  Sub-femtogram resolution of an in-liquid cavity optomechanical mass sensor
based on the twin-microbottle glass resonator is demonstrated. An evaluation of
the frequency stability using an optomechanical phase-locked loop reveals that
this cavity optomechanical sensor has the highest mass resolution of
$(7.0\times2.0)\times 10^{-16}$ g in water, which is four orders of magnitude
better than that in our first-generation setup [Sci. Adv. 8, eabq2502 (2022)].
This highly sensitive mass sensor provides a free-access optomechanical probe
in liquid and could thus be extended to a wide variety of in-situ chemical and
biological metrology applications.
