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zinc oxide non-woven textile
Accepted Manuscript Title: Electrospinning/electrospraying vs. electrospinning: A comparative study on the design of poly(L-lactide)/zinc oxide non-woven textile Author: Daniela Virovska Dilyana Paneva Nevena Manolova Iliya Rashkov Daniela Karashanova PII: DOI: Reference:
S0169-4332(14)01233-1 http://dx.doi.org/doi:10.1016/j.apsusc.2014.05.192 APSUSC 28011
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-4-2014 25-5-2014 26-5-2014
Please cite this article as: D. Virovska, D. Paneva, N. Manolova, I. Rashkov, D. Karashanova, Electrospinning/electrospraying vs. electrospinning: a comparative study on the design of poly(L-lactide)/zinc oxide non-woven textile, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.192 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 Electrospinning/electrospraying vs. electrospinning: a comparative study on the design of poly(L-lactide)/zinc oxide non-woven textile Daniela Virovska,1 Dilyana Paneva,1* Nevena Manolova,1 Iliya Rashkov,1*, Daniela
1
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Karashanova2
Laboratory of Bioactive Polymers, Institute of Polymers, Bulgarian Academy of Sciences,
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Acad. G. Bonchev St, bl. 103A, BG-1113 Sofia, Bulgaria 2
Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Acad. G.
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Bonchev St, bl. 109, BG-1113 Sofia, Bulgaria
E-mail: [email protected] (D. Paneva)
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E-mail: [email protected] (I. Rashkov)
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* Corresponding authors. Tel./fax: +359 02 9793289/+359 02 8700309
Abstract
New hybrid fibrous materials from the biocompatible and biodegradable aliphatic
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polyester poly(L-lactide) (PLA) and pristine or surface-functionalized nanosized zinc oxide
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were prepared. The application of the techniques: (i) electrospinning of a suspension of ZnO in PLA solution, or (ii) simultaneous electrospinning of PLA solution and electrospraying of a
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ZnO suspension in PLA solution (at low PLA concentration) enabled the fabrication of hybrid materials of diverse design: non-woven textile consisting of fibers in which ZnO was deposited on the fibers’ surface (designated as type “on”) or was mainly in the fibers’ bulk (designated as type “in”). The photocatalytic activity of the new fibrous materials was estimated in respect to Methylene Blue (MB) and Reactive Red (RR) dyes. Type “on” hybrid materials had higher photocatalytic activity as compared to type “in” materials. It was shown that type “on” materials preserved their photocatalytic activity in respect to MB even after three repeated uses, while for the RR dye the same held true for ZnO-on-PLA mats only. The type “on” materials exhibited antimicrobial activity against the pathogenic microorganism Staphylococcus aureus as evidenced by the performed microbiological tests. Keywords: nanosized zinc oxide; poly(L-lactide); electrospinning; electrospraying; photocatalytic activity; antibacterial activity
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2 1. Introduction The nanosized zinc oxide exhibits photocatalytic and antibacterial activities, and is therefore a very attractive component for incorporation in new hybrid materials. Such hybrid materials can find diverse applications in a wide range of fields: from medicine through environmental protection to the needs of optoelectronics [1-3]. The electrospinning is a very
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promising technique for preparation of micro- and nanofibrous materials in which diverse in nature drugs, as well as metal or metal oxide nanoparticles can be incorporated applying a
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single-step procedure [4-5]. Electrospinning results in obtaining fibers in which the additive is distributed mainly in their bulk. Electrospinning performed in conjunction with
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electrospraying enables fabrication of hybrid materials consisting of fibers and filler nanoparticles deposited on the fibers’ surface while distributed in the entire volume of the mat
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[7,8]. Nanosized zinc oxide has been incorporated in fibrous materials by electrospinning, and synthetic polymers such as poly(ethylene oxide) [9], nylon [10,11], poly(vinyl pyrrolidone) [12], polyacrylonitrile [13], and poly(vinyl alcohol) [14] have been mainly used. The studies
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on the incorporation of nanosized zinc oxide in fibrous materials from bio-based polymers are still scarce. ZnO nanoparticles have been incorporated by electrospinning in poly(3hydroxybutyrate-co-3-hydroxyvalerate) [15,16], alginate [17] and cellulose [18] fibers. To the
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best of our knowledge preparation of hybrid materials from ZnO by applying simultaneous
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electrospinning and electrospraying has been reported only for nylon 6 [10]. Till now, the properties of polymer/ZnO hybrid materials prepared by electrospinning and those obtained
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by electrospinning/electrospraying have not been compared. Poly(L-lactide) (PLA) is regarded as the most attractive polymer and it is expected that
PLA will replace the widely used synthetic polymers for the manufacturing of daily used in the household plastic devices as well as for elaboration of new materials for medicine and environmental protection [19,20]. PLA is synthesized from annually renewable source and its manufacturing has been increased considerably during the past decade. It is biocompatible in respect to cells, tissues and organs and is a biodegradable polymer. The hybrid fibrous materials
from
PLA
and
zinc
oxide
prepared
by
electrospinning
or
electrospinning/electrospraying are of outstanding interest since they can combine the beneficial properties of the polymer and the inorganic filler, namely: biodegradability, photocatalytic and antibacterial activities. In the present study new hybrid fibrous materials from PLA/nanosized zinc oxide were prepared by applying the electrospinning and electrospinning/electrospraying techniques. The obtained materials are designated as type “in” and type “on”, respectively. The morphology of
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3 the materials was observed by scanning electron microscopy (SEM), and the nanosized zinc oxide distribution – by transmission electron microscopy (TEM) and EDX mapping of zinc. The ZnO content and the thermal degradation of the fibrous materials were evaluated by thermogravimetric analysis (TGA). The surface elemental composition of the different types of ZnO used, as well as that of the type “in” and “on” mats was determined by X-ray
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photoelectron spectroscopy (XPS). The photocatalytic activity of the type “in” and “on” materials was evaluated using Methylene Blue (MB) and Reactive Red (RR) as model dyes.
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The antibacterial activity of the materials against the pathogenic microorganism
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Staphylococcus aureus (S. aureus) was tested.
2. Materials and methods
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2.1. Materials
Poly(L-lactide) (PLA) was a commercial product (INGEO™ Biopolymer 4032D, NatureWorks LLC – USA) having M W = 259 000 g/mol and M W / M n = 1.94 as determined
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by size-exclusion chromatography using polystyrene standards. Four types of commercial nanosized zinc oxides available under the trade mark Zano®20 of Umicore Zink Chemicals – Belgium were used, namely: nanosized zinc oxide (Zano®20), zinc oxide with silanized
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surface (Zano®20 Plus), zinc oxide with aminofunctionalized surface (Zano®20 Plus-2) and
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zinc oxide with esterfunctionalized surface (Zano®20 Plus-3). They are further denoted as ZnO, ZnO(Si), ZnO(amino) and ZnO(ester), respectively. These oxides are rod-like with
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diameter of 10-30 nm and length of 100 nm [21,22]. Chloroform, N,N-dimethylformamide (DMF) and ethanol (EtOH) were of analytical grade, purchased from Merck. Methylene Blue (MB, Scheme 1A) dye was purchased from Merck, and Reactive Red (CI Reactive Red 1, RR, Scheme 1B) was kindly supplied by Prof. T. Konstantinova, UCTM-Sofia.
N
S
+ N Cl
Cl SO3Na N N
H OH N
N N Cl
N
SO3Na
NaO3S
A
N
B
Scheme 1. Formulae of Methylene Blue (A) and Reactive Red (B) dyes 2.2. Preparation of fibrous materials from PLA/nanosized zinc oxide by electrospinning (type “in” mats) and by electrospinning/electrospraying (type “on” mats)
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4 For preparation of type “in” materials the corresponding oxide: ZnO, ZnO(Si), ZnO(amino) or ZnO(ester), was dispersed in PLA solution and the obtained suspension was subjected to electrospinning. PLA concentration was 10 wt.%, and zinc oxide content was 23 wt.% in respect to the total solids. Depending on zinc oxide type PLA (0.74 g) was dissolved in 3.70 mL CF for ZnO, ZnO(amino) or ZnO(ester), and in 3.40 mL chloroform for ZnO(Si).
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After the complete dissolution of the polymer a preliminary sonicated (30 min in an ultrasonic bath Bandelin Sonorex, 160/640 W, 35 kHz) suspension of nanosized zinc oxide (1 mL) was
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added and then the obtained mixture was stirred using magnetic stirrer. As a continuous phase DMF was used for ZnO, ZnO(amino) or ZnO(ester) dispersions, and chloroform - for ZnO(Si)
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dispersions. The concentration of zinc oxide in the suspension in DMF or chloroform of 22.2 % (w/v) in 10 mL suspension was selected in such a manner so as the oxide content in 1 mL
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of the suspension added into PLA solution to be 23 wt.% in respect to the total solids. The spinning suspension of PLA and zinc oxide was transferred in a syringe equipped with a metal needle connected to the positively charged electrode of the high-voltage power supply.
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Electrospinning was conducted at a constant applied voltage of 25 kV and constant tip-tocollector distance of 17 cm using a grounded rotating aluminum collector (1800 rpm). The spinning suspensions were delivered at a constant rate of 3 mL/h enabled by the use of a
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pump Syringe Pump NE-300 (New Era Pump Systems, Inc.).
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For preparation of the type “on” hybrid materials the following equipment was used for electrospinning/electrospraying: two pumps for delivering (i) PLA solution [10 wt.%
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solution of PLA in chloroform/DMF = 3.7/1 (v/v)] for electrospinning, and (ii) 30 % (w/v) suspension of zinc oxide prepared in the presence of 0.05 % (w/v) PLA using the following solvents: DMF for ZnO, EtOH for ZnO(amino) and for ZnO(ester), and chloroform for ZnO(Si) (3 mL). PLA dissolved in 3 mL chloroform was added to the suspension of zinc oxide to serve as a sticking agent for the nanosized zinc oxide onto PLA fibers. The zinc oxide suspensions were sonicated for 30 min. A common collector was used for deposition of PLA fibers and zinc oxide (rotating rate: 1800 rpm). The pumps for delivering the spinning solution and the zinc oxide suspension were located at an angle of 180° in respect to the collector. The delivery rate of the spinning solution and of the suspension was 3 mL/h. The electrospinning/electrospraying was conducted at a tip-to-collector distance of 17 cm for the electrospinning of PLA solution and 15 cm for electrospraying of the PLA/zinc oxide suspension. The applied voltage was provided using a common high-voltage power supply.
2.3. Characterization of the prepared PLA/zinc oxide mats
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5 The morphology of the prepared fibrous materials was evaluated by scanning electron microscopy (SEM). The samples were vacuum-coated with gold prior to observation by SEM (Jeol JSM-5510 or Philips 515). The mean fiber diameter was estimated by ImageJ software, and their morphology was assessed applying the criteria for overall evaluation of electrospun materials as described in details in [23]. TEM observations were carried out by JEM 2100
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(JEOL Co. Ltd.) operating at a voltage of 200 kV. The samples were prepared by depositing on a copper grid. In order to demonstrate that zinc oxide is incorporated into PLA/zinc oxide
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fibers EDX mapping of zinc was applied.
The thermal stability of the fibrous materials and the content of zinc oxide were
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determined by thermogravimetry using Perkin Elmer TGA 4000 Thermogravimetric Analyzer in nitrogen. The heating was from r.t. to 600°C at a heating rate of 10 °C/min.
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The surface chemical composition of the materials was determined by X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out in the UHV chamber of an ESCALAB-MkII (VG Scientific) spectrometer using Mg Kα excitation with a
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total resolution of ca. 1 eV. Energy calibration was performed taking the C 1s line at 285 eV as a reference.
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and Reactive Red dyes
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2.4. Photocatalytic activity of the prepared new hybrid materials in respect to Methylene Blue
For assessment of the photocatalytic activity of the prepared new hybrid fibrous
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materials Methylene Blue (MB) and Reactive Red (RR) were used as model dyes. Samples of approximate size of 10×10 mm were cut so as to the zinc oxide content in the mats to be 0.6 mg. The samples were immersed in 7 mL aqueous solution of MB (2×10-5 mol/L) or RR (6.3×10-5 mol/L) and kept for 30 min in dark. After that they were irradiated with UV-visible light (UVASPOT 400/T, Dr. Honle AG; UV lamp UV 400 F/2; 400 W; wavelength range: from 260 to 600 nm) for 5 h. During the experiments the temperature of the solutions was kept at 20 °С using a LAUDA PE 104 thermostat. For comparison of the photocatalytic activity of type “in” and type “on” materials, the weight of the mats was selected in such a manner so as the weight of the zinc oxide to be one and the same (0.6 mg). The photocatalytic decomposition of the MB and RR was followed using DU800 spectrophotometer (Beckman Coulter, Inc.) at 660 nm and 532 nm, respectively, by recording the decrease in the absorbance of MB or RR in the presence of the hybrid materials as a function of the irradiation time. The presented data are average values from 3 measurements.
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6 The apparent rate constant (Kapp) was calculated from the kinetic curves of the dyes degradation: ln(CoMB or RR/CMB or RR) = f(t), where CoMB or RR was the initial concentration of MB or RR, CMB
or RR
– the concentration of MB or RR at the moment t, and t was the
irradiation time (Figure S1). Kapp was calculated from the dependence ln(CoMB or RR/CMB or RR) = Kapp × t [24].
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The reusability of the materials was tested by immersing them into a fresh MB or RR solution after a catalysis run; before each replacement of the dye solution the mats were
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washed three times with distilled water. This procedure was repeated three times for each
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sample.
2.5. Microbiological tests against S. aureus
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Initial suspension of S. aureus with optical density at 600 nm (OD600) of 0.05 was transferred into tubes containing 2 ml Luria-Bertani medium. One of the tubes served as a control. PLA, ZnO-in-PLA and ZnO-on-PLA mats with zinc oxide content 0.5 mg/mL cell
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culture were placed in the other tubes. The tubes were cultivated under agitation at 205 rpm for 8 h. Aliquots were taken per hour and their OD600 was determined. A total of 3
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measurements were averaged.
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7 3. Results and discussion 3.1. Surface composition of the nanosized ZnO. Finding of an appropriate continuous phase for preparation of stable dispersions of nanosized zinc oxide.
TGA analyses showed that while ZnO powder did not contain any organic phase, ZnO(amino), ZnO(ester) and ZnO(Si) contained 0.4 %, 0.4%, and 2.0 %, respectively. The
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surface composition of the four types of nanosized zinc oxide powders was determined by
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XPS (Table 1).
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Table 1. Surface composition of zinc oxide powder as determined by XPS survey spectrum. Zinc oxide type C (%) O (%) Zn (%) N (%) Si(%) (powder) ZnO 55.00 45.00 ZnO(amino) 9.20 43.90 45.70 1.20 ZnO(ester) 12.86 44.30 42.84 ZnO(Si) 22.50 41.50 34.50 1.50
Peaks for zinc (Zn 2p3/2 at 1022 eV) and oxygen (O 1s at 530 eV) (Figure 1) were detected on
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ZnO surface. As seen from Table 1, the oxygen content is higher than the theoretical one of 50/50. Similarly to other studies [25], this can be attributed to the presence of two types of oxygen atoms on ZnO surface: one participating in a bond with Zn, and a second one engaged
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in –OH groups (Figure 1B). For the other types of zinc oxide again peaks for Zn 2p3/2 at 1022
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eV and for O 1s at 530 eV was registered, however (and as expected) presence of carbon was detected as well. ZnO(amino) surface contains also 1.20% nitrogen, and the registered peak
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for N 1s is at 400 eV and can be attributed to nitrogen atom engaged in C-NH2 bond. For ZnO(Si) a peak at 104 eV was detected characteristic for presence of Si 2p on the surface of the zinc oxide with silanized surface. As seen from Table 1, the silicon content is 1.50%. Since there are no data on the stability of dispersions from the used in the present
study commercial products nanosized zinc oxide in any solvent (up to date these products have been utilized only for preparation of polymer materials by melt extrusion) at a first stage studies were performed for finding of the appropriate continuous phase for preparation of stable dispersions PLA/zinc oxide. It was found that chloroform (which is a good solvent for PLA) although was suitable for ZnO(Si) dispersions, was not appropriate for ZnO, ZnO(amino), or for ZnO(ester) dispersion. ZnO(amino) and ZnO(ester) gave stable dispersions in DMF and EtOH, while ZnO – only in DMF. It was found that stable dispersions could be obtained even at inorganic filler content as high as 30 % (w/v). In the present study DMF was used as continuous phase for ZnO dispersions, and DMF or EtOH for ZnO(amino) and ZnO(ester) or chloroform – for ZnO(Si) dispersions.
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A B Figure 1. Zn 2p3/2 (A) and O 1s (B) spectrum of ZnO powder.
3.2. Preparation of type “in” hybrid materials from PLA/nanosized zinc oxide by electrospinning and estimation of their photocatalytic activity
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Type “in” materials containing nanosized zinc oxide in the bulk and on the surface of
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the fibers were prepared by electrospinning of suspensions of the corresponding zinc oxide in PLA solution at a content of the nanosized inorganic filler of 23 wt.% in respect to the total
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solids.
Representative SEM and TEM images of the obtained fibrous materials are shown in
Figure 2. The mean diameter value of PLA fibers prepared in absence of zinc oxide depended on the solvent system. In the case when chloroform was used as a solvent, two populations of fibers were formed: thinner fibers with mean diameter of 700 ± 21 nm, and thicker ones with mean diameter of 8.40 ± 1.24 µm. It was 1.70 ± 0.26 µm when chloroform/DMF = 3.7/1 (v/v) was used as a solvent system. The chloroform/DMF ratio was the same as that one used for the spinning dispersions from PLA/ZnO, PLA/ZnO(amino) and PLA/ZnO(ester) pairs. The smaller mean diameter value of PLA fibers prepared using chloroform/DMF system may be attributed to the presence of DMF which is a poor solvent for PLA. It was found that the incorporation of ZnO, ZnO(amino) and ZnO(ester) did not affect considerably the mean fiber diameter value, and it was ca. 2.00 ± 0.44 µm.
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B
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A
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C D Figure 2. SEM micrographs of fibers from ZnO-in-PLA (A) and ZnO(Si)-in-PLA (B);
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magnification: × 1000. ТЕМ micrograph of ZnO-in-PLA (C). A schematic representation of
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the cross-section of a type “in” fiber (D).
For ZnO(Si)-in-PLA mats the mean diameter of the thinner fibers population was not affected by the presence of ZnO(Si), while the diameter of the thicker fibers population increased from 8.40 ± 1.24 µm [in absence of ZnO(Si)] up to 11.50 ± 2.22 µm [for ZnO(Si)-in-PLA mat]. As seen from the TEM micrograph (Figure 2C), the zinc oxide was distributed mainly in the fibers’ bulk, however, there was zinc oxide on the fibers’ surface as well. EDX mapping images of Zn showed that the zinc oxide was distributed uniformly along the fibers (Figure S2B). A schematic representation of the cross-section of a type “in” fiber is shown in Figure 2D. Since PLA was totally destructed in the TGA experiments (100 % weight loss at 370 °C, Figure 3), and moreover, zinc oxide has excellent thermal stability [21], it was possible to
determine the content of zinc oxide in the mats. It was determined also for the materials containing nanosized zinc oxide with a modified surface since the organic fraction was
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10 negligible. TGA thermograms of PLA mat as well as of mats that contain different types of
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zinc oxide are presented in Figure 3.
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Figure 3. TGA thermograms of mats from: PLA (1); ZnO(Si)-in-PLA (2), ZnO(ester)-in-PLA
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(3), ZnO-in-PLA (4) and ZnO(amino)-in-PLA (5).
ZnO-in-PLA, ZnO(amino)-in-PLA and ZnO(ester)-in-PLA mats destructed thermally
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in two steps, the first stage was at ca. 285 °C. In the second stage the degradation temperature did not depend significantly on the type of the zinc oxide and was 398 °C for ZnO-in-PLA,
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395 °C for ZnO(ester)-in-PLA, and 397 °C for ZnO(amino)-in-PLA. Similarly to PLA mats, ZnO(Si)-in-PLA mats degraded in one step, however at a lower temperature (284 °C). It can be concluded that the incorporation of the different types of zinc oxide lead to a decrease of the thermal stability of PLA mats. This result is consistent with data obtained for materials from PLA/ZnO and PLA/ZnO(Si) prepared by melt extrusion [21]. Similar decrease of the degradation temperature has been detected for nanofibrous materials from nylon 6,6/nanosized zinc oxide prepared by electrospinning as well [26]. This is attributed to the catalytic activity of zinc oxide that leads to thermal degradation of the polymer at lower temperature. From the performed TGA studies it was also found that the zinc oxide content (23 wt.%) in the mats prepared by electrospinning corresponded to that one in the feed for all types of zinc oxide thus corroborating the stability of the PLA/zinc oxide suspensions for the electrospinning process duration.
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11 It is widely known that nanosized zinc oxide is an excellent agent for photocatalytic degradation of diverse in nature organic pollutants, such as e.g. wastewaters from the textile
A
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industry.
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Figure 4. MB photocatalytic degradation: dependence on irradiation time in the presence of a
mat from PLA (blank) (◊), ZnO-in-PLA (∇), ZnO(amino)-in-PLA (▼), ZnO(ester)-in-PLA (□), ZnO(Si)-in-PLA (■), and blank control (♦) (A); and Kapp value for type “in” mats of the first (empty bars) and the second (gray bars) stages of the degradation process (B).
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12 Its incorporation in fibrous materials by electrospinning or electrospinning/electrospraying enables the design of new zinc oxide carriers that can be applied for heterogenic photocatalytic mineralization of organic pollutants. In the present study the photocatalytic degradation of Methylene Blue dye (MB) was followed. MB is widely used as a model dye; in addition, it is a known pollutant of wastewaters from the textile industry. The dependence of
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MB photocatalytic degradation on UV-vis irradiation time for type “in” mats is presented in Figure 4А. As seen, the MB degradation on UV-vis irradiation time did not depend
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significantly on the type of zinc oxide. This result may be attributed to the presence of approximately one and the same content of zinc oxide on the surface of type “in” mats (ca. 0.4
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% as determined by XPS). Almost complete decolorization of the MB aqueous solutions occurred after a 5-h UV-vis irradiation under the action of the mats that contain zinc oxide. The apparent rate constant (Kapp) of MB degradation process was calculated from the linear
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dependence of ln(C0MB/CMB) as a function of the UV-vis irradiation time of the aqueous solution of the dye in the presence of the type “in” mats. It was found that MB degradation
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occurred in 2 steps: slow one (from 0 to 20 min of contact of the mat with the dye) followed by a fast one. As seen from Figure 4B, Kapp value of the slow (ca. 0.0030 min-1) as well as of the fast (ca. 0.0045 min-1) stage did not depend on the type of the zinc oxide.
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Blank experiments (in absence of a zinc oxide-containing mat) showed that MB alone
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was also sensitive under UV-vis irradiation of its aqueous solutions. As seen from Figure 4A, ca. 25 % and ca. 40% of the dye degraded after 2 h and after 5 h irradiation, respectively.
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Therefore, further the stable under UV-vis irradiation Reactive Red azo dye (RR) was subjected to photocatalytic degradation. It is known that azo dyes are highly toxic and they can be considered as carcinogens since they can form toxic aromatic amines [27]. It is also known that the reactive dyes are among of the most widely spread pollutants in wastewaters from the textile industry due their wide application for textile dyeing; moreover, they are very stable to degradation. While RR was stable in aqueous solution under UV-vis irradiation, mats containing zinc oxide were found to lead to dye degradation (Figure 5). The decolorization in 5 h was not complete; at that the amount of the residual non-degraded dye is higher as compared to that of MB. The best results were obtained using ZnO-in-PLA mats; in this case the residual non-degraded RR was 30%. For the rest of the type “in” mats the non-degraded residue was higher reaching 50%. During the first minutes of contact of RR with type “in” mats any degradation did not occur. This initial stage depended on the type of the nanosized oxide: ca. 50 min for mats that contain ZnO, ZnO(amino) or Zn(ester) and 80 min for ZnO(Si)-in-PLA mats. After that degradation occurred; the calculated Kapp value was ca.
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13 0.0010 min-1 for type in” mats that contained ZnO, ZnO(amino) or ZnO(ester), and ca. 0.0020
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min-1 for ZnO(Si)-in-PLA mats.
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Figure 5. RR photocatalytic degradation: dependence on irradiation time in the presence of a
mat from PLA (blank) (♦), ZnO-in-PLA (∇), ZnO(amino)-in-PLA (▼), ZnO(ester)-in-PLA
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(□) or ZnO(Si)-in-PLA (■).
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The obtained results for relatively slow photocatalytic MB and RR degradation under the action of type “in” mats can be ascribed to the fact that the zinc oxide in these mats is
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distributed mainly in the fibers’ bulk, thus hindering its contact with the dye in its aqueous solution. It was suggested that using electrospinning in conjunction with electrospraying would allow fabrication of materials in which the nanosized zinc oxide would be deposited on the fibers’ surface. Therefore, studies were performed on the preparation of materials PLA/zinc oxide by combined electrospinning/electrospraying techniques.
3.3. Preparation of type “on” hybrid materials from PLA/nanosized zinc oxide by electrospinning/electrospraying and estimation of their photocatalytic activity
Type “on” mats from PLA/nanosized zinc oxide were prepared by simultaneous electrospinning of a PLA solution and electrospraying of a suspension containing ZnO, ZnO(amino), ZnO(ester) and ZnO(Si) [zinc oxide content 30 % (w/v)] in chloroform/DMF = 1/1 (v/v) for ZnO, chloroform/EtOH = 1/1 (v/v) for ZnO(amino) or ZnO(ester), or chloroform for ZnO(Si). A small amount of PLA: 0.05 % (w/v) was added to the zinc oxide dispersions in
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14 order to serve as a sticking agent for the oxide on the PLA fibers surface in order zinc oxide loss to be avoided. SEM and TEM images of fibers from ZnO-on-PLA and ZnO(Si)-on-PLA mats are presented in Figure 6, and EDX mapping of Zn of ZnO(amino)-on-PLA mat is
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shown in Figure S2D.
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C C’ D Figure 6. SEM (A and B, magnification: × 1000) and TEM (C, magnification: × 25000; and
D, magnification: × 15000) micrographs of ZnO-on-PLA (A and C) and ZnO(Si)-on-PLA (B and D) fibers. A schematic representation of a cross-section of a type “on” fiber (C’). The mean diameter of the PLA fibers was 1.70 ± 0.26 µm. As seen, the nanosized zinc oxide was deposited mainly as spherical nanoporous agglomerates. For ZnO-on-PLA, ZnO(amino)on-PLA and ZnO(ester)-on-PLA mats small (mean diameter of ca. 1 µm) and large (mean
diameter of ca. 5 µm) spherical aggregates were observed. A schematic presentation of the cross-section of a fiber with a spherical aggregate from mat type “on” is given in Figure 6C’. For ZnO(Si)-on-PLA mat the mean diameter of the spherical aggregates was larger, and ranged from 2 to 14 µm.
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C D Figure 7. Photocatalytic degradation of MB (A) and RR (C): dependence on irradiation time
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in the presence of a mat from PLA (blank) (◊),ZnO-on-PLA (∇), ZnO(amino)-on-PLA (▼), ZnO(ester)-on-PLA (□) or ZnO(Si)-on-PLA (■).Kapp values of the degradation process of MB
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(B) or RR (D) for mats type “on” for the first (empty bars) and the second (gray bars) stage.
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This may be attributed to the use of highly volatile continuous medium (chloroform) for the nanosized ZnO(Si). The microporous structure of the spherical aggregates for ZnO(Si)-onPLA observed by SEM may be also explained by its use. Similar porous microbeads have been obtained by electrospraying of PLA from its solution in chloroform [28]. The zinc oxide content was 45% for ZnO-on-PLA mat, 40% for ZnO(amino)-on-PLA and ZnO(ester)-onPLA mats, and 60% for ZnO(Si)-on-PLA mat as evidenced by TGA. The results obtained from the studies on the photocatalytic activity of the mats type
“on” in respect to MB and RR are presented in Figure 7. As seen from Figure 7А, they had different behavior as compared to type “in” materials: MB degraded completely for 3 h for ZnO-on-PLA, ZnO(amino)-on-PLA and ZnO(ester)-on-PLA, and for 4 h in the case of ZnO(Si)-on-PLA mats, thus confirming our hypothesis that the deposition of the nanosized zinc oxide on the fibers surface by applying combined electrospinning/electrospraying
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16 techniques would lead to enhancement of the photocatalytic activity of the hybrid PLA/zinc oxide mats. Similarly to the case of type “in” materials, MB degradation occurred in two stages (Figure 7B), however, Kapp values of the first as well as of the second stage were significantly higher as compared to those calculated for MB degradation under the action of type “in” mats.
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For type “on” mats Kapp value and the duration of the first stage depended on the type of zinc oxide (Figure 7B). While during the first stage of MB degradation Kapp value was 0.0090 minfor ZnO-on-PLA, ZnO(amino)-on-PLA and ZnO(ester)-on-PLA mats, for ZnO(Si)-on-PLA
cr
1
mat Kapp value it was lower (0.0022 min-1). The duration of the first stage was 20 min for
us
ZnO-on-PLA mat and ca. 30 min for the rest of the type “on” mats. The second stage of MB degradation was faster; and the dye degradation by ZnO(amino)-on-PLA mat (0.0200 min-1) was with the highest Kapp value while the lowest value was calculated for ZnO(Si)-on-PLA
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mat (0.0094 min-1). ZnO(amino)-on-PLA and ZnO(Si)-on-PLA mats had close in value content of zinc oxide on their surface as evidenced by XPS (6.50% and 7.60%, respectively).
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Thus, the slower degradation of MB cannot be due to lower zinc oxide content on the fibers surface. As already mentioned, the mean diameter of spherical aggregates in ZnO(Si)-on-PLA materials were significantly larger (up to 14 µm) as compared to the rest of the type “on”
d
materials. The zinc oxide incorporated in the aggregates become inaccessible for the water
te
molecules. This in turn results in a delayed formation of hydroxyl radicals under the UV-vis irradiation in the presence of ZnO(Si)-on-PLA mat. The hydroxyl radicals are responsible for
Ac ce p
the ability of the zinc oxide to degrade organic pollutants to formation of harmless carbon dioxide as well to the conversion of the nitrogen and sulfur in the heterocycle in MB structure to inorganic ions [29].
The dependence of the photocatalytic degradation of RR on the UV-vis irradiation
time for type “on” mats is presented in Figure 7C. As seen, using combined electrospinning/electrospraying technique resulted in obtaining ZnO-on-PLA, ZnO(amino)on-PLA and ZnO(ester)-on-PLA mats that led to complete RR degradation after a 4-h contact
with the aqueous solution of the dye. The presence of a triazine group in RR makes its degradation under the action of zinc oxide more difficult [30]; and the longer degradation time of RR as compared to that of MB can be attributed to triazine group presence. In the case of ZnO(Si)-on-PLA mats the reduction of the photocatalytic activity was even more pronounced than that tendency when using MB as a model dye. ZnO(Si)-on-PLA mats did not degrade RR completely after a 5-h contact with the aqueous solution of RR. This is additional evidence that the sizes of the aggregates on the
Page 16 of 24
17 fibers’ surface of ZnO(Si)-on-PLA mats have an effect on the delay the photocatalytic effect
A
B
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te
d
M
an
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cr
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of the zinc oxide.
Figure 8. Reusability in the frames of three consecutive cycles of ZnO-on-PLA (∇),
ZnO(amino)-on-PLA (▼) and ZnO(ester)-on-PLA (□) mats in respect to photocatalytic degradation of MB (A) and of ZnO-on-PLA mats (∇) for degradation of RR (B). Noteworthy, as compared to ZnO(Si)-in-PLA mats the non-degraded residue for ZnO(Si)-onPLA mats is 2-fold lower (51.0±2.4%, vs. 24.0±2.4% %, respectively). For ZnO(amino)-onPLA and ZnO(Si)-on-PLA mats a lag time of 10 and 20 min, respectively, was detected. This
Page 17 of 24
18 lag time was shorter as compared to the determined for mats type “in”. In contrast, in the case of ZnO-on-PLA and ZnO(ester)-on-PLA mats the photocatalytic degradation started from the beginning of the contact. This difference is most likely due to the highest content of zinc oxide on the surface of these mats: 14.60% and 11.00% zinc for ZnO-on-PLA and ZnO(ester)-on-PLA mats, respectively, as determined by XPS. As seen from Figure 7D, for
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the type “on” mats again two stages (a slow and a fast one) were registered, and the Kapp value of the corresponding stage depended on the mat composition. The highest Kapp values of the
cr
corresponding stage were determined for ZnO-on-PLA mats, and the lowest ones – for ZnO(Si)-on-PLA mats.
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Studies on the reusability of ZnO-on-PLA, ZnO(amino)-on-PLA and ZnO(ester)-onPLA mats in respect to MB and RR were performed as well. The mats were subjected to three
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subsequent cycles using one and the same mat and after a triple change with fresh solution of the dye. The obtained results are presented in Figure 8. As seen from Figure 8А, the efficiency of the used mats in respect to the photocatalytic degradation of MB was similar for
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the first, second and the third cycle, thus indicating that the prepared new fibrous materials can be used repeatedly for degradation of MB. The photocatalytic activity did not depend on the type of the nanosized zinc oxide. Concerning the photocatalytic degradation of RR, the
d
activity of the mats depended on the type of the nanosized zinc oxide. The type “on” mats
te
with incorporated ZnO(amino) and ZnO(ester) reduced their photocatalytic activity after the first stage in respect to this dye. There was non-degraded dye: 45% and 60%, at the second
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and the third cycle for ZnO(amino)-on-PLA and ZnO(ester)-on-PLA, respectively. As seen from Figure 8B, for ZnO-on-PLA mats the amount of the non-degraded dye at the second and third cycle is lower: 11% and 18%, respectively.
3.4. Microbiological tests against S. aureus
It is known that nanosized zinc oxide has antibacterial activity against pathogenic
microorganisms and its minimal inhibitory concentration in respect to S. aureus is 0.5 mg/mL [31]. It may be anticipated that its incorporation in mats from PLA will impart them antibacterial activity. Thus, in the present study we checked whether the hybrid materials from
PLA
and
nanosized
zinc
oxide
prepared
by
electrospinning
or
by
electrospinning/electrospraying were able to inhibit the development of the pathogenic microorganism S. aureus. The dynamics of the bacterial growth was studied in liquid broth that contained S. aureus cells. The time-dependent alterations in the bacterial growth were monitored at regular time intervals up to 8 h by registration of OD600 of the bacterial
Page 18 of 24
19 suspension of S. aureus, which was in contact with a mat from PLA, от ZnO-in-PLA or ZnO-
an
us
cr
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on-PLA.
Figure 9. Dependence of OD600 on incubation time of S. aureus in the presence of PLA, ZnO-
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in-PLA or ZnO-on-PLA mats; blank bar – control (bacterial suspension of S. aureus); content
of the zinc oxide in the bacterial suspension: 0.5 mg/mL.
d
As seen from Figure 9, the presence of PLA mat did not significantly diminish the absorbance
te
in respect to the control (i.e. did not lead to considerable inhibition of S. aureus development). While ZnO-in-PLA mats exerted some inhibitory effect, a significant inhibition of S. aureus
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development was caused by the presence of ZnO-on-PLA mats and after an 8-h contact of the mat with the pathogenic microorganism the percentage of inhibition was ca. 30 %. Therefore, the incorporation of nanosized zinc oxide in PLA mats by electrospinning/electrospraying is more suitable in terms of antibacterial activity of PLA/zinc oxide mats as compared to the application of electrospinning of PLA/zinc oxide suspension.
Conclusion
For
the
first
time
the
application
of
electrospinning
and
electrospinning/electrospraying techniques was compared in terms of the photocatalytic (in respect to MB and RR) and antibacterial activity of the mats from PLA/nanosized zinc oxide. It was found that the use of electrospinning/electrospraying (type “on” mats) is more efficient for preparation of hybrid materials in terms of the above mentioned activities. This due to the fact that for the type “on” mats the zinc oxide is deposited only on the fibers’ surface, while for the type “in” mats it is mainly in the fibers’ bulk. The new type “on” hybrid materials may
Page 19 of 24
20 find application for biomedical applications (as antibacterial scaffolds) as well as for heterogeneous degradation of organic pollutants such as the used MB and RR model dyes.
Acknowledgements
Financial support from the Bulgarian Science Fund (Grant DCVP 02/2/2009) is gratefully
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acknowledged. D.V. acknowledges the OP-HRD Grant BG051PO001-3.3.06-006 of the European Social Fund. The authors appreciate highly the contribution of Assoc. Prof. A.
us
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Kujumdzieva from the Sofia University for the performance of the microbiological tests.
References
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[5] R. Jayakumar, M. Prabaharan, K.T. Shalumon, K.P. Chennazhi, S.V. Nair, Biomedical applications of polymer/silver composite nanofibers, Adv. Polym. Sci. 246 (2012) 263-282. [6] D. Crespy, K. Friedemann, A.-M. Popa, Colloid-Electrospinning: Fabrication of multicompartment nanofibers by the electrospinning of organic or/and inorganic dispersions and emulsions, Macromol. Rapid Commun. 33 (2012) 1978-1995. [7] A. Jaworek, A. Krupa, M. Lackowski, A.T. Sobczyk, T. Czech, S. Ramakrishna, S. Sundarrajan, D. Pliszka, Electrospinning and electrospraying techniques for nanocomposite non-woven fabric production, Fibres Text East. Eur. 17 (2009) 77-81. [8] E. Korina, O. Stoilova, N. Manolova, I. Rashkov, Multifunctional hybrid materials from poly(3-hydroxybutyrate), TiO2 nanoparticles, and chitosan oligomers by combining electrospinning/electrospraying and impregnation, Macromol. Biosci. 13 (2013) 707-716. [9] X. Sui, C. Shao, Y. Liu, Photoluminescence of polyethylene oxide-ZnO composite electrospun fibers, Polymer 48 (2007) 1459-1463.
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21 [10] N. Vitchuli, Q. Shi, J. Nowak, K. Kay, J.M. Caldwell, F. Breidt, M. Bourham, M. McCord, X. Zhang, Multifunctional ZnO/Nylon 6 nanofiber mats by an electrospinningelectrospraying hybrid process for use in protective applications, Sci. Technol. Adv. Mater. 12 (2011) no. 055004, 1-7. [11] H.J. Kim, H.R. Pant, C.H. Park, L.D. Tijing, N.J. Choi, C.S. Kim, Hydrothermal growth
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[12] Z. Zhang, C. Shao, F. Gao, X. Li, Y. Liu, Enhanced ultraviolet emission from highly dispersed ZnO quantum dots embedded in poly(vinyl pyrrolidone) electrospun nanofibers, J.
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[13] B. Pant, H.R. Pant, N.A.M. Barakat, M. Park, K. Jeon, Y. Choi, H.-Y. Kim, Carbon
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[14] T. Jamnongkan, S. Ryo, S.K. Sukumaran, M. Sugimoto, K. Koyama, Effect of ZnO nanoparticles on the electrospinning of poly(vinyl alcohol) from aqueous solution: influence of particle size, Polym. Eng. Sci. (2013) DOI 10.1002/pen.23730.
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[15] W. Yu, C.-H. Lan, S.-J. Wang, P.-F. Fang, Y.-M. Sun, Influence of zinc oxide
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[16] R. Naphade, J. Jog, Electrospinning of PHBV/ZnO membranes: structure and properties, Fiber. Polym. 13 (2012) 692-697.
[17] K.T. Shalumon, K.H. Anulekha, S.V. Nair, Nair S.V., K.P. Chennazhi, R. Jayakumar, Sodium alginate/poly(vinyl alcohol)/nano ZnO composite nanofibers for antibacterial wound dressings, Int. J. Biol. Macromol. 49 (2011) 247-254. [18] S. Ye, D. Zhang, H. Liu, J. Zhou, ZnO nanocrystallites/cellulose hybrid nanofibers fabricated by electrospinning and solvothermal techniques and their photocatalytic activity, J. Appl. Polym. Sci. 121 (2011) 1757-1764. [19] G.-Q. Chen, M.K. Patel, Plastics derived from biological sources: Present and future: A technical and environmental review, Chem. Rev. 112 (2012) 2082-2099. [20] J.-M. Raquez, Y. Habibi, M. Murariu, Ph. Dubois, Polylactide (PLA)-based nanocomposites, Prog. Polym. Sci. 38 (2013) 1504-1542. [21] M. Murariu, A. Doumbia, L. Bonnaud, A.L. Dechief, Y. Paint, M. Ferreira, C. Campagne, E. Devaux, Ph. Dubois, High-performance polylactide/ZnO nanocomposites
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22 designed for films and fibers with special end-use properties, Biomacromolecules 12 (2011) 1762-1771. [22] R. Pantani, G. Gorrasi, G. Vigliotta, M. Murariu, Ph. Dubois, PLA-ZnO nanocomposite films: Water vapor barrier properties and specific end-use characteristics, Eur. Polym. J. 49 (2013) 3471-3482.
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[26] F. Kayaci, C. Ozgit-Akgun, I. Donmez, N. Biyikli, T. Uyar, Polymer-inorganic core-shell
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acid nanofibrous microcapsules for tissue engineering, Biomed. Microdevices 14 (2012) 35[29] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, Ch. Guillard, J.-M. Herrmann, Photocatalytic degradation pathway of methylene blue in water, Appl. Catal. B: Environ. 31 (2001) 145-157.
[30] W.N.A. Guerra, J.M.T. Santos, L.R.R. de Araujo, Decolorization and mineralization of reactive dyes by a photocatalytic process using ZnO and UV radiation, Water Sci. Technol. 66 (2012) 158-164.
[31] Z. Emami-Karvani, P. Chehrazi, Antibacterial activity of ZnO nanoparticle on grampositive and gram-negative bacteria, Afr. J. Microbiol. Res. 5 (2011) 1368-1373.
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*Highlights (for review)
Highlights
Electrospinning/electrospraying vs. electrospinning: a comparative study on the design of poly(L-lactide)/zinc oxide non-woven textile Daniela Virovska,1 Dilyana Paneva,1* Nevena Manolova,1 Iliya Rashkov,1*, Daniela
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Karashanova2
cr
New fibrous materials from poly(L-lactide) and nanosized zinc oxide have been prepared.
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Electrospinning and electrospinning/electrospraying techniques are compared. It has been found that the electrospinning/electrospraying technique is more efficient.
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ce pt
ed
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The mats have photocatalytic and antibacterial activity.
Page 23 of 24
Graphical Abstract (for review)
Graphical Abstract for the Table of contents only
Electrospinning/electrospraying vs. electrospinning: a comparative study on the design of poly(L-lactide)/zinc oxide non-woven textile Daniela Virovska,1 Dilyana Paneva,1* Nevena Manolova,1 Iliya Rashkov,1*, Daniela
Ac
ce pt
ed
M
an
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cr
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Karashanova2
Page 24 of 24
S0169-4332(14)01233-1 http://dx.doi.org/doi:10.1016/j.apsusc.2014.05.192 APSUSC 28011
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-4-2014 25-5-2014 26-5-2014
Please cite this article as: D. Virovska, D. Paneva, N. Manolova, I. Rashkov, D. Karashanova, Electrospinning/electrospraying vs. electrospinning: a comparative study on the design of poly(L-lactide)/zinc oxide non-woven textile, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.192 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 Electrospinning/electrospraying vs. electrospinning: a comparative study on the design of poly(L-lactide)/zinc oxide non-woven textile Daniela Virovska,1 Dilyana Paneva,1* Nevena Manolova,1 Iliya Rashkov,1*, Daniela
1
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Karashanova2
Laboratory of Bioactive Polymers, Institute of Polymers, Bulgarian Academy of Sciences,
cr
Acad. G. Bonchev St, bl. 103A, BG-1113 Sofia, Bulgaria 2
Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Acad. G.
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Bonchev St, bl. 109, BG-1113 Sofia, Bulgaria
E-mail: [email protected] (D. Paneva)
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E-mail: [email protected] (I. Rashkov)
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* Corresponding authors. Tel./fax: +359 02 9793289/+359 02 8700309
Abstract
New hybrid fibrous materials from the biocompatible and biodegradable aliphatic
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polyester poly(L-lactide) (PLA) and pristine or surface-functionalized nanosized zinc oxide
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were prepared. The application of the techniques: (i) electrospinning of a suspension of ZnO in PLA solution, or (ii) simultaneous electrospinning of PLA solution and electrospraying of a
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ZnO suspension in PLA solution (at low PLA concentration) enabled the fabrication of hybrid materials of diverse design: non-woven textile consisting of fibers in which ZnO was deposited on the fibers’ surface (designated as type “on”) or was mainly in the fibers’ bulk (designated as type “in”). The photocatalytic activity of the new fibrous materials was estimated in respect to Methylene Blue (MB) and Reactive Red (RR) dyes. Type “on” hybrid materials had higher photocatalytic activity as compared to type “in” materials. It was shown that type “on” materials preserved their photocatalytic activity in respect to MB even after three repeated uses, while for the RR dye the same held true for ZnO-on-PLA mats only. The type “on” materials exhibited antimicrobial activity against the pathogenic microorganism Staphylococcus aureus as evidenced by the performed microbiological tests. Keywords: nanosized zinc oxide; poly(L-lactide); electrospinning; electrospraying; photocatalytic activity; antibacterial activity
Page 1 of 24
2 1. Introduction The nanosized zinc oxide exhibits photocatalytic and antibacterial activities, and is therefore a very attractive component for incorporation in new hybrid materials. Such hybrid materials can find diverse applications in a wide range of fields: from medicine through environmental protection to the needs of optoelectronics [1-3]. The electrospinning is a very
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promising technique for preparation of micro- and nanofibrous materials in which diverse in nature drugs, as well as metal or metal oxide nanoparticles can be incorporated applying a
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single-step procedure [4-5]. Electrospinning results in obtaining fibers in which the additive is distributed mainly in their bulk. Electrospinning performed in conjunction with
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electrospraying enables fabrication of hybrid materials consisting of fibers and filler nanoparticles deposited on the fibers’ surface while distributed in the entire volume of the mat
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[7,8]. Nanosized zinc oxide has been incorporated in fibrous materials by electrospinning, and synthetic polymers such as poly(ethylene oxide) [9], nylon [10,11], poly(vinyl pyrrolidone) [12], polyacrylonitrile [13], and poly(vinyl alcohol) [14] have been mainly used. The studies
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on the incorporation of nanosized zinc oxide in fibrous materials from bio-based polymers are still scarce. ZnO nanoparticles have been incorporated by electrospinning in poly(3hydroxybutyrate-co-3-hydroxyvalerate) [15,16], alginate [17] and cellulose [18] fibers. To the
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best of our knowledge preparation of hybrid materials from ZnO by applying simultaneous
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electrospinning and electrospraying has been reported only for nylon 6 [10]. Till now, the properties of polymer/ZnO hybrid materials prepared by electrospinning and those obtained
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by electrospinning/electrospraying have not been compared. Poly(L-lactide) (PLA) is regarded as the most attractive polymer and it is expected that
PLA will replace the widely used synthetic polymers for the manufacturing of daily used in the household plastic devices as well as for elaboration of new materials for medicine and environmental protection [19,20]. PLA is synthesized from annually renewable source and its manufacturing has been increased considerably during the past decade. It is biocompatible in respect to cells, tissues and organs and is a biodegradable polymer. The hybrid fibrous materials
from
PLA
and
zinc
oxide
prepared
by
electrospinning
or
electrospinning/electrospraying are of outstanding interest since they can combine the beneficial properties of the polymer and the inorganic filler, namely: biodegradability, photocatalytic and antibacterial activities. In the present study new hybrid fibrous materials from PLA/nanosized zinc oxide were prepared by applying the electrospinning and electrospinning/electrospraying techniques. The obtained materials are designated as type “in” and type “on”, respectively. The morphology of
Page 2 of 24
3 the materials was observed by scanning electron microscopy (SEM), and the nanosized zinc oxide distribution – by transmission electron microscopy (TEM) and EDX mapping of zinc. The ZnO content and the thermal degradation of the fibrous materials were evaluated by thermogravimetric analysis (TGA). The surface elemental composition of the different types of ZnO used, as well as that of the type “in” and “on” mats was determined by X-ray
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photoelectron spectroscopy (XPS). The photocatalytic activity of the type “in” and “on” materials was evaluated using Methylene Blue (MB) and Reactive Red (RR) as model dyes.
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The antibacterial activity of the materials against the pathogenic microorganism
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Staphylococcus aureus (S. aureus) was tested.
2. Materials and methods
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2.1. Materials
Poly(L-lactide) (PLA) was a commercial product (INGEO™ Biopolymer 4032D, NatureWorks LLC – USA) having M W = 259 000 g/mol and M W / M n = 1.94 as determined
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by size-exclusion chromatography using polystyrene standards. Four types of commercial nanosized zinc oxides available under the trade mark Zano®20 of Umicore Zink Chemicals – Belgium were used, namely: nanosized zinc oxide (Zano®20), zinc oxide with silanized
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surface (Zano®20 Plus), zinc oxide with aminofunctionalized surface (Zano®20 Plus-2) and
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zinc oxide with esterfunctionalized surface (Zano®20 Plus-3). They are further denoted as ZnO, ZnO(Si), ZnO(amino) and ZnO(ester), respectively. These oxides are rod-like with
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diameter of 10-30 nm and length of 100 nm [21,22]. Chloroform, N,N-dimethylformamide (DMF) and ethanol (EtOH) were of analytical grade, purchased from Merck. Methylene Blue (MB, Scheme 1A) dye was purchased from Merck, and Reactive Red (CI Reactive Red 1, RR, Scheme 1B) was kindly supplied by Prof. T. Konstantinova, UCTM-Sofia.
N
S
+ N Cl
Cl SO3Na N N
H OH N
N N Cl
N
SO3Na
NaO3S
A
N
B
Scheme 1. Formulae of Methylene Blue (A) and Reactive Red (B) dyes 2.2. Preparation of fibrous materials from PLA/nanosized zinc oxide by electrospinning (type “in” mats) and by electrospinning/electrospraying (type “on” mats)
Page 3 of 24
4 For preparation of type “in” materials the corresponding oxide: ZnO, ZnO(Si), ZnO(amino) or ZnO(ester), was dispersed in PLA solution and the obtained suspension was subjected to electrospinning. PLA concentration was 10 wt.%, and zinc oxide content was 23 wt.% in respect to the total solids. Depending on zinc oxide type PLA (0.74 g) was dissolved in 3.70 mL CF for ZnO, ZnO(amino) or ZnO(ester), and in 3.40 mL chloroform for ZnO(Si).
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After the complete dissolution of the polymer a preliminary sonicated (30 min in an ultrasonic bath Bandelin Sonorex, 160/640 W, 35 kHz) suspension of nanosized zinc oxide (1 mL) was
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added and then the obtained mixture was stirred using magnetic stirrer. As a continuous phase DMF was used for ZnO, ZnO(amino) or ZnO(ester) dispersions, and chloroform - for ZnO(Si)
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dispersions. The concentration of zinc oxide in the suspension in DMF or chloroform of 22.2 % (w/v) in 10 mL suspension was selected in such a manner so as the oxide content in 1 mL
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of the suspension added into PLA solution to be 23 wt.% in respect to the total solids. The spinning suspension of PLA and zinc oxide was transferred in a syringe equipped with a metal needle connected to the positively charged electrode of the high-voltage power supply.
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Electrospinning was conducted at a constant applied voltage of 25 kV and constant tip-tocollector distance of 17 cm using a grounded rotating aluminum collector (1800 rpm). The spinning suspensions were delivered at a constant rate of 3 mL/h enabled by the use of a
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pump Syringe Pump NE-300 (New Era Pump Systems, Inc.).
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For preparation of the type “on” hybrid materials the following equipment was used for electrospinning/electrospraying: two pumps for delivering (i) PLA solution [10 wt.%
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solution of PLA in chloroform/DMF = 3.7/1 (v/v)] for electrospinning, and (ii) 30 % (w/v) suspension of zinc oxide prepared in the presence of 0.05 % (w/v) PLA using the following solvents: DMF for ZnO, EtOH for ZnO(amino) and for ZnO(ester), and chloroform for ZnO(Si) (3 mL). PLA dissolved in 3 mL chloroform was added to the suspension of zinc oxide to serve as a sticking agent for the nanosized zinc oxide onto PLA fibers. The zinc oxide suspensions were sonicated for 30 min. A common collector was used for deposition of PLA fibers and zinc oxide (rotating rate: 1800 rpm). The pumps for delivering the spinning solution and the zinc oxide suspension were located at an angle of 180° in respect to the collector. The delivery rate of the spinning solution and of the suspension was 3 mL/h. The electrospinning/electrospraying was conducted at a tip-to-collector distance of 17 cm for the electrospinning of PLA solution and 15 cm for electrospraying of the PLA/zinc oxide suspension. The applied voltage was provided using a common high-voltage power supply.
2.3. Characterization of the prepared PLA/zinc oxide mats
Page 4 of 24
5 The morphology of the prepared fibrous materials was evaluated by scanning electron microscopy (SEM). The samples were vacuum-coated with gold prior to observation by SEM (Jeol JSM-5510 or Philips 515). The mean fiber diameter was estimated by ImageJ software, and their morphology was assessed applying the criteria for overall evaluation of electrospun materials as described in details in [23]. TEM observations were carried out by JEM 2100
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(JEOL Co. Ltd.) operating at a voltage of 200 kV. The samples were prepared by depositing on a copper grid. In order to demonstrate that zinc oxide is incorporated into PLA/zinc oxide
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fibers EDX mapping of zinc was applied.
The thermal stability of the fibrous materials and the content of zinc oxide were
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determined by thermogravimetry using Perkin Elmer TGA 4000 Thermogravimetric Analyzer in nitrogen. The heating was from r.t. to 600°C at a heating rate of 10 °C/min.
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The surface chemical composition of the materials was determined by X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out in the UHV chamber of an ESCALAB-MkII (VG Scientific) spectrometer using Mg Kα excitation with a
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total resolution of ca. 1 eV. Energy calibration was performed taking the C 1s line at 285 eV as a reference.
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and Reactive Red dyes
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2.4. Photocatalytic activity of the prepared new hybrid materials in respect to Methylene Blue
For assessment of the photocatalytic activity of the prepared new hybrid fibrous
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materials Methylene Blue (MB) and Reactive Red (RR) were used as model dyes. Samples of approximate size of 10×10 mm were cut so as to the zinc oxide content in the mats to be 0.6 mg. The samples were immersed in 7 mL aqueous solution of MB (2×10-5 mol/L) or RR (6.3×10-5 mol/L) and kept for 30 min in dark. After that they were irradiated with UV-visible light (UVASPOT 400/T, Dr. Honle AG; UV lamp UV 400 F/2; 400 W; wavelength range: from 260 to 600 nm) for 5 h. During the experiments the temperature of the solutions was kept at 20 °С using a LAUDA PE 104 thermostat. For comparison of the photocatalytic activity of type “in” and type “on” materials, the weight of the mats was selected in such a manner so as the weight of the zinc oxide to be one and the same (0.6 mg). The photocatalytic decomposition of the MB and RR was followed using DU800 spectrophotometer (Beckman Coulter, Inc.) at 660 nm and 532 nm, respectively, by recording the decrease in the absorbance of MB or RR in the presence of the hybrid materials as a function of the irradiation time. The presented data are average values from 3 measurements.
Page 5 of 24
6 The apparent rate constant (Kapp) was calculated from the kinetic curves of the dyes degradation: ln(CoMB or RR/CMB or RR) = f(t), where CoMB or RR was the initial concentration of MB or RR, CMB
or RR
– the concentration of MB or RR at the moment t, and t was the
irradiation time (Figure S1). Kapp was calculated from the dependence ln(CoMB or RR/CMB or RR) = Kapp × t [24].
ip t
The reusability of the materials was tested by immersing them into a fresh MB or RR solution after a catalysis run; before each replacement of the dye solution the mats were
cr
washed three times with distilled water. This procedure was repeated three times for each
us
sample.
2.5. Microbiological tests against S. aureus
an
Initial suspension of S. aureus with optical density at 600 nm (OD600) of 0.05 was transferred into tubes containing 2 ml Luria-Bertani medium. One of the tubes served as a control. PLA, ZnO-in-PLA and ZnO-on-PLA mats with zinc oxide content 0.5 mg/mL cell
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culture were placed in the other tubes. The tubes were cultivated under agitation at 205 rpm for 8 h. Aliquots were taken per hour and their OD600 was determined. A total of 3
Ac ce p
te
d
measurements were averaged.
Page 6 of 24
7 3. Results and discussion 3.1. Surface composition of the nanosized ZnO. Finding of an appropriate continuous phase for preparation of stable dispersions of nanosized zinc oxide.
TGA analyses showed that while ZnO powder did not contain any organic phase, ZnO(amino), ZnO(ester) and ZnO(Si) contained 0.4 %, 0.4%, and 2.0 %, respectively. The
ip t
surface composition of the four types of nanosized zinc oxide powders was determined by
cr
XPS (Table 1).
an
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Table 1. Surface composition of zinc oxide powder as determined by XPS survey spectrum. Zinc oxide type C (%) O (%) Zn (%) N (%) Si(%) (powder) ZnO 55.00 45.00 ZnO(amino) 9.20 43.90 45.70 1.20 ZnO(ester) 12.86 44.30 42.84 ZnO(Si) 22.50 41.50 34.50 1.50
Peaks for zinc (Zn 2p3/2 at 1022 eV) and oxygen (O 1s at 530 eV) (Figure 1) were detected on
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ZnO surface. As seen from Table 1, the oxygen content is higher than the theoretical one of 50/50. Similarly to other studies [25], this can be attributed to the presence of two types of oxygen atoms on ZnO surface: one participating in a bond with Zn, and a second one engaged
d
in –OH groups (Figure 1B). For the other types of zinc oxide again peaks for Zn 2p3/2 at 1022
te
eV and for O 1s at 530 eV was registered, however (and as expected) presence of carbon was detected as well. ZnO(amino) surface contains also 1.20% nitrogen, and the registered peak
Ac ce p
for N 1s is at 400 eV and can be attributed to nitrogen atom engaged in C-NH2 bond. For ZnO(Si) a peak at 104 eV was detected characteristic for presence of Si 2p on the surface of the zinc oxide with silanized surface. As seen from Table 1, the silicon content is 1.50%. Since there are no data on the stability of dispersions from the used in the present
study commercial products nanosized zinc oxide in any solvent (up to date these products have been utilized only for preparation of polymer materials by melt extrusion) at a first stage studies were performed for finding of the appropriate continuous phase for preparation of stable dispersions PLA/zinc oxide. It was found that chloroform (which is a good solvent for PLA) although was suitable for ZnO(Si) dispersions, was not appropriate for ZnO, ZnO(amino), or for ZnO(ester) dispersion. ZnO(amino) and ZnO(ester) gave stable dispersions in DMF and EtOH, while ZnO – only in DMF. It was found that stable dispersions could be obtained even at inorganic filler content as high as 30 % (w/v). In the present study DMF was used as continuous phase for ZnO dispersions, and DMF or EtOH for ZnO(amino) and ZnO(ester) or chloroform – for ZnO(Si) dispersions.
Page 7 of 24
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8
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A B Figure 1. Zn 2p3/2 (A) and O 1s (B) spectrum of ZnO powder.
3.2. Preparation of type “in” hybrid materials from PLA/nanosized zinc oxide by electrospinning and estimation of their photocatalytic activity
d
Type “in” materials containing nanosized zinc oxide in the bulk and on the surface of
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the fibers were prepared by electrospinning of suspensions of the corresponding zinc oxide in PLA solution at a content of the nanosized inorganic filler of 23 wt.% in respect to the total
Ac ce p
solids.
Representative SEM and TEM images of the obtained fibrous materials are shown in
Figure 2. The mean diameter value of PLA fibers prepared in absence of zinc oxide depended on the solvent system. In the case when chloroform was used as a solvent, two populations of fibers were formed: thinner fibers with mean diameter of 700 ± 21 nm, and thicker ones with mean diameter of 8.40 ± 1.24 µm. It was 1.70 ± 0.26 µm when chloroform/DMF = 3.7/1 (v/v) was used as a solvent system. The chloroform/DMF ratio was the same as that one used for the spinning dispersions from PLA/ZnO, PLA/ZnO(amino) and PLA/ZnO(ester) pairs. The smaller mean diameter value of PLA fibers prepared using chloroform/DMF system may be attributed to the presence of DMF which is a poor solvent for PLA. It was found that the incorporation of ZnO, ZnO(amino) and ZnO(ester) did not affect considerably the mean fiber diameter value, and it was ca. 2.00 ± 0.44 µm.
Page 8 of 24
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9
B
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an
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A
d
C D Figure 2. SEM micrographs of fibers from ZnO-in-PLA (A) and ZnO(Si)-in-PLA (B);
te
magnification: × 1000. ТЕМ micrograph of ZnO-in-PLA (C). A schematic representation of
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the cross-section of a type “in” fiber (D).
For ZnO(Si)-in-PLA mats the mean diameter of the thinner fibers population was not affected by the presence of ZnO(Si), while the diameter of the thicker fibers population increased from 8.40 ± 1.24 µm [in absence of ZnO(Si)] up to 11.50 ± 2.22 µm [for ZnO(Si)-in-PLA mat]. As seen from the TEM micrograph (Figure 2C), the zinc oxide was distributed mainly in the fibers’ bulk, however, there was zinc oxide on the fibers’ surface as well. EDX mapping images of Zn showed that the zinc oxide was distributed uniformly along the fibers (Figure S2B). A schematic representation of the cross-section of a type “in” fiber is shown in Figure 2D. Since PLA was totally destructed in the TGA experiments (100 % weight loss at 370 °C, Figure 3), and moreover, zinc oxide has excellent thermal stability [21], it was possible to
determine the content of zinc oxide in the mats. It was determined also for the materials containing nanosized zinc oxide with a modified surface since the organic fraction was
Page 9 of 24
10 negligible. TGA thermograms of PLA mat as well as of mats that contain different types of
an
us
cr
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zinc oxide are presented in Figure 3.
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Figure 3. TGA thermograms of mats from: PLA (1); ZnO(Si)-in-PLA (2), ZnO(ester)-in-PLA
d
(3), ZnO-in-PLA (4) and ZnO(amino)-in-PLA (5).
ZnO-in-PLA, ZnO(amino)-in-PLA and ZnO(ester)-in-PLA mats destructed thermally
te
in two steps, the first stage was at ca. 285 °C. In the second stage the degradation temperature did not depend significantly on the type of the zinc oxide and was 398 °C for ZnO-in-PLA,
Ac ce p
395 °C for ZnO(ester)-in-PLA, and 397 °C for ZnO(amino)-in-PLA. Similarly to PLA mats, ZnO(Si)-in-PLA mats degraded in one step, however at a lower temperature (284 °C). It can be concluded that the incorporation of the different types of zinc oxide lead to a decrease of the thermal stability of PLA mats. This result is consistent with data obtained for materials from PLA/ZnO and PLA/ZnO(Si) prepared by melt extrusion [21]. Similar decrease of the degradation temperature has been detected for nanofibrous materials from nylon 6,6/nanosized zinc oxide prepared by electrospinning as well [26]. This is attributed to the catalytic activity of zinc oxide that leads to thermal degradation of the polymer at lower temperature. From the performed TGA studies it was also found that the zinc oxide content (23 wt.%) in the mats prepared by electrospinning corresponded to that one in the feed for all types of zinc oxide thus corroborating the stability of the PLA/zinc oxide suspensions for the electrospinning process duration.
Page 10 of 24
11 It is widely known that nanosized zinc oxide is an excellent agent for photocatalytic degradation of diverse in nature organic pollutants, such as e.g. wastewaters from the textile
A
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te
d
M
an
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cr
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industry.
B
Figure 4. MB photocatalytic degradation: dependence on irradiation time in the presence of a
mat from PLA (blank) (◊), ZnO-in-PLA (∇), ZnO(amino)-in-PLA (▼), ZnO(ester)-in-PLA (□), ZnO(Si)-in-PLA (■), and blank control (♦) (A); and Kapp value for type “in” mats of the first (empty bars) and the second (gray bars) stages of the degradation process (B).
Page 11 of 24
12 Its incorporation in fibrous materials by electrospinning or electrospinning/electrospraying enables the design of new zinc oxide carriers that can be applied for heterogenic photocatalytic mineralization of organic pollutants. In the present study the photocatalytic degradation of Methylene Blue dye (MB) was followed. MB is widely used as a model dye; in addition, it is a known pollutant of wastewaters from the textile industry. The dependence of
ip t
MB photocatalytic degradation on UV-vis irradiation time for type “in” mats is presented in Figure 4А. As seen, the MB degradation on UV-vis irradiation time did not depend
cr
significantly on the type of zinc oxide. This result may be attributed to the presence of approximately one and the same content of zinc oxide on the surface of type “in” mats (ca. 0.4
us
% as determined by XPS). Almost complete decolorization of the MB aqueous solutions occurred after a 5-h UV-vis irradiation under the action of the mats that contain zinc oxide. The apparent rate constant (Kapp) of MB degradation process was calculated from the linear
an
dependence of ln(C0MB/CMB) as a function of the UV-vis irradiation time of the aqueous solution of the dye in the presence of the type “in” mats. It was found that MB degradation
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occurred in 2 steps: slow one (from 0 to 20 min of contact of the mat with the dye) followed by a fast one. As seen from Figure 4B, Kapp value of the slow (ca. 0.0030 min-1) as well as of the fast (ca. 0.0045 min-1) stage did not depend on the type of the zinc oxide.
d
Blank experiments (in absence of a zinc oxide-containing mat) showed that MB alone
te
was also sensitive under UV-vis irradiation of its aqueous solutions. As seen from Figure 4A, ca. 25 % and ca. 40% of the dye degraded after 2 h and after 5 h irradiation, respectively.
Ac ce p
Therefore, further the stable under UV-vis irradiation Reactive Red azo dye (RR) was subjected to photocatalytic degradation. It is known that azo dyes are highly toxic and they can be considered as carcinogens since they can form toxic aromatic amines [27]. It is also known that the reactive dyes are among of the most widely spread pollutants in wastewaters from the textile industry due their wide application for textile dyeing; moreover, they are very stable to degradation. While RR was stable in aqueous solution under UV-vis irradiation, mats containing zinc oxide were found to lead to dye degradation (Figure 5). The decolorization in 5 h was not complete; at that the amount of the residual non-degraded dye is higher as compared to that of MB. The best results were obtained using ZnO-in-PLA mats; in this case the residual non-degraded RR was 30%. For the rest of the type “in” mats the non-degraded residue was higher reaching 50%. During the first minutes of contact of RR with type “in” mats any degradation did not occur. This initial stage depended on the type of the nanosized oxide: ca. 50 min for mats that contain ZnO, ZnO(amino) or Zn(ester) and 80 min for ZnO(Si)-in-PLA mats. After that degradation occurred; the calculated Kapp value was ca.
Page 12 of 24
13 0.0010 min-1 for type in” mats that contained ZnO, ZnO(amino) or ZnO(ester), and ca. 0.0020
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min-1 for ZnO(Si)-in-PLA mats.
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Figure 5. RR photocatalytic degradation: dependence on irradiation time in the presence of a
mat from PLA (blank) (♦), ZnO-in-PLA (∇), ZnO(amino)-in-PLA (▼), ZnO(ester)-in-PLA
d
(□) or ZnO(Si)-in-PLA (■).
te
The obtained results for relatively slow photocatalytic MB and RR degradation under the action of type “in” mats can be ascribed to the fact that the zinc oxide in these mats is
Ac ce p
distributed mainly in the fibers’ bulk, thus hindering its contact with the dye in its aqueous solution. It was suggested that using electrospinning in conjunction with electrospraying would allow fabrication of materials in which the nanosized zinc oxide would be deposited on the fibers’ surface. Therefore, studies were performed on the preparation of materials PLA/zinc oxide by combined electrospinning/electrospraying techniques.
3.3. Preparation of type “on” hybrid materials from PLA/nanosized zinc oxide by electrospinning/electrospraying and estimation of their photocatalytic activity
Type “on” mats from PLA/nanosized zinc oxide were prepared by simultaneous electrospinning of a PLA solution and electrospraying of a suspension containing ZnO, ZnO(amino), ZnO(ester) and ZnO(Si) [zinc oxide content 30 % (w/v)] in chloroform/DMF = 1/1 (v/v) for ZnO, chloroform/EtOH = 1/1 (v/v) for ZnO(amino) or ZnO(ester), or chloroform for ZnO(Si). A small amount of PLA: 0.05 % (w/v) was added to the zinc oxide dispersions in
Page 13 of 24
14 order to serve as a sticking agent for the oxide on the PLA fibers surface in order zinc oxide loss to be avoided. SEM and TEM images of fibers from ZnO-on-PLA and ZnO(Si)-on-PLA mats are presented in Figure 6, and EDX mapping of Zn of ZnO(amino)-on-PLA mat is
us
cr
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shown in Figure S2D.
B
Ac ce p
te
d
M
an
A
C C’ D Figure 6. SEM (A and B, magnification: × 1000) and TEM (C, magnification: × 25000; and
D, magnification: × 15000) micrographs of ZnO-on-PLA (A and C) and ZnO(Si)-on-PLA (B and D) fibers. A schematic representation of a cross-section of a type “on” fiber (C’). The mean diameter of the PLA fibers was 1.70 ± 0.26 µm. As seen, the nanosized zinc oxide was deposited mainly as spherical nanoporous agglomerates. For ZnO-on-PLA, ZnO(amino)on-PLA and ZnO(ester)-on-PLA mats small (mean diameter of ca. 1 µm) and large (mean
diameter of ca. 5 µm) spherical aggregates were observed. A schematic presentation of the cross-section of a fiber with a spherical aggregate from mat type “on” is given in Figure 6C’. For ZnO(Si)-on-PLA mat the mean diameter of the spherical aggregates was larger, and ranged from 2 to 14 µm.
Page 14 of 24
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15
B
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A
C D Figure 7. Photocatalytic degradation of MB (A) and RR (C): dependence on irradiation time
d
in the presence of a mat from PLA (blank) (◊),ZnO-on-PLA (∇), ZnO(amino)-on-PLA (▼), ZnO(ester)-on-PLA (□) or ZnO(Si)-on-PLA (■).Kapp values of the degradation process of MB
te
(B) or RR (D) for mats type “on” for the first (empty bars) and the second (gray bars) stage.
Ac ce p
This may be attributed to the use of highly volatile continuous medium (chloroform) for the nanosized ZnO(Si). The microporous structure of the spherical aggregates for ZnO(Si)-onPLA observed by SEM may be also explained by its use. Similar porous microbeads have been obtained by electrospraying of PLA from its solution in chloroform [28]. The zinc oxide content was 45% for ZnO-on-PLA mat, 40% for ZnO(amino)-on-PLA and ZnO(ester)-onPLA mats, and 60% for ZnO(Si)-on-PLA mat as evidenced by TGA. The results obtained from the studies on the photocatalytic activity of the mats type
“on” in respect to MB and RR are presented in Figure 7. As seen from Figure 7А, they had different behavior as compared to type “in” materials: MB degraded completely for 3 h for ZnO-on-PLA, ZnO(amino)-on-PLA and ZnO(ester)-on-PLA, and for 4 h in the case of ZnO(Si)-on-PLA mats, thus confirming our hypothesis that the deposition of the nanosized zinc oxide on the fibers surface by applying combined electrospinning/electrospraying
Page 15 of 24
16 techniques would lead to enhancement of the photocatalytic activity of the hybrid PLA/zinc oxide mats. Similarly to the case of type “in” materials, MB degradation occurred in two stages (Figure 7B), however, Kapp values of the first as well as of the second stage were significantly higher as compared to those calculated for MB degradation under the action of type “in” mats.
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For type “on” mats Kapp value and the duration of the first stage depended on the type of zinc oxide (Figure 7B). While during the first stage of MB degradation Kapp value was 0.0090 minfor ZnO-on-PLA, ZnO(amino)-on-PLA and ZnO(ester)-on-PLA mats, for ZnO(Si)-on-PLA
cr
1
mat Kapp value it was lower (0.0022 min-1). The duration of the first stage was 20 min for
us
ZnO-on-PLA mat and ca. 30 min for the rest of the type “on” mats. The second stage of MB degradation was faster; and the dye degradation by ZnO(amino)-on-PLA mat (0.0200 min-1) was with the highest Kapp value while the lowest value was calculated for ZnO(Si)-on-PLA
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mat (0.0094 min-1). ZnO(amino)-on-PLA and ZnO(Si)-on-PLA mats had close in value content of zinc oxide on their surface as evidenced by XPS (6.50% and 7.60%, respectively).
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Thus, the slower degradation of MB cannot be due to lower zinc oxide content on the fibers surface. As already mentioned, the mean diameter of spherical aggregates in ZnO(Si)-on-PLA materials were significantly larger (up to 14 µm) as compared to the rest of the type “on”
d
materials. The zinc oxide incorporated in the aggregates become inaccessible for the water
te
molecules. This in turn results in a delayed formation of hydroxyl radicals under the UV-vis irradiation in the presence of ZnO(Si)-on-PLA mat. The hydroxyl radicals are responsible for
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the ability of the zinc oxide to degrade organic pollutants to formation of harmless carbon dioxide as well to the conversion of the nitrogen and sulfur in the heterocycle in MB structure to inorganic ions [29].
The dependence of the photocatalytic degradation of RR on the UV-vis irradiation
time for type “on” mats is presented in Figure 7C. As seen, using combined electrospinning/electrospraying technique resulted in obtaining ZnO-on-PLA, ZnO(amino)on-PLA and ZnO(ester)-on-PLA mats that led to complete RR degradation after a 4-h contact
with the aqueous solution of the dye. The presence of a triazine group in RR makes its degradation under the action of zinc oxide more difficult [30]; and the longer degradation time of RR as compared to that of MB can be attributed to triazine group presence. In the case of ZnO(Si)-on-PLA mats the reduction of the photocatalytic activity was even more pronounced than that tendency when using MB as a model dye. ZnO(Si)-on-PLA mats did not degrade RR completely after a 5-h contact with the aqueous solution of RR. This is additional evidence that the sizes of the aggregates on the
Page 16 of 24
17 fibers’ surface of ZnO(Si)-on-PLA mats have an effect on the delay the photocatalytic effect
A
B
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of the zinc oxide.
Figure 8. Reusability in the frames of three consecutive cycles of ZnO-on-PLA (∇),
ZnO(amino)-on-PLA (▼) and ZnO(ester)-on-PLA (□) mats in respect to photocatalytic degradation of MB (A) and of ZnO-on-PLA mats (∇) for degradation of RR (B). Noteworthy, as compared to ZnO(Si)-in-PLA mats the non-degraded residue for ZnO(Si)-onPLA mats is 2-fold lower (51.0±2.4%, vs. 24.0±2.4% %, respectively). For ZnO(amino)-onPLA and ZnO(Si)-on-PLA mats a lag time of 10 and 20 min, respectively, was detected. This
Page 17 of 24
18 lag time was shorter as compared to the determined for mats type “in”. In contrast, in the case of ZnO-on-PLA and ZnO(ester)-on-PLA mats the photocatalytic degradation started from the beginning of the contact. This difference is most likely due to the highest content of zinc oxide on the surface of these mats: 14.60% and 11.00% zinc for ZnO-on-PLA and ZnO(ester)-on-PLA mats, respectively, as determined by XPS. As seen from Figure 7D, for
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the type “on” mats again two stages (a slow and a fast one) were registered, and the Kapp value of the corresponding stage depended on the mat composition. The highest Kapp values of the
cr
corresponding stage were determined for ZnO-on-PLA mats, and the lowest ones – for ZnO(Si)-on-PLA mats.
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Studies on the reusability of ZnO-on-PLA, ZnO(amino)-on-PLA and ZnO(ester)-onPLA mats in respect to MB and RR were performed as well. The mats were subjected to three
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subsequent cycles using one and the same mat and after a triple change with fresh solution of the dye. The obtained results are presented in Figure 8. As seen from Figure 8А, the efficiency of the used mats in respect to the photocatalytic degradation of MB was similar for
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the first, second and the third cycle, thus indicating that the prepared new fibrous materials can be used repeatedly for degradation of MB. The photocatalytic activity did not depend on the type of the nanosized zinc oxide. Concerning the photocatalytic degradation of RR, the
d
activity of the mats depended on the type of the nanosized zinc oxide. The type “on” mats
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with incorporated ZnO(amino) and ZnO(ester) reduced their photocatalytic activity after the first stage in respect to this dye. There was non-degraded dye: 45% and 60%, at the second
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and the third cycle for ZnO(amino)-on-PLA and ZnO(ester)-on-PLA, respectively. As seen from Figure 8B, for ZnO-on-PLA mats the amount of the non-degraded dye at the second and third cycle is lower: 11% and 18%, respectively.
3.4. Microbiological tests against S. aureus
It is known that nanosized zinc oxide has antibacterial activity against pathogenic
microorganisms and its minimal inhibitory concentration in respect to S. aureus is 0.5 mg/mL [31]. It may be anticipated that its incorporation in mats from PLA will impart them antibacterial activity. Thus, in the present study we checked whether the hybrid materials from
PLA
and
nanosized
zinc
oxide
prepared
by
electrospinning
or
by
electrospinning/electrospraying were able to inhibit the development of the pathogenic microorganism S. aureus. The dynamics of the bacterial growth was studied in liquid broth that contained S. aureus cells. The time-dependent alterations in the bacterial growth were monitored at regular time intervals up to 8 h by registration of OD600 of the bacterial
Page 18 of 24
19 suspension of S. aureus, which was in contact with a mat from PLA, от ZnO-in-PLA or ZnO-
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on-PLA.
Figure 9. Dependence of OD600 on incubation time of S. aureus in the presence of PLA, ZnO-
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in-PLA or ZnO-on-PLA mats; blank bar – control (bacterial suspension of S. aureus); content
of the zinc oxide in the bacterial suspension: 0.5 mg/mL.
d
As seen from Figure 9, the presence of PLA mat did not significantly diminish the absorbance
te
in respect to the control (i.e. did not lead to considerable inhibition of S. aureus development). While ZnO-in-PLA mats exerted some inhibitory effect, a significant inhibition of S. aureus
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development was caused by the presence of ZnO-on-PLA mats and after an 8-h contact of the mat with the pathogenic microorganism the percentage of inhibition was ca. 30 %. Therefore, the incorporation of nanosized zinc oxide in PLA mats by electrospinning/electrospraying is more suitable in terms of antibacterial activity of PLA/zinc oxide mats as compared to the application of electrospinning of PLA/zinc oxide suspension.
Conclusion
For
the
first
time
the
application
of
electrospinning
and
electrospinning/electrospraying techniques was compared in terms of the photocatalytic (in respect to MB and RR) and antibacterial activity of the mats from PLA/nanosized zinc oxide. It was found that the use of electrospinning/electrospraying (type “on” mats) is more efficient for preparation of hybrid materials in terms of the above mentioned activities. This due to the fact that for the type “on” mats the zinc oxide is deposited only on the fibers’ surface, while for the type “in” mats it is mainly in the fibers’ bulk. The new type “on” hybrid materials may
Page 19 of 24
20 find application for biomedical applications (as antibacterial scaffolds) as well as for heterogeneous degradation of organic pollutants such as the used MB and RR model dyes.
Acknowledgements
Financial support from the Bulgarian Science Fund (Grant DCVP 02/2/2009) is gratefully
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acknowledged. D.V. acknowledges the OP-HRD Grant BG051PO001-3.3.06-006 of the European Social Fund. The authors appreciate highly the contribution of Assoc. Prof. A.
us
cr
Kujumdzieva from the Sofia University for the performance of the microbiological tests.
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21 [10] N. Vitchuli, Q. Shi, J. Nowak, K. Kay, J.M. Caldwell, F. Breidt, M. Bourham, M. McCord, X. Zhang, Multifunctional ZnO/Nylon 6 nanofiber mats by an electrospinningelectrospraying hybrid process for use in protective applications, Sci. Technol. Adv. Mater. 12 (2011) no. 055004, 1-7. [11] H.J. Kim, H.R. Pant, C.H. Park, L.D. Tijing, N.J. Choi, C.S. Kim, Hydrothermal growth
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[12] Z. Zhang, C. Shao, F. Gao, X. Li, Y. Liu, Enhanced ultraviolet emission from highly dispersed ZnO quantum dots embedded in poly(vinyl pyrrolidone) electrospun nanofibers, J.
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[14] T. Jamnongkan, S. Ryo, S.K. Sukumaran, M. Sugimoto, K. Koyama, Effect of ZnO nanoparticles on the electrospinning of poly(vinyl alcohol) from aqueous solution: influence of particle size, Polym. Eng. Sci. (2013) DOI 10.1002/pen.23730.
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[15] W. Yu, C.-H. Lan, S.-J. Wang, P.-F. Fang, Y.-M. Sun, Influence of zinc oxide
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*Highlights (for review)
Highlights
Electrospinning/electrospraying vs. electrospinning: a comparative study on the design of poly(L-lactide)/zinc oxide non-woven textile Daniela Virovska,1 Dilyana Paneva,1* Nevena Manolova,1 Iliya Rashkov,1*, Daniela
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New fibrous materials from poly(L-lactide) and nanosized zinc oxide have been prepared.
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Electrospinning and electrospinning/electrospraying techniques are compared. It has been found that the electrospinning/electrospraying technique is more efficient.
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The mats have photocatalytic and antibacterial activity.
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Graphical Abstract (for review)
Graphical Abstract for the Table of contents only
Electrospinning/electrospraying vs. electrospinning: a comparative study on the design of poly(L-lactide)/zinc oxide non-woven textile Daniela Virovska,1 Dilyana Paneva,1* Nevena Manolova,1 Iliya Rashkov,1*, Daniela
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Karashanova2
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