# Photocatalytic Lithography
## 4 Applications of Direct and Remote Photocatalytic Lithography  
Patterning of Self-Assembled Monolayers and Generation of Superhydrophilic-Superhydrophobic Patterns  
Molecules carrying functional \"head\" groups capable of physical and/or chemical interactions with a surface can form, under proper conditions, ordered assemblies called self-assembled monolayers (SAMs). Typical examples of \"head\" groups include thiols, silanes, and phosphonates. The \"tail\", generally an organic moiety such as an alkyl chain, is almost as important as the \"head\" for determining the final properties of the functionalized surface and for directing the interactions between the different molecules. SAMs are characterized by the presence of relatively ordered domains and can be used as molecular photoresists in lithography, as their selective modification and/or degradation can lead to the formation of patterns with different chemical functionalities [20]. Such an approach, however, typically requires high power and short wavelength (\(\leq\)254 nm) UV light, as it is supposed to directly break chemical bonds.  
By exploiting photo-generated reactive oxygen species, photocatalytic lithography makes it possible to use near-UV (\(\geq\)365 nm) or even visible light sources. This is a remarkable advantage, especially on the side of safety and user-friendliness [21; 22]. SAMs on titanium dioxide surfaces rapidly degrade upon exposure to UV light, meaning that direct photocatalytic lithography can be easily carried on. Fewer applications have been reported for remote photocatalysis. The fabrication of large-area nanopatterned alkyl phosphonate self-assembled monolayers on titanium oxide surfaces has been achieved by means of interferometric lithography [23].  
The generation of surface patterns with different wettability properties is of interest for many different applications, and the offset printing plate is one of the prime candidates for the application of superhydrophilic-superhydrophobic patterns on titanium dioxide. State-of-the-art offset printing plates use hydrophilic-hydrophobic patterns on aluminum substrates, which are fabricated as follows: (i) a positive photoresist is coated on an anodized aluminum substrate; (ii) the substrate is irradiated with UV light through a photomask featuring the desired pattern; (iii) the exposed photoresist is removed, leaving a pattern of hydrophobic, photoresist-coated areas, and hydrophilic, bare aluminum areas. This conventional process suffers from many drawbacks, including: the aluminum substrate is single-use, chemical waste is generated from the developing step, and the printing resolution is limited by the wettability contrast between hydrophilic and hydrophobic areas, which is of course lower than superhydrophilic-superhydrophobic ones.  
The application of a titanium dioxide layer to improve the performance of conventional aluminum offset printing plates has been described by [PERSON] et al. [24; 25]. The patterns consisted of organic compounds susceptible to photochemical degradation by TiO\({}_{2}\) under UV irradiation (usually SAMs forming long-chain alkyl-based molecules such as octadecylalkoxy- or chloro-silanes). After the printing process, the offset plate could be cleaned and restored to its original state simply by UV irradiation of the entire surface, which could then be patterned again with superhydrophilic-superhydrophobic areas, thus providing a reusable printing plate. To overcome the mechanical stress-related problems associated with the removal of the titania coating, the application of oxidized titanium offset plates has been successfully demonstrated, using water-based ink patterns deposited with an ink-jet printer as an unconventional photomask.  
Examples of more sophisticated applications of photocatalytically-produced wettability patterns include the development of multiscale chemical gradients [26], the patterning of thin oxide films [27], the formation of patterns for the immobilization of algae [28], and mammalian cells [29].