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The metal-organic coordination polymers at the nanoscale have emerged as attractive nanomaterials due to their tunable nature. In this work, we for the first time prepared an adenine-based lanthanide coordination polymer nanoparticle (CPNP) with fluorescence sensing function. This kind of CPNP was composed of adenine, terbium ion (Tb(3+)), and dipicolinic acid (DPA) as an auxiliary linking molecule that can sensitize the fluorescence of Tb(3+). The fluorescence of the CPNPs is very weak due to the existence of photoinduced electron transfer (PET) from adenine to DPA, which prevents the intramolecular energy transfer from DPA to Tb(3+), leading to the quench of fluorescence of the CPNPs. In the presence of Hg(2+), however, significant enhancement in the fluorescence of CPNPs was observed because of the suppression of the PET process by the coordination of Hg(2+) with adenine. As a kind of Hg(2+) nanosensor, the CPNPs exhibit excellent selectivity and ultrahigh sensitivity up to the 0.2 nM detection limit. The CPNPs also possess an approximately millisecond-scale-long fluorescence lifetime due to the inclusion of Tb(3+) ions. We envision that the CPNPs could find great potential applications in ultrasensitive time-resolved fluorometric assays and biomedical imaging in the future owing to their long emission lifetimes, excellent dispersion, and stability in aqueous solution.