A hydrofluoric acid-free method to dissolve and quantify silica nanoparticles in aqueous and solid matrices

As the commercial use of synthetic amorphous silica nanomaterials (SiO₂-NPs) increases, their effects on the environment and human health have still not been explored in detail. An often-insurmountable obstacle for SiO₂-NP fate and hazard research is the challenging analytics of solid particulate silica species, which involves toxic and corrosive hydrofluoric acid (HF). We therefore developed and validated a set of simple hydrofluoric acid-free sample preparation methods for the quantification of amorphous SiO₂ micro- and nanoparticles. To circumvent HF, we dissolved the SiO₂-NPs by base-catalyzed hydrolysis at room temperature or under microwave irradiation using potassium hydroxide, replacing the stabilizing fluoride ions with OH^-, and exploiting the stability of the orthosilicic acid monomer under a strongly basic pH. Inductively coupled plasma - optical emission spectroscopy (ICP-OES) or a colorimetric assay served to quantify silicon. The lowest KOH: SiO₂ molar ratio to effectively dissolve and quantify SiO₂-NPs was 1.2 for colloidal Stöber SiO₂-NPs at a pH >12. Fumed SiO₂-NPs (Aerosil^®) or food grade SiO₂ (E551) containing SiO₂-NPs were degradable at higher KOH: SiO₂ ratios >8000. Thus, hydrofluoric acid-free SiO₂-NP digestion protocols based on KOH present an effective (recoveries of >84%), less hazardous, and easy to implement alternative to current methods.