Abstract
Silica nanoparticles (SNPs) belong to the most widely produced nanomaterials nowadays. Particle size distribution (PSD) is a key property of SNPs that needs to be accurately determined for a successful application. Many single particle and ensemble characterization methods are available for the determination of the PSD of SNPs, each having different advantages and limitations. Since most preparation protocols for SNPs can yield bimodal or heterogeneous PSDs, the capability of a given method to resolve bimodal PSD is of great importance. In this work, four different methods, namely transmission electron microscopy (TEM), dynamic light scattering (DLS), microfluidic resistive pulse sensing (MRPS) and small-angle X-ray scattering (SAXS) were used to characterize three different, inherently bimodal SNP samples. We found that DLS is unsuitable to resolve bimodal PSDs, while MRPS has proven to be an accurate single-particle size and concentration characterization method, although it is limited to sizes above 50 nm. SAXS was found to be the only method which provided statistically significant description of the bimodal PSDs. However, the analysis of SAXS curves becomes an ill-posed inverse mathematical problem for broad size distributions, therefore the use of orthogonal techniques is required for the reliable description of the PSD of SNPs.
Highlights
Silica nanoparticles (SNPs) represent one of the most widely utilized engineered nanomaterials.Due to the versatile properties of SNPs, their application is widespread in many fields ranging from chemical and food industry, through environmental sciences, to medical applications [1,2,3,4,5]
We investigated the capabilities of various methods for the determination of the Particle size distribution (PSD)
transmission electron microscopy (TEM) images of the prepared SNPs shown in Figure 1a,c,e reveal that all samples are bimodal
Summary
Silica nanoparticles (SNPs) represent one of the most widely utilized engineered nanomaterials.Due to the versatile properties of SNPs, their application is widespread in many fields ranging from chemical and food industry, through environmental sciences, to medical applications [1,2,3,4,5]. The most commonly used process for the preparation of SNPs is based on the controlled hydrolysis and condensation of alkoxysilanes [1]. Controlled growth of monodisperse SNPs in the 50 nm to 2000 nm size range in water-ethanol solution using ammonia as a base catalyst was first reported by Stöber et al [6]. This method was further optimized and widely used nowadays [7,8]. In this method, the particle growth rate is limited by the hydrolysis of alkoxysilane molecules and proceeds through the surface condensation of hydrolyzed monomers or small oligomers. An analogous method was introduced by Yokoi et al [9], which uses L-lysine instead of ammonia as the base catalyst and dosing the alkoxysilane from a top organic layer (octane) to ensure slow saturation of the aqueous phase
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