Microviscosity in Multiple Regions of Complex Aqueous Solutions of Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide)

Amphiphilic Fluorinated Unimer Micelles as Nanocarriers of Fluorescent Probes for Bioimaging

The unique self-assembly properties of unimer micelles are exploited for the preparation of fluorescent nanocarriers embedding hydrophobic fluorophores. Unimer micelles are constituted by a (meth)acrylate copolymer with oligoethyleneglycol and perflurohexylethyl side chains (PEGMA90-co-FA10) in which the hydrophilic and hydrophobic comonomers are statistically distributed along the polymeric backbone. Thanks to hydrophobic interactions in water, the amphiphilic copolymer forms small nanoparticles (<10 nm), with tunable properties and functionality. An easy procedure for the encapsulation of a small hydrophobic molecule (C153 fluorophore) within unimer micelles is presented. UV-vis, fluorescence, and fluorescence anisotropy spectroscopic experimental data demonstrate that the fluorophore is effectively embedded in the nanocarriers. Moreover, the nanocarrier positively contributes to preserve the good emissive properties of the fluorophore in water. The efficacy of the dye-loaded nanocarrier as a fluorescent probe is tested in two-photon imaging of thick ex vivo porcine scleral tissue.

A tyrosinase fluorescent probe with large Stokes shift and high fluorescence enhancement for effective identification of liver cancer cells

Tyrosinase is widely regarded as an important biomarker for melanocytic and liver cancer. However, most currently reported tyrosinase probes have been focused on malignant melanoma study, and few tyrosinase probe have been applied for liver cancer investigation. Herein, we developed a novel probe HFC-TYR for sensitive and selective tracking of tyrosinase activity at enzyme and cellular level, and investigated its application for liver cancer diagnosis. As expected, HFC-TYR has excellent response ability for tyrosinase sensing at enzyme level, such as large Stokes shift (170 nm), high fluorescence enhancement (178-fold), low detection limit (0.12 U/mL), which indicates its potential for efficient identification of endogenous tyrosinase activity at cellular levels. Unsurprisingly, HFC-TYR is proved to be able detect endogenous tyrosinase levels in various living cells. More importantly, HFC-TYR is successfully used to distinguish HepG2 cells from other cells (SKOV3, HeLa and 293T), indicating that tyrosinase is overexpressed in HepG2 cells and HFC-TYR can specifically identify HepG2 cells at cellular level. Meanwhile, HFC-TYR is able to further monitor the endogenous tyrosinase activity in zebrafish models. Therefore, all the findings confirm that HFC-TYR has the application potential of liver cancer diagnosis.

Dynamics of small molecules within the F127 PEO-PPO-PEO triblock copolymer gel and sol phases studied at the molecular scale

Triplet excited states of guest molecules with different hydrophobicities were used to probe the association and dissociation dynamics of these guests with F127 micelles in the gel and sol phases. The dynamics probed was on a longer length scale than amenable with fluorescence techniques, but at a shorter length scale than probed in translational diffusion studies. The mobility of the guests at the molecular scale showed that subtle changes in the guest's structure affect the guest's release time from the micelles, where the structural features of the guest are more important than the phase, gel vs. sol, of the system.

Micelle Heterogeneity from the 2D Kinetics of Solute Rotation

The chemical and physical properties of microstructured materials vary with position. The photophysics of solute molecules can measure these local properties, but they often show multiple rates (rate dispersion), which complicates the interpretation. In the case of micelles, rate dispersion in a solute's anisotropy decay has been assigned to either local anisotropy or heterogeneity in the local viscosity. To resolve this conflict, the rotation of PM597 molecules in SDS micelles has been measured by polarized MUPPETS (multiple population-period transient spectroscopy). This 2D technique shows that heterogeneity is strong and that local anisotropy is minimal. The results suggest that on a subnanosecond time scale, the solute sees only one strong fluctuation of the micelle structure. The anisotropic, average structure emerges on longer time scales.

Characterization of chlorophyll derivatives in micelles of polymeric surfactants aiming photodynamic applications

The spectrophotometric properties of chlorophylls' derivatives (Chls) formulated in the Pluronics® F-127 and P-123 were evaluated and the results have shown that the Chls were efficiently solubilized in these drug delivery systems as monomers. The relative location of the Chls in the Pluronics® was estimated from the Stokes shift and micropolarity of the micellar environment. Chls with phytyl chain were located in the micellar core, where the micropolarity is similar to ethanol, while phorbides' derivatives (without phytyl chain) were located in the outer shell of the micelle, i.e., more polar environment. In addition, the thermal stability of the micellar formulations was evaluated through electronic absorption, fluorescence emission and resonance light scattering with lowering the temperature. The Chls promote the stability of the micelles at temperatures below the Critical Micellar Temperature (CMT) of these surfactants. For F-127 formulations, the water molecules drive through inside the nano-structure at temperatures below the CMT, which increased the polarity of this microenvironment and directly affected the spectrophotometric properties of the Chls with phytyl chain. The properties of the micellar microenvironment of P-123, with more hydrophobic core due to the small PEO/PPO fraction, were less affected by lowering the temperature than for F-127. These results enable us to better understand the Chls behavior in micellar copolymers and allowed us to design new drug delivery system that maintains the photosensitizer's properties for photodynamic applications.

Study on binding and fluorescence energy transfer efficiency of Rhodamine B with Pluronic F127-gold nanohybrid using optical spectroscopy methods

This work focuses on the binding efficiency and fluorescence resonance energy transfer (FRET) of fluorescent dye Rhodamine B (Rh B) to Pluronic F127-gold nanohybrid. The formation of gold nanoparticles inside Rh B doped Pluronic F127 copolymer have been characterized using dynamic light scattering study, HR-TEM images, UV-visible spectra and fluorescence studies. Fluorescence quenching and the constant fluorescence lifetime of the Rhodamine B present in the cavity of Pluronic F127-gold nanohybrid suggested a strong binding ability (3.5×10³Lmol^-1), static nature of quenching and better energy transfer efficiency of fluorescent dye towards Pluronic F127-gold (Au) nanohybrids.

On the role of a coumarin derivative for sensing applications: Nucleotide identification using a micellar system

The recognition of nucleotides is of crucial importance because they are the basic constituents of nucleic acids. The present study is focused on the selective interaction between a novel amphiphilic fluorophore containing coumarin and imidazole, CI (1-methyl-3-(12-((2-oxo-2H-chromen-7-yl)oxy)dodecyl)-1H-imidazol-3-ium bromide), and different nucleotide-monophosphates (NMPs). It was supposed that the solubilization of the low water soluble CI in a micelle system of hexadecyltrimethylammonium chloride (CTAC) would make the coumarin moiety of CI available to the interaction with the water-soluble NMPs. Changes in CTAC critical micelle concentration suggested that CI strongly interacted with the host cationic surfactant, thus forming a positively charged interface enriched with coumarin able to interact with the anionic NMPs. Steady-state fluorescence quenching revealed that CI/CTAC system was capable of distinguish between purine- and pyrimidine-based nucleotides. A modified Stern-Volmer equation permitted the use of a quenching model that accounted for the possible interactions between the micelles and the nucleotides. The data analysis allowed calculating selective parameters that differentiated according to the type of nucleotide either at 25 or 50°C. Our results established the utility of the novel coumarin derivative fluorophore, supported by the simple and suitable micellar systems, as a tool for DNA sensing applications.

Characterization of dimethylsulfoxide/glycerol mixtures: a binary solvent system for the study of “friction-dependent” chemical reactivity

The properties of binary mixtures of dimethylsulfoxide and glycerol, measured using several techniques, are reported.

Solvation Dynamics in Different Phases of the Lyotropic Liquid Crystalline System

Reverse hexagonal (HII) liquid crystalline material based on glycerol monooleate (GMO) is considered as a potential carrier for drugs and other important biomolecules due to its thermotropic phase change and excellent morphology. In this work, the dynamics of encapsulated water, which plays important role in stabilization and formation of reverse hexagonal mesophase, has been investigated by time dependent Stokes shift method using Coumarin-343 as a solvation probe. The formation of the reverse hexagonal mesophase (HII) and transformation to the L2 phase have been monitored using small-angle X-ray scattering and polarized light microscopy experiments. REES studies suggest the existence of different polar regions in both HII and L2 systems. The solvation dynamics study inside the reverse hexagonal (HII) phase reveals the existence of two different types of water molecules exhibiting dynamics on a 120-900 ps time scale. The estimated diffusion coefficients of both types of water molecules obtained from the observed dynamics are in good agreement with the measured diffusion coefficient collected from the NMR study. The calculated activation energy is found to be 2.05 kcal/mol, which is associated with coupled rotational-translational water relaxation dynamics upon the transition from "bound" to "quasi-free" state. The observed ∼2 ns faster dynamics of the L2 phase compared to the HII phase may be associated with both the phase transformation as well as thermotropic effect on the relaxation process. Microviscosities calculated from time-resolved anisotropy studies infer that the interface is almost ∼22 times higher viscous than the central part of the cylinder. Overall, our results reveal the unique dynamical features of water inside the cylinder of reverse hexagonal and inverse micellar phases.

How does bile salt penetration affect the self-assembled architecture of pluronic P123 micelles? – light scattering and spectroscopic investigations

The triblock copolymer of the type (PEO)₂₀–(PPO)₇₀–(PEO)₂₀ (P123) forms a mixed supramolecular aggregate with different bile salts, sodium deoxycholate (NaDC) and sodium taurocholate (NaTC), having different hydrophobicity.

Solvation and protonation of coumarin 102 in aqueous media: a fluorescence spectroscopic and theoretical study

The ground- and excited-state protonation of Coumarin 102 (C102), a fluorescent probe applied frequently in heterogeneous systems with an aqueous phase, has been studied in aqueous solutions by spectroscopic experiments and theoretical calculations. For the dissociation constant of the protonated form in the ground state, pKa = 1.61 was obtained from the absorption spectra; for the excited-state dissociation constant, pKa* = 2.19 was obtained from the fluorescence spectra. These values were closely reproduced by theoretical calculations via a thermodynamic cycle (the value of pKa* also by calculations via the Förster cycle) using an implicit–explicit solvation model (polarized continuum model + addition of a solvent molecule). The theoretical calculations indicated that (i) in the ground state, C102 occurs primarily as a hydrogen-bonded water complex, with the oxo group as the binding site, (ii) this hydrogen bond becomes stronger upon excitation, and (iii) in the ground state, the amino nitrogen atom is the protonation site, and in the excited state, the carboxy oxygen atom is the protonation site. A comprehensive analysis of fluorescence decay data yielded the values kpr = 3.27 × 10(10) M(–1) s(–1) for the rate constant of the excited-state protonation and kdpr = 2.78 × 10(8) s(–1) for the rate constant of the reverse process (kpr and kdpr were treated as independent parameters). This, considering the relatively long fluorescence lifetimes of neutral C102 (6.02 ns) and its protonated form (3.06 ns) in aqueous media, means that a quasi-equilibrium state of excited-state proton transfer is reached in strongly acidic solutions.

Solute dynamics in block-copolymer reverse micelles: do water content and copolymer concentration alter the microenvironment?

Solute dynamics has been explored in reverse micelles formed with the triblock copolymer (EO)13-(PO)30-(EO)13 (L64), where EO and PO represent ethylene oxide and propylene oxide units, respectively, with small amounts of water in p-xylene. To this effect, nonradiative rate constants (knr) and reorientation times (τr) of two carbocyanine derivatives, 3,3'-diethyloxadicarbocyanine iodide (DODCI) and merocyanine 540 (MC 540) have been measured at different mole ratios of water to copolymer (W) and also at three copolymer concentrations. By examining the nonradiative rate constants and the reorientation times of the two solutes, the microenvironment offered by L64/water/p-xylene reverse micellar system has been investigated. It has been observed that there is no variation in the nonradiative rate constants as well as in the reorientation times of both DODCI and MC 540 with an increase in W and [L64]. Since knr represents activated twist motion about the double bonds for these solutes, it is sensitive to the local friction and likewise, τr also provides information about the microenvironment. Thus, the results of this study indicate that DODCI and MC 540 are located in the cores of the L64 reverse micelles that are made up of hydrated ethylene oxide blocks and the hydration levels are not altered despite an increase in the water content and copolymer concentration. In other words, there is no variation in the microenvironment offered by L64/water/p-xylene reverse micellar system upon increasing W and [L64].

Microheterogeneity and microviscosity of F127 micelle: the counter effects of urea and temperature

F127 is the most widely studied triblock copolymer and due to the presence of very long polypropylene oxide (PPO) and polyethylene oxide (PEO) groups, F127 micelle has different microenvironments clearly separated into core, corona, and peripheral regions. Urea has been known to have adverse effects on the micellar properties and causes demicellization and solvation; on the other hand, rise in temperature causes micellization and solvent evacuation from the core and corona regions. In the present study, we have investigated the microheterogeneity of the core, corona, and peripheral regions of the F127 micelle using red edge excitation shift (REES) at different temperatures and urea concentrations and correlated the effect of both on the micellar system. It was found that the temperature counteracts the effect of urea and also that the counteraction is more prominent in the core region with respect to corona, and the peripheral region is least affected. Also, the core and corona regions are very much heterogeneous, while the peripheral region is more of a homogeneous nature. Using time-resolved fluorescence anisotropy, we found that the microviscosity within the micelles vary in the order of core > corona > peripheral region, and urea has a general tendency to reduce the microviscosity, especially for core and corona regions. On the other hand, rise in temperature initially increases and then decreases the microviscosity throughout, and at elevated temperatures the effect of urea is being dominated by the effect of temperature, thereby establishing the counter effects of temperature and urea on the F127 micellar system.

Application of time-resolved fluorescence for direct and continuous probing of release from polymeric delivery vehicles

Though accurately evaluating the kinetics of release is critical for validating newly designed therapeutic carriers for in vivo applications, few methods yet exist for release measurement in real time and without the need for any sample preparation. Many of the current approaches (e.g. chromatographic methods, absorption spectroscopy, or NMR spectroscopy) rely on isolation of the released material from the loaded vehicles, which require additional sample purification and can lead to loss of accuracy when probing fast kinetics of release. In this study we describe the use of time-resolved fluorescence for in situ monitoring of small molecule release kinetics from biodegradable polymeric drug delivery systems. This method relies on the observation that fluorescent reporters being released from polymeric drug delivery systems possess distinct excited-state lifetime components, reflecting their different environments in the particle suspensions, i.e., confined in the polymer matrices or free in the aqueous environment. These distinct lifetimes enable real-time quantitative mapping of the relative concentrations of dye in each population to obtain precise and accurate temporal information on the release profile of particular carrier/payload combinations. We found that fluorescence lifetime better distinguishes subtle differences in release profiles (e.g. differences associated with dye loading) than conventional steady-state fluorescence measurements, which represent the averaged dye behavior over the entire scan. Given the method's applicability to both hydrophobic and hydrophilic cargo, it could be employed to model the release of any drug-carrier combination.

Rotational diffusion of coumarin 153 in nanoscopic micellar environments of n-dodecyl-β-D-maltoside and n-dodecyl-hexaethylene-glycol mixtures

The microstructure of mixed micelles containing n-dodecyl-β-D-maltoside and n-dodecyl-hexaethylene-glycol, two nonionic surfactants belonging to the alkyl polyglucoside and polyoxyethyelene alkyl ether families, respectively, has been investigated. With the aim of understanding how the micellar composition affects the microenvironmental properties of micelles, we have examined the photophysics and dynamics of the neutral probe coumarin 153 in the binary mixtures of the surfactants across the entire composition range. We present data on the steady-state absorption and emission spectra of the probe, as well as fluorescence lifetimes and both steady-state and time-resolved fluorescence anisotropies. These data indicate that the participation of the ethoxylated surfactant in the mixed micelle induces an increasing hydration in the palisade layer of the micelle, which forces the probe to migrate toward the inner micellar region, where it senses a slightly less polar environment. The time-resolved fluorescence anisotropy data were analyzed on the basis of the two-step and wobbling-in-cone model. The average reorientation time of the probe molecule was found to decrease with the presence of the ethoxylated surfactant, in good agreement with steady-state fluorescence anisotropy data, suggesting a reduction of the microviscosity in the solubilization site of the probe. The behavior of all diffusion reorientation parameters was analyzed on the basis of two factors: the micellar hydration and the headgroup flexibility of both surfactants. It was concluded that the increasing participation of the ethoxylated surfactant induces a greater hydration in the micellar palisade layer, producing the formation of a less compact microenvironment where the probe experiences a faster rotational reorientation.

Mobility of molecules and ions solubilized in protein gels: diffusion in the thick fraction of hen egg white

The thick fraction of hen egg white is a protein hydrogel with an immeasurably high viscosity composed of ∼90% water that can serve as a model system for mammalian mucous membrane. Measurements of the rate constants of diffusion-controlled reactions occurring within the gel (and corresponding activation energies) and electric conductivity revealed that the thick fraction of egg white can be envisioned as a 3D network comprising hydrated protein molecules (held by intermolecular S-S bridges) surrounded by water pools and channels (of nonuniform diameters) that have a microviscosity that is very similar to that of bulk water. This was corroborated by differential scanning calorimetry measurements that revealed that 16% of water is bound to proteins. The melting kinetics of ice crystallites (produced from the freezable water) indicates nonhomogeneous water pool size.

Local viscosity analysis of triblock copolymer micelle with cyanine dyes as a fluorescent probe

The local viscosity of Pluronic F127 triblock copolymer micelles in water was determined with cyanine dyes as fluorescent probes. These dyes show very weak fluorescence at a low temperature, but show enhanced fluorescence at a temperature higher than the critical micellization temperature (T(cm)). This is because a viscous environment within the micelle suppresses the formation of a nonradiative twisted intramolecular charge transfer (TICT) excited state of the dyes. The good correlation between the fluorescence quantum yields of the dyes and the viscosity and the temperature of the media allows a determination of local viscosity of micelle based on the fluorescence quantum yields. The local viscosity of both core and corona regions of micelles increases at >T(cm) and shows a maximum at a temperature 7-9 °C higher than T(cm), and decreases at higher temperature due to the increased fluidity. The core viscosity is larger than that of the corona, and the corona viscosity increases toward the micelle center. The polymer concentration has different effects on the core and corona viscosity: the corona viscosity increases with a polymer concentration increase at the entire temperature range, whereas the core viscosity increases only at a low temperature. The corona viscosity increase is due to the condensation of a large number of polyethylene oxide (PEO) blocks. In contrast, the dehydration degree of polypropylene oxide (PPO) blocks in the core scarcely changes, and the core has a similar composition regardless of polymer concentration. The larger polymer concentration promotes a micelle formation at lower temperature where the fluidity increase is very weak, resulting in larger core viscosity.

Study of diffusion of organic dyes in a triblock copolymer micelle and gel by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) has been used to study translational diffusion of three fluorescent dyes in a micelle and a gel. It was demonstrated that a highly hydrophobic dye, DCM, remains confined to a particular micelle during the passage of the micellar aggregation through the confocal volume. As a result, DCM exhibits slow diffusion of the large micellar aggregate with a diffusion coefficient (D(t)) approximately 25 times slower compared with that of water. In contrast, a hydrophilic probe (C343 or C480) occasionally diffuses out of the micelle into bulk water and displays a large D(t) (twofold smaller in F127 and approximately six times smaller in the P123 micelle compared with that in bulk water). In a gel, diffusion of the individual micelles is completely arrested and hence, the autocorrelation in FCS arises solely from the diffusion of the dye in the gel. In this case, all the three dyes exhibit extremely slow diffusion (300, 45, and 20 times slower than that in water for DCM, C480, and C343 in F127 gel, respectively). In a P123 and F127 gel, diffusion of DCM is respectively, seven and 29 times slower compared with that of the ionic probe C343. The relatively small value of red-edge excitation shift (REES) of the emission maximum, suggests that DCM is confined within the core of the triblock copolymer micelles and gels. The hydrophilic probes (C343 or C480) exhibit fast diffusion in the micelles and gels. However, their REES is very different. The large REES of C480 suggests that it is distributed over a large region of the micelle, whereas the low REES of C343 indicates that it is located primarily in the peripheral corona region.

Ultrafast dynamics in reverse micelles

Recent advances in ultrafast laser technology have spurred investigations of microheterogeneous solutions. In particular, researchers have explored details of reverse micelles (RMs), which present isolated droplets of polar solvent sequestered from a continuous nonpolar phase by a surfactant layer. This review explores recent studies utilizing a variety of ultrafast laser techniques to uncover details about structure and dynamics in various RMs. Using ultrafast vibrational spectroscopy, researchers have probed hydrogen-bond dynamics and vibrational energy relaxation in RMs. These studies have developed our understanding of reverse micellar structure, identifying varying water environments in the RMs. In a plethora of experiments employing probe molecules, researchers have explored the confined environment presented by RMs and their impact on a range of chemical reactions. These studies have shown that confinement, rather than the specific interactions with surfactants, is an important factor determining the impact of the reverse micellar environment on the chemistry.

Fluorescence spectroscopic investigation to identify the micelle to gel transition of aqueous triblock copolymer solutions

Steady-state and time-resolved fluorescence anisotropy measurements using probes coumarin 153 (C153) and 4-heptadecylumbelliferon (HUF) have been carried out to understand the micelle to gel transition of an aqueous triblock copolymer P123 ((EO)(20)-(PO)(70)-(EO)(20)) (EO = ethylene oxide; PO = propylene oxide) solution. Anisotropy results with a normal fluorescent probe, C153, do not show a characteristic change due to the micelle to gel transition. However, the probe HUF having a long hydrocarbon chain that helps its strong association with the micelle shows an increase in anisotropy above the sol-gel transition point. This difference has been explained as invoking a substantial contribution from the micellar structural fluctuations to the depolarization of HUF as its hydrocarbon chain is embedded in the micellar structure, which is not sensed significantly by the normal probe C153. That the extent of change in anisotropy for HUF upon gelation is not that large is possibly caused by the collective motion of the physically interconnected nodes, as observed from the dynamic light scattering studies, which acts in favor of a relatively faster depolarization in the gel phase. Similar studies in other copolymers, such as P85 ((EO)(26)-(PO)(40)-(EO)(26)) and F127 ((EO)(100)-(PO)(65)-(EO)(100)), further demonstrate the potential of probes latched with hydrocarbon chains in displaying a characteristic change for the micelle to gel transition which otherwise remains obscured for normal fluorescent probes.