A surfactant type fluorescence probe for detecting micellar growth

We report on the detection of micellar growth in anionic, cationic, and catanionic surfactant systems using a novel surfactant type fluorescence probe, sodium 12-(N-dansyl)amino-dodecanate (12-DAN-ADA). The fluorescent group was incorporated in the tail of the surfactant which tethers the fluorescent group deep inside the apolar micellar cores. The fluorescence anisotropy of 12-DAN-ADA was found to be very sensitive for directly detecting the micellar growth in micelles containing oppositely charged surfactants, including cationic CTAB systems and mixed systems of oppositely charged surfactants (DEAB/SDS); in regard to the like charged SDS micellar systems, the sensitivity can be greatly enhanced by addition of a water soluble quencher which quenches the background fluorescence from the equilibrium population of free 12-DAN-ADA.     

The use of acridine orange base (AOB) as molecular probe to characterize nonaqueous AOT reverse micelles

The behavior of acridine orange base (AOB) in nonaqueous reverse micelles composed of n-heptane/AOT/polar solvent has been performed. Ethylene glycol (EG), propylene glycol (PG), glycerol (GY), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA) were employed as water substitutes. The studies were performed by static and time-resolved emission spectroscopy. Thus, the distribution of AOB between the two pseudophases of the aggregates was quantified by measuring the partition constants from emission spectra at different surfactant concentration. Similar values to those obtained by means of absorption spectroscopy were obtained. This match is indicating that AOB is not experiencing partition during the lifetime of the excited state. Partitioning to the micelles is strongly favored in micelles containing hydrogen-bond donor (HBD) solvents rather than non-HBD solvents. Variations of fluorescence lifetimes with AOT concentration confirm these results. By the solvatochromic behavior of AOB in the different systems it is shown that the microenvironment at the interface is distinct from that of the bulk polar solvent, indicating that the probe senses no "free" solvent. The steady state anisotropy (r) was measured for EG/AOT/n-heptane and DMF/AOT/n-heptane systems as representatives for HBD and non-HBD polar solvents, respectively. The value of r is higher in the micelles containing EG than that obtained with DMF, and increases with AOT concentration. This is explained as due to highly structured polar solvents in the inner core. EG is interacting with the polar heads of AOT through hydrogen-bond interaction, while DMF can only interact with the Na+ counterions. This is confirmed by the time-resolved emission spectra (TRES) of the probe in the micellar systems, in comparison with the bulk solvents.     

STATIC and TIME-RESOLVED FLUORESCENCE OF AN AMPHIPHILIC FLAVIN IN AEROSOL OT REVERSED MICELLES

Spectral and time-resolved fluorescence studies have been carried out on N(3)-undecyllumi-flavin dispersed in reversed micelles composed of the surfactant sodium di(2-ethylhexyl) sulfosuccinate (Aerosol OT, AOT), various amounts of water and n -heptane as continuous phase. The fluorescence spectral properties (spectral distribution, quantum efficiencies and lifetimes) as function of the water to AOT molar ratio suggest that the flavin occupies a position within the surfactant boundary layer in close contact with water. The fluorescence anisotropy exhibits biexponential decay with a short (0.3 ns) and a longer (1.6–2.4 ns) correlation time. The contribution of the short component increases with the growth of the droplet providing evidence for enhanced flexibility of the flavin in the interfacial layer.     

反胶束法合成CdS纳米粒子及其表征

以AOT为保护剂,采用反胶束法合成CdS纳米粒子。利用水洗法洗去保护剂AOT,通过加入不同量的无水乙醇调节分散介质的极性,改变CdS纳米粒子在分散介质中的"溶解度",从而实现不同尺寸粒子的分离。采用紫外-可见(UV-vis)吸收光谱、透射电子显微镜(TEM)、荧光光谱法对其进行表征。     

Surfactant self-assembling in gas phase: electrospray ionization- and matrix-assisted laser desorption/ionization-mass spectrometry of singly charged AOT clusters

The self-assembling of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) in gas phase has been investigated by electrospray ionization- and matrix-assisted laser desorption/ionization mass spectrometry. Large surfactant clusters with an aggregation number close to that found in apolar media have been observed either as positive or negative ions. Moreover, the marked predominance of singly charged species as well as preliminary theoretical calculations strongly suggest an aggregate structure characterized by an internal hydrophilic core hosting the extra charge surrounded by an apolar shell constituted by the surfactant alkyl chains. This structure is similar to that of the more familiar reversed micelles formed when an appropriate surfactant is solubilized in apolar solvents. Finally, similar trends are observed independently either on the ionization technique or the polarity of the solvent used. This, together with the large dependence of the aggregation number on the flow rates, strongly indicates that self-assembling of the surfactant molecules occurs during the evaporation step.     

An example of how to use AOT reverse micelle interfaces to control a photoinduced intramolecular charge-transfer process

6-Propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe due to its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer (ICT) states which can be particularly useful as sensors. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES), and time-resolved area normalized emission (TRANES) spectroscopies on PRODAN dissolved in nonaqueous reverse micelles. The reverse micelles are composed of polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane. Sequestered polar solvents included ethylene glycol (EG), propylene glycol (PG), glycerol (GY), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA). The experiments were performed with varying surfactant concentrations at a fixed molar ratio W(S) = [polar solvent]/[AOT]. In every reverse micelle studied, the results show that PRODAN undergoes a partition process between the external solvent and the reverse micelle interface. The partition constants, K(p), are quantified from the changes in the PRODAN emission and/or absorption spectra with the surfactant concentration. The K(p) values depend strongly on the encapsulated polar solvent and correlate quite well with the AOT reverse micelle interface's zones where PRODAN can exist and emits. Thus, the partition toward the reverse micelle interface is strongly favored in DMF and DMA containing micelles where the PRODAN emission comes only from an ICT state. For GY/AOT reverse micelles, the K(p) value is the lowest and only emission from the local excited (LE) state is observed. On the other hand, for EG/AOT, PG/AOT, and water/AOT reverse micelles, the K(p) values are practically the same and emission from both states (LE and ICT) is simultaneously detected. We show here that it is possible to control the PRODAN state emission by simply changing the properties of the AOT reverse micelle interfaces by choosing the appropriate polar solvent to make the reverse micelle media. Indeed, we present experimental evidence with the answer to the long time question about from which state does PRODAN emit, a process that can be controlled using the unique reverse micelle interfaces properties.     

Effect of the addition of a nonaqueous polar solvent (glycerol) on enzymatic catalysis in reverse micelles. Hydrolysis of 2-naphthyl acetate by alpha-chymotrypsin

The kinetics of hydrolysis of 2-naphthyl acetate (2-NA) catalyzed by alpha-chymotrypsin (alpha-CT), in reverse micellar solutions formed by glycerol (GY)-water (38% v/v) mixture/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane has been determined by spectroscopic measurements. To compare the efficiency of this reaction with that observed in micelles with water in the core, as well as in the corresponding homogeneous media, the reaction was also studied in water/AOT/n-heptane reverse micellar solutions and in both homogeneous media (water and GY-water, 38% v/v mixture). In every media, alpha-CT was characterized by the absorption and emission spectra, the fluorescence lifetimes, and the fluorescence anisotropy of its tryptophan residues. The effect of AOT concentration on the kinetic parameters obtained in the micellar systems was determined, at a constant molar ratio of the inner polar solvent and surfactant. Moreover, the data obtained allowed the evaluation of the 2-NA partition constant between the organic and the micellar pseudophase. It is shown that the addition of GY to the micelle interior results in an increase in the catalytic properties of alpha-CT. The fluorescence anisotropy studies in the different media show that the addition of GY increases the viscosity as compared with the aqueous systems. It seems that the GY addition to the reverse micellar aggregates results in a decrease of the conformational mobility of alpha-CT, which leads to an increase of the enzyme stability and activity.     

Correlating proton transfer dynamics to probe location in confined environments

The dramatic impact of differing environments on proton transfer dynamics of the photoacid HPTS prompted us to investigate these systems with two highly complementary methods: ultrafast time-resolved transient absorption and two-dimensional NMR spectroscopies. Both ultrafast time-resolved transient absorption spectroscopy and time-resolved anisotropy decays demonstrate the proton transfer dynamics depend intimately on the specific reverse micellar system. For w(0) = 10 reverse micelles formed with anionic AOT surfactant, the HPTS proton transfer dynamics are similar to dynamics in bulk aqueous solution, and the corresponding (1)H 2D NOESY NMR spectra display no cross peaks between HPTS and AOT consistent with the HPTS residing well hydrated by water in the interior of the reverse micelle water pool. In contrast, ultrafast transient absorption experiments show no evidence for HPTS photoinduced proton transfer reaction in reverse micelles formed with the cationic CTAB surfactant. In CTAB reverse micelles, clear cross peaks between HPTS and CTAB in the 2D NMR spectra show that HPTS embeds in the interface. These results indicate that the environment strongly impacts the proton transfer reaction and that complementary experimental techniques develop understanding of how location critically affects molecular responses.     

The role of molecular size in the excited state behavior of aminocoumarin dyes in restricted media--2: study of BC I in AOT-formamide reversed micelles

Ground and excited state properties of a pre-twisted 7-diethylaminocoumarin dye (BC I) belonging to the family of coumarinyl benzopyrano pyridines are reported in isooctane-AOT-formamide reversed micelles. In reversed micelles, BC I, albeit soluble in formamide, is found to remain out of the polar solvent pool. But the photophysical properties of the probe dye are sensitive to the changes in the polarity of the interfacial region caused by increase in F0 = [formamide]/[AOT]. The spectroscopic properties and dynamics are indicative of dual emission due to the solubilization of the dye in two different environments (the nonpolar solvent and the interfacial region). Results of the steady-state fluorescence anisotropy experiments also support the presence of two different environments. The present study once again proves that molecular size is an important parameter in the study of the photophysical properties of the flexible aminocoumarin dyes in reversed micelles.     

New insights on the photophysical behavior of PRODAN in anionic and cationic reverse micelles: from which state or states does it emit?

6-propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe because of its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer states (ICT) that can be particularly useful as a sensor. The nature of the emissive states has not yet been established despite the detailed experimental and theoretical investigations done on this fluorophore. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES) and time-resolved area normalized emission (TRANES) spectroscopies on the molecular probe PRODAN in the anionic water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane and the cationic water/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/benzene reverse micelles (RMs). The experiments were done by varying the surfactant concentrations at a fixed molar ratio (W = [H2O]/[Surfactant]) and changing the water content at a constant surfactant concentration. The results obtained varying the surfactant concentration at W = 0 show a bathochromic shift and an increase in the intensity of the PRODAN emission band due to the PRODAN partition process between the external solvent and the RMs interface. The partition constants, Kp, are quantified from the changes in the PRODAN emission spectra and the steady-state anisotropy () with the surfactant concentration in both RMs. The Kp value is larger in the BHDC than the AOT RMs, probably due to the interaction between the cationic polar head of the surfactant and the aromatic ring of PRODAN. The partition process is confirmed with the TRES experiments, where the data fit to a continuous model, and with the time-resolved area normalized emission spectroscopy (TRANES) spectra, where only one isoemissive point is detected. On the other hand, the emission spectra at W = 10 and 20 show a dual fluorescence with a new band that emerges in the low-energy region of the spectra, a band that was previously assigned to the PRODAN emission from the water pool of RMs. Our studies demonstrate that this band is due to the emission from an ICT state of the molecular probe PRODAN located at the interface of the RMs. These results are also confirmed by the lifetime measurements, the TRES experiments where the results fit to a two-state model, and the time-resolved area normalized emission spectroscopy (TRANES) spectra where three or two isoemissive points are detected in the AOT and BHDC RMs, respectively. In the AOT RMs, Kp values obtained at W = 10 and 20 are almost independent of the water content; the values are higher for the BHDC RMs due to the higher micropolarity of this interface.     

Enzymes Hosted in Reverse Micelles in Hydrocarbon Solution

Reverse micelles are spheroidal aggregates formed by certain surfactants in apolar media. In contrast to normal micelles in water, the polar head groups of the surfactant molecules are directed towards the interior of the aggregate and form a polar core which can solubilize water (the “water pool”); the lipophilic chains are exposed to the solvent. The water of the water pool exhibits properties that (depending on the mole ratio of water to surfactant) differ from those of bulk water. Surprisingly, these reverse micelles are able to solubilize in hydrocarbon solvents hydrophilic molecules, e.g., enzymes and even plasmids, that are much larger than the original water-pool diameter. These biopolymer-containing reverse micelles can be viewed as novel microreactors, whose physical properties can be controlled through the water content. Remarkable is the ability of enzyme-containing micelles to react with water-insoluble, hydrocarbon-soluble substrates, as in the example of lipoxygenase with linoleic acid.     

Influence of Confined Water on the Photophysics of Dissolved Solutes in Reverse Micelles

The photophysical parameters of two probes with largely different hydrophobic character, namely, coumarin 1 and coumarin 343, are investigated in sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT)/hexane/water reverse micelles at various water/AOT molar ratio w₀. Correlation of photophysical parameters such as fluorescence quantum yield, fluorescence lifetime, and emission maxima with w₀ indicate distinctly different trends below and above w₀≈7 for both probes. The variation of the average rotational correlation times obtained from fluorescence anisotropy decays for both probes in reverse micelles further corroborate the above observation. Similar studies were also performed in nonaqueous reverse micelles with acetonitrile as polar solvent. Similar to aqueous reverse micelles, breaks in the photophysical parameters with increasing acetonitrile/AOT molar ratios w′₀ were also observed in these cases, although at a much lower w′₀ value of 3. The present results indicate that around w₀≈7 for aqueous reverse micelles (and around w′₀≈3 for nonaqueous reverse micelles) a distinct change occurs in the probe microenvironment, which is rationalized on the basis of the relative populations of interfacial and core water. We propose that until the ionic head groups and counterions are fully solvated by polar solvents, that is, up to w₀≈7 (or w′₀≈3), the interfacial water population dominates. Above these molar ratios coalescence of excess water molecules with each other to form truncated H‐bonded water clusters leads to a sizable population of core water. This is further substantiated by changes in the IR absorption spectra for the O? D stretching mode of diluted D₂O in reverse micelles with varying w₀. Critical comparison of the present results with relevant literature reports provide clear support for the proposals made on water structure in reverse micelles. The role of relative size of the probe and the reverse micelles for differences in polar solvent to AOT ratios (w₀=7 and w′₀=3) in the observed breaks in the two types of reverse micelles is also discussed.     

Fluorescence anisotropy of ionic probes in AOT reverse micelles: influence of water droplet size and electrostatic interactions on probe dynamics

Fluorescence anisotropies of two structurally similar ionic probes, rhodamine 110 and fluorescein, were measured in di(2-ethylhexyl) sodium sulfosuccinate (AOT) reverse micelles as a function of the mole ratio of water to surfactant W. This study was undertaken to explore the influence of water droplet size and electrostatic interactions on the rotational diffusion of the probe molecules. It was noticed that at W = 1 and 2, the anisotropy decays of both the probes display single-exponential behavior and for a particular value of W, the time constants sensed by rhodamine 110 and fluorescein are identical. Moreover, an increase in the reorientation time was observed from W = 1 to 2. These observations indicate that, at W = 1 and 2, it is the overall rotation of micelle which is responsible for the decay of the anisotropy and also rule out the possibility of internal rotation of the probes within the reverse micelles. However, from W = 4 to 20, the anisotropy decays of the probes could only be described by a biexponential function with two time constants. The rotational diffusion of rhodamine 110 and fluorescein in the above-mentioned range of W was rationalized using the two-step model. The average reorientation time decreases with an increase in W for both the probes, and this decrease is pronounced in the case of fluorescein compared to that in rhodamine 110. The decrease in the average reorientation time with W is due to the change in the micellar packing within the core. The significant reduction in the average reorientation time of fluorescein is a consequence of repulsive electrostatic interactions between the negatively charged probe and the anionic head groups of the surfactant AOT.     

Characterization of the fluorescence emission properties of prodan in different reverse micellar environments

We have examined the steady state and time resolved fluorescence emission properties of the hydrophobic fluorescence probe, prodan, in three representative reverse micellar systems formed by the surfactants poly(oxyethylene) (tetramethylbutyl) phenylether (Triton X-100, neutral), cetyl trimethylammonium bromide (CTAB, cationic) and sodium bis-(2-ethylhexyl) sulfosuccinate (AOT, anionic) in organic solvent media containing different concentrations of water. The results obtained from the experiments indicate conspicuous dependence of the emission behaviour of prodan on the type of surfactant used and the water/surfactant molar ratio (w0). The nature of the emission profiles, along with relevant parameters namely emission maximum (lambda(em)max), anisotropy (r) and lifetime (tau) data are used to infer the distribution and microenvironments of the prodan molecules in the reverse micelles at different w0 values. Furthermore, quantitative estimates have been obtained for the polarities (in terms of the empirical polarity parameter E(T)(30)) of the sites of solubilization of the fluorophore in different reverse micellar systems.     

The influence of water on the photophysical and photochemical properties of Piroxicam in AOT/iso-octane/water reversed micelles

The photophysical properties of Piroxicam , a nonsteroidal anti-inflammatory drug (NSAID), were investigated at different pH_ext values in reversed micelles of Aerosol-OT (AOT) in iso -octane, using both steady-state and picosecond time-resolved fluorescence spectroscopy. In contrast with the very complex data obtained in aqueous media, where several prototropic species are in equilibrium, the reversed micellar system essentially favors two species. The absorption spectra shows only one isosbestic point at λ= 348 nm. Excited-state intramolecular proton transfer (ESIPT), also detected in water, is promoted at low water pool contents measured by ω₀= [H₂O]/[AOT]. A strongly shifted (λ_em= 470 nm) tautomeric emission is found. Upon the gradual increase of ω₀, striking differences with pH_ext are found. At pH_ext= 4, the drug preferentially locates itself in the interfacial region partitioning between a hydrophobic and a hydrophilic domain. Global analysis was applied to the decay data and the results were interpreted by the "two-state excited-state" formalism. At pH_ext= 7, the anionic species is prevalent and the probe locates itself deeper inside the water core of the reversed micelles. Thus, a strong dependence on water content is detected, approaching a behavior similar to that observed in free aqueous solutions.     

溶剂对反胶束法合成CdS纳米粒子粒径的影响

以磺基琥珀酸二辛酯钠盐(AOT)为保护剂,利用反胶束法在不同烷烃溶液中合成了CdS纳米粒子.采用紫外-可见光谱、透射电子显微镜、荧光光谱法对其进行表征.研究表明:在不同烷烃溶液中合成的CdS纳米粒子,其粒子大小和荧光强度都随溶剂而改变.     

Synthesis,Separation, and Characterization of Au@CdS Nanoparticles

The mixture of Au@CdS and CdS nanoparticles were obtained by reversed micelles method. n-Dodecanethiol was used to replace dioctylsulfosuccinate sodium salt (AOT) as a protector, and the surfactant AOT was washed away by water. By adding enthanol of different quantity, the polarity of dispersed medium was regulated and the solubility of Au@CdS and CdS nanoparticles in dispersed medium was adjusted. Consequently, the Au@CdS and CdS nanoparticles were separated effectively. Ultraviolet absorption spectra, transmission electronic microscopy (TEM), energy-dispersive X-ray analysis (EDX), and fluorescence spectra were used to characterize Au@CdS nanoparticles.     

AOT反相胶束介质中敏化稀土离子荧光: 嘌呤类化合物为能量给体

研究了反相胶束体系中敏化Tb^3^+的离子荧光。在AOT/C6H12H2O反相胶束溶液中[AOT: 琥珀酸二(2-乙基己基)酯磺酸钠]发生了非常有效的从茶碱的三重态到Tb^3^+的4f能层的能量转移, 并敏化稀土离子Tb^3^+产生离子荧光。而在阳离子表面活性剂CTAB形成的反相胶束体系中, 只能观察到较弱的Tb^3^+的离子荧光。表明在AOT反相胶束中Tb^3^+是键合在磺酸头基上, 有利于能量转移过程, 显著增强Tb^3^+的离子荧光。通过发光光谱和寿命测量, 详细讨论了AOT浓度和水泡大小(W值)等对敏化离子荧光的影响, 表明与Tb^3^+离子水合的水分子的高频OH振动猝灭Tb^3^+的离子发光, 该猝灭过程属静态猝灭。在较低的AOT浓度和较小的W值下, 可观察到较强的Tb^3^+离子荧光, 并建立了AOT反相胶束中五种嘌呤类化合物的分析方法, 检出限在8.0×10^-^9～8.0×10^-^7mol.dm^-^3之间。     

Confinement or the nature of the interface? Dynamics of nanoscopic water

The dynamics of water confined in two different types of reverse micelles are studied using ultrafast infrared pump-probe spectroscopy of the hydroxyl OD stretch of HOD in H2O. Reverse micelles of the surfactant Aerosol-OT (ionic head group) in isooctane and the surfactant Igepal CO 520 (nonionic head group) in 50/50 wt % cyclohexane/hexane are prepared to have the same diameter water nanopools. Measurements of the IR spectra and vibrational lifetimes show that the identity of the surfactant head groups affects the local environment experienced by the water molecules inside the reverse micelles. The orientational dynamics (time-dependent anisotropy), which is a measure of the hydrogen bond network rearrangement, are very similar for the confined water in the two types of reverse micelles. The results demonstrate that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.     

Nanostructure of open water-channel reversed micelles. I. 1H NMR spectroscopy and molecular modeling

A recently proposed model for the rodlike reversed micelles of nickel(II) bis(2-ethylhexyl)phosphate is examined in greater detail using 1H NMR spectroscopy and molecular modeling. 1H NMR spectra show that the solubilized water molecules are situated in a different environment compared with the water molecules in classical (AOT) reversed micelles. Geometry optimization and molecular dynamics simulation clearly indicate that the water molecules are not located in the interior core of the reversed micelles, but instead the water molecules exist in compartments or channels in the surface of these rodlike reversed micelles, thereby confirming the open water-channel model of reversed micelles. Molecular modeling was also employed to examine the effects of surfactant molecular structure, cosurfactant, solvent aromaticity, and temperature on the nanostructure of the reversed micellar aggregates. It is significant that molecular modeling provides an interpretation of the nanostructure of reversed micellar aggregates that is consistent with a variety of known experimental observations reported in the liquid/liquid extraction literature. These findings show that the structure of reversed micelles is much richer at the nanoscale level than previously recognized.