Non-Woven Fabrics Based on Nanocomposite Nylon 6/Zno Obtained by Ultrasound-Assisted Extrusion for Improved Antimicrobial and Adsorption Methylene Blue Dye Properties

Article Non-Woven Fabrics Based on Nanocomposite Nylon 6/ZnO Obtained by Ultrasound-Assisted Extrusion for Improved Antimicrobial and Adsorption Methylene Blue Dye Properties

M. Andrade-Guel 1 , C. A. Ávila-Orta 1 , C. Cabello-Alvarado 1,2,* , G. Cadenas-Pliego 1,* , S. C. Esparza-González 3 , M. Pérez-Alvarez 1 and Z. V. Quiñones-Jurado 4

1 Centro de Investigación en Química Aplicada, Saltillo 25294, Mexico; [email protected] (M.A.-G.); [email protected] (C.A.Á.-O.); [email protected] (M.P.-A.) 2 CONACYT—Centro de Investigación y de Innovación del Estado de Tlaxcala, Tlaxcala 90000, Mexico 3 Facultad de Odontología, Unidad Saltillo, Universidad Autónoma de Coahuila, Saltillo 25125, Mexico; [email protected] 4 Facultad de Ciencias Químicas, Universidad Juárez del Estado de Durango, Durango 34120, Mexico; [email protected] * Correspondence: [email protected] (C.C.-A.); [email protected] (G.C.-P.) Abstract: Approximately 200,000 tons of water contaminated with dyes are discharged into effluents Citation: Andrade-Guel, M.; annually, which in addition to infectious diseases constitute problems that afflict the population Ávila-Orta, C.A.; Cabello-Alvarado, worldwide. This study evaluated the mechanical properties, surface structure, antimicrobial perfor- C.; Cadenas-Pliego, G.; mance, and methylene blue dye-contaminant adsorption using the non-woven fabrics manufactured Esparza-González, S.C.; by melt-blowing. The non-woven fabrics are composed of nylon 6 (Ny 6) and zinc oxide nanoparti- Pérez-Alvarez, M.; Quiñones-Jurado, cles (ZnO NPs). The polymer nanocomposites were previously fabricated using variable frequency Z.V. Non-Woven Fabrics Based on ultrasound assisted-melt-extrusion to be used in melt-blowing. Energy dispersion spectroscopy Nanocomposite Nylon 6/ZnO Obtained by Ultrasound-Assisted (SEM-EDS) images showed a homogeneous dispersion of the ZnO nanoparticles in nylon 6. The Extrusion for Improved mechanical properties of the composites increased by adding ZnO compared to the nylon 6 matrix, Antimicrobial and Adsorption and sample Ny/ZnO 0.5 showed the best mechanical performance. All fabric samples exhibited Methylene Blue Dye Properties. antimicrobial activity against S. aureus and fungus C. albicans, and the incorporation of ZnO nanopar- Polymers 2021, 13, 1888. https:// ticles significantly improved this property compared to pure nylon 6. The absorption efficiency of doi.org/10.3390/polym13111888 methylene blue (MB), during 60 min, for the samples Ny/ZnO 0.05 and Ny/ZnO 0.25 wt%, were 93% and 65%, respectively. The adsorption equilibrium data obeyed the Langmuir isotherm. Academic Editor: Antonio Zuorro Keywords: non-woven fabric; nylon 6; zinc oxide; antimicrobial Received: 28 April 2021 Accepted: 27 May 2021 Published: 6 June 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in There has been a continued increase in environmental problems due to the contami- published maps and institutional affil- nation of water by at least 10,000 different dyes and pigments that are used in the textile iations. industry [1]. Methylene blue is a cationic dye used for dyeing cotton, wool, and silk. This dye can cause impacts when reaching water resources due to the reduction in sunlight infiltration, in addition to health problems if ingested [2]. The spread of infectious diseases generated by pathogenic microorganisms in water has become one of the most severe public health problems worldwide. This issue has triggered social, scientific, and industrial Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. interest in the development of practical alternatives that contribute to the decrease in This article is an open access article harmful agents. Recently, the design and application of polymers with ceramics nanoparti- distributed under the terms and cles has generated valuable results in areas related to health and personal care, since the conditions of the Creative Commons incorporation of nanometric charges drastically modifies the antibacterial properties of the Attribution (CC BY) license (https:// materials obtained [3,4]. creativecommons.org/licenses/by/ Different materials have been reported for the adsorption of dyes between them mag- 4.0/). netite nanoparticles [5], urea calcium alginate xerogel beads [6], metal/mineral materials

Polymers 2021, 13, 1888. https://doi.org/10.3390/polym13111888 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1888 2 of 13

such as CuO, zinc, iron, and zeolite [7]. Bioadsorbents have also been used as bivalve shells of anadara uropigimelana [8], sepia shells (cuttlefish bones) [9], to fabricate monolithic algal green powder (MAGP) based on the marine green macroalga enteromorpha flexu- osa [10] as well as magnetically modify products of the coffee industry [11]. Nowadays, methodologies based on energy sources such as microwave or ultrasound radiation are ex- ploited to accelerate different sorption processes. These technologies are considered green, efficient and more time efficient. Most of the adsorbent materials mentioned above need to be regenerated, which does not solve the problem of water contamination since organic solvents are needed for the regeneration of said material and only transfer the problem of contaminants to the eluent in this case, the solvent used for the regeneration [12]. Filtration methods are promising technologies to remove contaminants in water [13]; an alternative is the use of non-woven fabric obtained from a polymer nanocomposite. Nylon 6 is a common raw material for textile fiber or fabrics that can be elastic, resistant, and prevents moth attack. These fibers are used in the manufacture of socks, fabrics, knitted fabrics, and bristles [14]. Moreover, cotton fabrics coated with ZnO nanoparticles showed satisfactory results against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. However, these fabric coatings are on the surface of the fibers so they can be lost over time or after several washing cycles [15]. Constant washing affects both the antimicrobial properties and the toxicity of the antimicrobial agents in commercial antimicrobial fabrics. ZnO is a ceramic with antimicrobial, antifungal properties, environmental stability, and its use is approved by the Food and Drug Administration (FDA) [16]. The exact mechanism of antibacterial activity remains controversial, since there are still doubts as to how ZnO nanoparticles act [17]. Some of the mechanisms that have been proposed in the literature are: direct contact of ZnO nanoparticles with cell walls, which results in the destruction of bacterial cells and the disruption of DNA replication [18], the release of antimicrobial ions, mainly Zn2+ [19], and ROS formation [20], whilst other authors they attribute it to the effects of different conditions on the antibacterial activity such as the size of the nanomaterials, concentration/dosage, temperature, capping agent, and reducing agent [21]. Most studies have shown the strong ROS generating potential of ZnO nanoparticles, suggesting it has an important role in bacterial killing. On the other hand, one of the methods for the production of non-woven fabric is melt- blowing, which is a technique with several advantages compared to the electrospinning method, such as it is scalable, it does not require solvents, and it is a single-process step that produces fiber diameters from 0.3 to 20 µm [22]. The originality of the study is based on the fact that the ultrasound-assisted extrusion process homogenizes the distribution of the nanometric particles in the polymer matrix, requesting to have a non-woven fabric with isotropic properties. In other studies, the incorporation of ZnO in textiles is carried out by functionalization with a textile substrate by dip coating methods, sol–gel and electrodeposition, among others [23]. This work reports the manufacture of non-woven nylon 6 fabric with ZnO nanoparticles forming nanocomposites prepared by an ultrasound-assisted extrusion process, using different concentrations of nanometric ZnO particles (0.25, 0.5, and 1.0 wt%). Morphology, the mechanical properties, antibacterial activity, and methylene blue adsorption of these non- woven fabrics were evaluated leading to the proposal of not only the adsorption of the dye but also its degradation.

2. Materials and Methods Zinc oxide nanoparticles were purchased from SkySpring Nanomaterials Inc. (Hous- ton, TX, USA), with an average diameter of 15–30 nm, hemispherical geometry, and 99.98% purity. A commercial nylon 6 resin from DuPont Zytel 7301 NC010 (Wilmington, DE, USA) was used as the polymer matrix. Methyl blue (MB) was purchased from Sigma Aldrich (Saint Louis, MO, USA), classiﬁcation Acute Tox. 4; H302. Polymers 2021, 13, x FOR PEER REVIEW 3 of 15

Polymers 2021, 13, 1888 3 of 13 DE, USA) was used as the polymer matrix. Methyl blue (MB) was purchased from Sigma Aldrich (Saint Louis, MO, USA), classification Acute Tox. 4; H302.

2.1.2.1. Synthesis Synthesis ofof NylonNylon 6/ZnO6/ZnO Nanocomposites NylonNylon 6/ZnO 6/ZnO nanocompositenanocomposite preparationpreparation waswas carriedcarried outout inin the the following following way: way: nylon ny- 6lon polymer 6 polymer with with three three charges charges of ZnO of ZnO nanoparticles nanoparticles (0.25%, (0.25%, 0.5%, 0.5%, and and 1%) 1%) were were prepared pre- bypared an ultrasound-assisted by an ultrasound-assisted melt extrusion melt extrusion process. process. A laboratory-size A laboratory-size twin-screw twin-screw extruder ex- fromtruder Thermo from Thermo Scientific Scientific (model, (model, Prism TSE-24MC) Prism TSE-24MC) (Thermo (Thermo Fisher Scientific, Fisher Scientific, Waltham, Wal- MA, USA)tham, with MA, a USA) screw with diameter a screw of diameter 24 mm and of 24 L/D mm ratio andof L/D 40:1 ratio was of assisted 40:1 was by assisted a catenoidal by a ultrasoniccatenoidaltip ultrasonic (Branson tip Ultrasonics (Branson Ultrasonics Corp., Brookfield, Corp., Brookfield CT, USA) , CT, [24 USA)]. The [24]. temperature The tem- ◦ profileperature was profile 235 Cwas and 235 the °C rotational and the rotational speed was speed 100 rpm. was 100 The rpm. extruder The extruder was connected was con- to a homemadenected to a ultrasonichomemade generator ultrasonic (15 generator to 50 kHz, (15 to 100% 50 kHz, of 750 100% W). of Previously, 750 W). Previously, the resin the was ◦ driedresin forwas 12 dried h at 100for 12C h before at 100 the°C before preparation the preparation of nanocomposites. of nanocomposites. 2.2. Synthesis of Non-Woven Fabrics 2.2. Synthesis of Non-Woven Fabrics TheThe non-wovennon-woven fabricfabric was manufactured using using a a Fiber Fiber Extrusion Extrusion Technology Technology (FET- (FET- UK)UK) pilot pilot machine,machine, equippedequipped withwith two single spindle extruders. extruders. The The process process conditions conditions of non-woven fabric made from nanocomposites nylon 6/ZnO, were: extrusion zone 1, of non-woven◦ fabric made from◦ nanocomposites nylon ◦6/ZnO, were: extrusion zone◦ 1, 245 245°C, extrusionC, extrusion zone zone 2, 250 2, 250 °C, C,extrusion extrusion zone zone 3, 3,270 270 °C,C, extrusion extrusion zone zone 4, 4,270 270 °C,C, flange flange zone, 275 ◦C, melt pump zone 275 ◦C, dual heat zone 275 ◦C, melt-blown adapter zone, zone,◦ 275 °C, melt pump zone 275 °C,◦ dual heat zone 275 °C, melt-blown adapter zone, 280280 °C,C, melt-blownmelt-blown hothot airair zone,zone, 280280 °C.C. The speed of of the the collecting collecting band band was was 0.6 0.6 m/min, m/min, andand the the speedspeed ofof thethe dosingdosing pumppump was 6 and 8 rpm. The The use use of of two two different different extruders extruders allowsallows thethe preparationpreparation of of the the nanocomposite nanocomposite se separatelyparately to tosubject subject it to it an to ultrasound-as- an ultrasound- assistedsisted extrusion extrusion that that disperses disperses the the nanopartic nanoparticlesles in in the the polymeric polymeric matrix. matrix. Once Once that that the the nanocompositenanocomposite is is obtained;obtained; thethe non-woven fabric is is manufactured manufactured in in the the FET-UK FET-UK extruder extruder byby melt-blow. melt-blow. SamplesSamples of of pure pure nylon nylon 6 6 non-woven non-woven fabric fabric and and nylon nylon 6/ZnO 6/ZnO nanoparticles nanoparticles non-woven non-wo- fabricven fabric at 0, 0.25,at 0, 0.50.25, and 0.5 1.0and wt%, 1.0 wt%, were were identified identified as: Ny; as: Ny/ZnONy; Ny/ZnO 0.25; 0.25; Ny/ZnO Ny/ZnO 0.5; 0.5; and Ny/ZnOand Ny/ZnO 1, respectively. 1, respectively. Figure Figure1 shows 1 shows the twothe two types types of extruders of extruders used used to manufactureto manufac- theseture these materials: materials: first thefirst nanocomposites; the nanocomposites; and and later later the non-woventhe non-woven fabrics. fabrics.

FigureFigure 1. 1.Images Images of of extruders extruders used used in in to to manufacture manufacture nonwoven nonwoven fabric: fabric: (a )( twin-screwa) twin-screw extruder extruder con- nectedconnected to a homemadeto a homemade ultrasonic ultrasonic generator; generator; and (b and) fiber (b) extrusion fiber extrusion technology technology (FET-UK) (FET-UK) pilot machine. pilot machine. 2.3. Characterization 2.3. TheCharacterization non-woven nylon 6/ZnO fabrics were observed using a Carl Zeiss HD HAL/LED opticalThe microscope. non-woven nylon The surface 6/ZnO fabrics analysis were was ob doneserved by using scanning a Carl Zeiss electron HD microscopyHAL/LED andoptical energy microscope. dispersion The spectroscopy surface analysis (SEM-EDS) was done through by scanning an electron electron microscope microscopy model and JEOLenergy JSM-7401F; dispersion the spectroscopy microscope (SEM-EDS) acceleration through voltage an was electron 3.0 kV microscope using the LEI model secondary JEOL electronsJSM-7401F; as athe detector. microscope The acce mechanicalleration voltage tests were was carried 3.0 kV using out by the the LEI universal secondary machine elec- CEF-122,trons as a according detector. toThe ASTM-D-5034-09 mechanical tests andwere ASTM-D-2261-13. carried out by the universal machine CEF- 122,Accelerated according to washingASTM-D-5034-09 tests were and carried ASTM-D-2261-13. out under the AATCC-TEST METHOD 61 standard for all the non-woven fabrics obtained. The antimicrobial activity of the materials was evaluated through the agar diffusion technique based on Kirby Bauer. This method determines the sensitivity of a microorganism against a speciﬁc material: in this case, non-woven fabric [25]. Polymers 2021, 13, 1888 4 of 13

The antimicrobial tests were carried out as follows: strains Candida albicans (ATCC® 10231™), and Staphylococcus aureus (ATCC® 23235™) were grown in a liquid medium— nutritious broth for C. albicans and soy trypticase broth for S. aureus. The bacteria were incubated at 37 ◦C and the fungus, at 33 ◦C for 24 h, until turbidity was obtained in the middle of 0.5 Mc Farland each of the inoculums. Grown cultures were prepared and smeared onto the surface of nutrient agar and soy trypticase in Petri plates. Specimens of approximately 5 × 3 mm in size were cut from the textile samples under aseptic conditions and placed in the agar surface and the plates were incubated at 37 ◦C and 33 ◦C, respectively, for 24 h. and the diameter of the formed zone of inhibition (in mm) was determined. For evaluating the adsorption of dye, the fabrics were activated with a UV lamp (Numak model GL-9406) at a wavelength of 365 nm irradiating for 10 min. The adsorption experiments were carried out by filtering 20 mL of the 200 mg/L solution (methylene blue) with non-woven fabric (5 × 5 cm) with a weight of 20 mg at room temperature during 60 min. The non-woven fabric was fixed in a glass funnel and the methylene blue solution was poured into the non-woven fabric slowly for 60 min. An aliquot (solution methylene blue) of the experiment was taken every 15 min to be analyzed by UV–Vis spectroscopy at 664 nm. All the experiments were made in triplicate. The percentage of adsorption efficiency was calculated according to Equation (1):

C − C % Adsorption ef ficienty = i e × 100 (1) Ci

where Ci and Ce are the initial and ﬁnal concentrations, respectively. The adsorption capacity of the non-woven fabric was calculated with Equation (2) in equilibrium: (C − C )V q = i e (2) e m where V is the volume of the solution in liters (L) and m is the amount of mass in mg of absorbent.

Adsorption Isotherm The Langmuir and Freundlich models were used to describe the adsorption equilibrium data. For the assessment of both models, the absorption isotherms’ data were ﬁtted and the correlation coefﬁcient (R2) was calculated using the trendline command in Microsoft Excel. The Langmuir isotherm was calculated by Equation (3):

C C 1 e = e + (3) qe qm KLqm

−1 −1 where qe (mg·g ) and Ce (mg·L ) are the concentrations of the solid and liquid phases of adsorbate in equilibrium, respectively, qm is the maximum adsorption capacity, and KL is the constant obtained from the graph of Ce/qe against Ce. The Freundlich isotherm was calculated by Equation (4):

1 ln q = ln K + ln C (4) e F n e

−1 −1 where KF (mg·g ) (L·mg ) and 1/n are the Freundlich constants related to the adsorption capacity and n is the heterogeneity factor calculated by linearly plotting ln·qe against ln·Ce.

2.4. Nonwoven Fabric Regeneration After the adsorption of methylene blue, the non-woven fabric turns blue. To regenerate the non-woven fabric and reuse it, it was exposed to the sunlight for 3 h or under a UV lamp (Model Numak GL-9406) with a wavelength of 365 nm for one hour. After treatment, the non-woven fabric removed the blue color, which indicates the degradation of the MB. Polymers 2021, 13, x FOR PEER REVIEW 5 of 15

2.4. Nonwoven Fabric Regeneration After the adsorption of methylene blue, the non-woven fabric turns blue. To regenerate the non-woven fabric and reuse it, it was exposed to the sunlight for 3 h or under a Polymers 2021, 13, 1888 UV lamp (Model Numak GL-9406) with a wavelength of 365 nm for one hour. After treat-5 of 13 ment, the non-woven fabric removed the blue color, which indicates the degradation of the MB.

3.3. ResultsResults andand DiscussionDiscussion 3.1. Microscopy Characterization of Nylon 6 and Nanocomposites 3.1. Microscopy Characterization of Nylon 6 and Nanocomposites Optical microscopy images of nylon 6 and the nanocomposites (Figure2a,d,g) show Optical microscopy images of nylon 6 and the nanocomposites (Figure 2a,d,g) show that the fibers maintain a random orientation and present a cylindrical shape with a that the fibers maintain a random orientation and present a cylindrical shape with a di- diameter of 15–18.1 microns, though no significant differences were observed between ameter of 15–18.1 microns, though no significant differences were observed between the the samples. SEM micrographs show that the fibers did not exhibit damage on their samples. SEM micrographs show that the fibers did not exhibit damage on their surface, surface, observing only some roughness (Figure2b,e,h). Nanocomposite micrographs observing only some roughness (Figure 2b,e,h). Nanocomposite micrographs show a ZnO show a ZnO NPs homogeneous dispersion on the fiber surface, which is according to the NPs homogeneous dispersion on the fiber surface, which is according to the concentration concentration employed. EDS spectra (Figure2f,i) confirm the presence of ZnO NPs. The employed. EDS spectra (Figure 2f,i) confirm the presence of ZnO NPs. The evidence ob- evidence observed by the SEM technique suggests that NPs can play an important role served by the SEM technique suggests that NPs can play an important role in the degra- in the degradation of MB because their surface can interact with the dye and cause its dation of MB because their surface can interact with the dye and cause its degradation. degradation. Raza et al. [26] incorporated ZnO with chitosan into cotton fabric, finding a Raza et al. [26] incorporated ZnO with chitosan into cotton fabric, finding a prevalence of prevalence of the zinc element, which is in agreement with this study they also observed the zinc element, which is in agreement with this study they also observed the agglomer- the agglomeration of ZnO particles due to the impregnation method used. ation of ZnO particles due to the impregnation method used.

FigureFigure 2.2. OpticalOptical microscopymicroscopy ofof ((aa)) nylon;nylon; ((dd)) Ny/ZnONy/ZnO 0.5; and ( g) Ny/ZnO 1. 1. SEM SEM images images of of (b (b) ) nylon;nylon; ( e()e) Ny/ZnO Ny/ZnO 0.5; 0.5; and and ( h(h)) Ny/ZnO Ny/ZnO 1. 1. EDS EDS spectra spectra of of (c ()c nylon;) nylon; (f )(f Ny/ZnO) Ny/ZnO 0.5; 0.5; and and (i )(i Ny/ZnO) Ny/ZnO 1. 1. 3.2. Mechanical Properties 3.2. MechanicalFigure3 shows Properties the results of the mechanical analysis of maximum breaking strength, elongationFigure percentage, 3 shows the and results tear of force. the mechanical Nylon 6 presented analysis a of maximum maximum breaking breaking strength strength, of 31.87elongation MPa, which percentage, increased and to tear 35.67 force MPa. Nylon for the sample6 presented with 0.5%wt.a maximum of ZnO. breaking This behavior strength is relatedof 31.87 to previousMPa, which studies, increased which refersto 35.67 that MPa a low for amount the sample of ZnO with and 0.5%wt. an adequate of ZnO. dispersion This ofbehavior the nanoparticles is related improvedto previous the studies, mechanical which properties refers that of nanocomposites a low amount of from ZnO polyamide and an andadequate polyurethane dispersion [27 ].of While the nanoparticles the behavior im forproved elongation the mechanical and tear force properties was similar of nano- to the previouscomposites measurement, from polyamide the sample and polyurethane Ny/ZnO 0.5 [27]. showed While the the best behavior performance. for elongation and

3.3. Antimicrobial Properties The antimicrobial activity of non-woven fabrics was tested using the Kirby Bauer agar diffusion method. The macroscopic results reveal a contact inhibition effect using Ny/ZnO materials. Figure4 shows a complete inhibition for S. aureus; however, nylon without nanoparticles also presents a growth inhibition, which has already been reported by Mowery et al. [28]. These authors attribute the biological activity of nylon to the fact that the polymer has a structural basis of the oligomers of β-peptides made up of amino acids, Polymers 2021, 13, 1888 6 of 13

which damage the cell wall. Any sample of non-woven fabric shows inhibition against E. coli, however, the samples of Ny/ZnO 0.25, Ny/ZnO 0.5, and Ny/ZnO 1.0 of non-woven fabric show inhibition against the fungus C. albicans (Figure5). Janaki et al. [ 29] report activity against C. albicans of ZnO using the diffusion method in agar; they attribute the Polymers 2021, 13, x FOR PEER REVIEW 6 of 15 − inhibition to the formation of reactive oxygen species (ROS) such as superoxide anion O2 , · hydrogen peroxide H2O2 and hydroxyl radical (HO ), generated by the ZnO. These reactive tearspecies force damagewas similar the to cellsthe previous of the meas microorganismsurement, the sample resulting Ny/ZnO in their0.5 showed decomposition the and entire +2 bestdestruction performance. and the Zn ions have a minor inﬂuence on the antibacterial activity [30].

Figure 3. 3.MechanicalMechanical properties properties of non-woven of non-woven fabrics: (a fabrics:) maximum (a) maximumbreaking strength; breaking (b) elonga- strength; (b) elongation %; tionand %; (c and) tear (c) force.tear force.

3.3. AntimicrobialDural Erem Properties et al., when obtaining fibers with nylon 6 and ZnO nanocomposites in a 16The cm antimicrobial long microcomponder, activity of non-woven feeding fabrics 10 gwas of tested the nanocomposite. using the Kirby Bauer They found that the agar diffusion method. The macroscopic results reveal a contact inhibition effect using Ny/ZnOsensitivity materials. barrier Figure in its 4 fibersshows a can complete vary dependinginhibition for onS. aureus the ZnO; however, content nylon and also by the type withoutof bacteria. nanoparticles In addition, also presents they a growth observed inhibition, that thewhich bacteriostatic has already been effect reported of the fibers against byGram-positive Mowery et al. [28]. bacteria These wasauthors presented attribute atthe 0.5 biological wt% ZnO activity particles of nylon and to the this fact same effect against Polymers 2021, 13, x FOR PEER REVIEW 7 of 15 thatGram-negative the polymer has bacteria a structural was basis obtained of the oligomers at a concentration of β-peptides lower made thanup of amino 0.5 wt%. These authors acids,tested which concentrations damage the cell up wall. to 5 wt%Any sample [31]. of non-woven fabric shows inhibition against E. coli, however, the samples of Ny/ZnO 0.25, Ny/ZnO 0.5, and Ny/ZnO 1.0 of non- woven fabric show inhibition against the fungus C. albicans (Figure 5). Janaki et al. [29] report activity against C. albicans of ZnO using the diffusion method in agar; they attribute the inhibition to the formation of reactive oxygen species (ROS) such as superoxide anion O2−, hydrogen peroxide H2O2 and hydroxyl radical (HO·), generated by the ZnO. These reactive species damage the cells of the microorganisms resulting in their decomposition and entire destruction and the Zn+2 ions have a minor influence on the antibacterial activity [30]. Dural Erem et al., when obtaining fibers with nylon 6 and ZnO nanocomposites in a 16 cm long microcomponder, feeding 10 g of the nanocomposite. They found that the sensitivity barrier in its fibers can vary depending on the ZnO content and also by the type of bacteria. In addition, they observed that the bacteriostatic effect of the fibers against Gram- positive bacteria was presented at 0.5 wt% ZnO particles and this same effect against Gram-negative bacteria was obtained at a concentration lower than 0.5 wt%. These authors tested concentrations up to 5 wt% [31].

Figure 4. Effect of control samples, antibiotic, nylon, Ny/ZnO 1, Ny/ZnO 0.5, Ny/ZnO 0.25 on the Figure 4. Effect of control samples, antibiotic, nylon, Ny/ZnO 1, Ny/ZnO 0.5, Ny/ZnO 0.25 on the growthgrowth of S. of aureusS. aureus. .

Figure 5. Effect of antibiotic, nylon, Ny/ZnO 1, Ny/ZnO 0.5, Ny/ZnO 0.25 on the growth of C. albicans.

3.4. Methylene Blue Adsorption After an accelerated wash treatment using the AATCC-TEST METHOD 61 standard to evaluate the color fastness and changes in the surface of the fabric, using detergents and washes, the fabrics were characterized to determine the adsorption efficiency of methylene blue, when using nylon 6 non-woven fabric with ZnO nanoparticles. Figure 6 shows the adsorption efficiency percentage, where we can observe that the pristine nylon non- woven fabric only presented 13% adsorption efficiency, indicating that methylene blue could not be removed from the water by the non-woven fabric without nanoparticles. This agrees with that reported by Lee et al., in the adsorption of methylene blue using nylon-6 nanofibers entangled with graphene flakes. The nylon 6 presents low adsorption, and by increasing the concentration of graphene, it presents a higher performance [32].

Polymers 2021, 13, x FOR PEER REVIEW 7 of 15

Polymers 2021, 13, 1888 7 of 13 Figure 4. Effect of control samples, antibiotic, nylon, Ny/ZnO 1, Ny/ZnO 0.5, Ny/ZnO 0.25 on the growth of S. aureus.

Figure 5. Effect of antibiotic, nylon, Ny/ZnO 1, Ny/ZnO 0.5, Ny/ZnO 0.25 on the growth of Figure 5. Effect of antibiotic, nylon, Ny/ZnO 1, Ny/ZnO 0.5, Ny/ZnO 0.25 on the growth of C. albi- cansC.. albicans.

3.4.3.4. Methylene Methylene Blue BlueAdsorption Adsorption AfterAfter an accelerated an accelerated wash treatment wash treatment using the usingAATCC-TEST the AATCC-TEST METHOD 61 METHODstandard 61 standard toto evaluate evaluate the thecolor color fastness fastness and changes and changes in the surface in the surfaceof the fabric, of the using fabric, detergents using detergents and andwashes, washes, the the fabricsfabrics were were characterized characterized to determine to determine the adsorption the adsorption efficiency efficiency of meth- of methylene yleneblue, blue, when when using using nylonnylon 6 6 non-woven non-woven fabric fabric with withZnO nanoparticles. ZnO nanoparticles. Figure 6 show Figures 6 shows the the adsorption efficiency percentage, where we can observe that the pristine nylon non- adsorption efficiency percentage, where we can observe that the pristine nylon non-woven woven fabric only presented 13% adsorption efficiency, indicating that methylene blue couldfabric not only be removed presented from 13% the water adsorption by the non-woven efficiency, fabric indicating without that nanoparticles. methylene This blue could not be agreesremoved with that from reported the water by Lee by et the al., non-wovenin the adsorption fabric of methylene without nanoparticles.blue using nylon-6 This agrees with nanofibersthat reported entangled by Leewith etgraphene al., in theflakes. adsorption The nylon of6 presents methylene low adsorption, blue using and nylon-6 by nanofibers increasingentangled the withconcentration graphene of graphene, flakes. The it presents nylon a 6 higher presents performance low adsorption, [32]. and by increasing the concentration of graphene, it presents a higher performance [32]. Non-woven fabrics assigned as Ny/ZnO 0.25, Ny/ZnO 0.5 and Ny/ZnO 1.0 presented percentages of adsorption of methylene blue of 42%, 82% and 61%, respectively. Ummarty- otin et al. reported the synthesis of ZnO nanoparticles which were deposited on the surface of nylon 6 by the spin-coated method: they reported only 60% adsorption of methylene blue using different percentages of the weight of nanoparticles 1.0, 3.0 and 5.0 wt% [33]. In this study, we report 82% adsorption of methylene blue for the Ny/ZnO 0.5 sample which contains 0.5% of ZnO nanoparticles. This is attributed to the nanocomposite preparation by means of ultrasound-assisted extrusion that allows a uniform distribution and a good dispersion of the nanoparticles in non-woven fabric. On the other hand, with the other studies, they used impregnation methods, dip-coating, spin-coating that creates agglomer- ates on the surface and with washing the coating that was acquired can be eliminated. A different situation occurs in the non-woven fabrics prepared in this study: the ZnO NPs are inside the polymer matrix and are not removed. In another study, the color removal of textile wastewater was achieved up to 81%, with only ZnO nanoparticles synthesized using a polyvinylpyrrolidone (PVP)-assisted co-precipitation route [34]. This method can be limited because the nanoparticles are not supported in a polymeric matrix and have to directly come into contact with the wastewater from the textile industry. The method proposed in this paper is considered a method that can be very useful at an industrial level, since it uses low amounts of ZnO nanoparticles and can be adaptable to large volumes of wastewater. The advantage of using non-woven fabric for dye removal in water is that it is used as a reusable filter and can also retain solids and micrometer-sized particles.

3.4.1. Effect of UV Radiation on MB Adsorption Figure7 shows the adsorption efficiency when the non-woven fabric is activated for 10 min with a UV lamp, an increase in the percentage of adsorption efficiency is observed in all samples. Ny/ZnO 0.5 presents 93% adsorption efficiency, and an increase in the Polymers 2021, 13, x FOR PEER REVIEW 8 of 15

Non-woven fabrics assigned as Ny/ZnO 0.25, Ny/ZnO 0.5 and Ny/ZnO 1.0 presented percentages of adsorption of methylene blue of 42%, 82% and 61%, respectively. Um- martyotin et al. reported the synthesis of ZnO nanoparticles which were deposited on the surface of nylon 6 by the spin-coated method: they reported only 60% adsorption of methylene blue using different percentages of the weight of nanoparticles 1.0, 3.0 and 5.0 wt% [33]. In this study, we report 82% adsorption of methylene blue for the Ny/ZnO 0.5 sample which contains 0.5% of ZnO nanoparticles. This is attributed to the nanocomposite preparation by means of ultrasound-assisted extrusion that allows a uniform distribution and a good dispersion of the nanoparticles in non-woven fabric. On the other hand, with the Polymers 2021, 13, 1888 other studies, they used impregnation methods, dip-coating, spin-coating that creates ag- 8 of 13 glomerates on the surface and with washing the coating that was acquired can be eliminated. A different situation occurs in the non-woven fabrics prepared in this study: the ZnO NPs are inside the polymer matrix and are not removed. percentageIn another of study, adsorption the color efﬁciency removal of is textile also observed wastewater in was the Ny/ZnOachieved up 0.25% to 81%, sample from 40% with only ZnO nanoparticles synthesized using a polyvinylpyrrolidone (PVP)-assisted co- precipitationto 65%. Although route [34]. the This exposure method can time be limited to UV lightbecause is short,the nanoparticles the non-woven are not fabricsup- that contains portedthe ZnO in a polymeric nanoparticles matrix canand behave activated to directly even come though into contact the ZnOwith the concentrations wastewater are low and fromthese the aretextile dispersed industry. in The the me polymericthod proposed matrix. in this Another paper is workconsidered reported a method by Ashraf that et al. details canthe be use very of useful long at exposure an industrial times level, to UVsince radiation it uses low (4 amounts h), for theof ZnO polyester nanoparticles fabric functionalized andwith can thebe adaptable ZnO nanorods to large volumes system. of In wastewater. the Ny/ZnO The advantage 1 sample, of thereusing non-woven is only an increase of 4% fabricregarding for dye adsorptionremoval in water efﬁciency is that when it is used the sampleas a reusable is UV filter irradiated, and can whichalso retain may be due to the solids and micrometer-sized particles. concentration of ZnO and a blockage caused by more molecules adsorbing active sites [35].

Polymers 2021, 13, x FOR PEER REVIEW 9 of 15

tionalized with the ZnO nanorods system. In the Ny/ZnO 1 sample, there is only an in-

creaseFigure of 4% 6. The regarding adsorption adsorption efﬁciency efficiency of nylon, when Ny/ZnO the sample 1, Ny/ZnO is UV irradiated, 0.5, Ny/ZnO which 0.25 washed non- Figure 6. The adsorption efficiency of nylon, Ny/ZnO 1, Ny/ZnO 0.5, Ny/ZnO 0.25 washed non- maywoven be due fabric to the (MB concentration concentration of =ZnO 200 and mg/L; a blockage nanocomposite caused by content more =molecules 20 mg/20 ad- mL; T = 25 ◦C and sorbingwoven fabric active (MB sites concentration [35]. = 200 mg/L; nanocomposite content = 20 mg/20 mL; T = 25 °C and t = 60t = min). 60 min).

3.4.1. Effect of UV Radiation on MB Adsorption Figure 7 shows the adsorption efficiency when the non-woven fabric is activated for 10 min with a UV lamp, an increase in the percentage of adsorption efficiency is observed in all samples. Ny/ZnO 0.5 presents 93% adsorption efficiency, and an increase in the percentage of adsorption efficiency is also observed in the Ny/ZnO 0.25% sample from 40% to 65%. Although the exposure time to UV light is short, the non-woven fabric that contains the ZnO nanoparticles can be activated even though the ZnO concentrations are low and these are dispersed in the polymeric matrix. Another work reported by Ashraf et al. details the use of long exposure times to UV radiation (4 h), for the polyester fabric func-

Figure 7. The adsorption efﬁciency of nylon, Ny/ZnO 1, Ny/ZnO 0.5, Ny/ZnO 0.25 UV activated Figure 7. The adsorption efficiency of nylon, Ny/ZnO 1, Ny/ZnO 0.5, Ny/ZnO 0.25 UV activated ◦ non-wovennon-woven fabric fabric (MB concentration (MB concentration = 200 mg/L, = 200 nanocomposite mg/L, nanocomposite content = 20 mg/20 content mL, = T 20 = 25 mg/20 °C mL, T = 25 C andand t = 60 t = min). 60 min).

3.4.2.3.4.2. Adsorption Adsorption Efficiency Efﬁciency of MB as of a MB Function as a Functionof Time of Time TheThe adsorption adsorption efficiency efficiency of methylene of methylene blue according blue to according the time of to the the nylon time and of the nylon and Ny/ZnO 0.5 samples is shown in Figure 8. The non-woven nylon fabric without nanopar- Ny/ZnO 0.5 samples is shown in Figure8. The non-woven nylon fabric without nanoparticles ticles showed low levels of adsorption, and after 60 min, the maximum adsorption was 18%.showed For non-woven low levels fabric of adsorption,with 0.5 wt% andof ZnO after nanoparticles, 60 min, the there maximum is a higher adsorption adsorp- was 18%. For tionnon-woven efficiency (95%). fabric The with great 0.5 adsorption wt% of ZnO efficiency nanoparticles, of the non-woven there isNy/ZnO a higher fabric adsorption can efficiency be(95%). attributed The to great a good adsorption dispersion efficiencyof the NPs ofin the polymer non-woven matrix, Ny/ZnO and the synthesis fabric can of be attributed to nanocompositesa good dispersion of nylon of 6 the using NPs an in ultrasound the polymer-assisted matrix, extrusion and theprocess synthesis is an effective of nanocomposites of methodnylon of 6 dispersing using an ultrasound-assisteddifferent nanoparticles extrusion[36,37], which process is also is conserved an effective in the method non- of dispersing wovendifferent fabric. nanoparticles The adsorption [36 properties,37], which are is preserved also conserved even after in the washing, non-woven and the fabric. dis- The adsorption coloration can be observed in the image of Figure 8 with the exposure time of the Ny/ZnO 0.5properties sample. are preserved even after washing, and the discoloration can be observed in the image of Figure8 with the exposure time of the Ny/ZnO 0.5 sample.

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Polymers 2021 13 , , 1888 Polymers 2021, 13, x FOR PEER REVIEW 10 of 159 of 13

Figure 8. The adsorption efficiency of nylon and Ny/ZnO 0.5 (MB concentration = 200 mg/L, nanocomposite content = 20 mg/20 mL, T = 25 °C and t = 60 min). Figure 8. The adsorption efficiency of nylon and Ny/ZnO 0.5 (MB concentration = 200 mg/L, Figure 8.3.4.3. nanocompositeThe adsorption Adsorption efficiency content Isothermof nylon = 20 and mg/20 Ny/ZnO mL, 0.5 T (MB = 25 concentratio◦C and tn = = 60200 min). mg/L, nanocomposite content = 20 mg/20 mL, T = 25 °C and t = 60 min). 3.4.3.The Adsorption results of Isotherm the Langmuir and Freundlich isotherms are presented in Figure 9. The 3.4.3. Adsorption Isotherm Ny/ZnOThe 0.5 results isotherm of the has Langmuir a homogeneous and Freundlich adsorption isotherms behavior, are presented when presenting in Figure9. the co- The results of the2 Langmuir and Freundlich isotherms are presented in Figure 9. The efficientThe Ny/ZnO of determinationNy/ZnO 0.5 isotherm 0.5 isotherm has (R ahas) homogeneousvalue a homogeneous of 0.95 adsorption(Tableadsorption 1), behavior, which behavior, when agrees when presenting with presenting thethat co- reported the by Jawadcoefficient et al. of efficient determination[38]. The of determination Langmuir (R2) value (R isotherm2) value of 0.95 of 0.95 (Table is (Tablebased1), 1), which onwhich the agrees agrees assumption with with that that reported reportedthat byan byactivation pointJawad on et the al. [ Jawad38surface]. The et al. Langmuirof [38]. the The adsorbent Langmuir isotherm isotherm is is capable based is based on of on the adsorbing the assumption assumption a thatmolecule, that an anactivation activation indicating the point on the surfacepoint on the of thesurface adsorbent of the adsorbent is capable is capable of adsorbing of adsorbing a a molecule,molecule, indicating indicating the the formation of formationa monolayer of a monolayer [39,40]. [39,40]. On Onthe the othe other hand,hand, the the nylon nylon sample sample has a heterogene- has a heterogene- ousformation adsorption of aous monolayer adsorptionbehavior [behavior39 when,40]. On whenpresenti the presenti otherngng hand, the the coefficientcoefficient the nylon of sampledetermination of determination has a (R heterogeneous2) value of(R 2) value of adsorption behavior when presenting the coefficient of determination (R2) value of 0.98. 0.98. The Freundlich0.98. The Freundlich isotherm isotherm is based is based on on a a multilayer multilayer adsorption, adsorption, and the andconcentration the concentration The Freundlichon isothermthe surface of is the based adsorbent on a multilayerincreases as the adsorption, adsorbate concentration and the concentration increases [36]. on the onsurface the surface of the adsorbent of the adsorbent increases increases as the adsorbate as the concentrationadsorbate concentration increases [36]. increases [36].

Figure 9. Langmuir and Freundlich model of adsorption for methylene blue of the nylon and Ny/ZnO 0.5 (MB concentration = 200 mg/L, nanocomposite content = 20 mg/20 mL, T = 25 ◦C and t = 60 min).

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Table 1. Parameters of the isotherm constants and correlation coefficients calculated for urea adsorption.

Langmuir Freundlich Sample 2 2 k qmax R n KF R Nylon 1.20 196.92 0.9387 9.61 52.17 0.9817 Ny/ZnO 0.5 0.023 0.33 0.9527 0.30 5.41 0.8346

3.4.4. Effect of pH on the Adsorption MB The pH is a parameter that can affect the adsorption capacity of the material since it plays an important role in protonation/deprotonation, though it can also affect the affinity of the adsorbent surface. Figure 10a shows the effect of pH on the adsorption capacity of the Ny/ZnO non-woven fabric for methylene blue: from pH 2, to 4 and 6, the adsorption increases progressively; at pH 11, the adsorption of the dye is reduced. In Figure 10b, the adsorption efficiency increases from 80% to 93% when the pH increases from 2 to 7, and in the case of a basic pH such as 11, there is a decrease of 60%. Elwakeel et al. reported sorption efficiency, >92% of MB for chitosan functionalized with 2-mercaptobenzimidazole, they present a linear Polymers 2021, 13, x FOR PEER REVIEW 12 of 15 behavior [41]. Instead of another study for the removal of methylene blue using bivalve shells of anadara uropigimelana as a biosorbent at pH 11 and 12, there is a decrease in the percentage of the removal or adsorption similar to that reported in this work [8].

FigureFigure 10. 10. (a) Effect(a) Effect of pH ofon pHmethylene on methylene blue of adsorp bluetion of onto adsorption non-woven onto fabric non-woven Ny/ZnO 0.5; fabric (b) Ny/ZnO adsorption0.5; (b) adsorption efficiency % efﬁciencya different %pH a (MB different concentration pH (MB = 200 concentration mg/L, nanocomposite = 200 mg/L, content nanocomposite = 20 mg/20content mL, = T 20 = 25 mg/20 °C and mL, t = 60 T =min). 25 ◦ C and t = 60 min).

3.4.5.3.4.5. Mechanism Mechanism of the of theAdsorption Adsorption First,First, a dye a dye adsorption adsorption occurs occurs on the non-wo on theven non-woven fabric as seen fabric in Figure as seen 11a; in the Figure non- 11a; the wovennon-woven fabric turns fabric blue turns due blueto the due contaminant to the contaminant (dye). In this (dye). case, the In non-woven this case, thefabric non-woven is transferred, then it is irradiated by sunlight for 3 h or using the UV lamp for an hour and as shown in Figure 11b, the colorant degrades and is totally eliminated. Unlike the other materials previously reported with the Ny/ZnO nonwoven fabric, no solvent is required for regeneration. Figure 12 shows the methylene blue degradation scheme. The non-woven fabric is irradiated with a UV lamp, which generates free radicals ˙OH and ˙O2 due to the presence of the ZnO nanoparticles in the polymeric matrix. The photocatalytic activity of ZnO increases when irradiating for 10 min with the lamp. These two types of radicals decompose methylene blue [42], and in addition to that, they can break the cell membrane of bacteria, prevent their growth and inhibit them from water. Unlike other materials, this non-woven fabric can be produced at an industrial level as a water filter, since it can eliminate the microorganisms and dyes present in contaminated water.

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fabric is transferred, then it is irradiated by sunlight for 3 h or using the UV lamp for an hour and as shown in Figure 11b, the colorant degrades and is totally eliminated. Unlike Polymers 2021, 13, x FOR PEER REVIEWthe other materials previously reported with the Ny/ZnO nonwoven fabric, no solvent13 of is15

required for regeneration.

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a b

Figure 11. (a) Non-woven fabric with MB; and (b) non-woven fabric after one hour irradiation with Figure 11. (a) Non-woven fabric with MB; and (b) non-woven fabric after one hour irradiation with UV lamp. UV lamp. Figure 12 shows the methylene blue degradation scheme. The non-woven fabric is irradiated with a UV lamp, which generatesb free radicals ˙OH and ˙O2 due to the presencea of the ZnO nanoparticles in the polymeric matrix. The photocatalytic activity

of ZnO increases when irradiatingO for 10 min with the lamp. These two types of radicals 2 − decomposeFigure 11. (a methylene) Non-woven blue fabric [42 with], and MB; in and addition (b) non-woven to that, they fabric can after break one thehour cell irradiation membrane with ofUV bacteria, lamp. prevent their growth and inhibit them.O from2 water.

O 2 − .O2 .OH

H O 2 .OH

Figure 12. Schematic representations of the degradation mechanism of methylene blue dye on non- woven fabric under UV irradiation.

4. Conclusions H O Non-woven fabrics from nylon 6 and ZnO were successfully2 manufactured by melt-

blowing. SEM images showed that the ZnO nanoparticles were dispersed homogeneously FigureintoFigure the 12.12. polymer. Schematic The representations representations sample Ny/ZnO of of the the 0.5 degradatio degradation presentedn mechanism the mechanism best mechanical of ofmethylene methylene performance. blue blue dye dyeon non- onAll non-wovensampleswoven fabric inhibited fabric under under UVthe UVirradiation. growth irradiation. of S. aureus, including the nylon matrix. The non-woven fabric prepared with just 0.25 wt% ZnO prevented the growth of C. albicans. The meth- ylene4. ConclusionsUnlike blue absorption other materials, efficiency this non-woven was 93% in fabric a time can of be 60 produced min., and at the an industrialadsorption level equilib- as a water filter, since it can eliminate the microorganisms and dyes present in contaminated water. riumNon-woven data obeyed fabrics the Langmuir from nylon isotherm. 6 and ZnO were successfully manufactured by melt- blowing. SEM images showed that the ZnO nanoparticles were dispersed homogeneously Author Contributions: Conceptualization, C.C.-A., M.A.-G. and G.C.-P.; methodology, C.A.Á.-O. into the polymer. The sample Ny/ZnO 0.5 presented the best mechanical performance. All and Z.V.Q.-J.; performed the experiments, M.A.-G. and S.C.E.-G.; writing—original draft preparation,samples M.A.-G., inhibited M.P.-A. the and growth C.C.-A.; of writing—review S. aureus, including and editing, the nylon C.C.-A. matrix. and G.C.-P.; The non-woven All authors havefabric read prepared and agreed with to justthe pub0.25lished wt% version ZnO prevented of the manuscript. the growth of C. albicans. The methylene blue absorption efficiency was 93% in a time of 60 min., and the adsorption equilibrium data obeyed the Langmuir isotherm.

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4. Conclusions Non-woven fabrics from nylon 6 and ZnO were successfully manufactured by melt- blowing. SEM images showed that the ZnO nanoparticles were dispersed homogeneously into the polymer. The sample Ny/ZnO 0.5 presented the best mechanical performance. All samples inhibited the growth of S. aureus, including the nylon matrix. The non-woven fabric prepared with just 0.25 wt% ZnO prevented the growth of C. albicans. The methylene blue absorption efﬁciency was 93% in a time of 60 min., and the adsorption equilibrium data obeyed the Langmuir isotherm.

Author Contributions: Conceptualization, C.C.-A., M.A.-G. and G.C.-P.; methodology, C.A.Á.-O. and Z.V.Q.-J.; performed the experiments, M.A.-G. and S.C.E.-G.; writing—original draft preparation, M.A.-G., M.P.-A. and C.C.-A.; writing—review and editing, C.C.-A. and G.C.-P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by (FORDECYT-CONACYT), grant number 296356 and (FOMIX- CONACYT) grant number TLAX-2018-01-01-43129. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors are also grateful Leticia Melo López, Jesus Gilberto Rodriguez Velazquez, Myrna Salinas Hernández, Jesús G. Quiroz López and Jesús A. Cepeda Garza for their technical support. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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