  Biocompatible fluorescent agents, such as phenylenediamine carbon dots (CDs),
are key contributors to the theragnostic paradigm, enabling real-time in vivo
imaging of drug delivery cargoes. This study explores the optical properties of
these CDs, demonstrating their potential for two-photon fluorescence imaging in
brain vessels. Using an open aperture z-scan technique, we measured the
wavelength-dependent nonlinear absorption cross-section of the CDs, achieving a
peak value near 50 GM. This suggests the potential use of phenylenediamine CDs
for efficient multiphoton excitation in the 775 - 895 nm spectral range.
Mesoporous vaterite nanoparticles were loaded with fluorescent CDs to examine
the possibility of a simultaneous imaging and drug delivery platform. Efficient
one and two-photon imaging of the CD-vaterite composites, uptaken by macrophage
and genetically engineered C6-Glioma cells, was demonstrated. For an in vivo
scenario, vaterite nanoparticles loaded with CDs were directly injected into
the brain of a living mouse, and their flow was monitored in real-time within
the blood vessels. The facile synthesis of phenylenediamine carbon dots, their
significant nonlinear responses, and biological compatibility show a viable
route for implementing drug tracking and sensing platforms in living systems.
