In this work we investigated, theoretically and experimentally, the magneto-optical properties of a magnetic fluid consisting of core-shell nanoparticles, where the core is made of magnetite, while the shell is silver. The theoretical model used was based on Mie s theory, under the electrostatic approximation, i.e. for nanoparticles with diameters much less than the incident wavelength (lambda). A Clausius-Mosotti for a core-shell system was used to calculate the electrical susceptibility of the core-shell nanoparticle for equals to 632 nm. The susceptibility was shown to be strongly dependent on the core diameter and the shell thickness. Nevertheless, a maximum value of 7.20 (greater than isolated nanoparticles of silver, which has 0 = 4.30, or magnetite with 0 = 1.47) was obtained for a fraction f, defined as f = (Dcore/Dcore−shell)3, equal to 0.36. This result suggest that there exist an ideal fraction f for nanocomposites with enhanced optical properties. In order to compare our theoretical results with experimental data a core-shell magnetic fluid was synthesized on the Institute of Chemistry of UFG by the group of Dr. Em´ılia Celma de Oliveira Lima. The nanoparticles were suspended in water at fisiological pH and recovered by a double layer of lauric acid (dodecanoic acid). The nanoparticles were characterized by X-ray diffraction, high resolution electron transmission, energy dispersive X-ray spectroscopy, and vibrating sample magnetometer. The Sturges method was used to obtain the nanoparticle diameter histogram. The data revealed the existence of a bimodal nanoparticle distribution. Both distributions were curve fitted using a lognormal function. The modal diameter of one of them was 9.24 ± 0.03 nm with a dispersity of 0.27 ± 0.02, while for the other one we found a modal diameter of 23.0 ± 0.2 nm with disperisty 0.2 ± 0.1. The energy dispersive X-ray spectroscopy confirmed the existence of magnetite and silver only for larger particle diameters, while the lower ones only magnetite was found. From the experimental analysis we confirmed the synthesis of a magnetic fluid containing 10% of core-shell nanoparticles. Magnetization data was used to estimate the magnetic particle volume fraction. The magneto-optical properties were obtained using a magnetotransmissivity technique, where the polarizer and analyser axis are positioned on the magnetic field direction. The sample containing 10% of core-shell nanoparticles, with a total particle volume fraction of 0.18%, had shown an extinction of light of 100% for a magnetic field of only 500 Oe, while a magnetic fluid with 100% of core nanoparticles, at a similar particle concentration (0.15%), had shown a 50% extinction of light at the same field range. The magnetotransmissivity data were curve fitted with a theoretical model containing only two parameters, one related to the electrical susceptibility and the other to the formation of self-organized nanostructures in the colloid. The mean agglomerate size (nanoparticles forming linear chains) had changed from 2.09 to 3.36 for a particle volume fraction increasing from 0.06% to 0.18%. Using the estimative of the double layer lenght of lauric acid, approximately 2 nm, and analyzing the magnetotransmissivity data for several particle concentrations, we were able to obtain the fraction f of core-shell nanoparticles of 0.17. This result, together with TEM data, allowed us to calculate the core diameter of the core-shell nanoparticle as 13 nm. Indeed such result suggest that in order to be suscessful in coating the nanoparticle with the shell element one might need monodisperse-like nanoparticle systems.

Aiming to develop a nanoparticle-based optical biosensor using gold nanoparticles (AuNPs) synthesized using green methods and supported by carbon-based nanomaterials, we studied the role of carbon derivatives in promoting AuNPs localized surface plasmon resonance (LSPR), as well as their morphology, dispersion, and stability. Carbon derivatives are expected to work as immobi-lization platforms for AuNPs, improving their analytical performance. Gold nanoparticles (AuNPs) were prepared using an eco-friendly approach in a single step by reduction of HAuCl4·3H2O using phytochemicals (from tea) which act as both reducing and capping agents. UV–Vis spectroscopy, transmission electron microscopy (TEM), zeta potential (ζ-potential), and X-ray photoelectron spectroscopy (XPS) were used to characterize the AuNPs and nanocomposites. The addition of reduced graphene oxide (rGO) resulted in greater dispersion of AuNPs on the rGO surface compared with carbon-based nanomaterials used as a support. Differences in morphology due to the nature of the carbon support were observed and are discussed here. AuNPs/rGO seem to be the most promising candidates for the development of LSPR biosensors among the three composites we studied (AuNPs/G, AuNPs/GO, and AuNPs/rGO). Simulations based on the Mie scattering theory have been used to outline the effect of the phytochemicals on LSPR, showing that when the presence of the residuals is limited to the formation of a thin capping layer, the quality of the plasmonic resonance is not affected. A further discussion of the application framework is presented.

Aiming to develop a nanoparticle-based optical biosensor using gold nanoparticles (AuNPs) synthesized using green methods and supported by carbon-based nanomaterials, we studied the role of carbon derivatives in promoting AuNPs localized surface plasmon resonance (LSPR), as well as their morphology, dispersion, and stability. Carbon derivatives are expected to work as immobilization platforms for AuNPs, improving their analytical performance. Gold nanoparticles (AuNPs) were prepared using an eco-friendly approach in a single step by reduction of HAuCl4·3H2O using phytochemicals (from tea) which act as both reducing and capping agents. UV–Vis spectroscopy, transmission electron microscopy (TEM), zeta potential (ζ-potential), and X-ray photoelectron spectroscopy (XPS) were used to characterize the AuNPs and nanocomposites. The addition of reduced graphene oxide (rGO) resulted in greater dispersion of AuNPs on the rGO surface compared with carbon-based nanomaterials used as a support. Differences in morphology due to the nature of the carbon support were observed and are discussed here. AuNPs/rGO seem to be the most promising candidates for the development of LSPR biosensors among the three composites we studied (AuNPs/G, AuNPs/GO, and AuNPs/rGO). Simulations based on the Mie scattering theory have been used to outline the effect of the phytochemicals on LSPR, showing that when the presence of the residuals is limited to the formation of a thin capping layer, the quality of the plasmonic resonance is not affected. A further discussion of the application framework is presented.

The development and realization of a plasmonic structure based on the LSP interaction of metal nanoparticles with an embedding matrix of amorphous silicon is proposed. As a planned application, this structure will need to be usable as the basis for a sensor device applied in biomedical applications, after proper functionalization with selective antibodies. The final sensor structure needs to be low-cost, compact, and disposable. The study reported in this paper aims to analyze varied materials for nanoparticles embedded in an amorphous silico matrix. Metals of interest for nanoparticles composition are aluminum and gold. As a comparison term, a non-plasmonic material like alumina, resulting from oxidation of Al nanoparticles, is also considered. As a preliminary approach to this device, we study in this work the optical properties of spherical metal nanoparticles embedded in an amorphous silicon matrix, as a function of size and metal type. Following an analysis based on the exact solution of the Mie theory, experimental measurements realized with arrays of metal nanoparticles are compared with the simulations.

This paper reports about a study of the local plasmonic resonance (LSPR) produced by metal nanoparticles embedded in a dielectric or semiconductor matrix. It is presented an analysis of the LSPR for different nanoparticle metals, shapes, and embedding media composition. Metals of interest for nanoparticle composition are Aluminum and Gold. Shapes of interest are nanospheres and nanotriangles. We study in this work the optical properties of metal nanoparticles diluted in water or embedded in amorphous silicon, ITO and ZnO as a function of size, aspect-ratio and metal type. Following the analysis based on the exact solution of the Mie theory and DDSCAT numerical simulations, it is presented a comparison with experimental measurements realized with arrays of metal nanospheres. Simulations are also compared with the LSPR produced by gold nanotriangles (Au NTs) that were chemically produced and characterized by microscope and optical measurements.

We propose the development and realization of a plasmonic structure based on the LSP interaction of metal nanoparticles with an embedding matrix of amorphous silicon. This structure need to be usable as the basis for a sensor device applied in biomedical applications, after proper functionalization with selective antibodies. The final sensor structure needs to be low cost, compact and disposable. The study reported in this paper aims to analyze diferente materials for nanoparticles and embedding medium composition. Metals of interest for nanoparticles composition are Aluminum, Gold and Alumina. As a preliminary approach to this device, we study in this work the optical properties of metal nanoparticles embedded in an amorphous silicon matrix, as a function of size, aspect-ratio and metal type. Following an analysis based on the exact solution of the Mie theory, experimental measurements realized with arrays of metal nanoparticles are compared with the simulations.