There has been an extensive development of nano-materials with biocide and probiotic properties and, in particular, of nano-magnesia MgO and nano-zinc oxide ZnO. An example of a biocide is “Antibacterial characteristics of magnesium oxide powder,” J. Sawei et al., World Journal of Microbiology and Biotechnology 16, Issue 2, pp. 187-194 (2000), and T. Yin and Y. He, “Antibacterial activities of magnesium oxide nanoparticles against foodborne pathogens,” J. Nanopart. Res. 13:6877-6885.
In the study by Sawai et al., the objective was to make high surface area MgO with particle sizes below about 50 nm. In trials of these materials, the MgO particles rapidly react with water to form nano-magnesium hydroxide Mg(OH)2. Prior art references to nano-MgO are ascribed herein to nano-Mg(OH)2. These hydrated nano-materials exhibit broad-spectrum bioactivity response to virus, bacteria and fungi. The powder, and the hydrated nano-powder, has as an ability to deactivate toxic materials such as chemical warfare agents.
In a paper published by T. Yin and Y. Lu, it was demonstrated that nano-MgO particles had a strong biocide activity against two foodborne pathogens, namely Escherichia Coli and Salmonella. This work is important because nano-MgO/Mg(OH)2 is not believed to be toxic to humans or animals, and has a positive impact on plants through the supply of magnesium as a fertilizer. For example, seven log reductions in E. Coli were observed at a dosage rate of 8 g/liter solids, and dosages of 1 g/liter suppressed growth, and that 3 g/liter would kill all cells within 24 hours. While Mg(OH)2 is relatively insoluble, it rapidly dissolves in low pH environments, especially at the pH of digestive systems. This would be true of nano-MgO/Mg(OH)2 because the dissolution rate is faster the higher the surface area.
U.S. Pat. No. 6,827,766 B2 claims a decontamination product comprising nano-particles including MgO and Mg(OH)2, selective biocides and a liquid carrier, including water. The biocide properties are significantly enhanced by the presence of the nano-particles. The decontamination processes include a liquid spray, fog, aerosol paste, gel, wipe, vapor or foam. While the claims are limited to the requirement of adding an existing biocide as an adjuvant to the product, the examples disclosed teach that the nano-particles, in the liquid carriers, had an effective, long-term biocide activity without the adjuvant. Specifically, their example 3 shows that a ratio of 5/1 water/oil emulsion with 2% nano-MgO, CaO, and ZnO solids had such properties, notably without the requirement of a biocide.
The impact of the particle size would seem to be important. U.S. Pat. No. 2,576,731 (Thomsen) discloses the use of magnesium hydroxide slurry, made from a standard magnesium oxide, as the basis for a foliar spray as a carrier for active biocides for both insects and fungi where the benefits are associated with the ability of the alkaline particles to absorb active biocides to render them insoluble, and the strong adherence of the particles on the leaves of the plants such that the biocide can act over many washings of the leaf. That patent describes the role of the magnesium hydroxide as having no insecticidal or germicidal activity. In the context of this invention, the important teaching of that patent is the adherence of magnesium hydroxide.
This view was supported by a paper published by Motoike et al., “Antiviral activities of heated dolomite powder,” Biocontrol Sci. 13(4):131-8 (2008), in which processed dolomite is shown to exhibit anti-viral activity. U.S. Patent Publication 2009/0041818 A1 claims an anti-viral agent that is a mixture of an oxide and a hydroxide, in which it is taught that hydroxide ions are produced by the reaction of the oxide with a hydroxide. It is claimed that many materials can provide the hydroxide, among which is Mg(OH)2, and the oxide is preferably MgO. The relevant disclosure of this prior art is that the biocide activity of such conventional slurries is primarily transient and thus a manufactured magnesium hydroxide, or hydrated calcined dolomite slurry, does not have a significant long-term biocide effect. Without being limited by theory, this work suggests that the active chemical species in such a hydroxide slurry are naturally present, but their concentration is too low for a sustained impact on microbes. This disclosure seeks to overcome this limitation.
Insight into how the nano-Mg(OH)2 has a significant bioactivity compared to standard materials is gained at two levels.
First, at the biological level, the most plausible theory of why pathological fungal growth is suppressed by chemical processes is the presence of Reactive Oxygen Species (ROS). ROS have a high redox-potential, and include the superoxide ion O22−, which is known to generate hydroxyl radicals OH, perhydroxyl anions HO2— and hydrogen peroxide H2O2 by hydrolysis with water. There are equilibria between these species in water that is largely regulated by the pH, and at the pH near a nano-Mg(OH)2 grain, around 10.4, the perhydroxyl anion dominates. Plants can ramp up the production of ROS as a defense against pathogenic microbial attack, with the ROS attacking the primitive cell walls of pathogenic fungus and bacteria. In response, fungus can produce chemical species that react and neutralize the ROS, and the ROS attacks and destroys the cell walls of pathogenic microbes. The same model for the activity is true of pathogenic bacteria, in particular, the anaerobic gram-negative bacteria. The ROS symbiosis is associated with the relationship between the plant ROS and the beneficial gram-positive bacteria, which are essential to a healthy environment for growth. Gram-positive bacteria are generally beneficial and aerobic, and the ROS increases the oxygen level in the environment. For example, as demonstrated in the case of rice blast fungus: Kun Huang, Kirk J. Czymmek, Jeffrey L. Caplan, James A. Sweigard and Nicole M. Donofrio (2011).
Second, at the atomic level, it is evident that the long-term biological activity of nano-Mg(OH)2 slurry is associated with is ability to produce, and stabilize ROS. In general terms, small crystal grains have, by definition, a high proportion of their crystalline surfaces, which are formed at the high energy surfaces, and it is well understood that such surfaces are the source of energetic oxidants, such as the ROS species. In the case of Mg(OH)2, techniques such as Electron Paramagnetic Resonance has detected all of the radical species described above on normal crystals, albeit at low concentrations. ROS radicals in solution can recombine, and the bio-activity impact of ROS would degrade by radial recombination. In the presence of Mg(OH)2, the ROS rate of dissipation can be substantially reduced, if not suppressed, by the generation of magnesium peroxide MgO2. Magnesium peroxide is a stable crystalline material, and is usually formed in a mixture with hydrogen peroxide H2O2, water and excess MgO. It is stable in this form at ambient temperature (I. I. Vol'nov and E. I. Latysheva, “Thermal stability of magnesium peroxide,” Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 13-18, January, 1970). Therefore, nano-Mg(OH)2 can not only form ROS at the grain boundaries but also the ROS species can be stabilized on the grain surfaces. The ROS species are stored on the nano-grain surfaces, and would be released by the change in the equilibria associated with a pathogen attack, and general dissolution of the nano-Mg(OH)2 to supply magnesium to the plant as a fertilizer.
In summary, a reasonable model for the bioactivity of nano-Mg(OH)2 is that each particle is a nanoscale crystalline grain that has a high concentration of ROS, which is stabilized on the energetic surfaces of the grain, and the bioactivity arises from the enhancement of the plant's own natural defense systems that form ROS to provide the aerobic environment that suppresses pathogenic microbes. This effect is enhanced by the pH of the Mg(OH)2 at 10.4, which may neutralize acids extruded by pathogens; the net positive particle charge from hydrolysis, which attracts the particles to negatively charged surfaces of certain microbes and cells; and the adherence of the particles onto the surfaces of the microbes and cells of plants. By contrast, normal Mg(OH)2 with grain sizes of 0.1 to 100 microns generally have surfaces that are dominated by the stable (001) surface, and the concentration of ROS would be small.
The same mechanisms ascribed above to nano-Mg(OH)2 may apply to other bioactive materials based on metal oxides, such as nano-ZnO and AgO. Their nano-grains will also support a range of ROS species that depend on the specific defects at the respective grain boundaries. For example, nano-ZnO is known to produce peroxyl and hydroxyl radicals.
The mechanism for bio-activity of nano-grain particles is substantially different from most other fungicides and bactericides, which use toxic compounds to target pathogenic microbes. First, the mechanism of ROS lies at the core differentiation between aerobic and anaerobic microbes, and genetic evolution to limit the impact of the bio-activity is unlikely. Second, the mechanism is an enhancement of the natural processes whereby plants defend themselves against pathogenic attack. No new chemical species are involved, and the products of the decomposition are essential nutrients or micro-nutrients and, in the case of magnesium, it is an essential nutrient for the production of chlorophyll. Plants absorb magnesium through stomata on the leaves, and the aerobic/anaerobic contests between fungi, gram-positive and gram-negative microbes and the plant cells take place, both within the soil and on the leaves, for example, as described by Susan S. Hirano and Christen D. Upper, Microbiol. Mol. Biol. Rev. 64:3624-653 (2000).
A probiotic has been defined in a proceeding of the U.S. Patent and Trademark Office, Trademark Trial and Appeals Board, Serial No. 77758863 (2013), as a generic name for a fertilizer using friendly bacteria in the soil-producing microbial ecology means to bring back symbiotic relationships to the soil. In this application, the definition is extended to include symbiotic relationships on the plant leaves, and the symbiosis is specifically associated with the relationship between the plant and the beneficial gram-positive bacteria, which are essential to a healthy environment for growth. Indeed, when nano-Mg(OH)2 is applied onto leaves as a folia spray, the impact of magnesium absorption as a fertilizer is noticeable through both the color from increased chlorophyll, and the increased leaf thickness. Thus, at a technical level, the properties of nano-Mg(OH)2 satisfies the requirements of being a probiotic soil or plant amendment.
The means of production of nano-materials use chemical synthesis, and the materials are expensive to produce. Furthermore, the handling of very fine powders is difficult because these powders have a tendency to readily float in air. Most importantly, nano-materials are very difficult to filter from air using conventional air filters. Thus, the production processing of these materials requires expensive handling equipment to avoid loss of the materials and to meet safety, health and environmental regulations. These costs are such that nano-materials have not made a substantial impact in the markets for biocides. Equally important, there are concerns about nano-particles arising from their ability to be absorbed through the skin, and inhaled into the lungs, by virtue of their small size.
There is a need for a product that has the same desirable intrinsic biological activity of nano-materials using a process that can produce significant volumes of product, but also avoiding the handling issues of nano-materials, and their potential for absorption and inhalation.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.