Evaluation of Physico-Chemical Characteristics of Cement Superplasticizer Based on Polymelamine Sulphonate.

Cementitious materials are used to construct an engineered barrier in repositories for radioactive waste. The cement matrix may contain a variety of organic compounds, some of which are polymeric admixtures used as plasticizers. Superplasticizers (SPs) are highly effective organic cement additives for reducing water amount, increasing workability, homogeneity, plasticity and the non-segregation of mortars and grouts, improving mechanical properties and resistance to destructive environments. SPs in cement could have an impact on the long-term safety of the disposals of radioactive waste. These organic agents can leach from the cementitious matrix into groundwater and may affect the migration behaviour of radionuclides. The detailed chemical composition and other characteristics of the cement (CEM I 42.5 R, Sweden) used for the leaching experiments were evaluated. It contained mainly CaO (52.51 ± 1.37, %), and the surface area of the cement particles was 13.2 ± 1.3 m2/g. An insignificant increase in pH (from 12.6 ± 0.1 to 12.8 ± 0.1) was observed for the leachates over 10 days. A commercially available cement superplasticizer based on polymelamine sulphonate (PMS) Peramin SMF10 (Peramin AB, Sweden) was chosen for the research. The product's chemical composition was analysed using wavelength-dispersive X-ray fluorescence (WD-XRF) spectroscopy, while other physico-chemical properties of the PMS superplasticizer were assessed by Raman spectroscopy and thermo-gravimetric analysis. In aqueous solutions and powders of PMS, the same most intensive features were observed at 774 cm-1 (ring out-of-plane deformation), 977 cm-1 (C-N-C bending, SO stretching) and 1055 cm-1 (C-N=C bending) in the Raman spectra. At up to 270 °C, the polymer was thermally stable. Raman and UV/Vis spectroscopies were used to assess the rate of the alkaline degradation of PMS superplasticizer in different aqueous solutions. No changes were observed in the hydrolytic solutions with any of the above analytical methods over a period of 3 years. The results obtained revealed a good thermal and chemical stability (in highly alkaline media, pH = 9.9-12.9) of the PMS polymer.

Epitaxial Lateral Overgrowth of GaN on a Laser-Patterned Graphene Mask.

Epitaxial lateral overgrowth (ELO) of GaN epilayers on a sapphire substrate was studied by using a laser-patterned graphene interlayer. Monolayer graphene was transferred onto the sapphire substrate using a wet transfer technique, and its quality was confirmed by Raman spectroscopy. The graphene layer was ablated using a femtosecond laser, which produced well-defined patterns without damaging the underlying sapphire substrate. Different types of patterns were produced for ELO of GaN epilayers: stripe patterns were ablated along the [1¯100]sapphire and [112¯0]sapphire directions, a square island pattern was ablated additionally. The impact of the graphene pattern on GaN nucleation was analyzed by scanning electron microscopy. The structural quality of GaN epilayers was studied by cathodoluminescence. The investigation shows that the laser-ablated graphene can be integrated into the III-nitride growth process to improve crystal quality.

Green Removal of DUV-Polarity-Modified PMMA for Wet Transfer of CVD Graphene.

To fabricate graphene-based high-frequency electronic and optoelectronic devices, there is a high demand for scalable low-contaminated graphene with high mobility. Graphene synthesized via chemical vapor deposition (CVD) on copper foil appears promising for this purpose, but residues from the polymethyl methacrylate (PMMA) layer, used for the wet transfer of CVD graphene, drastically affect the electrical properties of graphene. Here, we demonstrate a scalable and green PMMA removal technique that yields high-mobility graphene on the most common technologically relevant silicon (Si) substrate. As the first step, the polarity of the PMMA was modified under deep-UV irradiation at λ = 254 nm, due to the formation of ketones and aldehydes of higher polarity, which simplifies hydrogen bonding in the step of its dissolution. Modification of PMMA polarity was confirmed by UV and FTIR spectrometry and contact angle measurements. Consecutive dissolution of DUV-exposed PMMA in an environmentally friendly, binary, high-polarity mixture of isopropyl alcohol/water (more commonly alcohol/water) resulted in the rapid and complete removal of DUV-exposed polymers without the degradation of graphene properties, as low-energy exposure does not form free radicals, and thus the released graphene remained intact. The high quality of graphene after PMMA removal was confirmed by SEM, AFM, Raman spectrometry, and by contact and non-contact electrical conductivity measurements. The removal of PMMA from graphene was also performed via other common methods for comparison. The charge carrier mobility in graphene films was found to be up to 6900 cm2/(V·s), demonstrating a high potential of the proposed PMMA removal method in the scalable fabrication of high-performance electronic devices based on CVD graphene.

Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy for Probing Riboflavin on Graphene.

Graphene research and technology development requires to reveal adsorption processes and understand how the defects change the physicochemical properties of the graphene-based systems. In this study, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and graphene-enhanced Raman spectroscopy (GERS) coupled with density functional theory (DFT) modeling were applied for probing the structure of riboflavin adsorbed on single-layer graphene substrate grown on copper. Intense and detailed vibrational signatures of the adsorbed riboflavin were revealed by SHINERS method. Based on DFT modeling and detected downshift of prominent riboflavin band at 1349 cm-1 comparing with the solution Raman spectrum, π-stacking interaction between the adsorbate and graphene was confirmed. Different spectral patterns from graphene-riboflavin surface were revealed by SHINERS and GERS techniques. Contrary to GERS method, SHINERS spectra revealed not only ring stretching bands but also vibrational features associated with ribityl group of riboflavin and D-band of graphene. Based on DFT modeling it was suggested that activation of D-band took place due to riboflavin induced tilt and distortion of graphene plane. The ability to explore local perturbations by the SHINERS method was highlighted. We demonstrated that SHINERS spectroscopy has a great potential to probe adsorbed molecules at graphene.

MOVPE Growth of GaN via Graphene Layers on GaN/Sapphire Templates.

The remote epitaxy of GaN epilayers on GaN/sapphire templates was studied by using different graphene interlayer types. Monolayer, bilayer, double-stack of monolayer, and triple-stack of monolayer graphenes were transferred onto GaN/sapphire templates using a wet transfer technique. The quality of the graphene interlayers was examined by Raman spectroscopy. The impact of the interlayer type on GaN nucleation was analyzed by scanning electron microscopy. The graphene interface and structural quality of GaN epilayers were studied by transmission electron microscopy and X-ray diffraction, respectively. The influence of the graphene interlayer type is discussed in terms of the differences between remote epitaxy and van der Waals epitaxy. The successful exfoliation of GaN membrane is demonstrated.

Gold Nanoparticles Generated Using the Nanosecond Laser Treatment of Multilayer Films and Their Use for SERS Applications.

Surface-enhanced Raman spectroscopy (SERS) substrates fabricated using a repeated laser treatment of thin gold films are demonstrated. The presented SERS substrates consist of the gold nanoparticles, whose density and size depend on the used film thickness and number of treated films. The larger number of the treated gold film layers increases the amount of larger nanoparticles (size >20 nm). However, a large number of small nanoparticles (5-20 nm) in all cases is also observed. The manufactured SERS substrates exhibit a high enhancement factor, which is in the range of 106. The enhancement factor can be increased by adding an additional Au coating on the top of nanoparticles generated from a single gold layer. The demonstrated laser-based fabrication approach of large-scale SERS substrates is simple, reliable, without the use of chemicals for the reduction and stabilization of nanoparticles, and cost-effective.

Shell-isolated nanoparticle-enhanced Raman spectroscopy for characterization of living yeast cells.

Studying the biochemistry of yeast cells has enabled scientists to understand many essential cellular processes in human cells. Further development of biotechnological and medical progress requires revealing surface chemistry in living cells by using a non-destructive and molecular structure sensitive technique. In this study shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was applied for probing the molecular structure of Metschnikowia pulcherrima yeast cells. Important function of studied cells is the ability to eliminate iron from growth media by precipitating the insoluble pigment pulcherrimin. Comparative SERS and SHINERS analysis of the yeast cells in combination with bare Au and shell-isolated Au@SiO2 nanoparticles were performed. It was observed that additional bands, such as adenine ring-related vibrational modes appear due to interaction with bare Au nanoparticles; the registered spectra do not coincide with the spectra where Au@SiO2 nanoparticles were used. SHINERS spectra of M. pulcherrima were significantly enhanced comparing to the Raman spectra. Based on first-principles calculations and 830-nm excited Raman analysis of pulcherrimin, the SHINERS signatures of iron pigment in yeast cells were revealed. Being protected from direct interaction of metal with adsorbate, Au@SiO2 nanoparticles yield reproducible and reliable vibrational signatures of yeast cell wall constituents.

Crowding enhances lipase turnover rate on surface-immobilized substrates.

Utilizing surface-immobilized synthetic lipid substrates containing the redox-active ferrocene groups, the enzymatic activity of lipase from Thermomyces lanuginosus was measured by the cyclic voltammetry method. The activity was correlated with the surface density of the protein by the ATR-IR spectroscopy and the total internal reflection ellipsometry. It was found that the lipase turnover rate significantly increases with its surface density. Despite expected hindrance effects due to the crowding of the enzyme molecules in the near surface-saturation range of concentrations, the turnover rate was consistently higher compared with the values measured at low concentrations. The effect was explained by the change in the surface arrangement of the enzyme. In the low concentration range, lipase adsorbs onto a surface adopting a predominantly horizontal position. At high concentrations, as the surface density approaches saturation, the enzyme molecules due to crowding are forced into the predominantly vertical position, which is more favorable for the activation of the lipase through the interaction between the "hydrophobic lid" of the lipase and the hydrophobic adsorbate surface.

Effect of potential on temperature-dependent SERS spectra of neuromedin B on Cu electrode.

Adsorption of decapeptide neuromedin B (NMB) on copper electrode has been investigated by in situ surface-enhanced Raman scattering (SERS) spectroelectrochemistry in the temperature interval from 12 to 72 °C at -0.600 and -1.000 V potentials. It was found that intensities of peptide bands decrease at temperatures above 30 °C with higher decrease slope at -1.000 V. Frequency of F12 mode (1004 cm(-1)) of non-surface-interactive phenylalanine residue was found to be insensitive to temperature variation at both studied electrode potentials, while frequency-temperature curves for surface-interactive groups (Amide-III, methylene) were found to be controlled by the potential. In particular, opposite frequency-temperature trends were detected for Amide-III (Am-III) mode indicating decrease in H-bonding interaction strength of amide C[double bond, length as m-dash]O and N-H groups above 38 °C for -0.600 V, and increase in H-bonding interaction strength between 12 and 72 °C for -1.000 V. Anomalous Am-III temperature-dependence of the frequency at -1.000 V was explained by temperature-induced transformation of a disordered secondary structure to a helix-like conformation. The potential-difference spectrum revealed interaction of methylene groups with Cu surface at sufficiently negative potential values because of the appearance of a soft C-H stretching band near 2825 cm(-1) and a broad band near 2904 cm(-1) assigned to vibration of a distal C-H bond of the surface-confined methylene group. Consequently, a rapid decrease in frequency of CH(2)-stretching band with temperature was observed at -1.000 V, while no essential frequency changes were detected for this mode at -0.600 V. The results show that electrode potential controls the temperature-dependence of the frequency for vibrations associated with surface-interactive molecular groups.

Potential induced changes in neuromedin B adsorption on Ag, Au, and Cu electrodes monitored by surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS), electrochemistry, and generalized two-dimensional correlation analysis (G2DCA) methods were used to define neuromedin B (NMB) ordered superstructures on Ag, Au, and Cu electrode surfaces at different applied electrode potentials in an aqueous solution at physiological pH. The orientation of NMB and the adsorption mechanism were determined based on the analysis of enhancement, broadness, and shift in wavenumber of particular bands, which allow drawing some conclusions about NMB geometry and changes in this geometry upon change of the electrode type and applied electrode potential. The presented data demonstrated that NMB deposited onto the Ag, Au, and Cu electrode surfaces showed bands due to vibrations of the moieties that were in contact/close proximity to the electrode surfaces and thus were located on the same side of the polypeptide backbone. These included the Phe(9) and Trp(4) rings, the sulfur atom of Met(10), and the -CCN- and -C═O units of Asn(2). However, some subtle variations in the arrangement of these fragments upon changes in the applied electrode potential were distinguished. The Amide-III vibrations exhibited an electrochemical Stark effect (potential dependent frequencies) with Stark tuning slope sensitive to the electrode material. Potential-difference spectrum revealed that the imidazole ring of His(8) was bonded to the Cu electrode surface at relatively positive potentials.

Structure of functional Staphylococcus aureus alpha-hemolysin channels in tethered bilayer lipid membranes.

We demonstrate a method for simultaneous structure and function determination of integral membrane proteins. Electrical impedance spectroscopy shows that Staphylococcus aureus alpha-hemolysin channels in membranes tethered to gold have the same properties as those formed in free-standing bilayer lipid membranes. Neutron reflectometry provides high-resolution structural information on the interaction between the channel and the disordered membrane, validating predictions based on the channel's x-ray crystal structure. The robust nature of the membrane enabled the precise localization of the protein within 1.1 A. The channel's extramembranous cap domain affects the lipid headgroup region and the alkyl chains in the outer membrane leaflet and significantly dehydrates the headgroups. The results suggest that this technique could be used to elucidate molecular details of the association of other proteins with membranes and may provide structural information on domain organization and stimuli-responsive reorganization for transmembrane proteins in membrane mimics.

Probing of lipase activity at air/water interface by sum-frequency generation spectroscopy.

The infrared-visible sum-frequency generation (SFG) vibrational spectroscopy was used to probe enzymatic activity of Thermomyces lanuginosus lipase (TLL) at air/water interface. A monolayer of amphiphilic O-palmitoyl-2,3-dicyanohydroquinone (PDCHQ), containing target ester group and two CN groups serving as vibrational markers, was utilized as an enzyme substrate. SFG data revealed the detailed molecular scale structure and properties of the PDCHQ layer at the interface. In particular, we demonstrate that hydrophilic headgroup of PDCHQ is mainly in the form of an oxyanion, and the enzyme-induced cleavage of the ester bond could be spectroscopically monitored by the disappearance of the intense C tripple bond N resonance at 2224 cm(-1). The enzymatic nature of the ester bond cleavage was confirmed by the control experiments with deactivated S146A mutant variant of TLL. By comparing action of wild type (WT) TLL and its inactive S146A mutant, it was shown that two effects take place at the interface: disordering of the lipid monolayer due to the adsorption of enzyme and enzymatic cleavage of the ester bond. The concentration of enzyme as low as 10 nM could be easily sensed by the SFG spectroscopy. We present spectroscopic evidence that upon hydrolysis one of the products, 2,3-dicyanohydroquinone, leaves the surface, while the other, palmitic acid, remains at air/water interface in predominantly undissociated form with the mono-hydrogen-bonded carbonyl group. Strong amide I (1662 cm(-1)) and amide A (3320 cm(-1)) SFG signals from TLL suggest that enzyme molecules position themselves at air/water interface in an orderly fashion. Presented work demonstrates the potential of SFG spectroscopy for in situ real-time monitoring of enzymatic processes at air/water interface.

Computational Modeling of the Electrochemical System of Lipase Activity Detection.

This paper presents computational modeling of response kinetics of bioelectroanalytical system based on solid supported lipase substrate and lipase interaction. The model assumes that lipase substrate is formed by dripping and drying a small amount of the ethanol solution of 9-(5'-ferrocenylpentanoyloxy)nonyl disulfide (FPONDS) and that lipase is capable of cleaving FPONDS ester bonds via hydrolysis mechanism. Two mathematical models have been developed and evaluated trough computational simulation series by comparing them to experimental data. The results of simulation demonstrate that a good fitting might be obtained only taking into account non-linear substrate wash off process.

Electrochemical method for the detection of lipase activity.

A novel electrochemical technique for the general assay of lipase activity is described. The method utilizes a solid-supported lipase substrate, which is formed by dripping and drying a small amount of an ethanol solution of 9-(5'-ferrocenylpentanoyloxy)nonyl disulfide (FPONDS) onto gold modified by a hexanethiol self-assembled monolayer. The redox ferrocene group of FPONDS generates the electrochemical signal, the intensity of which is proportional to the number of FPONDS molecules at the interface. Electrochemical and surface-enhanced infrared absorption spectroscopic data, as well as control experiments with an engineered, deactivated mutant enzyme, demonstrate that the wild-type lipase from Thermomyces lanuginosus is capable of cleaving the ester bonds of FPONDS molecules via an enzymatic hydrolysis mechanism, which includes the adsorption of the lipase onto the substrate surface. The hydrolysis liberates the ferrocene groups from the interface triggering a decay of the electrochemical redox signal. The rate of the electrochemical signal decrease is proportional to the lipase activity/concentration. These data suggest a general method for the direct measure of enzymatic activity of lipases.