Strong overtones and combination bands in ultraviolet resonance Raman spectroscopy

Ultraviolet resonance Raman spectroscopy is carried out using a continuous wave frequency-doubled argon ion laser operated at 229, 244, and 257 nm in order to characterize the overtones and combination bands for several classes of organic compounds in liquid solutions. Contrary to what is generally anticipated, for molecules such as pyrene and anthracene, strong overtones and combination bands can show up; it is demonstrated that their intensity depends critically on the applied laser wavelength. If the excitation wavelength corresponds with a purely electronic transition--this applies to a good approximation for 244-nm excitation in the case of pyrene and for 257-nm excitation in the case of anthracene--mostly fundamental vibrations (up to 1700 cm(-1)) are observed. Overtones and combination bands are detected but are rather weak. However, if the laser overlaps with the vibronic region--as holds for 229- and 257-nm excitation for pyrene and 244-nm excitation for anthracene--very strong bands are found in the region 1700-3400 cm(-1). As illustrated for pyrene at 257 nm, all these bands can be assigned to first overtones or binary combinations of fundamental vibrations. Their intensity distribution can roughly be simulated by multiplying the relative intensities of the fundamental bands. Significant bands can also be found in the region 3400-5000 cm(-1), corresponding with second overtones and ternary combinations. It is shown that these findings are not restricted to planar and rigid molecules with high symmetry. Substituted pyrenes exhibit similar effects, and relatively strong overtones are also observed for adenosine monophosphate and for abietic acid. The reasons for these observations are discussed, as well as the potential applicability for analytical purposes.     

Measurement of the Docking Time of a DNA Molecule onto a Solid-State Nanopore

We present measurements of the change in ionic conductance due to double-stranded (ds) DNA translocation through small (6 nm diameter) nanopores at low salt (100 mM KCl). At both low (<200 mV) and high (>600 mV) voltages we observe a current enhancement during DNA translocation, similar to earlier reports. Intriguingly, however, in the intermediate voltage range, we observe a new type of composite events, where within each single event the current first decreases and then increases. From the voltage dependence of the magnitude and timing of these current changes, we conclude that the current decrease is caused by the docking of the DNA random coil onto the nanopore. Unexpectedly, we find that the docking time is exponentially dependent on voltage (t ∝ e(-V/V(0))). We discuss a physical picture where the docking time is set by the time that a DNA end needs to move from a random location within the DNA coil to the nanopore. Upon entrance of the pore, the current subsequently increases due to enhanced flow of counterions along the DNA. Interestingly, these composite events thus allow to independently measure the actual translocation time as well as the docking time before translocation.     

Application of bacteria involved in the biological sulfur cycle for paper mill effluent purification

In anaerobic wastewater treatment, the occurrence of biological sulfate reduction results in the formation of unwanted hydrogen sulfide, which is odorous, corrosive and toxic. In this paper, the role and application of bacteria in anaerobic and aerobic sulfur transformations are described and exemplified for the treatment of a paper mill wastewater. The sulfate containing wastewater first passes an anaerobic UASB reactor for bulk COD removal which is accompanied by the formation of biogas and hydrogen sulfide. In an aeration pond, the residual COD_organic and the formed dissolved hydrogen sulfide are removed. The biogas, consisting of CH₄ (80-90 vol.%), CO₂ (10-20 vol.%) and H₂S (0.8-1.2 vol.%), is desulfurised prior to its combustion in a power generator thereby using a new biological process for H₂S removal. This process will be described in more detail in this paper. Biomass from the anaerobic bioreactor has a compact granular structure and contains a diverse microbial community. Therefore, other anaerobic bioreactors throughout the world are inoculated with biomass from this UASB reactor. The sludge was also successfully used in investigation on sulfate reduction with carbon monoxide as the electron donor and the conversion of methanethiol. This shows the biotechnological potential of this complex reactor biomass.     

Fabrication and characterization of nanopore-based electrodes with radii down to 2 nm

We report on the fabrication and characterization of gold nanoelectrodes with carefully controlled nanometer dimensions in a matrix of insulating silicon nitride. A focused electron beam was employed to drill nanopores in a thin silicon nitride membrane. The size and shape of the nanopores were studied with high-resolution transmission electron microscopy and electron-energy-loss two-dimensional maps. The pores were subsequently filled with gold, yielding conically shaped nanoelectrodes. The nanoelectrodes were examined by atomic and electrostatic force microscopy. Their applicability in electrochemistry was demonstrated by steady-state cyclic voltammetry. Pores with a radii down to 0.4 nm and electrodes with radii down to 2 nm are demonstrated.     

Hybrid pore formation by directed insertion of α-haemolysin into solid-state nanopores

Most experiments on nanopores have concentrated on the pore-forming protein α-haemolysin (αHL) and on artificial pores in solid-state membranes. While biological pores offer an atomically precise structure and the potential for genetic engineering, solid-state nanopores offer durability, size and shape control, and are also better suited for integration into wafer-scale devices. However, each system has significant limitations: αHL is difficult to integrate because it relies on delicate lipid bilayers for mechanical support, and the fabrication of solid-state nanopores with precise dimensions remains challenging. Here we show that these limitations may be overcome by inserting a single αHL pore into a solid-state nanopore. A double-stranded DNA attached to the protein pore is threaded into a solid-state nanopore by electrophoretic translocation. Protein insertion is observed in 30-40% of our attempts, and translocation of single-stranded DNA demonstrates that the hybrid nanopore remains functional. The hybrid structure offers a platform to create wafer-scale device arrays for genomic analysis, including sequencing.     

Patterning: Strategies for Patterning Biomolecules with Dip‐Pen Nanolithography (Small 8/2011)

Dip‐pen nanolithography (DPN) is an atomic force microscopy (AFM)‐based lithography technique, which has the ability to fabricate patterns with a feature size down to approximately 15 nm using both top‐down and bottom‐up approaches. DPN utilizes the water meniscus formed between an AFM tip and a substrate to transfer ink molecules onto surfaces. A major application of this technique is the fabrication of micro‐ and nano‐arrays of patterned biomolecules. To achieve this goal, a variety of chemical approaches has been used. This review concisely describes the development of DPN in the past decade and presents the related chemical strategies that have been reported to fabricate biomolecular paterns with DPN at micrometer and nanometer scale, classified into direct‐ and indirect DPN methodologies, discussing tip‐functionalization strategies as well.     

Atomic-scale electron-beam sculpting of near-defect-free graphene nanostructures

In order to harvest the many promising properties of graphene in (electronic) applications, a technique is required to cut, shape, or sculpt the material on the nanoscale without inducing damage to its atomic structure, as this drastically influences the electronic properties of the nanostructure. Here, we reveal a temperature-dependent self-repair mechanism that allows near-damage-free atomic-scale sculpting of graphene using a focused electron beam. We demonstrate that by sculpting at temperatures above 600 °C, an intrinsic self-repair mechanism keeps the graphene in a single-crystalline state during cutting, even though the electron beam induces considerable damage. Self-repair is mediated by mobile carbon ad-atoms that constantly repair the defects caused by the electron beam. Our technique allows reproducible fabrication and simultaneous imaging of single-crystalline free-standing nanoribbons, nanotubes, nanopores, and single carbon chains.     

Modeling the conductance and DNA blockade of solid-state nanopores

We present measurements and theoretical modeling of the ionic conductance G of solid-state nanopores with 5-100 nm diameters, with and without DNA inserted into the pore. First, we show that it is essential to include access resistance to describe the conductance, in particular for larger pore diameters. We then present an exact solution for G of an hourglass-shaped pore, which agrees very well with our measurements without any adjustable parameters, and which is an improvement over the cylindrical approximation. Subsequently we discuss the conductance blockade ΔG due to the insertion of a DNA molecule into the pore, which we study experimentally as a function of pore diameter. We find that ΔG decreases with pore diameter, contrary to the predictions of earlier models that forecasted a constant ΔG. We compare three models for ΔG, all of which provide good agreement with our experimental data.     

Lithographically fabricated nanopore-based electrodes for electrochemistry

We report a new technique for fabricating electrodes for electrochemical applications with lateral dimensions in the range 15-200 nm and a reproducible, well-defined geometry. This technique allows determining the electrode size by electron microscopy prior to electrochemical measurements and without contamination of the metal electrode. We measured the diffusion-limited current with stepped-current voltammetry and showed that its dependence on electrode size can be quantitatively understood if the known geometry of the electrodes is explicitly taken into account.