Intracellular pH sensing and targeted imaging of lysosome by a galactosyl naphthalimide-piperazine probe

An acidic pH independent piperazine–TPE AIEgen as a unique bioprobe for lysosome tracing

“…With proton-pumping vacuolar ATPases in lysosomes or stimulation caused by a lysosomotropic agent that can inhibit autophagy and protein degradation by raising the lysosomal pH, lysosomes can maintain their luminal environment under acidic conditions within pH 3.8–6.6. 3 , 17 , 18 Thus, lysosomes are the most acidic organelles compared to other subcellular components (pH ca. 7.4).…”

“… 10 , 13 , 23 Except for commercial lysosomal probes, morpholine and piperazine units are widely employed as lysosome specific targeting functional groups. 1 , 12 – 14 , 17 , 18 , 24 – 29 To guarantee lysosome selectivity, the functional groups, hydrophilicity as well as the polarity of the targeting probe play important roles. Since the dipole moment of piperazine is similar to that of morpholine, 30 and one more N atom in piperazine endows it with a better ability to make hydrogen bonds, piperazine is more hydrophilic than morpholine.…”

“…The fluorescence of LysoTracker Red, LysoTracker Green and most pH responsive lysosomal probes is turned on due to the protonation of the N atom which effectively eliminates photoinduced electron transfer (PET), the phenomenon that is responsible for fluorescence quenching. 11 , 17 , 18 , 21 , 24 , 29 Thus, the lysosomal probes with the working mechanism of protonation prohibiting PET are highly reliant on the pH value and their emission intensity is unstable and varies significantly with a varying acidity of the microenvironment. 17 , 21 , 22 , 29 On the other hand, a high background luminescent noise signal can be observed due to insufficient fluorescence quenching through PET outside of the lysosome, and some LysoTracker probes in such a case 22 , 31 do not allow a high signal to noise ratio to be obtained.…”

“…31 , 32 However, there are only a few AIEgens developed for targeting lysosomes and for most AIE lysosomal probes developed up till now, their photoluminescence (PL) changes within the lysosomal pH range, which results in low localizing stability against pH changes. 26 , 31 Although the fluorescence emission of most lysosomal probes is pH-dependent to a varying degree, and some of them do allow for monitoring lysosomal pH changes, 12 , 17 , 18 , 27 , 29 there are few studies that have focused on the development of intralysosomal pH stable fluorescent AIE probes. However, a pH-independent lysosomal probe that would allow monitoring of other fundamental microenvironment parameters, such as the viscosity (67–170 ± 20 cP at 25 °C) of the lysosome, would be of great use for sensing microviscosity which can reflect the status, integrity and function of this organelle.…”

Photochromism and molecular logic gate operation of a water-compatible bis-glycosyl diarylethene

“…The PET effect can be blocked by protonation of piperazine in an acidic environment, thereby recovering the strong emission of 1,8-naphthalimide. 31 After addition of 6.0 eq.…”