John Kiwi

Antibacterial effects of silver nanoparticles on gram-negative bacteria: Influence on the growth and biofilms formation, mechanisms of action

Antibacterial action of silver nanoparticles (AgNP) on Gram-negative bacteria (planctonic cells and biofilms) is reported in this study. AgNP of 8.3 nm in diameter stabilized by hydrolyzed casein peptides strongly inhibited biofilms formation of Escherichia coli AB1157, Pseudomonas aeruginosa PAO1 and Serratia proteamaculans 94 in concentrations of 4-5 μg/ml, 10 μg/ml and 10-20 μg/ml, respectively. The viability of E. coli AB1157 cells in biofilms was considerably reduced by AgNP concentrations above 100 to -150 μg/ml. E. coli strains with mutations in genes responsible for the repair of DNA containing oxidative lesions (mutY, mutS, mutM, mutT, nth) were less resistant to AgNP than wild type strains. This suggests that these genes may be involved in the repair of DNA damage caused by AgNP. E. coli mutants deficient in excision repair, SOS-response and in the synthesis of global regulators RpoS, CRP protein and Lon protease present similar resistance to AgNP as wild type cells. LuxI/LuxR Quorum Sensing systems did not participate in the control of sensitivity to AgNP of Pseudomonas and Serratia. E. coli mutant strains deficient in OmpF or OmpC porins were 4-8 times more resistant to AgNP as compared to the wild type strain. This suggests that porins have an important function related AgNP antibacterial effects.

Fast kinetic spectroscopy, decoloration and production of H2O2 induced by visible light in oxygenated solutions of the azo dye Orange II

A fast kinetics study by pulsed laser spectroscopy allows a mechanism for Orange II photobleaching in the presence of O2 to be proposed. Evidence is presented that the hydroperoxy radical HO2 is the main oxidative species for Orange II in oxygenated aqueous solution under visible light irradiation. The probability of reaction between O2 and Orange II leading to radical formation (≈5×10-4) is observed to be very low. The lifetime and amplitude of the radicals anions and cations in solution are compared in O2-, air- and Ar-containing solutions. Singlet oxygen (1O2) is shown not to be present during the initial photobleaching stages of Orange II. Steady-state irradiation of Orange II in aqueous solution in the presence of O2 causes two simultaneous processes: photodegradation to more biodegradable intermediates with CO2 evolution and photoinduced generation of peroxides in solution. The observed peroxide generation stops when initial Orange II reaches a low concentration and does not occur from the intermediates during the photodegradation of Orange II. Direct electron transfer from excited Orange II to O2 provides the driving force for the accelerated photodegradation of the dye. No electron transfer to O2 was observed in the dark involving the ground state of the azo dye. The oxidation potential Orange II/Orange+ is determined by cyclic voltammetry to be 0.76 V(NHE) and a value of -1.54 V(NHE) could be estimated for the oxidation potential of the couple Orange II*/Orange+.

Photo-Assisted ImmobilizedFenton Degradation up to pH 8 of Azo Dye Orange II Mediated by Fe3+/Nafion/Glass Fibers

Fe3+ Ions have been immobilized into very thin Nafion films cast onto a glass-fiber mat immersed in an alcoholic solution of Nafion oligomers. This immobilized Fenton catalyst was used to abate/mineralize the azo dye Orange II, taken as a model organic compound. The abatement of Orange II on the Fe3+/Nafion/glass fibers was observed to proceed within the same time period as when Nafion alone was used to immobilize the Fe3+ ions during the photo-Fenton reaction. The amount of Nafion in the Nafion Fe3+/Nafion/glass fibers was ca. 15 times less per unit surface area compared to Fe3+-exchanged on conventional Nafion membranes used to immobilize Fe3+ ions. Orange II solutions under visible-light irradiation in the presence of H2O2 were mineralized up to pH 8 with a kinetics comparable to that found during the degradation runs at pH 3. Repetitive mineralization cycles mediated by the Fe3+/Nafion/glass fibers under visible light did not show any decrease in the activity of the immobilized catalysts. A reaction mechanism consistent with the experimental data is suggested. The morphology of the Fe3+/Nafion/glass fibers was characterized by scanning electron microscopy (SEM) showing thin Nafion films cast deposited on the glass fibers. Transmission-electron-microscopy (TEM) micrographs reveal Fe3+-oxy-hydroxide particles of 3 – 6 nm before and after repetitive Orange II photodegradation. X-Ray photoelectron spectroscopy (XPS) provided the evidence for the existence of Fe clusters on the topmost layer of the catalyst mainly as FeIII. The improvements brought by the glass fibers are a) the use of low quantities of expensive Nafion supported on glass mats to achieve dye degradation rates comparable to Nafion alone and b) Fenton-mediated degradation of azo dyes at pH 8 without the costly initial acidification usually needed for this type of treatment.

Photoassisted carbon dioxide reduction on aqueous suspensions of titanium dioxide

Abstract The reduction of carbon dioxide to formic acid, formaldehyde and methanol was carried out by illuminating aqueous suspensions of titanium oxide with a high pressure mercury lamp. The production rate of the organic products was enhanced by titanium oxide, which was doped with RuO 2 . The highest initial energy conversion efficiency obtained was 0.04%. However, the efficiency declined with prolonged illumination, suggesting that the observed photoreduction is not a truly photocatalytic reaction.

Light-induced hydrogen formation and photo-uptake of oxygen in colloidal suspensions of α-Fe2O3

Hydrogen evolution and oxygen reduction as a function of pH of the electrolyte have been investigated for α-Fe2O3 powders and colloids by means of a solar simulator. The discrete particle dimensions and surface dynamics of iron oxide seem to be the dominant factors leading to H2 generation despite the unfavourable position of the conduction band in Fe2O3. Evidence is presented that photo-uptake of O2 occurs in α-Fe2O3 suspensions.

Efficient photo-assisted Fenton catalysis mediated by Fe ions on Nafion membranes active in the abatement of non-biodegradable azo-dye

Highly dispersed Fe ions on Nafion membranes are shown to decompose H2O2 with similar kinetics as found for homogeneous solutions containing Fe3+ ions during the photo-assisted Fenton degradation of Orange II, avoiding the drawbacks of the homogeneous treatment.

Evidence for immobilized photo-Fenton degradation of organic compounds on structured silica surfaces involving Fe recycling

The present study reports on the use of novel structured inorganic silica fabrics loaded with Fe ions by exchange-impregnation as a heterogeneous photocatalyst. These Fe–silica fabrics are denoted EGF/Fe(0.4%). Experimental evidence shows that Fe ions are released from the silica fabrics and react with H2O2 to form oxidative radicals in solution; the Fe ions are reduced to Fe(II) during oxalic acid and oxalate oxidation. The Fe3+ is extracted from the support to the aqueous medium where it is re-oxidized by being re-adsorbed onto the silica fabric. The contributions of the homogeneous and heterogeneous photocatalysis processes during the degradation of oxalic acid and oxalates were quantified as a function of the solution pH and the results presented agree with the modeling of the iron oxide surface in the presence of oxalates at different pH values. By attenuated total reflection infrared spectroscopy (ATRIR), the asymmetric stretching vibration doublet band at 1740 cm−1 and the satellite peaks corresponding to surface carboxylates were followed during the photocatalytic destruction of the oxalates. The results obtained indicate that the oxalate decomposition channel involves a fast light-activated decarboxylation of the Fe complex: [RCO2Fe]2+ → [R˙] + CO2 + Fe2+. The structural features of the EGF/Fe(0.4%) surfaces were investigated before and after the oxalate photocatalysis by high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and gas adsorption studies (BET).

Comparison of Methods for Evaluation of the Bactericidal Activity of Copper-Sputtered Surfaces against Methicillin-Resistant Staphylococcus aureus

ABSTRACT Bacteria can survive on hospital textiles and surfaces, from which they can be disseminated, representing a source of health care-associated infections (HCAIs). Surfaces containing copper (Cu), which is known for its bactericidal properties, could be an efficient way to lower the burden of potential pathogens. The antimicrobial activity of Cu-sputtered polyester surfaces, obtained by direct-current magnetron sputtering (DCMS), against methicillin-resistant Staphylococcus aureus (MRSA) was tested. The Cu-polyester microstructure was characterized by high-resolution transmission electron microscopy to determine the microstructure of the Cu nanoparticles and by profilometry to assess the thickness of the layers. Sputtering at 300 mA for 160 s led to a Cu film thickness of 20 nm (100 Cu layers) containing 0.209% (wt/wt) polyester. The viability of MRSA strain ATCC 43300 on Cu-sputtered polyester was evaluated by four methods: (i) mechanical detachment, (ii) microcalorimetry, (iii) direct transfer onto plates, and (iv) stereomicroscopy. The low efficacy of mechanical detachment impeded bacterial viability estimations. Microcalorimetry provided only semiquantitative results. Direct transfer onto plates and stereomicroscopy seemed to be the most suitable methods to evaluate the bacterial inactivation potential of Cu-sputtered polyester surfaces, since they presented the least experimental bias. Cu-polyester samples sputtered for 160 s by DCMS were further tested against 10 clinical MRSA isolates and showed a high level of bactericidal activity, with a 4-log10 reduction in the initial MRSA load (106 CFU) within 1 h. Cu-sputtered polyester surfaces might be of use to prevent the transmission of HCAI pathogens.

TiO2 nanoparticles suppress Escherichia coli cell division in the absence of UV irradiation in acidic conditions

TiO(2) nanoparticles (NPs) activated by UV irradiation are known to have a bactericidal effect. In this study we report the details of TiO(2) NPs influence on the colony-forming capacity of E. coli in the dark at pH 4.0-4.5. At this pH the bacterial cells are negatively charged and TiO(2) NPs present a positive charge. A 60 min contact between E. coli with TiO(2) at concentrations of 0.02-0.2 mg/mL led to a reduction of E. coli cell number from 10(8) to 10(4)CFU/mL. After the reduction the system remains unchanged during the subsequent incubation. The observed reduction was a function on the initial E. coli concentration. In the presence of 0.04 mg/mL TiO(2) the colony-forming units (CFU) reduction after 60 min was of four-five orders of magnitude when the initial concentration was 10(8) cells/mL. But when starting with an E. coli concentration of 10(7) cells/mL the cell number reduction was less than one order of magnitude. Less than one order of magnitude cell number reduction was also observed for suspensions of E. coli 10(8) cells/mL and 0.002 mg/mL of TiO(2). The bacteria number reduction was always accompanied by the formation of cell aggregates. During cell incubation with TiO(2), the pH of the suspension increased, but did not reach the TiO(2) isoelectric point (IEP). E. coli cells stained with the fluorescent dye acridine orange (AO) showed that the fluorescence of single cells remained unchanged after incubation in the presence of TiO(2). The color change of fluorescence was revealed only in aggregated cells. This suggests changes in the physiologic state of E. coli incorporated into the aggregates. Aggregates of E. coli occur due to the electrostatic interaction between TiO(2) NPs and the bacterial cell surface. A hypothesis is suggested in this study to explain the CFU reduction and the retention of a certain irreducible number of cells capable of further division in the suspension in the presence of TiO(2) in the dark.

Preparation and Mechanism of Cu-Decorated TiO2–ZrO2 Films Showing Accelerated Bacterial Inactivation

Antibacterial robust, uniform TiO2-ZrO2 films on polyester (PES) under low intensity sunlight irradiation made up by equal amounts of TiO2 and ZrO2 exhibited a much higher bacterial inactivation kinetics compared to pure TiO2 or ZrO2. The TiO2-ZrO2 matrix was found to introduce a drastic increase in the Cu-dopant promoter enhancing bacterial inactivation compared to Cu sputtered in the same amount on PES. Furthermore, the bacterial inactivation was accelerated by a factor close to three, by Cu- on TiO2-ZrO2 at extremely low levels ∼0.01%. Evidence is presented by X-ray photoelectron spectroscopy for redox catalysis taking place during bacterial inactivation. The TiO2-ZrO2-Cu band gap is estimated and the film properties were fully characterized. Evidence is provided for the photogenerated radicals intervening in the bacterial inactivation. The photoinduced TiO2-ZrO2-Cu interfacial charge transfer is discussed in term of the electronic band positions of the binary oxide and the Cu TiO2 intragap state.