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

Area of Science:

• Environmental Science
• Analytical Chemistry
• Materials Science

Background:

• Commercial use of synthetic amorphous silica nanomaterials (SiO2-NPs) is rising, but their environmental and health impacts require further investigation.
• Analyzing SiO2-NPs is challenging due to the need for toxic hydrofluoric acid (HF) in traditional methods.
• Existing analytical methods for SiO2-NPs are often hazardous and complex, hindering research into their fate and risks.

Purpose of the Study:

• To develop and validate hydrofluoric acid-free sample preparation methods for quantifying amorphous SiO2 micro- and nanoparticles.
• To provide a safer, simpler alternative to HF-based digestion protocols for SiO2-NP analysis.
• To facilitate more detailed research on the environmental and human health effects of SiO2-NPs.

Main Methods:

• Developed base-catalyzed hydrolysis using potassium hydroxide (KOH) at room temperature or under microwave irradiation to dissolve SiO2-NPs.
• Replaced fluoride ions with hydroxide ions (OH-) and utilized the stability of orthosilicic acid monomers in strongly basic conditions (pH >12).
• Quantified silicon using Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES) or a colorimetric assay.

Main Results:

• Established effective SiO2-NP dissolution and quantification using KOH with molar ratios as low as 1.2 for colloidal Stöber SiO2-NPs.
• Determined higher KOH:SiO2 ratios (>8000) were needed for fumed SiO2-NPs (Aerosil®) and food-grade SiO2 (E551).
• Achieved effective digestion with high recovery rates (>84%) using the KOH-based protocols.

Conclusions:

• Hydrofluoric acid-free SiO2-NP digestion protocols using KOH are effective, less hazardous, and easy to implement.
• These new methods offer a viable alternative to traditional HF-based techniques for SiO2-NP analysis.
• The developed methods will aid in advancing research on the environmental and health implications of SiO2-NPs.