Reversible Tuning of SERS Hot Spots with Aptamers A reversible surface-enhanced Raman spectroscopy (SERS)-based sensor with stem loop aptamers is reported. It is capable of detecting its target molecule at concentrations below the micromolar level and of delivering robust signals from events involving fewer than ten target molecules. Washing the sensor with buffer causes the SERS spectrum to revert to its pre-detection-event form, fully restoring its capability as a sensor. [GRAPHICS]