Plasma‑Assisted Green Synthesis of ZnO Directly on Polyethylene Terephthalate Fabric

This article presents green in situ synthesis of ZnO directly on polyethylene terephthalate (PET) fabrics using pomegranate peel extract. The surface of PET was activated by environmentally friendly oxygen plasma, and the process was compared to alkali treatment where the extract of wood ash was used instead of classical chemicals. Sorption analysis showed that the hydrophilic character of the plasma-treated sample was much better than that of the alkali-treated and untreated samples. Both treatments slightly decreased the breaking strength and elongation of the fabric. Scanning electron microscopy, colour measurements, dispersive X-ray spectroscopy, and antioxidant activity investigations showed that the ZnO particles were successfully synthesised on alkali-and plasma-treated PET. The liquid chromatography-mass spectrometry results showed that ellagic acid and punicalagin were the most abundant compounds in the pomegranate peel extract that enabled the synthesis of ZnO. The UV protection, amount of ZnO, and uniformity were the highest for the ZnO prepared on the PET samples by the plasma-assisted green synthesis. Additional optical assessment of fabric porosity and thickness measurements confirmed that these fabrics have excellent UV protection due to the presence of ZnO.


Introduction
Researching and designing textiles that provide increased protection against ultraviolet (UV) radiation are currently an important objective in research and industry. The demand for such material is present in the military, construction and architecture, agriculture, protective garment, and everyday clothing production sectors. Prolonged exposure to UV radiation causes skin damage that can lead to the development of skin cancer [1]. One of the prospective materials that is UV stable, recyclable, and has good mechanical properties is polyethylene terephthalate (PET); however, PET provides minimum protection against UV radiation. Zinc oxide (ZnO) is known as an excellent UV blocking material that can be applied on textiles [2,3]. It is also biologically safe and biocompatible [4]. The application of ZnO to textiles is predominantly performed using already prepared particles [5][6][7]. However, research on the functionalization of textiles with in situ synthesised (formed directly on the material) nanostructures is increasing due to the reduced number of processing steps, saving time, and costs [8][9][10]. Classic reducing agents, such as sodium hydroxide and urea, are usually used for the in situ synthesis of ZnO on PET textiles [11,12]. However, the use of green reducing agents for nanoparticle synthesis is increasing due to health and environmental concerns [13]. In some cases, both classical and plant-derived reducing agents have been used for the in situ synthesis of nanoparticles on textiles [14,15]. Moreover, it was shown that classical reducing agents are not necessary when synthesis methods with the proper design are used [16,17]. Pomegranate peels were found to be a great reducing agent for the synthesis of various nanoparticles [18][19][20]. The peels represent 43% of the fruit and are the waste components after pomegranate juice extraction, and approximately 1.62 million tons of waste are generated worldwide [21,22]. The peel extracts are rich in phenolic compounds, and the highest content was found in aqueous extracts compared to 1 3 methanol, ethanol, and acetate extracts [23]. Since pomegranate peel extract has excellent antioxidant power due to phenolic compounds, it is ideal for nanoparticle synthesis, which is a step towards more sustainable, green, and ecologically friendly processes in the functionalization of textiles with in situ synthesised ZnO. Alternative alkaline sources to NaOH are ashes obtained from heating pellets and Seidlitzia rosmarinus [24][25][26]. S. rosmarinus have been used as the alkaline source for polyester hydrolysis (pretreatment) and in situ synthesis of ZnO/Ag composites on PET fabric [26]. The activation of polyester with alkali leads to the formation of hydrophilic active sites such as -COOH and -OH due to the partial hydrolysis of the ester linkage [27]. Hydrophilicity is also increased after treatment with oxygen plasma for only 3 s [28]. The oxygen concentration after oxygen plasma treatment increased from 21 to 39 at.%, mostly due to surface enrichment with C-O, C = O, and O = C-O groups. Consequently, the water contact angle on the PET dropped from 72° to below 20°. Plasma is an ecologically friendly textile pretreatment that allows surface chemical and/or morphological changes to the substrate without affecting its bulk properties [29]. The use of plasma is favourable for increasing the adsorption of various substances on polyester, from microcapsules, nanoparticles, dyes, and finishing coatings [30,31]. Moreover, regarding environmental concerns, there is no need for the disposal of chemicals after a treatment, as in the case of wet chemical modifications [32].
The aim of our research was to develop a completely green and sustainable in situ synthesis of ZnO on PET fabric with natural plant extract and activation of the polymer with oxygen plasma instead of the already established alkali activation to avoid the use of alkali in the synthesis process. Both pretreatments were compared in terms of ZnO formation and UV protection.

Preparation of Plant Extract
The pomegranate fruit peels were washed with distilled water and air-dried. The dried peels were then ground into a powder with a kitchen blender. The pomegranate peel powder was mixed in bidistilled water at a concentration of 50 g/L. The mixture was heated to the boiling point and boiled for 5 min. The mixture was left aside to cool for 2 h and then centrifuged at approximately 4000 rpm for 1 min. Finally, the remaining solution was vacuum filtered and used as a reducing agent (Extr).

Alkali Synthesis Process
First, the aqueous wood ash extract was prepared as an alkali source. The wood ash was collected from the pellet heating system and used as received. The alkali was prepared at room temperature in bidistilled water by adding 10 g/L wood ash, mixing, and vacuum filtering after 10 min. The in situ synthesis of ZnO on PET fabric was performed by three sequence dips of the fabric for 1 min in alkali, 0.5 M ZnAc, and Extr, respectively. Between the dips, the fabric was dried in a continuous dryer at 100 °C for 2 min after immersion in alkali and for 5 min when immersed in ZnAc or Extr. After synthesis, the samples were dried in a laboratory oven at 100 °C for 30 min and cured at 150 °C for 5 min. The schematic presentation of the procedure is presented in Fig. 1.

Plasma Synthesis Process
When plasma pretreatment was used as the surface activation procedure, the PET samples were first functionalized in an inductively coupled plasma system at a frequency of 27.12 MHz, and a nominal forward power 10 kW. Oxygen gas with a purity of 99.99% was introduced into the plasma system at 56 sccm flow and 20 Pa of pressure. The PET samples were treated with oxygen plasma for 4 s. The plasmatreated fabric was sequentially dipped in 0.5 M ZnAc and Extr for 1 min each. Drying between the dips and after synthesis was performed as described previously. The schematic presentation of the procedure is presented in Fig. 2.

Scanning Electron Microscopy (SEM)
The morphology of the samples was examined using a JSM-6060 LV scanning electron microscope (JEOL, Japan). To ensure sufficient electrical conductivity, the samples were coated with a layer of gold using a JEOL JFC-1300 auto-fine coater for 60 s at 30 mA.

Dispersive X-Ray Spectroscopy (EDS)
PET fabrics were probed by energy-dispersive X-ray spectroscopy (EDS) mounted on an environmental SEM Quanta 650 equipped with the latest state-of-the-art Oxford Live EDS Ultim max 40 mm 2 SDD detector. Analysed samples were not coated, because this SEM and EDS instruments are capable of operating in low-vacuum mode at 70 Pa.

Colour Measurements
The CIELAB values of the samples were measured using a Datacolor Spectro 1050 reflectance spectrophotometer (Datacolor, Luzern, Switzerland). Five consecutive measurements were performed on each sample using a 3 mm aperture on four layers of fabric.

Ultraviolet Protection Factor (UPF)
The ultraviolet protection factor (UPF) of the samples was determined according to test method AATCC TM 183 [33]. Ultraviolet transmission measurements were performed using a Lambda 850 + UV/Vis spectrophotometer (Perkin Elmer, Waltham, MA, USA) in the spectral range between 290 and 400 nm. The UPF rating and UVR protection categories were determined according to the Australian/New Zealand Standard: Sun Protective Clothing-Evaluation and Classification [34].

Antioxidant Activity (DPPH Assay)
The functionalized PET samples were analysed for antioxidant activity with the free radical scavenging method using 1,1-diphenyl-2-picrylhydrazyl (DPPH) [35][36][37]. The PET samples were immersed in 0.1 mM DPPH solution and shaken in the dark at 37 °C for 30 min. After removing the PET samples, the absorbance values of the remaining reaction solutions were measured at a wavelength of 517 nm using a Lambda 850 + UV/Vis spectrophotometer (Perkin Elmer, Waltham, MA, USA). The antioxidant activity was calculated from the absorbance measurements according to Eq. (1) where Ac is the absorbance of the blank DPPH solution and As is the absorbance of the DPPH solution in contact with the functionalized PET fabric.

Liquid Chromatography-Mass Spectrometry (LC-MS)
LC-MS analysis was performed on an UltiMate 3000 UHPLC system (Thermo Scientific, U.S.A.) coupled with a triple quadrupole/linear ion trap mass spectrometer (4000 QTRAP LC-MS/MS System; Applied Biosystems/MDS Sciex, Ontario, Canada). Methanol (Chromasolv LC-MS grade, Fluka, Switzerland) and water purified on a Milli-Q system from Millipore (Bedford, MA, USA) were used for the preparation of the mobile phases, and formic acid from Fluka was used as a modifier. An analytical HPLC column Hypersil GOLD Aq (2.1 × 150 mm, 3 µm particle size, Thermo) was used at a flow rate of 0.3 ml/min. All of the analytes were quantified in MRM mode, and the declustering potential and collision energy were optimized for the individual analytes. A mobile phase consisting of acetonitrile and water, both modified with 0.1% formic acid, was used throughout the work. The injection volume and column temperature were 10 µL and 30 °C, respectively.

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
The samples were analysed by mass spectrometry with inductively coupled plasma (ICP) after microwave digestion using an Agilent Technologies 7900 ICP-mass spectrometry (MS) instrument. All reagents used were of analytical grade or better. For sample dilution and preparation of standards, ultrapure water (Milli-Q, Millipore) and ultrapure acids (HNO 3 and HCl, Merck-Suprapure) were used. Standards were prepared in-house by dilution of certified, traceable, ICP-grade single-element standards (Merck CertiPUR). An Agilent Technologies 7900 ICP-mass spectrometry (MS) instrument equipped with a MicroMist glass concentric nebuliser and a Peltier-cooled Scott-type spray chamber was used. Prior to ICP-MS analysis, each sample was weighed (approximately 100 mg) and digested using a microwaveassisted digestion system (CEM MDS-2000) in a solution of 6 ml of hydrochloric acid and 2 ml of nitric acid. The digested samples were cooled to room temperature and then diluted with 2% v/v nitric acid until their concentration was within the desired concentration range prior to analysis.

Absorptiveness of Fabrics
The absorbency of the untreated, ash, and plasma-treated PET fabrics was tested using the capillary rise method according to the standard DIN 53,924 [38]. Each sample was hung vertically and aligned parallel to the ruler with millimetre divisions. The lower end of the sample was immersed 10 mm deep into lightly coloured distilled water. After 30, 60, and 300 s, the height of the wicking above the water surface was measured. The results are given as the mean and standard deviation of the four measurements.

Mechanical Properties
The breaking strength (N) and elongation (%) of the untreated and functionalized PET samples were measured with Instron 5567 according to the ISO 13934-1:2013 standard [39] with slight modifications. PET samples with dimensions of 50 mm × 25 mm were analysed using a preloading of 1 N and a speed of 50 mm/min. Three measurements of each sample were performed. Samples were conditioned at 21 °C and 65% air humidity for 24 h before analysis. The breaking strength and elongation were read at the breaking point of the test sample, and the average value was calculated.

Optical Assessment of Fabric Porosity
The porosity of the untreated and functionalized samples was determined using the optical assessment protocol described in Kostajnšek et al. 2021 [40]. Using a stereomicroscope (Leica Microsytems GmbH, Wetzlar, Germany), images of the samples were taken at 30 × magnification. The images were further analysed using the ImageJ program, where the percentage of open area was determined.

Fabric Thickness
The thickness of the untreated and functionalized samples was measured in accordance with the SIST EN ISO 5084 standard using a circular pressure foot of 25 cm 2 and 20 cN/ cm 2 . Ten measurements of each sample were made, and the average value and standard deviation were calculated.

Results and Discussion
The colour of the raw, untreated fabric is white, with CIELAB values of CIEL* = 93.06, CIEa* = − 0.44 and CIEb* = 0 (Fig. 3). Functionalization of PET fabric, regardless of the treatment, leads to darkening of the substrate, and the CIEL* values decrease. The darkest sample was PET fabric functionalized by the Zn precursor and pomegranate peel extract (sample "ZnAc-Extr"), and the lightest was the sample where ZnO particles were synthesised by plasmaassisted green synthesis (sample "Plasma-synthesis"). The colours on the green-red axis (CIE a*) and blue-yellow axis (CIE b*) were: • reddish-yellow for the sample functionalized only by reducing agent (sample "Extr"), • less red and yellower for the sample functionalized by Zn precursor and reducing agent (sample "ZnAc-Extr"), • less red and yellower for the sample where the in situ synthesis of ZnO was alkali-assisted, and • greenish and yellower for the sample where the in situ synthesis of ZnO was plasma-assisted.
In the bioreduction process of Zn salt to ZnO using plant extracts, the change in the colour of the solution indicates the presence of ZnO particles, which is attributed to the excitation of surface plasmon vibrations of ZnO particles [41,42]. It was observed by H. Agarwal [43] that samples where the synthesis of ZnO was successful became yellow, as also indicated by our samples (Fig. 3).
The success of synthesised ZnO directly on PET fabric was evaluated by SEM (Fig. 4) and EDS (Fig. 5). Compared to the untreated sample (Fig. 4a), all functionalized samples have a visible layer on the surface of the fibres resulting from either Zn-acetate, plant extract, or the formation of ZnO. The samples where the in situ synthesis of ZnO was performed, either using alkali or plasma assistance, have numerous visible particles present, which are evenly distributed on the fibres (Figs. 4b and c). The lower magnification of the SEM image of the sample where synthesis was performed using alkali ( Fig. 4b) revealed that particles were not evenly distributed on the entire fabric's surface. In contrast, in the case of synthesis by plasma assistance, the particles were evenly distributed on the entire surface of the fabric (Fig. 4c). The reason for this result is that the zinc precursor and the plant extract (sample "ZnAc-Extr", Fig. 4d) do not form particles on the PET surface which is due to the lack of free polar functional groups on the PET fabric that would enable the nucleation and growth of ZnO [44,45]. Therefore, activating the textile surface is essential for the formation of ZnO directly on the textile substrate when using natural extracts as reducing agents, providing active sites for zinc precursor adsorption. Treating polyester in an alkaline medium [11] or with oxygen plasma [46] is an effective method for producing hydrophilic groups such as hydroxyl and carboxyl groups on the fabric's surface.
Samples functionalized with only alkali or oxygen plasma were prepared and analysed for their wicking capability ( Table 1). The PET fabric gains higher hydrophilicity after alkali or plasma treatment, as the wicking height of water is higher than that of the untreated sample. The wicking height of water on the plasma-treated sample was higher than that on the ash-treated sample, which indicates higher functionalization with oxygen-rich groups on the polymer surface. As reported by Ibrahim et al. [27], the activation of PET with oxygen-rich groups after alkali treatment is due to the partial hydrolysis of the ester linkage and the formation of -COOH and -OH active sites. Similar activation was observed after oxygen plasma treatment, whereas the surface of PET was enriched with C-O, C = O, and O = C-O groups [28].
To determine how alkali (ash) or oxygen plasma treatments influence the mechanical properties of PET fabric, the breaking strength and elongation of samples were measured and compared to the untreated sample (  [47,48]. A reduction in tensile strength was also observed after alkaline treatment [49]. EDS analysis offered proof of ZnO particle synthesis on the PET surface (Fig. 5). The surface of the untreated sample consists mainly of carbon (C) and oxygen (O) elements (Fig. 5a). The sample treated with zinc precursor and pomegranate peel extract (Fig. 5b) contains a minimal amount of Zn, which is present due to zinc acetate. Higher Zn and O contents were found on samples where the in situ synthesis of ZnO was successful (Figs. 5c and 5d). Both pretreatments successfully promoted the synthesis of ZnO, whereas the plasma-assisted green synthesis was greater. The relative weight percentages for C, O, and Zn for plasma-synthesised ZnO were 42.5, 39.8, and 14.3%, respectively, while for alkali-synthesised ZnO, they were 48.00, 40.0, and 9.2%.
Our previous research showed that pomegranate peel extract has a very high content of phenolic compounds [25]. To determine which phenolic compounds are abundant and responsible for the successful reduction of zinc salt to ZnO, HPLC analysis was performed (Table 3). Ellagic acid was the most abundant compound detected, with a concentration of 1001.29 µg/ml. A very high concentration (543.00 µg/ ml) of punicalagin was also measured, which is a typical compound found in pomegranate peels [50]. Punicalagin has a large number (16) of hydroxyl groups (− OH), and it was previously reported that this is the reason for the compound's antioxidative activity [51]. Catechins and epicatechins were also detected (37.7 and 11.10 µg/ml, respectively). Namal Senanayake et al. [52] performed a study on catechins and reported again that their antioxidant activity is due to the aromatic ring and hydroxyl groups, to which free radicals can bind. They described that catechins can also bind transition metal ions. Gallic acid is one of the most common phenolic acids found in plants and fungi [53,54], and it    was detected at a concentration of 33.0 µg/ml. Interestingly, gallic acid has been proposed to be a compound that has a prooxidative effect. This means that it can either promote or inhibit the formation of free radicals, depending on the concentration and other compounds present [54]. Small concentrations of quercetin, rutin, kaempferol, chlorogenic acid, caffeic acid, neochlorogenic acid, resveratrol, ferulic acid, piceid, vitexin, and orientin were also detected. It was reported that phenolic compounds are crucial for the successful synthesis of ZnO, since they have good antioxidant properties, and antioxidants are reductants, while reductants are not necessary antioxidants [51,[55][56][57][58][59]. The proposed mechanism of ZnO formation on PET is presented in Fig. 6.
The formation of ZnO particles on the polyester begins with the activation of the inert polymer with alkali (ash) or oxygen plasma, which improves the surface reactivity due to newly formed hydrophilic end groups that form a negative charge on the fabric due to partial anionisation in the solution. The ionised functionalized groups act as adsorption sites for Zn 2+ ions from ZnAc solution and together form nucleation sites for ZnO particles. In the third step of the synthesis, the polyphenols from the extract (containing a large number of − OH groups) are introduced, which reduce and stabilize the ZnO in the drying and curing process [60][61][62][63].
Additional confirmation of ZnO synthesis on the alkali and plasma preactivated PET fabric was the antioxidant activity results of differently functionalized samples ( Table 4). It was previously reported that textiles where nanoparticles were directly synthesised have lower antioxidant activity than samples where nanoparticles did not form [64][65][66]. The antioxidant activity of PET fabrics functionalized only with a pomegranate peel extract (sample "Extr") or a zinc precursor and a pomegranate peel extract (sample "ZnAc-Extr"), on which ZnO nanoparticles did not form as previously shown by SEM and EDS, have high antioxidant activity due to the presence of phenolic compounds found in pomegranate peel extract [21]. Phenolic compounds are active ingredients and enable natural plant extracts to act as reducing agents in the process of nanoparticle formation [67,68]. During the process of nanoparticle formation, the phenolics are consumed,  meaning that their concentrations are reduced, resulting in lower antioxidant activity of either of the nanoparticles or the textiles functionalized with nanoparticles [64][65][66]69]. ZnO nanoparticles play an important role in providing protection against UV radiation [70]. PET fabrics can be used for various products, from everyday garments, protective clothing, awnings, camping tents, carriage covers, etc., where additional protection against UV radiation is necessary. The UV/Vis transmission spectra are presented in Fig. 7. The transmission (%) in the UV region is very low for samples containing ZnO. From the transmission results, the UV protection factor (UPF) was calculated [33]. These results are shown in Table 5. The untreated sample has a minimum protection category with a UPF value of 16.48. The transmittance of UVA was 28.20% and that of UVB was 2.21%. The PET samples where ZnO particles were synthesised directly on the PET fabric have very high UPF values (177.91 and 186.92), putting the fabrics into the excellent protection category. The UPF value of the PET fabric functionalized by the in situ plasma-assisted synthesis of ZnO was higher than that of the fabric where ZnO was alkaliassisted. These results are in accordance with the SEM and EDS results. The fabric functionalized with plasma-synthesised ZnO displayed transmittance of UVA and UVB radiation of only 0.84 and 0.49%, respectively.
The concentration of zinc in the untreated, alkali-synthesised, and plasma-synthesised samples was determined by ICP-MS ( Table 6). The alkali-synthesised sample exhibited   2.08% of zinc present in the sample, while the plasmasynthesised sample presented an even higher value, 3.52%. These results are in accordance with the EDS and UPF results.
The results of fabric thickness and porosity are further presented, since they can affect the UV protective properties of the fabric [71][72][73]. The value of the samples thickness increases after in situ synthesis of ZnO, regardless of the pretreatment used ( Table 7). The thickness increased from 0.134 mm (sample "Untreated") to 0.144 and 0.145 mm (samples "Alkali-synthesis" and "Plasmasynthesis", respectively). Since the fabric porosity can provide more details about how the synthesis of ZnO directly on PET fabric influences the coverage of the fabric's surface and consequently sample's transmission, the optical assessment of fabric porosity was performed, and the percentage of open area was determined (Fig. 8). The untreated PET fabric had an open area of 8.43%, and the samples where ZnO synthesis was alkali or plasmaassisted had 0.35 and 0.81% open areas. The difference in open area percentage between the untreated PET and samples where ZnO synthesis was alkali or plasma-assisted was not large enough to cause a great difference in UV blocking ability ( Table 5). The UV blocking ability of samples "Akali-synthesis" and "Plasma-synthesis" was due to the presence of ZnO on the fabric.

Conclusions
Treating PET fabric with oxygen plasma for in situ ZnO green synthesis using pomegranate peel extract was proven to be a very effective method to achieve excellent protection against UV radiation. Plasma treatment was compared to alkali treatment where the ecologically friendly approach was considered using wood ash instead of classical chemicals. Both treatments increased the hydrophilicity of PET and slightly decreased the breaking strength of fabric. The PET sample with plasma-synthesised ZnO provided better protection against UV radiation with a UV protection factor of 186.92, a higher amount, and a more uniform distribution of ZnO particles than the PET sample with alkalisynthesised ZnO. Activation of the polymer is a necessary step in the in situ synthesis process, as shown by the PET samples where no activation was performed. The ZnO particles did not form on the surface of the nonactivated samples. The formation of ZnO particles on the activated PET was also confirmed by colour measurements, antioxidant activity analysis, EDS, and ICP-MS. HPLC results showed that ellagic acid and punicalagin were the most abundant compounds in the pomegranate peel extract that enabled the synthesis of ZnO. Changes in fabric porosity and thickness had no influence on the UV protection. The use of plasma for the green in situ synthesis of nanoparticles directly on the textile or polymer surface is advantageous over wet chemical treatments, because it is an environmentally friendly modification technique that does not generate waste and does not affect the bulk properties of the polymer. Acknowledgements This work was financially supported by the Slovenian Research Agency (Project J2-1720, Programme P2-0213, and a grant for the doctoral student A.V.).

Conflict of Interest The authors declare no conflicts of interest.
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