Impact of surface functional group modification on cellular internalization and cytotoxicity of silica nanoparticles

Silica nanoparticles (SiO₂NPs) are widely used in industrial products. Surface modification of SiO₂NPs is one of the promising strategies to develop safer nanomaterials by design. The present study was designed to determine the effects of amino or carboxyl functionalization of rhodamine-labeled SiO₂NPs on cellular uptake and cytotoxicity.

In the in vivo arm of the study, male mice were randomly divided into seven groups (n = 6, each) and exposed to either amino (NH₂)- or carboxyl (COOH)-functionalized, or non-functionalized (OH)-rhodamine-labeled SiO₂NPs at 2 or 10 mg/kg bw, or endotoxin-free water as a control, by pharyngeal aspiration. At 24 h after administration, the mice were euthanized and bronchoalveolar lavage fluid (BALF) was collected for differential cell count and assessment of silica nanoparticle uptake using confocal microscopy. In the in vitro arm of the study, murine RAW264.7 macrophages were exposed to NH₂-or COOH-functionalized or OH- rhodamine-labeled SiO₂NPs. Nonspecific caspase inhibitor, necroptosis inhibitor, pyroptosis inhibitor and autophagy inhibitor were used to determine the roles of cell death signaling in cytotoxicity.

The in vivo studies demonstrated significant increase in lung weight at 2 and 10 mg/kg bw by OH-SiO₂NPs but not the other two SiO₂NPs. At 10 mg/kg bw, COOH-SiO2NPs induced a significant increase in BALF macrophages, whereas OH- SiO₂NPs significantly decreased macrophages. OH-SiO₂NPs at 2 mg/kg bw and NH₂- and COOH-SiO₂NPs at 10 mg/kg bw significantly increased BALF neutrophiles. The in vitro studies showed greater NH₂-SiO₂NPs internalization into RAW264.7 macrophages than OH-SiO₂NPs, while OH-SiO₂NPs induced cytotoxicity and upregulation of IL-1β and TNF-α to greater extent than the other two types. Co-treatment with pan-caspase inhibitor and necroptosis inhibitor attenuated (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) (MTS) cytotoxicity of OH-SiO₂NPs.

NH₂- or COOH-functionalization reduced the harmful changes observed with OH- SiO₂NPs, which included increase in lung weight and BALF neutrophils at low dose in mice as well as decrease in cell viability and upregulation of proinflammatory cytokines in RAW264.7 macrophages. The results suggested that OH-SiO₂NPs-induced cytotoxicity against macrophages was mediated at least in part through apoptotic/necroptotic signaling but was not related to internalization of particles. The results imply possible development of safer silica nanoparticles by amino- or carboxyl-functionalization of their silanols.