Herein　a　rapid　and　sensitive　fluorometric　bioanalysis　platform　for　mercury(ii)　(Hg2+)　detection　was　innovatively　developed　using　ultrathin　two-dimensional　MXenes　(Ti3C2)　as　fluorescence　quencher　and　Hg2+-induced　exonuclease　III　(Exo　III)-assisted　target　recycling　strategy　for　efficient　signal　amplification.　Initially,　fluorophore-labeled　single-stranded　DNA　(FAM-labeled　probe)　can　be　easily　adsorbed　onto　the　surface　of　ultrathin　Ti3C2　nanosheets　by　hydrogen　bonding　and　metal　chelating　interaction,　and　the　fluorescence　signal　emitted　by　the　FAM-labeled　probe　is　quenched　strongly　owing　to　the　fluorescence　resonance　energy　transfer　between　the　FAM　and　ultrathin　Ti3C2　nanosheets.　Upon　sensing　the　target　Hg2+,　the　protruding　DNA　fragment　at　the　3′　end　of　hairpin　will　hybridize　with　primer　(hairpin-Hg2+-primer),　and　then　further　digested　by　Exo　III　to　produce　a　probe　(nicker).　The　released　target　Hg2+　and　primer　continue　to　participate　in　the　next　recycling,　resulting　in　more　hairpin　probes　becoming　nickers.　The　combination　of　a　large　number　of　nickers　and　FAM-probe　resulted　in　a　significant　increase　in　the　fluorescence　signal　of　the　system,　which　was　attributed　to　the　fact　that　the　double　helix　DNA　was　more　rigid　and　separated　from　the　surface　of　the　ultrathin　Ti3C2　nanosheets.　The　obvious　fluorescence　signal　change　of　the　Ti3C2-based　Exo　III-assisted　target　recycling　can　be　accurately　monitored　by　fluorescence　spectrometry,　which　is　also　proportional　to　the　concentration　of　Hg2+.　Under　optimum　operating　conditions,　the　peak　intensity　(520　nm　wavelength)　of　fluorescence　increased　with　increasing　Hg2+　within　a　wide　dynamic　working　range　from　0.05　nM　to　50　nM　(R2　=　0.9913)　with　a　limit　of　detection　down　to　42.5　pM.　The　proposed　strategy　uses　ultrathin　MXenes　as　a　platform　for　binding　nucleic　acids,　which　contributes　to　its　potential　in　nucleic　acid　hybridization-based　biosensing　and/or　nucleic　acid　signal　amplification　bio-applications.　©　The　Royal　Society　of　Chemistry.