Two 25 base-pair cDNA strands are encapsulated within an optically trapped nanodroplet, and we observe the kinetics of their hybridization in dynamic equilibrium via single-molecule fluorescence resonance energy transfer (FRET) as a function of temperature and of the solution's NaCl concentration. We have observed the duplex unfolding and refolding, and we have observed quasistable partially unfolded states under low salinity conditions. Furthermore, our measurements reveal that, even in conditions under which the duplex is stable, it undergoes conformational fluctuations in solution.