**1. Introduction**

The presence and spread of pathogenic microorganisms in our environment is still a present and growing problem. This problem is exacerbated by the development of microbial resistance to antibiotics and disinfectants. Among the many ways of spreading microorganisms, one of the most widespread is their transfer through tactile contact, in particular through all solid surfaces [1]. Microorganisms living on the touch surfaces can transfer onto the human body during interaction and contribute to the spread of infections [2]. One of the preconditions for the presence of microorganisms on the surfaces of materials is their ability to adhere, which allows the colonization of the surface and the development of a bacterial biofilm [3,4], that is, a source of potential infection associated with the use of materials [5,6]. In addition, microorganisms colonizing surfaces of materials may change their functionality and structure, and even cause degradation (biocorrosion phenomenon) [7].

There are different concepts and strategies for providing antibacterial properties of surfaces [8,9]. These properties, especially increased resistance to colonization and multiplication by bacteria, can be obtained by modifying the surface layer of the material [1,10–12] or by depositing antibacterial coatings with appropriate chemical and physical properties [13–16]. Particularly important, along with free surface energy and surface charge [3,4,17], are hydrophobic properties of the surface, preventing bacterial adhesion and precluding their multiplication. Very important is the degree of surface hydrophobicity [17] and the hydrophobic or hydrophilic properties of the cell walls of the bacteria themselves [18]. The more hydrophobic cells adhere more strongly to hydrophobic surfaces, while hydrophilic cells strongly adhere to hydrophilic surfaces [18]. Insufficient hydrophobicity of the surface, combined with the hydrophobicity of the bacteria, can increase the propensity of microorganisms to adhesion [17].

The solution, consisting in the production of a coating with antimicrobial properties, is a widely studied and frequently applied approach [13,14,19]. A particularly interesting method for producing such coatings can be the sol-gel method, which makes it possible to produce ceramic oxide coatings from the liquid phase in simple and convenient ways and at a relatively low temperature [20]. Colloidal solution, from which coatings are deposited in this method, allows the easy introduction of a wide spectrum of substances modifying the properties of the resulting thin oxide films, such as in terms of bactericidal activity [21–23]. In order to obtain anti-bacterial properties of such coatings, in addition to providing an appropriate surface wettability condition, it is possible to use the antibacterial properties of metal additives, such as Ag [24–26], Zn [21,27] or Cu [27,28], among others [29]. The impact of metals on bacteria, though widely described and proven [24,30–32], is very diverse and not fully understood, as it is most likely a combination of many di fferent mechanisms [33,34].

The combination of the bactericidal e ffect of metal ions with the hydrophobicity of the surface of the coating can provide an antibacterial e ffect through the synergy of both phenomena.

In this work, studies of SiO2 coatings prepared by sol-gel method, with additions of hydrophobizing compounds and zinc compounds, were carried out. The aim was to determine the impact of these additives on the structure and properties of SiO2 coatings, in particular, on their antibacterial properties against *Escherichia coli—*understood as limiting the susceptibility of the coatings to colonization and survival of bacteria on their surfaces.

#### **2. Materials and Methods**

#### *2.1. Preparation of Sols and Deposition of Coatings*

A precursor for the preparation of sols was tetraethoxysilane (TEOS) and Si(OC2H5)4 (98% Fluka). In this study, three types of sols were used: basic SiO2 sol and two sols of SiO2 modified with additives, increasing the hydrophobicity of coatings made from these sols. In order to increase hydrophobicity, the additives of methyltrimethoxysilane (MTMS), CH3Si(OCH3)3 (98%, Aldrich) and hexamethyldisilazane (HMDS) HN[Si(CH3)3]2 (98%, Fluka) were used.

The SiO2 basic sol (sol S) was prepared by dissolving the precursor (TEOS) in anhydrous ethyl alcohol (EtOH) and adding 36% hydrochloric acid (HCl) as a catalyst. The molar ratios of TEOS/EtOH/HCl components were 1:20:0.6.

The sol labeled as M was obtained by adding to the sol S methyltrimethoxysilane (MTMS) so that the molar ratio of TEOS/MTMS was 1:0.64.

The third type of sol was obtained in two stages. In the first stage, the precursor (TEOS) was dissolved in anhydrous ethyl alcohol (EtOH) and 25% NH4OH was added as the catalyst. The TEOS/EtOH/NH4OH molar ratios were 1:20:0.1. Then, hexamethyldisilazane (HMDS) was added to the above mixture. The TEOS/HMDS molar ratio was 1:0.5. The second step was mixing the prepared liquid with the same volume of sol M (with the addition of MTMS). In the sol thus prepared, the MTMS/HMDS molar ratio was 1.28:1. This sol was labeled as MH.

After preparing the sols MH and M (hydrophobically modified) the amount of solvent (EtOH) was doubled to give the molar ratio TEOS/EtOH 1:40 to ensure a longer life (extended gelation time in the vessel) of these sols.

Zinc nitrate Zn(NO3)2·6H2O (98%, Chempur) was used as additive for each type of prepared sol in a molar ratio of TEOS/Zn = 1:0.01, 1:0.05 and 1:0.1. After the addition of zinc nitrate, the sol was stirred continuously using a magnetic stirrer (Wigo, Piastów, Poland) for about 10 h at room temperature.

In addition, in order to compare the properties of the coatings doped with Zn derived from di fferent chemical compounds, zinc acetate Zn(CH3COO)2·2H2O (98%, Chempur), was used instead of zinc nitrate additive to the sol M. The molar ratio of TEOS/Zn was preserved and was 1:0.01, 1:0.05 and

1:0.1. After the addition of zinc acetate, the sol was stirred for 2 h at 60◦C (to accelerate the dissolution of Zn compound), then 8 h at room temperature.

The priority of choosing the amount of additives was the balance between obtaining the properties of hydrophobic coatings and preserving the stability of the sol as well as the possibility of dissolving the appropriate amount of Zn compounds in the sol. The sols obtained in the manner described above remained stable (i.e., they did not show a tendency to gelate in the vessel or a distinct change in viscosity) for several months.

The labeling of the prepared sols and the chemical compounds used to prepare them are shown in Table 1.


**Table 1.** Labeling of prepared sols.

Prior to the deposition of coatings, the sols were aged at room temperature for 7 days. The coatings were applied to basic laboratory glass slides with a thickness of 1 mm, as well as on wafers of monocrystalline silicon with an orientation (100) and thickness of 0.3 mm. The surface of the substrates was about 2 cm2. Before the deposition process, the substrates were washed in an ultrasonic washer (SONIC-3, Polsonic, Warszawa, Poland) in ethyl alcohol and dried with a stream of compressed air.

Coatings were deposited by dip-coating method using dip-coater (TLO 0.1, MTI Corporation, Richmond, CA, USA), by immersing the substrate in a sol and then withdrawing it at a constant rate of 0.2 mm/s. After applying the coating it was dried at room temperature.

#### *2.2. Characterization of Coatings*

The evaluation of coating morphology was carried out using optical microscopy (MO) (MM100, Optatech, Warszawa, Poland) and scanning electron microscopy (SEM) (S-3000M, Hitachi, Tokyo, Japan). Magnifications, 200× (MO) for coatings deposited on glass and silicon substrates and 1000× (SEM) for coatings deposited on a silicon substrates, were applied.

Surface topography of chosen coatings deposited on silicon substrate were measured using atomic force microscope (AFM) (MULTIMODE 5, Bruker Corporation, Billerica, MA, USA) working in tapping mode. All investigations were performed under ambient conditions. Image acquisition was performed with the use of Nanoscope 7.3 software and further image processing was done using Nanoscope Analysis 1.5 software (Bruker Corporation). For each sample, the area of 10 × 10 μm<sup>2</sup> was scanned. From topography images, commonly used roughness parameters were determined (average values taken from 512 surface profiles).

The thickness of the coatings deposited on silicon substrates was determined using X-ray reflectometry by measuring the intensity of the X-ray beam reflected from the surface of the sample. The tests were carried out using an X-ray di ffractometer (EMPYREAN, Malvern Panalytical, Worcestershire, UK) using characteristic CoKα (1,7903 A) radiation. Measurements were carried out in the parallel beam geometry in the omega–2theta angle range of 0.1 to 3 degrees. The value of the critical incidence angle was 0.19–0.21 degrees.

The contact angle, defined as the angle formed between the surface on which the drop of water has been deposited and the surface tangent to the drop at the point of contact with the surface, was determined using the drop shape analyzer (DSA100, Kruss GmBH, Hamburg, Germany). The measurements were carried out for coatings deposited on glass substrates.

The chemical structure of the coatings was studied using an Fourier-transform infrared (FTIR) spectrometer (Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA) in the range of 4000–400 cm<sup>−</sup>1. The study was carried out in transmission mode, recording the absorbance of IR radiation passing through coatings deposited on silicon.
