In situ electron microscopy techniques for nanoparticle dispersion analysis of commercial sunscreen

Nanoparticles are common active ingredients within many commercial products including sunscreen. Consequently, accurate characterisation of nanoparticles in these products is vital to enhance product design, whilst also understanding the toxicological implications of these nanoparticles. Whilst bulk techniques are useful in providing some information, they often cannot resolve individual particles, and therefore electron microscopy can be used for high-resolution nanoparticle characterisation. However, conventional high vacuum dry TEM does not accurately represent nanoparticle dispersions and other in situ methods must be used. Here, we use a combination of techniques including liquid cell transmission electron microscopy (LCTEM), cryogenic (cryo)-TEM and cryo-scanning electron microscopy (SEM) to characterise a commercial sunscreen containing titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles. Our work illustrates that whilst LCTEM does not require any sample preparation more beam artefacts can occur causing ZnO dissolution with only TiO2 nanoparticles visualised. Comparatively, cryo-TEM allows characterisation of both ZnO and TiO2, yet only cryo-SEM could be used to analyse the pure product (without dilution) but biased the characterisation to the larger fraction of nanoparticles and agglomerates. Ultimately, only with a combination of different in situ EM techniques can an accurate characterisation of commercial products be achieved in order to ensure effective and safe product design and manufacture.