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

Ferrous ion (Fe) 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 Fe, it is still a huge challenge to sensitively and accurately determine Fe especially in interstitial water. Herein, we prepared a ferrozine gel rod for valence fixation during sampling and subsequent smartphone-based detection of Fe. The electrode potential of the redox pair can be varied through the formation of Fe-ligand complexes, and when E was higher than [Formula: see text] , the oxidation of Fe by O was hindered, thus achieving the valence fixation of Fe. Six ligands were screened, and it was found that ferrozine could effectively increase the redox potential after complexing with Fe, and also exhibits an obvious color change while fixing the valence of Fe. To facilitate Fe 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 Fe 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 Fe 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 Fe 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 Fe 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 Fe in environmental waters.