Effect of Different Silica Coatings on the Toxicity of upconversion nanoparticles on RAW 264.7 macrophage cells

Abstract

Upconversion nanoparticles (UCNP) consisting of NaYF₄ doped with 18% Yb and 2% Er were coated with microporous silica shells of 7±2 nm and 21±3 nm thickness. Subsequently, the initially negatively charged particles were optionally functionalized with N-(6-aminohexyl)-aminopropyltrimethoxysilane (AHAPS), providing a positive charge onto the nanoparticle surface. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements revealed that the particles with the thicker shells release fewer lanthanide ions in 24 h than particles with a thinner shell but that even a 21±3 nm thick silica layer does not entirely block the disintegration process of the UCNP. MTT tests and cell cytometry measurements with macrophages (RAW 264.7 cells) indicate that the cells treated with amino-functionalized particles with a thicker silica shell have higher viability than those incubated with UCNP with a thinner silica shell even if more particles with a thicker shell are taken up. This effect is less significant for negatively charged particles. A cell cycle analysis with amino-functionalized particles also confirms that a thicker silica shell reduces the cytotoxicity. Thus, growing silica shells of sufficient thickness is a simple approach to minimize the cytotoxicity of UCNP.