All-optical nanoscale sensor of force changes intensity when pushed or pulled

A team of Columbia University researchers and collaborators are designing all-optical (probed via light) nanoscale sensors of force—that can detect piconewton to micronewton forces—based on luminescent nanocrystals that change intensity when pushed or pulled.

The optically active components within the team’s nanocrystals are lanthanide ions, a.k.a. rare earth elements, which are doped into a nanocrystal. And the nanosensors the team designed offer 100x better force sensitivity than other rare-earth-based nanosensors and an optical range 10 to 100x greater than any previous optical nanosensor—it’s a technology disruption for everything ranging from robotics to medicine to space travel.

Jim Schuck, an associate professor of mechanical engineering, and Natalie Fardian-Melamed, a postdoctoral researcher within his group, are leading the work and collaborating with the Cohen and Chan groups at Lawrence Berkeley National Lab’s Molecular Foundry.

“Our work started almost by accident,” says Fardian-Melamed. “We were initially planning different experiments, and when we pressed on our avalanching nanoparticles with an atomic force microscope (AFM) tip it was actually as a control measurement to see if mechanical force has any influence on the optical signal we get from these nanoparticles.”

Mechanical force

It turns out, mechanical force showed a drastic effect on the optical signal of the avalanching nanoparticles. “So we planned different experiments to verify whether mechanical force did indeed lead to these huge optical signal changes—and once we knew for sure, we leveraged our newfound knowledge to design different particle configurations that react to force in different ways,” Fardian-Melamed says. “Now we have nanosensors that get brighter with applied force, get dimmer, or change their emission color.”

These nanosensors rely on a photon avalanche—in which a single photon absorbed within a lanthanide-doped crystal sets off a chain reaction that culminates in a burst of upconverted photons.

This chain reaction “relies on parameters such as the energy transfer processes between the lanthanides (and interionic distances) and the phonon energies of the host crystal lattice—both of which can be modified by applying force,” explains Fardian-Melamed. “Because the photon avalanche is such a nonlinear effect, a tiny change in any of these parameters translates into a dramatic change in the avalanche optical signal.”

All-optical and all-infrared nanosensors

One of the key benefits of the team’s sensor design is it’s not only all-optical but all-infrared as well—meaning its input and output are benign, deeply penetrating, and biologically safe infrared light. It enables researchers to peer deep into physiological and technological systems to detect the mechanical forces there—from afar. Importantly, these sensors can provide early detection of malfunctioning or failing systems.

Another key benefit is “the ultrawide force range you can detect with them,” Fardian-Melamed says. “There are many ultrasensitive force sensors out there, or sensors that can detect larger, microscale forces—but, in our case, we can detect more than four orders of magnitude of force with the same sensor.”

And since these are nanoscale sensors, the “possibilities of systems and sites you can detect with them are endless,” Fardian-Melamed adds. “You can reach deeply subsurface or interfacial nanoscale sites—without large extensions or wires.” All readouts are fully remote.

Serendipity of discovery

When the team first discovered photon avalanching nanoparticles are so sensitive to force, they didn’t quite believe it. “We initially thought the signal changes we observed were due to quenching by the AFM tip material, or something else,” says Fardian-Melamed. “So we designed different types of particles (different coating shell thicknesses, different concentrations, etc.), and set out to see if it was indeed the mechanical force influencing the avalanche process.”

The team had a hunch that if they designed a pre-avalanching particle—with a doping concentration just below what would sustain an avalanche—and pressed on it, it would decrease the interionic distances, increase phonon energies, and eventually lead to a brighter avalanching particle under force.

“One dark night during the holiday season, when everyone was off on vacation and the lab was pretty empty, I pressed on one of these particles,” recalls Fardian-Melamed. “We had turned a pre-avalanching particle into an avalanching particle—with mechanical force.”

This moment was a turning point for the team. They knew they had total control over the avalanching process with mechanical force—and it enabled them to design different force-sensing modalities, including mechanobrightening ones.

Sky’s the limit

What’s next? There’s still work to be done, and the team is currently trying to make the nanoparticles involved more uniform, adding self-calibrating functionality into them, and exploring how they react to other environmental stimuli such as temperature.

“Our nanosensors will have a great impact on various fields where you can’t measure multiple forces scales from interstitial and/or substrate sites remotely,” Fardian-Melamed says. “There are many open questions in cancer research or energy and sustainability—and we think our sensors will help to find answers. We foresee these nanosensors revolutionizing areas spanning medicine to robotics to battery design.”

These nanosensors are truly a technology disruption. “There are endless possibilities; the sky really is the limit,” Fardian-Melamed says.

FURTHER READING

N. Fardian-Melamed et al., Nature, 637, 70–75 (2025); https://doi.org/10.1038/s41586-024-08221-2.