Metamaterials gained renown as a way of creating invisibility cloaks — devices that could make an object 'disappear' before one's eyes. Less well known is that they can also act as detectors for biological compounds. Writing in Optics Express, Bingham et al. describe two-dimensional metamaterials designed so that, when exposed to electromagnetic radiation, their resonant frequencies coincide with those of vitamin H (C. M. Bingham et al. Optics Express 16, 18565–18575; 2008). The resonant frequencies of vitamin H occur in the terahertz range, and these results thus provide an example of biodetection in that frequency regime.
The properties of metamaterials lie in their structure rather than their chemical composition. One asset of these man-made materials is that they can be engineered to possess a precise response to electromagnetic radiation. Bingham and colleagues created metamaterials with designs that mimic several types of symmetry observed in nature, using both square and hexagonal tiles. Their tiles, the unit cells of metamaterial structures (shown on left of picture), consist of up to three different subunits. The overall structures (shown on right) look rather like a Persian carpet.
To maximize the electromagnetic response of a metamaterial, the unit cells must be tightly tessellated — that is, the gaps between tiles must be minimized. But why incorporate more than one subunit into a tile? The advantage is that the metamaterial preserves the different electromagnetic properties of each subunit: a material formed with three distinct subunits is resonant at three different frequencies. A triple-resonator metamaterial allows a biological compound to be identified more accurately because there are three frequency-match points of comparison.
With this in mind, the authors simulated metamaterial structures computationally to find the best materials for the job. They then made the best designs, shone terahertz radiation on them and recorded the electromagnetic response. As predicted, metamaterials with structures that combined three distinct subunits (such as that pictured on the lower right panel) resonated at three distinct frequencies, the individual frequencies of the different subunits.
As the authors had hoped, the simulated and experimental resonances of their metamaterials were a good match for those of vitamin H. This match could therefore form the basis of a biodetector. Bingham et al. have found that their multi-subunit tiling techniques can create multi-resonator metamaterials that can be used as biodetectors. But that is not all. Their metamaterials could potentially detect hazardous chemicals.
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Advanced Science (2018)