Detecting free radicals in sunscreens exposed to UVA radiation using chemiluminescence

• PICL is an easy test method for free radicals and takes 5–10 min per sample.
• ZnO and TiO2 nanoparticle sunscreens exposed to UVA emit PICL.
• PICL intensity is much higher from sunscreens containing P25 and nano-anatase.
• Particles extracted from a sunscreen caused severe photocatalytic damage.
• Photocatalysts such as P25 and nano-anatase should not be used in sunscreens.

One of the current concerns with the application of nanoparticles in sunscreens, and in particular nano-TiO2 and ZnO, is their potential to photogenerate free radicals and reactive oxygen species (ROS) when they absorb ultraviolet wavelengths from sunlight. Free radicals and ROS are known to be associated with UV-induced skin damage and oxidative stress, from which sunscreens are expected to offer significant protection. Here we describe a simple method, based on chemiluminescence emission, for detecting free radicals generated in commercial sunscreens alone, and when applied to various substrates, following exposure to UVA (320–400 nm) radiation. This photo-induced chemiluminescence (PICL) technique could be used to optimise sunscreen formulations so as to minimise free radical photogeneration during exposure to sunlight.

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