Infrared External Reflection Spectroscopy Research Articles

Towards the direct detection of viral materials at the surface of protective face masks via infrared spectroscopy

The ongoing COVID-19 pandemic represents a considerable risk for the general public and especially for health care workers. To avoid an overloading of the health care system and to control transmission chains, the development of rapid and cost-effective techniques allowing for the reliable diagnosis of individuals with acute respiratory infections are crucial. Uniquely, the present study focuses on the development of a direct face mask sampling approach, as worn (i.e., used) disposable face masks contain exogenous environmental constituents, as well as endogenously exhaled breath aerosols. Optical techniques—and specifically infrared (IR) molecular spectroscopic techniques—are promising tools for direct virus detection at the surface of such masks. In the present study, a rapid and non-destructive approach for monitoring exposure scenarios via medical face masks using attenuated total reflection infrared spectroscopy is presented. Complementarily, IR external reflection spectroscopy was evaluated in comparison for rapid mask analysis. The utility of a face mask-based sampling approach was demonstrated by differentiating water, proteins, and virus-like particles sampled onto the mask. Data analysis using multivariate statistical algorithms enabled unambiguously classifying spectral signatures of individual components and biospecies. This approach has the potential to be extended towards the rapid detection of SARS-CoV-2—as shown herein for the example of virus-like particles which are morphologically equivalent to authentic virus—without any additional sample preparation or elaborate testing equipment at laboratory facilities. Therefore, this strategy may be implemented as a routine large-scale monitoring routine, e.g., at health care institutions, nursing homes, etc. ensuring the health and safety of medical personnel.

Morphological Behavior of DPPC-DMPC Two-dimensional Mixed Monolayer on Water Surface by Dropping Method

We have investigated the morphology of dipalmitoyl phosphatidyl choline (DPPC)-dimyristoyl PC (DMPC) mixed monolayer formed on the water surface by the dropping method using surface tension measurement (STm), Brewster angle microscopy (BAM), and infrared external reflection spectroscopy (IERS). The limiting molecular area (A0) obtained by STm showed that the negative deviation at < mole fraction (xDMPC)= 0.4 whereas the positive deviation at > xDMPC =0.6 when DMPC was added to the DPPC monolayer. BAM images also showed that the condensed rigid monolayer at xDMPC < 0.4 changed to the expandable monolayer at xDMPC > 0.5. Moreover, IER spectra showed that the alkyl chains of both DPPC and DMPC molecules are in trans conformation in mixed monolayer at xDMPC =0.2 whereas gauche conformation existed at xDMPC = 0.8. Above findings have justified the transition of flexible gauche conformation alkyl chains of DMPC molecule into rigid trans conformation by the addition of DPPC molecule at xDMPC < 0.4 whereas alkyl chains retained flexible gauche conformation in both DPPC and DMPC molecules at xDMPC > 0.6.

Structural Change in Dimyristoyl Phosphatidyl Choline (DMPC) Molecule Alkyl Chains on the Addition of Cholesterol to DMPC Monolayer: Specific Addition Effect of Cholesterol

We have investigated the morphology of dimyristoyl phosphatidyl choline (DMPC)—cholesterol mixed monolayer formed on the water surface by dropping method using surface tension measurement (STm), Brewster angle microscopy (BAM), and infrared external reflection spectroscopy (IERS). STm results showed negative deviation of the limiting molecular area (A0) of cholesterol occurred when cholesterol was added to the DMPC monolayer. BAM images showed the expandable DMPC monolayer changed to the condensed rigid monolayer at more than cholesterol mole fraction (xChol) 0.4. IERS recordings showed that the addition of cholesterol at xChol = 0.4 occurred structural change from gauche- to trans- conformation of two DMPC molecule alkyl chains. From these results, it is found that cholesterol molecule has specific properties that cause structural transition of DMPC molecule alkyl chains.

Analysis of the Surface Coverage of a Self-Assembled Monolayer of Octadecyl Silane on a Si(100) Surface by Infrared External-Reflection Spectroscopy

The coverage of a self-assembled monolayer (SAM) of octadecyl silane on a silicon surface is analyzed by infrared external-reflection spectrometry via molecular orientation analysis. A commercially available octadecyl trimethoxy silane (ODS) solution is employed, which readily reacts with an oxidized surface of Si(100) prepared by a UV/ozone treatment in an ambient condition to yield a highly reproducible SAM. With the present technique, a very flat surface with no holes is obtained in the entire area of the SAM. IR external-reflection spectra of the SAM apparently indicate that the alkyl chains are highly ordered having the all-trans zigzag conformation, and the molecular orientation agrees with that of a Langmuir-Blodgett (LB) monolayer film of octadecanoic acid. Since the orientation analysis is based on an assumption that the surface is fully covered by the SAM as the LB film, the agreement of the orientation confirms that the surface coverage of the present SAM on Si(100) is on the same level as the LB film.

Structures of Langmuir-Gibbs Films Consisting of Long-Chain Fatty Acid and Water-Soluble Surfactants

The structures of Langmuir-Gibbs (LG) films at the air-solution interface were studied using surface tensiometry, Brewster angle microscopy (BAM), and infrared external reflection spectroscopy (IR-ERS). The LG films were fabricated by forming Langmuir films of deuterated arachidic acid (D19A) and then injecting a water-soluble surfactant, sodium dodecyl sulfate or octaethylene glycol monododecyl ether, into the subphase. The presence of the LG films at the air-solution interface affected the surface tension of water. BAM observations revealed the formation of phase-separated monolayers consisting of D19A monolayers and expanded monolayers of water-soluble surfactant at low concentrations of the water-soluble surfactant. The presence of phase-separated structures was supported by the IR-ERS results. At high concentrations, the water-soluble surfactants adsorbed to the domains of D19A monolayers, forming bilayers under the monolayers. The IR-ERS results suggest the penetration of the water-soluble surfactants into the domains of D19A monolayers and the diffusion of D19A molecules into the bilayer regions of the water-soluble surfactants. At concentrations greater than the critical aggregate concentration and/or critical micelle concentration, D19A molecules were solubilized in the solution. In some cases, the multilayers were kinetically stabilized and were present even 24 h after the injection of the water-soluble surfactants into the subphase.

Adsorbed Monolayers of Mixed Surfactant Solutions of Sodium Dodecylsulfate and Cetylpyridinium Chloride Studied by Infrared External Reflection Spectroscopy

Infrared external reflection (IER) spectra of monolayers adsorbed on mixed surfactant solutions of sodium dodecylsulfate (SDS) and cetylpyridinium chloride were measured as a function of mole fraction of SDS (X SDS ). The alkyl chains of the surfactants in the adsorbed monolayer (Gibbs monolayer) at the air-solution interface were in all-trans conformations in the solid state, regardless of X SDS . The adsorbed monolayers at any X SDS had a single layer structure, except for X SDS ≈ 0.6-0.7, which had multilayer structures. Furthermore, the time dependency of the IER spectra revealed that the formation of the multilayer structures occurs in two processes: a faster single layer formation followed by subsequent slower multilayer formation.

Effects of Concentration and Temperature on SDS Monolayers at the Air−Solution Interface Studied by Infrared External Reflection Spectroscopy

Infrared external reflection (IER) spectra of sodium dodecyl sulfate (SDS) monolayers at the air-solution interface and infrared transmission spectra of the corresponding aqueous solutions were measured at various SDS concentrations and temperatures. A comparison between the spectra of adsorbed monolayers and bulk solutions revealed that the conformational order of the SDS alkyl-chain at the air-solution interface improved with increasing the SDS concentrations, up until the saturation adsorption, and that the conformational order of the adsorbed SDS monolayer was higher than those of monomers and micelles. In addition, below the Krafft point temperature, the adsorbed SDS was maintained in the liquid crystal state, while SDS in the bulk solution was in the crystalline state. Furthermore, the SDS adsorption density was evaluated based on the IER band intensities of the insoluble monolayer of tridecanoic acid with an identical alkyl chain length to SDS.

Characteristics of Long-Chain Fatty Acid Monolayers Studied by Infrared External-Reflection Spectroscopy

Unique monolayer properties of a long-chain fatty acid, hexacosanoic acid (C26), have been studied by surface pressure−area (π−A) isotherms, Brewster angle microscopy (BAM), and a new analytical technique using infrared external-reflection (IR-ER) spectroscopy, in comparison to other shorter long-chain fatty acids (C23−C25). The shorter fatty acids gave an identical limiting molecular area, although the shape of the isotherms was different from one another. Nevertheless, C26 yielded a significantly larger limiting molecular area than the shorter ones, with a relatively simple isotherm in shape. The IR-ER spectra of the C26 monolayer deposited on GaAs suggested that the molecules were ordered well in the film, which is not consistent with the large limiting molecular area. The dielectric dispersion analysis of the spectra, which has recently been developed, supported that the C26 monolayer had a highly ordered molecular conformation. The key to understanding the inconsistency was found in the degree of lat...

Phase Transition of Octadecylurea at the Air−Water Interface Studied by Infrared External Reflection Spectroscopy

External infrared reflection spectroscopy was applied to study a phase transition of the monolayer of octadecylurea (OU) at the air−water interface. Upon compression at a constant temperature (6 °C in this study) the monolayer undergoes the transition from a condensed phase (β phase) with a larger limiting area (0.250 nm2/molecule) to another condensed phase (α phase) with a smaller limiting area (0.195 nm2/molecule). The formation of the β phase and the phase transition accompany discrete IR spectral changes in the νas(CH2), νs(CH2), amide I, amide II, and δ(NH2) vibration regions. The spectral changes indicated that (i) upon compression of the monolayer to form the β phase the alkyl side chain is converted from an irregular state containing gauche conformations to an ordered one consisting of the all-trans conformation and at the same time the peptide bond in the head group is transformed from a fully hydrated state to a hydrogen-bonded one and (ii) upon further compression to the α phase and/or a colla...

Molecular Orientation in Langmuir Films of 12-Hydroxystearic Acid Studied by Infrared External-Reflection Spectroscopy

12-Hydroxystearic acid is known to exhibit a wide plateau region between 0.3 and 0.95 nm2/molecule in the π−A isotherm. In situ polarized FT-IR external-reflection spectra of Langmuir (L) films of 12-hydroxystearic acid on the water surface were recorded under various surface areas, and the molecular-orientation angles were quantitatively evaluated. At the final stage of the plateau region (0.4 nm2/molecule), the orientation angle of the hydrocarbon chain was about 55° from the surface normal. Upon monolayer compression, however, the angle decreased to about 28° in the solid-phase region. This change corresponds well to the result in Langmuir−Blodgett films that was reported earlier. In addition, it was elucidated that the hydrocarbon chain tended to become random during the surface pressure relaxation.

Molecular-Orientation Change in Langmuir Films of Stearic Acid and Cadmium Stearate upon Surface Compression, as Studied by Infrared External-Reflection Spectroscopy

Abstract For interpretating the various phases appearing in a two-dimensional system, we have recorded in situ polarized FT-IR external reflection spectra of Langmuir films of stearic acid and cadmium stearate on the water surface at various surface areas, and evaluated the molecular orientation angles quantitatively. In the stearic acid monolayer, the orientation angle of the hydrocarbon chain from the surface normal decreased from 20° to almost 0° upon monolayer compression. In the cadmium stearate monolayer, on the other hand, the orientation angle (near 0°) did not change upon monolayer compression, though it was widely scattered at large surface areas, because of the formation of a rigid crystalline island. These results correspond well to the π–A isotherms.

Structure Disordering during Surface Pressure Relaxation of Langmuir Films of Stearic Acid as Studied by Infrared External Reflection Spectroscopy

Abstract In situ FT-IR external reflection spectra of Langmuir films of stearic acid on the water surface were recorded duing the surface pressure relaxation at two surface areas, corresponding to the liquid condensed (L2) and super-liquid (LS) phases. In the L2 phase, the surface pressure relaxation was minimal and so was the spectral change. In the LS phase, however, the structural change from the trans to gauche conformation was detected.

An IR-External Reflection Spectroscopy Sample Holder Which Facilitates Reproducible Substrate Positioning

Infrared external reflection spectroscopy (IR-ERS) has become an important tool for probing the molecular architecture of chemically modified surfaces. Combined with considerations based on classical electromagnetic theory, this technique has been used to characterize monomolecular and thin polymeric films at surfaces with a variety of reflectivities. Research efforts in the areas of adhesion, electrocatalysis, corrosion inhibition, and microelectronics have all benefitted from the information obtained by IR-ERS.

Quantitative aspects of infrared external reflection spectroscopy: polymer/glassy carbon interface

The quantitative aspects of Infrared external reflection spectroscopy (IR-ERS) for the characterlzatlon of thln polymer fibns (<OS pm) on glassy carbon (GC) have been developed. Classical electromagnetlc theory has been appHed to caiculate the mean-square electrlc fleld (MSEF) as a function of angle of Incidence, polarization, and substrate optical properties. These data were used to determine conditions for optlmum Sensitivity. Maximum MSEF occur at 60' from the surface normal for p-polarized llght and at normal Incidence for s-polarlzed light. Spectral band shapes due to the low reflectivity of GC for films of poly(methy1 methacrylate) (PMMA) at GC are severely distorted with respect to those for transmlsslon spectra. For example, the band shape for the carbonyl stretch for a 78-A PMMA film, analyzed with p-polarized irradiation that was lncldent at 60°, Is very asymmetric with an absorption maximum shifted several wavenumbers higher in energy. Irradiation at 20' with spolarlzed light results in derlvative-ilke band shapes that strongly resemble the mirror-Image of the anomalous dlspersion of the real part of the PMMA refractive Index. Integrated absorption strengths for spectra calculated via the parallel layer model for classical electromagnetlc theory agree, In almost every Instance, to within about 7% of those measured; absorbance maxima and minima agree to within 1-2 cm-'. This agreement Indicates that the observed dlstortions restdl from the Inherent d#ferences between ERS and transmlsslon measurements and, more Importantly, demonstrates the quantltatlve capabilities of IR-ERS.