## 1 Introduction  
With progress in human industrialization and urbanization, although human living standards have continually improved, challenges such as environmental pollution, resource depletion, and ecosystem destruction have emerged. Phosphorus, a crucial trace element in plant photosynthesis, plays a vital role in maintaining ecological balance [1]. However, the accelerated pace of human industrialization has disrupted the balance of trace elements in the ecosystem. In particular, the discharge of phosphorus-containing substances into rivers and lakes has led to frequent occurrences of algal bloom and water eutrophication [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. Given these concerns, it has become essential to monitor the concentration of total phosphorus (TP) in aquatic ecosystems to prevent eutrophication. As phosphorus typically exists in nature in compound forms, it is challenging to directly detect the TP concentration. This necessitates the conversion of organic and inorganic compounds containing phosphorus into easily detectable phosphates (PO4\({}^{3-}\)) through pretreatments in advance, a process known as TP pretreatment. Phosphorus-containing compounds are separated into phosphates via pretreatment and colored blue by a coloring agent. The concentration of TP is determined by measuring the absorbance of the blue compounds post-dyeing. As mentioned above, as a crucial stage in the TP analysis process, pretreatment significantly influences the accuracy and reliability of TP concentration testing. Currently, phosphorus-containing compounds are pretreated using a conventional piece of pretreatment equipment called an autoclave. Autoclave pretreatment necessitates operation under high temperature and pressure conditions. Given its substantial size and considerable cost, it is primarily employed in laboratories, research institutes, and similar facilities [16; 17; 18]. Compared with the conventional method of using an autoclave for pretreatment, photocatalysis technology has emerged as a promising solution to address the above-mentioned challenges. Through photocatalytic reactions using potassium persulfate (K\({}_{2}\)S\({}_{2}\)O\({}_{8}\)) as the oxidizing agent, organic and inorganic compounds containing phosphorus can be pretreated and transformed into easily detectable PO\({}_{4}\)\({}^{3-}\), and the phosphorus content in water can be monitored through changes in the PO\({}_{4}\)\({}^{3-}\) concentration. The process flows of the two types of TP pretreatment and measurement methods are shown in Figure 1. Both types of method include the pretreatment and measurement of TP samples that differ mainly in terms of TP pretreatment strategy. The first type of approach uses a conventional autoclave to pretreat the organic and inorganic compounds containing phosphorus and transform them into easily detectable PO\({}_{4}\)\({}^{3-}\) under high temperature (121 \({}^{\circ}\)C) and high pressure (1.1 kg/cm\({}^{2}\)) conditions, adopting K\({}_{2}\)S\({}_{2}\)O\({}_{8}\) as the oxidizing agent. This conventional TP pretreatment method requires that the sample is oxidized for least 30 mins under harsh experimental conditions (high temperature and high pressure). The alternative TP pretreatment method involves a portable and inexpensive LOC device prepared using micro-electro-mechanical system technology [19; 20; 21]. Using the principle of photocatalysis, a K\({}_{2}\)S\({}_{2}\)O\({}_{8}\) oxidizing agent is used to pretreat phosphorus-containing compounds and transform them into PO\({}_{4}\)\({}^{3-}\) under UV (365 nm) light irradiation. Owing to its high photocatalytic activity, titania (TiO\({}_{2}\)) is widely used in the field of photocatalysis. After pretreatment, a mixture (colorant) containing ammonium mybdate (NH\({}_{4}\))\({}_{6}\)Mo\({}_{7}\)O\({}_{24}\).4H\({}_{2}\)O) and ascorbic acid (C\({}_{6}\)H\({}_{8}\)O\({}_{6}\)) is added to stain the samples. As the PO\({}_{4}\)\({}^{3-}\) concentration increases, the color of the dyed sample transitions from light blue to dark blue.