A facile two-photon red-emission fluorescent marker with a large Stokes shift for in vivo brain imaging of amyloid-beta aggregates

Efficient fluorescent probes for brain imaging of the progressive changes of amyloid-beta (Aβ) aggregates in Alzheimer’s disease (AD) are highly desired to track the occurrence and development of AD. Herein, a donor-acceptor-donor-type fluorescent probe (BTD-Aβ) is prepared using benzothiadiazole (BTD) scaffold as the acceptor, together with p-dimethylaminobenzene and p-hydroxybenzene as donors. The probe is an electrically neutral molecule with low molecular weight of 348.1 Da, large two-photon absorption cross-section of 240 Goeppert-Mayer, red emission centered at 630 nm, and large Stokes shift of 200 nm. In addition, the probe displays improved sensing and imaging capabilities to Aβ aggregates including faster response (within 12 min), higher binding affinity (Kd = 374 nM), higher selectivity, and good blood-brain barrier penetration (16.0%). The probe is successfully used for in situ observation of Aβ42 aggregating in brain-derived endothelial cells and in vivo brain imaging in AD mice.