A　growing　interest　has　emerged　in　carbon　dot　(CDs)　fluorescent　probes,　as　they　are　non-destructive,　non-contact,　sensitive,　fast,　and　low-cost.　However,　the　low　fluorescence　quantum　yield　(QY)　and　the　uncertainty　of　fluorescence　quenching　mechanism　are　the　main　bottlenecks　restricting　its　application.　Here,　we　used　mustard　seeds　as　carbon　source,　CDs　were　synthesized　by　a　simple　hydrothermal　method　without　further　surface　treatment　with　the　QY　reached　a　value　as　high　as　17.92　%　in　468　nm　excitation　energy.　As　a　label-free　fluorescent　probe,　CDs　can　be　used　to　sense　Fe3+　ions　in　environmental　water　and　human　oral　keratinocytes　(Hok)　cells,　with　a　detection　limit　of　0.51　μM　and　the　linear　range　of　0–75　μM.　Moreover,　we　have　conducted　an　in-depth　study　on　the　mechanism　of　fluorescence　quenching　of　CDs　caused　by　Fe3+　ions.　Our　study　provides　powerful　evidences　that　the　mechanism　of　fluorescence　quenching　to　Fe3+　ions　is　a　synergistic　effect　of　static　quenching　(SQ)　and　aggregation-induced　effect　(AIE),　which　accompanied　by　forming　a　non-fluorescent　ground　state　complex　between　CDs　and　Fe3+　ions.　This　work　promises　to　provide　new　ideas　for　the　most　appropriate　detection　strategy　and　design　the　best　performing　sensor,　allowing　their　effective　application　in　the　sensing　field.　©　2022