# Reflective Fiber Surface Plasmon Resonance Sensor for High-Sensitive Mercury Ion Detection
## 1 Introduction  
The problem of heavy metal pollution is becoming more and more serious and has always been one of the key issues of environmental pollution [1; 2]. Mercury, as one of the heavy metal contaminants, causes serious health issues, affecting neurological, renal organ, and gastrointestinal systems [3]. The main form of mercury as a pollutant is mercury ion (Hg\({}^{2+}\)), which has high chemical activity in solution [4]. Therefore, a reliable and convenient Hg\({}^{2+}\) detection method is of great significance for the safety of drinking water. There are many traditional methods for Hg\({}^{2+}\) detection, such as those based on colorimetry and fluorescence [5; 6], nanoparticles [7] or functional polymers [8]. These methods typically require time-consuming assay procedures and complex specialized laboratory equipment and samples.  
In contrast, fiber optic sensors have unique advantages, such as multiplexing, high flexibility, high sensitivity, remote sensing, low cost, and immunity to electromagnetic interference [9; 10; 11], which play an increasingly important role in the detection of heavy metal ions. In recent years, researchers proposed many fiber sensing technologies to detect heavy metal ions, including: Evanescent wave absorption [12], Mach-Zehnder interferometer [13], fluorescence [14], and surface plasmon resonance (SPR) [15]. Compared with other methods, the optical fiber SPR sensor has higher sensitivity. It has been reported that fiber optic SPR sensors have been used to detect the concentration of manganese ions [16], cadmium ions [17], and mercury ions [18] in solution. However, the sensitive films of the presented mercury ion sensors usually rely on gold nanoparticles, the use of which does not ensure uniformity of the film, and thus affecting the linearity and even measurement accuracy of such sensors [18; 19]. Besides, the sensing heads of the present sensors often work in the transmission type [18; 20], which is not convenient for the insertion of the sensing head into the analysis solution and not suitable for remote measurement of this kind of sensor. It has been demonstrated that fiber SPR sensors could also work in the reflective type by coating an Au mirror [21] or Ag mirror [18] onthe fiber end. However, the additional coating process of the metal film on the fiber end increases the fabrication complexity of the sensing head. Besides, the Ag film is easily oxidized, which then influences the reflectivity.  
In order to solve the above problems, this paper proposes a reflective fiber SPR sensor for measuring Hg\({}^{2+}\) concentrations. The sensor head is fabricated by splicing a single mode fiber (SMF) and a multimode fiber (MMF). A gold film for exciting SPR and a gold mirror for reflecting optical resonance are simultaneously coated on the surface of the SMF by using a sputtering apparatus. This reflective sensing structure can be used as a plug-in probe and is suitable for remote measurement. Also, a multilayered chitosan (CS)/polyacrylic acid (PAA) was successfully and uniformly coated on the side surface of the gold film, which was demonstrated as a low cost and excellently sensitive film for the detection of the Hg\({}^{2+}\) concentrations [20]. When Hg\({}^{2+}\) is adsorbed on the sensitive film, the refractive index of the sensitive film changes with the Hg\({}^{2+}\) concentration to affect the resonant peak wavelength of the SPR. In addition, the fiber optic sensor designed in this paper has relatively good recyclability and stability while being simple to manufacture and operate.