Selected MRI and organic fluorescence sensors for the recognition of Zn2+ions in biological matrixes: A review

Metal ions play a vital role in the maintenance of life, and their levels must be carefully regulated. Both defi-ciency and excessive accumulation of metal ions can lead to significant health consequences such as diabetes, cardiovascular diseases, cancer, neurological disorders, etc. Among these metal ions, Zn2+ is the second most abundant essential element in the human body. Zn2+ ion is the structural cofactor of different enzymes and DNA-binding proteins. It is very important for various biological processes like immune function, brain function, gene transcription, mammalian reproduction, and many others. Therefore, the recognition of Zn2+ in living cells and bio-imaging is gaining significant attention. Chemical sensors are molecular devices that convert chemical in-formation into measurable analytical signals, such as optical, electrical, magnetic, and electronic signals. A chemical sensor contains a receptor linked to the signal source, such as an MRI contrast agent, chromophore, or fluorophore. When an analyte interacts with a sensor, it changes the relaxivity, absorption, or fluorescence in-tensity of the sensor. From these changes, we can detect a specific analyte in various matrixes. In this review article, we discussed various MRI contrast agents and organic fluorescence probes for the recognition of Zn2+ ions. We focus especially on the designing of fluorimetric probes and MRI contrast agents for sensing Zn2+ in various living cells (J774A.1, MCF7, L929, HepG2, PC12, PC-3, C6, HeLa, A549, HK2, BHK-21 and E. coli cells), Zebrafish embryos/larvae, living mouse and hippocampal tissues.