CdS quantum dots (QDs) modified with bismuthiol II potassium salt is prepared in one step. Based on the characteristic fluorescence enhancement of CdS QDs at 480 nm by silver ions, simultaneously, a red shift of fluorescence emission bands of CdS QDs from 460 to 480 nm is observed. A simple, rapid, sensitive and specific detection method for silver ion is proposed. Under optimum conditions, the fluorescence intensity of CdS QDs was linearly proportional to silver ion concentration from 0.01 to 5.0 micromol L(-1) with a detection limit of 1.6 nmol L(-1). In comparison with single organic fluorophores, functionalized CdS quantum dots are brighter, more stable against photobleaching and do not suffer from blinking. Furthermore, the proposed method shows higher sensitivity and selectivity. A possible fluorescence enhancement mechanism is also studied.