Nanostructures derived from expired drugs and their applications toward sensing, security ink, and bactericidal material.

In this research, a facile and economical route is introduced for the transformation of pharmaceutical waste (i.e., expired medicines) into value-added fluorescent carbon quantum dots (pharmaceutically derived CQDs abbreviated as 'P-CQDs'). The synthesized P-CQDs were identified to have surface functionalities of -OH, C=O, and C=C with an average size of ~2-3 nm and a high quantum yield of 35.3%. The photoluminescence of P-CQDs recorded a maximum optical emission intensity at 2.8 eV (425 nm). The binding of Cu (II) ions by -COOH functionalities on the surface of P-CQDs led to its fluorescence quenching (turn-off) over a wide Cu (II) concentration range of 0.25-50 ppm. The P-CQDs exhibited the detection limit of 0.66 ppm (well below the WHO permissible limit of 2 ppm). The fluorescence intensity of the P-CQDs-Cu (II) complex was recovered from NaHCOHence, their "off-on" behavior was also explored for security ink applications for information encryption and decryption. Moreover, the rich oxygenated groups on the surface of the P-CQDs were utilized for green synthesis of plasmonic Ag@P-CQDs nanostructures, which were also demonstrated to have enhanced potential as bactericidal materials (e.g., against both E. coli and S. aureus). The overall results of this study are demonstrated to help create new and diverse routes for converting expired drugs into value-added nanostructures.