A simple bioassay was designed for the detection of adenosine, a human metabolite involved in several physiological processes, on the basis of the functionalization of gold nanoparticles with DNA oligonucleotides selectively binding adenosine. The test was possible thanks to the selective and sensitive binding of adenosine by two DNA oligonucleotides forming an aptamer. Each DNA strand was grafted on the surface of gold nanoparticles, which formed an insoluble aggregate within a few minutes in the presence of millimolar concentrations of adenosine. The experiments described are suitable for undergraduate and graduate students developing skills in handling and functionalizing nanomaterials for use in biosensors. Besides the handling of nanomaterials, this laboratory training enabled the students to familiarize themselves with the use of nucleic acids in novel techniques, for example, as efficient probes for biosensing purposes.