Researchers have used nanoparticles doped with light-emitting lanthanide ions to detect a wide range of very weak forces. [Image: Andrew Mueller / Columbia Engineering]
Scientists in the United States say they have used infrared nanosensors exploiting the phenomenon of photon avalanching to detect miniscule forces across a wide range of magnitudes and with unprecedented sensitivity (Nature, doi: 10.1038/s41586-024-08221-2). They claim that the lanthanide-doped devices would be ideal for applications requiring accurate non-invasive sensing over multiple length scales—in areas from cellular biophysics to space travel.
Less is more
Photon avalanching is a chain reaction in which a single photon absorbed by a suitable material yields multiple photons at a higher frequency. This nonlinear process is characteristic of elements within the periodic table's lanthanide series and allows the detection of otherwise very weak signals. However, it requires large amounts of lanthanide ions and until recently was only possible in bulk materials.
Jim Schuck and colleagues at Columbia University reported in 2021 that they had instead achieved photon avalanching using 25-nm-diameter crystals of sodium yttrium fluoride doped with high concentrations of the lanthanide thulium. The group has now shown how these nanoparticles can be used as exceptionally sensitive force sensors, demonstrating three distinct types of sensing by varying the dopant concentration.
The work, led by Columbia’s Natalie Fardian-Melamed and carried out in collaboration with researchers at Lawrence Berkeley National Laboratory and the University of Utah, used an atomic force microscope (AFM) placed on top of an inverted optical microscope. Using this setup, the researchers exposed single nanoparticles to infrared laser light and measured the emitted radiation while tapping the particles very gently with the AFM's cantilever arm.
The full four orders of magnitude, they say, is 10 to 100 times larger than the range of any previous optical nanosensor.
Three types of sensing
Employing particles with between 4.5% and 8% of yttrium ions replaced by thulium ions, Schuck and coworkers first demonstrated the existence of photon avalanching without applying a force, measuring a steep rise in output intensity as they ramped up the pump laser. They then called the AFM into action and revealed the expected shift in the excitation–emission curve, which showed that for forces as small as 200 nanonewtons, a given emission intensity requires a substantially higher excitation intensity. This implies that bigger forces will generate smaller emissions for a given pump power.
After that, the researchers demonstrated the dynamic range of their setup by exposing each nanoparticle to forces between 0 and 2.5 micronewtons—establishing in each case how much of the range they could detect. They found that all particles could sense forces over about three orders of magnitude, with the laser pump power determining the range's end points. Lower powers enabled the detection of hundreds of piconewtons to hundreds of nanonewtons, while higher powers allowed sensing of single-digit nanonewtons to single-digit micronewtons. The full four orders of magnitude, they say, is 10 to 100 times larger than the range of any previous optical nanosensor.
Measuring the smallest forces—the system's resolution—was made possible by reducing the dopant concentration to 4%. Left alone, such particles do not exhibit avalanching. But when pressed by the AFM, they too revealed the characteristic nonlinear rise in emission—in contrast to the more lanthanide-rich particles, whose emission drops when prodded. This behavior made it possible to detect forces as small as 475 piconewtons inside 3 seconds, as opposed to 620 piconewtons for thulium concentrations above 4.5%.
Finally, Schuck, Fardian-Melamed and colleagues tested what they term “piezochromic” sensors. With thulium concentrations of 15%, these objects emit significant avalanches of photons at two wavelengths: 800 nm, as with lower concentrations, and also 700 nm. The researchers found that when applying force, the ratio of these emissions (the sensor's “color”) clearly shifts—radiation at the longer wavelength intensifies compared with that at the shorter wavelength. The fact that the output of the devices is in the form of a ratio, they argue, could make them robust against environmental interference (which makes calibration a problem).
The fact that the output of the devices is in the form of a ratio, they argue, could make them robust against environmental interference (which makes calibration a problem).
A new sensing paradigm
According to Schuck, the new results help to “transform the paradigm of sensing” by allowing sensitive and dynamical measurement of forces and pressures in real-world environments that are “currently unreachable with today’s technologies.” Among the complex physical or biological systems that could be probed with these sensors, he and his colleagues cite energy storage units, migrating cells and developing embryos. In the last case, he says, nanosensors attached to cells' membranes could potentially reveal the spatial variation of forces involved in an embryo's development.
But the researchers caution that challenges remain before their technology can be widely used. Among these, they say, are the innate variation between nanoparticles, as well as the sensors' susceptibility to fluctuations in external parameters such as temperature. They say that the latter problem is less of an issue for the piezochromic scheme, and it might be overcome by using other lanthanide elements to ensure correct calibration.