Advances in probe crossing the blood–brain barrier strategies and fluorescent imaging for brain dynamics

The blood–brain barrier (BBB) is a selectively permeable barrier that plays a crucial role in protecting the brain. However, the dual nature of its protective mechanisms presents significant challenges for the development of real-time imaging tools targeting active substances within the brain. With the emergence of novel diseases and the worsening of pre-existing brain disorders such as epilepsy, Alzheimer's disease (AD), and glioblastoma, there is an urgent need to expand the repertoire of chemical tools to refine diagnostic and therapeutic strategies for these conditions. In this context, fluorescent probes have shown remarkable potential for the development and optimization of novel diagnostic tools capable of crossing the BBB. Their advantages include adjustable molecular weight, controllable lipid water partition coefficients, and the capability for real-time imaging in living organisms. In this review, we discuss the evaluation methods and physicochemical parameters of fluorescent probes designed to cross the BBB. We also summarize recent studies on fluorescent probes for detecting active substances in the brain, including reactive nitrogen species (RNS), reactive oxygen species (ROS), reactive sulfur species (RSS), proteins, and neurotransmitters. These studies highlight the methodologies and practicalities of developing efficient fluorescent probes for BBB penetration. Furthermore, this paper briefly addresses the challenges associated with fluorescent probes crossing the BBB and proposes potential strategies for future technological advancements. These insights provide valuable references for the development of new optical probes for in situ brain imaging.