Green Synthesis of Tannic Acid‐Modified Gold Nanozymes for Rapid and Selective Colorimetric Detection of Fe ³⁺ , H ₂ O ₂ , and I ⁻

A cost‐effective and rapid colorimetric sensing platform was developed to overcome the high cost and operational complexity of traditional analytical methods for detecting ferric ions (Fe 3+ ), hydrogen peroxide (H 2 O 2 ), and iodide ions (I − ). Tannic acid‐modified gold nanoparticles (TA‐AuNPs) were synthesized via a one‐pot method, where tannic acid (TA) acted as both reducing and stabilizing agent. The resulting TA‐AuNPs exhibited excellent stability under acidic, alkaline, and high‐salinity conditions and demonstrated peroxidase‐like catalytic activity. In the presence of Fe 3+ , TA‐AuNPs catalyzed the H 2 O 2 ‐mediated oxidation of 3,3′, 5,5′‐tetramethylbenzidine (TMB) to its blue oxidized form (ox‐TMB), showing a linear response from 20 to 100 µM with a limit of detection (LOD) of 1.34 µM. For H 2 O 2 , a linear range of 30–150 µM and an LOD of 10.1 µM were achieved. TA‐AuNPs also catalyzed the oxidation of I − to triiodide (I 3 − ), producing a color change from pink to yellow with a peak at 350 nm. The I − sensor showed an LOD of 2.38 μM and >100.8% recovery in real water samples. This multifunctional TA‐AuNP nanozyme platform provides a simple, sensitive, and selective approach for Fe 3+ , H 2 O 2 , and I − detection, with promising applications in environmental monitoring, public health, and food safety.