Langmuir and Langmuir-Blodgett films of Cl-PPV mixed with stearic acid: implication of the morphology on the surface and spectroscopy properties

The Past and the Future of Langmuir and Langmuir-Blodgett Films

The Langmuir-Blodgett (LB) technique, through which monolayers are transferred from the air/water interface onto a solid substrate, was the first method to allow for the controlled assembly of organic molecules. With its almost 100 year history, it has been the inspiration for most methods to functionalize surfaces and produce nanocoatings, in addition to serving to explore concepts in molecular electronics and nanoarchitectonics. This paper provides an overview of the history of Langmuir monolayers and LB films, including the potential use in devices and a discussion on why LB films are seldom considered for practical applications today. Emphasis is then given to two areas where these films offer unique opportunities, namely, in mimicking cell membrane models and exploiting nanoarchitectonics concepts to produce sensors, investigate molecular recognitions, and assemble molecular machines. The most promising topics for the short- and long-term prospects of the LB technique are also highlighted.

Conjugated polymers as Langmuir and Langmuir-Blodgett films: Challenges and applications in nanostructured devices

Initially developed for classic systems composed of fatty acids and phospholipids, the Langmuir and Langmuir-Blodgett (LB) techniques allow the fabrication of nanometer-scale devices at self-assembly interfaces with high control over the thickness and molecular architecture. Their application in the research and production of new plastic materials has grown considerably over the past few decades due to the efficiency of conjugated polymers (CPs) for the production of light-emitting diodes, flexible displays, solar cells, and other photoelectronic devices. The structuring of polymers at different interfaces is not trivial as this class of macromolecules can undergo through different processes of folding/unfolding, which hinders the formation of stable Langmuir monolayers and, consequently, the production of Langmuir-Blodgett films. With these ideas in mind, the present article aims to review a series of elements related to the formation of stable Langmuir and Langmuir-Blodgett films of CPs, especially those based on poly(phenylene vinylene)s, polyfluorenes, and polythiophenes. This review is divided into two parts where we first discuss the formation of neat CP films, and then the strategies for the formation of stable CP films based on the co-immobilization with fatty acids, other polymers, and enzymes as mixed films.

Langmuir and Langmuir-Blodgett Films of Poly[(9,9-dioctylfluorene)-co-(3-hexylthiophene)] for Immobilization of Phytase: Possible Application as a Phytic Acid Sensor

In this work, the copolymer poly[(9,9-dioctylfluorene)-co-(3-hexylthiophene)] was employed as a matrix for immobilizing phytase, aiming at the detection of phytic acid. The copolymer was spread on the air-water interface forming Langmuir monolayers and phytase adsorbed from the aqueous subphase. The interactions between the copolymer and the enzyme components were investigated with surface pressure and surface potential-area isotherms, Brewster angle microscopy, and polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). The enzyme could be incorporated in the monolayers from the aqueous subphase, expanding the copolymer films and maintaining its secondary structure. The polymeric films presented a morphological heterogeneous pattern at the air-water interface because of the ability of their chains to fold and entangle, causing inherent defects in the organization as well as unbalanced lateral distribution at the air-water interface because of the formation of aggregates. The interfacial films were transferred to solid supports as Langmuir-Blodgett films and characterized by PM-IRRAS and scanning electronic microscopy, which showed not only the co-transfer of the enzyme but also the maintenance of their heterogeneous morphological pattern. The enzymatic activity of the blended film was analyzed by UV-vis spectroscopy and allowed the estimation of the value of the Michaelis constant (13.08 mM), demonstrating the feasibility of the system to selectively detect phytic acid for biosensing purposes.

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.