Chemistry

In view of the prospective applications of polyamine coatings in functional gold nanoparticles for use as carriers in gene delivery systems, in tissue repair and as bactericidal and virucidal non-toxic vehicle, we have investigated the interactions of poly-L-lysine (PLL) with gold nanoparticles (AuNP). Since direct binding of PLL to AuNP is not strong at neutral pH, we have focused on PLL interactions with carboxylated self-assembled monolayers (SAM) on AuNP, such as the citrate-capped AuNP. The double-shell nanoparticles AuNP@Cit/PLL thus produced do not contain any toxic thiols. We have observed strong electrostatic interactions between polycationic chains of PLL and AuNP@Cit in weakly acidic to weakly alkaline solutions (pH 5e9), as evidenced by the bathochromic shift of the local surface plasmon (SP) band and strong increase in resonance elastic light scattering (RELS) intensity. The stoichiometry of interactions evaluated on the basis of RELS data indicates on a hyper-Langmuirian type of interactions with stoichiometric coefficient n ¼ 1.35 (PLL : AuNP@Cit). From the RELS titration data, a shift of the deprotonation constant for the bound PLL has been determined (pK a ¼ 11.6 for the bound PLL vs. 10.48 for the free PLL). The deprotonation of PLL leads to AuNP aggregate disassembly, evidenced by sharp RELS decline and hypsochromic shift of SP band. We have found that under these conditions, a residual aggregation due to the interparticle interactions between b-sheets of PLL overcoat become predominant.

The improved highly sensitive piezoelectric immunosensor has been developed and evaluated using a model interaction of antibody with the model hapten-herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). For immobilization of 2,4-D, the self-assembled layers of cystamine, 4-aminothiophenol or 3,3%-dithio-bis(propionic acid N-hydroxysuccinimide ester) were formed on smooth and rough crystals coated with gold or silver electrodes. The immunochemical interactions performed well in all cases, the aminothiophenol on gold was chosen as the optimum with regard to regeneration of immunosensing surfaces. The kinetics of interaction of surface-bound 2,4-D with free antibody provided significantly higher kinetic parameters (kinetic association rate constant) when using optically smooth crystals compared to common rough crystal. Therefore, the smooth crystal should be preferred for future kinetic studies. The competitive assay of the herbicide 2,4-D achieved a limit of detection of 10 ng/l using the monoclonal anti-2,4-D antibody F6C10. Finally, a direct assay format has been evaluated using a thicker layer of glutaraldehydecrosslinked antibody on the sensing surface. The direct binding of a small herbicide molecule was followed in real time. The detected concentration of 2,4-D (5 mg/l) was low enough for future direct monitoring of this herbicide in water.

New dynamic aspects of the catalysis of methanol oxidation reaction have been studied. The submonolayer films of bi-functional catalysts (PtRu) were deposited on novel nanostructured supporting materials, designed with the goal of achieving high variability of their electronic and chemical properties to influence the catalytic activity of sub-monolayer catalyst. The mesoscopic TiO 2 supporting film formation was investigated using EQCN, voltammetric, and AFM techniques. The conditions for the formation of monodispersed TiO 2 nanoparticles with regular nanopores, 20-80 nm in diameter, were described. It follows from EQCN and voltammetric measurements and AFM image analysis that the nanopores are formed by a dissolution-precipitation mechanism. The activity of submonolayer catalysts PtRu deposited on nanoporous TiO 2Àn films was investigated using the point defect formation voltammetry. A lower poisoning effect for cluster PtRu on a TiO 2 support film than that for unsupported PtRu and bare Pt catalysts has been observed. The implications of the support-catalyst interactions are discussed in details.

New dynamic aspects of the catalysis of methanol oxidation reaction (MOR) have been studied using quantum mechanical calculations applied to the support-catalyst cluster interactions and surface diffusivity of adsorbed intermediates. For very small catalyst-support clusters, we have found a strong enhancement of the ligand effect for bimetallic catalysts of the type Pt n M m attributed to the decreased local density of states near the Fermi level of Pt atoms neighboring the additive metal atom M. This enhancement results in a decreased barrier for surface diffusion of adsorbed CO ad through the cooperative diffusion mechanism, based on structural relaxation of the catalyst-support cluster, proposed in this work. The strong ligand effect dominates over the Schwoebel potential and trapping well effects, being responsible for accumulation of poisoning intermediates at step sites on the catalyst surface and gradual decrease of catalytic activity with decreasing size of catalyst nanoparticles. The lattice relaxation and strong ligand effects in small catalyst-support clusters lead to lower adsorption energy for CO ad and thus, to higher reactivity and mobility of reactants and intermediates. The experimental investigations included submonolayer films of bi-functional catalysts (PtRu, PtFe) deposited on novel nanostructured supporting materials, designed with the goal of achieving high variability of their electronic and chemical properties to influence the catalytic activity of sub-monolayer catalyst. The mesoscopic TiO 2 supporting film formation was investigated using EQCN, pulse voltammetric and AFM techniques. The conditions for the formation of monodispersed TiO 2 nanoparticles with regular nanopores (nanotubes), 20-80 nm in diameter, were described. It follows from EQCN and voltammetric measurements and AFM image analysis that the nanopores are formed by a dissolution-precipitation mechanism. The catalysts, Pt and PtRu, deposited on supporting nanoporous TiO 2−x films, were used to study MOR. A lower poisoning effect for cluster PtRu on a TiO 2−x support film than that for unsupported PtRu or bare Pt catalysts has been observed. These effects have been attributed to differences in CO ad binding energy and lowering of activation energy for surface mobility leading to a more facile 2D diffusion of CO ad from Pt sites to Ru and the supporting TiO 2−x . The substrate-catalyst interactions were further investigated using quantum mechanical calculations performed for a model TiO 2 nano-ring (representing an orifice of a TiO 2−x nanotube studied experimentally) with adlayers of Pt, Ru and Fe catalysts. We have found unusually strong electron delocalization effects for Pt 2 Fe 2 clusters on (TiO 2 ) 4 as compared to (TiO 2−x ) 4 Pt 2 Ru 2 . We have also analyzed various states in surface diffusion of CO ad on bimetal clusters supported on (TiO 2 ) n and observed considerable dynamic widening of metal-to-metal atom distances induced by CO adsorption (up to 9% for Pt-Pt distance and up to 15% for Fe-Fe distance). We propose that this new dynamic effect leading to cooperative surface diffusion may be further explored in designing novel nanoparticle catalysts. Published by Elsevier Ltd.

Photoelectrochemical degradation of a textile diazo dye, naphthol blue black (NBB), has been investigated using different semiconductor electrodes. A higher photoelectrocatalytic activity has been observed for WO 3 film electrodes, prepared by electrodeposition, than for TiO 2 nanoparticulate film electrodes. By annealing of the WO 3 film deposited on a Pt substrate, we were able to increase the photodegradation rate of NBB, owing to the reduced losses due to the electron-hole recombination. This treatment also increased the long-term photostability of the electrode. The effect of the supporting electrolyte was examined using NaCl, NaClO 4 , KNO 3 , and Na 2 SO 4 solutions. The highest NBB degradation rate was found in the acidic Cl − medium. With increasing solution pH, the degradation rate decreased to a lower level indicating a change in the NBB photodecomposition mechanism. The reaction is of the first order with respect to NBB in the concentration range up to ca. 6 ×10 − 5 M NBB. The apparent rate constants for the photoelectrocatalytic degradation of NBB in 0.5 M NaCl +8.8× 10 − 5 M NBB solution, on a WO 3 film electrode at E = 1.08 V versus SCE, are 1.302 × 10 − 4 and 3.41 × 10 − 5 cm − 2 s − 1 , under illumination with ultraviolet and visible light, respectively. It has been found that the mechanism of the sub-band excitation with the dye self-sensitization is a minor degradation path for NBB, while the main path involves the electron-hole generation and heterogeneous oxidation with participation of valence-band electron holes. The latter reactions involve oxidation of solution components with generation of powerful oxidants, such as the OH , Cl 2 − , and Cl radicals, being directly responsible for the fast dye decomposition.

The ionic permeation and discharge processes at a controllable-gate ion-channel electrode were investigated. The ionically transmissible films were formed on Au piezoelectrodes by surface modification performed using self-assembled monolayers (SAM) of glutathione (GS) molecules adsorbed onto gold. The gating state of a AuSG film was controlled by utilizing uncomplexed Cu 2+ ions acting as the controller cations through their interactions with terminal carboxylic groups of AuSG, which results in a conformational transition leading to the film ion-channel opening. The open-gate conformation of AuSG is stabilized by lowering the solution pH. Under these conditions, we were able to record Cu 2+ ion transport and discharge at a Au substrate surface accessible through the open ion gates. We have found that despite of a very limited space of this access, in the first stage of the Cu 0 deposition process, a Cu-upd on Au is formed, with LSV peak potential E p = 0.385 V versus Ag/AgCl, though the rate of upd formation is lower than that for a upd on an unmodified Au, as evidenced by the decrease in the Cu deposited mass and the deposition charge. The value of the apparent charge transfer coefficient α app is also smaller (α app = 0.36 compared to 0.52 for bare Au). The 3D nucleation and growth of copper nanocrystals was observed using the potential step i-t and m-t transients. The size of Cu 0 nuclei at the beginning of their overlapping process was in the range of d = 2-3 nm, which is larger than the ion-channel height (h ≈ 1.22 nm) determined for the open-gate conformation of AuSG. We found no dynamic film instabilities associated with fast potential step programs.

The anodic TiO 2 films on Ti 0 substrates were synthesized using different electrochemical techniques including potential step, potentiodynamic, and galvanostatic technique, from a 1 M Na 2 SO 4 +10 mM NaF solution. The AFM imaging of film surfaces revealed that thin films (h < 20 nm) are composed of small spherical-shaped nanocrystals, up to 20 nm in size. In thicker TiO 2 films (h > 20 nm), cylindrical nanopores with diameter of ca. 30 nm, are formed. In contrast to the nanocrystalline nature of anodic TiO 2 films, thermally grown TiO 2 films at 850 • C are composed of oriented large pyramidal crystallites (200-500 nm in size), which are transformed into even larger moulds (2-3 m in size) at 1050 • C. Thermal films of this type have been known to show inferior surface properties leading to large reflectance losses and the presence of surface states, which promote the electron-hole recombination. In this paper, we propose a new way to remedy these disadvantages of thermal films by special low temperature electrochemical anodic treatment. We have found that by the anodic treatment of thermally grown TiO 2 films, it is possible to control the surface properties, such as the nanocrystalline structure and nanopore formation. These features are desirable for solar energy conversion devices and solar hydrogen production, where the enhancement of electrocatalytic activity by increasing real surface area, reduction of reflectance losses, and removal of surface states created by thermal growth, are of primary importance. The tested electrochemical treatment included program waveforms inducing nanocrystallinity and nanopore formation. The pulse-voltammetric procedures are proposed to control surface non-stoichiometry of TiO 2 films and surface-states density of the photoelectrodes. Published by Elsevier Ltd on behalf of the International Association for Hydrogen Energy.

The current-voltage characteristics of n-TiO2 semiconductor electrodes were determined in fluoride-containing solutions under illumination and in the dark, and found to exhibit electrochemical behavior similar to that reported for other media. Increased solubility of the TiO2 is found to occur over a narrow range of solution pH values near 3.2. A photoetching process is recommended for the n-TiO2 surface&amp;#39;s preparation

The improved highly sensitive piezoelectric immunosensor has been developed. The gold electrodes of the piezoelectric quartz crystals were modified with self-assembled layer of cysteamine and carboxyderivative of atrazine was covalently attached after activation with O-(Nsuccinimidyl)-N,N,N 0 ,N 0 -tetramethyluronium tetrafluoroborate (TSTU). The competitive immunoassay for atrazine was developed using antiatrazine monoclonal antibody (MAb) D6F3. The limit of detection for atrazine was 0.025 ng/ml, the total time of analysis being 25 min. The repeated use of the immunosensors was possible after regeneration using pepsin at pH 2.0. The direct immunosensor for atrazine was constructed by oriented immobilization of anti-atrazine MAb to Protein A covalently attached to the gold surface activated with 3,3 0 -dithiobis(propionic acid N-hydroxysuccinimide ester) (DTSP). The MAb-Protein A complex was stabilized by cross-linking with dimethyl pimelimidate. Thus developed immunosensor was able to specifically respond to atrazine present in solution, providing limit of detection of 1.5 ng/ml, one assay was completed within 10 min (5 min binding, 5 min washing of surface). The observed changes of frequency are expected to result from conformational changes of the immobilized antibody after formation of the immunocomplex with atrazine. Furthermore, kinetic studies were carried out in different arrangements, the obtained kinetic association equilibrium constant was 2:3 Â 10 7 l mol À1 for interaction between immobilized antibody and atrazine in solution. #

This paper describes the results of an investigation of the evolution and the reconstitution of core-shell assembled gold nanoparticles in electrocatalytic activation for methanol electrooxidation. The aim is to probe the structural and morphological reconstitution upon the catalytic activation. Gold nanoparticles of 2-nm core size are linked by 1,9-nonanedithiolates into a network thin film on planar substrate, and are explored

The catechol-mediated DNA damage in the presence of Cu(II) ions involves oxidation of guanine to 8-oxoguanine (8-oxoG) and DNA strand scission. It proceeds through the reactive oxygen species (ROS) generation. The mutagenicity of 8-oxoG lesions is due to its miscoding propensity reflected in GC→TA transversion taking place during the DNA repair process. To gain new insights into the nature of catechol-mediated DNA damage and its prevention, we have investigated the changes in DNA melting characteristics and 8-oxoG formation as the indicators of DNA damage in a model calf-thymus DNA system. A novel fluorescence method for DNA melting temperature determination, based on DAPI fluorescent-probe staining, has been proposed. The DNA melting-onset temperature has been found to be more sensitive to DNA damage than the standard melting temperature due to the increased width of the melting transition observed in oxidatively damaged DNA. We have found that the efficiency of Fenton cascade in generating DNA-damaging ROS is higher for catechol than for GSH, two strong antioxidants, mainly due to the much longer distance between ROS-generating radical group in GS to nucleobases than that of semiquinone radical group to nucleobases (2.1nm vs. 0.27nm), making the ROS transport from GSH an order of magnitude less likely to damage DNA because of short lifetime of HO radicals. The antioxidant and DNA-protecting behaviors of GSH have been elucidated. We have found that the redox potential of GSH/GSSG couple is lower than that of catechol/semiquinone couple. Hence, GSH keeps catechol in the reduced state, thereby shutting down the initial step of the catechol-mediated Fenton cascade. The catechol-induced DNA damage in the presence of Cu(II) ions has also been confirmed in studies of ON-OFF hairpin-oligonucleotide beacons.

The adsorption of a model protein, bovine serum albumin (BSA), on Au electrodes was investigated using the Cu adatom probe method and Electrochemical Quartz Crystal Nanobalance (EQCN) technique. The adsorption of BSA was confirmed by AFM imaging and has been found to be controlled by kinetics. Using the Cu adatom probe method, we were able to reconstruct the entire BSA adsorption transient Θ BSA vs. t. The adsorption rate constant k 1 , determined from this transient is k 1 = 2.45 × 10 5 L mol − 1 s − 1 . We have found that the bulk Cu 0 deposition process is blocked by BSA adsorption and it decays exponentially with time during BSA adsorption. It ceases completely when a full monolayer of BSA is formed. In contrast to that, the mass associated with Cu-u.p.d. decreases only to ca. 50% of that in the absence of BSA, indicating that Cu adatoms can penetrate (wedge) into the space between the surface Au atoms and the adsorbed BSA molecules. In addition to that, we have found that the degree of penetration of Cu adatoms can be controlled by the applied deposition potential. By selecting a sufficiently cathodic potential, we were able to deposit a full Cu-u.p.d. monolayer, independent of the BSA surface coverage extending from Θ BSA = 0 to Θ BSA ≈ 1. The positive shift of Cu ad desorption peak potential E p , observed in the presence of adsorbed BSA, has been interpreted in terms of Frumkin exchange interaction forces between Cu ad and BSA ad , on the basis of our earlier theoretical model, expanded here to include adsorbed species in two monolayers. This expansion is possible owing to the fast rate of Cu adatom penetration in the interfacial region. From the plots of E p vs. Θ BSA , the presence of strong attractive interactions between Cu ad and BSA ad was deduced. These interactions result in a super-shift of the Cu-u.p.d. desorption peak potential, corresponding to the exchange interaction coefficient g M,X b −4, indicating on a possibility of the formation of a stable interface complex.