Adapted from this Columbia Engineering press release
Mechanical force is an essential feature for many physical and biological processes. Remote measurement of mechanical signals with high sensitivity and spatial resolution is needed for a wide range of applications, from robotics to cellular biophysics and medicine and even to space travel. Nanoscale luminescent force sensors excel at measuring piconewton forces, while larger sensors have proven powerful in probing micronewton forces. However, large gaps remain in the force magnitudes that can be probed remotely from subsurface or interfacial sites, and no individual, non-invasive sensor has yet been able to make measurements over the large dynamic range needed to understand many systems.
In a paper published in Nature, a team of staff and users from Columbia Engineering report that they have invented new nanoscale sensors of force. They are luminescent nanocrystals that can change intensity and/or color when you push or pull on them. These “all-optical” nanosensors are probed with light only and therefore allow for fully remote read-outs — no wires or connections are needed.
The researchers, led by Columbia professor Jim Schuck, along with the Cohen and Chan groups at the Foundry, developed nanosensors that have attained both the most sensitive force response and largest dynamic range ever realized in similar 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,” Schuck says.
The new nanosensors achieve high-resolution, multiscale function with the same nanosensor for the first time. This is important as it means that just this nanosensor, rather than a suite of different classes of sensors, can be employed for the continuous study of forces, from the subcellular to the whole-system level in engineered and biological systems, such as developing embryos, migrating cells, batteries, or integrated NEMS, very sensitive nanoelectromechanical systems in which the physical motion of a nanometer-scale structure is controlled by an electronic circuit, or vice versa.
The team was able to build these nanosensors by exploiting the photon-avalanching effect within nanocrystals. In photon-avalanching nanoparticles, the absorption of a single photon within a material sets off a chain reaction of events that ultimately leads to the emission of many photons. So: one photon is absorbed, many photons are emitted. It is an extremely nonlinear and volatile process that Schuck likes to describe as “steeply nonlinear,’ playing on the word “avalanche.”
The optically active components within the study’s nanocrystals are atomic ions from the lanthanide row of elements in the periodic table, also known as rare-earth elements, which are doped into the nanocrystal. For this paper, the team used thulium.
The researchers found that the photon avalanching process is very, very sensitive to several things, including the spacing between lanthanide ions. With this in mind, they tapped on some of their photon avalanching nanoparticles (ANPs) with an atomic force microscopy (AFM) tip, and discovered that the avalanching behavior was greatly impacted by these gentle forces — much more than they had ever expected.
“We discovered this almost by accident,” Schuck says. “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 didn’t believe it at first; we thought the tip may be having a different effect. But then we did all the control measurements and discovered that the response was all due to this extreme force sensitivity.”
Knowing how sensitive the ANPs were, the team then designed new nanoparticles that would respond to forces in different ways. In one new design, the nanoparticle changes the color of its luminescence depending on the applied force. In another design, they made nanoparticles that do not demonstrate photon avalanching under ambient conditions, but do begin to avalanche as force is applied — these have turned out to be extremely sensitive to force.
Read the full press release
