Method for preparation of attapulgite-based pH-responsive antibacterial material

This patent application claims the benefit and priority of Chinese Patent Application No. 202010280575.9, entitled “Method for preparation of attapulgite-based pH-responsive antibacterial material” filed on Apr. 10, 2020, the disclosure of which is incorporated by reference herein in its entirety as part of the present application.

The present disclosure relates to a method for preparing an attapulgite-based pH-responsive antibacterial material, which is mainly used to replace antibiotics in the process of livestock and poultry breeding, and belongs to the technical field of nano-mineral functional materials and animal feed additives.

On Jul. 9, 2019, the Ministry of Agriculture and Rural Affairs of the People's Republic of China issued Announcement No. 194, which announced the withdrawal plan and related management policies of medicated feed additives, and clearly proposed a “comprehensive ban on antibiotics” on Jul. 1, 2020. Therefore, it urgently needs to develop the safe and efficient feed antibiotic substitutes for healthy animal breeding.

Among of many antibiotic substitutes, natural antibacterial agents such as plant extracts exhibit the unlimited potential for the substitution of antibiotics, which are also one of the most common functional additive products for feed antibiotic substitute applications in the EU (Northern Animal Husbandry, 2019, 09, 17). Plant extracts are mainly intended to directly inhibit and kill pathogenic bacteria, in which aldehydes such as cinnamaldehyde are a commonly used feed additive with a good antibacterial effect after being added into the feed (Chinese Journal of Animal Nutrition, 2019, 1-8). However, aldehydes need to be stabilized due to the disadvantages of easy oxidization, strong volatility, and a certain pungent odor.

Attapulgite is a natural hydrated magnesium aluminum silicate clay mineral with one-dimensional nanorod-like morphology and zeolite-like pores of 0.37×0.64 nm. It has been reported that organic molecules could be resided in the pores of attapulgite under the mechanochemical action. However, the aldehydes residing in the attapulgite pores still present a pungent odor. In order to further solve the shortcomings of natural aldehyde-based antibacterial agents, the present disclosure is to prepare a pH-responsive antibacterial material by adopting the principle of forming Schiff bases between amino groups of chitosan and aldehydes on the basis of aldehydes residing in the attapulgite pores.

The object of the present disclosure is to provide a method for preparing an attapulgite-based pH-responsive antibacterial material. On the one hand, the method makes it possible to solve the disadvantages of natural aldehyde-based antibacterial agents of easy oxidization, strong volatility and pungent odor. On the other hand, the method makes it possible to solve the sustained release of natural aldehyde-based antibacterial agents in animal gutswith pH-responsiveness of chitosan, which could be directly used as animal feed for safe animal breeding.

Preparation of an Attapulgite-Based pH-Responsive Antibacterial Material

The present disclosure provides a method for preparing an attapulgite-based pH-responsive antibacterial material, including:

directly spraying a natural aldehyde-based antibacterial agent onto an attapulgite powder under stirring, and constantly stirring the attapulgite powder for 20-30 min;

grinding the attapulgite powder in a ball mill for 30-60 min to obtain a ground attapulgite powder;

placing the ground attapulgite powder in a stirred tank, and spraying a chitosan-citric acid aqueous solution onto the ground attapulgite powder;

after spraying, constantly stirring the ground attapulgite powder for 30-120 min; and

finally, drying the ground attapulgite powder to obtain a dried attapulgite powder, sieving the dried attapulgite powder to obtain a sieved attapulgite powder, and packaging the sieved attapulgite powder to obtain the antibacterial material.

In some embodiments, the attapulgite is smashed, acidified and degritted to have a purity of not less than 90% and a particle size of not lower than 200 mesh.

In some embodiments, the natural aldehyde-based antibacterial agent is at least one selected from the groups consisting of citral, lauryl aldehyde, cinncloval, cinnamaldehyde, benzaldehyde, perillaldehyde, vanillin, furfural, 2,4-octadienal, 2,4-nonadienal, 2,4-heptadienal, and 2,4-decadienal, and the natural aldehyde-based antibacterial agent is used in an amount of 10-50% of the mass of the attapulgite.

In some embodiments, the ball mill has a grinding ball-to-powder mass ratio of 1:10 in grinding state.

In some embodiments, the chitosan-citric acid aqueous solution has a mass concentration of chitosan of 5-10%, and a mass concentration of citric acid of 1-5%; the chitosan-citric acid aqueous solution is used in an amount of 7.5-20% of the mass of the attapulgite.

Structural Characterization of the Antibacterial Material

Taking a cinnamaldehyde-chitosan Schiff base prepared in Example 2 as an example, the structure of the antibacterial material prepared in the present disclosure is characterized. FIG. 1 is a diagram of thermogravimetric analysis curves of the attapulgite and as-prepared antibacterial material in Example 2 of the present disclosure. Compared with attapulgite, it can be seen that the antibacterial material has an organic loading ratio of 22.9%, showing that the attapulgite as a support material can effectively realize the reaction of aldehydes with chitosan to prepare a Schiff base antibacterial material.

FIG. 2 is a diagram of the Fourier Transform Infrared spectra of the attpaulgite and antibacterial material obtained in Example 2 of the present disclosure. It can be seen that there is an absorption peak at 1456 cm −1 attributing to C—H and an absorption peak at 874 cm −1 attributing to C—H of a benzene ring, indicating that there is a benzene ring structure in aldehyde-chitosan Schiff base structure; there are absorption peaks at 1632 and 1656 cm′ attributing to —C═N, indicating that the aldehyde is bound to the chitosan through carbon-nitrogen double bonds. Therefore, it can be concluded that a Schiff base antibacterial material has been prepared successfully.

3. Evaluation of Antibacterial Activity of the Antibacterial Material

The antibacterial activity is evaluated using a minimal inhibitory concentration (MIC) assay. Herein, an experimental group, a negative control group, a positive control group, and a blank control group are set up.

(1) Preparation of standard strains: Standard Escherichia coli ( E. coli , ATCC 25922) strains and Staphylococcus aureus ( S. aureus , ATCC 25923) strains were selected. E. coli and S. aureus stored at −20° C. were thawed and inoculated into Luria-Bertani (LB) broth, and incubated in a 35° C. incubator for 12 h at a speed of 160 r/min. After resurrecting the strains, the strains were transferred to nutrient agar media again, and incubated in a constant temperature incubator at 37° C. After 3 h, the bacteria were subcultured to enter the logarithmic phase. A single colony of the standard strain entering the logarithmic phase was selected to prepare a bacterial suspension, the turbidity of the bacterial suspension was adjusted to 0.5 McFarland standard turbidity, and the bacterial suspension was diluted with PBS in a ratio of 1:10 to achieve a bacterial count of 10 7 CFU/mL, 10 6 CFU/mL, 10 5 CFU/mL, and 10 4 CFU/mL, respectively.

(2) Preparation of agar plates with different concentrations of unknown samples: The samples treated at different pH values were evenly dispersed into the nutrient agar media, E. coli groups were divided into 2.5 mg/mL, 1.5 mg/mL, 1 mg/mL, and 0.5 mg/mL groups; S. aureus groups were divided into 2.5 mg/mL, 1.5 mg/mL, 1 mg/mL, and 0.5 mg/mL groups.

(3) Plating: 1 μL each of the above-mentioned bacterial suspensions (with a bacterial content of about 1×10 4 CFU/mL) was pipetted and plated in agar plates containing unknown samples; the samples were plated in three different positions in each plate, and the test was repeated twice. All the plates were transferred to a constant temperature incubator for culture, and the growth of the bacteria was observed after culturing at 37° C. for 24 h.

Table 1 shows the results of the MIC assay of the samples obtained in Examples 1 to 6 treated at different pH values against E. coli and S. aureus . The results showed that the samples obtained in Examples 1 to 6 of the present disclosure have MICs of 0.5 mg/mL and 1 mg/mL against E. coli and S. aureus , respectively, and exhibit strong antibacterial activity against both Gram-negative and Gram-positive bacteria. Compared with the MICs of samples treated at different pH values, it can be seen that the samples prepared by the present disclosure show weak antibacterial activity under acidic conditions and strong antibacterial activity under neutral conditions, respectively.

To sum up, in the present disclosure the mechanochemical method is used to reside aldehydes in the attapulgite pores, and then a chitosan solution is sprayed onto an attapulgite powder, so that the aldehydes react with chitosan to form Schiff bases. The present disclosure makes it possible to overcome the shortcomings of easy volatility and strong pungent odor of natural aldehyde-based antibacterial agents, improve the palatability of animals when they are used in safe animal breeding, thereby increasing the utilization efficiency of natural aldehyde-based antibacterial agents. The present disclosure makes it possible to improve the utilization efficiency of natural aldehyde-based antibacterial agents. In the present disclosure, chitosan is used to confer pH responsiveness on antibacterial materials: amino groups of the chitosan are protonated and antibacterial substances are not released under acidic conditions, while the hydrolysis of Schiff base leads to continuous release of antibacterial substances under neutral and weakly alkaline conditions. Therefore, the antibacterial agent could be directly used as animal feed for safe animal breeding.

FIG. 1 is a diagram of thermogravimetric analysis curves of the attapulgite and antibacterial material obtained in Example 2 of the present disclosure.

FIG. 2 is a diagram of Fourier Transform Infrared spectra of the antibacterial material obtained in Example 2 of the present disclosure.

The method for preparing an attapulgite-based pH-responsive antibacterial material according to the present disclosure and antibacterial activity of the attapulgite-based pH-responsive antibacterial material will be described in detail below with reference to specific examples.