Electrical properties of outer membrane extensions from Shewanella oneidensis MR-1

Shewanella oneidensis MR-1 is a metal-reducing bacterium that is able to exchange electrons with solid-phase minerals outside the cell. These bacterial cells can produce outer membrane extensions (OMEs) that are tens of nanometers wide and several microns long. The capability of these OMEs to transport electrons is currently under investigation. Tubular chemically fixed OMEs from S. oneidensis have shown good dc conducting properties when measured in an air environment. However, no direct demonstration of the conductivity of the more common bubble-like OMEs has been provided yet, due to the inherent difficulties in measuring it. In the present work, we measured the electrical properties of bubble-like OMEs in a dry air environment by Scanning Dielectric Microscopy (SDM) in force detection mode. We found that at the frequency of the measurements (∼2 kHz), OMEs show an insulating behavior, with an equivalent homogeneous dielectric constant ε_OME = 3.7 ± 0.7 and no dephasing between the applied ac voltage and the measured ac electric force. The dielectric constant measured for the OMEs is comparable to that obtained for insulating supramolecular protein structures (ε_protein = 3-4), pointing towards a rich protein composition of the OMEs, probably coming from the periplasm. Based on the detection sensitivity of the measuring instrument, the upper limit for the ac longitudinal conductivity of bubble-like OMEs in a dry air environment has been set to σ_OME,ac < 10^-5 S m^-1, a value several orders of magnitude smaller than the dc conductivity measured in tubular chemically fixed OMEs. The lack of conductivity of bubble-like OMEs can be attributed to the relatively large separation between cytochromes in these larger OMEs and to the suppression of cytochrome mobility due to the dry environmental conditions.

Depth mapping of metallic nanowire polymer nanocomposites by scanning dielectric microscopy

Polymer nanocomposite materials based on metallic nanowires are widely investigated as transparent and flexible electrodes or as stretchable conductors and dielectrics for biosensing. Here we show that Scanning Dielectric Microscopy (SDM) can map the depth distribution of metallic nanowires within the nanocomposites in a non-destructive way. This is achieved by a quantitative analysis of sub-surface electrostatic force microscopy measurements with finite-element numerical calculations. As an application we determined the three-dimensional spatial distribution of ∼50 nm diameter silver nanowires in ∼100 nm-250 nm thick gelatin films. The characterization is done both under dry ambient conditions, where gelatin shows a relatively low dielectric constant, ε_r∼ 5, and under humid ambient conditions, where its dielectric constant increases up to ε_r∼ 14. The present results show that SDM can be a valuable non-destructive subsurface characterization technique for nanowire-based nanocomposite materials, which can contribute to the optimization of these materials for applications in fields such as wearable electronics, solar cell technologies or printable electronics.

Charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride

When two-dimensional crystals are brought into close proximity, their interaction results in reconstruction of electronic spectrum and crystal structure. Such reconstruction strongly depends on the twist angle between the crystals, which has received growing attention due to interesting electronic and optical properties that arise in graphene and transitional metal dichalcogenides. Here we study two insulating crystals of hexagonal boron nitride stacked at small twist angle. Using electrostatic force microscopy, we observe ferroelectric-like domains arranged in triangular superlattices with a large surface potential. The observation is attributed to interfacial elastic deformations that result in out-of-plane dipoles formed by pairs of boron and nitrogen atoms belonging to opposite interfacial surfaces. This creates a bilayer-thick ferroelectric with oppositely polarized (BN and NB) dipoles in neighbouring domains, in agreement with our modeling. These findings open up possibilities for designing van der Waals heterostructures and offer an alternative probe to study moiré-superlattice electrostatic potentials.

Mapping the dielectric constant of a single bacterial cell at the nanoscale with scanning dielectric force volume microscopy

Mapping the dielectric constant at the nanoscale of samples showing a complex topography, such as non-planar nanocomposite materials or single cells, poses formidable challenges to existing nanoscale dielectric microscopy techniques. Here we overcome these limitations by introducing Scanning Dielectric Force Volume Microscopy. This scanning probe microscopy technique is based on the acquisition of electrostatic force approach curves at every point of a sample and its post-processing and quantification by using a computational model that incorporates the actual measured sample topography. The technique provides quantitative nanoscale images of the local dielectric constant of the sample with unparalleled accuracy, spatial resolution and statistical significance, irrespectively of the complexity of its topography. We illustrate the potential of the technique by presenting a nanoscale dielectric constant map of a single bacterial cell, including its small-scale appendages. The bacterial cell shows three characteristic equivalent dielectric constant values, namely, ε_r,bac1 = 2.6 ± 0.2, ε_r,bac2 = 3.6 ± 0.4 and ε_r,bac3 = 4.9 ± 0.5, which enable identifying different dielectric properties of the cell wall and of the cytoplasmatic region, as well as, the existence of variations in the dielectric constant along the bacterial cell wall itself. Scanning Dielectric Force Volume Microscopy is expected to have an important impact in Materials and Life Sciences where the mapping of the dielectric properties of samples showing complex nanoscale topographies is often needed.

Dielectric constant of flagellin proteins measured by scanning dielectric microscopy

The dielectric constant of flagellin proteins in flagellar bacterial filaments ∼10-20 nm in diameter is measured using scanning dielectric microscopy. We obtained for two different bacterial species (Shewanella oneidensis MR-1 and Pseudomonas aeruginosa PAO1) similar relative dielectric constant values εSo = 4.3 ± 0.6 and εPa = 4.5 ± 0.7, respectively, despite their different structure and amino acid sequence. The present results show the applicability of scanning dielectric microscopy to nanoscale filamentous protein complexes and to general 3D macromolecular protein geometries, thus opening new avenues to study the relationship between the dielectric response and protein structure and function.

Anomalously low dielectric constant of confined water

The dielectric constant ε of interfacial water has been predicted to be smaller than that of bulk water (ε ≈ 80) because the rotational freedom of water dipoles is expected to decrease near surfaces, yet experimental evidence is lacking. We report local capacitance measurements for water confined between two atomically flat walls separated by various distances down to 1 nanometer. Our experiments reveal the presence of an interfacial layer with vanishingly small polarization such that its out-of-plane ε is only ~2. The electrically dead layer is found to be two to three molecules thick. These results provide much-needed feedback for theories describing water-mediated surface interactions and the behavior of interfacial water, and show a way to investigate the dielectric properties of other fluids and solids under extreme confinement.

Direct mapping of the electric permittivity of heterogeneous non-planar thin films at gigahertz frequencies by scanning microwave microscopy

We obtained maps of electric permittivity at ∼19 GHz frequencies on non-planar thin film heterogeneous samples by means of combined atomic force-scanning microwave microscopy (AFM-SMM). We show that the electric permittivity maps can be obtained directly from the capacitance images acquired in contact mode, after removing the topographic cross-talk effects. This result demonstrates the possibility of identifying the electric permittivity of different materials in a thin film sample irrespectively of their thickness by just direct imaging and processing. We show, in addition, that quantitative maps of the electric permittivity can be obtained with no need for any theoretical calculation or complex quantification procedures when the electric permittivity of one of the materials is known. To achieve these results the use of contact mode imaging is a key factor. For non-contact imaging modes the effects of local sample thickness and of the imaging distance make the interpretation of the capacitance images in terms of the electric permittivity properties of the materials much more complex. The present results represent a substantial contribution to the field of nanoscale microwave dielectric characterization of thin film materials with important implications for the characterization of novel 3D electronic devices and 3D nanomaterials.

Full-wave modeling of broadband near field scanning microwave microscopy

A three-dimensional finite element numerical modeling for the scanning microwave microscopy (SMM) setup is applied to study the full-wave quantification of the local material properties of samples. The modeling takes into account the radiation and scattering losses of the nano-sized probe neglected in previous models based on low-frequency assumptions. The scanning techniques of approach curves and constant height are implemented. In addition, we conclude that the SMM has the potential for use as a broadband dielectric spectroscopy operating at higher frequencies up to THz. The results demonstrate the accuracy of previous models. We draw conclusions in light of the experimental results.

Internal Hydration Properties of Single Bacterial Endospores Probed by Electrostatic Force Microscopy

We show that the internal hydration properties of single Bacillus cereus endospores in air under different relative humidity (RH) conditions can be determined through the measurement of its electric permittivity by means of quantitative electrostatic force microscopy (EFM). We show that an increase in the RH from 0% to 80% induces a large increase in the equivalent homogeneous relative electric permittivity of the bacterial endospores, from ∼4 up to ∼17, accompanied only by a small increase in the endospore height, of just a few nanometers. These results correlate the increase of the moisture content of the endospore with the corresponding increase of environmental RH. Three-dimensional finite element numerical calculations, which include the internal structure of the endospores, indicate that the moisture is mainly accumulated in the external layers of the endospore, hence preserving the core of the endospore at low hydration levels. This mechanism is different from what we observe for vegetative bacterial cells of the same species, in which the cell wall at high humid atmospheric conditions is not able to preserve the cytoplasmic region at low hydration levels. These results show the potential of quantitative EFM under environmental humidity control to study the hygroscopic properties of small-scale biological (and nonbiological) entities and to determine its internal hydration state. A better understanding of nanohygroscopic properties can be of relevance in the study of essential biological processes and in the design of bionanotechnological applications.

Nanoscale dielectric microscopy of non-planar samples by lift-mode electrostatic force microscopy

Lift-mode electrostatic force microscopy (EFM) is one of the most convenient imaging modes to study the local dielectric properties of non-planar samples. Here we present the quantitative analysis of this imaging mode. We introduce a method to quantify and subtract the topographic crosstalk from the lift-mode EFM images, and a 3D numerical approach that allows for extracting the local dielectric constant with nanoscale spatial resolution free from topographic artifacts. We demonstrate this procedure by measuring the dielectric properties of micropatterned SiO2 pillars and of single bacteria cells, thus illustrating the wide applicability of our approach from materials science to biology.

Nanoscale Electric Permittivity of Single Bacterial Cells at Gigahertz Frequencies by Scanning Microwave Microscopy

We quantified the electric permittivity of single bacterial cells at microwave frequencies and nanoscale spatial resolution by means of near-field scanning microwave microscopy. To this end, calibrated complex admittance images have been obtained at ∼19 GHz and analyzed with a methodology that removes the nonlocal topographic cross-talk contributions and thus provides quantifiable intrinsic dielectric images of the bacterial cells. Results for single Escherichia coli cells provide a relative electric permittivity of ∼4 in dry conditions and ∼20 in humid conditions, with no significant loss contributions. Present findings, together with the ability of microwaves to penetrate the cell membrane, open an important avenue in the microwave label-free imaging of single cells with nanoscale spatial resolution.

On the initial propagation of dental caries

A multi-dimensional model for dental caries is applied to study the shape of caries lesions in a realistic tooth geometry and to examine the rate of progress of caries. An upgraded model, taking into account the outer prismless enamel layer, is derived and solved. The model demonstrates the importance of this layer in delaying the onset of caries. The conclusions are discussed in light of experimental results.

Comparison of two-dimensional echocardiography and pulsed Doppler tissue imaging during dobutamine-atropine stress testing to detect coronary artery disease

In order to compare the diagnostic accuracy of two-dimensional (2-D) echocardiography and pulsed Doppler tissue imaging (pDTI) during dobutamine-atropine stress testing (DAST) to detect significant coronary lesions, 41 patients underwent DAST (up to 40 microg/k/min of dobutamine with additional atropine during submaximal heart rate responses) and coronary angiography. Pulsed Doppler tissue sampling of territories corresponding to the left anterior descending (LAD), left circumflex (LCx), and right coronary arteries (RCAs) were performed in the apical four-chamber plus aorta and two-chamber apical views. The measurements were repeated at rest, at low dose (10 microg/k/min), and at peak stress. Pulsed DTI measurements included peak early systolic (Vs), peak early diastolic (Ve), and peak late diastolic (Va) velocities. Harmonic 2-D echocardiography was recorded at rest, low dose, peak stress, and recovery, and compared with pDTI assessment. Positive 2-D echocardiography was considered as infarction or ischemic response. The results were evaluated for the prediction of significant coronary stenosis (50% luminal narrowing). Feasibility of pDTI was 100%, 95%, and 98% for the LAD, the LCx, and RCA territories, respectively. At rest, Vs in territories supplied by arteries with coronary artery disease (CAD) (6.3 +/- 2.0 cm/sec) was not different from those without (6.6 +/- 2.1 cm/sec). Vs increased less in territories supplied by arteries with than without CAD (75 +/- 107% vs 102 +/- 69%, P = NS). Ve was lower in territories with CAD at rest (6.0 +/- 2.1 cm/sec vs 8.2 +/- 3.4 cm/sec, P < 0.0001) and low dose (7.2 +/- 2.1 cm/sec vs 8.8 +/- 3.6 cm/sec, P < 0.01), but similar at peak stress (7.6 +/- 3.5 cm/sec vs 8.1 +/- 3.3 cm/sec). Ve increase was similar in territories with (36 +/- 74%) than without CAD (15 +/- 6 4%). Va was similar at rest and low dose in territories with and without CAD (9.2 +/- 2.7 cm/sec vs 9.1 +/- 2.3 cm/sec and 10.9 +/- 3.1 vs 10.3 +/- 3.6 cm/sec, respectively), but lower at peak stress in territories with CAD (13.3 +/- 4.6 cm/sec vs 15.3 +/- 4.5 cm/sec, P = 0.05). The Va increase was lower in territories with CAD (43 +/- 37% vs 77 +/- 72%, P < 0.05). In a territory-based analysis, a failure to achieve Vs > or =10.5 cm/sec at peak stress in the LAD and LCx, and > or =10.0 cm/sec in the RCA territory, was found to be the more accurate limit to detect CAD in the corresponding arteries: sensitivity (95% confidence intervals): 63% (55-71), P = NS vs 2-D echocardiography: 59% (51-67); specificity 76% (68-84), P < 0.01 vs. 2-D echocardiography: 95% (89-100); and accuracy 69% (63-75), P = NS vs 2-D echocardiography: 76% (70-82). Thus, pDTI is feasible during DAST but not more accurate than 2-D echocardiography for the detection of significant CAD in a territory-based study.

Nodular hyperplasia of the bartholin gland increasing in size during sexual intercourse

OBJECTIVE

A case of Bartholin gland nodular hyperplasia with review of the literature and diagnostic criteria is reported.

MATERIAL AND METHODS

In a 31-year-old woman, a nodular mass was found arising in the left labium minus. The mass size increased during sexual intercourse.

RESULTS

The mass was well-circumscribed but not encapsulated and showed a proliferation of acini lined by mucussecreting columnar epithelium. The lesion retained the lobular pattern and the relationship between acini and ducts as seen in the normal Bartholin gland.

CONCLUSION

Nodular hyperplasia of the Bartholin gland should be differentiated from adenoma, in which architecture is disrupted. Variation in size during sexual intercourse probably is mediated by vascular engorgement. Surgical excision is curative.

Comparison of treadmill exercise echocardiography before and after exercise in the evaluation of patients with known or suspected coronary artery disease

OBJECTIVES

We sought to compare the feasibility and accuracy of peak treadmill exercise echocardiography versus postexercise echocardiography imaging.

BACKGROUND

Although peak exercise echocardiography has been reported for both supine and orthostatic bicycle exercise and has shown higher sensitivity than postexercise imaging, acquiring images at peak exercise with treadmill has not been explored.

METHODS

Peak and post-treadmill exercise echocardiography and coronary angiography were performed on 89 patients with known or suspected coronary artery disease. Positive exercise echocardiography was defined as necrosis or ischemic response. Positive coronary angiography was defined as >/=1 diseased vessels (>/=50% luminal narrowing). Images were analyzed in a blind manner by an expert observer.

RESULTS

Postexercise images were acquired within 80 seconds after exercise (40 +/- 14). Mean heart rate (bpm) was 139 +/- 22 at peak versus 118 +/- 25 at postexercise imaging (P <.001). Interpretable peak and postexercise images were obtained for all 89 patients. Of the 72 classified as having positive exercise echocardiography, 23 had new regional wall motion abnormality at peak (21 with positive angiography), which resolved at postexercise imaging. Sensitivity was higher with peak than with postexercise imaging (94% vs 73%, P <.001). Specificity was similar (68% vs 79%), as was predictive positive value (92% vs 93%). Negative predictive value was again higher with peak imaging (76% vs 44%, P <.05). Total accuracy was higher with peak imaging (89% vs 74%, P <.05).

CONCLUSIONS

Peak treadmill exercise echocardiography is technically feasible and has higher sensitivity and accuracy than post-treadmill exercise echocardiography. Therefore in the clinical setting peak exercise echocardiography should be performed to diagnose ischemia.

Paradoxic phenomenon of premature beats with narrow QRS in the presence of bundle-branch block

In contrast to the functional bundle-branch block (BBB) which is frequently rate-related and disappears when heart rate slows, we have encountered seven cases of a paradoxic disappearance of an existing, ischemia-induced BBB in premature beats. Supernormal conduction within the blocked bundle branch was not present in any of the cases. His bundle electrography in three of the cases demonstrated His spikes before each of the narrow-QRS premature beats. The His-Q intervals of the premature beats were considerably shorter than those of the control sinus beats. This finding clearly indicated an ectopic origin midway between the ventricular myocardium and the His bundle, i.e., within the ischemic fascicles of the left bundle, or the ischemic right bundle. From their points of origin such fascicular impulses propagate antegradely toward the respective area of the ventricular myocardium, and retrogradely toward the main His bundle, and at the same time down the remaining two fascicles. The antegrade conduction time in the short pathway down the ischemic fascicle is equaled by the faster conduction through the longer pathways of the remaining, uninjured fascicles, thereby accounting for the almost simultaneous activation of the myocardium of the two ventricles and the resultant narrow-QRS complexes.