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Carbon dots (CD) generated from biomass were fabricated on g‐C3N4 to construct a metal‐free nanozyme with inherent peroxidase mimic functions. The formation of CD/g‐C3N4 was confirmed by different characterization techniques. The CD/g‐C3N4 displays distinct peroxidase mimic activities, as evidenced by the catalyzed oxidation of TMB (3,3',5,5', ‐tetramethylbenzidine) in the presence of H2O2, as well as color alteration from transparent to blue. The mechanistic analysis of peroxidase activity indicated that H2O2 was catalyzed by CD/g‐C3N4 to produce reactive oxygen species. In an acidic solution, the presence of Cr (VI) accelerates the breakdown of H2O2 to ⋅OH radicals thereby increasing the rate of TMB oxidation. Hence, a visual colorimetric sensor was developed for the purpose of detecting Cr(VI) with limits of detection (LOD) 0.31 μM, based on the accelerated electron transfer between TMB and H2O2 with the assistance of CD/g‐C3N4. [ABSTRACT FROM AUTHOR]