Dependence of SARS-CoV-2 (COVID-19) inactivation ability on the crystallinity level of transparent Cu₂O thin films

The inactivation ability of SARS-CoV-2 (COVID-19) was examined using two types of transparent Cu₂O thin films with different crystallinities on a Na-free glass substrate. The low-crystallinity Cu₂O thin film, which was fabricated by irradiating 254 nm ultraviolet (UV)-light with an intensity of 6.72 mW cm${}^{-2}$ onto a spin-coated precursor film involving Cu${}^{2+}$ complexes at room temperature, exhibited an outstanding COVID-19 inactivation ability to reduce 99.999% of the virus after 1 h of incubation. The X-ray diffraction results of the UV-irradiated thin film indicated a cubic Cu₂O lattice with a small crystallite size of 2 ± 1 nm. Conversely, the high-crystallinity Cu₂O thin film with a crystallite size of 16 ± 3 nm, obtained by heating a spin-coated precursor film containing another Cu${}^{2+}$ complex, showed a negligibly low inactivation activity at the same level as the Na-free glass substrate. The eluted concentrations of Cu ions from both Cu₂O thin films were analyzed after immersion in Dulbecco’s modified Eagle’s medium (DMEM) for 0.25–2 h. The eluted Cu–ion concentration of 1.16 ppm was observed for the UV-irradiated thin film by DMEM immersion after 1 h, but that of 0.04 ppm was observed for the heat-treated thin film. This indicated that an important factor of virus inactivation on Cu₂O thin films is highly related to the elution of Cu ions that occurred from the surface in the medium.