Upconversion ratiometric fluorescence and colorimetric dual-readout assay for uric acid

A new upconversion colorimetric and ratiometric fluorescence detection method for uric acid (UA) has been designed. Yb(3+), Er(3+) and Tm(3+) co-doped NaYF4 nanoparticles (UCNPs) was synthesized. The co-doped NaYF4 nanoparticles, emit upconversion fluorescence with four typical emission peaks centered at 490nm, 557nm, 670nm and 705nm under the 980nm near-infrared (NIR) irradiation. The ZnFe2O4 magnetic nanoparticles (MNPs) possessing excellent peroxidase-like activity was prepared and used to catalyze oxidation the coupling of N-ethyl-N-(3-sulfopropyl)-3-methylaniline sodium salt (TOPS) and 4-amino-antipyrine (4-AAP) in the presence of H2O2 to form purple products (compound 1) which has a characteristic absorption peak located at 550nm. The upconversion fluorescence at 557nm was quenched by the compound 1 while the upconversion emission at 705nm was essentially unchanged, the fluorescence ratio ((I557/I705)0/(I557/I705)) is positively proportional to UA concentration in existence of uricase. More importantly, colorimetric signal can be easily observed and applied to directly distinguish the concentration of UA by the naked eye. Under the optimized conditions, the linear range of colorimetric and ratiometric fluorescence sensing towards UA was 0.01-1mM, the detection limits were as low as 5.79μM and 2.86μM (S/N=3), respectively. The proposed method has been successfully applied to the analysis of UA in human serum. These results indicate that the colorimetric and ratiometric fluorescence dual-readout assay method has great potential for applications in physiological and pathological diagnosis.