Styrylcyanine-based fluorescent probes with red-emission and large Stokes shift for the detection of viscosity

Aggregation-induced emission-twisted intramolecular charge transfer-activated fluorescent probe for analyzing mitochondrial viscosity in cells and zebrafish

Mitochondria are crucial energy-supplying organelles that support cellular activities and play vital roles in cell metabolism, aging, autophagy, and apoptosis. Abnormal viscosity can alter the mitochondrial microenvironment, disrupt normal mitochondrial function, and lead to disease. To address this, we designed and developed two aggregation-induced emission-twisted intramolecular charge transfer fluorescent probes, namely, (E)-1,1,3-trimethyl-2-(4-(1,2,2-triphenylvinyl)styryl)-1H-benzo[e]indol-3-ium (HSL-1) and (E)-2-(4-(di-p-tolylamino)styryl)-1,3,3-trimethyl-1H-benzo[e]indol-3-ium (HSL-2). In vitro fluorescence detection revealed that both HSL-1 and HSL-2 were sensitive to viscosity and demonstrated a strong log-linear relationship, with linear coefficients of 0.982 and 0.980, respectively. Notably, the responses of HSL-1 and HSL-2 to viscosity changes were unaffected by pH, polarity, or interfering ions. HSL-1 exhibited stronger resistance to background interference than HSL-2 and significantly enhanced fluorescence intensity; thus, it was selected for cell experiments and animal fluorescence intensity assessments. Furthermore, HSL-1 showed excellent biocompatibility, enabling real-time detection of mitochondrial viscosity changes and identification of viscosity abnormalities triggered by mitophagy in HeLa cells. It could also monitor changes in mitochondrial viscosity in zebrafish. In conclusion, HSL-1 is a valuable tool for studying viscosity and understanding diseases associated with abnormal mitochondrial viscosity.

A bifunctional sensitive fluorescence probe based on pyrene for the detection of pH and viscosity in lysosome

Lysosome is one of the important organelles in intracellular transport. It plays a significant role in the physiological process. The lysosomal microenvironment affects the functions of lysosome. When the original acidic environment of lysozyme is destroyed or the fluid viscosity increases gradually, various diseases are easily induced. However, most fluorescent probes can only locate in cells. The fewer probes of subcellular organelles were found and their functions are often single. So, it is of great importance to design multifunctional fluorescent probes with the capable of localizing in lysosome. In this study, a novel lysosome probe, 4-(4-Pyren-1-yl-but-3-enyl)-morpholine (PIM), was synthesized using pyrene as a fluorescent group and morpholine as a target group. The introduction of morpholine group made PIM localize in lysosome with high selectivity. The fluorescence will be enhanced with the increased viscosity because of restricting the rotation of CC bond and CN in PIM, and the detecting linear range is from 4.05 cP to 393.48 cP, which qualified the requirement of the viscosity monitoring in body. Meanwhile, the fluorescence intensity of PIM declines with the decrease of pH because the Schiff base of PIM is hydrolyzed, which was affirmed by ¹H NMR, LC-MS and fluorescence spectra. Moreover, cell imaging and MTT experiments confirmed that PIM as a novel bifunctional probe can be used to detect pH and endogenous viscosity in lysosome.

A deep-red emission fluorescent probe with long wavelength absorption for viscosity detection and live cell imaging

Intracellular viscosity is closely related to a series of biological processes and could be a biomarker for various diseases. Herein, we reported a deep-red emission viscosity probe ACI, which showed a turn-on fluorescence effect with excellent selectivity encountering high viscous medium. To assure the practical biological application, ACI demonstrated not only a long wavelength emission at 634 nm but also a long wavelength excitation at 566 nm, which were crucial to afford deeper penetration depth and higher sensitivity in bioimaging. The photophysical properties and viscosity recognition mechanism of the probe were carefully discussed here. Theoretical calculations furtherly confirmed that high viscous medium could inhibit the twisted intramolecular charge transfer (TICT) process of the probe which quenched the fluorescence in low viscous media, and restore the emission. More importantly, it was successfully applied to visualize the viscosity in living cells. Graphical abstract.

A deep-red fluorescent molecular rotor based on donor-two-acceptor modular system for imaging mitochondrial viscosity

A new donor-two-acceptor modular fluorescence rotor DpCy7 involving a phenolate donor unit and two benzothiazolium acceptor moieties was designed and synthesized. The DpCy7 underwent an internal charge transfer to form a Cy7-like longer conjugated system fluorochrome at a physiological pH. The probe exhibited a strong turn-on (8.5-fold) deep-red emission with a larger Stokes shift in glycerol aqueous solutions with restriction of rotation. Both the fluorescence intensity and fluorescence lifetime displayed the linear relationship of viscosity changes in the logarithmic plots. Furthermore, the HeLa cell imaging experiments of DpCy7 indicated that the rotor could be used to monitor the mitochondrial viscosity in living cells. This new type of deep-red fluorescence rotor provides a potential platform for determining viscosity at subcellular levels.

A deep-red fluorescence molecular rotor DpCy7 based on donor-two-acceptor modular system has been designed logically and synthesized for sensitive and selective response to viscosity changes and imaging of mitochondrial viscosity in living cells.

Quinone-Derived π-Extended Phenazines as New Fluorogenic Probes for Live-Cell Imaging of Lipid Droplets

We describe a new synthetic methodology for the preparation of fluorescent π-extended phenazines from the naturally-occurring naphthoquinone lapachol. These novel structures represent the first fluorogenic probes based on the phenazine scaffold for imaging of lipid droplets in live cells. Systematic characterization and analysis of the compounds in vitro and in cells led to the identification of key structural features responsible for the fluorescent behavior of quinone-derived π-extended phenazines. Furthermore, live-cell imaging experiments identified one compound (P1) as a marker for intracellular lipid droplets with minimal background and enhanced performance over the lipophilic tracker Nile Red.