## 4 Conclusions  
This paper introduces a TiO\({}_{2}\)/Au deposition method to address the challenges related to gold film detachment and the oxidation of gold particles during experiments. The Schottky barrier formed by the direct connection of metal nanoparticles and TiO\({}_{2}\) film helped inhibit the recombination of electron-hole pairs [27; 28; 47]. The use of back irradiation increased the utilization rate of incident light and avoided the potential obstruction effects of PDMS, TP, or RhB reagents on incident light.  
Subsequently, different heat-treatment temperatures were applied to the deposited gold film and TiO\({}_{2}\) film to minimize the influence of TiO\({}_{2}\) heat treatment on the size and distribution of the gold nanoparticles. Lastly, the photocatalytic degradation performance of TiO\({}_{2}\) and its enhancement by gold-film doping were verified using RhB dye and K\({}_{2}\)S\({}_{2}\)O\({}_{8}\) oxidizing agent. A comparative analysis revealed that the photocatalytic performance of TiO\({}_{2}\) is enhanced the most when the doping thickness of gold films was 2 nm. A further increase in the deposition thickness of the gold film deteriorated the photocatalytic efficiency, owing to a reduced direct contact surface between the gold particles and TiO\({}_{2}\), which reduced the reaction rate. Moreover, multilayer stacking reduces light absorption and utilization [48; 49].  
An LOC test device with a deposition thickness of 2 nm and an autoclave were used to pretreat phosphorus-containing compounds with identical concentrations. The photocatalytic time was modified by adjusting the flow rate of the micropump, and the absorbance of the pretreated sample under different photocatalytic durations was compared with that of the sample pretreated by the conventional method (autoclave). The results indicate that when the photocatalytic reaction time is 15 min, the TP pretreatment with the LOC device achieves a pretreatment performance that is nearly identical to the traditional autoclave method. Finally, the results obtained by the LOC device (reaction time: 15 min) for pretreating phosphorus-containing solutions with concentrations ranging from 0 mg/1000 mL to 1 mg/1000 mL were compared with those of the autoclave. The experimental results further verify the feasibility of the LOC equipment replacing traditional autoclave equipment.  
Figure 12: Comparative results of the LOC device and autoclave used to determine the relationship between the TP concentrations and absorbance. (wavelength = 880 nm; photocatalytic reaction time of 15 min).  
Notably, although noble metal deposition can enhance the photocatalytic performance of TiO2, various factors such as the preparation method, film deposition thickness, crystal structure, and the size and shape of the noble metal particles also significantly influence the photocatalytic degradation performance of TiO2 and must be considered.  
Conceptualization, J.W. and S.-D.K.; Formal analysis, J.-Y.L. and J.-S.K.; Investigation, J.W.; Supervision, S.-D.K., M.H. and S.-H.K.; LOC device fabrication, N.J. and H.K.; Writing--Review and editing, J.W. and S.-H.K.; Sample analysis, D.-Y.K. and Y.N. All authors have read and agreed to the published version of the manuscript.  
This research was supported by The Korea Innovation Foundation (INNOPOLIS) grant funded by the Korea government (MSIT) (2020-DD-UP-0348), and Ministry of Environment as \"The Eco-technopia 21 projects\" (RS-2023-00218010), and the BK21 FOUR project funded by the Ministry of Education, Republic of Korea (4199990113966).  
Not applicable.  
Data availability Statement: Data are contained within the article.  
The authors declare no conflicts of interest.