# Cellulose and Vanadium Plasmonic Sensor to Measure Ni\({}^{2+}\) Ions
## 3 Results and Discussion  
### Sensor Surface Functionalization  
Figure 2 shows the comparison of the SPR reflectance curve observed after the introduction of Ni(II) ion at two different concentrations, i.e., 8 ppb and 10 ppb. These concentrations were chosen as they represent the concentrations that are near the detection limit obtained from previous work [36]. First, from Figure 2a, there was no increment in resonance angle when a bare Au film interacted with two respective concentrations of  
Figure 1: Schematic of the surface plasmon resonance device. Ni(II) ion. This is because the refractive indices of 8 and 10 ppb of Ni(II) ion concentrations were almost similar to the refractive index of deionized water (DW), thus did not change the refractive index of the metal surface. A similar phenomenon was observed for MPA-self-assembled monolayer-modified Au film when tested with Ni(II) ion concentration, as depicted in Figure 2b. When the V layer assisted the Au film surface (Figure 2c), an increase in resonance angle was observed upon introduction of the first lowest concentration of Ni(II) ion, 8 ppb. However, when 10 ppb of Ni(II) ion was introduced to the prior Au/V thin film, there was no increment in resonance angle. This was due to the unsuccessful binding of Ni(II) ion due to no interaction force between Ni(II) ion and the sensing layer, thus tends to cause the degradation of the sensing layer after twice introduction of Ni(II) ion solution. Next, the Au/CNC and Au/CNC-CTA thin film were also tested with 8 ppb and 10 ppb of Ni(II) ion. The obtained reflectance curve, as shown in Figure 2d,e, respectively, resulting in a very small increase in resonance angle when detecting low concentrations of Ni(II) ion. Figure 2: _Cont._  
Compared to the individual V, CNCs, CNCs-CTA, the composite layer of V-CNCs and V-CNCs-CTA was done. Figure 2f shows no increment in resonance angle with the addition of CNCs, indicating the non-functional of CNCs onto Au/V thin film because of electrostatic repulsion of the resultant positive charge of V-CNCs composite against Ni(II) ion. On the other hand, V-CNCs-CTA caused a small increase in resonance angle upon the binding of 8 ppb of Ni(II) ion (Figure 2g). However, an introduction of 10 ppb of Ni(II) ion, a similar resonance angle value was observed. This suggests that the interaction of Ni(II) ion does not change the refractive index of Au/V-CNCs-CTA thin film. It is, therefore, likely that a self-assembled monolayer of MPA is required to support the deposition of a thin sensing layer for increased surface refractive index. The first self-assembled monolayer of MPA for the development of Au/MPA/V-CNCs gives a small increase in resonance angle for detection of 8 ppb and 10 ppb of Ni(II) ion (Figure 2h). Yet, with the modification of CNCs with CTA, the Au/MPA/V-CNCs-CTA shows a greater increase in resonance angle to the higher value due to the excellent plasmonic enhancement (Figure 2i). The interactions with 8 ppb of Ni(II) ion successfully resulted in an increase in resonance angle of 0.534\({}^{\circ}\) from the reference signal and again, increased further by 0.655\({}^{\circ}\) with the addition of 10 ppb of Ni(II) ion. The increase in resonance angle was proportional to the concentration of Ni(II) binding to the surface functionalization of MPA/V-CNCs-CTA on the gold film. This phenomenon, also relevant to the conformational change in the analytes bound to the sensor film, suggests the increase in the thickness of the absorbed sensor film. Therefore, the self-assembled monolayer of MPA with the attached layer of V-CNCs-CTA showed a better performance than other thin films. Figure 2: The surface plasmon resonance (SPR) reflectance curve for (**a**) Au; (**b**) Au/3-mercaptoethanol-topropionic acid (MPA); (**c**) Au/V; (**d**) Au/cellulose nanocrystals (CNCs): (**e**) Au/CNCs– hexadecyltrimethylammonium bromide (CTA); (**f**) Au/V-CNCs; (**g**) Au/V–CNCs–CTA; (**h**) Au/MPA/V-CNCs; (**i**) Au/MPA/V-CNCs-CTA thin film when detecting 8 ppb and 10 ppb of Ni(II) ion solution.