The precipitate structure of copper-based antibacterial and antiviral agents enhances their longevity for kitchen use

The global concern over infectious diseases, amplified by the COVID-19 pandemic, highlights the need for effective disinfection methods in food environments. Current methods include ethanol-based disinfectants, sodium hypochlorite, photocatalysts, and silver or copper ions. However, bacterial transmission from frequently uncleaned cooking surfaces like tables, hot plate handles, and cookware remains a significant issue. Stainless steel, commonly used in kitchens, can harbor bacteria if not properly sanitized. Copper and silver possess well-documented antibacterial and antiviral properties attributed to mechanisms such as cell membrane damage, active oxygen generation, DNA damage, and photocatalytic effects. This study focuses on SKYBE-783™, a novel CBMS, to investigate its long-lasting antibacterial and antiviral activity and the underlying mechanisms responsible for its efficacy. The CBMS consists of only inorganic ions including Cu²⁺ (140 mg/l), Ca²⁺ (0.11 mg/l), Cl⁻ (33 mg/l), and HPO₄²⁻ (12.0 mg/l), and has a pH value close to 3.0. The enduring effect is attributed to the precipitates formed upon drying of the liquid agent. This study aims to examine the structure and composition of these precipitates to elucidate the mechanism behind the observed antibacterial and antiviral properties of the CBMS.

Extensive research supports the antibacterial properties of copper and silver ions. Mechanisms of action include cell membrane damage (18-24), the generation of reactive oxygen species (ROS) (20-24), DNA damage (25-26), and photocatalytic effects (27-30). Antiviral properties of copper and copper oxide have also been documented (31), including studies on SARS-CoV-2 inactivation (32). The existing literature establishes a foundation for understanding the potential of copper-based agents, however, a comprehensive understanding of the precipitate structure and its contribution to long-term efficacy was lacking. This gap in knowledge motivated this study.

The study investigated the structure and composition of CBMS precipitates formed on various substrates (PMMA, SUS304, SUS316L, Zr) after drying for different periods (8 days to 1 year). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to analyze the morphology and elemental composition of the precipitates. Scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDS) with a higher resolution (5 nm) was employed for detailed compositional analysis. X-ray photoelectron spectroscopy (XPS) provided information on the chemical states of elements in the precipitates. Electron diffraction patterns analyzed the crystal structures of the precipitates. Antibacterial properties were assessed using various bacteria (*E. coli*, *S. aureus*, MRSA, etc.) against textile products and other materials following ISO 20743:2013 and JIS Z2801 (ISO 22196:2007) standards. Antiviral activity was evaluated using various viruses (influenza A, FCV, SARS-CoV-2 variants) on plastics and other non-porous surfaces according to the ISO 21702 method. Sustainability of the antibacterial effect was examined over time (8 days, 14 days, 3 months, 6 months, 1 year) on PMMA, SUS304, and SUS316L substrates. MRSA sterilization performance was evaluated on SUS304, PMMA, and a PC keyboard over 1-12 weeks. The safety of CBMS was evaluated through acute oral toxicity, acute skin irritation, skin sensitization, and reverse mutation tests. Finally, a trial at a sushi restaurant assessed the effect in a real-world setting by measuring ATP levels before and after CBMS application.

SEM revealed that CBMS forms SBSCD structures with deposits measuring approximately 5 µm, exhibiting varied microstructures including scaly skin-like agglomerates and flower-shaped precipitates. SEM-EDX analysis identified Cu, Cl, and O in the submicron particles, while flower-shaped precipitates contained Cu, P, and O, resembling Cu₂(OH)PO₄ and CuO. STEM-EDS confirmed the presence of Cu, P, Cl, and O in precipitates on both PMMA and Zr substrates. Electron diffraction patterns suggested the presence of Cu₂P₂O₇, Cu₂O, and Cl₂Cu₂O. XPS analysis confirmed the presence of copper, chlorine, and phosphorus in the precipitates, showing changes in the relative amounts of these elements over time (8 days vs. 1 year). The amount of P decreased, and the amount of Cl increased with prolonged drying time, while Cu content increased up to 6 months and then decreased. Antibacterial tests showed that CBMS was effective against various bacteria, and antiviral tests showed its efficacy against several viruses, including SARS-CoV-2 variants. The antibacterial effect was sustained for up to 6 months on PMMA, SUS304, and SUS316L against *E. coli* and *S. aureus*. The MRSA sterilization performance was good on SUS304 up to 4 weeks but decreased thereafter. Toxicity tests showed CBMS to be highly safe. The trial at a sushi restaurant demonstrated a significant reduction in ATP levels after CBMS application.

The formation of the SBSCD structure with its varying copper compounds is crucial for the long-lasting antibacterial and antiviral effects of CBMS. The gradual transformation of the initial copper compounds to CuO, which exhibits antibacterial properties, contributes to the sustained efficacy, although the overall efficacy might decrease over a long period due to factors such as decreased specific surface area and the weaker antibacterial properties of divalent copper ions compared to monovalent copper ions. The variation in antibacterial performance across different substrates may be related to differences in bonding strength between the precipitate and the substrate material, differences in the nanoparticulate structure of the precipitate, and the nanostructure of the substrate itself. While the study showed effectiveness against various bacteria and viruses, variations in antibacterial properties depending on the bacterial species were observed, highlighting the need for further investigation into species-specific responses. The ATP reduction in the sushi restaurant trial indicates improved hygiene and food safety, corroborating the findings from laboratory tests.

This study demonstrates that CBMS, a novel copper-based agent, exhibits long-lasting antibacterial and antiviral activity due to the formation of a unique SBSCD precipitate structure. The agent demonstrates high safety and efficacy against various bacteria and viruses. The sustained antibacterial performance is attributed to the gradual transformation of copper compounds and the inherent properties of CuO. Future research could focus on optimizing the composition of CBMS to enhance its long-term performance and to investigate species-specific antimicrobial effects further.

The study primarily focused on laboratory settings and one real-world trial. A more extensive real-world evaluation is needed to confirm the long-term efficacy and performance under diverse conditions. While the safety profile of CBMS is promising, further toxicological studies are recommended for comprehensive risk assessment. The ATP assay, while useful as a rapid indicator, does not directly measure bacterial counts and might not capture the full antimicrobial spectrum.