Illustration of the atomic arrangement within a single lanthanide-doped nanocrystal. Each lanthanide ion can emit light. Image credit: Dr. Andrew Mueller
Mechanical force is central to many physical and biological phenomena, and remote high-sensitivity measurements of mechanical signals with fine spatial resolution are increasingly sought after. These measurements would be pertinent to applications ranging from robotics and medicine to space exploration.
While nanoscale luminescent force sensors can measure forces at the piconewton range, and larger sensors of this type can detect forces at the micronewton level, there remains a significant gap in the ability to remotely measure forces over a much broader range, particularly from subsurface or interfacial sites.
In a groundbreaking study, researchers have successfully developed new nanoscale force sensors made from luminescent nanocrystals. These “all-optical” sensors change their intensity and/or color in response to mechanical stress and can be probed entirely with light, requiring no wires or physical connections for remote readouts.
This innovation is expected to significantly enhance optical force sensors’ sensitivity and dynamic range, transforming fields such as robotics, cellular biophysics, medicine, and space travel.
For the first time, the new nanosensors allow for high-resolution multiscale functionality with a single sensor. This is a significant breakthrough in that a single type of nanosensor can now be used to perform continuous force monitoring at the scale of subcellular structures all the way up to whole systems.
This versatility makes the sensor ideal for studying forces in both engineered and biological systems, such as developing embryos, migrating cells, batteries, and integrated NEMS (nanoelectromechanical systems), where an electronic circuit controls the physical movement of nanometer-scale structures, or vice versa.
Fardian-Melamed, a postdoctoral scholar in his group, along with the Cohen and Chan groups at Lawrence Berkeley National Lab (Berkeley Lab), said, “What makes these force sensors unique – apart from their unparalleled multiscale sensing capabilities – is that they operate with benign, biocompatible, and deeply penetrating infrared light.”
“This allows one to peer deep into various technological and physiological systems and monitor their health from afar. Enabling the early detection of malfunction or failure in these systems, these sensors will profoundly impact fields ranging from human health to energy and sustainability.”
The researchers created these nanosensors by harnessing the photon-avalanching effect in nanocrystals. First identified by Schuck’s lab at Columbia Engineering, photon-avalanching nanoparticles have a singular property: when just one photon is absorbed, it creates a chain reaction that leads to the emission of many photons. Extremely nonlinear and volatile, what Schuck calls “steeply nonlinear,” this process is similar to an avalanche, where one photon spawns many more being emitted.
The optically active components of the nanocrystals in this study are atomic ions from the lanthanide series of elements, also known as rare-earth elements. The team used thulium, a specific lanthanide, to enhance the nanocrystals’ optical properties for this research.
The researchers found that this photon-avalanching process was highly sensitive to several parameters, including the spacing between lanthanide ions. When they tested these photon-avalanching nanoparticles, or ANPs, using an atomic force microscopy (AFM) tip, they found that even gentle forces significantly affected the avalanching behavior—much more than they had expected.
“We discovered this almost by accident,” says Schuck. “We suspected these nanoparticles might be sensitive to force, so we measured their emission while tapping on them. To our surprise, they were far more sensitive than we expected! At first, we didn’t believe it, thinking the AFM tip might be causing a different effect. But after Natalie conducted thorough control measurements, we confirmed that the response was due to the extreme force sensitivity.”
Recognizing that ANPs are extremely force-sensitive, the team then created new nanoparticles that responded differently to forces. One causes the nanoparticle’s luminescence color to change with force. Another design—though not normally subject to photon avalanching—begins to avalanche under the application of force, meaning it is incredibly force-sensitive.
The Berkeley lab team developed custom ANPs based on Columbia’s feedback, synthesizing and characterizing dozens of samples to understand and optimize the particles’ optical properties.
The group will next seek to integrate these ultrasensitive force sensors into critical systems, such as developing embryos—a research domain of Columbia’s Mechanical Engineering Professor Karen Kasza.
“For the sensor design, the team seeks to integrate self-calibrating capabilities directly into the nanocrystals, enabling each to act as a standalone sensor. That addition should be readily achievable with an extra thin shell introduced in the synthesis process for the nanocrystals,” says Schuck.
Journal Reference:
- Fardian-Melamed, N., Skripka, A., Ursprung, B. et al. Infrared nanosensors of piconewton to micronewton forces. Nature 637, 70–75 (2025). DOI: 10.1038/s41586-024-08221-2
