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. 2020 Jul 27;5(30):19174–19180. doi: 10.1021/acsomega.0c02551

Highly Water-Soluble Luminescent Silica-Coated Cerium Fluoride Nanoparticles Synthesis, Characterizations, and In Vitro Evaluation of Possible Cytotoxicity

Jun Wang †, Anees A Ansari ‡, Abdul Malik §, Rabbani Syed §, Mohammad Shamsul Ola ∥, Ashok Kumar ⊥, Khalid M AlGhamdi ⊥,#, Shahanavaj Khan §,¶,*
PMCID: PMC7409243  PMID: 32775919

Abstract

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A coprecipitation process was utilized for the preparation of terbium fluoride nanocrystals by cerium fluoride. Silica was used to modify the surface of these core/shell nanocrystals. The synthesized CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 nanoparticles (NPs) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV/vis spectrophotometry, and photoluminescence spectrophotometry. XRD patterns showed resolved reflection planes with broad widths, confirming the nanocrystalline nature of the CeF3:Tb@LaF3@SiO2 NPs. Fourier transform infrared spectra clearly revealed a uniform, smooth silica layer encapsulating the luminescent seed core and confirmed the polycrystalline nature of the CeF3:Tb@LaF3@SiO2 NPs. The TEM result showed an average crystalline size of 18 nm, which illustrated good agreement with the XRD results. The results of photoluminescence spectrophotometry confirmed the doping of terbium ions in the CeF3 crystal lattice. The cytotoxicity results of the MTT assay showed that CeF3:Tb@LaF3@SiO2 NPs have minimum toxicity with respect to CeF3:Tb@LaF3 NPs and the control drug dasatinib on HT-29 and HepG2 cell lines. Moreover, results of inverted microscopy confirmed the nontoxic and biocompatible nature of CeF3:Tb@LaF3@SiO2 NPs. These findings show that CeF3:Tb@LaF3@SiO2 NPs are promising candidates for applications in biomedical science in the future, such as bioimaging, biolabeling, biodetection or bio-probing, labeling of cells and tissue, drug delivery, cancer therapy, and multiplexed analysis.

Introduction

Great advancements have been made in the design and synthesis of nanomaterials for application in various fields in biomedical research.1 Nanomaterials, in particular bionanomaterials, are currently of great interest to many scientists because of their huge potential for applications in medical sciences.2 The emphasis of nanobiotechnology on multidisciplinary research to develop novel systems for different biomedical implications ranging from treatment, diagnosis, or prevention of various diseases and even future applications potentially in therapy + diagnosis.2−4 However, nanoparticles (NPs) containing heavy metal ions such as Pb2+ or Cd2+ can be toxic to living organisms.5,6 Inorganic lanthanide-doped NPs are a nontoxic alternative with potential for use in biomedical fields.7

Luminescent nanocrystals (NCs) activated through lanthanide Ln3+ ions have been the interest of research at present because of their unique characteristics including long luminescence lifetimes, high resistance to photochemical degradation and photobleaching, and high chemical and physical stability.8−10 Furthermore, these NCs have the capability to form the stable colloids.11 The energy/charge transfer processes can increase the luminescence intensity efficiency, allowing for the use of NCs at low concentration in biomedical research. Research on core/shell structures has recently uncovered useful insights into their biological compatibility and luminescence intensity.12−14 The attractiveness of core/shell structures is due to their excellent physicochemical properties, including their high thermal stability, surface coating, and improved water solubility.14,15 The silica coating method is a common technique used to increase the solubility of water and shield cores of NC from the circumferential environment.16 Simple and cost-effective methods to obtain spherical, chemically inert NPs higher luminescence, enhanced optical responses, and extended decay times have been detected while the cores of doped NPs are shielded through SiO2 shells.17 Consequently, the biological influence of silica NPs has become a topic of interest in present time.18

Nanomaterials based on CeF3:Tb@LaF3 are very promising because of the above-mentioned features.19,20 Upon coating CeF3:Tb3+ NPs with LaF3 shells, the emission intensity has been shown to improve by more than 25% compared to CeF3:Tb3+ core NPs. The loss of energy from the surface luminescence centers was considerably reduced through the LaF3 shell, which acted as a barrier for energy transfer to the upper surface of the shell, and therefore the lifespan and luminescence intensity of the core/shell NPs were significantly improved. Alteration in of the surface of silica core/shell NPs was shown to significantly increase the solubility of CeF3:Tb@LaF3 in an aqueous environment.19,20 Recently, we prepared CeF3:Tb@LaF3@SiO2 with enhanced excitation and emission intensity.19 There are various reports on the magnetic and luminescent behavior of multimodal particles, including CeF3:Tb@LaF3.10,19,20 The luminescent and magnetic properties of lanthanide NPs can be principally valuable in designing multifunctional nanomaterials by allowing them to be used in MRI and other luminescence techniques.21,22 Such properties are often required for biomedical uses and could be helpful in overcoming various problems in medicine, including deep tissue imaging and low resolution imaging. In the current study, we synthesized and investigated the anticancer properties of CeF3:Tb@LaF3, CeF3Tb@ LaF3, and CeF3:Tb@LaF3@SiO2 NPs. Our results showed that the coating of surface changed the crystalline property without altering the NPs crystal structure. Notably, the silica surface coating provided excellent photostability and great solubility, which is ideal for various biological and biomedical applications.

Results and Discussion

Preparation of CeF3:Tb@LaF3@SiO2 NPs

CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 NPs were successfully synthesized using Ce(NO3)36H2O and 4.5 mL (2 M) of Tb(NO3)36H2O as precursor materials.

Characterization of CeF3:Tb@LaF3@SiO2 NPs

The synthesized CeF3:Tb@LaF3@SiO2 NPs were characterized using various techniques.

Figure 1a shows the X-ray diffraction (XRD) pattern of the CeF3:Tb@LaF3@SiO2 NPs at room temperature. The XRD pattern shows that every diffraction peaks are well-matched with the hexagonal phase of bulk CeF3 (JCPDS card no. 08-0045), as published in the literature.23,24 Broad, well-resolved reflection planes revealed the nanocrystalline nature of the materials.

Figure 1.

Figure 1

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(a) XRD pattern and (b–d) TEM micrographs of core/shell NPs.

The image of transmission electron microscopy (TEM) in Figure 1b,c clearly illustrates the polycrystalline nature of the material. As seen in Figure 1b,d, a uniform, smooth silica layer was effectively encapsulated over the luminescent seed core NPs. Figure 1c shows the uncoated silica layer on NPs. The average crystalline size observed in the TEM image was 18 nm, which agreed well with the XRD results. The expansion in the amorphous silica layer increased the crystalline size of the nanomaterials.

The Fourier transform infrared (FTIR) spectra in Figure 2a clearly display the well-known infrared absorption peaks of amorphous silica at 1092, 952, and 805 cm–1, which are ascribed to Si–O–Si, Si–O, and Si–OH, respectively.25,26 This verified the surface coating of silica, surrounding the luminescent seed CeF3:Tb@LaF3 NPs. The peak monitored at 537 cm–1 was pointed to a metal–oxygen stretching vibrational mode. A broad band with a low intensity was observed between 300 and 3800 cm–1 and pointed to the stretching vibration of Si–O–H molecules. Figure 2b shows the absorption spectra of core/shell NPs at room temperature in watery media. The peak observed at 250 nm corresponded to a charge transfer transition from 2F5/2(4f1) Ce3+ to the Ce3+ (5d1) excited state.27

Figure 2.

Figure 2

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(a) FTIR spectrum, (b) absorption spectrum, and (c) photoluminescence spectrum of the core/shell NPs, with the inset showing the excitation spectrum of core/shell NPs.

Photoluminescence spectrophotometry was performed to examine the doping of terbium ions in the CeF3 crystal lattice. The emission spectrum in Figure 2c shows four emission transitions at 487, 541, 574–584, and 619 nm equivalents to 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7F3, which are 4f → 4f transitions of the Tb3+ ion, correspondingly. The inset in Figure 2c shows the excitation spectrum of core/shell NPs.10,23,24,28 Several narrow, intense bands were observed, corresponding to 4f–4f intraconfigurational excitation transitions of the Tb ion.29

In Vitro Anticancer Activity

In general, the toxicity and safety of NPs is a main interest in biomedical purposes. Various research works have been performed to analyze the in vitro cytotoxicity of various types of NPs, and the outcomes demonstrate that the cytotoxic effect may be connected with different factors including the size, shape, physical appearance, agglomeration state, and degree of functionalization of the NPs.2,17,30 In the current work, MTT assays were employed to evaluate the possible cytotoxic effect of CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 NPs on HT-29 and HepG-2 cell lines. These cancer cell lines were exposed with various NPs concentrations.

Estimation of the Possible Cytotoxicity of CeF3:Tb@LaF3 (Core/Shell) and Silica-Coated CeF3:Tb@LaF3@SiO2 (Core/Shell/SiO2) on the HT-29 Cell Line

The possible cytotoxicity of CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 NPs on the HT-29 cell line after the treatment for 24 h was evaluated and compared with positive (anticancerous drug dasatinib) and negative (untreated cells) controls. The results showed a dose-dependent cytotoxic effect for core/shell CeF3:Tb@LaF3 NPs on the HT-29 cell line (Figure 3a). As shown in the Figure 3a, the HT-29 cell was retained about 60% viability after 24 h exposure of CeF3:Tb@LaF3 NPs with high concentration (100 μg/mL).

Figure 3.

Figure 3

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In vitro evaluation of the % cell viability of HT-29 after 24 h of treatment at 37 °C by an MTT assay. (a) Graph showing the % cell viability of HT-29 against 3.125, 6.25, 12.5, 25, 50, and 100 μg/mL concentrations of CeF3:Tb@LaF3@SiO2 (blue bar) and CeF3:Tb@LaF3 (red bar) NPs. (b) Graph showing the % cell viability of HT-29 using 3.125, 6.25, 12.5, 25, 50, and 100 μg/mL concentrations of the control drug dasatinib. Error bars show the standard deviation for triplicate measurements of each sample.

Furthermore, we demonstrated that CeF3:Tb@LaF3@SiO2 NPs gave approximately 75% HT-29 cell viability with 100 μg/mL concentration after 24 h of exposure in the MTT assay (Figure 3a). These results indicated the lower cytotoxicity and thus good biocompatibility of CeF3:Tb@LaF3@SiO2 NPs, which may be helpful for applications in imaging and diagnostic fields of advanced biomedical science. In contrast, dasatinib was shown to be highly cytotoxic to the HT-29 cell line (Figure 3a).

Estimation of the Possible Cytotoxicity of CeF3:Tb@LaF3 (Core/Shell) and Silica-Coated CeF3:Tb@LaF3@SiO2 (Core/Shell/SiO2) on the HepG-2 Cell Line

Similarly, the possible cytotoxicity of CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 NPs on the HepG-2 cell line was evaluated and the results compared with the anticancerous drug dasatinib. Nontreated cancerous cells were utilized as negative control in each experiment. The results showed a dose-dependent cytotoxic effect of core/shell CeF3:Tb@LaF3 NPs on the HepG-2 cell line (Figure 4a). As shown in Figure 4a, the viability of HepG-2 cell was retained about 70% after 24 h of treatment with CeF3:Tb@LaF3 NPs at the highest concentration (100 μg/mL).

Figure 4.

Figure 4

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In vitro evaluation of the % cell viability of HepG2 after 24 h of treatment at 37 °C by an MTT assay. (a) Graph showing the % cell viability of HepG2 against 3.125, 6.25, 12.5, 25, 50, and 100 μg/mL concentrations of CeF3:Tb@LaF3@SiO2 (blue bar) and CeF3:Tb@LaF3 (red bar) NPs. (b) Graph showing the % cell viability of HepG2 using 3.125, 6.25, 12.5, 25, 50, and 100 μg/mL concentrations of the control drug dasatinib. Error bars show the standard deviation for triplicate measurements of each sample.

Furthermore, we demonstrated that CeF3:Tb@LaF3@SiO2 NPs gave a HepG-2 cell viability of approximately 78% with the 100 μg/mL concentration after exposure for 24 h in the MTT assay (Figure 4a). These results indicated that CeF3:Tb@LaF3@SiO2 NPs had a low level of cytotoxicity on both cell lines. In contrast, the results for dasatinib showed a highly cytotoxic effect on the HepG-2 cell line (Figure 4b). These results indicated the low cytotoxicity and good biocompatibility of CeF3:Tb@LaF3@SiO2 NPs, which may be helpful for applications in imaging and diagnostic fields of advanced biomedical science.

Microscopy of Cells Treated with CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2

CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 NPs did not have any considerable effect on the morphology of HT-29 cancer cells in comparison with untreated control cells. Figure 5a–c shows the negligible effect of CeF3:Tb@LaF3 on the HT-29 cell line after 24 h of incubation at concentrations of 3.125, 25, and 100 μg/mL. The morphology of the treated HT-29 cells remains same as untreated HT-29 cells (Figure 5d).

Figure 5.

Figure 5

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Photomicrograph of HT-29 cells treated with various concentrations of CeF3:Tb@LaF3 NPs, as well as an untreated negative control. HT-29 cells treated with (a) 3.12, (b) 25, and (c) 100 μg/mL. (d) Normal untreated HT-29 cells as a control.

As shown in Figure 6a–c, no effect of CeF3:Tb@LaF3@SiO2 on HT-29 cells on 24 h of exposure was observed as compared with untreated HT-29 cells (Figure 6d). All pictures of NPs exposed and unexposed cells were captured at 100× magnification.

Figure 6.

Figure 6

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Photomicrograph of HT-29 cells treated with various concentrations of CeF3:Tb@LaF3@SiO2 NPs, as well as an untreated negative control. HT-29 cells treated with (a) 3.12, (b) 25, and (c) 100 μg/mL. (d) Normal untreated HT-29 cells as a control.

The process of contact inhibition is responsible for arresting cell growth in normal cells as the growing cells come in contact with each other and stop proliferating upon monolayer formation. Contact inhibition is an important anticancer activity mechanism that is diminished in growing cancer cells.31 Our CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 NPs did not disrupt contact inhibition in HT-29 cells to any significant extent. Therefore, our results showed that CeF3:Tb@LaF3@SiO2 NPs may be useful in diagnostics and imaging because of their low cytotoxicity and good biocompatibility.

Conclusions

CeF3:Tb@LaF3 (core/shell) and CeF3:Tb@LaF3@SiO2 (core/shell/silica) NPs were successfully synthesized, and characterization revealed a uniform, smooth silica layer encapsulating the luminescent seed core NPs. The average crystalline size was observed by TEM to be approximately 18 nm, which corroborated the results of XRD. In vitro cytotoxicity experiments demonstrated that CeF3:Tb@LaF3@SiO2 NPs had outstanding biocompatibility and were less toxic than CeF3:Tb@LaF3 NPs, attributable to the silica layer modification. Silica is readily accessible for further surface modification for applications such bio-probing or biolabeling. Furthermore, the results of the microscopic study showed that silica-modified core/shell/silica NPs could be used in diagnostic and imaging in biomedical fields in the near future. The predicted improvement in biocompatibility upon surface coating with functionalized silica was confirmed.

From these results, we propose that CeF3:Tb@LaF3@SiO2 is a promising candidate for utilization in various areas in biomedical research, including biolabeling, biodetection, or bio-probing, labeling of cells and tissue, bioimaging, drug delivery, cancer therapy, and multiplexed analysis.

Materials and Methods

Materials

Cerium nitrate hexahydrate, lanthanum oxide, terbium oxide, ammonium fluoride, C2H5OH, NH4OH, HNO3, ethylene glycol (EG), and tetraethyl-orthosilicate (TEOS), Milli-Q water, Dulbecco’s modified Eagle’s medium (DMEM), streptomycin, penicillin, MTT assay kit were used in the study. Lanthanum nitrate heptahydrate and terbium nitrate hexahydrate were synthesized by mixing the corresponding oxides in diluted nitric acid.

Preparation of CeF3:Tb and CeF3:Tb@LaF3@SiO2 NPs

For preparation of CeF3:Tb and CeF3:Tb@LaF3 NPs, 2.3 ml of a 2 M stock solution of Ce(NO3)36H2O and 4.5 mL (2 M) of Tb(NO3)36H2O were mixed with 7.5 mL of EG in 50 mL of distilled water. An equimolar aqueous solution of NH4F was then injected into the dissolved solution, and the reaction mixture was put on to a hot plate with constant stirring for 2 h24 at 80 °C. Separation was induced by centrifugation and the precipitate was rinsed with Milli-Q water and dehydrated on room temperature. A similar procedure was used to coat the LaF3 shell. For deposition of a silica shell over the CeF3:Tb@LaF3 NPs, the Stober method was followed.32−34 As-prepared CeF3:Tb@LaF3 NPs (25 mg) were mixed in an aqueous solution containing 45 mL of ethanol, 10 mL of distilled water, and 0.5 mL of NH4OH solution by ultrasonication for 30 min. TEOS (0.5 mL) was then injected dropwise into the dissolved solution with constant magnetic stirring at room temperature for 6 h. The final formulation was collected by centrifugation and rinsed with Milli-Q water and dehydrated in an oven at 60 °C.

Characterizations

The prepared product of CeF3:Tb@LaF3@SiO2 NPs was characterized with various techniques.

X-ray Diffraction

Powder XRD was performed using a PANalytical X’Pert X-ray diffractometer with Cu Kα1 radiation (λ = 1.5405 c5).

Transmission Electron Microscope

Field-emission TEM (FE-TEM), JEM-2100F, JEOL, transmission electron microscope was utilized to analyze the shape of the synthesized NPs.

UV/Vis Spectrophotometry

A Cary 60 (Agilent technologies) UV/vis spectrophotometer was utilized to measure the absorption spectra.

Photoluminescence Spectrophotometry

A Vertex 80 spectrometer (Bruker, USA) was utilized to record the FTIR spectra. Photoluminescence spectra were recorded with a Fluorolog-3 spectrophotometer (model: FL 3-11, HORIBA, Jobin Yvon, Edison, MJ, USA).

In Vitro Anticancer Activity

Cell Culture

Two cell lines, HT-29 (human colon carcinoma) and HepG-2 (Human liver carcinoma) were purchased for experiments. HT-29 and HepG-2 cells were grown in DMEM containing 10% fetal bovine serum. The medium also contained 100 units/mL of penicillin and 100 mg/mL of streptomycin. The cell medium was replaced every 48 h and cells were trypsinized to reduce the cell population by half once 90% confluence was obtained.

In Vitro Cell Viability Evaluation Test

HT-29 and HepG-2 cells were transferred into 96-well plates (cell-culture) at a density of 2 × 104 cells and incubated in a humidified incubator at 37 °C with 5% CO2 for 24 h. The medium was then changed to a new medium having different concentrations of CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 NPs (3.125, 6.25, 12.5, 25, 50, and 100 μg/mL) and further incubated with 5% CO2 at 37 °C for 24 h. The drug dasatinib was utilized as a positive control against to two cancer cell lines. The untreated cancer cell lines acted as a negative control in each experiment. All experiments were done in triplicate.

MTT Assay for the Analysis of Possible Cytotoxicity

To evaluate the possible cytotoxicity of CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 NPs on three cell lines, the cell’s viability was analyzed using a standard MTT assay (Sigma-Aldrich). The medium was discarded from 96-well plate after 24 h incubation and the cells were washed with phosphate-buffered saline to eliminate NPs from the wells. A new medium having 10 μL of the MTT reagent was then supplied into each well. The absorbance was analyzed at 595 nm through an automated microplate reader (ELx800, BioTek, US).

The percentage viability (in terms of the IC50) of different cell lines treated with CeF3:Tb@LaF3, CeF3:Tb@LaF3@SiO2, and the control drug were calculated using triplicate results for every concentration. The untreated cell line acted as a negative control in each experiment.

Microscopy of Cells Treated with CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2

Briefly, HT-29 was seeded in DMEM containing 10% fetal bovine serum, 100 units/mL of penicillin and 100 mg/mL of streptomycin on a 6-well plate and kept in an incubator overnight at 37 °C supplied with 5% CO2. Three different concentrations of CeF3:Tb@LaF3 and CeF3:Tb@LaF3@SiO2 (3.125, 25, and 100 μg/mL) were used to observe the changes in the cell morphology. Nontreated cells were acted as negative control. Pictures were taken after 24 h of incubation with a charge-coupled device camera assembled with an inverted phase-contrast microscope at 100× magnification (Olympus, Japan).

Statistical Analysis

The data were examined to utilize SPSS version 22 statistics software. The results are illustrated as the mean ± SD and the data were analyzed by one-way analysis of variance (ANOVA). P values < 0.05 were supposed statistically significant.

Acknowledgments

This research work was supported by the Research Groups Program (Research Group number RG-1440-070), Deanship of Scientific Research, King Saud University, Riyadh, KSA.

The authors declare no competing financial interest.

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