Uric acid (UA) optical biosensor was developed in this study. Nanocomposites were synthesized using NaYF₄: Er³⁺/Yb³⁺ as the matrix and iron, cobalt, and copper (as dopants) at different rations, in a one-pot hydrothermal treatment. Nanocomposites were prepared with different ratios of Co and Cu while concentration of Fe was constant for all the variants. The detection of UA is based on measuring change in fluorescence signals of the nanocomposites in presence and absence of UA. Fe, Co, and Cu shows peroxidase-like activity which was used for oxidation of o-phenylenediamine (OPD) to 2,3-Diaminophenazine (DAP) in presence of uricase (UAO) to catalyze UA. The co-doped NaYF₄: Er³⁺/Yb³⁺ nanoparticles emit upconversion fluorescence with four typical emission peaks centered at 490 nm, 540 nm, 660 nm, and 800 nm under 980 nm near-infrared (NIR) irradiation which are attributed to transition from ²H_9/2, ⁴H_11/2, ⁴F_9/2 and ⁴I_9/2 to ground state ⁴I_15/2. DAP shows absorbance near 450 nm, the emitted fluorescence at 490nm gets quenched due to the inner filter effect. All the variants show quenching of fluorescence signal at 490 nm with different intensities. Intensity of blank signal with respect to (I_490(o)/I₄₉₀) were measured and are in proportion to UA concentration. Under optimized conditions, colorimetric and ratiometric linear range sensing for UA was 0.1-1 mM. Ratiometric fluorescence detection limit (S/N=3) was 0.66 µM. These results shows that the synthesized nanocomposites have great capability for the detection of UA using colorimetric and ratiometric dual-readout assay methods.