Biogenic Ceria Nanoparticles (CeO2 NPs) for Effective Photocatalytic and Cytotoxic Activity

Ceria nanoparticles (CeO2 NPs) are generally considered in various functional applications, such as catalysts in fuel cells, sensors, and antioxidant and oxidase-like enzymes in the biological environment. The CeO2 NPs were synthesized using the E. globulus leaf extract-mediated hydrothermal technique. The synthesized NPs were characterized by various analytical instruments including powder X-ray diffractometer (PXRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and dynamic light scattering (DLS) analysis. The XRD results showed an average NPs sizes of 13.7 nm. Cytotoxic study results showed an IC50 value of 45.5 µg/L for A549 and 58.2 µg/L for HCT 116, indicating that CeO2 NPs are more toxic to A549 compared to HCT116 cell lines. The generation of ROS was responsible for its cytotoxic activity against cancer cell lines. Specific surface area (40.96 m2/g) and pore diameter (7.8 nm) were measured using Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption isotherms. CeO2 NPs with a high surface area were used as photocatalyst in degrading sunset yellow (SY) dye under UV-irradiation and 97.3% of the dye was degraded within 90 min. These results suggest that the synthesized CeO2 NPs could be used as a good photocatalyst as well as a cytotoxic agent against human cancer cell lines.


Introduction
Nanomaterials having a high surface area, smaller size and different shapes, allow them to be used in different applications with enhanced properties [1,2]. Ceria nanoparticles (CeO 2 NPs) are rare earth oxide NPs that have several important inherent properties, such as oxygen storage capacity, catalysis, optics, magnetic, high thermal and chemical stability [3,4]. Currently various synthetic methods such as sol-gel, micro-emulsion, precipitation, sono-chemical, surfactant template synthesis and glycine-nitrate combustion [5,6] are adopted to synthesize NPs; however, all these methods use expensive, toxic and hazardous chemicals and solvents. Green synthesis of nanomaterials is the most favorable method because it is environmentally friendly, low-cost and non-toxic without using substances that are hazardous to the environment and mankind [7,8]. Moreover, the size of the NPs is easily controlled by varying the extract volume since phytochemicals act as a reducing as well as stabilizing agent. Green synthesis also results in a narrow size distribution of NPs. In addition, green-synthesized NPs are biocompatible due to the capping of biomolecules on the surface of the NPs.

Preparation of Extract
E. globulus leaves were washed thrice with Millipore water and dried at ambient temperature in a dust-free chamber without sunlight exposure until constant weight was obtained. Ten grams of dried powder of the leaves mixed with 100 mL Milli-Q water was heated at exposure to 80 • C for 2 h, and the resulting light yellow solution was filtered, centrifuged and stored at 4 • C for further studies.

Green Synthesis of CeO 2 NPs
Required amount of cerous nitrate hexahydrate (0.1N) was dissolved in 100 mL water, and then 100 mL aqueous plant extract was added dropwise through a peristaltic pump with stirring over 2-3 h. The pH of the mixture (initial pH 6.8) was adjusted to 8.2 by the addition of NaOH. After 24 h, pH of the mixture increased to 7.5 due to the formation of cerium hydroxide in the solution. The brownish yellow colored solution formed was centrifuged at 10,000 rpm for 15 min followed by ethanolic wash to remove impurities from solution. The obtained brownish yellow colored residue was dried at 80 • C for 2 h in a hot air oven and the dried powder was properly pulverized and then annealed at 400 • C for 3 h to obtain crystalline particles.
CeO 2 NPs were prepared in two steps. In the first step, cerium nitrate was mixed with E. globulus leaf extract, which formed a three-dimensional bridge complex due to interaction of phytochemicals with cerium ions. In this step, E. globulus extract containing 9, 12 octadeca trienoic acid chains reacted with tetravalent Ce +4 via hydroxyl groups from two different chains to form network complex bridge with Ce +4 which conjugates to all the compounds present in the extract caused synergistic effect resulting in a complex structure. In the second step, the polymeric network chains suffered slow decomposition during calcination. The mechanism of CeO 2 NPs formation is shown schematically in Figure 1.

Preparation of Extract
E. globulus leaves were washed thrice with Millipore water and dried at ambient temperature in a dust-free chamber without sunlight exposure until constant weight was obtained. Ten grams of dried powder of the leaves mixed with 100 mL Milli-Q water was heated at exposure to 80 °C for 2 h, and the resulting light yellow solution was filtered, centrifuged and stored at 4 °C for further studies.

Green Synthesis of CeO2 NPs
Required amount of cerous nitrate hexahydrate (0.1N) was dissolved in 100 mL water, and then 100 mL aqueous plant extract was added dropwise through a peristaltic pump with stirring over 2-3 h. The pH of the mixture (initial pH 6.8) was adjusted to 8.2 by the addition of NaOH. After 24 h, pH of the mixture increased to 7.5 due to the formation of cerium hydroxide in the solution. The brownish yellow colored solution formed was centrifuged at 10,000 rpm for 15 min followed by ethanolic wash to remove impurities from solution. The obtained brownish yellow colored residue was dried at 80 °C for 2 h in a hot air oven and the dried powder was properly pulverized and then annealed at 400 °C for 3 h to obtain crystalline particles.
CeO2 NPs were prepared in two steps. In the first step, cerium nitrate was mixed with E. globulus leaf extract, which formed a three-dimensional bridge complex due to interaction of phytochemicals with cerium ions. In this step, E. globulus extract containing 9, 12 octadeca trienoic acid chains reacted with tetravalent Ce +4 via hydroxyl groups from two different chains to form network complex bridge with Ce +4 which conjugates to all the compounds present in the extract caused synergistic effect resulting in a complex structure. In the second step, the polymeric network chains suffered slow decomposition during calcination. The mechanism of CeO2 NPs formation is shown schematically in Figure 1.
Surface morphology was checked by FE-SEM and elemental composition was monitored by energy dispersive X-ray spectroscopy (EDAX). In addition, XRD was used to check phase purity which was cross-checked with selected area electron diffraction (SAED) pattern. SAED patterns were recorded using high resolution TEM (HR-TEM) equipped with Gatan CCD camera. DLS experiment identified particle size distribution of CeO 2 NPs which was compared with XRD and HR-TEM results. Zeta potential was measured to predict stability using Horiba Scientific Nano Particii (SZ-100). CeO 2 NPs dispersion was prepared by sonicating 1 mg NPs in 100 mL water at pH 7.5 for DLS and zeta potential measurement. Thermal analysis of the synthesized CeO 2 NPs was carried out to determine weight loss with temperature. ATR-FTIR (Jasco-4100) at wave number range of 4000-400 cm −1 was used to trace functional groups attached to NPs as well as the extracts. UV-Vis-DRS study measured absorption intensity and band gap of the NPs in the wavelength ranges of 200-800 nm. The band gap of CeO 2 NPs was calculated by using the following equation as α = c(hν − E bulk ) 1/2 /hv, where α is absorption coefficient, c is constant, hν is the photon energy and E bulk is bulk 'band gap'.

Cell Culture
Cell culture study was carried out using conventional method where Dulbecco's modified Eagle's medium (DMEM) containing 100 µg penicillin/mL, 100 U streptomycin/mL) and 10% heat-inactivated FBS was used in cell culture T25 flasks. Human A549 and HCT116 cell lines were cultured at 37 • C and 5% CO 2 one day prior to the exposure studies with CeO 2 NPs.

Measurement of Cytotoxicity by MTT Assay
Cell viability was determined using MTT assay which is based on the conversion of the tetrazolium salt [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2 tetrazolium] to formazan crystals by mitochondrial NAD(P)H-dependent oxidoreductase enzymes. Briefly, cells were seeded at a density of 1 × 10 4 cells/well into a 96 well plate and incubated overnight at 37 • C with 5% CO 2. Then the cells were washed with PBS and media was discarded. The cells were then exposed to test chemical for 24 h at 37 • C with 5% CO 2 and RH more than 80%. After removal of medium, 100 µL PBS was added to each well followed by addition of 100 µL MTT solution (0.5 mg/mL MTT in DMEM without FBS) and incubated for 1 h at 37 • C and 5% CO 2 . The formed formazan crystals were dissolved in 10% SDS (sodium dodecyl sulfate in 0.01 N HCl). MTT solution was then removed and 100 µL DMSO was added to each well and absorbance was measured using ELISA reader at 570 nm (EL800, Bio-Tech Instruments, Inc., Winooski, VT, USA). Additionally, one blank was used without CeO 2 NPs for comparison.

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Cell viability calculation was used to check the cytotoxic effect of the toxicants. MTT assay was performed in 96 well plate for negative control (only cell culture medium and cells were added), positive control (having cells and standard drug cisplatin, CDDP), experimental wells (having cells with CeO NPs), and blank samples (having only medium). All the experiments were carried out in triplicate. Cell viability was calculated as the ratio of the mean absorbance of replicated wells compared to that of the negative control wells. The experimental wells were compared to negative control with 100% growth. The IC 50 values were obtained from the graph plotted using concentrations of CeO 2 NPs along X-axis vs. cell viability (%) along Y-axis. Cell viability calculation was used to check the cytotoxic effect of the toxicants. MTT assay was performed in 96 well plate for negative control (only cell culture medium and cells were added), positive control (having cells and standard drug cisplatin, CDDP), experimental wells (having cells with CeO NPs), and blank samples (having only medium). All the experiments were carried out in triplicate. Cell viability was calculated as the ratio of the mean absorbance of replicated wells compared to that of the negative control wells. The experimental wells were compared to negative control with 100% growth. The IC50 values were obtained from the graph plotted using concentrations of CeO2 NPs along X-axis vs. cell viability (%) along Y-axis.   Figure 3a represents the SEM image which shows spherical morphology of the particles. Figure  3b reveals the EDAX spectrum of CeO2 NPs with elemental analysis confirming the presence of "Ce" and "O" atoms with atomic percentage (79.87% and 14.49%) and weight percentage (43.30% and 46.56%), respectively. A very less intense carbon peak is seen due to the contribution of carbon tape attached on the stub surface.  [20]. The crystallite size of CeO2 NPs is 20.72 nm as per calculation by the Scherrer's formula [21].

Dynamic light Scattering (DLS) Analysis
The stability of CeO2 NPs is stated in terms of zeta potential. It is defined as the potential difference around surface charge groups associated on the NPs surfaces, and dispersed solvent medium containing groups of opposite charge. Basically, higher negative value of zeta potential The optical band gap (E g ) is found by the extrapolation of a Kubelka-Munk function fitted in a Tauc plot along the X-axis, as shown in Figure 2b. The band gap energy value indicates the semiconducting nature of the synthesized CeO 2 NPs. Figure 3a represents the SEM image which shows spherical morphology of the particles. Figure 3b reveals the EDAX spectrum of CeO 2 NPs with elemental analysis confirming the presence of "Ce" and "O" atoms with atomic percentage (79.87% and 14.49%) and weight percentage (43.30% and 46.56%), respectively. A very less intense carbon peak is seen due to the contribution of carbon tape attached on the stub surface.  [20]. The crystallite size of CeO 2 NPs is 20.72 nm as per calculation by the Scherrer's formula [21].

Dynamic light Scattering (DLS) Analysis
The stability of CeO 2 NPs is stated in terms of zeta potential. It is defined as the potential difference around surface charge groups associated on the NPs surfaces, and dispersed solvent medium containing groups of opposite charge. Basically, higher negative value of zeta potential indicates more repulsion between the particles which minimizes aggregation and results in higher stability of bio-fabricated NPs [22]. Figure 3d shows the particle size distribution with an average size of CeO 2 NPs as 20.9 nm. Figure 3e describes the measured zeta potential value of CeO 2 NPs as −35.5 mV at pH 7.5, which indicates higher dispersibility of the NPs with high stability.
Bioengineering 2019, 6, x FOR PEER REVIEW 6 of 12 indicates more repulsion between the particles which minimizes aggregation and results in higher stability of bio-fabricated NPs [22]. Figure 3d shows the particle size distribution with an average size of CeO2 NPs as 20.9 nm. Figure3e describes the measured zeta potential value of CeO2 NPs as −35.5 mV at pH 7.5, which indicates higher dispersibility of the NPs with high stability.

Transmission Electron Microscopy (TEM) Analysis
Figure 4a-c shows the transmission electron micrographs representing the particles sizes of 15-20 nm. Figure 4d specifies HRTEM micrograph which confirms the formation of CeO2 particles having fringe space of 3.1 Å The results are in close agreement with XRD data with (111) plane (3.24 Å) of CeO2 NPs and the low crystalline nature of particles resembles its high reactivity to radicals [23]. Figure 4f represents the SAED pattern and hkl values which supports the XRD pattern of polycrystalline particles.   Figure 4d specifies HRTEM micrograph which confirms the formation of CeO 2 particles having fringe space of 3.1 Å The results are in close agreement with XRD data with (111) plane (3.24 Å) of CeO 2 NPs and the low crystalline nature of particles resembles its high reactivity to radicals [23]. Figure 4f represents the SAED pattern and hkl values which supports the XRD pattern of polycrystalline particles.

Fourier-Transform Infrared Spectroscopy (FT-IR) Analysis
FTIR analysis is carried out to find out the functional groups attached to NPs surface as well as in phytochemicals present in E. globulus extract (Figure 5a). FTIR spectra show the presence of phenolic OH group (absorption band at 3350 cm −1 ), and carboxyl (C=O) group (absorption band at 1690 cm −1 ) which reduces Ce 4+ ions into CeO 2 NPs [24]. Other absorption bands such as 2954 and 2833 cm −1 correspond to the stretching vibrations of alkanes, 1390 cm −1 for the CH 2 bending, 1110.8 cm

Fourier-Transform Infrared Spectroscopy (FT-IR) Analysis
FTIR analysis is carried out to find out the functional groups attached to NPs surface as well as in phytochemicals present in E. globulus extract (Figure 5a). FTIR spectra show the presence of phenolic OH group (absorption band at 3350 cm −1 ), and carboxyl (C=O) group (absorption band at 1690 cm −1 ) which reduces Ce 4+ ions into CeO2 NPs [24]. Other absorption bands such as 2954 and 2833 cm −1 correspond to the stretching vibrations of alkanes, 1390 cm −1 for the CH2 bending, 1110.8 cm −1 for C-O bending vibrations. Additionally, lower shift functional groups correspond to 940.3 cm −1 for C-O and 640.3 cm −1 for Ce-O bond respectively. It resembles the formation of CeO2 NPs and the polyphenol groups present in the E. globulus extract behave both as reducing agent to cerium ions and stabilizing agent of CeO2 NPs.

Surface Area and Porosity Analysis
Specific surface area and pore size distribution of the synthesized CeO2 NPs are calculated using nitrogen adsorption-desorption isotherms. Figure 5b shows the nitrogen adsorptiondesorption isotherm at 77 K and the measured Brunauer-Emmett-Teller (BET) specific surface area of CeO2 NPs is 40.96 m 2 /g, which is much higher than commercial CeO2 NPs (8.50 m 2 /g) [25]. Additionally, Barrett-Joyner-Halenda (BJH) pore size distribution curve displays a narrow pore size distribution with a calculated pore diameter of 7.8 nm. These isotherms are considered as Type IV isotherms [26].

Thermogravimetric Analysis (TGA)
Thermogravimetric analysis shows weight loss of the compound with respect to temperature. Figure 5c displays the TGA thermogram of the bio-synthesized CeO2 NPs against temperature (@ 5 °C/min) under nitrogen gas atmosphere, with two-step weight losses within the temp range of 0-800 °C. The first step is associated with 11.6% weight loss in the temperature range of 20-200 °C due to elimination of water molecules and absorbed moisture. Second major weight loss of 18.6% occurred in 300-400 °C due to oxidative decarboxylation of Ce(NO3)3 to CeO2 [4].

Photocatalytic Degradation of SY Dye
The photocatalytic activity of the synthesized CeO2 NPs was studied during the degradation of SY dye under UV-irradiation. It was observed that the intensity of SY dye colour did not change in the dark, during a time period of 1-2 h, with or without CeO2 NPs. SY dye was found to degrade only under UV-irradiation, which was confirmed by UV-Visible spectroscopy (no appearance of dye peak) (Figure 6). Figure 6a describes the UV-Visible spectra for degradation kinetics of SY at different time intervals (from 0 to 90 min) at λmax 520 nm. It clearly shows that the colour intensity of SY decreases with time in the presence of CeO2 NPs. Initially, SY was found to degrade up to 90.7% after 90 min of UV irradiation. Figure 6b represents the plot of ln (Ct/C0) versus irradiation time

Surface Area and Porosity Analysis
Specific surface area and pore size distribution of the synthesized CeO 2 NPs are calculated using nitrogen adsorption-desorption isotherms. Figure 5b shows the nitrogen adsorption-desorption isotherm at 77 K and the measured Brunauer-Emmett-Teller (BET) specific surface area of CeO 2 NPs is 40.96 m 2 /g, which is much higher than commercial CeO 2 NPs (8.50 m 2 /g) [25]. Additionally, Barrett-Joyner-Halenda (BJH) pore size distribution curve displays a narrow pore size distribution with a calculated pore diameter of 7.8 nm. These isotherms are considered as Type IV isotherms [26].

Thermogravimetric Analysis (TGA)
Thermogravimetric analysis shows weight loss of the compound with respect to temperature. Figure 5c displays the TGA thermogram of the bio-synthesized CeO 2 NPs against temperature (@ 5 • C/min) under nitrogen gas atmosphere, with two-step weight losses within the temp range of 0-800 • C. The first step is associated with 11.6% weight loss in the temperature range of 20-200 • C due to elimination of water molecules and absorbed moisture. Second major weight loss of 18.6% occurred in 300-400 • C due to oxidative decarboxylation of Ce(NO 3 ) 3 to CeO 2 [4].

Photocatalytic Degradation of SY Dye
The photocatalytic activity of the synthesized CeO 2 NPs was studied during the degradation of SY dye under UV-irradiation. It was observed that the intensity of SY dye colour did not change in the dark, during a time period of 1-2 h, with or without CeO 2 NPs. SY dye was found to degrade only under UV-irradiation, which was confirmed by UV-Visible spectroscopy (no appearance of dye peak) ( Figure 6). Figure 6a describes the UV-Visible spectra for degradation kinetics of SY at different time intervals (from 0 to 90 min) at λ max 520 nm. It clearly shows that the colour intensity of SY decreases with time in the presence of CeO 2 NPs. Initially, SY was found to degrade up to 90.7% after 90 min of UV irradiation. Figure 6b represents the plot of ln (C t /C 0 ) versus irradiation time (min), and a linear relationship is obtained based on the following equation as Ln(C/C 0 ) = −kt, where C 0 is the concentration of SY at time t "0 min", C is the concentration of SY at time "t min" and k is the slope constant. Slope value obtained for SY with optimized catalyst dosage (35 mg) is k = 0.0250 min −1 , which follows pseudo first-order kinetic rate. The recyclability of the catalyst is worked out and up to three cycles, whereby no significant change in percentage of degradation is seen, as shown in Figure 6c.

Cytotoxicity Study
Cytotoxicity study of CeO2 NPs is performed against A549 and HCT 116 cancer cell lines. The cell viability is analyzed by MTT assay and IC50 (Half maximal inhibitory concentration) value obtained is 45.5 µg/L for A549 cell line and 58.2 µg/L for HCT 116, which clearly suggests that the biosynthesized CeO2 NPs are more toxic to A549 cell lines compared to HCT 116 cell lines. It can be

Cytotoxicity Study
Cytotoxicity study of CeO 2 NPs is performed against A549 and HCT 116 cancer cell lines. The cell viability is analyzed by MTT assay and IC 50 (Half maximal inhibitory concentration) value obtained is 45.5 µg/L for A549 cell line and 58.2 µg/L for HCT 116, which clearly suggests that the biosynthesized CeO 2 NPs are more toxic to A549 cell lines compared to HCT 116 cell lines. It can be noted that it is toxic to both the cell lines suggesting its potential as anticancer agent. The cytotoxicity of CeO 2 NPs is compared with standard drug cisplatin (CDDP), which is mostly similar at high dose of CeO 2 NPs (80 µg/mL) (Figure 7a).

Conclusion
In summary, CeO2 NPs were synthesized using E. globulus leaves extract via a green method. Synthesized NPs show photoluminescence, cubic structure, and spherical shape with particle size ranging from 8-20 nm, which are confirmed by various analytical techniques like XRD, SEM and TEM. The bio-synthesized CeO2 NPs are a superior photocatalyst compared to other photocatalysts due to oxygen vacancies, decreased band gap, surface defects and lower recombination tendency of the electron-hole pair. The synthesized nanoparticles remain stable up to four recycles without change in photocatalytic efficiency. Additionally, CeO2 NPs are cytotoxic to human lung A549 and colon HCT 116 carcinoma cell lines, with IC50 values of 45.5 and 58.2 µg/L respectively. The current results suggest the promising potential of CeO2 NPs for potential anticancer therapy.
Author Contributions: All authors contributed equally to the research work presented in this research article.
Funding: This research received no external funding.

Acknowledgement:
The authors gratefully acknowledge Vellore Institute of Technology, Vellore for providing

Estimation of Reactive Oxygen Species (ROS) Generation by DCF Method
To estimate intracellular ROS generation, a cell-permeable non-fluorescent dye 2, 7 dichlorofluorescein diacetate (DCFH2-DA) is used, which can be de-esterified intracellularly to highly fluorescent 2-dichloro fluorescein (DCF+) upon oxidation [27]. It is commonly observed that metal oxide NPs interact with cell membranes and proteins inside the cells and release ROS species leading to the imbalance in redox state of cell, which causes oxidative stress and cell death [17]. Pro-oxidant functional clusters present on the reactive surface of NPs play a major role in kinetics of toxicity, and disrupts the cell signaling and immune defense mechanism [18]. Recently, our research group has reported the death of cancer cell lines by SnO 2 NPs due to dose-dependent production of ROS species [19]. Basically, reactive oxygen species (i.e., free radicals) generated in cells under stress conditions interact with cytoplasm, proteins and mitochondria resulting in cell destruction [28]. Figure 7b explains the comparative fluorescence emission spectra of both control and CeO 2 NPs at different concentrations (20, 40, 60, 80 µg/mL) at an emission wavelength of 523 nm. It clearly exhibits ROS generation by CeO 2 NPs with increasing intensities of DCF dye, which is directly proportional to the dose. Compared to the control, bio-synthesized CeO 2 NPs show higher emission intensity due to more generation of ROS species or more cell deaths. ROS is generated from reduced nicotinamide adenine dinucleotide dehydrogenase II in the respiratory chain by the auto-oxidation process [28][29][30][31][32]. In addition to the emission spectra of ROS, fluorescence microscopy images are captured for negative control and exposure of IC 50 concentration of CeO 2 NPs dispersion shows the ROS emission with green colour spotted images, which indicate both cell lines generated ROS in the presence of CeO 2 NPs (Figure 8a

Conclusion
In summary, CeO2 NPs were synthesized using E. globulus leaves extract via a green method. Synthesized NPs show photoluminescence, cubic structure, and spherical shape with particle size ranging from 8-20 nm, which are confirmed by various analytical techniques like XRD, SEM and TEM. The bio-synthesized CeO2 NPs are a superior photocatalyst compared to other photocatalysts due to oxygen vacancies, decreased band gap, surface defects and lower recombination tendency of the electron-hole pair. The synthesized nanoparticles remain stable up to four recycles without change in photocatalytic efficiency. Additionally, CeO2 NPs are cytotoxic to human lung A549 and colon HCT 116 carcinoma cell lines, with IC50 values of 45.5 and 58.2 µg/L respectively. The current results suggest the promising potential of CeO2 NPs for potential anticancer therapy.
Author Contributions: All authors contributed equally to the research work presented in this research article.
Funding: This research received no external funding.

Acknowledgement:
The authors gratefully acknowledge Vellore Institute of Technology, Vellore for providing a platform and basic characterization facilities to carry out this work. Additionally, SB is sincerely thankful to UGC-NRC (Networking Resource Center) of School of Chemistry, University of Hyderabad, for providing all necessary facilities and financial assistance to complete this work. Heartfelt gratitude to ESAB INDIA

Conclusions
In summary, CeO 2 NPs were synthesized using E. globulus leaves extract via a green method. Synthesized NPs show photoluminescence, cubic structure, and spherical shape with particle size ranging from 8-20 nm, which are confirmed by various analytical techniques like XRD, SEM and TEM. The bio-synthesized CeO 2 NPs are a superior photocatalyst compared to other photocatalysts due to oxygen vacancies, decreased band gap, surface defects and lower recombination tendency of the electron-hole pair. The synthesized nanoparticles remain stable up to four recycles without change in photocatalytic efficiency. Additionally, CeO 2 NPs are cytotoxic to human lung A549 and colon HCT 116 carcinoma cell lines, with IC 50 values of 45.5 and 58.2 µg/L respectively. The current results suggest the promising potential of CeO 2 NPs for potential anticancer therapy.