The synthesis of carbon dots by folic acid and utilized as sustainable probe and paper sensor for Hg2+ sensing and cellular imaging

•A facile one-step solvothermal synthesis of FA-CDs using folic acid as raw materials.•FA-CDs and corresponding paper sensor can detect Hg2+ with LOD of 1.29 nM.•Both FA-CDs and paper sensor can be recovered by EDTA for detection of Hg2+.•FA-CDs exhibit excellent cellular imaging performance due to low cytotoxicity.