Biocompatible nanoparticles based on diketo-pyrrolo-pyrrole (DPP) with aggregation-induced Red/NIR emission for in vivo two-photon fluorescence imaging

Compared with traditional one-photon fluorescence imaging, two-photon fluorescence imaging techniques have shown advantages such as increased penetration depth, lower tissue autofluorescence, and reduced photodamage, and therefore are particularly useful for imaging tissues and animals. In this work, the design and synthesis of two novel DPP-based compounds with large two-photon absorption (2PA) cross-sections (σ ≥ 8100 GM) and aggregation-induced emission (AIE) properties are reported. The new compounds are red/NIR emissive and show large Stokes shifts (Δλ ≥ 3571 cm^-1). 1,2-Distearoyl-sn-glycero-3-phosphoethanol amine-N-[maleimide(polyethylene glycol)-2000 (DSPE-PEG-Mal) is used as the encapsulation matrix to encapsulate DPP-2, followed by surface functionalization with cell penetrating peptide (CPP) to yield DPP-2-CPP nanoparticles with high brightness, good water dispersibility, and excellent biocompatibility. DPP-2 nanoparticles have been used for cell imaging and two-photon imaging with clear visualization of blood vasculature inside mouse ear skin with a depth up to 80 μm.