Fluorescent Imaging of Amyloid-β Deposits in Brain: An Overview of Probe Development and a Highlight of the Applications for In Vivo Imaging

The β-amyloid (Aβ) plaques presented within the brain parenchyma have been widely proved to be one of the hallmarks of Alzheimer's disease (AD). According to the amyloid cascade hypothesis, the accumulation of Aβ plaques in the brain is intrinsic and fundamental for disease onset, and much research about the early diagnosis of AD is based on this. A recent development in Aβ detection has focused on the mapping of the molecule events in the brain using an exquisite, noninvasive, and inexpensive optical imaging technique, which has stimulated the rapid development of Aβ-specific fluorescent probes. Among them, nearinfrared (NIR) fluorophores have gained adequate attention due to the weak light attenuation in tissues and avoidance from auto-fluorescence of biological matter. In this review, we showcase the current developments of fluorescent probes that are subject to in vitro or in vivo detection of Aβ plaques in the brain, and give an emphasis on the probes used for in vivo twophoton microscopy and NIR imaging by highlighting their biological and photochemical properties.