Amine-functionalized lanthanide-doped KGdF4 nanocrystals, synthesized via a facile one-step solvothermal route by employing polyethylenimine as the surfactant and capping ligand, have been demonstrated to be sensitive time-resolved FRET bioprobes to detect a trace amount of biomolecules such as avidin at a concentration of 5.5 nM and to be potential T-1-MRI contrast agents due to a large longitudinal relaxivity of Gd3+ (5.86 S-1 center dot mM(-1) per Gd ion and 3.99 X 10(5) S(-1 center dot)mM(-1) per nanocrystal).