Recent Development of Photofunctional Transition Metal-Peptide Conjugates for Bioimaging and Therapeutic Applications

Peptides are attractive for diagnostic and therapeutic applications due to their small molecular size, ease of synthesis and chemical modifications, high target affinity and specificity, efficient tissue penetration, and low immunogenicity. In the past decade, there has been enormous interest in the utilization of photofunctional transition metal complexes as cellular probes, bioimaging reagents, and therapeutic agents. The interest stems from the intriguing photophysical and photochemical properties of the complexes, including large Stokes' shifts, long emission lifetimes, high photostability, and high singlet oxygen-photosensitization efficiency. In this Review, we summarize recent advances in the development of photofunctional transition metal-peptide conjugates for bioimaging and therapeutic applications, with a focus on the cellular uptake, intracellular localization, and (photo)cytotoxic activity of the conjugates. Our perspective on the current challenges and future development of photofunctional transition metal-peptide conjugates as diagnostic and therapeutic tools is also provided.