# Hg\({}^{2+}\) Optical Fiber Sensor Based on LSPR with PDDA-Templated AuNPs and CS/PAA Bilayers
## 1 Introduction  
Mercury ions (Hg\({}^{2+}\)) are one of the main sources of heavy metal pollution in water. Contamination by Hg\({}^{2+}\), not only causes serious environmental problems, but also central nervous system disorders, kidney or liver disorders due to the toxicity and carcinogenicity [1]. Various sensing technologies have been developed for Hg\({}^{2+}\) detection in an aqueous solution. Spectroscopic-based techniques, such as atomic absorption spectroscopy [2] and inductively coupled plasma spectroscopy [3], are amongst the most commonly used methods and have accurate Hg\({}^{2+}\) detection capability. However, these spectroscopic techniques have the same drawback of high cost due to the maintenance required for this sophisticated equipment. Additionally, the sample preparation process makes these methods time consuming. All of these undoubtedly urge the generation of new sensing technologies with simplicity, rapidity, low cost, and high sensitivity.  
Optical fiber sensing technology has attracted extensive attention and great research enthusiasm in the field of metal ion monitoring due to its unique advances such as low cost, light weight, ease of use, small in size, multiplexing, remote sensing, and high sensitivity [4; 5; 6; 7]. Up to now, optical fiber heavy metal ion sensors have been reported by utilizing fluorescence [8], surface-enhanced Raman scattering [9], surface plasmon resonance [10; 11; 12], and localized surface plasmon resonance (LSPR) [13; 14; 15; 16; 17]. LSPR is usually excited near the surface of the metal nanoparticles to enhance the local electromagnetic fields, and the basic principle of heavy metal ion detection based on LPSR is highly responsive to changes in the refractive index around metal nanoparticles [18; 19]. In view ofenhancing the plasmon resonance and the sensitivity of the refractive index sensing, the deposition of well-dispersed metallic nanoparticles is extremely important to realize LSPR sensor with high sensitivity and good reproducibility. Colloidal gold nanoparticles (AuNPs) are commonly deposited on an optical fiber surface through a substrate via the electrostatic self-assembly method. The first reported deposition method was based on positively charged silane to assemble negatively charged nanoparticles [20]. This method usually requires a complex operation and lengthy deposition time. For instance, a total of 13 h was required when depositing AuNPs on the hydroxylated optical fibers through (3-mercapopropyl) trimethoxy silane [21]. In addition to a lengthy deposition time, a large number of aggregates and low nanoparticle coverage were also observed in the silence method [22; 23]. In order to reduce the deposition time, a polyelectrolyte assembly method was then proposed to simplify the preparation of optical fiber LSPR sensors [24; 25; 26]. At high nanoparticle coverage, this method was also reported to have aggregation [27]. A clear aggregation was also observed as directly depositing the AuNPs onto the surface of poly (methyl methacrylate) chips and optical fibers [28]. The aggregation of nanoparticles broadens the LSPR band and hence strongly affects the refractive index resolution, sensitivity, and experimental reproducibility [29].  
In this paper, we proposed and experimentally demonstrated a heavy metal Hg\({}^{2+}\) sensor based on the optical fiber LSPR. We developed an alternative route to deposit well-dispersed AuNPs on bare optical fiber by fast flame-brushing the PDDA-templated nanoparticles. Chitosan (CS)/poly acrylic acid (PAA) multilayer film was employed to functionalize the optical fiber surface and then to facilitate Hg\({}^{2+}\) ions adsorption on the sensor for detection. LSPR spectrum shifts were monitored to characterize the sensor performance with various Hg\({}^{2+}\) concentrations.