Biomedical Optical Sensing Using Nano-/Micro-Structured Metamaterials

Work on optical sensing techniques continues to advance - and optical sensing techniques and related technologies are becoming a well-established approach for, inter alia, applications in biomedical and environmental sensing. Both structured surfaces and arrays of integrated devices that are fabricated by means of classical planar technologies are likely to have a central role in the development of low-cost and reliable biosensors, at both the molecular and cell levels. One approach that has been demonstrated as viable in biomedical sensing is the use of fluorescent labelling, e.g. for carrying out competition immunoassays to identify the possible presence of specific analyte molecules in suitably prepared fluid samples. Many bio-materials exhibit a finite level of fluorescence that could possibly be exploited in sensing, but is also potentially problematic when it forms an undesired background, thereby limiting sensitivity. But the alternative approach of label-free biomedical sensing seems likely to be favoured in future applications. A 'standard' approach to label-free sensing exploits the detection of changes in the (complex) refractive index that occur when bio-material is added to the local environment of a designed resonant structure, thereby changing its resonance frequency. Designed tuning of reflection, transmission and absorption resonances can be used to help identify specific molecules, through selection of the known bond resonances of the molecules of interest. Since it is typically possible to organise resonant structures in arrays that consist of thousands of individual resonant 'atoms', thereby forming a metasurface, it has become possible to select and quantify various characteristic molecular bond resonances simultaneously - and to identify possible molecular compositions in composite bio-material.