Highly Sensitive Colorimetric Assay for Determining Fe³⁺ Based on Gold Nanoparticles Conjugated with Glycol Chitosan

A highly sensitive and simple colorimetric assay for the detection of Fe³⁺ ions was developed using gold nanoparticles (AuNPs) conjugated with glycol chitosan (GC). The Fe³⁺ ion coordinates with the oxygen atoms of GC in a hexadentate manner (O-Fe³⁺-O), decreasing the interparticle distance and inducing aggregation. Time-of-flight secondary ion mass spectrometry showed that the bound Fe³⁺ was coordinated to the oxygen atoms of the ethylene glycol in GC, which resulted in a significant color change from light red to dark midnight blue due to aggregation. Using this GC-AuNP probe, the quantitative determination of Fe³⁺ in biological, environmental, and pharmaceutical samples could be achieved by the naked eye and spectrophotometric methods. Sensitive response and pronounced color change of the GC-AuNPs in the presence of Fe³⁺ were optimized at pH 6, 70°C, and 300 mM NaCl concentration. The absorption intensity ratio (A₇₀₀/A₅₁₀) linearly correlated to the Fe³⁺ concentration in the linear range of 0-180 μM. The limits of detection were 11.3, 29.2, and 46.0 nM for tap water, pond water, and iron supplement tablets, respectively. Owing to its facile and sensitive nature, this assay method for Fe³⁺ ions can be applied to the analysis of drinking water and pharmaceutical samples.