In the present study, we report a novel aptasensor based on silver nanoparticle enhanced fluorescence for the detection of adenosine. First, the distance dependence nature of silver nanoparticle enhanced fluorescence was investigated through fluorescent dyes modified oligonucleotides to control the spacing distance between dyes and AgNP. The results showed that the fluorescence intensity reached the maximum value with the spacing distance of dyes about 8. nm from AgNP surface. The fluorescence intensity decreases when the spacing distance is either above or below this value. Based on this result, a fluorescence switch is constructed. In the " OFF" state, without the target molecules, there is a greater spacing distance between the Cy3 dyes and the AgNP giving comparatively lower fluorescence intensity. While in the " ON" state, in the presence of target molecules, the fluorescence signals increased for the conformation structure change of the aptamer which shorten the spacing distance between the Cy3 dyes and the AgNP to 8. nm. Using adenosine as target, the aptasensor produced a linear range from 200. nM to 200μM with a correlation coefficient of 0.9949 and the detection limit was 48. nM estimated using 3. σ. The aptasensor was also found to be specific in targeting adenosine. The presented method shows a new strategy of combining aptamer recognition and silver nanoparticle for fluorescence signal enhancement and increasing sensitivity. © 2011 Elsevier B.V.