Surface state-regulated redox carbon nanodots for plasmonic morphology-dependent ratiometric sensing

•Novel CNDs with tunable redox ability and different color emission are synthesized.•The increased graphitization of core causes the red-shift of CNDs fluorescence.•The redox abilities of CNDs are regulated by the change of surface states.•Redox CNDs can be visually distinguished by the colorful plasmonic variation.•Based on fluorescence and scattering changes, CNDs are used for ratiometric Au3+ sensing.