Comparison of Responses among Non-coated, PE-only Coated and PE-AuNP Coated LPFGs towards Mercury (II) Ions

Next, in order to confirm the reaction between mercury (II) ions and gold nanoparticles layer, the experimental procedures were repeated with uncoated LPFG and PE-only coated LPFG (with optimized number of PE bilayers only). The purpose of this experiment is to compare the responses of the LPFGs with and without AuNPs coated towards mercury (II) ions. Figure 5.8(a) and (b) show the comparisons of the normalized transmission power and resonance wavelength shift of the first PE–AuNP coated LPFG (LPFG 1), PE-only coated LPFG and the uncoated LPFG. From Figure 5.8, both the normalized transmission power and the resonance wavelength shift of the PE–

AuNP coated LPFG increased as the concentration of mercury solution increased, up to a point where the saturation state was reached. On the contrary, for the uncoated LPFG, both the normalized transmission power and resonance wavelength shift did not show significant responses towards the mercury solution. As the concentration increased, both the normalized transmission power and resonance wavelength shift remained almost constant.

The shift of resonance wavelength of PE–AuNP coated LPFG in mercury solution was almost 33.5 times compared to the uncoated LPFG. The increase in transmission power of PE–AuNP coated LPFG in mercury solution was around 14.5 times of the uncoated LPFG.

Similar to the uncoated LPFG, the normalized transmission power and resonance wavelength shift of the PE-only coated LPFG did not show prominent changes with increasing mercury (II) ions concentration compared to the PE–AuNP coated LPFG as shown in Figure 5.8 (a) and (b). The increase in normalized transmission power of PE–AuNP coated LPFG was 10.3 times compared to PE-only coated LPFG while the resonance wavelength shift of the PE–AuNP coated LPFG was 4.5 times compared to the wavelength shift of PE-only coated LPFG. The response of the PE-only coated LPFG towards different mercury (II) concentrations compared to the uncoated LPFG was relatively better due to the improved sensitivity of the LPFG caused by the deposition of PE layers.

(a)

(b)

Figure 5.8 First comparison of (a) resonance wavelength shift of non-coated LPFG, PE-only coated LPFG and PE-AuNP coated LPFG; (b) normalized transmission power of non-coated LPFG, PE-only coated LPFG and PE-AuNP

0.5 ppm 1.0 ppm 2.0 ppm 5.0 ppm 10.0 ppm

0.5 ppm 1.0 ppm 2.0 ppm 5.0 ppm 10.0 ppm

Again, the comparison was conducted with the second PE-AuNP coated LPFG to confirm the consistency of findings. The comparison of the normalized transmission power and resonance wavelength is shown in Figure 5.9(a) and (b). Both the uncoated LPFG and PE-only coated LPFG did not show a prominent response towards the increment in concentrations of mercury (II) solutions. On the contrary, both the normalized transmission power and the resonance wavelength shift of the PE–AuNP coated LPFG increased as the concentration of mercury solution increased, until it reached to a point where the response plateaued. The shift of resonance wavelength of PE–AuNP coated LPFG in mercury solution was almost 28.1 times compared to the uncoated LPFG. If compared to the PE-only coated LPFG, the wavelength shift of PE-AuNP coated LPFG was 4.7 times higher. On the other hand, the increase in transmission power of PE–AuNP coated LPFG in mercury solution was around 15.8 times the uncoated LPFG, and 7.2 times the PE-only coated LPFG. From these comparisons, it can be concluded that gold nanoparticles are suitable to be used as the sensing agent towards mercury (II) ions due to their unique reaction in forming amalgam.

(a)

(b)

Figure 5.9 Second comparison of (a) resonance wavelength shift of non-coated LPFG, PE-only coated LPFG and PE-AuNP coated LPFG; (b) normalized transmission power of non-coated LPFG, PE-only coated LPFG and PE-AuNP

0.5 ppm 1.0 ppm 2.0 ppm 5.0 ppm 10.0 ppm

0.5 ppm 1.0 ppm 2.0 ppm 5.0 ppm 10.0 ppm

5.6 Summary

In this chapter, a novel PE–AuNP coated LPFG sensor that is applicable for real-time monitoring had been successfully demonstrated and tested for the detection of mercury (II) ions in water. It had proven that the coated LPFG was able to detect mercury (II) solutions. The deposition of PE–

AuNP layers further modified the surface of arc-induced LPFG where AuNPs were used as the sensing agents that captured mercury (II) ions. Generally, the resonance wavelength of the PE–AuNP coated LPFG shifted to a longer wavelength and the transmission power increased throughout the duration of 5 hours exposure to varied concentrations. For both resonance wavelength and transmission power, the rate of change was the highest in the first 10 to 30 minutes of soaking in the mercury (II) solutions until the saturation point was achieved. These responses of the PE–AuNP coated LPFGs were then compared with uncoated and PE-only coated LPFGs. Negligible changes in the resonance wavelength and transmission power were observed for the uncoated and PE-only coated LPFGs. The response of PE–AuNP coated LPFG compared to other LPFGs was caused by the formation of amalgam on the PE–AuNP LPFG surface due to the reaction of mercury (II) ions with AuNP. Nonetheless, the lifespan of LPFG is one of the main drawbacks if employed LPFG as a sensor in water bodies as the structure of LPFG is very easily broken by any external force caused by environmental factors.

Therefore, there is a need to construct a structure which can protect the LPFG sensor to prolong its lifespan for long-term monitoring of mercury (II) ions.

The structure to strengthen and protect the LPFG is discussed in next chapter.

CHAPTER 6

A NOVEL HYBRID LPFG-DGT SENSOR SYSTEM FOR