L. J. Whitman

The BARC biosensor applied to the detection of biological warfare agents.

The Bead ARray Counter (BARC) is a multi-analyte biosensor that uses DNA hybridization, magnetic microbeads, and giant magnetoresistive (GMR) sensors to detect and identify biological warfare agents. The current prototype is a table-top instrument consisting of a microfabricated chip (solid substrate) with an array of GMR sensors, a chip carrier board with electronics for lock-in detection, a fluidics cell and cartridge, and an electromagnet. DNA probes are patterned onto the solid substrate chip directly above the GMR sensors, and sample analyte containing complementary DNA hybridizes with the probes on the surface. Labeled, micron-sized magnetic beads are then injected that specifically bind to the sample DNA. A magnetic field is applied, removing any beads that are not specifically bound to the surface. The beads remaining on the surface are detected by the GMR sensors, and the intensity and location of the signal indicate the concentration and identity of pathogens present in the sample. The current BARC chip contains a 64-element sensor array, however, with recent advances in magnetoresistive technology, chips with millions of these GMR sensors will soon be commercially available, allowing simultaneous detection of thousands of analytes. Because each GMR sensor is capable of detecting a single magnetic bead, in theory, the BARC biosensor should be able to detect the presence of a single analyte molecule.

Direct deposition of continuous metal nanostructures by thermal dip-pen nanolithography

We describe the deposition of continuous metal nanostructures onto glass and silicon using a heated atomic force microscope cantilever. Like a miniature soldering iron, the cantilever tip is coated with indium metal, which can be deposited onto a surface forming lines of a width less than 80 nm. Deposition is controlled using a heater integrated into the cantilever. When the cantilever is unheated, no metal is deposited from the tip, allowing the writing to be registered to existing features on the surface. We demonstrate direct-write circuit repair by writing an electrical connection between two metal electrodes separated by a submicron gap.

A DNA array sensor utilizing magnetic microbeads and magnetoelectronic detection

We describe a multi-analyte biosensor that uses magnetic microbeads as labels to detect DNA hybridization on a micro-fabricated chip. The beads are detected by giant magnetoresistance (GMR) magnetoelectronic sensors embedded in the chip. The prototype device is a tabletop unit containing electronics, a chip carrier with a microfluidic flow cell, and a compact electromagnet and is capable of simultaneous detection of eight different analytes.

Manipulation of adsorbed atoms and creation of new structures on room-temperature surfaces with a scanning tunneling microscope

A general method of manipulating adsorbed atoms and molecules on room-temperature surfaces with the use of a scanning tunneling microscope is described. By applying an appropriate voltage pulse between the sample and probe tip, adsorbed atoms can be induced to diffuse into the region beneath the tip. The field-induced diffusion occurs preferentially toward the tip during the voltage pulse because of the local potential energy gradient arising from the interaction of the adsorbate dipole moment with the electric field gradient at the surface. Depending upon the surface and pulse parameters, cesium (Cs) structures from one nanometer to a few tens of nanometers across have been created in this way on the (110) surfaces of gallium arsenide (GaAs) and indium antimonide (InSb), including structures that do not naturally occur.

Magnetic labeling, detection, and system integration

Among the plethora of affinity biosensor systems based on biomolecular recognition and labeling assays, magnetic labeling and detection is emerging as a promising new approach. Magnetic labels can be non-invasively detected by a wide range of methods, are physically and chemically stable, relatively inexpensive to produce, and can be easily made biocompatible. Here we provide an overview of the various approaches developed for magnetic labeling and detection as applied to biosensing. We illustrate the challenges to integrating one such approach into a complete sensing system with a more detailed discussion of the compact Bead Array Sensor System developed at the U.S. Naval Research Laboratory, the first system to use magnetic labels and microchip-based detection.

The structure of silicon surfaces from (001) to (111)

Abstract We describe the structure of silicon surfaces oriented between (001) and (111) as determined by scanning tunneling microscopy (STM) and first-principles, total-energy calculations. In addition to reviewing and reproducing the structures reported for the few surfaces previously studied, we describe a number of additional surfaces in order to provide a complete overview of the (001)-to-(111) surface morphology. As the sample orientation is titled from (001) to (111) (ϑ=0 to 54.7°), the surface morphology varies as follows: (1) Si(001) to Si(114) = (001)-like surfaces composed of dimers separated by steps (both rebonded and nonrebonded); (2) Si(114) to Si(113) =mesoscale sawtooth facets composed of the stable (114)−2 × 1 and (113)−3 × 2 planes; (3) Si(113) to Si(5 5 12) =mesofacets composed of (113)−3 × 2 and (5 5 12)-like planes; (4) Si(5 5 12) to ∼Si(223) =nanoscale sawtooth facets composed of (5 5 12)-like and unit-cell-wide (111)−7 × 7 planes; and (5) ∼Si(223) to Si(111)=(111)−7 × 7 terraces separated primarily by single- and triple-layer steps. The change in the surface morphology is accompanied by a change in the composition of surface structural units, progressing from (001)-like structures (e.g. dimers, rebonded steps, and tetramers) to (111)-like structures (π-bonded chains, adatoms and dimer-chain walls). The resultant morphology is a delicate balance between the reduction of dangling bond density achieved by the formation of these structural units, and the resulting surface stress associated with their unusual bond angles and bond lengths.

Nanoscale deposition of solid inks via thermal dip pen nanolithography

We demonstrate that nanolithography can be performed using a heated atomic force microscope (AFM) cantilever tip to control the deposition of a solid organic “ink.” The ink, octadecylphosphonic acid (OPA), has a melting temperature near 100°C and can self-assemble on mica. Postdeposition analysis shows that deposition occurs only when the cantilever tip is heated above OPA’s melting temperature, that the deposited structure does not spread significantly while cooling, and that a cool tip coated with OPA does not contaminate the substrate during subsequent imaging. Single lines were written with a width of 100nm. This approach greatly expands the potential of dip pen nanolithography, allowing local control of deposition and deposition of materials typically immobile at room temperature, while avoiding potential problems arising from inadvertent deposition and postdeposition diffusion.

The chemisorption of chlorosilanes and chlorine on Si(111)7 × 7

Abstract The chemisorption of SiCl4, Si2Cl6, and chlorine on Si(111)7 × 7 has been characterized using soft X-ray photoemission with synchrotron radiation, thermal desorption spectroscopy, and Auger electron spectroscopy. SiCl4 dissociatively chemisorbs on room temperature Si(111)7 × 7 with an extremely low sticking coefficient, with only SiCl remaining on the surface. In contrast, Si2Cl6 chemisorbs with ∼ 500 times greater probability and then partly dissociates into SiClx (x = 1, 2, 3) fragments. A monolayer of Cl deposited directly also contains SiCl, SiCl2, and SiCl3 surface species, but they are created via reaction with substrate Si atoms and have lower Si2p core level binding energies. Upon heating the surface all the adsorbed Cl is removed via desorption of silicon chlorides, primarily SiCl2, indicating that SiCl4, Si2Cl6, and chlorine will etch Si(111)7 × 7 if an additional reactant is not avail to remove the surface Cl. Interestingly, the different reactivities of SiCl4 and Si2Cl6 upon adsorption can be explained by the dynamics of different adsorption mechanisms.

Independent control of grafting density and conformation of single-stranded DNA brushes

We describe self-assembly of ssDNA brushes that exploits the intrinsic affinity of adenine nucleotides (dA) for gold surfaces. The grafting density and conformation of these brushes is deterministically controlled by the length of the anchoring dA sequences, even in the presence of thymine nucleotides (dT). We produce and characterize brushes of model block-oligonucleotides, d(Tm-An), with systematically varied lengths m and n of the thymine and adenine blocks [denoted d(Tm) and d(An), respectively]. The hairpin conformation, dominant for self-complementary d(Tm-An) oligos in solution, is disrupted by the high preferential affinity of dA for gold surfaces. As a result, the d(Tm-An) oligos adsorb as a brush of d(T) strands immobilized via the d(A) blocks. Quantitative analysis by FTIR spectroscopy and x-ray photoelectron spectroscopy (XPS) reveals a unique feature of DNA immobilization via d(A) blocks: The surface density of dA nucleotides is close to saturation and is nearly independent of d(A) block length. Accordingly, the lateral spacing (grafting density) of the d(T) blocks is determined by the length of the d(A) blocks. The d(T) blocks extend away from the surface in a brush-like conformation at a lateral spacing 2–3 times larger (a grafting density 5–10 times lower) than in analogous films immobilized via standard thiol linkers. This combination of brush-like conformation and low saturation grafting density is expected to increase the efficiency of DNA hybridization at surfaces. Therefore, immobilization via d(A) blocks offers a method of producing DNA brushes with controlled properties for applications in biotechnology and nanotechnology.

W-structured type-II superlattice long-wave infrared photodiodes with high quantum efficiency

Results are presented for an enhanced type-II W-structured superlattice (WSL) photodiode with an 11.3μm cutoff and 34% external quantum efficiency (at 8.6μm) operating at 80K. The new WSL design employs quaternary Al0.4Ga0.49In0.11Sb barrier layers to improve collection efficiency by increasing minority-carrier mobility. By fitting the quantum efficiencies of a series of p-i-n WSL photodiodes with background-doped i-region thicknesses varying from 1to4μm, the authors determine that the minority-carrier electron diffusion length is 3.5μm. The structures were grown on semitransparent n-GaSb substrates that contributed a 35%–55% gain in quantum efficiency from multiple internal reflections.