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A new nanofabrication approach is reported for the scalable production of plasmonically modulated upconverting nanoparticles, with the potential for force sensing. 

Abstract A novel force sensor exploiting the interaction between plasmonic nanostructures and upconversion nanoparticles (UCNPs) is reported. The nanosensor is composed of a gold nanodisk and UCNPs separated by a flexible polymer layer. The gold nanodisk is designed to exhibit a plasmon resonance that selectively enhances one of the emission bands of the UCNPs while leaving the other ones largely unaffected. As the nanosensor is compressed or stretched by an external force, the polymer layer thickness changes, modulating the plasmon‐UCNP coupling. The resulting changes in the luminescence intensity provide the basis for sensing. Furthermore, the nanosensor employs ratiometric sensing, which makes it highly robust against any environmental variations. The nanosensors exhibit two orders of magnitude higher responsivity than previously reported UCNP‐based force sensors. They can be prepared as an on‐chip sensor array or in a colloidal solution, making them suitable for a variety of applications in biology and robotics. 

Abstract Förster resonance energy transfer (FRET)‐based devices have been extensively researched as potential biosensors due to their highly localized responsivity. In particular, dye‐conjugated upconverting nanoparticles (UCNPs) are among the most promising FRET‐based sensor candidates. UCNPs have a multi‐modal emission profile that allows for ratiometric sensing, and by conjugating a biosensitive dye to their surface, this profile can be used to measure localized variations in biological parameters. However, the complex nature of the UCNP energy profile as well as reabsorption of emitted photons must be taken into account in order to properly sense the target parameters. To the authors’ knowledge, no proposed UCNP‐based sensor has accurately taken care of these intricacies. In this article, the authors account for these complexities by creating a FRET‐based sensor that measures pH. This sensor utilizes Thulium (Tm³⁺)‐doped UCNPs and the fluorescent dye fluorescein isothiocyanate (FITC). It is first demonstrated that photon reabsorption is a serious issue for the 475 nm Tm³⁺emission, thereby limiting its use in FRET‐based sensing. It is then shown that by taking the ratio of the 646 and 800 nm emissions rather than the more popular 475 nm one, it is possible to measure pH exclusively through FRET.