A new selective fluorescent sensor for detection of dopamine (DA) using Co$^{2+}$ -decorated thioglycolic acidcapped CdS QDs (Co$^{2+}$@TGA–CdS QDs) was proposed. Basically, the fluorescent intensity of TGA–CdS QDs can bequenched by Co$^{2+}$. However, with the addition of dopamine into the solution of Co$^{2+}@TGA–CdS QDs, the fluorescent intensity was efficiently quenched due to the formation of the efficient quencher (Co$^{2+}$–dopamine complex). Thus, the concentration of dopamine can be determined by tracking the decrease in the fluorescent intensity of the Co$^{2+}$@TGA–CdSQDs. The proposed sensor showed high selectivity towards the detection of dopamine over other catecholamine derivatives and related compounds. Under optimal conditions, the quenching efficiency of dopamine on the Co$^{2+}$@TGA–CdS QDs system was linearly proportional to the concentration of dopamine in the range of 20−100 $\mu$mol l$^{−1}$. The limits of detectionand quantification were 1.8 and 12.7 $\mu$mol l$^{−1}$, respectively. This sensor system was demonstrated to detect dopamine in real urine samples with satisfactory results.