prodrug, which released one equivalent of caffeic acid and chlorambucil upon phototriggering, showed reasonable biocompatibility, cellular uptake and photoregulared drug release in vitro. Photoresists During the 1980s, AT&T Bell Laboratories explored the use of nitrobenzyl-based PPGs as photoresists. Over the course of the decade, they developed a deep UV positive-tone photoresist where the protected substrate was added to a copolymer of poly(methyl methacrylate) and poly(methacrylic acid). Initially, the blend was insoluble. However, upon exposure to 260 ± 20 nm light, the PPG would be removed yielding 2-nitrosobenzaldehyde and a carboxylic acid that was soluble in aqueous base. Surface modification When covalently attached to a surface, PPGs do not exhibit any surface-induced properties (i.e. they behave like PPGs in solution, and do not exhibit any new properties because of their proximity to a surface). Consequently, PPGs can be patterned on a surface and removed in manner analogous to lithography to create a multifunctionalized surface. This process was first reported by Solas in 1991; protected nucleotides were attached to a surface and spatially-resolved single stranded polynucleotides were generated in a step-wise “grafting from” method. In separate studies, there have been multiple reports of using PPGs to enable the selective separation of blocks within block-copolymers to expose fresh surfaces. Furthermore, this surface patterning method has since been extended to proteins. Caged etching agents (such as hydrogen fluoride protected with 4-hydroxyphenacyl) allows to etch only surfaces exposed to light. Gels Various PPGs, often featuring the 2-nitrobenzyl motif, have been used to generate numerous gels. In one example, researchers incorporated PPGs into a silica-based sol-gel. In a second example, a hydrogel was synthesized to include protected Ca2+ ions. Finally, PPGs have been utilized to cross-link numerous photodegradable polymers, which have featured linear, multi-dimensional network, dendrimer, and branched structures. References Protecting groups Photochemistry