## 3 Results and Discussion
### Photocatalytic Degradation Performance  
Figure 8a shows the absorbance of RhB by the LOC device with different gold film deposition thicknesses for a photocatalytic reaction time of 1 min. The peak absorbance after photocatalytic degradation was 550 nm. Thus, from comparing the peak values at 550 nm, the photocatalytic degradation performance of different LOC detection devices could be discerned. The experimental results (Figure 8b) show that the deposition of gold films enhanced the photocatalytic degradation efficiency compared to that attained using pure TiO\({}_{2}\). When the photocatalytic reaction time was 1 min, the performance difference between the various LOC detection devices was the most pronounced. As the photocatalytic reaction time increased, this gap gradually decreased, because each detection device demonstrated an almost complete degradation of RhB. When the photocatalytic time was 6 min, RhB was completely degraded, and there was no significant difference in the photocatalytic degradation performances of the LOC device with different noble metal film deposition thicknesses. However, regardless of the photocatalytic reaction period, the LOC detection device with a metal film deposition thickness of 2 nm exhibited the highest photocatalytic degradation efficiency. This observation indicates that when the thickness of the gold film is 2 nm, the UV light absorption and interface of the TiO\({}_{2}\) and gold nanoparticles are optimized, and the highest photocatalytic degradation performance can be achieved. With further increases in the deposition thickness of the gold film, the photocatalytic efficiency decreased, which is attributable to a reduction in the direct contact surface area of the gold particles and TiO\({}_{2}\), lowering the reaction rate. Additionally, multilayer stacking reduced light absorption and utilization. The transmittance test results of TiO\({}_{2}\)/Au nanocomposite film (Figure 9) confirm these observations. Although noble metal deposition can enhance the photocatalytic performance of TiO\({}_{2}\), it was necessary to consider the influence of its preparation method, film deposition thickness, crystal structure, and the size and shape of noble metal particles on the photocatalytic degradation performance of TiO\({}_{2}\)[11]. Figure 8: (**a**) Absorbance of RhB when the photocatalytic reaction time was 1 min, and (**b**) RhB degradation graph at 550 nm with different LOC devices under different photocatalytic reaction durations. Figure 10a shows the absorbance of a standard solution with different PO4\({}^{3-}\) concentrations (0.0, 0.1, 0.5, 1, 2 mg/1000 mL) after coloring. With the increase in the PO4\({}^{3-}\) concentration, the absorbance gradually increases, verifying the positive correlation between PO4\({}^{3-}\) concentration and absorbance. Figure 10b, showing the images of different PO4\({}^{3-}\) solution concentrations after coloring, further confirms the absorbance test results discussed above. Moreover, the peak absorbance of the PO4\({}^{3-}\) solution after coloring was mainly 880 nm; thus, the concentration of the PO4\({}^{3-}\) solution can be determined by comparing the absorbance values at 880 nm. Overall, the photocatalytic degradation experiment results of RhB indicate that when the deposition thickness of the noble metal film is 2 nm, the TiO2/Au nanocomposite film exhibits the highest photocatalytic degradation efficiency. Moreover, the absorbance test of PO4\({}^{3-}\) standard solution with different concentrations verifies the positive correlation between the PO4\({}^{3-}\) concentration and absorbance.