Quantum dots (QDs) are usually used as fluorescent probe, and they are difficult to use in colorimetric detection. However, in this report carboxyl-functionalized CdS (COF-CdS) QDs were synthesized in aqueous solution for colorimetric detection following a classic method. On the basis of inducing the aggregation of COF-CdS QDs, a simple naked eye colorimetric method with high sensitivity and selectivity was developed for the sensing of Co(2+) ions in aqueous solutions. The Co(2+) ions induced COF-CdS QDs results in a marked enhancement of the UV-vis absorption spectra at 360 nm, and the process was accompanied by a visible color change from colorless to yellowish brown within 5 min, which proves a sensitive detection of Co(2+) ions. The sensing of Co(2+) ions can therefore be easily achieved by a UV-vis spectrophotometer or even by the naked eye. Under the optimized circumstances, this method yields excellent sensitivity (LOD = 0.23 μg mL(-1)) and selectivity toward Co(2+) ions. The calibration plot of (A - A(0)) at 360 nm against concentration of Co(2+) ions was linear over the range from 0.5 to 14 μg mL(-1) with a correlation coefficient of 0.9996. The accuracy and reliability of the method were further ascertained by recovery studies via standard addition method with percent recoveries in the range of 99.63-102.46%. The plausible mechanism for the color change reaction has also been discussed. Our attempt may provide a cost-effective, rapid, and simple solution for the inspection of Co(2+) ions in the presence of a complex matrix from environmental aqueous samples.