Coprecipitation Method of Synthesis, Characterization, and Cytotoxicity of Pr3+:LaF3 (CPr = 3, 7, 12, 20, 30%) Nanoparticles
The Pr3+:LaF3 (CPr = 3, 7, 12, 20, 30%) nanoparticles were characterized by means of high-resolution transmission electron microscopy, X-ray diffraction, optical spectroscopy, energy dispersive X-ray spectroscopy, dynamic light scattering, and MTT assay. It was revealed that the average diameter of all the NPs is around 14–18 nm. The hydrodynamic radius of the Pr3+:LaF3 (CPr = 7%) nanoparticles strongly depends on the medium. It was revealed that hydrodynamic radii of the Pr3+:LaF3 (CPr = 7%) nanoparticles in water, DMEM, and RPMI-1640 biological mediums were 18 ± 5, 41 ± 6, and 186 ± 8 nm, respectively. The Pr3+:LaF3 (CPr = 7%) nanoparticles were nontoxic at micromolar concentrations toward COLO-320 cell line. The lifetime curves were fitted biexponentially, and for the Pr3+:LaF3 (CPr = 7%) NPs, the luminescence lifetimes of Pr3+ ions were 480 ± 2 and 53 ± 5 nanosec.
Bionanomaterials engineering is one of today’s most promising fields of materials science . Nanomaterials are utilized in different fields of biomedicine and biology [2, 3]. Among the huge number of nanomaterials, rare-earth doped trifluoride nanoparticles (NPs) have a special place mainly because of their excellent photostability, long luminescent lifetimes, and sharp emission bands which are highly important for industrial and biomedical applications . Particular success has been achieved in the development of luminescent NPs based on rare-earth doped trifluorides which are utilized as nanoscintillators for “hybrid” radiotherapy—photodynamic therapy (PDT) . Also, NPs are used for in vivo imagining of biological objects . In this case, they should operate in the so-called biological window (650–1300 nm). In case of photothermal therapy and subtissue thermal sensing [6–8], the luminescent intensity ratio of some luminescent peaks of NPs should be dependent on temperature in the physiological temperature range (20–50°C). Due to these unique properties of fluoride NPs, the methods of NP synthesis providing excellent optical properties and low toxicity are under the intense investigation [9, 10] Hence, NPs which are expected to be used in different areas including biomedicine should be characterized very carefully in order to guarantee the excellent physical and chemical properties and low toxicity.
Among the lanthanides, Pr3+ as a dopant seems to be very attractive. It has been reported in [11–13] that, because of thermally coupled 3P1 to 3P0 electronic states of Pr3+ ions, Pr3+ doped nanomaterials can be used as nanothermometers operating in broad temperature range including physiological one which paves the way toward thermal sensing of a single cell in vitro and toward thermal imaging of integrated circuits . Also, the biological activity of PrF3 nanoparticles is under investigation. For these reasons, Pr3+ doped fluoride NPs are considered as a very promising material for different applications.
The main purpose of the work is to characterize Pr3+:LaF3 (CPr = 3, 7, 12, 20, 30%) NPs (molar ratio of Pr3+ and La3+ ions are 3 : 97, 7 : 93, 3 : 22, 1 : 4, and 3 : 7, resp.) by different methods in order to provide foundation for further studying of temperature sensing properties and biological activity of Pr3+:LaF3 NPs.
The NPs were characterized by means of HR TEM, X-ray diffraction, and optical spectroscopy. For chosen Pr3+:LaF3 (CPr = 7%) NPs, we additionally study their hydrodynamic radii in water, DMEM, and RPMI-1640 biological mediums and cytotoxicity toward human cancer cells (COLO-320 cell line) via the MTT assay.
2. Materials and Methods
2.1. Sample Requirements and Methods of Characterization
The size and shape of NPs can be studied via high-resolution transmission electron microscopy (HR TEM) and the phase composition is commonly studied via X-ray diffraction .
NPs which are expected to be used in biomedical application should be nontoxic and stay chemically stable under light irradiation so that toxic components of reaction product are not delivered to the cells . Generally, toxicity of inorganic substances depends on their solubility in water . Also, rare-earth doped fluorides demonstrate the lowest solubility among other rare-earth based materials (oxides and so on). For instance, the CeF3 and other trifluorides with tysonite crystal structure demonstrate solubility around 10−5-10−6 mol/L and, as a consequence, low toxicity . Also, NPs itself can affect the cell life cycle . Hence, the toxicity of NPs must be studied very carefully. Commonly, the toxicity of NPs can be estimated in vitro via colorimetric MTT assay, described in .
In such important works [6–8] where rare-earth doped LaF3 NPs demonstrate their efficiency in biomedical application, coprecipitation method of synthesis of NPs is used. Indeed, this method is usually user-friendly and inexpensive. It allows synthesizing NPs in 10–100 nm size range. Use of water as the exchange reaction medium instead of other solvents which could be toxic also can be important for the protection of the environment and for biomedical application . But one of the most prominent disadvantages of the coprecipitation method is that the NPs usually contain large amount of adsorbed water , which essentially affects spectral properties of the synthesized nanoparticles, including their luminescent features [18, 19]. Moreover, in [17, 20], it is reported that the NPs synthesized via coprecipitation method contain captured water molecules in the lattice because of fast growth of NPs in chemical reaction in an aqueous solution. Anyway, regardless of its disadvantages, this simple method gives the opportunity to study properties of NPs for many research teams without using expensive equipment.
After the synthesis, the residues of precursors which were used during the synthesis should be removed very carefully. The degree of purification of NPs from the precursors is controlled by means of energy-dispersive X-ray spectroscopy (EDX spectroscopy) . But, in , the absence of nitrate ion impurities in fluoride NPs is determined by standard qualitative reaction with diphenylamine (diphenylamine test). Hence, the degree of purification of NPs should be controlled very carefully.
One of the unique properties of NPs is protein corona formation phenomenon . After suspending of NPs in biological medium or physiological liquids, they will inevitably come into contact with a huge variety of biomolecules including proteins and sugars. These biomolecules immediately coat the NP surfaces and form the so-called “protein corona.” This fact increases the sizes of NPs in a biological medium and causes its tendency to form agglomeration . The “protein corona” is difficult to resolve via transmission electron microscopy because of low electron density of surface organic ligands [24–26]. On the other hand, the average sizes of NPs and their agglomerations in different mediums can be measured by means of DLS [27, 28]. In this method, the information about size can be extracted from the Stocks–Einstein equation  and size is classified as a hydrodynamic radius of NPs. The hydrodynamic radius depends on biological medium in which the NPs are suspended . In , it is shown that the values of the hydrodynamic radii of gold NPs demonstrate significant difference for DMED and RPMI-1640 biological mediums.
In order to study optical properties of NPs, conventional techniques for bulk crystal are used.
2.2. Synthesis and Characterization of Pr3+:LaF3 Nanoparticles
Pr3+:LaF3 (CPr = 3, 7, 12, 20, 30%) NPs (molar ratio of Pr3+ and La3+ ions are 3 : 97, 7 : 93, 3 : 22, 1 : 4, and 3 : 7, resp.) were synthesized via coprecipitation method using common chemical reaction for rare-earth elements described in . For example, in order to synthesize Pr3+:LaF3 (CPr = 30%) NPs, 0.6 g of Pr2O3 and 1.2 g of La2O3 were added to 110 mL of 10% nitric acid in a glass beaker. The mixture was heated to 50°C and stirred for 45 min; then, a transparent light-green solution appeared. The pH of the solution was 1. Then, the mixture was filtered, poured in a polypropylene glass, and placed in an ultrasonic cleaner (model UD100SH-2LQ, ultrasonic power 100 W), and a solution of 0.69 g of NaF in 100 mL of distillated water was added. Then, the pH was adjusted to 4 by adding 25% solution of ammonium hydrate. Then, the mixture was stirred for 10 minutes under the ultrasonic treatment. The precipitate was purified with distillated water by centrifugation (12,000 rpm, centrifugation time was 15 min) 8 times. In order to synthesize Pr3+:LaF3 NPs with other Pr:La ratio, the suitable ratio of Pr2O3 and La2O3 and stoichiometric amount of NaF were taken. Then, nanoparticles were dried in air. The structure of the material was characterized by X-ray diffraction method with Shimadzu XRD-7000S X-ray diffractometer. Analysis of samples was carried out in a transmission electron microscope Hitachi HT7700 Exalens. Samples were prepared as follows: 10 microliters of the suspension was placed on a formvar/carbon lacey 3 mm copper grid and drying was performed at room temperature. After drying, the grid was placed in a transmission electron microscope using a special holder for microanalysis. The analysis was held at an accelerating voltage of 100 kV in the TEM mode; the elemental analysis was carried out in the STEM mode, at the same parameters using Oxford Instruments X-Max™ 80T detector. The control of amount of nitrates in colloidal solution of the NPs after each stage of centrifugation was performed by identification test using diphenylamine (diphenylamine test).
2.3. Measurement of Hydrodynamic Radii of Pr3+:LaF3 (CPr = 7%) NPs and/or Their Agglomerations in Distilled Water, DMEM, and RPMI-1640 Biological Mediums
In order to measure NP sizes and the rate of NP agglomeration in different mediums the size distributions and average hydrodynamic radii in water, DMEM, and RPMI-1640 biological mediums were characterized by means of DLS with Photocor-FC spectrometer. DMEM and RPMI-1640 biological mediums were purchased from Paneco (Moscow, Russia) and were used without further modification. 1 ml of 10 mM water colloid solution of Pr3+:LaF3 (CPr = 7%) NPs was added to 4 ml of distilled water, DMEM, and RPMI-1640 biological mediums, and then after 10 minutes of sonication (ultrasonic cleaner model UD100SH-2LQ, ultrasonic power 100 W), all necessary measurements were done.
2.4. Cells Preparation and Cytotoxicity of Pr3+:LaF3 (CPr = 7%) NPs
COLO-320 cells were cultured in RPMI-1640 medium supplemented with 10% fetal calf serum at 37°C and 5% CO2 in a humidified atmosphere.
The cytotoxicity of the NPs was analyzed via the colorimetric MTT assay. The test protocol for cytotoxicity evaluation was adopted from elsewhere . Nanoparticle suspension in distilled water was added to the cultural medium in a ratio of 1/10 (v/v) for each concentration. Then, the obtained suspension was sonicated for 10 min until the suspension appeared homogeneous to the naked eye. COLO-320 cells were treated with the NPs at 0.5, 1, 2.5, and 5 mM. Exposure time was 24 h at 37°C in humid air (98%) containing 5% CO2. Three hours prior to the end of the exposure period, MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2 H-tetrazolium bromide, Sigma-Aldrich, #M5655) solution in PBS (5 mg/ml, 20 l/well) was added to the cells. After the completion of the exposure period, the supernatant was removed and 100 l/well solution containing 10% SDS (Sigma-Aldrich, #L3771) in PBS was added. Absorbance at 570 nm of each well was measured using a microplate reader (Biorad, xMark). Each experiment was repeated 2 times, with five replications.
2.5. Optical Spectroscopy of Pr3+:LaF3 NPs
The luminescence spectra were recorded using CCD spectrometer (StellarNet), which detects the emission in 200–1100 nm spectral range with a spectral resolution of 0.5 nm. The optical parametric oscillator laser system (420–1200 nm) from JV LOTIS TII was used for excitation of the luminescence of the samples. The pulse width and the pulse repetition rate were 10 ns and 10 Hz, respectively. The spectral width of laser radiation was less than 0.15 nm. The luminescent lifetimes of Pr3+ ions were detected using BORDO 211А (10 bit, 200 MHz band-width) digital oscillograph. The experiments were carried out at room temperature.
3. Results and Discussion
3.1. NPs Characterization
Transmission electron microscopy (TEM) data indicate that obtained Pr3+:LaF3 (CPr = 3, 30%) (Pr3+:LaF3 (CPr = 7, 12, 20%) samples are not shown here for the sake of brevity) samples consist of nearly monodisperse well-crystallized NPs. An average diameter of all the samples is around 14–18 nm (Figures 1(a)–1(d)), and it does not depend on the concentration of Pr3+ ions. The average diameters of the NPs are listed in Table 1. Also, the sample has regular almost spherical shape. Selected area electron diffraction (SAED) patterns correspond to hexagonal crystal structure and do not contain any reflections from impurity phases. The presence of circular rings in the SAED patterns indicates polycrystallinity of Pr3+:LaF3 samples. No signs of orientation ordering or oriented attachment of the nanoparticles are observed. According to the X-ray diffraction data (Figure 2), all the Pr3+:LaF3 (CPr = 3, 7, 12, 20, 30%) NPs were hexagonal structured crystals. Sharp peaks of the patterns confirm good crystallinity of the NPs. The peaks exhibit a little right shift for the samples with the Pr doping level enhancing because of the crystal lattice distortion. The radius of Pr3+ (1.05 Å) is smaller than that of La3+ (1.13 Å) due to the lanthanide contraction, so the cell volume of Pr3+:LaF3 reduces with more Pr3+ replacing La3+, which results in the XRD peaks shifting to higher degree. The lattice parameters are listed in Table 1. The lattice parameters for PrF3 (JCPDS–46–1167) and for LaF3 (JCPDS–32–0483) are a = 0.7079 nm and c = 0.7238 nm and a = 7.7186 Å and c = 7.352 Å, respectively. The average nanoparticle size was calculated using the Debye–Scherrer formula:antat.ruwhere is the mean size of an NP, is the shape factor (we used ), is the X-ray wavelength (0.15418 nm), is the line broadening at half the maximum intensity (FWHM) in radians, and is the Bragg angle (in degrees). The values of all the NPs are around 13 nm which is in good accordance with HR TEM data ( values are listed in Table 1). It can be concluded that the peak broadening of the XRD spectra is mainly related to the nanoscale dimensionality of the crystalline particles.
3.2. Control of Purification of the NPs
EDX spectroscopy indicates that the Pr3+:LaF3 (CPr = 7%) NPs contain Pr, F, O, and trace quantity of Si as it is shown in Figure 3. The presence of Cu and C is related to using the copper grid during the HR TEM measurements. Presence of O can be attributed to the presence of adsorbed water , and trace quantity of Si is probably related to using glass beakers during the reaction of oxides with nitric acid. The chemical reactions with NaF were performed in polypropylene glasses. Additionally, the diphenylamine test did not reveal presence of nitrates in the NPs colloidal solution after the third stage of centrifugation (Figure 4(a)). Also, the sensitivity of the diphenylamine test was estimated. For this purpose, NaNO3 was taken as a source of nitrates. It was revealed that the threshold concentration of nitrates which can be detected via the diphenylamine test was around 0.01 M in distilled water (Figure 4(b)). According to both methods, 4 stages of centrifugation (12,000 revolutions/min 10 min) in water guarantee thorough absence of nitrates and other precursors.
3.3. Measurement of Hydrodynamic Radii of Pr3+:LaF3 (CPr = 7%) NPs and/or Their Agglomerations in Distilled Water, DMEM, and RPMI-1640 Biological Mediums
According to DLS data, in water, more than 90% of Pr3+:LaF3 (CPr = 7%) NPs have the mean hydrodynamic radius in distilled water around 18 ± 5 nm and it does not change for at least 21 days. The quantity of the NPs agglomeration with mean hydrodynamic radius more than 400 nm is less than 10%. Those facts indicate a very low degree of huge particle agglomeration in distilled water colloid solution. The value of the mean hydrodynamic radius is two times more than the mean NPs radius, obtained via TEM. It can be suggested that NPs form small agglomerates consisting of a few NPs as it was shown in  via TEM. Also, it should be noted that unlike TEM, hydrodynamic radius does not determine real NP size but can be calculated from the Stocks–Einstein equation in DLS method . The time dependence of the hydrodynamic radii of Pr3+:LaF3 (CPr = 7%) NPs in water, RPMI-1640, and DMEM biological mediums is shown in Figure 5. It was revealed that, in distilled water, the hydrodynamic radius is stable for at least 21 days which can be attributed to suitable value of Z-potential of the NPs . Hydrodynamic sizes of Pr3+:LaF3 (CPr = 7%) NPs in RPMI-1640 and DMEM biological mediums were also measured (Figure 5). In RPMI-1640 biological medium, 75.0% of Pr3+:LaF3 (CPr = 7%) NPs have the hydrodynamic radii about 186 ± 60 nm, and it slightly increases up to 200 nm during 20 hours and then demonstrates stability. In DMEM biological medium 79, 4% of Pr3+:LaF3 (CPr = 7%) NPs have the hydrodynamic radii about 43 ± 3, and it does not change significantly for 72 hours. It was also detected that the rate of huge agglomerations with diameter more than 800 nm is less than 20% for both DMEM and RPMI 1640. These facts mean that colloidal solutions of the NPs in both biological milieus are stable but the NPs form agglomerations. As it was shown in [28, 31, 32], the stability of colloidal NPs can collapse due to screening of the electrostatic interactions. This screening can result in aggregation, and multivalent electrolytes were found to be more efficient than monovalent ions at suppressing the stabilizing effect of the electric double layer . According to the chemical content of DMEM and RPMI-1640 biological mediums and , RPMI-1640 contains approximately 5-fold more phosphate ions (PO43−) (5.63 mM) compared to DMEM (0.92 mM), and these multivalent phosphate ions suppress the colloid stability of the NPs more efficiently in RPMI-1640 than in DMEM. As it was already mentioned above, in RPMI-1640, the hydrodynamic radius of the NPs increases during 20 hours unlike DMEM, which probably can be attributed to the dynamic behavior of the protein corona in RPMI-1640 [30, 34], and some proteins with low affinity in RPMI-1640 can be adsorbed and then can be desorbed or/and replaced after a while . For example, unlike DMEM, RPMI-1640 contains human plasma which serves as a main source of proteins with low affinity .
It can be concluded that although the hydrodynamic radii of the NPs differ between each other in water, DMEM, and RPMI-1640, the colloidal solutions of the NPs in water and DMEM are stable at least for 72 hours. In case of RPMI-1640, a slight change in the hydrodynamic radius takes place which at least can be attributed to the complicated physicochemical processes at the NP-medium interface . Probably, the DMEM medium demonstrating minor change in the value of hydrodynamic radii of the NPs is more appropriate for toxicity study of LnF3-based NPs.
3.4. Toxicity of Pr:LaF3 (CPr = 7%) NPs
The toxicity of Pr3+:LaF3 (CPr = 7%) NPs was studied in a 0.05–25 mM concentration range toward COLO-320 cell line. The viability ranges of COLO-320 are shown in Figure 6. The NPs are nontoxic at micromolar concentrations. It was mentioned above  that the toxicity of materials generally depends on their solubility in the water. As it was mentioned above, the lanthanum fluoride has the lowest solubility among other rare-earth entities (oxides, phosphors, and so on) around 10−5-10−6 mol/L, which correlates substantially with that of fluorspar CaF2, which is ranked as safe . This is unlike with quantum dots (CdSe) having higher solubility and releasing toxic Cd and Se ions . It can be suggested that the main mechanism of toxicity is still under the discussion, but at least it is not related to release of La, Pr, and F ions or residues of precursors, or their impact on the cells is neglectable.
3.5. Optical Spectroscopy and Luminescence Lifetime Pr3+:LaF3 (CPr = 3, 7, 12, 20, 30%) NPs
Room temperature luminescence spectra of the Pr3+:LaF3 (CPr = 3, 7, 12, 20, 30%) NPs excited by laser beam at 444 nm are presented in Figure 7. The emission from 1D2 state was not found under the excitation condition and at the studied temperature range. Probably, the emission from 1D2 is not observed because of the lack of nonradiative relaxation of 3Pj to 1D2 due to the low cutoff phonon frequency in LaF3 (350–400 cm−1), which is 2 times less than the one for YAG (700–865 cm−1). . The analogic case is observed in the Pr3+:NaYF4 system .
The lifetime curves of 3P0 state of Pr3+ ions are shown in Figure 8. Although the emission from 1D2 is not observed, the curves are fitted biexponentially. A given possible explanation for this is a different probability of nonradiative decay for ions at or near the surface and ions (τ2) in the core of the particles (τ1) . It is clearly seen that the luminescence lifetime τ1 of Pr3+ ions decreases while the concentration of Pr3+ ions increases (Table 2)
This τ1 lifetime decrease can be related to migration of energy to quenching centers (defects) . Also, in , it was shown that in PrF3 NPs uniform distribution of defects over the particle structure takes place and this factor determines the luminescence lifetime of Pr3+ ions in the core of the NP (τ1). For the Pr3+ ions located into the core of the NP, the migration energy process to these defects is probably the main mechanism of luminescence quenching although, in  it was shown that the NPs synthesized via co-precipitation method contain captured water molecules in the lattice because of fast growth of NPs in chemical reaction in an aqueous solution. The role of this captured water molecules in luminescence quenching process is still under the discussion and we suggest that in the core of the NPs migration energy process dominants over quenching by captured water molecules. On the other hand, one of the most significant differences between nanosized crystals and bulk crystals is the increased role of surface and, as a consequence, interaction of ions with surface ligands becomes very significant . The adsorbed OH groups can act as main quenching units on the surface of the NPs. It can be suggested that at the surface area the luminescence quenching can be attributed to nonradiative transitions from rare-earth exited state (3P0 in case of Pr3+) to vibrational states of OH molecule [19, 39] and this process dominates over the energy migration one. The τ2 lifetime does not change significantly while the concentration of Pr3+ ions increases from 3 to 30%. As we proved via HR TEM, the size of the NPs does not change while the concentration of Pr3+ ions increases from 3 to 30%. Hence, the number of adsorbed water molecules probably stays constant and their contribution to the luminescence quenching process is also constant approximately.
Low doped nanoparticles (CPr = 3, 7%) have the largest lifetimes around 560 ± 4 and 480 ± 2 nanosec, respectively, and these values are at least 10 times less than lifetimes for bulk Pr3+:LaF3 crystals . Probably, in the case of NPs, these small values of lifetimes are related to nonradiative relaxation on the OH group  of adsorbed and captured water molecules, and in order to remove these water molecules, the appropriate measurements such as microwave-assisted treatment  or treatment with high temperature  should be done.
The Pr3+:LaF3 (CPr = 3, 7, 12, 20, 30%) NPs were synthesized via coprecipitation method. The NPs were characterized by means of HR TEM, X-ray diffraction, optical spectroscopy, EXD spectroscopy, DLS, and MTT assay. It was revealed that all the NPs are well-crystallized hexagonal structured nanosized particles and are almost spherical in shape with the average diameter of all the NPs around 15–20 nm and it is regardless of concentration of Pr3+ ions. EXD spectroscopy and the diphenylamine test did not reveal any presence of nitrates and other precursors after 4 stages of centrifugation (12,000 revolutions/min 10 min). Also, the sensitivity of the diphenylamine test was estimated. The hydrodynamic radius of the Pr3+:LaF3 (CPr = 7%) NPs strongly depends on the medium. It was revealed that hydrodynamic radii of the Pr3+:LaF3 (CPr = 7%) NPs in water, DMEM, and RPMI-1640 are 18 ± 5, 41 ± 6, and 186 ± 8 nm, respectively, after 2 hours. The Pr3+:LaF3 (CPr = 7%) NPs are nontoxic at micromolar concentrations toward COLO-320 cell line. The lifetime curves are fitted biexponentially, and for the Pr3+:LaF3 (CPr = 7%) NPs, the luminescence lifetimes of Pr3+ ions are 0.56 ± 0.04 sec and 51 ± 4 nanosec.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Optical spectroscopy and the synthesis were supported by the state assignment in the sphere of scientific activities (Project no. 3.1156.2017/4.6 and no. [3.5835.2017/6.7]). Microscopy studies were carried out at the Interdisciplinary Center of Analytical Microscopy of Kazan Federal University. The microscopy studies were funded by the subsidy of the Russian government (Agreement no. 02.A03.21.0002) to support the Program of Competitive Growth of Kazan Federal University among World’s Leading Academic Centers. A. Maksim S. Pudovkin was supported by the Foundation for Assistance to Small Innovative Enterprises (FASIE) (Agreement no. 11271GU/2017). The authors thank Ajrat Kiyamov (Kazan Federal University, Kazan, Tatarstan 420008, Russia) for performing the X-ray diffraction experiments.
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