Salah Boussaad

High resolution surface plasmon resonance spectroscopy

A method for detecting surface plasmon resonance with high resolution (∼10−5 degrees or ∼10−8 refractive index units) and fast response time (1 μs) is described. In the method, light is focused through a prism onto a metal film on which molecules to be detected are adsorbed. The total internal reflection of the incident light is collected with a bicell photodetector instead of a single cell or an array of photodetectors that are widely used in previous works. The ratio of the differential signal to the sum signal of the bicell photodetector provides an accurate measurement of shift in surface plasmon resonance angle caused by the adsorption of molecules onto the metal films or by conformational changes in the adsorbed molecules. Using the method, we have studied subtle conformational changes in redox protein, cytochrome c, due to an electron transfer reaction.

In situ detection of cytochrome c adsorption with single walled carbon nanotube device

We report on the in situ detection of cytochrome c adsorption onto individual SWNT transistors via the changes in the electron transport properties of the transistors.

Atom-size gaps and contacts between electrodes fabricated with a self-terminated electrochemical method

We describe a method to fabricate atomic-scale gaps and contacts between two metal electrodes. The method uses a directional electrodeposition process and has a built-in self-termination mechanism. The final gap width and contact size are preset by an external resistor (Rext) that is connected in series to one of the electrodes. If 1/Rext is chosen to be much smaller than the conductance quantum (G0=2e2/h), a small gap with conductance determined by electron tunneling is formed. If 1/Rext is comparable or greater than G0, a contact with conductance near a multiple of G0 is fabricated.

Surface plasmon resonance enhanced optical absorption spectroscopy for studying molecular adsorbates

We present an automated setup to measure the surface plasmon resonance (SPR)-enhanced optical absorption spectra of molecular adsorbates. The setup detects the reflectivity at the SPR resonance angle as a function of the incident light wavelength. Because the resonance angle is wavelength dependent, a feedback mechanism adjusts the photodetector position to follow the resonance angle when the wavelength varies. Both theoretical calculations and experimental measurements show a signal enhancement of up to ∼40 times over the conventional absorption spectroscopy. The SPR-based absorption spectroscopy is surface specific because the optical field is localized near the surface at resonance. In addition, the SPR angular shift is simultaneously measured, which provides adsorbate coverage and adsorption kinetic information. We anticipate that with our automated system, the method could be used in the study of adsorbed molecules and in chemical and biosensor applications.

High-performance differential surface plasmon resonance sensor using quadrant cell photodetector

We describe a simple, stable, and high-resolution surface plasmon resonance (SPR) sensor using a quadrant cell photodetector. The sensor focuses a diode laser through a prism onto a gold film that is divided into two areas, one for reference and the other for sensing analyte. The angular shifts of the SPR generated in the two areas are detected with a quadrant cell photodetector. Because signals from the two areas are produced and detected with the same laser, optics, and photodetector, the difference in the SPR angular shifts eliminates errors due to thermal drift, mechanical noise, and laser fluctuations. It also removes the SPR angular shift due to the change in the refractive index of the bulk solution as an analyte solution is introduced into the sample cell each time, and thus gives an accurate detection of the specific adsorption of analyte.

Electrochemical fabrication of atomically thin metallic wires and electrodes separated with molecular-scale gaps

This article summarizes our recent effort to fabricate electrochemically metallic nanowires and electrodes separated with molecular scale nanogaps. The nanowires were fabricated by etching a small portion of a micron-scale metallic wire supported on a solid substrate. The etching was controlled by continuously monitoring the conductance of the wire. When the thinnest portion of the wire reached the atomic scale, the conductance decreased in a stepwise fashion. By further etching away the last few atoms, a molecular-scale gap between two electrodes was created and the ballistic electron transport through the nanowire was replaced with quantum tunneling. By depositing atoms back, the above processes could be reversed, allowing us to achieve a desired nanowire or gap. The nanowires may be used for chemical sensor applications and the nanogaps may be used to wire small molecules to the outside world for molecular electronics applications.

Influence of Redox Molecules on the Electronic Conductance of Single-Walled Carbon Nanotube Field-Effect Transistors : Application to Chemical and Biological Sensing

In an effort to develop sensitive nanoscale devices for chemical and biological sensing, we have examined, using liquid gating, the conductance of semiconducting single-walled carbon nanotube-based field-effect transistors (SWCNT-FETs) in the presence of redox mediators. As examples, redox couples K3Fe(CN)6/K4Fe(CN)6 and K2IrCl6/K3IrCl6 are shown to modulate the SWCNT-FET conductance in part through their influence via the electrolyte gate on the electrostatic potential of the solution, as described by Larrimore et al. (Nano Lett. 2006, 6, 3129-1333) and in part through electron transfer between the redox mediators and the nanotubes. In the latter case, the rate of electron transfer is determined by the difference in chemical potential between the redox mediator and the SWCNTs and by the concentrations of the oxidized and reduced forms of the redox couple. Furthermore, these devices can detect the activity of redox enzymes through their sensitivity to the change in oxidation state of the enzyme substrate. An example is given for the blue copper oxidase, Trametes versicolor laccase, in which the rate of change of the SWCNT device conductance is linearly proportional to the rate of oxidation of the substrate 10-(2-hydroxyethyl)phenoxazine, varied over 2 orders of magnitude by the laccase concentration in the picomolar range. The behavior described in this work provides a highly sensitive means with which to do chemical and biological sensing using SWCNTs that is different from the amperometric, capacitive, and field-effect type sensing methods previously described in the literature for this material.

Magnetic-field-assisted assembly of metal/polymer/metal junction sensors

We present a method to assemble Au/polyaniline/Au junctions and demonstrate a chemical sensor application. The building blocks consist of an array of microelectrodes on a silicon chip, microfabricated metallic bars, and a thin polyaniline layer deposited on the microelectrodes or on the bars. The individual bars suspended in solution are placed, with the help of a magnetic field, across the microelectrodes to form the junctions. The polyaniline layer is ∼30 nm thick and modified with glycine-glycine-histidine oligopeptides. Strong binding of Cu2+ to the oligopeptide is converted into a conductance change of the junctions, allowing selective detection of trace amounts of Cu+2 ions.

Electrochemical properties of atomic-scale metal wires

We have fabricated atomic-scale metal wires with quantized conductance in electrolytes and studied double layer charging, anionic adsorption and surface stress effects on the quantum transport in the wires. In the double layer charging regime, the conductance changes linearly with the potential, which can be attributed to a change in the amount of electron spill-out of the wire. A large change in the potential results in a substantial stress in the wires, causing atoms in the wires to rearrange. Upon anionic adsorption, the conductance decreases due to the scattering of conduction electrons by the adsorbed anions. # 2003 Elsevier Ltd. All rights reserved.

Discrete tunneling current fluctuations in metal–water–metal tunnel junctions

We have studied electron tunneling through water between two metal electrodes supported on a solid substrate and observed random fluctuations in the tunneling current between two discrete levels. The two-level fluctuations persist when changing the concentration and the valency of the ions, and pH of the water solutions. A given two-level fluctuation is, in general, not affected by the applied bias voltage, but it is usually disrupted by changing the width of the tunnel gap. We attribute the discrete conductance fluctuations to random trapping or escaping of a single electron in or from a localized state in the tunnel gap.