Simultaneous coupling of metal removal and visual detection by nature-inspired polyphenol-amine surface chemistry

The interplay between polyphenols, amines, and metals has broad implications for surface chemistry, biomaterials, energy storage, and environmental science. Traditionally, polyphenol-amine combinations have been recognized for their ability to form adhesive, material-independent thin layers that offer a diverse range of surface functionalities. Herein, we demonstrate that a coating of tannic acid (TA) and polyethyleneimine (PEI) provides an efficient platform for capturing and monitoring metal ions in water. A unique feature of our PEI/TA-coated microbeads is the 'Detection-Capture' (Detec-Ture) mechanism. The galloyl groups in TA coordinate with Fe(III) ions (capture), initiating their oxidation to gallol-quinone. These oxidized groups subsequently react with PEI amines, leading to the formation of an Fe(II/III)-gallol-PEI network that produces a vivid purple color, thereby enabling visual detection. This mechanism couples metal capture directly with detection, distinguishing our approach from existing studies, which have either solely focused on metal removal or metal detection. The metal capturing capacity of our materials stands at 0.55 mg g^-1, comparable to that of established materials like alginate and wollastonite. The detection sensitivity reaches down to 0.5 ppm. Our findings introduce a novel approach to the utility of metal-polyphenol-amine networks, presenting a new class of materials suited for simultaneous metal ion detection and capture in environmental applications.