The　authors　describe　a　fluorometric　method　for　the　determination　of　the　activity　of　the　enzyme　DNase　I.　Gold　nanoparticles　were　functionalized　with　a　Ru(II)bipyridyl　complex　to　which　dsDNA　chains　were　covalatently　linked　via　carboxy　groups.　The　emission　of　the　Ru(II)　complex　(measured　at　excitation/emission　wavlengths　of　450/629　nm)　is　quenched　by　the　gold　nanoparticles　due　to　surface　energy　transfer.　The　emission　is　restored　as　soon　as　the　Ru(II)-labeled　dsDNA　is　cut　off　by　the　action　of　DNase　I.　Based　on　this　mechanism,　the　DNase　I　activity　in　vitro　was　detected　with　high　sensitivity.　The　effect　of　the　different　lengths　of　DNA　chains　on　luminescence　quenching　and　sensing　ability　was　studied.　The　intensity　of　restored　luminescence　is　linearly　related　to　the　activity　of　DNase　I　in　the　range　from　4　to　400　mU·mL−1　with　a　detection　limit　of　50　μU·mL−1.　[Figure　not　available:　see　fulltext.].　©　2017,　Springer-Verlag　Wien.