Salmon is one of the most favorite nutrient fish species in the food industry. It is often required for efficient and sensitive real-time freshness monitoring, due to the highly perishable fish with high moisture content. Total Volatile Basic Nitrogen (TVB-N) is one of the indicators to assess the fish freshness, because the volatile biogenic amines can be released during spoilage, such as putrescine and cadaverine. Therefore, amine compounds can serve as the key volatile biomarkers for the freshness levels among various targets. However, conventional detection has restricted the on-site or single-mode application, such as a destructive nature, time-consuming, complex sample pretreatment, and less portability. It is of great significance to rapidly and visually detect the amines for national food safety. Multimodal sensing can be expected to apply to the key stages of food production, storage, and transportation, due to cross-validated signals and high reliability. Sensing strategies can also be integrated with the colorimetric and fluorescent responses to visually assess and precisely quantify the salmon freshness using portable devices. This study aims to synthesize the chemical sensor ((E)-5-(4-hydroxy-3,5-dimethoxybenzylidene) thiazolidine-2,4-dione, HTYW) using syringaldehyde and 2,4-thiazolidinedione as raw materials, and then immobilized onto filter paper or polyvinyl alcohol (PVA) to fabricate sensing labels. Furthermore, a smartphone sensing platform was integrated to monitor the freshness of salmon. The results demonstrate that the probe exhibited 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. In practical implementation, HTYW was further integrated into two substrate formats, filter paper and polyvinyl alcohol (PVA) films, thus resulting in versatile sensing labels for diverse environmental conditions. The porous nature of filter paper also facilitated the vapor diffusion, ideal for rapid screening. While the PVA matrix was used to enhance mechanical stability and humidity resistance for prolonged monitoring. Experimental results showed that both labels exhibited pronounced colorimetric changes under ultraviolet and ambient light, thereby enabling straightforward, naked-eye visual assessment of fish freshness levels. The visual output significantly lowered the technical barrier for on-site use. A portable system was further developed to integrate a smartphone platform with advanced image recognition and real-time data processing. Ultimately, the experiment verified that the HTYW sensing system achieved a non-destructive, efficient, and rapid assessment of fish freshness via robust dual-mode signaling. A reliable, user-friendly tool was also provided for on-site monitoring from production and storage to transportation in the food supply chain. Its practical utility and broad applicability also underscore the significant potential in both food safety assurance and smart material applications. Furthermore, the HTYW also demonstrated considerable promise as a functional material in food monitoring. It can be readily processed into flexible films, printable inks, and patternable substrates for advanced anti-counterfeiting and secure information storage. The dual-purpose function can bridge the gap between food science and materials engineering in intelligent packaging and anti-counterfeiting technologies. The finding can also highlight its prospective utility in real-time food safety surveillance in interdisciplinary applications.

In order to make up for the misjudgment of fish freshness by colorimetry due to the unclear color distinction, a naphthol-benzothiazole derivative (HBO) was synthesized by the condensation reaction of naphthol as a fluorophore with benzothiazole salt. HBO could recognize organic amines by colorimetric and fluorescent responses, and allow nondestructive detection of salmon freshness. HBO showed an obvious response for all 12 amines tested in an EtOH/H₂O (1:9, V/V) system, had a low limit of detection (0.4 μmol/L for spermine), and could image for spermine in living cells. The sensing gel HBO agarose gel (HBOAL) was prepared by loading the probe onto agarose gels. When salmon flesh is stored with HBOAL, the fish can be judged to be spoiled if HBOAL exhibits a light red color in daylight and a yellow color in ultraviolet light. HBOAL distinguished clearly among the three grades of salmon fish freshness with accurate evaluation results, and therefore could be used to monitor the freshness of salmon fish in real time.