Critical micelle concentration fluorescence emission

While the photosensitizer DCM serves as a probe into the micellar interior, DODCI provides a probe of a small region near the water-micelle interface. The time-resolved emission spectra for DODCI in TX-lOO, CTAB, and SDS above the critical micelle concentration showed a marked increase in the emission quantum yield (two to three times greater than in water). Also, a significant increase in the excited-state lifetime of DODCI was observed, from 0.70 ns in water to 2.25 ns in SDS, 2.36 ns in CTAB, and 2.55 ns in TX-lOO. The observations of increased fluorescence quantum yield are attributed to a decrease in the photoisomerization rate of DODCI as a result of the micellar environment. 

Different parameters describing the principal properties of self-assemblies are mentioned in the literature. Critical micellization concentration (CMC) is described using the pyrene fluorescence method. From a stock solution of pyrene dissolved in chloroform, an aliquot is transferred with a micropipette into a series of dry test tubes the solvent is allowed to evaporate under vacuum by protecting it from light to obtain dry pyrene. Then, a series of polymer solutions in buffer solution (pH 7.4) are added to the pyrene. Mixtures are stirred in the dark for 24 h at 25°C and then filtered through a membrane filter for separation of undissolved pyrene crystals. The concentration of solubilized pyrene in the micellar phase is determined spectrofluorometrically at 339 nm wavelengths of excitation and 390 nm of emission. 

In order to determine uneqnivocaUy if these micellar aggregates were capable of di-8-ANEPPS nptake leading to a change in its fluorescence spectrum, a series of experiments was conducted. 44 was added (as a 33 mM solution in DMSO) to a solution of di-8-ANEPPS at time = 0 min. The fluorescence excitation and emission spectrum was then recorded at 10 min time intervals. This was done for additions of 44 at both pre- and post-critical micelle concentration (CMC) levels. The results of these experiments are shown in Fig. 3.13. 

Students then prepare three aqueous samples - one below the critical micelle concentration (or cmc, the concentration required for micelles to form), one above cmc, and one significantly above cmc. The addition of a fluorescent probe (with a visible emission that is sensitive to the polarity of its environment) presents students with a visible cue of the formation of micelles using a handheld ultraviolet lamp. At low surfactant concentration, the luminescent probe is in a polar environment (water), while above cmc it becomes trapped within the non-polar interior of the micelle. Comparison of the color and intensity of samples below and above cmc provides evidence for the notable difference of environment in the immediate vicinity of the probe (see Figure 2 b-c). The third sample becomes quite viscous, which provides evidence for the presence of elongated worm-like micelles that become entangled at high concentration (55). 

Figure 3. Fluorescence emission spectra ofpyrene in aqueous preparations of a gemini amphiphile above (broken line) and below (solid line) the critical micelle concentration. Note that the 11/13 ratio changes from 1.23 (above cmc) to 1.72...

---