An Efficient In Situ Paper‐Based Sensor for Iron <scp>II</scp> and <scp>III</scp> in Real Samples

ABSTRACT Herein, we report a colorimetric detection of iron II and III through intensity and wavelength modulation of the localized surface plasmon resonance (LSPR) signaling at maximum intensity of 450 nm, while presenting red (482 nm) and blue (400 nm) shifts for the two iron forms, respectively. Silver nanoparticles functionalized with Bougainvillea (BV) leaves aqueous extract (BV‐AgNPs) having spherical to hexagonal shape (as affirmed by LSPR, dynamic light scattering (DLS), and transmission electron microscopy (TEM)), and an average size of 19.5 ± 0.46 nm were synthesized. Interactions of the fabricated probe with various heavy metal ions provided rapid detection, higher sensitivity, and significant selectivity for only iron II and III. It was found that after iron interaction, bleaching and red color change were observed, which likely arises from Fe III‐induced oxidation/dissolution of AgNPs, and iron II‐induced controlled aggregation through surface coordination with phytochemical capping agents on BV‐AgNPs. That is why BV‐AgNPs offer the lowest detection limit for iron II and III (0.123 and 3.89 nM, respectively) with significant linearity, recoveries in real samples, and higher sensitivity than the Environmental Protection Agency recommended values. Moreover, BV‐AgNPs‐coated paper can also colorimetrically differentiate iron II and III in aqueous solutions enabling very simple in situ monitoring.