Recognition tunneling

Electrical detection of single pollen allergen particles using electrode-embedded microchannels

We report on the development of a microfluidic system for the electrical detection of single pollen allergen particles. Our device consists of 500 nm electrode gaps fabricated in an 800 nm wide fluidic channel. We flowed pollen allergen particles of average size 330 nm along the channel via fluid pumping and simultaneously monitored temporal change in dc current flowing through the sensing electrodes. Current spikes were detected, which can be attributed to a capacitance discharging upon trapping/detrapping of single allergens in the electrode gap. This sensing mechanism may open new avenues for a highly sensitive pollen allergen sensor.

Stability of an aqueous quadrupole micro-trap

The recently demonstrated functionality of an aqueous quadrupole micro- or nano-trap opens a new avenue for applications of Paul traps, like confinement of a charged biomolecule which requires a water environment for its chemical stability. Besides the strong viscosity forces, the motion of a charged particle in the aqueous trap is subject to dielectrophoretic and electrophoretic forces. In this study, we describe the general conditions for stability of a charged particle in an aqueous quadrupole trap. We find that for typical micro-trap parameters, the effects of both dielectrophoresis and electrophoresis significantly influence the trap stability. In particular, an aqueous quadrupole trap could play the role of a synthetic virtual nanopore for the third generation of DNA sequencing technology.

Fabrication and characterization of a solid-state nanopore with self-aligned carbon nanoelectrodes for molecular detection

Stochastic molecular sensors based on resistive pulse nanopore modalities are envisioned as facile DNA sequencers. However, recent advances in nanotechnology fabrication have highlighted promising alternative detection mechanisms with higher sensitivity and potential single-base resolution. In this paper we present the novel self-aligned fabrication of a solid-state nanopore device with integrated transverse graphene-like carbon nanoelectrodes for polyelectrolyte molecular detection. The electrochemical transduction mechanism is characterized and found to result primarily from thermionic emission between the two transverse electrodes. Response of the nanopore to Lambda dsDNA and short (16-mer) ssDNA is demonstrated and distinguished.

Insulated gold scanning tunneling microscopy probes for recognition tunneling in an aqueous environment

Chemically functionalized probes are required for tunneling measurements made via chemical contacts ("Recognition Tunneling"). Here, we describe the etching of gold STM probes suitable for chemical functionalization with moieties bearing thiol groups. Insulated with high density polyethylene, these probes may be used in aqueous electrolytes with sub pA leakage currents. The area of the exposed probe surface was characterized via the saturation current in an electroactive solution (0.1 M K(3)Fe(CN)(6)). Twenty five percent of the probes had an exposed region of 10 nm radius or less.

Gap distance and interactions in a molecular tunnel junction

The distance between electrodes in a tunnel junction cannot be determined from the external movement applied to the electrodes because of interfacial forces that distort the electrode geometry at the nanoscale. These distortions become particularly complex when molecules are present in the junction, as demonstrated here by measurements of the AC response of a molecular junction over a range of conductivities from microsiemens to picosiemens. Specific chemical interactions within the junction lead to distinct features in break-junction data, and these have been used to determine the electrode separation in a junction functionalized with 4(5)-(2-mercaptoethyl)-1H-imidazole-2-carboxamide, a reagent developed for reading DNA sequences.

Electrical detection of single methylcytosines in a DNA oligomer

We report label-free electrical detections of chemically modified nucleobases in a DNA using a nucleotide-sized electrode gap. We found that methyl substitution contributes to increase the tunneling conductance of deoxycytidines, which was attributed to a shift of the highest occupied molecular orbital level closer to the electrode Fermi level by methylation. We also demonstrate statistical identifications of methylcytosines in an oligonucleotide by tunneling current. This result suggests a possible use of the transverse electron-transport method for a methylation level analysis.

Recognition tunneling measurement of the conductance of DNA bases embedded in self-assembled monolayers

The DNA bases interact strongly with gold electrodes, complicating efforts to measure the tunneling conductance through hydrogen-bonded Watson Crick base pairs. When bases are embedded in a self-assembled alkane-thiol monolayer to minimize these interactions, new features appear in the tunneling data. These new features track the predictions of density-functional calculations quite well, suggesting that they reflect tunnel conductance through hydrogen-bonded base pairs.

Identifying single bases in a DNA oligomer with electron tunnelling

It has been proposed that single molecules of DNA could be sequenced by measuring the physical properties of the bases as they pass through a nanopore. Theoretical calculations suggest that electron tunnelling can identify bases in single-stranded DNA without enzymatic processing, and it was recently experimentally shown that tunnelling can sense individual nucleotides and nucleosides. Here, we report that tunnelling electrodes functionalized with recognition reagents can identify a single base flanked by other bases in short DNA oligomers. The residence time of a single base in a recognition junction is on the order of a second, but pulling the DNA through the junction with a force of tens of piconewtons would yield reading speeds of tens of bases per second.