Eradication of Gram‐Positive and Gram‐Negative Bacteria by Photosensitizers Immobilized in Polystyrene

Immobilization of photosensitizers in polymers opens prospects for their continuous and reusable application. Methylene blue (MB) and Rose Bengal were immobilized in polystyrene by mixing solutions of the photosensitizers in chloroform with a polymer solution, followed by air evaporation of the solvent. This procedure yielded 15–140 μm polymer films with a porous surface structure. The method chosen for immobilization ensured 99% enclosure of the photosensitizer in the polymer. The antimicrobial activity of the immobilized photosensitizers was tested against Gram‐positive and Gram‐negative bacteria. It was found that both immobilized photosensitizers exhibited high antimicrobial properties, and caused by a 1.5–3 log₁₀ reduction in the bacterial concentrations to their total eradication. The bactericidal effect of the immobilized photosensitizers depended on the cell concentration and on the illumination conditions. Scanning electron microscopy was used to prove that immobilized photosensitizers excited by white light caused irreversible damage to microbial cells. Photosensitizers immobilized on a solid phase can be applied for continuous disinfection of wastewater bacteria.     

Nanostructure of complexes between cationic lipids and an oppositely charged polyelectrolyte

The morphology of aqueous solutions of polyelectrolytes and oppositely charged lipids is the subject of extensive colloid science research, because of their application in industry and medicine, the latter especially for gene therapy. In this work, we show that complexes of two different cationic lipids with the polyelectrolyte sodium poly(acrylic acid), PAA, share similar morphology with the complexes of those lipids with nucleic acids, implying a broader and universal packing phenomenon. We characterized by direct-imaging cryogenic-temperature transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and zeta (ζ)-potential two cationic lipids, 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and bis(11-ferrocenylundecyl) dimethylammonium bromide (BFDMA), which are used in gene transfection, at equivalent lipid/polyelectrolyte charge ratio. Our results revealed that, for both types of complexes, onion-like multilamellar nanostructures formed, which exhibited similar morphology as in complexes of DNA or oligonucleotides (lipoplexes), based on the same lipids. Our findings suggest that the onion-like packing may be energetically favorable for a wide range of polyelectrolyte-liposome systems, from oligonucleotides and DNA to PAA.     

Molecular optoelectronics: the interaction of molecular conduction junctions with light

The interaction of light with molecular conduction junctions is attracting growing interest as a challenging experimental and theoretical problem on one hand, and because of its potential application as a characterization and control tool on the other. It stands at the interface between two important fields, molecular electronics and molecular plasmonics and has attracted attention as a challenging scientific problem with potentially important technological consequences. Here we review the present state of the art of this field, focusing on several key phenomena and applications: using light as a switching device, using light to control junction transport in the adiabatic and non-adiabatic regimes, light generation in biased junctions and Raman scattering from such systems. This field has seen remarkable progress in the past decade, and the growing availability of scanning tip configurations that can combine optical and electrical probes suggests that further progress towards the goal of realizing molecular optoelectronics on the nanoscale is imminent.     

A Planar 3-Convex Set is Indeed a Union of Six Convex Sets

Suppose S is a planar set. Two points $a,b$ in S  see each other via S if $[a,b]$ is included in S . F. Valentine proved in 1957 that if S is closed, and if for every three points of S, at least two see each other via S, then S is a union of three convex sets. The pentagonal star shows that the number three is the best possible. We drop the condition that S is closed and show that S is a union of (at most) six convex sets. The number six is best possible.     

INACTIVATION OF GRAM-NEGATIVE BACTERIA BY PHOTOSENSITIZED PORPHYRINS

Photosensitization of Escherichia coli and Pseudomonas aeruginosa cells by deuteroporphyrin (DP) is shown to be possible in the presence of the polycationic agent polymyxin nonapeptide (PMNP). Previous studies established complete resistance of Gram-negative bacteria to the photodynamic effects of porphyrins. The present results show that combined treatment of E. coli or P. aeruginosa cultures with DP and PMNP inhibit cell growth and viability. No antibacterial activity of PMNP alone could be demonstrated and cell viability remained unchanged. Spectroscopically, PMNP was found to bind DP, a mechanism which probably assists its penetration into the cell's membranes. Insertion of DP into the cells was monitored by the characteristic fluorescence band of bound DP at 622 nm. Binding times were 5-40 min and the extent of binding increased with decreasing the pH from 8.5 to 6.5. DP binding constants, as well as the concentrations of PMNP which were required for maximal effect on the various Gram-negative bacteria, were determined fluorometrically. By the treatment of DP, PMNP and light the growth of E. coli and P. aeruginosa cultures was stopped and the viability of the culture was dramatically reduced. Within 60 min of treatment the survival fraction of E. coli culture was 9 x 10(-6) and that of P. aeruginosa was 5.2 x 10(-4). Electron microscopy depicted ultrastructural alterations in the Gram-negative cells treated by DP and PMNP. The completion of cell division was inhibited and the chromosomal domain was altered markedly.     

Diffusion of vibrationally excited molecules

A treatment is presented for the effect of intermolecular vibrational energy transfer on the diffusion coefficient of vibrationally excited molecules. An analytic treatment based on random walk statistics and a Monte Carlo type calculation have been performed. Both methods yield very similar results which correlate well with existing experimental studies. A hard sphere collision model is treated extensively with comparisons made to other internmolecular potentials. The results support the involvement of long range energy transfer in V → V interactions. The effect of temeprature on the diffusion coefficient of vibrationally excited molecules is calculated, with applications to the CO^*₂CO₂ system.     

Molecular transport junctions: asymmetry in inelastic tunneling processes

Inelastic electron tunneling spectroscopy (IETS) measurements are usually carried out in the low-voltage ("Ohmic", i.e., linear) regime where the elastic conduction/voltage characteristic is symmetric to voltage inversion. Inelastic features, normally observed in the second derivative d(2)I/dV(2) are also symmetric (in fact antisymmetric) in many cases, but asymmetry is sometimes observed. We show that such asymmetry can occur because of different energy dependences of the two contact self-energies. This may be attributed to differences in contact density of states (different contact material) or different energy dependence of the coupling (STM-like geometry or asymmetric positioning of molecular vibrational modes in the junction). The asymmetry scales with the difference between the energy dependence of these self-energies and disappears when this dependence is the same for the two contacts. Our nonequilibrium Green function approach goes beyond proposed WKB scattering theory in properly accounting for Pauli exclusion, as well as providing a path to generalizations, including consideration of phonon dynamics and higher-order perturbation theory.     

Inelastic electron tunneling spectroscopy in molecular junctions: peaks and dips

We study inelastic electron tunneling through a molecular junction using the nonequilibrium Green's function formalism. The effect of the mutual influence between the phonon and the electron subsystems on the electron tunneling process is considered within a general self-consistent scheme. Results of this calculation are compared to those obtained from the simpler Born approximation and the simplest perturbation theory approaches, and some shortcomings of the latter are pointed out. The self-consistent calculation allows also for evaluating other related quantities such as the power loss during electron conduction. Regarding the inelastic spectrum, two types of inelastic contributions are discussed. Features associated with real and virtual energy transfer to phonons are usually observed in the second derivative of the current I with respect to the voltage Phi when plotted against Phi. Signatures of resonant tunneling driven by an intermediate molecular ion appear as peaks in the first derivative dI/dPhi and may show phonon sidebands. The dependence of the observed vibrationally induced lineshapes on the junction characteristics, and the linewidth associated with these features are also discussed.     

Direct cryogenic-temperature transmission electron microscopy imaging of phospholipid aggregates in soybean oil

Cryogenic-temperature transmission electron microscopy (cryo-TEM) was fine-tuned to make it applicable to the study of a natural oil-based system. While liquid ethane, the cryogen of choice for specimen preparation, was found to dissolve the oil, we observed that the triglycerides found in many natural oils serve as cryoprotectants that allow vitrification of such oil systems in liquid nitrogen, a cryogen that provides relatively low cooling rates. Using the modified technique combined with digital imaging by a cooled CCD camera, we were able to directly visualize inverse micellar and liquid-crystalline aggregates formed in the soybean oil/soybean phospholipids/water/hexane system, which is commercially important in soybean oil processing. The method developed here should be widely applicable to other organic solvent-based systems.