Synthesis of biocompatible uniform NaYF4:Yb3+,Er3+ nanocrystals and their characteristic photoluminescence

► A viable and efficient procedure for producing biocompatible NaYF4:Yb3+,Er3+ nanocrystals. ► Emission intensity of PEG coated NaYF4:Yb,Er varies nonlinearly with increasing PEG coating. ► Intensity ratio of red to green emission of PEG coated NaYF4:Yb,Er depends uniquely on the excitation power. ► The excitation and emission mechanisms related to PEG coating were discussed.