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. 2019 May 28;9:7938. doi: 10.1038/s41598-019-44128-z

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Results of digestion methods tested to find the optimal KOH: SiO₂ ratio that allowed for a complete dissolution of SiO₂-NPs. Dynamic light scattering (DLS) particle count rates and transmission electron microscopy (TEM) images are shown. High count rates represent undigested or partially digested colloidal SiO₂-NPs. The lowest possible potassium hydroxide concentration for complete solubilization of the SiO₂-NPs (digestion method: High_SiO₂) was 0.092 M. In these or higher tested KOH concentrations (molar ratio of 1.2–60.1 KOH: SiO₂), all SiO₂-NPs dissolved. The high particle count rate in the 48 mM KOH sample with a molar KOH: SiO₂ ratio of 0.6 shows that the lower KOH concentration did not fully dissolve the 50 mg of SiO₂-NPs. For these incompletely digested SiO₂-NP samples, the TEM micrographs revealed a broadening of the size, an increase in surface roughness, and the presence of nanopores compared to the original SiO₂-NPs.