Upconversion emissive ceramic nanophosphors like Er³⁺-doped LaOCl are useful for advanced security printings and fluorescence bioimaging techniques. For such applications, functional polymers are necessary for surface modification in order to provide adsorptivity, steric stability as well as other processes. Surface modification, however, brings about a potential degradation of the emission intensity of doped rare earth ions contingent upon the vibration energy of the surrounding environment. It is necessary to understand the structure of surface modification in relation to emission intensity. In this study, we demonstrate the surface modification of Er³⁺-doped LaOCl nanophosphor with simple acetate anions using the compounds acetyl chloride and aqueous sodium acetate, respectively. We report that the surface modification using acetyl chloride has little effect upon degrading emission intensity as it does not increase surface hydroxy groups.