Femtosecond/picosecond time-resolved fluorescence study of hydrophilic polymer fine particles

Femtosecond/picosecond time-resolved fluorescence study of hydrophilic polymer fine particles (polyacrylamide, PAAm) was reported. Ultrafast fluorescence dynamics of polymer/water solution was monitored using a fluorescent probe molecule (C153). In the femtosecond time-resolved fluorescence measurement at 480 nm, slowly decay components having lifetimes of tau(1) approximately 53 ps and tau(2) approximately 5 ns were observed in addition to rapid fluorescence decay. Picosecond time-resolved fluorescence spectra of C153/PAAm/H2O solution were also measured. In the time-resolved fluorescence spectra of C153/PAAm/H2O, a peak shift from 490 to 515 nm was measured, which can be assigned to the solvation dynamics of polymer fine particles. The fluorescence peak shift was related to the solvation response function and two time constants were determined (tau(3) approximately 50 ps and tau(4) approximately 467 ps). Therefore, the tau(1) component observed in the femtosecond time-resolved fluorescence measurement was assigned to the solvation dynamics that was observed only in the presence of polymer fine particles. Rotational diffusion measurements were also carried out on the basis of the picosecond time-resolved fluorescence spectra. In the C153/PAAm/H2O solution, anisotropy decay having two different time constants was also derived (tau(6) approximately 76 ps and tau(7) approximately 676 ps), indicating the presence of two different microscopic molecular environments around the polymer surface. Using the Stokes-Einstein-Debye (SED) equation, microscopic viscosity around the polymer surface was evaluated. For the area that gave a rotational diffusion time of tau(6) approximately 76 ps, the calculated viscosity is approximately 1.1 cP and for tau(7) approximately 676 ps, it is approximately 10 cP. The calculated viscosity values clearly revealed that there are two different molecular environments around the polyacrylamide fine particles.     

Surface enhanced fluorescence and Raman imaging of Langmuir-Blodgett azopolymer films

The spectroscopic properties and surface-enhanced spectra of Langmuir-Blodgett (LB) films of methacrylic homopolymer (HPDR13) are presented. It is shown that LB film displays strong fluorescence attributed to the spatial restrictions imposed by its structure. The emission is observed in conjunction with photoisomerization, a process clearly demonstrated by the formation of surface-relief gratings in the LB film [C.R. Mendon?a et al., Macromolecules 32 (1999) 1493]. Surface-enhanced Raman scattering (SERS), Surface-enhanced resonance Raman scattering (SERRS) and surface-enhanced fluorescence (SEF) were observed for LB films of HPDR13 deposited onto silver island films. SERS measurements were also carried out on a sample fabricated with one monolayer LB film deposited onto silver islands followed by one overlayer of silver (LB sandwiched between two layers of silver islands). The polymer interacts very weakly with the metal surface (physisorption), and the enhancement effect is determined by the local electric field enhancement. The strong SERS and SERRS signals were suitable for micro-Raman imaging. Line, area mapping and global images of the LB monolayer on silver island are reported. The transfer ratio in the fabrication of the LB suggests a homogeneous coating of the silver islands, thereby the chemical images show the variation of the SERS intensity due to surface enhancement.     

Characterization of Langmuir-Blodgett films of a ferroelectric liquid crystal

Molecular orientation, structure, and phase transition behaviors in Langmuir-Blodgett (LB) and cast films of a ferroelectric liquid crystal of sec-butyl 6-(4-(nonyloxy)benzoyloxy)-2-naphthoate (FLC-1) are determined by ultraviolet (UV) spectroscopy, X-ray diffraction, and infrared (IR) spectroscopy. It is found that the orientation angle of chromophores theta in LB films is 41 degrees from the surface normal. The tilt angle of the chromophore changes at 56, 70, and 88 degrees C, respectively, which denotes the presence of phase transitions. Two kinds of layered or isomeric crystal structures of the LB films with layer spacings of 3 and 3.5 nm at room temperature have been found while the latter disappears above 45 degrees C, as confirmed by measurement of temperature-dependent IR spectra.     

Characterization of IgG Langmuir-Blodgett films immobilized on functionalized polymers

The bioactivity of anti-human IgG Langmuir-Blodgett (LB) films, the non-specific adsorption of protein and the topography of anti-IgG LB films have been studied for application in immunosensors. The antibody (AB) LB films were horizontally deposited on glass and functionalized polymers, such as carboxy-poly(vinyl chloride) (PVC-COOH), chloropropyl and aminopropyl sol-gel. The LB films were characterized by means of ellipsometry, atomic force microscopy (AFM) and bicinchoninic acid (BCA) protein test. The interpretation of ellipsometric data was performed using a one-layer model. Non-specifically adsorbed protein was desorbed by washing the IgG film in 0.5 M NaCl, 2 M NaCl and 1% N-cetyl-N,N,N-trimethylammoniumbromide detergent solution resulting in a 50% reduction of the film thickness. The mean thickness of an anti-IgG film on glass measured by ellipsometry, PVC-COOH and aminopropyl sol-gel was 9+/-2, 11+/-1 and 23+/-8 nm, respectively. According to the BCA test 6-8 mug antibody (AB) per slide was bound to the functionalized polymers, but only 3 mug AB per slide was adsorbed on glass. The average distance of anti-IgG granules as indicated by AFM measurements on PVC-COOH, chloropropyl and aminopropyl sol-gel was 42+/-20, 34+/-3 and 23+/-4 nm. The average distance of granular AB structures on glass, however, was 150+/-50 nm.     

Polyelectrolyte templating strategy for the fabrication of multilayer hemicyanine Langmuir-Blodgett films showing enhanced and stable second harmonic generation

Polyelectrolyte templating effectively suppresses the aggregation of cationic hemicyanine-based amphiphiles in monolayer Langmuir-Blodgett (LB) films leading to enhanced and stable optical second harmonic generation (SHG). The current study explores the impact of different polyelectrolytes (salts of poly(4-styrenesulfonic acid), deoxyribonucleic acid, and carboxymethylcellulose) on the mode of formation of multilayer LB films of the hemicyanine amphiphile and their SHG response. Pressure-area isotherms and Brewster angle microscopy reveal the impact of the polyelectrolyte complexation on the Langmuir films. Transfer ratios observed during film deposition, supported by electronic absorption spectra and atomic force microscope images of the multilayer LB films, suggest that the polyanions influence the deposition sequence, leading to significant variations in the SHG. Carboxymethylcellulose is identified as an optimal template that induces favorable z-type deposition, leading to the formation of stable multilayer films. These films exhibit the expected quadratic increase of SHG with the extent of deposition; significantly the film response is very stable under extended laser irradiation. It is proposed that structural adjustments of the sandwiched polymer layer lead to the observed deposition sequence and film stability. Polyelectrolyte templating is demonstrated to be a simple and effective strategy for the fabrication of multilayer LB films to elicit efficient quadratic nonlinear optical response.     

The ultra-fast process of picosecond time-resolved energy transfer in liquid cyclohexane by picosecond single-pulse radiolysis

Two formation processes of singlet states of solute molecules in irradiated liquid cyclohexane have been observed by using a single picosecond electron pulse-radiolysis technique. The slower process corresponds to the very fast formation process reported by Beck et al. The faster process is an ultra-fast process which finishes immediately after a picosecond electron pulse, and is hardly quenched by CCl₄ on triethylamine. The ratio of the faster component to the slower component in liquid cyclohexane is very different from that in liquid toluene.     

Persistence of ordering in 4-n-pentyl-4'-cyanobiphenyl above the nematic-isotropic transition as detected by picosecond time-resolved fluorescence spectroscopy

The orientational ordering of 4-n-pentyl-4'-cyanobiphenyl (5CB) was explored with the aid of picosecond, time-resolved fluorescence spectroscopy by monitoring the time dependence of excimer formation of the mesogen at various temperatures. It was found that in the nematic phase the excimer is formed with high efficiency, while in the isotropic phase the excimer formation was less effective. Immediately after pulse excitation the time-resolved fluorescence spectra revealed that short range ordering persists just above the N-I transition, although the macroscopic order had disappeared.     

Europium speciation by time-resolved laser-induced fluorescence

Time-resolved laser-induced fluorescence has been used to investigate Eu complexes formed with a few main ligands encountered in natural waters: hydroxide, carbonate and humic substances. By varying pH and concentrations of ligands at fixed europium concentration and ionic strength, it was possible, together with free europium Eu³⁺, to identify spectrally and temporally carbonate complexes, namely Eu(CO₃)⁺, Eu(CO₃)₂⁻ and Eu(CO₃)₃³⁻ and humate complexes EuHA. For hydroxide complexes, no differences were found in terms of fluorescence spectra and lifetimes. A spectral interpretation is described by using deconvolution for all systems.     

Characterization of Langmuir and Langmuir-Blodgett films of a thiomacrocyclic ionophore by surface pressure and AFM

The new synthesized thiomacrocyclic ionophore 4-phenyl-4-sulfide-11-(1-oxodecyl)-1,7-dithia-11-aza-4-phosphacyclotetradecane has proven to form Langmuir and Langmuir-Blodgett (LB) films. This ionophore shows a large affinity for copper(II) ions. Thus, the influence of the subphase composition on the surface pressure-area isotherms has been studied. The LB films have been observed by AFM and the effect of the subphase composition and the deposition surface pressure on the LB films is discussed. AFM image morphology has been correlated to the ionophore molecular structure. Surface pressure-area isotherms and AFM images show that the presence of copper(II) ions has an important role in the film structure.     

Distance-dependent metal-enhanced fluorescence from Langmuir-Blodgett monolayers of alkyl-NBD derivatives on silver island films

We described the effect of fluorophore distance from the silver island films (SIFs) on the metal-enhanced fluorescence (MEF) from two newly developed long-chain nitrobenzoxadiazole derivatives (NBD-C16 and NBD-C18). The well-established Langmuir-Blodgett technique is used to deposit the fluorophores at defined distances from the SIFs surface, and an inert amphiphilic stearic acid is used to control the distance. NBD probes deposited directly on the SIFs surface show the highest metal-enhanced fluorescence of approximately 32-fold, and both of the probes that were studied show a consistent decrease in metal-enhanced fluorescence when increasing the distance from the fluorophore to the SIFs surface. The lowest fluorescence enhancement of approximately 4-fold is observed for the probes located 90 nm from the SIFs surface. Additionally, we also have noticed the shortest fluorescence lifetimes for the NBD probes deposited directly onto the SIFs surface, and the lifetimes are consistently increased when increasing the distances between the fluorophore and SIFs surfaces. These contrasting spectral changes, enhanced fluorescence, and decreased fluorescence lifetimes are in accordance with an increase in the rate of radiative decay for fluorophores near the silver particles. The present study provides significant information on the effect of fluorophore distance on the metal-enhanced fluorescence phenomenon.     

Langmuir-Blodgett films formed by continuously varying surface pressure. Characterization by IR spectroscopy and epifluorescence microscopy

Monolayer films of phospholipids at the air-water interface have been transferred to solid substrates under conditions of continuously varying surface pressure, an approach termed COVASP. The molecular and supramolecular properties of the film constituents have been characterized with two complementary techniques. IR spectroscopy was used to monitor chain conformation as a function of transfer surface pressure. Results were compared to those from Langmuir films determined directly at the A/W interface by IR reflection-absorption spectroscopy (IRRAS). The methylene stretching frequencies for both proteated and acyl chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC and DPPC-d62) in the transferred molecules indicate that the phospholipids retain at least, in part, their surface pressure-dependent chain-conformational order characteristics. The line widths of these modes are somewhat reduced, suggestive of slower rates of reorientational motion in the Langmuir-Blodgett (LB) films. Epifluorescence microscopy reveals a progressive condensation gradient, including nucleation and growth of probe-excluding condensed domains along the transfer line. DPPC condensation, observed along a single LB film, was qualitatively comparable to compression-driven condensation as observed in situ or in conventional LB films transferred at constant pressures. However, condensation along the compression isotherm in COVASP-LB films was reduced by 15-20% as compared to films equilibrated at different constant pressures, probably the result of kinetic differences in equilibration processes. As a preliminary demonstration of the utility of this new approach, the monolayer --> multilayer transition known to occur (Eur. Biophys. J. 2005, 34, 243) in a four-component model for pulmonary surfactant has been examined. IR parameters from both the lipid and the protein constituents of the film all indicate that the transition persists during the transfer process. This new approach for the study of transferred films will permit the efficient characterization of lipid-protein interactions and structural transitions occurring in pulmonary surfactant films subjected to dynamic compression.     

Don't forget Langmuir-Blodgett films

Langmuir-Blodgett (LB) films have been investigated, extensively, over the past 70 years. Despite considerable efforts aimed at exploiting their nonlinear optical, piezoelectric, pyroelectric, semiconducting, sensing and barrier properties, problems associated with film quality and stability have hampered their practical development. This feature article highlights one recent advance in the LB area (i.e., the ionic cross-linking of LB films or "gluing"), which has resulted in single bilayer membranes possessing extraordinary quality and stability. The gluing of LB films provides new opportunities for an old technology that has been waning in recent years.     

Dicyanopyrazine-linked porphyrin Langmuir-Blodgett films

We investigated the influence of arachidic acid/cadmium dication (AA/Cd(2+)) as a transfer promoter for the deposition of dicyanopyrazine-linked porphyrin (2-DCPP) Langmuir-Blodgett (LB) films on both hydrophobic and hydrophilic substrates. In the case of LB deposition on a hydrophilic substrate, the presence of AA/Cd(2+) does not improve 2-DCPP LB deposition. The poor transfer in the case of the hydrophilic surface is believed to be due to 2-DCPP not wetting the surface during the down-stroke deposition, and this is not improved by the transfer agent. However, on a hydrophobic substrate, deposition of 2-DCPP is significantly improved by the presence of AA/Cd(2+). Comparison of the UV-visible spectrum of a 2-DCPP LB film with that of 2-DCCP dissolved in chloroform reveals that the Soret and Q bands for the 2-DCPP LB film are broadened and red-shifted due to aggregation of porphyrin rings in the LB film. UV-visible spectral changes and ellipsometry as a function of the number of deposition layers suggest continuous transfer of 2-DCPP/AA onto the hydrophobic substrate and reproducibility in the deposition process. The Soret and Q bands of the 2-DCPP LB film upon acid vapor exposure have also been investigated, and these measurements may have chemical sensor applications.     

Characterization of Langmuir-Blodgett organoclay films using X-ray reflectivity and atomic force microscopy

Monolayers of organoclay platelets were formed at the air/water interface using the Langmuir technique and were then investigated either by in situ or lifted onto Si wafers and studied ex situ, using X-ray reflectivity (XR) methods. The XR data showed that the surfactant molecules on the clay platelets formed a dense, self-assembled monolayer where the molecules were tilted at an angle of 35 degrees +/-6 degrees from the normal to the dry clay surface. The surfactant layers only covered a fraction of the clay platelet surface area, where the fractional surface coverage for the three clays studied (C6A, C15A, and C20A) was found to be 0.90, 0.86, and 0.73, respectively. These values were significantly higher than those estimated from the cation exchange capacity (CEC) values. Rather than being uniformly distributed, the surfactant was clustered in patchy regions, indicating that the surface of the clay platelets had both polar and non-polar segments. This heterogeneity confirmed the hypothesis which was previously invoked to explain the distribution of the clay platelets in melt mixed homopolymer and polymer blend nanocomposites.     

Langmuir-Blodgett films of diazobenzene molecules

Langmuir-Blodgett (LB) films from diazobenzene Sudan III have been investigated using surface potential measurements as a function of number of layers and deposition pressures, with the surface potential data being related to molecular dipole moments obtained from theoretical electronic structure calculations. The surface potential increased with the number of layers for SIII LB films, and then tended to saturate. Results from density functional theory (DFT) and UV-vis spectroscopy indicated that the increase is due to addition of layers with oriented molecular dipoles, with the saturation tendency being attributed to a decrease in the amount of material deposited in each layer. The surface potential increased with the surface pressure used for deposition, probably owing to a higher contribution from the vertical component of the dipole moment as a closer molecular packing, which is associated with decreasing conformational entropy, was reached.     

Investigation of adsorption behavior and energy transfer of cationic porphyrins on clay surface at low loading levels by picosecond time-resolved fluorescence measurement

Time-resolved fluorescence and steady-state spectroscopic measurements were performed with +4-charged cationic porphyrins adsorbed on an anionic-type clay (Sumecton SA; SSA) surface at a low molecular loading level (10 % vs. cation-exchange capacity of clay) corresponding to an occupied area of ca 50 nm² per molecule. Absorption spectra indicated no interaction between transition moments of the porphyrins on the clay surface. An efficient energy-transfer process from donor to acceptor porphyrin was observed on the clay surface even under low porphyrin loading conditions. The efficiency of energy transfer obtained from the steady-state measurement was 65 %. Real-time behavior of the porphyrins was successfully captured during energy transfer. The rate constant of the energy transfer obtained from time-resolved fluorescence measurements was found to be 5.3 × 10⁸ s^?1. According to the efficiency and the rate constant, it is proposed that the adsorbed porphyrins did not have a uniform and fixed distribution.     

Characterization of carboxylate sites in surface oxidized polyethylene films by fluorescence techniques

Oxidized reactive sites introduced by controlled chemical reactions in non-crystalline regions of semi-crystalline polyolefin films are located mainly on the film surface or in adjacent subsurface regions. The so-formed reactive sites in low density polyethylene (LDPE) blown films were labelled by fluorescent groups and investigated by various fluorescence techniques. The surface or subsurface location of the labelled site is indicated by a solvatochromic spectral shift and confirmed by the polarity dependent fluorescence lifetime of the labelled LDPE film immersed in a polar liquid. Fluorescence concentration quenching and concentration depolarization demonstrate the high density of oxidized sites near the film surface. The measurement of rotational mobility and orientation during film stretching adds further experimental tools to distinguish reactive oxidized sites in the surface or in the bulk.     

Elastic theory for Langmuir-Blodgett films

An elastic theory for Langmuir-Blodgett films is presented. The expression for the free energy density of the film is obtained in terms of the deformation tensor by means of symmetry considerations and by making use of a quasi-microscopic model. The influence of a linear term in the deformation tensor in the elastic energy density is analysed. It is shown that when the elastic constant of the linear term is larger than a critical value, the ground state of the film is periodically distorted.     

Polyoxometalate monolayers in Langmuir-Blodgett films

Langmuir and Langmuir-Blodgett (LB) films of a variety of polyoxometalates of different shapes, sizes, and charges were prepared by taking advantage of the adsorption properties of these polyanions on a positively charged monolayer of an organic surfactant spread on water. Three different aspects were investigated. 1) The electrochemical and electrochromic properties of LB films containing the easily reducible polyoxoanion [P2Mo18O62]6-. Absorbance changes of these LB films deposited onto an ITO substrate have been induced by repeated switching of the applied potential. These changes are due to the formation of the colored reduced forms of the polyanion. Coloration and bleaching of the LB film occur very quickly and are reversible. 2) The preparation of LB films based on magnetic polyoxometalates, such as the Keggin anions, [CoW12O40]6- and [SiMn(OH2)W11O39]6-, or containing magnetic clusters of increasing nuclearities such as [Co4(H2O)2(PW9O34)2]10- and [Co4(H2O)2(P2W15O62)2]16- based on a Co4O16 ferromagnetic cluster, and the polyoxometalates [Co9(OH)3(H2O)6(HPO4)2(PW9O34)3]16- and [Ni9(OH)3-(H2O)6(HPO4)2(PW9O34)3]16- based on a nonanuclear M9O36 cluster. 3) The preparation of LB films of the giant heteropolyoxomolybdate, [Na3(NH4)12][Mo57Fe6(NO)6O174(OH)3-(H2O)24]76 H2O.