3.4. TEM analysis
Particle size, shape, and composition of commercial sunscreens
were investigated by combining TEM with EDS.
Samples needed dilution before TEM/EDS imaging. Electron
micrographs of the samples clearly showed the NPs because
the resolution was not affected by the rest of formulation
(Fig. 6). Differences in particle shape were detected between
samples. In particular, needle-shaped particles were found in
COM 1, 2, and 4, while both needle shaped and spherical
particles were observed in COM 5. Compositional analysis of
inorganic residues by EDS demonstrated the presence of Si or
Al signal closely match inorganic filter's signal in most samples.
Reports have noted that TiO2 NP surfaces have been
modified by silicon dioxide and aluminum oxide to reduce
photoreactivity and minimize the formation of reactive oxygen
species [14]. Scanning electron microscopy (SEM) was
coupled with EDS to analyze the commercial sunscreens. The
samples needed dilution before SEM/EDS imaging but the
charging of the surrounding organic matrix appeared to
reduce the resolution (data not shown). Due to its high resolution,
this suggests that TEM is more suitable than SEM for NP
in these samples. In agreement with the International Cooperation
on Cosmetic Regulation criteria [7], the six samples
contained TiO2 and ZnO particles exhibiting at least one
dimension smaller than 100 nm. COM 1 and COM 2 consisted
of a mixture of TiO2 and ZnO NPs. XRD and TEM measurements
provided consistent sizes and complementary nanomaterial
characteristics (Table 2). Rutile TiO2 NPs exhibited
needle-like shapes in COM 1, COM 2, and COM 4. Therefore, in
COM 5, anatase and rutile TiO2 NPs were deduced to show
spherical and needle-like shapes, respectively. These results
demonstrate that XRD data were overlaid onto TEM data. Both
XRD and TEM are suitable for size-related analysis in
sunscreens.