In　this　work,　we　proposed　a　new　method　based　on　carbon　dots　(named　m-CDs)　for　selective　and　efficient　detection　of　nitrite　(NO2-),　which　was　based　on　the　interaction　between　the　amine　group　of　m-CDs　and　NO2-　via　a　diazo　reaction　that　produced　diazonium　salts　and　induced　the　fluorescence　quenching　of　m-CDs.　The　concentration　of　NO2-　shows　a　good　linear　relationship　with　a　quenched　fluorescence　intensity　from　0.063　to　2.0　mu;M　(R2　=　0.996)　with　a　detection　limit　of　0.018　mu;M.　In　addition,　a　ratiometric　fluorescence　probe　(m-CDsRu(bpy)3]2+)　was　constructed　via　electrostatic　interaction　by　introducing　Ru(bpy)3Cl2middot;6H2O　as　an　internal　reference　fluorescent　reagent.　Interestingly,　a　transition　of　the　fluorescent　color　of　the　ratiometric　probe　from　cyan　to　red　could　be　visually　observed　upon　increasing　the　concentration　of　NO2-.　Based　on　these　findings,　a　ratiometric　fluorescent-based　portable　agarose　hydrogel　test　kit　was　fabricated　and　applied　for　on-spot　assessment　of　NO2-　content　within　10　min.　As　far　as　we　know,　this　is　the　first　ratiometric　fluorescent　sensor　for　visual　detection　of　NO2-.　It　has　broad　application　prospects　in　environmental　monitoring　and　food　safety　assessment.　©　2019　American　Chemical　Society.