Fluorescence probing of block copolymeric micelles using Coumarin 153 (original) (raw)
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Collection of Czechoslovak Chemical Communications, 2002
The micellization behavior of a hydrophobically modified polystyrene-block-poly(methacrylic acid) diblock copolymer, PS-N-PMA-A, tagged with naphthalene between blocks and with anthracene at the end of the PMA block, was studied in 1,4-dioxane-methanol mixtures by light scattering and fluorescence techniques. The behavior of a single-tagged sample, PS-N-PMA, and low-molar-mass analogues was studied for comparison. Methanol-rich mixtures with 1,4-dioxane are strong selective precipitants for PS. Multimolecular micelles with compact PS cores and PMA shells may be prepared indirectly by dialysis from 1,4-dioxane-rich mixtures, or by a slow titration of copolymer solutions in 1,4-dioxane-rich solvents with methanol under vigorous stirring. In tagged micelles, the naphthalene tag is trapped in nonpolar and fairly viscous core/shell interfacial region. In hydrophobically modified PS-N-PMA-A micelles, the hydrophobic anthracene at the ends of PMA blocks tends to avoid the bulk polar solvent and buries in the shell. The distribution of anthracene tags in the shell is a result of the enthalpy-to-entropy interplay. The measurements of direct nonradiative excitation energy transfer were performed to estimate the distribution of anthracene-tagged PMA ends in the shell. The experimental fluorometric data show that anthracene tags penetrate into the inner shell in methanol-rich solvents. Monte Carlo simulations were performed on model systems to get reference data for analysis of time-resolved fluorescence decay curves. A comparison of experimental and simulated decays indicates that hydrophobic traps return significantly deep into the shell (although not as deep as in + The study is a part of the long-term Research Plan of the School of Science No. MSM 113100001. aqueous media). The combined light scattering, fluorometric and computer simulation study shows that the conformational behavior of shell-forming PMA blocks in non-aqueous media is less affected by the presence of nonpolar traps than that in aqueous media.
Probing solubilization sites in block copolymer micelles using fluorescence quenching
Journal of Photochemistry and Photobiology A: Chemistry, 2011
The solubilization sites provided by micelles formed by a diblock copolymer with one neutral hydrophobic block, polystyrene, and one charged hydrophilic block, poly(acrylic acid) or poly(methacrylic acid), have been studied by fluorescence quenching of pyrene by polar and nonpolar quenchers. Pyrene solubilized into these micelles is distributed between the inner corona and the micelle core. The fraction of pyrene residing in the inner corona is almost unity for star micelles, where the corona-forming blocks are larger than the core-forming blocks, and around 0.5 for crew-cut micelles where the opposite situation prevails. The kinetics of the quenching process depends on the pyrene location, i.e. is static in the micelle core, and largely dynamic in the inner corona at low quencher concentration. The rate constants for fluorescence quenching by nitromethane are shown to increase with increasing pH.
Macromolecules, 1991
A-B-A block copolymers (A = poly(methacry1ic acid), B = polystyrene) have been prepared by anionic polymerization. These amphiphilic copolymers can form stable micelles in water/l,l-dioxane mixtures as well as in water or an aqueous buffer. These micelles are presumed to have a polystyrene core and poly(methacry1ic acid) shell. The ability of the micelles to solubilize and release hydrophobic species was studied by fluorescence methods, primarily using pyrene as a fluorescence probe. The following processes were studied: (1) the effect of pyrene loading on monomer/excimer fluorescence ratio and quenching by Cu2+; (2) the rate of exchange between micelles containing pyrene and other aromatic species by the time dependence of either their monomer/excimer ratio or sensitized fluorescence after mixing micelles; (3) the time dependence of the fluorescence quenching of pyrene following the addition of small molecules (Nfl-dimethylaniline, CC4). The following conclusions were obtained (1) a significant fraction (ca. 20-30%) of the pyrene molecules were on or near the polystyrene-water interface (this depends on loading); (2) diffusion of the probe out of the micelle is the rate-determining step in the release and exchange of large hydrophobes. This process is very slow in ita later phases and probably represents slow diffusion from the core of the polystyrene region of the micelle.
Inorganica Chimica Acta, 2012
We describe a method to determine the critical micelle concentration (CMC) of low molecular weight surfactants and amphiphilic diblock copolymers in water based on the use of the fluorescent dye coumarin 153 (C153). The method is based on the measurement of the fluorescence intensity, solvatochromic shift and fluorescence anisotropy of C153 and was tested with the low molecular weight surfactants SDS, CTAB and Triton X-405 in water, for which we obtained, within the experimental error, CMC values identical to those previously published. The method was further used to determine the CMC of a family of poly(Ndecylacrylamide)-b-poly(N,N-diethylacrylamide) amphiphilic block copolymers synthesized by RAFT polymerization, with an equal poly(N-decylacrylamide) hydrophobic block length and increasing poly(N,N-diethylacrylamide) hydrophilic block lengths. By measuring the fluorescence anisotropy of C153 in block copolymer aqueous solutions, it is also possible to detect the formation of pre-micellar aggregates at concentrations below the CMC. The dye C153 is more appropriate than pyrene to study self-assembly in water, where pyrene forms ground and excited state aggregates at very low concentrations. Furthermore, C153 is an excellent anisotropy probe with a very high limiting anisotropy (0.375) that can be used to determine the CMC, detect the presence of pre-micellar aggregates, and evaluate the fluidity of the hydrophobic core of block copolymer micelles.
Time-resolved fluorescence study of micellizing block copolymers
Journal of Molecular Structure, 1990
ABSTRACT We have studied the dynamics of polystyrene-block-hydrogenated polyisoprene samples, fluorescently labelled on the polystyrene block, by steady-state and time-resolved fluorometry. In selective precipitants for the labelled block, fluorescent probes are trapped and immobilized in compact micellar cores. The rotation of pendant fluorophors is frozen. However, the fast torsional vibrations depolarize partially the fluorescence. As the rotation of micelles is slow as compared with the fluorescence life-time, a significant residual anisotropy is observed. In good solvents for both blocks, fluorescent probes in expanded copolymer coils are exposed to solvent molecules and free to rotate.
The Journal of Physical Chemistry B, 2010
Fluorescent probes, coumarin 153 (C153) and octadecylrhodamine B (ORB), were used to study the selfassembly in water of poly(N-decylacrylamide)-block-poly(N,N-diethylacrylamide), (PDcA 11-block-PDEA 295 ; M n) 40 300 g mol-1 ; M w /M n) 1.01). From the variation of both the fluorescence intensity and the solvatochromic shifts of C153 with polymer concentration, the critical micelle concentration (CMC) was determined as 1.8 (0.1 µM. On the other hand, steady-state anisotropy measurements showed the presence of premicellar aggregates below the CMC. Time-resolved fluorescence anisotropy evidenced that ORB is located in the premicellar aggregates and the micelle core, while C153 is partitioned between the aggregates and the water phase. The micelle core contains both semicrystalline and amorphous regions. In the semicrystalline regions the probes cannot rotate, while in the amorphous regions the rotational correlation times correlate well with the hydrodynamic volume of the probes. The amorphous region of the micelle core is relatively fluid, reflecting the large free-volume accessible to the probes.
Macromolecules, 1998
The dynamic stability of the micellar aggregates formed by amphiphilic block copolymers, i.e., poly((dimethylamino)alkyl methacrylate-b-sodium methacrylate), has been investigated by steadystate fluorescence spectroscopy and size exclusion chromatography. The dynamics of exchange of block copolymer molecules between the micelles, formed in aqueous solution, depend on a manifold of factors, e.g., alkyl substituent in the hydrophobic block, the relation between the hydrophobic and hydrophilic blocks, and the architecture of the block copolymer. All copolymers investigated show a slow unimer exchange with an exchange rate constant on the order of 10-3 s-1 , with a difference of a factor of 20 between the fastest and the slowest exchange. It is possible to tune the exchange rate in a controlled way; for instance, an extension or branching of the alkyl chain slows down the exchange rate. The same effect is observed when the hydrophobic/hydrophilic balance of these copolymers is increased or when either the hydrophilic or the hydrophobic moiety of diblocks is divided into two external blocks, leading to a triblock copolymer.
Journal of Physical Chemistry B, 2009
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.
Block Copolymer Micelles in Aqueous Media
Collection of Czechoslovak Chemical Communications, 1993
Micellization of di- and triblock copolymers, poly(methacrylic acid)-block-polystyrene and poly(methacrylic acid)-block-polystyrene-block-poly(methacrylic acid), varying in molecular weight and composition, has been studied by static and dynamic light scattering, and sedimentation velocity. Micelles with polystyrene cores were prepared in water-dioxane mixtures, rich in dioxane, and transferred into water-rich mixtures, water, and aqueous buffers via stepwise dialysis. It has been shown that, in dioxane-rich mixtures, the micellar system was in dynamic equilibrium, while in water-rich solvents, water, and aqueous buffers the micellization equilibrium was frozen and micelles behaved like autonomus particles. Under certain conditions, micelles were accompanied by independent large particles. This phenomenon, known from other micellar systems as an "anomalous micellization", is discussed.
Macromolecules, 1999
Amphiphilic diblock copolymers were synthesized based on poly(2-ethyl-2-oxazoline) (PEtOz) as a hydrophilic block and aliphatic polyesters such as poly(L-lactide) (PLA) or poly(-caprolactone) (PCL) as a hydrophobic block. Their micellar characteristics in an aqueous phase were investigated by using dynamic light scattering and fluorescence techniques. The block copolymers formed micelles in the aqueous phase with critical micelle concentrations (cmcs) in the range of 1.0-8.1 mg/L. The cmc values become lower upon increasing the length of the hydrophobic block. The mean diameters of the micelles were in the range of 108-192 nm, with a narrow distribution. In general, the micelle size increased as the hydrophobic PLA or PCL block became larger. The partition equilibrium constants, K v, of pyrene in the micellar solutions of the block copolymers were from 1.79 × 10 5 to 5.88 × 10 5. For each block copolymer system of PEtOz-PLA or PEtOz-PCL, the Kv value increased as the length of the hydrophobic block increased. The steady-state fluorescence anisotropy values (r) of 1,6-diphenyl-1,3,5-hexatriene (DPH) were 0.265-0.284 in PEtOz-PLA solution and 0.189-0.196 in PEtOz-PCL solution. The anisotropy values of PEtOz-PLAs were higher than those of PEtOz-PCLs. The anisotropy values were independent of the length of the hydrophobic block when the chemical structures of the hydrophobic blocks were identical. The micelles underwent hydrogen bonding at pH <3.5 with poly(acrylic acid), which produced polymer complex precipitates that could be reversibly dispersed as micelles at pH >3.8.