ABSTRACT
Bifunctional magneto-plasmonic nanoparticles that exhibit synergistically magnetic and plasmonic properties are advanced substrates for surface-enhanced Raman spectroscopy (SERS) because of their excellent controllability and improved detection potentiality. In this study, composite magneto-plasmonic nanoparticles (Fe3O4@AgNPs) were formed by mixing colloid solutions of 50 nm-sized magnetite nanoparticles with 13 nm-sized silver nanoparticles. After drying of the layer of composite Fe3O4@AgNPs under a strong magnetic field, they outperformed the conventional silver nanoparticles during SERS measurements in terms of signal intensity, spot-to-spot, and sample-to-sample reproducibility. The SERS enhancement factor of Fe3O4@AgNP-adsorbed 4-mercaptobenzoic acid (4-MBA) was estimated to be 3.1 × 107 for a 633 nm excitation. In addition, we show that simply by changing the initial volumes of the colloid solutions, it is possible to control the average density of the silver nanoparticles, which are attached to a single magnetite nanoparticle. UV-Vis and SERS data revealed a possibility to tune the plasmonic resonance frequency of Fe3O4@AgNPs. In this research, the plasmon resonance maximum varied from 470 to 800 nm, suggesting the possibility to choose the most suitable nanoparticle composition for the particular SERS experiment design. We emphasize the increased thermal stability of composite nanoparticles under 532 and 442 nm laser light irradiation compared to that of bare Fe3O4 nanoparticles. The Fe3O4@AgNPs were further characterized by XRD, TEM, and magnetization measurements.
ABSTRACT
The synthesis of magnetic particles triggers the interest of many scientists due to their relevant properties and wide range of applications in the catalysis, nanomedicine, biosensing and magnetic separation fields. A fast synthesis of iron oxide magnetic particles using an eco-friendly and facile microwave-assisted solvothermal method is presented in this study. Submicron Fe3O4 spheres were prepared using FeCl3 as an iron source, ethylene glycol as a solvent and reductor and sodium acetate as a precipitating and nucleating agent. The influence of the presence of polyethylene glycol as an additional reductor and heat absorbent was also evaluated. We reduce the synthesis time to 1 min by increasing the reaction temperature using the microwave-assisted solvothermal synthesis method under pressure or by adding PEG at lower temperatures. The obtained magnetite spheres are 200-300 nm in size and are composed of 10-30 nm sized crystallites. The synthesized particles were investigated using the XRD, TGA, pulsed-field magnetometry, Raman and FTIR methods. It was determined that adding PEG results in spheres with mixed magnetite and maghemite compositions, and the synthesis time increases the size of the crystallites. The presented results provide insights into the microwave-assisted solvothermal synthesis method and ensure a fast route to obtaining spherical magnetic particles composed of different sized nanocrystallites.
ABSTRACT
The results of colossal magnetoresistance (CMR) properties of La1-xSrxMnyO3 (LSMO) films grown by the pulsed injection MOCVD technique onto an Al2O3 substrate are presented. The grown films with different Sr (0.05 ≤ x ≤ 0.3) and Mn excess (y > 1) concentrations were nanostructured with vertically aligned column-shaped crystallites spread perpendicular to the film plane. It was found that microstructure, resistivity, and magnetoresistive properties of the films strongly depend on the strontium and manganese concentration. All films (including low Sr content) exhibit a metal−insulator transition typical for manganites at a certain temperature, Tm. The Tm vs. Sr content dependence for films with a constant Mn amount has maxima that shift to lower Sr values with the increase in Mn excess in the films. Moreover, the higher the Mn excess concentration in the films, the higher the Tm value obtained. The highest Tm values (270 K) were observed for nanostructured LSMO films with x = 0.17−0.18 and y = 1.15, while the highest low-field magnetoresistance (0.8% at 50 mT) at room temperature (290 K) was achieved for x = 0.3 and y = 1.15. The obtained low-field MR values were relatively high in comparison to those published in the literature results for lanthanum manganite films prepared without additional insulating oxide phases. It can be caused by high Curie temperature (383 K), high saturation magnetization at room temperature (870 emu/cm3), and relatively thin grain boundaries. The obtained results allow to fabricate CMR sensors for low magnetic field measurement at room temperature.
