As one kind of the xanthene, rhodamine fluorophores have attracted much attention because of its good water solubility, high fluorescence quantum yield, molar extinction coefficient and mitochondrial targeting compared with other fluorophores. Moreover, due to the unique "on-off" property of the screw ring, this type of dye has become one of the most extensive dye platforms for the construction of various fluorescent probes.
Fig. 1 Chemical structure of rhodamine fluorophore
Rhodamine fluorophores and their derivatives have been used in metal ion detection, analytical chemistry, PH spectral probe and other fields.
Metal ion detection: Rhodamine fluorophore is an ideal chromophore for metal ion probe due to its cheap price, easy modification and rich spectral properties. The molar absorption coefficient and the fluorescence quantum yield of rhodamine lactam which has closed-loop structure is very low, they almost have no fluorescence. Then, when identifying groups under the synergy of carbonyl with certain ions, can lead to probe lactam ring fracture and form open loop structure, fluorescence intensity significantly enhanced, so as to realize the selective recognition of the material. At present, Cu2+, Fe3+, Hg2+ and other mental ions elective recognition probe is widely used.
Analytical chemistry: Rhodamine fluorophore aqueous solutions have high molar absorbance coefficient, and their molecular structure has a large rigid plane, so the monomer aqueous solutions can produce strong fluorescence, they also have good REDOX capacity. We can perform absorbance spectrophotometry, fluorescence spectrophotometry, or catalytic kinetic analysis for many substances taking advantage of these strengths. At present, more than fifty kinds of substances can be determined by rhodamine compounds with high sensitivity.
pH spectral probe (RGP): The change of pH value of biological fluid is one of the important indexes to measure the physiological changes of organisms. Monitoring the changes of pH in body fluids and cells is of great significance for studying the metabolic, physiological and pathological processes of organisms. In recent years, some rhodamine-based probes have been reported and successfully applied to the study of pH in the environment, lysosomes and cells. Its recognition mechanism is shown in the figure below:
Fig. 2 Two structures for Hg2+detection probes
Fig. 3 Possible recognition mechanism of RGP 
- Dong-Mei, L.; et al. Synthesis and spectral characterization of a novel rhodamine-based pH probe. Hua xue Shi ji, 2020, 42(3): 311-314.