A Biomineral-Inspired Approach of Synthesizing Multicolor Colloidal Persistent Luminescent Nanophosphors and Mechanoluminescent Fluids as Intravital Light Sources

Light is used in a wide range of biotechnologies, such as fluorescence imaging and optogenetics, yet a critical challenge of applying light <i>in vivo</i> arises from the poor penetration of photons in biological tissue due to scattering and absorption. Therefore, delivering photons deep into the body from an external light source requires invasive procedures, such as craniotomy, to surgically remove overlying tissues. Besides conventionally used external light sources, trap-engineered materials with persistent luminescence (PerL, as known as afterglow) and mechanoluminescence (ML) represent an arising opportunity for light delivery in deep tissue owing to their ability to store photon energy in their host structures. However, materials with strong short-wavelength PerL and ML are usually bulk particles (&gt; 10 μm) synthesized via solid-state reactions at &gt; 1000 ^oC to facilitate uniform doping and desirable polymorphs. Their large sizes prevent the formation of stable colloids in aqueous environment and prohibit their biological applications.<br/>To address this challenge, here we report a bioinspired demineralization (BID) approach to synthesize stable colloidal solutions of solid-state PerL and ML nanoparticles with tunable wavelengths and remarkable luminescence intensity. Specifically, the BID approach is inspired by the strategy of biomineralization in nature: Biominerals, such as apatite in the dental enamel, can be gradually dissolved to nanostructures in a mildly acidic environment yet are resistant to further dissolution. This unconventional dissolution process, unlike the self-accelerating dissolution of soluble salts, is facilitated by the formation and growth of pits on the surface. The low solubility and thus high interfacial tension of biominerals results in a large critical size of 10 to 100 nm, below which the growth of dissolution pit is kinetically suppressed. In the BID approach, we demonstrated that sparingly soluble solid-state PerL and ML particles, similar to biominerals, also exhibited kinetically suppressed dissolution. Specifically, we used a citrate buffer to mimic the undersaturated environment and produce colloidal nanoparticles down to 20 nm from their solid-state precursors with emission wavelengths covering the entire visible spectrum, including Sr₂MgSi₂O₇:Eu,Dy (470 nm), Sr₄Al₁₄O₂₅:Eu,Dy (490 nm), ZnS:Cu,Al (534 nm), ZnS:Mn (578 nm), CaTiO₃:Pr (610 nm), and Ca_0.85Sr_0.15S:Eu,Tm (650 nm). Furthermore, these BID-produced nanoparticles can be stably suspended in aqueous solution, while preserving the optical properties of their solid-state precursors, including strong PerL or ML intensities.<br/>Additionally, we demonstrated the utility of BID-produced colloids in biology via excitation-free imaging and ultrasound-mediated non-invasive optogenetics. On one hand, owing to their exceptionally bright luminescence, BID-produced PerL nanophosphors exhibited the highest signal-to-background ratio for <i>in vivo</i> imaging among reported materials, and yielded the first example of transcranial afterglow imaging of cerebral vessels in the mouse brain. Furthermore, the bright short-wavelength PerL nanophosphors acted as an intravital light source and provided internal excitation for genetically encoded fluorescent reporters in a mouse brain through the intact skull, thus enabling afterglow imaging of fluorescent proteins for the first time. On the other hand, the BID-produced ML fluids acted as an ultrasound-mediated light source in the body after systemic delivery. We were able to visualize the ultrasound-mediated light emission from the ML fluids in the vascular systems of multiple mouse organs (including brain, liver and kidney) for the first time, and used this light emission for non-invasive optogenetics stimulation in transgenic mice expressing light sensitive ion channel ChR2. <b>The manuscripts based on these results were recently published in <i>Sci. Adv.</i> (DOI: 10.1126/sciadv.abo6743) and <i>JACS</i> (DOI: 10.1021/jacs.2c06724).</b>