Perovskites as Multiphoton Fluorescence Contrast Agents for In Vivo Imaging

Multiphoton fluorescence microscopy is a powerful tool for imaging and exploring biological tissue at subcellular spatial resolution while minimizing photobleaching and autofluorescence. For optimal performance in multiphoton microscopy, materials exhibiting a large multiphoton absorption cross section (σ_n) and fluorescence quantum yield are desired. Notably, perovskite nanocrystals (CsPbX₃, PNCs) exhibit exceptionally large two-, three-, up to five photon absorption cross section (σ₂ ∼ 10⁶ GM, σ³ ∼ 10^-73 cm⁶s² photon^-2, σ₅ ∼ 10^-136 cm¹⁰s⁴ photon^-4), along with near unity fluorescence quantum yield, making them desirable for deep tissue applications. Here, we employed PNCs as contrast agents to image mesenchymal stromal cells in a living mouse. The PNCs were stabilized by encapsulating them in a SiO₂ matrix (∼60-70 nm in diameter), offering versatility for subsequent surface modification to target specific biological entities for both diagnostic and therapeutic applications. Multiphoton imaging of PNCs offers substantial benefits for dynamic tracking of cells in deep tissue, such as in understanding immune cell migration and other biological processes in both healthy and diseased tissues.