Biosensors of a proposed type would exploit scattering of light by surface plasmon resonance (SPR). Related prior biosensors exploit absorption of light by SPR. Relative to the prior SPR biosensors, the proposed SPR biosensors would offer greater sensitivity — in some cases, enough sensitivity to detect bioparticles having dimensions as small as nanometers.

A surface plasmon wave can be described as a light-induced collective oscillation in electron density at the interface between a metal and a dielectric. At SPR, most incident photons are either absorbed or scattered at the metal/dielectric interface and, consequently, reflected light is greatly attenuated. The resonance wavelength and angle of incidence depend upon the permittivities of the metal and dielectric.

Light Scattered by SPR from a bioparticle/ligand binding site would be focused to a bright spot on an image detector.

An SPR sensor of the type most widely used heretofore includes a gold film coated with a ligand — a substance that binds analyte molecules. The gold film is thin enough to support evanescent-wave coupling through its thickness. The change in the effective index of refraction at the surface, and thus the change in the SPR response, increases with the number of bound analyte molecules. The device is illuminated at a fixed wavelength, and the intensity of light reflected from the gold surface opposite the ligand-coated surface is measured as a function of the angle of incidence. From these measurements, the angle of minimum reflection intensity is determined.

These measurements and the determination of the angle of minimum reflection intensity are performed before and after (and can be performed during) exposure of the sensor to a sample containing the analyte molecules. Any shift in the angle between such successive determinations is indicative of a change in the concentration of analyte molecules in the sample. This type of sensor is characterized by low sensitivity for the following reasons:

  • A small number of analyte molecules gives rise to a small shift in the angle of minimum reflection intensity.
  • Because one is measuring a reflection dip rather than a reflection peak, the measurement can be strongly affected by noise. The difficulty of determining the small angular shift is analogous to the difficulty of measuring the shift of a dark spot on a bright background.

A biosensor according to the proposal would afford a much greater signal-to-noise ratio by exploiting SPR in a different way that would involve, literally, a bright spot on a dark background. A proposed sensor (see figure) would include a coupling prism, an index-of-refraction matching liquid, a glass slide, and a metal film thin enough to support evanescent wave coupling. The metal surface to be exposed to the specimen would be coated with ligand in a regular array of patches. The array of patches would be observed by a miniature microscope that would include a lens and a complementary oxide/semiconductor (CMOS) image detector. The microscope would be designed so that each ligand patch would occupy many CMOS pixels and the resolution of the microscope would be close to the optical limit (about one wavelength of the incident light).

The sensor would be illuminated with collimated light at a wavelength and angle of incidence chosen so that SPR would occur whenever and wherever analyte molecules became bound to the ligand. In the absence of such binding, there would be little scattered light. In the presence of such binding at any spot on the ligand, the strong SPR scattering from that spot would cause the spot to be imaged brightly in the microscope. Even a bioparticle smaller than a wavelength of light could induce sufficient SPR scattering to be detectable.

This work was done by Yu Wang, Bedabrata Pain, Thomas Cunningham, and Suresh Seshadri of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-40683, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Coatings, colorants and finishes Coatings, colorants, and finishes Glass Imaging and visualization Metals Noise Optics Physical Sciences Semiconductors Sensors and actuators
This Brief includes a Technical Support Package (TSP).
Document cover
Scattering-Type Surface-Plasmon-Resonance Biosensors

(reference NPO-40683-) is currently available for download from the TSP library.

Don't have an account?

More From SAE Media Group

    Magazine cover
    NASA Tech Briefs Magazine

    This article first appeared in the January, 2005 issue of NASA Tech Briefs Magazine (Vol. 29 No. 1).

    Read more articles from the archives here.

    Overview

    The document is a technical support package from NASA's Jet Propulsion Laboratory, focusing on Scattering-Type Surface-Plasmon-Resonance (SPR) Biosensors. It outlines advancements in SPR technology, which has been utilized for biosensing applications for over a decade. SPR technology is notable for its non-invasive detection capabilities, allowing real-time monitoring of molecular interactions without direct contact with the sample.

    The document highlights the limitations of conventional SPR biosensors, such as the BIAcore, which primarily measure the mass of material binding to the sensor surface. This method can yield low signal-to-noise ratios, particularly for small analytes, as the detection relies on shifts in reflection dips, akin to observing a dark spot against a bright background. To address these challenges, the document proposes enhancements to SPR biosensors that utilize scattering techniques, which can significantly improve sensitivity and detection capabilities.

    One key advancement discussed is the use of evanescent wave illumination combined with scattering to capture and manipulate bio-particles as small as a few nanometers in diameter. This approach allows for the detection of nano-sized particles through strong SPR scattering, even when the particles are smaller than the wavelength of light used in the detection process. The intensity of the scattered light is proportional to the size of the bio-particle, enabling precise measurements.

    The document also describes an experimental setup that incorporates a CMOS microscope to enhance the detection of small particles. This setup includes a glass slide with metal films and pixelized ligands, which, when bound to analytes, generate detectable SPR scattering. The CMOS imager captures microscopic images of the ligand pixels, allowing for high-resolution detection.

    Additionally, the document references a phenomenon reported by Yu Wang regarding voltage-induced color-selective absorption with surface plasmons, which can tune the absorption spectrum by changing the coupling distance of two SP waves. This tunability broadens the potential applications of SPR technology.

    Overall, the document emphasizes the potential of scattering-type SPR biosensors to revolutionize biosensing by improving sensitivity and enabling the detection of smaller biological particles, thus expanding the scope of applications in various scientific and commercial fields.