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ResearchPhotonic Crystal BiosensorsA new class of optical biosensors based on the unique properties of Photonic Crystals has been recently developed by the Cunningham Group and by SRU Biosystems, a company co-founded by Prof. Cunningham in 2000. A Photonic Crystal label-free biosensor is comprised of a periodic arrangement of dielectric material in two or three dimensions. If the periodicity and symmetry of the crystal and the dielectric constants of the materials used are chosen appropriately, the Photonic Crystal will selectively couple energy at particular wavelengths, while excluding others. To create a biosensor, a Photonic Crystal may be optimized to provide an extremely narrow resonant mode whose wavelength is particularly sensitive to modulations induced by the deposition of biochemical material on its surface. A sensor structure, shown below, consists of a low refractive index plastic material with a periodic surface structure that is coated with a thin layer of high refractive dielectric material. Device structures based on linear gratings and two-dimensional gratings (i.e. arrays of holes, posts, or veins arranged in checkerboard or hexagonal close-packed grids along the sensor surface) have been demonstrated. The sensor is measured by illuminating the surface with white light, and collecting the reflected light from different locations on the sensor. The biosensor design enables a simple manufacturing process to produce sensor sheets in continuous rolls of plastic film that are hundreds of meters in length. The mass manufacturing of a biosensor structure that is measurable in a noncontact mode over large areas enables the sensor to be incorporated into single-use disposable consumable items such as 96, 384, and 1536-well standard microplates and microarray slides, thereby making the sensor compatible with standard fluid handling infrastructure employed in most laboratories. For label-free detection, the sensor operates by measuring changes in the wavelength of reflected light as biochemical binding events take place on the surface. For example, when a DNA spot is deposited on the sensor surface, an increase in the reflected wavelength occurs only on the locations on the Photonic Crystal surface where the DNA deposited mass density results in a change in surface dielectric permittivity, and the amount of wavelength shift is proportional to the deposited mass density. The readout instrument is able to detect deposited mass changes on the surface with resolution less than 1 pg/mm2, and a spatial resolution of ~4 microns per pixel.
Figure 2. Schematic of the photonic crystal biosensor readout method, in which a broadband light source (such as a light bulb or LED) illuminates the biosensor surface at normal incidence and a narrow band of wavelength are reflected. A spectrometer records changes in the reflected wavelength as biomaterial attaches to the photonic crystal surface.
Figure 3. Photo of nanoreplica molded plastic photonic crystal biosensors incorporated into standard format 96-, 384-, and 1536-well microplates. Microplates are the most commonly used liquid handling format in life science research, and integration of biosensors with microplates enables biosensor experiments to be performed at high throughput and low cost per assay. The biosensors are manufactured from long, continuous sheets of plastic film with manufacturing methods that allow the sensor surface to produced on a square yardage basis – thus allowing the sensor to be used for single-use disposable applications. References:
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