Biophototheranostic Story of Nile Blue Families: O, S, Se-Atom Engineering

Cancer remains a leading cause of death worldwide. Phototheranostic tools, known for their rapid response and high precision, have opened new avenues for cancer diagnosis and treatment. As a pioneering agent, Nile Blue (NB) dye has attracted significant interest due to its excellent chemical stability, lipophilicity, and near-infrared (NIR) excitation/emission properties, establishing itself as a reference molecular platform in biomedicine. Our research journey with NB began with its remarkable potential as a fluorescence probe in the field of bioimaging for disease diagnosis. As research progressed, we began to explore structural modifications to expand its functional boundaries. A key breakthrough from our team in 2018 revealed that replacing the central oxygen (O) atom in the NB structure with a sulfur (S) atom significantly enhanced the intersystem crossing (ISC) rate of the resulting analog ENBS, enabling its use as a specific superoxide anion (O₂^•-) photosensitizer (PS) for low O₂-dependent photodynamic therapy (PDT) (J. Am. Chem. Soc. 2018, 140, 14851-14859).This discovery opened the door to optimizing ENBS via "chalcogen atomic engineering" and inspired our intensive research into systematic modifications of the NB framework, such as by conjugating drug molecules, targeting groups and activation sites. We contributed a series of ENBS derivatives widely used in cancer PDT. Another representative study involved replacing the central O in NB with a selenium (Se) atom. The resulting compound, ENBSe, capable of functioning as a photocatalyst (PC), exhibited a significantly improved triplet state efficiency (J. Am. Chem. Soc. 2022, 144, 163-173). By triggering cellular biomolecular conversion (e.g., nicotinamide adenine dinucleotide (NADH) oxidation and cytochrome c (Cyt c (Fe³⁺)) reduction) and interfering with the mitochondrial respiratory chain, ENBSe effectively addressed certain challenges in biocatalysis. Importantly, the entire process is O₂-independent, offering a new strategy for treating hypoxic tumors. These systematic and in-depth research efforts have earned our research group a distinctive "Nile Blue" label in the field, resulting in a plethora of NB analogs developed by many groups around the world.During the past 10 years, our research group has employed a "chalcogen atom substitution" strategy to systematically develop O/S/Se-modified NB derivatives, making significant contributions to the design of phototheranostic tools. Throughout this evolutionary process, NB has progressed from a simple fluorescent bioimaging probe to an innovative therapeutic agent and ultimately into a groundbreaking biophotoredox catalyst. In this comprehensive Account, we begin with a brief historical overview of NB dyes, then trace the developmental trajectory of NB, ENBS, and ENBSe to thoroughly present the innovative achievements. Finally, we conclude with a critical perspective on the clinical translation potential of NB-based analogs. We are convinced that our summarized work of the "single molecular scaffold, multiple theranostic functions" paradigm will redefine the development of next-generation phototheranostic agents, driving transformative advances in healthcare.