Gold Nanoparticles coated LPFG

5.4.1 Interaction of Gold Nanoparticles with Mercury

Gold is known as the neighbour element of mercury in the periodic table. The unique physical properties of these two metals have led to the

inspiration of many investigations and research, especially after the discovery of the adsorption and reaction in between these two metals. (James et al., 2012) The process of bringing gold particles into contact with mercury is known as amalgamation, where a new compound called amalgam is formed after the reaction between gold and mercury (Levlin et al., 1999). As discussed earlier, gold particles were used widely in various kinds of sensors to detect mercury due to their reactions. In this research, gold nanoparticles were used as the sensing agents to be coated onto LPFG to capture mercury ions in water as well.

5.4.2 Synthesis of Gold Nanoparticles

The gold nanoparticles, AuNP, that were employed in this research were synthesized according to the Frens method developed in 1973 (Frens et al.). Generally, the nanoparticles were formed through the basic reduction between gold (III) chloride trihydrate (HAuCl4⋅3H2O) and trisodium citrate dihydrate (Na3C6H5O⋅2H2O). One advantage of using the Frens method in synthesizing gold nanoparticles is that the size of nanoparticles can be controlled by adjusting the mole ratio between the gold and citrate. Initially, both gold (III) chloride trihydrate and trisodium citrate dehydrate were prepared before the synthesis process. 0.01 wt% of gold (III) chloride trihydrate was prepared by dissolving 0.01 g of gold (III) chloride in 100 ml of deionised water. On the other hand, 1 wt% of trisodium citrate dehydrate was prepared by dissolving 1 g of sodium citrate 2-hydrate in 100 ml of deionized water. After that, 50 ml of 0.01 wt% gold (III) chloride trihydrate solution was

heated and stirred to boiling point. Subsequently, a small amount of 1 wt%

trisodium citrate dehydrate solution was added gradually to the boiled gold (III) chloride trihydrate solution in a drop-wise method. The solution was then kept heated and stirred until a colour change was discovered. The yellow colour of the solution changed to blue and finally into reddish purple, indicating the formation of nanoparticles. The citrate ions were used as both reducing and capping agents in stopping the particle growth and agglomeration so that the nano size of gold can be attained. Gold nanoparticles that were synthesized through this method will be in the nanosphere shape. In addition, the sizes of the gold nanoparticles produced were in the range from 50 nm to 80 nm.

5.4.3 Coating process of LPFG

Chapter 4 discussed thin film coating as one of the methods adopted to sensitize the arc-induced LPFG, where polyelectrolytes layers were deposited onto the fiber gratings to modify the cladding index and enhance its sensitivity.

In this chapter, the PE-coated LPFG was further modified with the deposition of gold nanoparticles (AuNPs) onto the PE layers. The AuNPs were immobilized onto the LPFG surface using the ESA technique. In the previous chapter, a desired number of polyelectrolytes bilayers (PDDA/PSS)n, had already been deposited onto the gratings surface and the outermost layer of the coating was negatively-charged PSS. Before further modifying the LPFG surface with AuNP, another layer of positively-charged PDDA needed to be coated above the outermost PSS layer. This is because the surface of the AuNP synthesized using citrate-reduction method was covered with citrate

anions, which caused the surface to become negatively-charged (Turkevich et al., 1951; Lim et al., 2014) Hence, an extra layer of positively-charged PDDA must be coated to attract the opposite charges of the nanoparticles towards the gratings surface. The electrostatic forces between the negatively charged AuNP and the positively charged PDDA resulted in the formation of a uniformly distributed self-assembly monolayer of gold nanoparticles on the fiber gratings (Tan et al., 2018). The structure of the PE-AuNP coated LPFG is as shown in Figure 5.1.

After the deposition of AuNPs, the coating of gold nanoparticles on the surface of the LPFG was investigated by using a field emission scanning electron microscope (FESEM) (JEOL JSM-6701F). The coated section of LPFG was cut to less than 1 cm length and was adhered to the holder by using carbon tape. After that, the sample was sputtered with gold palladium alloy and mounted onto a stage and the surface morphology of coated section was observed.

Figure 5.1 Schematic diagram of the PE-AuNP coated LPFG

5.4.4 Experimental Setup

The mercury (II) solutions used in this experiment were prepared from the mercury standard solution (Wako, 100 mg/L, Japan). Five different concentrations of mercury (II) solutions were tested, ranging from 0.5 ppm to 10.0 ppm. The mercury solutions were prepared by diluting the standard solution according to the ratio of standard solution and water as shown in Table 5.1.

AuNP

(negatively-charged) PDDA

(positively-charged) (PDDA/PSS)_n

LPFG

Table 5.1 Volume of mercury standard solution and deionised water required to prepare mercury (II) solutions with different concentrations

Concentrations which was same as the configuration discussed in Chapter 4, with the purpose of further improving the sensitivity of LPFG (Loh et al., 2015). Similarly, two optical circulators were used in the setup so that the light passes through the LPFG twice to increase the transmission attenuation of resonance wavelength.

The position of LPFG was fixed by two fiber clamps, and a constant tension of 18cN was attached to the LPFG throughout the experiments to ensure the deionized water until the response reached to a stable state. Subsequently, the coated LPFG was soaked in every concentration of mercury (II) solution for

an hour (starting from lowest concentration to highest concentration) until the response of LPFG plateaued. In between different concentrations of mercury (II) solutions, the LPFG was rinsed with deionized water to remove the residue from the previous measurand. Experiments were conducted in a controlled room with a constant temperature of 24.2 ◦C ±0.2 ◦C to avoid the temperature cross sensitivity of the LPFG. The power fluctuation of the broadband light source was recorded to be within the range of ±0.01 dBm.