Revolutionary All-Optical Nanoscale Sensors Unlock Hidden Realms of Force Detection

Mechanical force is a crucial aspect for various physical and biological functions. There is a requirement for remote monitoring of mechanical signals with exceptional sensitivity and spatial accuracy for numerous applications, including robotics, cellular biophysics, medicine, and even space exploration. Nanoscale luminescent force sensors excel in detecting piconewton forces; meanwhile, larger sensors have proven effective in examining micronewton forces.

Nevertheless, significant gaps persist in terms of force magnitudes that can be examined from subsurface or interfacial locations, and no single non-invasive sensor has yet accomplished measurements across the extensive dynamic range necessary for understanding many systems.

Novel, highly responsive nanoscale force sensors

In a publication released today in Nature, a group led by researchers from Columbia Engineering and their collaborators announce the innovation of new nanoscale force sensors. These are luminescent nanocrystals that alter their intensity and/or color under the influence of applied force. These “all-optical” nanosensors are examined exclusively through light, permitting fully remote readings—no wires or connections necessary.

The team, spearheaded by Jim Schuck, associate professor of mechanical engineering, and Natalie Fardian-Melamed, a postdoctoral researcher in his team, along with the Cohen and Chan groups from Lawrence Berkeley National Lab (Berkeley Lab), developed nanosensors that have achieved both unparalleled force response sensitivity and the broadest dynamic range ever realized in similar nanoprobes.

They demonstrate an astounding 100 times greater force sensitivity compared to existing nanoparticles that utilize rare-earth ions for their optical responses, and a functional range exceeding four orders of magnitude in force, a much larger span—10–100 times more—than any prior optical nanosensor.

“We anticipate our discovery will transform the sensitivities and dynamic ranges attainable with optical force sensors, immediately impacting technologies across areas from robotics to cellular biophysics and medicine to space exploration,” Schuck asserts.

New nanosensors can function in previously unreachable environments

The newly developed nanosensors achieve enhanced resolution and multiscale functionality utilizing the same nanosensor for the first time. This is significant as it indicates that this singular nanosensor, rather than multiple classes of sensors, can be utilized for the ongoing exploration of forces, from subcellular to system-wide levels in engineered and biological frameworks, such as developing embryos, migrating cells, batteries, or integrated NEMS, highly sensitive nanoelectromechanical systems where the physical movement of a nanometer-scale structure is governed by an electronic circuit, or the reverse.

“The uniqueness of these force sensors—beyond their unmatched multiscale sensing capabilities—lies in their operation with benign, biocompatible, and deeply penetrating infrared light,” states Fardian-Melamed. “This allows for in-depth observation of diverse technological and physiological systems and monitoring their wellbeing from a distance. By facilitating early detection of malfunctions or failures in these systems, these sensors will significantly influence areas ranging from human health to energy and sustainability.”

Employing the photon-avalanching effect to construct the nanosensors

The team successfully created these nanosensors by harnessing the photon-avalanching effect within nanocrystals. In photon-avalanching nanoparticles, first identified by Schuck’s group at Columbia Engineering, the capture of a single photon in a material triggers a sequence of reactions culminating in the emission of numerous photons.

Thus, the absorption of one photon leads to the emission of many photons. It is an exceedingly nonlinear and volatile process that Schuck often describes as “steeply nonlinear,” playing on the term “avalanche.”

The optically active components within the nanocrystals studied are atomic ions from the lanthanide series in the periodic table, commonly referred to as rare-earth elements, which are embedded into the nanocrystal. In this research, the team utilized thulium.

The scientists discovered that the photon avalanching phenomenon is extremely sensitive to various factors, including the distance between lanthanide ions. With this understanding, they gently tapped some of their photon avalanching nanoparticles (ANPs) using an atomic force microscopy (AFM) tip and realized that this tapping dramatically influenced the avalanching behavior—far more than they had anticipated.

“We stumbled upon this almost accidentally,” Schuck recounts. “We suspected these nanoparticles should respond to force, so we monitored their emission while tapping them. They turned out to be significantly more responsive than we expected! Initially, we doubted the results; we thought that perhaps the tip was causing a different effect. However, Natalie conducted all the control measurements and found that the response was entirely due to this extreme force sensitivity.”

Aware of how responsive the ANPs were, the team proceeded to design new nanoparticles that would react to forces in distinct manners. In one novel design, the nanoparticle alters its luminescence color based on the applied force. In another configuration, they crafted nanoparticles that do not exhibit photon avalanching under normal conditions but begin to avalanche when force is applied—these have proven to be exceptionally sensitive.to compel.

In this research, Schuck, Fardian-Melamed, along with other members of the Schuck nano-optics group collaborated intimately with a group of investigators at the Molecular Foundry at Lawrence Berkeley National Lab (Berkeley Lab), led by Emory Chan and Bruce Cohen. The Berkeley lab group created the tailored ANPs utilizing insights from Columbia, synthesizing and analyzing numerous samples to comprehend and enhance the particles’ optical qualities.

Future Directions

The group now intends to utilize these force detectors in a significant system where they can make a notable contribution, such as in a developing embryo, akin to those examined by Columbia’s Mechanical Engineering Professor Karen Kasza. Concerning the design of the sensors, the researchers are eager to incorporate self-calibrating capabilities into the nanocrystals, enabling each one to operate as an independent sensor. Schuck is confident that this can be easily achieved by introducing another thin layer during the synthesis of the nanocrystals.

“The significance of inventing new force detectors was recently highlighted by Ardem Patapoutian, the 2021 Nobel Laureate who stressed the challenges in examining environmentally responsive processes within multiscale systems—that is, within most physical and biological processes,” Schuck remarks.

“We are thrilled to contribute to these discoveries that revolutionize the sensing paradigm, enabling the precise and dynamic mapping of critical variations in forces and pressures in real-world settings that are currently inaccessible with contemporary technologies.”