## 1 Introduction  
Given the direct impact of fish quality on consumer health, the assessment of fish quality warrants significant attention. Moreover, the growing consumption of fish globally [1], coupled with increasing concerns about food hygiene and safety among both regulatory bodies and consumers, underscores the urgency of addressing the challenges in fish quality control. According to statistics from the Institute of Marine Research, Vietnam's fishery production from 2011 to 2015 included 493.9 thousand tons of mackerel, 42.6 thousand tons of tuna, and 344 thousand tons of pompano [2]. Thus, it is evident that tuna, mackerel, and pompano are important seafood products in Vietnam's fishing grounds. These three types of fish are used for both domestic markets and export, making them crucial to enhancing quality monitoring [2]. Urea is a type of chemical fertilizer used in agriculture to increase nitrogen content for crops and is not a food preservative. However, in Vietnam, urea is mixed with ice to preserve seafood because when urea dissolves in water, the water becomes cold due to an endothermic reaction, thereby keeping the fish meat fresh for longer. The misuse of urea in preservation arises from several factors: prolonged sea trips, insufficient ice supplies, ease of accessibility and use, low cost of urea, and a lack of knowledge and awareness and poor attitudes of those involved in the seafood supply chain regarding food safety [2]. These reasons create the risk of urea contamination in seafood. Urea is not included in the list of food additives permitted for use in food issued under Circular No. 27/2012/TT-BYT [3], the Circular on food additive management published by the Ministry of Health of Vietnam in 2012, nor is it included in the Codex General Standard for Food Additives (GSFA, Codex STAN 192-1995) [4]. Regular consumption of food containing urea, even in low amounts, can lead to chronic poisoning with symptoms such as prolonged insomnia, headaches, bodyaches, memory loss, frequent cramps, loss of appetite leading to malnutrition, intestinal ulcers, and imbalances in calcium and phosphorus causing osteoporosis [5]. Eating food with high urea residue can result in acute poisoning with symptoms like abdominal pain, nausea, diarrhea, difficulty breathing, heart failure, and arteriosclerosis, which can lead to death [5]. Standard methods for analyzing urea in the laboratory are often time-consuming, sample-destructive, expensive, and require skilled personnel [6]. To promptly detect urea in seafood at markets, rapid analysis methods are needed to screen samples at risk of urea contamination without requiring sample destruction and complicated sample preparation. To the best of our knowledge, no research has been conducted on rapid and non-destructive analysis methods specifically for determining urea content in raw fish samples. We also find that most of the currently available rapid test kits for urea concentration are not specifically designed for fish samples. For example, the Urea/Ammonia Assay Kit (Rapid) produced by Neogen's Megazyme is suitable for the rapid analysis of urea in water, beverages, milk, and food products [7]. This kit utilizes an enzymatic spectrophotometric method for accurate determination of urea concentrations [8]. However, it requires a spectrophotometer and two supplied enzymes, with a reaction time of approximately 10 min [7; 8]. In Vietnam, a rapid detection kit for urea in frozen and fresh fish samples has been developed and commercialized by the Institute of Science and Technology, Ministry of Public Security of Vietnam. This kit, named UT12, has a limit of detection (LOD) of 1000 ppm [9]. While this test kit offers the advantages of being cost-effective and capable of non-destructive urea detection in fish samples, the sample preparation process remains somewhat cumbersome, involving the application of water droplets to the fish body and gills. Additionally, the time required to obtain results is still relatively long, ranging from 1 to 5 min [9].