Abstract
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.
Highlights
IntroductionWhile providing excellent detection limits in simple matrices in the low μmol L−1 or even nmol L−1 range[6,21], the detection of the colored silicomolybdic acid complexes[15,16,17,22] can suffer from interferences with a broad palette of contaminants such as iron, phosphates, nitrate, ammonia, or sugars, making the quantification of Si in this manner challenging in complex mixtures[20]
The same volume of 0.05 M KOH still dissolved up to 30 mg colloidal SiO2 nanoparticle (SiO2-NP), but did not dissolve 50 mg SiO2-NPs anymore, apparent from the high particle counts per second detected by DLS in that particular sample (Fig. 2)
The threshold pH of >12 corresponds to the 14 mM KOH that are not neutralized by Si(OH)[4] in the sample digested with 1.2 KOH: SiO2 (82 mM KOH, 68 mM SiO2), and is in agreement with the pH of 9–12 reported by Croissant et al to dissolve SiO2-NPs31
Summary
While providing excellent detection limits in simple matrices in the low μmol L−1 or even nmol L−1 range[6,21], the detection of the colored silicomolybdic acid complexes[15,16,17,22] can suffer from interferences with a broad palette of contaminants such as iron, phosphates, nitrate, ammonia, or sugars, making the quantification of Si in this manner challenging in complex mixtures[20] For such matrices, the more recent and robust approaches used to detect Si are inductively coupled plasma – optical emission spectrometry (ICP-OES) or - mass spectrometry (MS). Despite these first successes using KOH, the state of the art remains to dissolve SiO2 in acidic protocols using hydrofluoric acid, largely due to a lack of validation and knowledge on the application scope of this non-traditional approach
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