No abstract

The upper triplet states of Merocyanine 540 and related cyanine dyes have been generated by a two-color, two-pulse method. The decay of this state partitions between internal conversion to T1 and remarkably efficient reverse intersystem crossing to the singlet manifold. Thus, UV or visible laser generation of the T1 state (either directly or by a sensitized route) followed by excitation with a 640 nm pulse ultimately results in fluorescence and isomerization from the lowest excited singlet state (S1), as evidenced by the production of the ground state photoisomer absorption and emission coincident with the second laser pulse. Since identical behavior (isomerization and fluorescence) is observed for direct excitation into the singlet manifold, we conclude that both the singlet and triplet states are of the same geometry and that isomerization does not occur directly from either T1 or T n .

Staphylococcus epidermidis is among the most commonly isolated microbes from medical implant infections, particularly in the colonization of blood-contacting devices. We explored the relationships between surface wettability and root-mean-square roughness (Rq) on microbial adhesive strength to a substrate. Molecular-level interactions between S. epidermidis and a variety of chemically and texturally distinct model substrata were characterized using a cellular probe and atomic force microscopy (AFM). Substrata included gold, aliphatic and aromatic self-assembled monolayers, and polymeric and proteinaceous materials. Substrate hydrophobicity, described in terms of the water contact angle, was an insufficient parameter to explain the adhesive force of the bacterium for any of the surfaces. Correlations between adhesion forces and Rq showed weak relationships for most surfaces. We used an alternate methodology to characterize the texture of the surface that is based on a fractal tiling algorithm applied to images of each surface. The relative area as a function of the scale of observation was calculated. The discrete bonding model (DBM) was applied, which describes the area available for bonding interactions over the full range of observational scales contained in the measured substrate texture. Weak negative correlations were obtained between the adhesion forces and the area available for interaction, suggesting that increased roughness decreases bacterial adhesion when nano- to micrometer scales are considered. We suggest that modification of the DBM is needed in order to include discontinuous bonding. The adhesive strength is still related to the area available for bonding on a particular scale, but on some very fine scales, the bacteria may not be able to conform to the valleys or pits of the substrate. Therefore, the bonding between the bacterium and substrate becomes discontinuous, occurring only on the tops of ridges or asperities.