To determine if a fish is fresh, people usually look at its fish eyes and gills, or smell its scent. But a more accurate method is to search for compounds that appear when fish first start to spoil. According to the latest issue of ACS Sensors magazine, an international research team led by Monash University in Australia has developed an electronic sensor based on microneedles that can determine the freshness of fish by detecting the level of hypoxanthine (HX) in their bodies within two minutes, providing a new tool for food quality and safety testing. The signs of fish spoilage may take hours or even days to manifest, and compounds like HX begin to form almost immediately after the fish die. Therefore, HX can be used as a reliable indicator to determine the freshness of fish. At present, measuring HX requires a time-consuming process and specialized laboratory equipment. To make HX detection simpler and more portable, the research team designed a 4 × 4 microneedle array sensor and coated the microneedle surface with gold nanoparticles and enzymes that can decompose HX. When in use, press the sensor onto the surface of the fish body, fix the microneedle device, and the potential changes caused by enzyme decomposition of HX will be captured in real time by the sensor to determine the freshness of the fish. The research team conducted a validation experiment using thick cut salmon slices, and the fish meat was left at room temperature for up to 48 hours. Experiments have shown that the sensor can detect HX concentrations as low as one in 50 billion, corresponding to "very fresh" fish samples. The detection results can be obtained in about 100 seconds, and the sensitivity is comparable to existing laboratory detection kits. The research team stated that although further development is needed for this sensor to be put into portable applications, experimental results show its potential in real-time food quality monitoring. (New Society)