In a recent study published in the journal Nature, researchers from Columbia Engineering have developed new nanoscale force sensors. These sensors are luminescent nanocrystals that can change color and/or intensity when pushed or pulled. Since these “all-optical” nanosensors are only probed by light, they can be read out completely remotely without cables or other connections.
Illustration of the atomic arrangement within a single lanthanide-doped nanocrystal. Each lanthanide ion can emit light. Image Credit: Andrew Mueller/Columbia Engineering
Mechanical force is crucial to many physical and biological processes. Remote measurement of mechanical signals with high sensitivity and spatial resolution is required for various applications, including robotics, cellular biophysics, medicine, and even space travel. Nanoscale luminescent force sensors are excellent at measuring piconewtons, although larger sensors have shown effectiveness at probing micro newtons.
Nevertheless, there are still significant gaps in the force magnitudes that may be remotely probed from interfacial or subsurface sites, and no single non-invasive sensor has been able to measure the wide dynamic range required to comprehend many systems.
New, Highly Responsive Nanoscale Sensors of Force
Together with the Cohen and Chan groups at Lawrence Berkeley National Lab (Berkeley Lab), the researchers, under the direction of Jim Schuck, associate professor of mechanical engineering, and Natalie Fardian-Melamed, a postdoctoral scholar in his group, created nanosensors that achieved the largest dynamic range and the most sensitive force response ever achieved in comparable nanoprobes.
They have 100 times better force sensitivity than the existing nanoparticles that utilize rare-earth ions for their optical response and an operational range that spans more than four orders of magnitude in force, a much larger range -- 10-100 times larger -- than any previous optical nanosensor.
We expect our discovery will revolutionize the sensitivities and dynamic range achievable with optical force sensors, and will immediately disrupt technologies in areas from robotics to cellular biophysics and medicine to space travel.
Jim Schuck, Associate Professor, School of Mechanical Engineering, Columbia Engineering
New Nanosensors can Operate in Previously Inaccessible Environments
For the first time, the new nanosensors can perform multiscale, high-resolution functions. In biological and engineered systems, such as developing embryos, migrating cells, batteries, or integrated NEMS, very sensitive nanoelectromechanical systems in which an electronic circuit controls the physical motion of a nanometer-scale structure, or vice versa, this is significant because it means that only this nanosensor, not a collection of different classes of sensors, can be used for the continuous study of forces, from the subcellular to the whole-system level.
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 have a profound impact on fields ranging from human health to energy and sustainability.
Natalie Fardian-Melamed, Postdoctoral Scholar, Columbia Engineering
Using the Photon-Avalanching Effect to Build the Nanosensors
The team was able to construct these nanosensors by using the photon-avalanching action in nanocrystals. The absorption of a single photon within a material initiates a series of reactions that culminate in the release of several photons in photon-avalanching nanoparticles, initially identified by Schuck’s group at Columbia Engineering.
Thus, several photons are released when one photon is absorbed. Schuck likes to refer to this highly explosive and nonlinear process as “steeply nonlinear,” playing on the term “avalanche.”
Atomic ions from the periodic table's lanthanide row, sometimes referred to as rare-earth elements, are doped into the study’s nanocrystals and serve as their optically active constituents. The team used thulium for this study.
Team Investigates a Surprising Observation
The researchers discovered that the distance between lanthanide ions is one factor that significantly impacts the photon avalanching process. In light of this, they used an atomic force microscopy (AFM) tip to tap on some of their photon avalanching nanoparticles (ANPs). They found that these mild stresses had a far greater influence on the avalanching behavior than they had ever anticipated.
Schuck added, “We discovered this almost by accident. We suspected these nanoparticles were sensitive to force, so we measured their emission while tapping on them. And they turned out to be way more sensitive than anticipated! We actually did not believe it at first; we thought the tip may be having a different effect. But then Natalie did all the control measurements and discovered that the response was all due to this extreme force sensitivity.”
After realizing how sensitive the ANPs were, the scientists created new nanoparticles that would react differently to forces. In one novel design, the applied force causes the nanoparticle's luminescence to change color. In another design, they created nanoparticles that, when force is applied, start to avalanche even though they do not exhibit photon avalanching in ambient conditions. These nanoparticles have proven to be incredibly sensitive to force.
Collaborative Effort with Lawrence Berkeley National Lab
Schuck, Fardian-Melamed, and other members of the Schuck nano-optics team collaborated closely with a group of scientists led by Bruce Cohen and Emory Chan at the Molecular Foundry at Lawrence Berkeley National Lab (Berkeley Lab) for this project. Based on Columbia's input, the Berkeley lab team created the unique ANPs, synthesizing and analyzing thousands of samples to comprehend and enhance the particles' optical characteristics.
What is Next
The team's current goal is to use these force sensors in a crucial system where they can have a big influence, such a developing embryo, like the ones that Columbia Mechanical Engineering Professor Karen Kasza has examined. In terms of sensor design, the researchers want to enable self-calibrating capabilities in the nanocrystals so that each one may operate independently as a sensor. Schuck thinks that adding a second thin shell during nanocrystal formation will make this task simple.
“The importance of developing new force sensors was recently underscored by Ardem Patapoutian, the 2021 Nobel Laureate who emphasized the difficulty in probing environmentally sensitive processes within multiscale systems – that is to say, in most physical and biological processes,” Schuck noted.
Schuck concluded, “We are excited to be part of these discoveries that transform the paradigm of sensing, allowing one to sensitively and dynamically map critical changes in forces and pressures in real-world environments that are currently unreachable with today’s technologies.”
Journal Reference:
Fardian-Melamed, N., et al. (2025) Infrared nanosensors of piconewton to micronewton forces. Nature. doi.org/10.1038/s41586-024-08221-2.