Rhodamine B- and coumarin-modified chitosan as fluorescent probe for detection of Fe³⁺ using quenching effect

Iron ion is an essential metal ion that plays a crucial role in various metabolic processes in living organisms. It is involved in cellular metabolism, acts as a catalyst for enzymatic reactions, and functions as an oxygen carrier in hemoglobin. Insufficient levels of iron in the body can lead to adverse health conditions such as diabetes, anemia, liver and kidney damage, and heart diseases. Considering these factors, the development of an effective fluorescent probe for the detection of excessive Fe³⁺ levels is necessary to protect both environment and human health. Numerous fluorescent probes have been reported thus far for Fe³⁺ detection. However, the most of these probes rely on a single emission intensity alteration mechanism, such as photoinduced electron transfer (PET). In this study, we have developed fluorescent probes using chitosan functionalized with rhodamine B and coumarin derivatives, for efficient detection of Fe³⁺ and Zn²⁺ ions. The hybrid chitosan/rhodamine B (CS/RB) and hybrid chitosan/coumarin (CS/MCMC) fluorescent probes exhibit strong fluorescence intensity under ultraviolet light. When iron ions are introduced to CS/RB and CS/MCMC solution, they coordinate with weak-field ligands (such as N and O) in the structure of CS/RB through a high-spin method. This coordination leads to the self-assembly of Fe³⁺ on the CS/RB surface, generating single electrons and resulting in high paramagnetism, which in turn quenches the fluorescence. The quenching effect of Fe³⁺ on the CS/RB and CS/MCMC fluorescent probes enables efficient detection of Fe³⁺ ions. Importantly, this fluorescence quenching effect is specific to Fe³⁺ and is not affected by other metal ions. In summary, the developed fluorescent probes based on chitosan derivatives offer a promising approach for the sensitive and specific detection of iron ions in aqueous solutions.