Our study aims to develop a coumarin-based fluorescence-enhanced Fe³⁺ (AFY) probe for the selective and sensitive detection of Fe³⁺. As an essential trace element in biological systems, abnormal levels of Fe³⁺ have been associated with various diseases, making it critical to developing efficient detection methods. Compared to traditional analysis methods, fluorescence sensors offer advantages such as high selectivity, sensitivity, and ease of use. However, most existing probes for detecting Fe³⁺ ions are quenched probes, which have suboptimal performance. Therefore, designing and developing a new “OFF-ON” Fe³⁺ fluorescent probe based on coumarin is of great significance.

We employ high-resolution mass spectrometry (HR-MS), hydrogen nuclear magnetic resonance (¹H NMR), and carbon nuclear magnetic resonance (¹³C NMR) to characterize the structure of the AFY probe. We evaluate the performance of the probe in identifying Fe³⁺ using UV and fluorescence spectroscopy. We determine the complexation constant and detection limit of the probe and investigate the response mechanism of AFY to Fe³⁺. Fe³⁺ levels in real water samples are measured using mobile phone-based intelligent recognition, fluorescence spectrometry, and atomic absorption spectrometry, demonstrating the practical application potential of AFY.

The AFY probe enables “naked eye” detection of Fe³⁺ through a color change in the solution from purple to pink, accompanied by the emission of strong red fluorescence in the presence of Fe³⁺. This fluorescence enhancement is attributed to the fluorescence chelation enhancement (CHEF) effect. The detection limit for Fe³⁺ with AFY is found to be 0.35 μmol/L, indicating good sensitivity. Both solution colorimetric methods and AFY-based test paper are employed for semi-quantitative Fe³⁺ detection in actual water samples. In addition, a smartphone-based color recognition App is used to digitally process the color of the AFY test paper. A good linear correlation is observed between the color value (R+G)/B and the Fe³⁺ concentration, enabling the quantitative detection of Fe³⁺ in water samples. The content of Fe³⁺ in water samples is further determined by fluorescent calibration curves, atomic absorption spectrometry, and smartphone-based colorimetry. The results show that the AFY probe is capable of rapid and accurate quantitative detection of Fe³⁺ in real water samples.

In this paper, we develop a coumarin-based red fluorescence-enhanced Fe³⁺ probe, AFY, which allows for the “naked eye” detection of Fe³⁺. Due to its good water solubility and rapid response, the probe is formulated into a test paper for semi-quantitative detection of Fe³⁺ in both pure water and tap water. The Fe³⁺ content in these water samples is determined using smartphone-based colorimetry, fluorescence spectrometry, and atomic absorption spectrometry. The rapid and accurate detection of Fe³⁺ in real water samples using the AFY probe presents a promising application for environmental monitoring and water quality assessment.