Although cadmium has been recognized as a highly toxic heavy metal and poses many detrimental effects on human health, the Cd²⁺-uptake and nosogenesis mechanisms are still insufficiently understood, mainly because of the lack of facile analytical methods for monitoring changes in the environmental and intracellular Cd²⁺ concentrations with high spatial and temporal reliability. To this end, we present the design, synthesis, and photophysical properties of a cadmium sensor, DQCd1 based on the fluorophore 4-isobutoxy-6-(dimethylamino)-8-methoxyquinaldine (model compound 1). Preliminary investigations indicate that 1 could be protonated under neutral media and yield a resonance process over the quinoline fluorophore. Upon excitation at 405 nm, 1 shows a strong fluorescence emission at 554 nm with a quantum yield of 0.17. Similarly, DQCd1 bears properties comparable to its precursor. It exhibits fluorescence emission at 558 nm (Φ_f = 0.15) originating from the monocationic species under physiological conditions. Coordination with Cd²⁺ causes quenching of the emission at 558 nm and simultaneously yields a significant hypsochromic shift of the emission maximum to 495 nm (Φ_f = 0.11) due to inhibition of the resonance process. Thus, a single-excitation, dual-emission ratiometric measurement with a large blue shift in emission (Δλ = 63 nm) and remarkable changes in the ratio (F_{495 nm}/F_{558 nm}) of the emission intensity (R/R₀ up to 15-fold) is established. Moreover, the sensor DQCd1 exhibits very high sensitivity for Cd²⁺ (K_d = 41 pM) and excellent selectivity response for Cd²⁺ over other heavy- and transition-metal ions and Na⁺, K⁺, Mg²⁺, and Ca²⁺ at the millimolar level. Therefore, DQCd1 can act as a ratiometric fluorescent sensor for Cd²⁺ through inhibition of the resonance process. Confocal microscopy and cytotoxicity experiments indicate that DQCd1 is cell-permeable and noncytotoxic under our experimental conditions. It can indeed visualize the changes of intracellular Cd²⁺ in living cells using dual-emission ratiometry.