Grinding Materials Unlocks Virus-Trapping Ability

Professor Akira Nakajima and his research team at Institute of Science Tokyo (Science Tokyo) have been developing inorganic antiviral materials that can work even in the dark. Their previous work includes antiviral materials offering both potency and durability by combining copper with rare-earth elements such as lanthanum and yttrium.

Rare-earth elements, however, are unevenly distributed geographically, raising concerns about stable supplies. To reduce dependence on these elements, the researchers turned their attention to complex oxides that combine manganese with more widely available elements such as zinc and copper. They also explored whether changing the surface of the material itself-not just the combination of elements-could improve its performance.

In inorganic materials, applying mechanical force can change the surface area, crystal structure, and chemical states of the constituent elements, sometimes improving catalytic performance. Using mechanical force to alter a material's structure or chemical properties is known as mechanochemical processing. Until now, however, its effects on the antiviral performance of inorganic materials had not been well understood.

The researchers therefore tested ball milling, a type of mechanochemical processing. In ball milling, a material is placed in a container with hard balls and rotated so that repeated impacts and friction grind the material into finer particles. In this study, the team ball-milled several complex oxides in ethanol and examined how the treatment affected their antiviral properties.

The researchers compared the antiviral activity of several manganese-based complex oxides before and after ball milling. Antiviral activity increased in every material after treatment, but the degree of improvement varied. A complex oxide containing copper and manganese, CuMn₂O₄, showed the strongest antiviral performance.

By contrast, a bismuth-manganese complex oxide, BiMn₂O₅, gained a larger surface area and became better at oxidatively breaking down organic compounds after milling, yet its antiviral activity remained lower than that of the other materials. This suggested that simply making the particles smaller and increasing their surface area was not the only reason for the improved antiviral performance.

Closer analysis revealed that in ZnMn₂O₄ and CuMn₂O₄, milling caused some oxygen atoms to leave the material surface, increasing the number of Lewis acid sites-surface sites that readily interact with molecules on the virus surface. The researchers propose that these sites bind to molecules oi the virus surface and help immobilize the virus on the material. Oxidation reactions involving manganese can then occur nearby, damaging components such as the membrane surrounding the virus.

In milled BiMn₂O₅, however, Lewis acid sites did not form, even though the material had strong oxidative decomposition activity. In other words, the material had the ability to damage viruses but was less able to hold them firmly at its surface. The researchers believe this helps explain its lower antiviral activity.

The findings show that antiviral performance can be improved not only by choosing which elements to combine, but also by modifying how a material's surface behaves. A better understanding of the optimal balance between the ability to immobilize viruses and the ability to inactivate them through oxidation could lead to antiviral materials that work in the dark while reducing reliance on rare-earth elements. This study used bacteriophage Φ6, a model enveloped virus. Further studies will be needed to determine how well the materials work against other viruses.

When we first synthesized the manganese-based complex oxides and measured their antiviral activity, the results were not as strong as we had hoped. I remember thinking that perhaps the project was not going to work out. Then Kotaro Miyazaki, a master's student working on the study, discovered that grinding the samples in ethanol dramatically increased their antiviral activity. That observation ultimately led to the findings reported here.

This study reminded me once again that we understand only a small part of how nature works. Unexpected phenomena often hold clues to discoveries we have yet to make. That was one of the lessons this research taught us.

(Akira Nakajima, Professor, Department of Materials Science and Engineering, School of Materials and Chemical Technology, Institute of Science Tokyo)

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