A dual-mode colorimetric and fluorometric nanosensor for detection of uric acid based on N, P co-doped carbon dots and in-situ formation of Au/Ag core-shell nanoparticles

Green emissive N,P co-doped carbon dots (N,P-CDs) were prepared by a simple hydrothermal method from citric acid, urea, and phosphoric acid as carbon, nitrogen and phosphorous pre-sources, respectively. The prepared N, P-CDs were applied to establish a dual-mode colorimetric and fluorometric sensor for monitoring uric acid based on in-situ formation of Ag shell on the surface of AuNPs. We observed that the fluorescence intensity of N, P-CDs was quenched by adding AuNPs due to overlap between the emission peak of N,P-CDs and surface plasmon resonance peak (SPR) of AuNPs. With adding Ag+ and uric acid, the SPR peak intensity of AuNPs increased due to the reduction of Ag+ to Ag-0 and Ag shell formation on the surface of AuNPs. Also, a blue shift in the SPR peak position of AuNPs was observed at the higher concentration of uric acid, resulting in the color variation of the solution from red to orange. These variations in the SPR peak intensity and position caused more fluorescence quenching of N,P-CDs because of the increasing spectral overlap between N,P-CDs and Au@AgNPs. We indicated that there is a logarithmic relationship between increasing SPR peak intensity of AuNPs as well as fluorescence quenching of N,P-CDs and uric acid concentration in the range of 0.1-10 and 0.5-10 mu M, respectively. Regarding the mentioned facts, we developed a dual colorimetric and fluorometric sensor to determine uric acid. The detection limit was 40 and 400 mu M for colorimetric and fluorometric methods, respectively. The established sensor was exploited for uric acid analysis in human urine samples.