Times Higher Signal Research Articles

First-principles simulation of light-ion microscopy of graphene

The extreme sensitivity of 2D materials to defects and nanostructure requires precise imaging techniques to verify presence of desirable and absence of undesirable features in the atomic geometry. Helium-ion beams have emerged as a promising materials imaging tool, achieving up to 20 times higher resolution and 10 times larger depth-of-field than conventional or environmental scanning electron microscopes. Here, we offer first-principles theoretical insights to advance ion-beam imaging of atomically thin materials by performing real-time time-dependent density functional theory simulations of single impacts of 10–200 keV light ions in free-standing graphene. We predict that detecting electrons emitted from the back of the material (the side from which the ion exits) would result in up to three times higher signal and up to five times higher contrast images, making 2D materials especially compelling targets for ion-beam microscopy. This predicted superiority of exit-side emission likely arises from anisotropic kinetic emission. The charge induced in the graphene equilibrates on a sub-fs time scale, leading to only slight disturbances in the carbon lattice that are unlikely to damage the atomic structure for any of the beam parameters investigated here.

High\u2013Bias\u2013Field Operation of GaAs Photoconductive Terahertz Emitters

We demonstrate experimentally the increase of optical-to-terahertz conversion efficiency for GaAs-based photoconductive terahertz emitters. This increase is achieved by preventing device breakdown through series resistors, which act as a current limiter. Pulsed photoexcitation and potential current fluctuations result in heat dissipation leading to local heating, which further increases the current and may lead to device breakdown. We manage to increase the maximum bias field before device breakdown by a factor of 3 under illuminated conditions. For a laser system with 250-kHz repetition rate, the terahertz emission amplitude increases linearly with applied bias field up to 120 kV/cm bias field, which results in 3 times higher signal as compared to the standard device. Furthermore, we have also achieved this expanded breakdown prevention at 78-MHz repetition rate, where an integrated on-chip resistance leads to an enhancement of the terahertz field amplitude by 70%. This simple technique can increase the performance of almost all photoconductive terahertz emitters by using appropriate resistances according to the emitter capacitance and laser repetition rate.

Fano Metamaterials on Nanopedestals for Plasmon-Enhanced Infrared Spectroscopy

We report a sensing platform for surface-enhanced infrared absorption (SEIRA) spectroscopy, based on Fano metamaterials (FMMs) on dielectric nanopedestals. FMMs consist of two parallel gold (Au) nanorod antennas, with a small horizontal coupler attached to one of the nanorod antenna. When placed on SiO2 dielectric nanopedestals, which exhibit strong field enhancements caused by the interference between subradiant and superradiant plasmonic resonances, they provide the highly enhanced E-field intensities formed near the Au nanoantenna, which can provide more enhanced molecular detection signals. Here, the sensing characteristics of FMMs on nanopedestals structure was confirmed by comparison with FMMs on an unetched SiO2 substrate as a control sample. The control FMMs and the FMMs on nanopedestals were carefully designed to excite Fano resonance near the target 1-octadecanethiol (ODT) fingerprint vibrations. The FMMs were fabricated by using nanoimprint lithography and the nanopedestal structures were formed by isotropic dry-etching. The experimental reflection spectra containing the enhanced absorption signals of the ODT monolayer molecules was analyzed using temporal coupled-mode theory. The FMMs on nanopedestals achieved over 7% of reflection difference signal, which was 1.7 times higher signal than the one from the control FMMs. Based on the FMMs on nanopedestal structures proposed in this study, it may be widely applied to future spectroscopy and sensor applications requiring ultrasensitive detection capability.

Abstract TP129: Aging is Associated with Vulnerability of Carotid Plaques in Patients With Type 2 Diabetes Mellitus

Introduction: Type 2 diabetes mellitus (T2DM) is a well-known risk factor for atherosclerosis. A previous carotid ultrasound study found diabetic patients had more echolucent plaques compared with nondiabetic patients. Another MRI-based carotid plaque imaging study revealed diabetes was associated with development of vulnerable plaques. Therefore diabetes is considered as a predictor for the presence of unstable plaques. The aim of this study is to investigate the association between vulnerability of carotid plaques and clinical characteristics in patients with T2DM. Methods: This study included type 2 diabetic patients with moderate to severe carotid artery stenosis (n=38, mean age 67.8, range 40-83). Using black-blood MRI of carotid arteries, the signal intensity of carotid plaques were measured in all patients. Univariable and multivariable analysis was conducted to examine the association between signal intensity of carotid plaques and clinical characteristics. Results: Vulnerable plaques (VP) , defined as plaques presenting more than one and a half times higher signal intensity than that of ipsilateral sternocleidomastoid muscle, were found in 8 patients. The rates of symptomatic lesions were significantly higher in patients with VP compared to patients without VP [odds ratio (OR) 17.4, 95% confidence interval (CI) 1.495-202.470, p=0.024]. In univariable analysis, VP was associated with age (p=0.0016) and ischemic heart disease (OD 10.5, 95% CI 1.142-96.576, p=0.0422). The association between other factors (sex, hypertension, hyperlipidemia, smoking habit, HbA1c, diabetic retinopathy, diabetic nephropathy) and VP was not significant. Multivariate logistic regression analysis proved that age was an independent factor associated with VP (OD 1.319, 95% CI 1.072-1.939, p=0.0455). Conclusion: This study suggests that there is a significant correlation between presence of VP detected by carotid MRI and ischemic symptoms, and that aging contributes to vulnerability of carotid plaques in patients with T2DM.

Fabrication of plasmon length-based surface enhanced Raman scattering for multiplex detection on microfluidic device.

The length of bioreceptors plays an important role in signal enhancement of surface-enhanced Raman scattering (SERS) due to amplification of electromagnetic fields generated by the excitation of localized surface plasmons. Herein, intact antibodies (IgG) and Fab fragments conjugated onto gold nanostar were used to fabricate two kinds of immunosensors for measurement of their SERS signals. Using CA125 as the antigen and Rhodamine-6G (R6G)-conjugated immunogolds, a SERS immunosensor was self-assembled by antigen-antibody interaction. The results showed that the SERS signal from the Fab immunosensor was 2.4 times higher than that of the IgG immunosensor. Furthermore, increased hot-spots by silver atom deposition onto the IgG and Fab immunosensor showed 2.1 and 1.4 times higher signals than before enhancement, respectively. For application, based on the Fab immunosensor, a SERS-compatible microfluidic system was designed for multiplex assays to overcome the drawbacks of conventional assays. This system can measure biological specimens directly from bio fluids instead of using a complex microfluidic device containing separation and detection elements. Four approved biomarkers of breast cancer, including cancer antigen (CA125), HER2, epididymis protein (HE4), and Eotaxin-1, were detected from patient-mimicked serum with limits of 15 fM, 17 fM, 21 fM, and 6.5 fM, respectively. The results indicated that the lengths and geometry of the bioreceptors determined the intensity of SERS signal from the interface and cavity of the sandwich immunosensor. Silver atom deposition at the cavity of the immunosensor increased the SERS signal. Finally, the SERS immunosensor built-in microfluidic system improved the performance of multiplex diagnostics.

Molecular Imprinting on the Nanoscale – Rapid Detection of Ag Nanoparticles by QCM Sensors

Within this paper, we report first attempts of molecular imprinting with man-made nanoparticles as templates of the imprinting process. In a first step, we screened different polymers for their respective affinities towards silver nanoparticles by the respective frequency shifts observed with quartz crystal microbalance (QCM) sensors. Polyurethanes and polystyrenes show almost similar effects, where the latter reach about 80% of the effects on the former. In comparison, polyvinyl alcohol for this system yields only 5% signal compared to the polyurethane. Sedimentation surface imprinting turned out successful: NP MIP reveal cavities of the correct dimensions (20nm in this case) on their surfaces, which cannot be seen on the respective NIP. Successful imprinting is further corroborated by first mass-sensitive measurements on QCM revealing ∼6 times higher signals on the MIP compared to the NIP and concentration-dependent, reversible sensor signals. To the best of our knowledge, this constitutes first successful MIP sensors for silver nanoparticles.

A micro blood sampling system for catheterized neonates and pediatrics in intensive care unit

A new micro blood sampling system has been designed, fabricated, and characterized to reduce iatrogenic blood loss from the catheterized neonates and pediatrics in intensive care unit by providing micro-volume of blood to analytical biomedical microdevices which can do point-of-care testing for their critical care. The system can not only save enormous iatrogenic blood loss through 1 to 10 μL of blood sampling and re-infusion of 1 to 5 mL of discard blood but also reduce the infection risk through the closed structure while satisfying the key criteria of the blood sampler. The sampled blood preserved its quality without rupturing of red blood cells verified by blood potassium concentrations of 3.86 ± 0.07 mM on the sampled blood which is similar to 3.81 ± 0.04 mM measured from the blood which did not go through the system. The sampling volume among the sampling channels showed consistency with the relative standard deviation of 1.41 %. In addition to the micro blood sampling capability, the sampling system showed negligible sample cross-contamination. The analyte-free samples collected after aspirating 7,500 times higher signal sample showed the same output signal as blank. The system was also demonstrated not to cause air-embolism by having no bubble generation during flushing procedure and the system was verified as leak-free since there was no fluid leakage under 30 times higher pressure than central venous pressure for 24 h.

Retrieval of tropospheric CO column from hyperspectral infrared sounders – application to four years of Aqua/AIRS and MetOp-A/IASI

Abstract. Four years of tropospheric integrated content of CO were retrieved from infrared hyperspectral observations of AIRS onboard Aqua and IASI onboard MetOp-A, for the period July 2007–June 2011. The retrieval method is based on a double differential approach that relies on the difference between brightness temperatures observed by the sounder and BT simulated by the Automatised Atmospheric Absorption Atlas (4A) radiative transfer model on colocated ECMWF reanalyses, for several couples of channels located in the 4.67 μm CO band. AIRS and IASI give access to similar integrated contents of CO with a maximum sensitivity near 450 hPa and a half-height width of the weighting function between 200 and 750 hPa depending on the thermal contrast (i.e., the difference between the surface temperature and the temperature of the first pressure level). However, differences in their spectral and radiometric characteristics yield differences in the retrieval characteristics with AIRS selected couples of channels being more sensitive to surface characteristics. Moreover, IASI covers the whole CO absorption band, with a 3 times better spectral resolution, giving access to channels presenting a 3 times higher signal to noise ratio. This results in a better precision and lower standard deviation of the IASI retrievals. Conservatively, comparisons with CARIBIC aircraft measurements yield an averaged relative difference of 3.4% for IASI and 4.9% for AIRS. On average, AIRS and IASI retrievals are in very good agreement, showing the same seasonality, seasonal amplitudes, interannual variability and spatial distribution. The analysis of the monthly evolution of CO particularly highlights the expected strong influence of biomass burning on the evolution of CO in several tropical regions. In particular, a sharp increase in CO in 2010 in the southern tropics, especially over South America and South Africa, is observed, and is shown to be related to El Niño and to the Atlantic Multidecadal Oscillation.

Anodic stripping voltammetry of zinc at boron-doped diamond electrodes in ammonia buffer solution

This paper describes the deposition of zinc(II) with anodic stripping voltammetry on the boron-doped diamond electrode. We illustrate the dependency of several parameters on the magnitude of the oxidation peak and try to optimize the method. The supporting electrolyte was found to influence the oxidation peak magnitude. Compared with acetic acid, the most frequently used supporting electrolyte, ammonia buffer solution leads to a four times higher signal. We assume that the formation of zinc complexes, primarily tetraaminezinc(II), are responsible for the better response. Further factors studied and assessed include buffer pH, buffer concentration, deposition potential, deposition time and scan rate. With the improved conditions, a final detection limit of 5 ppb was accomplished.

Quantum-optical coherence tomography with classical light

Quantum-optical coherence tomography (Q-OCT) is an interferometric technique for axial imaging offering several advantages over conventional methods. Chirped-pulse interferometry (CPI) was recently demonstrated to exhibit all of the benefits of the quantum interferometer upon which Q-OCT is based. Here we use CPI to measure axial interferograms to profile a sample accruing the important benefits of Q-OCT, including automatic dispersion cancellation, but with 10 million times higher signal. Our technique solves the artifact problem in Q-OCT and highlights the power of classical correlation in optical imaging.

Sequential conversion by catalytically active MIP and immobilized tyrosinase in a thermistor

To amplify the heat-signal generated by MIP catalyzed solvolysis of phenylacetate the reaction has been combined for the first time in a reactor with the subsequent oxidation by immobilized tyrosinase. The polymer cleaves the substrate and the released phenol is afterwards converted to o-benzoquinone by the tyrosinase. The separated and sequentially coupled reactions are characterized by the heat generated in a thermistor. The sequential substrate conversion results in a combined heat generation which results a five times higher signal than compared to the polymer alone.

Tandem Dye Acceptor Used To Enhance Upconversion Fluorescence Resonance Energy Transfer in Homogeneous Assays

In fluorescence resonance energy transfer (FRET)-based assays, spectral separation of acceptor emission from donor emission is a common problem affecting the assay sensitivity. The challenge derives from small Stokes shifts characteristic to conventional fluorescent dyes resulting in leakage of donor emission to the measurement window intended only to collect the acceptor emission. We have studied a FRET-based homogeneous bioaffinity assay utilizing a tandem dye acceptor with a large pseudo-Stokes shift (139 nm). The tandem dye was constructed using B-phycoerythrin as an absorber and multiple Alexa Fluor 680 dyes as emitters. As a donor, we employed upconverting phosphor particles, which uniquely emit at visible wavelengths under low-energy infrared excitation enabling the fluorescence measurements free from autofluorescence even without time-resolved detection. With the tandem dye, it was possible to achieve four times higher signal from a single binding event compared to the conventional Alexa Fluor 680 dye alone. Tandem dyes are widely used in cytometry and other multiplex purposes, but their applications can be expanded to fluorescence-based homogeneous assays. Both the optimal excitation and emission wavelengths of tandem dye can be tuned to a desired region by choosing appropriate fluorophores enabling specifically designed acceptor dyes with large Stokes shift.

Ultraviolet Spectral Variability and the Lyα Forest in the Lensed Quasar Q0957+561

Far-ultraviolet spectra of the gravitational lens components Q0957+561A, B were obtained with the Hubble Space Telescope Faint Object Spectrograph (FOS) at five equally spaced epochs, one every 2 weeks. We confirm the flux variability of the quasar's Lyα and O VI λ1037 emission lines in IUE spectra reported in earlier work of Dolan et al. The fluxes in these lines vary on a timescale of weeks in the observer's rest frame, independently of each other and of the surrounding continuum. The individual spectra of each image were co-added to investigate the properties of the Lyα forest along the two lines of sight to the quasar. Absorption lines having equivalent width Wλ ≥ 0.3 A in the observer's frame not previously identified by Michalitsianos et al. as interstellar lines, metal lines, or higher order Lyman lines were taken to be Lyα forest lines. The existence of each line in this consistently selected set was then verified by its presence in two archival FOS spectra with ~ 1.5 times higher signal to noise than our co-added spectra. Lyα forest lines with Wλ ≥ 0.3 A appear at 41 distinct wavelengths in the spectra of the two images. One absorption line in the spectrum of image A has no counterpart in the spectrum of image B, and one line in image B has no counterpart in image A. Based on the separation of the lines of sight over the redshift range searched for Lyα forest lines, the density of the absorbing clouds in the direction of Q0957+561 must change significantly over a distance R = 160 h kpc in the simplified model where the absorbers are treated as spherical clouds and the characteristic dimension, R, is the radius. (We adopt H0 = 50 h50 km s-1 Mpc-1, q0 = , and Λ = 0 throughout this paper.) The 95% confidence interval on R extends from 50 to 950 h kpc. We show in the Appendix that the fraction of Lyα forest lines that appear in only one spectrum can be expressed as a rapidly converging power series in 1/r, where r the ratio of the radius of the cloud to the separation of the two lines of sight at the redshift of the cloud. This power series can be rewritten to give r in terms of the fraction of Lyα forest wavelengths that appear in the spectrum of only one image. A simple linear approximation to the solution that everywhere agrees with the power series solution to better than 0.8% for r ≥ 2 is derived in the Appendix.

Capillary zone electrophoresis of linear and branched oligosaccharides

The electrophoretic behavior of derivatized linear and branched oligosaccharides from various sources was examined in capillary zone electrophoresis with polyether-coated fused-silica capillaries. Two UV-absorbing (also fluorescent) derivatizing agents (2-amino-pyridine and 6-aminoquinoline) were utilized for the electrophoresis and sensitive dtection of neutral oligosaccharides, e.g., N-acetylchitooligosaccharides, high-mannose glycans and xyloglucan oligosaccharides. The oligosaccharides labelled with 6-aminoquinoline yielded eight times higher signal than those tagged with 2-aminopyridine. Plots of logarithmic electrophoretic mobilities of labelled N-acetylchitooligosaccharides with 6-aminoquinoline or 2-aminopyridine versus the number of sugar residues in the homologous series yielded straight lines in the size range studied, the slopes of which were independent of the tagging functions. The slopes of these lines are referred to as the N-acetylglucosaminyl group mobility decrement. The oligosaccharides were better resolved in the presence of tetrabutylammonium bromide in the running electrolyte. Furthermore, the electrophoretic mobilities of branched oligosaccharides were indexed with respect to linear homooligosaccharides, an approach that may prove valuable in correlating and predicting the mobilities of complex oligosaccharides.