Synthesis of NaYF4:Ho3+/Yb3+ colloidal upconversion phosphor and its application for OCT-based imaging, temperature sensing, fingerprinting and security ink

In this work, an NaYF4:Ho3+/Yb3+ upconversion phosphor in colloidal form was synthesized and then its suitability for image contrast enhancement in optical coherence tomography (OCT) and photothermal (PT) OCT imaging was analysed. A prepared sample showed intense upconversion emission in the green region (541 nm) due to the F-5(4)(S-5(2)) -> I-5(8) transition of the Ho3+ ion. The colloidal stability of the particles was tested in several organic media and the best stability of the particles was found in dimethylformamide (DMF) medium. The applicability of the prepared particles is also demonstrated for temperature sensing, fingerprint detection, and security ink.