Infrared nanosensors of piconewton to micronewton forces

Mechanical force is an essential feature for many physical and biological processes 1 , 2 , 3 , 4 , 5 , 6 – 7 , and remote measurement of mechanical signals with high sensitivity and spatial resolution is needed for diverse applications, including robotics 8 , biophysics 9 , 10 , energy storage 11 and medicine 12 , 13 . Nanoscale luminescent force sensors excel at measuring piconewton forces, whereas larger sensors have proven powerful in probing micronewton forces 14 , 15 – 16 . However, large gaps remain in the force magnitudes that can be probed remotely from subsurface or interfacial sites, and no individual, non-invasive sensor is capable of measuring over the large dynamic range needed to understand many systems 14 , 17 . Here we demonstrate Tm 3+ -doped avalanching-nanoparticle 18 force sensors that can be addressed remotely by deeply penetrating near-infrared light and can detect piconewton to micronewton forces with a dynamic range spanning more than four orders of magnitude. Using atomic force microscopy coupled with single-nanoparticle optical spectroscopy, we characterize the mechanical sensitivity of the photon-avalanching process and reveal its exceptional force responsiveness. By manipulating the Tm 3+ concentrations and energy transfer within the nanosensors, we demonstrate different optical force-sensing modalities, including mechanobrightening and mechanochromism. The adaptability of these nanoscale optical force sensors, along with their multiscale-sensing capability, enable operation in the dynamic and versatile environments present in real-world, complex structures spanning biological organisms to nanoelectromechanical systems. An avalanching-nanoparticle force sensor that can operate in the piconewton-to-micronewton range with exceptional force responsiveness is achieved by using the mechanical sensitivity of the photon-avalanching process.