Cerium Phosphate–Cerium Oxide Heterogeneous Composite

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Cerium Phosphate−Cerium Oxide Heterogeneous Composite Nanozymes with Enhanced Peroxidase-Like Biomimetic Activity for Glucose and Hydrogen Peroxide Sensing G. Vinothkumar, Arun I. Lalitha, and K. Suresh Babu* Centre for Nanoscience and Technology, Madanjeet School of Green Energy Technology, Pondicherry University, R V Nagar, Kalapet, Puducherry 605 014, India

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ABSTRACT: In the present work, we focus on the development of CePO4−CeO2 composite nanorods with peroxidase mimetic activity for the sensitive detection of hydrogen peroxide and glucose. The Ce3+/PO43− molar ratio (CP10:1, CP5:1, CP2:1) in the hydrothermal reaction controlled the formation of pure CePO4, CePO4−CeO2 composite nanozymes with different percentages of CeO2, and its crystal structure. A higher Ce3+/PO43− molar ratio (CP10:1 or CP5:1) was required to obtain CePO4−CeO2 composite nanostructure, while a lower Ce3+/PO43− molar (CP2:1) ratio was sufficient to fabricate pure CePO 4 nanorods. In the presence of hydrogen peroxide, the prepared nanozymes catalyze the oxidation of chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB). Steady state kinetic analysis based on the Michaelis−Menten model revealed that CP10:1 showed excellent affinity toward the TMB (Km = 0.236 mM and Vmax = 8.78 × 10−8 M s−1) in comparison to the catalytic activity of CP5:1 and CP2:1 and horseradish peroxidase (Km = 0.434 mM and Vmax = 10.0 × 10−8 M s−1). The superior peroxidase activity of CePO4−CeO2 composite nanozymes can be ascribed to the enhanced redox switching between Ce3+ ↔ Ce4+ sites from the CePO4 and CeO2 lattice, respectively. The colorimetric detection of hydrogen peroxide and glucose showed a linear response around 150 μM concentration with the limits of detection (LOD) of 2.9 and 4.1 μM, respectively. microcrystals,17 CePO4 nanorods decorated with quantum dots,18 and CePO4@Gd:TbPO4 core−shell structure19 for the luminescent and bioimaging applications due to their unique optical property that emerges from the Ce 4f level transitions. Moreover, cerium phosphate and cerium oxide nanomaterials are well-known for their technological applications in sensors,10,20 catalysis,21,22 luminescent host materials,23 fuel cells,24 bioimaging, and biomedicine.25 Highly reactive cerium assumes a trivalent Ce3+ oxidation state in CePO4 lattice, while it exists in tetravalent Ce4+ state in the stoichiometric ceria structure in its bulk form. Interestingly, the presence of mixed oxidation state and its reversible redox switching between the oxidation state (Ce3+ ↔ Ce4+) in the nano dimension is the fundamental reason for the catalytic activity of CePO4 and CeO2 nanozymes. In CeO2, the reduction of Ce4+ to Ce3+ state is accompanied by the creation of positively charged oxygen vacancies at the lattice.26 A similar explanation was given by Kitsuda et al. for the presence of a minor amount of Ce4+ species through the oxidation of Ce3+ to Ce4+ ions present in the CePO4 involves the incorporation of negatively charges oxygen species without affecting the crystal structure.27 Wang et

1. INTRODUCTION Natural enzymes are biocatalysts that regulate every biological process in living organisms. Inspired by its remarkable functions, the development of novel artificial enzymes has become an active field of research. The artificial enzymes based on inorganic nanomaterials termed as “nanozymes” have enormous advantages over natural enzymes such as high specificity, sensitivity, stable performance in a wide range of pH and temperature, low cost, mass production, and biocompatibility.1−6 Recently, rareearth-based nanomaterials such as cerium phosphate7 and ceria8,9 have been shown to exhibit peroxidase-like biomimetic activity. The peroxidase-like activity of the nanomaterials is not only an alternative to natural enzyme horseradish peroxidase but also is well-extended to applications like detection of hydrogen peroxide, glucose,10,11 lead ions,12 mercury ions,13 and DNA methylation,14 as well as immunoassays and immunostaining,15,16 among others. Cerium phosphate (CePO4) is a polymorphic material known to crystallize in rhabdophane type hexagonal structure at low temperature and transform into monoclinic structure at higher temperature. Much attention has been provided toward the improvement of luminescent properties of CePO4 nanomaterials doped with rare earth ions, and other hierarchical structures such as CePO4 nanorods attached to cerium oxide (CeO2) © XXXX American Chemical Society

Received: August 29, 2018

A

DOI: 10.1021/acs.inorgchem.8b02423 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

For all the samples, the volume remained the same, while the Ce/PO43− molar ratio changed. The cerium nitrate solution was added dropwise to the phosphate solution under stirring at room temperature which subsequently formed a white colored dense slurry. The pH of the solution was maintained acidic (pH ∼1) using 1 N of HCl. After 0.5 h of continuous stirring, the slurry was then transferred into a Teflon-lined autoclave and maintained at 180 °C for 24 h. The precipitated white colored powder was collected, washed in a centrifuge with distilled water/ethanol mixture, and dried at 80 °C overnight for the final product. 2.3. Characterizations. The X-ray diffraction pattern of the cerium phosphate samples were obtained using a XRD diffractometer (Rigaku, Ultima IV, Cu Kα, λ = 1.5406, 40 kV, and 30 mA) in the range of 10− 70° with a step size of 0.02° at a rate of 2° per minute. The morphology of the synthesized nanomaterials was determined by high-resolution transmission electron microscope (HRTEM) operated at 200 kV (FEI, TECNAI G2). X-ray photoelectron spectroscopy (XPS) was used to identify the oxidation state of the cerium and quantification of Ce3+/ Ce4+ ratio on the surface using Omicron ESCA+ (Omicron nanotechnology, Oxford Instrument, Germany) with an Al Kα source (E ∼ 1486.7 eV). UV−vis absorbance spectra were recorded on a UV−vis spectrometer (PerkinElmer, Lambda 650S) with a resolution of 1 nm. The thermal behavior of the materials was analyzed with a thermogravimetric analyzer (Q600 STD, TA Instruments) coupled with differential scanning colorimetry at a heating rate of 10 °C up to to 1000 °C in nitrogen atmosphere. Raman spectra of the powder samples were collected between 200 and 1100 cm−1 using confocal Raman spectrometer (Renishaw RM 200, United Kingdom) using 785 nm laser from an Ar+ laser source. The vibrational spectra of the powder samples were acquired on a FTIR spectrometer (NICOLET 6700) from 2800 to 400 cm−1 with a resolution of 0.1 cm−1. A Hitachi-7000 spectrofluorimeter equipped with 150 W xenon Lamp was used to study the photoluminescence emission spectra of the samples at a desired excitation wavelength. The Brunauer−Emmett−Teller (BET) method was used to estimate the surface area of the samples. N2 adsorption− desorption isotherms were recorded at 77 K on a BELSORP-max, MicrotracBEL surface area analyzer. 2.4. Peroxidase Assay. The peroxidase activity of the pure cerium phosphate (hexagonal and monoclinic) and ceria−cerium phosphate nanozymes were tested using hydrogen peroxide and TMB at different concentrations. In a typical measurement, the reaction volume containing 100 μL of citric acid buffer (pH 4.0), 400 μL of TMB (0.8 mM), and 15 μL of H2O2 (100 mM) were used. For Michaelis− Menten kinetic analysis, the concentrations of TMB (0−1000 μM) or H2O2 (0−1000 μM) was varied. The changes in the absorbance spectra were monitored at a wavelength of 653 nm using UV−visible spectrophotometer. All the experiments were performed in triplicate. 2.5. Hydrogen Peroxide and Glucose Sensing. Hydrogen peroxide sensing was carried out with a reaction mixture of 400 μL of citric acid buffer (pH 4.0), 200 μL of TMB (0.8 mM), and nanocomposites (100 μg/mL) as mentioned above by varying the concentration of H2O2 from 0 to 400 μM. Similarly, for the detection of glucose, the reaction mixture contains 400 μL of citric acid buffer (pH 4.0), 200 μL of TMB (0.8 mM), and nanocomposites (100 μg/mL) and 100 μL of glucose oxidase (2 mg/mL) were added and incubated at 37 °C for 30 min. As controls in the experiment, 5 mM solutions of galactose, maltose, fructose, sucrose, and buffer were used.

al. demonstrated that the residual Ce3+/Ce4+ sites in CePO4 nanospheres are responsible for peroxidase mimetic activity.7 Similarly, many authors have explored the oxidation−reduction cycle in CePO4 for luminescent switching applications.27,28 Lv et al. also demonstrated the hydrogen peroxide and glucose sensing of Tb:CePO4 nanostructures based on redox cycling capacity.10 Li et al. demonstrated the transfer of electrons between the CePO4 (Ce3+) rods and CeO2 (Ce4+) micrometer crystals in enhancing the luminescence property in the ultraviolet (UV) region.17 However, no efforts have been taken to improve the peroxidase activity of cerium phosphate nanostructures. Recently, a variety of nanocomposites with novel compositions and structures such as Au@Pt, Pd@Au, Au-CNT hybrids, and Pt-CeO2,29−32 among others, were developed as peroxidase mimetics. However, the cost of such noble materials is a major drawback for practical applications. As a cost-effective approach, various hierarchical structures based on CeO2 such as TiO2@ CeO2 core−shell, Fe-Ceria, Fe3O4@CeO2, Co3O4@CeO2, and graphene/CeO233−37 have been employed to enhance peroxidase activity. In order to improve peroxidase-like enzymatic activity for sensing applications, we present a radical approach to form cerium phosphate−ceria composite nanostructures with different compositions to manipulate the redox cycling between Ce3+ and Ce4+ at the phosphate−oxide interface. Besides the role of incorporating Ce4+ sites from CeO2 on the surface of CePO4, the multienzyme mimetic activity of cerium oxide38 (catalase, peroxidase, oxidase, superoxide dismutase, etc.) is an added advantage for the enzymatic activity and such oxide−phosphate composite 1D nanostructure as an enzyme mimetic, to the best of our knowledge, has not been reported in the literature. Nevertheless, the morphology, crystallite size, chemical composition, surface-to-volume ratio, and crystal structure of the nanomaterials are also known to influence the catalytic properties in general. CePO4 can be prepared in various morphologies like spheres,39 nanofibers,40 nanowires,41−43 nanorods,44 and nanotubes45 based on the synthesis methods such as hydrothermal,46 sol−gel,47 precipitation,48 microemulsion49 and microwave50 methods. In the present work, cerium phosphate−ceria composite nanorods were synthesized to enhance the redox cycling between Ce3+ and Ce4+ at the interface in comparison to that in pure CePO 4. The biocompatible nature of both ceria and cerium phosphate and its significant peroxidase activity will be the key for a wide range of applications such as antioxidants and biosensors.

2. EXPERIMENTAL METHODS 2.1. Chemicals. Cerium nitrate hexahydrate (CeNO3)3·6H2O, Himedia, India), diammonium hydrogen phosphate ((NH4)2HPO4, Fischer Scientific, India), 70% hydrochloric acid (Himedia, India), 30% hydrogen peroxide (H2O2, Himedia), citrate buffer pH 4.0, glucose oxidase (GOx), and 3,3′,5,5′-tetramethylbenzidine (TMB, SRL, India) have been utilized as received. Sugars such as glucose, galactose, maltose, sucrose, and fructose (Himedia, India) were used as controls in the glucose-sensing experiment. 2.2. Synthesis. The cerium phosphate (CP) nanomaterials (hexagonal h-CP, monoclinic m-CP) and ceria−cerium phosphate (ceria-CP) composites were synthesized by hydrothermal method using cerium nitrate hexahydrate and diammonium hydrogen phosphate as the precursors. Ce/PO43− molar ratios of 10:1, 5:1, 2:1, 1:1, 1:2, and 1:5 were used to synthesize h-CP, m-CP, and CeO2−CP nanomaterials. In a typical synthesis of CP (CP1:1), an equimolar concentration of 60 mL of cerium nitrate solution and 60 mL of diammonium hydrogen phosphate solutions were prepared separately.

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction. The XRD patterns of pure CePO4 and CePO4−CeO2 composite nanomaterials synthesized by controlling the Ce3+/PO43− ratio using hydrothermal method are shown in Figure 1 The Ce3+/PO43− ratio has been varied over a broad range in order to understand the growth mechanism and crystalline nature of CePO4. The samples are labeled as CP1:1, CP1:2, CP1:5, CP2:1, CP5:1, and CP10:1 according to the Ce3+/PO43− molar ratio. At an equimolar ratio (CP1:1), the obtained XRD pattern resembles a monazite type B

DOI: 10.1021/acs.inorgchem.8b02423 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 1. Crystal Structure, Average Crystallite Size, Lattice Strain, and Phase Percent of CeO2 Present in Various CePO4 Nanozymes

monoclinic phase of the cerium phosphate (m-CP) matching with the Powder Diffraction File (PDF) no. 01−073−6439 (International Centre for Diffraction Data (ICDD)) with a space group of (P21/c1). By increasing the phosphate concentration (CP1:2 and CP1:5), no change in the peak position was observed; however, variation in the peak intensities of (021) and (212̅) planes indicate preferential growth of CePO4 along particular directions at higher phosphate concentration. Particularly, the intensity of the (212̅) peak increases gradually, while the intensity of (021) was suppressed. An equimolar or higher PO43− anionic concentration stabilized the material in monoclinic phase in the as-prepared condition without any post heat treatments. Under phosphate-deficient condition (CP2:1), CePO4 crystallized in a rhabdophane type hexagonal close packed structure (h-CP) with a space group of P6222 (PDF no. 01−075−1880, ICDD). The thermal analyses of hexagonal and monoclinic CePO4 are shown in Figure S1. On increasing the Ce3+/PO43− ratio further (CP5:1 and CP10:1), additional peaks in the XRD pattern were observed that are not related to monoclinic or hexagonal structure of cerium phosphate. The additional peaks observed at 28.51, 33.03, 47.43, 56.37, and 59.17° positions can be attributed to (111), (200), (220), (311), and (222) planes, respectively, of cubic fluorite structured ceria (f m3m) matching with PDF no. 01−073−6328 (ICDD).51 The peak intensity of the ceria gradually increases on increasing the cerium concentration, and the (111) peak corresponding to CeO2 becomes dominant at CP10:1. Apart from ceria and cerium phosphate phases, no sign of other secondary phases like cerium metaphosphate or cerium polyphosphates were observed, which indicates the successful fabrication of CePO4−CeO2 composite nanostructure. Phase analysis (FullProf 3.0) of samples CP10:1 and CP5:1 revealed a phase percentage of 14.83 and 8.45% of CeO2 present in the samples, respectively. Table 1 shows the mean crystallite size and lattice strain of the cerium phosphate nanorods using Williamson−Hall equation:52 kλ + 4ε sin θ D

crystal structure

mean crystallite size (nm)

lattice strain (ε)

CP10:1 CP5:1 CP2:1 CP1:1 CP1:2 CP1:5

h-CP + CeO2 h-CP + CeO2 h-CP m-CP m-CP m-CP

15.72 ± 0.12 16.2 ± 0.37 18.09 ± 0.19 19.87 ± 0.12 19.53 ± 0.43 20.14 ± 0.29

−0.00347 −0.00395 0.00129 0.00278 0.00261 0.00204

% CeO2 14.83% 6.45%

where β is the full width at half-maxima (fwhm), k is the Scherrer’s constant (k = 0.91), ε is the lattice strain, θ is the Bragg’s angle, λ is the wavelength of X-ray (0.14517 nm), and D is the mean crystallite size (nm). Figure 1c shows the Williamson−Hall plot used for the calculation of mean crystallite size and lattice strain of the samples. Both hexagonal and monoclinic samples exhibit a mean crystallite size of 15−20 nm. The W−H plots show a positive slope for the pure h-CP and m-CP that indicates the presence of tensile strain, while the negative slope corresponding to the composite structure indicates the compressive strain in the lattice. Also, the compressive strain was found to be marginally increasing with the increase in the CeO2 composition. It is well-known that the crystal structure, composition, morphology, and surface defects can influence the lattice strain. In our case, the lattice mismatch between the CePO4 (hexagonal) and CeO2 (face-centered cubic) in the composite structure can be attributed to the changes observed in the lattice strain compared to that of pure hexagonal and monoclinic phases. 3.2. Transmission Electron Microscopy. The representative TEM images of the hexagonal CP10:1 and monoclinic CP1:1 nanozymes are shown in the Figure 2a−h, along with the corresponding SAED pattern and EDS spectra. Both the samples exhibit a rodlike morphology with an average diameter of 20 nm (approximately) and length ranging from 200 to 500 nm. The 1D rodlike morphology of CP10:1 and CP2:1 is consistent with FE-SEM analysis (Figure S2). The d-spacing values calculated from the HR-TEM image (inset of Figure 2b) showed 0.61 nm which is in close agreement with the d-spacing value of the (100) plane with hexagonal structure. Moreover, the d-spacing value of 0.31 nm observed from the HR-TEM and SAED image corresponds to the (111) plane of ceria phase confirms the formation of composite nanostructure which is in agreement with the XRD results. A d-spacing value of 0.52 nm as shown in Figure 2f can be attributed to the (100) plane of monoclinic CePO4. The CeO2−CePO4 composite phase was observed only when CePO4 crystallized in hexagonal structure, under cerium excess (CP5:1 and CP10:1) conditions. The interaction between the precursors and its concentration controls the growth of crystal structure and morphology. Bao et al. obtained hexagonal CePO4 rodlike morphology with an average diameter of 20−30 nm at a molar ratio PO43−/Ce3+ of 10. When the ratio increased to 600, the nanorods aggregate to form monoclinic phase with an average diameter of 60−70 nm.53 However, in our case, the phosphate-rich conditions only stimulate the growth of monoclinic structure rather than the stable low-temperature phase of hexagonal structure. The difference in the observation can be ascribed to the differences in the temperature and concentration of PO43−. However, the mechanism of structural

Figure 1. XRD pattern of cerium phosphate samples showing hexagonal h-CP, ceria-CP composite (a), monoclinic m-CP (b), Williamson−Hall plot (selected samples) for the calculation of average crystallite and lattice strain (c), and crystal structure of CePO4 (monoclinic and hexagonal) and cubic CeO2 (d).

β cos θ =

sample code

(1) C

DOI: 10.1021/acs.inorgchem.8b02423 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Low- (a) and high-resolution (b) TEM image of the cerium phosphate composite nanorods with its corresponding d-spacing values. The inset in (b) shows the growth of hexagonal cerium phosphate along [100] direction (c) SAED pattern and (d) EDS spectra of the CePO4−CeO2 composite nanorods, (e−h) show the monoclinic CePO4 nanorods and corresponding SAED and EDS patterns.

Figure 3. Possible mechanism for the growth of h-CP, m-CP, and CePO4−CeO2 composite nanorods. At CP5:1 and CP10:1, the cerium ions reacts with the dissolved oxygen to form CeO2 phase in addition to CePO4.

transformation and composite structure formation based on the precursor concentration can be explained from the available literature as follows. The dissolution of Ce3+ and PO43− from the precursors in the reaction media are attracted by Coulombic interaction which leads to the formation of CePO4. The inset of Figure 2b indicates the hexagonal CePO4 nanorods grown along the [100] direction where the Ce3+ and PO43− tetrahedron are alternatively arranged, providing a net charge and strong polarity leads to the attraction of more ions. However, in CP1:1, the excess PO43− ions absorbed on the stable hexagonal CePO4 at low temperature creates metastable phase which increase the electrostatic potential at the surface. The rearrangement of the atoms becomes essential in order to minimize the surface energy which leads to the growth of monoclinic structure. On increasing the PO43− concentration further to CP1:5 and CP1:10, the oriented growth pattern as observed from the XRD pattern is evident in that the system tends to minimize the

surface energy by growing along other planes. However, the possibility of direct formation of monoclinic seeds cannot be excluded as the excess PO43− in the medium might increase the supersaturation of the solution which could alter the reaction mechanism. At a very low PO43− concentration, formation of hexagonal phase is the natural step, but the excess Ce3+ ions in the media reacts with the residual dissolved oxygen in the reaction media is the only way to form ceria phase (Figure 3). In a very similar experimental condition, Li et al. demonstrated that the Ce3+ ions in the solution react initially with HPO42− to form CePO4 by homogeneous nucleation.17 Once the PO43− ions are depleted, Ce3+ reacts with the dissolved oxygen to form ceria. Ce3 + + HPO4 2 − → CePO4 ↓ + H+

(2)

4Ce3 + + 6H 2O + O2 → 4CeO2 ↓ + 12H+

(3)

However, under PO43−-rich condition, all the Ce3+ ions in the solution are depleted; hence, the formation of CeO2 phase was D

DOI: 10.1021/acs.inorgchem.8b02423 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry not observed under Ce3+-deficient conditions such as CP1:1, CP1:2, and CP1:5. 3.3. Raman Spectra. The Raman spectra was recorded to investigate the structural aspects of the samples and is shown in Figure 4. The bands at 576 cm−1 (Ag) and 624 cm−1 (Ag, Bg) at

Figure 5. UV−vis spectra of cerium phosphate with hexagonal h-CP, ceria-CP composite (a) and monoclinic m-CP (b).

nm originates from the 2F5/2 of Ce3+ to the 5d states of the Ce3+ levels split by the crystal field in the hexagonal and monoclinic structure.50 A broad absorbance band in the region between 320 to 450 nm were also observed (CP5:1 and CP10:1) corresponding to the Ce4+ to O2− charge transfer transitions emerge from the ceria.26 The broad absorption extending up to 450 nm is also usually observed in the pure CePO4 phases.55 However, the absorption of the broad peak increases with the increase in the ceria content (relative to the 275 nm band) which is consistent with XRD and Raman analysis. For monoclinic CePO4, we observed an additional peak at 257 nm. However, in the monoclinic phase, the broad humplike absorption band centered around 360 nm decreases with the increase in the phosphate concentration. Further photoluminescence spectra and the band gap values estimated for the samples are shown in Figures S4 and S5 3.5. X-ray Photoelectron Spectra. The X-ray photoelectron spectra of the CePO4−CeO2 composite and CePO4 nanorods are shown in Figures 6 and S6. The valence state of cerium ions on the surface of the nanorods can be deduced from the X-ray photoelectron spectra. The Ce 3d spectrum of CP2:1 showed four components at 881.5 eV (vo), 885.5 eV (v′), 900.4 eV (uo), and 904.1 eV (u′) corresponding to the main peaks and satellites of Ce3+ state of cerium phosphate. In addition to the Ce3+ lines in CP10:1, energy states located at 883.4 eV (v), 887.5 eV (v′′), 898.8 eV (v′′′), 901.8 eV (u), 907.1 eV (u′′), and 916.7 eV (u′′′) can be attributed to Ce4+ state. Generally, the valence state of cerium is trivalent in CePO4 and tetravalent in CeO2.56,57 It can be observed that the mixed oxidation state of Ce3+/Ce4+ in CP10:1 emerges from the presence of cerium oxide in the CePO4−CeO2 composite phase which is in agreement with XRD, Raman and UV−visible absorption spectra. The % Ce3+ and % Ce4+ species on the surface of the CePO4−CeO2 composite nanorods were estimated using the following equations:58

Figure 4. Raman spectra of cerium phosphate samples showing hexagonal h-CP, ceria-CP composite (a), and monoclinic m-CP (b).

the low-frequency region can be assigned to the symmetric bending vibrations (v4) of PO43− bonds of CePO4. The weak bands seen around 310 and 396 cm−1 (Bg) and 412 and 466 cm−1 (Ag) can be attributed to the asymmetric bending vibrations (v2) of PO43− bonds. Similarly, in the high-frequency region, an intense peak at 977 cm−1 (Ag) and weak bands at 1047 cm−1 (Ag) and 1086 cm−1 (Bg) in hexagonal structure can be attributed to the symmetric and asymmetric stretching vibrations of P−O bonds. For CP1:1, CP1:2, and CP1:5, new peaks appear at 969 and 991 cm−1 (Ag) in the high-frequency region which confirms the formation of monoclinic phase. In particular, the band observed at 977 cm−1 corresponding to the PO43− group in the hexagonal structure splits into multiple bands (symmetric P−O mode at 969 cm−1 and asymmetric P− O mode at 991 cm−1) in the monoclinic crystal structure that resulted in the formation of additional peaks in the region (Figure 4b). In hexagonal CePO4, Ce3+ ions are connected to six PO43− tetrahedron, while it is connected to seven PO43− tetrahedron in the monoclinic structure.53 The PO43− tetrahedron has C2 symmetry which is pseudotetrahedral symmetry (Td). However, in the monoclinic structure, the PO43− tetrahedron are arranged in a lower symmetry C1 (Td → C1), which leads to the splitting of the 977 cm−1 band into multiple bands in the higher frequency region. Interestingly, the samples CP10:1 and CP5:1 show an intense vibrational band around 460 cm−1 as shown in Figure 4a. In general, ceria nanoparticles with fcc crystal structure exhibit a sharp and intense peak around 464 cm−1 corresponding to the F2g mode of vibrations from Ce−O8 bonds, while the vibrational mode is also possible for the CePO4 structure around the same region corresponding to the bending vibration of PO43− group as shown in the Figure 4a. However, in CePO4, (pure hexagonal and monoclinic phase) the intensity of 466 cm−1 band is always weaker than the dominant 977 and 969 cm−1 peaks of hexagonal and monoclinic structure, respectively. In CP10:1 and CP5:1, the intensity of the 460 cm−1 peak is very high compared to that of the other possible vibrational modes confirming the formation of mixed ceria-CePO4.26,54 The vibrational properties of samples studied using Raman spectra is in good agreement with the FT-IR spectra (Figure S3). 3.4. Optical Absorption. The UV−vis absorption spectra of the CePO4 nanozymes are shown in the Figure 5a,b. The observed peaks at 214, 235, and 275 nm arise from the hexagonal CePO4. The absorption peaks observed between 200 and 280

% Ce 4 + = % Ce3 + =

∑ (A u + A u ″ + A u ‴ + A v + A u ″ + A v ‴) ∑ Ai

(4)

∑ (A uo + A vo + A u′ + A v′) ∑ Ai

(5)

where Auo, Avo, Au, Au′′′, Av, Au′, and Av′ are the area of the respective bands of the Ce3+/Ce4+ peaks and Ai area of integrated peaks. The concentration of Ce3+ and Ce4+ species on CP10:1 were found to be 68.7% and 31.3%, respectively. Generally, in cerium-based compounds, the redox switching between the oxidation state of cerium (Ce3+/Ce4+) plays a crucial role in the catalytic and biological applications.59 E

DOI: 10.1021/acs.inorgchem.8b02423 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. (a) Colorless TMB+H2O2 solution turned blue-green color when incubated with CP10:1 nanorods (100 μg mL−1), (b) the oxidation of chromogenic substrate by H2O2 in the presence of pure and composite nanozymes, (c) pH-dependent measured at room temperature, and (d) temperature-dependent peroxidase-like activity measured at pH 4.0.

respectively, with type-IV adsorption−desorption isotherms. The decrease in the surface area can be attributed to the presence of a higher percentage of ceria on CP10:1 nanozyme. Furthermore, the performance of CP10:1 (sample with best activity (CP10:1)) was tested at different pH and temperature conditions as shown in Figure 7c,d. CP10:1 nanorods showed good activity in the acidic pH range of 2.0−6.0 and gradually decreased at physiological and basic pH conditions, similar to natural enzyme HRP. The CP10:1 nanorods revealed excellent stability in the range of 20−80 °C which is superior compared to the HRP enzyme. 3.7. Kinetics and Mechanism. For further understanding, steady-state kinetic analyses of pure CePO4 and CePO4−CeO2 composite nanozymes were compared using Michaelis−Menten model. Figure 8 shows the Michaelis−Menten kinetics of TMB and H2O2 as substrates in the presence of nanozymes. To calculate the kinetic parameters, the Lineweaver−Burk plot or double-reciprocal plot was (shown as insets in Figure 8) was calculated using

Figure 6. X-ray photoelectron spectra of CePO4 (CP2:1) and CePO4− CeO2 (CP10:1) composite nanorods (a) Ce 3d, (b) O 1s, and (c) P 2p core levels.

The O 1s core level spectra of cerium phosphate nanorods shown in Figure 6b. was deconvoluted using Gaussian distribution. Two peaks at binding energies 530.3 and 532.0 eV were observed for O 1s. The dominant peak at lower binding energy (530.3 eV) can be ascribed to lattice oxygen.27 The higher binding energy peak observed at 532.1 eV may arise from the chemisorbed oxygen or hydroxyl species. The P 2p core level spectra (Figure 6c) of cerium phosphate exhibit two peaks located at 133.5 eV (P 2p1/2) and 132.4 eV (P 2p3/2), characteristic of pentavalent phosphorus (PO43− group) in the lattice.60 3.6. Peroxidase Mimetic Activity and Optimization. The peroxidase-like activity of the CP nanorods were evaluated by a colorimetric method that involves hydrogen peroxide and TMB. When TMB was added to H2O2, the solution remained transparent. Upon addition of CP nanorods, the transparent solution transformed into blue-green color with an absorption maximum at 653 nm as shown in Figure 7a. The color change can be attributed to the formation of charge transfer diamine complex61,62 as shown in the Figure 7a. The incubation of TMB with CePO4 nanorods separately for 30 min could not produce any color change in the solution, which indicates the requirement of both to oxidize TMB. The catalytic performance of selected CP nanorods (CP10:1, CP5:1, and CP2:1) have been compared as shown in Figure 7b. The maximum absorption was observed for CP10:1 compared to other samples indicates the role of CeO2 concentration on the peroxidase-like activity (14.83% in CP10:1 and 6.45% in CP5:1, refer Table 1). To check the role of surface area on the catalytic activity, surface area measurements were carried out. The measured BET surface areas of CP10:1 and CP5:1 were found to be 57 and 63 m2 g−1,

K ij 1 1 1 yzz = m jjj + z V Vmax jk [S] K m zz{ (6) where V is the initial velocity, Vmax is the maximal reaction velocity, [S] represents the concentration of the substrate, and Km is the Michaelis−Menten constant. All the measurements were carried out at room temperature. The estimated kinetic parameters Vmax and Km are listed in Table 2. The Km value obtained for the composite nanozymes CP10:1 (0.236 mM) was found to be lower compared to that of horseradish peroxidase enzyme (HRP) (0.434 mM). A lower Km value indicates good affinity between the enzyme and substrate molecules. The lower Km obtained for CP10:1 compared to those of CP5:1 and CP2:1 indicates excellent affinity of CP10:1 toward TMB substrate.63 However, a higher concentration of hydrogen peroxide is required for the efficient oxidation of TMB molecules for CP10:1 (Vmax = 8.78 × 10−8 M s−1) compared to that for CP2:1 (Vmax = 2.76 × 10−8 M s−1). F

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like activity occurring through reversible transformation between the Ce3+ and Ce4+ sites can be written as follows.64 Ce3 + + H 2O2 + H+ → Ce 4 + + OH• + H 2O −

(7)

OH• + H 2O2 → HO2 + H 2O

(8)

Ce 4 + + HO2− → Ce3 + + O2 + H+

(9)

The generation of hydroxyl radicals in the reaction was confirmed using the fluorescenct emission observed at 424 nm in the presence of terephthalic acid (Figure S7). In our case, composite nanozymes CP10:1 and CP5:1 were enriched with Ce4+ sties due to the presence of ceria which is in perfect agreement with the Ce4+ → O2− transition observed from the optical absorption analysis and enhanced intensity of the F2g mode of vibrations. The Ce4+-enriched nanorods facilitate the charge transfer between the CePO4 (Ce3+) → CeO2 (Ce4+). The enhanced charge transfer interaction between the Ce3+ and Ce4+ sites within the cerium oxide−cerium phosphate nanorods showed better catalytic activity when compared to that of pure CePO4. The mechanism for the enhanced peroxidase-like biomimetic activity of the CePO4−CeO2 nanorods is illustrated in Figure 9.

Figure 8. Steady-state kinetic analysis of the cerium phosphate and cerium phosphate-ceria composite nanozymes at different concentrations of TMB (left) and hydrogen peroxide concentration (right).

Table 2. Kinetic Parameters Calculated from the Lineweaver−Burk Plot Corresponding to the PeroxidaseLike Activity of the Samples Km (m M)

Vmax (× 10−8 M s−1)

sample

CeO2 wt %

TMB

H2O2

TMB

H2O2

CP10:1 CP5:1 CP2:1

14.83 6.45

0.236 0.314 0.437

4.766 2.5118 1.977

8.78 4.57 2.76

29.79 26.44 26.01

Figure 9. Schematic representation the peroxidase-like activity of the CePO4−CeO2 composite nanozymes.

3.8. Hydrogen Peroxide and Glucose Sensing. In order to demonstrate the potential application of CePO4−CeO2 composite material, we demonstrate hydrogen peroxide sensing based on the peroxidase activity of superior nanozyme, CP10:1. Hydrogen peroxide not only is a byproduct of various biological enzymatic reactions but also is involved in intracellular signaling, oxidizing biomolecules, and denaturing proteins resulting in pathological conditions such as oxidative stress, cancer, Alzheimer’s disease, and so on.65 The concentration of hydrogen peroxide was varied over a broad range from 5 to 400 μM. As expected, the absorption maxima gradually increased with respect to H2O2 concentration (as shown in the inset of Figure 10). The absorption maxima plotted against H2O2 concentration show a linear trend up to 150 μM concentration. Further increase in H2O2 concentration leads to the deviation from the straight-line behavior as indicated in Figure 10. Thus, a visually observable color change at different concentrations of hydrogen peroxide can be used to construct a colorimetric biosensor. Moreover, the limit of detection (LOD) of hydrogen peroxide has been evaluated using the relation LOD = 3σ/k, where σ is the standard deviation of absorbance blank and k is the slope of the calibration curve. The LOD value was found to be 2.9 μM for H2O2 sensing. The linearity and LOD resulting from this and other works have been summarized in Table S2. In addition, the

Similarly, comparing the Km values of both TMB and H2O2 substrates also suggest CP10:1 possess the enhanced reaction rate constant. In order to compare the catalytic activity of the samples, the Kcat value of the nanozymes was calculated. The Kcat values obtained for CP10:1 (Kcat = 2.95 × 103 s−1) was found to be around 2- and 3-fold higher than CP5:1 (Kcat = 1.7 × 103 s−1) and CP2:1 (Kcat = 1.09 × 103 s−1), respectively. The results demonstrate that the CePO4−CeO2 composite nanorods exhibit excellent peroxidase activity compared to pure CePO4. Moreover, a comparison table corresponding to various kinetic parameters obtained in the present work and results reported in the literature are shown in Table S1. From the Table 2, the peroxidase-like activity of the samples follow the order CP10:1 > CP5:1> CP2:1 which can be associated with the presence of CeO2. As an individual component, the ceria and CePO4 nanozymes inherently possess peroxidase-like activity due to the presence on the surface of a small amount of Ce4+ and Ce3+ active sites, respectively. However, the relatively lower number of active sites severely limits the redox cycling between the +3 and +4 states in a reversible manner as explained by many authors for ceria and cerium phosphate. The general mechanism for the peroxidaseG

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4. CONCLUSION The CePO4−CeO2 nanorods synthesized by hydrothermal method exhibit excellent peroxidase-like activity. Control of Ce3+/PO43− in the hydrothermal synthesis alters the formation of crystal structure and chemical composition of the end product. Higher concentrations of cerium (CP10:1 and CP5:1) resulted in CePO4−CeO2 composite nanorods, while only pure CePO4 was observed for other ratios. Particularly, at equimolar or lower ratio of Ce3+/PO43−, monoclinic phase was formed, while at higher Ce3+/PO43− ratios, CePO4 crystallized in hexagonal phase. Both pure and composite nanorods exhibit peroxidase-like activity in the presence of H2O2 and TMB. Among the prepared nanozymes, CP10:1 shows superior catalytic activity, and the catalytic activity of the samples follows the order CP10:1 > CP5:1 > CP2:1. The steady-state kinetic analysis based on the Michaelis−Menten model also revealed that CP10:1 exhibits excellent affinity toward the TMB in comparison to those of CP5:1 and CP2:1. The superior peroxidase activity of CePO4−CeO2 composite nanozymes can be ascribed to the enhance redox switching between Ce3+ ↔ Ce4+ sites from the CePO4 and CeO2 lattice, respectively. The colorimetric detection of hydrogen peroxide and glucose showed a linear response around 150 μM concentration with LODs of 2.9 and 4.1 μM, respectively. Thus, our present study demonstrates the improved peroxidase mimetic activity of CePO4−CeO2 composite nanorods and their application in antioxidant and biosensing applications.

Figure 10. Linear calibration curve for H2O2 detection using CP10:1 nanorods. The absorption maxima monitored at 653 nm (from inset) for different concentration of hydrogen peroxide varied from 5 to 400 μM. Linear trend was observed up to 150 μM concentration of H2O2 with χ2 = 0.998.

selectivity of the hydrogen peroxide sensor was carried out in the presence of various interfering metal ions like K+, Na+, Ca2+, Mg2+, Ni2+, Fe2+, and Zn2+ and glycine (Figure S8). The sensor showed good selectivity toward hydrogen peroxide despite the presence of interferences. In addition to hydrogen peroxide sensing, CePO4−CeO2 composite nanorods were used for glucose sensing in the range from 1 to 500 μM by monitoring the absorbance at 653 nm (Figure 11a). It is well-known that the oxidation of glucose in the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02423. Details of TG-DTA isotherm, FE-SEM images, EDAX spectra, FT-IR, Photoluminescence spectra, Tauc’s Plot, X-ray photoelectron spectra (survey scan), detection of hydroxyl radicals and selectivity of sensor (PDF)

■

Figure 11. (a) Linear calibration curve for glucose detection using CP10:1 nanorods. The absorption maxima monitored at 653 nm for different concentration of glucose from 0 to 5 mM. Linear trend was observed from 0 to 100 μM concentration of glucose with χ2 = 0.992 and (b) the selectivity of CP10:1 toward 5 mM of galactose, maltose, fructose, sucrose, and glucose sensing.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-4132654976. ORCID

G. Vinothkumar: 0000-0002-4942-4440 Notes

presence of oxygen can produce gluconic acid and hydrogen peroxide.66 The formation of hydrogen peroxide subsequently oxidized the TMB molecules in the presence of nanozyme and changes its color from transparent to blue as shown in Figure 9. As illustrated in the inset of Figure 11a, the absorbance changes linearly up to 100 μM concentration of glucose. Furthermore, the LOD was found to be 4.1 μM for glucose sensing. The LOD value and detection range of CePO4−CeO2 nanorods toward glucose sensing are compared in Table S3. The selectivity of CePO4−CeO2 nanorods toward glucose sensing was tested using different sugars like galactose, maltose, fructose,and sucrose as shown in Figure 11b. The inset of Figure 11b shows the color change with respect to different sugars. The CePO4−CeO2 composite nanorods showed excellent sensitivity and selectivity toward glucose compared to that toward other sugars.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support provided through Startup grant (PU/PC/Start-Up Grant/2011-12/312) of Pondicherry University. We also thank Central Instrumentation Facility (CIF), Pondicherry University, for the characterization of the samples.

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REFERENCES

(1) Gao, L.; Zhuang, J.; Nie, L.; Zhang, J.; Zhang, Y.; Gu, N.; Wang, T.; Feng, J.; Yang, D.; Perrett, S.; Yan, X. Intrinsic Peroxidase-like Activity of Ferromagnetic Nanoparticles. Nat. Nanotechnol. 2007, 2, 577−583. (2) Tan, B.; Zhao, H.; Wu, W.; Liu, X.; Zhang, Y.; Quan, X. Fe3O4AuNPs Anchored 2D Metal-Organic Framework Nanosheets With DNA Regulated Switchable Peroxidase-Like Activity. Nanoscale 2017, 9, 18699−18710. H

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GdPO4/CePO4:Tb Nanophosphors and Their Photoluminescence Properties. Appl. Surf. Sci. 2013, 266, 22−26. (20) Zhai, Y.; Zhang, Y.; Qin, F.; Yao, X. An Electrochemical DNA Biosensor for Evaluating the Effect of Mix Anion in Cellular Fluid on the Antioxidant Activity of CeO2 Nanoparticles. Biosens. Bioelectron. 2015, 70, 130−136. (21) Kang, J.; Byun, S.; Nam, S.; Kang, S.; Moon, T.; Park, B. Synergistic Improvement of Oxygen Reduction Reaction on Gold/ cerium-Phosphate Catalysts. Int. J. Hydrogen Energy 2014, 39, 10921− 10926. (22) Vickers, S. M.; Gholami, R.; Smith, K. J.; MacLachlan, M. J. Mesoporous Mn- and La-Doped Cerium Oxide/cobalt Oxide Mixed Metal Catalysts for Methane Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 11460−11466. (23) Chen, G.; Zhao, H.; Rosei, F.; Ma, D. Effect of Redox Reaction Products on the Luminescence Switching Behavior in CePO4:Tb Nanorods. J. Phys. Chem. C 2013, 117, 10031−10038. (24) Pusztai, P.; Haspel, H.; Tóth, I. Y.; Tombácz, E.; László, K.; Kukovecz, Á .; Kónya, Z. Structure-Independent Proton Transport in Cerium(III) Phosphate Nanowires. ACS Appl. Mater. Interfaces 2015, 7, 9947−9956. (25) Meng, L.; Yang, L.; Zhou, B.; Cai, C. Cerium Phosphate Nanotubes: Synthesis, Characterization and Biosensing. Nanotechnology 2009, 20, 035502. (26) Vinothkumar, G.; Amalraj, R.; Babu, K. S. Fuel-Oxidizer Ratio Tuned Luminescence Properties of Combustion Synthesized Europium Doped Cerium Oxide Nanoparticles and Its Effect on Antioxidant Properties. Ceram. Int. 2017, 43, 5457−5466. (27) Kitsuda, M.; Fujihara, S. Quantitative Luminescence Switching in CePO4:Tb by Redox Reactions. J. Phys. Chem. C 2011, 115, 8808− 8815. (28) Chen, H.; Ren, J. Selective Detection of Fe2+ by Combination of CePO4:Tb3+ nanocrystal−H2O2 Hybrid System with Synchronous Fluorescence Scan Technique. Analyst 2012, 137, 1899−1903. (29) He, W.; Liu, Y.; Yuan, J.; Yin, J.; Wu, X.; Hu, X.; Hu, X.; Zhang, K.; Liu, J.; Chen, C.; et al. Au@Pt Nanostrucutres as Oxidase and Peroxidase mimetics for Use in Immunoassays. Biomaterials 2011, 32, 1139−1147. (30) Nangia, Y.; Kumar, B.; Kaushal, J.; Suri, C. R. Palladium@Gold Bimetalliec Nanostrucutres as Peroxidase mimic for Development of Sensitive Fluoroimmunoassay. Anal. Chim. Acta 2012, 751, 140−145. (31) Ahmed, S. R.; Kim, J.; Suzuki, T.; Lee, J.; Park, E. Y. Enhanced Cataytic Activity of Gold Nanoparticle-Carbon Nanotube Hybrids for Influenza Virus Detection. Biosens. Bioelectron. 2016, 85, 503−508. (32) Li, Z.; Yang, X.; Yang, Y.; Tan, Y.; He, Y.; Liu, M.; Liu, X.; Yuan, Q. Peroxidase-Mimicking Nanozyme With Enhanced Activity and High Stability Based on Metal-Support Interactions. Chem. - Eur. J. 2018, 24, 409−415. (33) Artiglia, L.; Agnoli, S.; Paganini, M. C.; Cattelan, M.; Granozzi, G. TiO2@CeOX Core−Shell Nanoparticles as Artificial Enzymes with Peroxidase-Like Activity. ACS Appl. Mater. Interfaces 2014, 6, 20130− 20136. (34) Jampaiah, D.; Srinivasa Reddy, T.; Kandjani, A. E.; Selvakannan, P. R.; Sabri, Y. M.; Coyle, V. E.; Shukla, R.; Bhargava, S. K. Fe-Doped CeO2 Nanorods for Enhanced Peroxidase-like Activity and Their Application towards Glucose Detection. J. Mater. Chem. B 2016, 4, 3874−3885. (35) Huang, F.; Wang, J.; Chen, W.; Wan, Y.; Wang, X.; Cai, N.; Liu, J.; Yu, F. Synergistic Peroxidase-Like Activity of CeO2-Coated Hollow Fe3O4 Nanocomposites as an Enzyme Mimic for Low Detection Limit of Glucose. J. Taiwan Inst. Chem. Eng. 2018, 83, 40−49. (36) Jampaiah, D.; Reddy, T. S.; Coyle, V. E.; Nafady, A.; Bhargava, S. K. Co3O4@CeO2 Hybrid Flower Like Microspheres: a Strong Synergistic Peroxidase-Mimicking Artificial Enzyme With High Sensitivity for Glucose Detection. J. Mater. Chem. B 2017, 5, 720−730. (37) Radhakrishnan, S.; Kim, S. J. An Enzymatic Biosensor for Hydrogen Peroxide Based One-Pot Preparation of CeO2-rediced Graphene Oxide Nanocomposite. RSC Adv. 2015, 5, 12937−12943.

(3) Jia, H.; Yang, D.; Han, X.; Cai, J.; Liu, H.; He, W. Peroxidase-Like Activity Of Co3O4 Nanoparticles Used for Biodetection and Evaluation of Antioxidant Behavior. Nanoscale 2016, 8, 5938−5945. (4) Chen, T. M.; Tian, X. M.; Huang, L.; Xiao, J.; Yang, G. W. Nanodiamonds as pH-Switchable Oxidation and Reduction Catalysts With Enzyme-Like Activities for Immunoassay and Antioxidant Applications. Nanoscale 2017, 9, 15673−15684. (5) Zhang, W.; Sun, Y.; Lou, Z.; Song, L.; Wu, Y.; Gu, N.; Zhang, Y. InVitro Cytotoxicity Evaluation of Graphene Oxide from the PeroxidaseLike Activity Perspective. Colloids Surf., B 2017, 151, 215−223. (6) Cho, S.; Shin, H. Y.; Kim, M. I. Nanohybrids Consisting of Magnetic Nanoparticles and Gold Nanoclusters as Effective Peroxidase Mimics and Their Application for Colorimetric Detection of Glucose. Biointerphases 2017, 12, 01A401. (7) Wang, W.; Jiang, X.; Chen, K. CePO4:Tb,Gd Hollow Nanospheres as Peroxidase Mimic and Magnetic-Fluorescent Imaging Agent. Chem. Commun. (Cambridge, U. K.) 2012, 48, 6839−6841. (8) Tian, Z.; Li, J.; Zhang, Z.; Gao, W.; Zhou, X.; Qu, Y. Highly Sensitive and Robust Peroxidase-like Activity of Porous Nanorods of Ceria and Their Application for Breast Cancer Detection. Biomaterials 2015, 59, 116−124. (9) Liu, Q.; Ding, Y.; Yang, Y.; Zhang, L.; Sun, L.; Chen, P.; Gao, C. Enhanced Peroxidase-like Activity of Porphyrin Functionalized Ceria Nanorods for Sensitive and Selective Colorimetric Detection of Glucose. Mater. Sci. Eng., C 2016, 59, 445−453. (10) Lv, C.; Di, W.; Liu, Z.; Zheng, K.; Qin, W. Luminescent CePO4:Tb Colloids for H2O2 and Glucose Sensing. Analyst 2014, 139, 4547−4555. (11) Tanaka, S.; Kaneti, Y. V.; Bhattacharjee, R.; Islam, M. N.; Nakahata, R.; Abdullah, N.; Yusa, S.; Nguyen, N. T.; Shiddiky, M. J. A.; Yamauchi, Y.; Hossain, M. S. A. Mesoporous Iron Oxide Synthesized Using Poly(styrene-b-acrylicacid-b-ethylene glycol) Block Copolymer Micelles as Templates for Colorimetric and Electrochemical Detection of Glucose. ACS Appl. Mater. Interfaces 2018, 10, 1039−1049. (12) Dezfuli, A. S.; Ganjali, M. R.; Norouzi, P. Facile Sonochemical Synthesis and Morphology Control of CePO4 Nanostructures via an Oriented Attachment Mechanism: Application as Luminescent Probe for Selective Sensing of Pb2+ Ion in Aqueous Solution. Mater. Sci. Eng., C 2014, 42, 774−781. (13) Han, K. N.; Choi, J.-S.; Kwon, J. Gold Nanozyme-Based Paper Chip for Colorimetric Detection of Mercury Ions. Sci. Rep. 2017, 7, 2806. (14) Bhattacharjee, R.; Tanaka, S.; Moriam, S.; Masud, M. K.; Lin, J.; Alshehri, S. M.; Ahamad, T.; Salunkhe, R. R.; Nguyen, N. T.; Yamauchi, Y.; Hossain, M. S. A.; Shiddiky, M. J. A. Porous Nanozymes: Peroxidase-Mimetic Activity of Mesoporous Iron Oxide for Calorimetric and Electrochemical Detection of Global DNA Methylation. J. Mater. Chem. B 2018, 6, 4783−4791. (15) Maji, S. K.; Mandal, A. K.; Nguyen, K. T.; Borah, P.; Zhao, Y. Cancer Cell Detection and Therapeutics Using Peroxidase-Active Nanohybrid of Gold Nanoparticle-Loaded Mesoporous Silica-Coated Graphene. ACS Appl. Mater. Interfaces 2015, 7, 9807−9816. (16) Masud, M. K.; Yadav, S.; Islam, M. N.; Nguyen, N. T.; Salomon, C.; Kline, R.; Alamri, H. R.; Alothman, Z. A.; Yamauchi, Y.; Hossain, M. S. A.; Shiddiky, M. J. A. Gold-Loaded Nanoporous Ferric Oxide Nanocubes with Peroxidase-Mimicking Activity for Electrocatalytic and Calorimetric Detection of Autoantibody. Anal. Chem. 2017, 89, 11005−11013. (17) Li, G.; Chao, K.; Peng, H.; Chen, K.; Zhang, Z. Facile Synthesis of CePO4 Nanowires Attached to CeO2 Octahedral Micrometer Crystals and Their Enhanced Photoluminescence Properties. J. Phys. Chem. C 2008, 112, 16452−16456. (18) Fang, J.; Evans, C. W.; Willis, G. J.; Sherwood, D.; Guo, Y.; Lu, G.; Raston, C. L.; Iyer, K. S. Sequential Microfluidic Flow Synthesis of CePO4 Nanorods Decorated With Emission Tunable Quantum Dots. Lab Chip 2010, 10, 2579−2582. (19) Fan, Y. Y.; Hu, Z. C.; Yang, J.; Zhang, C.; Zhu, L. UltrasonicAssisted Synthesis of Core-Shell Structure CePO4:Tb/GdPO4 and I

DOI: 10.1021/acs.inorgchem.8b02423 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (38) Singh, S. Cerium Oxide Based Nanozymes: Redox Phenomenon at Biointerfaces. Biointerphases 2016, 11, 04B202. (39) Yang, R.; Qin, J.; Li, M.; Liu, Y.; Li, F. Redox Hydrothermal Synthesis of Cerium Phosphate Microspheres with Different Architectures. CrystEngComm 2011, 13, 7284−7292. (40) Guan, M.; Sun, J.; Shang, T.; Zhou, Q.; Han, J.; Ji, A. A Facile Synthesis of Cerium Phosphate Nanofiber by Solution-Solid Method. J. Mater. Sci. Technol. 2010, 26, 45−48. (41) Ye, C.; Guo, H.; Zhang, M.; Zhu, H.; Hu, J.; Lai, X.; Li, A. Synthesis and EnhancedElectrochemical Property of Au-Doped Cerium Phosphate Nanowires. Mater. Lett. 2014, 131, 141. (42) Zhang, F.; Wong, S. S. Ambient Large-Scale Template-Mediated Synthesis of High-Aspect Ratio Single-Crystalline, Chemically Doped Rare-Earth Phosphate Nanowires for Bioimaging. ACS Nano 2010, 4, 99−112. (43) Zhang, Y.; Guan, H. Hydrothermal Synthesis and Characterization of Hexagonal and Monoclinic CePO4 Single-Crystal Nanowires. J. Cryst. Growth 2003, 256, 156−161. (44) Ansari, A. A. Effect of Surface Coating on Structural and Photophysical Properties of. Mater. Sci. Eng., B 2017, 222, 43−48. (45) Chen, G.; Sun, S.; Zhao, W.; Xu, S.; You, T. Template Synthesis and Luminescence Properties of CePO4:Tb Nanotubes. J. Phys. Chem. C 2008, 112, 20217−20221. (46) Vinothkumar, G.; Lalitha, A I.; Arunkumar, P.; Ahmed, W.; Ryu, S.; Cha, S. W.; Babu, K. S. Strucutre Dependent Luminescence, Peroxidase Mimetic and Hydrogen Peroxide Sensing of Samarium Doped Cerium Phosphate Nanorods. J. Mater. Chem. B 2018, 6, 6559− 6571. (47) Sumaletha, N.; Rajesh, K.; Mukundan, P.; Warrier, K. G. K. Environmentally Benign Sol−gel Derived Nanocrystalline Rod Shaped Calcium Doped Cerium Phosphate Yellow-Green Pigment. J. Sol-Gel Sci. Technol. 2009, 52, 242−250. (48) Verma, S.; Bamzai, K. K. Preparation of Cerium Orthophosphate Nanosphere by Coprecipitation Route and its Structural, Thermal, Optical, and Electrical Characterization. J. Nanopart. 2014, 2014, 1−12. (49) Xing, Y.; Li, M.; Davis, S. A.; Mann, S. Synthesis and Characterization of Cerium Phosphate Nanowires in Microemulsion Reaction Media. J. Phys. Chem. B 2006, 110, 1111−1113. (50) Palma-Ramírez, D.; Domínguez-Crespo, M. A.; Torres-Huerta, A. M.; Dorantes-Rosales, H.; Ramírez-Meneses, E.; Rodríguez, E. Microwave-Assisted Hydrothermal Synthesis of CePO4 Nanostructures: Correlation between the Structural and Optical Properties. J. Alloys Compd. 2015, 643, S209−S218. (51) Al-Agel, F. A.; Al-Arfaj, E.; Al-Ghamdi, A. A.; Stein, B. D.; Losovyj, Y.; Bronstein, L. M.; Shokr, F. S.; Mahmoud, W. E. Structure and Magnetic Properties of Diluted Magnetic Metal Oxides Based on Cu-doped CeO2 Nanopowders. Ceram. Int. 2015, 41, 1115−1119. (52) Mahmoud, W. E.; Al-Ghamdi, A. A.; Al-Heniti, A.; Al-Ameer, S. The Influence of Temperature on the Structure of Cd-doped ZnO Nanopowders. J. Alloys Compd. 2010, 491, 742−746. (53) Bao, J.; Yu, R.; Zhang, J.; Yang, X.; Wang, D.; Deng, J.; Chen, J.; Xing, X. Low-Temperature Hydrothermal Synthesis and Structure Control of Nano-Sized CePO4. CrystEngComm 2009, 11, 1630−1634. (54) Pusztai, P.; Simon, T.; Kukovecz, Á ; Kónya, Z. Structural Stability Test of Hexagonal CePO4 Nanowires Synthesized at Ambient Temperature. J. Mol. Struct. 2013, 1044, 94−98. (55) Tang, C.; Bando, Y.; Golberg, D.; Ma, R. Cerium Phosphate Nanotubes: Synthesis, Valence State, and Optical Properties. Angew. Chem. 2005, 117, 582−585. (56) Kumar, A.; Babu, S.; Karakoti, A. S.; Schulte, A.; Seal, S. Luminescence properties of Europium-doped Cerium Oxide nanopartilces: Role of vacancy and oxidation States. Langmuir 2009, 25, 10998−11007. (57) Glorieux, B.; Berjoan, R.; Matecki, M.; Kammouni, A.; Perarnau, D. XPS analyses of Lanthanides Phosphates. Appl. Surf. Sci. 2007, 253, 3349−3359. (58) Asuvathraman, R.; Gnanasekar, K. I.; Clinsha, P. C.; Ravindran, T. R.; Govindan Kutty, K. V. Investigations on the charge compensation

on Ca and U Substitution in CePO4 by using XPS, XRD and Raman Spectroscopy. Ceram. Int. 2015, 41, 3731−3739. (59) Gupta, A.; Das, S.; Neal, C. J.; Seal, S. Controlling the surface chemistry of Cerium Oxide nanoparticles for biological Applications. J. Mater. Chem. B 2016, 4, 3195−3202. (60) Celebioglu, A.; Vempati, S.; Ozgit-Akgun, C.; Biyikli, N.; Uyar, T. Water-Soluble Non-Polymeric Electrospun Cyclodextrin Nanofiber Template for the Synthesis of Metal Oxide Tubes by Atomic Layer Deposition. RSC Adv. 2014, 4, 61698−61705. (61) Liu, Y.; Zhu, G.; Yang, J.; Yuan, A.; Shen, X. Peroxidase-Like Catalytic Activity of Ag3PO4 Nanocrystals Prepared by a Colloidal Route. PLoS One 2014, 9, e109158. (62) Liu, Z.; Wang, J.; Li, Y.; Hu, X.; Yin, J.; Peng, Y.; Li, Z.; Li, Y.; Li, B.; Yuan, Q. Near-Infrared Light Manipulated Chemoselectvie Reductions Enabled by an Upconversional Supersandwich Nanostrucutre. ACS Appl. Mater. Interfaces 2015, 7, 19416−19423. (63) Zhao, H.; Dong, Y.; Jiang, P.; Wang, G.; Zhang, J. Highly Dispersed CeO2 on TiO2 Nanotube: A Synergistic Nanocomposite with Superior Peroxidase-Like Activity. ACS Appl. Mater. Interfaces 2015, 7, 6451−6461. (64) Xu, C.; Qu, X. Cerium Oxide Nanoparticle: A Remarkably Versatile Rare Earth Nanomaterial for Biological Applications. NPG Asia Mater. 2014, 6, e90. (65) Lisanti, M. P.; Martinez-Outschoorn, U. E.; Lin, Z.; Pavlides, S.; Whitaker-menezes, D.; Pestell, R. G.; Howell, A.; Sotgia, F. Hydrogen Peroxide Fuels Aging, Inflammation, Cancer Metabolism and Metastasis. Cell Cycle 2011, 10, 2440−2449. (66) Dong, Y.; Zhang, H.; Rahman, Z. U.; Su, L.; Chen, X.; Hu, J.; Chen, X. Graphene Oxide-Fe3O4 Magnetic Nanocomposites with Peroxidase-Like Activity for Calorimetric Detection of Glucose. Nanoscale 2012, 4, 3969−3976.

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