Antibacterial Inorganic Coating of Calcium Silicate Hydrate Substrates by Copper Incorporation

Abstract

Under environmental conditions, biofilms can oftentimes be found on different surfaces, accompanied by the structural degradation of the substrate. Since high-copper-content paints were banned in the EU, a solution for the protection of these surfaces has to be found. In addition to hydrophobation, making the surfaces inherently biofilm-repellent is a valid strategy. We want to accomplish this via the metal exchange in calcium silicate hydrate (CSH) substrates with transition metals. As has been shown with Europium, even small amounts of metal can have a great influence on the material properties. To effectively model CSH surfaces, ultrathin CSH films were grown on silicon wafers using Ca(OH)₂ solutions. Subsequently, copper was incorporated as an active component via ion exchange. Biofilm development is quantified using a multiple-resistant Pseudomonas aeruginosa strain described as a strong biofilm former cultivated in the culture medium for 24 h. Comprehensive structural and chemical analyses of the substrates are done by environmental scanning electron microscopy (ESEM), transmission Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Results do not show any structural deformation of the substrates by the incorporation of the Cu combined with three-dimensional (3D) homogeneous distribution. While the copper-free CSH phase shows a completely random distribution of the bacteria in biofilms, the samples with copper incorporation reveal lower bacterial colonization of the modified surfaces with an enhanced cluster formation.

Keywords: antibacterial; calcium silicate hydrate; cement; coating; copper; inorganic.

Figure 1

SEM gives an overview of the film’s uniformity on a micrometer scale. Images (A, C) represent a sample without any copper incorporation, while images (B, D) represent a sample after copper incorporation. A fracture edge (E) makes a local cross section available.

Figure 2

FT-IR absorption spectra token before (blue line) and after (red line) the incorporation of Cu into the CSH samples, referenced to the initial piranha-cleaned surface.

Figure 3

Core XP spectra of the surface after the incorporation of Cu into the CSH samples.

Figure 4

Lateral distribution of all components is very homogeneous, and this applies to all samples. The copper signal is correlated with CaO of the respective samples along the sputtering time. This points to the incorporation of copper into the CSH volume.

Figure 5

Two exemplary images of CSH (upper line) and Cu–CSH-coated Si-wafer surfaces (middle line): Living bacteria appear green fluorescent by Syto 9 and dead/damaged bacteria appear red fluorescent by the propidium iodide staining procedure. In the bottom line, one can see the radial distribution function (RDF) determined from several images: While the gray line of the PA49 (on CSH) generates a random distribution, certain peaks are found in the RDF of the PA49 on Cu–CSH. Due to the rod shape of the PA, two different distances can be seen in the cluster formation; one below 1 μm and one around 2 μm. The other peaks appearing in this distribution point to cluster-to-cluster distance.