Supramolecular Large Nanosheets Assembled at Air/Water Interfaces and in Solution from Amphiphilic Heptagon-Containing Nanographenes

We report the synthesis of a new set of amphiphilic saddle-shaped heptagon-containing polycyclic aromatic hydrocarbons (PAHs) functionalized with tetraethylene glycol chains and their self-assembly into large two-dimensional (2D) polymers. An in-depth analysis of the self-assembly mechanism at the air/water interface has been carried out, and the proposed arrangement models are in good agreement with the molecular dynamics simulations. Quite remarkably, the number and disposition of the tetraethylene glycol chains significantly influence the disposition of the PAHs at the interface and conditionate their packing under pressure. For the three compounds studied, we observed three different behaviors in which the aromatic core is parallel, perpendicular, and tilted with respect to the water surface. We also show that these curved PAHs are able to self-assemble in solution into remarkably large sheets of up to 150 μm2. These results show the relationship, within a family of curved nanographenes, between the monomer configuration and their self-assembly capacity in air/water interfaces and organic-water mixtures.

Amphiphilic polymers for aggregation-induced emission at air/liquid interfaces

Polymersomes and related self-assembled nanostructures displaying Aggregation-Induced Emission (AIE) are highly relevant for plenty of applications in imaging, biology and functional devices. Experimentally simple, scalable and universal strategies for on-demand self-assembly of polymers rendering well-defined nanostructures are highly desirable. A purposefully designed combination of amphiphilic block copolymers including tunable lengths of hydrophilic polyethylene glycol (PEG_m) and hydrophobic AIE polymer poly(tetraphenylethylene-trimethylenecarbonate) (P(TPE-TMC)_n) has been studied at the air/liquid interface. The unique 2D assembly properties have been analyzed by thermodynamic measurements, UV-vis reflection spectroscopy and photoluminescence in combination with molecular dynamics simulations. The (PEG)_m-b-P(TPE-TMC)_n monolayers formed tunable 2D nanostructures self-assembled on demand by adjusting the available surface area. Tuning of the PEG length allows to modification of the area per polymer molecule at the air/liquid interface. Molecular detail on the arrangement of the polymer molecules and relevant molecular interactions has been convincingly described. AIE fluorescence at the air/liquid interface has been successfully achieved by the (PEG)_m-b-P(TPE-TMC)_n nanostructures. An experimentally simple 2D to 3D transition allowed to obtain 3D polymersomes in solution. This work suggests that engineered amphiphilic polymers for AIE may be suitable for selective 2D and 3D self-assembly for imaging and technological applications.

Mimicking the bioelectrocatalytic function of recombinant CotA laccase through electrostatically self-assembled bioconjugates

Unprecedented 3D nanobiosystems composed of recombinant CotA laccases and citrate-stabilised gold nanoparticles have been successfully achieved by an electrostatic self-assembly strategy. The bioelectrochemical reduction of O2 driven by CotA laccase at the spore coat was mimicked. Consequently key insights into its bioelectrocatalytic function were unravelled.

Subtle chemical modification for enrichment of Fmoc-amino acid at a phospholipid interface

Amino acids including the Fmoc group (9-fluorenylmethyloxycarbonyl) are bioinspired molecules that display intriguing features in self-assembly and biological applications. The influence of a delicate chemical modification between Fmoc-F and Fmoc-Y on the interaction with a phospholipid surface was analyzed. Langmuir monolayers of the 1,2-dimyristoyl-sn-glycero-3-phosphate (DMPA) phospholipid were used to mimic the eukaryotic cell membrane. In situ Brewster angle microscopy and UV-vis reflection spectroscopy provided insights on the effect of the Fmoc-amino acid derivatives on the DMPA phospholipid. The formation of H-bonds between the Fmoc-Y and the DMPA molecules was assessed, demonstrating the crucial role of the hydroxyl group of Fmoc-Y in enhancing the interaction with biosurfaces.

A modest chemical modification of the Fmoc-amino acids led to enhanced interaction with a model surface for biomembrane.

Unravelling the 2D self-assembly of Fmoc-dipeptides at fluid interfaces

Dipeptides self-assemble into supramolecular structures showing plenty of applications in the nanotechnology and biomedical fields. A set of Fmoc-dipeptides with different aminoacid sequences has been synthesized and their self-assembly at fluid interfaces has been assessed. The relevant molecular parameters for achieving an efficient 2D self-assembly process have been established. The self-assembled nanostructures of Fmoc-dipeptides displayed significant chirality and retained the chemical functionality of the aminoacids. The impact of the sequence on the final supramolecular structure has been evaluated in detail using in situ characterization techniques at air/water interfaces. This study provides a general route for the 2D self-assembly of Fmoc-dipeptides.

Surface-Active Fluorinated Quantum Dots for Enhanced Cellular Uptake

Fluorescent nanoparticles, such as quantum dots, hold great potential for biomedical applications, mainly sensing and bioimaging. However, the inefficient cell uptake of some nanoparticles hampers their application in clinical practice. Here, the effect of the modification of the quantum dot surface with fluorinated ligands to increase their surface activity and, thus, enhance their cellular uptake was explored.

Fluorinated CdSe/ZnS quantum dots: Interactions with cell membrane

Fluorescent inorganic quantum dots are highly promising for biomedical applications as sensing and imaging agents. However, the low internalization of the quantum dots, as well as for most of the nanoparticles, by living cells is a critical issue which should be solved for success in translational research. In order to increase the internalization rate of inorganic CdSe/ZnS quantum dots, they were functionalized with a fluorinated organic ligand. The fluorinated quantum dots displayed an enhanced surface activity, leading to a significant cell uptake as demonstrated by in vitro experiments with HeLa cells. We combined the experimental and computational results of Langmuir monolayers of the DPPC phospholipid as a model cell membrane with in vitro experiments for analyzing the mechanism of internalization of the fluorinated CdSe/ZnS quantum dots. Surface pressure-molecular area isotherms suggested that the physical state of the DPPC molecules was greatly affected by the quantum dots. UV-vis reflection spectroscopy and Brewster Angle Microscopy as in situ experimental techniques further confirmed the significant surface concentration of quantum dots. The disruption of the ordering of the DPPC molecules was assessed. Computer simulations offered detailed insights in the interaction between the quantum dots and the phospholipid, pointing to a significant modification of the physical state of the hydrophobic region of the phospholipid molecules. This phenomenon appeared as the most relevant step in the internalization mechanism of the fluorinated quantum dots by cells. Thus, this work sheds light on the role of fluorine on the surface of inorganic nanoparticles for enhancing their cellular uptake.

Tailoring a compact and stable Langmuir bi-dimensional PbX-based layered perovskite film at the air-water interface and on solid support

The present work studies the stability of Langmuir organic-inorganic superlattice materials thin films consisting of layered perovskite-based films with controlled 2D framework as well as to design experimental conditions for increasing the efficiency of the organic-inorganic perovskite motif by mechanical stimulus. Therefore, a whole covering of the air/water interface by a compact and stable lead-based layered perovskite structure is pursued. A 2D layered perovskite-type hybrid structure of the form [(CH₃(CH₂)₁₉NH₃)₂(PbX₄)], X=Cl, and Br, in which, two-dimensional sheets stabilized by a inner bilayer of organic monoammonium cation matrix, is mechanically tailored by successive compression-expansion cycles. The formation of 2D molecular patterns has been characterized by ΔR, BAM, XRD and XPS.

Octadecyl-viologen Photooxidation in Surface Films: Macroscopic Contraction of Langmuir Monolayer by UV Irradiation

The effects of UV radiation on a viologen derivative, octadecylviologen (OV), in Langmuir monolayers at the air-aqueous solution interface and in Langmuir-Blodgett (LB) films have been investigated. Langmuir monolayers suffer a sharp contraction after UV irradiation, clearly visible by the drop in surface pressure or the loss of surface area observed in the surface pressure-area isotherms. The UV-vis reflection measurements reveal a deep change in the OV monolayer caused by a photochemical reaction, which suggests the pyridones formation as photoreaction products. LB films (Z type), before and after being irradiated with UV light, have been studied by using UV-vis absorption and infrared and X-ray photoelectron spectroscopy. The results confirm that after the photodegradation of the viologen films, the presence of oxygen results in the appearance of pyridones as reaction products. This article demonstrates that, in the absence of catalysts, the photooxidation of viologen surface films occurs only under a particular molecular organization imposed by the air-water interface.

UV-Vis Reflection-Absorption Spectroscopy at air-liquid interfaces

UV-Visible Reflection-Absorption Spectroscopy (UVRAS) technique is reviewed with a general perspective on fundamental and applications. UVRAS is formally identical to IR Reflection-Absorption Spectroscopy (IRRAS), and therefore, the methodology developed for this IR technique can be applied in the UV-visible region. UVRAS can be applied to air-solid, air-liquid or liquid-liquid interfaces. This review focuses on the use of UVRAS for studying Langmuir monolayers. We introduce the theoretical framework for a successful understanding of the UVRAS data, and we illustrate the usage of this data treatment to a previous study from our group comprising an amphiphilic porphyrin. For ultrathin films with a thickness of few nm, UVRAS produces positive or negative bands when p-polarized radiation is used, depending on the incidence angle and the orientation of dipole absorption. UVRAS technique provides highly valuable information on tilt of chromophores at the air-liquid interface, and moreover allows the determination of optical parameters. We propose UVRAS as a powerful technique to investigate the in situ optical properties of Langmuir monolayers.

Direct observation by using Brewster angle microscopy of the diacetylene polimerization in mixed Langmuir film

Mixed Langmuir monolayers of 10,12-Pentacosadiynoic acid (DA) and amphiphilic hemicyanine (HSP) have been fabricated at the air-water interface. The mixed monolayer has been proved to be completely homogeneous. The DA molecules are arranged in a single monolayer within the mixed Langmuir monolayer, as opposed to the typical trilayer architecture for the pure DA film. Brewster angle microscopy has been used to reveal the mesoscopic structure of the mixed Langmuir monolayer. Flower shape domains with internal anisotropy due the ordered alignment of hemicyanine groups have been observed. Given the absorption features of the hemicyanine groups at the wavelength used in the BAM experiments, the enhancement of reflection provoked by the absorption process leads to the observed anisotropy. The ordering of such groups is promoted by their strong self-aggregation tendency. Under UV irradiation at the air-water interface, polydiacetylene (PDA) has been fabricated. In spite a significant increase in the domains reflectivity has been observed owing to the modification in the mentioned enhanced reflection, the texture of the domains remains equal. The PDA polymer chain therefore grows in the same direction in which the HSP molecules are aligned. This study is expected to enrich the understanding and design of fabrication of PDA at interfaces.

Diacetylene mixed Langmuir monolayers for interfacial polymerization

Polydiacetylene (PDA) and its derivatives are promising materials for applications in a vast number of fields, from organic electronics to biosensing. PDA is obtained through polymerization of diacetylene (DA) monomers, typically using UV irradiation. DA polymerization is a 1-4 addition reaction with both initiation and growth steps with topochemical control, leading to the "blue" polymer form as primary reaction product in bulk and at interfaces. Herein, the diacetylene monomer 10,12-pentacosadiynoic acid (DA) and the amphiphilic cationic N,N'-dioctadecylthiapentacarbocyanine (OTCC) have been used to build a mixed Langmuir monolayer. The presence of OTCC imposes a monolayer supramolecular structure instead of the typical trilayer of pure DA. Surface pressure, Brewster angle microscopy, and UV-vis reflection spectroscopy measurements, as well as computer simulations, have been used to assess in detail the supramolecular structure of the DA:OTCC Langmuir monolayer. Our experimental results indicate that the DA and OTCC molecules are sequentially arranged, with the two OTCC alkyl chains acting as spacing diacetylene units. Despite this configuration is expected to prevent photopolymerization of DA, the polymerization takes place without phase segregation, thus exclusively leading to the red polydiacetylene form. We propose a simple model for the initial formation of the "blue" or "red" PDA forms as a function of the relative orientation of the DA units. The structural insights and the proposed model concerning the supramolecular structure of the "blue" and "red" forms of the PDA are aimed at the understanding of the relation between the molecular and macroscopical features of PDAs.

Aggregation and structural study of the monolayers formed by an amphiphilic thiapentacarbocyanine

Exotic assemblies with unique photophysical features can be built based on organic dyes at the air–liquid interface.

From two-dimensional to three-dimensional at the air/water interface: the self-aggregation of the acridine dye in mixed monolayers

The formation of well-defined supramolecular structures on the nanoscopic scale is a fundamental step in nanotechnology. The fine control of the layer-by-layer growth of the supramolecular assemblies at interfaces is most desirable. The collapse of a mixed monolayer composed of two surfactants in an equimolar ratio (the organic dye N-10-dodecyl acridine (DAO) and stearic acid (SA)) is analyzed herein. The collapse process of the DAO/SA mixed monolayer has been monitored using surface pressure-molecular area (π-A) and surface potential isotherms, UV-visible reflection spectroscopy, polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), Brewster angle microscopy (BAM), and synchrotron-based in situ X-ray reflectivity (XRR) measurements. The collapse of the DAO/SA mixed monolayer leads to an ordered trilayer. The growth of anisotropic 2D domains of micrometric size is observed during the formation of the trilayer, related to the ordering of the acridine polar headgroups. The trilayer is organized with the first and third monolayers displaying the polar headgroups pointing to the aqueous subphase, whereas the intermediate layer displays the polar headgroups pointing to the air. The trilayer is stabilized by the strong self-aggregation acridine dye group of the DAO molecule. The controlled transition from a monolayer to a trilayer described herein is proposed as a model for further interfacial supramolecular structures of tunable thickness comprising organic dyes.

Revisiting the Brewster Angle Microscopy: the relevance of the polar headgroup

The Brewster Angle Microscopy (BAM) is a powerful microscopy technique allowing the in situ visualization of the morphology of Langmuir monolayers at the air/water interface. The use of the BAM for attaining structural insights in the molecular arrangement of the Langmuir monolayers is widespread. In this review, we examine the reflection of a Langmuir monolayer under a rather different perspective than classical: the influence of the polar headgroup of the amphiphiles in the BAM images is taken into account. The relevance of the polar headgroup as the main cause of the BAM features has been the focus of a reduced number of BAM studies. An emerging experimental and theoretical framework from recent bibliography is discussed. Different theoretical scenarios are considered, concerning the size and absorption of radiation of the polar headgroup. Two qualitative examples showing physical phenomena regarding the reflectivity changes in a BAM experiments are discussed. The anisotropy in the BAM images as inner textures is of special interest. Quantitative structural information of the molecular arrangement of the monolayer is obtained by simulating the textures of the domains observed. The quantitative assessment of the detailed molecular arrangement of the polar headgroup by BAM is highly valuable, as this information can hardly be obtained from other experimental techniques. The procedure for extracting quantitative structural data from the experimental BAM pictures is revised in detail from the recent bibliography for further application of this model to different Langmuir monolayers.

Controlling the molecular organization of porphyrins by hosting in amphiphilic matrix

Mixed films containing ionic water-soluble or -insoluble porphyrins and amphiphilic anchors have been prepared at the air/water interface and on solid support (Langmuir-Blodgett, LB). Their molecular organization has been controlled by different strategies. Monomer, twisted dimer and slipped dimers, and tetramer patterns have been organized at those interfaces as a function of the central metal ion of water-soluble porphyrins, the lipid host-matrix and the applied surface pressure. In addition, stable monomolecular or tri-layers are possible to fabricate. Moreover, for water-insoluble porphyrins, dilution by increasing molar ratio of the amphiphilic matrix in mixed films facilitates the disaggregation, i.e. the monomer porphyrin moieties increases. As a result, the response to the presence of the toxic gas, i.e. the speed of the response and recovery phases, the temperature and concentration ranges has been notably enhanced.

Self-assembly of Acridine Orange into H-aggregates at the air/water interface: tuning of orientation of headgroup

The surface active derivative of the organic dye Acridine Orange (N-10-dodecyl-acridine orange (DAO)) has been included in mixed Langmuir monolayers with stearic acid (SA). The maximum relative content on DAO for a stable mixed monolayer is a molar ratio of X(DAO) = 0.5. Brewster angle microscopy (BAM) reveals a high homogeneity at the micrometer level for the mixed monolayer in equimolar proportion (X(DAO) = 0.5), whereas the appearance of domains occurs for lower content of DAO, i.e., X(DAO) = 0.2 and 0.1. The aggregation of the DAO headgroup leads to well-defined H-aggregates at the air/water interface for those mixed monolayers with a low content of DAO. However, for the mixed monolayers enriched in DAO, e.g., X(DAO) = 0.5, the molecular crowding prevents the formation of defined supramolecular structures. Molecular organization and tilting of the DAO headgroup is quantitatively analyzed by in situ UV-visible reflection spectroscopy. The formation of H-aggregates of the DAO headgroup can be reversibly tuned with the applied surface pressure. A molecular mechanism for the conformational rearrangement of the DAO molecule is proposed using RM1 quantum semiempirical calculations.

A DMPA Langmuir monolayer study: from gas to solid phase. An atomistic description by molecular dynamics Simulation

In this work, a DMPA Langmuir monolayer at the air/water interface was studied by molecular dynamics simulations. Thus, an atomistic picture of a Langmuir monolayer was drawn from its expanded gas phase to its final solid condensed one. In this sense, some properties of monolayers that were traditionally poorly or even not reproduced in computer simulations, such as lipid domain formation or pressure-area per lipid isotherm, were properly reproduced in this work. Thus, the physical laws that control the lipid domain formation in the gas phase and the structure of lipid monolayers from the gas to solid condensed phase were studied. Thanks to the atomistic information provided by the molecular dynamics simulations, we were able to add valuable information to the experimental description of these processes and to access experimental data related to the lipid monolayers in their expanded phase, which is difficult or inaccessible to study by experimental techniques. In this sense, properties such as lipids head hydration and lipid structure were studied.

Molecular organization of a water-insoluble iridium(III) complex in mixed monolayers

In this work, organized mixed monolayers containing a cationic water-insoluble iridium(III) complex, Ir-dye, [Ir(ppy)(2)(tmphen)]PF(6), (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline, and ppy = 2-phenylpyridine), and an anionic lipid matrix, DMPA, dimyristoyl-phosphatidic acid, with different molar proportions, were formed by the co-spreading method at the air-water interface. The presence of the dye at the interface, as well as the molecular organization of the mixed films, is deduced from surface techniques such as pi-A isotherms, Brewster angle microscopy (BAM) and reflection spectroscopy. The results obtained remark the formation of an equimolar mixed film, Ir-dye/DMPA = 1:1. BAM images reveal a whole homogeneous monolayer, with gradually increasing reflectivity along the compression process up to reaching the collapse of this equimolecular monolayer at pi approximately equal to 37 mNm(-1). Increasing the molar ratio of DMPA in the mixture, the excess of lipid molecules organizes themselves forming dark flower-like domains of pure DMPA at high surface pressures, coexisting with the mixed Ir-dye/DMPA = 1:1 monolayer. On the other hand, unstable mixed monolayers are obtained by using an initial dye surface concentration higher than the equimolecular one. These mixed Langmuir monolayers have been successfully transferred onto solid substrates by the LB (Langmuir-Blodgett) technique.

J-aggregation of a water-soluble tetracationic porphyrin in mixed LB films with a calix[8]arene carboxylic acid derivative

The molecular organization of a mixed film, containing a water-soluble tetracationic porphyrin (TMPyP) and a p-tert-butyl calix[8]arene octacarboxylic acid derivative (C8A), at the air-water interface and on a solid support (LB film), has been investigated. Although the TMPyP aggregation was not detected at the air-water interface, TMPyP J-aggregates have been found in the LB films (Y-type). Unlike tetraanionic porphyrins, for example TSPP, the TMPyP J-aggregates are not induced by a zwitterion formation. The TMPyP J-aggregation is a result of a "double comb" configuration, where porphyrins from opposite layers are interwoven in a linear infinite J-aggregate. Our results confirm that TMPyP molecules tend to self-aggregate strongly, provided the electrostatic repulsions of their peripheral groups are cancelled by the anionic groups of the C8A matrix.

Magnetic Langmuir-Blodgett films of ferritin with different iron contents

Magnetic Langmuir-Blodgett films of four ferritin derivatives with different iron contents containing 4220, 3062, 2200, and 1200 iron atoms, respectively, have been prepared by using the adsorption properties of a 6/1 mixed monolayer of methyl stearate (SME) and dioctadecyldimethylammonium bromide (DODA). The molecular organization of the mixed SME/DODA monolayer is strongly affected by the presence of the water-soluble protein in the subphase as shown by pi-A isotherms, BAM images, and imaging ellipsometry at the water-air interface. BAM images reveal the heterogeneity of this mixed monolayer at the air-water interface. We propose that the ferritin is located under the mixed matrix in those regions where the reflectivity is higher whereas the dark regions correspond to the matrix. Ellipsometric angle measurements performed in zones of different brightness of the mixed monolayer confirm such a heterogeneous distribution of the protein under the lipid matrix. Transfer of the monolayer onto different substrates allowed the preparation of multilayer LB films of ferritin. Both infrared and UV-vis spectroscopy indicate that ferritin molecules are incorporated within the LB films. AFM measurements show that the heterogeneous distribution of the ferritin at the water-air interface is maintained when it is transferred onto solid substrates. Magnetic measurements show that the superparamagnetic properties of these molecules are preserved. Thus, marked hysteresis loops of magnetization are obtained below 20 K with coercive fields that depend on the number of iron atoms of the ferritin derivative.

Structural Investigation of Langmuir and Langmuir−Blodgett Monolayers of Semifluorinated Alkanes

The behavior of a semi-fluorinated alkane (C(10)F(21)C(19)H(39)) has been studied at the air-water interface by using surface pressure and surface potential-area isotherms as well as infrared spectroscopy for the Langmuir-Blodgett films. In addition, based on the quantum chemical PM3 semiempirical approach, the dimer structure was investigated, and the double helix was found to be the most stable conformation of the dimer. The obtained results allow us to imply that the phase transition observed in the course of the surface pressure/area isotherm is due to a conformational change originating from the double helix to a vertical, single helix configuration.

Anodic electrodeposition of NiTSPP from aqueous basic media

The oxidative electrodeposition of NiTSPP (tetrakis(4-sulfonatophenyl) Ni porphyrin) on ITO electrode from 0.1 M NaOH aqueous solution has been studied, and UV-visible and reflection FTIR spectroscopies have been used to analyze the composition of such film. By use of UV-vis spectroscopy, small absorbance of the film and an almost nulling effect on the Soret band of the porphyrin along the Ni(III)/Ni(II) redox process were observed. The reflection FTIR spectroscopy detected the presence of Ni-OH groups in the reduced film and as well the state of the porphyrin molecules as radical cation. Moreover, the porphyrin has been quantified by means of the area of the vibration bands assigned to the sulfonate groups by using as reference a Langmuir-Blodgett film containing a known surface concentration of NiTSPP. These results lead us propose the formation of a conductor salt by electrocrystallization, with stoichiometries TSPP/Ni(II)(OH)2 and TSPP/Ni(III)OOH, for its reduced and oxidized forms, respectively. In these two forms, the porphyrin rings will be present as radical cation, which may be stabilized through its dimerization or polymerization.

Conformational Changes of a Calix[8]arene Derivative at the Air−Water Interface

The particular behavior of a p-tert-butyl calix[8]arene derivative (C8A) has been studied at the air-water interface using surface pressure-area isotherms, surface potential-area isotherms, film relaxation measurements, Brewster angle microscopy (BAM), and infrared spectroscopy for Langmuir-Blodgett films. Thus, it is observed that the properties of the film, for example, isotherms, domain formation, and FTIR spectra, recorded during the first compression cycle differ appreciably from those during the second compression and following cycles. The results obtained are interpreted on the basis of the conformational changes of the C8A molecules by surface pressure, allowing us to inquire into the inter- and intramolecular interactions (hydrogen bonds) of those molecules. Thus, the compression induces changes in the kind of hydrogen bonds from intra- and intermolecular with other C8A molecules to hydrogen bonds with water molecules.