Perovskites as Multiphoton\nFluorescence Contrast Agents\nfor In Vivo Imaging

Multiphoton fluorescence microscopy is a powerful tool\nfor imaging\nand exploring biological tissue at subcellular spatial resolution\nwhile minimizing photobleaching and autofluorescence. For optimal\nperformance in multiphoton microscopy, materials exhibiting a large\nmultiphoton absorption cross section (σ<sub>n</sub>) and fluorescence\nquantum yield are desired. Notably, perovskite nanocrystals (CsPbX<sub>3</sub>, PNCs) exhibit exceptionally large two-, three-, up to five\nphoton absorption cross section (σ<sub>2</sub> ∼ 10<sup>6</sup> GM, σ<sup>3</sup> ∼ 10<sup>–73</sup> cm<sup>6</sup>s<sup>2</sup> photon<sup>–2</sup>, σ<sub>5</sub> ∼ 10<sup>–136</sup> cm<sup>10</sup>s<sup>4</sup> photon<sup>–4</sup>), along with near unity fluorescence quantum yield,\nmaking them desirable for deep tissue applications. Here, we employed\nPNCs as contrast agents to image mesenchymal stromal cells in a living\nmouse. The PNCs were stabilized by encapsulating them in a SiO<sub>2</sub> matrix (∼60–70 nm in diameter), offering versatility\nfor subsequent surface modification to target specific biological\nentities for both diagnostic and therapeutic applications. Multiphoton\nimaging of PNCs offers substantial benefits for dynamic tracking of\ncells in deep tissue, such as in understanding immune cell migration\nand other biological processes in both healthy and diseased tissues.