The PAA-stabilized CeO2 nanoparticles were synthesized using the low-temperature precipitation method. A mixed solution containing 30 mM cerium(III) nitrate hexahydrate salt and ammonium cerium(IV) nitrate salt with 10% by weight of PAA (MW 5100) was prepared, and 30% ammonium hydroxide solution was added to this in a dropwise manner. Eu-doped cerium nanoparticles were synthesized similarly. During the synthesis of Eu-doped CeO nanoparticles, the concentration of ammonium cerium(IV) nitrate salt ((NH4)2[Ce(NO3)6]) remained constant and amounted to 30 mM, whereas the concentration of cerium(III) nitrate hexahydrate salt (Ce(NO3)3·6H2O) was changed depending on the degree of doping. The amount of europium(III) nitrate hydrate salt (Eu(NO3)3·5H2O) was added so that the total concentration of cerium nitrate hexahydrate and europium nitrate hydrate salts was 30 mM. For example, for 20% Eu doping, 0.006 M Eu salt and 0.024M Ce salt were used and similarly for other levels of Eu doping. Several syntheses of Eu-CeONPs with 5%, 10%, 15% and 20% doping were conducted to compare properties and select the most optimal suspensions. After continuous stirring for 24 h, the obtained suspensions of doped Eu-CeO nanoparticles were dialyzed against 5L of distilled water at pH 7 for 2 days. The water was changed 3 times a day.
The hydrodynamic particle size and zeta potential of Eu-CeONPs were determined by Dynamic Light Scattering (DLS) (Zetasizer Nano Series, Malvern Instruments, Malvern, Worcestershire, UK). The sample was measured at 25 °C in triplicate with at least 20 measurements using water as dispersant with parameters set for cerium oxide (refractive index = 2.2 and absorption = 0.001). The zeta potential measurements were conducted using LDE (Laser Doppler Electrophoresis) and Zetasizer Nano Series (Malvern Instruments, Malvern, Worcestershire, UK). The X-ray Photoelectron Spectroscopy (XPS) measurements were carried out in a multi-chamber UHV system equipped with a hemispherical analyzer (SES R4000, Gammadata Scienta, Uppsala, Sweden), and the numerical analysis was performed with CasaXPS 2.3.23 software after subtracting the Shirley-type background. The experimental results were fitted using a profile with a variable ratio (70:30) of Gaussian and Lorentzian lines [35 (link)]. The absorption spectra were recorded with a UV-Vis spectrometer (Shimadzu Corporation, Duisburg, Germany) in the range of 200–800 nm. Photoluminescence spectra of Eu-CeONPs were recorded using a Fluorolog®-3 spectrofluorometer (HORIBA Jobin Yvon, Longjumeau, France) at the excitation wavelength of 380 nm and emission spectra with the scan range from 400 to 660 nm with 5 nm slit widths and an integration time of 0.1 s. All samples were characterized as synthesized. To confirm the imaging abilities of synthesized nanoparticles by optical modalities, the nanoprobes were deposited on the surface of positively charged latex microparticles and visualized by Carl Zeiss LSM780 (Carl Zeiss, Jena, Germany) confocal microscopy.
The hydrodynamic particle size and zeta potential of Eu-CeONPs were determined by Dynamic Light Scattering (DLS) (Zetasizer Nano Series, Malvern Instruments, Malvern, Worcestershire, UK). The sample was measured at 25 °C in triplicate with at least 20 measurements using water as dispersant with parameters set for cerium oxide (refractive index = 2.2 and absorption = 0.001). The zeta potential measurements were conducted using LDE (Laser Doppler Electrophoresis) and Zetasizer Nano Series (Malvern Instruments, Malvern, Worcestershire, UK). The X-ray Photoelectron Spectroscopy (XPS) measurements were carried out in a multi-chamber UHV system equipped with a hemispherical analyzer (SES R4000, Gammadata Scienta, Uppsala, Sweden), and the numerical analysis was performed with CasaXPS 2.3.23 software after subtracting the Shirley-type background. The experimental results were fitted using a profile with a variable ratio (70:30) of Gaussian and Lorentzian lines [35 (link)]. The absorption spectra were recorded with a UV-Vis spectrometer (Shimadzu Corporation, Duisburg, Germany) in the range of 200–800 nm. Photoluminescence spectra of Eu-CeONPs were recorded using a Fluorolog®-3 spectrofluorometer (HORIBA Jobin Yvon, Longjumeau, France) at the excitation wavelength of 380 nm and emission spectra with the scan range from 400 to 660 nm with 5 nm slit widths and an integration time of 0.1 s. All samples were characterized as synthesized. To confirm the imaging abilities of synthesized nanoparticles by optical modalities, the nanoprobes were deposited on the surface of positively charged latex microparticles and visualized by Carl Zeiss LSM780 (Carl Zeiss, Jena, Germany) confocal microscopy.
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