Currently, control of the quality and composition of consumed drinking water has become extremely popular. One of its important parameters is the content of iron ions, an excess of which causes toxic effects. The detection of low concentrations of iron ions in drinking water requires highly sensitive and fast methods with the possibility of mass application by a wide range of consumers.

This work demonstrates the use of monodispersed gold nanoparticles (AuNP) coated with mercaptosuccinic acid (MSA) as a reagent for the determination of Fe³⁺ ions. Mercaptosuccinic acid as a thiol compound provides reduction and stabilization in the synthesis of gold nanoparticles. In addition, the presence of two carboxyl groups makes it possible to bind Fe³⁺ ions. In the proposed method, the binding of Fe³⁺ ions by AuNP-MSA complexes causes aggregation in a colloidal solution, accompanied by recorded spectral changes.

The synthesize AuNP-MSA preparations were characterized by transmission electron microscopy and dynamic light scattering; the stated average particle diameter is 19.8 nm. The absorption ratios of A₅₃₀/A₆₅₀ was selected as an analytical signal and corresponded to a change in the color of the solution from red to blue. The effect of the pH of the system on changes in the analytical signal in the presence of Fe³⁺ ions was studied; the maximum detection sensitivity was achieved at pH 4.5. The optimal ratio of the volumes of the AuNP-MSA colloidal solution (with the optical density around 1.0) and the volumes the test sample was established, equal to 1:30. The proposed approach provides reliable detection of Fe³⁺ ions in concentrations up to 10 ng/ml, which is 30 times lower than the maximum allowable concentration for drinking water. The analytical system is also highly selective; samples containing other heavy metal ions did not cause significant optical changes. To confirm the practical application of the developed system, samples of drinking, spring and tap water with different contents of Fe³⁺ ions were analyzed.

The obtained results indicate that the use of gold nanoparticles functionalized with mercaptosuccinic acid as an analytical reagent is promising. The developed technique makes it possible to determine ultra-low concentrations of Fe³⁺ ions quickly and without using expensive equipment.

This work was supported by the Russian Science Foundation (Grant No. 19-44-02020) and Ministry of Science and High Education.