Metamaterials are defined by Wikipedia as “–artificial materials engineered to provide properties which may not be readily available in nature.” Now a research group has used one such metamaterial, created using gold nanorods, for designing a sensitive label free biosensor. Metamaterial biosensor consists of an array of gold nanorods of 20-700nm in length, 10-50nm in diameter with inter-rod distance of 40-70nm giving around 1010-1011 rods/cm2. Each sensor element is around 2 cm2 in area
The plasmonic sensor chip is mounted on a prism for light coupling as is done for conventional gold film based SPR sensors. For biological assay a flow cell is attached at the top. Using aqueous solution of glycerine of different concentrations (hence different refractive index) sensitivity of 32,000nm per refractive index unit (RIU) was established for these sensors. Corresponding number for gold nanoparticle/nanostructure based sensor is two order of magnitude lower and that of conventional film based sensor is more than 10 fold lower. In addition, compared to probe depth of 50nm for gold nanoparticle based sensor the probe depth for gold nanorod arrays sensor is 500nm hence making them useful to probe biological interactions between large proteins or for sandwich type immunoassays. The two reasons for higher sensitivity, as given by authors, include a) better overlap between Plasmon sensing field and sensed substance and b) higher surface area for capture of biological material.
Using streptavidin-biotin model system, the metamaterial sensors were able to detect biotin at 300nM concentration that is two log order of magnitude more sensitive than conventional SPR instrument.
A ten fold improvement over the current “gold” standard – a gold film based SPR sensor like Biacore- is a dramatic jump certain to bring new applications within the realm of label-free sensing. I can foresee the use of this sensor for small scale interaction and screening. A million dollar question is can the sensor chips be made reproducibly and inexpensively?