To date, surface plasmon resonance (SPR) spectroscopy identifies molecules via specific bindings with their ligands immobilized on a surface. We demonstrate here that a high-resolution multiwavelength SPR technique can measure the electronic states of the molecules and thus allow direct identification of the molecules. Using this new capability, we have studied the electronic and conformational differences between the oxidized and reduced states of cytochrome c immobilized on a modified gold electrode. When the wavelength of the incident light is far away from the optical absorption bands of the protein, a approximately 0.008 degree decrease in the resonance angle, due to a conformational change, occurs as the protein is switched from the oxidized to reduced states. When the wavelength is tuned to the absorption bands, the resonance angle oscillates at the wavelengths of the absorption peaks, which provides electronic signatures of the protein.

A method for detecting surface plasmon resonance with high resolution (ϳ10 Ϫ5 degrees or ϳ10 Ϫ8 refractive index units͒ and fast response time ͑1 s͒ 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.

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 (R ext) that is connected in series to one of the electrodes. If 1/R ext is chosen to be much smaller than the conductance quantum (G 0 ϭ2e 2 /h), a small gap with conductance determined by electron tunneling is formed. If 1/R ext is comparable or greater than G 0 , a contact with conductance near a multiple of G 0 is fabricated.

We have measured the mechanical properties of a polymer wire bridged across the two prongs of a microfabricated tuning fork. The polymer wire is formed by stretching so that it has a built-in mechanical stress. Upon exposure to organic vapors, the stress in the polymer wire diminishes, which is sensitively detected from the changes in the resonance of the tuning fork. We have also shown that by trimming the polymer wire from micrometers down to nanometers the sensitivity and response improve considerably because of an increased surface-to-volume ratio and sharper resonance of the polymer wire-tuning fork system.

No abstract