NOVELTY - Preparation of rare earth coordination polymer nanoparticles involves dissolving europium nitrate and isophthalic acid in purified water and ethanol, respectively, dissolving imidazole-dicarboxylic acid in water, adding sodium hydroxide solution, ultrasonically dispersing, adding hydrochloric acid to adjust pH to neutral, adding 8 ml N,N'-dimethylformamide, 2 mL europium nitrate solution and 2 mL isophthalic acid solution, reacting at 170 degrees C for 8 hours, centrifuging the reaction solution, washing with ethanol and resuspending obtained particles in ethanol.    USE - Preparation of rare earth coordination polymer nanoparticles used as mercury ion fluorescent probe in chemical sensing, drug delivery and cell imaging applications.    ADVANTAGE - The method enables efficient preparation of rare earth coordination polymer nanoparticles with uniform size, and high fluorescence intensity, sensitivity, dispersibility and selectivity, by simple and environmentally-friendly process.    DETAILED DESCRIPTION - Preparation of rare earth coordination polymer nanoparticles involves dissolving europium nitrate and isophthalic acid in purified water and ethanol, respectively, dissolving imidazole-dicarboxylic acid in water, adding sodium hydroxide solution, ultrasonically dispersing until imidazole-dicarboxylic acid is completely dissolved, adding hydrochloric acid to adjust pH to neutral, adding 8 ml N,N'-dimethylformamide, 2 mL europium nitrate solution and 2 mL isophthalic acid solution, reacting at room temperature for 30 minutes, transferring resultant solution to a reactor, reacting at 170 degrees C for 8 hours, centrifuging the reaction solution, washing with ethanol and resuspending obtained particles in ethanol. The concentration of each europium nitrate solution and isophthalic acid solution is 40 mM. The concentration of imidazole-dicarboxylic acid solution is 10 mM. An INDEPENDENT CLAIM is included for usage method of rare earth coordination polymer nanoparticles, which involves adding 1 mu L rare earth coordination polymer nanoparticle tris-hydrochloric acid buffer with concentration of 25 mM to 400 mu L solution of imidazole-4,5-dicarboxylic acid, reacting at room temperature for 5 minutes, adding mercury ions with different concentration, and measuring fluorescence intensity of the reaction solution.