The increasing global demand for food has sparked global attention to food safety issues. Food is susceptible to various safety hazards, including external factors such as temperature, humidity, and heavy metals, as well as internal factors caused by microorganisms that disrupt normal metabolism. Freshness is an important attribute to consider when evaluating food quality, especially for perishable foods such as meat, seafood, fruits, and vegetables.When food deteriorates, under the action of microorganisms, acid and alkaline substances such as H2S, HCHO, biogenic amines, nitrogen-containing compounds, and harmful acids are produced. Multiple methods have been developed to detect these putrefactive substances, including high-performance liquid chromatography (HPLC), liquid chromatography tandem mass spectrometry (LC-MS), and enzyme-linked immunosorbent assay (ELISA).However, these methods typically rely on large devices and may require preprocessing. By developing new sensing technologies, these limitations can be overcome, making food safety assessments more convenient. As lipids gradually decompose, compounds such as ammonia and amines accumulate and evaporate into the surrounding air, causing changes in the pH value of the aquatic products themselves and their packaging. Therefore, monitoring the pH value of food or the pH value of the air inside food packaging is crucial for ensuring product freshness. The methods based on colorimetry and fluorescence detection have become a promising approach to address these challenges.
In the field of food pH detection, researchers widely use natural pigments and synthetic dyes to develop acid-base indicators. In addition, some of these indicators have been successfully integrated into the new intelligent food packaging, providing non-destructive real-time monitoring of food quality. Especially natural pigments, due to their non-toxic or low toxicity, ensure safety and have significant advantages over dyes.Among them, Alizarin (C14H8O4), as an anthraquinone compound, shows a color change from yellow to red as the acidity level moves towards the alkaline direction. It has been used as an acid-base indicator for pH detection in food and even in smart packaging films. However, Alizarin has certain limitations as it is a fat soluble pigment that is insoluble in water and difficult to react with the pH value of aqueous solutions.Alizarin complex ketone can compensate for this deficiency. It is a yellow brown crystalline powder formed by the reaction of Alizarin and iminodiacetic acid, which is easily soluble in alkaline aqueous solutions. In acidic aqueous solutions, the solubility of Alizarin complex ketone decreases, but it can still meet the detection requirements.
Compared with more accurate colorimetric sensors, fluorescent sensors are low-cost, easy to operate, and easy to carry. Therefore, they have become powerful tools for detecting various chemical substances in food. However, most sensors have slow response speed, low sensitivity, and poor specificity. In addition, some sensors can only sense the pH value in the solution, which limits their applicability in real-time food detection. It is highly necessary to develop pH fluorescence sensors with excellent sensitivity, fast response, and portability for on-site evaluation of food freshness.Metal organic frameworks (MOFs) are porous crystalline materials with three-dimensional structural characteristics, composed of self-assembled metal ions or clusters with organic ligands. Due to their unique advantages, they have attracted great attention in recent decades. These advantages include high specific surface area, rich skeleton structure, excellent crystallinity and activity.The rapid development of technology promotes the integration of MOF and non-destructive spectroscopy, providing a fast, cost-effective, and portable tool for food safety analysis. Among them, fluorescent MOFs have become a promising chemical sensor, which can easily introduce fluorescence and have various advantages in structure, functional components, and various detection mechanisms.However, most fluorescent MOFs are unstable in aqueous systems, especially in buffer solutions. Some MOF sensors have poor correlation between fluorescence intensity and pH value. Therefore, it is necessary to develop MOF based fluorescent pH sensors that are easy to synthesize, stable, and accurate for practical applications in food safety.UiO-66-NH2, as a member of the UiO-66 family, has advantages such as large specific surface area, uniform pore size, easy functionalization, and good stability. Its combination with ligands, enzymes, etc. has been widely used in the food industry for safety. Therefore, as a carrier for detecting the freshness of food, it is also reliable and safe. In addition, as a stable MOF, UiO-66-NH2 can maintain a stable chemical structure under both acidic and alkaline conditions.
Du Xiaoyu and others from the School of Food Science and Technology of Shanghai Ocean University, Shanghai Aquatic Products Processing and Storage Engineering Technology Research Center, Shanghai Ocean University Key Laboratory for High Quality Utilization and Storage of Aquatic Products (jointly built by departments and provinces), and Lingang New Area Marine Biomedical Technology Innovation Platform have developed a new dual-mode response sensor for pH value detection of aqueous solutions.Choose low-cost Alizarin complex ketone as a colorimetric indicator to form a colorimetric sensor, and introduce the easily synthesized UiO-66-NH2 to provide fluorescence to improve the accuracy of the sensor.
Source: Sensor Expert Network