Ultrafast and Efficient Detection of Formaldehyde in Aqueous Solutions Using Chitosan-based Fluorescent Polymers

Detection of a toxic formaldehyde (HCHO) pollutant in aqueous solutions is of significant importance, because HCHO is widely found in aquatic food because of illicit addition or improper storage. Many small-molecule-based fluorescent probes, which rely on HCHO-specific formaldehyde-amine condensation or the aza-Cope rearrangement reaction, have been developed in terms of facile operation and high selectivity. However, some primary challenging issues are the restricted sensitivity and long equilibrium response time caused by the slow chemical reaction between these small-molecule-based sensors and low-concentration HCHO pollutant in testing samples. Herein, a robust hydrophilic hydrazino-naphthalimide-functionalized chitosan (HN-Chitosan)-based polymeric probe is reported, which takes advantage of specific chemical reaction between HCHO and grafted hydrazino-naphthalimide groups to trigger a "turn-on" fluorescence response. Superior to its small-molecule analogs, HN-Chitosan is based on random coil polymer chains of biopolymeric chitosan, which is thus capable of employing the cooperative binding effect of multiple hydrazino-naphthalimide recognition sites and adjacent hydroxyl groups to "enrich" the low-concentration HCHO pollutant around the polymer chains via weak supramolecular interactions. Therefore, the HCHO-specific chemical reaction with grafted hydrazino-naphthalimide groups is significantly accelerated, resulting in the unprecedented ultrafast equilibrium fluorescence response (less than 1 min) and high sensitivity. Encouraged by its satisfying sensitivity, selectivity, fast response, and wide linear detection range, we successfully expand its application to real-world food and water analysis. In view of the modular design principle of our polymeric probe, the proposed strategy could be generally applicable to construct powerful polymeric probes for ultrafast detection of other important pollutants.