Multifunctional Flax Fibres Based on the Combined Effect of ... - MDPI

nanomaterials Article

Multifunctional Flax Fibres Based on the Combined Effect of Silver and Zinc Oxide (Ag/ZnO) Nanostructures Sofia M. Costa 1 , Diana P. Ferreira 1, * , Armando Ferreira 2 , Filipe Vaz 2 Raul Fangueiro 1,3 1 2 3

*

and

Centre for Textile Science and Technology (2C2T), University of Minho, 4800 Guimarães, Portugal; [email protected] (S.M.C.); [email protected] (R.F.) Center of Physics, University of Minho, 4710-057 Braga, Portugal; [email protected] (A.F.); [email protected] (F.V.) Department of Mechanical Engineering, University of Minho, 4800 Guimarães, Portugal Correspondence: [email protected]; Tel.: +35-1253510982

Received: 20 November 2018; Accepted: 16 December 2018; Published: 19 December 2018

Abstract: Cellulosic fibre-based smart materials exhibiting multiple capabilities are getting tremendous attention due to their wide application areas. In this work, multifunctional flax fabrics with piezoresistive response were developed through the combined functionalization with silver (Ag) and zinc oxide (ZnO) nanoparticles (NPs). Biodegradable polyethylene glycol (PEG) was used to produce AgNPs, whereas ZnONPs were synthetized via a simple and low-cost method. Flax fabrics with and without NPs were characterized by Ground State Diffuse Reflectance (GSDR), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and Thermogravimetric analysis (TGA). After creating a conductive surface by flax functionalization with AgNPs, ZnONPs were synthetized onto these fabrics. The developed fibrous systems exhibited piezoresistive response and the sensor sensitivity increased with the use of higher ZnO precursor concentrations (0.4 M). Functionalized fabrics exhibited excellent antibacterial activity against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria, higher hydrophobicity (WCA changed from 00 to >1000 ), UV radiation resistance, and wash durability. Overall, this work provides new insights regarding the bifunctionalization of flax fabrics with Ag/ZnO nanostructures and brings new findings about the combined effect of both NPs for the development of piezoresistive textile sensors with multifunctional properties. Keywords: smart and multifunctional materials; cellulosic fibres; silver and zinc oxide nanoparticles; piezoresistive response; antibacterial effect; hydrophobicity

1. Introduction Over the last few years, the research and development of fibre-based smart materials that are capable of detecting and responding to external stimuli has increased dramatically. These materials can be applied in several areas including healthcare, electronics, sports, water treatment, military and aerospace [1,2]. The modification of cellulosic fibres’ surfaces with nanostructures allows the possibility to build sensors (e.g., piezoresistive sensors) directly onto the fabrics, creating wearable electronic devices [3,4]. Besides sensing properties, these fibre-based materials can also present multiple functions such as ultraviolet (UV) protection, flame retardancy, hydrophobicity/self-cleaning, and antibacterial effect, depending on the surface functionalization. Nanoparticles (NPs) are very attractive

Nanomaterials 2018, 8, 1069; doi:10.3390/nano8121069

www.mdpi.com/journal/nanomaterials

Nanomaterials 2018, 8, 1069

2 of 21

materials to be used in the development of functional fibrous systems, due to their higher surface area and exclusive optical, electrical, and catalytic properties [5–7]. Silver (Ag) and zinc oxide (ZnO) NPs exhibit numerous interesting properties, such as antibacterial effect, UV-blocking, piezoresistive properties, electrical conductivity, and photocatalytic activity, being widely used to functionalize different cellulosic fibres [6,8–11]. Hence, the combined effect of these NPs can be extremely useful for multifunctional materials development. Despite the strong research in this area [12], there are few problems related with sustainability and mass production that are still not well addressed. Thus, there is a huge need to explore and improve the processes sustainability, the cost and easiness of experimental methodologies, the industrial scalability, and the systems durability/washability. Taking into account the materials sustainability, natural cellulosic fibres (NCF) have emerged as valuable alternatives to replace synthetic ones, leading to a reduction in environmental pollution [13,14]. Moreover, NCF present numerous advantages, including a high abundance, low-cost, biodegradability, biocompatibility, low-weight, and good mechanical properties, making them very suitable materials to develop environmentally friendly fibrous systems. Among NCF, flax fibres are considered as one of the most promising and valuable, due to their high specific mechanical properties, being one of the most used as reinforcement in composite materials [13–16]. Besides the use of green materials, avoiding extremely high temperatures and choosing non-toxic and biodegradable solvents is also of particular importance. In the present study, the potential of Ag/ZnO nanostructures for multifunctional fibrous systems development was evaluated, taking into consideration the sustainability of the materials and methodologies involved. The development of piezoresistive sensors requires a conductive surface, which can be achieved by the functionalization of flax fabrics with AgNPs, since these NPs have electrical properties. In this way, AgNPs were synthetized using an eco-friendly and very easy methodology with polyethylene glycol (PEG) (as reducing and stabilizing agent). Afterwards, the in-situ synthesis of ZnONPs was performed onto AgNPs-treated flax fabric, using zinc acetate as precursor, with water as solvent, and a minimal concentration of sodium hydroxide (NaOH). At all stages, the processes scalability has always been considered, as well as the methodologies costs. All of the developed samples were characterized by Ground State Diffuse Reflectance (GSDR), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and Thermogravimetric analysis (TGA). Finally, the fabrics multiple properties were evaluated, including the electrical conductivity, piezoresistive behaviour, antibacterial effect, hydrophobicity, UV resistance, and washability. 2. Materials and Methods 2.1. Materials The flax fabrics were supplied by RCS (Braga, Portugal) with an areal mass of 315 g/m2 . Silver nitrate (AgNO3 ) and PEG Mr ~200 were purchased from Sigma Aldrich. Zinc acetate dehydrated (Zn(CH3 COO)2 ·2H2 O) and NaOH were obtained from Akzo Nobel. An UV blacklight lamp was used (15 W) from HQTM . 2.2. Flax Fabrics Pretreatment Flax fabrics (4 cm × 4 cm) were washed with 5 % (v/v) of non-ionic detergent at 80 ◦ C for 30 min to remove impurities (waxes, fats, etc). The fabrics were further cleaned in distilled water at 70 ◦ C for 30 min and then dried at 100 ◦ C for 10 min.


3 of 21

2.3. Functionalization of Flax Fabrics with NPs 2.3.1. Green Synthesis and Deposition of AgNPs onto Flax Fabrics AgNPs were synthetized and incorporated onto flax fabrics accordingly with the method that was previously described by Ferreira et al. [5]. Briefly, a 0.1 M AgNO3 and PEG200 solution was prepared and kept under stirring for one hour at 25 ◦ C. The solution colour change to yellow indicates the reduction of Ag ions into Ag metal (Ag0 ). After this, the fabrics were impregnated with this solution and kept overnight. The dip-pad-dry method was used to improve the incorporation of the AgNPs into the fabrics. The samples were dried at 100 ◦ C for 20 min, washed with water, and dried again at 100 ◦ C for 6 min. 2.3.2. In-Situ Synthesis of ZnONPs onto AgNPs-Treated Flax Fabrics ZnONPs were synthetized following the method that was described in [17], with few modifications. Briefly, AgNPs-treated flax samples were immersed on aqueous solutions of Zn(CH3 COO)2 ·2H2 O with different concentrations (0.01 M, 0.05 M, 0.1 M, 0.2 M and 0.4 M). The solutions were heated to 50 ◦ C under constant stirring (1 h), and 10 mL of NaOH (1 M) was added (1 mL/min). Finally, Ag/ZnO-treated flax samples were removed from the solution and heated to 100 ◦ C for 6 h. The samples will be described accordingly with the precursor (Zn(CH3 COO)2 ·2H2 O) solution concentration under use (example: flax + AgNPs + xMZnONPs, x = 0.01, 0.05, 0.1, 0.2 or 0.4 M). 2.4. Flax Fabrics Samples Characterization 2.4.1. Ground State Diffuse Reflectance (GSDR) The fabric samples’ GSDR spectra were recorded in the 250 to 700 nm wavelength range, using a Spectrophotometer UV 2501PC Shimadzu. Each sample was analysed in three different places to ensure homogeneity. The remission function (F(R)) was calculated accordingly with the Kubelka-Munk equation: K (1 − R ) 2 = , (1) F ( R) = 2R S where K is the absorption coefficient and S is the dispersion coefficient. 2.4.2. Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive Spectroscopy (EDS) The samples surface morphology was analysed by FESEM using a NOVA 200 Nano SEM from FEI Company (Hillsboro, OR, USA). All of the samples were coated with a very thin film (20 nm) of Gold (Au)-Palladium (Pd) before the experiment. Images were taken in topographic mode with an accelerated voltage of 10 kV. The EDS technique (Hillsboro, OR, USA) coupled to FESEM was performed to evaluate the elemental composition of the samples, using an EDAX Si (Li) detector with 15 kV of acceleration voltage. The solution of PEG containing AgNPs was analysed in transmission electron mode and the samples were installed in Cu-C grids by immersion in the solution. After this, the samples were analysed with an acceleration voltage of 15 kV, using a scanning transmission electron detector (STEM) (Hillsboro, OR, USA). A Zeiss Ultra 55 FESEM (Hillsboro, OR, USA) was used to observe the morphology of the samples using an acceleration voltage of 3 kV. Three samples for each condition was evaluated. 2.4.3. X-ray Diffraction (XRD) The crystallinity of the NPs was evaluated using a Bruker D8 Discover diffractometer, operated at a voltage of 40 kV and a current of 40 mA with Cu-Kβ radiation. Data were collected for 2θ values ranging from 10◦ to 90◦ .


4 of 21

2.4.4. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) Chemical composition of the fabric samples was studied by ATR-FTIR analysis using an IRAffinity-1S, SHIMADZU equipment (Kyoto, Japan). Each spectrum was obtained in transmittance mode using a diamond ATR crystal cell by accumulation of 45 scans with a resolution of 8 cm−1 from Nanomaterials 2018, 8, x FOR PEER REVIEW 4 of 22 400 to 4000 cm−1 .

2.4.5. Thermogravimetric Thermogravimetric Analysis Analysis (TGA) (TGA) 2.4.5. TGA analysis analysiswas wasperformed performed order to estimate the amount ofincorporated NPs incorporated onto flax TGA in in order to estimate the amount of NPs onto flax fabrics fabricsausing a STA 700 SCANSCI. Untreated and treated were subjected to a heating process using STA 700 SCANSCI. Untreated and treated fabricsfabrics were subjected to a heating process from from room temperature to 600 °C under nitrogen flow and with a heating rate of 20 °C/min. The ◦ ◦ room temperature to 600 C under nitrogen flow and with a heating rate of 20 C/min. The percentage percentage of NPs that wereondeposited on the flax fabric was calculated using the following system of NPs that were deposited the flax fabric was calculated using the following system of equations: of equations: ( f lax f abric residues% % NPs %+ f lax 𝑓𝑎𝑏𝑟𝑖𝑐 f abric % ==sample %; %; 100 𝑁𝑃𝑠 % + 𝑓𝑙𝑎𝑥 % 𝑠𝑎𝑚𝑝𝑙𝑒total 𝑡𝑜𝑡𝑎𝑙residues 𝑟𝑒𝑠𝑖𝑑𝑢𝑒𝑠 (2) (2) NPs lax f𝑓𝑎𝑏𝑟𝑖𝑐 abric %%==100% 100% 𝑁𝑃𝑠%%++f𝑓𝑙𝑎𝑥 Considering that that the the nanoparticles nanoparticles have have residues residues of of 100% 100% and and the the sample sample with NPs after thermal Considering treatment is composed of NPs and flax fabric residue. The samples were tested triplicate order treatment is composed of NPs and flax fabric residue. The samples were tested in in triplicate in in order to to diminish associate error to natural fibres (heterogeneous surface). diminish thethe associate error to natural fibres (heterogeneous surface). 2.5. Multifunctional Properties Evaluation 2.5.1. 2.5.1. Resistivity Resistivity Measurements Measurements and and Electromechanical Electromechanical Characterization Characterization The The electrical electrical conductivity conductivity of of the the flax flax samples samples depends depends on on the the formation formation of of aa continuous continuous conductive network in the insulating matrix. In order to evaluate pressure sensing properties of conductive network in the insulating matrix. In order to evaluate pressure sensing properties of flax flax fabric, the samples were subjected to a compression of 20%. The sensing experiments were fabric, the samples were subjected to a compression of 20%. The sensing experiments were performed performed in a compression configuration using a Shimadzu-AG-IS universal testingwith machine in a compression configuration using a Shimadzu-AG-IS universal testing machine a loadwith cell −1 on aofload cell of 500 N, a z-deformation of 0.5 mm, and a compression velocity of 4 mm · min −1 500 N, a z-deformation of 0.5 mm, and a compression velocity of 4 mm.min on samples with a samples diameter mm, during 10 cycles. The sensitivity value was calculated diameterwith of 10amm, duringof1010cycles. The sensitivity average value wasaverage calculated from four different from four different measurements for electro-mechanical each sample. The electro-mechanical testsbywere performed measurements for each sample. The tests were performed measuring the by measuring the electrical resistance of the sample, through the electrodes placed into the clamps electrical resistance of the sample, through the electrodes placed into the clamps (Figure 1). The (Figure 1). Theof measurement of the surface resistance mechanical experiment 0 in each measurement the surface resistance change ∆R/R0 change in each∆R/R mechanical experiment was obtained was obtained from the measurement of the electrical resistance, with a Keithley 2700 digital multimeter. from the measurement of the electrical resistance, with a Keithley 2700 digital multimeter.

Schematic representation the experimental configuration of the clamps for the Figure 1.1.Schematic representation of theofexperimental configuration of the clamps for the compression compressionwith experiments with simultaneous electrical measurements for electro-mechanical experiments simultaneous electrical measurements for electro-mechanical response evaluation of response evaluation of the samples. the samples.

Before that, the electrical resistivity of the flax samples was measured by the two-wire method, where 10 V V to to 10 10 V, V, step of 1, and the current measured where the the voltage voltage was was applied applied with with aa range range from from − −10 by a Keithley 487 picoammeter/voltage source. All of the measurements were performed in direct current (DC) mode, at room temperature. The electrical resistivity (ρ) was calculated by:

𝜌=𝑅

(3)

where R is the electrical resistance, A is the area of the electrode (6 × 1 mm2), and L is the distance


5 of 21

by a Keithley 487 picoammeter/voltage source. All of the measurements were performed in direct current (DC) mode, at room temperature. The electrical resistivity (ρ) was calculated by: ρ=R

A L

(3)

where R is the electrical resistance, A is the area of the electrode (6 × 1 mm2 ), and L is the distance between the electrodes (2 mm). The electrical conductivity is given by the inverse of the electrical resistivity (σ = 1/ρ). The sensing properties of the flax samples was quantified by the gauge factor (GF), which is defined as the ratio between the variation in the electrical resistance and the mechanical deformation applied [18]: ∆R ∆l = GF (4) R0 l0 where R0 is the steady-state material electrical resistance before deformation, ∆R is the resistance change that is caused by the mechanical deformation, l0 is the initial thickness of the flax sample, and ∆l the thickness variation. Three replicates of each condition were evaluated. 2.5.2. Antibacterial Tests The antibacterial activity of the solutions containing NPs and the functionalized flax fabrics was assessed, following the Agar Well Diffusion Method [19] and the JIS L 1902–Halo Method [20], respectively, against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. Firstly, two different inoculums containing each bacteria tested were prepared using nutrient broth (NB) and incubated for 12–18 h at 37 ◦ C. Regarding the agar well diffusion method, nutrient agar (NA) was prepared, autoclaved, and applied onto petri dishes until agar solidification. Subsequently, 100 µL of inoculum with a concentration of 1 × 106 cells/mL was spread over the entire agar surface. The agar medium was punched to create five wells and each well was filled with different solutions, in equal amounts. The agar dishes were incubated for 24 h at 37 ◦ C [19]. According to the JIS L 1902–Halo method, 0.33 mL of inoculum containing 1 × 107 cells/mL was added to 4.67 mL of NA, and finally, distributed uniformly onto sterilized petri dishes. After agar solidification, square flax fabric samples of 1 cm × 1 cm were placed over the agar dishes and incubated for 24 h at 37 ◦ C [20]. After samples incubation, the antibacterial activity was evaluated by the halo formation (inhibition zone) around the edges of the samples, for both methods. The appearance of the inhibition zone indicates the absence of bacteria growth, demonstrating the samples’ antibacterial effect. The diameter of inhibition zones formed around the edges of the samples was measured in order to infer about the potency of antibacterial activity. Three replicates of each condition were tested. 2.5.3. Water Contact Angle Measurement The water contact angle (WCA) was determined using a Contact Angle System (dataphysics) coupled to a high-resolution camera. A volume of 5 µL of distilled water was dispensed from the syringe onto the sample’s surface. Each sample was measured in 10 different places and the results were averaged to obtain both mean and standard deviations. 2.5.4. UV Radiation Resistance and Washability In order to evaluate the UV resistance ability of functionalized flax samples, they were placed under UV light (distance = 10 cm) for a total period of 80 h and colour measurements were assessed in distinct time points with a Datacolor spectrophotometer using the Cielab coordinates D65/10 software (Lucerne, Switzerland). The necessary CieLab parameters were determined: L* is the lightness from black (0) to white (100); a* indicates if the sample is redder (positive values) or greener (negative values); and, b* indicates if the sample is yellower (positive values) or bluer (negative


6 of 21

values). Finally, the overall/total difference (∆E*) between the evaluated samples (exposure to different times of UV radiation) and the initial ones (no exposure to UV radiation) were calculated using the following equation: ∆E = [(∆L∗)2 + (∆b∗)2 + (∆a∗)2 ]

1/2

(5)

To evaluate the wash durability of the Ag/ZnO NPs active coating onto flax fabric, the samples were placed in contact with distilled water and centrifuged at 200 rpm for 100 h, continuously. This procedure mimics the domestic washing programmes accordingly with the rpm values that were described in the standard ISO6330–Textiles, Domestic washing and drying, procedures for textile testing. In this standard, the agitation speed could range from 119 rpm to 179 rpm (delicate and durable press parameters, respectively). The main goal of this test was to show that the NPs are fixed onto the fabrics even with a continuous contact with water under rotation and not to detach the NPs from the fabrics. Therefore, a small volume of the washing solution was collected at specific time points (15 min, 30 min, 1 h, 2 h, 24 h, 72 h, and 100 h) and the absorbance was measured (spectrophotometer UV-1800, Shimadzu, wavelength range: 200–800 nm). 2.6. Statistical Analysis Statistical analysis was performed using a GraphPad prism (version 6.0). The Shapiro–Wilk test was used to evaluate the normal distribution of the samples. A parametric ANOVA was performed to evaluate the differences between the samples. Data are presented by mean ± SD. Statistical significance was considered if p < 0.05. 3. Results and Discussion 3.1. Synthesis and Characterization of Flax Fabrics Functionalized with AgNPs and ZnONPs As mentioned before, the main goal of this work was to functionalize cellulosic fibres, namely flax fabrics with Ag/ZnO hierarchical nanostructures to introduce multiple new properties onto these fabrics. In addition, always considering the sustainability of the processes, the methodologies scalability, and the durability. Initially, Ag NPs were used to develop a conductive surface and the ZnO NPs to introduce a better piezoresistive response. Overall, both NPs presented other functionalities that will be described further, such as: antibacterial activity and higher hydrophobicity. In this way, AgNPs were synthesized by an eco-friendly method that was previously described by authors [5]. In this method, the non-toxic and biodegradable PEG polymer was used as a reducing and stabilizing agent. The PEG hydroxyl groups are responsible for the progressive reduction of Ag+ ions into Ag0 , as demonstrated by the following equation [5]: CH2 CH2 OH + Ag+ → CH2 CHO + Ag0

(6)

PEG also acts as stabilizer and dispersing agent, because this polymer covers the surface of AgNPs, preventing their aggregation. Furthermore, hydrogen bonds can be formed between PEG groups and cellulose hydroxyl groups from flax, improving the incorporation of AgNPs onto the flax fabric. After this, in-situ synthesis of ZnONPs was performed onto AgNPs-treated flax fabrics, using a simple and low-cost method. In this process, NaOH acted as the reducing agent of zinc acetate (precursor) to produce ZnONPs, accordingly with the following reactions [17]: Zn(CH3 COO)2 ·2H2 O + 2NaOH → Zn(OH)2 + 2CH3 COONa + 2H2 O Zn(OH)2 + 2H2 O → Zn(OH)24− + 2H+ Zn(OH)24− → ZnO + H2 O + 2OH−

(7)


7 of 21

Several precursor (zinc acetate) concentrations ranging from 0.01 M to 0.4 M were used in order to infer about their influence in the NPs synthesis, deposition, and dispersion, as well as in the final properties, including piezoresistive response, antibacterial effect, hydrophobicity, and UV resistance. 3.1.1. Ground State Diffuse Reflectance (GSDR) GSDR was used to evaluate the fabrics functionalization with both NPs. The Kubelka–Munk remission function of flax fabrics functionalized with AgNPs and different concentrations of ZnONPs are shown in Figure 2. Each spectrum represents the mean of R values that were obtained for the measurement in three different places of samples. Nanomaterials 2018, 8, x FOR PEER REVIEW 7 of 22

2. Ground State Diffuse Reflectance (GSDR)spectra spectra of of flax silver Figure Figure 2. Ground State Diffuse Reflectance (GSDR) flax fabric fabrictreated treatedwith with silver nanoparticles (AgNPs) and different concentrations of zincnanoparticles oxide nanoparticles (ZnONPs). nanoparticles (AgNPs) and different concentrations of zinc oxide (ZnONPs).

Flax fabrics functionalized with AgNPs exhibit a peak atat approximately 437 nm, which indicates Flax fabrics functionalized with AgNPs exhibit a peak approximately 437 nm, which indicates the surface plasmon resonance (SPR) band formation of AgNPs. In the flax fabrics’ spectra treated with the surface plasmon resonance (SPR) band formation of AgNPs. In the flax fabrics’ spectra treated both Ag andAg ZnO NPs it is visible the appearance of a new band atband UV region (~380 nm), with both and ZnO NPs it is visible the appearance of absorption a new absorption at UV region (~380 characteristic of ZnONPs. Furthermore, the sharpthe shape of the ZnONPs band indicates nm), characteristic of ZnONPs. Furthermore, sharp shape of theabsorption ZnONPs absorption band that these NPs are monodispersed [21,22]. Nevertheless, as the concentration of ZnONPs precursor indicates that these NPs are monodispersed [21,22]. Nevertheless, as the concentration of ZnONPs increases (from 0.05 M(from to 0.40.05 M), there intensity,in broadening, andbroadening, shifting of the precursor increases M to is 0.4a decrease M), thereinisthe a decrease the intensity, and AgNPs absorption band.absorption This phenomenon canphenomenon be related with size ofincreasing NPs (as will beof shifting of the AgNPs band. This canthe be increasing related with the size shown with analysis), with the highestwith quantity of ZnONPs as compared AgNPs and NPs (as willFESEM be shown with FESEM analysis), the highest quantity of ZnONPswith as compared with with the presence agglomerates/nanoclusters [5,23]. At the same withtime, the second AgNPs and with of theAgNPs presence of AgNPs agglomerates/nanoclusters [5,23]. Attime, the same with the process functionalization), AgNPs that are not strongly the fabric surface second(ZnONPs process (ZnONPs functionalization), AgNPs thatsoare not so attached strongly to attached to the fabric are lost, decreasing the absorption band intensity. The in-situ of ZnONPs onto flaxonto fabric surface are lost, decreasing the absorption band intensity. Thesynthesis in-situ synthesis of ZnONPs flax is fabric performed after synthesis deposition of AgNPs onto flax. Firstly, AgNPs arethe incorporated is performed after and synthesis and deposition of AgNPs onto the flax. Firstly, AgNPs are onto the flax fabric, samples arethe washed and After theAfter flax fabrics with AgNPs incorporated onto the flax fabric, samples aredried. washed andthis, dried. this, the flaxthe fabrics with are subjected to the in-situ to synthesis of ZnONPs and using NaOH as reducing It is the AgNPs are subjected the in-situ synthesisinofsolution ZnONPs in solution and using NaOHagent. as reducing normal that not sothat strongly attached to theattached surface to arethe lost due agent.that It isduring normalthis thatprocess, during the thisAgNPs process, theare AgNPs are not so strongly surface toare thelost usedue of NaOH andofalso due and to the stirring which is why the intensity ofintensity the plasmon to the use NaOH also due toprocess, the stirring process,which is why the of the band decreases. plasmon the band is still there the fabrics stillthe present thestill strong yellow plasmon bandHowever, decreases.the However, plasmon bandand is still there and fabrics present the colour correspondent to the AgNPs presence. Moreover, with the minimum concentration of ZnONPs strong yellow colour correspondent to the AgNPs presence. Moreover, with the minimum precursor (flax +ofAgNPs + 0.01 MZnONPs), the absorption band of AgNPs is visible, indicating concentration ZnONPs precursor (flax +only AgNPs + 0.01 MZnONPs), only the absorption band of that this concentration is too low produce ZnONPs.is too low to produce ZnONPs. AgNPs is visible, indicating thattothis concentration

3.1.2. Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive Spectroscopy (EDS) The surface morphology of untreated and AgNPs-treated flax fabrics was further evaluated by FESEM and EDS analyses (Figure 3).


8 of 21

3.1.2. Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive Spectroscopy (EDS) The surface morphology of untreated and AgNPs-treated flax fabrics was further evaluated by FESEM and2018, EDS8,analyses (Figure 3). Nanomaterials x FOR PEER REVIEW 8 of 22

Figure 3. Scanning transmission electron detector (STEM) micrograph of the AgNPs synthesized in polyethylene glycol (PEG) solution with magnifications of 500 nm (A). Field Emission Scanning Figure 3. Scanning transmission electron detector (STEM) micrograph of the AgNPs synthesized in Electron Microscopy imageswith of untreated flax (B) of and500 flaxnm + AgNPs (C) with magnifications polyethylene glycol (FESEM) (PEG) solution magnifications (A). Field Emission Scanning of 1 mm,Microscopy 50 µm and 10 µm (from the left to the right). images in topographic mode. Energy Electron (FESEM) images of untreated flax The (B) and flax are + AgNPs (C) with magnifications Dispersive (EDS) spectrum with AgNPs (D). of 1 mm, 50Spectroscopy µm and 10 µm (from the leftof toflax the fabric right).treated The images are in topographic mode. Energy

Dispersive Spectroscopy (EDS) spectrum of flax fabric treated with AgNPs (D).

Figure 3A presents the STEM analysis of the AgNPs that were synthesized using the PEG solution. This Figure image reveals the successful withthat sizes ranging from 23.4 to 120.9 nm. 3A presents the STEMsynthesis analysis of of the theNPs, AgNPs were synthesized using the PEG After the in-solution synthesis, the NPs were incorporated onto the flax fabric, as can be seen in solution. This image reveals the successful synthesis of the NPs, with sizes ranging from 23.4 to 120.9 Figure 3C. The presence of the AgNPs onto the flax surface is visible when compared with the nm. After the in-solution synthesis, the NPs were incorporated onto the flax fabric, as can be seen in untreated fabric showed 3B.onto Besides thecompared flax fabric,with and the the Figure 3C.flax The presence of in theFigure AgNPs thethe flaxpresence surfaceofisAgNPs visible onto when presence of an excessive quantity of PEG coating the flax surface is also notable. The use of PEG is untreated flax fabric showed in Figure 3B. Besides the presence of AgNPs onto the flax fabric, and very useful inofthe to their dispersive character. However, during ZnONPs the presence ansynthesis excessiveprocess quantitydue of PEG coating the flax surface is also notable. Thethe use of PEG in-situ synthesis, the excess of PEG is eliminated, as can be seen in Figure 4A. EDS spectra (Figure 3D) is very useful in the synthesis process due to their dispersive character. However, during the ZnONPs shows the presence of several peaks, namely those of elemental carbon and oxygen, which are the in-situ synthesis, the excess of PEG is eliminated, as can be seen in Figure 4A. EDS spectra (Figure main constituents of NCF.ofAn additional peak appeared at 2.99 keV, which be attributed to 3D) shows the presence several peaks,strong namely those of elemental carbon andcan oxygen, which are silver [5,9,24], confirming the presence of Ag onto the flax fabric surface. the main constituents of NCF. An additional strong peak appeared at 2.99 keV, which can be After to in-situ of ZnONPsthe onto AgNPs-treated FESEM and EDS analyses attributed silversynthesis [5,9,24], confirming presence of Ag ontoflax thefabric, flax fabric surface. (Figure 4) were performed to evaluate the surface morphology and elemental composition of the After in-situ synthesis of ZnONPs onto AgNPs-treated flax fabric, FESEM and EDS analyses samples, respectively. (Figure 4) were performed to evaluate the surface morphology and elemental composition of the

samples, respectively.

Nanomaterials 2018, 8, 1069 Nanomaterials 2018, 8, x FOR PEER REVIEW

9 of 21 9 of 22

Figure images of of flax Figure4.4. FESEM FESEM images flaxfabric fabricfunctionalized functionalizedwith withboth bothAgNPs AgNPsand andZnONPs ZnONPs(A). (A). Magnifications ofof 1 mm, 50 50 µm, 10 µm, andand 4 µm (from the left thetoright). EDS spectra of Z1 zone Magnifications 1 mm, µm, 10 µm, 4 µm (from theto left the right). EDS spectra of Z1 (B) and(B) Z2and zone zone Z2(C). zone (C).

Figure4 4clearly clearlydemonstrates demonstratesthe thepresence presenceofofNPs NPson onthe thesurface surfaceofofflax flaxfabrics, fabrics,showing showinga adense dense Figure anduniform uniformdeposition deposition of of Ag and distinct morphologies of NPs can can be seen: and and ZnO ZnONPs. NPs.Moreover, Moreover,two two distinct morphologies of NPs be spheres and platelets, which which can be attributed to ZnONPs AgNPs, respectively, as confirmedasby seen: spheres and platelets, can be attributed to and ZnONPs and AgNPs, respectively, EDS analysis thatanalysis was performed each zone:inZ1 (platelets: 4B) and Z2 4B) (spheres: confirmed by EDS that wasspecifically performed in specifically each zone: Z1Figure (platelets: Figure and Figure 4C). Figure As shown spherical sizeshave ranging 58.3from to 223.9 Z2 (spheres: 4C). in AsFigure shown4A, in Figure 4A, particles sphericalhave particles sizesfrom ranging 58.3 nm, to while platelets present diameter in the range from 600 to 684.2 nm and thickness of 91 nm. These 223.9 nm, while platelets present diameter in the range from 600 to 684.2 nm and thickness of 91 nm. results suggest that ZnONPs synthesis process, namely the use ofuse temperature, influences the AgNPs These results suggest that ZnONPs synthesis process, namely the of temperature, influences the shape, shape, which which changed from relatively spherical (Figure (Figure 3) to platelets (Figure 4). It has4). been described AgNPs changed from relatively spherical 3) to platelets (Figure It has been that temperature could have a key role in the growth,growth, size, and shape of AgNPs. In fact, described that temperature could have a key roleformation, in the formation, size, and shape of AgNPs. al. [25] demonstrated that thethat increase of reaction temperature promoted an increase AgNPs InJiang fact,et Jiang et al. [25] demonstrated the increase of reaction temperature promoted an in increase Moreover, the authors increasing temperature led to a reduction quantityinof insize. AgNPs size. Moreover, thereported authors that reported that increasing temperature led to a in reduction particlesofwith spherical shape andshape an increase platelets particles,particles, which iswhich in agreement with the quantity particles with spherical and an of increase of platelets is in agreement results are that shown this work (Figure The4A). increase in NPs size is probably due to thedue fusion with the that results arein shown in this work 4A). (Figure The increase in NPs size is probably to growth process of small particles into larger ones, which is promoted by the heating process [25]. the fusion growth process of small particles into larger ones, which is promoted by the heating process [25]. In this way, the temperature that is used in ZnONPs synthesis (50 °C) could be responsible for the formation of AgNPs with increased size and platelets shape.


10 of 21

used in ZnONPs synthesis (50 ◦ C) could be responsible 10 forofthe 22 formation of AgNPs with increased size and platelets shape. In In EDS EDS analysis analysis (Figure (Figure 4C), 4C), besides besides the the presence presence of of Ag Ag related related peaks, peaks, there there is is also evidence evidence of of strong strong signals signals of of Zn Zn and andO, O,thus thusconfirming confirmingthe thepresence presenceof ofZnONPs ZnONPsonto ontoAgNPs-treated AgNPs-treatedflax flaxfabrics. fabrics. Three Three new peaks appeared at 1.03, 8.67, and 9.69 keV that are attributed to Zn. These These results results are are concordance concordance with previous studies and confirm the formation of ZnONPs [26].

In this way,2018, the8,temperature that is Nanomaterials x FOR PEER REVIEW

3.1.3. X-ray X-ray Diffraction Diffraction (XRD) (XRD) 3.1.3. Figure 55 shows shows the theXRD XRDpattern patternof ofuntreated untreatedflax flaxfabric fabric(A), (A),AgNPs-treated AgNPs-treatedflax flaxfabric fabric(B), (B),and and Figure Ag/ZnONPs-treatedflax flaxfabric fabric(C). (C). Ag/ZnONPs-treated

Figure Figure 5. XRD XRD spectra spectra of of flax flax fabric: fabric: untreated untreated (A), (A), treated treated with with AgNPs AgNPs (B) (B) and and treated treated with with Ag Ag and and ZnO NPs NPs (C). (C). ZnO ◦ ◦ 22.96◦ , and Flax fabric fabric presents presents four four diffraction diffraction patterns patterns that that are located at 2θ == 14.92 Flax 14.92°,, 16.68 16.68°,, 22.96°, and ◦ 34.41 , which correspond to the ( − 1 1 0), (1 1 0), (2 0 0), and (0 0 4) planes, respectively, of crystalline 34.41°, which correspond to the (−1 1 0), (1 1 0), (2 0 0), and (0 0 4) planes, respectively, of crystalline domains of of cellulose cellulose II [17], [17], the the main main constituent constituent of of NCF. NCF. Flax Flax fabric fabric treated treated with with AgNPs AgNPs clearly clearly domains ◦ ◦ ◦ 77.86◦ , demonstrates the the appearance appearance of of new peaks at 2θ 82.14◦ , which which demonstrates 2θ == 38.01 38.01°,, 44.29 44.29°,, 64.69 64.69°,, 77.86°, and 82.14°, correspond to (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of the face-centered cubic (FCC) lattice of correspond 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of the face-centered cubic (FCC) lattice ◦ silver [8].[8]. TheThe synthesis of ZnONPs ontoonto this this fabric revealed the formation of newofpeaks at 2θ =at32.21 of silver synthesis of ZnONPs fabric revealed the formation new peaks 2θ =, ◦ ◦ ◦ ◦ ◦ 35.09 , 35.09°, 37.05 , 37.05°, 47.61 , 47.61°, 57.33 , and 67.89 patternspatterns correspond to (1 0 0),to (0(1 0 2), (1 (0 0 1), 32.21°, 57.33°, and. These 67.89°.diffraction These diffraction correspond 0 0), 0 (1 0(12), (1 1(10), and(1(11 10),2)and planes wurtzite ZnO structure, [17,27]. Moreover, 2), 0 1), 0 2), (1 1of 2) hexagonal planes of hexagonal wurtzite ZnO respectively structure, respectively [17,27]. Ag diffraction arepatterns maintained high crystalline nature of both nature particles Moreover, Ag patterns diffraction are confirming maintainedthe confirming the high crystalline of when both incorporated onto flax fabrics. particles when incorporated onto flax fabrics.

3.1.4. Attenuated Attenuated Total Total Reflectance-Fourier Reflectance-Fourier Transform 3.1.4. Transform Infrared Infrared Spectroscopy Spectroscopy (ATR-FTIR) (ATR-FTIR) The ATR-FTIR ATR-FTIR spectra spectra of of flax, flax, flax flax coated coated with with AgNPs, AgNPs, and and flax flax functionalized functionalized with with Ag Ag and and ZnO ZnO The NPs are shown in Figure 6. NPs are shown in Figure 6.


11 of 21 11 of 22

Figure Figure 6. 6. Attenuated Attenuated Total Total Reflectance-Fourier Reflectance-Fourier Transform Transform Infrared Infrared Spectroscopy Spectroscopy (ATR-FTIR) (ATR-FTIR) spectra spectra of of flax flax fabric fabric (A), (A), flax flax functionalized functionalized with with AgNPs AgNPs (B), (B), and and flax flax functionalized functionalized with with AgNPs AgNPs and and ZnONPs (C). ZnONPs (C).

The The ATR-FTIR ATR-FTIR spectra spectra of of the the untreated untreated flax flax fabric fabric show show the the typical typical band band peaks peaks of of the the main main − 1 constituents constituents of of NCF: NCF:cellulose, cellulose,hemicellulose, hemicellulose,and andlignin. lignin.The Theband bandatat3333 3333cm cm−1 corresponds corresponds to to O-H O-H stretching, which indicates the presence of absorbed water molecules or hydroxyl groups from cellulose stretching, which indicates the presence of absorbed water molecules or hydroxyl groups from −1 can be attributed to the asymmetric C-H stretching vibration and ligninand [5,28], the [5,28], band at 2916 cmat cellulose lignin the band 2916 cm−1 can be attributed to the asymmetric C-H stretching 1 is related with the C=O stretching vibration of of celluloseofand hemicellulose, the band at the 1732band cm−at vibration cellulose and hemicellulose, 1732 cm−1 is related with the C=O stretching the hemicellulose and lignin, the band at approximately cm−1 represents vibration of the hemicellulose and lignin, the band at1636 approximately 1636 C=C cm−1 stretching represents from C=C −1 was attributed to C-H wagging vibration, the band at 1366 cm−1 might lignin, the band at 1427 cm stretching from lignin, the band at 1427 cm−1 was attributed to C-H wagging vibration, the band at −1 represented C-O-C vibration, while the band at be assigned to thebe C-H bending, the C-H bandbending, at 1157 cm 1366 cm−1 might assigned to the the band at 1157 cm−1 represented C-O-C vibration, − 1 1026 cm was due to C-O stretch vibration [5,9,29]. −1 while the band at 1026 cm was due to C-O stretch vibration [5,9,29]. ATR-FTIR ATR-FTIR spectra spectra of of AgNPs-treated AgNPs-treated flax flax fabrics fabrics revealed revealed the the appearance appearance of of several several new new peaks, peaks, −1 . Moreover, the band at 1636 cm−1 which are attributed to PEG, namely 1246, 945, and 887 cm −1 −1 which are attributed to PEG, namely 1246, 945, and 887 cm . Moreover, the band at 1636 cm shifted shifted cm−1corresponds , which corresponds to carbonyl groupformation. (C=O) formation. Ag ions be −1, which to 1647 to cm1647 to carbonyl group (C=O) Some AgSome ions could becould reduced 0 reduced Aghydroxyl by the hydroxyl groups fromcomponents cellulose components of flax fabrics. If this oxidation to Ag0 bytothe groups from cellulose of flax fabrics. If this oxidation occurs, then occurs, then the band correspondent to the carbonyl group (C=O) formation of cellulose will appear the band correspondent to the carbonyl group (C=O) formation of cellulose will appear at at −1 [5,30]. approximately approximately 1647 1647 cm cm−1 [5,30]. Synthesis Synthesis of of ZnONPs ZnONPs onto onto flax flax fabrics fabrics led led to to the the formation formation of of new new peaks. peaks. The The bands bands peaking peaking −1 are related to asymmetric and symmetric stretching vibrations of C=O around 1558 and 1400 cm −1 around 1558 and 1400 cm are related to asymmetric and symmetric stretching vibrations of C=O group, ZnONPs precursor (zinc acetate) thatthat waswas usedused in the [31]. group, which whichmay maybebeattributed attributedtoto ZnONPs precursor (zinc acetate) in reaction the reaction −1 represents Zn–O stretching vibration, The appearance of new peaks at 671, 613, and 417 cm −1 [31]. The appearance of new peaks at 671, 613, and 417 cm represents Zn–O stretching vibration, confirming confirming the the presence presence of of ZnONPs ZnONPs onto onto flax flax fabric fabric surface surface [6,28,29,32,33]. [6,28,29,32,33]. Since Since the the band band peak peak related related with with AgNPs AgNPs formation formation is is very very close close to to the the peaks peaks of of zinc zinc acetate acetate (precursor), (precursor), these these peaks peaks can can mask Ag peaks. Moreover, it is important to refer that FTIR spectrum shown in Figure 6C corresponds mask Ag peaks. Moreover, it is important to refer that FTIR spectrum shown in Figure 6C to the sample to treated with the treated highest with zinc acetate concentration (0.4 M), which originates with corresponds the sample the highest zinc acetate concentration (0.4 peaks M), which higher intensity. conclusion, all of these clearly demonstrate the presence of both Ag and originates peaksIn with higher intensity. In findings conclusion, all of these findings clearly demonstrate the ZnONPs onto the flax fabrics. presence of both Ag and ZnONPs onto the flax fabrics.


12 of 21 12 of 22

3.1.5. Thermogravimetric Analysis (TGA) TGA analysis was performed in order to estimate the residual weight of the untreated (without NPs) and treated treated (with (withNPs) NPs)samples, samples,and andtherefore, therefore,totoinfer infer about percentage NPs deposited about thethe percentage of of NPs deposited on on the flax fabric. Flax samples without any treatment were analysed as well as the samples the flax fabric. Flax samples without any treatment were analysed as well as the samples containing containing 0.1+ MAgNPs + 0.2 After MZnONPs. up residues to 600 °Cwere the residues were 0.1 MAgNPs 0.2 MZnONPs. heatingAfter up toheating 600 ◦ C the considered. Theconsidered. amount of The NPs on the flax fabric was quantified using equation 2. The flax fabric NPs amount depositedofon the deposited flax fabric was quantified using equation 2. The flax fabric without NPs revealed without NPs revealed 17.72% wt of residues (average value) and the treated samples with NPs 17.72% wt of residues (average value) and the treated samples with NPs exhibited 30.59% wt. While exhibited 30.59% wt. 2While considering 2 and assuming that the by final is 100% considering equation and assuming that equation the final sample is 100% composed flaxsample fabric and NPs, composed by flax fabric and NPs, the percentage of NPs15.6 onto is approximately 15.6 % wt. the percentage of NPs onto the sample is approximately % the wt. sample (considering the flax fabric residues (considering thesample flax fabric and the sample total residues 30.59%). 17.72% and the totalresidues residues17.72% 30.59%). 3.2. Multifunctional Flax Fabrics The development of fibre-based sensors requires a conductive surface. In fact, the main goal of to take take advantage advantage of of their their electrical electrical conductivity. conductivity. The The functionalization functionalization using AgNPs in this system is to values of the fibres, as already discussed in a of NCF with AgNPs reduces the electrical resistivity values previous research work in which functionalization of jute fabrics with AgNPs decreased the fabrics Ω·mto to 1.02 1.02 ××10 103 3Ω·m Ω·m(using (using0.1 0.1M M precursor precursor concentration) concentration) [5]. [5]. resistivity values from 1.5 1.5 × × 1077 Ω·m Therefore, in this work, we decided to use the same concentration of silver precursor in order to obtain conductive flax fabrics and introduce the ZnONPs for the piezoresistive behaviour and other functionalities. In In this this way, way, the functionalization of flax fabrics with AgNPs resulted in an electrical functionalities. 1033Ω·m. Ω·m. resistivity value of 3.33 × × 10 3.2.1. Strain Strain Sensing Sensing Mechanism Mechanism of of Flax Flax Fabrics Fabrics 3.2.1. To evaluate To evaluate the the possibilities possibilities of of using using flax flax fabrics fabrics in inpiezoresistive piezoresistivesensors, sensors,Ag/ZnO-modified Ag/ZnO-modified samples were were evaluated evaluated by by measuring It has has samples measuring the the electrical electrical response response under under mechanical mechanical compression. compression. It been shown that the percolation threshold corresponds to the region with the largest GF in carbon been shown that the percolation threshold corresponds to the region with the largest GF in carbon nanotubes/polymercomposites composites [34], [34], due due to to the the fact fact that that close close to to the the percolation percolation threshold threshold an an applied applied nanotubes/polymer deformation is is able able to to induce induce strong strong and and reversible reversible variations variations in in the the nanotube nanotube network network configuration configuration deformation (e.g., variations of the nanotubes relative distance) [35], which has a strong influence in the variation variation (e.g., variations of the nanotubes relative distance) [35], which has a strong influence in the of the electrical resistivity. Far from the percolation threshold, network variations are smaller and of the electrical resistivity. Far from the percolation threshold, network variations are smaller and therefore their Following aa similar similar idea, idea, conducting conducting therefore their effect effect in in the the electrical electrical response response is is also also small small [36]. [36]. Following AgNPs should also reveal strain sensitivity, opening new possibilities to be explored within the field field AgNPs should also reveal strain sensitivity, opening new possibilities to be explored within the of sensing sensing applications. applications. For For that, that, all all of of the the samples samples (thickness (thickness 11 mm) mm) were were compressed compressed 0.5 0.5 mm, of mm, and and the change of the electrical resistance was recorded during 10 loading and unloading cycles. Figure the change of the electrical resistance was recorded during 10 loading and unloading cycles. Figure7 precursor 7shows showsthe theelectromechanical electromechanicalresponse responseand andGF GFvalues valuesof offlax flaxfabrics, fabrics, according according to to the the ZnO ZnO precursor concentration under under use use (from concentration (from 0.05 0.05 to to 0.4 0.4 M). M).

Figure 7. Cont.


Nanomaterials 2018, 8, x FOR PEER REVIEW

13 of 21

13 of 22

Electromechanical response Figure 7. Electromechanical response of flax treated with AgNPs (A), flax treated with AgNPs and different ZnONPs ZnONPsprecursor precursorconcentration concentrationfrom from 0.05 M (B–E), respectively, gauge different 0.05 to to 0.40.4 M (B–E), respectively, and and gauge factorfactor (GF) (GF) values (F). values (F).

application of of mechanical mechanicalpressure pressureonto ontothe theAg/ZnO-treated Ag/ZnO-treated fabrics causes a reduction The application fabrics causes a reduction in in flax thickness, which induces electro-conductive particles movement, leading a change in flax thickness, which induces thethe electro-conductive particles movement, leading to a to change in flax flax electrical resistance [4]. shown As shown in Figure treated with only AgNPs is able electrical resistance [4]. As in Figure 7A, 7A, flaxflax thatthat waswas treated with only AgNPs is able to to change electrical resistanceunder undercompression. compression.However, However,the thevariation variationon on resistance resistance is is very change itsits electrical resistance small/weak, sensors. OnOn thethe other hand, doping Ag small/weak, making this this sample samplenot notsuitable suitablefor forpiezoresistive piezoresistive sensors. other hand, doping with ZnO significantly increases the variation in electrical resistance under strain, improving the flax Ag with ZnO significantly increases the variation in electrical resistance under strain, improving the piezoresistive response, leading to antoincrease of theofGF 0.7 ±0.7 0.2±to (Figure 7F).7F). flax piezoresistive response, leading an increase thefrom GF from 0.21.2 to± 1.20.2 ± 0.2 (Figure The physical phenomenon behind the electromechanical response can be explained by the interactions between the flax fabrics and ZnO Because of of the the much higher Young’s ZnO NPs NPs fillers. fillers. Because modulus of ZnONPs (~127 GPa) [37] as compared to that of the flax fabrics (486 MPa) [38], ZnONPs can be regarded as rigid elements elements during during the the strain/release strain/releasecycles. cycles.During Duringthe thestrain straincycle, cycle,the the flax+ flax +AgNPs AgNPs+ +ZnONPs ZnONPscomposite compositeisisunder undercompressive compressivestress stressin inthe the transverse transverse direction direction of of stretching, stretching, causing ZnONPs to separate out of the compressive plane plane and and the electrical electrical resistivity resistivity to to increase. increase. However, for all samples, except the flax + AgNPs + 0.2 MZnONPs, after repeated cycles, a slight drop However, for all samples, except the flax + AgNPs + 0.2 MZnONPs, after repeated cycles, a slight in both intensity and offset of theofmaximum of theofpeak is seen in theinnext few cycles, due drop in the bothpeak the peak intensity and offset the maximum the peak is seen the next few cycles, possibly, to stress relaxation. The ∆R/R -time0curve seems seems to be stabilized after a few cycles, is due possibly, to stress relaxation. The0∆R/R -time curve to be stabilized after a fewwhich cycles, synchronized with the response of stress under cyclic strain. which is synchronized with the response of stress under cyclic strain. + AgNPs + 0.05 MZnONPs sample, increase electricalresistance resistance with with applied For flax flax+AgNPs+0.05MZnONPs sample, thethe increase ininelectrical compression maybe interpreted, as follows. With applied loading, discontinuities in the conductive pathways start to appear within the the flax flax ++ AgNPs AgNPs ++ ZnONPs composite, and the amount of discontinuities increases with an increase of applied load, which results in the increasing of electrical resistance. After After releasing releasingthe theflax flax++AgNPs AgNPs++ZnONPs ZnONPscomposite composite strain-free condition, most toto itsits strain-free condition, most of of disconnected conductive pathways recovertototheir theirinitial initialstates. states. However, However, the the some broken thethe disconnected conductive pathways recover AgNPs a permanent contact disruption, which manifested as the as increase of overall AgNPs ++ZnONPs ZnONPsresult resultinin a permanent contact disruption, which manifested the increase of electrical resistance indicated by the by ∆R/R offset. On the hand, thethe sample flax + AgNPs overall electrical resistance indicated the 0∆R/R 0 offset. On other the other hand, sample flax + AgNPs+ + 0.2 MZnONPs show a reversible electrical resistance and this reversibility is maintained after the


Nanomaterials 2018, 8, x FOR PEER REVIEW

14 of 21

14 of 22

0.2 MZnONPs show a reversible electrical resistance and this reversibility is maintained after the 10 10 strain/release cycles, which means thatamount this amount of ZnONPs could be capable of detecting strain/release cycles, which means that this of ZnONPs could be capable of detecting different different movements/pressures. movements/pressures. To evaluatethe thesensitivity sensitivityofoffunctionalized functionalizedflax flaxsamples, samples,GF GF were calculated through slope To evaluate were calculated through thethe slope of of the fraction between the variation in the electrical resistance and the mechanical deformation the fraction between the variation in the electrical resistance and the mechanical deformation applied, applied, accordingly with number equation4 (Figure number7F). 4 (Figure 7F). From these it is clearly visible that accordingly with equation From these results, it is results, clearly visible that the addition the addition of ZnONPs increases the sensor sensitivity, while it also improves the response of ZnONPs increases the sensor sensitivity, while it also improves the response and recovery timeand by recovery time by increasing the charge transport speed. Moreover, the use of the highest ZnO increasing the charge transport speed. Moreover, the use of the highest ZnO precursor concentration precursor concentration to higherthe GFflax values, increasing theand flaxmaking fabric sensitivity making it led to higher GF values,led increasing fabric sensitivity it suitable and to be used as suitable to be used as piezoresistive textile sensors. piezoresistive textile sensors.

3.2.2. Antibacterial 3.2.2. Antibacterial Activity Activity Several studies studies report report that that AgNPs antibacterial activity activity against against Several AgNPs and and ZnONPs ZnONPs exhibit exhibit remarkable remarkable antibacterial broad spectrum spectrum of of bacteria bacteria [6,8,9,17,39]. [6,8,9,17,39]. Although Although the the mechanisms mechanisms underlying underlying the the antibacterial antibacterial aa broad activity of of these these NPs NPs are are not not yet yet fully fully understood, understood, the the proposed proposed main main ones ones include include the the disruption disruption of of activity bacterial cell cell membrane, membrane, induction induction of ROS production production leading leading to oxidative stress, stress, metal metal ion ion release release bacterial of ROS to oxidative + and Zn2+ + 2+ (Ag ), damage to DNA and proteins, among others [39–42]. (Ag and Zn ), damage to DNA and proteins, among others [39–42]. In the present present study, study, antibacterial antibacterial activity activity of of NPs NPs solutions solutions (Figure (Figure 8) 8) and and functionalized functionalized flax flax In the fabrics (Figure Gram-negative (E. (E. coli)coli) andand Gram-positive (S. aureus). The fabrics (Figure 9) 9)was wasevaluated evaluatedagainst against Gram-negative Gram-positive (S. aureus). antibacterial effect was assessed by the formation of a clear inhibition zone (halo) around the samples, The antibacterial effect was assessed by the formation of a clear inhibition zone (halo) around the which indicates the absence bacteria growth. growth. Moreover, the size the of the zone, which samples, which indicates the of absence of bacteria Moreover, sizeinhibition of the inhibition zone, indicates the antibacterial effect level, calculated (Table 1)(Table [20]. Firstly, agar well diffusion which indicates the antibacterial effectwas level, was calculated 1) [20].the Firstly, the agar well method was used to test the antibacterial activity of the solutions containing AgNPs and ZnONPs. diffusion method was used to test the antibacterial activity of the solutions containing AgNPs and The resultsThe areresults shownare in Figure ZnONPs. shown 8. in Figure 8.

Figure 8. Comparison Comparisonof ofinhibition inhibitionzone zoneofofdifferent different solutions against S. aureus bacteria. solutions against E. E. colicoli andand S. aureus bacteria. (1) (1) PEG; AgNPs solution AgNO with PEG); (3) Distilled water; (4) ZnONPs solution (0.2 PEG; (2) (2) AgNPs solution (0.1(0.1 MM AgNO 3 with PEG); (3) Distilled water; (4) ZnONPs solution (0.2 M 3 M Zn(CH distilled water); (5) ZnONPs solution M Zn(CH ·2H 3COO) 2.2H O with distilled water); and,and, (5) ZnONPs solution (0.4 (0.4 M Zn(CH 3COO) 2.2H22O with Zn(CH 3 COO) 2 ·22H 2 O with 3 COO) 2O with distilled water). distilled water).

As shown 8, 8, thethe solvents thatthat werewere used used for thefor synthesis of AgNPs (well 1) and As shownininFigure Figure solvents the synthesis of AgNPs (wellZnONPs 1) and (well 3) show a dense population of bacterial colonies around the samples for both bacteria, indicating ZnONPs (well 3) show a dense population of bacterial colonies around the samples for both bacteria, no antibacterial effect. On the other the suspensions containingcontaining AgNPs (well 2) and ZnONPs indicating no antibacterial effect. Onhand, the other hand, the suspensions AgNPs (well 2) and (wells 4 and 5) clearly display the formation of an inhibition zone. The zone. inhibition zone size similarity ZnONPs (wells 4 and 5) clearly display the formation of an inhibition The inhibition zone size suggests the antibacterial activity ofactivity AgNPsof solutions is identical both E.for coli andE.S.coli aureus. similaritythat suggests that the antibacterial AgNPs solutions is for identical both and In contrast, to have a greater effect on S.effect aureus S. aureus. InZnONPs contrast, solutions ZnONPs seem solutions seem to haveantibacterial a greater antibacterial onasS.compared aureus as to E. coli, since halo diameter is higher for Gram-positive bacteria, whichbacteria, can be explained, even compared to E.the coli, since the halo diameter is higher for Gram-positive which can be partially, by the difference in composition of each bacteria cell wall [17,43,44]. Another important explained, even partially, by the difference in composition of each bacteria cell wall [17,43,44].

Another important assumption is that the antibacterial effect of ZnONPs depends on physicochemical properties of NPs, including size, shape, solubility, and the ability to form free biocidal metal ion [39].


15 of 21

assumption is that the antibacterial effect of ZnONPs depends on physicochemical properties of NPs, including size, and the ability to form free biocidal metal ion [39]. Nanomaterials 2018,shape, 8, x FORsolubility, PEER REVIEW 15 of 22

Figure 9. Antibacterial Antibacterial activity activity of of flax (A), flax + AgNPs (B), flax + AgNPs ++ 0.2 0.2 MZnONPs MZnONPs (C) (C) and flax ++ AgNPs AgNPs ++0.4 0.4MZnONPs MZnONPs(D) (D)for forE. E.coli coliand and S. S. aureus. aureus. Table 1. Inhibition zone diameter of functionalized flax fabrics against E. coli and S. aureus bacteria Table 1. Inhibition zone diameter of functionalized flax fabrics against E. coli and S. aureus bacteria accordingly with the JIS L 1902–Halo Method. Values are represented by mean ± standard deviation accordingly with the JIS L 1902–Halo Method. Values are represented by mean ± standard deviation of triplicates. of triplicates.

Samples

Samples

Flax FlaxFlax + AgNPs Flax + AgNPs Flax + AgNPs + 0.2 MZnONPs Flax + AgNPs + 0.2 MZnONPs Flax AgNPs++0.4 0.4MZnONPs MZnONPs Flax ++ AgNPs

Mean Diameter of the Halos (mm) Mean Diameter of the Halos (mm) E. coli S. aureus E. coli S. aureus 0 0 0 4 ± 0.08 6.33 ± 00.33 46 ± 0.08 6.33 ± 0.33 ± 0.22 9 ± 0.28 6 ± 0.22 9 ± 0.28 14.33 ± 0.09 0.09 13.67 14.33 ± 13.67±±0.09 0.09

As expected, untreated flax fabrics were unable to inhibit the growth of both bacteria tested As 9A). expected, untreated flax fabrics were unable to inhibit thetreated growthsamples of both(Figure bacteria9 tested (Figure Otherwise, inhibition zones are clearly visible around B–D), (Figure 9A). inhibition zones visible around treated (Figure 9B–D), showing the Otherwise, efficacy of both Ag and ZnOare NPsclearly as antibacterial agents, whensamples incorporated onto flax showingInterestingly, the efficacy of Ag and ZnO NPsonto as antibacterial agents, incorporated onto flax fabrics. theboth addition of ZnONPs AgNPs-treated flaxwhen induces an improvement in fabrics. Interestingly, the addition of ZnONPs onto AgNPs-treated flax induces an improvement in the the antibacterial activity of the fabric, as shown by the halo size increase (Table 1). In addition, with antibacterial activity of the fabric, as shown by halo sizeeffect increase (Table 1).against In addition, with the the increase of the precursor concentration, thethe inhibitory is stronger both bacteria. increase of the precursor concentration, the inhibitory effect is stronger against both bacteria. These These results show that, besides the antibacterial effect of synthesized NPs, when these NPs are results show that, besides the antibacterial effect of synthesized NPs, when these NPs are combined combined onto flax fabrics, the antibacterial properties are maintained, allowing for the production onto flax fabrics, the antibacterial properties are maintained, of a natural fibre-based textile with antibacterial properties. allowing for the production of a natural fibre-based textile with antibacterial properties. 3.2.3. Hydrophobicity Properties 3.2.3. Hydrophobicity Properties The wettability of the flax fabrics’ surface is dependent of chemical composition and geometrical The wettability of the flax fabrics’ surface is dependent of chemical composition and geometrical structure (related with roughness) of the surface [6,24,45]. This property was examined by measuring structure (related with roughness) of the surface [6,24,45]. This property was examined by measuring the WCA in samples’ different locations. Figure 10 shows the images (A-C) and the mean (D) of WCA the WCA in samples’ different locations. Figure 10 shows the images (A-C) and the mean (D) of obtained for untreated flax, flax coated with AgNPs, and flax functionalized with both AgNPs and WCA obtained for untreated flax, flax coated with AgNPs, and flax functionalized with both AgNPs ZnONPs. If the WCA is smaller or higher than 90°, ◦the surface is considered as hydrophilic or and ZnONPs. If the WCA is smaller or higher than 90 , the surface is considered as hydrophilic or hydrophobic, respectively. hydrophobic, respectively.

Nanomaterials 2018, 8, x8,FOR Nanomaterials 2018, 1069PEER REVIEW

16 of 16 22 of 21

Figure 10. Representative images of water contact angle (WCA) of untreated flax (A), flax + AgNPs Figure 10. Representative images of water contact angle (WCA) of untreated flax (A), flax + AgNPs (B) and flax + AgNPs + 0.4 MZnONPs (C). Comparison between the WCA values (D). Values are (B)represented and flax + by AgNPs MZnONPs (C). Comparison between the WCA (D). Values are mean+±0.4 SD. Flax + AgNPs vs. flax + AgNPs + ZnONPs: ****:values p < 0.0001. represented by mean ± SD. Flax + AgNPs vs. flax + AgNPs + ZnONPs: ****: p < 0.0001.

As shown in Figure 10A, the water drops that were released onto untreated flax fabrics were As shown absorbed. in Figure 10A, water dropsspread that were released flax fabrics were immediately Waterthe drops quickly on the surfaceonto anduntreated the samples were completely ◦ immediately absorbed. Water drops quickly spread on the surface and the samples were completely wetted, resulting in a WCA of 0 . These results were as expected due to the abundance of hydroxyl wetted, resulting in a WCAstructure of 0°. These results were as expected the due fabrics to the abundance of hydroxyl groups in the cellulose of flax fabrics. Moreover, pretreatment before the groups in the led cellulose structureof of flax fabrics. Moreover, thelignin, fabricsfat, pretreatment the experiments to a removal several impurities, including and waxes before contributing experiments led to a removal of several impurities, including lignin, fat, and waxes contributing to to the hydrophilic character of the flax surface [46]. the hydrophilic character of the flax surface [46]. With the addition of AgNPs and ZnONPs, the WCA is continuously increasing, as it can be seen With addition of AgNPs and ZnONPs, theofWCA is continuously increasing, as it can besurface seen in Figurethe 10B,C. As previously mentioned, one the factors that influences the WCA is the in roughness. Figures 10B,C. As previously mentioned, one of the factors that influences the WCA is the surface Untreated flax fabrics present a smooth surface, as shown in Figure 3B. On the other side, roughness. flax fabrics surface, as shown in Figure4A). 3B. In Onfact, the the other side, when NPsUntreated are incorporated ontopresent fabricsaasmooth new roughness is created (Figure synthesis when NPs are ontodemonstrating fabrics a new roughness created (Figure 4A). In fact, the synthesis of AgNPs ledincorporated to higher WCA, the increaseisin the fabrics’ hydrophobicity. Since the PEG of polymer AgNPs led to higher WCA, demonstrating the increase in the fabrics’ hydrophobicity. the is hydrophilic, the increase in WCA is only due to the addition of AgNPs, which Since contributes PEG polymer hydrophilic, the increase in WCA is only duecharacter to the addition of AgNPs, which to the surfaceisroughness improving the surface hydrophobicity [8,47]. However, the obtained ◦ contributes to the surface roughness improving the surface hydrophobicity character [8,47]. WCA was around 58 , which means that the fabrics still have hydrophilic properties. This result However, the obtained WCA was around 58°, which means that the fabrics still have hydrophilic is consistent with the study of Rajavel et al. [47], which also demonstrates a strong influence of the properties. result is consistent study of Rajavel et al. [47], which also demonstrates a precursorThis concentration (AgNO3 ) with on thethe hydrophilic/hydrophobic properties. strong influence of the precursor concentration (AgNO 3) on the hydrophilic/hydrophobic properties. In contrast, the incorporation of ZnONPs induces a significant increase in WCA (higher than contrast, the of ZnONPs induces a significant than 90◦In ), indicating theincorporation hydrophobic surface character (Figure 10D). Theincrease additioninofWCA these(higher NPs produces 90°), indicating the hydrophobic surface The addition of thesewith NPs produces a uniform coating covering the entirecharacter fabric, as(Figure shown10D). in Figure 4A (in contrast the coating a uniform coating covering the entire fabric, as shown in Figure 4A (in contrast with the coating with only AgNPs), which contributes to the increase of all surface roughness, leading to higherwith WCA only AgNPs), which contributes to the of all surface roughness, leading to higher WCA values. This finding is in agreement withincrease several studies reporting the efficiency of ZnONPs to produce values. This finding is in [6,11,48]. agreementTherefore, with several studies reporting the efficiency of ZnONPs hydrophobic structures the coating with ZnONPs significantly improvestothe produce hydrophobic structures [6,11,48]. Therefore, the coating with ZnONPs significantly hydrophobicity of the flax fabric, making it suitable for several applications, such as self-cleaning, improves the hydrophobicity of the flax fabric, making it suitable for several applications, such as easy-cleaning, and water-proof surfaces. self-cleaning, easy-cleaning, and water-proof surfaces. 3.2.4. UV Radiation Resistance and Wash Durability 3.2.4. UV Radiation Resistance and Wash Durability UV radiation induces a continuous damage of fibrous materials’ surface, including colour-fading, UV radiation induces properties, a continuous damage of fibrous materials’ surface, including colourreduction in mechanical and fibre deterioration [49]. To understand the effect of UV fading, reduction in mechanical properties, and fibre deterioration Towas understand theIneffect of radiation onto fabrics, the monitoring of fabric colour change over[49]. time performed. this way, UV radiation fabric colour time was performed. Infrom this 2, the samplesonto werefabrics, exposedthe to monitoring UV radiationoffor a total of 80change h, with over irradiation intervals ranging way, the10 samples werethe exposed to UV radiation for were a total of 80 h, with irradiation intervals ranging 5 and h, in which samples’ colour changes evaluated. Figure 11 shows the overall colour from 2, 5 and h, in which the samples’ changes were Figure 11 shows the overall change (∆E)10between samples exposedcolour to different times of evaluated. UV radiation, as compared to the initial colour change (∆E) between samples exposed to different times of UV radiation, as compared to the samples (time of UV radiation = 0 h), as a function of irradiation time. initial samples (time of UV radiation = 0 h), as a function of irradiation time.


17 of 21 17 of 22

Figure Figure11. 11.Total Totalcolour colourchange changewith withirradiation irradiationtime timeofofflax flaxfabrics fabrics(black (blackcurve) curve)flax flaxfabrics fabricscoated coated with withAgNPs(red AgNPs(redcurve) curve)and andwith withAg/ZnO Ag/ZnONPs NPs(blue (bluecurve). curve).

From FromFigure Figure1111ititisisclearly clearlyvisible visiblethat thatflax flaxtreated treatedwith withboth bothAg/ZnO Ag/ZnONPs NPswas wasthe thesample samplewith with the smallest ∆E values over the total of 80 h of UV radiation, indicating the ability of NPs to avoid UV the smallest ∆E values over the total of 80 h of UV radiation, indicating the ability of NPs to avoid radiation damage effectseffects onto fabrics. UV radiation damage onto fabrics. Regarding the AgNPs-treated Regarding the AgNPs-treatedflax flaxsample, sample,an anabrupt abruptcolour colourdifference difference(∆E) (∆E)during duringthe thefirst first1515hh ofofirradiation is observed. This is probably due to the continuous formation of AgNPs through irradiation is observed. This is probably due to the continuous formation of AgNPs throughthe the reduction of remaining AgNO . In fact, several studies [5,9] report that UV radiation induces the 3 reduction of remaining AgNO3. In fact, several studies [5,9] report that UV radiation induces the + 0 onto the NCF surface. After 15 h of exposure to UV radiation, photoreduction photoreductionofofAg Ag+ ions ions into into Ag Ag0 onto the NCF surface. After 15 h of exposure to UV radiation, ∆E ∆E values remain relativelyidentical, identical,suggesting suggestingno noadditional additional change change in in colour values remain relatively colour with withan anincreasing increasing irradiation time. irradiation time. On other hand, ZnONPs incorporation onto AgNPs-treated flax promotes a dramaticareduction Onthe the other hand, ZnONPs incorporation onto AgNPs-treated flax promotes dramatic inreduction ∆E valuesinand decrease is also observed whenobserved compared to untreated flax. results suggest ∆Ethis values and this decrease is also when compared to These untreated flax. These that ZnONPs can act as UV filtering, resulting in less UV radiation being available to change NCF results suggest that ZnONPs can act as UV filtering, resulting in less UV radiation being available to colour and making resistant to the agingtoprocess [29,49–52]. Moreover,Moreover, with the change NCF colourthis andcoating makingmore this coating more resistant the aging process [29,49–52]. synthesis ZnONPs AgNPs-treated flax fabrics, remaining AgNO3 was totally with the process synthesisofprocess of onto ZnONPs onto AgNPs-treated flaxthe fabrics, the remaining AgNO 3 was removed, preventing the appearance of high initial ∆E values obtained for flax coated with totally removed, preventing the appearance of high initial ∆E values obtained for flax coatedonly with AgNPs. In conclusion, coatingcoating with Ag/ZnO NPs restricted the colour observed in the fabrics only AgNPs. In conclusion, with Ag/ZnO NPs restricted thechanges colour changes observed in the when exposure to UV radiation. fabrics when exposure to UV radiation. To infer about To infer aboutthe thedurability durabilityofofAg/ZnO Ag/ZnONPs NPscoating, coating,the thefibres fibreswere wereimmersed immersedininwater waterand and subjected subjectedtotocentrifugation. centrifugation.The Thewashing washingsolution solutionwas wasmonitored monitoredby byUV-Vis UV-Visspectroscopy spectroscopyatatdifferent different time min, 11 h, h, 22h, h,24 24hh7272h,h,and and100 100 Accordingly, with GSDR results from timepoints: points: 15 15 min, min, 30 30 min, h.h. Accordingly, with GSDR results from flax flax functionalized with both NPs (Figure 2), the absorption bands of Ag and ZnO NPs appear at functionalized with both NPs (Figure 2), the absorption bands of Ag and ZnO NPs appear at 437 and 437 nm, respectively. As shown in 12, Figure 12, UV-Vis of washing solutions not 380and nm,380 respectively. As shown in Figure UV-Vis spectraspectra of washing solutions did notdid present present any absorption band at those wavelengths. Moreover, these results were observed for all time any absorption band at those wavelengths. Moreover, these results were observed for all time points points evaluated, demonstrating remained attached thefabric fabricafter after100 100 h h of evaluated, demonstrating that that bothboth NPsNPs remained attached to to the of washing. washing. Therefore, these results show the efficient and prolonged NPs adsorption onto fabrics, creating Therefore, these results show the efficient and prolonged NPs adsorption onto fabrics, creatingaa durable durableactive activecoating. coating.

Nanomaterials 2018, 2018, 8, 8, x1069 Nanomaterials FOR PEER REVIEW

21 1818ofof 22

1 0.9 0.8

15min 30min 1h 2h 24h 72h 100h

0.7

Absorbance (a.u.)

0.6 0.5 0.4 0.3 0.2 0.1 0 200

300

400

500

600

700

800

Wavelength (nm) Figure 12. UV-Vis spectra of Ag/ZnO-treated flax washing solutions evaluated at different time points. Figure 12. UV-Vis spectra of Ag/ZnO-treated flax washing solutions evaluated at different time points. 4. Conclusions

In this work, for the first time, flax fabrics were functionalized with two different types of 4. Conclusions nanostructures (Ag and ZnO) to obtain multifunctional smart materials. Ag and ZnO nanostructures this work,synthetized for the firstand time, flax fabrics functionalized with two different typescost, of wereIn successfully adsorbed in flaxwere fabrics, taking into account the sustainability, nanostructures (Ag ZnO) to obtain multifunctional materials. Ag andofZnO and simplicity of theand methodologies used. GSDR analysissmart revealed the presence SPR nanostructures band of AgNPs were successfully synthetized and adsorbed in flax fabrics, taking into account sustainability, peaking at 437 nm and the absorption band at UV region (~380 nm) of ZnONPs. the FESEM, EDS, and cost, and simplicity of the methodologies used. GSDR analysis revealed the presence of SPR band of ATR-FTIR analyses confirmed the synthesis and adsorption of both Ag and ZnO nanostructures onto AgNPs at 437 nm the absorption band UV crystalline region (~380 nm) of when ZnONPs. FESEM, EDS, the flax peaking fabrics surface. Theand XRD pattern revealed theatNPs structure incorporated onto and ATR-FTIR analyses confirmed the synthesis and adsorption of both Ag and ZnO nanostructures flax fabrics. TGA analysis showed that the quantity of NPs deposited onto the flax fabric was 12.87% wt. onto thetoflax fabrics surface. The sensor, XRD pattern revealed thewas NPs crystalline structure when In order develop a piezoresistive a conductive surface created by functionalizing flax incorporated onto flax fabrics. TGA analysis showed that the quantity of NPs deposited onto flax 7 fabrics with AgNPs, which decreased the fabrics resistivity from 1.5 × 10 to 3.33 × 103 Ω Besides ·m. the fabric was 12.87% wt.fabric In order to develop piezoresistive sensor, a conductive surface was created AgNPs-treated flax changing their aelectrical resistance under mechanical compression, the 7 to by functionalizing flax fabrics with AgNPs, which decreased the fabrics resistivity from 1.5 × 10 addition of ZnONPs enhanced the piezoresistive behaviour. At the same time, synthesis of ZnONPs led 3 Ω·m. Besides AgNPs-treated flax fabric changing their electrical resistance under 3.33 × 10GF to higher values as the ZnO precursor concentration increased, improving the sensor’s sensitivity. mechanical compression, the addition of also ZnONPs enhanced piezoresistive behaviour. At the Furthermore, the introduction of ZnONPs improved other the properties, including the antibacterial same synthesis of S. ZnONPs led flax to higher GF values as the ZnO precursor concentration effect time, against E. coli and aureus and fabric hydrophobicity (increasing the WCA above 90◦ ). increased, improving the sensor’s sensitivity. Furthermore, the introduction of ZnONPs also Simultaneously, functionalized flax fabrics exhibited UV resistance and wash durability. Overall, improved other properties, including the of antibacterial effectNCF, against E. coli S. aureus and flax this work demonstrates the development multifunctional which canand be used in a variety of fabric hydrophobicity (increasing the WCA above 90°). Simultaneously, functionalized flax fabrics monitoring/sensing applications, namely as piezoresistive sensors. Due to the antibacterial activity exhibited UV resistance anditwash durability. Overall, work demonstrates the development and hydrophobic character, is possible to expand the this application range of these fibrous systems of to multifunctional NCF, which can be used in a variety of monitoring/sensing applications, namely as several other areas. piezoresistive sensors. Due to the antibacterial activity and hydrophobic character, it is possible to Author Contributions: D.P.F. andof R.F. conceived and designed experiments; S.M.C., D.P.F. and A.F. performed expand the application range these fibrous systems tothe several other areas. the experiments; All the authors analysed the data and wrote the paper.

Author Contributions: andtoR.F. conceived designed experiments; S.M.C., D.P.F. supported and A.F. Funding: The authors areD.P.F. thankful TSSiPRO project,and operation codethe NORTE 01-0145-FEDER-000015, by the “Programa Operacional do Norte” number NORTE-45-2015-02 and the FCT (Portuguese Science performed the experiments; AllRegional the authors analysed the data and wrote the paper. Foundation) for Armando Ferreira grant: SFRH/BPD/102402/2014. Funding: The authors are thankful to TSSiPRO project, operation code NORTE 01-0145-FEDER-000015, Conflicts of Interest: The authors declare no conflict of interest. supported by the “Programa Operacional Regional do Norte” number NORTE-45-2015-02 and the FCT (Portuguese Science Foundation) for Armando Ferreira grant: SFRH/BPD/102402/2014. Conflicts of Interest: The authors declare no conflict of interest.


19 of 21

References 1. 2. 3. 4. 5. 6. 7.

8. 9.

10. 11. 12.

13. 14.

15. 16. 17.

18. 19. 20. 21.

22.

Koncar, V. 1—Introduction to Smart Textiles and Their Applications. In Woodhead Publishing Series in Textiles; Koncar, V., Ed.; Woodhead Publishing: Oxford, UK, 2016; pp. 1–8. ISBN 978-0-08-100574-3. Stoppa, M.; Chiolerio, A. Wearable Electronics and Smart Textiles: A Critical Review. Sensors 2014, 14, 11957–11992. [CrossRef] [PubMed] Zeng, W.; Shu, L.; Li, Q.; Chen, S.; Wang, F.; Tao, X.-M. Fiber-Based Wearable Electronics: A Review of Materials, Fabrication, Devices, and Applications. Adv. Mater. 2014, 26, 5310–5336. [CrossRef] [PubMed] Wang, Y.; Ali, S.; Wijekoon, J.; Gong, R.H.; Fernando, A. A Wearable Piezo-Resistive Sensor for Capturing Cardiorespiratory Signals. Sens. Actuators A Phys. 2018, 282, 215–229. [CrossRef] Ferreira, D.; Ferreira, A.; Fangueiro, R. Searching for Natural Conductive Fibrous Structures via a Green Sustainable Approach Based on Jute Fibers and Silver Nanoparticles. Polymers 2018, 10, 63. [CrossRef] Shaban, M.; Mohamed, F.; Abdallah, S. Production and Characterization of Superhydrophobic and Antibacterial Coated Fabrics Utilizing ZnO Nanocatalyst. Sci. Rep. 2018, 8, 3925. [CrossRef] Yetisen, A.K.; Qu, H.; Manbachi, A.; Butt, H.; Dokmeci, M.R.; Hinestroza, J.P.; Skorobogatiy, M.; Khademhosseini, A.; Yun, S.H. Nanotechnology in Textiles. ACS Nano 2016, 10, 3042–3068. [CrossRef] [PubMed] Mahmud, S.; Sultana, M.; Pervez, M.; Habib, M.; Liu, H.-H. Surface Functionalization of “Rajshahi Silk” Using Green Silver Nanoparticles. Fibers 2017, 5, 35. [CrossRef] Rehan, M.; Barhoum, A.; Van Assche, G.; Dufresne, A.; Gätjen, L.; Wilken, R. Towards Multifunctional Cellulosic Fabric: UV Photo-Reduction and in-Situ Synthesis of Silver Nanoparticles into Cellulose Fabrics. Int. J. Biol. Macromol. 2017, 98, 877–886. [CrossRef] Pandimurugan, R.; Thambidurai, S. UV Protection and Antibacterial Properties of Seaweed Capped ZnO Nanoparticles Coated Cotton Fabrics. Int. J. Biol. Macromol. 2017, 105, 788–795. [CrossRef] Seki, Y. Conductive Cotton Fabrics Coated with Myristic Acid/Zinc Oxide Nanoparticles. Polym. Plast. Technol. Eng. 2018, 57, 766–774. [CrossRef] Lim, Z.H.; Chia, Z.X.; Kevin, M.; Wong, A.S.W.; Ho, G.W. A Facile Approach towards ZnO Nanorods Conductive Textile for Room Temperature Multifunctional Sensors. Sens. Actuators B Chem. 2010, 151, 121–126. [CrossRef] Fangueiro, R.; Rana, S. Natural Fibres: Advances in Science and Technology Towards Industrial Applications, 1st ed.; Springer: Dordrecht, The Netherlands, 2016; p. 456. ISBN 978-94-017-7513-7. Sanjay, M.R.; Madhu, P.; Jawaid, M.; Senthamaraikannan, P.; Senthil, S.; Pradeep, S. Characterization and Properties of Natural Fiber Polymer Composites: A Comprehensive Review. J. Clean. Prod. 2018, 172, 566–581. [CrossRef] Fangueiro, R.; Rana, S. Advances in Natural Fibre Composites: Raw Materials, Processing and Analysis, 1st ed.; Springer International Publishing: New York, NY, USA, 2017; p. 280. ISBN 978-3-319-64640-4. Shaker, K.; Ashraf, M.; Jabbar, M.; Shahid, S.; Nawab, Y.; Zia, J.; Rehman, A. Bioactive Woven Flax-Based Composites: Development and Characterisation. J. Ind. Text. 2015, 46, 549–561. [CrossRef] Borda d’ Agua, R.; Branquinho, R.; Duarte, M.P.; Mauricio, E.; Fernando, A.L.; Martins, R.; Fortunato, E. Efficient Coverage of ZnO Nanoparticles on Cotton Fibres for Antibacterial Finishing Using a Rapid and Low Cost in Situ Synthesis. New J. Chem. 2018, 42, 1052–1060. [CrossRef] Beeby, S.; Ensel, G.; Kraft, M.; White, N. MEMS Mechanical Sensors; Artech House: Boston, MA, USA, 2004; p. 282. ISBN 9781580535366. Roy, A.; Gauri, S.S.; Bhattacharya, M.; Bhattacharya, J. Antimicrobial Activity of CaO Nanoparticles. J. Biomed. Nanotechnol. 2013, 9, 1570–1578. [CrossRef] [PubMed] Pinho, E.; Magalhães, L.; Henriques, M.; Oliveira, R. Antimicrobial Activity Assessment of Textiles: Standard Methods Comparison. Ann. Microbiol. 2011, 61, 493–498. [CrossRef] Gogurla, N.; Sinha, A.K.; Santra, S.; Manna, S.; Ray, S.K. Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices. Sci. Rep. 2014, 4, 6483. [CrossRef] Muthuchamy, M.; Muthusamy, A.; Govindhan, M.; Santhanakrishnan, S.; Akbar Sait Hameedha, B.; Radhakrishnan Jeeva, P. Investigation on the Electrical Conductivity of ZnO Nanoparticles-Decorated Bacterial Nanowires. Adv. Nat. Sci. Nanosci. Nanotechnol. 2016, 7, 45011.


23.

24. 25.

26.

27. 28.

29. 30.

31.

32. 33. 34.

35.

36. 37. 38. 39.

40.

41. 42. 43.

20 of 21

Shameli, K.; Ahmad, M.B.; Jazayeri, S.D.; Shabanzadeh, P.; Sangpour, P.; Jahangirian, H.; Gharayebi, Y. Investigation of Antibacterial Properties Silver Nanoparticles Prepared via Green Method. Chem. Cent. J. 2012, 6, 73. [CrossRef] Shateri-Khalilabad, M.; Yazdanshenas, M.E.; Etemadifar, A. Fabricating Multifunctional Silver Nanoparticles-Coated Cotton Fabric. Arab. J. Chem. 2017, 10, S2355–S2362. [CrossRef] Jiang, X.C.; Chen, W.M.; Chen, C.Y.; Xiong, S.X.; Yu, A.B. Role of Temperature in the Growth of Silver Nanoparticles Through a Synergetic Reduction Approach. Nanoscale Res. Lett. 2011, 6, 32. [CrossRef] [PubMed] Shaban, M.; Abdallah, S.; Khalek, A.A. Characterization and Photocatalytic Properties of Cotton Fibers Modified with ZnO Nanoparticles Using Sol–gel Spin Coating Technique. Beni-Suef Univ. J. Basic Appl. Sci. 2016, 5, 277–283. [CrossRef] Pelicano, C.M.; Rapadas, N.J.; Magdaluyo, E., Jr. X-Ray Peak Profile Analysis of Zinc Oxide Nanoparticles Formed by Simple Precipitation Method. AIP Conf. Proc. 2017, 1901, 20016. [CrossRef] Saoud, K.; Alsoubaihi, R.; Bensalah, N.; Bora, T.; Bertino, M.; Dutta, J. Synthesis of Supported Silver Nano-Spheres on Zinc Oxide Nanorods for Visible Light Photocatalytic Applications. Mater. Res. Bull. 2015, 63, 134–140. [CrossRef] Shao, D.; Gao, Y.; Cao, K.; Wei, Q. Rapid Surface Functionalization of Cotton Fabrics by Modified Hydrothermal Synthesis of ZnO. J. Text. Inst. 2017, 108, 1391–1397. [CrossRef] Fahmy, A.; El-Zomrawy, A.; Saeed, A.M.; Sayed, A.Z.; El-Arab, M.A.E.; Shehata, H.A.; Friedrich, J. One-Step Synthesis of Silver Nanoparticles Embedded with Polyethylene Glycol as Thin Films. J. Adhes. Sci. Technol. 2017, 31, 1422–1440. [CrossRef] Khan, M.F.; Ansari, A.H.; Hameedullah, M.; Ahmad, E.; Husain, F.M.; Zia, Q.; Baig, U.; Zaheer, M.R.; Alam, M.M.; Khan, A.M.; et al. Sol-Gel Synthesis of Thorn-like ZnO Nanoparticles Endorsing Mechanical Stirring Effect and Their Antimicrobial Activities: Potential Role as Nano-Antibiotics. Sci. Rep. 2016, 6, 27689. [CrossRef] Alireza, S.; Ismail, A.F.; Hadi, N.; Othaman, Z.; Mustafa, M.K. Spectral Features and Antibacterial Properties of Cu-Doped ZnO Nanoparticles Prepared by Sol-Gel Method. Chin. Phys. B 2016, 25, 77803. Kumar, S.; Sharma, V.; Bhattacharyya, K.; Krishnan, V. Synergetic Effect of MoS2-RGO Doping to Enhance the Photocatalytic Performance of ZnO Nanoparticles. New J. Chem. 2016, 40, 5185–5197. [CrossRef] Ferrreira, A.; Rocha, J.G.; Ansón-Casaos, A.; Martínez, M.T.; Vaz, F.; Lanceros-Mendez, S. Electromechanical Performance of Poly(Vinylidene Fluoride)/Carbon Nanotube Composites for Strain Sensor Applications. Sens. Actuators A Phys. 2012, 178, 10–16. [CrossRef] Paleo, A.J.; van Hattum, F.W.J.; Pereira, J.; Rocha, J.G.; Silva, J.; Sencadas, V.; Lanceros-Méndez, S. The Piezoresistive Effect in Polypropylene—Carbon Nanofibre Composites Obtained by Shear Extrusion. Smart Mater. Struct. 2010, 19, 65013. [CrossRef] Mano, J.F.; Sencadas, V.; Costa, A.M.; Lanceros-Méndez, S. Dynamic Mechanical Analysis and Creep Behaviour of β-PVDF Films. Mater. Sci. Eng. A 2004, 370, 336–340. [CrossRef] Yogamalar, R.; Srinivasan, R.; Vinu, A.; Ariga, K.; Bose, A.C. X-Ray Peak Broadening Analysis in ZnO Nanoparticles. Solid State Commun. 2009, 149, 1919–1923. [CrossRef] Huang, X.; Netravali, A. Characterization of Flax Fiber Reinforced Soy Protein Resin Based Green Composites Modified with Nano-Clay Particles. Compos. Sci. Technol. 2007, 67, 2005–2014. [CrossRef] Khan, S.T.; Musarrat, J.; Al-Khedhairy, A.A. Countering Drug Resistance, Infectious Diseases, and Sepsis Using Metal and Metal Oxides Nanoparticles: Current Status. Colloids Surf. B Biointerfaces 2016, 146, 70–83. [CrossRef] Sirelkhatim, A.; Mahmud, S.; Seeni, A.; Kaus, N.H.M.; Ann, L.C.; Bakhori, S.K.M.; Hasan, H.; Mohamad, D. Review on Zinc Oxide Nanoparticles: Antibacterial Activity and Toxicity Mechanism. Nano-Micro Lett. 2015, 7, 219–242. [CrossRef] Wang, L.; Hu, C.; Shao, L. The Antimicrobial Activity of Nanoparticles: Present Situation and Prospects for the Future. Int. J. Nanomed. 2017, 12, 1227–1249. [CrossRef] Yan, X.; He, B.; Liu, L.; Qu, G.; Shi, J.; Hu, L.; Jiang, G. Antibacterial Mechanism of Silver Nanoparticles in Pseudomonas Aeruginosa: Proteomics Approach. Metallomics 2018, 10, 557–564. [CrossRef] Basha, S.K.; Lakshmi, K.V.; Kumari, V.S. Ammonia Sensor and Antibacterial Activities of Green Zinc Oxide Nanoparticles. Sens. Bio-Sens. Res. 2016, 10, 34–40. [CrossRef]


44. 45. 46.

47.

48. 49. 50.

51.

52.

21 of 21

Shateri-Khalilabad, M.; Yazdanshenas, M.E. Bifunctionalization of Cotton Textiles by ZnO Nanostructures: Antimicrobial Activity and Ultraviolet Protection. Text. Res. J. 2013, 83, 993–1004. [CrossRef] Duta, L.; Popescu, A.C.; Zgura, I.; Preda, N.; Mihailescu, I.N. Wettability of Nanostructured Surfaces. In Wetting and Wettability; Popescu, A.C., Ed.; IntechOpen: Rijeka, Croatia, 2015. [CrossRef] Abdel-Halim, E.S.; Konczewicz, W.; Zimniewska, M.; Al-Deyab, S.S.; El-Newehy, M.H. Enhancing Hydrophilicity of Bioscoured Flax Fabric by Emulsification Post-Treatment. Carbohydr. Polym. 2010, 82, 195–201. [CrossRef] Rajavel, K.; Gomathi, R.; Pandian, R.; Rajendra Kumar, R.T. In Situ Attachment and Its Hydrophobicity of Size- and Shape-Controlled Silver Nanoparticles on Fabric Surface for Bioapplication. Inorg. Nano-Metal Chem. 2017, 47, 1196–1203. [CrossRef] Pal, S.; Mondal, S.; Maity, J. In Situ Generation and Deposition of ZnO Nanoparticles on Cotton Surface to Impart Hydrophobicity: Investigation of Antibacterial Activity. Mater. Technol. 2018, 1–8. [CrossRef] Takuya, T.; Xungai, W. Nanoparticle Coatings for UV Protective Textiles. Res. J. Text. Appar. 2010, 14, 9–20. [CrossRef] Xuan, L.; Han, G.; Wang, D.; Cheng, W.; Gao, X.; Chen, F.; Li, Q. Effect of Surface-Modified TiO2 Nanoparticles on the Anti-Ultraviolet Aging Performance of Foamed Wheat Straw Fiber/Polypropylene Composites. Materials 2017, 10, 456. [CrossRef] [PubMed] El-Naggar, M.E.; Shaarawy, S.; Hebeish, A.A. Multifunctional Properties of Cotton Fabrics Coated with in Situ Synthesis of Zinc Oxide Nanoparticles Capped with Date Seed Extract. Carbohydr. Polym. 2018, 181, 307–316. [CrossRef] [PubMed] Desuo, Z.; Guangyu, Z.; Ling, C.; Yanfen, L.; Yuyue, C.; Hong, L. Multifunctional Finishing of Cotton Fabric Based on in Situ Fabrication of Polymer-Hybrid Nanoparticles. J. Appl. Polym. Sci. 2013, 130, 3778–3784. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).