Bifunctional Fluorescent AuNPs/g-C₃N₄ Nanomatrix for Sequential Detection of Cu²⁺ and Ciprofloxacin Assists the Construction of a Three-Way Molecular Logic Gate

An excitation-independent (λem = 434 nm; λex = 370 nm) fluorescent AuNPs/g-C3N4 nanomatrix developed for the selective, sensitive, and sequential fluorescence turn-off/on detection of Cu2+ and ciprofloxacin (CIP) in water and urine, respectively, is presented here. The AuNPs/g-C3N4 was synthesized via an in situ self-deposition method, with comprehensive structural characterization confirming the successful deposition of AuNPs (∼22 nm) onto g-C3N4 sheets. The AuNPs/g-C3N4 exhibited highly sensitive fluorescence quenching in response to Cu2+, displaying a linear concentration range of 0 → 55 μM and LoD of 2.33 μM. Stern–Volmer analysis indicated a static quenching mechanism for Cu2+ binding, with a KSV of 2.4 × 10–2 LM–1. Time-resolved fluorescence lifetime decay profiles revealed a slight decrease in the average lifetime of AuNPs/g-C3N4 (from 4.48 to 4.05 ns) upon successive additions of Cu2+ (0.5, 1.0, and 1.5 μM), further supporting a static quenching interaction. Furthermore, the AuNPs/g-C3N4 probe demonstrated good sensitivity toward CIP in the 0 → 100 μM concentration range, with an LoD of 4.65 μM. Notably, the addition of CIP to the AuNPs/g-C3N4·Cu2+ solution resulted in the reduction of Cu2+ to Cu+, leading to a near 100% recovery of the AuNPs/g-C3N4 fluorescence. In contrast, the average lifetime of AuNPs/g-C3N4 (4.48 ns) significantly changed to 1.22 × 10–8, 4.91 × 10–10, and 1.51 × 10–10 s upon the addition of CIP (0.5, 1.0, and 1.5 μM, respectively). The practical utility of this fluorescence nanomatrix was validated by its accurate detection of Cu2+ in real-water samples and CIP in human urine samples, achieving high recovery rates (101–106%). These findings confirm the AuNPs/g-C3N4 probe as an effective and rapid turn-off/on fluorescence sensor for the concurrent detection of Cu2+ and CIP in aqueous and biological matrices. Moreover, the observed fluorescence turn-off/on switching behavior induced by Cu2+ and CIP enabled the construction of a three-input molecular logic gate keypad lock system for potential information storage applications.