Valence fixable ferrozine gel rod combined with smartphone for facile determination of redox-active Fe2+ in environmental water.

Ferrous ion (Fe²⁺) can indicate the redox situation of water and also plays an important role in maintaining the ecological balance of water bodies. However, due to the redox-active property of Fe²⁺, it is still a huge challenge to sensitively and accurately determine Fe²⁺ especially in interstitial water. Herein, we prepared a ferrozine gel rod for valence fixation during sampling and subsequent smartphone-based detection of Fe²⁺. The electrode potential of the redox pair can be varied through the formation of Fe²⁺-ligand complexes, and when E_complex was higher than [Formula: see text] , the oxidation of Fe²⁺ by O₂ was hindered, thus achieving the valence fixation of Fe²⁺. Six ligands were screened, and it was found that ferrozine could effectively increase the redox potential after complexing with Fe²⁺, and also exhibits an obvious color change while fixing the valence of Fe²⁺. To facilitate Fe²⁺ detection, a cross-linked porous polymer gel rod prepared by acrylamide and sodium alginate was used to encapsulate the ferrozine molecules. The ferrozine gel rod enabled fixation the valence of Fe²⁺ longer than 30 days, and the resulted purple-red color was pictured and analyzed by a smartphone. Ultimately, the developed ferrozine gel rod sensing system was able to achieve sensitive and linear detection of Fe²⁺ in the range of 1-200 μM with the limit of detection as low as 0.33 μM, and it also exhibited excellent selectivity and anti-interference ability. The accuracy and reliability of the method was verified by the determination of Fe²⁺ in spiked water samples and certified standard reference water samples. Finally, the ferrozine gel rod sensing system was successfully applied to in-situ detection of Fe²⁺ in interstitial water, overlying water and upper water of lake and river. This facile system that enabled valence fixation and fast detection is promising for detection of Fe²⁺ in environmental waters.