This PhD thesis is focused on DNA and RNA templated reactions, using PNAs as reacting partners. The work deals with the application and improvement of a previously known reaction, in which a phenylazide is reduced by a Ru(II) photocatalyst in presence of a sacrificial reductant. We first demonstrated that this reaction can serve as a method to sense oligonucleotides in vivo. The same chemistry was applied to the development of a lateral flow assay for the enzyme-free detection of DNA from food samples. The combination of the use of a new substrate for the photoreduction and a precipitating dye enabled precise localization of the fluorophore and high speed of release. Bioluminescence was used as the source of photons to enable Ru(II) photocatalysis via BRET, for the release of small molecules at biological effective concentrations. Finally, a novel Ru(II) photocatalyst was prepared from an inactive Ru(II) precursor via oligonucleotide templated ligation.