Aptamers are short, synthetic, single-stranded nucleic acid segments that bind with strong affinity and selectivity for a target molecule of interest. The main requirements of effective biosensors are high selectivity and sensitivity for the target, making aptamers ideal building blocks for biosensors. In this study, we used the ATP aptamer in a Fluorescence Resonance Energy Transfer (FRET)-based biosensor. The ATP aptamer sequence was divided into two segments: one of the segments (the FRET donor) was modified with a CdSe/ZnS core-shell quantum dot (QD), and the other (the FRET acceptor) was modified with an organic dye (Dy-Light 647). The size of the QDs was chosen to have maximum FRET overlap with the dye. This poster will highlight a series of experiments with these components. First, the optimal coverage of quantum dots by aptamers will be examined. FRET experiments with the donor and the acceptor in the presence of the target (ATP) and potential interferences (e.g. GTP) will also be discussed.