Chemically characterizing the cortical cell nano-structure of human hair using atomic force microscopy integrated with infrared spectroscopy (AFM-IR).

OBJECTIVE: The use of conventional microscopy and vibrational spectroscopy in the optical region to investigate the chemical nature of hair fibres on a nanometre scale is frustrated by the diffraction limit of light, prohibiting the spectral elucidation of nanoscale sub-structures that contribute to the bulk properties of hair. The aim of this work was to overcome this limitation and gain unprecedented chemical resolution of cortical cell nano-structure of hair. METHODS: The hybrid technique of AFM-IR, combining atomic force microscopy with an IR laser, circumvents the diffraction limit of light and achieves nanoscale chemical resolution down to the AFM tip radius. In this work, AFM-IR was employed on ultra-thin microtomed cross-sections of human hair fibres to spectrally distinguish and characterize the specific protein structures and environments within the nanoscale components of cortical cells. RESULTS: At first, a topographical and chemical distinction between the macrofibrils and the surrounding intermacrofibillar matrix was achieved based on 2.5 × 2.5 μm maps of cortical cell cross-sections. It was found that the intermacrofibrillar matrix has a large protein content and specific cysteine-related residues, whereas the macrofibrils showed bigger contributions from aliphatic amino acid residues and acidic-/ester-containing species (e.g. lipids). Localized spectra recorded at a spatial resolution of the order of the AFM tip radius enabled the chemical composition of each region to be determined following deconvolution of the Amide-I and Amide-II bands. This provided specific evidence for a greater proportion of α-helices in the macrofibrils and correspondingly larger contributions of β-sheet secondary structures in the intermacrofibrillar matrix, as inferred in earlier studies. Analysis of the parallel and antiparallel β-sheet structures, and of selected dominant amino acid residues, yielded further novel composition and conformation results for both regions. CONCLUSION: In this work, we overcome the diffraction limit of light using atomic force microscopy integrated with IR laser spectroscopy (AFM-IR) to characterize sub-micron features of the hair cortex at ultra-high spatial resolution. The resulting spectral analysis shows clear distinctions in the Amide bands in the macrofibrils and surrounding intermacrofibrillar matrix, yielding novel insight into the molecular structure and intermolecular stabilization interactions of the constituent proteins within each cortical component.EPSRC and Unilever on Grant EP/R511870/

The Adsorption of Synovene on ZDDP Wear Tracks: A Sum Frequency Generation (SFG) Vibrational Spectroscopy Study

The adsorption of the lubricant additive Synovene on steel and on ZDDP/steel wear tracks from base oil has been investigated by sum frequency generation (SFG) vibrational spectroscopy, an interface specific technique. SFG spectra (resonances) were investigated in the C-H stretching region and arise from the aliphatic chains of the palm oil constituent of Synovene. The observation of SFG spectra means that Synovene is adsorbed at the oil/metal and at the oil/ZDDP/metal interfaces and that the aliphatic chains of Synovene have a net polarisation order with respect to the surface. The intense spectrum observed when the film is first formed decreases in intensity with increasing temperature. It is proposed that this is due to a decrease in film thickness as the film tends towards monolayer thickness. A dependence of the intensity and shape of SFG resonances on film thickness due to a thickness-dependent interference effect has been observed in other thin film systems, most notably lipid films on gold. Supporting evidence for the film thickness hypothesis comes from examining the spectra of different thickness films of palmitic acid on steel, one of the constituents of Synovene. The spectra on the wear track are less intense and less reproducible than on the bare metal. After periods of several days at room temperature the spectra on both surfaces gain in intensity implying a return to thicker layers of Synovene under cold conditions.BP Castro

Thermal Behaviour of Synovene and Oleamide in Oil Adsorbed on Steel

AbstractOleamide and Synovene lubricant additives when mixed together show a clear co-operative effect leading to friction and wear reduction. Sum Frequency Generation vibrational spectroscopy has been used to record in situ spectra of these additives with the aim of understanding the behaviour of these molecules when adsorbed on steel immersed in a model base oil at pre-selected temperatures. The spectra of the individual components and of mixtures have been recorded up to 130 °C. Individual spectra from both molecules have been distinguished using per-deuterated oleamide. The temperature at which maximum ordered adsorption of pure Synovene molecules occurs drops from ~ 130 to ~ 70 °C in the mixture with oleamide. Our results show that co-adsorption occurs, which causes a change in net polar orientation of the oleamide component suggesting the hydrocarbon chains of the oleamide molecules reverse their polar orientation when Synovene is present. The net effect of co-adsorption and change in orientation as well as conformation of the two molecules could explain the reduction of friction and wear observed at the metal–metal interface.</jats:p

SFG Study of the Potential-Dependent Adsorption of the p-Toluenesulfonate Anion at an Activated Carbon/Propylene Carbonate Interface

Sum frequency generation spectroscopy has been used to characterize the potential-dependent adsorption of the p-toluenesulfonate anion at the activated carbon/propylene carbonate interface of the commercial carbon YP50F. Spectra recorded from the interface between YP50F and a 1 M tetraethylammonium p-toluenesulfonate in propylene carbonate solution showed no ordered anion adsorption without an applied potential. In contrast, there is clear evidence of increasingly ordered anion adsorption with applied potential. Furthermore, there is evidence of hysteresis such that the anion remains adsorbed when the applied potential was decreased back to 0 V. Significant reversal of polarity was required before the anion signal was lost. Changes to the propylene carbonate solvent peaks during the electrochemical cycle were also observed. The data indicate that the positive electrode charges either via a counterion adsorption mechanism or via an ion-exchange mechanism.E.K.H. acknowledges the European Research Council ERC Grant ERC-2009-AdG-247411 for funding

Investigating Bénard-Marangoni migration at the air-water interface in the time domain using sum frequency generation (SFG) spectroscopy of palmitic acid monolayers.

Sum-frequency generation (SFG) spectroscopy is frequently used to investigate the structure of monolayer films of long-chain fatty acids at the air-water interface. Although labeled a non-invasive technique, introducing intense SFG lasers onto liquid interfaces has the potential to perturb them. In the present work, narrowband picosecond SFG is used to study the structural changes that occur in palmitic acid and per-deuterated palmitic acid monolayers at the air-water interface in response to the high field strengths inherent to SFG spectroscopy. In order to determine structural changes and identify measurement artifacts, the changes in specific resonance intensities were measured in real-time and over a broad range of surface concentrations from films spread onto a stationary Langmuir trough. Using narrowband instead of broadband SFG minimizes the overlap of the incident infrared beam in the lipid C-H stretching region with resonances from the water sub-phase. Nevertheless, narrowband SFG still generates a thermal gradient at the surface, which produces a significant decrease in local concentration in the area of the laser spot caused by Bérnard-Marangoni convection originating in the sub-phase. The decrease in concentration results in an increase in the conformational disorder and a decrease in the tilt angle of lipid tails. Crucially, it is shown that, even at the highest monolayer concentrations, this gives rise to a measurement effect, which manifests itself as a dependence on the spectral acquisition time. This effect should be taken into account when interpreting the structure of monolayer films on liquid surfaces deduced from their SFG spectra.Unileve

In situ investigation of the oxidation of a phospholipid monolayer by reactive oxygen species

The oxidation of membrane lipids has been widely studied for several decades due to its significance in biological systems. However, despite its damaging physiological impact and its known role in many diseases, relatively little is understood about the specific structural consequences of oxidative action, particularly in vivo. In this work, a combination of sum-frequency generation (SFG) spectroscopy, surface tensiometry, and surface-selective infrared spectroscopies are used to gain deeper insight into the oxidation of phospholipids by reactive oxygen species (ROS) generated in situ. Oxidation is achieved by employing the Fenton reaction to convert physiological levels of H2O2 into OH and HO2 radicals in proximity to the head-groups of lipid monolayers at the air-water interface. By temporally monitoring the surface tension and spectroscopic changes at the interface as the oxidation proceeds, the impact of oxidation on the structure, conformation, and intermolecular interactions within the membrane has been revealed

Orientation analysis of sum frequency generation spectra of di-chain phospholipids: Effect of the second acyl chain

Sum Frequency Generation (SFG) spectroscopy is widely used for studying the di-chain phospholipid monolayers incorporated in model cell membranes. In this context it is frequently assumed, without justification, that the chains are identical and so their individual contribution to the SFG spectra are indistinguishable. However, the combination of both attractive and repulsive Van der Waals’ interactions between the chains results in a finite angle between their two terminal methyl groups resulting in non-equivalent contributions to the non-linear susceptibility. This work describes the application of the underlying non-linear theory required to produce the accurate SFG spectral simulations needed to test this assertion, and therefore provide the necessary quantitative validation. For phospholipids comprising two identical saturated chains, which typically have small angles of divergence, these simulations predict only small deviations in the SFG intensities from those calculated assuming a single methyl orientation. Non-identical tails, however, with differences in the degree or type of chain unsaturation, or in the parity of the chain lengths give much larger discrepancies when compared to the assumption of single chain. In these cases, the two tails must be treated as separate entities and that their structural relationship incorporated into the interpretation of their SFG spectra. A second important result from the simulations arises from the systematic nature of the deviations which shows that even small intensity changes should not be quickly dismissed on the basis of being subsumed by the uncertainties associated with spectral noise.Unilever R&

Mechanistic Insights into the Challenges of Cycling a Nonaqueous Na–O2 Battery

Superoxide-based nonaqueous metal–oxygen batteries have received considerable research attention as they exhibit high energy densities and round-trip efficiencies. The cycling performance, however, is still poor. Here we study the cycling characteristic of a Na–O2 battery using solid-state nuclear magnetic resonance, Raman spectroscopy, and scanning electron microscopy. We find that the poor cycling performance is primarily caused by the considerable side reactions stemming from the chemical aggressiveness of NaO2 as both a solid-phase and dissolved species in the electrolyte. The side reaction products cover electrode surfaces and hinder electron transfer across the electrode–electrolyte interface, being a major reason for cell failure. In addition, the available electrode surface and porosity change considerably during cell discharging and charging, affecting the diffusion of soluble species (superoxide and water) and resulting in inhomogeneous reactions across the electrode. This study provides insights into the challenges associated with achieving long-lived superoxide-based metal–O2 batteries

A time domain study of surfactin penetrating a phospholipid monolayer at the air-water interface investigated using sum frequency generation spectroscopy, infrared reflection absorption spectroscopy, and AFM-nano infrared microscopy.

We have investigated the interaction of surfactin with a monolayer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air-water interface as a function of time, following its injection into the sub-phase, using non-linear Sum Frequency Generation (SFG) vibrational spectroscopy and Infrared Reflection Absorption Spectroscopy (IRRAS). SFG resonances from the phospholipid and from the surfactin were distinguished from each other by using selective deuteration. The surface pressure at the interface was measured concurrently for up to 8 h. After an induction period, the spectra from the lipid diminished and those of surfactin gradually appeared whilst at the same time the surface pressure increased. However, eventually the surfactin signals disappeared and those of the lipid reappeared. Although the SFG spectra of the lipid disappeared at intermediate times, the IRRAS spectra of the lipid were always present at the interface. Variation in the temporal SFG behaviour was investigated as the pH of the sub-phase, the initial surface pressure of the lipid, and the surfactin concentration were changed. Samples of the surface film were transferred onto mica substrates at selected times along the temporal profile and imaged by Atomic Force Microscopy - nano Infrared Spectroscopy (nano-IR). A model is proposed to account for the results from the four different experimental techniques used