Angelos Malliaris

Poly(propyleneimine) Dendrimers as pH-Sensitive Controlled-Release Systems

Molecular composites were prepared by solubilizing pyrene in di- aminobutane poly(propyleneimine) den- drimers having 32 or 64 primary amine end groups (DAB-32 or DAB-64). The dendrimer - pyrene binding constants were determined as Kpy/DAB-32a 16 725 200m ˇ1 and Kpy/DAB-64a 33 858 663m ˇ1 by fluorescence spec- troscopy. Fluorescence studies were also employed to probe the release of pyrene from the interior of dendrimers as a function of pH. When the pH value of the system was decreased from pH 11 by addition of HCl, the fluorescence inten- sity of the system was found to increase by approximately tenfold at pH 2 - 4. In addition, at pH 2, the ratio of the first to the third vibrational peak of pyrene (I1/I3) increased from 0.9, the value typical for pyrene solvated in dendrimer solution, to 1.60, the value characteristic of pyrene in water. Pyrene release from the interior of dendrimers was con- firmed by fluorescence quenching when sodium iodide was added, since NaI does not affect the emission of dendrim- er-solubilized pyrene. Finally, fluores- cence quenching was used to locate the solubilization sites of pyrene within the dendrimer microcavities. These sites are close to the core of the dendrimer, near the tertiary amino groups which are also responsible for quenching the fluores- cence of the dendrimer-bound pyrene.

Solubilization of organic molecules in SDS micelles studied by static fluorescence methods

Abstract The effect of a number of polar and non-polar organic molecules on the micellization of sodium dodecyl sulfate at low surfactant and additive concentration has been studied by fluorescence probing methods. The solubilization and its effect on micellar parameters and structure is examined and some general trends are discussed. Special emphasis is focused on the application of static fluorescence methods, particularly static fluorescence quenching, since these methods can provide important and unique information albeit their simplicity.

Dynamic behaviour of fluorescence quenchers in cetyltrimethylammonium chloride micelles

The kinetics of quenching of the fluorescence of pyrene solubilized in cetyltrimethylammonium chloride (CTAC) micelles by three different types of quenchers [pyrene or cetylpyridinium chloride (immobile quenchers), dodecylpyridinium chloride (mobile, hydrophobic quencher) and iodide ion (mobile, hydrophilic quencher)] has been investigated by means of time-resolved fluorescence measurements. The results obtained with the two immobile quenchers have permitted the first determination of the mean micellar aggregation number N of CTAC at high concentration up to 1.2 mol dm–3. These N-values have then been used for a quantitative evaluation of the kinetic and equilibrium quenching parameters for the other two types of quenchers. The results concerning the quenching by the iodide ion are discussed on the basis of the available models proposed for this type of quenching. They appear to favour the electrostatic model of Almgren et al.

Polymerization of Micelle-Forming Surfactants

Abstract In this review we discuss the principal aspects of the polymerization of micelle-forming surfactants in the aggregated state. The effect of monomer aggregation-organization on polymerization and polymer products has been studied on several occasions [1–3], such as thermotropic and lyotropic liquid crystals [4, 5], liquid—liquid interfaces [6–8], micelles [9–11], vesicles [12], etc. Such monomer aggregation-organization may produce important modifications in polymer structure and tacticity, in rates of polymerization kinetics, etc. In this sense, micellar polymerization means that the micelle is not merely the medium in which polymerization takes place, as is the case in emulsion polymerization, but instead the micelle-forming surfactant monomers themselves underero polymerization.

Effect of substituents on the spectroscopic properties of all-trans-1,6-diphenyl-1,3,5-hexatriene

Abstract An extensive spectroscopic study of two derivatives of the all- trans -1,6-diphenyl-1,3,5-hexatriene (DPH) has been conducted. The one derivative carries, at the para position of one of the phenyl rings of the parent molecule, the electron donating –N–(CH 3 ) 2 group, while the other carries at the same position the electron withdrawing –NO 2 substituent. The absorption–fluorescence spectra, quantum yields, lifetimes and steady state fluorescence anisotropies of these derivatives, were studied in various solvents, at different temperatures and at different viscosities. The findings are presented and discussed along with the exceptional fluorescence probing properties of these two particular derivatives. For comparative purposes, the main spectroscopic properties of the parent molecule are also included.

Simultaneous analysis of single-photon timing data with a reference method: Application to a poisson distribution of decay rates

Abstract The performance of the simultaneous nonlinear least-squares analysis of multiple fluorescence decay curves is examined. The physical model describing the system corresponds to a Poisson distribution of decay rates. Such a model is assumed to describe the fluorescence decay of an excited probe, solubilized in the micelle and quenched by quenchers that are Poisson distributed over the micelles. Using synthetic data sets, it is shown that the global analysis with the reference convolution method is superior to the single curve analysis both in the recovery of model parameters and in its model testing capability. The applicability of this simultaneous analysis approach is further demonstrated by real experimental data obtained for quenching of the 1-methylpyrene fluorescence by the immobile quencher N-tetradecylpyridinium chloride in aqueous dodecyltrimethylammonium chloride (DTAC) micelles. The mean aggregation number of DTAC at 20 °C determined by global analysis was 47±1.

Simultaneous analysis of fluorescence decay curves for the one-step determination of the mean aggregation number of aqueous micelles

Abstract The practical applicability of the global non-linear least-squares analysis of multiple fluorescence decay curves is examined using real micellar decay data. Fluorescence decays of 1-methylpyrene quenched by the “immobile” quencher N-tetradecylpyridinium chloride in sodium dodecyl sulphate (SDS) micelles were measured using the single-photon timing method with reference convolution. Decays with 103 and 104 peak counts were analyzed individually and simultaneously. The globally determined mean aggregation number (Nagg = 69) of SDS is in excellent agreement with literature values determined from single curve analyses.

Limitations on the determination of the stoichiometry and equilibrium constants of weak complexes by computer fitting methods: experimental verification

Abstract We present experimental evidence showing that computer fitting methods are inadequate for the study of weak complexations, whenever a certain equation relates the reaction parameters. In such cases, one must resort to direct (non-computational) methods for the determination of the stoichiometry of the interaction and the equilibrium constants. Our data refer to complex formation between cyclodextrins and diphenylpolyenes, and we show that differentiation between the 1/1 and 2/1 stoichiometries, by computer fitting, is not always possible. The direct method employed here to determine the correct interaction parameters was primarily time-resolved fluorescence spectroscopy.

On the determination of the stoichiometry and equilibrium constants of weak complexations

Abstract In this Letter, we present a criticism of the use of the (non-linear) least-squares fitting methods, as well as the so-called double-reciprocal plots, for determination of the stoichiometry and equilibrium constants of reactions involving weak complex formations. We show that, contrary to what is generally believed, these methods do not always suggest the correct parameters of such reactions. In particular, differentiation between 1/1 and 2/1 stoichiometries is impossible in many cases. The reason is that, in these cases, the fitting equations corresponding to the 1/1 and 2/1 stoichiometries turn out to be equivalent. Additional independent experimental evidence is therefore necessary in order to obtain reliable values for the complexation parameters.

The micelle-water monomer exchange process in solutions of ionic surfactants measured by transient fluorescence quenching

Abstract The effect of various structural factors on the dissociation rate constant k − of a quencher surfactant from a host micelle has been examined by the time-resolved fluorescence method.