Facile one-step synthesis of multifunctional carbon dots for rapid fingerprint recognition, information encryption, and Fe3+ ion detection

Fingerprint recognition plays a crucial role due to its uniqueness. However, conventional detection methods suffer from significant drawbacks, including high consumption, health risk and sample contamination, which restrict their practical application in the crime investigations and forensic analysis. This necessitates the development of simple, rapid, safe and environmentally-friendly alternative methods for fingerprints detection. In this work, we report the synthesis of novel cyan-emitting carbon dots (CDs) via a facile one-step hydrothermal reaction using polyethyleneimine and coumarin as precursors. The CDs enabled the clear visualization of fingerprint patterns on various substrates through immersion and spraying methods with pure water solutions, without the need for any cosolvents. Level 2 and level 3 details of fingermarks were evidently visible for individual identification within a short exposure time under 365 nm UV light irradiation. Additionally, fluorescent 3D fingerprint images revealed direct and specific evidence regarding pressing habits. Interestingly, the fluorescence intensity of CDs was tunable by adjusting the pH. We further fabricated invisible CDs ink for information encryption and decryption via UV irradiation and acid/alkali treatment. Moreover, CDs have the specific ability to sense Fe 3+ ions in actual water samples, with a detection limit of 0.613 μM and a linear range of 0–27 μM. We conducted an in-depth study on the fluorescence quenching mechanism of CDs caused by Fe 3+ ions, proposing a plausible mechanism based on a synergistic effect of dynamic quenching, static quenching and inner filter effect. Therefore, this study provides a promising multifunctional nanomaterial for fingerprint recognition, data encryption, and Fe 3+ detection.