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Quantum-dot (QD) based nanosensors can be used to detect a wide range of molecules. This study examined a nanosensor comprised of thrombin binding aptamer (TBA) with 700NC InGaP QD on the 5 ' terminus and an Au nanoparticle quencher on the 3 ' terminus. Both K+ and Pb2+ bind to TBA, resulting in a conformational change that brings the Au quencher closer to the QD. Photoluminescence measurements indicated a decrease in fluorescence corresponding to an increase in either K+ or Pb2+ concentration. For healthy blood serum K+ concentrations (3.5-5 mM), the beacon exhibited 15-17% quenching efficiency. Pb2+ concentration of 0.48 μM, the threshold for toxicity in serum, yielded 14% quenching. The beacon's ability to detect changes in ion levels in a critical range of concentrations can make it an effective diagnostic tool.