In natural environments, biofilms are the predominant mode of bacterial growth [1], [2]. Biofilm formation happens in two stages; first the cells adhere to an inert surface which is a spontaneous and rapid action and in the second stage the formation of multi-layered bacterial cell aggregation happens, which is enmeshed in exopolymeric matrix produced by bacteria [3]. The initial bacterial cell adhesion process depends on different strains and species that interact with the inert surface [1]. Bacterial cell adhesion and biofilm control are an important aspect to food, pharmaceutical and medical sectors. The sessile bacterial consortium can become resistant to antimicrobial compounds and disinfectant agents due to horizontal gene transfer [4]. Surface modification if carefully tailored is capable of preventing cell adhesion and biofilm formation [5]. The surface properties can be practically modified to reduce bacterial adhesion and also reduce biofouling process of the surfaces [6]. Surface modification in case of metals refers to any alteration of physico-chemical properties of a surface (hydrophobicity, etc.). This, will lead to specific biochemical interactions that prevent bacterial attachment and thus hampering biofilm formation [7].

Copper is an important commercial material of use in various industries, it has very high conductivity, strength and excellent thermal property. Copper and its various alloys are widely used in cooling water systems, for potable water pipelines, industrial heating and in decorative metallic works [8]. The electronic industry also needs copper for its high value components manufacturing. Several studies on corrosion of copper and its alloys were carried out in varied conditions and were reported in literature [9], [10], [11], [12], [13]. Industrially, microbial biofilms cause several issues in cooling water systems and in distribution system pipelines, where the biofilm growth impairs the heat transfer efficiency, that reduces the function of condenser system resulting in the turbine back pressure as well as impairing the functioning of ancillary units [14], [15]. Microbes, that cause corrosion include algae, bacteria and fungi, primarily, there are five types of bacteria that are identified to cause corrosion. They include biopolymer producing bacteria, iron reducing, iron oxidizing, nitrate and sulphate reducing bacteria [16], [17]. The main factors/metabolites, which promote copper corrosion, are; slime (exopolymers), ammonia, hydrogen sulphide, and organic acids. The primary source of sulphide is from sulphate reducing bacteria (SRB), which grow under anaerobic conditions. In addition, putrefaction of plant and animal matter within the distribution system pipelines during periods of shutdown and stagnancy release organo-sulphur compounds and augment SRB growth. Furthermore, corrosion of copper-based alloys in water distribution systems is aggravated by the presence of exopolymer producing bacteria. Exopolymer producing bacteria (such as Pseudomonas sp) attach to the metal surfaces by forming a gel matrix that entrap other metal ions and create innumerable tiny galvanic corrosion cells, which act in syndicate corroding the metal [18].

Self-assembled molecules (SAMs) protect materials from corrosion and influence surface properties such as adhesion, adsorption and wetting [8]. One of the biggest advantages of SAMs are their molecular (crystalline) perfection, resistance to contamination and ease of preparation. The self-assembly monolayer formation depends on weaker van der Waals interactions and directional bonds such as ionic, hydrogen bonds organize atoms or molecules in an array [19]. The molecules or ions adjust their own atomic positions to reach a thermodynamic minimum to create stable films. The performance of the SAM monolayer is based on its ability to stop electron transmission from the metal substrate to the reducing molecules present in the surrounding medium [20]. The alkanethiol (R-SH) SAMs interact with metals to form self-assembled monolayers, this molecular interaction has attracted considerable interest [21].

Among the various molecular coatings organo-functional silanes are a probable candidate for surface modification. Silanes can be used to modify the surface energy or wettability of substrates through the interaction of boundary layers of solids, effecting variable degrees of hydrophobicity or hydrophilicity [22], [23]. Monomeric silicon chemicals are known as silanes and when they contain at least one silicon carbon bond (e.g., Si-CH₃) are called organo-silanes. Organo-functional silanes are molecules carrying two different reactive groups so that they can react with inorganic substrates such as glass and metals and form stable covalent bonds and organic substitution [24]. In addition, the organo-silane products were found to prevent the adhesion of the pathogenic bacteria on glass surfaces, however, their effect is relatively influenced by bacterial species [25].

Generally, silanes act as adhesion promoters between an inorganic substrate and an organic coating. Silanes are identified by the structure XnSiR4-n, where R is an organo ligand compatible with the organic layer and X forms a metal-siloxane bond (Si-O-metal). Silane molecules are bi-functional, the resulting direct and cross-linking bonds repel moisture and offer tremendous adhesion, as measured by salt spray, vapor humidity, condensing humidity, bending, and influence tests [26], [27]. Modification of surfaces with organo-silanes usually increases the hydrophobic quality and low surface free energy of native surfaces. Hydrophobicity of surfaces has been reported as an important factor affecting the attachment of bacteria on surfaces. Specifically, hydrophobicity seems to decrease the adhesion of microorganisms on to metallic surfaces [28] and in the same time enhances the detachment of sessile cells [24]. Recently, antifouling-coatings with SAM technology are being developed for antibacterial activity [29].

In this study the investigators assayed some SAM coatings for their antibacterial activity as well as in prevention of copper metal corrosion. Two important corrosion-causing bacteria were evaluated for their adhesion and biofilm formation properties with respect to SAM layers of alkanethiol and silane on copper. In addition, tests were conducted to study the hydrophobicity of the SAM monolayers and their corrosion resistant property.

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