Optical force sensor based on plasmon modulated upconversion luminescence

We report a novel force sensor exploiting the interaction between plasmonic nanostructures and upconversion nanoparticles (UCNPs). The nanosensor is composed of a gold nanodisk and UCNPs separated by a flexible polymer layer. The gold nanodisk is designed to exhibit a plasmon resonance that selectively enhances one of the emission bands of the UCNPs while leaving the other ones largely unaffected. As the nanosensor is compressed or stretched by an external force, the polymer layer thickness changes, modulating the plasmon-UCNP coupling. The resulting changes in the luminescence intensity provides the basis for sensing. Furthermore, the nanosensor employs ratiometric sensing which makes it highly robust against any environmental variations. Our nanosensors exhibit two orders of magnitude higher responsivity than previously reported UCNP-based force sensors. They can be prepared as an on-chip sensor array or in a colloidal solution, making them suitable for a variety of applications in biology and robotics.