Narrow Results

Optical imaging in vivo holds significant implications for disease diagnosis, and nanoprobes with near-infrared (NIR) emission leverage the deep tissue penetration and high spatiotemporal resolution provided by NIR light, demonstrating considerable application potential. This study presents the design and synthesis of three nitrogen-doped boron–dipyrrin (Aza-BODIPY) molecules: Aza–BDP–OCH₃, Aza–BDP–OH, and Aza–BDP-I. Leveraging the strong electron-accepting properties of the Aza-BODIPY core, we developed a donor–acceptor–donor (D-A-D) structure for Aza–BDP–OCH₃ through modifications with triphenylamine and methoxy groups, resulting in NIR fluorescence. Aza–BDP–OH was obtained via demethylation using boron tribromide, whereas Aza–BDP-I was synthesized by introducing iodine into Aza–BDP–OCH₃. These three molecules self-assemble with the amphiphilic polymer PMHC${}_{18}$-mPEG to form nanoparticles (NPs), yielding optical nanoprobes. The resulting NPs exhibit NIR emission, good water solubility, and biocompatibility. At a concentration of 100$\mu \mathrm{\text{g}}\cdot {\mathrm{\text{mL}}}^{-1}$, these NPs demonstrate low biological toxicity, highlighting their potential for biological applications. Following tail vein injection, Aza–BDP-I NPs accumulate in tumors and effectively illuminate them via the enhanced permeability and retention (EPR) effect. Furthermore, these organic NPs were metabolized by the liver. Therefore, Aza-BODIPY-based NIR fluorescent NPs offer a promising platform for the development of in vivo optical nanoprobes.

BODIPY and aza-BODIPY fluorophores linked to steroids are being developed as multimodal-imaging agents to monitor the mechanism of action of biologically active components in living systems.

Two ferrocene substituted aza-BODIPYs and their corresponding aza-dipyrrins were synthesized and studied. All the compounds were characterized by HRMS, NMR, IR, absorption spectroscopy and cyclic voltammetry techniques. Absorption spectra indicated intramolecular electron transfer from the ferrocene to the aza-BODIPY core. The X-ray crystal structure of 1,7-bisferrocenyl-aza-BODIPY suggested moderate interactions between the ferrocene moieties and aza-BODIPYs core. Ferrocenyl-aza-BODIPYs were non-emissive due to electron transfer from ferrocene moieties to boron aza-dipyrrin core. However the emission of these compounds was dramatically enhanced by oxidizing the ferrocene moieties to ferrocenium ions, which prevents electron transfer between these moieties. Cyclic voltammetry studies suggested that aza-BODIPYs were easier to be reduced as compared to their corresponding aza-dipyrrins.

A pH-driven self-assembly of a simple aza-BODIPY was discovered in PBS solution, whereby ion-specific J-aggregated nanostructures were generated at very low dye concentration (2.5–20 $\mu$M). The aggregation process was investigated in different conditions (pH, temperature and time) by monitoring absorption spectral shifts and associated nanostructure morphological changes. The pH-driven self-assembly process demonstrated an instantaneous thermodynamic phenomenon associated with three characteristic structures, each with distinctive optical properties. When the sample was first formulated within a short time window, a thermodynamically less stable intermediate with an unusual morphology of triangular nanoplates and broad absorption was observed. The formation of these structures was independent of the ions in PBS solution (Na${}^{+}$, K${}^{+})$, thus indicating that the triangular structure was inherent to the anisotropic structure of aza-BODIPY scaffolds. The second structure associated with a metastable pathway generated a uniform population of spherical nanovesicles, while the third structure, generated through a more thermodynamically stable pathway consisted of fibers. The absorption spectra suggested that both spherical and fiber structures contributed to the J-aggregation band at 735 nm in the near infrared optical spectrum and their population in each formulation was concentration dependent. The results highlighted the significance of ion effects in self-assembly of aza-BODIPY and the mechanistic structural changes of the morphology. Furthermore, this fundamental discovery offers a versatile method for the self-assembly of aza-BODIPY J-aggregates as a new nanoplatform with potential photonic applications.

A pH-driven self-assembly of a simple aza-BODIPY was discovered in PBS solution, whereby ion-specific J-aggregated nanostructures were generated at very low dye concentration (2.5–20 μM). The aggregation process was investigated in different conditions (pH, temperature and time) by monitoring absorption spectral shifts and associated nanostructure morphological changes. The pH-driven self-assembly process demonstrated an instantaneous thermodynamic phenomenon associated with three characteristic structures, each with distinctive optical properties. When the sample was first formulated within a short time window, a thermodynamically less stable intermediate with an unusual morphology of triangular nanoplates and broad absorption was observed. The formation of these structures was independent of the ions in PBS solution (Na⁺, K⁺), thus indicating that the triangular structure was inherent to the anisotropic structure of aza-BODIPY scaffolds. The second structure associated with a metastable pathway generated a uniform population of spherical nanovesicles, while the third structure, generated through a more thermodynamically stable pathway consisted of fibers. The absorption spectra suggested that both spherical and fiber structures contributed to the J-aggregation band at 735 nm in the near infrared optical spectrum and their population in each formulation was concentration dependent. The results highlighted the significance of ion effects in self-assembly of aza-BODIPY and the mechanistic structural changes of the morphology. Furthermore, this fundamental discovery offers a versatile method for the self-assembly of aza-BODIPY J-aggregates as a new nanoplatform with potential photonic applications.