Metal-Doped TiO₂ Optical Fiber Textiles for Concurrent Removal of Airborne Bacteria and Volatile Organic Compounds

Abstract

This study focuses on the application of photocatalysis for air pollution, targeting both chemical and biological contaminants. The selected target compounds were 3-methylbutan-1-ol (C₅H₁₂O), a volatile organic compound abundantly generated in the food industry, and Escherichia coli, representing a relevant bacterial indicator commonly encountered in such industrial environments and effectively embodying a biological threat. In this work, a series of experiments was conducted in a batch reactor using a novel TiO₂-based photocatalytic system integrating metal wires, namely copper (Cu) and silver (Ag), woven into an optical fiber support. A comparative evaluation of photocatalytic performance across different media was carried out for the removal of 3-methylbutan-1-ol, as well as for E. coli deactivation. The results demonstrated notable performance of the TiO₂-Cu medium for chemical treatment, achieving 97% removal efficiency after 85 min at an inlet concentration of 28 mg·m⁻³. Similarly, significant antibacterial activity was observed with 5.50 log reduction in colony-forming units (CFU) after 2.5 h. The photocatalytic performance of TiO₂-Cu supports was further validated under different operating conditions, including relative humidity levels ranging from 20% to 60% and concentration range from 5–30 mg·m⁻³. Finally, this study also includes a comparison between the TiO₂-Cu support and conventional photocatalytic systems based on TiO₂, particularly for simultaneous (combined) treatment of chemical and biological contaminants, with promising and encouraging outcomes.

1. Introduction

Indoor air contaminants such as volatile organic compounds (VOCs) and bacteria, generated by food processing and preparation, can present serious risks to public health. Many of these microbial isolates are considered potential contributors to sick building syndrome and are often associated with clinical manifestations such as allergies, rhinitis, asthma, and conjunctivitis [1]. These adverse health effects can be caused by bacteria, viruses, molds, and pollens. Therefore, to protect the health of occupants and workers, it is essential to control the environmental factors that promote microbial growth and proliferation in indoor environments, particularly in public and food-handling areas. Consequently, bacterial inactivation and VOC removal remain critical objectives for indoor air quality management [2].

Promising advanced technologies offer cost-effective solutions with good removal efficiency, such as photocatalysis, which has shown effectiveness in reducing chemical and microbiological contaminants, particularly when treating polluted air [3,4,5]. In this context, recent studies have focused particularly on the synthesis of TiO₂-based composite materials and on improving their photocatalytic disinfection efficiency by combining TiO₂ with noble metals as promising prospects for more practical designs in this field [6,7]. Such metal–semiconductor heterojunctions enhance the photoactivity of TiO₂ and improve its efficiency the degradation of air pollutants. Depending on their intrinsic properties, the deposited metal nanoparticles can limit the recombination of photogenerated electron/hole pairs in the semiconductor and thereby promote overall photodegradation through the reactive species produced under irradiation (e.g., $\cdot$OH, $\cdot$O₂⁻, H₂O₂) [8,9]. Current research focuses on synthesizing materials that combine high photoactivity with adequate adhesion, making them suitable for indoor air treatment applications.

Table 1. Some relevant studies focused on synthesizing TiO₂-based composites and improving their photocatalytic disinfection efficiency by combining TiO₂ with noble metals.

Type of Catalyst	Mass/Loading	Irradiation Conditions	Target Microorganisms	Main Outcome (Abatement)	Ref.
Platinum nanoparticles on TiO2 nanotubes (Pt-NPs/TiO2-NTs)	Total deposit (TiO2 + Pt) fixed at 0.4 mg·cm−2	Pt photodeposition using a UVC lamp (254 nm), followed by degradation tests under continuous visible light (400–700 nm)	Escherichia coli, 2 × 104 CFU·mL−1	100% inactivation of E. coli after 180 min of irradiation; electrodeposition for 120 s (0.91 wt.% Pt) outperforms 3 h photodeposition (5.78 wt.% Pt) due to finer, more homogeneously dispersed Pt nanoparticles	[10]
Cu–Ag/TiO2 on optical fibers	Catalyst loading maintained at 1 mg·cm−2 of TiO2	In situ UVA LED (365 nm) irradiation on optical fibers; light intensity set at 1.5 W·m−2	Escherichia coli, 3 × 103 CFU·mL−1	E. coli inactivation of 0.25 and 0.37 log CFU for TiO2-Ag and TiO2-Cu, respectively, under the tested conditions	[11]
Indium sulfide (In2S3)	Catalyst dose set at 1.0 g·L−1 (optimal)	Visible-light-driven photocatalysis under fluorescent tubes (5.2 mW·cm−2), xenon lamp (192 mW·cm−2), and blue LED (100 mW·cm−2)	Escherichia coli K-12, 2 × 107 CFU·mL−1	Complete inactivation after 5 h under fluorescent tubes; inactivation time reduced to 2.5 h under xenon lamp and 10 h under blue LED	[12]
TiOₓ–Au nanocomposites (TiOₓ–Au NCs)	Low TiOₓ–Au NCs loading (catalyst concentration 0.5 mg·mL−1)	Simulated solar irradiation using a xenon arc lamp, 230 W·m−2	Escherichia coli and methicillin-resistant Staphylococcus aureus (MRSA); fungus Candida albicans	TiOₓ–Au NCs (50:1) with light exposure yield a marked reduction (>80%) in bacterial counts and significant antifungal activity	[13]
TiO2/SiO2/Au thin film	Mass indirectly controlled by the number of printed layers; the kinetic constant kmax increases with layer number	Fixed irradiance of 22 mW·cm−2 (UVA-LED) or natural sunlight; only a short UVC pretreatment (1.2 mW·cm−2) after synthesis to remove organic residues	Escherichia coli (102 CFU·mL−1); total coliforms (102–103 CFU·mL−1)	With 8 cm2 of thin film and 7.5 mm water height, complete E. coli disinfection is achieved after 30 min and total coliform removal after 45 min	[14]
Au/TiO2	Different Au loadings tested: 0.5, 1, 3, and 5 mol% Au relative to Ti	Antibacterial tests under continuous visible light (400–700 nm)	Escherichia coli, 107 CFU·mL−1	1 mol% Au/TiO2 shows the best antibacterial performance: survival after 5 h drops to 32.4%, compared to 90.9% in the control without catalyst	[15]
Au-decorated TiO2 nanotubes (Au/TiO2 NTs)	Au mass increases with the number of photo-induced deposition cycles (1, 2, or 3)	Constant LED irradiation (420–480 nm, 12 000 nW·m−2)	Porphyromonas gingivalis, 105 CFU·mL−1; Fusobacterium nucleatum, 105 CFU·mL−1	Maximum antibacterial efficacy obtained for the TNT–Au sample (≈5.5 wt.% Au), with 97.34% inactivation of P. gingivalis and 92.13% of F. nucleatum	[16]
Pt/TiO2	Photocatalyst concentration fixed at 1 g·L−1; Pt content varied from 0.5 to 2 wt.%	Irradiation under a sun-like spectrum, 120 W·m−2	Escherichia coli, 2.9 × 103 CFU·mL−1	Using 2 wt.% Pt–TiO2 and 120 W·m−2 irradiance results in complete removal of coliforms and full E. coli disinfection within 3 h	[17]

summarizes selected recent studies addressing this objective, highlighting the photocatalytic systems employed and the corresponding antibacterial performance.

As such, and in this work, innovative photocatalytic media are employed to enhance bacterial inactivation and VOC degradation, based mainly on the impregnation of antibacterial metals such as Ag and Cu, in photocatalytic supports based on optical fibers coated with TiO₂. The novelty of this work lies in the use of specific textile-like optical fiber technology (TiO₂, TiO₂-Cu, and TiO₂-Ag), adapted to chemical/microbiological treatment applications. The objective of this configuration is to provide a new generation of more compact filters with regular, homogeneous in situ illumination and improved contact between the gaseous phase and the active surface.

The novelty of this work lies in the development of an innovative photocatalytic system based on TiO₂-coated optical fibers integrated with metal wires (Cu and Ag), designed for the simultaneous removal of VOCs and airborne bacteria in a single reactor configuration. Unlike conventional photocatalytic materials, this study introduces a textile-inspired luminous catalyst architecture, enabling uniform in situ light distribution and enhanced contact between pollutants and active sites. Furthermore, the incorporation of copper and silver within the fiber structure creates metal semiconductor heterojunctions, improving charge separation and boosting photocatalytic efficiency. The system exhibits a dual-function capability (chemical + biological treatment) in a compact configuration, while also revealing and analyzing the competition effects between VOC degradation and bacterial inactivation, an aspect that is rarely addressed in such integrated air treatment systems.

2. Results and Discussion

2.1. Photocatalytic Study

2.1.1. Biological Treatment

To investigate the antibacterial activity of the different photocatalytic supports (TiO₂-Cu, TiO₂-Ag, and TiO₂ alone), E. coli was chosen as the target. To approximate realistic contamination levels, a bacterial suspension with an initial concentration of approximately 1.85 × 10⁸ CFU·mL⁻¹ was prepared, then diluted to obtain an E. coli solution at 2.3 × 10⁵ CFU·mL⁻¹. Subsequently, 100 μL of the bacterial suspension was spread on the catalyst surface and illuminated by the same radiation source. After pre-selecting photocatalysis times (0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h), the treated strain was recovered from the catalyst surface using a Tween solution. Serial dilutions were made with tryptone salt broth to facilitate bacterial counting.

After dilution, 100 μL of each solution was plated on Petri dishes containing nutrient solution and placed in an incubator for 24 h at 37 °C to determine the number of bacterial colonies. After incubation, colonies formed on the surface of the culture medium were counted using Equation (1). The efficiency of the different treatments was evaluated using the log inactivation approach, which is a standard method in microbiological and photocatalytic studies to quantify the reduction in viable bacterial concentration.

Figure 1 shows the log inactivation of E. coli resulting from photocatalytic treatment under the same UV intensity at different times with the photocatalytic supports TiO₂-Cu, TiO₂-Ag and TiO₂ alone all containing the same amount of TiO₂, namely 1 mg·cm⁻². The results show that after 2 h, there is a significant inactivation of E. coli with the Cu-impregnated support of the order of 5.52 log CFU, compared with 3.5 and 3.86 log CFU for TiO₂ alone and TiO₂-Ag, respectively.

To model the kinetics of bacterial disinfection, a simplified form of the Langmuir–Hinshelwood (L–H) equation is often used. For a heterogeneous surface reaction, the reaction rate r is defined by

r = −dC/dt = (k_rK_eC₀)/(1 + K_eC₀)

(1)

where k_r is the intrinsic rate constant and K_e is the adsorption equilibrium constant. At low concentrations (K_e C < 1), the equation simplifies to pseudo-first-order kinetics:

$$-\mathit{ln}\left({C/C}_0\right)=kapp t$$

(2)

Table 2. Kinetic constants (k) of bacterial disinfection with photocatalytic media determined with the Langmuir–Hinshelwood model.

Media	TiO2	TiO2-Cu	TiO2-Ag
k (min−1)	0.029	0.046	0.032
R2 (%)	96	98	92

lists the kinetic constants $k$ for bacterial inactivation obtained with the different photocatalytic media, determined from the L–H model.

According to the results obtained in Table 2, the TiO₂-only substrate is the least effective. This behavior indicates that direct contact between silver, copper and bacteria plays an important role in enhancing bactericidal activity [18], where researchers have shown that silver [19] and copper [20] accelerate bacterial inactivation through their disinfectant properties. As mentioned in the literature, the effectiveness of E. coli inactivation under photocatalytic treatment is attributed to oxidative damage, mainly induced by photogenerated reactive oxygen species, produced by redox reactions between adsorbed species and photogenerated electrons/holes.

Cu²⁺ ions incorporated into TiO₂ and Ag⁺ nanoparticles present on the material surface act as electron traps, improving electron/hole separation and decreasing the recombination rate. As a result, they contribute to increasing the inactivation rate compared to undoped TiO₂ [21,22]. The superior performance of TiO₂-Cu (k = 0.046 min⁻¹) can be partly explained by an additional chemical mechanism: copper can reversibly switch between its oxidation states (Cu²⁺/Cu⁺), participating in redox cycles that promote ROS generation.

Assessment in dark mode is essential for distinguishing between photocatalytic activity (induced by UV light) and the intrinsic antibacterial activity of metals (Ag and Cu) through contact or ion release. Figure 2 below shows the results of bacterial inactivation using various media in dark mode at the same concentrations.

The comparative study between dark and UV modes demonstrates that bacterial inactivation by pure TiO₂ is strictly dependent on photonic activation (approximately 0.4 log CFU after 3 h in dark mode), confirming a purely photocatalytic mechanism. Conversely, the doped samples exhibit significant intrinsic toxicity independent of light, with Cu showing the most pronounced activity (2.6 log CFU) due to the release of (Cu²⁺/Cu⁺) ions that disrupt vital cellular functions, followed by Ag with moderate biocidal activity (1.9 log CFU). In conclusion, while the total efficacy observed under UV light (5.5 log CFU) results solely from the photocatalytic activity, for the TiO₂–Cu and TiO₂–Ag composites the overall effect results from a strong synergy combining the intrinsic biocidal properties of the metals with the photonic activation of the semiconductor.

Based on the results in Figure 1 and Figure 2, the remarkable performance of the TiO₂-Cu photocatalyst is driven by a synergistic effect resulting from copper incorporation. This doping optimizes charge separation, limits electron–hole recombination and increases the production of reactive species. Activity is further enhanced by the formation of Cu₂O/CuO-TiO₂ heterojunctions that facilitate charge transfer, as well as the Cu⁺/Cu²⁺ redox cycle, which stimulates surface oxidation reactions. Finally, copper’s intrinsic antibacterial properties, coupled with TiO₂, provide this material with superior catalytic and antimicrobial performance compared to standard catalysts.

2.1.2. Oxidation of Organic Molecules

The photocatalytic performance of the different substrates investigated here was studied for the degradation of C₅H₁₂O at the same input concentration. Figure 3 shows the evolution of C₅H₁₂O concentrations during photocatalytic reactions at the same UV intensity, with different substrates as a function of time.

The results show that, in terms of performance, almost complete removal of C₅H₁₂O is achieved with all photocatalytic media after a certain time. However, in terms of efficiency, a comparison expressed by the degradation rate factor is necessary to highlight the real performance of the photocatalytic media.

Table 3. Kinetic constants (k) for C₅H₁₂O degradation with photocatalytic media determined with the Langmuir–Hinshelwood model.

Media	TiO2	TiO2-Cu	TiO2-Ag
k (min−1)	0.3	0.046	0.027
R2 (%)	99	98	98

shows the kinetic constants (k) for C₅H₁₂O degradation with photocatalytic media. The results highlight the photocatalytic performance of TiO₂ media alone in the face of chemical agents. This can be interpreted by the saturation of several of the catalyst’s active sites responsible for pollutant degradation due to the loading of Ag or Cu on the impregnated support, which reduces the active surface area, and consequently the degradation rate. In addition, the literature reveals that the metals Ag and Cu mainly present considerable antibacterial, antifungal and photocatalytic performances, but they are more resistant to oxidation [23].

However, in our case, the TiO₂-Cu photocatalyst shows more interesting photocatalytic activity than the TiO₂-Ag. This behavior can be attributed to several phenomena. First, added Cu generally forms CuO and/or Cu₂O domains in contact with TiO₂, which create a p-n heterojunction at the Cu₂O/CuO–TiO₂ interface. This junction promotes charge separation by directing photogenerated electrons towards TiO₂ and holes towards the Cu-based oxides, thus reducing electron–hole recombination and increasing the ROS yield. Sassi et al. demonstrated the efficiency of the Cu₂O/CuO–TiO₂ heterojunction for removing organic pollutants, achieving 94% degradation of amido black under UV irradiation (256 nm) [24]. Second, the presence of Cu⁺/Cu²⁺ redox couples can participate in additional surface reactions, including Fenton-like pathways in the presence of H₂O₂, enhancing oxidative capacity [25]. In contrast, Ag nanoparticles act primarily by forming a Schottky barrier at the Ag-TiO₂ interface, but in the absence of significant plasmonic excitation under UV irradiation, their contribution to the injection of hot electrons and the formation of reactive oxygen species remains limited. Duan et al. reported that Ag nanoparticle deposition markedly enhances light absorption and charge separation under visible light; Ag–TiO₂ nanocomposites exhibited superior visible-light photocatalytic NO removal compared with commercial TiO₂ P25, primarily due to the plasmonic resonance of silver nanoparticles [26].

2.2. Effect of Operating Parameters on Chemical Treatment

To further assess the photocatalytic performance of the TiO₂–Cu substrate, a series of tests was carried out over a concentration range from 5 to 28 mg·m⁻³, as well as over a relative humidity (RH) range from 30 to 60%. According to Figure 4a, the results show a trend consistent with data from the literature: increasing inlet concentration leads to higher surface coverage and thus a larger absolute abatement at higher concentrations, indicating that the process becomes predominantly reaction-controlled rather than mass transfer-limited [27,28]. In terms of efficiency, almost complete elimination of the pollutant is achieved even at high concentrations, with a degradation rate of around 97% after 85 min at an inlet concentration of 28 mg·m⁻³.

Regarding the effect of relative humidity, the results shown in Figure 4b exhibit the expected behavior with an optimum performance around 40% RH. At this value, the balance between adsorbed water (source of hydroxyl radicals), pollutant adsorption, and the number of available active photocatalytic sites is favorable for achieving high abatement, in agreement with previous observations reported in our earlier work. At lower or higher RH, either insufficient hydroxyl availability or competitive adsorption of water molecules can limit the degradation [29,30,31].

2.3. Combination Treatment

The efficiency of the Cu/TiO₂ catalyst was also evaluated for bacterial inactivation under the simultaneous removal of Escherichia coli and 3-methylbutan-1-ol. This study could reveal a wealth of information, such as the ability of copper nanoparticles embedded with catalysts to oxidize more than one pollutant at a time, and the effect of multiple pollutants on both photocatalytic activity and the antibacterial performance of the TiO₂-Cu support. To achieve this objective, an initial E. coli concentration of approximately 2.3 × 10⁵ CFU·mL⁻¹ of the strain was deposited on the catalyst in the presence of 3-methylbutan-1-ol ([C₅H₁₂O] = 5 mg·m⁻³). The experiment was carried out in the same batch reactor under UV intensity.

Figure 5 shows the simultaneous elimination of E. coli and 3-methylbutan-1-ol as a function of irradiation time. Bacterial inactivation is shown by the columns, where we can see a decrease in E. coli concentrations, reaching total bacterial elimination in the single pollutant case after 180 min of irradiation.

The results show that the competition effect dominates the combined treatment in our case (E. coli and 3-methylbutan-1-ol), where a clear decrease in the E. coli inactivation rate occurred during the simultaneous VOC treatment, or a better rate appeared only after about 3 h, without reaching total elimination. This could be interpreted by the fact that the degradation of 3-methylbutan-1-ol seems to take the upper hand by occupying the photoactive sites. Indeed, a degradation rate of up to 97% is recorded after around 3 h, at which point the sites appear to be freed up to treat E. coli. However, the degradation rate of 3-methylbutan-1-ol achieved is still far from that of the VOC-only treatment mentioned in the previous results, where this figure was reached after 85 min, clearly highlighting the competition effect described by several authors [32,33].

To develop a competitive kinetic model that quantifies the influence of VOC/bacteria ratios on overall removal efficiency, we use Equations (2) and (4) for VOC/bacteria, where inhibition by bacteria is low because their molar concentration is negligible compared to the gas molecules, and bacteria/VOC, where inhibition by the VOC is significant.

To quantify the interaction between the two species, we use the inhibitory factor “$\mathsf{\alpha }=(kcompound-kmix)/kcompound$” [34], which mathematically expresses how the presence of one pollutant inhibits the degradation of the other. In a Langmuir–Hinshelwood-type competitive kinetics model, the competition factor for bacterial inactivation is calculated as the ratio of the rate constants in the presence and absence of the competitor. The results are summarized in

Table 4. Kinetic constants (k) and inhibitory factor (α) for VOC/E. coli degradation with TiO₂–Cu support, determined using the Langmuir–Hinshelwood model.

Competition Effect	k (min−1)	α	R2 (%)
Bacteria on VOC degradation	0.058	−0.26	98
VOC on bacteria inactivation	0.024	0.48	95

.

The VOC/bacteria competition effect is closely linked to the affinity of each species for active sites and to microorganism robustness. The results show that the degradation kinetics of VOCs are faster in the presence of E. coli, suggesting a synergistic effect where the generated oxidation byproducts or ROS facilitate inactivation. However, this dynamic varies depending on polarity, as highly polar pollutants compete directly with membrane moisture for adsorption on the catalyst. Furthermore, cell wall structure plays a decisive role; while a Gram-negative bacterium such as E. coli is more sensitive, a thicker cell wall (Gram-positive or mold) would mobilize more ⋅OH radicals, thereby slowing VOC degradation due to increased demand for reactive species [32,33]. Conversely, as illustrated in Figure 5 the presence of VOCs can initially inhibit bacterial inactivation (comparison at t = 1 h and 2 h) by occupying reactive sites, before equilibrium is restored over the long term [32,33].

2.4. Mechanisms of Photocatalytic Activity

The simultaneous removal of VOC and E. coli using Cu/TiO₂ photocatalyst involves several phenomena, including charge generation, reactive species formation, and competitive adsorption. Under UV irradiation, TiO₂ generates electron/hole pairs that migrate to the surface. In the presence of copper species, additional charge separation pathways and redox reactions become possible due to the Cu⁺/Cu²⁺ cycling and the formation of Cu₂O/CuO–TiO₂ heterojunction, which improve electron production and extend charge carrier lifetimes [35]. Electrons react with surface Cu²⁺ which are partially reduced to Cu⁺, while holes accumulate in the valence bands of TiO₂ and Cu₂O, increasing the rate of surface oxidation reactions [36]. Under these conditions, reactive oxygen species such as hydroxyl radicals ($\cdot$OH), superoxide radicals ($\cdot$O₂⁻), are generated. These species are responsible for the oxidation of both the VOC and the bacterial cell components. However, the presence of two pollutants induces a strong competitive adsorption and consumption of reactive species [10]. The alcohol (3-methylbutan-1-ol) has higher affinity for surface OH groups and reacts faster with photogenerated $\cdot$OH radicals, resulting in preferential VOC degradation. This reduces the availability of ROS for bacterial attack, explaining the lower inactivation rate observed in the combined system compared with bacteria alone [37]. Furthermore, bacterial inactivation requires a sequence of complex steps, first membrane oxidation, then protein and enzymes denaturation, leakage of intracellular (DNA) content, and eventual cell death (Figure 6). These bacterial degradation steps were discussed by Vu et al. who showed SEM images for E. Coli after photocatalytic treatment [38]. Adding to that, Khraisheh et al. demonstrated the degradation of bacteria based on ROS and the results showed that TiO₂ doped with 10% Cu prepared by wet-impregnation method resulted in a complete 100% E. coli inactivation within one hour (from ∼10⁶ to 0 CFU·mL⁻¹) [39].

Since bacterial removal requires sustained exposure to ROS, this becomes critical after consumption of most of the generated ROS by VOCs. This behavior is consistent with previous studies reporting that simultaneous photocatalytic treatment of organics and microorganisms leads to competitive adsorption and shared consumption of reactive species on catalytic sites [32,33]. The overall mechanisms of pollutant degradation in this system are summarized schematically in Figure 6.

3. Materials and Methods

3.1. Target Pollutants

The biological treatment study was conducted using the Escherichia coli strain E. coli DSM 10198-0307-001, supplied by Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (DSMZ, Braunschweig, Germany), and stored in a cold room at approximately 4 °C. As a model VOC, methylbutan-1-ol (C₅H₁₂O) was tested. This compound belongs to the family of alcohol-based VOCs. Alcohol-type VOCs are commonly emitted from industrial processes and can also originate from household products [1]. Because of their high volatility and potential impact on indoor air quality, C₅H₁₂O compounds are used as model pollutants in photocatalytic and air treatment studies.

3.2. Photocatalytic Supports and Irradiation Modes

Photocatalysis experiments were conducted in a 110 × 10 cm light-emitting textile panel (UVtex^®) supplied by Brochier Technologies (Villeurbanne, France), equipped with a UVA-LED (365 nm) to provide in situ illumination. The spectral characteristics of the light source are presented in Figure S1.

These new materials are double-sided photocatalytic substrates based on TiO₂-coated optical fiber metal wires (copper (Cu) and silver (Ag)) woven using the Jacquard process. The textile surface (polyester, Trevira CS TM fibers) was treated to ensure uniform light output across the entire surface of the optical fibers [11]. Manufacturing details and characterization results can be found in our previous works [40,41]. Figure S2 presents SEM images of the photocatalytic substrates based on an optical fiber with the metal wire.

Figure S3 presents the X-ray diffraction (XRD) spectrum of the TiO₂ coating, confirming its crystalline structure. The diffractogram exhibits distinct anatase and rutile phases consistent with JCPDS standards, highlighted by anatase peaks at 25.3° (101) and 74.0° (215), and a rutile peak between 41° and 42° (111). Additionally, metallic copper (Cu) is identified by a prominent peak in the 89–90° range corresponding to the (311) plane. The reflection observed at approximately 15° is attributed to the underlying optical fiber substrate rather than to the deposited photocatalyst.

3.3. Experimental Setup and Analytical Methods

3.3.1. Experimental Setup

A luminous textile reactor was used, whose detailed configuration has been described elsewhere. A UVA (365 nm) LED connected to a dimmer was used to distribute the radiation across the surface of the catalyst through the fiber filaments. The pollutant was injected into the reactor in the form of liquid droplets which evaporated to generate the gaseous pollution, which was then homogenized using a magnetic stirrer.

The E. coli experiments were carried out using dilution and extraction solutions prepared in advance. Dilution solutions were prepared from tryptone salt broth (Biokar Diagnostics, Allonne, France), while extraction solutions were prepared from NaCl and Tween 80. An agar solution was also prepared to solidify the diluted suspensions in Petri dishes: this agar medium was Tryptic Soy Agar (TSA, GranuCult) supplied by Merck KGaA (Darmstadt, Germany).

First, a stock solution of Escherichia coli (DSM 10198-0307-001) with a concentration on the order of 10⁸ CFU·mL⁻¹ was first calibrated. This stock was then diluted using sterile tryptone salt solution to obtain an initial suspension concentration of approximately 10⁴ CFU·mL⁻¹, representative of environmental conditions. The bacterial suspension was introduced into the reactor via a nebulizer. After photocatalytic treatment, bacteria were collected on a membrane filter and extracted using a solution containing NaCl and Tween 80. Serial decimal dilutions were then performed, and aliquots were plated in duplicate. Tryptic Soy Agar was poured into Petri dishes, followed by incubation at 30 °C for 24 h before colony enumeration.

In the contact-based tests, after exposure the irradiated catalyst was immersed in the extraction solution and shaken. Then, 1 mL of this solution was transferred into 9 mL of dilution solution to obtain the 10⁻¹ dilution. From this, 2 × 1 mL were pipetted into two Petri dishes for colony counting. Several successive dilutions were performed in the same way to ensure countable plates within the optimal range. The agar was then poured into the dishes, which were placed in the incubator for 24 h. After incubation, colonies on each plate were counted using a colony counter device.

The experimental system is illustrated schematically in Figure 7.

3.3.2. Analytical Methods

(a)

Biological treatment part

To calculate the concentration N in CFU·mL⁻¹ of E. coli, the following calculation method is used [33]:

For a colony count ≥ 15 in at least one dish, N (CFU·mL⁻¹) is calculated using the following formula (Equation (3)):

$$N=(\sum C)/(V\times (n_1+0.1\times n_2)\times d)$$

(3)

where C is the total number of colonies counted on all selected plates (plates from two successive dilutions); V is the volume of suspension spread in each Petri dish (1 mL); $n_1$ and $n_2$ are the numbers of plates retained at the lowest and the next higher dilution, respectively, corresponding to the lowest dilution retained; and d is the dilution factor of this lowest retained dilution.

The log inactivation expresses the ratio between the initial bacterial concentration (N₀) and the remaining bacteria after treatment (N), according to the following equation [11]:

$$\mathrm{Log} \mathrm{inactivation} = {\log }_{10}(N_0/N)$$

(4)

(b)

Chemical treatment

To ensure a credible assessment, we consider the efficiency of the media in terms of the photocatalytic degradation rate. And because of the complex mechanism of the photocatalytic reaction, we consider only the beginning of the treatment to avoid the influence of intermediate byproducts on the photocatalytic removal of the pollutant. As indicated in the literature, the results will be represented by the Langmuir–Hinshelwood equation (L-H), which can be obtained from the initial slope of the C = f (time) curve, where the influence of byproducts is assumed to be negligible [31,34].

4. Conclusions

This study shows the effect of UV-activated, Ag and Cu-modified TiO₂ optical fibers on the removal of chemical and biological contaminants from air. The results indicate that the nature of added metal strongly influences photocatalytic performance. Among the modified materials, Cu-loaded TiO₂ showed superior performance compared to the Ag-loaded TiO₂ for the removal of 3-methylbutan-1-ol with degradation rates of 0.028 and 0.016 mol.m⁻³.s⁻¹ respectively. This improvement is attributed to the formation of TiO₂–Cu heterojunctions and the Cu⁺/Cu²⁺ redox cycle, which promote more efficient charge separation and stimulate ROS production.

In addition, antibacterial tests revealed the high efficiency of TiO₂-Cu compared with Ag-modified and unmodified TiO₂. This enhanced antimicrobial activity results from the combined action of ROS generated by photocatalysis and the natural antibacterial properties of copper. However, in the simultaneous presence of E. coli and 3-methylbutan-1-ol, the TiO₂-Cu catalyst proved less effective at degrading pollutants and inactivating bacteria, demonstrating competition for the same adsorption sites and reactive species.

Despite its promising results, this study presents several limitations that should be addressed in future work. First, only a single VOC (3-methylbutan-1-ol) and one bacterial model (E. coli) were investigated, which restricts the generalization of the observed trends to other indoor air contaminants. Extending the approach to additional VOC families (e.g., ketones, aldehydes, aromatics) and to more resistant microorganisms (Gram-positive bacteria, fungal spores, or viruses) would provide a broader validation of the Cu/TiO₂ luminous textile concept. The competition between VOC oxidation and bacterial inactivation was clearly observed but not fully quantified. Future studies should integrate detailed kinetic modeling, surface coverage analysis, and ROS consumption measurements to better resolve the underlying mechanisms of this competition. In addition, long-term stability tests, potential metal leaching assessment, and scale-up considerations in realistic indoor environments will be essential for the practical implementation of this photocatalytic textile technology.