Synthesis And Properties Of Fe²⁺ Fluorescent Probe Based On Nitrogen Oxides

Iron is an important metal element needed in human body and the most abundant transition metal element in living system.Under physiological conditions,it exists in a stable Fe²⁺/Fe³⁺redox state,and plays the important physiological and pathological roles in biological system.However,excessive iron can induce the production of highly reactive oxygen species（ROS）,which can cause severe cell damage.In addition,the destruction of the iron regulatory system not only leads to a series of diseases,but also leads to cell iron death.Therefore,it is of great significance to develop an effective method for monitoring intracellular Fe²⁺.In this study,Fe²⁺fluorescent probe based on nitrogen oxides was designed and synthesized,based on the two characteristics of nitrogen oxides,which are water-soluble and bioreducible under low oxygen.The details are summarized as follows:Using 2-acetyl pyridine as the material,the intermediate 2-acetyl pyridine nitrogen oxide（R1）was synthesized by peroxidation reaction.The intermediate R1was further condensed with 4-diethylaminobenzaldehyde to obtain probe（E）-2-(3-（4-diethylamino benzaldehyde）phenyl（acryloyl）pyridine nitrogen oxide（CR1）.After CR1 sensing Fe²⁺,the nitrous oxide structure in the probe was reduced and the fluorescence at 617 nm wavelength was enhanced at 435 nm excitation wavelength.The probe exhibited a large Stokes shift of 182 nm and orange-red fluorescence emission.Under physiological conditions,the probe CR1 was highly selective in response to Fe²⁺and could complete the sensing for Fe²⁺within 5 minute.It has good identification ability and biological applicability under physiological conditions,and has a good application potential in clinical.The intermediate R1 was condensed with 6-hydroxy-2-naphthalene formaldehyde to obtain the probe（E）-2-（3-（6-hydroxy-2-methyl）acryloyl）pyridine nitrogen oxide（CR2）.After CR2 sensing Fe²⁺,the nitrous oxide group in the probe was reduced and the fluorescence at 549 nm was enhanced at 381 nm excitation wavelength.The probe exhibited a large Stokes shift of 168 nm and yellow-green fluorescence emission.The probe CR2 has a high selectivity for the response of Fe²⁺,which can complete the recognition of Fe²⁺in 4 minute,and the fluorescence intensity has a good linear relationship with the Fe²⁺concentration,and the detection limit is1.33μM.Under physiological conditions,the probe shows stable recognition and good biological applicability.Density functional theory calculations showed that the structures of probes CR1and CR2 have the non-coplanar structures between the pyridine ring and the electron-donating group due to the influence of the N-O bond,showing weak fluorescence.After Fe²⁺sensing,the N-O bond is removed,forming an intramolecular charge transfer with good electron push-pull conjugated structure,which results in the enhanced fluorescence.