Mercury (II) ions Detection with Hybrid LPFG-DGT

The first experiment conducted in this final stage of research aimed to determine whether the proposed hybrid LPFG-DGT sensor system is able to detect the presence of mercury (II) ions. The hybrid sensor was first exposed to deionized water until its response stabilized. Consequently, the experiment continued with five different concentrations of mercury (II) ion solutions, i.e.

increasing from 0.5 ppm to 10 ppm. The results of the experiments are shown in Figure 6.4 and Figure 6.5.

From the results, it can be observed that the resonance wavelength of the hybrid LPFG-DGT structure encountered a total red shift of 0.8 nm to a longer wavelength and a total increment of 1.071 dBm in the transmission power. For each different concentration, the output spectra of the LPFG-DGT hybrid sensor reached stability around 70 minutes of exposure to the different concentrations of mercury (II) ion solutions. It was also observed that for the hybrid LPFG-DGT structure, the rate of reaction was the highest in the first 40 minutes to 50 minutes of exposure to every concentration of mercury (II) solution. After the first 40 to 50 minutes, the reaction started to slow down and in the end reached a plateau.

For 0.5 ppm of mercury (II) ion solution, the transmission power of the resonance notch encountered a total increment of 0.354 dBm in the first 50

minutes, whereas for the next 20 minutes, the transmission power remained the same. In the first 60 minutes of exposure to 1.0 ppm of mercury (II) ion solution, the increment of transmission power was around 0.274 dBm but it remained at the same level after 60 minutes of exposure in 1.0 ppm of mercury (II) ion solution. On the other hand, for 2.0 ppm of mercury (II) solution, the sensor encountered an increment in transmission power of 0.385 dBm in the first 50 minutes and remained at the same level beyond this duration. The response of the hybrid sensor became slower in 5.0 ppm of mercury (II) solutions, i.e. encountered only 0.032 dBm of increment in transmission power.

Finally, as the LPFG-DGT hybrid sensor was exposed to 10.0 ppm of mercury (II) ion solution, the response became saturated, i.e. increased only 0.022 dBm in the first 10 minutes of exposure and remained the same afterwards.

On the other hand, in 0.5 ppm and 1.0 ppm of mercury (II) ion solutions, the resonance wavelength encountered a total shift of 0.24 nm and 0.20 nm respectively in the first 40 minutes of exposure time and remained at the same wavelength after that. In 2.0 ppm and 5.0 ppm of solutions, the notch shifted for a total of 0.24 nm and 0.12 nm in the first 50 minutes of exposure, respectively. However, the resonance notch of the hybrid sensor remained the same in 10.0 ppm of mercury (II) solutions because the saturation state had been reached and no further reaction occurred in between the gold nanoparticles and mercury (II) ions.

(b)

(a)

(b)

Figure 6.4 (a) Transmission power response; (b) Transmission wavelength shift of the first hybrid sensor towards mercury (II) solutions

Figure 6.5 Transmission response of the first hybrid sensor towards mercury solutions

Notch shifted Transmission power

increased

A similar experiment was repeated with another hybrid LPFG-DGT structure to confirm the findings. The results of the second hybrid structure are shown in Figure 6.6. From the results, it can be observed that the sensor had a similar response as the first hybrid structure. The resonant wavelength of the second hybrid structure encountered a total red shift of 0.68 nm to a longer wavelength and a total increment power of 0.864 dBm.

For 0.5ppm of mercury (II) ion solution, the transmission power of the resonance notch encountered a total increment of 0.233 dBm in the first 40 minutes, whereas for the next 30 minutes, the transmission power remained almost the same as the response had stabilised in the mercury (II) solution.

When the hybrid sensor was exposed to 1.0 ppm of mercury (II) ion solution, the increment of transmission power was around 0.298 dBm. On the other hand, for 2.0 ppm of mercury (II) solution, the sensor encountered an increment in transmission power of 0.272 dBm in the first 60 minutes and remained at same level beyond this duration. The response of the hybrid sensor became smaller in 5.0 ppm of mercury (II) solution; it encountered only 0.030 dBm of increment in transmission power. Lastly, as the second LPFG-DGT hybrid sensor was exposed to 10.0 ppm of mercury (II) solution, the response became saturated; its transmission power had increased only 0.012 dBm in the first 20 minutes of exposure and remained the same afterwards.

On the other hand, in 0.5 ppm of mercury (II) solutions, the resonance wavelength encountered a total shift of 0.24 nm to a longer wavelength in the first 40 minutes of exposure time and remained at the same wavelength after

that. When the sensor was exposed to 1.0 ppm of mercury (II) solution, the resonant wavelength shifted a total of 0.24 nm to a longer wavelength and the position of the notch remained the same for the remaining 30 minutes. In 2.0 ppm of mercury (II) solution, the notch shifted for a total of 0.16 nm in the first 30 minutes and the response plateaued after that. In addition, the resonant wavelength encountered a total red shift of 0.08 nm in the first 20 minutes of exposure, respectively. However, the resonance notch of the hybrid sensor remained the same in 10.0 ppm of mercury (II) solution.

The results of both hybrid structures show that the proposed sensor system is able to detect mercury as its resonant wavelength had shifted to a longer wavelength and its minimum transmission power had increased when the sensors were exposed to mercury (II) solutions.

(a)

(b)

Figure 6.6 (a) Transmission power response; (b) Transmission wavelength

6.5.2 Comparison of Performances between Hybrid and Open Structure