Highlights　We　report　a　simple　solution-process　route　to　realize　ammonia　(NH3)　gas　sensor　based　on　graphene　quantum　dots　(GQDs).　Transmission　electron　microscopy　analysis　confirmed　that　the　8-10　nm　GQDs　were　formed　from　multi-walled　carbon　nanotubes　by　using　ultrasonication　treatment.　The　as-fabricated　gas　sensor　showed　promising　selectivity　response　when　expose　to　NH3　ambient　at　room　temperature.　It　is　indicated　that　by　adjusting　the　pH　value　of　the　aqueous　GQDs　in　acidic　and　neutral,　two　types　of　gas　sensors　with　contrary　current　responses　could　be　obtained,　which　might　be　resulted　from　quantum　confinement,　edge　effects　and　presence　of　functional　groups　on　GQDs.　©　2015　Elsevier　B.V.