Ultraviolet Extinction of Metal Oxide Nanoparticles Imaged at the Single Nanoaggregate Level

ABSTRACT Metal oxide nanoparticles are at the core of many nanotechnology applications, notably UV blocking filters, sunscreen creams and photocatalyzers. However, the tendency of metal oxide nanoparticles to form complex nanoaggregates complicates their analysis and imposes the development of alternative characterization methods with a sensitivity down to the level of a single nanoaggregate to reveal the size and shape dependences hidden in the ensemble‐based bulk measurements. Here we use bright‐field UV microscopy to monitor the extinction cross‐sections of TiO 2 , SnO 2 and Fe 2 O 3 nanoparticles with a resolution of a single nanoaggregate with dimensions well below 200 nm. Correlative electron and atomic force microscopy images of the same nanoaggregates provide information about the dimensions and fill fractions of each sample. Our experimental results are put in regard to numerical simulations based on discrete dipole approximation (DDA), demonstrating correct qualitative and quantitative consistency with theoretical predictions, given the assumptions and simplifications of the model. Our results provide critical information characterizing the material and morphology parameters determining the UV response of metal oxide nanoaggregates, with a direct relevance for cosmetic sunscreen applications and related areas of nanotechnology.