Dual-Responsive and Reusable Optical Sensors Based on 2,3-Diaminoquinoxalines for Acidity Measurements in Low-pH Aqueous Solutions.

This work is focused on the age-old challenge of developing optical sensors for acidity measurements in low-pH aqueous solutions (pH < 5). We prepared halochromic (3-aminopropyl)amino-substituted quinoxalines QC1 and QC8 possessing different hydrophilic-lipophilic balance (HLB) and investigated them as molecular components of pH sensors. Embedding the hydrophilic quinoxaline QC1 into the agarose matrix by sol-gel process allows for fabrication of pH responsive polymers and paper test strips. The emissive films thus obtained can be used for a semi-quantitative dual-color visualization of pH in aqueous solution. Being exposed to acidic solutions with pH in the range of 1-5, they rapidly give different color changes when the analysis is performed in daylight or under irradiation at 365 nm. Compared with classical non-emissive pH indicators, these dual-responsive pH sensors allow for an increase in the accuracy of pH measurements, particularly in complex environmental samples. pH indicators for quantitative analysis can be prepared by the immobilization of amphiphilic quinoxaline QC8 using Langmuir-Blodgett (LB) and Langmuir-Schäfer (LS) techniques. Compound QC8 possessing two long alkyl chains (n-C8H17) forms stable Langmuir monolayers at the air-water interface, and these monolayers can be successfully transferred onto hydrophilic quartz and hydrophobic polyvinylchlorid (PVC) substrates using LB and LS techniques, respectively. The 30-layer films thus obtained are emissive, reveal excellent stability, and can be used as dual-responsive pH indicators for quantitative measurements in real-world samples with pH in the range of 1-3. The films can be regenerated by immersing them in basic aqueous solution (pH = 11) and can be reused at least five times.

Multistate Aggregation-Induced Chiroptical Properties of Enantiopure Disulfide-Mediated Bispyrene Macrocycles.

A chiral bispyrene macrocycle designed for exclusive intermolecular excimer fluorescence upon aggregation was synthesized by a double hydrothiolation of a bis-enol ether macrocycle followed by intramolecular oxidation of free thiols. Unusually high stereoselectivity was achieved for the thiol-ene additions under templated conditions and Et3 B/O2 radical initiation. After enantiomer separation (chiral stationary phase HPLC), aqueous conditions provoked aggregation. Detailed structural evolution was afforded by ECD/CPL monitoring. Three regimes can be observed and characterized by strong modifications in chiroptical patterns under, at, or above a 70 % H2 O : THF threshold. In luminescence, high glum dissymmetry factors values were obtained, up to 0.022, as well as a double sign inversion of CPL signals during the aggregation, a behavior rationalized by time-dependent density functional theory (TDDFT) calculations. Langmuir layers of enantiopure disulfide macrocycles were formed at the air-water interface and transferred onto solid substrates to afford Langmuir-Blodgett films, which were then studied by AFM and UV/ECD/fluorescence/CPL.

Deposition of Extended Ordered Ultrathin Films of Au38 (SC2 H4 Ph)24 Nanocluster using Langmuir-Blodgett Technique.

Langmuir-Blodgett technique is utilized to deposit ultrathin films of Au38 (SC2 H4 Ph)24 nanocluster onto solid surfaces such as mica and silicon. The morphologies of the films transferred at various surface pressures within the mono/bi/trilayer regime are studied by atomic force microscopy (AFM). The time spent on the water surface before the deposition has a decisive effect on the final ordering of nanoclusters within the network and is studied by fast AFM, X-ray reflectivity, and grazing-incidence wide-angle X-ray scattering.

Ultrathin Films of Porous Metal-Organic Polyhedra for Gas Separation.

Ultrathin films of a robust RhII -based porous metal-organic polyhedra (MOP) have been obtained. Homogeneous and compact monolayer films (ca. 2.5 nm thick) were first formed at the air-water interface, deposited onto different substrates and characterized using spectroscopic methods, scanning transmission electron microscopy and atomic force microscopy. As a proof of concept, the gas separation performance of MOP-supported membranes has also been evaluated. Selective MOP ultrathin films (thickness ca. 60 nm) exhibit remarkable CO2 permeance and CO2 /N2 selectivity, demonstrating the great combined potential of MOP and Langmuir-based techniques in separation technologies.

SnO2/TiO2 Thin Film n-n Heterostructures of Improved Sensitivity to NO2.

Thin-film n-n nanoheterostructures of SnO2/TiO2, highly sensitive to NO2, were obtained in a two-step process: (i) magnetron sputtering, MS followed by (ii) Langmuir-Blodgett, L-B, technique. Thick (200 nm) SnO2 base layers were deposited by MS and subsequently overcoated with a thin and discontinuous TiO2 film by means of L-B. Rutile nanopowder spread over the ethanol/chloroform/water formed a suspension, which was used as a source in L-B method. The morphology, crystallographic and electronic properties of the prepared sensors were studied by scanning electron microscopy, SEM, X-ray diffraction, XRD in glancing incidence geometry, GID, X-ray photoemission spectroscopy, XPS, and uv-vis-nir spectrophotometry, respectively. It was found that amorphous SnO2 films responded to relatively low concentrations of NO2 of about 200 ppb. A change of more than two orders of magnitude in the electrical resistivity upon exposure to NO2 was further enhanced in SnO2/TiO2 n-n nanoheterostructures. The best sensor responses RNO2/R0 were obtained at the lowest operating temperatures of about 120 °C, which is typical for nanomaterials. Response (recovery) times to 400 ppb NO2 were determined as a function of the operating temperature and indicated a significant decrease from 62 (42) s at 123 °C to 12 (19) s at 385 °C A much smaller sensitivity to H2 was observed, which might be advantageous for selective detection of nitrogen oxides. The influence of humidity on the NO2 response was demonstrated to be significantly below 150 °C and systematically decreased upon increase in the operating temperature up to 400 °C.

Large-area alignment of tungsten oxide nanowires over flat and patterned substrates for room-temperature gas sensing.

Alignment of nanowires over a large area of flat and patterned substrates is a prerequisite to use their collective properties in devices such as gas sensors. In this work, uniform single-crystalline ultrathin W18 O49 nanowires with diameters less than 2 nm and aspect ratios larger than 100 have been synthesized, and, despite their flexibility, assembled into thin films with high orientational order over a macroscopic area by the Langmuir-Blodgett technique. Alignment of the tungsten oxide nanowires was also possible on top of sensor substrates equipped with electrodes. Such sensor devices were found to exhibit outstanding sensitivity to H2 at room temperature.

Unsaturation of serine lipids modulating the interaction of a cytosporone with models of the external leaflet of tumorigenic cell membranes.

Cytosporone-B was isolated from fungi and incorporated in models of tumorigenic cell membranes using palmitoyloleoylglycerophosphoserine (POPS) and dipalmitoyl glycerophosphoserine (DPPS) lipids. While for DPPS, the compound condensed the monolayer and decreased the surface compressional modulus, it expanded and kept the compressional modulus for POPS. Hysteresis for compression-expansion cycles was more sensitive for POPS than for DPPS, while a high degree of destabilization was observed for POPS. As observed with infrared spectroscopy and Brewster angle microscopy, specific changes were selective regarding molecular organization and morphology. Atomic force microscopy for transferred monolayers as Langmuir-Blodgett films also confirmed such specificities. We believe these data can help understand the mechanism of action of bioactive drugs in lipid interfaces at the molecular level.

Physical behavior of KR-12 peptide on solid surfaces and Langmuir-Blodgett lipid films: Complementary approaches to its antimicrobial mode against S. aureus.

Biophysical characterization of antimicrobial peptides helps to understand the mechanistic aspects of their action. The physical behavior of the KR-12 antimicrobial peptide (e.g. orientation and changes in secondary structure), was analyzed after interactions with a Staphylococcus aureus membrane model and solid surfaces. We performed antimicrobial tests using Gram-positive S. aureus (ATCC 25923) bacteria. Moreover, Langmuir-Blodgett experiments showed that the synthetic peptide can disturb the lipidic membrane at a concentration lower than the Minimum Inhibitory Concentration, thus confirming that KR-12/lipid interactions are involved. Partially- and fully-deactivated KR-12 hybrid samples were obtained by physisorption and covalent immobilization in chitosan/silica and glyoxal-rich solid supports. The correlation of Langmuir-Blodgett data with the α-helix formation, followed by FTIR-ATR in a frozen-like state, and the antimicrobial activity showed the importance of these interactions and conformation changes on the first step action mode of this peptide. This is the first time that material science (immobilization in solid surfaces assisted by FTIR-ATR analysis in frozen-like state) and physical (Langmuir-Blodgett/Schaefer) approaches are combined for exploring mechanistic aspects of the primary action mode of the KR-12 antimicrobial peptide against S. aureus.

Hybrid System for Local Drug Delivery and Magnetic Hyperthermia Based on SPIONs Loaded with Doxorubicin and Epirubicin.

Cancer is one of the most common causes of death worldwide, thus new solutions in anticancer therapies are highly sought after. In this work, superparamagnetic iron oxide nanoparticles (SPIONs) conjugated with anticancer drugs are synthesized and investigated as potential magnetic drug nanocarriers for local drug delivery and mild magnetic hyperthermia. We have obtained a hybrid system loaded with holmium and anticancer drugs and thoroughly studied it with respect to the size, morphology, surface modifications and magnetic properties, and interactions with the model of biological membranes, cytotoxicity. We present that nanoparticles having a round shape and size 15 nm are successfully stabilized to avoid their agglomeration and modified with doxorubicin or epirubicin within a controlled way. The number of drugs loaded into the SPIONs was confirmed with thermogravimetry. The hybrid based on SPIONs was investigated in touch with model biological membranes within the Langmuir-Blodgett technique, and results show that modified SPION interacts effectively with them. Results obtained with magnetic hyperthermia and biological studies confirm the promising properties of the hybrid towards future cancer cell treatment.

Biocatalysts immobilized in ultrathin ordered films.

The immobilization of enzymes and other proteins into ordered thin materials has attracted considerable attention over the past few years. This research has demonstrated that biomolecules immobilized in different [Langmuir-Blodgett (LB)/Langmuir-Schaefer (LS)] matrixes retain their functional characteristics to a large extent. These new materials are of interest for applications as biosensors and biocatalysts. We review the growing field of oxidases immobilized onto ordered Langmiur-Blodgett and Langmuir-Schaefer films. Strategies for the preparation of solid supports and the essential properties of the resulting materials with respect to the envisaged applications are presented. Basic effects of the nature of the adsorption and various aspects of the application of these materials as biosensors, biocatalysts are discussed. Outlook of potential applications and further challenges are also provided.

Photosensitive Thin Films Based on Drop Cast and Langmuir-Blodgett Hydrophilic and Hydrophobic CdS Nanoparticles.

Comparative photoelectrochemical studies of cadmium sulfide (CdS) nanoparticles with either hydrophilic or hydrophobic surface properties are presented. Oleylamine organic shells provided CdS nanoparticles with hydrophobic behavior, affecting the photoelectrochemical properties of such nanostructured semiconductor. Hydrophilic CdS nanoparticles were drop-cast on the electrode, whereas the hydrophobic ones were transferred in a controlled manner with Langmuir-Blodgett technique. The substantial hindrance of photopotential and photocurrent was observed for L-B CdS films as compared to the hydrophilic, uncoated nanoparticles that were drop-cast directly on the electrode surface. The electron lifetime in both hydrophilic and hydrophobic nanocrystalline CdS was determined, revealing longer carrier lifetime for oleylamine coated CdS nanoparticles, ascribed to the trapping of charge at the interface of the organic shell/CdS nanoparticle and to the dominant influence of the resistance of the organic shell against the flux of charges. The "on" transients of the photocurrent responses, observed only for the oleylamine-coated nanoparticles, were resolved, yielding the potential-dependent rate constants of the redox processes occurring at the interface.

1.34 µm Q-Switched Nd:YVO4 Laser with a Reflective WS2 Saturable Absorber.

In this work, a Tungsten disulfide (WS2) reflective saturable absorber (SA) fabricated using the Langmuir-Blodgett technique was used in a solid state Nd:YVO4 laser operating at 1.34 µm. A Q-switched laser was constructed. The shortest pulse width was 409 ns with the repetition rate of 159 kHz, and the maximum output power was 338 mW. To the best of our knowledge, it is the first time that short laser pulses have been generated in a solid state laser at 1.34 µm using a reflective WS2 SA fabricated by the Langmuir-Blodgett method.

Controlling the Formation of Peptide Films: Fully Developed Helical Peptides are Required to Obtain a Homogenous Coating over a Large Area.

The influence of conformational dynamics on the self-assembly process of a conformationally constrained analogue of the natural antimicrobial peptide Trichogin GA IV was analysed by spectroscopic methods, microscopy imaging at nanometre resolution, and molecular dynamics simulations. The formation of peptide films at the air/water interface and their deposition on a graphite or a mica substrate were investigated. A combination of experimental evidence with molecular dynamics simulation was used to demonstrate that only the fully developed helical structure of the analogue promotes formation of ordered aggregates that nucleate the growth of micrometric rods, which give rise to homogenous coating over wide regions of the hydrophilic mica. This work proves the influence of helix flexibility on peptide self-organization and orientation on surfaces, key steps in the design of bioinspired organic/inorganic hybrid materials.

Introducing double polar heads to highly fluorescent Thiazoles: Influence on supramolecular structures and photonic properties.

HYPOTHESIS: Supramolecular structures determine properties of optoelectronically active materials and can be tailored via the Langmuir-Blodgett (LB) technique. Interactions between dyes can cause high crystallinities of Langmuir monolayers, thus rendering retaining their integrity during the LB-deposition challenging. However, increasing degrees of freedom exclusively at the polar anchoring moieties of dyes might improve processability without perturbing the dye's optoelectronic properties nor the function-determining crystallinity of the layer. EXPERIMENTS: (Amphiphilic) thiazole dyes without, with a mono-polar, and with a double-polar anchor were synthesized, whereas the two constituting polar moieties of the latter derivate are separated by a flexible alkyl chain. The supramolecular structures and crystallinities of Langmuir and LB monolayers were characterized by means of LB isotherms, atomic force microscopy and polarization-resolved fluorescence spectroscopy. FINDINGS: As compared to the mono-polar reference the introduction of a flexible double-polar head did not deteriorate UV-vis absorption, emission or electrochemical properties of the thiazole but significantly extended the range of constant compressibility modulus, thus indicating improved processability of the Langmuir monolayers. Indeed, AFM studies revealed that the integrity of the monolayers could be retained during LB-deposition. Additionally, also the underlying supramolecular structure of the chromophore moieties is largely identical to those obtained from the mono-polar reference thiazoles.

Interfacial self-assembly of functional bilayer templates comprising porphyrin arrays and graphene oxide.

Fabricating of solid-supported hybrid nanostructures remains a challenging problem because it is difficult to control all interfacial interactions influencing the structure and stability of these systems. The most widely used approach to solving this problem is a bottom-up assembly on the surface templates such as self-assembled monolayers (SAMs). Herein we suggest an alternative approach to tailoring solid surfaces by a formation of an interlayer anchoring the nanostructured film to the solid substrate. We formed a multifunctional bilayer template (MBT), comprising an adhesive monolayer of graphene oxide and a functional ordered monolayer of metal organic compound (Zinc-tetra(4-pyridyl)porphyrin) directing further bottom-up growth of the nanostructures. The one-step assembly of MBT proceeded spontaneously at the air/water interface and was monitored by an in-situ fiber optic absorption and fluorescence spectroscopy in a Langmuir trough. Dilatation surface rheology was applied to study the evolution of molecular organization of the monolayers upon adding the zinc ions, GO and their mixture into the subphase. The MBT templates were used for the assembly of porphyrin-based SURMOFs with two different structures. Our strategy makes it possible to assemble surface-anchored nanostructures avoiding the use of SAMs and it can be extended to other types of ultrathin hybrid systems.

Oriented collagen nanocoatings for tissue engineering.

Collagens are among the most widely present and important proteins composing the human total body, providing strength and structural stability to various tissues, from skin to bone. In this paper, we report an innovative approach to bioactivate planar surfaces with oriented collagen molecules to promote cells proliferation and alignment. The Langmuir-Blodgett technique was used to form a stable collagen film at the air-water interface and the Langmuir-Schaefer deposition was adopted to transfer it to the support surface. The deposition process was monitored by estimating the mass of the protein layers after each deposition step. Collagen films were then structurally characterized by atomic force, scanning electron and fluorescent microscopies. Finally, collagen films were functionally tested in vitro. To this aim, 3T3 cells were seeded onto the silicon supports either modified or not (control) by collagen film deposition. Cells adhesion and proliferation on collagen films were found to be greater than those on control both after 1 (p<0.05) and 7 days culture. Moreover, the functionalization of the substrate surface triggered a parallel orientation of cells when cultured on it. In conclusion, these data demonstrated that the Langmuir-Schaefer technique can be successfully used for the deposition of oriented collagen films for tissue engineering applications.