Order in phospholipid Langmuir-Blodgett monolayers determined by total internal reflection fluorescence

Total internal reflection spectroscopy for studying soft matter

Total internal reflection (TIR) spectroscopy is a widely used technique to study soft matter at interfaces. This tutorial review aims to provide researchers with an overview of the principles, experimental design and applications of TIR spectroscopy to enable them to understand how this class of techniques might be used in their research. It also highlights limitations and pitfalls of TIR techniques, which will assist readers in critically analysing the literature. Techniques covered include attenuated total reflection infrared spectroscopy (ATR-IR), TIR fluorescence, TIR Raman scattering and cavity-enhanced techniques. Other related techniques are briefly described.

A quantitative assessment of inhaled drug particle-pulmonary surfactant interaction by atomic force microscopy

To date limited consideration has been given to the physical interaction between inhaled drug particles and pulmonary surfactant (PS). This study combines atomic force microscopy (AFM) with a Langmuir-Blodgett (LB) approach to quantify the force of adhesion between micronised budesonide particles and simulated PS monolayers. A LB approach was used to prepare Survanta monolayers at pre-determined surface pressures and AFM was employed to facilitate their visualisation. Adhesion measurements between drug particles and PS monolayers were executed via AFM. Contact angle measurements were performed to probe material wetting characteristics, the data confirmed that budesonide is hydrophobic and Survanta films at increasing surface pressure exhibit a rising hydrophobic character. AFM revealed that PS properties were governed by applied surface pressure and that the degree of interaction of budesonide was greater at higher surface pressure, where packing of the lipid film was increased; consistent with the point of exhalation. This correlates well with the accepted inhaler technique. The increasing hydrophobicity of the PS film, on increased pressure, was believed to be the primary reason for increased interaction with the hydrophobic budesonide. Surface chemistries of the drug particles and PS interface are considered to be important for inhaled drug delivery.

Combination of Polarized TIRF and ATR Spectroscopies for Determination of the Second and Fourth Order Parameters of Molecular Orientation in Thin Films and Construction of an Orientation Distribution Based on the Maximum Entropy Method

This article describes two mathematical formalisms for the determination of the second and fourth order parameters of molecular films using optical spectroscopy. Method A uses polarized total internal reflection fluorescence (TIRF) to calculate the second and fourth order parameters, {P2(cos theta)} and {P4(cos theta)}, using an independently determined value for the angle between the absorption and emission dipoles, gamma. Method B uses {P2(cos theta)} obtained from attenuated total reflectance (ATR) data, along with polarized TIRF measurements to calculate {P4(cos theta)} and {cos2 gamma}. The choice of a specific method should rely on experimental considerations. We also present a method to separate the contributions of substrate surface roughness and dipole orientation with respect to the molecular axis from the spectroscopically determined second and fourth order parameters. Finally, a maximum entropy approach for construction of an orientation distribution from order parameters is compared with the commonly used delta and Gaussian distributions.

Orientation distribution functions based upon both (P1), (P3) order parameters and upon the four (P1) up to (P4) values: application to an electrically poled nonlinear optical azopolymer film

The most probable orientational distribution functions of rod-like polar molecules contained in a noncentrosymmetric uniaxial system are established using the first-rank and third-rank Legendre polynomials, (P1(cos theta)) and (P3(cos theta)) order parameters, and the maximum entropy method. Emphasis is put on the different domains of existence in the ((P1), (P3)) plane for the various shapes of the distributions: it is thus shown that, for any positive (P1(cos theta)) value and for decreasing (P3(cos theta)) values, the distribution function may exhibit either a distorted oblate form with an intense maximum at 0 degrees, or a three-leaved rose curve with maxima at 60 degrees, 180 degrees, and 300 degrees, and finally another markedly oblate shape with a strong maximum at 180 degrees. As an illustrative example, we have considered the azobenzene molecular orientations in an electrically poled p(DR1M) homopolymer thin film after a thermal process and several relaxation periods. We have made use not only of the (P1) and (P3) parameters determined from polarized second-harmonic generation (SHG) measurements, but also of the (P2) values extracted from UV-visible spectra and of the (P4) values adjusted according to the information entropy theory. In such a thin film with very large nonlinear properties (d33 coefficients were varying from 437.0 to 117.0 pm/V at 1064 nm) it is evidenced that a strong polar order is maintained even after a long relaxation period of 42 days. So, the distribution functions demonstrate that the poling treatment was quite efficient and they emphasize the importance in the determination of both couples of odd and even order parameters in such uniaxially oriented optical elements.

Single molecule research on surfaces: from analytics to construction and back

The study of single molecules opens a new dimension in understanding nature down to its finest ramifications. While much progress was achieved in the last decade concerning the detection techniques, suitable techniques for manipulating and handling the biomolecules still bear a challenge. Primarily, the task is keeping an individual, active molecule of a certain lifespan in the spot. Here, we will focus on techniques for the functional immobilization of (single) molecules on surfaces to enable their observation at one position over a time period. Presenting the main methods of reversible immobilization we will accentuate the chelator lipid concept as combining all features prerequisite for functional, reversible and well-defined immobilization. This will also show that single molecule research in principle is the synthesis of an insight into the function of nature and nano-biotechnology (manipulation): thus of analytics, construction, and back.

Order in phospholipid Langmuir-Blodgett layers and the effect of the electrical potential of the substrate

The ordering in dipalmitoylphosphatidylcholine (DPPC) Langmuir-Blodgett monolayers and bilayers on a semiconducting indium tin oxide (ITO) surface has been investigated at the equilibrium potential of the interface and at various externally applied potentials. Second- and fourth-rank order parameters of a diphenylhexatriene (DPH) containing phospholipid probe were derived from total internal reflection fluorescence measurements, and orientation distributions were calculated using the maximum-entropy method. Generally, bimodal orientation distributions were obtained, suggesting that only part of the probes is aligned with the DPPC molecules. The effect of applied potentials is small for DPPC layers on unmodified (hydrophilic) ITO; with decreasing potential the ordering changes slightly to more random distributions, possibly because of the onset of hydrogen evolution at the substrate surface. For monolayers on hydrophobized ITO, where the phospholipids are initially with their tails directed toward the surface, the changes are more significant. At the highest positive potential applied, the derived order parameters indicate that nearly all probes are flat on the surface. This can be understood as a result of enhanced competition between headgroups and tails for access to the surface as it becomes more polarized. On unmodified ITO the electrochemistry of Fe(CN)6(3-/4-) and Ru(bipyridyl)3(2+/3+) is hardly hindered by the presence of DPPC monolayers or bilayers. On hydrophobized ITO a DPPC monolayer enhances the redox reactions.