A cotton fabric modified with a hydrogel containing ZnO nanoparticles. Preparation and properties study

Applied Surface Science 345 (2015) 72–80

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A cotton fabric modified with a hydrogel containing ZnO nanoparticles. Preparation and properties study Desislava Staneva a,∗ , Daniela Atanasova a , Evgenia Vasileva-Tonkova b , Varbina Lukanova a , Ivo Grabchev c,d a

University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria Institute of Microbiology, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria c Sofia University St. Kliment Ohridski, Faculty of Medicine, 1407 Sofia, Bulgaria d Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia b

a r t i c l e

i n f o

Article history: Received 14 January 2015 Received in revised form 3 March 2015 Accepted 20 March 2015 Available online 30 March 2015 Keywords: Zinc oxide nanoparticles Textile Antimicrobial activity Nanocomposite Hydrogel Photopolymerization

a b s t r a c t Two different methods were used to obtain composite materials based on a ZnO nanoparticles–hydrogel–cotton fabric. The hydrogels, synthesized by photopolymerization, were utilized to provide uniform distribution and binding of the nanoparticles to the fiber surface and to prevent their agglomeration. N-methyldiethanolamine (MDEA) was used as a co-initiator in hydrogel photopolymerization and as an alkaline agent in the synthesis of ZnO nanoparticles. Due to the difference in size, shape and morphology of the nanoparticles, examined by a TEM and SEM, it was found that the materials have distinct photoluminescence properties, e.g. in the entire visible or UV range. The composite materials with small size nanoparticles and photoluminescence in near UV range were investigated for antibiotic activity against the bacterial strains Pseudomonas aeruginosa and Acinetobacter johnsonii known as important pathogens in clinical infections. Significantly high antibacterial effect on the bacteria tested was achieved, especially on A. johnsonii. This suggests potential application of the new formulations as effective wound dressings to control the spread of infections. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The addition of nanoparticles to obtain materials of novel properties has been a current trend in textile coating and finishing. Nanoparticles can behave differently from bulk materials due to their small size and resulting large surface area. The other useful advantages of nanoparticles specified by Buyle et al. [1] are the need of smaller amounts of active product and the need of its very uniform distribution in coatings. Research on ZnO based nanostructures has drawn considerable attention in the last few years as a multi-functional material due to its versatile properties such as UV absorption, near UV and visible (green, blue and violet) emission, optical transparency, electrical conductivity, piezoelectricity, and antibacterial properties [2]. That is why those nanoparticles have a vast area of applications in sensors, drug-delivery, cosmetics, optical and electrical devices, photovoltaic devices and solar cells [3]. The use of nanoparticles of ZnO has been seen as a viable solution to stop infectious diseases due to their antimicrobial properties.

∗ Corresponding author. Tel.: +359 2 8163 265. E-mail address: [email protected] (D. Staneva). http://dx.doi.org/10.1016/j.apsusc.2015.03.141 0169-4332/© 2015 Elsevier B.V. All rights reserved.

Many research groups work in the field of ZnO nanoparticles synthesis and their application to textile materials where ZnO nanoparticles have been used for imparting antibacterial properties, UV-blocking and self-cleaning properties to textile materials [4–7]. The main problem when coating textiles with formulations containing nanoparticles is the control over the dispersion and surface distribution of the nanoparticles. If agglomeration occurs, the actual particle sizes lie in the range of several micrometers (␮m) or even higher, so that the typical properties of nanosized objects might be lost. Such particles no longer behave like nanomaterials. [1]. There are two possible ways for preparing coatings of textile materials with nanoparticles. One is ex situ, e.g. at first the nanoparticles are synthesized and then applied over a textile material [8]. Agglomeration is prevented by stabilization of the nanoparticles with a high molecular binding agent during the preparation of the nanoparticles and their application over fabric. Chitosan [9], acrylic binder [10,11] or soluble starch [12] is used as a stabilizing agent. The other method, which ensures the more uniform distribution of nanoparticles, is their in situ preparation and binding to a textile surface. ZnO nanostructures were in situ synthesized on the surface of a cotton fabric through a simple and efficient wet chemical

D. Staneva et al. / Applied Surface Science 345 (2015) 72–80

method of Shateri-Khalilabad et al. [13]. However, further agglomeration is observed when using the approach they proposed. The synthesized particles are in two different structures: bundle-like and flower-like, and are composed of a few rods. This is the reason for the actual particle sizes to be higher than 100 nm or even in the range of several ␮m. So, to prevent the agglomeration there is need of a stabilizing agent [14]. For the past decade the different synthetic methods have been used to attain precise control over the properties of ZnO and the other nanoparticles. The wet chemical methods applied for the purpose are the most popular due to their low cost, reliability, and environmentally friendly synthetic routes [15]. Broadly the wet chemical method is based on the conversion of zinc ions into Zn(OH)2 in the presence of alkaline agent and subsequent obtaining of ZnO nanoparticles upon annealing. However, small changes in the conditions may lead to the production of particles with very different properties as size, shape, composition, crystallinity, and morphology. This may be the reason for their different optical, antimicrobial, catalytic, etc. characteristics. So, the interesting area for investigation is to understand how the growth and agglomeration of nanoparticles can be controlled. Small changes in temperature, solvent or pH control or addition of some auxiliary can yield textile materials with very different properties and with application in a very different area. In the recent years, the use of nanoparticles in antimicrobial applications has gradually increased as they are one of the promising means in coping with antibiotic resistance. Nanoparticle metal oxides are a new class of important materials that are being increasingly developed in research and health-related spheres [16]. Although in vitro antibacterial activity and efficacy of ZnO have been investigated, little is known about the antibacterial activity of ZnO nanoparticles [17]. It has been shown that ZnO nanoparticles have selective toxicity to bacteria but exhibit minimal effects on human cells [18]. After attaching to the cell membranes ZnO nanoparticles disturb their functions such as permeability and respiration [19]. Another possibility could be induction of intercellular reactive oxygen species including hydrogen peroxide (H2 O2 ), a strong oxidizing agent harmful to bacterial cells [17,20]. It has also been reported that ZnO can be activated by UV and visible light to generate highly reactive oxygen species such as OH− , H2 O2 and O2 2− . The negatively charged hydroxyl radicals and superoxides cannot penetrate into the cell membrane and are likely to remain on the cell surface, whereas H2 O2 can penetrate into bacterial cells [21]. The present work addresses the photopolymerization synthesis of a hydrogel with ZnO nanoparticles dispersed uniformly on the cotton fabric surface. Two approaches are used to obtain such composite materials. The difference in their properties was evaluated viewing the antibiotic activity against the bacterial strains Pseudomonas aeruginosa and Acinetobacter johnsonii. 2. Materials and methods

73

60 °C x 30 min 40 °C x 15 min

40 °C

washing & drying

dye fabric CH3COONa Fig. 1. Dyeing procedure.

2.2. Reaction of Eosin Y with chloroacetyl chloride (MEC) 0.692 g (0.001 mol) of Eosin Y was dissolved in 30 ml dioxan and 0.11 ml (0.001 mol) of chloroacetyl chloride dissolved in 10 ml dioxan was added drop wise into solution at room temperature for over 1 h. After that the solution was stirred at 50 ◦ C for 1 h and the precipitate was filtered, washed with water and dried. Yield was 76%. FTIR (cm−1 ): 2954, 1770, 1521, 1409, 1205, 1107, 1082, 869, 746, 694. 1 H NMR (600 MHz, CDCl ): ı 8.26 (d, 1H, J = 8.2 Hz); 7.84 (d, 3 1H, J = 7.6 Hz); 7.52 (m, 3H,); 7.26 (d, 1H, J = 7.9 Hz); 7.67 (t, 1H, J = 7.8 Hz); 4.31 (t, 2H, CH2 ); API-ES-MS C22 H8 Br4 NaClO5 (730.1 g mol−1 ) (positive) m/z: calcd: 730.1; found: 731.3 ([M+H]+ ); analysis: calculated (%): C 36.16, H 1.09; found (%): C 36.38, H 1.03. 2.3. Synthesis of modified Eosin Y (MEY) 0.356 g (0.0005 mol) of MEC was dissolved in 20 ml of N,Ndimethylformamide and 1 ml of pyridine was added. The solution was stirred vigorously at 70 ◦ C for 5 h. The precipitate was filtered off, washed with acetone, and dried in vacuum at room temperature. Yield was 96%. FTIR (cm−1 ): 3057, 2956, 1761, 1558, 1456, 1340, 1228, 1196, 974, 874, 762, 694. 1 H NMR (600 MHz, CDCl ): ı 8.97 (d, 1H, J = 7.3 Hz); 8.62 (d, 1H, 3 J = 7.6 Hz); 8.14 (m, 1H,); 8.04 (m, 1H,); 7.89 (t, 1H, J = 7.1 Hz); 7.82 (t, 1H, J = 7.0 Hz); 7.76 (t, 1H, J = 6.9 Hz); 7.46 (m, 2H); 6.89 (m, 2H); 4.49 (s, 2H, CH2 ); API-ES-MS C27 H13 Br4 NaNClO5 (positive) m/z: calcd: 809.1; found: 810.2 ([M+H]+ ); analysis: C27 H13 Br4 NaNClO5 (809.1 g mol−1 ) calculated (%): C 40.04, H 1.60, N 1.73; found (%): C 40.30, H 1.66, N 1.79. 2.4. Composite sample preparation The procedure for cotton surface functionalization involves dyeing with reactive dye, which is the modified Eosin Y (MEY), and this acts as photoinitiator (step 1) and subsequently there is hydrogel formation by surface initiated photopolymerization with dispersed ZnO nanoparticles under visible light (step 2). The MEY (photo-sensitizer) is used with a tertiary amine co-initiator such as N-methyldiethanolamine (MDEA).

2.1. Materials A bleached and unmercerized, plain-woven 100% cotton fabric with a surface weight of 140 g/m2 was used throughout the work. Acryl amide, N,N -methylenebisacrylamide (bis-AAm), Zn(NO3 )2 × 6H2 O and N-methyldiethanolamine (MDEA) were used without further purification as obtained from Aldrich. Poly(vinyl alcohol) (PVA) (Mw 15,000, 86–89% hydrolyzed) was obtained from Fluka. All solutions were made with distilled water. The light sources for photopolymerization were two energy saving lamps: 1. HL 8325, 25 w, 1230 Lumen, 6400 K, Horoz Electric. 2. VO 11,211, 11 w, 550 Lumen, 6400 K, Vitoone.

2.4.1. Dyeing of cotton fabric with modified Eosin Y (step 1) Dyeing was carried out at a liquor-to-goods ratio of 1:25. After setting out the dyebath to the calculated volume, the dye (MEY), at two concentrations (0.5% and 1.0% owf) was added to the dyebath and the temperature raised to 40 ◦ C. The fabric was dipped and treated at this temperature for 15 min. Next the sodium acetate (100 g/l) was added and the temperature was raised to 60 ◦ C. A fixation cycle was run for 30 min. At the end the fabric was removed, rinsed in warm and cold water to remove any unfixed dye and dried in air at a dark place, because the dye is photosensitive and the fabric color bleaches under exposure to visible light. The obtained samples are marked as EO5 and E1. The dyeing procedure is presented in Fig. 1.

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Fig. 2. Routes to the preparation of a ZnO nanoparticles–hydrogel–cotton fabric composite material with PVA as a capping agent.

2.4.2. Hydrogel formation through photopolymerization (step 2) The hydrogel with dispersed ZnO nanoparticles was synthesized on the textile surface by two methods. 2.4.2.1. The first method. The procedure for the synthesis of ZnO nanoparticles and hydrogel is briefly summarized in Fig. 2. PVA was dissolved in distilled water (1% solution) under vigorous stirring at ∼50 ◦ C. Zn(NO3 )2 ·6H2 O (0.1 mol l−1 ), acrylamide (15%), and crosslinker N,N -methylenebisacrylamide (bis-AAm) (1.0 wt% to acrylamide) were dissolved in distilled deionized water under vigorous stirring at room temperature. All solutions were mixed and MDEA was added to it. The concentrations of MDEA were 2.0, 8.0 and 70.0 wt% to monomers and pH of the solutions was, respectively, pH 6, pH 8 and pH 11. MDEA was a co-initiator for the photopolymerization and an alkaline agent for the reaction with Zn(NO3 )2 . The starting volume of the solution was of a liquor-togoods ratio 20:1. The samples of the dyed with MEY cotton fabric (E1) were dipped into the above prepared solutions, squeezed and immediately illuminated with two lamps emitting visible light for 5 h. Then the samples were dried in an oven at 80 ◦ C for 30 min and annealed at 140 ◦ C for 3 min in air. 2.4.2.2. The second method. The fabric dyed with Eosin Y (EO5) was used in the second method. First each fabric sample was impregnated with an aqueous solution of Zn(NO3 )2 at different concentrations. Then the samples were dried in oven at 80 ◦ C for 10 min. Next each sample was overlaid with an aqueous solution of acrylamide (25%), crosslinker N,N -methylenebisacrylamide (bis-AAm) (5.0 wt% to acrylamide) and MDEA at different concentrations. The starting volume of solutions in each step was of liquor-to-goods ratio 1.7:1. The samples were illuminated with two lamps, emitting visible light for 5 h. Then the samples were dried in oven at 80 ◦ C for 30 min and annealed at 140 ◦ C for 3 min in air. The procedure for the synthesis of ZnO nanoparticles and hydrogel using the second method is summarized briefly in Fig. 3. 2.5. Characterization of the prepared ZnO nanoparticles–hydrogel–cotton fabric composite material The photopolymerization is evaluated by determination of gel fraction. The samples of composite materials were measured in the dried state after photopolymerization and drying in oven. They were soaked in distilled water for 18 h up to a constant weight and

taken out from water in order to remove the soluble parts. The samples were dried again and measured. The gel fraction percentage was calculated by the following equation: Gel fraction (%) =

W  1

W2

× 100

(1)

where W2 and W1 are the weights of composite materials in the dry state before and after soaking in distilled water, respectively. The fluorescence spectra were taken on a fluorescence spectrophotometer Cary Eclipse with resolution of 2 nm. The color characteristics of the fabrics were determined on a Texflach ACS/DATACOLOR with a Spectraflash 600 spectrophotometer using D65 illuminant and 10◦ observer. The reflectance spectra (R%) were used to evaluate the dyeing of the cotton fabric with modified Eosin Y. The shape and size of the particles were obtained through TEM, using a transmission electron microscope JEOL JEM 2100. The fiber from the composite sample was placed between two metal greeds. A beam of electrons was transmitted through a specimen and interacting with the hydrogel led to hydrogel swelling. This allowed observing the nanoparticles distribution in the hydrogel and determining their size and shape. The surface morphology of untreated and treated fabrics was analyzed by scanning electron microscope (SEM) JSM-5510 (JEOL), operated at 10 kV of acceleration voltage. Before imaging, the investigated samples were coated with gold by JFC-1200 fine coater (JEOL). The abrasion resistance of fabrics was investigated by a Martindale test according to BDS EN ISO 12947-3 (determination of mass loss) with the use of M235 Martindale apparatus (SDL Atlas). A standard wool woven fabric was used as the abrasive element. The bursting strength of samples was measured on Dynamometer FD-03 according to ISO 1328-2:1999-Bursting properties of fabrics. The hygroscopic capacity of the fabrics was determined by moisture absorption–desorption behavior (Eqs. (2) and (3)). The percentage of moisture absorption in the samples was calculated by the following equation: A (%) =

Mb − Ma × 100 Ma

(2)

where Mb is the weight of the sample placed in constant temperature and humidity (T = 20 ◦ C and relative air humidity – 100% for

D. Staneva et al. / Applied Surface Science 345 (2015) 72–80

75

Fig. 3. Routes to the preparation of a ZnO nanoparticles–hydrogel–cotton fabric composite material.

4 h); Ma is the weight of the sample, dried in a constant temperature oven at 105 ◦ C for 1 h. The percentage of the moisture release from the samples was calculated by the following equation: B (%) =

Mb − Mc × 100 Mb − Ma

(3)

where Mb is the weight of the sample, placed in constant temperature and humidity (T = 20 ◦ C and relative air humidity – 100% for 4 h); Mc is the weight of sample, placed in a constant temperature T = 20 ◦ C and 0% humidity for 4 h; Ma is the weight of the sample, dried in a constant temperature oven at 105 ◦ C for 1 h. 2.6. Antimicrobial activity assays The textile samples were investigated for antimicrobial activity by the agar diffusion method against gram-negative bacteria P. aeruginosa and A. johnsonii as test microorganisms. Suspensions of overnight grown cultures were prepared and smeared onto the surface of nutrient agar in Petri plates. Specimens of about 9/12 mm in size were cut from the textile samples under aseptic conditions and placed onto the agar surface. A non-impregnated cotton sample was used as a control. The plates were incubated for 24 h at 28 ◦ C, and the diameter of the formed zone of inhibition (in mm) was determined. The antimicrobial effect of the cotton samples was tested also in a liquid nutrient broth medium against P. aeruginosa. The test tubes with 2.5 ml sterile nutrient broth were inoculated with overnight bacterial culture and the specimens were inserted into the test tubes. Test tubes without inserted specimens were also prepared. After 24 h incubation at 28 ◦ C under shaking at 240 rpm, the specimens were removed, and the bacterial growth was determined by measuring the turbidity of the medium at 570 nm (OD570 ).

photopolymerization and as an alkaline agent in the synthesis of ZnO nanoparticles. The eosin radical was responsible for the covalent attachment of the gel onto the substrate surface and its quantity could affect the hydrogel structure [22]. The photoinitiation mechanism was previously investigated [23] and it was found that the molar concentration ration of MDEA/eosin is an important factor influencing the conversion of monomers in photopolymerization reaction. In this study the cotton fabric was dyed at two concentration of MEY (0.5% and 1.0% owf) to obtain samples E1 and EO5. Fabric E1 was used by the first method of ZnO nanoparticles synthesis and E05 was used by the second method. Fig. 4 shows the reflection spectra of the initial cotton fabric and both fabrics E05 and E1. The minimum reflectance (R%) is observed at about 520–530 nm in the spectra of the dyed fabrics, where the MEY has an absorption maximum. There is also a band with a maximum in the 575–620 nm range since MEY is fluorescent. The minimum and the maximum in the spectrum of sample E1 are more intensive than those of E05 due to the higher concentration of MEY bounded to the fabric. Two different routes to the synthesis of a hydrogel on the fabric surface with ZnO nanoparticles distributed in its structure have been implemented. 3.2. Preparation of ZnO nanoparticles–hydrogel–cotton fabrics by the first method The first is the process proposed by Kundu et al. [24]. A key step in this technique is the introduction PVA as a capping agent for the preparation of ZnO nanoparticles. PVA molecules offer plenty of active OH groups to adsorb the metal cations in a specific pattern. A

R, %

90 80

3. Results and discussion 3.1. Dyeing of cotton fabric with modified Eosin Y

70 cotton fabric E05 E1

60 50

The hydrogel was used both as a binder of ZnO nanoparticles onto cotton surface and for the prevention of their agglomeration. It was obtained by surface initiate photopolymerization in two steps. First the cotton fabric was dyed with MEY at two different concentrations, which in alkaline medium was able to react with the hydroxyl groups of the cellulose macromolecules of cotton fabric. The dyeing with photosensitive MEY was a way to obtain a stable hydrogel layer on the substrate surface. Next the fabric was treated with a water solution containing MDEA, acrylamide and bis-AAm. MDEA was used as a co-initiator in the hydrogel

40 30 20 10 450

500

550

600

650

700

Wavelength, nm Fig. 4. Reflection spectra of untreated cotton fabric; E05 – cotton fabric, dyed with 0.5% MEY owf; E1 – cotton fabric, dyed with 1.0% MEY owf.

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Table 1 Preparation of ZnO nanoparticles–hydrogel–cotton fabrics by the first method with different concentrations of MDEA and obtained gel fraction. Sample

MDEA (%)

pH of solution

Gel fraction (%)

P1 P2 P3

2 8 70

6.0 8.0 11.0

89.8 89.9 85.5

Photoluminescence intensity (A.U.)

metal ion–polymer complex forms via a kind of ligand reaction with active OH groups of surfaced PVA molecules. The polymer skeleton eventually limits the growth in a specific shape of the lattice. Tang et al. prepared ZnO nanoparticles with various emission colors: blue, green, yellow and orange. The emission color of nanoparticles was tuned via adjustment of the pH of precipitation solution [25]. This was the reason to prepare three samples at different MDEA concentrations and different pH of the working solution. The data are summarized in Table 1. For the assessment of the photopolymerization process, the gel fraction synthesized on fabric surface was determined by weighing the textile sample after the polymerization and after 18 h immersion in distilled water. It has been found that the increasing quantity of MDEA leads to a decrease of the quantity of the obtained gel. The different pH of the solution and the presence of the PVA can be considered as two factors that influence the growth and size of the ZnO nanoparticles on which depends their photoluminescent emission [2]. The spectrum of ZnO nanoparticles has emission peaks in a UV region, corresponding to the near band gap excitonic emission [26], and also different peaks in the visible region. The visible emissions from nanostructured ZnO originates mainly from different defect states (several oxygen vacancies, Zn interstitials, etc.) [26–29]. The photoluminescence spectra in the visible region of samples P3, P2 and P1 are shown in Fig. 5. They are complex and have bands with maxima at 423, 462, 520, and 545 nm. Kundu et al. [24] also found the emission of ZnO nanoparticles, prepared by sol–gel process in support of polyvinyl alcohol (PVA) molecules to have different peaks. The most intense peak lies in the violet band in the 417–424 nm range. A blue emission band occurs in the 447–455 nm range (a) (Band-I) and in the 485–486 nm range (b) (Band-II). A green emission band occurs at 500–504 nm. The spectra of the samples P1, P2 and P3 are distinguished in the photoluminescent intensity and no shift of the maxima is observed. As the photoluminescence emission is in the entire visible region, the feel is for white emission color. The increase in pH of starting solution leads to decrease of the gel fraction and to an increase in photoluminescence intensity due to the higher concentration of ZnO nanoparticles.

P3 P2 P1

1000 423 nm 462 nm

800

520 nm 545 nm

600 400 200 400

450

500

550

600

Wavelength, nm Fig. 5. Photoluminescence spectra of composite materials P1, P2 and P3, obtained by the first method.

To examine the crystallographic nature of the ZnO nanoparticles, TEM and selected area electron diffraction (SAED) were used. Fig. 6 gives TEM images of sample P3 at different magnifications (Fig. 6a–d) and the SAED of the nanoparticles (Fig. 6e). As seen, the nanoparticles are incorporated into the hydrogel and have different e and shape (round, square and rod-like). The size of the particles varies from 20 to over 100 nm. Some nanorod shaped particles are with a high diameter/length ratio (1:16). The deviation of the lattice parameters is caused by the presence of various point defects such as zinc antisites, oxygen vacancies, and extended defects, such as threading dislocations. PVA restricts the growth of ZnO particle and the defect states are modified and this influences the optical properties of the material. The selected area electron diffraction (SAED) spectra (Fig. 6d) confirm the polycrystalline nature of ZnO nanoparticles consisting of much smaller subcrystals. 3.3. Preparation of ZnO nanoparticles–hydrogel–cotton fabrics by the second method Although the use of PVA as an agent limits the growth of ZnO nanoparticles, the results have shown the need of a more precise control over the process. It has been interesting to find conditions under which particles with a uniform distribution in shape and dimensions could be obtained. As mentioned above, the role of PVA is to form a metal ion-polymer complex via its OH groups. The cellulose macromolecules have also plenty OH groups. The second method proposed uses them as a factor managing the nanoparticles size and shape. The hydrogel has been used to prevent the agglomeration and to provide the uniform distribution over fabric surface. ZnO nanoparticles have been synthesized in situ on the fabric surface via a wet chemical reaction. In this case fabric E05 has been used. It was dyed at a MEY concentration lower than when the first method was applied. The reason was the observation that the color bleaching was slower than the rate of hydrogel formation. The conversion of the MEY to its colorless form was an indicator for the radical generation and the process of the photopolymerization. [23,28] It is also supposed that eosin molecules are the link between the hydrogel and cellulose macromolecule. The higher MEY concentration influences the gel structure and can hinder the processes of photopolymerization and the formation of nanoparticles. The other advantage of the second method is the reduction of the used water for treating the fabric and thus the wastewater is also reduced. The fabric samples were first impregnated with the solution of Zn(NO3 )2 and after drying were impregnated again with the solution of monomers and MDEA. Next the fabrics were treated like they were according to the first method. As the reaction of formation of ZnO nanoparticles and a hydrogel are connected via MDEA, which is involved in both processes, it was interesting to study how changes in the concentration of zinc nitrate and MDEA affect the properties of the resulting composite materials. Six samples were prepared varying the quantity of MDEA and zinc nitrate. The data are summarized in Table 2. The fraction of the gel, synthesized on the fabric surface, was determined for the assessment of the photopolymerization process and the results are also shown in Table 2. The dependence of gel formation on the concentration of zinc nitrate on one hand and on MDEA concentration on the other hand is presented in Fig. 7. The higher concentration of both precursors (Zn(NO3 )2 and MDEA) decreases the gel fraction but increases the concentration of ZnO nanoparticles. Fig. 8 shows photoluminescence excitation spectra of the samples over a wavelength of 240–360 nm. The intensity for the samples m3, m5 and m8 in region from 300 to 350 nm is higher

D. Staneva et al. / Applied Surface Science 345 (2015) 72–80

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Fig. 6. TEM micrographs of material P3 and SAED pattern of the nanoparticles at different magnifications.

Table 2 Preparation of ZnO nanoparticles–hydrogel–cotton fabrics by the second method with different concentrations of Zn(NO3 )2 and MDEA and the obtained gel fraction. Reaction conditions

m1 m2 m3 m4 m5 m8

Gel fraction (%)

Zn(NO3 )2 (%)

MDEA, % of monomers and zinc nitrate

2 2 5 5 5 10

20 50 20 50 100 50

97.4 95.5 87.6 89.1 81.7 81.3

Reldtive intensity (A.U.)

Sample

250

m8 m5 m4 m3 m2 m1

200 150 100 50 0 240

100

zinc nitrate concentration (2,5,10 %) MDEA concentration (20,50,100 %)

Gel fraction, %

95

90

85

260

280 300 320 Wavelength, nm

340

360

Fig. 8. Excitation spectra for samples m1, m2, m3, m4, m5, and m8 (em = 380 nm).

obtained by the first method and their size distribution to be relatively narrow [28,30]. Probably the particles have fewer defects in their crystal lattice [31]. The intensity of luminescence spectra in the band at 378 nm is similar for samples m3, m5 and m8

Photoluminescence Intensity (A.U.)

than that for the samples m1, m2 and m4 and coincides with UVA region of ultraviolet radiation. The most intense peak at 320 nm has been chosen for excitation of the photoluminescence spectra for all samples. Fig. 9 shows the photoluminescence spectra of the textile samples (m1–m8), obtained by the second proposed method. An intensive band with a maximum at 378 nm has been obtained in all spectra. The shift of the photoluminescence in UV region reveals the size of the nanoparticles to be smaller than that of the nanoparticles

250 m8 m5 m4 m3 m2 m1

200 150 100 50 0 350

400

450

500

550

600

Wavelength, nm

80 m2/m3

m4/m4

1

m8/m5

Fig. 7. Gel fraction dependence on the concentrations of zinc nitrate and MDEA.

Fig. 9. Photoluminescence spectra for samples m1, m2, m3, m4, m5, and m8 (ext = 320 nm).

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D. Staneva et al. / Applied Surface Science 345 (2015) 72–80

Fig. 10. TEM micrographs of material m8 at different magnifications.

7 untreated P3 m8

6 %, weight loss

and for the samples m1, m2 and m4, respectively, as it has been observed in the excitation spectra. This can be explained by the difference in the nanoparticles concentration. The data in Table 2 and Fig. 7 show that at the smaller gel fraction the intensity of the photoluminescence is higher due to the quantity of ZnO nanoparticles. That is why samples m8 and m2 have been chosen for the antimicrobial investigations. TEM images of ZnO nanoparticles microstructure in sample m8 are presented in Fig. 10a–c. The nanoparticles of a round shape are mostly sized approximately under 10 nm and are distributed evenly in the hydrogel.

5 4 3 2 1 0

3.4. SEM studies on treated and untreated fabrics Fig. 11 shows SEM images of the untreated and ZnO-hydrogel coated cotton fibers, prepared by the first (sample P3) and the second method (sample m8) (Fig. 11c). As seen, the surface of the original cotton fiber is folded (Fig. 11a). The treatment by the first method results in coating the surfaces of the modified cotton fibers with a rough thick film of granular substances and in dispersing some agglomerated particles (Fig. 11b). The higher magnification image of the treated fiber in Fig. 11d shows that the surface has a foam-like morphology due to the presence of ZnO nanoparticles inside the gel networks (sample P3). In the case of sample m8 (Fig. 11e), the morphology is a more homogenous one with

0

1000

2000

3000 4000 5000 abrasion cycles

6000

Fig. 12. The weight reduction versus number of abrasion cycles.

ganglia-like hills similar to the morphology of the film deposited onto the glass surface [32]. The initial surface of the cotton fiber is well visible. 3.5. Mechanical properties of treated and untreated fabrics 3.5.1. Bursting strength of samples The bursting strength of coated fabrics is often used as a measure of the multidirectional modulus of the material, as opposed

Fig. 11. SEM images of (a) untreated cotton fibers, (b) sample P3, (c) sample m8, (d) sample P3 at magnification ×27,000, (e) sample m8 at magnification ×30,000.

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Fig. 13. Zones of inhibition of P. aeruginosa and A. johnsonii by the materials m2 and m8.

to tensile properties which only provide guidance to the strength of coated-fabrics in a single plane. The bursting strength of the untreated cotton fabric is 309 N and that of sample m8 – 310 N. However, the bursting strength of sample P3 is 255 N. The lowered bursting strength is likely due to the different morphology of the surface film, as seen from SEM images. The agglomeration of nanoparticles could be responsible for the ductility loss and these results are similar to previous observations [33]. 3.5.2. Determination of abrasion resistance Weight reduction of samples during the abrasion test is presented in Fig. 12. The results revealed that fabric P3 coated with a rough thick film has improved wear properties, if compared to those of the untreated fabric. Fabric m8 also showed a lower weight loss, as compared to the one of untreated cotton fabric, but the difference is small and appears after 3000 abrasion cycles. 3.5.3. Hygroscopic capacity of the fabrics The moisture absorption–desorption tests have shown that the hydroscopic swelling degree of composite materials P3 (18%) and m8 (20%) is higher than that of the untreated cotton fabric (13%). Moisture penetrates the fibers through their pores and capillaries, and is bound by polar functional groups of the cotton fibers. Inside the hydrogel modified fibers there is bound moisture as well [34]. The desorption of the moisture is a slower process and for 4 h the weight change for pure fabric is 16.6%, while for P3 it is 5.9% and for m8 is 5.7%. The water is more firmly bound inside the fibers modified with hydrogel than in the untreated cotton fibers. 3.6. Antimicrobial activity against the bacterial strains P. aeruginosa and A. johnsonii The textile samples m2 and m8 were investigated for antimicrobial activity against the bacterial strains P. aeruginosa and A. johnsonii known as important pathogens in clinical infections. As seen from Fig. 13, the samples exhibit good inhibitory activity against A. johnsonii with zones of inhibition around m2 and m8 specimens about 28 mm and 33 mm, respectively. Moderate inhibition activity of the samples against P. aeruginosa was observed, with zones of inhibition around m2 and m8 about one mm and six mm, respectively. In the tests in liquid medium, samples m2 and m8 caused a significant inhibition of the growth of P. aeruginosa expressed by decreasing the turbidity of the medium – about 62% and 81%, respectively (Fig. 14). The antimicrobial effect of the tested samples should be due to the release of ZnO from the cotton textile by diffusion or by induction of intercellular reactive oxygen species including hydrogen peroxide (H2 O2 ).

Fig. 14. Effect of materials (untreated cotton fabric (KS ), m2 and m8), on the growth of P. aeruginosa tested in liquid nutrient broth medium (NB). KO , NB without added sample.

Members of the genus Acinetobacter are recognized as significant nosocomial pathogens including wound infections [35]. Significantly high antibacterial effect of ZnO nanoparticles on the bacteria tested in this study, especially on A. johnsonii, suggests potential application of the new formulations as effective wound dressings to control the spread of infections.

4. Conclusion Composite materials containing ZnO nanoparticles have been prepared using two different approaches. According to the first method PVA has been used as a capping agent. The nanoparticles obtained have different shape (round, square and rod) and different size and form rough thick film of granular substances and results in dispersing of some agglomerated particles. Their photoluminescence properties and SAED analysis confirm the polycrystalline nature of the nanoparticles with different defects in their crystal structure. ZnO nanoparticles synthesized by the second method are of a round shape and size approximately under 10 nm. They are distributed evenly in the hydrogel structure. The advantage of the second method is the reduction of the used water for treatment of the fabric and thus the wastewater also is reduced. The textile samples m2 and m8 have exhibited a good inhibitory activity against A. johnsonii and have caused a significant inhibition of the growth of P. aeruginosa. However, sample m8 obtained at a higher concentration of Zn(NO3 )2 has shown better results. The obtained composite textile materials comprising ZnO nanoparticles are suitable for luminescent, antimicrobial, UV-absorption and other applications.

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