  Optomechanical crystal cavities have rich perspectives for detecting and
indirectly analysing biological particles, such as proteins, bacteria and
viruses. In this work we demonstrate the working principle of an optomechanical
crystal cavity operating under ambient conditions as a sensor of submicrometer
analytes by optically monitoring the frequency shift of thermally activated
mechanical modes. The resonator has been specifically designed so that the
cavity region supports a particular family of low modal-volume mechanical
modes, commonly known as -pinch modes-. These involve the oscillation of only a
couple of adjacent cavity cells that are relatively insensitive to
perturbations in other parts of the resonator. The eigenfrequency of these
modes decreases as the deformation is localized closer to the centre of the
resonator. Thus, by identifying specific modes that undergo a frequency shift
that amply exceeds the mechanical linewidth, it is possible to infer if there
are particles deposited on the resonator, how many are there and their
approximate position within the cavity region.
