Introduction to the themed issue ‘Nano- and Molecular Engineering of Photosensitisers’

Like many English words, photosensitiser arises from Greek roots and literally means “something sensitive to light”, clearly a very broad definition. In this themed issue of Photochemical & Photobiological Sciences, photosensitiser stands only for systems which generate Reactive Oxygen Species upon appropriate excitation and in suitable conditions. Most of the time (but not exclusively) the species is singlet oxygen.

Photosensitising systems involve an infinite variety of molecular and nano-structures and involve multiple mechanisms of action. They are used in medicine as photodynamic therapy treatment for cancer but also have activity against infections and optical diseases, and are used in the environment for water decontamination, etc. Depending on the excitation mode, specific photosensitising structures are needed.

The role of the engineer, often but not always a chemist, is crucial in the development of new photosensitisers, which can be molecular or nano. Structure–activity relationship studies and good communication with the other scientists in the field – photodynamic applications being by nature multi-disciplinary – are essential to be able to design new photosensitising systems likely to exhibit improved efficacy. These reviews and papers focus on the design, synthesis and evaluation of different families of photosensitisers.

The different strategies used to control the generation of singlet oxygen are described by Senge et al. through the design of photosensitizers and delivery systems, humorously reported as the Good, the Bad and the Ugly (DOI: 10.1039/C8PP00008E). Several reviews focus on various photosensitising systems used in the photodynamic therapy of cancer: the role of azaphthalocyanines is reported by Zimcik, Novakova et al. (DOI: 10.1039/C8PP00106E), the evolution of aminolevulinic acid derivatives is summarized by Eggleston et al. (DOI: 10.1039/C8PP00362A), while the most recent Bodipy derivatives are compiled by Chung, Lee et al. (DOI: 10.1039/C8PP00113H). Habermeyer et al. provide an industrial point of view on the chemistry of photosensitisers (DOI: 10.1039/C8PP00222C), and a full glycoporphyrin story from the Institut Curie is narrated by Thomas et al. (DOI: 10.1039/C8PP00123E). Finally, the importance of two types of nanoparticles is evidenced, gold nanoparticles specifically in anticancer PDT by Russell et al. (DOI: 10.1039/C8PP00271A), and Mesoporous Silica Nanoparticles by Durand et al. (DOI: 10.1039/C8PP00143J), these later being used not only against cancer but also in antibacterial treatments. The pros and cons of fullerenes, with the same broad usage, are discussed by Hamblin (DOI: 10.1039/C8PP00195B). Focusing on antimicrobial applications, Wainwright (DOI: 10.1039/C8PP00145F) details the importance and superiority of synthetic small-molecule photoantimicrobials. Another review by Frochot et al. (DOI: 10.1039/C8PP00112J) highlights the promises of X-ray irradiation, an emerging excitation technique, and the advantages of combining photosensitizers with NO photodononors is featured by Sortino et al. (DOI: 10.1039/C8PP00272J). Last but not least, leaving the biomedical field, the environmental benefits of the remediation by advanced oxidation processes to eliminate chemical contaminants in wastewaters is reviewed by Faustino, Almeida et al. (DOI: 10.1039/C8PP00249E).

Five research articles are also part of this special issue. A two-part article by Lange et al. (DOI: 10.1039/C8PP00318A and DOI: 10.1039/C8PP00317C) deals with cyclopeptidic photosensitizer prodrugs used as proteolytically triggered drug delivery systems of pheophorbide for selective photodiagnosis and photodynamic therapy; the factors affecting the photothermal therapy potential of small iron oxide nanoparticles at NIR wavelengths are reported by Yağcı Acar et al. (DOI: 10.1039/C8PP00203G); the antibacterial activity of a photosensitive hybrid cellulose fabric is demonstrated by Sol et al. (DOI: 10.1039/C8PP00212F); and finally the effect of thiophene substitution on the intersystem crossing of arene photosensitizers is investigated by Zhao et al. (DOI: 10.1039/C8PP00230D).

I wish to express my appreciation to all the authors for their enthusiasm in contributing to this special issue, which is a collection of reviews and research articles from all around the world. As for light, the topic has indeed no frontiers! I hope readers from different research fields and levels, whether it be students becoming acquainted with the subject, or more experienced colleagues, will appreciate the content and find it useful.

Finally, I wish to thank the editorial staff of Photochemical & Photobiological Sciences for assistance, and Professor Rex Tyrrell, Editor-in-Chief, for his support and help during the whole process.

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