Solvatochromic Probe Research Articles

Investigating the organisation of the asymmetric plasma membrane using a novel solvatochromic reporter.

The plasma membrane (PM) of an animal cell is a heterogenous mixture of lipids and proteins that exhibits both lateral heterogeneity and transverse asymmetry in composition. The complex mixture of lipid species with a range of headgroups and degrees of acyl-chain unsaturation influences the properties of the bilayer such as ordering and consequently the function of embedded membrane proteins. In this study, we have probed the presence of liquid-ordered (lo) domains in each leaflet of the PM and their transbilayer registry in live cells by designing a membrane localised probe that is sensitive to lipid order. This solvatochromic probe, C3L, uses Laurdan as a template and reports on changes in lipid order of complex lipid compositions such as Giant Plasma Membrane Vesicles (GPMVs), while exhibiting a preference for lo-domains. C3L distributes across both leaflets in the live cell membrane, regulated by flippases and scramblases and may be selectively and robustly confined to the inner leaflet after back extraction. C3L indicates the inner leaflet to be significantly disordered compared to the outer leaflet wherein the depletion of sphingomyelin (SM) leads to a more outer-leaflet restricted effect but phosphatidylserine (PS) depletion leads to effects on both leaflets, especially the inner leaflet. This suggests that PS and SM play different roles in building and maintaining domains and coupling them across the bilayer. Further, C3L reported lipid organisation indicates domains of smaller scale and broader order distribution in the PM of live cells at the same temperature compared to homogenous and disordered domains in GPMVs derived from them providing support for active de-mixing in membranes of living cells. Using this tool, we intend to explore the impact of membrane composition and cytoskeletal activity in tuning cellular signalling by regulating transbilayer properties of the PM.

Multi-harmonic spectral phasor analysis of LAURDAN hyperspectral imaging enables the in vivo study of membrane dynamics of multiple membrane-bound organelles.

Hyperspectral imaging (HSI) is a paramount technique in biomedical science, however, unmixing and quantification of each spectral component is a challenging task. Traditional unmixing relies on algorithms that need spectroscopic parameters from the fluorescent species in the sample. The phasor-based multi-harmonic unmixing method requires only the empirical measurement of the pure species to compute the pixel-wise photon fraction of every spectral component. Using simulations, we demonstrate the feasibility of the approach for up to 5 components and explore the use of adding a 6th unknown component representing autofluorescence. The simulations show that the method can be successfully used in typical confocal imaging experiments (with pixel photon counts between 101 and 103). As a proof of concept, we tested the method in living cells, using 5 common commercial dyes for organelle labeling and we easily and accurately separate them. Finally, we challenged the method by introducing a solvatochromic probe, 6-Dodecanoyl-N,N-dimethyl-2-naphthylamine (LAURDAN), intended to measure membrane dynamics on specific subcellular membrane-bound organelles by taking advantage of the linear combination between the organelle probes and LAURDAN. We succeeded in monitoring the membrane order in the Golgi apparatus, mitochondria, and plasma membrane in the same in-vivo cell and quantitatively comparing them. The phasor-based multi-harmonic unmixing method can help expand the outreach of HSI and democratize its use by the community for it does not require specialized knowledge.

Demonstration of Solute-specific Synergism in Binary Solvents.

The structure and solvation behavior of binary liquid mixtures of Methanol (MeOH) and N, N-Dimethylformamide (DMF) are explored by ascertaining their intermolecular interactions with either Rhodamine-B (RhB) or Rhodamine101 (Rh101) dye through steady-state absorption, emission, and two-photon induced fluorescence. Specifically, in the present investigation, we examine the strong synergistic solvation observed for the combinations of hydrogen bond donating (MeOH) and accepting (DMF) solvent pairs. Solvatochromism causes the solvatochromic probe molecules to sense increased polarity compared to their bulk counterparts. The origin of synergism was explained in terms of solute-solvent and solvent-solvent interactions in binary solvent mixtures interactions, as evidenced by probe dependence. The solvation behavior of the Methanol and DMF binary solvent mixture shows strong probe dependence, with Rh101 showing synergism while RhB does not.

Evaluation of the Polarity in Binary Liquid Mixtures as a Function of Volume Fraction

The variation of solvatochromic polarity parameters as a function of mole fraction data may not represent the realistic environment of the solvatochromic probe molecule. This is because of the fact that the solvation environment of the solvatochromic probe molecules is not only determined by the number of individual solvent molecules around it but also by the size of those individual solvent molecules. In this regard we wish to understand the variation of ET N polarity parameter of binary liquid mixtures as a fuction of volume fractions. Representation of the ET N parameter as a function of volume fraction data leads to a completely different dataset for binary liquid mixtures where water is used as one of the solvents. These mixtures usually display negative deviations from linearity for the ET N values, when ET N values are plotted as a function of mole fraction values of the solvents. On the other hand, if the ET N values are plotted as a function of volume fraction values of the solvents in binary liquid mixtures containing water, these deviations are minimized to a large extent and even reversed to the positive deviations in some of these mixtures. This study gives an insight into a long lasting problem of understanding the negative deviations of the ET N values from linearity observed in binary liquid mixtures of water, as these deviations are either minimized or reversed into positive deviations when ET N values are plotted versus volume fraction of solvents. This study opens up a whole new discussion about the solvatochromic data of the solvent mixtures, whether this data must be represented as a function of mole fraction values or as a function of volume fraction of the solvents. The representation of the solvatochromic data as a function of volume fraction of solvents might lead to new insights in studying the solute-solvent interactions in binary solvent mixtures.

Fluorescence of pyrene and its derivatives to reveal constituent and composition dependent solvation within hydrophobic deep eutectic solvents.

Depending on their constituents and composition, deep eutectic solvents (DESs) may exhibit widely varying physicochemical properties. Based on the miscibility of water in a DES, it can broadly be classified as 'hydrophobic' or 'hydrophilic'. The polarity offered by hydrophobic DESs as compared to common organic solvents in the context of solute solubilization thus becomes of the utmost importance. A versatile fluorescence probe, pyrene (Py), its aldehyde derivative, pyrene-1-carboxaldehyde (PyCHO), and a termini end-tagged dipyrenyl polydimethylsiloxane polymer (Py-PDMS-Py) are employed to assess the solvation environment afforded by DESs composed of thymol (Thy), (-)-menthol (Men), and n-decanoic acid (DA). To decipher the effect of the constituents and composition on solute solvation, DESs composed of different pairs of constituents in different molar ratios, namely Thy : Men (1 : 1 and 1 : 2), DA : Men (1 : 1 and 1 : 2), and Thy : DA (2 : 1, 1 : 1, and 1 : 2), are investigated. Pyrene, through its band 1-to-band 3 emission intensity ratio (Py I1/I3), displays higher cybotactic region dipolarity in Thy-containing DESs due to the phenyl ring of Thy; the sensitivity of Py I1/I3 towards temperature is also higher in Thy DESs. The fluorescence lifetime of pyrene and its temperature dependence are higher in Men-containing DESs in comparison. Quenching of pyrene fluorescence by nitromethane in these DESs is dynamic in nature and the recovered bimolecular quenching rate constants (kq) reveal efficient diffusion of the fluorophore-quencher pair in these DESs as compared to other iso-viscous media. The kq adheres to the Stokes-Einstein relation, implying inherent homogeneity associated to these DESs. PyCHO emission spectra show a high energy structured band in Thy : Men DESs, whereas the band shifts bathochromically and becomes broad in DA-containing DESs. The PyCHO cybotactic region is relatively nonpolar in Thy : Men DESs as compared to Thy : DA and Men : DA DESs. The extent of intramolecular excimer formation by Py-PDMS-Py reveals these DESs to be "good" solvents for polymer solvation where DES-polymer interactions are maximized. The microviscosity (ημ) surrounding Py-PDMS-Py correlates with the bulk dynamic viscosity (ηbulk) within the investigated DESs, further corroborating the absence of microheterogeneity. Overall, the observations reveal the similarity of these hydrophobic DESs to common organic solvents in the context of solute solubilization.

Frequency Changes in Terminal Alkynes Provide Strong, Sensitive, and Solvatochromic Raman Probes of Biochemical Environments.

The C≡C stretching frequencies of terminal alkynes appear in the "clear" window of vibrational spectra, so they are attractive and increasingly popular as site-specific probes in complicated biological systems like proteins, cells, and tissues. In this work, we collected infrared (IR) absorption and Raman scattering spectra of model compounds, artificial amino acids, and model proteins that contain terminal alkyne groups, and we used our results to draw conclusions about the signal strength and sensitivity to the local environment of both aliphatic and aromatic terminal alkyne C≡C stretching bands. While the IR bands of alkynyl model compounds displayed surprisingly broad solvatochromism, their absorptions were weak enough that alkynes can be ruled out as effective IR probes. The same solvatochromism was observed in model compounds' Raman spectra, and comparisons to published empirical solvent scales (including a linear regression against four meta-aggregated solvent parameters) suggested that the alkyne C≡C stretching frequency mainly reports on local electronic interactions (i.e., short-range electron donor-acceptor interactions) with solvent molecules and neighboring functional groups. The strong solvatochromism observed here for alkyne stretching bands introduces an important consideration for Raman imaging studies based on these signals. Raman signals for alkynes (especially those that are π-conjugated) can be exceptionally strong and should permit alkynyl Raman signals to function as probes at very low concentrations, as compared to other widely used vibrational probe groups like azides and nitriles. We incorporated homopropargyl glycine into a transmembrane helical peptide via peptide synthesis, and we installed p-ethynylphenylalanine into the interior of the Escherichia coli fatty acid acyl carrier protein using a genetic code expansion technique. The Raman spectra from each of these test systems indicate that alkynyl C≡C bands can act as effective and unique probes of their local biomolecular environments. We provide guidance for the best possible future uses of alkynes as solvatochromic Raman probes, and while empirical explanations of the alkyne solvatochromism are offered, open questions about its physical basis are enunciated.

Fluorescent Probes for Lipid Membranes: From the Cell Surface to Organelles

Biomembranes are ubiquitous lipid structures that delimit the cell surface and organelles and operate as platforms for a multitude of biomolecular processes. The development of chemical tools─fluorescent probes─for the sensing and imaging of biomembranes is a rapidly growing research direction, stimulated by a high demand from cell biologists and biophysicists. This Account focuses on advances in these smart molecules, providing a voyage from the cell frontier─plasma membranes (PM)─toward intracellular membrane compartments─organelles. General classification of the membrane probes can be based on targeting principles, sensing profile, and optical response. Probes for PM and organelle membranes are designed based on multiple targeting principles: conjugation with natural lipids or synthetic targeting ligands and in situ cell labeling by bio-orthogonal chemistry, conjugation to protein tags, and receptor-ligand interactions. Thus, to obtain membrane probes targeting PM with selectivity to one leaflet, we designed membrane anchor ligands based on a charged group and an alkyl chain. According to the sensing profile, we define basic membrane markers with constant emission and probes for biophysical and chemical sensing. The markers are built from classical fluorophores, exemplified by a series of bright cyanines and BODIPY dyes bearing the PM anchors (MemBright). Membrane probes for biophysical sensing are based on environment-sensitive fluorophores: (1) polarity-sensitive solvatochromic dyes; (2) viscosity-sensitive fluorescent molecular rotors; (3) mechanosensitive fluorescent flippers; and (4) voltage-sensitive electrochromic dyes. Our solvatochromic probes based on Nile Red (NR12S, NR12A, NR4A), Laurdan (Pro12A), and 3-hydroxyflavone (F2N12S) through polarity-sensing can visualize liquid ordered and disordered phases of lipid membranes, sense lipid order and its heterogeneity in cell PM, detect apoptosis, etc. Chemically sensitive probes, combining a dye, membrane-targeting ligand, and molecular recognition unit, enable the detection of pH, ions, redox species, lipids, and proteins at the biomembrane surface. In terms of the optical response profile, we can identify (1) fluorogenic (turn-on) probes, allowing background-free imaging; (2) ratiometric probes, e.g., solvatochromic probes, which enable ratiometric imaging by changing their emission/excitation color; (3) fluorescence lifetime-responsive probes, e.g., fluorescence molecular rotors and flippers, suitable for fluorescence lifetime imaging (FLIM); and (4) switchable probes, important for single-molecule localization microscopy. We showed that combining solvatochromic probes with on-off switching through a reversible binding specifically to cell PM enables the mapping of their biophysical properties with superior resolution. While the majority of efforts have been focused on PM, the probes for cellular organelles, such as endoplasmic reticulum, mitochondria, Golgi apparatus, etc., emerge rapidly. Thus, nontargeted solvatochromic probes can distinguish organelles by the emission color. Targeted solvatochromic probes based on Nile Red revealed unique signatures of polarity and lipid order of individual organelles and their different sensitivities to oxidative or mechanical stress. Lipid droplets, which are membraneless lipidic structures, constitute another interesting organelle target for probing the cell stress. Currently, we stand at the beginning of a long route with big challenges ahead, in particular (1) to achieve superior organelle specificity; (2) to label specific biomembrane leaflets, notably the inner leaflet of PM; (3) to detect lipid organization in a proximity of specific proteins; and (4) to probe biomembranes in tissues and animals.

Imaging of lipid droplets using coumarin fluorophores in live cells and C. elegans.

Fluorescent probes offer incredibly effective tools for visualizing the dynamic morphology of lipid droplets (LDs) and investigating their physiological interactions. In this work, we have utilized solvatochromic coumarin probes bearing nitrile and ester substituents for live-cell imaging. The fluorescence probes are characterized by a donor (diethylamino) and acceptor (nitrile and/or ester) substituents and a rotatable double bond. The designed architecture allows investigation of environmental sensitivity apart from providing excellent ability to target sub-cellular organelles. The synthesized fluorophores showed low cytotoxicity and excellent localization within the lipid droplets. Further, the fluorophores were also utilized to study viscosity changes within the LDs induced by Nystatin. More importantly, we also demonstrate imaging of LDs in multi-cellular animal models such as C. elegans.

Influence of pH and low/high- methoxy pectin complexation on the hydrophobic binding sites of β-lactoglobulin studied by a fluorescent probe method

β-lactoglobulin(β-Lg)-polysaccharide soluble complexes formed in a specific pH range through electrostatic attraction have attracted a growing interest in the design of food-grade encapsulation systems for hydrophobic compounds, which is mainly ascribed to the ligand-binding properties of β-Lg. However, it remains unclear whether pH-induced conformational changes in β-Lg and its electrostatic complexation with anionic pectin affect their ability to bind hydrophobic compounds. Here, a fluorescent probe method was employed to provide useful insights into the field. Three solvatochromic fluorescent probes (i.e. Nile red, retinol and curcumin) were selected as representative models of hydrophobic compounds that were bound to the inner cavity or/and the outer surface of β-Lg. Binding with β-Lg or β-Lg/pectin complexes largely enhanced the fluorescence of the hydrophobic probes. Especially, the polarity difference of different binding sites on β-Lg was revealed by the fluorescence spectra of β-Lg-Nile red as a function of pH. Both Nile red and retinol were bound more favorably to β-Lg at neutral pH than at acidic pH, possibly due to the accessibility of the inner cavity in the former case. Upon acidification, the gradual reduction in fluorescence intensity of the pre-formed protein-ligands complexes (i.e. at pH 7.0) was ascribed to the dissociation between Nile red (or retinol) and the inner cavity of β-Lg. β-Lg-curcumin interactions were less affected by pH variations, suggesting curcumin mainly binding to the outer surface of β-Lg. For the three tested hydrophobic compounds, the formation of soluble complexes between β-Lg with pectin had no adverse effects on their interactions with the protein. These results may provide useful insights into the binding of hydrophobic compounds to β-Lg or β-Lg/pectin complexes. The methodology may also be extended to study the encapsulation performance of other biopolymers or particles.

Phasor-based multi-harmonic unmixing for in-vivo hyperspectral imaging

Hyperspectral imaging (HSI) is a paramount technique in biomedical science, however, unmixing and quantification of each spectral component is a challenging task. Traditional unmixing relies on algorithms that need spectroscopic parameters from the fluorescent species in the sample. The phasor-based multi-harmonic unmixing method requires only the empirical measurement of the pure species to compute the pixel-wise photon fraction of every spectral component. Using simulations, we demonstrate the feasibility of the approach for up to 5 components and explore the use of adding a 6th unknown component representing autofluorescence. The simulations show that the method can be successfully used in typical confocal imaging experiments (with pixel photon counts between 101 and 103). As a proof of concept, we tested the method in living cells, using 5 common commercial dyes for organelle labeling and we easily and accurately separate them. Finally, we challenged the method by introducing a solvatochromic probe, 6-Dodecanoyl-N,N-dimethyl-2-naphthylamine (LAURDAN), intended to measure membrane dynamics on specific subcellular membrane-bound organelles by taking advantage of the linear combination between the organelle probes and LAURDAN. We succeeded in monitoring the membrane order in the Golgi apparatus, Mitochondria, and plasma membrane in the same in-vivo cell and quantitatively comparing them. The phasor-based multi-harmonic unmixing method can help expand the outreach of HSI and democratize its use by the community for it does not require specialized knowledge.

A Specific Interaction between Ionic Liquids' Cations and Reichardt's Dye.

Solvatochromic probes are often used to understand solvation environments at the molecular scale. In the case of ionic liquids constituted by an anion and a cation, which are designed and paired in order to obtain a low melting point and other desirable physicochemical properties, these two indivisible components can interact in a very different way with the probe. This is the case with one of the most common probes: Reichardt's Dye. In the cases where the positive charge of the cation is delocalized on an aromatic ring such as imidazolium, the antibonding orbitals of the positively charged aromatic system are very similar in nature and energy to the LUMO of Reichardt's Dye. This leads to an interesting, specific cation-probe interaction that can be used to elucidate the nature of the ionic liquids' cations. Parallel computational and experimental investigations have been conducted to elucidate the nature of this interaction with respect to the molecular structure of the cation.

Solvatochromic fluorescent styryl pyrene probes for the quantitative determination of water content in organic solvents

A series of D-π-A fluorescent styryl pyrene probes were synthesized through the Wittig reaction. The introduction of an electron-withdrawing substituent on the phenyl ring creates a push-pull system that shows solvent-dependent intramolecular charge transfer (ICT) and solvatochromic properties. Among these, the styryl pyrene probes with nitro (Py-NO2) and aldehyde (Py-CHO) substituents exhibited remarkable sensitivity to environment polarity. They show remarkable fluorescence changes from green to red, and blue to orange in nonpolar to polar organic solvents, respectively. The results were explained by the Lippert-Mataga and ET(30) models, which were supported by theoretical calculations by the density functional theory (DFT). Importantly, the fluorescence emission of Py-NO2 in organic solvents is sensitive to the presence of water in the range of 0–34.2% (v/v) in THF, 0–9.1% (v/v) in DMF, and 0–39.7% (v/v) in CH3CN. The limits of detection of water were established as low as 0.007%, 0.033%, and 0.036% (v/v) in THF, DMF, and CH3CN, respectively. Py-CHO showed somewhat higher limits of detection than Py-NO2 for the same solvents but exhibited a broad linear response to water content in EtOH between 3.3 and 63.6% (v/v) water content with the limit of detection at 2.350% (v/v). Thus, it is possibly useful as a sensor for the wide range detection of alcohol content in beverages. These push-pull styryl pyrene probes not only served as a new supersensitive fluorescent sensor for the quantitative detection of low to medium-level water content in organic solvents but may also be useful in a broad range of applications.

Viscosity-Sensitive Solvatochromic Fluorescent Probes for Lipid Droplets Staining.

Lipid droplets (LDs) are simple intracellular storage sites for neutral lipids and exhibit important impact on many physiological processes. For example, the changes in the polar microenvironment inside LDs could affect physiological processes, such as lipid metabolism and storage, protein degradation, signal transduction, and enzyme catalysis. Herein, a new fluorescent chemo-sensor (Couoxo-LD) was formulated by our molecular design strategy. The probe could be applied to effectively label intracellular lipid droplets. Intriguingly, Couoxo-LD demonstrated positive sensitivity to both polarity and viscosity, which might be attributed to its D-π-A structure and the twisted rotational behavior of the carbon–carbon double bond (TICT). Additionally, Couoxo-LD was successfully implemented in cellular imaging due to its excellent selectivity, pH stability, and low biotoxicity. In HeLa cells, the co-localization curve between Couoxo-LD and commercial lipid droplet dyes overlapped at 0.93. The results indicated that the probe could selectively sense LDs in HeLa cells. Meanwhile, Couoxo-LD can be applied for in vivo imaging of zebrafish.

Covalent Solvatochromic Proteome Stress Sensor Based on the Schiff Base Reaction.

Covalent-type probes or sensors have been seldom reported for aggregated proteins. Herein, we reported a series of covalent solvatochromic probes to selectively modify and detect aggregated proteomes through the Schiff base reaction. Such covalent modification was discovered by serendipity using the P1 probe with an aldehyde functional group, exhibiting enhanced fluorescence intensity and unusually large blue shift upon protein aggregation. Supported by the biochemical and mass spectrometry results, we identified that this probe can modify the lysine residue of aggregated proteins selectively over folded ones via the Schiff base reaction. The generality of designing such a covalent-type probe was demonstrated in multiple probe scaffolds using different model proteins. Finally, we exploited the distinct solvatochromism of P1 after Schiff base linkage with aggregated proteins to visualize the distinct morphology of aggregated proteomes, as well as to quantify the polarity heterogeneity inside it. This work may intrigue the exploration of other chemical reaction types to covalently functionalize aggregated proteins that were difficult to analyze.

Arrangement of Hydrogen Bonds in Aqueous Solutions of Different Globular Proteins.

This work presents the first evidence that dissolved globular proteins change the arrangement of hydrogen bonds in water, with different proteins showing quantitatively different effects. Using ATR-FTIR (attenuated total reflection—Fourier transform infrared) spectroscopic analysis of OH-stretch bands, we obtain quantitative estimates of the relative amounts of the previously reported four subpopulations of water structures coexisting in a variety of aqueous solutions. Where solvatochromic dyes can measure the properties of solutions of non-ionic polymers, the results correlate well with ATR-FTIR measurements. In protein solutions to which solvatochromic dye probes cannot be applied, NMR (nuclear magnetic resonance) spectroscopy was used for the first time to estimate the hydrogen bond donor acidity of water. We found strong correlations between the solvent acidity and arrangement of hydrogen bonds in aqueous solutions for several globular proteins. Even quite similar proteins are found to change water properties in dramatically different ways.

Decanoic Acid Based Deep Eutectic Solvents: Constituent-Controlled Physicochemical Properties and Solvatochromic Probe Behavior

Decanoic Acid Based Deep Eutectic Solvents: Constituent-Controlled Physicochemical Properties and Solvatochromic Probe Behavior

4-Dimethylamino-beta-nitrostyrene, a fluorescent solvatochromic probe to estimate the apparent dielectric constant in serum albumin: Experimental and molecular dynamics studies

The spectroscopic properties of 4-dimethylamino-beta-nitrostyrene (DANS) were theoretically and experimentally correlated with the polarity of organic solvents and applied to estimate the overall apparent dielectric constant in the binding sites of bovine serum albumin (BSA). The linear correlation between McRae's solvent polarity function and the wavelength of maximum emission (R2 = 0.8899) showed the suitability of DANS as a fluorescent solvatochromic probe. The interaction between BSA and DANS was evaluated (Ka = 3.1 × 104 mol−1 L), and based on the DANS/BSA emission, the apparent dielectric constant of BSA was estimated (ε = 5.5 ± 0.8). The experimental and theoretical absorption and emission maxima were correlated using density-functional theory. Molecular dynamics was used to calculate the mean electrostatic energy between DANS and the organic solvent molecules. These results were associated with the experimental fluorescence and applied to the BSA/DANS system, resulting in electrostatic energy (−37 kJ mol−1). The application of this model generated a dielectric constant of 5.8 ± 1.3 to BSA, which was consistent with the values obtained by McRae's solvent polarity function. The mean electrostatic energy of DANS in BSA binding sites was calculated directly from molecular dynamics simulations and resulted in −40 ± 2, −47 ± 3, and −48 ± 3 kJ mol−1 for sites I, II, and III. These results showed that the environment offered by the site I presented the value of electrostatic energy closest to the obtained with the solvatochromic model. In conclusion, DANS can be used to estimate the apparent polarity of albumins.

Binary mixtures with deep eutectic Solvents: Comparing properties with a Non–Ideality approach

• A simple methodology compares properties of related binary mixtures of deep eutectic solvents (DESs). • The non-ideality approach, using plots of experimental data, does not require any postulated model. • Comparison of non-ideality indices of related systems can reveal useful trends, not evident from isolated reports. • Besides offering an interpretation of the data, these trends can also have a predictive value. • Different properties of related systems were analyzed, with a total of 70 examples from the literature. The non–ideality approach for the comparison of different properties of related binary mixtures of deep eutectic solvents (DESs) is illustrated with seventy examples from the literature. A preferential–solvation index ( PSI ) is used to compare polarities and the preferential solvation of solvatochromic probes in these systems. A non–ideality index ( NII ) and/or area ( NIA ) allow the comparison of thermodynamic properties of related mixtures. The approach, which is of easy application from experimental plots, can in principle be used for the comparison of any property of related binary mixtures of DESs. It is not based on theoretical assumptions and complements existing models, by comparing scattered data from different sources and revealing unexpected trends or patterns with a predictive power. It thus represents a simple alternative to the problem of interpreting and predicting the behavior of such complex, multi-component mixtures, which defy the application of theoretical models to their rationalization.

Structure Rigidification Promoted Ultrabright Solvatochromic Fluorescent Probes for Super-Resolution Imaging of Cytosolic and Nuclear Lipid Droplets.

Lipid droplets (LDs) containing cytosolic and nuclear LDs have recently received increasing attention because of their diverse biological roles in living systems. However, developing fluorescent probes for super-resolution visualization of these subcellular LDs still remains challenging due to insufficient fluorescence brightness and poor nuclear membrane permeability. Herein, we rationally synthesized a series of ultrabright solvatochromic fluorescent probes based on benzoboranils (BBAs) for LD-specific super-resolution imaging using structured illumination microscopy (SIM). The rigidly structured probes exhibit ultrahigh fluorescence quantum yields of up to 99.9% in low-polar solvents. They also show more significant fluorescence enhancements in lipid environments than commercial LD probes. Owing to these excellent merits, our lipophilic fluorescent probes can specifically light up subcellular LDs at ultralow concentrations down to 10 nM. Further use of BBA-CF3 for super-resolution SIM imaging of cytosolic and nuclear LDs and their fusion process was successfully achieved. The unprecedented spatial resolution for nuclear LDs with an FWHM value of 142 nm was also acquired. Collectively, our ultrabright fluorescent probes hold tremendous potential to unveil the mysterious roles of cytosolic and nuclear LDs in biological research using SIM.

A Solvatochromic Fluorescent Probe for Solvent Polarity Detection Using a Smartphone

AbstractA solvatochromic fluorescent probe (probe 1) with D‐π‐A molecular configuration was developed. Probe 1 exhibited excellent polarity sensitivity. The ET (30) value of solvents demonstrated a good linear relationship with the Stokes shift of probe 1 except for DMSO. Moreover, the linear correlation coefficient value reached 0.97. The probe demonstrated different colors based on the type of solvents. With the increase in solvent polarity, the color gradually changed from blue to earthy yellow. It has a linear functional relationship between the B value (Blue, the blue primary color of light in RGB) and ET (30) values of the solvent existed. It provided strong support for the rapid determination of solvent polarity. In addition, probe 1 can be used for the determination of alcohol levels in Baijiu by smartphone.