Nanotechnology promises a renaissance in sensor design
02 April 2012
Les Hunt takes a look at recent examples of research in the field of nanotechnology that offers encouraging results for the development of the next generation of advanced thermal imaging sensors and sensors that are able to detect minute defects in ferromagnetic materials.
Ever noticed how the surface of a soap bubble or the inside of a seashell changes colour depending on the angle of the light? It’s a phenomenon called iridescence. Scientists at GE Global Research have been studying the iridescence of Morpho butterfly wings, hoping to find 21st century applications inspired by this age-old natural technology.
By combining those iridescent properties of Morpho wings with more recent discoveries in nanotechnology, GE’s research team has created what could become the next generation of thermal imaging sensors, components critical to night vision goggles, advanced medical diagnostic devices and surveillance cameras, to name just a few examples. The development could mean cheaper thermal sensors with higher sensitivity and faster response times.
Back in 2010, GE’s work on the chemical sensing capabilities of tiny nanostructures on the Morpho butterfly’s wings caught the attention of DARPA, the Pentagon’s innovation lab. DARPA subsequently awarded GE’s photonics team a four-year, $6.3m grant to develop better sensors for detecting dangerous warfare agents and explosives. While that project continues, these recent discoveries could have even wider implications for thermal imaging.
“This new class of thermal imaging sensors promises significant improvements over existing detectors in their image quality, speed, sensitivity, size, power requirements and cost,” says Radislav Potvrailo, the GE Global Research principal scientist who is leading the photonics program. Potvrailo and his team found that when subjected to infrared radiation, the wing’s nanostructures experience a temperature rise and expand as a result, causing the colour change, or iridescence. Using nanotechnology to add tiny nanotubes to the wings, the team was able to increase the amount of radiation the wings were able to absorb, improving their sensitivity. A more detailed account of this work is to be found in the journal, Nature Photonics.
Meanwhile, researchers in India have hit upon a new optical sensor that can image internal defects in ferromagnetic materials. The photonic eye, as it has been dubbed, is based on a magnetically polarisable nano-emulsion that changes colour when it comes into contact with a defective region in a sample.
According to team leader John Philip of the Metallurgy and Materials Group at Indira Gandhi Centre for Atomic Research (IGCAR) in Tamilnada, the sensor will be able to locate defects and discontinuities such as fatigue, cracks, corrosion pits, metallic inclusions and abrasion in ferromagnetic components and structures.
Current techniques, such as ‘magnetic flux leakage’ (MFL), that detect defects in ferromagnetic materials work by measuring stray fields near cracks using a sensor. However, such methods require sophisticated instrumentation and complex signal processing to correlate the MFL signal with the defect signature. Moreover, sophisticated algorithms are then needed to recreate a 2D or 3D image of the defect. Alternative techniques that employ magnetic particles to visualise flux lines around an inclusion are messy because the magnetic particles must adhere to the component being tested and are difficult to remove afterwards.
“Our new sensor overcomes all of these challenges because it does not come into contact with a sample – it is placed a few millimetres above – and does not involve any complex signal collection or analysis," says Philip. "It is a simple visual test for relatively large defects (a few millimetres in size) buried inside a sample.”
Philip's team developed a special nano-emulsion to make its sensor. This consists of a colloidal suspension of single-domain superparamagnetic magnetite nanoparticles of approximately 6.5nm in size that respond to a weak magnetic field. The particles are capped with a monolayer of anionic surfactant to prevent agglomeration.
In the absence of a magnetic field, the nanodroplets are randomly oriented but when the nanofluid comes into contact with a magnetic defect, the droplets align in a chain-like fashion along the field created by the defective region. This chain of particles diffracts incident white light to produce bright colours and the colours obtained depend on the defect features. "This approach allows us directly to visualise the defect, with a violet colour being seen at the point where the magnetic flux leakage reaches its maximum – that is at the edge of the defect," says Philip.
Spurred on by these results, the team now plans to use its nanofluid to make a large-area, flexible-film-based sensor, and to develop the appropriate pattern recognition software. Such a device could be used by non-specialists to analyse defective regions rapidly in a wide variety of ferromagnetic materials. A more detailed account of IGCAR’s current work in this area is to be found in the journal, Applied Physics Letters (apl.aip.org).