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A holographic sensor consists of a smart material embedded with a hologram for detecting certain metabolites or molecules.
Molecules are detected as they interact with the material, which causes a change in the structure of the hologram and its holographic reflection properties like spacing between the holographic fringes or refractive index.
The specificity of the holographic sensor can be controlled by adding specific molecules to the polymer film that interact selectively with the analytes of interest.
The holographic sensor is integrated with a display, a transducer, and a sensor component, which measures the wavelength of the light diffracted from the holographic material and can be used to determine the molecular concentration.
While holographic sensors might not be the most widely used sensors, many groups are exploring the possibilities in sensor design for different applications. AJ Marshall et al. from the University of Cambridge, UK, developed a holographic sensor in 2003 using ionizable monomers incorporated into thin, polymeric, hydrogel films.
These films were converted into volume holograms by a diffusion method using an Nd:YAG laser (neodymium-doped yttrium aluminum laser - a crystal structure typically used as a gain medium in solid-state lasers).
The replay wavelength of this holographic sensor can be varied by controlling the exposure conditions. The research team evaluated the effects of temperature, ionic strength, hydrogel composition, and the parameters affecting the response time and reversibility.
In 2000, Owen RB et al from the Worcester Polytechnic Institute, USA proposed an in-line digital holographic sensor used for the detection and characterization of marine particulates.
The system captures each particle individually deep inside the water at a resolution of 5 µm, by transmitting a collimated beam over a lens-less charge-coupled device (CCD) array via the water column. In this device, the light which is diffracted by particulates forms an object beam, and the undeflected light acts as a reference beam.
The two beams are combined to form an in-line hologram, which is then reconstructed numerically. The CCD material eliminates vibration and is sufficiently sensitive to reduce the exposure time.
Rapid numerical reconstruction of the hologram also avoids the need for optical reconstruction and photographic processing. The test results suggested that a single particle can be traced from one hologram to the next to form a velocity vector for marine mass transport. Therefore, it was concluded that the digital holographic sensor can provide in situ ground-truth measurements for environmental remote sensing.
Holographic sensors are used for determining the following:
- The acidity of the medium
- Ionic strength
- Alcohol content
- Identification of bivalent metal salts
- Quality of water
- Blood glucose
There are several well-established methods for using biosensor-based analytical devices for characterizing the molecules and recognizing specific biological substances. However, there have been fewer commercial applications of these systems as early biosensors were expensive and not suited for large-scale manufacturing.
These problems are now resolved through the development of bio-recognition systems integrated with transducer technologies, which make biosensor analytical devices feasible for applications in printing and photography industries and microelectronics. A simple reflection holographic sensor is one such approach. Although holographic sensors have potential applications in various markets, non-invasive biosensing, pathogen detection, and drug discovery are some of the areas in which these can have more significant uses.
However, one of the major challenges involved in holographic sensor applications includes the noise artifacts resulting from temporal and spatial coherence when the sample is illuminated with lasers at oblique angles.
This coherence-induced noise forms speckling patterns that obscure sample structure images. This issue is now being addressed by replacing laser illumination with partially coherent light from a large aperture having a diameter of ~0.05-0.1mm and a bandwidth of 1 to 10 nm. The in-line holograms using partial coherence provide a gating function that filters the noise beyond a defined resolution level.
Sources and Further Reading
- Kraiskil AV, et al. Holographic sensors for diagnostics of solution components. Quantum electronics. 2010;40(2):178.
- Owen, RB and Zozulya AA. In-line digital holographic sensor for monitoring and characterizing marine particulates. Optical Engineering. 2000;39(8):2187-2197.
- Marshall AJ, et al. Holographic sensors for the determination of ionic strength. Analytica Chimica Acta. 2004;527(1):13-20.
- Holographic advances boost diagnostic sensor development - Scientist live
This article was updated on 13th February, 2020.