Abstract :
[en] Nitrite is a commonly used food additive in the meat industry that cannot be completely replaced. Its main functions include promoting color development, inhibiting microbial growth, providing antioxidant effects, and imparting a distinctive flavor to meat products. However, nitrite is also a highly toxic substance, and an excessive amount of nitrite in food can pose significant health risks. When interacting with proteins (such as primary and secondary amines), nitrite can produce carcinogenic nitrosamines. Additionally, food safety problems related to nitrite have occured in recent years. Thererfore, the rapid detection of trace concentrations of nitrite in food is extremely essential. Great efforts have been made to establish new methods to determine nitrite in food, however, the existing detection methods are not comprehensive enough. In comparison to other methods, fluorescence spectrometry has become an effective method for detecting nitrite ions due to its high sensitivity, good selectivity, low detection limit and simple detection conditions. The key to determining nitrite by the fluorescence method lies in designing and fabricating fluorescent nanoprobes with good selectivity and high sensitivity.
Metal-organic frameworks (MOFs), a novel class of multifunctional materials, have demonstrated excellent potential in various fields such as gas storage, adsorption and separation, catalysis, sensing, drug delivery and more in recent years. Due to their chemical design flexibility and tunable pore structures, MOFs exhibit inherent advantages in the field of fluorescence sensing. Therefore, it is both feasible and innovative to explore their application in nitrite detection and food safety protection. This study aims to design and synthesize several novel luminescent metal-organic framework (LMOF) based fluorescence nanoprobes. These probes are created by integration MOFs with different dyes and successfully applying them to detect nitrite in real meat samples. Moreover, to expand the application scope of powdered LMOF and develop the LMOFs-based sensing device, LMOFs were combined with natural hydrogel polymers to form a composite hydrogel test kit. This innovation approach opens a new avenue for the application of LMOF materials in the field of fluorescence sensing, particularly in the sensing and detecting of nitrite.
In the first part, a single-emission LMOF-based nanoprobe, denoted as Rh6G@MOF-5, was fabricated through a facile solvethermal synthesis method. The porous structure and chemical designability of MOF-5 material facilitate the good diffusion of the targeted analyte through the pre-gathering process, with rhodamine 6G as a recognition element for nitrite. This enables Rh6G@MOF-5 to serve as an ideal fluorescent sensing material with high selectivity and sensitivity during nitrite detection process. It exhibits a linear ranges of 0–200 μmol/L and a low detection limit of 0.2 μmol/L. Moreover, the sensing platform can also be use for the nitrite detection in real meat samples. The results demonstrate that this material may be a promising candidate for nitrite detection in food.
In addition, to overcome limitations associated with single-emission sensing measurements, a ratiometric LMOF-based fluorescence sensor (Rh6G@UIO-66-NH2) which coordinated by Zr metal ions, 2-amino-terephthalic acid ligands, and Rh6G dye molecular, was synthesized. Combining the sensing advantages of UIO-66-NH2 with the chemical activity of Rh6G, the Rh6G@UIO-66-NH2 composite exhibited excellent sensitivity for nitrite (LOD = 0.021 μM), a short response time, anti-interferece and stability. Althrough powder samples displayed certain properties, challenges persisted, including difficulties in recovery and utilization, high reagent consumption, and sensitity to environmental conditions. To address these issues, a portable dual-mode sensing test kit, which denoted as the Gel/Rh6G@UIO-66-NH2 hydrogel kit, was further developed. This kit was prepared by immobilizing MOFs powders within gelatin hydrogel. Beyond the quantitative detection of nitrite by the Rh6G@UIO-66-NH2 sensor using a ratiometric fluorescence method, the Gel/Rh6G@UIO-66-NH2 hydrogel kit also qualitatively recognized nitrite with the naked eye through a colorimetric method. In the presence of nitrite, the visible colors of this dual-mode sensing test kit changed from pale yellow to golden yellow, and the corresponding fluorescence colors changed from bright to dark. This research could offer valuable prospect for well-designed LMOFs materials in the fluorescence sensing of nitrite ions in the food system.
Based on the above results, another facile ratiometric fluorescent sensor, named as 5-AF@UIO-66, was proposed by post-modificating 5-AF dye molecules within the framework. Under appropriately acidic conditions, nitrite was oxidized into nitrosyl cation (NO+), and it reacted with 5-AF to generate diazotized substances, leading to fluorescence quenching and the original resonance energy transfer from Zr-MOF to 5-AF. Due to its good water stability and the presence of amino groups on the framework, 5-AF@UIO-66 was successfully applied for nitrite turn-off fluorescence detection. The nanocomposite offers rapid response, low detection limit, good reproducibility, and naked-eye recognition of nitrite, making 5-AF@UIO-66 a promising material for practical applications. Based on that, a stimuli-responsive LMOF-based hydrogel test kit (Aga/5-AF@UIO-66) was fabricated for precisely quantifying nitrite content, combining it with a chemiluminescence imaging system. After exposure to nitrite for 10 min, the photo image of the Aga/5-AF@UIO-66 hydrogel test kit could be converted into digital data, creating a direct quantitative and visual method for nitrite identification. The Aga/5-AF@UIO-66 hydrogel test kit, as a colorimetric sensor, performed equally well for detecting nitrite in meat products with satisfactory recoveries.
In summary, this dissertation broadens the application scope of LMOFs, particularly in food science, offering a new approach for creating high-sensitivity LMOF-based sensing platforms. It introduces novel materials and technologies for reliable on-site nitrite detection, showcasing the promising application potential of stable LMOF nanoprobes and their hydrogel test kit in food safety monitoring. This research holds significant value in advancing the application of LMOFs in food research fields.
