Fluorescent graphitic carbon nitride and graphene oxide quantum dots as efficient nanozymes: Colorimetric detection of fluoride ion in water by graphitic carbon nitride quantum dots

•Fluorescent g-CNQDs and GQDs are synthesized using simple cost-effective method.•Peroxidase mimic activity of prepared g-CNQDs and GQDs (nanozymes) were established.•A simple colorimetric system is established for ultrasensitive detection of fluoride ions.•High sensitivity and selectivity of g-CNQDs for fluoride ions detection with LOD of 0.53 mg/L.