As　recently　emerging　nanomaterials,　boron　nanosheets　(BNSs)　have　attracted　more　and　more　attention　in　various　fields　such　as　supercapacitors,　photodetectors,　bioimaging,　and　electrocatalysis　due　to　their　advantages　of　good　biological　compatibility,　environmental　friendliness,　and　good　electro-optical　properties.　However,　the　study　and　application　of　BNSs　in　chemical　and　biological　sensing　are　still　in　the　infant　stage,　mainly　due　to　the　requirement　of　complicated,　high-cost,　and　time-consuming　preparation　strategies.　In　this　work,　a　new　class　of　BNSs,　namely　oxidized-BNSs　(i.e.,　ox-BNSs),　were　easily　and　rapidly　synthesized　by　chemically　treating　boron　powder　with　diluted　HNO3　in　a　very　short　time　(less　than　15　min).　The　composition,　morphology,　optical　property,　and　peroxidase　mimetic　activity　of　obtained　ox-BNSs　were　investigated　in　detail.　The　prepared　ox-BNSs　were　several-layered　nanosheets　with　abundant　oxygen-containing　groups,　emitted　blue　fluorescence,　and　possessed　good　intrinsic　peroxidase　mimetic　activity,　based　on　which　a　sensitive　and　selective　colorimetric　sensor　was　developed　for　detection　of　H2O2　and　glucose.　The　new　easy　preparation　strategy　and　good　sensing　performances　of　the　prepared　ox-BNSs　would　greatly　stimulate　the　study　and　application　of　BNSs　in　chemo-　and　biosensing.　©　2023　American　Chemical　Society.