We present a method to determine the local dielectric permittivity of thin insulating layers. The measurement is based on the detection of force gradients in electric force microscopy by means of a double pass method. The proposed experimental protocol is simple to implement and does not need any modification of standard commercial devices. Numerical simulations based on the equivalent charge method make it possible to carry out quantification whatever the thickness of film, the radius of the tip, and the tip-sample distance. This method has been validated on a thin SiO 2 sample for which the dielectric permittivity at the nanoscale has been characterized in the literature. We also show how we can quantitatively measure the local dielectric permittivity for ultrathin polymer film of poly͑vinyl acetate͒ and polystyrene.
The electrochemical reduction of CO2 in a highly selective and efficient manner is a crucial step towards its reuse for the production of chemicals and fuels. Nanostructured Ag catalysts have been found to be effective candidates for the conversion of CO2 to CO. However, the ambiguous determination of the intrinsic CO2 activity and the maximization of the density of exposed active sites has greatly limited the use of Ag towards the realization of practical electrocatalytic devices.Here, we report a superstructure design strategy prepared by the self-assembly of two-dimensional Ag nanoprisms for maximizing the exposure of active edge ribs. The vertically stacked Ag nanoprisms allow exposure of > 95% of the edge sites, resulting in an enhanced selectivity and activity towards the production of CO from CO2 with an overpotential of 152 mV. The Ag superstructures also demonstrates a selectivity over 90% for 100 hours together with a current retention of ≈ 94% at -600 mV versus the reversible hydrogen electrode and a partial energy efficiency for CO production of 70.5%. Our electrochemical measurements on individual Ag nanoprisms with various edge-to-basal plane ratios and the Ag superstructures led to the identification of the edge ribs as the active sites thanks to the ≈ 400 mV decrease in the onset potential compared to that of the Ag (111) basal planes and a turnover frequency of 9.2 × 10 -3 ± 1.9 × 10 -3 s -1 at 0 V overpotential.
The control of the EOF direction and magnitude remains one of the more challenging issues for the optimization of separations in CE. In this work, we investigated the possibility to use variously charged polyanions for a fine-tuning of the EOF using polyelectrolyte multilayers. For that purpose, polyanions of poly(acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonate) (PAMAMPS) with different chemical charge rates varying between 3 and 100% were used. These copolymers are statistic hydrophilic copolymers of acrylamide (AM) and 2-acrylamido-2-methyl-1-propanesulfonate (AMPS). The study of the influence of the chemical charge rate (AMPS molar proportion in the copolymer) on the electroosmotic mobility (mu(eo)) of a capillary modified by a polyelectrolyte bilayer (polycation/PAMAMPS) revealed that the fine-tuning of the EOF was possible, at least for cathodic or slightly anodic EOF (micro(eo) from -5 x 10(-5) to +35 x 10(-5) cm(2)V(-1)s(-1)). Electroosmotic mobility values were compared with the free-draining electrophoretic mobilities of the PAMAMPS constituting the last layer of the capillary coating. The stability of the EOF is discussed in detail on the basis of successive determinations of electroosmotic mobility and migration times. The application to the separation of a model peptide mixture demonstrated the interest (and the simplicity) of this approach for optimizing resolution and analysis time. Experimental resolutions were compared to the theoretical ones that we would obtain on a fused-silica capillary having the same EOF as the coated capillary.
We present a study about AFM imaging of living, moving or self-immobilized bacteria in their genuine physiological liquid medium. No external immobilization protocol, neither chemical nor mechanical, was needed. For the first time, the native gliding movements of Gram-negative Nostoc cyanobacteria upon the surface, at speeds up to 900 µm/h, were studied by AFM. This was possible thanks to an improved combination of a gentle sample preparation process and an AFM procedure based on fast and complete force-distance curves made at every pixel, drastically reducing lateral forces. No limitation in spatial resolution or imaging rate was detected. Gram-positive and non-motile Rhodococcus wratislaviensis bacteria were studied as well. From the approach curves, Young modulus and turgor pressure were measured for both strains at different gliding speeds and are ranging from 20±3 to 105±5 MPa and 40±5 to 310±30 kPa depending on the bacterium and the gliding speed. For Nostoc, spatially limited zones with higher values of stiffness were observed. The related spatial period is much higher than the mean length of Nostoc nodules. This was explained by an inhomogeneous mechanical activation of nodules in the cyanobacterium. We also observed the presence of a soft extra cellular matrix (ECM) around the Nostoc bacterium. Both strains left a track of polymeric slime with variable thicknesses. For Rhodococcus, it is equal to few hundreds of nanometers, likely to promote its adhesion to the sample. While gliding, the Nostoc secretes a slime layer the thickness of which is in the nanometer range and increases with the gliding speed. This result reinforces the hypothesis of a propulsion mechanism based, for Nostoc cyanobacteria, on ejection of slime. These results open a large window on new studies of both dynamical phenomena of practical and fundamental interests such as the formation of biofilms and dynamic properties of bacteria in real physiological conditions.
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