# Photocatalytic Lithography
## 5 Singlet Oxygen-Based Photocatalytic Lithography  
Singlet oxygen, O\({}_{2}\)(\({}^{1}\)\(\Delta_{\mathrm{g}}\)), the lowest excited electronic state of molecular oxygen (Figure 9a), has a characteristic chemistry which sets it apart from other reactive oxygen species. It can be prepared in several ways, including chemical reactions [59; 60], but one of the most convenient ways involves electronic energy transfer from an excited state of a given molecule, a so-called sensitizer. This approach has great relevance for cell biology and biotechnology (e.g., cellular stress) as well as for medical applications (e.g., photodynamic therapy of tumors), and has been applied for the chemical patterning of surfaces. One of the first applications of singlet oxygen to photoimaging processes was reported  
Figure 8: Photocatalytic patterning of graphene. (**a**) Schematic illustration of the fabrication procedure of layer-by-layer hybrid films consisting of PDDA/GO/PDDA/TiO\({}_{\mathrm{x}}\). (**b**) Photographs of (PDDA/GO/PDDA/TiO\({}_{\mathrm{x}}\))\({}_{\mathrm{n}}\) hybrid films with different layers, before and after illumination with a 300 W Xe lamp for 30 min. (**c**) Model setup for the photocatalytic reduction lithography (PRL) approach and the corresponding PRL patterns on the (PDDA/GO/PDDA/TiO\({}_{\mathrm{x}}\))\({}_{\mathrm{20}}\) hybrid film after illumination. Reproduced by permission of the Royal Society of Chemistry from ref. [54]. (**d**) Schematic illustration of the photocatalytic approach to engineering single- or few-layer graphene. (**e,g**) Optical and (**f-i**) SEM images of patterned reduced graphene oxide (RGO) illustrating the feasibility of complex structural design. Scale bars: (**e–h**) 50 \(\upmu\)m, inset of (**e**) 100 \(\upmu\)m, (**i**) 10 \(\upmu\)m. Reproduced with permission from [55], American Chemical Society, 2011.  
in 1987 by [PERSON] et al., who described the use of singlet oxygen to develop a photosensitive lithographic plate. Zinc tetraphenylporphyrin was selected as the photosensitizer [61]. [PERSON] et al. and, one year later in 1988, [PERSON] et al. described processes for the recording of holograms produced by near-infrared emitting lasers, which involved the sensitization of singlet oxygen and oxidation of an adapted trap. [PERSON] et al. used tricarbocyanine dyes as photosensizers, while different dyes (thionine, methylene blue, eosin, rose Bengal, acridine orange) were chosen by [PERSON] et al. [62]. Porphyrins and phthalocyanines are among the most versatile photosensizers for the production of singlet oxygen [63]. They are a large family of pyrrole-based molecules with remarkable chemical and thermal stability. Owing to their large conjugated electron system, they display intense absorption bands in the visible range, a property which is shared by their metal complexes. The optical spectrum of porphyrins is characterized by a very strong absorption in the 400-450 nm region (Soret band, due to a \(\pi\)-\(\pi^{\bullet}\) transition) and in the 500-700 nm region (Q band) [64]. Porphyrin-based photocatalytic lithography was proposed by [PERSON] et al. in 2008 [65] as an alternative approach to photocatalytic oxide-based lithographic approaches. Suitable photosensizers, such as copper chlorophyll, hematoporphyrin IX, and magnesium phthalocyanine, were applied from their solutions on the surface of a patterned stamp mask made of poly(dimethyl siloxane) (PDMS). PDMS was chosen as a convenient stamp matrix for its transparency, flexibility, and chemical inertness. In this approach, the stamp is a mask, not in the sense of blocking light exposure to certain regions of the substrate, but rather in the sense of localizing chemical reactivity to the areas of contact between the stamp and the surface to be patterned. In other words, in contrast to usual stamping or contact printing, mass is not transferred from the stamp to the substrate.