## 1 Introduction
The phosphorus compounds and oxidizing agent are introduced into the LOC testing device via a micropump. The photocatalytic reaction time is controlled by adjusting the flow rate of the micropump. Finally, the absorbance of samples subjected to different photocatalytic reaction durations are measured through UV-vis spectrophotometry. TiO\({}_{2}\)--an important semiconductor material with applications in various fields, such as photocatalytic water splitting for hydrogen production, sensors, solar cells, self-cleaning  
Figure 1: Process flow of total phosphorus (TP) analysis using an autoclave and an LOC device. coating, degradation of organic pollutants, and TP or total nitrogen photocatalyst--has attracted substantial research attention owing to its non-toxic, strong oxidation properties, earth abundance, and stable chemical structure [22; 23; 24]. However, the wide bandgap (3.2 eV) of TiO\({}_{2}\) limits the separation and transport efficiency of its photogenerated carriers. Moreover, the rapid recombination of photogenerated carriers restricts the photocatalytic activity of TiO\({}_{2}\). Considering these aspects, it has become necessary to identify techniques that will improve the photocatalytic performance of TiO\({}_{2}\). At present, doping with noble metals (e.g., Au, Ag, and Pt) is the most prevalent method of improving the photocatalytic performance of TiO\({}_{2}\)[25]. Therefore, in this work, TiO\({}_{2}\)/Au nanocomposite film test devices with different gold film thicknesses were prepared through various methods, such as electron beam (e-beam) evaporation and atomic layer deposition (ALD). The photocatalytic activity of TiO\({}_{2}\) was enhanced through the synergistic activity of TiO\({}_{2}\) and gold nanoparticles [26; 27]. The degradation properties of TiO\({}_{2}\)/Au nanocomposite film under different photocatalytic reaction durations were evaluated using rhodamine B (RhB) as a pollutant in microfluidic channels composed of polydimethylsiloxane (PDMS). The principle of the photocatalytic reaction of TiO\({}_{2}\)[28] can be explained as follows: When incident light with photon energy greater than the bandgap width of TiO\({}_{2}\) irradiates the sample surface, it leads to the formation of electron-hole pairs which cause redox reactions to occur with the substances on the surface. Electrons combine with the oxygen molecules dissolved in water to produce superoxide ion-free radicals (\(\bullet\)O\({}_{2}\)\({}^{-}\)), while holes convert water molecules or hydroxide ions adsorbed on the TiO\({}_{2}\) surface into hydroxyl radicals (\(\bullet\)OH). Superoxide ions and hydroxyl radicals are highly oxidizing and can decompose the organic compounds in water into inorganic compounds such as carbon dioxide (CO\({}_{2}\)) and water (H\({}_{2}\)O) [29; 30]. Chemical equations representing the roles of the electron-hole pairs are presented below. \[\text{TiO}_{2}+\text{hv}=>\text{Electrons (e${}^{-}$)}+\text{ Holes (h${}^{+}$)} \tag{1}\]  
\[\text{Electrons (e${}^{-}$)}+\text{O}_{2}=>\bullet\text{O}_{2}{}^{-} \tag{2}\]  
\[\text{Holes (H${}^{+}$)}+\text{H}_{2}\text{O}=>\text{H}^{+}+\bullet\text{OH} \tag{3}\]  
\[\text{Holes (H${}^{+}$)}+\text{OH}^{-}=>\bullet\text{OH} \tag{4}\]  
The synergistic reaction of doped TiO\({}_{2}\) and gold nanoparticles is shown in Figure 2. In the TiO\({}_{2}\)/Au nanocomposite film, electrons migrating to valence bands flow from TiO\({}_{2}\) to gold nanoparticles due to the energy band difference between them, thereby increasing carrier concentration and impeding their recombination. In addition, gold nanoparticles and possible defect sites at the interface of TiO\({}_{2}\) promote electron migration. Figure 2: Schematic of the pollutant degradation mechanism of TiO\({}_{2}\)/Au nanocomposite film under UV irradiation. The LOC pretreatment equipment discussed in this article aims to employ photocatalytic technology as a substitute for conventional autoclaves.