Fig. 1

Characterization of SiO2 NPs of different origins. a Typical SEM image of a SiO2 NP standard. b, c Typical SEM (b) and TEM (c) images of engineered SiO2 NPs. d SEM image of natural quartz particles. e, f SEM images of intact (e) and fragmentary (f) diatomite particles. g, XRD patterns of engineered and natural SiO2 NPs. h EDX patterns of engineered and natural SiO2 NPs. i The atomic ratio of O to Si (RO/Si) of engineered and natural SiO2 NPs based on the EDX measurements. The error bars represent 2s.d. (n = 4-15). EP, EF, ES, ND, and NQ represent precipitated silica, fumed silica, sol–gel silica, diatomite, and quartz, respectively