Dodecyltrimethylammonium Bromide Micelles Research Articles

Conductometric, tensiometric and spectroscopic studies on the interaction of congo red with dodecyltrimethylammonium bromide: Effects of electrolyte and non-electrolyte

To investigate the interactions between congo red (CR) dye and dodecyltrimethylammonium bromide (DTAB) surfactant in aqueous, electrolytic (MgCl2) and non-electrolytic (urea) solutions, conductometric, tensiometric and spectroscopic methods were used. Conductivity and surface tension measurements were performed to evaluate the critical micelle concentration (cmc) values of the cationic surfactant, DTAB. The change in Gibb’s energy (ΔGm0), enthalpy (ΔHm0), and entropy (ΔSm0) of micellization were also calculated. The cmc value of DTAB was found to drop in the presence of CR dye and a further decrease was noticed in the presence of MgCl2, while the values augmented when urea was present. Due to the interruption of the structured water molecules encasing the hydrophobic groups of the DTAB, the cmc values augmented with an increase in temperature. The presence of MgCl2 led to a greater degree of ionization (α) of DTAB micelles. For all of the systems under study, the values of ΔGm0and ΔHm0 were found to be negative. The -TΔSm0 values, which were noted to be higher than ΔHm0 values suggest that the gain in entropy possibly be the prime factor that governs the process of micellization. Many of the CR dye molecules were drawn into DTAB micelles as their concentrations in the system were amplified, as evidenced by the absorbance values in UV–Vis spectroscopic studies.

Comparative study of the co-assembly behaviour of 3-chloro-4-hydroxy-phenylazo dyes with DTAB.

The co-assembly of three one-fold negatively charged 3-chloro-4-hydroxy-phenylazo dyes (Yellow, Blue and Red) with the cationic surfactant dodecyltrimethylammoniumbromide (DTAB) was studied to probe dye-DTAB binding stoichiometry and assembly morphology. For each dye, phase separation was observed above a given dye : DTAB ratio with the ratio depending on the dye. While Yellow and DTAB showed liquid/liquid phase separation above Yellow : DTAB = 1 : 1.67, crystalline dye-DTAB complexes were observed for Blue-DTAB and Red-DTAB above Blue : DTAB = 1 : 2.56 and Red : DTAB = 1 : 2.94 respecively. In homogeneous solution, UV/vis spectroscopic investigations suggest stochiometries of Yellow : DTAB = 1 : 2, Blue : DTAB = 1 : 3 and Red : DTAB = 1 : 4. It was concluded, that Yellow exhibits the highest dye : DTAB binding stoichiometry in both, dye-surfactant complexes in the 2-phase region and in solution, whereas the lowest dye : DTAB binding stoichiometry was observed for Red-DTAB in both cases. The observed stoichiometries are inversely correlated to the impact dye addition has on the morphology of DTAB micelles. Generally, addition of dye to DTAB micelles leads to a reduction in spontaneous curvature of these micelles and to the formation of triaxial ellipsoidal or cylindrical micelles from oblate ellipsoidal DTAB micelles. At a DTAB concentration of 30 mM and a dye concentration of 5 mM, this effect was most pronounced for Red and least pronounced for Yellow, whilst Blue showed an intermediate effect.

Exploring cationic polyelectrolyte-micelle interaction via excited-state proton transfer. Signatures of probe transfer.

Excited-state proton transfer (ESPT) is a sensitive tool for the delicate monitoring of structural reorganization, hydration level, and confinement within surfactant and polymer assemblies. Here, we investigate the interaction of a cationic polyelectrolyte, poly(diallyl dimethylammonium chloride) (PDADMAC), with micelles of differently charged surfactants using 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) as an ESPT probe. We used three surfactants: anionic sodium dodecyl sulfate (SDS), cationic dodecyl trimethylammonium bromide (DTAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12), possessing the same alkyl (dodecyl) chain but varying headgroup charges. The fluorescence of HPTS residing initially within the micellar medium modulates differently in the presence of PDADMAC. For the anionic SDS and cationic DTAB micelles, the emission spectrum of HPTS does not alter significantly; however, for SB12 micelles, the emission spectrum undergoes a strong modulation upon adding the polyelectrolyte. The emission intensities quench strongly at a low concentration of PDADMAC but recover at a higher concentration. The emission intensity ratio of the two emission bands also changes significantly, implying strong modulation of the ESPT process with varying PDADMAC concentrations. The time-resolved area normalized emission spectra (TRANES) disclose single isoemissive points in the SB12 micelle at low and high concentrations of PDADMAC but two different isoemissive points (one characteristic of the SB12 micelle at 500 nm and another characteristic of the PDADMAC interface at 480 nm) in the mixed assembly at an intermediate concentration. Detailed analysis suggests that the polyelectrolyte can enforce the transfer of the anionic probe HPTS from the zwitterionic micelle to the PDADMAC assembly above a specific PDADMAC concentration. The transfer of the molecular probe between two assemblies resembles a drug sequestration event, and the study reveals necessary emission signatures.

SANS contrast matching for the unambiguous localization of anionic dye in cationic surfactant micelles.

Contrast variation in small-angle neutron scattering (SANS) was successfully applied to localize the anionic azo dye Blue in co-assemblies with the cationic surfactant dodecyltrimethylammoniumbromide (DTAB). For this purpose, the scattering contrast between DTAB and the aqueous solvent was eliminated by SANS contrast matching, leaving only the scattering signal from Blue to be detected. Results obtained by contrast matching were confirmed by NOESY NMR-spectroscopy, showing that Blue interacts with the positively charged DTAB head groups and with up to the 4th neighbouring methylene group of the DTAB C12-alkyl chain. Its localization in the outer layer of the Blue-DTAB co-assembly explains the uniaxial growth of spheroidal DTAB micelles to wormlike micelles with increasing [Blue] : [DTAB] ratio from 0 : 1 to 1 : 3. This is in line with the concept of the packing parameter for amphiphilic substances.

Influence of micellar size on the structure of surfactant-DNA complexes.

We have studied the structure of complexes of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) with DNA as a function of surfactant to DNA base molar ratio (R) and salt concentration. Small-angle x-ray scattering data show the formation of nematic gels at lower and higher salt concentrations, irrespective of the value of R. Two crystalline phases are observed over intermediate salt concentrations; a square (S) phase for R>3 and a hexagonal (H_{S}) phase for lower R. Electron density maps of these phases show intercalated structures, with DTAB micelles sandwiched between long DNA strands. The composition of these complexes, estimated using elemental analysis, indicates that the micelles are not very long, and they occupy only about half of the interstitial volume between the DNA strands. This phase behavior is strikingly different from that of complexes of DNA with longer chain surfactants cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB), which show only a hexagonal (H) phase over similar ranges of R and salt concentration, the H_{S} structure observed in the present study being a sqrt[3]×sqrt[3] superlattice of the H structure. Madelung energies of the S and H structures, calculated from the electrostatic interaction between their cylindrical constituents, suggest that the former is preferred in DTAB-DNA complexes due to the smaller micellar radius of DTAB. The propensity of DTAB to form short micelles seems also to favor the H_{S} phase at lower R. These results illustrate the important role of micellar size in determining the structure of these two-dimensional macro-ion crystals.

Influence of n-alcohols on aqueous DTAB micelles studied by ultrasonic analysis

Abstract The influence of chain length of n-alcohols such as 1-butanol, 1-pentanol, 1-hexanol and 1-heptanol on cationic dodecyl trimethyl ammonium bromide (DTAB) micelles has been investigated. The effect of concentration was determined at alcohol concentrations of (10, 20, 30, 40 and 50) mM and at temperatures of 298.15 K, 303.15 K, 308.15 K and 313.15 K using ultrasonic velocity, density, viscosity and conductivity measurements. To study molecular interactions in micelles of various mixtures of DTAB and n-alcohols by using acoustical parameters, such as adiabatic compres-sibility (β ad), intermicellar free length (L f ), acoustic impedance (Z), molar volume (V M ) have been calculated by using ultrasonic velocity (U) and density (ρ). With the help of the trends observed when varying these parameters, the molecular interactions and thus the micellar growth of mixed systems of DTAB and n–alcohol were discussed. Viscosity data such as absolute viscosity, viscous relaxation time, oil solubilization, foam stability and conductance data complemented the observed ultrasonic data.

Thermo-acoustic, spectroscopic, and electrochemical investigation of sparfloxacin–ionic surfactant interactions

The rising antibiotic resistance crisis poses a severe threat in the medicinal field and thus, underlines an urgent need for developing new and more effective antibiotics to overcome this crisis. However, one of the significant challenges in developing more effective antibiotics that can diffuse passively across the biomembranes is the limited understanding of their physicochemical interactions with these membranes. There has been a recent renaissance of interest in using surfactant systems as potential model biomembranes to correlate the physicochemical interactions of pharmaceutical drugs and biomolecules with the cell membranes at the molecular level. In this study, the effect of adding a cationic surfactant, dodecyltrimethylammonium bromide (DTAB), and an anionic surfactant, sodium dodecyl sulfate (SDS), to a solution containing sparfloxacin (SPRF), a fluoroquinolone drug, was investigated using volumetric and acoustic measurements under physiological conditions (phosphate buffer, pH 7.4) over a temperature range (298.15 to 313.15 K) at an interval of 5 K. The obtained density and sound velocity data were used to compute and interpret various volumetric and acoustic parameters based on cosphere overlap model and electrostriction effect for comprehension of solute–solute and solute–solvent interactions prevailing in these systems. The parameters, partitioning and binding coefficients of SPRF with ionic surfactants, are measured employing UV–visible spectroscopy and cyclic voltammetry. The results revealed that SPRF shows more significant partitioning and binding with SDS micelles than with DTAB micelles.

Microscopic insights on the structural and dynamical aspects of Imidazolium-based surface active ionic liquid micelles

Ionic liquids have drawn vast attention due to their extraordinary physicochemical properties and their relevance in a wide range of applications. As a result of their inherent amphiphilic nature, many ionic liquids are emerging as novel surfactants, being referred to as surface active ionic liquids (SAILs). Under suitable conditions in aqueous solution, these SAILs self-assemble to form nanostructures such as micelles and vesicles. Here, we report on the structure and dynamics of one such SAIL micellar system, 1-Decyl-3-methylimidazolium Bromide (DMIMBr), as studied using fully atomistic molecular dynamics (MD) simulation and quasi-elastic neutron scattering (QENS). Results are compared with a conventional surfactant, dodecyltrimethylammonium bromide (DTAB) micelles. MD simulations showed that while the structure of the DMIMBr micelle is very similar to that of DTAB, the conformational flexibility of the alkyl chains is starkly different. The alkyl chains in the SAIL are more ordered, having fewer gauche defects. This is consistent with experimental results obtained using QENS which also showed slower alkyl chain segmental dynamics. MD results also show restricted dynamics of the hydration water in DMIMBr micelles, of a sub-diffusive nature, suggesting stronger affinity of water molecules to the imidazolium moiety. Thus the decreased flexibility in the alkyl chains could arise from the slower hydration dynamics. In contrast, the lateral motion of the surfactant is found to be relatively faster in DMIMBr micelles than in DTAB micelles. The understanding gained from this study provides a microscopic insight about the dynamical aspects of SAIL micelles, which might be useful for related applications including micellar catalysis and nanoparticle synthesis.

Gatifloxacin–Ionic Surfactant Interactions: Volumetric, Acoustic, Voltammetric, and Spectroscopic Studies

AbstractSurfactant systems have been frequently used as pseudomodels for investigating interactions of drugs with biological membranes because of their structural similarities with the latter. This helps to understand complicated yet very important biological processes like diffusion of bioactive moieties through biomembranes. The current study deals with voltammetric and spectroscopic studies to evaluate the interaction of a potential antibacterial drug, gatifloxacin (GTF), with a cationic surfactant, dodecyltrimethylammonium bromide (DTAB), and an anionic surfactant, sodium dodecyl sulfate (SDS), under physiological conditions (phosphate buffer, pH 7.4). For more detailed insight into the GTF–ionic surfactant interactions, density and acoustic data were also recorded and used to calculate several important parameters, namely, apparent molar volume (ɸV), isentropic compressibility (Ks), and apparent molar isentropic compressibility (ɸK) at T = 298.15, 303.15, 308.15, and 313.15 K. Values for partial molar volume (, partial molar expansivity , specific acoustic impedance (Z), relative association (RA), intermolecular free length (Lf), and sound velocity number (U) were also obtained. The interpretation of the concentration dependence of the above‐mentioned quantities using a cosphere overlap model led to a better apprehension of solute–solute and solute–solvent intermolecular interactions present in the investigated system, whereas cyclic voltammetry and ultra violet (UV)–visible spectroscopic studies assisted in predicting the location of adsorbed GTF molecules within the DTAB and SDS micelles.

DTAB micelle formation in ionic liquid/water mixtures is determined by ionic liquid cation structure

HypothesisThe high CMCs and low aggregation numbers of ionic micelles in the extreme electrolyte environment of ionic liquids (ILs) seem to be at odds with the effect of dilute aqueous electrolytes, which lower CMCs and promote elongated micelles. We hypothesise that the driving force for micellisation in ILs is determined by their underlying amphiphilic nanostructure, and that this can be controlled by mixing with water. ExperimentsCMCs and micelle sizes of dodecyltrimethylammonium bromide (DTAB) are determined in mixed solvents comprising water and the ionic liquids ethylammonium nitrate (EAN), ethanolammonium nitrate (EtAN), and propylammonium nitrate (PAN) over a wide composition range. Their behaviour is compared with aqueous electrolytes up to their solubility limit. CMCs are determined by a variety of techniques, and their relative strengths critically evaluated. Micelle morphology is determined by small-angle neutron scattering. FindingsIn water-rich mixtures, ILs do behave like simple electrolytes. Counterion binding dominates, both lowering the aqueous CMC and favouring a sphere-rod transition. However, even at modest concentrations, IL cations become incorporated into the micelle, causing the CMC to pass through a minimum, and arresting the sphere-rod transition. The efficiency of the cation depends on its amphiphilicity. As the IL content increases further, its role as a component of the bulk solvent becomes dominant: Only here does IL nanostructure influence micellization, as it increases alkyl chain solubility (EAN, PAN) and hence raises the CMC.

Features of the Interaction between Dodecyltrimethylammonium Bromide Micelles and Sodium 4-Styrene Sulfonate

Spin probe EPR spectroscopy and UV spectroscopy have been employed to study the interaction between a hydrophobic anionic monomer, sodium styrene sulfonate, and micelles of a cationic surfactant, dodecyltrimethylammonium bromide. It has been shown that the interaction with the monomer decreases the critical micelle concentration of the surfactant in aqueous solutions. It has been found that the monomer is incorporated into micelles; therefore, the presence of the surfactant may hinder polymerization of the monomer.

Concentration-dependent changes to diffusion and chemical shift of internal standard molecules in aqueous and micellar solutions.

Sodium 4,4-dimethyl-4-silapentane-1-sulfonate (DSS) is the most widely accepted internal standard for protein NMR studies in aqueous conditions. Since its introduction as a reference standard, however, concerns have been raised surrounding its propensity to interact with biological molecules through electrostatic and hydrophobic interactions. While DSS has been shown to interact with certain proteins, membrane protein studies by solution-state NMR require use of membrane mimetics such as detergent micelles and, to date, no study has explicitly examined the potential for interaction between membrane mimetics and DSS. Consistent with its amphipathic character, we show DSS to self-associate at elevated concentrations using pulsed field gradient-based diffusion NMR measurements. More critically, DSS diffusion is significantly attenuated in the presence of either like-charged sodium dodecyl sulfate or zwitterionic dodecylphosphocholine micelles, the two most commonly used detergent-based membrane mimetic systems used in solution-state NMR. Binding to oppositely charged dodecyltrimethylammonium bromide micelles is also highly favourable. DSS-micelle interactions are accompanied by a systematic, concentration- and binding propensity-dependent change in the chemical shift of the DSS reference signal by up to 60ppb. The alternative reference compound 4,4-dimethyl-4-silapentane-1-ammonium trifluoroacetate (DSA) exhibits highly similar behaviour, with reversal of the relative magnitude of chemical shift perturbation and proportion bound in comparison to DSS. Both DSS and DSA, thus, interact with micelles, and self-assemble at high concentration. Chemical shift perturbation of and modulation of micellar properties by these molecules has clear implications for their use as reference standards.

Tuning Nanoparticle-Micelle Interactions and Resultant Phase Behavior.

The evolution of the interaction between an anionic nanoparticle and a nonionic surfactant and their resultant phase behavior in aqueous solution in the presence of electrolyte and ionic surfactants have been studied. The mixed system of anionic silica nanoparticles (Ludox LS30) with nonionic surfactant decaethylene glycol monododecylether (C12E10) forms a highly stable clear phase over a wide concentration range of surfactant. Small-angle neutron scattering (SANS) and dynamic light scattering data show that the surfactant micelles adsorb on the surface of the nanoparticle, resulting in micellar-decorated nanoparticle structures. With the addition of a small amount of electrolyte into this system, the stability gets disturbed substantially and turns to a two-phase (turbid) system. The evolution of interaction in this system has been examined, and it was found that micelle-induced long-range depletion attraction (modeled by a double Yukawa potential) between nanoparticles leads to their aggregation. Interestingly, the addition of anionic surfactant sodium dodecyl sulfate (SDS) in this two-phase system transforms it to a transparent one-phase state, suppressing the depletion-mediated aggregation of nanoparticles. This is attributed to the formation of anionic C12E10-SDS mixed micelles, and it is their repulsive micelle-micelle interaction that disrupts the depletion phenomenon. On the other hand, the addition of cationic surfactant dodecyl trimethylammonium bromide (DTAB) to the turbid LS30-C12E10 electrolyte system shows no change in the turbidity arising from an aggregated nanoparticle system. The driving interaction, in this case, is different from that of the surfactant-mediated depletion attraction; it is due to the attraction between the nanoparticles mediated by the presence of oppositely charged DTAB micelles between them, resulting in a charge-driven bridging aggregation of nanoparticles. Each of these multicomponent systems has been investigated using contrast variation SANS measurements for different contrast conditions where the role of individual components (nanoparticle or surfactant) in the mixed system has been selectively studied. These results thus show that nanoparticle-surfactant micelle interactions can be tuned by the presence of electrolyte and/or choice of surfactant combination.

Interfacial Hydration Dynamics in Cationic Micelles Using 2D-IR and NMR.

Using the thiocyanate anion as a vibrational probe chromophore in conjunction with infrared and NMR spectroscopy, we find that SCN- strongly associates with the cationic head group of dodecyltrimethylammonium bromide (DTAB) micelles, both in normal-phase and reverse micelles. In competition with chloride and iodide ions, we find no evidence for displacement of thiocyanate, in accord with the chaotropicity of the Hofmeister ordering, while lending support to a direct interaction picture of its origin. Ultrafast 2D-IR spectroscopy of the SCN- probe in a range of DTAB micelle sizes (w0 = 4 to w0 = 12) shows little if any size dependence on the time scale for spectral diffusion, which is found to be ∼3.5 times slower than in bulk water (both D2O and H2O). Normal-phase micelles studied with 2D-IR exhibit essentially the same spectral dynamics as do reverse micelles, indicating a lack of sensitivity to interfacial curvature. Combined with 1H NMR chemical shift perturbations, we conclude that the SCN- ions tightly associate with the head groups and are partially buried. The 3-4-fold slowdown in spectral diffusion is consistent with the excluded volume model for interfacial perturbation to hydrogen bond reorientation dynamics. On the basis of these observations and comparisons to previous studies of zwitterionic interfaces probed with phosphate transitions, we conclude that the SCN- spectral dynamics in both reverse- and normal-phase micelles is largely dominated by hydration contributions, and offers a promising probe of interfacial hydration at cationic interfaces. Addition of competitive anions alters neither the IR spectra nor the ultrafast dynamics, indicating that SCN- is robustly associated with the head groups.

Effects of Hydrotropic Salt on the Nanoscopic Dynamics of DTAB Micelles.

Effects of a hydrotropic salt, sodium salicylate (NaSal), on the dynamic behavior of cationic dodecyltrimethylammonium bromide (DTAB) micelles as studied using dynamic light scattering (DLS) and quasielastic neutron scattering (QENS) techniques are reported here. DLS study showed that the addition of NaSal leads to a decrease in the apparent diffusion coefficient of the whole micelle indicating micellar growth. QENS data analysis suggested that observed dynamics involves two distinct motions, lateral motion of the surfactant over the curved micellar surface and localized segmental motion of the surfactant. It is found that the addition of NaSal slows down the lateral motion of DTAB while the localized segmental motion of the DTAB chain is not affected much. An atomistic molecular dynamics (MD) simulation was performed to gain further insight into the underlying phenomena. MD simulation results are found to be consistent with the experimental observations. MD simulation revealed that location of the salicylate ions on the micellar surface and their strong electrostatic association with their oppositely charged surfactant headgroup are the major factors in slowing down the lateral motion of the DTAB molecule. In the present work, a quantitative description of the effects of NaSal on the nanoscopic dynamics of DTAB micelles and its correlation with the microstructure of the micelle is provided.

Effect of Hydroxyl Groups on Solubilization States of Alkandiols in DTAB Micelles: Application of a Differential Conductivity Technique.

The partition coefficients, Kx, of 1,2-alkanediols and α,ω-alkanediols between dodecyltrimethylammonium bromide (DTAB) micelles and water, and the change of the degree of counterion dissociation, dα/dXam, on DTAB micelles associated with solubilization of the alkanediols are determined by a differential conductivity technique. The standard Gibbs energy change of transfer per methylene group, ΔG0(CH2), is estimated from a dependence of logarithmic value of the Kx on the carbon number of alkyl chain in the alkanediols. The dα/dXam is independent of the carbon number of alkyl chain in the alkanediols. Comparing the present results with those of DTAB/1-alkanols system, it is suggested that solubilization states of the alkanediols in DTAB micelles are as follows: 1,2-alkanediols are solubilized as its alkyl chain is oriented to micellar interior just like 1-alkanols, while α,ω-alkanediols are solubilized as its alkyl chain is partly located in micellar surface region.

Measurement of the interaction enthalpy for mixed micelles of dodecyltrimethylammonium bromide and 1-dodecyl-3-methylimidazolium bromide

In order to clarify the nature of the inter-micellar interactions, we developed an approach to obtain the intermicellar interaction enthalpy through measuring the mixing heats of dodecyltrimethylammonium bromide (DTAB)/1-dodecyl-3-methylimidazolium bromide (C12mimBr) mixed micelles by using flow-mixing calorimetry. It was found that the inter-micellar interaction enthalpies for this system were negative, representing the attractive enthalpy interactions between the mixed micelle droplets. These results are contradictory to those reported in the literature and interpreted by the predominant attractive dehydration effect of counterion. Static light scattering experiment revealed that the repulsive nature of the interaction characterized by the Gibss free energy should be dominated by the entropy interaction.

Rheological properties of associates of ionic monomers with micelles of oppositely charged surfactants

Rheological properties of associates formed by interaction of trimethyl[methacryl-oxyethyl]ammonium methyl sulfate with sodium octyl- and dodecyl sulfate micelles, as well as associates of sodium 4-styrenesulfonate with dodecyltrimethylammonium bromide micelles in aqueous solutions were studied by steady shear and oscillatory (dynamic) shear measurements with Fourier transform. It was shown that viscosity depends on the composition and achieves maximum value at equimolar ratio of components for two of studied systems. The extremal dependence of the viscosity vs. composition is not observed for systems with sodium octyl sulfate due to weak interactions between the components. The systems exhibiting the anomalous dependencies of concentration to viscosity are viscoelastic fluids due to the physical entanglements between the associates.

Denaturant effects on HbGp hemoglobin as monitored by 8-anilino-1-naphtalene-sulfonic acid (ANS) probe

Glossoscolex paulistus extracellular hemoglobin (HbGp) stability has been monitored in the presence of denaturant agents. 8-Anilino-1-naphtalene-sulfonic acid (ANS) was used, and spectroscopic and hydrodynamic studies were developed. Dodecyltrimethylammonium bromide (DTAB) induces an increase in ANS fluorescence emission intensity, with maximum emission wavelength blue-shifted from 517 to 493nm. Two transitions are noticed, at 2.50 and 9.50mmol/L of DTAB, assigned to ANS interaction with pre-micellar aggregates and micelles, respectively. In oxy-HbGp, ANS binds to protein sites less exposed to solvent, as compared to DTAB micelles. In DTAB–HbGp–ANS ternary system, at pH 7.0, protein aggregation, oligomeric dissociation and unfolding were observed, while, at pH 5.0, aggregation is absent. DTAB induced unfolding process displays two transitions, one due to oligomeric dissociation and the second one, probably, to the denaturation of dissociated subunits. Moreover, guanidine hydrochloride and urea concentrations above 1.5 and 4.0mol/L, respectively, induce the full HbGp denaturation, with reduction of ANS-bound oxy-HbGp hydrophobic patches, as noticed by fluorescence quenching up to 1.0 and 5.0mol/L of denaturants. Our results show clearly the differences in probe sensitivity to the surfactant, in the presence and absence of protein, and new insights into the denaturant effects on HbGp unfolding.

Micelle-induced versatile sensing behavior of bispyrene-based fluorescent molecular sensor for picric acid and PYX explosives.

The effect of surfactant micelles on the photophysical properties of a cationic bispyrene fluorophore, Py-diIM-Py, was systemically examined. The results from series of measurements including UV-vis absorption, steady-state fluorescence emission, quantum yield, fluorescence lifetime, and time-resolved emission spectra reveal that the cationic fluorophore is only encapsulated by the anionic sodium dodecyl sulfate (SDS) surfactant micelles and not incorporated in the cationic dodecyltrimethylammonium bromide (DTAB) and neutral Triton X-100 (TX100) surfactant micelles. This different fluorophore location in the micellar solutions significantly influences its sensing behavior to various explosives. Fluorescence quenching studies reveal that the simple variation of micellar systems leads to significant changes in the sensitivity and selectivity of the fluorescent sensor to explosives. The sensor exhibits an on-off response to multiple explosives with the highest sensitivity to picric acid (PA) in the anionic SDS micelles. In the cationic DTAB micelles, it displays the highest on-off responses to PYX. Both the sensitivity and selectivity to PYX in the cationic micelles are enhanced compared with that to PA in the anionic micelles. However, the poor encapsulation in the neutral surfactant TX100 micelles leads to fluorescence instability of the fluorophore and fails to function as a sensor system. Time-resolved fluorescence decays in the presence of explosives reveal that the quenching mechanism of two micellar sensor systems to explosives is static in nature. The present work demonstrates that the electrostatic interaction between the cationic fluorophore and differently charged micelles plays a determinative role in adjusting its distribution in micellar solutions, which further influences the sensing behavior of the obtained micellar sensor systems.