Polarization Modulation IR Reflection Absorption Research Articles

Spectroscopic Metrics for Alkyl Chain Ordering in Lying-Down Noncovalent Monolayers of Diynoic Acids on Graphene

Noncovalent monolayer chemistries are widely used to control physical properties of 2D materials. For many applications (e.g., energy conversion, sensing), molecular ordering across a range of length scales is important in determining the physical properties of the interface. Scanning probe microscopy can resolve details of molecular packing and orientation over nanoscopic areas of graphene, graphite, and other 2D materials; however, evaluating molecular ordering over larger scales is also key. Such ordering is especially challenging to characterize at large scales for lying-down phases (thickness <0.5 nm) on topographically rougher materials such as graphene (vs flatter graphite). Here, we combine scanning electron microscopy and polarization-modulated IR reflection absorption spectroscopy to evaluate alkyl chain ordering in lying-down monolayers of diynoic acids on few-layer graphene and graphite substrates with areas ∼1 cm2. The ability to assess ordering in this widely used class of molecules reinforc...

The Scope of Direct Alkylation of Gold Surface with Solutions of C1-C4 n-Alkylstannanes.

Treatment of cleaned gold surfaces with dilute tetrahydrofuran or chloroform solutions of tetraalkylstannanes (alkyl = methyl, ethyl, n-propyl, n-butyl) or di-n-butylmethylstannyl tosylate under ambient conditions causes a self-limited growth of disordered monolayers consisting of alkyls and tin oxide. Extensive use of deuterium labeling showed that the alkyls originate from the stannane and not from ambient impurities, and that trialkylstannyl groups are absent in the monolayers, contrary to previous proposals. Methyl groups attached to the Sn atom are not transferred to the surface. Ethyl groups are transferred slowly, and propyl and butyl rapidly. In all cases, tin oxide is codeposited in submonolayer amounts. The monolayers were characterized by ellipsometry, contact angle goniometry, polarization modulated IR reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy with ferrocyanide/ferricyanide, which revealed a very low charge-transfer resistance. The thermal stability of the monolayers and their resistance to solvents are comparable with those of an n-octadecanethiol monolayer. A preliminary examination of the kinetics of monolayer deposition from a THF solution of tetra-n-butylstannane revealed an approximately half-order dependence on the bulk solution concentration of the stannane, hinting that more than one alkyl can be transferred from a single stannane molecule. A detailed structure of the attachment of the alkyl groups is not known, and it is proposed that it involves direct single or multiple bonding of one or more C atoms to one or more Au atoms.

Side chain assisted nanotubular self-assembly of cyclic peptides at the air–water interface

Langmuir monolayers of two artificial cyclic peptides with an alternating sequence of L-glutamic acid and 3-aminobenzoic acid subunits, a cyclohexapeptide (C6G) and a cyclooctapeptide (C8G), were investigated using a variety of techniques, including π–A isotherms, in situ surface sum-frequency generation (SFG) and Brewster angle microscopy. The monolayers were also transferred onto a solid substrate by the Langmuir–Blodgett technique and characterized by grazing incident X-ray diffraction (GIXD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The investigations indicated that C6G forms 2D crystallite structures at the air–water interface, whereas no such structures were observed for C8G. Being amphiphilic, both peptides attain a horizontal orientation on the water surface after spreading. Surface compression causes the molecules to flip to a perpendicular state, thus minimizing the molecular area. The measurements also indicate that, in the perpendicular state, self-assembly of C6G leads to a tubular arrangement of the peptide rings. According to GIXD and TEM data, pairs of tubes arrange in a well defined and oriented order producing 2D crystals. Surface vibrational spectroscopic methods (sum-frequency generation and polarization modulation IR reflection–absorption) combined with molecular modeling gave insight into the arrangement of individual C6G molecules in the tubes. Overall, our results indicated that the tubular assembly of C6G is most likely due to intermolecular H-bonding between the CO group in the glutamic acid side chains and peptide NH group of a neighboring peptide ring.

Isotopic Exchange of CO Adsorbed on Pt(111)

The isotopic exchange of CO adsorbed on Pt(111) was studied using polarization modulation IR reflection absorption spectroscopy (PM-IRRAS) and temperature programmed desorption. It was found that the rate constants for the exchange reaction are much higher than would be expected from previous investigations of CO adsorbed on Pt nanoparticles. The adsorption of CO on Pt(111) under elevated pressures of CO and H(2) was also studied using PM-IRRAS. It was seen that CO pressures above 1 mbar lead to a shift in the absorption peak arising from CO adsorbed on a bridge site from 1850 to 1875 cm(-1). Exposing the CO-covered Pt(111) surface to 1000 mbar H(2) did not lead to any significant desorption of CO at room temperature, whereas at 363 K H(2) exposure did lead to a significant desorption of CO, due to the increased chemical potential of H(2). In a mixture of CO and H(2) with partial pressures of 0.01 mbar and 1000 mbar, respectively, no significant effect of H(2) on the PM-IRRAS spectrum was seen at temperatures below 423 K.

Structure of Human Annexin A6 at the Air-Water Interface and in a Membrane-Bound State

We postulate the existence of a pH-sensitive domain in annexin A6 (AnxA6), on the basis of our observation of pH-dependent conformational and orientation changes of this protein and its N- (AnxA6a) and C-terminal (AnxA6b) halves in the presence of lipids. Brewster angle microscopy shows that AnxA6, AnxA6a, and AnxA6b in the absence of lipids accumulate at the air-water interface and form a stable, homogeneous layer at pH below 6.0. Under these conditions polarization modulation IR absorption spectroscopy reveals significant conformational changes of AnxA6a whereas AnxA6b preserves its α-helical structure. The orientation of protein α-helices is parallel with respect to the interface. In the presence of lipids, polarization modulation IR reflection absorption spectroscopy experiments suggest that AnxA6a incorporates into the lipid/air interface, whereas AnxA6b is adsorbed under the lipid monolayer. In this case AnxA6a regains its α-helical structures. At a higher pressure of the lipid monolayer the average orientation of the α-helices of AnxA6a changes from flat to tilted by 45° with respect to normal to the membrane interface. For AnxA6b no such changes are detected, even at a high pressure of the lipid monolayer—suggesting that the putative pH-sensitive domain of AnxA6 is localized in the N-terminal half of the protein.

Formation of Metallic Langmuir Film on the Water Surface with Fatty Acid and BEDO-TTF Molecules

The formation process of metallic Langmuir film on the water surface fabricated from BEDO-TTF (BO) and stearic acid (SA) is described. The in-situ observation by Brewster angle microscope revealed a creation of small islands after spreading the mixing solution on the water surface and then a formation of homogeneous Langmuir film by a coalescence of the islands with decreasing of the surface area. An appearance of mixed valence state of BO in both islands and Langmuir film was confirmed by polarization-modulated IR reflection absorption spectroscopy. A theoretical simulation of the spectra supports the bilayer model on the water surface, in which a metallic BO layer underlies between the SA layer and the water surface. An investigation on the Langmuir−Blodgett (LB) films transferred onto substrates by infrared spectroscopy implies the existence of hydrogen-bonded carboxylate groups of (R−COO···H···OOC−R)- in the SA layer. Finally, the optical conductivity in the LB films can be compared with those of the superconductor crystal (BO)2ReO4(H2O) through the Drude optical parameters obtained from the IR spectra fitting.

The mechanism of charge transfer salt formation in the LB films of BEDO-TTF and fatty acid

The mixed valence state formation in a metallic Langmuir film on the water surface is reported. The mixing molecular system of BEDO-TTF (BO) and stearic acid (SA) forms a bilayer film, where a SA layer is lying on the BO layer, on the water surface. The polarization-modulated IR reflection absorption spectroscopy (PM-IRRAS) reveals a metallic optical property in the BO layer described by Drude model. Hydrogen bonding (R-COOHOOC-R) - anions in the SA layer are suggested by the comparison of IR spectrum of carboxylic salt of BEDT-TTF.

Covalent Modification of Carbon Surfaces by Aryl Radicals Generated from the Electrochemical Reduction of Diazonium Salts

Electrochemical reduction of a wide variety of aromatic diazonium salts on carbon electrodes (glassy carbon, highly oriented pyrolytic graphite) leads to the covalent attachment of the corresponding aromatic radicals. The films thus deposited on glassy carbon surfaces require mechanical abrasion to be removed. Cyclic voltammetry, X-ray photoelectron spectroscopy, polarization modulation IR reflection absorption spectroscopy, Auger spectroscopy, and Rutherford backscattering spectroscopy allow the characterization of the overlayer and an estimate of the surface coverage. The latter can be controlled through diazonium concentration and electrolysis duration. The mechanism of derivatization is discussed on the basis of the kinetic data obtained from cyclic voltammetry and preparative electrolysis. This versatile method of surface modification may find applications in the field of carbon−epoxy composites as attested by the successful binding of grafted p-aminophenyl groups with epichlorhydrin.

Investigations at the air/water interface using polarization modulation IR spectroscopy

The ability of polarization modulation IR reflection absorption spectroscopy (PM–IRRAS) to study the air/water interface is presented. A brief description of the set-up and of the experimental procedure is given. Theoretical simulations accounting for the uniaxial nature of the spread monolayer lead to optimum experimental conditions (71 ° for the angle of incidence) and to a specific surface selection rule. Application to the study of cadmium arachidate, dimyristolyl phosphatidylcholine (DMPC) and polypeptidic Langmuir films illustrates the potential uses of this method.

IR active and inactive CO adsorbed on a Pt electrode in acidic solution observed by reflection absorption spectroscopy

Monolayer and submonolayers of CO adsorbed in the double-layer region on a Pt electrode were investigated in 1M HClO 4 by using in situ polarization-modulated IR reflection absorption spectroscopy (IRRAS) and an electrochemical oxidation. As for the monolayer of CO, it was concluded that linear CO accounted tor only 10% of all the adsorbed CO, and the remainder was RAS-inactive CO (reflection absorption spectroscopically inactive CO). As for the submonolayers of CO, the average number of adsorption sites per CO molecule changed from 1.7 to 1.2 with an increase in the total coverage from 0.1 to 0.9. The bridged CO could not be detected for these coverages by the IRRAS method. The band intensity of linear CO was not proportional to the total coverage. Therefore, it was concluded that the RAS-inactive CO were present, and these were one-site occupying and two-site occupying species. The adsorbed species predominating was the two-site occupying RAS-inactive CO for low coverage; and for high coverage the one-site occupying RAS-inactive CO. The RAS-inactive CO are described by a model in which CO is adsorbed on two Pt atoms with the C-O axis parallel to the electrode surface.

IR active and inactive CO adsorbed on a Pt electrode in acidic solution observed by reflection absorption spectroscopy

Monolayer and submonolayers of CO adsorbed in the double-layer region on a Pt electrode were investigated in 1M HClO4 by using in situ polarization-modulated IR reflection absorption spectroscopy (IRRAS) and an electrochemical oxidation. As for the monolayer of CO, it was concluded that linear CO accounted tor only 10% of all the adsorbed CO, and the remainder was RAS-inactive CO (reflection absorption spectroscopically inactive CO). As for the submonolayers of CO, the average number of adsorption sites per CO molecule changed from 1.7 to 1.2 with an increase in the total coverage from 0.1 to 0.9. The bridged CO could not be detected for these coverages by the IRRAS method. The band intensity of linear CO was not proportional to the total coverage. Therefore, it was concluded that the RAS-inactive CO were present, and these were one-site occupying and two-site occupying species. The adsorbed species predominating was the two-site occupying RAS-inactive CO for low coverage; and for high coverage the one-site occupying RAS-inactive CO. The RAS-inactive CO are described by a model in which CO is adsorbed on two Pt atoms with the C-O axis parallel to the electrode surface.

Diode laser spectroscopy of adsorbed gas on metal surfaces

Electroreflectance vibrational spectroscopy (EVS) and polarization-modulated IR reflection absorption spectroscopy (PS) are compared and analyzed. Both techniques have been used with a tunable diode laser to obtain quantitative reflectance spectra of adsorbed CO on single crystal Ni. System calibration for absolute accuracy is described. The reproducibility of EVS and PS is compared by using both to study the same sample on two different days. The spectra cover the 1800-2160-cm(-1) range and span the CO vibrational frequency. For spectroscopy of CO, EVS is more sensitive. The integrated IR absorption of c(2 x 2) CO on Ni(100) measured with PS agrees with previous measurement.