Nanowire sensors penetrate cells

In nano-scale optomechanical cavities, photons bounce off mirrors. Their momentum becomes amplified enough to cause mechanical deflection of an oscillator. Exploitation of cavity nanomechanics to sense deflections has led to realisation of mass and force detectors with unprecedented sensitivity. However, when the mechanical system is shrunk beyond the optical wavelength (to sub-wavelength scales), diffraction emerges and the amplification effect is diminished.

With EU funding, the project 'Operation of cavity optomechanics in fluids for ultrasensitive mass detection' (OPTONANOMECH) addressed this and other challenges to pave the way to ultrasensitive measurements inside individual living cells. Scientists pushed the frontiers through use of active oscillation to achieve high sensitivity at ambient temperature and without vacuum conditions.

The nanomechanical resonator is part of a photonic circuit in such a way that actuation and detection is all-optical. The active oscillation resulting from regenerative cavity back-action eliminates the need for a constant alternating current driving force. This approach not only eliminates the size restriction imposed by electrical connections, more importantly it solves the diffraction problem and enables unprecedented sensitivity. This opens the door to novel cavity designs that could achieve measurements at the quantum limit, the limit on measurement accuracy at quantum scales due to back-action effects. Without the need for external excitation, novel sensor designs can be greatly simplified.

Using a semiconductor nanowire-based optical detection system, scientists were able to detect a few zeptograms (close to the mass of a proton or hydrogen atom) in fluid with short nanowires. Such sensitivity would enable detection of single ligand-receptor events, the binding of molecules to receptors in a lock and key type fashion. Such events are a pillar of intra- and intercellular signalling. Given that the nanowires can penetrate the cell membrane, the system is also well-suited to intracellular drug and gene delivery and intracellular monitoring.

Ultrasensitive mass and force detection at room temperature and in fluids paves the way to detection of dynamic biological events under realistic conditions and in real time. Taking that capability out of the lab and into the clinic will provide a revolutionary new tool for diagnosis, monitoring and therapy. OPTONANOMECH is paving the way.