A ratiometric AIE probe for dual color imaging lipid droplets and lysosomes and real-time quantitative monitoring lipophagy

Lipophagy, one of the interactions between lipid droplets (LDs) and lysosomes, is essential for maintaining intracellular homeostasis. In this work, a pH-sensitive ratiometric fluorescent probe (LD-L) with aggregation -induced emission (AIE) characteristics was developed to simultaneously stain LDs and lysosomes with dual -color emission and to real-time quantitatively monitor the lipophagy process. LD-L was constructed by intro-ducing a proton accepter of N,N '-diethylamino unit into AIE-active pyrazoline-based skeleton. Upon protonation, bright emission at 570 nm of LD-L blue shifted to 475 nm, resulting in a significant ratiometric fluorescent response towards pH change. A good linear relationship between fluorescent intensity ratio (I475/I570) and pH range of 4.85-5.05 was obtained while the pKa of LD-L was determined to be 4.97. Importantly, LD-L could simultaneously stain the LDs and lysosomes with different emission under single 405 nm excitation and image the intracellular pH with a ratiometric fluorescence mode. LD-L was further applied to real-time monitor lip-ophagy process and to quantify the pH change of LDs at later stage of lipophagy.