Towards Room Temperature Phase Transition of W-Doped VO2 Thin Films Deposited by Pulsed Laser Deposition: Thermochromic, Surface, and Structural Analysis

Vanadium dioxide (VO₂) with an insulator-to-metal (IMT) transition (∼68 °C) is considered a very attractive thermochromic material for smart window applications. Indeed, tailoring and understanding the thermochromic and surface properties at lower temperatures can enable room-temperature applications. The effect of W doping on the thermochromic, surface, and nanostructure properties of VO₂ thin film was investigated in the present proof. W-doped VO₂ thin films with different W contents were deposited by pulsed laser deposition (PLD) using V/W (+O₂) and V₂O₅/W multilayers. Rapid thermal annealing at 400-450 °C under oxygen flow was performed to crystallize the as-deposited films. The thermochromic, surface chemistry, structural, and morphological properties of the thin films obtained were investigated. The results showed that the V⁵⁺ was more surface sensitive and W distribution was homogeneous in all samples. Moreover, the V₂O₅ acted as a W diffusion barrier during the annealing stage, whereas the V+O₂ environment favored W surface diffusion. The phase transition temperature gradually decreased with increasing W content with a high efficiency of -26 °C per at. % W. For the highest doping concentration of 1.7 at. %, VO₂ showed room-temperature transition (26 °C) with high luminous transmittance (62%), indicating great potential for optical applications.

Ultrafast Laser Processing of Nanostructured Patterns for the Control of Cell Adhesion and Migration on Titanium Alloy

Femtosecond laser texturing is a promising surface functionalization technology to improve the integration and durability of dental and orthopedic implants. Four different surface topographies were obtained on titanium-6aluminum-4vanadium plates by varying laser processing parameters and strategies: surfaces presenting nanostructures such as laser-induced periodic surface structures (LIPSS) and ‘spikes’, associated or not with more complex multiscale geometries combining micro-pits, nanostructures and stretches of polished areas. After sterilization by heat treatment, LIPSS and spikes were characterized to be highly hydrophobic, whereas the original polished surfaces remained hydrophilic. Human mesenchymal stem cells (hMSCs) grown on simple nanostructured surfaces were found to spread less with an increased motility (velocity, acceleration, tortuosity), while on the complex surfaces, hMSCs decreased their migration when approaching the micro-pits and preferentially positioned their nucleus inside them. Moreover, focal adhesions of hMSCs were notably located on polished zones rather than on neighboring nanostructured areas where the protein adsorption was lower. All these observations indicated that hMSCs were spatially controlled and mechanically strained by the laser-induced topographies. The nanoscale structures influence surface wettability and protein adsorption and thus influence focal adhesions formation and finally induce shape-based mechanical constraints on cells, known to promote osteogenic differentiation.

2D reproduction of the face on the Turin Shroud by infrared femtosecond pulse laser processing

Femtosecond pulse laser processing concentrates a huge quantity of light energy in extremely short pulses of a few tens to hundreds of femtoseconds, enabling superficial laser machining or marking of any kind of materials, with a reduced or insignificant heat affected area. A digitized paper printed image of the face on the Turin Shroud was used to monitor a scan head intercalated between a femtosecond pulsed laser source and a linen fabric sample, enabling the direct 2D reproduction of the image of the face with a laser beam size corresponding to one pixel of the digitized image. The contrast in the marked image was controlled by adjusting the energy density, the number of superimposed pulses per pixel, and the distance between successive impacts. The visual aspect of the laser-induced image is very similar, at naked eye, to the source image. The negative photograph of the marked linen fabric reveals a face remarkably close to the well-known negative picture of the face on the Turin Shroud. Analyses by infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy were performed to characterize the laser marked areas.

Electroanalytical Performance of Nitrogen-Doped Graphene Films Processed in One Step by Pulsed Laser Deposition Directly Coupled with Thermal Annealing

Graphene-based materials are widely studied to enable significant improvements in electroanalytical devices requiring new generations of robust, sensitive and low-cost electrodes. In this paper, we present a direct one-step route to synthetize a functional nitrogen-doped graphene film onto a Ni-covered silicon electrode substrate heated at high temperature, by pulsed laser deposition of carbon in the presence of a surrounding nitrogen atmosphere, with no post-deposition transfer of the film. With the ferrocene methanol system, the functionalized electrode exhibits excellent reversibility, close to the theoretical value of 59 mV, and very high sensitivity to hydrogen peroxide oxidation. Our electroanalytical results were correlated with the composition and nanoarchitecture of the N-doped graphene film containing 1.75 at % of nitrogen and identified as a few-layer defected and textured graphene film containing a balanced mixture of graphitic-N and pyrrolic-N chemical functions. The absence of nitrogen dopant in the graphene film considerably degraded some electroanalytical performances. Heat treatment extended beyond the high temperature graphene synthesis did not significantly improve any of the performances. This work contributes to a better understanding of the electrochemical mechanisms of doped graphene-based electrodes obtained by a direct and controlled synthesis process.

Review of Graphene Growth From a Solid Carbon Source by Pulsed Laser Deposition (PLD)

Graphene is a remarkable two-dimensional (2D) material that is of great interest to both academia and industry. It has outstanding electrical and thermal conductivity and good mechanical behavior with promising applications in electronic devices, supercapacitors, batteries, composite materials, flexible transparent displays, solar cells, and sensors. Several methods have been used to produce either pristine graphene or doped graphene. These include chemical vapor deposition (CVD), mechanical exfoliation, decomposition of SiC, liquid-phase exfoliation, pulsed laser deposition (PLD). Among these methods, PLD, which is routinely used for growing complex oxide thin films has proved to be an alternative to the more widely reported CVD method for producing graphene thin films, because of its advantages. Here we review the synthesis of graphene using PLD. We describe recent progress in preparing pristine graphene and doped graphene by PLD, including deposition processes and characterization. The goal of this complete survey is to describe the advantages of using the technique for graphene growth. The review will also help researchers to better understand graphene synthesis using the PLD technique.

Robust Electrografting on Self-Organized 3D Graphene Electrodes

Improving graphene-based electrode fabrication processes and developing robust methods for its functionalization are two key research routes to develop new high-performance electrodes for electrochemical applications. Here, a self-organized three-dimensional (3D) graphene electrode processed by pulsed laser deposition with thermal annealing is reported. This substrate shows great performance in electron transfer kinetics regarding ferrocene redox probes in solution. A robust electrografting strategy for covalently attaching a redox probe onto these graphene electrodes is also reported. The modification protocol consists of a combination of diazonium salt electrografting and click chemistry. An alkyne-terminated phenyl ring is first electrografted onto the self-organized 3D graphene electrode by in situ electrochemical reduction of 4-ethynylphenyl diazonium. Then the ethynylphenyl-modified surface efficiently reacts with the redox probe bearing a terminal azide moiety (2-azidoethyl ferrocene) by means of Cu(I)-catalyzed alkyne-azide cycloaddition. Our modification strategy applied to 3D graphene electrodes was analyzed by means of atomic force microscopy, scanning electron microscopy, Raman spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy (XPS). For XPS chemical surface analysis, special attention was paid to the distribution and chemical state of iron and nitrogen in order to highlight the functionalization of the graphene-based substrate by electrochemically grafting a ferrocene derivative. Dense grafting was observed, offering 4.9 × 10(-10) mol cm(-2) surface coverage and showing a stable signal over 22 days. The electrografting was performed in the form of multilayers, which offers higher ferrocene loading than a dense monolayer on a flat surface. This work opens highly promising perspectives for the development of self-organized 3D graphene electrodes with various sensing functionalities.

Structural similarities of Na,K-ATPase and SERCA, the Ca(2+)-ATPase of the sarcoplasmic reticulum

The crystal structure of SERCA1a (skeletal-muscle sarcoplasmic-reticulum/endoplasmic-reticulum Ca(2+)-ATPase) has recently been determined at 2.6 A (note 1 A = 0.1 nm) resolution [Toyoshima, Nakasako, Nomura and Ogawa (2000) Nature (London) 405, 647-655]. Other P-type ATPases are thought to share key features of the ATP hydrolysis site and a central core of transmembrane helices. Outside of these most-conserved segments, structural similarities are less certain, and predicted transmembrane topology differs between subclasses. In the present review the homologous regions of several representative P-type ATPases are aligned with the SERCA sequence and mapped on to the SERCA structure for comparison. Homology between SERCA and the Na,K-ATPase is more extensive than with any other ATPase, even PMCA, the Ca(2+)-ATPase of plasma membrane. Structural features of the Na,K-ATPase are projected on to the Ca(2+)-ATPase crystal structure to assess the likelihood that they share the same fold. Homology extends through all ten transmembrane spans, and most insertions and deletions are predicted to be at the surface. The locations of specific residues are examined, such as proteolytic cleavage sites, intramolecular cross-linking sites, and the binding sites of certain other proteins. On the whole, the similarity supports a shared fold, with some particular exceptions.

Thermal denaturation of the Na,K-ATPase provides evidence for alpha-alpha oligomeric interaction and gamma subunit association with the C-terminal domain

Thermal denaturation can help elucidate protein domain substructure. We previously showed that the Na,K-ATPase partially unfolded when heated to 55 degrees C (Arystarkhova, E., Gibbons, D. L., and Sweadner, K. J. (1995) J. Biol. Chem. 270, 8785-8796). The beta subunit unfolded without leaving the membrane, but three transmembrane spans (M8-M10) and the C terminus of the alpha subunit were extruded, while the rest of alpha retained its normal topology with respect to the lipid bilayer. Here we investigated thermal denaturation further, with several salient results. First, trypsin sensitivity at both surfaces of alpha was increased, but not sensitivity to V8 protease, suggesting that the cytoplasmic domains and extruded domain were less tightly packed but still retained secondary structure. Second, thermal denaturation was accompanied by SDS-resistant aggregation of alpha subunits as dimers, trimers, and tetramers without beta or gamma subunits. This implies specific alpha-alpha contact. Third, the gamma subunit, like the C-terminal spans of alpha, was selectively lost from the membrane. This suggests its association with M8-M10 rather than the more firmly anchored transmembrane spans. The picture that emerges is of a Na,K-ATPase complex of alpha, beta, and gamma subunits in which alpha can associate in assemblies as large as tetramers via its cytoplasmic domain, while beta and gamma subunits associate with alpha primarily in its C-terminal portion, which has a unique structure and thermal instability.

Chemical modification reveals involvement of different sites for nucleotide analogues in the phosphatase activity of the red cell calcium pump

The calcium pump of plasma membranes catalyzes the hydrolysis of ATP and phosphoric esters like p-nitrophenyl phosphate (pNPP). The latter activity requires the presence of ATP and/or calmodulin, and Ca2+ [22, 25]. We have studied the effects of nucleotide-analogues and chemical modifications of nucleotide binding sites on Ca2+-pNPPase activity. Treatment with fluorescein isothiocyanate (FITC), abolished Ca2+-ATPase and ATP-dependent pNPPase, but affected only 45% of the calmodulin-dependent pNPPase activity. The nucleotide analogue eosin-Y had an inhibitory effect on calmodulin-dependent pNPPase (Kieosin-Y = 2 microM). FITC treatment increased Kieosin-Y 15 times. Acetylation of lysine residues with N-hydroxysuccinimidyl acetate inactivates Ca2+-ATPase by modifying the catalytic site, and impairs stimulation by modulators by modifying residues outside this site [9]. Acetylation suppressed the ATP-dependent pNPPase with biphasic kinetics. ATP or pNPP during acetylation cancels the fast component of inactivation. Acetylation inhibited only partially the calmodulin-dependent pNPPase, but neither ATP nor pNPP prevented this inactivation. From these results we conclude: (i) ATP-dependent pNPPase depends on binding of ATP to the catalytic site; (ii) the catalytic site plays no role in calmodulin-dependent pNPPase. The decreased affinity for eosin-Y of the FITC-modified enzyme, suggests that the sites for these two molecules are closely related but not overlapped. Acetimidation of the pump inhibited totally the calmodulin-dependent pNPPase, but only partially the ATP-pNPPase. Since calmodulin binds to E1, the E1 conformation or the E2 if E1 transition would be involved during calmodulin-dependent pNPPase activity.

Acetylation with succinimidyl acetate affects both the catalytic site and the regulation of the erythrocyte Ca2+ pump

Acetylation of lysine residues of the erythrocyte Ca2+ pump using succinimidyl acetate (SA) led to its complete inactivation. In the absence of any of the major activators of the pump (namely calmodulin and acidic phospholipids), ATP fully protected the pump from inactivation by SA, with a K0.5 of 13 microM. This value is very close to the Km of the high-affinity site for ATP, thus suggesting that the residue(s) involved is(are) near or at the catalytic site of the Ca(2+)-ATPase. Furthermore, the presence of 500 microM ATP prevented the acetylation of about two residues per molecule of enzyme. Acetylation by SA also prevented the activation of the Ca2+ pump by calmodulin, acidic phospholipids or controlled trypsin proteolysis. This effect of SA treatment was not avoided by the presence of ATP in the preincubation medium, indicating a second set of modified residues. The fact that the three modes of activation were cancelled in a similar fashion by SA suggests that, although acting via different mechanisms, they share at least a common step in which SA-sensitive lysine residues may participate. Moreover, modification of the pump by SA plus ATP decreased the KCa when the activity was measured in both the absence and presence of calmodulin, suggesting that the residue(s) modified in this case is(are) involved directly in the regulation of the affinity for Ca2+.

Differential reactivity of lysine residues of the red blood cell Ca2+ pump involved in the E1-E2 conformational equilibrium

1. Modification of Lys residues of the Ca(2+)-ATPase from human red blood cells with methyl acetimidate (MA) inhibited up to 70% of the Ca(2+)-ATPase activity. Furthermore, calmodulin-activated p-nitrophenyl phosphatase activity was fully inhibited at non-limiting concentrations of MA. 2. Treatment with MA inhibited phosphorylation of the Ca(2+)-ATPase. 3. When the enzyme was treated with 7.2 mM-MA in the presence of 100 microM-Ca2+, Ca(2+)-ATPase activity was decreased by 33%, whereas when the membranes were treated with MA in the presence of 50 microM-VO4(3-), this activity was decreased by only 8%. 4. When membranes were either proteolysed or preincubated with 1 mM-Ca2+, MA quickly inactivated the Ca(2+)-ATPase (k = 1.2 min-1). On the other hand, inactivation of membranes preincubated in the absence of Ca2+ and Mg2+ was slow (k = 0.08 min-1). 5. When the activity was measured in the absence of calmodulin, MA decreased to the same extent the values of KCa (the apparent dissociation constant for Ca2+) and Vmax, but in the presence of calmodulin the treatment decreased Vmax. only. 6. The results are consistent with the idea that MA reacts readily with the Ca(2+)-ATPase when the enzyme is in an E1 conformation, but not an E2 conformation, and that, reciprocally, treatment of the enzyme with MA shifts the enzyme to E1. 7. Provided that Ca2+ is present, ATP, with low apparent affinity (K0.5 = 195 microM), protected against inactivation by MA. However, MA treatment did not change the Km values of either the high-affinity or the low-affinity site for ATP, suggesting that protection results from a shift to a conformation in which the Lys residues are inaccessible to MA.

Characteristics of a presynaptic plasma membrane Ca2(+)-ATPase activity from electric organ

Ca2(+)-ATPase activity was measured in electric organ synaptosomal homogenates and their derived presynaptic plasma membranes using a low ionic strength medium, low in Ca2+ and Mg2+, and devoid of K+. The enzyme activity showed a high apparent affinity for Ca2+ (KCa:0.5 microM) and was: (1) 5-fold stimulated by 120 nM calmodulin, (2) highly sensitive to LaCl3 inhibition, and (3) not affected by 20 mM NaN3 or 0.1 mM ouabain. The addition of Mg2+ promoted the disappearance of Ca2(+)-ATPase activity. Incubation of synaptosomal homogenates in the above-mentioned assay medium with [gamma -32P]ATP resulted in the appearance of a 140 kDa band as revealed by SDS-gel electrophoresis. Labeling of this band with 32P was inhibited by 1 mM EGTA or 10 mM NH2OH, indicating that the isotope incorporation required the presence of Ca2+ and the formation of an acyl-phosphate derivative. The results indicate that the Ca2(+)-ATPase activity from synaptosomal homogenates had characteristics corresponding to those of the enzyme that catalyzes an outward transport of Ca2+ in nerve terminals. Preincubation of synaptosomes in Ca2+ plus K+, a depolarizing procedure, induced a large and rapid decrease in the Ca2(+)-ATPase activity, possibly mediated via Ca2+ entry through voltage-gated Ca2+ channels. Furthermore, the muscarinic cholinergic agonist oxotremorine (at 15 microM concentration) did not significantly affect either the enzyme activity or the intensity of the Ca2(+)-dependent 32P incorporation into the 140 kDa band, suggesting that the enzyme is not coupled to muscarinic binding sites.