A very simple molecule derived from salicylaldehyde and N-phenyl ethylenediamine (L1) functions as dual-mode ratiometric fluorescence “turn on” sensors for Zn²⁺ and Al³⁺ at two different wavelengths. The sensing is based on the combined effect of inhibition of excited-state intra-molecular proton transfer (ESIPT), CHN isomerization and chelation-enhanced fluorescence (CHEF). Moreover, the [L1–Zn²⁺] system functions as a better Al³⁺ sensor where Al³⁺ ratiometrically displaces Zn²⁺ from the [L1–Zn²⁺] complex. Emission wavelength dependent differentiation of Zn²⁺ and Al³⁺ using L1 allows us to develop a binary logic gate that functions as a molecular switch. L1 efficiently detects Zn²⁺ and Al³⁺ in human breast cancer cells (MCF7) while the [L1–Zn²⁺] complex specifically detects Al³⁺ in the said cells.