Hypochlorite　(ClO-)　is　one　of　the　most　important　reactive　oxygen　species　(ROS),　which　plays　an　important　role　in　sustaining　human　innate　immunity　during　microbial　invasion.　Moreover,　ClO-　is　a　powerful　oxidizer　for　water　treatment.　The　safety　of　drinking　water　is　closely　related　to　its　content.　Herein,　m-phenylenediamine　(mPD)　is　used　as　a　precursor　to　prepare　carbon　dots　(named　m-CDs)　with　highly　fluorescent　quantum　yield　(31.58%　in　water),　and　our　investigation　shows　that　the　strong　fluorescent　emission　of　m-CDs　can　be　effectively　quenched　by　ClO-.　Based　on　these　findings,　we　developed　a　novel　fluorescent　nanoprobe　(m-CDs)　for　highly　selective　detection　of　ClO-.　The　linear　range　was　from　0.05　to　7　μM　(R2　=　0.998),　and　the　limit　of　detection　(S/N　=　3)　was　as　low　as　0.012　μM.　Moreover,　a　portable　agarose　hydrogel　solid　matrix-based　ratiometric　fluorescent　nanoprobe　(m-CDs　at[Ru(bpy)3]2+)　sensor　was　subsequently　developed　for　visual　on-site　detection　of　ClO-　with　the　naked　eyes　under　a　UV　lamp,　suggesting　its　potential　in　practical　application　with　low　cost　and　excellent　performance　in　water　quality　monitoring.　Additionally,　intracellular　detection　of　exogenous　ClO-　was　demonstrated　via　ratiometric　imaging　microscopy.　©　2017　American　Chemical　Society.