In recent years, several publications on microfluidic devices have focused on the process of electroporation, which results in the poration of the biological cell membrane. The devices involved are designed for cell analysis, transfection or pasteurization. The high electric field strengths needed are induced by placing the electrodes in close proximity or by creating a constriction between the electrodes, which focuses the electric field. Detection is usually achieved through fluorescent labeling or by measuring impedance. So far, most of these devices have only concerned themselves solely with the electroporation process, but integration with separation and detection processes is expected in the near future. In particular, single-cell content analysis is expected to add further value to the concept of the microfluidic chip. Furthermore, if advanced pulse schemes are employed, such microdevices can also enhance research into intracellular electroporation.

No abstract

No abstract

No abstract

We have studied the optical collisions of cold, metastable helium atoms in a magneto-optical trap. We have detected the rate of Penning and associative ionization of two metastable helium atoms at a temperature of 1 mK with and without nearly resonant light. We find that the associative ionization rate is increased with more than a factor 20 due to the presence of the light field. We present a simple, semiclassical model, which accounts for all the observed features, and which agrees, also on an absolute scale, with our experimental results. [S0031-9007(98)06429-1] PACS numbers: 32.80. Pj, 33.80.Eh, 34.50.Rk Because of the breakthrough of laser cooling techniques it is nowadays possible to study collision processes at ultralow temperatures [1]. The study of optical collisions, where during the collisions of two cold atoms a photon is absorbed, has been very successful. It has lead to a wealth of experimental data on the long range interactions in these systems and has been shown to be complementary to molecular spectroscopy [2]. While in the latter case one probes the molecular structures at short range, in the former case one obtains information on the interaction taking place at long range and is able to observe the molecular complex during the reaction.Optical collisions can be induced by irradiation of light detuned below atomic resonance. The transition energy at long range is lowered due to the dipole-dipole attraction C 3 ͞R 3 in the first excited state and the excitation point R c can be selected by the detuning of the light. The study of the ionization rate as a function of the detuning provides information on the dynamics and ionization of the molecular excited states involved. In general, the ionization rate will increase if the laser is tuned closer to resonance, since the number of atomic pairs that can be excited will increase by increasing R c . However, once the detuning becomes too small the ionization rate will decrease for two reasons. First of all, the atomic pairs are excited at such a large distance, that spontaneous emission during the time the molecular system reaches small internuclear distances leads to a "switching off" of the attractive 1͞R 3 potential, thus prohibiting a close collision in which ionization can occur. Second, the attractive potential becomes too weak compared to the relative kinetic energy, so that absorption does not lead to a close collision.We have studied optical collisions using cold, metastable He͑2 3 S͒ atoms. Although optical collisions have already been studied experimentally in several systems [3], the He͑2 3 S͒ system is in many aspects different from all other systems. He͑23 S͒ is a prototype system with only one electron active and no hyperfine structure. This makes it possible to obtain accurate potential curves for the He͑2 3 S͒-He͑2 3 S͒ system, which have been published recently [4]. It is therefore possible to obtain a direct comparison on an absolute scale between experiment and theory for this system. Another difference becomes clear, if one con...

No abstract

No abstract

Vegetative cells of Bacillus cereus were subjected to low doses of nisin (0.06 g/ml) and mild pulsed-electric field treatment (16.7 kV/cm, 50 pulses each of 2-s duration). Combining both treatments resulted in a reduction of 1.8 log units more than the sum of the reductions obtained with the single treatments, indicating synergy.Nisin, an antimicrobial protein produced by Lactococcus lactis subsp. lactis, is the only bacteriocin that is approved by the World Health Organization to be used as a food preservative today (1, 7). Its primary target is the cytoplasmic membrane of vegetative cells. Nisin interacts via electrostatic interactions with the phospholipids and increases the permeability of the membrane by pore formation, resulting in a rapid efflux of small molecules (3, 16). The efflux of cellular constituents results in a complete collapse of the proton motive force and cellular death (4, 5). The practical application of nisin is limited because its inhibition spectrum is restricted to gram-positive bacteria only (9) and its activity can be influenced by pH or food ingredients like fat particles (18). However, by combining nisin with other mild preservatives like essential oils (14) or nonthermal pasteurization techniques, these restrictions could be overcome.Pulsed-electric field (PEF) treatment is a nonthermal pasteurization technique which inactivates microorganisms (15,20) by irreversible structural changes in the membrane, resulting in pore formation and loss of the selective permeability properties of the membrane (6, 13, 17). The extent of the permeability increase depends on the strength and duration of the electric field pulse (2,6,11,19,20). An imposed electric field causes polarization and subsequently accumulation of free charges at both sides of the cell surface, leading to an increased transmembrane potential difference and a reduction of the membrane thickness which finally results in pore formation (2, 19). Since nisin and PEF both act on the membrane, an additive effect might be expected.Bacillus cereus IFR-NL94-25, obtained from the Institute of Food Research (Norwich, United Kingdom) was grown at 20°C in brain heart infusion broth (Oxoid) containing 0.5% (wt/vol) glucose. Cells were harvested in the exponential growth phase, washed, and resuspended in 5 mM potassium-HEPES buffer (pH 7.0) to an optical density at 660 nm of 0.2 (tube diameter ϭ 9 mm). Cell suspensions were checked for spores by microscopy before harvesting and subsequently by analysis of surviving spores after a standard heat treatment (80°C, 5 min).The PEF system used here was a custom-built batch system applying single square-shaped pulses. The treatment chamber (800 l) was formed by two stainless steel cylinders with a diameter of 12.6 mm which were tightly fitted into a Plexiglas