## 3 Results and Discussion
### Photocatalytic Degradation Performance
Figure 11a shows the absorbance of the photocatalytic degradation of a phosphorous-containing solution with a TP concentration of 1 mg/1000 mL under different photocatalytic reaction durations, using an LOC device with a gold-film deposition thickness of 2 nm  
Figure 10: (**a**) Measured absorbance of standard solution with different concentrations of phosphate and (**b**) images of solutions with different phosphate concentrations after coloring. Figure 9: Transmittance of TiO2 and TiO2/Au nanocomposite films with different deposition thicknesses of gold films. and an autoclave. The peak absorbance at 880 nm is shown in Figure 11b. The absorbance increased with the increase in photocatalytic reaction time. This observation indicates that, with the increase in the photocatalytic reaction time, more phosphorus-containing compounds decompose into PO4\({}^{3-}\). However, when the photocatalytic time was greater than 25 min, the absorbance became saturated. This finding suggests that, at 25 min, the phosphorous-containing compounds with a concentration of 1 mg/1000 mL were completely decomposed, and that further prolonging the photocatalytic reaction time does not increase the PO4\({}^{3-}\) concentration. From the absorbance test results of the phosphorus-containing compound solutions with a TP concentration of 1 mg/1000 mL under different photocatalytic reaction durations, we can conclude that the LOC test device with a gold-film deposition thickness of 2 nm could replace conventional TP pretreatment equipment (autoclave) when the photocatalytic reaction time is 15 min. Therefore, an LOC test device with a gold-film deposition thickness of 2 nm was used to pretreat a phosphorous compound with concentrations ranging from 0 mg/1000 mL to 1 mg/1000 mL for a photocatalytic reaction duration of 15 min, and the results have been compared with those obtained using an autoclave. The experimental results (Figure 12) show that when the duration is 15 min, a LOC device with a deposition thickness of 2 nm can replace the conventional TP pretreatment equipment (autoclave). The measured absorbance with variations in phosphorus concentration using the LOC test device with a gold film thickness of 2 nm exhibits similar trends to those of the conventional equipment (autoclave). In general, the variations in concentration and absorbance corresponding to the LOC device and autoclave can be expressed by the following equations:  
\[\text{Autoclave: }y_{\text{(Absorbance)}}=0.18427x\text{ (TP Concentration)}+0.00318 \tag{5}\]  
\[\text{LOC Device: }y_{\text{(Absorbance)}}=0.18384x\text{ (TP Concentration)}+0.00391 \tag{6}\]  
Thus, the phosphorus concentration of an unknown analyte can be calculated using the LOC device, based on the provided equation relating the phosphorus concentration to absorbance. The results from the two different TP pretreatment experiments show that the LOC detection device is a promising substitute to the traditional autoclave equipment, effectively addressing the limitations of high cost, large volume, and prolonged response time  
Figure 11: (**a**) Measured absorbance of phosphorus-containing analyte using the autoclave and LOC device under different photocatalytic reaction durations and (**b**) peak values at 880 nm. associated with traditional equipment. Compared to traditional autoclaves, the LOC device is cost-effective and compact, in line with the increasing demand for on-site testing.