The use of copper nanoparticles to prevent bacterial and fungal growth on leather

Introduction
Biocides and fungicides are used at different stages of the leather-making process. However, bacteria and fungi are still able to grow on finished leather, and especially vegetable tanned leather. The reason for this is that sweat from footwear can act as a carbon source for microbial growth, or that the concentration of fungicides used in the leather-making process are too low to inhibit fungal growth, or that the fungi have developed resistance to the fungicide.

It is well known that the majority of metals are lethal to all types of bacteria. Copper and its compounds have been used for a long time as disinfectants. Copper causes plasmid DNA degradation in gram-positive and gram-negative bacteria in a dose-dependent manner. Copper surfaces are known to kill bacteria, fungi and viruses.

This study was carried out to investigate the antimicrobial properties of copper nanoparticles in the post tanning and finishing leather processes.

Experimental
Copper nanoparticles were prepared according to a chemical process.

These copper nanoparticles were added to the retanning process in the dyehouse, as well as to the topcoat mixture when the leather was finished. The actual concentration of copper nanoparticles used in the experiments is not actually mentioned in the research paper.

The effect of the copper nanoparticles on the colour of the leather, the colour fastness, and the strength of the leather were all tested.

The anti-bacterial activity of the leather where the copper nanoparticles had been incorporated was tested by placing a 2cm radius piece of the leather onto petri dishes that were covered by bacterial (both gram-positive and gram-negative bacteria) growth.

Results
Tables 1 – 3 show the colour fastness values of the control and copper nanoparticle-treated leather. The rubfastness properties of both leathers are similar. Both leathers have excellent colour fastness, and the copper nanoparticles had no detrimental effect on the leather. From Table 3 it can be seen that the physical strength properties of the experimental leather are better than the control leather. The experimental leather’s organoleptic properties were also relatively better than the control leather.

The effect of incorporating the copper nanoparticles into the crust leather and the finishing topcoat on its anti-bacterial properties can be seen in Figure 1. It is evident that the sample of leather that contains the copper nanoparticles has the ability to kill both gram-positive and gram-negative bacteria surrounding the small piece of leather.

Figure 1. Effect of control finished leather and copper nanoparticle-coated finished leather against different bacterial species. (1 – control leather, 2 – copper nanoparticle-coated leather).rn

The antifungal activity of the copper nanoparticles was investigated by the agar diffusion method whereby a small quantity of the copper nanoparticle solution was placed in small wells cut into agar plates where a species of fungus (Aspergillus niger) had been allowed to grow over the surface of the agar. Antifungal activity is evident from zones of growth inhibition around the wells (Figure 2). It is evident from Figure 2 that the copper nanoparticle solution does have anti-fungal properties.

Figure 2. Growth inhibition of different concentrations of copper nanoparticle solution against a fungal species that commonly grows on leather.

Conclusion
The results of this study have shown that the use of copper nanoparticles in retanned leather imparts both antimicrobial and functional properties such as good mechanical strength to the experimental leather. The incorporation of copper nanoparticles into the topcoat of finished leather effectively inhibits both bacterial and fungal growth. This research paper paves the way for more studies on the use of metals to inhibit microbial growth on leather.

This article is a summary of the paper “Bactericidal and Fungicidal Action of Copper Nanoparticles on Leather Surface” Journal of the American Leather Chemists Association (JALCA) Vol 118, 519 - 528 (2023).