Journal of Photochemistry and Photobiology A: Chemistry 213 (2010) 158–163 Contents lists available at ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem Singlet oxygen generation in the presence of titanium dioxide materials used as sunscreens in suntan lotions Marta Buchalska, Gabriela Kras, Marcin Oszajca, Wiesław Lasocha, Wojciech Macyk ∗ Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland article info Article history: Received 15 February 2010 Received in revised form 13 May 2010 Accepted 25 May 2010 Available online 4 June 2010 Keywords: Titanium dioxide Suntan lotion Photosensitizer Reactive oxygen species Singlet oxygen abstract The goal of this study was to test photoreactivity of commercially available suntan lotions and their components (mainly TiO 2 ). Isolated water insoluble components containing titanium dioxide appeared almost non-active in the tests of 4-chlorophenol degradation, however a fast UV-light induced degra- dation of azur B and oxidation of ˛-terpinene to ascaridol in the presence of these materials was observed. The photoreactivity of suntan lotions and their components was compared to the photoactiv- ity of phenalenone (an efficient 1 O 2 photosensitizer). The results have proven a relatively low efficiency of hydroxyl radicals formation, however significant rates of reactions involving singlet oxygen were observed in the presence of either the components of the cosmetics or the suntan lotions used as received. Moreover, an efficient photocurrent generation by photoelectrodes made of isolated TiO 2 mate- rials reflects their photoredox properties. Although singlet oxygen scavengers used as additives in suntan lotions might decrease the risk related to generation of this reactive oxygen species, producers of cosmet- ics containing titanium dioxide should consider testing TiO 2 photoactivity in reference to 1 O 2 generation. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Topical sunscreens can be divided into two main categories: organic and inorganic. The category of inorganic sunscreens is rep- resented by titanium dioxide and zinc oxide. These compounds in the nano- or microcrystalline forms constitute an opaque bar- rier reflecting and scattering, but mainly absorbing ultraviolet light (both UV-A and UV-B). Titanium dioxide has been registered (US Federal Register, 43FR38206, 25 August 1978) to be a safe phys- ical sunscreen [1]. TiO 2 is photostable and inert towards organic components of suntans [2] and shows no skin irritating properties [3]. Micronisation and encapsulation of titanium dioxide improve quality of inorganic sunscreens since such materials show lowered scattering of visible light and shift of the onset of absorbed light to shorter wavelengths (quantum-size effect) [4]. Titanium dioxide occurs in three crystalline forms: anatase, rutile and brookite. Absorption properties of anatase and rutile are perfectly fitted to their applications as sunscreens [5–7] since their absorption onsets around 400 nm fall to the frontier between vis- ible and UV-light. Anatase shows usually a better photocatalytic activity as compared to rutile and is used as an active component of the self-cleaning and self-sterilizing surfaces [8,9]. Rutile, used ∗ Corresponding author. Tel.: +48 126632005; fax: +48 126335392. E-mail address: macyk@chemia.uj.edu.pl (W. Macyk). as a white paint pigment, whitener in a toothpaste and UV absorber in sunscreens [10] in general shows a lower photoactivity although the mixtures of anatase and rutile appear to be particularly effi- cient photocatalysts. Photocatalytic processes at TiO 2 are based on an electron–hole pair photogeneration followed by the interfacial electron transfer (IFET) or energy transfer processes resulting in reactive oxygen species (ROS) formation [8,9,11]. Hydroxyl rad- icals OH • , superoxide O 2 •- , hydrogen peroxide H 2 O 2 are main representatives of ROS formed as a result of IFET. Another reac- tive oxidant, singlet oxygen 1 O 2 , can be a product of the energy transfer to adsorbed oxygen molecule [12–14] or a superoxide- hole recombination [15]. ROS may be responsible for oxidation of organic components of suntan lotions [16] as well as for ageing and destruction of skin. Previous studies on TiO 2 isolated from com- mercial suntan lotions have proven, that titanium dioxide used as a sunscreen may be photoactive—photocatalyzed phenol oxidation and DNA damage were observed [1,17]. Therefore sunscreens based on titanium dioxide or zinc oxide should contain additional antiox- idants, e.g. glutathione, ˛-tocopherol (vitamin E), ascorbic acid or ˇ-carotene (an excellent 1 O 2 quencher [18]). A particularly good protection against unwanted ROS can be achieved by a smart tita- nium dioxide modification which might inhibit its photocatalytic activity. An example of such system was recently described by Lee et al. who applied a hydrophobic multicomponent polymer coat- ing for nanocrystalline TiO 2 , containing natural antioxidants from grape seeds [19]. The resulting composite material showed good 1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2010.05.019