.6Eu0.4VO4 nanoparticles after photoreduction and
addition of 100 µM H2O2. The luminescence signal is normalized to the initial value before
addition of H2O2 (t=0). Laser intensity: 0.3 kW/cm², 466 nm; acquisition time: 3s. The red line
represents the averaged signal.
Reversibility of the photoreduction and reoxidation. We performed a cycling photoreduction and
recovery experiment on single 30-nm Gd0.6Eu0.4VO4 nanoparticles. For practical reasons, we
kept the same illumination parameters for both reduction and oxidation steps (excitation
intensity, 1.6 kW/cm²; acquisition time, 1s). Figure S7 shows that the photoreduction and
oxidation processes are reversible. During the oxidation step, there is an interplay between
photoreduction due to laser excitation and reoxidation due to hydrogen peroxide7. Therefore,
higher illumination intensities are more suitable for the detection of higher hydrogen peroxide
concentrations (10 mM was used for this experiment). The slight luminescence decrease
observed after recovery is also a signature of the competition between oxidation and reduction
reactions.
10
Figure S7: Photoreduction and recovery cycles for a single normal-route Gd0.6Eu0.4VO4 particle.
Illumination intensity at 466 nm, 1.6 kW/cm²; acquisition time, 1s. Red arrows show additions of
10 mM hydrogen peroxide, green arrows show rinsing steps.
Hydrogen peroxide detection in physiological medium. We measured the response of normal-
route Gd0.6Eu0.4VO4 particles to 100 µM hydrogen peroxide in a physiological medium
composed of HBSS (Hank's buffered salt solution) with 10 mM HEPES and 10% fetal bovine
serum (FBS). The luminescence evolution is similar to that observed in phosphate buffer saline
(PBS) for the same nanoparticles (see Fig. S9) as well as to those in pure water (see Fig. 2E).
This demonstrates that the presence of proteins does not modify the oxidant detection response.
11
Figure S8: Luminescence recovery after photoreduction and subsequent addition of 100 µM
H2O2 in HBSS/HEPES (10 mM)/10% fetal bovine serum medium and in phosphate buffer saline
(PBS). Signal averaged for N=4 single nanoparticles for each condition. Photoreduction at 466
nm, 1.6 kW/cm-1
; recovery observation at 466 nm, 0.3 kW/cm-1
. Aquisition time : 3s.
Hydrogen peroxide detection with citrate-route nanoparticles. Citrate-route Gd0.6Eu0.4VO4
nanoparticles show similar photoreduction under strong illumination and subsequent recovery in
an oxidative medium. Figure S9 shows an average luminescence decrease of 25% under 1.6
kW/cm² excitation at 466 nm. The luminescence of 21 single particles is plotted in Fig. S9C and
the average decay signal can be fitted by a biexponential (T1=1.6±0.1 s and T2=15±0.3 s). The
photoreduction is less efficient and faster in this type of particles than in normal-route ones (Fig.
2). After photoreduction, the addition of hydrogen peroxide causes a luminescence recovery
(Fig. S9B and S9D). With respect to normal-route nanoparticles, the reduction is slightly lower
in citrate-route particles (25% versus 30%) and the luminescence recovery is slower: the
12
characteristic recovery time in 50 µM H2O2 is 80 s in citrate-route particles and 25 s in normal-
route particle (Fig. S7B and Fig. 2D). Moreover, the photoreduction and recovery signals in
citrate-route particles have a typical dispersion of 8% which is higher than in normal-route
particles (5%). Normal-route particles are thus more efficient for local oxidant sensing than the
citrate-route ones. The observed differences are probably due to the shorter distances between
particle center and surface in the case of the smaller particles or to differences in surface
properties or in crystallinity.
Figure S9. Hydrogen peroxide detection properties of citrate-route Gd0.6Eu0.4VO4 nanoparticles
(A) Luminescence intensity evolution of under strong illumination (1.6 kW/cm², 466 nm) during
t=200 s (average for ~20 nanoparticles). Acquisition time: 1 s. (B) Luminescence recovery (0.3
kW/cm², 466 nm, acquisition time: 3 s, average for ~20 particles for each concentration) in 5 µM
and 50 µM hydrogen peroxide after prior photoreduction. The luminescence decrease and
13
recoveries are fitted with a biexponential and a monoexponential, respectively (solid lines). (C)
Luminescence evolution upon photoreduction of 21 individually detected particles (same
conditions as in A). The red curve is the average signal shown in (A). (D) Luminescence
recovery signals of 19 individually detected particles in 50 µM hydrogen peroxide (same
conditions as in B). The red curve is the average signal shown in (B).
Magnetic properties of GdVO4 and Gd0.6Eu0.4VO4 nanoparticles. Bulk GdVO4 is known to
display a paramagnetic-antiferromagnetic transition at TN =2.50 K8. The temperature dependence
of the molar susceptibility recorded for GdVO4 and Gd0.6Eu0.4VO4 nanoparticles obtained by the
two coprecipitation methods confirms a paramagnetic behavior down to 5 K (See Fig. S10A for
citrate-route Gd0.6Eu0.4VO4 particles). Note that at low temperatures, the magnetic properties of
the particles are only dependent on Gd3+
and not on Eu3+
ions, which exhibit a zero electronic
spin configuration at their ground state. Fitting the molar susceptibility molχ to a Curie-Weiss
law θ
χ−
=T
Cmol
mol yields a Curie constant 510)02.007.9( −⋅±=molC Km3mol
-1 and a Weiss
temperature 2.1−=θ K, whereas a θ value of -3.2 K is found for bulk GdVO4. Similar results
were found for the normal-route GdVO4 particles where we measured a Weiss temperature of
4.2−=θ K . Both magnetic dilution in the doped samples and finite size effects9,10
should be
responsible for this depressed θ value with respect to bulk GdVO4. Indeed, recent investigations
on DyPO4 and GdPO4 antiferromagnetic compounds with Néel temperatures TN of 3.4 K and
0.77 K, respectively, in the bulk state confirmed a shift to low temperatures of the point of
discontinuity in the magnetic susceptibility for 2.6 nm nanoparticles with respect to the bulk
materials.10
14
The change in magnetization values per Gd3+
ion as a function of the magnetic field is shown
in Figure S10B. The data related to the pure and Eu-doped samples, superimpose fairly well. The
slight deviation, a somewhat steeper increase for the bulk reference, may arise from the 0.2 K
temperature difference between the measurements. This confirms that, in the low temperature
region, doping with Eu3+
ions does not alter the magnetic properties of the host material.
Figure S10. (A) Molar susceptibility of bulk GdVO4 and citrate-route Gd0.6Eu0.4VO4
nanoparticles as a function of temperature (open symbols) fitted to a Curie-Weiss law (solid
lines). (B) Magnetic moment per Gd3+
in citrate-route GdVO4 and Gd0.6Eu0.4VO4nanoparticles as
a function of the magnetic field.
Evaluation of Gd0.6Eu0.4VO4 nanoparticle cytotoxicity.
15
Figure S11. MTT tests on endothelial progenitor cells (N=3 for each condition, error bars are
s.e.m.). Cells were incubated with 200 µL of dextran-coated Gd0.6Eu0.4VO4 colloid (citrate or
normal route, 10 mM in vanadate concentration) added to 2 mL of their ordinary culture
medium. Incubation times were 3 hours and overnight.
Luminescence from urine and tissue homogenates.
16
Figure S12. Luminescence observed under a UV lamp in homogenates of organs of mice injected
with 0.06 mmol/kg 1: Citrate-route dextran-coated Gd0.6Eu0.4VO4 colloid, 2: normal-route
dextran-coated Gd0.6Eu0.4VO4 colloid, 3: distilled water. (A) Solid residues in urine after
centrifugations and washings. (B) Same samples as in (A) and photograph taken with an
interference filter centered at 617 nm in front of the camera objective. (C) Liver homogenates.
Green circles emphasize the characteristic pink emission of Gd0.6Eu0.4VO4 NPs. (D) Kidney
homogenates prepared following the same procedure as for (B) samples.
MRI imaging with 30 nm Gd0.6Eu0.4VO4 particles.
Figure S13. Contrast evolution in bladder, liver, and kidney after antero-orbital injection of 0.06
mmol/kg of normal-route Gd0.6Eu0.4VO4 nanoparticles.
Experimental Methods
17
Nanoparticle synthesis. Sodium orthovanadate Na3VO4 (99.9%, Alfa Aesar) was dissolved in
ultrapure water to a final concentration of 0.1 M. The pH was adjusted to the range 12.5–13.0
and the solution was filtered through a 0.22 µm syringe filter. Gd(NO3)3 · 6H2O (purity 99.9%,
Alfa Aesar), Eu(NO3)3 · 6H2O (99.9%, Alfa Aesar) [and sodium citrate dihydrate C6H5O7Na3 ·
2H2O (> 99%, Sigma Aldrich) for the citrate route] were dissolved in ultrapure water to a final
concentration of 0.1 M and used as prepared.
For the standard coprecipitation route,11
a volume of 0.1 M sodium vanadate solution was
stirred vigorously at ambient temperature. The same volume of 0.1 M rare-earth nitrate solution
was then added with a flow rate of about 1 mL/min. Depending on the intended composition, the
lanthanide solution was a mixture of 60% vol Gd(NO3)3 solution with 40% vol Eu(NO3)3
solution or a pure Gd(NO3)3 solution. During the addition, the pH was verified at regular time
intervals. When the pH approached 9.5, a 1 M NaOH solution was added until the pH reached
10.5. After completion of the addition, the stirring was maintained for 30 min.
For the citrate route,12
1 volume equivalent of 0.1 M lanthanide nitrate solution was stirred
vigorously at 60°C. 0.75 volume equivalents of 0.1 M sodium citrate solution was slowly poured
into the flask forming a white to slightly yellow precipitate. 0.75 equivalents of 0.1 M sodium
vanadate solution was then added resulting, after addition of about 2/3 of the sodium vanadate
solution, in the complete dissolution of the precipitate and the formation of a limpid dispersion.
The reaction medium is stirred for another 30 min at 60°C and then allowed to cool down.
In both protocols, the synthesis was followed by centrifugation at 26,300 g for 20 min for
particles prepared by the normal route or 2 h for those made by the citrate route, or/and dialysis
until the colloid conductivity decreased below 100 µS/cm.
18
Determination of the vanadate concentration. The colloidal solutions were diluted to about
5 mM vanadate concentration in 1 mL. 900 µL of this solution were placed in a glass vial and
100 µl of HCl 37% wt were added. Pristine nanoparticles were readily dissolved by vortexing,
while silica-coated ones need an additional heating step of about 1 min at 100°C for complete
dissolution. The vial was tightly closed during the thermal treatment to avoid water evaporation.
The mixture was then diluted with 6 mL of HCl at 1.15 M. The red-brown color of the
trichloromonoperoxovanadium (V) complex appeared after addition of 105 µL of 3% wt H2O2
and a final vortexing. The absorption was measured using a Carry 50 UV-VIS spectrophotometer
(Agilent Technologies). 1.15 M HCl was used as reference. The vanadate concentration of the
dispersion was obtained by comparing the absorption at 405 nm and 460 nm to those of
calibration samples prepared from sodium orthovanadate Na3VO4 (99.9%, Alfa Aesar) and
containing an equimolar quantity of rare earth chlorides (EuCl3, 99.99%, Sigma Aldrich and/or
GdCl3, pure, Prolabo) in the same ratio as in the nanoparticles.
Determination of rare earth leaching. Titration of the rare-earths in solution was performed
using xylenol orange following an adaptation of a procedure described by Barge et al.13
Xylenol
orange undergoes a change in the relative intensity of its two absorbance peaks in the presence of
free rare-earth ions (Fig. S14). This approach does not allow distinguishing between Eu3+
and
Gd3+
ions but is rather an indicator of the sum of Eu3+
and Gd3+
concentrations.
Immediately after synthesis, the colloids were spun at 13,000 rpm for 10 min. The supernatant
was extracted and the precipitate was dispersed in ultra-pure water. Colorimetric dosage of
100 µL of the extracted supernatant was performed adding 2.8 mL of acetate buffer (50 mM, pH
5.80), and 100 µL xylenol orange solution (36 mg/100 mL, Molecula). The absorption was
measured and the concentration was determined from the A573/A433 ratio, where A573 and
19
A433 are the absorption values at 573 nm and 433 nm, respectively, following a calibration
curve obtained with rare-earth nitrate samples with the same rare-earth Gd:Eu concentration ratio
as in the nanoparticles (Fig. S14). A new calibration was performed for each series of
experiments.
Figure S14. Calibration of free rare-earth titration. (A) Absorption spectra of the xylenol
orange solution in the presence of known rare-earth concentrations in the same Gd:Eu
concentration ratio as for the nanoparticles. (B) Calibration curve obtained from a series of
absorption spectra.
Cell culture and cell mortality assay. Endothelial progenitor cells (EPC) were grown in
endothelial cell growth medium (EGM-2 BulletKit with supplement, Lonza) added with 20%
bovine fetal serum and 1% penicillin-streptomycin. Cell culture medium was changed every two
days and cells were diluted and transferred before 80% confluence. EPC cells originate from a
primary cell line derived from mice endothelial glomerular tissue.
We assessed cell mortality with MTT tests. The reduction of the tetrazolium salt MTT (3-(4,5-
dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) into the blue-colored formazan by the
mitochondrial enzyme succinate dehydrogenase takes place only in living cells. Thus, the
20
produced amount of formazan is proportional to the number of living cells. We grew cells in six-
well plates with three wells for each incubation condition (N=3) with 2 mL of culture medium.
We added 200 µL of dextran- coated Gd0.6Eu0.4VO4 colloid (citrate or normal route, 10 mM in
vanadate concentration) to the culture medium described above. After the incubation time (3
hours or overnight), we measured the sample absorbance A570 nm-A630 nm. The N=3 measurements
were averaged and normalized to the control sample.
Luminescence from urine and tissue homogenates. Homogenates of liver and kidneys were
obtained by mechanical grinding of tissues. We added 1 mL of distilled water and sonicated 3 x
1 minute in order to obtain a homogeneous suspension. The suspensions were then spun at 600 g
during 5 minutes. The precipitates were discarded and the supernatants were spun again at 1500
g during 5 minutes. The obtained precipitates were observed under ultraviolet illumination
(Fisher Bioblock Scientific TCP-20.M, 6x8W 312 nm). Urine samples were sonicated 5 x 1
minute then spun at 5000 g. The supernatants were discarded and the precipitate dispersed in 1
mL of distilled water and sonicated again 5 x 1 minute. The suspensions were then spun at 600 g
and the precipitates were discarded. Finally, the extracted clear supernatants were spun at 5000 g
during 10 minutes and the precipitates were imaged under ultraviolet illumination.
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