Abstract: With the continuous expansion of the global food supply chain and the accelerated process of integration and increasing consumer demand for food safety and quality, the development of efficient and sensitive real-time food freshness monitoring technologies has become a significant issue in the food industry and scientific research. Fish, especially nutrient-rich species like salmon, are highly perishable due to their high moisture content. During the spoilage process, they release volatile biogenic amines such as putrescine and cadaverine, which are closely correlated with Total Volatile Basic Nitrogen (TVB-N) the core indicator for assessing freshness. Therefore, among various detection targets, amine compounds serve as key volatile biomarkers produced during the spoilage of protein-rich foods such as fish, with their concentrations closely correlated to freshness levels. Traditional detection methods have limitations such as being destructive, time-consuming, and involving complex sample pretreatment, which restricts the feasibility of their on-site application. Therefore, it is of great significance to realize rapid and visualized detection of amines to ensure food safety and reduce resource waste. In recent years, multimodal sensing technology has demonstrated considerable potential in key stages of food production, storage, and transportation due to its ability to provide cross-validated signals and enhance detection reliability. Compared with traditional single-mode detection methods, sensing strategies integrating colorimetric and fluorescent dual responses not only allow preliminary visual assessment but also enable precise quantification using portable devices, offering new pathways for developing next-generation intelligent packaging and real-time monitoring systems. In this study, a novel chemical sensor ((E)-5-(4-hydroxy-3,5-dimethoxybenzylidene)thiazolidine-2,4-dione, HTYW) was successfully synthesized using syringaldehyde and 2,4-thiazolidinedione as raw materials, and then immobilized onto filter paper or polyvinyl alcohol (PVA) to fabricate sensing labels, which were further integrated with a smartphone-based sensing platform for the freshness monitoring of salmon. The results demonstrate that the probe exhibits a rapid dual-mode response (4 s), high sensitivity, excellent pH adaptability (4-13), and a wide linear response range (0-300 μmol/L for diethylamine) toward 14 kinds of amine solutions.To further advance its practical implementation, HTYW was integrated into two distinct substrate formats, filter paper and polyvinyl alcohol (PVA) films, resulting in versatile sensing labels tailored for diverse environmental conditions. The porous nature of filter paper facilitates rapid vapor diffusion, ideal for quick screening, while the PVA matrix offers enhanced mechanical stability and humidity resistance for prolonged monitoring. Experimental results demonstrated that both labels exhibit pronounced and discernible colorimetric changes under ultraviolet and ambient light, thereby enabling straightforward, naked-eye visual assessment of fish freshness levels. This straightforward visual output significantly lowers the technical barrier for on-site use. Building upon this, the study further developed a comprehensive portable detection system by integrating a smartphone-based platform with advanced image recognition and real-time data processing algorithms. This integrated system effectively addresses the long-standing limitations of conventional detection methods—such as their destructive nature, operational complexity, and lack of portability—providing a reliable, user-friendly tool for on-site monitoring throughout the food supply chain, from production and storage to transportation. Its practical utility and broad applicability underscore its significant potential in both food safety assurance and smart material applications. Beyond its primary role in food monitoring, HTYW also demonstrates considerable promise as a functional material in interdisciplinary fields. It can be readily processed into flexible films, printable inks, and patternable substrates for applications in advanced anti-counterfeiting and secure information storage. This dual-purpose functionality not only bridges the gap between food science and materials engineering but also paves new avenues for the development of next-generation intelligent packaging and sophisticated anti-counterfeiting technologies. Ultimately, the experimental outcomes verify that the HTYW-based sensing system successfully achieves non-destructive, efficient, and rapid assessment of fish freshness via robust dual-mode signaling, highlighting its prospective utility not only in real-time food safety surveillance but also across a spectrum of interdisciplinary applications.