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Abstract

Textile materials can be easily used as a support for the nano-decoration with active particles in order to gain new features such as self-cleaning, antimicrobial efficiency, water repellency, mechanical strength, color change and protection against ultraviolet radiations. In this context, our present research reports the fabrication and characterization (physico-chemical analysis and surface morphology) of cotton fabrics treated with reduced graphene oxide decorated with two types of TiO₂ nanoparticles co-doped with 1% iron and nitrogen atoms (TiO₂/rGO NPs) and synthesized in different hydrothermal conditions by a simultaneous precipitation of Ti³⁺ and Fe³⁺ ions to achieve their uniform distribution or after a sequential precipitation of these two cations for obtaining a higher concentration of iron on the surface of Ti⁴⁺ oxyhydroxide. Further, the antimicrobial efficiency of these TiO₂/rGO-treated textiles and their influence on human cells were assessed. We demonstrated the successful development of TiO₂/rGO coating of cotton fabrics which are harmless for human skin cells and inhibit the growth of Staphylococcus aureus and Enterococcus faecalis. These findings confirm their great potential as novel graphene-based materials for biomedical and photocatalytic applications and this approach could be used for the large-scale fabrication of innovative self-cleaning and antimicrobial textiles.

Keywords

Cotton fabrics titanium dioxide reduced graphene oxide photocatalysis self-cleaning

Introduction

Considering the important role of fabrics in human life from today's society, a new interesting research field involving both textile industry and nanotechnology has been developed over the last decades [1]. Due to their unique high surface area, textile materials can be easily used as a support for the nano-decoration with active particles in order to attain multifunctional fabrics with enhanced properties [2 –4]. In this way, fibers properties are improved and the functionalized materials gain new features such as water repellency [5], mechanical strength [6], color change [7], UV protection [8], antimicrobial [9] or even catalysis [10].

Titanium dioxide turned out to be the most effective and cheap alternative for textile coatings [11,12]. Lately, many articles focused on the impregnation of textiles with titanium dioxide nanoparticles (TiO₂ NPs) and their self-cleaning properties [13 –16]. Two of the unique features of TiO₂-based nanomaterials are their photocatalytic activity and superhydrophilicity/superhydrophobicity [17]. The superhydrophilic surfaces coated with a thin layer of TiO₂ attract water forming a continuous free flowing film washing away the dust or other contaminants. On the superhydrophobic materials, TiO₂ NPs create a rough structure on the surface, easily removing water drops and dust particles from the surface, preventing it from wetting or soiling.

This phenomenon is inspired by nature and it was observed for the first time in Nelumbo nucifera leaves, an angiosperm plant, being called the “lotus effect” [18]. But unlike hydrophobic surfaces that are kept clean due to water-repellent structures, the hydrophilic coatings use photocatalysis to break down dirt and any other kind of impurities or contaminants under light exposure [19]. In addition, TiO₂ NPs show photocatalytic activity only under UV light, which is one of the major limitations for the development of their potential applications in photocatalysis [20].

On the other hand, graphene-related materials have received increasingly more attention among the researchers due to their excellent mechanical, optical, electrical, thermal and electronic properties with a wide range of applications [21]. For instance, graphene can act as an electron carrier in composites with enhanced catalytic performances and by combining it with nano-TiO₂, the limitation of the latter can be reduced [22]. Moreover, graphene can be used besides TiO₂ to intensify the hydrophobic, UV-protective, self-cleaning, antibacterial and antifungal properties of cotton fabrics without toxic effects [23].

Normally, there is no attraction between inorganic particles and polymeric materials such as textiles. Consequently, surface modification of textiles with nanoparticles or nanocomposites is not permanent, especially when considering washing. So, in the process of functionalization, textile surfaces need several steps of preparation which usually require different chemical agents [24]. For this reason, the development of such modified textiles requires a rigorous assessment of their impact on human health as well as the biosafety level they offer.

In a previous work, we decorated reduced graphene oxide (rGO) with two types of TiO₂ NPs co-doped with 1% iron and nitrogen atoms (TiO₂-Fe(1%)-N NPs) and synthesized in different hydrothermal conditions by a simultaneous precipitation of Ti³⁺ and Fe³⁺ ions to achieve their uniform distribution (particles noted S1) or after a sequential precipitation of these two cations for obtaining a higher concentration of iron on the surface of Ti⁴⁺ oxohydroxide (particles noted S2). The best photocatalytic efficiency was found for the sample with iron atoms localized near the nanoparticle surface, while both samples had a very good anti-inhibitory activity against Gram-positive and Gram-negative strains, without significant toxicity to human skin and lung cells [25]. As a next step, our present research reports the fabrication and characterization (physico-chemical analysis and surface morphology) of cotton fabrics impregnated with these novel rGO-based nanocomposites. Further, the photocatalytic performance and the antimicrobial efficiency of these TiO₂/rGO-treated textiles, as well as their biological effects on human cells were assessed.

Experimental

Synthesis of TiO₂/rGO nanocomposites

Reduced graphene oxide was decorated with TiO₂ NPs co-doped with 1% iron and nitrogen synthesized in hydrothermal conditions (200℃/2 h, using urea, in a Teflon lined autoclave) by simultaneous co-precipitation (particles S1) for uniform distribution of Ti³⁺ and Fe³⁺ ions, and by sequential co-precipitation (particles S2) with the aim to increase the probability for the iron ions to be localized on the particle surface. All the procedures were described in our previous work [25] with a detailed presentation of the physico-chemical properties of these nanocomposites. Briefly, for S1 synthesis, TiCl₃ (15 wt.% TiCl₃ in 10 wt.% HCl, Merck solution) and FeCl₃·6H₂O (p.a. Merck, Darmstadt, Germany) were solubilized together in water, and pH ∼9 was adjusted with NH₄OH (25 wt.%, Merck, Darmstadt, Germany). Next, Ti³⁺ was oxidized to Ti⁴⁺ under air bubbling to obtain Ti and Fe oxihydroxides. An amorphous co-precipitate was obtained after washing and drying, and it was subjected to the hydrothermal procedure in the presence of urea in a Teflon lined autoclave with stirring for 30 min at 105℃, and 120 min at 200℃. The final powder was calcined at 400℃ for 120 min. To obtain the sample S2, the precipitation process of Ti³⁺ and Fe³⁺ was performed in two steps. Firstly, the Ti³⁺ ions were precipitated and oxidized to Ti⁴⁺, and secondly, Fe³⁺ ions were mixed with the precipitate in a basic medium of reaction (pH ∼9).

TiO₂-Fe(1%)-N/rGO nanocomposites were hydrothermally obtained. A proper amount of sample S1 or S2 was mechanically stirred and ultrasonicated in water and poly diallyl dimethyl ammonium chloride (PDDA) (20 wt.%, molecular weight Mw = 100,000–200,000, Sigma Aldrich, St. Louis, MO, USA) as a dispersing, reducing and stabilizer reagent. After the elimination of PDDA excess, 2 wt.% GO (in respect to TiO₂) was added and incubated for 120 min at 150℃ in a Teflon-lined autoclave with stirring. In the end, the precipitate was was dried at 80℃ for 24 h. Taking into account the strong reducing effect of PDDA rGO decorated with TiO₂-Fe(1%)-N was obtained, which contains much less oxygen than GO, a monolayer material with high oxygen content (C/O ∼3 or less).

Cotton fabrics treatment and characterization

In this study, 100% cotton woven fabric (design pattern – till wave; weight – 253 g/m²; thickness – 0.568 mm fabric density – warp: 41 yarns/cm and weft: 21 yarns/cm) was used for the impregnation with TiO₂/rGO nanocomposites. The dispersions of rGO-based powders were prepared by the sonication of 0.5 g TiO₂-Fe(1%)-N + 2% rGO (S1 and S2 particles) in 1000 mL solution containing 0.19 g dodecyl hydrogen sulfate (DHS; Sigma-Aldrich, St. Louis, MO, USA), 15 mL ethanol and 0.6 mL poly(ethylene benzene sulfonic) acid (PBS; Sigma-Aldrich, St. Louis, MO, USA), for 3 h at 30 ºC. Afterwards, the cotton fabrics were immersed in the above prepared dispersions, maintained at 40℃ for 30 min and then, dried at 100℃ for 2 min. The treated fabrics were abbreviated S1G and S2G according to the used nanocomposite.

Further, the fabrics were immersed in 20 g/L perfluorocarbon polymer Nuva 4200liq (pH 5; Clariant, Muttenz, Switzerland), dried at 110℃ and thermofixed at 170℃ for 40 s. These final fabrics were abbreviated S1 and S2.

For an accurate characterization of the treated materials, especially for cytotoxicity and antimicrobial assays, the cotton fabrics were treated separately with the components of the final mixture, namely DHS + PBS (abbreviated DP) and Nuva 4200liq (abbreviated N). The samples' abbreviation and the treatment methods were: sample DP, cotton fabric immersed for 30 min at 40℃ in 50 mL aqueous solution containing 0.02 g DHS, 0.06 mL PBS and 15 mL ethanol and dried at 100℃ for 2 min; sample N, cotton fabric immersed for 30 min at 40℃ in 50 mL solution containing 20 g/L Nuva 4200liq (pH adjusted to 5 with acetic acid) and dried at 100℃ for 2 min.

The fabric morphology and chemical composition of deposited layers were investigated by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDX, Quanta 200, FEI, Netherlands) using the following conditions: a voltage of 10 kV, low vacuum, a working distance of 9.7 mm, and a gaseous secondary electron detector (GSED). The physical properties of the fabrics (weight, thickness, breaking strength, elongation, air/water permeability) were analyzed according to specific ISO standards. The wetting capacity was determined by measuring the contact angles with a 5 μL distilled water droplet on a VCA Optima (AST Products Inc., USA) instrument. The results are the average of 5–10 measurements in different points on the samples' surface. Fourier transform infrared spectroscopy (FTIR) measurements were done on a Bruker Tensor 27 spectrometer (Bruker, Germany) in attenuated total reflection (ATR) configuration. Powder (S1 and S2 NPs) and fabric samples (untreated cotton, S1G, S2G, S1 and S2) were deposited on the diamond crystal. ATR spectra in the frequency range of 4000–400 cm⁻¹, with a resolution of 4 cm⁻¹, were collected during 1 min. Spectra analysis, such as peaks identification, smoothing, and baseline correction, was performed with OPUS software (Bruker, Germany).

Evaluation of the photocatalytic performance

The photocatalytic efficiency was assessed by staining the TiO₂/rGO nanocomposites-treated fabrics with a 0.01 g/L methylene blue (MB) solution. These were half covered with paper and were exposed to UV light (λ=254 nm) in a completely closed cabinet, and to a visible light laboratory equipment (Xenotest, 1000 W xenon arc lamp, irradiance 4.5 mW/cm² at 300–400 nm, Heraeus Industrietechnik, Hanau, Germany). Chromaticity coordinates of exposed and non-exposed materials were measured with a UV-Vis Hunterlab spectrophotometer (Hunter Associates Laboratory, Reston, VA, USA), with a CIELAB 1976 color space and D65-light source. The photocatalytic efficiency was compared to stained cotton fabrics which were not treated with particles.

The nanocomposites' adherence on the fabric's surface and photocatalytic efficiency to washing was investigated by subjecting the fabrics to five washing cycles, staining with MB, exposing to UV and visible light and evaluation of the color changes. The washing cycles were done under the following conditions: materials (5 cm × 5 cm) were introduced into 100 mL distilled water containing 0.4 g of phosphate-free European Colourfastness Establishment (ECE) detergent and no optical bleaching agents. The materials were kept at 40℃ for 30 min, and then rinsed two times with 100 mL of distilled water at 40℃ for 3 min.

Antimicrobial activity assay

The antimicrobial activity of the modified textiles was assessed against Staphylococcus aureus ATCC 6538 and Enterococcus faecalis ATCC 29212. For this purpose, the microbial strains were purchased from American Type Culture Collection (ATCC, USA). Fresh cultures were obtained on tryptic soy agar (TSA) and used to prepare microbial suspensions of ∼10⁷ colony forming units (CFU)/mL in sterile saline. Circular samples of 8 mm diameter from the textile materials were sterilized by autoclaving at 121℃, for 15 min, immersed in 1 mL of microbial suspensions of ∼10⁷ CFU/mL and left in contact for 2 h and 24 h. Afterwards, the microbial suspensions incubated with the tested samples were vortexed and 10 µL of serial 10-fold dilutions of the obtained suspension were plated in triplicate on TSA. After 24 h of incubation at 37℃, viable cell counts were performed and the numbers of CFU/mL for each sample were established.

In vitro biocompatibility assessment

Normal human skin fibroblasts (CCD-1070Sk cell line, ATCC Cat. No. CRL-2091) were cultured in complete Eagle's minimum essential medium (MEM; Gibco/Invitrogen, Carlsbad, CA, USA) containing 10% fetal bovine serum (FBS; Gibco/Invitrogen, Carlsbad, CA, USA) at 37 ℃ in a humidified atmosphere with 5% CO₂. For biocompatibility assessment, the cells were seeded at a density of 2 × 10⁴ cells/cm² in a 24-well plate and left to adhere overnight. Further, the fibroblasts were exposed to the cotton fabrics treated with TiO₂/rGO composites, cut into 1 × 1 cm² pieces, sterilized at 120℃ for 20 min and activated with a light source for 30 min. After 4 and 24 h of incubation, cell morphology and viability were evaluated, while cytotoxicity tests were also performed. The results were expressed relative to the untreated cotton fabric used as a control.

The cell viability was measured using AlamarBlue cell viability assay (Invitrogen, Carlsbad, CA, USA) which is based on the ability of viable cells to reduce resazurin (the active ingredient of AlamarBlue reagent), a highly fluorescent red compound. After the end of each time exposure interval, the culture medium and cotton samples were removed and 100 μL of AlamarBlue stock solution were added directly to cells in culture medium and incubated for 1 h at 37℃ and 5% CO₂. Finally, the fluorescence intensity was recorded (excitation wavelength = 570 nm and emission wavelength = 585 nm) (FlexStation 3, Molecular Devices, USA).

The lactate dehydrogenase (LDH) amount released in culture medium was assessed as a measure of cell membrane integrity using a commercial kit (TOX7, Sigma-Aldrich, St. Louis, MO, USA). Volumes of 50 µL of culture supernatants were incubated for 30 min in dark with 100 µL mix composed from equal parts of dye, substrate and cofactor. The reaction was stopped by adding 15 µL of 1 N HCl and the absorbance was read at 490 nm using a microplate reader (FlexStation 3, Molecular Devices, USA).

The level of nitric oxide (NO) released in the culture medium as an indicator of inflammation was quantified using the Griess reagent (a stoichiometric solution (v/v) of 0.1% naphthylethylendiamine dihydrochloride and 1% sulphanilamide in 5% H₃PO₄). Culture supernatants were mixed with an equal volume of Griess reagent and absorbance was read at 550 nm using a microplate reader (FlexStation 3, Molecular Devices, USA).

Cell cytoskeleton morphology changes were visualized via fluorescence imaging using cells fixed with 4% paraformaldehyde for 20 min and permeabilized with 0.1% Triton X-100—2% bovine serum albumin for 1 h. Filamentous actin (F-actin) was labeled with 20 μg/mL phalloidin conjugated with fluorescein isothiocyanate (FITC) (Sigma-Aldrich, Munich, Germany) and nuclei were counterstained with 2 μg/mL 4′,6-diamidino-2-phenylindole (DAPI) (Molecular Probes, Life Technologies, Carlsbad, CA, USA). Images were captured using a fluorescence microscope Olympus IX71 (Olympus, Tokyo, Japan).

The intracellular reactive oxygen species (ROS) level was determined using a fluorescent compound 2′,7′-dichlorofluorescein diacetate (DCFH-DA, Sigma-Aldrich, City, State Abbrev, USA). The skin fibroblasts were washed with PBS and incubated with the dye for 30 min at 37℃. Further, the excess dye was removed and the cells were suspended in PBS and detached by scraping. The fluorescence was quantified using a fluorimeter (FP-750 Spectrofluorometer, Jasco, Tokyo, Japan) (excitation wavelength = 488 nm and emission wavelength = 515 nm). All results were expressed relative to control after fluorescence intensity was reported to the number of viable cells of each sample.

Statistical analysis

The antibacterial efficiency and cell culture assays were performed in triplicates, and data were shown as mean ± standard deviation (SD). The statistical significance was analyzed by Student's t-test or two-way ANOVA followed by Bonferroni post hoc test using GraphPad Prism 5, and a value of p < 0.05 was considered significant.

Results

Surface morphology investigation of coated fabric by SEM

SEM analysis (Figure 1) showed the modification of the cotton surfaces, which are non-uniformly coated by TiO₂/rGO nanocomposites. While the untreated cotton knit and the cotton sample treated with DHS and PBS (DP sample) have a smooth surface, the fluorocarbon polymer (Nuva 4200liq) embraces the fabric fibers in relatively thick layers, observable on samples S1, S2 and N. On fabrics treated with composite powders, a relatively high number of particles, unevenly dispersed, form clusters and the average sizes of these agglomerates are between 145 nm and 600 nm.

Figure 1.

SEM images of cotton fabrics: control – untreated cotton fabric (a), DP – cotton fabric treated with DHS and PBS (b), N – cotton fabric treated with Nuva 4200liq (c), S1G – cotton fabric treated with S1 powder (d), S1 – cotton fabric treated with S1 powder and Nuva 4200liq (e), S2G – cotton fabric treated with S2 powder (f) and S2 – cotton fabric treated with S2 powder and Nuva 4200liq (g).

More experiments are needed to scientifically demonstrate the relatively low number of NPs on the cotton fabrics. Some hypothesis regarding this issue could be the following listed below:

the method of deposition: during the impregnation on foulard machine, as no stirring was possible, the particles sedimented and remained at the vessel's bottom;

the aggregation of TiO₂ dispersions in aqueous conditions due to their low dimensions;

the surfactants used to prepare a “relatively” stable dispersion of TiO₂/rGO could hamper the adherence of particle on the material's surface;

even TiO₂ is positively charged below pH 6 (point of zero charge), it is enveloped in a layer of surfactants. As consequence, a slight repulsion between TiO₂ and cotton, negatively charged, could appear and, the number of particles deposited on the fabric remains low.

Moreover, the textiles are inherently non-uniform and, as the SEM images do not cover the whole surface, it is not possible to have a precise, accurate image of particles spreading.

Based on EDX spectra recorded in the same time with SEM analysis (Figure 2), the quantification of the elements present on the treated cotton fabrics was performed. Table 1 reveals the presence of Ti as the major element on the photocatalysts-treated fabrics, having almost the same weight percentage of Ti on both fabrics treated with the particles S1 and S2, namely 2.82% for S1G sample and 2.95% for S2G sample. After covering with fluorocarbon polymer Nuva 4200liq, the TiK percentage dramatically decreased to 1.13% and 1.12%, respectively, for S1 and S2 samples. This decrease of Ti amount, by more than 60%, could be attributed to the low adherence of the particles to the fabric surface; during the treatment, many of the particles migrated in the solution and formed large agglomerates that were unable to attach to the material's surface.

Figure 2.

EDX spectra recorded for cotton fabrics: control – untreated cotton fabric (a), DP – cotton fabric treated with DHS and PBS (b), N – cotton fabric treated with Nuva 4200liq (c), S1G – cotton fabric treated with S1 powder (d), S1 – cotton fabric treated with S1 powder and Nuva 4200liq (e), S2G – cotton fabric treated with S2 powder (f) and S2 – cotton fabric treated with S2 powder and Nuva 4200liq (g).

Table 1.

Energy-dispersive X-ray spectroscopy (EDX) analysis of cotton fabrics treated with TiO₂/rGO nanocomposites.

Element (wt.%)	Control	DP	N	S1G	S1	S2G	S2
C K	45.83	46.75	43.71	43.91	40.48	42.55	40.02
O K	54.17	53.25	51.96	53.27	58.39	54.50	58.86
F K	0	0	4.33	0	0	0	0
Ti K	0	0	0	2.82	1.13	2.95	1.12
Total	100	100	100	100	100	100	100

FTIR spectra recorded for S1 and S2 NPs (Figure 3(a)) showed a high intensity peak between 700 and 400 cm⁻¹, corresponding to Ti-O-Ti bonds. Within this region, there are also present the vibrations specific to FeO₆ and FeO₄ structural units. The low intensity peak at ∼1600 cm⁻¹ is attributed to bending O–H bond from hydroxyl groups of Ti–OH and water molecules, and the other one situated at 3420 cm⁻¹ corresponds to O–H stretching vibration. The bands specific for treated cotton fabrics (Figure 3(b)) are situated at la 3302 cm⁻¹ (O–H), 2898 cm⁻¹ (C–H), 1478 (H–O–H and C=O), 1311–1306 cm⁻¹ (C–O–C), 1142 cm⁻¹ (C–O and C–C, characteristic for β-glycosidic bond of pyranosic cycle), 950 cm⁻¹ (CH₂) and 700–400 cm⁻¹ (Ti–O–Ti). The broad band between 3600 and 3000 cm⁻¹ is specific for O–H stretching vibrations, indicating the high number of OH groups. Consequently, a larger band corresponds to a more hydrophile fabric, suggesting that the treated materials become hydrophobic, as it was confirmed by contact angle measurements (Table 2). These results prove the interaction between the TiO2/rGO nanocomposites and the cotton fabric.

Figure 3.

FTIR spectra recorded for S1 and S2 NPs (a) and cotton fabrics (b): control – untreated cotton fabric, S1 – cotton fabric treated with S1 powder and Nuva 4200liq, S2 – cotton fabric treated with S2 powder and Nuva 4200liq, S1G – cotton fabric treated with S1 powder, and S2G – cotton fabric treated with S2 powder.

Table 2.

Physico-mechanical properties of initial and treated cotton fabrics (S1 and S2) parameters.

		Results for each sample			Standard
		Cotton	S1	S2	Standard
Weight (g/m²)		253	253	249	SR EN ISO 12127:2003
Thickness (mm)		0.568	0.658	0.635	SR EN ISO 5084:2001
Water vapor permeability (%)		34.3	32.6	34.3	STAS 9005:1979
Air permeability (L/m²/s); 100 Pa		86.24	71.65	75.92	SR EN ISO 9237:1999
Fabric density (number of yarns/10 cm)	Warp	414	420	420	SR EN ISO 1049-2:2000
Fabric density (number of yarns/10 cm)	Weft	214	215	214	SR EN ISO 1049-2:2000
Breaking force (N)	Warp	710	812	1051	SR EN ISO 13934-1:2013
Breaking force (N)	Weft	470	507	511	SR EN ISO 13934-1:2013
Breaking elongation (%)	Warp	15.27	17.60	20.2	SR EN ISO 13934-1:2013
Breaking elongation (%)	Weft	15.95	17.31	16.01	SR EN ISO 13934-1:2013
Surface resistivity (× 10¹³ Ωsq)		18.9	6.52	6.96	SR EN ISO 1149-1:2006
Volume resistivity (× 10¹⁴ Ωcm)		11.5	4.66	18.8	SR EN ISO 1149-1:2006
Contact angle (º)		0 º	146.38 º	146.16 º	Sessile drop method

Note: SR EN ISO and STAS, European Standards of the International Organization for Standardization adopted as Romanian Standards.

The physical properties of the initial and treated cotton fabrics (weight, thickness, breaking strength, elongation, and air/water permeability) as well as their hydrophobicity and photocatalytic efficiency were already analyzed and reported in our two previous studies [26,27].

As shown in Table 2, except the weight, all the analyzed physical properties showed slight modifications. The results demonstrated an increase of thickness with 15.86% for sample S1 and 11.79% for sample S2, fabric density on warp (1.45%), breaking force on warp (14.36%, respectively 48%) and weft (7.87%, respectively 8.72%), and breaking elongation up to 33.77% on warp for sample S2. Instead, water vapor and air permeability decreased with approximately 5% and, respectively, 17% due to the deposited nanoparticles and fluorocarbon polymer layer on the fabric surface. Also, the decrease of surface and volume resistivity was mainly attributed to graphene. These results correlate very well with SEM images (Figure 1) and EDX spectra (Figure 2), demonstrating that a higher quantity of nanocomposites was deposited on S1 surface.

The initial cotton fabric is highly hydrophilic, the water being absorbed instantaneously. But, after coating with TiO₂/rGO nanocomposites and Nuva, it was observed that the materials become hydrophobic, due to the polymer, specially used to provide oil and water repellency to the textile materials.

Photocatalytic efficiency

The color differences between the treated samples and the control were measured via the CIE L*a*b* coordinates specified by the “Commission Internationale de l'Éclairage” (CIE) [28], where: L* represents lightness (dL* is the lightness difference between sample and control and indicates a lighter color if positive and darker if negative), a* signifies the red/green value (if da* is positive, the sample is redder than the control, otherwise is greener), and b* is the yellow/blue value (if db* is positive, the sample is yellower than the control, otherwise it is bluer), and dE* represents the total color difference and is calculated according to the following formula: dE* = $\sqrt{{dL}^{* 2} + {da}^{* 2} + {db}^{* 2}}$ .

Photocatalytic activity of the treated cotton fabrics and durability to wash assessed in terms of chromaticity coordinates, after staining with MB and exposure to UV and visible light, is shown in Table 3.

Table 3.

Color differences (dE*) of the cotton fabrics treated with TiO₂/rGO-based nanocomposites, stained with 0.01 g/L MB, and exposed to ultraviolet (UV) and visible (Vis) light.

Sample	Before washing		After five cycles of washing
Sample	dE* Vis	dE* UV	dE* Vis	dE* UV
Cotton	9.66	0.55	12.71	2.19
S1	10.16	1.91	13.96	1.76
S2	11.92	3.17	11.30	2.08

As it can be seen in Table 3, MB existing on the untreated material after UV light exposure suffers a very slight photo-degradation. However, if we consider the value of color differences (dE*), it is obvious that MB from the fabrics coated with TiO₂/rGO nanocomposites is more effectively degraded compared to the untreated ones, due to the deposited photocatalyst on the material's surface. Also, it is important to mention that MB on S2 fabric sample is more strongly photo-degraded than that of the cotton fabric treated with the first photocatalyst sample (S1). Under visible light, once again, S2 cotton sample proved to be the most efficient in photocatalytic degradation of MB.

After five washing cycles and exposure to UV and visible light, the color fading of MB is similar for treated and untreated fabrics, as it can be seen from the dE* values in Table 3. Therefore, the photocatalytic efficiency is lost, most probably due to the removal of TiO₂/rGO nanocomposites. Apparently, the MB stain on fabric sample S2 is less degraded than the untreated cotton. This behavior could be explained by the darker shade of the fabric due to the black color of graphene and the lower amount of TiO₂ present on the fabric's surface. Generally, the dark shades have a high degree of absorbance and low levels of reflected light, resulting in a greater degree of experimental error [29].

Antimicrobial activity

The antimicrobial activity of the obtained textiles was evaluated against S. aureus and E. faecalis strains using the standard method recommended by ASTM E2149-10 (Standard Test Method for Determining the Antimicrobial Activity of Immobilized Antimicrobial Agents under Dynamic Contact Conditions) for the assessment of the antimicrobial activity of agents immobilized in dynamic testing conditions. After 2 h of incubation, the obtained results were different between the two strains. A low inhibitory effect upon the microbial growth of S. aureus strain was observed for S1, S2 and DP materials, while the growth of E. faecalis was not decreased by the tested samples (Figure 2). This could be due to the intrinsic resistance of E. faecalis to photocatalytic samples. After 24 h of incubation, a significant inhibitory activity was observed for N, DP, S1G and S2G samples against both bacterial strains. All materials inhibited in different degrees the growth of E. faecalis and S. aureus strains (Figure 4), suggesting that a longer period of time exposure (one day) is required to induce an inhibitory effect on bacterial growth. This involves a multi-step process of three phases, including the photocatalysts' attack against cell wall, then against plasma membrane and destruction of structural components, and afterwards, the bacterial toxins decomposition.

Figure 4.

Graphic representation of the colony forming units (CFU)/mL values of microbial cells grown on the cotton knit samples after 2 h and 24 h of incubation.

In vitro biocompatibility of cotton knit treated with TiO₂/rGO

In vitro analysis of the effects induced by TiO₂-Fe(1%)-N + 2% rGO nanocomposites showed that cotton fabric impregnated with these photocatalysts did not reduce the viability of dermal fibroblasts (Figure 5(a)). In the case of LDH released in the culture medium (Figure 5(b)), it was observed that the final S1 and S2 samples did not induce cell membrane damage nor did they cause NO release (Figure 5(c)), being biocompatible for human dermal cells. The level of oxidative stress measured by the ROS (Figure 5(d)) confirmed the lack of cytotoxicity of these textile materials treated with nanoparticles with photocatalytic properties.

Figure 5.

Biocompatibility of cotton treated samples as shown by cell viability (a), lactate dehydrogenase (LDH) (b) and nitric oxide (NO) (c) release assays, as well as reactive oxygen species (ROS) levels (d) after 4 h and 24 h exposure on normal skin fibroblasts. Results are expressed as the mean ± SD (n = 3) and represented relative to the untreated cotton sample (control). *p < 0.05, **p < 0.01 and ***p < 0.001 compared to control.

Fluorescence microscopy images depicting the arrangement of actin filaments (Figure 6) in dermal cells exposed to untreated cotton knit and fabrics treated with photocatalytic nanoparticles revealed a morphology and cytoskeleton organization similar to those from control cells represented by untreated dermal fibroblasts. After 24 h of incubation, almost complete coverage of the culture vessel surface was observed, indicating that the presence of these materials did not inhibit cell proliferation.

Figure 6.

Actin cytoskeleton organization of dermal fibroblast cells after 4 h and 24 h of incubation with photocatalyst-treated cotton samples. F-actin (green) was labeled with FITC and nuclei (blue) were counterstained with DAPI. Scale bar: 100 µm.

Discussion

In the last decade, numerous studies have focused on the cotton functionalization with different inorganic particles in order to enhance its properties [30 –32]. But, the increasing use of nano-sized particles in all sorts of products, especially in textiles, has raised serious concerns about consumers' exposure and their safety. As the general labeling requirements in the textile industry are minimal, it is more difficult for the consumers to know if an item of clothing is nano-functionalized. For instance, in Europe nanomaterials, labeling includes so far only biozides like nano-Ag according to the new EU directive Nr. 528/2012 [33]. Therefore, we developed novel cotton fabrics coated with dispersions of TiO₂-Fe(1%)-N + 2% rGO nanocomposites and assessed their antimicrobial efficiency and toxic effects on human exposure.

The influence of the modified textiles on the microbial growth was assessed against two Gram-positive bacterial strains belonging to species commonly found in the hospital environment and transmitted to patients through contaminated clothes and linen, i.e. S. aureus and E. faecalis. After 24 h of contact, the growth of E. faecalis was inhibited by all modified textiles. The N, DP, S1 G and S2 G samples proved to be the most potent antimicrobial materials, inhibiting the growth of both strains. The most efficient material against S. aureus was N, while S1G exhibited the most intensive anti-E. faecalis effect. The fact that S1 and S2 samples exhibited an inhibitory effect on the S. aureus growth after 2 h of incubation and lost their efficiency after 24 h, is suggesting a rapid release of potential antimicrobial species from the treated textiles, taking place in the first hour of contact with the microbial cells.

In addition to the fact that cotton samples treated with photocatalytic TiO₂-Fe(1%)-N + 2% rGO nanocomposites could provide short-term protection against microbial colonization, these modified materials were harmless for skin cells, proving good biocompatibility. Our findings are in agreement with previous research regarding the safety profile of TiO₂-based nanofabrics revealed on human leukemic monocytes and normal human liver and kidney cells [34], highlighting the potential of these modified fabrics for the development of novel self-cleaning and antimicrobial rGO/TiO₂-based materials with biomedical applications.

Overview and conclusion

In this study, we demonstrated that the treatment of cotton fabrics with reduced graphene oxide decorated with nitrogen and iron co-doped TiO₂ NPs prepared by hydrothermal method is harmless for human skin cells and could be used for the development of innovative self-cleaning antimicrobial textiles. These findings confirm their great potential as novel graphene-based materials for biomedical and photocatalytic applications, improving human welfare.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to thank the Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI) in the frame of projects no. 87/2014 – CLEANTEX, PN-II-IN-EUK-2012-1-0013/ contract 334E/19.12.2013, no. 77/2018 PN-III-P1-1.1-PD-2016-1562 - NANO-BIO-INT and no. 13/2015 – NanoToxClass.

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Reduced graphene oxide/TiO 2 nanocomposites coating of cotton fabrics with antibacterial and self-cleaning properties

Abstract

Keywords

Introduction

Experimental

Synthesis of TiO2/rGO nanocomposites

Cotton fabrics treatment and characterization

Evaluation of the photocatalytic performance

Antimicrobial activity assay

In vitro biocompatibility assessment

Statistical analysis

Results

Surface morphology investigation of coated fabric by SEM

Photocatalytic efficiency

Antimicrobial activity

In vitro biocompatibility of cotton knit treated with TiO2/rGO

Discussion

Overview and conclusion

Footnotes

Declaration of conflicting interests

Funding

References

Synthesis of TiO₂/rGO nanocomposites

In vitro biocompatibility of cotton knit treated with TiO₂/rGO