As new class of fluorescent biolabels, upconversion nanoparticles (UCNPs) were very attractive for use as labels, compared with these traditional downconversion materials. It can eliminate problems associated with autofluorescence and scattered excitation light under near infrared light (NIR) excitation. In the present work, water-soluble NaYF4∶Yb3+, Er3+ UCNPs were successfully prepared by solvothermal synthesis, using branched polyethylenimine as the surfactants, and a generic design strategy for UCNPs conjugated aptamer based optical switch was presented. In the absence of the target thrombin protein, such a duplex structure brings the UCNPs and TAMRA close proximity of each other, leading to luminescence resonance energy transfer from UCNPs to TAMRA upon near-infrared light irradiation. When target is introduced, the quadruplex conformation of the aptamer should be preferentially stabilized, resulting in the displacement of the TAMRA labelled oligonucleotide conjugate and triggering UCNPs fluorescence intensity to increase and the corresponding TAMRA fluorescence intensity to decrease. Since the aptamer responds to its target molecule by switching on its fluorescent properties, the authors named this an “optical switch”.