Modulating the Catalytic Activity of Cerium Oxide Nanoparticles with the Anion of the Precursor Salt

Abstract

In this work, we tested our hypothesis that surface chemistry and antioxidant properties of cerium nanoparticles (CNPs) are affected by presence of counterions. We first employed various precursor cerium (III) (Ce(III)) salts with different counterions (acetate, nitrate, chloride, sulfate) to synthesize CNPs following the same wet chemical methodology. Electron spin resonance (ESR) studies provided evidence for the formation of radicals from counterions (e.g., NO₃•^2- from reduction of NO₃^- in CNPs synthesized from Ce(III) nitrate). Physicochemical properties of these CNPs, e.g., dispersion stability, hydrodynamic size, signature surface chemistry, SOD-mimetic activity, and oxidation potentials were found to be significantly affected by the anions of the precursor salts. CNPs synthesized from Ce(III) nitrate and Ce(III) chloride exhibited higher extent of SOD-mimetic activities. Therefore, these CNPs were studied extensively employing in-situ UV-Visible spectroelectrochemistry and changing the counterion concentrations affected the oxidation potentials of these CNPs. Thus, the physicochemical and antioxidant properties of CNPs can be modulated by anions of the precursor. Furthermore, our ESR studies present evidence of the formation of guanine cation radical (G•⁺) in 5'-dGMP via UV-photoionization at 77 K in the presence of CNPs synthesized from Ce(III) nitrate and chloride and CNPs act as the scavenger of radiation-produced electrons.

Figure 1

(A) The image demonstrates the solutions of CNPs synthesized using cerium acetate (CNPs-Ac), cerium ammonium nitrate (CNPs-AmN), cerium chloride (CNPs-Cl), cerium nitrate (CNPs-N), and cerium sulfate (CNPs-S) as precursors. All of the CNPs solutions appear to be stable as they do not exhibit any turbidity or precipitation except for CNPs-S which shows turbidity and precipitation indicating dispersion instability. (B) Transmission electron microscopy (TEM) images of CNPs-Ac, CNPs-AmN, CNPs-Cl, CNPs-N and CNP-S. The dimension of the bar represents a scale of 20 nm in all the TEM images. The images represent the crystallinity of CNPs synthesized from all different precursors. (C) The size and (D) the zeta potential of the nanoparticles have been investigated over a period of 30 days to observe the dispersion stability of the nanoparticles using dynamic light scattering (DLS).

Figure 2

(A) The UV-visible spectroscopy of different CNPs have been presented after 30 days of their synthesis. This indicates that the ratio of Ce³⁺/Ce⁴⁺ are more dominant in case of CNPs-Ac, CNPs-Cl and CNPs-N. In case of both CNPs-AmN and CNPs-S, the Ce³⁺ peak intensity is more prominent compared to Ce⁴⁺ peak which is almost insignificant in intensity. (B) The XPS results indicate that the %Ce³⁺ on the surface of CNPs is higher in case of CNPs-N followed by CNPs-AmN. CNPs-Cl and CNPs-S have approximately the same amount of Ce³⁺ concentration on the surface of CNPs. Furthermore, CNPs-Ac has the least amount of Ce³⁺ on the surface of CNPs.

Figure 3

(A) The SOD-mimetic activities of CNPs synthesized using different precursors, showing that CNPs-N and CNPs-Cl have the highest SOD-mimetic activity followed by those of CNPs-Ac. The CNPs-S and CNPs-AmN do not show high SOD-mimetic activity. Absorbance of WST-1 formazan dye at 440 nm in a 96 well plate is plotted against the time course of the reaction. (B) The % of antioxidant capacities of CNPs do correlate with the SOD-mimetic activities of CNPs thereby indicating the highest SOD-mimetic activities shown by CNPs-N and CNPs-Cl in (A).

Figure 4

Michaelis–Menten kinetics of CNPs (CNPs-Ac, CNPs-AmN, CNPs-Cl, CNPs-N and CNPs-S). The SOD-mimetic reactions were carried out by employing different concentrations of CNPs such as 0.1 mM, 0.5 mM, 1 mM and 1.5 mM.

Figure 5

(A) The SOD-mimetic activity of CeNO₃, NaNO₃ and CNPs-N. (B) The SOD-mimetic activity of CeCl₃, NaCl and CNPs-Cl. Note that the lower the curve, the higher is the SOD mimetic activity.

Figure 6

SOD-mimetic activity of CNPs-N with increasing concertation of NO₃⁻ concentration (by the addition of NaNO₃ at different concentrations; 0.03 mM, 0.3 mM, 3 mM and 30 mM) and of CNPs-Cl with increasing concertation of Cl⁻ concentration (by the addition of NaCl at different concentrations; 0.03 mM, 0.3 mM, 3 mM and 30 mM) at the 20^th minute of SOD assay.

Figure 7

The UV-Visible spectro-electrochemical results: (A) Open circuit potentials (OCP) values of CNPs-Cl with changing concentrations of Cl⁻ (0, 0.3 mM, 3 mM and 30 mM) and of CNPs-N with changing concentrations of NO₃⁻. (B) UV-Visible spectra of CNPs-Cl after applying varying voltages of 0 V, 0.1 V, 0.5 V, 0.8 V,1 V, 1.2 V, 1.5 V showing no change in the intensity of Ce³⁺ peak with increase in voltage. (C) UV-Visible spectra of CNPs-Cl with the addition of 30 mM Cl⁻ after applying similar varying voltages showing an increase in the intensity. The OCP calculations were performed based on three experimental repetitions.

Figure 8

(A) ESR spectrum (blue) was obtained after UV-photoionization at 254 nm (2 min, 77 K) of 5′-dGMP (2 mg/mL) in 7.5 M LiCl/H₂O and in the presence of 2.5 mM CNPs-N. Spectrum (pink) obtained after 77 K γ-irradiation (absorbed dose = 500 Gy) of NaNO₃ (1 mg/mL) in 7 M LiBr/D₂O. Green spectrum is the simulated spectrum (for simulation parameters, see text). (B) ESR spectrum (black) was obtained after UV-photoionization at 254 nm (2 min, 77 K) of 5′-dGMP (2 mg/mL) in 7.5 M LiCl/H₂O in the presence of 2.5 mM CNPs-Cl. Spectrum (red) was obtained after annealing the sample of 5′-dGMP in 7.5 M LiCl at 150 K for 10 min. This sample was originally γ-irradiated (77 K, absorbed dose = 2.5 KGy). All experimental spectra (blue, pink, black, red) were recorded at 77 K.

Scheme 1

Schematic representation of CNPs synthesis by wet chemical method employing H₂O₂ as the oxidizing agent.