Rational design of NIR fluorescence probes for sensitive detection of viscosity in living cells

Theoretical Insights into a Near-Infrared Fluorescent Probe NI-VIS Based on the Organic Molecule for Monitoring Intracellular Viscosity

So many biological functional disorders and diseases, such as atherosclerosis, hypertension, diabetes, Alzheimer's disease, as well as cell malignancy are closely related with the intracellular viscosity. A safe and effective intracellular viscosity detecting method is desired by the biomedical community. Recently, a novel near-infrared fluorescent probe NI-VIS with a twisting intramolecular charge transfer mechanism was developed. The capability of this probe to visualize the viscosity variation in cirrhotic liver tissues and map the micro viscosity in vivo were testified using an experiment. In this work, the twisting intramolecular charge transfer mechanism and fluorescent properties of the probe NI-VIS were studied in detail under quantum mechanical method. The low energy barrier among the different conformations of the probe indicated the occurrence of twisting intramolecular charge transfer due to the rotation of the aryl group in the probe molecule while within the low viscosity environment. The electronic structure analysis on different probe conformations revealed the electron transfer process of the probe under optical excitation. All these theoretical results could provide insights into understand in greater depth the principles and build highly effective fluorescent probe to monitor the viscosity in biological samples.

A near infrared two-channel fluorescent probe for the detection of hydrogen sulfide and viscosity with a negligible crosstalk influence

Both imbalance of H₂S production and the change of viscosity in cells are associated with many diseases such as inflammation, Alzheimer's disease, and Parkinson's disease. Thus, the development of two-channel fluorescent probes for the detection of H₂S and viscosity is of great significance for the study of pathogenic mechanisms. Herein, we design a two-channel NIR fluorescent probe RHO-DCO-DNP, which was able to selectively respond to H₂S in one channel (λex = 580 nm, λem = 760 nm) and to viscosity in another channel (λex = 400 nm, λem = 585 nm). It should be emphasized that there is a negligible impact from the crosstalk between the two optical channels and the two targets. In addition, with the low cytotoxicity and unique dual lysosome/mitochondria targeting capability, the probe was successfully applied to the sensing of H₂S and viscosity in normal cells and inflammation cells through fluorescent imaging. The probe could be a promising molecular tool for exploring the pathological role of H₂S, viscosity, and both of them.

Bio-compatible miniature viscosity sensor based on optical tweezers

Viscosity is a fundamental biomechanical parameter related to the function and pathological status of cells and tissues. Viscosity sensing is of vital importance in early biomedical diagnosis and health monitoring. To date, there have been few methods of miniature viscosity sensing with high safety, flexible controllability, and excellent biocompatibility. Here, an indirect optical method combining the significant advantages of both optical tweezers and microflows has been presented in this paper to construct a cellular micromotor-based viscosity sensor. Optical tweezers are used to drive a yeast cell or biocompatible SiO₂ particle to rotate along a circular orbit and thus generate a microvortex. Another target yeast cell in the vortex center can be controllably rotated under the action of viscous stress to form a cellular micromotor. As the ambient viscosity increases, the rotation rate of the micromotor is reduced, and thus viscosity sensing is realized by measuring the relationship between the two parameters. The proposed synthetic material-free and fuel-free method is safer, more flexible, and biocompatible, which makes the cellular micromotor-based viscosity sensor a potential detector of the function and pathological status of cells and tissues in vivo without introducing any exogenous cells.

Small molecule based fluorescent chemosensors for imaging the microenvironment within specific cellular regions

The microenvironment (local environment), including viscosity, temperature, polarity, hypoxia, and acidic-basic status (pH), plays indispensable roles in cellular processes. Significantly, organelles require an appropriate microenvironment to perform their specific physiological functions, and disruption of the microenvironmental homeostasis could lead to malfunctions of organelles, resulting in disorder and disease development. Consequently, monitoring the microenvironment within specific organelles is vital to understand organelle-related physiopathology. Over the past few years, many fluorescent probes have been developed to help reveal variations in the microenvironment within specific cellular regions. Given that a comprehensive understanding of the microenvironment in a particular cellular region is of great significance for further exploration of life events, a thorough summary of this topic is urgently required. However, there has not been a comprehensive and critical review published recently on small-molecule fluorescent chemosensors for the cellular microenvironment. With this review, we summarize the recent progress since 2015 towards small-molecule based fluorescent probes for imaging the microenvironment within specific cellular regions, including the mitochondria, lysosomes, lipid drops, endoplasmic reticulum, golgi, nucleus, cytoplasmic matrix and cell membrane. Further classifications at the suborganelle level, according to detection of microenvironmental factors by probes, including polarity, viscosity, temperature, pH and hypoxia, are presented. Notably, in each category, design principles, chemical synthesis, recognition mechanism, fluorescent signals, and bio-imaging applications are summarized and compared. In addition, the limitations of the current microenvironment-sensitive probes are analyzed and the prospects for future developments are outlined. In a nutshell, this review comprehensively summarizes and highlights recent progress towards small molecule based fluorescent probes for sensing and imaging the microenvironment within specific cellular regions since 2015. We anticipate that this summary will facilitate a deeper understanding of the topic and encourage research directed towards the development of probes for the detection of cellular microenvironments.

Facile mitochondria localized fluorescent probe for viscosity detection in living cells

Fluorescent probes act as a powerful tool to understand the function of intracellular viscosity, which are closely associated with many functional disorders and diseases. Herein we report a boron-dipyrromethene (4,4-difluoro-4-borata-3a,4a-diaza-s-indacene, BODIPY) group based new fluorescent probe (BV-1), which was synthesized facilely by a one-step Knoevenagel-type condensation reaction, to detect viscosity in living cells with high selectivity and sensitivity. DFT calculation demonstrated that the unsaturated moiety at the meso-position of BODIPY suppressed the fluorescence via twisted intramolecular charge transfer (TICT) mechanism in low viscosity media. By restricting the rotation of the molecular rotor, the fluorescence would be enhanced significantly with redshift in emission wavelength in high viscosity conditions. The fluorescence intensity ratio (log (I/I₀)) at 570 nm showed a good linearity (R² = 0.991) with the viscosity (log η) in the range of 2-868 cP. And the limit of detection (LOD) and limit of quantification (LOQ) for viscosity were calculated to be 0.16 cP and 0.54 cP, respectively. BV-1 was demonstrated to be mitochondria localized with low cytotoxicity. Utilizing the new probe BV-1, the changes in mitochondrial viscosity caused by monensin or nystatin have been monitored successfully in real time. This work will provide new efficient ways for the development of viscosity probes, which are expected to be used for the study of intracellular viscosity properties and functions.

A Xanthene Dye-based Sensor for Viscosity and Cell Imaging

A new xanthene dye, namely ImX, has been facilely prepared by reaction of 4-(1H-Imidazol-1-yl)benzaldehyde with N, N-diethyl-3-aminophenol in concentrated propionic acid, and then treated by p-chloranil. ImX presents the maximum absorption and emission band centered at 562 nm and 583 nm in water, respectively. Fluorescent spectra investigations demonstrate that ImX shows viscosity-selective fluorescent response and emission enhancement when the solvent viscosity increases from 1.1 cp. (water) to 1248 cp. (98 % glycerol). In addition, this viscosity-selective fluorescence response covers a wide pH range from 2.5 to 10.0. More significantly, ImX demonstrates low cytotoxicity and can be employed as tracer for the detection of Monensin-triggered viscosity enhancement by cell imaging.

Design, synthesis and application of a dual-functional fluorescent probe for reactive oxygen species and viscosity

A fluorescence probe based on cyanine fluorophore was designed and synthesized in this work, which can be used to determine viscosity and reactive oxygen species (e.g., OCl^-, ONOO^-) at different wavelengths. Under a low viscosity medium, the fluorescence quantum yield of the probe is very low; however, with the increase of the medium viscosity, the probe's emission at 571 nm is enhanced by nearly 25-fold due to the inhibition of intramolecular rotations. On the other hand, the probe shows a rapid and linear fluorescence response at 710 nm to OCl^- or ONOO^- within 1 min. The different spectral response regions of the probe permit the selective detection of both viscosity and reactive oxygen species. Furthermore, the probe is demonstrated to be cell permeable and capable of detecting the viscosity and the total amount of OCl^-/ONOO^- in living cells with the help of confocal microscope fluorescence imaging.

A fluoran-based viscosity probe with high-performance for lysosome-targeted fluorescence imaging

A new fluorescent probe Lyso-Fl has been facilely prepared by an esterification reaction of spironolactone fluoran dye Rdi with ethanol, which shows viscosity-selective response by fluorescence. The new probe delivers obvious fluorescence signal enhancement when environmental viscosity changes from 1.01 cP (water) to 1256 cP (98% glycerol). And, both the emission intensity (575 nm) and fluorescence lifetime of Lyso-Fl exhibit individually good linear relationships with the solution viscosity. Besides, Lyso-Fl gives a selective response to viscosity among various biological species and exhibits pH-independent (1-10) fluorescent signals towards viscosity. More importantly, Lyso-Fl shows low cytotoxicity and can be utilized for monitoring of dexamethasone-stimulated viscosity enhancement by cell imaging with excellent lysosome-targeted performance, promoting it a promising fluorescent probe for lysosomal viscosity detection.

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 red-emissive mitochondrial probe for imaging of the viscosity in living cells

L can exclusively stain mitochondria and monitor the mitochondrial viscosity changes.