3. Results and discussion
3.1. AFM analysis
Three untreated sunscreens (COM 1, COM 3, and COM 6) were
examined by AFM. AFM can detect inorganic NPs in the sunscreen
matrix and provide morphological information on the
metal oxides but requires another analytical technique to
compare NP size [12]. The AFM height images of the NPs in
sunscreen samples displayed unobvious contours (Fig. 1)
because the NPs were embedded in the soft cosmetic formulation,
precluding NP size measurement. Therefore, a phase
analysis was performed to distinguish different components
from the sunscreen matrix. The phase image of COM 1 presented
clearer NP features than height images (Fig. 2A). By
contrast, the 5 mm 5 mm topographic image of the TiO2 NP
standard solution showed some aggregates (Fig. 2B). Furthermore,
TiO2 NP sizes ranging from 14.4 nm to 27 nm were
measured in the standard solution for a 2 mm 2 mm scan
area. Because of the complexity of cosmetic formulation,
sunscreen samples required additional pretreatment and
calibration to obtain clear AFM images, and their elemental
composition was not analyzed, suggesting that AFM is not
suitable for ZnO and TiO2 NP characterization in commercial
products.