Emerging Applications of Fluorogenic and Non‐fluorogenic Bifunctional Linkers

Homo‐ and hetero‐bifunctional linkers play vital roles in constructing a variety of functional systems, ranging from protein bioconjugates with drugs and functional agents, to surface modification of nanoparticles and living cells, and to the cyclization/dimerization of synthetic polymers and biomolecules. Conventional approaches for assaying conjugation extents typically rely on ex situ techniques, such as mass spectrometry, gel electrophoresis, and size‐exclusion chromatography. If the conjugation process involving bifunctional linkers was rendered fluorogenic, then in situ monitoring, quantification, and optical tracking/visualization of relevant processes would be achieved. In this review, conventional non‐fluorogenic linkers are first discussed. Then the focus is on the evolution and emerging applications of fluorogenic bifunctional linkers, which are categorized into hetero‐bifunctional single‐caging fluorogenic linkers, homo‐bifunctional double‐caging fluorogenic linkers, and hetero‐bifunctional double‐caging fluorogenic linkers. In addition, stimuli‐cleavable bifunctional linkers designed for both conjugation and subsequent site‐specific triggered release are also summarized.