Seeing the Forces: Single Avalanching Upconverting Nanoparticles as Ultrasensitive Local Force Transducers

It has recently been demonstrated that ensembles of upconverting nanoparticles (UCNPs) exhibit changes in their emission spectra due to applied stress [1, 2]. Avalanching UCNPs (ANPs) [3], possessing steeply nonlinear optical responses, are ideally suited for sensing minute changes in their environments, as small perturbations are expected to usher large changes in signal. Because ANPs absorb and emit tissue-penetrating near-infrared (NIR) light, they are uniquely positioned for sensing within biological cells and fluids. To characterize and understand the response of these ANPs to local stress, it is important to study their mechano-opto-physics on a single particle level. Utilizing a custom designed combined AFM and inverted optical microscope system, we correlate sub-nano-Newton applied forces and observable changes in hyperspectral optical signals for isolated single UCNPs, demonstrating their potential as local force sensors.

[1] Lay A, et al. Nano Letters 2017, 17(7), 4172-4177

[2] Lay A, et al. ACS Central Science 2019, 5(7), 1211-1222

[3] Lee C, et al. Nature 2021, 589(7841), 230-235