Squaraine-hydrazine adducts for fast and colorimetric detection of aldehydes in aqueous media

Capitalizing on the nucleophilic addition of hydrazine toward the central cyclobutenyl core and affinity of aldehydes to hydrazine group, two water-soluble squaraine dyes (SQOH and SQPY) were synthesized as sensitive colorimetric and fluorescent chemosensors for aldehydes. The color change from blue to colorless and back to blue these sensors underwent was readily observed, even by the naked eyes. The response was fast (less than 1 s) and the detection limit for formaldehyde as an example was about 60 μM. In contrast to the previous nucleophilic attack to squaraines and the other amine references studied, this work underwent a whole ON-OFF-ON sensing circle based on the characteristics of hydrazine. Meanwhile, the different photophysical properties of the two squaraines related to their different structures were also demonstrated. Importantly, a possible sensing mechanism is proposed suggesting these types of dyes hold great potential in the area of rapid, sensitive, and convenient detection of aldehydes.