Viscosity Probe Research Articles

A novel NIR probe for detection of viscosity in cellular lipid droplets, zebra fishes and living mice

Lipid droplets (LDs) are organelles which storage the lipids for all organisms. They are involved in many biological processes. LDs stored neutral lipids consisting of triglycerides, steryl esters, and retinyl esters. Herein, we constructed a novel NIR viscosity probe NLV-1 for monitoring the viscosity changes in biological systems. The probe NLV-1 showed a very strong fluorescence emission at around 719 nm in viscous media, the fluorescence intensity of the probe NLV-1 increased 13.77-fold from methanol to 99% glycerol. Moreover, the probe NLV-1 have been successfully utilized for monitoring the viscosity changes in cellular lipid droplets, and in zebra fishes. More importantly, we utilized the probe for measuring the viscosity variations in living mice.

Construction of a ratiometric two-photon fluorescent probe to monitor the changes of mitochondrial viscosity

Abnormal viscosity variations in the mitochondrial matrix may induce changes in the mitochondrial network organization and further influence metabolite diffusion, which, consequently, are related to cellular aspects of several diseases and malfunctions, such as cell malignancy, atherosclerosis, diabetes, and Alzheimer’s disease. Studies have shown that mitochondrial viscosity changes are influenced by many factors, including disruption of ionic balance by drugs, over-production of reactive oxygen species, and cell apoptosis. However, the quantization of mitochondrial viscosity changes caused by these factors is uncertain, mainly because of the lack of a suitable fluorescent viscosity probe to accurately analyze changes in mitochondrial viscosity. By combining two fluorophores that have different sensitivities to viscosity, we designed and synthesized a new mitochondrial-targeted two-photon fluorescent viscosity probe (TM-V). The probe showed a direct linear relationship between the fluorescence intensity ratio of the two fluorophores and the media viscosity, which allowed us to determine the average mitochondrial viscosity in living cells. Using the novel TM-V probe, changes in mitochondrial viscosity under various conditions were quantified. In addition, as an excellent two-proton rotor probe, TM-V was able to report on viscosity in living cells and deeper tissues by ratiometric fluorescence imaging.

A lysosome targetable versatile fluorescent probe for imaging viscosity and peroxynitrite with different fluorescence signals in living cells.

The lysosome, which acts as the cellular recycling centre, is filled with numerous hydrolases that can degrade most cellular macromolecules. The abnormalities of the lysosome are closely associated with diseases, such as Heřmanský-Pudlák syndrome, Griscelli syndrome and Chédiak-Higashi syndrome. Studies have shown that abnormal viscosity and the accumulation of reactive oxygen species (ROS) in the lysosome will disorder the normal function of the lysosome. In this research, a versatile fluorescent probe Lyso-NA has been developed for the multi-channel imaging of lysosomal viscosity and peroxynitrite (ONOO-). When excited at 550 nm, the Lyso-NA exhibited about a 50-fold increase in fluorescence at 610 nm and also with the increasing viscosity from 1.0 cP to 1410 cP, and about a 3.5-fold increase in fluorescence at 510 nm (excitation at 440 nm) together with the increasing ONOO-. These satisfactory response properties make it possible to use Lyso-NA to monitor changes in both viscosity and ONOO- inside the lysosome. To achieve its practical application, it was further demonstrated that Lyso-NA exhibits low cytotoxicity, and good cell permeability, and could be used to monitor lysosomal viscosity and ONOO- in living cells.

A new GFP fluorophore-based probe for lysosome labelling and tracing lysosomal viscosity in live cells

Lysosomes, the main digestive compartment of live cells, are the most acidic organelles (pH 3.8-6.6) with the highest viscosity (47-190 cP at 25 °C). Lysosomal viscosity can reflect the status and function of the lysosomes, and abnormal lysosomal viscosity is responsible for the dysfunction of lysosomes, which is closely related to multiple diseases. In this work, a new GFP (green fluorescent protein) fluorophore-based probe (Lys-V) was designed and synthesized for mapping lysosomal viscosity in live cells. The increasing viscosity can restrict the two C[double bond, length as m-dash]C twisting double bonds between phenol and imidazolidinone, causing the fluorescence emission of Lys-V to enhance with increasing viscosity, and there is an excellent linear relationship between log FI (515 nm) and log viscosity. Notably, fluorescence of Lys-V is quite stable in different pH solvents at the same viscosity, demonstrating that Lys-V is suitable for quantifying viscosity over a wide pH range (from 4 to 10). Because of its ideal water solubility, low cytotoxicity and weak basic amino morpholine substituents, Lys-V can be utilized for lysosome labelling in live cells. Significantly, the dynamic viscosity changes in lysosomes under dexamethasone stimulus could also be captured by Lys-V. Our results will allow Lys-V to become a new chemical tool for detecting lysosomal viscosity in live cells as well as unveiling the underlying mechanism of lysosome behaviour during various biological processes, which is beneficial for studying lysosome-related diseases as well as the exploration of lysosome-related pharmacology.

Terbium(III) Luminescent Complexes as Millisecond-Scale Viscosity Probes for Lifetime Imaging.

Fluorescent probes that are able to directly measure viscosity are attractive candidates for the study of intracellular environments. We report a new class of luminescent rotors, based on the sensitized emission of a terbium(III) complex. A 4-fold increase in both quantum yield and luminescence lifetime was observed in viscous media for the studied complexes, with a lifetime ranging from 0.23 to 0.89 ms over a broad range of viscosities (0.6-1200 cP). The presented approach, relying on the millisecond-scale luminescence lifetime of the lanthanide ions, was applied to fixed T24 cancer cells using temporal sampling lifetime imaging microscopy.

Flapping viscosity probe that shows polarity-independent ratiometric fluorescence

Flapping fluorophores (FLAP) have been developed as a new series of molecular viscosity probes that show polarity-independent ratiometric fluorescence properties.

Development of a viscosity sensitive fluorescent probe for real-time monitoring of mitochondria viscosity

Through rational design, two new mitochondria-targeted fluorescent viscosity probes were developed, which exhibited favorable properties such as large turn on fluorescence signal, good selectivity, low cytotoxicity, and high colocation coefficient (>0.90).

Power-law electrokinetic behavior as a direct probe of effective surface viscosity

An exact solution to the Poisson-Boltzmann and Stokes equations is derived to describe the electric double layer with inhomogeneous dielectric and viscosity profiles in a lateral electric field. In the limit of strongly charged surfaces and low salinity, the electrokinetic flow magnitude follows a power law as a function of the surface charge density. Remarkably, the power-law exponent is determined by the interfacial dielectric constant and viscosity, the latter of which has eluded experimental determination. Our approach provides a novel method to extract the effective interfacial viscosity from standard electrokinetic experiments. We find good agreement between our theory and experimental data.

A Simple BODIPY-Based Viscosity Probe for Imaging of Cellular Viscosity in Live Cells.

Intracellular viscosity is a fundamental physical parameter that indicates the functioning of cells. In this work, we developed a simple boron-dipyrromethene (BODIPY)-based probe, BTV, for cellular mitochondria viscosity imaging by coupling a simple BODIPY rotor with a mitochondria-targeting unit. The BTV exhibited a significant fluorescence intensity enhancement of more than 100-fold as the solvent viscosity increased. Also, the probe showed a direct linear relationship between the fluorescence lifetime and the media viscosity, which makes it possible to trace the change of the medium viscosity. Furthermore, it was demonstrated that BTV could achieve practical applicability in the monitoring of mitochondrial viscosity changes in live cells through fluorescence lifetime imaging microscopy (FLIM).

Diffusion-Controlled Rotation of Triptycene in a Metal-Organic Framework (MOF) Sheds Light on the Viscosity of MOF-Confined Solvent.

Artificial molecular machines are expected to operate under conditions of very low Reynolds numbers with inertial forces orders of magnitude smaller than viscous forces. While these conditions are relatively well understood in bulk fluids, opportunities to assess the role of viscous forces in confined crystalline media are rare. Here we report one such example of diffusion-controlled rotation in crystals and its application as a probe for viscosity of MOF-confined solvent. We describe the preparation and characterization of three pillared paddlewheel MOFs, with 9,10-bis(4-pyridylethynyl)triptycene 3 as a pillar and molecular rotator, and three axially substituted dicarboxylate linkers with different lengths and steric bulk. The noncatenated structure with a bulky dicarboxylate linker (UCLA-R3) features a cavity filled by 10 molecules of N,N-dimethylformamide (DMF). Solid-state 2H NMR analysis performed between 293 and 343 K revealed a fast 3-fold rotation of the pillar triptycene group with the temperature dependence consistent with a site exchange process determined by rotator-solvent interactions. The dynamic viscosity of the MOF-confined solvent was estimated to be 13.3 N·s/m2 (or Pa·s), which is 4 orders of magnitude greater than that of bulk DMF (8.2 × 10–4 N·s/m2), and comparable to that of honey.

A Molecular Rotor that Measures Dynamic Changes of Lipid Bilayer Viscosity Caused by Oxidative Stress.

Oxidation of cellular structures is typically an undesirable process that can be a hallmark of certain diseases. On the other hand, photooxidation is a necessary step of photodynamic therapy (PDT), a cancer treatment causing cell death upon light irradiation. Here, the effect of photooxidation on the microscopic viscosity of model lipid bilayers constructed of 1,2‐dioleoyl‐sn‐glycero‐3‐phosphocholine has been studied. A molecular rotor has been employed that displays a viscosity‐dependent fluorescence lifetime as a quantitative probe of the bilayer's viscosity. Thus, spatially‐resolved viscosity maps of lipid photooxidation in giant unilamellar vesicles (GUVs) were obtained, testing the effect of the positioning of the oxidant relative to the rotor in the bilayer. It was found that PDT has a strong impact on viscoelastic properties of lipid bilayers, which ‘travels’ through the bilayer to areas that have not been irradiated directly. A dramatic difference in viscoelastic properties of oxidized GUVs by Type I (electron transfer) and Type II (singlet oxygen‐based) photosensitisers was also detected.

Cone–Plate Rheometer as Reactor and Viscosity Probe for the Detection of Transitional Phase Inversion of Brij30–Isopropyl Myristate–Water Model Emulsion

Brij30/isopropyl myristate/water model emulsions were used to study the inversion morphology change from O/W to W/O. The transitional phase inversion was detected by monitoring the electrical conductivity during a heating–cooling cycle while the viscosity of the emulsion was followed under constant shear rate with a cone–plate rheometer equipment. The two methods provide similar values of phase inversion temperature. However, different rheological profiles are observed depending on the surfactant concentration and water fraction. Viscosity maps of formulation-composition maps (specifically temperature–surfactant concentration and temperature–water fraction) pointed to the occurrence of both transitional and catastrophic inversion processes. A complete formulation–composition map with all different emulsion morphologies and isoviscosities contours for the 9% Brij30/IPM/water is presented. The use of a commercial cone–plate rheometer exhibits several advantages over the classical conductivity measurement; i...

Optical properties of two fluorene derived BODIPY molecular rotors as fluorescent ratiometric viscosity probes

Two fluorescent ratiometric fluorene derived BODIPY probes present a sensitive response to microviscosity changes.

A versatile fluorescent probe for imaging viscosity and hypochlorite in living cells

A novel fluorescent chemosensor was designed and synthesized based on a borondipyrromethene-hemicyanine dyad structure. This probe can be employed for the measurement of both viscosity and hypochlorite under different wavelengths. In non-viscous media, the dye had a very low fluorescence quantum yield. With the increase of viscosity, the fluorescence at 600 nm was enhanced significantly, which could be utilized for quantitative determination of viscosity. In addition, the probe exhibited a fast (within 1 min) ‘turn-on’ fluorescence response to hypochlorite with high selectivity. The fluorescence at 510 nm was directly proportional to hypochlorite concentration. Confocal fluorescence imaging experiments demonstrated the probe could permeate cell membranes and visualize viscosity and hypochlorite in living cells, where the action of hypochlorite caused the increase of intracellular viscosity.

Rate of shear of an ultrasonic oscillating rod viscosity probe

Ultrasonic oscillating rod probes have recently been used by researchers to measure viscosity and/or density in fluids. However, in order to use such probes to characterise the rheological properties of fluids, it is necessary to define the shear rate produced by the probe. This paper proposes an analytical solution to estimate the shear rate of ultrasonic oscillating rod viscosity probes and a method to measure their maximum operational shear rate. A relationship is developed which relates the torsional surface velocity of an oscillating cylindrical rigid body to the rate of shear in its vicinity. The surface displacement and torsional surface velocity of a torsional probe of length 1000mm and diameter 1mm were measured over the frequency range from 525 to 700kHz using a laser interferometer and the maximum shear rate estimated. The reported work provides the basis for characterising shear rate for such probes, enabling their application for rheological investigations.

Development of a new dual polarity and viscosity probe based on the foldamer concept.

Small molecular probes able to act as sensors are of enormous interest thanks to their multiple applications. Here, we report on the development of a novel supramolecular dual viscosity and polarity probe based on the foldamer concept, which increases the resolution limits of traditional probes at low viscosity values (0-4 mPa·s). The applicability of this new probe has been tested with a supramolecular organogel.

Quantitatively mapping cellular viscosity with detailed organelle information via a designed PET fluorescent probe.

Viscosity is a fundamental physical parameter that influences diffusion in biological processes. The distribution of intracellular viscosity is highly heterogeneous, and it is challenging to obtain a full map of cellular viscosity with detailed organelle information. In this work, we report 1 as the first fluorescent viscosity probe which is able to quantitatively map cellular viscosity with detailed organelle information based on the PET mechanism. This probe exhibited a significant ratiometric fluorescence intensity enhancement as solvent viscosity increases. The emission intensity increase was attributed to combined effects of the inhibition of PET due to restricted conformational access (favorable for FRET, but not for PET), and the decreased PET efficiency caused by viscosity-dependent twisted intramolecular charge transfer (TICT). A full map of subcellular viscosity was successfully constructed via fluorescent ratiometric detection and fluorescence lifetime imaging; it was found that lysosomal regions in a cell possess the highest viscosity, followed by mitochondrial regions.

Molecular Rotors As Conditionally Fluorescent Labels for Rapid Detection of Biomolecular Interactions

We demonstrate the use of fluorescent molecular rotors as probes for detecting biomolecular interactions, specifically peptide-protein interactions. Molecular rotors undergo twisted intramolecular charge transfer upon irradiation, relax via the nonradiative torsional relaxation pathway, and have been typically used as viscosity probes. Their utility as a tool for detecting specific biomolecular interactions has not been explored. Using the well characterized p53-Mdm2 interaction as a model system, we designed a 9-(2-carboxy-2-cyanovinyl) julolidine-based p53 peptide reporter, JP1-R, which fluoresces conditionally only upon Mdm2 binding. The reporter was used in a rapid, homogeneous assay to screen a fragment library for antagonists of the p53-Mdm2 interaction, and several inhibitors were identified. Subsequent validation of these hits using established secondary assays suggests increased sensitivity afforded by JP1-R. The fluorescence of molecular rotors contingent upon target binding makes them a versatile tool for detecting specific biomolecular interactions.

Fluorescence-based Broad Dynamic Range Viscosity Probes

We introduce two new fluorescent viscosity probes, SYBR Green (SG) and PicoGreen (PG), that we have studied over a broad range of viscosity and in collagen solutions. In water, both dyes have low quantum yields and excited state lifetimes, while in viscous solvents or in complex with DNA both parameters dramatically (300-1000-fold) increase. We show that in log-log scale the dependence of the dyes' quantum yield vs. viscosity is linear, the slope of which is sensitive to temperature. Application of SG and PG, as a fluorescence-based broad dynamic range viscosity probes, to the life sciences is discussed.

Dual mode monitoring probe for mitochondrial viscosity in single cell

Viscosity strongly influences intracellular transportation of mass and signal, and although bulk viscosity can be well measured with macroscopic samples quantities, microscale measurements in live cells still remain a challenge. By introducing a CHO group into the meso-position of the pentamethine chain, a thiazole-Cy5 dye based two-photon fluorescence probe, Mito-V, was developed. It exhibited a selective response to solution viscosity by fluorescence ratio and lifetime methods. More importantly, Mito-V can localize in mitochondria and quantitatively image its viscosity changes by the dual fluorescence mode during the apoptosis.