Noise-induced linewidth in frequency combs

Frequency combs generated by trains of pulses emitted from mode-locked lasers are analyzed when the center time and phase of the pulses undergo noise-induced random walk, which broadens the comb lines. Asymptotic analysis and computation reveal that, when the standard deviation of the center-time jitter of the nth pulse scales as n(p/2) where p is a jitter exponent, the linewidth of the kth comb line scales as k(2/p). The linear-dispersionless (p=1) and pure-soliton (p=3) dynamics in lasers are derived as special cases of this time-frequency duality relation. In addition, the linewidth induced by phase jitter decreases with power P(out), as (P(out))(-1/p).     

Line shape considerations in ultrafast 2D NMR

We have recently proposed and demonstrated an approach that enables the acquisition of 2D nuclear magnetic resonance (NMR) spectra within a single scan. The approach is based on spatially encoding the spins' evolution along the indirect domain with the aid of a magnetic field gradient, and subsequently decoding this information numerous times over the course of the signal acquisition while spins are subject to a train of gradient echoes. The present paper discusses further considerations pertaining the 2D line shapes arising from this new way of collecting NMR data. Specific issues that are hereby addressed include (i) the effects introduced by fast relaxation onto the spatial encoding process, particularly the line widths and line shapes that will then arise in the frequency domain; (ii) approaches capable of correcting for the mixed-phase kernels resulting in these fast-relaxation cases, corresponding in essence to spatially encoded analogs of the TPPI and hypercomplex time-domain acquisition procedures; (iii) the enveloping characteristics imposed by the use of discrete excitation pulses on the attainable spectral widths along the indirect domain; and (iv) an analysis of the signal-to-noise characteristics of the methodology, with experimental corroborations of theoretical predictions and an illustration of the method's capabilities to analyze protein solutions in the mM-range concentration.     

Arrayed acquisition of 2D exchange NMR spectra within a single scan experiment

We have recently demonstrated that magnetic field gradients in combination with frequency selective pulses, can be employed to collect a complete multi-dimensional NMR spectrum within a single scan. Following similar guidelines, field gradients could also be exploited to parallelize other types of NMR experiments where the final results arise from the collection and analysis of a series of time-incremented spectra. The present Communication exemplifies this concept by showing how a combination of gradients can be employed to monitor within a single continuous acquisition, a slow dynamic process which is in turn followed by systematic increments in the duration of a magnetization transfer time. Further, since 2D exchange NMR spectra can nowadays be themselves collected within one scan, the acquisition of a complete set of mixing-incremented 2D exchange patterns could be achieved within a single experiment entailing a total time of approximately 1 s.     

Microscopic flow measurements with optically trapped microprobes

The use of optical tweezers to measure micrometer-resolution velocity fields in fluid flow is demonstrated as an extension of a scanning confocal viscosity microscope. This demonstration is achieved by detection of the motion of an optically trapped microsphere in an oscillating laser trap. The technique is validated by comparison with an independent video-based measurement and applied to obtain a two-dimensional map of the flow past a microscopic wedge. Since the velocity is measured simultaneously with the trap relaxation time, the technique requires no fluid-dependent calibration and is independent of the trap stiffness and the particle size.     

Subtractive differential pulse anodic stripping voltammetry at a stationary mercury-coated glassy carbon electrode

The application of subtractive mode differential pulse anodic stripping voltammetry (SDPASV) at a stationary mercury-coated glassy carbon electrode for the analysis of labile Zn(II), Cd(II), Pb(II) and Cu(II) is described. It is shown that the method has an improved sensitivity to Cu(II) owing to elimination of high background currents normally encountered in normal mode DPASV at the TFME. The sensitivity limits of the present method to Cd(II) and Cu(II) is estimated to be 0.025 and 0.067 ppb respectively, when a 2 min deposition time is used. It is suggested that the high sensitivity of the method coupled to the relative simplicity of the stationary electrodes could make the method useful in environmental and natural water studies.     

Voltage-controlled metal binding on polyelectrolyte-functionalized nanopores

Most of the research in the field of nanopore-based platforms is focused on monitoring ion currents and forces as individual molecules translocate through the nanopore. Molecular gating, however, can occur when target analytes interact with receptors appended to the nanopore surface. Here we show that a solid state nanopore functionalized with polyelectrolytes can reversibly bind metal ions, resulting in a reversible, real-time signal that is concentration dependent. Functionalization of the sensor is based on electrostatic interactions, requires no covalent bond formation, and can be monitored in real time. Furthermore, we demonstrate how the applied voltage can be employed to tune the binding properties of the sensor. The sensor has wide-ranging applications and, its simplest incarnation can be used to study binding thermodynamics using purely electrical measurements with no need for labeling.     

Surface modification. II. Functionalization of solid surfaces with vinylic monomers

A new method to functionalize surfaces of solid substrates such as glass, silicon crystals, and silica microspheres with appropriate vinylic monomers, i.e., methyl vinyl ketone, methyl acrylate, methacrolein, and acrolein, is described. The surface modification process was performed through the following sequence of reactions: (a) derivatization of the surfaces with to-nitrile groups by interacting the substrates with SiCl₃(CH₂)₃CN; (b) subsequent reduction of the a)-nitrile groups with diborane to w-amine groups; (c) binding of the vinylic monomers to the surfaces via the to-amine groups. pK_1/2 of the surface primary amine groups, as determined by contact angle titration, was found to be 2–4 units lower than the pK_1/2 values of primary amine analogous in solution. Methyl vinyl ketone and methyl acrylate were covalently bound to the amine surfaces only under basic conditions via the Michael addition reaction. Methacrolein and acrolein were covalently bound to the amine surfaces under both acidic and basic conditions via two major reactions: the Michael addition reaction and Schiff base bond formation. The concentration of the aldehyde groups of the surfaces obtained by the reaction with methacrolein and acrolein was significantly higher than that obtained using the common, published method in which glutaraldehyde interacts with the amine surfaces.     

Quantum chemical studies of non-covalent interactions between the ethyl cation and rare gases

By taking into consideration the facts that rare gases (Ar, He, Kr, and Ne) are practically inert and that the structure of the ethyl cation (C₂H₅⁺) is stabilized through the hyperconjugation effect, a theoretical study at the B3LYP/6-311 + +G(d,p) level of calculation was carried out here in order to investigate the formation of the C₂H₅⁺···Ar, C₂H₅⁺···He, C₂H₅⁺···Kr, and C₂H₅⁺···Ne complexes. The charge transfers among H and Ar, He, Kr or Ne prove that the loss of electronic density on the rare gases are clearly noticeable. Additionally, the synergism between the structural changes and the vibration shifts have been demonstrated and justified on the basis of the Bent rule and QTAIM calculations. In complement, the interaction strength in H⁺···Ar, H⁺···He, H⁺···Kr, and H⁺···Ne was examined, although the covalent character is completely null because these contacts are very weak.     

Growth and characterization of a novel organic nonlinear optical material: l-alanine 2-furoic acid

l-alanine 2-furoic acid (LA2FA), a novel organic third order nonlinear optical material was grown by slow solvent evaporation technique at room temperature. The grown single crystals were characterized by XRD, spectral, thermal, optical, dielectric and third order nonlinear properties. LA2FA crystallizes into triclinic system with the space group P1. The cell parameters are found to be a = 3.97 Å, b = 7.09 Å, c = 10.69 Å, α = 73.61°, β = 83.57°, γ = 84.21° and V = 286 Å3. The modes of vibrations of different molecular groups present in LA2FA were identified by FTIR studies. The optical transparency of the grown crystals was investigated by UV–visible spectrum. The absorption spectrum reveals that the crystal has a high UV cut off of 245 nm and photonic band gap of 2.5 eV. The scanning electron microscope (SEM) study has been carried out to determine the surface morphology of the grown crystal. The thermal behavior of the crystal investigated using thermo gravimetric (TG) and differential thermal analysis (DTA) indicates that the material does not decompose before melting. The third order NLO property was studied in detail by z-scan technique.