Chemical characterisation of artists' spray-paints: A diagnostic tool for urban art conservation.

In this study the chemical characterisation of 24 commercial spray-paints in different colours as used in contemporary public murals, street art, and graffiti is presented. The analyses were focused on the identification of the binding media, pigments, and additives. In addition, four spray-paint samples were analysed in the form of bi-layered paint films to explore the possibility of determining the composition of multi-layered samples. The aim of the study was to provide a useful diagnostic tool for the conservation of spray-paints and the removal of overpaintings from both commissioned murals and any other form of cultural heritage. To achieve this goal, a multi-analytical approach was developed using Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) for the identification of the main binder, pigments, and fillers/extenders, while Raman spectroscopy and Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS) were used as complementary tools for the determination of organic and inorganic pigments, and fillers. Five kinds of binders were detected in this work: (1) acrylic resins combined with nitrocellulose, (2) acrylic resins modified with styrene and combined with nitrocellulose, (3) alkyd resins modified with styrene and combined with nitrocellulose, (4) combined acrylic and alkyd resins modified with styrene and blended with nitrocellulose, and (5) combined polystyrene and acrylic resins. Also, a wide variety of organic pigments and inorganic components were detected.

A self-assembly study of PNA-porphyrin and PNA-BODIPY hybrids in mixed solvent systems.

In this work a peptide nucleic acid (PNA) was covalently connected with two different chromophores, namely porphyrin and boron-dipyrromethene. To the best of our knowledge, this is the first example in the literature where a PNA unit is covalently linked to such chromophores. The self-assembly properties of the hybrids were examined through electron microscopy experiments by adopting the "good-bad" solvent self-assembly protocol. For both hybrids (PNA-TPP and PNA-BDP) we were able to observe distinctive supramolecular architectures. During these studies we investigated the influence of the solvent system, the concentration and the deposition method on the morphology of the formed nanostructures. In the case of PNA-TPP under all examined conditions well-formed nanospheres were obtained. Interestingly, in the PNA-BDP hybrid by simply altering the solvent mixture, self-assemblies of two different morphologies were formed (spherical and flake shaped). Absorption and emission studies suggested the formation of J-aggregates in all the obtained nanostructures. The nano-architectures assembled by PNA conjugates are capable of light-harvesting and producing hydrogen using Pt nanoparticles as a photocatalyst.

Multi-electron reduction of Wells-Dawson polyoxometalate films onto metallic, semiconducting and dielectric substrates.

The investigation of conditions allowing multi-electron reduction and reoxidation of polyoxometalate (POM) films onto solid substrates is considered an issue of critical importance for their successful incorporation in electronic devices, different types of sensors and catalytic systems. In the present paper, the rich multi-electron redox chemistry of films of Wells-Dawson ammonium salts, namely (NH4)6P2Mo18O62 and (NH4)6P2W18O62, on top of metallic (Al), semiconducting (ITO) and dielectric (SiO2) substrates under ambient conditions is investigated. The respective Keggin heteropolyacids, H3PMo12O40 and H3PW12O40, are also investigated for comparison. On Al substrates, the Wells-Dawson ammonium salts are found to be significantly more reduced (4-6e-) compared to the respective Keggin heteropolyacids (∼2e-), in accordance with their deeper lying lowest unoccupied molecular orbital (LUMO) level. Subsequent thermal treatment in air results in reoxidation of the initially highly reduced POM films. Similar behavior is found on ITO substrates, but in initially less reduced (2-4e-) Wells-Dawson POM films. On the other hand, on SiO2 substrates, the thermal reduction of (NH4)6P2Mo18O62 film is observed and attributed to the thermal oxidation of ammonium counterions by [P2Mo18O62]6- anions. Overall, the multi-electron reduction of Wells-Dawson ammonium salts onto metallic and semiconducting substrates (Al, ITO) is determined by the relative position of the LUMO level of POMs in relation to the Fermi level of the substrate (i.e. substrate work function) and affected in a synergistic way by the presence of ammonium counterions. In contrast, on dielectric substrates (SiO2) the reduction of Wells-Dawson POMs ((NH4)6P2Mo18O62) is attributed only to the oxidation of ammonium counterions.

Guided cell adhesion, orientation, morphology and differentiation on silicon substrates photolithographically micropatterned with a cell-repellent cross-linked poly(vinyl alcohol) film.

In this work, silicon substrates with poly(vinyl alcohol) (PVA) patterns created by a simple, low-cost and high-fidelity photolithographic procedure were evaluated with respect to cell adhesion and alignment, viability, metabolic activity, proliferation and cell cycle progression using the human glioblastoma cell-line U87MG and human skin fibroblasts. In addition, rat adrenal pheochromocytoma cells (PC-12) were employed to evaluate a modified photolithographic protocol appropriate for adhesion of cells requiring extracellular matrix components to adhere on the surface and to demonstrate that the proposed patterned substrates could provide unhindered cell differentiation. Regarding U87MG cells and skin fibroblasts, it was found that as the stripes width increased from 10 to 50 µm, the percentage of cells attached to Si versus the total area (Si + PVA) increased from 78% and 72% to 98.5% and 94.5% (p < 0.05), for U87MG cells and skin fibroblasts, respectively, with optimum cell alignment (≥95% of adherent cells with fidelity between 0.90 and 1.0; p < 0.05) for stripes width ranging between 20 and 22.5 µm. Concerning the viability, metabolic activity and proliferation of adherent cells, no statistically significant differences were observed compared to cells cultured onto non-patterned surfaces. Regarding PC-12 cells, a modification of the patterning procedure was followed involving coating of the substrate with type IV collagen prior to the photolithographic procedure, since they could not adhere on plain Si substrates. It was found that PC-12 cells adhere selectively (>95%) to collagen-coated Si stripes when the pattern width was equal to or wider than 10 µm. Following treatment with nerve growth factor, approximately 80% (p < 0.05) of the adherent cells differentiated to neuron-like cells extending neurites exclusively within the pattern. Given that the proposed patterning procedure allows highly selective cell adhesion without affecting cell proliferation, metabolic activity, and differentiation it could serve as a useful tool in various fields including tissue engineering, cell-based sensors and analytical microsystems.

Size-dependent single electron transfer and semi-metal-to-insulator transitions in molecular metal oxide electronics.

All-inorganic self-arranged molecular transition metal oxide hyperstructures based on polyoxometalate molecules (POMs) are fabricated and tested as electronically tunable components in emerging electronic devices. POM hyperstructures reveal great potential as charging nodes of tunable charging level for molecular memories and as enhancers of interfacial electron/hole injection for photovoltaic stacks. STM, UPS, UV-vis spectroscopy and AFM measurements show that this functionality stems from the films' ability to structurally tune their HOMO-LUMO levels and electron localization length at room temperature. By adapting POM nanocluster size in solution, self-doping and current modulation of four orders of magnitude is monitored on a single nanocluster on SiO2 at voltages as low as 3 Volt. Structurally driven insulator-to-semi-metal transitions and size-dependent current regulation through single electron tunneling are demonstrated and examined with respect to the stereochemical and electronic structure of the molecular entities. This extends the value of self-assembly as a tool for correlation length and electronic properties tuning and demonstrate POM hyperstructures' plausibility for on-chip molecular electronics operative at room temperature.

Self-assembly study of nanometric spheres from polyoxometalate-phenylalanine hybrids, an experimental and theoretical approach.

Herein, we report on the study of supramolecular assemblies based on polyoxometalates (POMs) upon their modification with amino acids. Two POM-amino acid hybrids were synthesized by coupling a functionalized Keggin type polyoxoanion [PW11O39{Sn(C6H4)C[triple bond, length as m-dash]C(C6H4)COOH}]4- with carboxyl-protected (methyl-ester) phenylalanine or diphenylalanine peptides. Surprisingly, all compounds, including the initial POM, formed supramolecular nanospheres in different solvent mixtures, which were examined by scanning electron microscopy (SEM). Molecular dynamics (MD) simulations for the POM-amino acid species revealed that the hydrophobic forces are mainly responsible for the initial aggregation into incipient micelle type structures, in which the organic arms are buried inside the aggregate while POM polar heads are more exposed to the solvent with tetrabutyl-ammonium counter cations acting as linkers.

Porphyrin-Sensitized Evolution of Hydrogen using Dawson and Keplerate Polyoxometalate Photocatalysts.

Hydrogen evolution using photocatalytic systems based on artificial photosynthesis is a major approach toward solar energy conversion and storage. In the polyoxometalate-based photocatalytic systems proposed in the past, middle/near UV light irradiation and noble-metal catalysts were mainly used. Although recently polyoxometalates were sensitized in visible light, photosensitizers or catalysts based on noble metals, and/or poor activity of polyoxometalates were generally obtained. Here we show the highly efficient [turnover number (TON)=215] hydrogen evolution induced by the zinc(II) mesotetrakis(N-methyl-pyridinium-4-yl)porphyrin (ZnTMPyP4+ ) sensitization of a series of polyoxometalate catalysts (two Dawson type, P2 Mo18 O626- and P2 W18 O626- anions, and one Keplerate {Mo132 } cluster) in a visible-light-driven, noble-metal-free, and fully water-soluble system. We attributed the high efficiency for hydrogen evolution to the multi-electron reduction of polyoxometalates and found that: (a) both Dawson polyoxometalates exhibit higher hydrogen evolution efficiency upon ZnTMPyP4+ sensitization in relation to the direct photoreduction of those compounds; (b) the P2 Mo18 O626- anion is more efficient (TON=65 vs. 38, respectively) for hydrogen evolution than the P2 W18 O626- anion; and (c) the high nuclearity Keplerate {Mo132 } cluster exhibits the highest efficiency (TON=215) for hydrogen evolution compared with the polyoxometalates studied.

Surface Modification of ZnO Layers via Hydrogen Plasma Treatment for Efficient Inverted Polymer Solar Cells.

Modifications of the ZnO electron extraction layer with low-pressure H plasma treatment increased the efficiency of inverted polymer solar cells (PSCs) based on four different photoactive blends, namely, poly(3-hexylthiophene):[6,6]-phenyl C71 butyric acid methyl ester (P3HT:PC71BM), P3HT:1',1″,4',4″-tetrahydro-di[1,4]methanonaphthaleno-[5,6]ullerene-C60 (P3HT:IC60BA), poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:PC71BM (PCDTBT:PC71BM), and (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]]):PC71BM (PTB7:PC71BM), irrespective of the donor:acceptor combination in the photoactive blend. The drastic improvement in device efficiency is dominantly attributable to the reduction in the work function of ZnO followed by a decreased energy barrier for electron extraction from fullerene acceptor. In addition, reduced recombination losses and improved nanomorphology of the photoactive blend in the devices with the H plasma treated ZnO layer were observed, whereas exciton dissociation also improved with hydrogen treatment. As a result, the inverted PSC consisting of the P3HT:PC71BM blend exhibited a high power conversion efficiency (PCE) of 4.4%, the one consisting of the P3HT:IC60BA blend exhibited a PCE of 6.6%, and our champion devices with the PCDTBT:PC71BM and PTB7:PC71BM blends reached high PCEs of 7.4 and 8.0%, respectively.

Old metal oxide clusters in new applications: spontaneous reduction of Keggin and Dawson polyoxometalate layers by a metallic electrode for improving efficiency in organic optoelectronics.

The present study is aimed at investigating the solid state reduction of a representative series of Keggin and Dawson polyoxometalate (POM) films in contact with a metallic (aluminum) electrode and at introducing them as highly efficient cathode interlayers in organic optoelectronics. We show that, upon reduction, up to four electrons are transferred from the metallic electrode to the POM clusters of the Keggin series dependent on addenda substitution, whereas a six electron reduction was observed in the case of the Dawson type clusters. The high degree of their reduction by Al was found to be of vital importance in obtaining effective electron transport through the cathode interface. A large improvement in the operational characteristics of organic light emitting devices and organic photovoltaics based on a wide range of different organic semiconducting materials and incorporating reduced POM/Al cathode interfaces was achieved as a result of the large decrease of the electron injection/extraction barrier, the enhanced electron transport and the reduced recombination losses in our reduced POM modified devices.

Protein-resistant cross-linked poly(vinyl alcohol) micropatterns via photolithography using removable polyoxometalate photocatalyst.

In the last years, there has been an increasing interest in controlling the protein adsorption properties of surfaces because this control is crucial for the design of biomaterials. On the other hand, controlled immobilization of proteins is also important for their application as solid surfaces in immunodiagnostics and biosensors. Herein we report a new protein patterning method where regions of the substrate are covered by a hydrophilic film that minimizes protein adsorption. Particularly, poly(vinyl alcohol) (PVA) cross-linked structures created by an especially developed photolithographic process are proved to prevent protein physisorption and they are used as a guide for selective protein adsorption on the uncovered areas of a protein adsorbing substrate such as polystyrene. The PVA cross-linking is induced by photo-oxidation using, as a catalyst, polyoxometalate (H3PW12O40 or α-(NH4)6P2W18O62), which is removed using a methyl alcohol/water mixed solvent as the developer. We demonstrate that the polystyrene and the cross-linked PVA exhibit dramatically different performances in terms of protein physisorption. In particular, the polystyrene areas presented up to 130 times higher protein binding capacity than the PVA ones, whereas the patterning resolution could easily reach dimensions of a few micrometers. The proposed approach can be applied on any substrate where PVA films can be coated for controlling protein adsorption onto surface areas custom defined by the user.

Radiation sensors based on the generation of mobile protons in organic dielectrics.

A sensing scheme based on mobile protons generated by radiation, including ionizing radiation (IonR), in organic gate dielectrics is investigated for the development of metal-insulator-semiconductor (MIS)-type dosimeters. Application of an electric field to the gate dielectric moves the protons and thereby alters the flat band voltage (VFB) of the MIS device. The shift in the VFB is proportional to the IonR-generated protons and, therefore, to the IonR total dose. Triphenylsulfonium nonaflate (TPSNF) photoacid generator (PAG)-containing poly(methyl methacrylate) (PMMA) polymeric films was selected as radiation-sensitive gate dielectrics. The effects of UV (249 nm) and gamma (Co-60) irradiations on the high-frequency capacitance versus the gate voltage (C-VG) curves of the MIS devices were investigated for different total dose values. Systematic improvements in sensitivity can be accomplished by increasing the concentration of the TPSNF molecules embedded in the polymeric matrix.

The influence of hydrogenation and oxygen vacancies on molybdenum oxides work function and gap states for application in organic optoelectronics.

Molybdenum oxide is used as a low-resistance anode interfacial layer in applications such as organic light emitting diodes and organic photovoltaics. However, little is known about the correlation between its stoichiometry and electronic properties, such as work function and occupied gap states. In addition, despite the fact that the knowledge of the exact oxide stoichiometry is of paramount importance, few studies have appeared in the literature discussing how this stoichiometry can be controlled to permit the desirable modification of the oxide's electronic structure. This work aims to investigate the beneficial role of hydrogenation (the incorporation of hydrogen within the oxide lattice) versus oxygen vacancy formation in tuning the electronic structure of molybdenum oxides while maintaining their high work function. A large improvement in the operational characteristics of both polymer light emitting devices and bulk heterojunction solar cells incorporating hydrogenated Mo oxides as hole injection/extraction layers was achieved as a result of favorable energy level alignment at the metal oxide/organic interface and enhanced charge transport through the formation of a large density of gap states near the Fermi level.

Characterization of a water-dispersible metal protective coating with Fourier transform infrared spectroscopy, modulated differential scanning calorimetry, and ellipsometry.

An ethylene-methacrylic acid copolymer, formulated by BASF as a waterborne suspension of its alkylammonium salt and used, among other applications, in art conservation as a temporary protective coating was characterized using Fourier transform infrared (FT-IR) spectroscopy aided by modulated differential scanning calorimetry (MDSC) and ellipsometry. The thermal conversion of thin copolymer films from the freshly applied state, where carboxylic acid and carboxylate ion functional groups co-exist, to a purely acidic working state was spectroscopically followed. Transmission mid-infrared data of the working state showed a 1 : 12 ratio of methacrylic acid towards ethylene units. The glass transition temperature (T(g)) in the same state was found at 45 °C. Copolymer films spin-coated on mechanically polished bronze and iron coupons were characterized with transflection infrared spectroscopy and compared to corresponding transmission mid-infrared spectra of copolymer films spin-coated on silicon wafers. In the case of bronze coupons, evidence for interaction of the carboxylate ion with the copper substrate was obtained. The chemical structure and the thermal behavior of the coating, as well as some implications on its protective capability towards iron and copper alloys, is discussed as this material has received considerable attention in the field of metal conservation and coatings.

Photochemically-induced acid generation from 18-molybdodiphosphate and 18-tungstodiphosphate within poly(2-hydroxyethyl methacrylate) films.

The capability of ammonium 18-molybdodiphosphate, (NH(4))(6)P(2)Mo(18)O(62) (Mo(18)(6-)), and ammonium 18-tungstodiphosphate, (NH(4))(6)P(2)W(18)O(62) (W(18)(6-)), to photochemically generate acid within films of a polymer with hydroxylic functional groups (namely, within poly(2-hydroxyethyl methacrylate) (PHEMA) films) is demonstrated. Upon UV irradiation, both 2:18 polyoxometalates (POMs) investigated are reduced with concomitant oxidation of PHEMA and generation of acid, which subsequently catalyzes the cross-linking of PHEMA. The photoacid generation is mainly evidenced by monitoring the protonation of an appropriate acid indicator (4-dimethylamino-4'-nitrostilbene) with UV spectroscopy and by photolithographic imaging experiments. By comparing the efficiency of both POMs to induce acid-catalyzed cross-linking of PHEMA under similar conditions, the W(18)(6-) ion is found to be more efficient in photoacid generation than the Mo(18)(6-) ion. Imaging of the POM-containing PHEMA films through UV photolithographic processing is demonstrated. In that process, both POMs can be entirely leached during the development step by using pure water as a developer, resulting in patterned PHEMA films. This characteristic renders the investigated POMs attractive materials for applications, especially in the area of biomaterials, where removal of the photoacid generator from the film at the end of the process is desirable.

Polyoxometalate-based layered structures for charge transport control in molecular devices.

Hybrid organic-inorganic films consisted of molecular layers of a Keggin-structure polyoxometalate (POM: 12-tungstophosphoric acid, H(3)PW(12)O(40)) and 1,12-diaminododecane (DD) on 3-aminopropyl triethoxysilane (APTES)-modified silicon surface, fabricated via the layer-by-layer (LBL) self-assembly method are evaluated as molecular materials for electronic devices. The effect of the fabrication process parameters, including primarily compositions of deposition solutions, on the structural characteristics of the POM-based multilayers was studied extensively with a combination of spectroscopic methods (UV, FTIR, and XPS). Well-characterized POM-based films (both single-layers and multilayers) in a controlled and reproducible way were obtained. The conduction mechanisms in single-layered and multilayered structures were elucidated by the electrical characterization of the produced films supported by the appropriate theoretical analysis. Fowler-Nordheim (FN) tunneling and percolation mechanisms were encountered in good correlation with the structural characteristics of the films encouraging further investigation on the use of these materials in electronic and, in particular, in memory devices.

A biomolecule friendly photolithographic process for fabrication of protein microarrays on polymeric films coated on silicon chips.

The last years, there is a steadily growing demand for methods and materials appropriate to create patterns of biomolecules for bioanalytical applications. Here, a photolithographic method for patterning biomolecules onto a silicon surface coated with a polymeric layer of high protein binding capacity is presented. The patterning process does not affect the polymeric film and the activity of the immobilized onto the surface biomolecules. Therefore, it permits sequential immobilization of different biomolecules on spatially distinct areas on the same solid support. The polymeric layer is based on a commercially available photoresist (AZ5214) that is cured at high temperature in order to provide a stable substrate for creation of protein microarrays by the developed photolithographic process. The photolithographic material consists of a (meth)acrylate copolymer and a sulfonium salt as a photoacid generator, and it is lithographically processed by thermal treatment at temperatures

157-nm Laser ablation of polymeric layers for fabrication of biomolecule microarrays.

A new methodology for protein microarray fabrication is proposed based on the ablation of polymer film using laser at 157 nm (F2). The polymer has been selected among others with the criterion of negligible protein adsorption. Improved results have been obtained by pretreatment of the polymer surface with an inert protein. The use of 157-nm laser radiation allowed very good depth control during the polymeric layer ablation process. In addition the importance of laser ablation at 157 nm is based on the fact that irradiated surfaces indicate limited chemical change due to the fact that laser ablation at 157 nm is only photochemical, thus avoiding excessive surface heating and damage. Results of protein microarray fabrication are presented to illustrate the viability of the proposed method.