A Copper-Based Coating for the Control of Airborne Viable Bacteria in a Prison Environment

Abstract

Infections in confined environments can spread by direct contact, contaminated surfaces, and airborne transmission. This is critical in prison facilities, where cleaning and sanitary conditions are inadequate. An alternative is the development of antimicrobial surfaces. A new antimicrobial coating was developed by incorporating copper microparticles into a standard commercial paint, aiming to reduce the concentration of bacteria on surfaces by granting antimicrobial properties to surfaces. The copper additive comprised Cu₂Cl(OH)₃ deposited on polyhedral zeolite. The efficacy of this coating was evaluated in detention cells in a police station, which are temporary prisons and inherently dirty environments. The experiment compared a cell painted with the copper additive coating and a control cell with the standard paint. Viable coliforms were measured on different surfaces and in the air for five months under normal usage. Bacterial load was reduced by ca. 68% by the copper-amended paint on cement surfaces. Surprisingly, airborne viable coliforms were reduced by ca. 87% in the detention cell treated with the copper coating. This research highlights the potential of antimicrobial coatings in controlling the spread of infections through contact with contaminated surfaces and emphasizes the significant reduction in airborne bacterial load. It is especially relevant for controlling infections where sanitization is limited but can be extended to other built environments, such as healthcare facilities.

1. Introduction

The global incarcerated population has increased by 24% since 2000. The total prison population has increased by 82% in Oceania, 43% in the Americas, 38% in Asia, and 32% in Africa. Europe is an exception, with a decrease of 27%. In contrast, the increase in South America is particularly significant, with a rise of 200% [1]. Latin American prison systems, including temporary detention cells, are characterized by overcrowding, poor hygienic conditions, and limited access to healthcare. These sub-optimal conditions promote the spread of infectious diseases among inmates [2,3]. Persons deprived of their liberty (PDLs) concentrated in these high-risk environments are exposed to infections such as COVID-19, HIV, hepatitis C, and tuberculosis [4]. The epidemiological impact of incarceration also extends to families, creating a health and human rights crisis for PDLs. In this context, it is essential to improve sanitary conditions.

In prison, infections can spread by consuming contaminated water, airborne transmission, or feces. Direct contact (i.e., physical contact between persons) is often required, but contaminated surfaces and objects can act as vehicles for pathogen dissemination. Frequent cleaning is thus required, which may not occur in most prison facilities. Undeniably, the inadequate sanitary conditions in the cells represent a challenge both from a public health perspective and a technological standpoint. The challenge lies in the need to develop and research materials with antimicrobial properties capable of replacing critical contact surfaces in the cells without altering their structure and infrastructure. Antimicrobial organic surface coatings such as paints, gels, and varnishes are versatile solutions that can be applied to various substrates in a standard manner, making them a highly adaptable option for improving sanitary conditions.

Antimicrobial materials such as copper and its alloys can reduce the bacterial load of pathogenic microorganisms in a few hours [5]. In 2008, the United States Environmental Protection Agency (EPA) approved the registration of copper alloys as antimicrobial materials, stating that they benefit public health. The mechanism of action is partly attributed to the formation of cupric ions (Cu²⁺). Cu²⁺ can form complexes with sulfur, nitrogen, or oxygen in functional groups, causing defects in the structure of nucleic acids and proteins, thus affecting microorganisms’ cell viability [6]. Copper or copper nanoparticles can be embedded in an organic matrix, such as paints, to create an antimicrobial surface coating. Incorporating copper into these coatings can significantly mitigate bacterial contamination. It has already been applied in hospitals in Europe, the United States, and Chile, and copper-coated surfaces have been shown to reduce pathogen loads [7].

Compared to other metals, copper stands out for its antimicrobial efficacy. It is preferred over silver and gold because it can eliminate bacteria, viruses, and fungi across various environmental conditions. This effectiveness remains intact even in dry or cold environments, where silver loses its efficiency [8,9]. Although silver is less toxic to fungi, its efficacy is limited compared with copper [10]. Gold has antimicrobial properties as a nanoparticle but is significantly less effective and much more expensive than copper and silver, making it a less practical option for infection control [11]. Copper’s affordability and abundance reinforce its use in settings like hospitals, where it has been shown to significantly reduce nosocomial infections when used on contact surfaces [12]. For these reasons, copper surpasses silver and gold’s cost and efficacy limitations in various contexts.

The efficacy of copper in reducing pathogenic microorganisms has been the subject of extensive research [13]; however, there is still an imperative for additional research and more comprehensive in vivo studies to determine the correlation between copper usage and decreased infections conclusively and to thoroughly authenticate the efficacy of copper to mitigate infections in critical environments like hospitals and clinics [14]. In situ clinical tests have been performed on toilet seats, brass tap handles, and brass doors, where copper and copper-based materials reduced around 100% of microorganism loads [15]. Copper alloy-surfaced objects can also reduce the risk of hospital-acquired infection in intensive care units [16]. The biocidal properties of copper have been extended to its particles. Various researchers have confirmed the size-dependent effect of copper nanoparticles, where lower diameters show better antimicrobial effects thanks to nanomaterials’ high specific surface area and surface energy [17,18]. Copper-based materials added to polymer/plastic matrices have also demonstrated antimicrobial properties, implying that copper-based materials can be incorporated into paints to be applied in real environments. For instance, an organic paint coating with nanostructured zeolite/copper particles has been produced and tested in a hospital environment, with a reduction of ~50% in microbial load [19]. Water distribution systems made of copper also have great potential to suppress some microorganisms’ growth compared to plastic or steel systems [20,21,22]. Many authors have demonstrated the antiviral properties of copper and its derivatives, including the inhibition of SARS-CoV-2 [23].

Although the effectiveness of antimicrobial surfaces preventing microbial growth on the treated surface is well known, the effect on indoor air has yet to be studied. Treating rooms with antimicrobial coatings can decrease—but not eliminate—the microbial load of airborne microorganisms such as the model bacteria Escherichia coli and Staphylococcus aureus [24] or Enterococcus faecalis [25]. The antimicrobial paints can also decrease viable airborne fungi in hospital wards [26]. However, the dissemination of airborne microorganisms has been mainly tackled by developing antimicrobial filters for air conditioners. Even when filters can be highly effective [27,28], air conditioners are obviously absent in detention cells.

This study developed an innovative antimicrobial coating that supports copper on an inert carrier such as zeolite, intended to provide cost-effective antimicrobial properties to surfaces. To achieve this, an experiment was conducted in detention cells with high PDL turnover and limited hygienic conditions. Cell surfaces and air coliform load were tested for five months to assess the coating’s effectiveness in reducing bacterial load. The copper coating significantly decreased the coliform bacteria on surfaces, with its most notable effect being in the air. This innovative approach seeks to enhance environmental hygiene in critical spaces where conventional cleaning is inadequate or impractical.

2. Materials and Methods

2.1. Synthesis and Characterization of Additive

Copper was supported on synthetic polyhedral zeolite by applying the procedure previously reported by Kim et al. [29]. The zeolite provided by Shanghai Jiuzhou Chemical has micropores (0.7–2 nm), which constitute 52% of the total pore volume, and mesopores (2–50 nm), which represent 48% of the total pore volume [30].

Briefly, 11.16 mmol of Copper (II) chloride (Sigma-Aldrich) in the presence of 11.74 mmol of ammonia solution (Sigma-Aldrich) was mixed in 100 mL of deionized water with 1.5 g of zeolite at room temperature for 6 h. Afterward, the solution was filtered and dried at room temperature for 2 h. Finally, the products were ground in an agate mortar. This process yields copper deposited on polyhedral zeolite, now referred to as copper additive (CuAd).

The morphology of CuAd was examined by scanning electron microscopy (SEM) in a Zeiss EVO MA10 equipment (Jena, Germany) with a voltage acceleration of 20 keV. Samples were coated with a thin layer of gold to avoid charging during measurements. The equipment is equipped with an Oxford X-Act energy dispersive spectrometer (EDS, Oxford, UK) for quantitative elemental analysis using point and area scan modes. The average particle sizes were determined by analyzing the SEM micrographs with the ImageJ 1.53 k software. The crystalline phases of the samples were identified using a Bruker D2 XRD equipment (30 kV, 10 mA, Billerica, MA, USA) with CuKα radiation (λ = 1.5418 Å). The diffraction patterns were obtained in the scan range of 10° < 2θ < 80°.

2.2. Preparation of Copper Coating

The base coating was a commercial oil paint based on an alkyd formulation in turpentine solvent (Óleo brillante, Tajamar, Santiago, Chile). This is the standard paint coating used in the routine maintenance of the detention cells used in this experiment. For the copper coating preparation, CuAd was ground in a ceramic mortar, sieved through a 200 Mesh (0.074 mm), mixed with the base paint at 0.1%, 2%, and 5% w/v, and mixed for two hours at 120 rpm using a 3-blade propeller stirrer. A test was conducted to determine the optimal CuAd concentration that inhibits bacterial growth by painting a square of 0.5 m × 0.5 m and measuring bacterial load as described in point 2.4.

2.3. Study Design

Two contiguous detention cells were used to compare the antimicrobial properties of the coating. The cells are in the 4th Police Station (33.4653 S, 70.6455 W) at the center of Santiago, Chile, with high people circulation, high PDL turnover, and limited sanitization. This study was limited to five months due to time constraints imposed by the operating conditions of the Chilean national police force, as it would not be possible to alter the regular use. Each cell was 2.8 × 2.8 × 2.6 m (20.4 m³), constructed in solid concrete on the floor, ceiling, and 3 walls; on one side, there was a structural seat made of concrete. The front consisted of steel bars with a 2 m × 1 m steel door (Figure 1). Cell #1 was painted with the 5% CuAd paint, while cell #2 was painted with the base oil paint as a control. Both coatings were applied using standard rollers. After 72 h, cells were used normally. Samples were collected 10 times during five months, roughly every two weeks. The average number of occupants was 80 PDL per month, and the duration of occupancy ranged from 12 to 24 h.

2.4. Antimicrobial Test: Sample Collection, Processing, and Analysis

The coliform load was assessed with the EnSURE luminometer (Hygiena LLC, Camarillo, CA, USA), based on the quantification of ATP via a chemiluminescent reaction [31]. The MicroSnap Coliform enrichment device (Cat. No. MS1-CEC) and detection device (Cat. No. MS2-coliform) were used following the manufacturer’s instructions. Briefly, surface samples were collected with the sterile swab included in the MicroSnap Coliform enrichment device. For each cell, 8 surface samples and 1 air sample were collected at each time point: 2 from the structural seat, 2 from the walls, 2 from the floor, and 2 from the steel bars (Figure 1). Surface samples were collected by applying the sterile swab 20 times in all directions, rotating it over a 30 × 30 cm (900 cm²) square surface. Samples from the steel bars were collected from the door (both the bars and the handle) using a sterile swab in the same manner but on an irregular surface of 900 cm². Air samples were collected using an industrial vacuum cleaner (Kärcher NT 30/1, Winnenden, Germany) to pump air through a 47 mm polycarbonate filter (Whatman Cyclopore, 1 µm pore size, Merck, Beaconsfield, UK) for 30 min. Afterward, the filter was wiped with another sterile swab. The location of each sample is shown in Figure 1. Cell temperature and humidity were recorded with a generic temperature and humidity meter. The 18 enrichment devices collected each time were incubated at 37 °C for 7 h to enrich coliforms. After this, 1 mL was transferred into the detection device, incubated for 30 min at 37 °C, and loaded into the EnSURE luminometer. Relative Luminescence Units (RLUs) were transformed into Colony-Forming Units (CFU) following the conversion table from the manufacturer’s protocol.

2.5. Statistical Analysis

Shapiro–Wilk normality tests (n < 50) were performed for the CFU distributions for each surface (air, bars, door, seat, and wall), segmented by the variable CuAd (with/without). Since the distribution was not normal, the non-parametric Wilcoxon-Mann–Whitney mean difference test (U Mann–Whitney) for independent samples was used. In all cases, a significance level of 5% was applied. For the descriptive analysis, boxplots were reported for the CFU of each of the surfaces (air, bars, door, seat, and wall), segmented by the variable CuAd (with/without), in addition to a table of means and standard deviation for CFU segmented by CuAd (with/without), indicating the average percentage reduction in CFU on CuAd surfaces. All analyses and graphs were performed using R software, version 4.1.2.

3. Results and Discussion

3.1. Morphology and Composition

CuAd was composed of Cu₂Cl(OH)₃ deposited on polyhedral zeolite. The size distribution shows microparticles of functionalized zeolite with a size distribution between 2 and 3 µm, with an average grain size of approximately 2.5 µm (Figure 2). The EDS data show the presence of Al (10.9%), Si (9.6%), Na (4.2%), and Cu (14.2%), confirming the functionalization of zeolite with copper. According to the EDS data and the formula of the zeolite (Shanghai Jiuzhou Chemical, Shanghai, China), which corresponds to a type 13X-APG, we can propose the following as a general formula: (Na⁺_(1−x)(Cu²⁺_x)_0.5Al₁Si_1.2O_4.5·XH₂O)(Cu₂Cl(OH)₃). This is because the expected ratio between the elements should be 1Al:1.2Si:1Na:1Cu, but as can be observed in the EDS experimental data, Cu²⁺ is higher and Na⁺ is lower, and the ratio between Al³⁺ and Si⁴⁺ is almost 1:1.2. Thus, we infer that Na⁺ ions have been exchanged with Cu²⁺ ions. These values confirm the typical zeolite structure, characterized by a high oxygen concentration, along with aluminum and silicon, and, as explained above, the low sodium proportion may be due to its exchange with copper.

Figure 3 shows the powder X-ray diffraction (PXRD). The sample is crystalline with some amorphous material. The observed peaks are consistent with the presence of Cu₂Cl(OH)₃ and zeolite [32,33], corroborating the SEM findings that the copper is supported on the zeolite. Micrometric zeolite was selected as the copper carrier, leveraging its high porosity and surface area, which enables a dense loading of active copper in a compact space [32]. This facilitated the efficient use of CuAd (5% v/w), preserving the usability and application characteristics of the base paint. The structure of CuAd, with microparticles of 2 to 3 µm and containing copper on its surface, might have enhanced its antimicrobial action by improving the interaction of copper with microorganisms without visibly affecting the homogeneous distribution of the additive in the base paint. This study did not address how CuAd is distributed within the paint matrix, but no clumping or sedimentation was detected.

Copper, as an antimicrobial agent on a zeolite carrier, is highly efficient due to the unique properties of zeolites. Zeolites possess a porous structure and an excellent ion exchange capacity, which allows for the controlled and sustained release of copper ions. This gradual release prolongs the antimicrobial action, ensuring continued effectiveness over time. The copper ions are well distributed across the zeolite surface, maximizing their exposure and enhancing their ability to neutralize a broad range of pathogens. Research has shown that materials like copper-loaded zeolites are particularly effective in releasing ions in a controlled manner, making them ideal for coatings, medical devices, and other antimicrobial uses [33,34].

3.2. CuAd Effectively Reduced Coliform Load

Before the experimental setup in detention cells, we tested the antimicrobial activity of different CuAd concentrations in oil paint. CuAd 0.1% yielded ~285 CFU, CuAd 2% yielded ~254 CFU, and CuAd 5% yielded ~8 CFU. Between 0.1% and 5%, there was a 97% reduction; therefore, 5% was used for the detention cell experiment.

Surfaces in cell #1, painted with 5% CuAd, had lower bacterial loads than those in control cell #2. This difference was significant through the five months (20 weeks) and consistent for the air and all surfaces (Figure 4). On 23 and 28 November, bacterial load was exceptionally high, but even in these days, cell #1 had a lower load than cell #2. Temperature was 25.8 ± 4.4 °C, and relative humidity was 39.4 ± 7.3% during this study. The increased bacterial load on these dates can be attributed to the violent clashes between high school students and police in Santiago’s central region during this period. The police station in the city’s heart saw a significant rise in detainees, leading to cell overcrowding, which likely contributed to the elevated bacterial contamination observed on these specific days. However, no correlation was found between bacterial load (CFU) and temperature or humidity (Supplementary Table S1).

CuAd reduced the coliform bacterial load by 68%–87%. The floor, seat, and wall had reductions of 73%, 71%, and 68%, respectively. Those three surfaces were painted, so these results were expected and in agreement with the experimental setup. Surprisingly, the most remarkable differences were for steel bars and air, with 81% and 87%, respectively (

Table 1. Reduction in viable coliform bacteria using coating with CuAd on air and surfaces, including steel bars, floor, seat, and wall.

Sample	n	Mean (Standard Error) for CFU		Reduction Percent
		with CuAd	without CuAd	%
Air	10	3.00 (1.69)	23.07 (5.13)	87
Bars	20	5.75 (1.46)	30.85 (6.89)	81
Floor	20	12.28 (2.75)	45.18 (10.10)	73
Seat	20	8.49 (1.89)	28.85 (5.78)	71
Wall	20	12.21 (2.73)	37.87 (8.47)	68

).

The lower bacterial load in the air and steel bars suggests that bacterial load reduction on painted surfaces influences the surroundings, probably by the re-dispersion of particles attached to surfaces by human movement and touching [35]. The notable decrease in airborne bacterial load, achieving an 87% reduction compared to surface levels, highlights the efficacy of antimicrobial polymer coatings and paints in confined spaces for controlling airborne pathogens. This innovative approach, still underexplored, shows significant potential for minimizing the risk of infections, especially in critical areas like hospitals, by preventing microbial contamination and improving air quality [24,26,27]. This is because biological particles in the air, such as bacteria, fungi, and viruses, can be responsible for infectious pathologies or immunoallergic symptoms [25]. Developing new surface antimicrobial materials has been a hot topic since the COVID-19 pandemic and has yet to be extensively investigated. For instance, organic-based materials such as tannic acid [36], chlorhexidine digluconate [37], and the surfactant Tween-20 with and without NaCl [38] have been applied as coatings with varying efficiencies against bacterial, fungal, and viral loads.

Considering the 5% of CuAd in the coating and the 14.2% of Cu in CuAd, the actual copper concentration in the paint was as low as 0.71% (w/v). This study underscores the antimicrobial efficacy of copper in paints with a reduced concentration of 0.71%, contrasting with previous research that recommended higher concentrations. For instance, in hospital settings, the recommended concentrations are 60% copper in health facility alloys [5], 16% copper oxide in acrylic polyester [39], and 60% for antimicrobial coatings in methyl methacrylate resin [40]. Similar studies on polyester surface coatings applied electrostatically, containing 5 wt% nanostructured zeolite/copper particles, showed a reduction of ~50% in microbial load [19]. Likewise, research on polypropylene composites with copper nanoparticles reports that even at low concentrations (e.g., 1%–10% by volume), copper is highly effective in reducing bacteria such as E. coli and S. aureus [7]. This aligns with our findings, where copper at 0.71% w/v effectively reduced bacterial load. The observed effectiveness in reducing coliform suggests that even lower concentrations of copper can effectively prevent infections, presenting a feasible, cost-effective option for infection control even in unhealthy environments such as detention cells.

Applying copper-based antimicrobial coatings in Chilean prison cells addresses the problem with cleaning protocols and high population density. Applying antimicrobial paints includes the use of copper as a water purifier, algicide, fungicide, molluscicide, antibacterial agent, and antifouling agent, and its proven efficacy against viruses like SARS-CoV-2 [7,23,24,41]. The potential of copper-based antimicrobial coatings transcends prisons. It encompasses a wide range of critical environments, including hospitals, healthcare institutions, food and textile industries, water purification systems, and schools, where their ability to inhibit the growth of microorganisms and prevent the spread of diseases provides a scalable and easily applicable solution. However, in this study, we only assessed coliform load, and further studies are needed to address other pathogens.

4. Conclusions

Based on the experimental data obtained, we demonstrated that antibacterial surfaces in a confined space can reduce the bacterial load throughout the entire space. This reduction is particularly relevant in detention cells, where cleaning protocols and hygienic conditions are often inadequate. Our study demonstrated that incorporating 0.71% w/v of copper into a standard commercial paint, where the copper additive consisted of Cu₂Cl(OH)₃ deposited on polyhedral zeolite, resulted in significant antimicrobial effects. This was tested in highly contaminated environments, such as detention cells. The results showed a ~68% reduction in viable coliforms on surfaces and an ~87% reduction in airborne coliforms in the treated cell, highlighting the effectiveness of copper in reducing bacterial load in both surface and airborne environments. Using zeolite as a support for copper was essential in enhancing the antimicrobial efficiency. Zeolite’s porous structure facilitates the controlled release of copper ions, increasing the longevity and effectiveness of the coating. This makes the combination of Cu₂Cl(OH)₃ and zeolite an ideal system for applications requiring prolonged antimicrobial activity, such as in detention cells and hospitals. This opens new possible applications for other spaces that need sanitizing, such as hospital patient rooms, where methicillin-resistant Staphylococcus aureus (MRSA) and multi-drug-resistant Gram-negative strains can be dispersed by nursing activities [42]. The more human activities occur in built environments, the more probable the dispersion of resistant strains. For instance, disseminating antibiotic resistance genes through the air constitutes an integral dimension of the human–animal–environment loop under the “One Health” perspective [43]. Our study only tested Enterobacteria, but copper materials have proven effective against Gram-positive bacteria such as S. aureus and Clostridium difficile, including spores [44,45].

Further experiments are needed to assess the effectiveness of CuAd against other bacterial groups. However, the coliform bacterial load was reduced with a low copper concentration under limited hygienic conditions, highlighting the potential of the coating to mitigate infection risks, especially against airborne pathogens. Applying these antimicrobial surfaces promises to significantly improve air sanitization, an aspect that has so far received limited attention.