# Cellulose and Vanadium Plasmonic Sensor to Measure Ni\({}^{2+}\) Ions
## 3 Results and Discussion
### SPR Reflectance Curve for Au/MPA/V-CNCs-CTA Thin Film  
Next, the proposed Au/MPA/V-CNCs-CTA thin film was tested with different concentrations of Ni(II), as shown in Figure 3. Note that the concentration of Ni(II) ion was reduced to 2 ppb to obtain the lowest detectable concentration. It can be observed from the figure that the corresponding resonance angle for DW was found to be 53.6592\({}^{\circ}\). Next, the surface-functionalization of MPA/V-CNCs-CTA manages to increase the resonance angle of each Ni(II) ion concentration detection, which was again associated with changes in the refractive index of the sensing medium. The increase in the resonance angles corresponding to 2 ppb, 4 ppb, 6 ppb, 8 ppb, 10 ppb, 20 ppb, and 50 ppb of Ni(II) ion was measured from the resonance angle of DW, yielded a value as shown in Table 1. The increase in the resonance angle was mainly due to the attachment of the Ni(II) ion onto the sensing surface. This means that when the sensing layer was exposed to the Ni(II) ion solution, the chemical absorption will induce a change in the sensing layer's refractive index, which changes the value of the resonance angle. It is believed that the change of resonance angle's value was associated with the change of the refractive index real part of the sensor surface [55; 56; 57]. It is also worth noting that the change in reflected intensity was associated with the change in the imaginary part and thickness of the sensor layer as the Ni(II) ion concentrations were varied.  
To validate this observation, the experimental reflectance curves were then fitted using the WinSpall program (Figure 4). The complex refractive index of the gold layer was obtained when contacted deionized water (refractive index = 1.333), with n and k values of  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Ni(II) ion Concentration**} & \multirow{2}{*}{**Resonance Angle (Degree)**} & \multicolumn{1}{c}{**Resonance Angle Increment**} \\ \multicolumn{1}{c}{**(ppb)**} & & **(Degree)** \\ \hline
0 & 53.6592 & 0 \\
2 & 53.7502 & 0.0910 \\
4 & 53.9389 & 0.2797 \\
6 & 54.1210 & 0.4618 \\
8 & 54.2120 & 0.5528 \\
10 & 54.3031 & 0.6439 \\
20 & 54.3941 & 0.7349 \\
50 & 54.4982 & 0.8390 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The increment of resonance angle for Au/MPA/V-CNCs–CTA thin film when detecting Ni(II) ion solution ranged from 2 ppb to 50 ppb.  
Figure 3: The SPR reflectance curves of Au/MPA/V-CNCs–CTA thin film when detecting Ni(II) ion solution ranged from 2 ppb to 50 ppb.  
0.1750 + 3.6796i and the thickness of 47 nm. This value is in good agreement with the one reported in [58]. The determination of these values thus allows further finding for refractive index real part and imaginary part and the thickness of the V-CNCs-CTA sensing layer. In this case, it is good to conclude that the real part of the refractive index of the sensing layer increases as the concentration of Ni(II) ion increases, which is contrary to the imaginary part. This is due to the strong interaction between the evanescent fields and sensing medium, which in turn increases the real part of the wave vector of the surface plasmon wave (ksp). The real part of ksp will be large when the refractive index is large to meet the resonance condition at the larger valuer of the resonance angle [59; 60]. Similarly, the observed effect of the binding can be attributed to the increase in layer thickness, as shown in Table 2. On the other hand, the imaginary part of the refractive index is slightly reduced, leading to less energy loss due to electronic damping. In this work, the formation of V-CNCs-CTA clearly remarkable changes in the reaction of Ni(II) ion, which was probably due to the easy oxidization of vanadium salts in the presence of modified cellulose nanocrystals. This is because the reaction between vanadium salts and a reducing agent such as cellulose nanocrystals could generate heat and products that may be flammable, combustible, or otherwise reactive to give vanadium oxide.