Unidirectional optical mechano-sensing with upconversion nanoparticles

Cells respond to forces, and their quantification can potentially inform on the role of mechanics in cell development, differentiation, tissue repair and homeostasis. Other force sensitive processes include cancer cell metastasis, heart development in embryos driven by fluid forces, and individual cell response to tension by enhancing microtubule growth and connections. Development of current mechano-sensing approaches has not yielded many options, especially in directional force measurement. We present a sharpened fiber-based approach for uniaxial forces. An upconversion nanoparticle (UCNP) is mounted on the tip of the fiber and optically accessed through the fiber, which is manipulated as a probe. In UCNPs, the modification of the crystal field via mechanical forces result in changes in emission intensity, spectral shifts, upconversion luminescence (UCL) lifetime and ratiometric UCL response. We report on a discernably large peak shift of between 5-10 nm, and an apparent phase transition, with increasing amount of applied force in the micro Newton regime, in a single direction. Moreover, the peak shift is linear to the applied compression force. We investigate the influence of the UCNP force sensing process using Raman spectroscopy.