An overview of Schiff base-based fluorescent turn-on probes: a potential candidate for tracking live cell imaging of biologically active metal ions

The development of new fluorescent turn-on probes has attracted much attention due to the selective and sensitive detection of biologically important metal ions and their applicability in live cell imaging. Since it has been established that fluorescent probes work well in cellular media, many intriguing investigations are under progress. It is desirable to use quick and easy methods for detecting metal ions in biological and environmental systems, and the probes should provide enhanced photophysical properties with sensitivity, selectivity, a low detection threshold, a quick response time, operational simplicity, and real-time analysis, which are non-lethal to living cells, demanding extensive research. There are several novel fluorescent turn-on probes reported thus far; however, Schiff base-based probes have gained particular interest because of their intriguing optical characteristics, great thermal stability, and ease of synthesis. Moreover, they stand out in the bioactive domain as promising fluorescent chemosensors for biologically prominent metal cations and are well known for their low cytotoxicity. Schiff bases exhibit strong luminescence properties in synergistic systems with many ideal fluorophores such as quinoline, anthracene, coumarin, fluorescein, rhodamine, boron-dipyrromethene (BODIPY), dansyl, and naphthalimide. As a result, these synthetically flexible compounds with coordination sites such as N and O can rigidly bind to a wide range of transition metal ions with turn-on fluorescence properties. This review compiles the detailed sensing properties of numerous Schiff base-based probes for biologically important metal cations such as Fe(ii), Zn(ii), Cu(ii), Ni(ii), Co(ii), Ag(i), Au(iii), Hg(ii), Cd(ii) and Pd(ii) via a fluorescence turn-on mechanism. Further, it covers the detection approaches elucidating the optoelectronic characteristics as well as their application in cellular imaging, which will provide a better understanding of the accumulation of metal ions in organelles. This review focused on Schiff base-based fluorescent turn-on probes for detecting biologically important metal ions, paying primary attention to their biological applications.