Hermetically Coated Nanosilver: No Ag + Ion Leaching
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Hermetically Coated Nanosilver: No Ag+ Ion Leaching G. Sotiriou, S. Gass, S.E. Pratsinis ETH Zurich (Swiss Federal Institute of Technology), Department of Process and Mechanical Engineering, Particle Technology Laboratory, Sonneggstrasse 3, 8092 Zurich, Switzerland ABSTRACT Dry-coated nanosilver is a promising material for bio-applications. Its inert and non-porous nanothin SiO2 coating preserves the plasmonic properties of the nanosilver and cures its toxicity by (1) preventing direct cell to silver contact and (2) blocking the release of toxic silver ions. However, fully hermetic coatings have, to date, not been produced. During the coating process, a certain number of core particles are either coated only partially, or escape the coating process entirely. Here, a systematic parametric study was undertaken in order to optimize an aerosol reactor for the synthesis and dry-coating of nanosilver. The reactor was optimized with respect to coating injection height, jet number, mixing flow rate. By synthesizing xAg/SiO2 composite particles, small silver sizes (9-11 nm) with relatively high Ag ion release were obtained. This enabled the quantitative evaluation of the coatings by Ag ion release measurements. INTRODUCTION Core-shell particles exhibit surface properties of their shell while preserving the bulk properties of their core material [1]. Careful selection of a particle shell facilitates the dispersion of functional materials into hosts, such as liquid suspensions and nanocomposites [2]. Coating titania with a silica shell, for example, enables its dispersion into inks, paints, toothpaste and sunscreen, while preserving the high scattering property of its core material [3]. Similarly, coating iron oxide with silica prevents magnetically self-induced agglomeration of functional particles and increases its thermal stability without affecting its magnetic properties [4]. In other cases, the coating of particles facilitates their surface functionalization because the surface chemistry of the shell material is well-understood [5]. As a result, functionalized particles can be used to target specific analytes [6]. Furthermore, the coating of functional materials also protects them from aggressive environments, such as alumina-coated oxidation-resistant Ni.[7] Frequently, however, it is the environment of the particles that needs to be protected from the core-material. In such cases, coatings offer a promising way of making otherwise harmful and toxic materials biocompatible. Due to its low plasmonic losses [8] in the UV-visible spectrum, nanosilver is an interesting material for biological sensing applications [9]. Moreover, due to its strong light absorbance and scattering, nanosilver can also be utilized to detect targeted cells as in-vivo biomarkers [10]. However, the environmental- and bioincompatibility of nanosilver limits its use in many applications [11]. In fact, nanosilver is toxic to biological systems upon direct contact with cells and/or toxic Ag ion release from its surface [12]. Especially for smaller Ag s
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