Significance An optical fibre surface plasmon resonance (SPR) sensor is one of the new type optical fibre sensors developed on the basis of optical fibre and sensing technologies. It has the inherent advantages of optical fibre and SPR sensors, such as high detection sensitivity and free labelling. Currently, it is used to measure physical parameters and detect chemical substances and biomass. Optical fibre SPR sensors make up for the shortcomings of conventional electrochemical sensors for substance detection and have a wide range of applications in the fields of chemistry, biology, medicine, and food safety.
With the rapid development of optical fibre SPR sensing technology, researchers have reviewed the current state of development in areas such as fibre SPR sensor structures, fibre substrates, and film materials. However, with the development and increased applications of fibre optic SPR sensors, sensitivity enhancement has gradually become one of the important parameters that limit the sensor’s performance. Up to date, there is no review report on this important issue. Based on the relevant literature, this paper reviewed the sensitivity-enhancement technology of fibre SPR sensing, summarised the sensitivity-enhancement methods from the perspectives of fibre substrate structure, film structure, and film material, and prospected the future development direction of these sensors.
Progress The intensity of evanescent waves can be effectively enhanced by designing different fibre substrate structures. The most commonly used evanescent-wave enhanced fibre substrate structures are the D-type, U-type, tapered-type, and fibre core mismatch structures. The D-type structure is the side polishing structure, which polishes one side of the optical fibre to a plane, making it easier for the sensing film to be deposited on the surface of the core. The U-shaped structure fibre SPR sensor has a bent sensing area, which reduces the incident angle of the optical signal transmitted in the sensing area and enhances the penetration depth, leading to an enhanced evanescent field and improved sensing sensitivity. The sensing area of an optical fibre SPR sensor with a tapered-type structure is sandwiched between two tapered fibre regions. Core mismatched fibre SPR sensors mainly include multimode fibre (MMF)-single mode fibre (SMF)-MMF, SMF-MMF-SMF, SMF-hollow-fibre-SMF, SMF-photonic-crystal-SMF, and other types of structures. In addition, the sensitivity of SPR sensors can be improved using new types of optical fibres such as photonic crystal or micro-structured fibres as sensing substrates.
The types of metal films commonly used in optical fibre SPR sensors include gold, silver, copper, and aluminium. The thickness of these metal films is generally about dozens of nanometres. Single-layer silver film SPR sensors have high resolution and sensitivity. However, the gold film is generally used in SPR sensors owing to the chemical activity of silver. The fibre optic SPR sensor with a silver-gold bimetallic film structure has a layer of gold film on the surface of the silver film, which makes the sensor have the advantages of high sensitivity and high resolution and enables the sensor to make full use of the advantages of stable chemical characteristics of the gold film. Because metal nanoparticles, particularly gold and silver nanoparticles, have strong plasma and catalytic effects, they are often used in the fabrication of optical devices. For some low molecular weight substances to be measured, metal nanoparticles can be combined with such substances, which amplifies the SPR signal and further improves the sensing sensitivity. Compared with other nanoparticles, gold nanoparticles have better biocompatibility and unique optical and electrical properties and are easy to prepare.
The surface of a common single metal film SPR sensor is relatively active, and it is easy for the sensor to react with the solution to be tested during the detection process, resulting in a reduction of the sensitivity. This problem can be solved by adding additional film materials on the surface of the metal film, which will improve the performance of the SPR sensor. As an important sensitive material, metal oxides are widely used. When the metal oxide film is used as the auxiliary film layer, the electric field distribution in the sensing film layer changes and the electric field intensity is enhanced. Thus, the sensitivity of the optical fibre SPR sensor can be effectively improved. Common auxiliary metal oxide films mainly include zinc oxide, indium tin oxide, and titanium dioxide. Transition metal dichalcogenides (TMDCs) have attracted considerable attention owing to their unique optical, electrical, and electrochemical properties. By chemical stripping, high-quality single or multilayer TMDC films can be prepared, which can be applied to optical fibre SPR sensors to improve their sensing performance.
Conclusions and Prospects Currently, the sensitivity-enhancement technologies have been used for the optical fibre SPR sensors. Furthermore, the optical fibre structure, film structure, and film material are focused on, which helps in effectively improving the fibre SPR sensor refractive index sensing sensitivity and providing practical and effective technical support for future development. Further breakthroughs are expected in the following areas. First, the sensitivity of the fibre SPR sensor can be improved using different sensitivity-enhancement methods, such as using optical fibre structure, film structure, and film material. Second, according to the target characteristics of the required sensor, the dielectric constant of the film material can be inversely calculated using reverse design and the composition and lattice structure design of the new thin-film material can be realized. Finally, by increasing the SPR coupling efficiency, the two-photon 3D micro-nano printing technology can be used to further improve the sensitivity.
