  We report narrow linewidth optomechanical oscillation of a silica microsphere
immersed in a buffer solution. Through a novel optomechanical transduction
sensing approach, single 10-nm-radius silica beads and Bovine serum albumin
(BSA) protein molecules with a molecular weight of 66 kDalton were detected.
This approach predicts the detection of 3.9 kDalton single molecules at a
signal-to-noise ration above unity.
