ORIGINAL CONTRIBUTION

Copper nanoparticle in cationized palm oil fibres:physico-chemical investigation

M. N. K. Chowdhury & M. D. H. Beg & M. R. Khan &

M. F. Mina & A. F. Ismail

Received: 13 September 2014 /Revised: 9 November 2014 /Accepted: 13 November 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract At ambient atmospheric condition copper nanopar-ticles (CuNPs) were prepared in aqueous medium using cop-per chloride, sodium borohydride, ascorbic acid and polyvinylalcohol (PVA) of two different molecular weights (Mws). Thephysical appearance of the prepared sol of CuNPs has beenfound to be stable for a couple of weeks when kept in ambientatmospheric condition, as confirmed by ultraviolet–visibleabsorption spectroscopy. Transmission electron microscopyexhibits spherical morphology of CuNPs with an average sizeof 3.5±1.1 nm developed in the sol, where PVA of high Mw

renders smaller size (2.5 nm) than that of low Mw. Thesynthesised CuNPs were embedded in palm oil fibres(POFs) via cationisation process. The cationisation of POFwas ascertained by Fourier-transformed infrared and zeropoint charge determination. The inclusion of nanoparti-cles (NPs) onto the fibres’ surface has been consistentlyproven by Fourier transformed infrared spectroscopy, X-ray diffraction study, field emission scanning electronmicroscopy, energy dispersive X-ray study and thermo-gravimetric analysis. CuNP-coated POF showed the in-crement of tensile strength (∼34 %) and antifungal ac-tivity (24 %) with respect to control fibres. The ob-served findings suggest that NPs can be effectively used

as reinforcing agents in natural fibres to improve theirproperty and durability.

Keywords Copper nanoparticles . Durability . Fibre .

Strength . Reinforcing agent

Introduction

Synthesis of novel nanomaterial is a significant focalpoint of numerous research spheres where continuousefforts are being made by scientists and engineers forboth industrial and technological advancements. In thisrespect, NPs have received extensive interests becauseof their outstanding physical, chemical, electronic, mag-netic, catalytic and surface properties in manifold appli-cations due to small particle size [1–5]. Among metals,copper NPs (CuNPs) have drawn much attention owingto their versatile applications, for instance, catalytic,electrical, optical and antifungal/antibacterial purposes[6, 7]. To synthesise CuNPs, notable commonly usedmethods are chemical reduction [8], co-precipitation [9],sol–gel processing [10, 11], template synthesis [12, 13],thermal reduction [14], microwave irradiation methods[15], vacuum vapor deposition [16] and laser ablation[17]. However, most of these works were performed inan inert atmosphere, where the synthesised CuNPsshowed less thermal stability and readily oxidising ten-dency. Therefore, the fabrication of these nanomaterialsin ambient atmospheric condition with a facile approachhas been a challenge of nano-research. To overcome thischallenge, nano-researchers are constantly providingtheir efforts to control over the size of NPs, where

M. N. K. Chowdhury (*) :A. F. IsmailAdvanced Membrane Technology Research Center (AMTEC),Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysiae-mail: chowdhurynajmul76@yahoo.com

M. D. H. Beg :M. R. Khan (*) :M. F. MinaFaculty of Chemical & Natural Resources Engineering, UniversitiMalaysia Pahang, 26300 Gambang, Kuantan, Malaysiae-mail: mrkhancep@yahoo.com

Colloid Polym SciDOI 10.1007/s00396-014-3462-y

further sustainable attempts are still needed to be givenby various synthesis routes. In this respect, the chemicalreduction process has emerged as an enthusiastic tech-nique to control the size of nanoparticles [8].

On the other hand, palm oil fibre (POF) is mainly acellulose-enriched natural product and an available re-newable resource in some countries like Malaysia, In-donesia, India, Brazil, etc. However, in spite of theirgreat potentials, POFs still remain fully unutilised in thefabrication of new biomaterials even though they haveenvironmentally friendly and biocompatible characteris-tics [18, 19]. Cellulose containing materials is exten-sively involved in the fields of medical supplies, tex-tiles, packaging, electronic devices [20], energy storage[21] and biomedical materials [22]. Although the POFscontain 42–65 % cellulose, their poor mechanicalstrength and easy degradation to bacterial attack makethem almost useless materials in various applications.Due to their no antibacterial activity, POFs are oftenvery prone to the growth of microorganisms when dis-posed to land, thereby causing various types of diseases.Consequently, improving the mechanical and antibacte-rial performance of cellulosic materials using a facileapproach intrigues many researchers. Cu is known toexhibit a broad-spectrum biocide and effectively inhibitthe growth of bacteria, fungi and algae [6, 7, 23].Recent studies revealed that nano-scaled CuNPs showeda good antibacterial activity [24, 25], and thesenanoparticle-coated surfaces suitably served medical re-quirements [26]. Consequently, in this work, at ambientatmosphere under the chemical reduction process, anovel formulation has to be necessarily employed fordeveloping CuNPs with a size of a few nanometres (1–10 nm). In order to expand the applications of NPs inrenewable natural resources to strengthen and sustainthe vastly used natural fibres technology, the synthesisedCuNPs are introduced with natural fibres. The synthe-sised CuNPs were embedded in POFs and expected toimprove fibres, mechanical strength and antimicrobialactivity so that metallic NPs reinforced fibres can beexploited in due applications. However, there is a draw-back of compatibility between natural fibres and metal-lic fillers. To surmount this incompatibility betweenCuNPs and POFs during impregnation, fibre surfaceswere necessary to modify using a cationic agent, whichcan serve as a compatibiliser by flanking both metal andfibres. In brief, the present paper focuses the detailedstudies on the synthesis of CuNPs; their application inPOFs to improve fibres’ performance, as examined byvarious experimental techniques, and the vital require-ment is also fulfilled by ensuring the reducing atmo-sphere of CuNPs up to the completion of its impregna-tion on/into the fibres.

Experimental

Materials

Copper(II) chloride dihydrate salt (CuCl2·2H2O) of 98 %purity and analytical grade sodium hydroxide were procuredfrom Merck, Germany. Fully hydrolysed poly(vinyl alcohol)(PVA) of molecular weights Mw=2000 and 70,000, sodiumborohydride (NaBH4) Reagent Plus (purity 99 %), ascorbicacid (purity 99.7 %) and 60 % solution of 3-chloro-2-hydroxypropyltrimethyl ammonium chloride (CHPTAC)were purchased from Sigma-Aldrich. Raw POFs were collect-ed from the LKPP Corporation Sdn. Bhd., Kuantan,Malaysia.The untreated POFwere golden-brown colour with an averagediameter of 0.19 mm.

Synthesis

Preparation of nanocopper sols

The CuNPs synthesis route follows from our own work,which was done previously [27], and the details of this syn-thesis and characterisations are discussed here.

Modification of POF surface

The POF surface modification was done by CHPTAC and thecationisation protocol was adopted from another own work[28], and the details of this modification and characterisationsare discussed here.

Adsorption of copper nanoparticles

The cationised palm oil fibres (CPOFs) were introduced in thesynthesised nanocopper sol, maintaining the fibre/sol ratio of1:30 with continuous shaking by a mechanical shaker. Toincrease the adsorption of NPs, they were immersed in thesols at the ambient temperatures with increasing copper con-centrations of 50, 100 and 214 mg/L for 12 h. Then, theCuNPs embedded CPOF (NP-POF) were removed from thebath, rinsed with water and dried in the dark place at roomtemperature.

Characterisations

Ultraviolet–visible absorption spectroscopy

Ultraviolet–visible (UV–vis) absorption spectroscopy wasperformed with a Perkin-Elmer Lambda 950 UV–visible dou-ble beam spectrophotometer to take the absorption spectra ofthe nanocopper sols over the wavelength range of 400–800 nm.

Colloid Polym Sci

Fourier-transformed infrared spectroscopy

Fourier-transformed infrared (FTIR) spectra of POF, CPOFand NP-POF were recorded using a Thermo Scientific ModelSmart Performer attenuated total reflectance (ATR) accessoryof Ge crystal, attached to a Thermo Scientific spectrophotom-eter (Model Nicolet Avatar-370) with a single bounce. TheFTIR parameters were as follows: angle of incidence, 45°;sampling area, 2 mm; number of background scans, 32; num-ber of scans, 32; and optical resolution, 4.00 cm−1. From theFTIR spectrum, the degree of cationisation (C) was calculatedaccording to the following relation [27]:

C ¼ I1648−I1495I1648

� �� 100% ð1Þ

where I1648 and I1495 are the maximum intensities of the peaksat 1648 and 1495 cm−1, respectively.

X-ray diffractometry

X-ray diffraction (XRD) data were collected using aRigakuMiniFlex II, Japan, operated at 30 kV and 15 mA aswell as equipped with computer-controlled software to set upthe apparatus and analyse the data. To prepare the disc spec-imen of the same thickness for each category of differentlytreated POFs, 1 g of chopped fibres was compressed in acylindrical mould with a pressure of 1 MPa. The specimenswere stepwise-scanned over the operational range of scatter-ing angle (2θ) between 10 and 50°, with a step of 0.02°, usingCuKα radiation of wavelength λ=1.541 Å. The data wererecorded in terms of the diffracted X-ray intensities (I) versus2θ. The X-ray crystallinity, XX-ray, was calculated by the helpof Segal’s method with the following equation:

XX‐ray ¼ I002−I amI002

� �� 100% ð2Þ

where I002 is the maximum intensity of 002 reflections fromthe crystalline and amorphous components, and Iam is theminimum intensity of diffraction from the amorphous part ofPOFs. During the calculation process, the height of the (002)peak was used at a 2θ≈22°, while the intensity of Iam wastaken from the minimum intensity between the (002) and(110) peaks at a 2θ≈16°. The average size of the cellulosecrystallites, Dhkl, was determined with the full width at half-maximum (FWHM) of the (002) peak using the followingScherer formula [29]:

Dhkl ¼ 0:9λβcosθ

ð3Þ

where β is the FWHM (in radians) and θ is the diffractionangle. The β value was determined by curve fitting aftersubtracting the amorphous background. The Gaussian curvewas fitted at the top of the peak for determining β and theposition using an appropriate program.

Morphological studies of nanoparticles and fibres

Transmission electron microscopy Transmission electron mi-croscopy (TEM) was performed with a LEO 912 AB EFTEMoperating at 120 kV. A small drop of the liquid sol was placedover a carbon-coated microscopic copper grid (200 mesh size)and dried under vacuum at room temperature beforemeasurements.

Scanning electron microscopy Surface morphologies of POF,CPOF and NP-POF were investigated using a field emissionscanning electron microscope (FE-SEM) of JEOL JSM-7600F, USA, equipped with an energy dispersive X-ray(EDX) system (OXFORD INCA). Fibres were mounted onsample holders with carbon tape and sputtered with gold.

Atomic absorption spectrophotometry

Atomic absorption spectrophotometry (AAS) was carried outwith a Perkin Elmer Analyst 400 apparatus to measure theamount of CuNPs in POF samples.

Determination of pH at zero point charge

The pH at the point of zero charge (pHPZC) of the fibres inaqueous solutions was determined with different pH valuesusing titration methods [30, 31], where 0.1 g of fibre wastaken in 50 mL of 0.1 M potassium nitrate (KNO3) solutionand agitated with a magnetic stirrer. The pH of the solutionswas measured after 30 mins (required time to reach at equi-librium) of fibre addition. The titration was carried out with0.1 M HCl and 0.1 M NaOH.

Antimicrobial activity

For testing the antimicrobial activity as per AATCC testmethod 30-2004, POF and NP-POF were buried under soil.After 7 and 30 days of exposition, the samples were unex-posed, washed with tap water and DI water, respectively, driedin air and then subjected to mechanical test. The mechanicalstrength was calculated from the average strength of 50 singlefibres where the diameters of individual fibres were used.

Mechanical test

The tensile properties of POF and NP-POF before and aftersoil burials were measured using a Shimadzu Universal

Colloid Polym Sci

Tensile Testing Machine (Japan) at a crosshead speed of3 mm/min. The buried samples were exposed from the soilafter 7 and 30 days and then tested. The single fibre was cut toa length of about 5 cm, and its tensile strengths (TS) weremeasured using a 100 N load. For each category of samples,50 individual fibres were tested, and the average values wereevaluated.

Results and discussion

Physical changes of sol

Pictures of prepared sols before and after the reactions areprovided in Fig. 1. A change of sol colour is clearly observedafter the addition of reducing agent solution, where the light-blue colour cupric chloride aqueous solution (Fig. 1a) changesto the wine-red colour sol (Fig. 1b). This change of colour is aconvincing indication of the formation of CuNPs in the sols.

UV–vis absorption analysis for CuNPs formation

The UV–vis spectroscopic results of copper chloride solution(a) and the synthesised CuNPs sol at initial stage (b) are shownin Fig. 2i. A strong broadening of surface plasmon resonance(SPR) observed at ∼516 nm for both spectra represents theexistence of metallic CuNPs. It was reported that when theparticle size is ∼5 nm or larger, the spectrum shows the SPRpeak, which becomes prominent when the particles growmuch larger in size [27, 32–34]. Since no clear SPR, peak isvisible in our observed spectrum, the size of synthesisedCuNPs is expected to be <5 nm. In addition with, TEMimages also confirmed that size distribution is more or less

even. Therefore, the absence of sharp SPR peak significantlyindicating the average particle size of CuNPs is <5 nm.

To study the CuNPs formation rate, the normalised absor-bance of CuNPs during the synthesis (maximum absorbanceat 516 nm is used for normalisation) is shown in Fig. 2ii. It wasobserved that copper crystal nuclei appear in aqueous solu-tions almost instantaneously once the reducing agent solutionsare combined with the reaction mixture. The obtained curveindicates that most of the CuNPs are formed within the firstminute of the synthesis.

Stability of CuNPs

Basically, the stability of CuNPs is analysed from the obtainedUV–vis absorption spectra as a function of synthesis time. Inthis case, the observed UV–vis spectroscopic results of coppersols synthesised at the initial stage, after 10min, and at 16 daysof synthesis are shown in Fig. 3i (a–c), respectively, which aresignificantly different from the spectrum of Cu2+ solution(shown in Fig. 2i (a)). However, comparing with the spectra(a) and (b), it is possible to say that the optical density stillincreases with time though it was in a lesser extent. It ismentioned that the colour of nanocopper sol sustained at leastFig. 1 Copper sols: a before and b after NPs formation

Fig. 2 i The UV–vis spectra of (a) Cu2+ solution and (b) nanocopper solsat initial stage of synthesis; ii evolution of normalised absorbance ofCuNPs during the synthesis (maximum absorbance at 516 nm is usedfor normalisation)

Colloid Polym Sci

for 15 days at ambient condition. This fact is also reflected inthe spectra (Fig. 3i (a and c)), which shows almost the samespectroscopic pattern of the same sol observed at 16 days fromthe synthesis. The observed findings indicate that the synthe-sised CuNPs sol was stable up to 15 days.

The sol remains stable even after keeping it in an ordinaryvial in contact with air, suggesting that the synthesised coppersol is resistant to oxidation by atmospheric air. This stability ofsol colour (shown in Fig. 3ii (a–c)) and UV–vis spectrumpattern may be attributed to the presence of anions, namelyborohydride and ascorbic acid. The presence of sodium boro-hydride, ascorbic acid and PVA played significant roles tostabilise the sol or resist from oxidation.

It is known that ascorbic acid can perform as an antioxidantvia scavenging the free radicals, and active oxygen molecules[35] can play this role with the donation of electrons andultimately to form the semi-dehydroascorbate radical anddehydroascorbic acid. In addition, borohydride anions(BH4

−) and OH− can protect CuNPs against atmosphericoxidisation by sustaining a reducing atmosphere [36]. Further-more, the polymeric capping agent, PVA, hinders the nucleifrom aggregation through its polar hydroxyl groups; rather, itcan absorb the CuNPs on the surface with coordination bonds[35].

Analysis of nanoparticle morphology

The morphologies and sizes of CuNPs monitored by TEMwith varying precursor concentrations and molecular weightsof PVA are shown in Fig. 4. TEM images (Fig. 4a–c) clarifythat the particle developed in the sol are in sizes in the order ofa few nanometres with a spherical shape. Analysis from 400 to500 particles of diameters within the range of 1.2–10.5 nmshows the average size of 3.5±1.1 nm (from number average)or 3.8±1.3 nm (from volume average). The notable variationsfound in the micrographs are that increasing the precursorconcentration increases the number of NPs with varying sizesand dispersions. Evidently, low concentration gives rise tofewer particles and high concentration to more particles. It isseen that 1000 mg/L sol renders almost homogenous

dispersion with nearly an equal size of particles. The averageparticle size determined from this sol is about 2.5 nm. On theother hand, the size and dispersion of nanoparticles preparedusing PVA of Mw=2000(Fig. 4d) are slightly different fromthat prepared using Mw=70,000 (Fig. 4b). Apparently, theparticle size of the former case is larger than that for the latter,clearly showing the size dependence of NPs on Mw of PVA.Basically, PVA chain length increases with increasing molec-ular weight and is generally invisible under an electron mi-croscope. Prior to the nucleation and growth of NPs, the Cuspecies are bonded to PVA in such a way that the lone pairs ofelectrons in the oxygen atoms of O–H groups of PVA unit aredonated to an sp-hybrid orbital of the Cu2+ ions to form theCu←O coordination. This bonding reduces the susceptibilityof the Cu ions to oxidation during the particle synthesis andprevents particle agglomeration, depending on PVAmolecularweights, which cause a slight difference in particle size. Theabove explanations are rational because longer PVA chainscan more effectively be adsorbed on the metallic surface thanshorter chains. A longer PVA chain can densely surround thecopper surface than the smaller one, and a densely packedlayer can hinder the oxidation of the metal atoms during NPsynthesis. Thus, the formation of smaller particle in the pres-ence of highMw PVA is plausible, as supported by the obser-vations of TEM.

Cationisation of fibres

The cationisation of fibres was confirmed by the surfacecharge determination and FTIR technique. The establishedtechnique for the determination of surface charge Q of cellu-losic material is followed to determine the pHPZC. In thisstudy, cationised and the untreated fibres surface charge werecalculated from the experimental titration data according toEq. (4) [30, 31]

Q ¼ 1

wCA−CB− Hþ½ �− OH−½ �ð Þ ð4Þ

wherew is the dry weight of fibres in aqueous system (g/L);CA and CB are the concentration (mol/L) of added acid and

Fig. 3 i The UV–vis spectra ofnanocopper sols (a) at initialstage, (b) after 10 min, and (c)after 16 days of synthesis; iiCuNPs sols: (a) at initial stage, (b)after 14 days and (c) after 16 daysof synthesis

Colloid Polym Sci

base (in aqueous system), respectively; [H+] and [OH−] are theconcentration (mol/L) of H+ and OH−, respectively. The pHvalue at the point of zero charge was then determined byplotting Q versus pH. Figure 5i (a) shows the surface chargeof the fibre as a function of pH. From this figure, it is obviousthat the surface charge of the POF is zero around pH 6.09 andthat of CPOF is around pH 5.20. Basically, the term Q indi-cates that the surface charge (indirectly the quantity of H+ orOH- adsorbed on the surface) of a material at a particular pH.Due to the cationisation, covalent bond is formed between thefibre and CHPTAC, leading to the attachment of permanentpositive charge species with the fibre. The existence of thistype of permanent positive charge on/into the surface could beretarded the adsorption of H+ ions on the fibre surface. There-fore, below the pHPZC, the surface charge of CPOF is lowerthan that of the POF. This finding of pHPZC changes indicatesthe successful cationisation of fibres.

Figure 5ii shows the FTIR spectra of POF, CPOF and NP-POF, respectively. While some of the characteristic cellulosepeaks are observed around 1000–1200 cm−1, another impor-tant characteristic bands from its functional groups were pre-viously attributed to the O–H stretching at 3450–3300 cm−1,the C–H stretching at 2926 cm−1 and the C–H bending(wagging) at 1318 cm−1 by others [37]. From the comparisonof the spectra of POF and CPOF, the latter one shows a new

peak at 1495 cm−1, which was ascribed by the quaternaryammonium moiety arising from the cationic agent CHPTAC[28, 37], ensuring the cationisation of POFs. A comparativeanalysis based on the absorption frequencies at 1495 cm−1

(characteristic absorption frequency of cationic agent attachedto the fibre) and 1648 cm−1 (absorption band related to thecellulose fibres) using Eq. (1) reveals ∼39 % cationisation offibres. On the other hand, the intensities at 1495 cm−1 forCPOF andNP-POF are observed to be different, indicating theabsorption of CuNPs by the fibres. Apart from this, a sharppeak observed in the spectra of CPOF and NP-POF at1048 cm−1 may correspond to the stretching frequency ofcarbon–nitrogen (C–N) single bond, which is absent in thespectrum of POF. This group comes from the cationic agentand also confirms the cationisation reaction.

Adsorption of copper nanoparticles

The amount of copper adsorptions on CPOF from 50, 100 and214 mg/L sols measured by AAS are found to be 1950, 2103and 2590 μg/g, respectively. The adsorption of copper onCPOF increases with increasing the sol concentrations. WhenCPOF are immersed in a nanocopper sol, CuNPs stabilised bynegatively charged PVA spheres can readily be adsorbed bythe positive sites of fibres. As per Scheme 1 (ii), due to the

Fig. 4 TEM images of CuNPsobserved from different sols: a 50,b 100, c 250 mg CuCl2 with PVAof Mw=70,000 and d 100 mgCuCl2 with PVA of Mw=2000

Colloid Polym Sci

cationisation (∼39% obtained from FTIR), it is confirmed thata significant amount of positive-charged moieties was at-tached with the fibres, leading to the reduction of negativecharge from the fibre surface and facilitating the attachment ofnegatively charged CuNPs (since they are PVA stabilised) viathe electrostatic attraction with quaternary ammonium moie-ties (Scheme 1 (iii)).

On the other hand, in the case of CuNPs, adsorption onCPOFs will be more favourable than the POFs; this is becausethe used sols pH was ∼3.0. At this pH, though the surface ofPOFs are showing a little bit more positive than that of theCPOFs (Fig. 5i), there is another internal fact that existsbehind the CuNPs attachment. At pH 3, the surface of POFsis positively charged by H+ ions attachment with the –C–O–H(fibre) sites, and this positively charged surface may be proneto the adsorption of negatively charged CuNPs in that solutionpH only. However, during the washing of CuNPs adsorbedfibres, the H+ ions were favoured to the desorption and willattract to the highly polar H–O–H, and the ultimate result ofadsorption of CuNPs on POFs was very low. Conversely, inthe case of CPOFs, permanent surface positive charge wasavailable, and by this site, CuNPs can be adsorbed easily on tothe surface.

XRD analysis

The X-ray diffractograms of POF, CPOF and NP-POF areillustrated in Fig. 6 from which the important peak positionsfor amorphous and crystalline parts of cellulose is observed.

The noticeable strong crystalline peak for POF is ob-served at around 21.1°, which represents the crystallo-graphic (002) plane of cellulose materials of POFs, asanalysed from the reported crystalline structure of celluloseIβ that shows the monoclinic form with a=7.784 Å, b=8.201 Å, c=10.380 Å, and γ=96.5°. The CPOF and NP-POF show a shifting of (002) peak towards a higherangle, indicating a decrease in interplanar spacing of(002) plane. This result strongly suggests that a closepacking takes place in the cellulose crystal, possibly bythe rearrangement of cellulose molecules after removal ofhemicellulose, lignin pectin, etc., due to cationic treatmentunder alkaline medium. However, an additional peak ap-pears at 2θ=42.3° for NP-CPOF and has been reportedlyattributed to the 111 reflection from the cubic coppercrystal [35]. This finding evidences CuNPs incorporationin the fibres. The average size of the CuNPs analysedfrom (111) peak is ∼17 nm, which is larger than thatfound by TEM. This higher size of CuNPs may be dueto the agglomeration in impregnating medium, oxidationon fibre surface and fibre drying to testing a bit long timewas required. On the other hand, the estimated percentageof crystallinities from CPOF to NP-POF exhibited anincrement trend upon CuNPs inclusion. Since metallicNPs are itself crystalline property [38], as well as due totheir smaller size and higher charge density, they have theextraordinary penetrating capacity/mobility [39–41] in thenatural fibre polymer matrix, leading to the overall crys-tallinity of the natural fibres.

Fig. 5 i Surface charge of fibres as a function of pH: (a) POF and (b)CPOF; ii FTIR spectra of: (a) POF, (b) CPOF and (c) NP-POF

Fig. 6 XRD pattern of (a) POF, (b) CPOF and (c) NP-POF

Colloid Polym Sci

Surface morphology of NP-POF

The NP adsorptions by fibres are noticeable in the FE-SEMimage and EDX spectrum (Fig. 7a and b respectively. In the

FE-SEM image (Fig. 7a), the CuNPs have beenmarked by redcolour and confirmed its incorporation in fibres, since a certainamount of Cu is also traced in the EDX spectrum of Fig. 7b. Inthe EDX spectrum (Fig. 7b), the initial peaks are attributed tothe most abundant elements of fibre i.e. carbon and oxygen,and the other peaks at ∼2.3 and ∼9.7 keV are due to the Au

Cl C CC N+

CH3

CH3

CH3

H

H H

H

H

OH

(CHPTAC)

+ NaOH

H

H

CH3

CH3

CH3

+NC CC

O

H2

H

+ NaCl

OH

H

H2

O

C CC N+

CH3

CH3

CH3

Cl-

H

H

+ O

OH

H

H

HH

H

CH3

CH3

CH3

+NC CC

(CAEFB)

(EFB Fiber )

(CAEFB)

C CC N+

CH3

CH3

CH3

H

H H

H

H

OH

O

OH-

+ Cu-

- ----

(CuNPs)

O

OH

H

H

HH

H

CH3

CH3

CH3

+NC CC

-- -

---Cu

(CuNP-CAEFB)

(i)

(ii)

(iii)

Cl - Cl -

-

-

-

-- - -

- --

Scheme 1 Mechanism ofimpregnation of CuNPs intofibres [28]

Fig. 7 Morphology of NP-POF: a FESEM image and b EDX profile

Fig. 8 Tensile strengths of untreated fibre (UF) and nanoparticleimpregnated fibre (NF) before burial (BB) and after burial (AB) undersoil at different days of exposition: i after 7 days and ii after 30 days

Colloid Polym Sci

particle, which was attributed to sputtering, and the peak forCl atom was ascribed from CHPTAC.

Mechanical performances due to soil burial up to 7and 30 days

Figure 8i and ii depicts the POF and NP-POF of before burial(BB) and after burial (AB) under soil after 7 and 30 days,respectively. It is observed that, in the case of 30 days of theexposition (after soil burials), the value decreases for both thePOF and NP-POF by 45 and 21 % from their respective σaveof before burial. That means the obtained (45−21)=24 %antifungal activity due to the CuNPs impregnation. However,after 7 days of exposition, the TS decreasing percentages forthe same fibres are 32 and 25 %, respectively, and ensuing inonly 7 % antifungal activity observed due to nanoparticleimpregnation. After 30 days of exposition, a large percentage(24 %) of antifungal activity for nanoparticle impregnation infibres is logical as compared to the 7 days of expositionbecause, within 30 days in POFs, the fungal attack as wellas the fibres degradations were highly significant, whereas inNP-POF, such attack and degradation were less prominent.Furthermore, in both of the expositions, σave decrease in POFis more than NP-POF, demonstrating that CuNP-reinforcedfibre is more durable than the control ones. The metallicnanoparticles can show a certain degree of sterilisation be-cause the catalytic properties of metallic species partly help tocreate active oxygen in water, which dissolves the organicsubstances to maintain the sterilising effect [42]. Moreover,CuNPs exhibit large relative surface area, thus creating morecontact with fungi and largely improving the fungicidal effec-tiveness, and they can adversely affect the cellular metabolismdue to their high reactivity with proteins and inhibit cellgrowth when contacting with fungus.

Conclusions

CuNPs are synthesised in the aqueous phase by means ofsodium borohydride, where the CuNPs are functionalisedwithPVA. TEM analysis confirms that the NP has an average sizeof 3.5 nm and is spherical in shape. PVA acts like both as astabiliser and a size controller because it prevents nano-Cuseeds from agglomeration due to its polar O–H groups. HighMw PVA is observed to inhibit the oxide formation more thanlow Mw PVA. UV–vis spectroscopy confirms the stability ofthe sols at least 15 days and indicates the formation of CuNPsof size <5 nm. The CuNPs impregnation onto POFs viacationisation has been verified by FE-SEM, EDX and XRDanalyses, showing a good consistency among various obser-vations. After 30 days of exposition, 24 % of antifungalactivity was found for CuNPs. The observed mechanical and

antimicrobial properties of POFs by low-embedded CuNPsapparently make them potentially ideal reinforcing agents toenhance their mechanical property and durability. The devel-oped material (NP-POF) may consider as an advanced mate-rial for developing the nanocomposite of indoor to outdoorapplications.

Acknowledgements The authors would like to acknowledge UniversitiMalaysia Pahang, Malaysia for providing financial support throughRDU100395 for this research.

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