Liquid-liquid extraction and purification of oil red O derived nitrogen-doped highly photoluminescent carbon dots and their application as multi-functional sensing platform for Cu2+ and tetracycline antibiotics

•NCDs were prepared using oil red O as precursor for the first time.•QY of NCDs reached 68% using liquid-liquid extraction and purification method.•NCDs demonstrate superior fluorescence property and high photostability.•Multi-functional sensing platform for Cu2+ and TCs detection were designed.•NCDs-AH provided an on-site visualization qualitative tool of Cu2+.