Grapefruit-type three-core fiber sensor for trace water based on silver-silk protein-silver structure

  Objective   Water impurity in organic solvent strongly affects the process of chemical reaction. However, current methods for detecting trace water still suffer from disadvantages such as complex operation, high toxicity of experimental reagents, low detection sensitivity, and the inability to monitor in real-time. In order to overcome these challenges, a D-type fiber sensor which combines silver and silk protein in the visible wavelength range is designed. It is designed for high-sensitivity detection of trace water in organic compounds, and the performance of the sensor is optimized and analyzed.

  Methods  First of all, the circular air hole in the middle of the fiber is replaced by the grapefruit type, which increases its size by about ten times (Fig.1), which can reduce the difficulty of the sensor in the actual production process with high efficiency. In addition, the lower metal silver film on the side profile of D-type fiber can form an energy channel with the nearby large air hole of grapefruit type, which promotes the energy leakage of the fiber core into the plasma and enhance the SPR effect. Secondly, a certain number of silver grating structures are added to the lower part of the top silver film, which can make the incident bright part confined to the slit cavity. Through reasonable structural design and size optimization, the local surface plasmon resonance (SPR) wave and surface plasmon wave can further resonate, thus realizing the enhancement of SPR phenomenon.

  Results and Discussions   Theoretical analysis shows that compared to a single-layer silver structure, the double-layer silver structure on the surface of the D-type fiber sensor can induce strong localized light. In the subsequent size selection process, the initial thickness of the silk protein and the height of the air hole (Tab.1), the height of the silver gratings (Tab.2), the amount of the silver gratings (Tab.3), the spacing of the silver gratings (Tab.4) and the thicknesses of the underlayer silver film (Tab.5) are optimized to obtain the optimal sensing structure. Through the above optimization, the optimal structural parameters of the sensor are as follows: the height of the fiber air hole is 4.5 μm, the height of the silver grating is 10 nm, the number of the silver grating is 58, the spacing of the silver grating is 50 nm, and the thickness of the underlying silver film is 31 nm. Compared with other structures or methods of trace water detection equipment in sensitivity or detection limit performance, the results show that the detection performance of the grapefruit-type three-core fiber sensor for trace water based on silver-silk protein-silver structure is much better than the previous design.

  Conclusions   Based on metal insulator metal structure and SPR principle, D-type fiber sensor is designed to detect trace water in organic matter. The sensing materials mainly use silver metal and silk protein, and the sandwich structure composed of silver metal can significantly enhance the local electric field, thus improving the sensitivity of the sensor. Silk protein has good optical properties and can quickly and accurately make reversible volume changes in response to external stimuli. Therefore, water absorption of silk protein can be calculated by its expanded volume combined with Darcy's law, and the water content in organic matter can be further obtained. The high sensitivity measurement of trace water in organic matter is realized, and the sensitivity can reach 1.39 nm/ppt (1 ppt=10⁻¹²). Moreover, the fitted line has an R² value greater than 0.999, achieving the expected performance. And the sensor is designed to have a long service life, and is not affected by temperature.