Self-Healing Organic Fluorophore of Cyanine-Conjugated Amphiphilic Polypeptide for Near-Infrared Photostable Bioimaging

Photobleaching and biotoxicity are the main bottlenecks for organic fluorescent dyes applied in real-time dynamic monitoring of living cells. Here, an unnatural amino acid, 4-nitro-3-phenyl-l-alanine (NPA), was used as a scaffold to covalently link a near-infrared fluorophore Cy5.5 and an amphiphilic polypeptide, poly[oligo(ethylene glycol) methyl ether methacrylate]-block-poly[2-amino-N4-(2-diisopropylamino-ethyl)-l-aspartic acid] (P(OEGMA)21-P(Asp)16-iPr), was then conjugated for increasing the photostability and improving the biocompatibility simultaneously. The protective agent of NPA can service as an effective triplet state quenching by intramolecular electron transfer between Cy5.5 and NPA. The less sensitivity of the electron-transfer process for molecular oxygen makes it an ideal photostabilized strategy for fluorophores applied in live-cell imaging. Bonding to copolymer is a common way for hydrophobic dyes to expand their application in biomedical imaging and increase their functionality, depending on the delivery system. The results indicate that Cy5.5–NPA-linked polypeptide copolymer exhibited an enhanced photostability and an excellent biocompatibility, which means this scaffolding strategy has a potential application in fluorescence-guided surgery, lived-cell imaging, and super-resolution microscopy