Examining Particle Size of Inorganic Active Ingredients within Sunscreens Using Dynamic Light Scattering

With the advancements in broad-spectrum sunscreens and the recent bans on benzene-based sunscreens due to their environmental toxicity, there has been a push toward broad-spectrum sunscreens containing in organic active ingredients. In this study, a procedure was developed to analyze the particle size and size distribution of inorganic active ingredients, titanium dioxide (TiO2) and/or zinc oxide(ZnO), of sunscreens with sun protection factor (SPF) values ranging from 15 to 50 using dynamic light scattering (DLS). These inorganic components are often engineered as nanoparticles in order to reduce visibility on the skin and retain UV scattering. Research suggests that the use of smaller nanoparticles to increase the efficacy of the inorganic filters may also be toxic to humans if it becomes permeable to the skin. This methodology allowed undergraduate students to workhands-on with particle sizing and compare sunscreen samples to nanopowder and dispersion standards using the effect of the hydrodynamic diameter. Students found that, due to agglomeration, the particle sizes for the nanopowder standards could exceed the manufacture's labeled size when dispersed in solution, which they then compared with their sunscreen data. The results also showed that some sunscreens had two distinct layers at the end of sample preparation, which could be correlated to the matrix components within the sunscreens. This study is intended for undergraduate analytical students and can be altered using the potential variations and scanning electron microscopy (SEM) with electron dispersive spectroscopy (EDS) to create a more challenging upper-level lab and allow for instrumentation comparisons.