Micro and nanostructures in optical fibre sensors
The purpose of this post is to explain in a simple and summarized manner the several existing micro and nanostructures employed to manufacture fibre optic sensors, which are described in depth in . In this post up to 10 different structures are distinguished. Each of them is defined, and their main advantages and applications are also mentioned.
1.Microstructured Optical Fibres (MOFs): light transmission is produced by a distribution of air holes manufactured in the fibre, as opposed to standard fibres, where transmission is possible due to the refractive index difference between the core and the cladding. MOFs are classified into Suspended Core Fibres (SSCs) and Photonic Crystal Fibres (PCFs), distinguishing in the last case between Solid Core Fibres (SC-PCFs) and Hollow Core Fibers (HC-PCFs) . MOFs are considered to be a good choice for liquid and gas sensing thanks to their air holes .
2. Nanostructures: both metals and non-metals are deposited with different techniques (sputtering, CVD), separately or combined, to manufacture structures at a nanometer level, pursuing high sensitivity to refractive index changes, periodic patterns or interaction over a large area. These structures include nanocolumns, nanorods or nanorings, which then are functionalized for biosensing applications .
3. Plastic Matrices: this technique involves depositing a mixture of plastic, an organic solvent and a sensing material (and sometimes, more substances) onto an optical fiber by dip coating. The optical fiber has to be cured to remove the organic solvent and harden the polymer, resulting in a robust sensing coating. This is their main advantage as well as their low reactivity to aging agents .
4. Carbon Nano Tubes (CNTs): they possess high conductivity and robustness, high surface/volume ratio and are almost a perfect black body. There are 2 main types: single walled (SWCNT), with a hollow nanostructure in which the C atoms are structured in single rolled tubes; and multi walled (MWCNT), in which various single tubes with several diameters are grouped concentrically . Both types are used for optical fibre sensors, especially for VOCs sensors in the case of SWCNT.
5. Sol Gel: this technique enables to control the porosity of the supporting matrix, which determines its adsorption properties. It can be distinguished between aerogels (manufactured in high T and pressure) and xerogels (atmospheric conditions). Fibres are dipped into the solution after the gelification has started. Silica sol gels are considered suitable for detecting VOCs .
6. Metal Oxides: they include ZnO, SnO2, ITO (indium tin oxide), In2O3 or titania. Sensors are based in the changes that some reagents produce to the luminescent properties of these materials or their refractive index. This last case includes LMRs generated by a thin layer of a metal oxide. These materials can also be combined with LPG and FBG structures. One of the main applications is the detection of VOCs .
7. Molecularly Imprinted Polymers (MIPs): they are synthetic molecules, developed as a more versatile alternative to biological molecules, which can quickly degrade if some physiological conditions are not met. Several chemicals reagents have to be mixed in a solvent to synthesize a MIP and their choice will determine its properties. The main advantage of the MIPs is their high selectivity .
8. Nanowires: this name is given for different structures, although 2 are the most relevant ones: stretched fibers with a diameter in the order of the nanometers (manufactured with the flame-brushing method or by electrical arch), and nanostructures with rod shape and certain properties. The term nanowire can only be employed if the diameter along the stretched section is lower than the working wavelength .
9. Layer-By-Layer (LbL) Nanostructures: this category includes structures at nm scale that have been coated employing the layer-by-layer (LbL) electrostatic self-assembly technique, which is based on the assembly of alternative polyelectrolyte chains with positive and negative electrical charge. LbL stands out for its versatility, enabling the use of different polymers, which results in nanostructures with different morphologies .
10. Electrospun Nano-Fibres: electrospinning is another highly versatile technique that can be utilized to generate a polymer nanoweb structure on optical fibres. It consists in depositing a material by using a viscoelastic solution which is stretched by electrostatic forces. The obtained nanomembranes possess a high surface/volume ratio, improving the interaction between the sensing material and the target analyte .
In conclusion, the employment of micro and nanostructured materials such as the ones explained in this post enables the development of optical fibre sensors with better characteristics for chemical and biosensing applications. Nevertheless, it has to be taken into account that the manufacturing and deposition processes are critical in determining the properties of these micro and nanostructures, and therefore, the performance of the corresponding optical fibre sensors.
 Elosua, C.; Arregui, F.J.; Del Villar, I.; Ruiz-Zamarreño, C.; Corres, J.M.; Bariain, C.; Goicoechea, J.; Hernaez, M.; Rivero, P.J.; Socorro, A.B.; et al. Micro and nanostructured materials for the development of optical fibre sensors. Sensors (Switzerland) 2017, 17, 2312.
 Lopez-Torres, D.; Elosua, C.; Arregui, F.J. Optical Fiber Sensors Based on Microstructured Optical Fibers to Detect Gases and Volatile Organic Compounds—A Review. Sensors2020, 20, 2555, doi:10.3390/s20092555
 Villatoro, J.; Kreuzer, M.P.; Jha, R.; Minkovich, V.P.; Finazzi, V.; Badenes, G.; Pruneri, V. Photonic crystal fiber interferometer for chemical vapor detection with high sensitivity. Opt. Express 2009, 17, 1447, doi:10.1364/oe.17.001447.