Mechanoluminescent materials, which emit light in response
to mechanical
stimuli, have recently been explored as promising candidates for photonic
skins, remote optogenetics, and stress sensing. All mechanoluminescent
materials reported thus far are bulk solids with micron-sized grains,
and their light emission is only produced when fractured or deformed
in bulk form. In contrast, mechanoluminescence has never been observed
in liquids and colloidal solutions, thus limiting its biological application
in living organisms. Here, we report the synthesis of mechanoluminescent
fluids via a suppressed dissolution approach. We demonstrate that
this approach yields stable colloidal solutions comprising mechanoluminescent
nanocrystals with bright emissions in the range of 470–610
nm and diameters down to 20 nm. These colloidal solutions can be recharged
and discharged repeatedly under photoexcitation and hydrodynamically
focused ultrasound, respectively, thus yielding rechargeable mechanoluminescent
fluids that can store photon energy in a reversible manner. This rechargeable
fluid can facilitate a systemically delivered light source gated by
tissue-penetrant ultrasound for biological applications that require
light in the tissue, such as optogenetic stimulation in the brain.
Many in vivo biological techniques, such as fluorescence imaging, photodynamic therapy, and optogenetics, require light delivery into biological tissues. The limited tissue penetration of visible light discourages the use of external light sources and calls for the development of light sources that can be delivered in vivo. A promising material for internal light delivery is persistent phosphors; however, there is a scarcity of materials with strong persistent luminescence of visible light in a stable colloid to facilitate systemic delivery in vivo. Here, we used a bioinspired demineralization (BID) strategy to synthesize stable colloidal solutions of solid-state phosphors in the range of 470 to 650 nm and diameters down to 20 nm. The exceptional brightness of BID-produced colloids enables their utility as multicolor luminescent tags in vivo with favorable biocompatibility. Because of their stable dispersion in water, BID-produced nanophosphors can be delivered systemically, acting as an intravascular colloidal light source to internally excite genetically encoded fluorescent reporters within the mouse brain.
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