Metal Doped Poly(tannic acid) Nanoparticles for Selective Detection and Discrimination of Phosphate Anions

Metal-mediated modulation of fluorescence provides a powerful route for developing highly selective fluorescent sensors. In this study, fluorescent polytannic acid nanoparticles (PTA Nps) were synthesized for phosphate detection, with and without metal-ion mediation. The nanoparticles exhibited a pronounced ratiometric fluorescence change (∼3.2-fold) in response to pyrophosphate (PPi), displaying excellent selectivity over other analytes and achieving a low detection limit of 3.93 μM. Mechanistic investigations indicated that PPi triggers deprotonation of phenolic −OH groups and forms hydrogen-bonding interactions with the PTA NPs. Additionally, the nanoparticles showed significant fluorescence quenching, ∼15.5-fold with Fe3+ and ∼2.5-fold with Cu2+ ions in aqueous medium. This quenching is attributed to the higher complexation affinity and binding constant for Fe3+ (∼48,310 M–1), compared to Cu2+ (∼3460 M–1). XPS, FT-IR, and EPR analyses confirmed metal–nanoparticle complexation as the mechanism underlying these responses. Upon PPi addition, Fe3+-incorporated nanoparticles displayed a turn-on fluorescence response (∼6.5-fold), while Cu2+-incorporated nanoparticles underwent further fluorescence quenching (∼2.0-fold) selectively in the presence of monohydrogen phosphate (HPO42–). Mechanistic studies revealed that PPi removes the Fe3+ complex from the nanoparticle surface, enabling fluorescence recovery, whereas the formation of a ternary Cu2+-HPO42– complex induces additional quenching. The sensor system demonstrated practical applicability by successfully detecting PPi in real samples, including tap water, wastewater, and soil extracts. Furthermore, a paper-based sensing platform was developed, enabling rapid and convenient on-site PPi detection and underscoring the potential of PTA NPs for environmental monitoring and diagnostic applications.