**1. Introduction**

The problematic relationships between environmental pollution and health preservation and prolongation are a subject of current interest and are, therefore, widely studied. Most health problems are associated with the presence of microorganisms such as bacteria, fungi, protozoa, and viruses that commonly infect humans in the living environment, leading to chronic infections and even leading to mortality [1–3]. The inanimate surfaces of different materials are often described as the source of hospital outbreaks because their contact with different microorganisms allows microbial permanence for a period of time favoring bacterial proliferation, and they are, therefore, reservoirs of bacteria that cause diseases transmitted through surfaces or fomites. This continues to be a cause of great concern for the medical domain, with a significant economic burden. One of the main causes of morbidity is infection with methicillin-resistant *Staphylococcus aureus* (MRSA) [4,5]. These microorganisms also contribute to severely deteriorating surfaces, greatly reducing the durability and increasing the repair costs of the material [6]. Considering this dynamic, a material with antibacterial surfaces would be ideal to prevent microbial proliferation and, likewise, contamination.

Therefore, surface materials and antibacterial systems are of the utmost importance not only in hospitals and sanitary environments but also for domestic, industrial, and marine applications, among others [7]. Various studies were carried out with the goal of improving the antibacterial capacity of construction materials as glazes on ceramic tiles and pastes based on alkali-activated slag [2,8,9]. Among these construction materials, the use of geopolymers is a great option due to their high alkalinity and ease of functionalization by incorporating semiconductor materials (ZnO, TiO2, CuO, and Fe2O3), which, when exposed to UV and UV–Vis radiation, exhibit a functional activity, i.e., photocatalytic properties [10,11]. One of the most studied semiconductors is titanium dioxide (TiO2), a harmless material that is highly resistant to photocorrosion, stable in aqueous solutions, inexpensive, and abundant in nature, which also exhibits a desired high photocatalytic activity under ultraviolet irradiation [9,12–14]. Semiconductor particles as TiO2, Fe3O4, ZnO, and CuO can cause inactivation of bacteria (i.e., elimination of bacteria) and viruses in different types of environments [2,15–17]. These metal oxides were incorporated into the surface of different materials using advanced deposition techniques, such as chemical vapor deposition, ion implantation, sputtering, and electrochemical deposition of a solution. However, these technologies are expensive and difficult to apply to large-volume particles or complex shapes [18].

In general, the antimicrobial activity of CuO and TiO2 metallic nanoparticles was studied with Gram-positive and Gram-negative bacteria [19–21]. Although the nanoparticles tended to inhibit the bacteria in all cases, it was observed that the effectiveness depends on the morphology and particle size. Hasmaliza et al. [9] evaluated the antibacterial properties of ceramic tiles coated with enamel mixed with anatase TiO2 by exposing the tiles for different times (0, 2, 4, and 8 h) to the bacterium *Escherichia coli*; the TiO2 content (5, 10, and 15 wt.%) and particle size of the oxide were also varied, and, at a longer exposure time, the number of colony-forming units (CFUs) decreased. At the same time, the nanometric particle sizes favored the antibacterial properties due to the greater surface area available to contact the bacteria; in contrast, in the presence of a greater amount of TiO2, the antibacterial yield was lower [9]. Kumar et al. [20] evaluated the antibacterial activity of a polymer nanocomposite containing TiO2 and CuO nanoparticles with satisfactory results. Haider et al. [19] evaluated the photocatalytic and antibacterial activity of TiO2 nanoparticles synthesized via the sol–gel method and calcined at different temperatures (400, 600, 800, and 1000 ◦C). TiO2 was exposed to two types of bacteria, *Pseudomonas aeruginosa* and *S. aureus*, and it was 100% effective in eliminating these bacteria under solar irradiation.

Geopolymers based on metakaolin, halloysite clay, and fly ash with TiO2 nanoparticles reported photocatalytic properties. These materials allow functional ceramics to be produced for self-cleaning ability, removing dyes as B-rhodamine and methylene blue, and leading to nitric oxide degradation [11,22–28]. Nanoparticles are also used in dental applications [10,29]. However, the information on the antibacterial effect of nanoparticles incorporated in geopolymeric pastes or mortars is limited [30]. TiO2 microparticles (20 and 50 wt.%) and ZnO nanoparticles were incorporated in geopolymer pastes based on metakaolin (MK) [31,32], where the authors reported a bacteriostatic effect in the presence of contaminated water. Similar results were obtained using MK-based geopolymer mortar doped with copper [6]. Geopolymers based on fly ash and calcined baluko shells incorporating Ag nanoparticles were also used to evaluate their antibacterial capacity [33,34]. These studies showed that the nanoparticles can be used to prepare geopolymers with satisfactory inhibition capacity for the growth of bacteria. Additionally, it was reported that Portland cement mortars with added glass containing 2 wt.% TiO2 and the incorporation of nanosilica exhibited the ability to inhibit *E. coli* growth in just 30 min [35].

The objective of this preliminary study was to determine the effects of incorporating nanoparticles of metallic oxides, such as TiO2 and CuO, on the antimicrobial potential of a geopolymeric binder based on metakaolin (MK), and its corresponding geopolymer mortar using glass waste as a fine aggregate. Geopolymer mortars were fabricated for their use as coatings of construction elements for environments susceptible to bacterial growth. Among the microorganisms potentially pathogenic to humans were selected Gram-negative bacteria such as *E. coli* and *Pseudomonas aeruginosa* and Gram-positive bacteria such as *S. aureus*. Infections with these bacteria are recurrent and transmissible to people close to each other [4,5,36,37]. In addition, this preliminary study proposes the direct incorporation of such nanoparticles by mechanically mixing the components of the material in order to directly apply the geopolymer mortar on the substrate surfaces instead of using conventional deposition processes to produce oxide coatings.

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

#### *2.1. Materials*

To obtain the geopolymer cement paste, the precursor used was metakaolin (MK MetaMax, BASF, Florham Park, NJ, USA). A mixture of a commercial potassium silicate (SiO2 = 26.38%, K2O = 13.06%, H2O = 60.56%), analytical grade potassium hydroxide (KOH), and water was used as an alkaline activator. The cementitious material was formulated using the following molar ratios of oxides, as determined by previous studies: SiO2/Al2O3 = 2.5 and K2O/SiO2 = 0.28 [38,39]. To produce the geopolymer composite with photocatalytic and antibacterial properties, titanium oxide (TiO2) and copper oxide (CuO) particles were added. The TiO2 used was high-purity and analytical grade (Merck, reference 1008081000). The CuO nanoparticles were synthesized from copper acetate ((CH3COO)2Cu·H2O) using the modified Pechini method [40] (Figure 1). To evaluate the cementitious properties, the pastes and mortars were prepared. The mortar was prepared using glass waste (G) as a fine aggregate.

To synthesize the CuO nanoparticles, citric acid and ethylene glycol were initially mixed in 100 mL of distilled water, and the mixture was heated to 70 ◦C. Then, copper acetate was added. These precursors were mixed with a molar ratio of 1:1:2, and the solution was constantly stirred using a magnetic stirrer. Simultaneously, an NH4OH solution was added until a neutral pH (pH = 7) was obtained. The resulting polymeric resin was subjected to an initial thermal treatment at 350 ◦C; the thermally treated powder was pulverized using a ceramic mortar. The resulting fine powder was thermally treated at 450 ◦C. The heating rate used in the two heat treatments was adjusted to 10 ◦C/min.

#### *2.2. Preparation of the Geopolymers*

To prepare the geopolymers, the components were firstly mixed in a solid state; then, the activating solution was added. The components were mixed for 15 min using a Hobart mixer. Next, the mixture was poured into silicone molds, and discs with 2.5 cm diameters were prepared. The liquid/solid ratio was 0.35 for the pastes and 0.40 for the mortar. The samples were wrapped with a plastic film to prevent moisture from evaporating and kept for 24 h in a chamber with a relative humidity (RH) >90% and a temperature of 25 ◦C. Subsequently, the samples were demolded and stored for 28 days. Figure 2 shows the experimental methodology followed in the study in order to prepare the geopolymer composites. Table 1 shows the compositions and codes of the mixtures evaluated. Two geopolymer pastes (GP, mGP) were prepared using 10 wt.% titanium oxide (TiO2) and 5 wt.% copper oxide (CuO) particles. The percentage of TiO2 incorporated was selected based on previous studies [11], where a high photocatalytic capacity for degrading B-rhodamine was achieved using 10 wt.% TiO2. Geopolymer mortars (GP-G) were manufactured with a binder/fine aggregate ratio of 1:2 by weight using glass waste (G) as fine aggregate. The photocatalytic capacity of MK-based geopolymer mortars (GP-G) increased up to 72% compared to that obtained with mortars using natural

sand [41]. Geopolymer mortars with CuO particles were not prepared because the results obtained with the proportion of the oxide used in mGP were negative, as discussed later.

**Figure 1.** Diagram of the methodology used to synthesize the CuO nanoparticles via the Pechini method.

**Figure 2.** Diagram of the methodology used to prepare the geopolymer composites.

**Table 1.** Compositions of the fabricated materials. ID—identifier.

