## 2 Materials and Methods  
### Design and Fabrication of the LOC Test Device  
Figure 3a shows the top view of the pre-designed microchannel for TP pretreatment, including a mixing channel and pretreatment channel. The length and width of the microchannel, excluding the input-output ports for connecting hoses, are 37 mm and 31 mm, respectively. The width and height of the mixing and pretreatment channels are 1 mm and 0.1 mm, respectively. To ensure thorough mixing of the oxidant and analyte, square obstacles with a width of 0.3 mm were placed within the mixing channel [20]. Two input ports in the mixing channel section facilitate the entry of the analyte and oxidant (K\({}_{2}\)S\({}_{2}\)O\({}_{8}\)), and an output port at the end of the pretreatment channel enables the collection of liquid after the photocatalytic reaction. Figure 3b presents a schematic of the photocatalytic reaction occurring within the microchannel. The analyte fed to the input port gradually flows towards the output under the influence of the micropump. When UV light with a wavelength of 365 nm irradiates the TiO\({}_{2}\)/Au nanocomposite film, the separated electrons react with oxygen molecules to produce superoxide ions (\(\bullet\)O\({}_{2}\)\({}^{-}\)), while the holes react with water molecules to form hydroxyl radicals (\(\bullet\)OH). The superoxide ions and hydroxyl radicals, as critical intermediates of photocatalytic reactions, significantly affect photocatalytic efficiency [31; 32]. Before preparing the microchannel, a PDMS mold was developed using a 6 in Si substrate using the photolithography method [19]. The fabrication processes of the PDMS mold and PDMS are illustrated in Figure 4a. First, a SU-8 2075-negative photoresist (PR) was used to prepare a PR coating on the Si substrate through the spin-coating method. Subsequently, the PR coating was soft-baked at different temperatures for different duration  
Figure 3: (a) Top view of the pre-designed microchannel of the LOC device and (b) schematic of the photocatalytic microfluidic reactor. tions. Next, the baked PR was exposed to a pre-designed photomask plate and subjected to post-exposure baking at different temperatures for different durations. Finally, the SU-8 developer was used to develop the baked PR for 10 mins (or more) to remove the PR outside of the exposure area, forming a microfluidic channel mold. After obtaining the PDMS mold, it was wrapped in tinfoil, and a mixture of 44 g of PDMS polymer base and curing agent (mixing ratio: 10 to 1) was poured into the mold. A 20 min vacuum treatment was implemented to remove the bubbles in the mixture. Next, the PDMS solution was baked at 70 \({}^{\circ}\)C for 2 h to transform the liquid into a transparent solid. The transparent solid PDMS layer was peeled off and cut to obtain a pattern inverse to that of the PDMS mold. The height and width of the microchannel are 0.1 mm and 1 mm, respectively. Inlet and outlet holes were punched in predetermined locations. The process of fabricating the TiO\({}_{2}\)/Au nanocomposite film is illustrated in Figure 4c. The pre-deposited 6 in quartz substrate was cleaned using acetone, isopropyl alcohol, and deionized water. Subsequently, the dried, contamination-free quartz substrate was placed in an e-beam evaporation chamber (SORONA SRN-200, Gyeonggi-do, Republic of Korea), and gold films with thicknesses of 1, 2, 3, and 4 nm were deposited. The deposited gold films were placed in a heating furnace for high-temperature annealing (600 \({}^{\circ}\)C, 1 h). This process transformed the shape of the deposited gold particles from an island-like shape to a nearly spherical one, thereby increasing the average interparticle distance [24].