Microfluidic device for concentration and SERS-based detection of bacteria in drinking water

There is a constant need for the development of easy-to-operate systems for the rapid and unambiguous identification of bacterial pathogens in drinking water without the requirement for time-consuming culture processes. In this study, we present a disposable and low-cost lab-on-a-chip device utilizing a nanoporous membrane, which connects two stacked perpendicular microfluidic channels. Whereas one of the channels supplies the sample, the second one attracts it by potential-driven forces. Surface-enhanced Raman spectrometry (SERS) is employed as a reliable detection method for bacteria identification. To gain the effect of surface enhancement, silver nanoparticles were added to the sample. The pores of the membrane act as a filter trapping the bodies of microorganisms as well as clusters of nanoparticles creating suitable conditions for sensitive SERS detection. Therein, we focused on the construction and characterization of the device performance. To demonstrate the functionality of the microfluidic chip, we analyzed common pathogens (Escherichia coli DH5α and Pseudomonas taiwanensis VLB120) from spiked tap water using the optimized experimental parameters. The obtained results confirmed our system to be promising for the construction of a disposable optical platform for reliable and rapid pathogen detection which couples their electrokinetic concentration on the integrated nanoporous membrane with SERS detection.     

N-terminal and C-terminal domains of arrestin both contribute in binding to rhodopsin

Visual arrestin terminates the signal amplification cascade in photoreceptor cells by blocking the interaction of light activated phosphorylated rhodopsin with the G-protein transducin. Although crystal structures of arrestin and rhodopsin are available, it is still unknown how the complex of the two proteins is formed. To investigate the interaction sites of arrestin with rhodopsin various surface regions of recombinant arrestin were sterically blocked by different numbers of fluorophores (Alexa 633). The binding was recorded by time-resolved light scattering. To accomplish site-specific shielding of protein regions, in a first step all three wild-type cysteines were replaced by alanines. Nevertheless, regarding the magnitude and specificity of rhodopsin binding, the protein is still fully active. In a second step, new cysteines were introduced at selected sites to allow covalent binding of fluorophores. Upon attachment of Alexa 633 to the recombinant cysteines we observed that these bulky labels residing in the concave area of either the N- or the C-terminal domain do not perturb the activity of arrestin. By simultaneously modifying both domains with one Alexa 633 the binding capacity was reduced. The presence of two Alexa 633 molecules in each domain prevented binding of rhodopsin to arrestin. This observation indicates that both concave sites participate in binding.     

Effects of perfluorocarbon gases on the size and stability characteristics of phospholipid-coated microbubbles: osmotic effect versus interfacial film stabilization

Micrometer-sized bubbles coated with phospholipids are used as contrast agents for ultrasound imaging and have potential for oxygen, drug, and gene delivery and as therapeutic devices. An internal perfluorocarbon (FC) gas is generally used to stabilize them osmotically. We report here on the effects of three relatively heavy FCs, perfluorohexane (F-hexane), perfluorodiglyme (F-diglyme ), and perfluorotriglyme (F-triglyme), on the size and stability characteristics of microbubbles coated with a soft shell of dimyristoylphosphatidylcholine (DMPC) and on the surface tension and compressibility of DMPC monolayers. Monomodal populations of small bubbles (~1.3 ± 0.2 μm in radius, polydispersivity index ~8%) were prepared by sonication, followed by centrifugal fractionation. The mean microbubble size, size distribution, and stability were determined by acoustical attenuation measurements, static light scattering, and optical microscopy. The half-lives of F-hexane- and F-diglyme-stabilized bubbles (149 ± 8 and 134 ± 3 min, respectively) were about 2 times longer than with the heavier F-triglyme (76 ± 7 min) and 4-5 times longer than with air (34 ± 3 min). Remarkably, the bubbles are smaller than the minimal size values calculated assuming that the bubbles are stabilized osmotically by the insoluble FC gases. Particularly striking is that bubbles 2 orders of magnitude smaller than the calculated collapse radius can be prepared with F-triglyme, while its very low vapor pressure prohibits any osmotic effect. The interface between an aqueous DMPC dispersion and air, or air (or N(2)) saturated with the FCs, was investigated by tensiometry and by Langmuir monolayer compressions. Remarkably, after 3 h, the tensions at the interface between an aqueous DMPC dispersion (0.5 mmol L(-1)) and air were lowered from ~50 ± 1 to ~37 ± 1 mN m(-1) when F-hexane and F-diglyme were present and to ~40 ± 1 mN m(-1) for F-triglyme. Also noteworthy, the adsorption kinetics of DMPC at the interface, as obtained by dynamic tensiometry, were accelerated up to 3-fold when the FC gases were present. The compression isotherms show that all these FC gases significantly increase the surface pressure (from ~0 to ~10 mN m(-1)) at large molecular areas (70 ?(2)), implying their incorporation into the DMPC monolayer. All three FC gases increase the monolayer's collapse pressures significantly (~61 ± 2 mN m(-1)) as compared to air (~54 ± 2 mN m(-1)), providing for interfacial tensions as low as ~11 mN m(-1) (vs ~18 mN m(-1) in their absence). The FC gases increase the compressibility of the DMPC monolayer by 20-50%. These results establish that, besides their osmotic effect, FC gases contribute to bubble stabilization by decreasing the DMPC interfacial tension, hence reducing the Laplace pressure. This contribution, although significant, still does not suffice to explain the large discrepancy observed between calculated and experimental bubble half-lives. The case of F-triglyme, which has no osmotic effect, indicates that its effects on the DMPC shell (increased collapse pressure, decreased interfacial tension, and increased compressibility) contribute to bubble stabilization. F-hexane and F-diglyme provided both the smallest mean bubble sizes and the longest bubble half-lives.     

Stacking of Self‐Assembled Surface Micelles in Ultrathin Films

Nonpolar fluorophilic/lipophilic tetrablock amphiphiles are investigated on the surface of water and on solid substrates using compression isotherms, Brewster angle microscopy, and atomic force microscopy. At low pressures, the tetrablocks form monolayers of closely packed surface hemimicelles. Further compression causes a 2D/3D transition. At the end of the plateau, half of the deposited material is expelled forming a second monolayer on top of the initially formed monolayer. Both layers of the films consist of surface micelles, thus providing the first example of spontaneous or compression‐driven stacking of self‐assembled nano‐objects.     

Solvent controlled mechanistic dichotomy in a Au(III)-catalyzed, heterocyclization triggered, Nazarov reaction

Tandem Au(III)-catalyzed heterocyclization/Nazarov cyclizations leading to substituted carbocycle fused furans are described. An interesting dichotomy of reaction pathways as a function of solvent, confirmed by the isolation and trapping of reaction intermediates, provided a basis for computational studies that supported the experimental findings.     

Fluorous materials in microbubble engineering science and technology—Design and development of new bubble preparation and sizing technologies

Microbubbles are being used in ultrasound diagnosis imaging, could serve as contrast agents for multiple imaging modalities and for parenteral oxygen delivery, and have potential in materials science as new multi-scale materials for magnetic, optical and magneto-optical devices. We designed new procedures for the preparation of narrowly size-distributed, monomodal microbubble populations, as well as a new multifrequency acoustical device for microbubble sizing. Methods for microbubble stabilization using fluorinated gases and surfactants are also discussed. Prospects include further use of fluorinated amphiphiles for bubble stabilization and size control, and decoration of bubble surface with functional particles (e.g. magnetic iron oxide nanoparticles).