<p>A portable and practical chemosensor, DQHP ((<i>E</i>)-2,4-dibromo-6-((2-(quinolin-2-yl)hydrazineylidene)methyl)phenol), was synthesized for the sensing of Cu²⁺. DQHP selectively detected Cu²⁺ via fluorescence turn-off response and a visible color variation from pale yellow to yellowish-brown in DMSO/PBS buffer (5:5). The calculated limits of detection for Cu²⁺ were 0.599 µM (fluorescence) and 1.34 µM (UV-vis), both well below the World Health Organization’s safe threshold of 31.5 µM. DQHP was effectively utilized to detect Cu²⁺ through fluorescence and UV-vis spectroscopy in a pH range of 5–9 and could quantify Cu²⁺ in real samples, including drinking and stream water. In addition, test strip and smartphone-based Cu²⁺ detection enabled portable and convenient on-site analysis. The sensing process of Cu²⁺ by DQHP was elucidated through ESI-MS, Job-plot, ¹H NMR titration, and TD-DFT calculations.</p>