A novel fluorescence and colorimetric dual probe based upon the inner filter effect (IFE) is reported to selectively and sensitively determine alkaline phosphatase (ALP). First, up-conversion nanoparticles (UCNPs) were prepared via a facile hydrothermal method and characterized by transmission electron microscopy (TEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), and zeta potential. Under near infrared (NIR) excitation at 980 nm, the prepared nanoparticles emit green light at 541 nm. The absorption band of p-phenylenediamine...