A multifunctional fluorescent chemosensor (PMN) for Al³⁺ and Zn²⁺ has been developed, which displays a dual-mode and “off–on” fluorescence response upon addition of Al³⁺ or Zn²⁺ at two different wavelengths. This was possibly due to the inhibition of PET and CN isomerism, and the activation of chelation-enhanced fluorescence (CHEF). Moreover, PMN exhibited a preferential second mode of selectivity for Al³⁺ as it ratiometrically displaces Zn²⁺ from the [PMN + Zn²⁺] complex. Such fluorescence modulation behavior mimics the performance of an “OR” logic gate. It also shows an excellent performance in the “dipstick” method.