Researchers from the Institute of Solid State Physics at the Hefei Institutes of Physical Science, Chinese Academy of Sciences, recently developed a fluorescence optical sensing platform for imaging and detecting gallic acid (GA) in teas and fruit juice. It enables more efficient and accurate food safety monitoring and quality control systems (see video).
“Gallic acid has not only strong anti-mutagenic, anti-carcinogenic, and antioxidant activity, but also is an important index to evaluate the antioxidant capacity of food,” says researcher Dr. Changlong Jiang, a professor at the Hefei Institutes. He worked alongside researcher Fan Yang, a doctoral student at the Institute of Solid State Physics Institute, as well as Lei Pan, first author of the study, and several others. GA is commonly found in various types of tea.
The researchers’ sensing platform combines ultraviolet (UV) light and a nonfluorescent porous quartz plate. It was designed using coordination polymerization—the process of reacting monomer molecules together in a chemical reaction to form chains or 3D networks—of europium ion (Eu3+) and 3,5-dicarboxyphenylboronic acid (BBDC) to construct a multi-emitting europium metal-organic fluorophore framework (Eu-MOF) with fluorescent sensing probes under single-wavelength excitation for rapid visual detection of GA.
The chemical element europium is a soft metal often used in the printing of Euro banknotes because of its luminescent properties under UV light. Dicarboxyphenylboronic acid is a biochemical used in proteomics (proteins) research.
The multi-emission Eu-MOF exhibits a variety of noteworthy luminescence properties, according to Jiang, “due to their rich emission sources of metal ions, ligands, and guest molecules. And they exhibit excellent performance in rapid detection and biological imaging.” This is an advantage over existing biological sensing systems, which have some limitations relating to sensitivity and stability, time constraints in the detection process, and cost.
In their work, the researchers dispersed the Eu-MOF suspension framework evenly into the grooves of the quartz plate, then added a GA solution (see Fig. 1 at top of this page). Fluorescent color changes were observed via UV light in a dark environment, and for the fluorescence images the system produced, corresponding color information about red-green-blue (RGB) values was obtained by a smartphone app that recognizes and distinguishes colors (see Fig. 2).
“It’s not easy for the human eye to distinguish subtle color changes,” Jiang says, noting that a smartphone app and its digitization of the RGB value information allowed the researchers to overcome this challenge and detect GA in the samples. The RGB ratio (red/blue) and the GA allows the researchers to calculate the concentration of GA.
“When the concentration of GA increases,” Jiang adds, “the Eu-MOF probes exhibit a continuous color change from red to blue under an excitation at 270 nm.”
They also explored the effects of different ligands (ions or molecules that bind to a central metal atom) and synthesis conditions on the structure and properties of luminescent MOFs, as well as the relationship between the luminescence, sensing mechanisms, morphology, internal material composition, and properties of MOFs, “to explore the intrinsic relationship between materials and luminescent properties,” Jiang says. “Synthesizing such MOFs and their composites, which can respond by color during the sensing process and be used in the multi-scene and multi-path detection environment, thereby improves the superiority of the sensing method.”
The researchers’ next steps will involve synthesizing white-light MOFs and MOF gels in the new system of metal framework optical sensing. They will also apply fluorescence spectroscopy analysis technology to the design and research of monochromatic-light MOFs under single-wavelength excitation.
“It is important to develop efficient preparation methods for visualization, patterning, and device-based luminescent MOFs,” Jiang says. “And we can promote the application of luminescent MOF devices and their sensing platforms in visual detection, food quality control, and biological anticounterfeiting.”
