Modification of radiation pressure due to cooperative scattering of light

Cooperative spontaneous emission of a single photon from a cloud of N atoms modifies substantially the radiation pressure exerted by a far-detuned laser beam exciting the atoms. On one hand, the force induced by photon absorption depends on the collective decay rate of the excited atomic state. On the other hand, directional spontaneous emission counteracts the recoil induced by the absorption. We derive an analytical expression for the radiation pressure in steady-state. For a smooth extended atomic distribution we show that the radiation pressure depends on the atom number via cooperative scattering and that, for certain atom numbers, it can be suppressed or enhanced. Cooperative scattering of light by extended atomic clouds can become important in the presence of quasi-resonant light and could be addressed in many cold atoms experiments.

Demographic characteristics and prevalence of serologic markers among blood donors who use confidential unit exclusion (CUE) in Sao Paulo, Brazil: implications for modification of CUE polices in Brazil

BACKGROUND: This study evaluated demographic profiles and prevalence of serologic markers among donors who used confidential unit exclusion (CUE) to assess the effectiveness of CUE and guide public policies regarding the use of CUE for enhancing safety versus jeopardizing the blood supply by dropping CUE. STUDY DESIGN AND METHODS: We conducted a cross-sectional analysis of whole blood donations at a large public blood center in Sao Paulo from July 2007 through June 2009, compared demographic data, and confirmed serologic results among donors who used and who have never used CUE (CUE never). RESULTS: There were 265,550 whole blood units collected from 181,418 donors from July 2007 through June 2009. A total of 9658 (3.6%) units were discarded, 2973 (1.1%) because CUE was used at the current donation (CUE now) and 6685 (2.5%) because CUE was used in the past (CUE past). The CUE rate was highest among donors with less than 8 years of education (odds ratio [OR], 2.78; 95% confidence interval [CI], 2.51-3.08). CUE now donations were associated with higher positive infectious disease marker rates than CUE never donations (OR, 1.41; CI, 1.13-1.77), whereas CUE past donations were not (OR, 1.04; CI, 0.75-1.45). CONCLUSION: The CUE process results in a high rate of unit discard. CUE use on an individual donation appears predictive of a high-risk marker-positive donation and, thus, appears to contribute modestly to blood safety. The policy of discarding units from donors who have previously CUE-positive donations does not improve safety and should be discontinued.

Chemical modification of a nanocrystalline TiO(2) film for efficient electric connection of glucose oxidase

A novel biosensor for glucose was prepared by adsorption of 1,1`-bis(4-carboxybenzyl)-4,4`-bipyridinium di-bromide compound (H(2)BpybcBr(2)) onto the surface of a nanocrystalline TiO(2) film deposited onto FTO glasses, which was used as a platform to assemble the enzyme glucose oxidase to the electrode surface. The H(2)BpybcBr(2)/TiO(2)/FTO modified electrode was characterized by scanning electron microscopy, X-ray fluorescence image, cyclic voltammograms and spectroelectrochemical measurements. The immobilization of GOD on functionalized TiO(2) film led to stable amperometric biosensing for glucose with a linear range from 153 mu mol L(-1) to 1.30 mmol L(-1) and a detection limit of 51 mu mol L(-1). The apparent Michaelis-Menten constant (K(m)) was estimated to be 3.76 mmol L(-1), which suggested a high enzyme-substrate affinity. The maximum electrode sensitivity was 1.25 mu A mmol L(-1). The study proved that the combination of viologen mediators with TiO(2) film retains the electrocatalytic activity of the enzyme, and also enhances the electron transfer process, and hence regenerating the enzyme in the reaction with glucose. (C) 2010 Elsevier Inc. All rights reserved.