Abstract: AuNCs modified by BSA were synthesized using chemical reduction method. Based on the fluorescence quenching effect of Co²⁺ on AuNCs, a fast, simple, and sensitive method for determining Co²⁺ was proposed. AuNCs were synthesized using citric acid as the reducing agent, BSA as the protective agent, and chloroauric acid as the raw material. An aliquot (0.20 mL) of the AuNCs solution was taken, and 0.6 mL of pH 9.0 sodium carbonate-sodium bicarbonate buffer solution was added. The mixture was diluted to 1.00 mL by water, and the fluorescence intensity F₀ of the above system (blank system) was measured at emmison wavelength of 630 nm. Ten vitamin B₁₂ tablets were taken and ground, and an aliquot (0.127 0 g) was taken, dissolved and diluted to 100 mL by water. The above solution of 10 μL was taken, and added into the blank system. The mixed solution was settled for 5 min, and the fluorescence intensity F was measured. The fluorescence intensity difference ΔF(ΔF=F₀-F) was used for quantification. As shown by the results, the synthesized AuNCs were evenly distributed, and Co²⁺had a fluorescence quenching effect on the AuNCs system, belonging to dynamic quenching process. Adding interference ions with a concentration of 10 times Co²⁺, ions of Fe²⁺, Pb²⁺, Al³⁺, Zn²⁺, Mg²⁺, Cr³⁺, Cu²⁺, Sr²⁺, Mn²⁺, Ca²⁺ did not cause interference. The interference of Fe³⁺ and Ni²⁺ could be masked by addition of triethanolamine and ortho phenanthrene, while the interference of Hg²⁺ was severe. Linear relationships between values of the concentration and ΔF of Co²⁺ were kept in the ranges of 2.0×10^-11-1.5×10^-10mol·L^-1 and 1.5×10^-10-9.9×10^-10mol·L^-1, with detection limit (3s/k) of 1.7×10^-12mol·L^-1. Test for recovery was made according to standard addition method on actual samples from two manufacturers, and determined values and label values were relatively close, giving recovery of Co²⁺ close to 100%, and RSDs (n=5) of the determined values less than 5.0%.