Structural Characterization of Monodisperse SiO₂ Spherical Nanoparticles Grown by Controlled Method to Develop Optical Phantoms

Abstract

Among the diverse applications of silica nanoparticles, the suitability of these to reproduce scattering and absorption properties in tissue simulating phantoms for diffuse optical imaging, through the use of Mie theory, has been recently proposed [1]. An important limitation to adjust the Mie theory to the pattern of scattered light by scattering spheres is the relative ratio between the wavelength of the incident light and the size of the scattering sphere, in this way, it is important to achieve monodisperse and controlled growth of the spheres diameter, so this ratio will be close to 1, allowing the Mie theory to work according to its own limitations.The development of diagnostic imaging systems has required the use of tissue simulating materials to mimic the optical properties of human or animal tissues [2,3]. The properties of these phantoms usually are inconsistent and change over time, making comparison of different imaging systems a serious problem when also different animal-derived phantoms are being used [4]. The use of a standard phantom suitable to be reproduced in any laboratory with constant and confident optical properties is the main goal of using silica nanoparticles to develop this standard.In this work a modified version of the Stöber Method [5,6] is used to obtain spherical silica particles by varying the proportion of water and ethanol contained in a solution while maintaining a constant proportion in the rest of the reagents: tetraethyl orthosilicate and hydroxide of ammonium. To analyze the structure of each sample the hydrodynamic diameters of SiO2 spheres were measured by dynamic light scattering (DLS) using a Malvern Zetasizer Nano Range analyzer. Then the nanoparticle diameters were measured by scanning electron microscopy (SEM) observing spherical and monodisperse particles; SEM analyses were carried on with a FE-SEM JEOL JSM-7800F.There are several methods for the synthesis of SiO2, we use a modified version of the Stöber Method. This method consists of controlled hydrolysis, catalyzed on alcoholic medium. The following precursors were used: tetraethylorthosilicate (TEOS, Si(OC2H5)4) as precursor of silica, ethanol and deionized water as solvents and ammonia hydroxide (NH4OH) as a catalyst; the synthesis took place at room temperature with magnetic stirring for 24 hours. Figure 1 shows the hydrodynamic diameter distribution of the particles as a function of the percentage of water diluted in ethanol, it is observed that in the range of 4 to 8% of water volume the diameter of the particle increases rapidly to around 750 nm, while outside this range the diameter of the nanoparticle decreases; these results are in relative agreement with others reported in the literature [6]. This controlled growth will allow to vary the wavelength used in accordance with de particle diameter. In Figure 2 the scattering silica nanoparticles SEM images are shown.

Controlled Growth of Silicon Dioxide Nanospheres by Regulation in the Addition Rate of the Precursor

“…In this work a modified version of the Stöber method is used [4], where by using tetraethylorthosilicate as precursor (TEOS) different sizes of silica spheres [5] are obtained in the presence of a catalyst (ammonium hydroxide), by means of the controlled addition of the precursor by dripping it into the reaction [6], in this way a monodisperse suspension of silica nanospheres can be achieved. These nanospheres are analyzed by dynamic light scattering (DLS, Malvern Zetasizer Nano Range analyzer) and by scanning electron microscopy (SEM, FE-SEM JEOL JSM-7800F) where, from a statistical analysis, its average diameter is calculated [7].…”

Embedding Silicon Dioxide Nanospheres for Homogeneous Incorporation onto a Resin Matrix

“…The optical properties of these phantoms are usually inconsistent and change over time, making comparison of different imaging systems a serious problem. The use of a reliable and reproducible phantom in any laboratory with constant and confident optical properties is the main goal of using silica (SiO2) nanoparticles to develop this standard [2]. There are several types of optical phantoms often consisting of a bulk material like silicone, epoxy resin, or poly(vinyl)-alcohol, with embedded scatterers and absorbers, allowing fine-tuning of the optical scattering and absorption properties of the sample as required for the intended application [3].…”