Table of ContentsIntroductionApplications Morphological Features of Monolayers During Compression/Decompression Inner Structure of 2D Condensed Phase Domains Formation Dynamics, Non Equilibrium Structures Monolayers & Chirality Monitoring 2D-Structures BiofluidsIntegration of Other Techniques Nanosecond Pulsed Lasers Imaging Ellipsometry and UV/VIS Reflection SpectroscopyAbout Accurion
Brewster angle microscopy has been established as a worldwide standard technique for the investigation of ultra thin films. Nanofilm´s BAM1 as shown in Figure 1 was the first commercial instrument available followed by different versions. Accurion now offers two different Brewster angle microscopes. The nanofilm_ep3bam as shown in Figure 2 is based on the ellipsometric platform nanofilm_ep3 and can be upgraded to an imaging ellipsometer.
Figure 1. BAM1 (1991) – first commercial Brewster Angle microscope..
Figure 2. nanofilm_ep3bam (since 2003)
The new nanofilm_ultrabam as shown in Figure 3 is based on a completely new optical pathway of light. The unique imaging optic provides fully focused images at max. 35 fps. Thus for the first time in a commercial instrument, high resolution and overall focused real time imaging of monolayers becomes possible. It enables the visualization of Langmuir monolayers or adsorbed films in real time.
Figure 3. nanofilm_ultrabam (since 2010).
Morphological Features of Monolayers During Compression/Decompression
The morphological study of monolayers has resulted in a comprehensive understanding of the two-dimensional condensed phase structure of monolayers. Brewster angle microscopy is a highly informative method for such a study. Brewster angle microscopy can be used for direct observations during compression of monolayers in a Langmuir trough. The advantage compared to epi fluoresence and AFM is that no markers are required and that the film need not be transferred to a solid substrate.
A number of papers report BAM images at specific points of the p- A compression-decompression isotherm. For such a study, a software integration of the LB-system is essential.
Inner Structure of 2D Condensed Phase Domains
Optical anisotropy induced by regions of different molecular orientations of alkyl chains within monolayers have been the subject of recent investigation. As mentioned, Brewster angle microscopy is a useful method to visualize the substructure with long-range orientational order by using an analyzer. Typical examples are methyl esters of fatty acids.
Figure 4. Ethyl stearate monolayer at p<1mN/m, Field of view ca. 600mm (nanofilm_ultrabam)
Formation Dynamics, Non Equilibrium Structures
The formation of a condensed phase in a fluid monolayer can occur far from equilibrium and growth kinetics can be so rapid that the stable phase does not have time to reach its lowest energy state at the microscopic level. Metastable microstructures are formed under non-equilibrium conditions and the growth patterns of these structures are mainly influenced by the complex interplay of microscopic interfacial dynamics and external driving forces.
Flores et al. studied how patterns formed by Langmuir monolayer domains of a stable phase, usually solid or condensed liquid propagate into a metastable one, which is usually liquid and gets expanded. During this propagation the interface between the two phases moves as the metastable phase is transformed into a highly stable phase.
The interface becomes unstable and forms a pattern between a chemical potential gradient that destabilizes the interface. During domain growth a morphology transition was discovered from tip splitting to side branching and doublons were also found. These morphological features were observed using Brewster angle microscopy in three different monolayers at the air/water interface: dioctadecylamine, ethyl palmitate, and ethyl stearate.
Figure 5. Monolayer of DMPE during first-order phase transition, contrast in domains caused by long range orientation order (nanofilm_ultrabam).
Monolayers & Chirality
Monolayers of enantiomers of amphiphilic molecules can be different. The interaction of enantiomers and diastereomer pairs can be studied for specific monolayer states. Figure 6 displays domains of D-Dipalmitoylphosphatidylcholin (D-DPPC), L-DPPC and the racemate.
Figure 6. Brewster angle micrograph of of D-Dipalmi-toylphosphatidylcholin and the racemate.
Langmuir blottget methods also provide the promising possibility of “bottom-up” self-assembly methods to realize novel 2D -structures. Present examples are using self-assembled monolayers at the air/water interface. Chen and Berman utilized Langmuir monolayers of diacetylene lipids along with a cytosinyl head-group (PDC) mixed with an alcohol derivative of the same lipid (PDOH). They used for example different film compositions and a guanosine-containing subphase. A specific base- pair formation at the air–solution interface between the diacetylene monolayer and the free complementary nucleoside in the solution is suggested. At the end the complex and beautiful structures were fixed by in situ UV polymerization.
Presently nanoparticles at the air/water interface are used in a large number of applications for example to produce two- dimesional colloidal crystals or nanowires. Gil et al. monitored the formation of 2D colloidal crystals by the Langmuir– Blodgett method using Brewster angle microscopy. In addition to Brewster angle microscopy (BAM), the monolayers have been characterized by surface pressure–area and surface potential–area isotherms at the air/water interface. After being transferred to a substrate the morphologies of the Langmuir–Blodgett films were studied using imaging ellipsometry and by scanning electron microscopy (SEM). Volinsky and Jalinek demonstrate the formation of a network of elongated Au “wires”. The laser-induced structured films exhibited superior stability and can be transferred from water onto solid substrates without disrupting the Au organization.
Currently Brewster angle microscopy is used in a wide range of applications. One class of applications is related to biological liquids such as tear fluids or more in detail Meibomian glad secretion lung surfactants in/from pulmonary fluid. Kärcher et al observed the dispersion process of Meibomian gland secretions in a Langmuir trough using a Brewster Angle Microscope (BAM). The secretion was characterized by a highly quick continuous spreading showing that sufficient material will be available to recover the superficial lipid layer of the tears between the blinks of the eye. Winsel et al. studied the thickness, morphology and surface pressure of the surfactant film of broncho-alveoalar lavage (BAL) fluid from patients with sarcoidosis. The surface films were studied during continuous adsorption of the BAL’s surface active material at the air/aqueous buffer interface. During the spontaneous adsorption of the pulmonary surfactant the surface pressure increased from 26 mN/m to 44 mN/m in the equilibrium state. Simultaneously to the increase of the surface pressure, a continuous increase of the reflectivity signal was observed by quantitative Brewster angle microscopy (BAM). The film thickness is calculated from the reflectivity values using an optical model.
Integration of Other Techniques
Nanosecond Pulsed Lasers
Time-resolved Brewster Angle Microscopy for photochemical and photothermal studies on thin-films and monolayers, as well as transient events in thin films and interfaces have been studied using time resolved pump-probe nanosecond Brewster angle microscopy. Based on a Brewster angle microscope EP³-BAM, Hobley et al. used two synchronized nanosecond pulsed lasers in the pump-probe arrangement. The time-resolved BAM has been shown to be a dynamic tool in studying dynamics of changes in monolayers and interfaces. Both morphological changes and photochemical transformations can be studied through the manifold of changes in both real and imaginary parts of the refractive index at the interface. Several effects such as high rates of domain growth due to interfacial effects or unique domain morphologies ensure for cooperative domain growth in molecular monolayers. It is anticipated that the method will have wide applications in the study of membranes, lipid bi-layers or ultra thin layers during materials processing.
Imaging Ellipsometry and UV/VIS Reflection Spectroscopy
Pérez-Morales et al. used imaging ellipsometry at the air–water interface to study the optical parameters of a mixed monolayer containing an anionic phospholipid matrix (DMPA), and a cationic porphyrin (Ni-TMPyP, Ni(II)-tetrakis(4- methylpyridyl) porphyrin) as a large counter ion. The nulling ellipsometric calculations were done on two phases seen directly by Brewster angle microscopy over particular regions with a size of few microns. Therefore values of the ellipsometric angles for different regions at the interface (domains and surrounding areas) have been obtained. The difference in reflectivity of the monolayer-covered water surface and the bare water surface under normal incidence was studied using a reflection spectroscope at normal incidence.
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