Synthesis and Antimicrobial Study of Nanoporous Metal-Organic Frameworks (MOFs) Loaded with Thymol

Introduction: With growing demands for food safety and public health, there has been large interest in developing food preservatives, especially using natural antimicrobials to improve the food safety and quality. Thymol is one of the essential oils found in oregano with antifungal, insecticidal, antimicrobial, and antioxidant abilities, and it can be potentially ideal as a food preservative if its major drawbacks can be addressed, including undesirable aroma and low solubility in water.

Purpose: To solve this limitation, loading and encapsulation are the main approaches to form micelles and nanoparticles, as it promotes better dispersing of thymol in water as well as maximizing the surface area of bacteria and thymol interaction.

Methods: In this study, for the first time, we have demonstrated a simple surfactant-free post-synthesis loading process of metal-organic frameworks (MOFs) with thymol. We have synthesized a MOFs (defined as Zn@MOF) from zinc nitrate hexahydrate and 2- aminoterephthalic acid (defined as Zn@MOF). Thymol was then loaded into the Zn@MOF pores. The whole process does did not require any surfactants or templates under relative mild conditions (105°C). The structure of porous crystal MOFs was confirmed using scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Results: Thymol was then loaded inside the MOFs with a loading rate of 3.95%. This method produced crystalline Zn@MOF with nanoscale porous cavities loaded with thymol, which were confirmed by multiple characterization methods. The antimicrobial experiment showed promising antimicrobial ability of T-Zn@MOF. Inhibition of E. coli O157:H7 was measured both in tryptic soy broth (TSB) medium and on TSA agar. An E. coli O157:H7 reduction of 4.4 log CFU/ml was achieved at a thymol to broth ratio of 0.04 g/100 g. An inhibition area of 223.73 mm² was observed after 12 hours incubation.

Significance: Although still at the very early stages of development, the T-Zn@MOF that we produced acts as a pioneer in the application of MOFs loading antimicrobial agents in the food.