Colorimetric detection of Fe³⁺ and Fe²⁺ and sequential fluorescent detection of Al³⁺ and pyrophosphate by an imidazole-based chemosensor in a near-perfect aqueous solution

A novel chemosensor was designed and synthesized for various analytes: Fe³⁺, Fe²⁺, Al³⁺ and pyrophosphate. The sensor showed a selective color change from yellow to orange toward both Fe³⁺ and Fe²⁺ in a near-perfect aqueous solution, which could be reusable simply through treatment with ethylenediaminetetraacetic acid. The detection limits (0.27 μM and 0.32 μM) for Fe³⁺ and Fe²⁺ were much lower than the environmental protection agency guideline (5.37 μM) in drinking water. The sensor could be used to quantify Fe³⁺ in real water samples. Moreover, this sensor acted as a ‘turn-on’ and ‘turn-off’ type fluorescent sensor toward Al³⁺ and pyrophosphate. The sensing mechanism of the sensor for Al³⁺ could be explained by chelation-enhanced fluorescence effect, which was supported by theoretical calculations. Through a metal-complex displacement method, the sensor-Al³⁺ complex selectively responded to pyrophosphate over various anions especially including phosphate-based anions. Interestingly, the sensor could be used to sequentially detect both Al³⁺ and pyrophosphate in the living cells.