Environmental analysis based on luminescence in organized supramolecular systems

The use of organized supramolecular systems—including micellar media and cyclodextrin inclusion complexes—combined with luminescence techniques in the study and determination of compounds and elements of environmental interest from 1990 to 2005 is reviewed. Analyses of environmental samples performed using fluorescence, photochemically induced fluorescence and phosphorescence spectroscopy as well as liquid chromatography, capillary electrophoresis and flow injection with luminescence detection in the presence of these organized media are described in detail.     

反相胶束介质中敏化双乙酰室温磷光——-α-萘乙酸为敏化剂

本文报道了AOT-C6H12-H2O反相胶束介质中α-萘乙酸(α-NAA)敏化双乙酰(BIAC)的室温磷光。详细讨论了琥珀酸二(2-乙基己基)酯磺酸钠(AOT)浓度和水泡大小(W值)对敏化磷光的影响。吸收、发光性质和微粘度性质的实验表明AOT浓度对敏化磷光强度的影响由敏化磷光寿命、能量转移效率和Poisson分布决定; 一定范围内, 随着水浓度增大, 由于粘度下降和内腔半径增大作用的相互抵销, 水泡大小仅有微弱影响。当W([H2O]/[AOT])大于20后, 内腔半径增大起主要作用, 敏化磷光强度快速下降。与普通SDS胶束相比, 磷光强度约增强13倍, 检出限约下降一个数量级。建立了灵敏的测定α-萘乙酸和双乙酰的敏化室温磷光法, 检出限分别达2.0×10^-^8mol.dm^-^3(α-NAA)和8.5×10^-^9mol.dm^-^3(BIAC)。     

Room temperature phosphorescence decay of metal chelates in micellar media

The characteristics of the decay of the triplet state of metal chelates have been investigated in micellar media using room temperature phosphorescence (RTP) measurements of the Al-ferron (7-iodo-8-hydroxyquinoline-5-sulfonic acid) complex in cetyltrimethylammonium bromide (CTAB) micelles. Applications of such decay measurements include discrimination between metal complex species and evaluation of the stoichiometry of phosphorescent metal chelates. A thorough study of the effect of foreign ions (interferences) on the photochemical characteristics of RTP of the aluminum complex leads to a tentative classification of foreign cations. Results are compared with those for the Nb(V)-ferron phosphorescent complex in CTAB micelles and possible reaction mechanisms in the presence of micellar media are outlined in an effort to contribute to basic knowledge of micelle stabilized RTP of such inorganic ions.     

Recent advances and future prospects in fluorescence and phosphorescence spectroscopy

Various approaches which manipulate the physical and chemical microenvironment of lumiphors are outlined and examples given of surfactant, microcrystalline and cyclodextrin molecular organization. These methodologies can give improved selectivity, and in some cases, enhanced fluorescence and/or phosphorescence intensities. Complimentary methods of generating luminescence via targeted energy transfer, namely, sensitized room temperature phosphorescence and chemiluminescence, are briefly discussed. The power of non-clasical luminescence techniques to produce more useful analytical results is illustrated for micelle-enhanced/sensitized room temperature phosphorescence, a liquid chromatographic/micelle-stabilized phosphorescence detector, and synchronous wavelength scanning/second derivative/micelle-stabilized phosphorescence. The combination of computer-assisted instrumentation and organized chemical microenvironments to obtain the total luminescence spectral profile should provide attomolar sensitivities, and selectivity without prior separation of mixtures on a more routine basis in the near future.     

Studies of cyclodextrin-enhanced room-temperature phosphorescence

The enhancement of room-temperature phosphorescence by α-, β- and γ-cyclodextrins in the presence of heavy atoms is described for p-aminobenzoic acid, anthracene and six of its derivatives, dibenzofuran and some other compounds. The sensitivity can be improved by treating the filter paper substrate with the cyclodextrin (preferably β-cyclodextrin) or by mixing the analyte with β-cyclodextrin prior to sample spotting on the substrate.     

Photolysis of 1,2-dihydroquinolines in micellar solutions of anionic and cationic surfactants

The kinetics of the photolysis of substituted 1,2-dihydroquinolines (DHQ) in micellar solutions was studied by steady-state and flash photolysis. The photolysis mechanism depends dramatically on the location of DHQ molecules in micelles, which is governed by the surfactant nature. In micellar solutions of the anionic surfactant sodium dodecyl sulfate (SDS), where the DHQ molecules are located in the Stern layer, the intermediate species decay kinetics follows a first-order law. When DHQ is in neutral form (pH 4–12), the rate constant of the intermediate carbocation decay increases from 25 to 198 s^?1 with an increasing concentration of DHQ in micelles. The positive micellar catalysis is caused by the acceleration of the final product formation with the DHQ molecule via proton abstraction from the intermediate cation. The formation of several types of intermediate species—carbocations in the aqueous phase and aminyl radicals in micelles—is observed in micellar solutions of the cationic surfactant cetyltrimethylammonium bromide (CTAB) due to the preferential location of DHQ molecules in the micellar core. The carbocation decays via a pseudofirst-order reaction with a rate constant close to that in the aqueous solution. The lifetime of the DHQ aminyl radicals in the micellar solutions is longer by several orders of magnitude than the lifetime observed for homogeneous solutions of hydrocarbons and alcohols.     

Some aspects of room-temperature phosphorescence in liquid solutions

Experimental requirements for room-temperature phosphorescence measurements in liquids (RTPL) are discussed. Phosphorescence quantum yields and triplet lifetimes of some brominated naphthalenes and halogenated biphenyls at 77 K in 2-methyltetrahydrofuran and at room temperature in hexane are reported and compared. Surprisingly the naphthalenes show only little loss in quantum yields in going from 77 K to room temperature. Sensitized phosphorescence is discussed as a means of expanding the analytical potential of RTPL. Results with a model system of benzophenone as a donor (analyte) and 1,4-dibromonaphthalene as an acceptor are presented.     

Determination of naproxen in pharmaceutical preparations by room-temperature phosphorescence. A comparative study of several organized media.

Different methods for the determination of naproxen by room-temperature phosphorescence (RTP) using organized media such as cyclodextrins (beta-CD and gamma-CD) and micelles (Triton X-100 and sodium dodecyl sulfate) are reported. The inclusion complexes formed between both beta- and gamma-cyclodextrins and naproxen were previously investigated at both acid and basic pH by spectrofluorimetry. In both cases, 1:1 guest-host stoichiometries were established and the corresponding association constants were calculated. Different systems were examined with the purpose of obtaining phosphorescent emission from naproxen solutions, and the best signals were obtained when naproxen was in the presence of beta-CD-cyclohexane-Tl(I), gamma-CD-1,3-dibromopropane, Triton X-100-Tl(I) and SDS-Tl(I), respectively. In all cases, sodium sulfite was used as deoxygenator. The use of an inorganic compound (thallium nitrate) as a heavy-atom source in a cyclodextrin system represents a novel finding. Surface response optimization approaches were carried out to optimize the chemical variables which have an influence on the RTP emission of naproxen. Based on the results obtained, univariate RTP calibration methods for the determination of the analyte in pharmaceutical preparations were satisfactorily developed. In one case, the standard additions method was applied to a mixture of naproxen and the antibiotic tetracycline.     

Structural characterization of micelles formed of cholesteryl-functionalized cyclodextrins

Amphiphilic cholesteryl 2,6-di-O-methyl-β-cyclodextrins (chol-DIMEB) can self-aggregate into spherical micelles of noteworthy potential for drug delivery. All-atom molecular dynamics simulations of chol-DIMEB micelles consisting of 3-24 monomers have been performed in aqueous solution. chol-DIMEB exhibits a pronounced tendency to self-assemble into core-shell structures. van der Waals interactions within the cholesteryl nucleus constitute the main driving force responsible for the formation of the micelle. The calculated radii of the hydrophobic core and of the hydrophilic shell for the micellar structure formed by 24 monomers agree well with the experiment. The cyclodextrin moieties are found to be exposed toward the aqueous medium and possess the appropriate flexibility to capture drugs in an effective fashion. Analysis of the solvent accessible surface area and hydration number indicates that the micelles are highly hydrosoluble species and can, therefore, enhance significantly the aqueous solubility of lipophilic drugs. In addition, the spatial structure of the micelles is suggestive of multiple potential drug binding sites. The present contribution unveils how micelles endowed with specific characteristics can form, while opening exciting perspectives for the design of novel micellar nanoparticles envisioned to be drug carriers of high potential.     

Synchronous wavelength scanning room temperature phosphorescence: Comparison of cyclodextrin and micellar media

Application of synchronous wavelength scanning to two fluid phase room temperature phosphorescence (RTP) techniques using β-cyclodextrin and micelles are evaluated and compared. The selectivity, sensitivity, susceptibility to light scattering interference and classes of compounds amenable to determination by the two RTP approaches are discussed. Synchronous wavelength scanning cyclodextrin RTP studies focus on its usefulness for identification of nitrogen heterocyclic phosphors. Spectral resolution is improved in both RTP techniques by second derivative manipulation of the digitally stored synchronous spectra. A significant scatter interference resulting from turbidity produced by the cyclodextrin inclusion complexes imposes constraints on the selectivity and sensitivity achievable with the cyclodextrin medium, especially when using synchronous wavelength scanning. Generally, micelle-induced RTP was found to be superior to cyclodextrin-induced RTP for analysis of various mixtures of the 12 heterocyclics and 15 carbocyclics studied, as well as for a mixture of the drug, propranolol, and its 4-hydroxy metabolite.     

Fluorescence and liquid room-temperature phosphorescence of the aluminium-ferron chelate in vesicles

In order to evaluate the analytical potential of vesicles for enhancing the room-temperature luminescence of metal chelates, the photoluminescence emission of the aluminium-ferron complex in vesicular solutions was examined. The presence of vesicles of didodecyldimethylammonium bromide produced a six-fold enhancement in the fluorescence intensity of the metal chelate. Vesicles also proved to be efficient stabilizers to obtain analytically useful liquid room-temperature phosphorescence for aluminium. Optimum experimental conditions and luminescence characteristics of the aluminium complex in the presence of vesicles are described and compared with those obtained for the same chelate in micellar media. As a result, a vesicle-enhanced fluorimetric procedure is proposed which has been successfully applied to the determination of low levels of aluminium in milk powders.     

Protolytic equilibria of 2-hydroxytropolone (4,5-d) thiophene in homogeneous and micellar solutions

The present study comprises an investigation of the optical absorption and fluorescence spectra of the title compound (HT) in homogeneous solutions of ethanol, cyclohexane, and sulphuric acid, and in aqueous micellar systems of anionic (NaLS), cationic (CTABr) and non-ionic (Triton X-100) surfactants.This compound behaves as monoprotic acid in buffer solutions of pH = 1–13 containing 3% v/v ethanol. It has pK = 8.2, but in the first excited singlet state the p_K* drops to 3.9. However, another protolytic equilibrium is established with pK₂ = 0.45 and pK*₂ = 2.15 in concentrated H₂SO₄ solutions.Contrary to all other media studied, the dissociated form of HT was observed in CTABr micellar solution with an apparent red shift indicating that the HT molecules are incorporated into the detergent layer of the micelles and at the interface of the aggregates.The influence of micellar solutions on the acid—base equilibrium of HT reveals that the effect of the charge distribution of the counter ions in the double layer is much larger than the effective dielectric constant at the site of solubilization.     

Fluorescence decay of singlet excited-state of safranine T and its interaction with ground-state of pyridinthiones in micelles and homogeneous media

The fluorescence decays of safranine T were studied in different homogeneous solvents and heterogeneous micellar solutions. It has been found that micellization leads to an increase in the lifetime. The lifetime distributions were studied in micelles and homogeneous media. It was found that the different half-width distributions of the dye in different micelles are related to the different orientation of the dye in different micelles. Also, the fluorescence quenching of safranine T by 4,6-disubstituted-3-cyanopyridin-2(1H)-thiones was studied in chloroform, methanol and acetonitrile as well as in different micelles. In heterogeneous media the kq values for quenching of safranine T by thiones in various micelles increase on the following order: kq(CTAB) < kq(TX-100) < kq(SDS). This is due to the electrostatic interactions between the anionic SDS and the cationic moiety of safranine T and therefore the quenching process will be less significant.     

Room temperature phosphorescence of 6-bromo-2-naphthol in micelles of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers and in their mixed aggregates with sodium dodecyl sulfate.

Room temperature phosphorescence (RTP) of 6-bromo-2-naphthol has been investigated in aqueous micellar solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers as well as in their mixed aggregates with sodium dodecyl sulfate. RTP of the phosphorophor was enhanced to some extent in the micelles of the block copolymers. However, marked enhancement of RTP was observed in the mixed aggregates. The enhancement of RTP is attributed to effective incorporation of the phosphorophor into the micelles and the aggregates, resulting in suppression of nonradiative deactivation of the phosphorescent state.     

Supramolecular assembly confined purely organic room temperature phosphorescence and its biological imaging

Purely organic room temperature phosphorescence, especially in aqueous solution, is attracting increasing attention owing to its large Stokes shift, long lifetime, low preparation cost, low toxicity, good processing performance advantages, and broad application value. This review mainly focuses on macrocyclic (cyclodextrin and cucurbituril) hosts, nanoassembly, and macromolecule (polyether) confinement-driven RTP. As an optical probe, the assembly and the two-stage assembly strategy can realize the confined purely organic RTP and achieve energy transfer and light-harvesting from fluorescence to delayed fluorescence or phosphorescence. This supramolecular assembly is widely applied for luminescent materials, cell imaging, and other fields because it effectively avoids oxygen quenching. In addition, the near-infrared excitation, near-infrared emission, and in situ imaging of purely organic room temperature phosphorescence in assembled confinement materials are also prospected.

Purely organic room temperature phosphorescence, especially in aqueous solution, is attracting increasing attention owing to its large Stokes shift, long lifetime, low preparation cost, low toxicity, good processing performance advantages, and broad application value.     

Correlation between phosphorescence lifetimes and ratios of steady state phosphorescence to fluorescence intensity for biacetyl in various solvents

The phosphorescence of biacetyl in fluid solutions is quenched not only by oxygen but also by impurities resulting from the distillation procedure and from the freeze-pump-thaw degassing technique. A linear correlation between the measured phosphorescence lifetimes and the ratios of the phosphorescence to the fluorescence intensities is established; the slope affords reliable calculations of the biacetyl triplet lifetimes from steady state measurements and additionally the determination of the rate constant k_p of radiative triplet deactivation. This constant is dependent on the solvent, whereas the fluorescence constant k_F is not.     

Pharmaceutical applications of micelles in chromatography and electrophoresis

This review surveys the use of micelles as separation media in chromatography and electrophoresis. Applications to pharmaceuticals whose molecular masses are relatively small are focused on in this review. In high-performance liquid chromatography (HPLC), chromatography using micelles and reversed-phase stationary phases such as octadecylsilylized silica gel (ODS) columns is known as micellar liquid chromatography (MLC). The main application of MLC to pharmaceutical analysis is the same as in ion-pair chromatography using alkylsulfonate or tetraalkylammonium. In most cases, selectivity is much improved compared with other short alkyl chain ion-pairing agents such as pentanesulfonate or octanesulfonate. Direct plasma/serum injection can be successful in MLC. Separation of small ions is also successful by using gel filtration columns and micellar solutions. In electrophoresis, especially capillary electrophoresis (CE), micelles are used as pseudo-stationary phases in capillary zone electrophoresis (CZE). This mode is called micellar electrokinetic chromatography (MEKC). Most of the drug analysis can be performed by using the MEKC mode because of its wide applicability. Enantiomer separation, separation of amino acids and closely related peptides, separation of very complex mixtures, determination of drugs in biological samples etc. as well as separation of electrically neutral drugs can be successfully achieved by MEKC. Microemulsion electrokinetic chromatography (MEEKC), in which surfactants are also used in forming the microemulsion, is successful for the separation of electrically neutral drugs as in MEKC. This review mainly describes the typical applications of MLC and MEKC for the analysis of pharmaceuticals.     

Micellar medium effects on the hydrolysis of phenyl chloroformate in ionic,zwitterionic, nonionic,and mixed micellar solutions

The spontaneous hydrolysis of phenyl chloroformate was studied in various anionic, nonionic, zwitterionic, and cationic aqueous micellar solutions, as well as in mixed anionic–nonionic micellar solutions. In all cases, an increase in the surfactant concentration results in a decrease in the reaction rate and micellar effects were quantitatively explained in terms of distribution of the substrate between water and micelles and the first‐order rate constants in the aqueous and micellar pseudophases. A comparison of the kinetic data in nonionic micellar solutions to those in anionic and zwiterionic micellar solutions makes clear that charge effects of micelles is not the only factor responsible for the variations in the reaction rate. Depletion of water in the interfacial region and its different characteristics as compared to bulk water, the presence of high ionic concentration in the Stern layer of ionic micelles, and differences in the stabilization of the initial state and the transition state by hydrophobic interactions with surfactant tails can also influence reactivity. The different deceleration of the reaction observed in the various micellar solutions studied was discussed by considering these factors. Synergism in mixed‐micellar solutions is shown through the kinetic data obtained in these media. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 445–451, 2002     

Cage reactions in the photolysis of 2,2,4,4-tetramethyl-3-pentanone

The product quantum yields in the photolysis of 2,2,4,4-tetramethyl-3-pentanone have been measured in homogeneous solvents of different viscosities, in micellar solutions of cetyltrimethylammonium chloride and sodium dodecyl sulfate, and in dioctadecyl ammonium chloride vesicles. The product quantum yield in n-heptane was found to be 1. This value decreases to 0.5 in paraffin oil as a consequence of geminate recombination. In the presence of free radical scavengers, the extent of geminate disproportionation can be evaluated from the yields of isobutene and 2,2-dimethyl propionaldehyde. From these yields and the geminate recombination yields the total amount of geminate processes and the disproportionation-to-combination ratio for caged radicals are estimated. It is found that micelles provide the most efficient cages. In these media only about 10% of the radicals avoid cage processes. The disproportionation-to-combination ratio of tert-butyl and pivaloyl radicals was found to be extremely media dependent. The measured values ranged from about 0.2 in paraffin oil to 0.8 in cetyltrimethylammonium chloride micelles.     

Micellar effects on photoprocesses in retinyl polyenes

The effects of micellar solubilization on excited-state properties of several retinyl polyenes have been examined primarily by nanosecond laser flash photolysis. The relative intensity of band system III (254–256 nm) in the ground state absorption spectrum of 11-cis retinal decreases significantly on going from methanol to micellar solutions, suggesting that the 12-s-trans form of 11-cis retinal is relatively favored in the organized media. In addition to microsecond transient phenomena due to triplets, the laser flash photolysis of all-trans and 11-cis retinal and all-trans retinyl Schiff base incorporated into micelles leads to ‘permanent’ absorption changes attributable to photoisomerization (in the case of retinals) and protonation and/or complexation with water (in the case of Schiff base). All-trans retinol and retinyl acetate in micellar solutions undergo ionic photodissociation leading to long-lived retinyl carbocation (λ_max = 585–600 nm), the process being monophotonic in the case of retinyl acetate and predominantly bipho-tonic in the case of retinol. The trends in the location of ground-state absorption maxima (^IB_u^+*←^IA_g) and triplet yield of retinals, and photodissociation yield of retinyl acetate suggest that the polarity of the environment probed by the polyene systems increases in the order: Triton X-100 < CTAB < NaLS.