Selective　oxidation　of　aromatic　alkanes　by　C-H　activation　is　one　of　the　key　reactions　in　organic　synthesis　and　the　chemical　industry.　Activation　of　C-H　bonds　to　obtain　high-value　added　products　under　mild　conditions　by　using　sustainable　carbocatalysts　is　of　particular　interest.　Herein,　we　report　a　sustainable,　green　and　template-free　strategy　towards　the　fabrication　of　N-doped　and　N/S　codoped　carbon　nanosheets　by　metal-free　carbonization　of　bioprecursors　guanine　and　guanine　sulfate.　The　formation　of　thin　and　N/S　codoped　carbon　nanosheets　was　induced　by　multiple　interactions　of　the　nucleobases.　Benefiting　from　the　unique　textural　structure　of　the　as-synthesized　carbons,　including　ultrathin　thickness,　optimal　porosity,　and　rich　structural　defects,　and　the　synergistic　coupling　effect　of　multiple　dopants,　the　carbon　nanosheets　show　a　high　catalytic　performance　with　85%　ethylbenzene　conversion　and　98%　selectivity　to　acetophenone　at　80　°C　after　4　h　reaction,　which　outperforms　other　equivalent　benchmarks　(e.g.　8.5　times　higher　conversion　and　3.2　times　higher　selectivity　than　those　of　oxidized　carbon　nanotubes).　Density　functional　theory　simulations　indicate　that　the　oxidation　of　ethylbenzene　is　catalyzed　by　the　synergistic　effect　of　p-N/S　and　g-N/S　catalysts　via　an　OH　radical　mechanism.　This　N/S　codoped　strategy　provides　guidance　for　the　design　of　carbon-based　catalysts　for　the　selective　oxidation　of　other　alkanes.　This　journal　is　©　The　Royal　Society　of　Chemistry.