Tuesday, April 27, 2010

French Atomic Agency Plasmon Nanosensors Exhibit Greater Sensitivity for Detecting Chemical and Biological Molecules

Commissariat A l'Energie Atomique (CEA) (Paris, FR) was awarded U.S. Patent 7,705,989 for microsensors and nanosensors with greater sensitivity which are used to detect chemical and biological species with surface plasmons. 

According to CEA inventors Patrick Chaton, Jean-Louis Bijeon, Pascal Royer and Pierre Michel Adam, a surface plasmon microsensor or a nanosensor for chemical or biological species detection includes pads distributed on the surface of a support with an electrically conductive material which are capable of immobilizing the chemical or biological species. The pads have a dimension less than 1 micron.   

Surface plasmons are electromagnetic modes that propagate on a metallic conductive surface and which are the consequence of a longitudinal oscillation of the charge density due to the collective movement of electrons located on the surface of the metal. Surface plasmons may be optically excited. One thus highlights resonances that are directly linked to the various propagation modes of the surface plasmons.

The CEA detector has been made to improve the resolution of existing sensors for chemical or biological species. It makes use of pads distributed on the surface of a support and capable of immobilising chemical or biological species. The size and the shape of the pads, as well as their distribution, may be provided so as to allow a resolution on the nanometric scale. Within the scope of the invention, localised surface plasmons are particularly used.

From a general point of view, the sensor detects modifications of the characteristics of the surface plasmons due to a change in the optical properties of the surrounding medium following the adsorption of chemical or biological species on the metallic substrates. Also according to the invention, the biochemical species adsorbed on the pads are identified by surface enhanced Raman spectroscopy, this enhancement and consequently this type of spectroscopy being possible thanks to the plasmon resonances of the metallic pads.

Advantageously, the microsensor or the nanosensor further comprises a means making it possible to increase the sensitivity of the sensor. These means may comprise a thin metallic film directly deposited on the surface of the support. A thin dielectric film may be intercalated between the thin metallic film and the pads in order to adjust the plasmon resonance as a function of the thickness of the dielectric layer.

These means may comprise a planar wave guide intended to convey a guided electromagnetic mode, said planar wave guide being formed on the surface or under the surface of the support and under the pads. They may be constituted by the grouping together of pads, the distance separating these grouped together pads being sufficiently small to allow an electromagnetic coupling between said grouped together pads. If the pads have a section in the shape of an ellipse, these means may be constituted by the small distance separating an end of a pad along the major axis of the ellipse from the end of the adjacent pad along the major axis of the ellipse, this small distance enabling an electromagnetic coupling between the pads.

The means make it possible to increase the sensitivity of the sensor may comprise at least one particle associated with a pad. This particle may be a particle fixed to said chemical or biological species. It may be fixed to an object intended to be placed near to a pad. This object may be the tip of a near field optical microscope. This particle may be metallic, the sensitivity is then reinforced by the coupling between the plasmon resonances of the pad and the particle. It may be constituted of a fluorescent material, the emission of fluorescence then being exacerbated by the plasmon resonance of the corresponding pad.

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