Design and synthesis of biodegradable nonconjugated S S PAMAM dendrimers with unexpected Deep-red/NIR emission and cell membrane targeting ability for biological imaging

Fluorescent non-conjugated polymers without any π-aromatic building blocks are of great significance in biological applications due to low cytotoxicity and good biocompatibility. However, it is still a great challenge to achieve bright and deep-red/near-infrared (NIR) emissions. Herein, we have designed and synthesized a series of deep-red/NIR non-conjugated polymers with aggregation-induced emission based on disulfide-linked poly(amidoamine) (SSPAMAM) dendrimers, which exhibit excellent biodegradation for bioimaging. These SSPAMAM dendrimers (G0.5-G2.0) containing disulfide bonds in their backbone were synthesized in high yield by incorporating cystamine instead of common diacrylamide. Unexpectedly, the SSPAMAM dendrimers prepared exhibited bright and excitation dependent multicolor-tunable emission. More interestingly, G2.0 SSPAMAM showed a deep-red/NIR emission (λem = 625 nm with a shoulder at 800 nm) originated from an extraordinarily rigid and compact conformation via strong intramolecular hydrogen bonds. Meanwhile, these dendrimers displayed excellent biodegradability because of the large number of disulfide linkages in backbone. Due to the excellent specific cytomembrane location, SSPAMAM dendrimers can be used as a potential cell membrane imaging reagent for multi-color (blue, green, and deep-red) imaging. Furthermore, they could also be used in zebrafish for bioimaging. This investigation has expanded the applications of fluorescent non-conjugated polymers to the field of long-wavelength in vivo biological imaging.