The multifaceted roles of peptides in "always-on" near-infrared fluorescent probes for tumor imaging.

Tumor-targeted near-infrared (NIR) fluorescence imaging by using NIR fluorescent probes has attracted extensive attentions in the field of tumor imaging and is becoming an attractive strategy for early cancer diagnosis and surgical guidance for the past few decades. Especially, due to the high affinity and low toxicity, the peptide-based NIR fluorescent probes play an important role in the field of tumor-targeted imaging and therapy. Extensive attempts have been made to develop a set of nontoxic and efficacious "always-on" peptide-based NIR fluorescent probes. In this review, we give a comprehensive analysis of the "always-on" peptide-based NIR fluorescent probes for tumor-targeting for the past 5 years, highlighting the design strategy, chemical synthesis, therapeutical applications, spectroscopic and pharmacological characterization, as well as the multifaceted roles of peptides. A comprehensive understanding of the tumor-targeted, "always-on" peptide-based NIR fluorescent probes will increase our knowledge of cancer diagnosis and benefit clinical cancer therapeutics.