Smartphone assisted fluorescent sensor for Fe3+ and ascorbic acid determination based on off-on carbon dots probe

•The sensor shows good sensitivity, selectivity, and can detect Fe3+ and AA.•Portable smartphone can be used to assist AA detection in resource-poor settings.•The “off-on” fluorescent C-dot probe is easily synthesized, stable, and recyclable.•The detection mechanism is inferred as inner filter effect and redox reaction.