<p>Excessive Cu²⁺ levels pose severe threats to human health and ecosystems, necessitating sensitive and selective detection methods. A novel ratiometric fluorescent probe (CDs@RBH) based on fluorescence resonance energy transfer (FRET) was developed through electrostatic assembly. This nanoplatform employs carbon quantum dots (CDs) as the energy donor and rhodamine hydrazide (RBH) as the acceptor. Upon Cu²⁺ addition, the spirolactam ring of RBH opens, triggering a distinct UV–Vis absorption peak at 550 nm, which overlaps with the emission spectrum of CDs (450 nm), enhancing FRET efficiency. This mechanism enables dual-signal readout: the fluorescence intensity of CDs at 450 nm decreases, while the RBH-Cu²⁺ emission at 575 nm increases, allowing for precise ratiometric detection. The probe exhibits high selectivity and exceptional sensitivity, with a detection limit (LOD) of 0.65 μM and quantification limit (LOQ) of 2.09 μM. Practical applications in tap water samples demonstrate excellent recovery rates (96–108%). This work presents a straightforward synthesis method, environmental stability, and dual-channel signal verification for detection, making it a promising tool for real-world Cu²⁺ monitoring in environmental and biological systems.</p>