Effect of Morphology and Size of Halloysite ... - ACS Publications

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Effect of Morphology and Size of Halloysite Nanotubes on Functional Pectin Bionanocomposites for Food Packaging Applications Maziyar Makaremi,† Pooria Pasbakhsh,*,† Giuseppe Cavallaro,‡ Giuseppe Lazzara,‡ Yoong Kit Aw,§ Sui Mae Lee,§ and Stefana Milioto‡ †

Advanced Engineering Platform, Mechanical Engineering Discipline, School of Engineering, Monash University Malaysia, Selangor 47500, Malaysia ‡ Department of Physics and Chemistry, University of Palermo, Viale delle Scienze, pad. 17, Palermo 90128, Italy § School of Science, Monash University Malaysia, Selangor 47500, Malaysia S Supporting Information *

ABSTRACT: Pectin bionanocomposite films filled with various concentrations of two different types of halloysite nanotubes were prepared and characterized in this study as potential films for food packaging applications. The two types of halloysite nanotubes were long and thin (patch) (200−30 000 nm length) and short and stubby (Matauri Bay) (50−3000 nm length) with different morphological, physical, and dispersibility properties. Both matrix (pectin) and reinforcer (halloysite nanotubes) used in this study are considered as biocompatible, natural, and lowcost materials. Various characterization tests including Fourier transform infrared spectroscopy, field emission scanning electron microscopy, release kinetics, contact angle, and dynamic mechanical analysis were performed to evaluate the performance of the pectin films. Exceptional thermal, tensile, and contact angle properties have been achieved for films reinforced by patch halloysite nanotubes due to the patchy and lengthy nature of these tubes, which form a bird nest structure in the pectin matrix. Matauri Bay halloysite nanotubes were dispersed uniformly and individually in the matrix in low and even high halloysite nanotube concentrations. Furthermore, salicylic acid as a biocidal agent was encapsulated in the halloysite nanotubes lumen to control its release kinetics. On this basis, halloysite nanotubes/salicylic acid hybrids were dispersed into the pectin matrix to develop functional biofilms with antimicrobial properties that can be extended over time. Results revealed that shorter nanotubes (Matauri Bay) had better ability for the encapsulation of salicylic acid into their lumen, while patchy structure and longer tubes of patch halloysite nanotubes made the encapsulation process more difficult, as they might need more time and energy to be fully loaded by salicylic acid. Moreover, antimicrobial activity of the films against four different strains of Gram-positive and Gramnegative bacteria indicated the effective antimicrobial properties of pectin/halloysite functionalized films and their potential to be used for food packaging applications. KEYWORDS: patch halloysite, halloysite nanotubes, pectin, bionanocomposites, food packaging, thermal resistance, antimicrobial film

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INTRODUCTION

materials with enhanced mechanical, thermal, transparency, and antimicrobial properties. Biopolymers are polymeric biomolecules produced by living organisms such as plants, trees, and bacteria.3 These polymers are capable of undergoing decomposition into water, carbon dioxide, and inorganic compounds without toxic residues, predominantly through the enzymatic action of microorganisms.4 Ever increasing concerns have been raised over environmental burdens and exhausting natural resources caused by nonbiodegradable plastic packaging materials, which led to

In the past three decades, increased usage of petroleum-based nondegradable plastic products has raised concerns in terms of their economic and environmental sustainability. As a consequence, in view of the current global demand, biodegradability is not only a functional requirement but also an essential environmental attribute.1 Over the years, extensive research has been done on biomaterials extracted from natural resources for their potential applications in numerous fields such as medical products, cosmetics, bioimplants, drug delivery, and textiles.2 Biodegradability as one of the unique characteristics of biomaterials led to a rapid growth in their usage for food packaging applications and also to an expression of demand to develop new and smart biodegradable composite © 2017 American Chemical Society

Received: March 27, 2017 Accepted: May 8, 2017 Published: May 8, 2017 17476

DOI: 10.1021/acsami.7b04297 ACS Appl. Mater. Interfaces 2017, 9, 17476−17488

Research Article

ACS Applied Materials & Interfaces

been proven by several in vitro and in vivo studies.21,22Various factors such as crystal structure, degree of alteration, chemical composition, and the effects of dehydration lead to morphological variability of HNTs to tubular, spheroidal, and plate-like particles, of which the tubular structure is the most common and valuable.23 It has been suggested that tubular morphology of HNTs resulted from deformation of platy kaolinite with the progressive alteration of kaolinite inducing a loss of structural rigidity at points along the crystal, which is interpreted as hydration to halloysite. As the alteration of kaolinite progressed, the halloysite developed on the kaolinite plate, curled smoothly, and rolling up part of the plate.24,25 Typically, the length of HNTs varies from 50 to 5000 nm, with an external diameter of 20−200 nm and internal diameter of 10−70 nm.26 HNTs can be employed as effective nanocarriers for the encapsulation and controlled release of chemically and biologically active compounds27−29 due to their biocompatible nature as well as their tubular shape. Among various types of HNTs, patch (PT) HNTs deposited from Western Australia are considered to be a very pure form of halloysite (HNTs content of 98%) with fibrous structure.26 These HNTs are very long (up to 30 μm), parallel, and tubular particles within bunches that form as patches and thin layers.26,30 PT HNTs occur as very pale white-blue translucent plastic clay within deeply weathered greenstones below a thick lateritic cap and apparently form in an environment that is relatively unconstrained physically and is chemically quite free of impurity solutes.31 The reported length of undisturbed PT HNTs is measured to be up to 30 μm,24 while other HNTs are in the range of 50−3000 nm.26,30 Furthermore, the reported surface area for PT HNTs is 81.6 m2/g, which is higher than reported values of other HNTs such as Matauri Bay (MB) (22.10), Dragonite (DG) (47.30), Jarrahdale (JA) (44.6), Camel Lake (CLA) (74.7),32 and Hallopure (HP) (20−30). In addition, the central void or lumen of PT HNTs is reported to occupy 39% of the total volume (due to its thin walls), while this value is lower for other HNTs.26 In addition to PT HNTs, various other tubular fibrous clays with various lengths have been previously studied. For instance, fibrous palygorskite clay has been reported to be less than 5 μm in length33 with potential to be used as a drug carrier.34 Moreover, attapulgite as a family of fibrous hydrous magnesium silicates consisting of a randomly oriented network of densely packed fibers with lengths between 1 and 2 μm35 has been reported to enhance mechanical and thermal properties of polymer composites36 as well as adsorption of various heavy metals.37,38 Furthermore, in separate studies performed by Hughes39 and Santos,40 fibrous tubular kaolinite was reported to be few micrometers in length, while fibrous sepiolite with high adsorption ability has lengths in the range from 20 nm to 2 μm.41 HNTs have succeeded repeatedly as the composite filler due to their unique properties. For instance, HNTs have been used to reinforce polymer matrixes such as epoxy resins, polypropylene, polyamide, and styrene rubber,42 which could be correlated to their rod-like morphology, high aspect ratio, and their unique surface chemical properties.23 De Silva et al.43 reported that incorporation of HNTs into chitosan membranes enhanced their mechanical and thermal properties. Furthermore, incorporation of HNTs in pectin films has been reported by Cavallaro et al.,14 while the obtained nanocomposite blends represented a sustainable alternative to the plastic materials synthesized from fossil fuels.

enlarged demand for biodegradable packaging materials from renewable sources (biopolymers).5 In food packaging, a major emphasis is on the development of a package that hinders the gain or loss of moisture, prevents microbial contamination, and acts as a barrier against permeation of water vapor, oxygen, carbon dioxide, and other volatile compounds such as flavors and taints in addition to the basic properties of packaging materials such as mechanical, optical, and thermal properties.5,6 The challenge is to target nanotechnology toward the development of natural structures for food protection, and hence edible films based on biopolymers are gaining a great interest in the scientific community due to their biodegradable nature and their potential use in food industry.7 Numerous studies have been done on potential applications of biopolymers such as chitosan, cellulose, starch, collagen, pectin, etc.8−10 Among these biopolymers, natural pectin is a green emerging material for food packaging applications, which is in need for further studies to be used as smart food packaging material. Pectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants and is thought to consist mainly of D-galacturonic acid (GalA) units, joined in chains by means of α-(1−4) glycosidic linkage.10 Pectins are divided into two classes with respect to their degree of methylation (DM), such as low-methoxylpectins (DM < 50%) and highmethoxylpectins (DM > 50%).11 DM determines the solubility of pectins, their gelling ability, and film forming properties, which are essential for industrial applicability to a large extent.12 The degree of methyl esterification depends on the source and the processing conditions, for example, storage, extraction, isolation, and purification. At present, apple pomace (low DM, 24%) and citrus peels (high DM, 74%) are the main sources of commercially acceptable pectins. It has been stated that food packaging using pristine pectin does not match the physical and thermal properties of conventional packaging, which limits their practical application.13 On the contrary, pectin composites containing reinforced pectin biofilms have been employed in food conservation due to their excellent mechanical properties and low oxygen barrier capability.14 For instance, Da Silva et al.15 prepared pectin nanocomposites incorporated by cellulose nanocrystal and glycerol. Results indicated that these nanocomposite films are viable alternatives to replace poly vinyl chloride (PVC) films for packaging and conservation of strawberries. Furthermore, it has been reported that addition of functional additives such as plasticizers improves the mechanical properties of pectin films, while addition of emulsifiers increases the stability and enhances the adhesion within the pectin matrix.16 Moreover, pectin composites have been utilized as coating material for food conservation. For instance, Placido et al.17 coated mangaba fruits with pectin biofilms, which resulted in an increase of their shelf life. In another study, Gorrasi et al.18 reinforced pectin matrix with layered double hydroxide (LDH). Results indicated the significant capability of these coatings on extending the preservation time of apricots. Addition of clay nanoparticles to polymer blends has paved the way for the development of new polymer nanocomposites with exclusive mechanical and chemical properties. Among these clay nanoparticles, halloysite nanotubes (HNTs) are new and uniqe-shaped nanomaterials with exceptional properties like biocompatibility, high surface area, and nontoxicity.19,20 The latter is of importance for food packaging material and has 17477


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ACS Applied Materials & Interfaces Thermal behavior of biopolymers reinforced with nanoclays such as HNTs has been considered in many studies due to its importance in further utilization of the new hybrid materials.44,45 It has been reported by Marney et al.46 that HNTs enhance the thermal stability of the PA6/HNTs composite by serving as a thermal insulation barrier at the surface. Furthermore, De Silva et al.19 reported enhancement in thermal stability of poly(lactic acid) bionanocomposite films by incorporation of HNTs. Similarly, Cavallaro et al.2 reported improvement in thermal behavior of pectin bionanocomposite films with addition of HNTs, while the dynamic mechanical properties of these bionanocomposites did not suffer any structural change as a consequence of temperature raise and were competitive with those of many traditional plastics such as polyethylene terephthalate (PET), polyvinyl chloride (PVC), polypropylene (PP), etc., which are typically utilized for packaging applications. In another study, Cavallaro et al.47 reported the effect of incorporation of HNTs to hydroxypropyl cellulose (HPC). Results stated that addition of a low amount of HNTs increased the energetic barrier and led to enhancement of the thermal stability of HPC.32 In this study, we prepared biofilms based on apple pectin and two different types of HNTs, MB with shorter tubes and lower surface area and PT with much longer tubes and higher surface area, to obtain a new biodegradable bionanocomposite with superior thermal, mechanical, and surface tension properties. Our hypothesis was that a higher surface area beside the longer and patchy HNTs should affect the thermal and mechanical properties of biofilms significantly. All bionanocomposites were extensively investigated from the physicochemical viewpoint by determining the thermal and mechanical properties as well as wettability. The morphological study was crucial to explain the nanostructural features. Furthermore, both types of HNTs were employed as nanocontainers for salicylic acid (SA), which is a well-known biocidal agent. The successful incorporation of SA into the HNTs cavity can be used to control its release mechanism. On this basis, the HNTs/SA hybrids were dispersed into the pectin matrix with the aim to develop functional biofilms with antimicrobial properties that can be extended over time. The acquired knowledge aimed at achieving deeper comprehension regarding the properties of pectin/nanoclay biodegradable biofilms relevant for the design of new hybrid sustainable materials, particularly for food packaging applications.

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Figure 1. Collected clay deposits containing patch halloysite. aqueous mixture was poured in Petri dishes and left to dry in an oven at 50 °C for 16 h. The average thicknesses of the bionanocomposite were between 0.04 and 0.07 mm. Preparation of Antimicrobial Bionanocomposite Films. Loading of Salicylic Acid into the HNTs Lumen. Loading of SA into the HNTs lumen was achieved by using the procedure described elsewhere for similar systems.48,49 Briefly, 0.8 g of HNTs was mixed with a saturated solution of SA (392 mg mL−1) in acetone and then was kept magnetically stirred for ca. 5 h. The obtained dispersion was transferred to a vacuum jar, which was then evacuated using a vacuum pump. The suspension was kept under vacuum for 5 h and then was cycled back to atmospheric pressure. This last step was repeated five times to increase the loading efficiency. Additionally, HNTs were kept in the solution overnight to further increase the loading efficiency. Finally, HNTs were separated from solution by centrifugation and were washed once with acetone and once with water. Filling of HNTs/SA into the Pectin Matrix. The obtained HNTs/SA hybrids were used as filler for the preparation of pectin-based films with antimicrobial activity. As previously described, the aqueous casting procedure was used to obtain the bionanocomposite films. A fixed filler concentration (10 wt %) was selected. For comparison, a film based on pristine pectin with SA (0.24 wt %) into its structure was prepared. It should be noted that the concentration of the biocide corresponds to that of APMB10 filled with the modified MB HNTs. Characterization of Bionanocomposite Films. Morphology Analysis. The morphologies of HNTs and bionanocomposite films containing different percentages of HNTs were observed under an ultrahigh resolution field emission scanning electron microscope (FESEM, Hitachi SU8010). To prevent electrostatic charging during observation, the samples were coated with a thin layer of platinum. Furthermore, the morphology of HNTs was observed under the transmission (STEM) mode of the same FE-SEM instrument. Fourier Transform Infrared (FTIR) Analysis. FTIR spectroscopy (Thermo Scientific IS10) was conducted to identify the chemical interactions occurring inside the bionanocomposite films and the chemical compositional homogeneity of the samples. All spectra were obtained for wavelengths of 800−3800 cm−1 with 32 scans per specimen at 0.4 cm−1 resolution. Water Contact Angle Analysis. Contact angle measurements were performed by using an optical contact angle apparatus (model 250, ramé-hart instrument). The films were fixed on top of a plane solid support and kept flat, while the contact angle of water in air was measured by the sessile drop method. The water droplet volume was 2 ± 0.2 μL, and at least five measurements were carried out at the instant of drop deposition (t = 0) and subsequently at intervals of 20 s on each sample, until 80 s. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was performed using Q5000 IR apparatus (TA Instruments) under the

EXPERIMENTAL SECTION

Materials. Apple pectin (HM pectin, degree of methyl esterification, 74%, Mw = 30−100 kg mol−1) and salicylic acid (SA) were obtained from Aldrich. Matauri Bay (MB) HNTs were supplied as a “processed product” by Imerys from their Matauri Bay operation, while patch (PT) HNTs were a field sample donated by K. Norrish (University of Adelaide) as illustrated in Figure 1. PT HNTs flocculated readily when dispersed in deionized water. Repeated washing and adjustment of pH to 7.5−8, with NaOH, was necessary to achieve a clay suspension. Separation of coarse impurities, including quartz sand and silt, was by gravity sedimentation and size separation following Stokes’ Law.26 Preparation of Bionanocomposite Films. The bionanocomposite films were prepared by using the casting method from water. Briefly, a 2 wt % aqueous solution of apple pectin was prepared under stirring at 50 °C for ca. 3 h. The solution was left to equilibrate at room temperature under stirring overnight. An appropriate amount of PT and MB HNTs (2.5, 5, 10, 15, and 30 wt %) then was added to the pectin solutions and kept under stirring overnight (16 h) followed by 30 min of sonication in a sonication water bath. The well-dispersed 17478


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Figure 2. FE-SEM images of (a,b) pure patch (PT) halloysite with a bird nest structure and (c) Matauri Bay (MB) halloysite. nitrogen flow of 25 cm3 min−1 for the sample and 10 cm3 min−1 for the balance. The measurements were conducted by heating the sample from room temperature to 700 °C with a rate of 10 °C min−1 while the weight of each sample was ca. 6 mg. TG experiments were also performed at different heating rates (β) with the aim to investigate the kinetics of the nonisothermal degradation of pectin by means of isoconversional procedures, which provide the activation energy (Ea) without making any assumption on the reaction mechanism.50,51 Particularly, β was changed from 5 to 25 °C min−1 with a step of 5 °C min−1. On the basis of the Kissinger− Akahira−Sunose (KAS) method,52 we determined Ea for the pectin degradation as follows:

ln

E β AR = ln − a 2 Eag (α) RT T

Table 1. Morphological Characteristics of HNTs Used in This Study dominant particle shapes

MB PT

tubular long-tubular

length (nm)

inner diameter (nm)

outer diameter (nm)

aspect ratio (L/D)

100−3000 200−20 000

15−70 12−22

50−200 40−55

12 200

with each other and bent where they created a bird nest structure (Figure 2a), but also they have smaller diameter. MB HNTs were tubular with length around 100 and 3000 nm, inner diameter between 15 and 70 nm, outer diameter around 50 and 200 nm, and aspect ratio (L/D) of 12.26 PT HNTs were in the form of long thin tubes and within the ranges of outer diameter around 40 and 55 nm, inner diameter between 12 and 22 nm, length around 200 and 30 000 nm, and aspect ratio of 200. Furthermore, as can be seen in Figure 2c, the sample of MB HNTs mostly consists of short and stubby tubes with lengths ranging from 100 nm to 2 μm, while some long and thin tubes with lengths up to 3 μm were also observed. For further understand the morphological properties of PT HNTs, scanning transmission electron microscopy (STEM) micrographs have been illustrated in Figure 3. PT clay may be close to the ideal fibrous form of halloysite. While the PT tubes are long and have regular thin diameter and are flexible, their thin walls result in their breakage into shorter fragments during the dispersion and processing. Undisturbed tubes were observed as bundles of closely packed parallel tubes with lengths up to 20 μm, while Norrish et al.30 observed longer PT tubes with length of 30 μm. In addition, disturbed tubes were observed as short tubes with lengths in the range of 200−5000 nm, which is consistent with findings of Pasbakhsh et al.26 FE-SEM micrographs of pectin bionanocomposites reinforced by MB and PT HNTs are illustrated in Figure 4. It can be seen from Figure 4a that pectin bionanocomposite reinforced by 5 wt % MB HNTs has a semirough cross-section surface, while the surface roughness of the bionanocomposites increased by further increase in HNTs content up to 30 wt % (Figure 4b). Individual HNTs tubes can be observed in these micrographs (Figure 4a and b), which indicates a good dispersion of MB HNTs inside the bionanocomposite. Previously it was reported that addition of high loadings of HNTs (i.e., 30 wt %) will lead to the formation of clusters and agglomerations (in the range of 10−50 μm or bigger) inside the nanocomposite,54,55 while this was not observed in this study for MB HNTs. Nevertheless, it is expected that bionanocomposites were reinforced by high loading of HNTs to have a higher magnitude of stiffness in comparison with pristine pectin films.

(1)

where g(α) is the integral conversion function, A is the pre-exponential factor, while R is the gas constant and T is the absolute temperature. The Ea values at each conversion degree (α) were calculated from the slope of the ln β2 versus 1/T plot. T

Mechanical Analysis. Mechanical properties of pectin bionanocomposite films were determined from stress−strain curves obtained by DMA Q800 instrument (TA Instruments). Bionanocomposite films of rectangular shape (10.00 × 6.00 × 0.060 mm3) were tested at a stress ramp of 1 MPa min−1, and values of the elastic modulus (E), tensile stress at which the material fractures (σr), and the percent elongation at break (ε%) were obtained at a constant temperature of 26.0 ± 0.5 °C. Release Experiments. Release experiments of SA from the antimicrobial bionanocomposite films were conducted in 50% v/v ethanol, which is a food simulant solvent according to the European Regulations.53 For these measurements, 2 × 2 cm film samples were immersed in 1 mL of simulant in sealed vials and in the dark. The amount of SA released over time was determined by using an Analytic Jena Specord S 600 BU UV−vis spectrophotometer. Antimicrobial Activity. The antimicrobial activity of bionanocomposite films was investigated against Gram-negative Escherichia coli ATCC 25922, Salmonella Typhimurium ATCC 14028, Pseudomonas aeruginosa ATCC 10145, and Gram-positive Staphylococcus aureus ATCC 29213 by disk diffusion method. The strains were cultured in nutrient broth (Merck, Germany) for 24 h at 37 °C. Each bacteria culture was swabbed uniformly across a nutrient agar (Merck, Germany) and allowed to be adsorbed for 15 min. Discs (6 mm in diameter) obtained from bionanocomposite films were placed in the middle of the agar plate aseptically, and the plates were placed in a 37 °C incubator for 24 h. Inhibitory action of tested samples on the growth of bacteria was determined by measuring the diameter of the inhibition zone.

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HNTs sample

RESULTS AND DISCUSSION

Morphological Analysis. FE-SEM images of pure MB and PT HNTs are illustrated in Figure 2. It can be seen that PT HNTs (Figure 2a and b, Table 1) are much longer in length in comparison with other types of HNTs, as they are entangled 17479


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Figure 3. STEM micrographs of pure PT HNTs.

Figure 4. FE-SEM micrographs of cross sections of pectin bionanocomposites containing 5 wt % (a) and 30 wt % (b) MB HNTs; 5 wt % (c) and 30 wt % (d) PT HNTs. Micrographs on the right side of (b) and (d) are magnified to reveal the arrangement of MB and PT HNTs inside the pectin matrix.

Bionanocomposites reinforced by PT HNTs have been illustrated in Figure 4c and d. It can be seen that most of the PT HNTs did not disperse individually inside the bionanocomposite, but were observed mostly in shape of closely packed bundles of long tubes. Furthermore, at high loading (30 wt %), surface roughness was increased, which could lead to stiffer bionanocomposite films. Overall, it was observed that PT HNTs were not as well dispersed as MB HNTs in the bionanocomposite matrix due to their bird nest structure, while MB HNTs were excellently dispersed in the bionanocomposite even at high loadings (30 wt %). Interestingly, this did not lead to inferior properties for bionanocomposites reinforced with PT, especially at lower PT loading. Fourier Transform Infrared Analysis. The Fourier transform infrared (FTIR) analysis spectra of pectin, HNTs, and their bionanocomposites are reported in Figure 5. It can be seen that in spectra of pure PT and MB HNTs, the main bands appearing in the spectrum include O−H deformation of inner hydroxyl groups at 910 and 909 cm−1, in-plane Si−O stretching at 1008 and 1004 cm−1, O−H stretching of inner hydroxyl groups at 3621 and 3620 cm−1, and O−H stretching of innersurface hydroxyl groups at 3696 and 3693 cm−1.56

Figure 5. Fourier transform infrared spectra of (a) PT HNTs and (b) MB HNTs. The remaining spectra are derived from pectin bionanocomposites reinforced by (c) 30 wt %, (d) 5 wt % PT HNTs, (e) 30 wt %, (f) 5 wt % MB HNTs, and (g) pure pectin.

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ACS Applied Materials & Interfaces The signals at 3697 and 3623 cm−1 attributed to the O−H stretching vibrations of inner-surface hydroxyl groups of HNTs are well evident in bionanocomposites containing loadings above 10 wt % of MB or PT HNTs. Furthermore, an appreciable shift toward to higher wavelength (from 3329 to 3353 cm−1) was observed due to the addition of 30 wt % of MB HNTs into the pristine pectin, while a higher magnitude of shift (from 3329 to 3362 cm−1) was observed in bionanocomposite films reinforced with 30 wt % of PT HNTs. In addition, incorporation of 30 wt % of MB HNTs led to a shift toward a lower wavelength in spectra of pristine pectin from 1012 to 1008 cm−1, while this value is shifted to 1007 cm−1 in bionanocomposite films containing 30 wt % of PT HNTs. This shift is attributed to O−H deformation of inner hydroxyl groups of HNTs. Although several shifts in peak values were observed in the reinforced bionanocomposite, no major chemical interaction was evident between HNTs and pectin molecular chains. Water Contact Angle Analysis. Table 2 illustrates the water contact angle of pectin bionanocomposite films loaded

with different percentages of MB and PT HNTs. It can be seen that as the loading of MB HNTs increases, the water contact angle gradually decreases. This could be attributed to the hydrophilic nature of HNTs, which enhanced the surface hydrophilicity of the films. HNTs enrichment at the surface observed in the morphology section earlier is consistent with this theory. Hence, excellent dispersion of HNTs with hydrophilic nature led to an increase in hydrophilicity of the films and reduced their water contact angle. Enhancement in hydrophilicity of bionanocomposite films due to addition of HNTs has been reported in the literature previously.14,47,57 Conversely, significant changes were not observed in water contact angles of bionanocomposite films reinforced with PT HNTs. This could be attributed to the difference in chemistry and morphology of PT HNTs in comparison with other HNTs. As discussed earlier, PT HNTs were mostly in the shape of closely packed bundles of long tubes, which are not dispersed as individual nanotubes like MB HNTs in the bionanocomposite matrix. Therefore, the existence of PT HNTs, even at high loading, did not enhance the surface hydrophilicity of the bionanocomposite films. The creation of a bird nest structure for PT HNTs makes it almost impossible for the water droplet to be diffused where the droplet is facing a nest and patchy structure. This could be important for applications in which a hydrophilic surface is not desired such as self-cleaning surfaces and coatings, which are growing in popularity. In addition, it was observed that the value of the water contact angle decreases over time (see Figure S1). This could be due to spreading and absorption of water droplet, occurring during the measurements. Similar results were reported for pectin-based bionanocomposites14,47 and other biopolymers.58 Thermogravimetric Analysis. TGA curves of pectin bionanocomposites reinforced by MB and PT HNTs are depicted in Figure 6. It can be seen that pure pectin sample

Table 2. Water Contact Angle of Bionanocomposite Films Loaded with Different HNTs water contact angle (deg) HNTs loading (wt %) 0 2.5 5 10 15 30

MB 92 86 81 77 59 42

± ± ± ± ± ±

PT 2 3 2 1 3 2

92 90 89 90 86 89

± ± ± ± ± ±

2 3 4 3 6 2

Figure 6. TGA curves of pectin bionanocomposites containing different wt % of HNTs. 17481


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ACS Applied Materials & Interfaces (AP) underwent a weight loss during heating from 25 to 120 °C and a further sharp decrease at ca. 215 °C. The first weight loss is due to the moisture loss (ML120), while the second one is generated by the degradation of pectin.2 The value associated with degradation of polymer shifted to 230 °C by incorporation of 30 wt % PT HNTs, while this value did not change in other bionanocomposite samples. The degradation trend of bionanocomposites became lower for samples reinforced by 30 wt % PT and MB HNTs at weight loss values of 32% and 33%, respectively, while this value for the AP sample was 41%. In terms of final weight loss at 700 °C, samples reinforced by 30 wt % of both HNTs showed a 61% weight loss, while this value was much lower in the AP sample (74%). This indicates that, as expected, incorporation of high amount of HNTs into pectin greatly enhanced the thermal stability. It has been reported that thermal properties of the nanocomposites can be strongly affected by dispersion of filler in polymer matrixes.59 As discussed in the Morphological Analysis, MB HNTs were well dispersed in bionanocomposite, where a low amount of agglomerations was observed. Hence, uniformly dispersed HNTs inside the bionanocomposite acted like a barrier to the passage of the volatile pyrolized products of pectin, leading to retardation in the thermal degradation of the bionanocomposite and their significant high thermal properties.60 Similar results were observed for nanocomposites based on poly(propylene) (PP)61 and polyethylene glycol (PEG).62 Particularly, the addition of 10% w/w of HNTs into the matrix induced an enhancement of the polymer degradation temperature as a consequence of the entrapment of volatile degraded products into the nanoparticles’ lumen.61,62 In case of samples reinforced with PT HNTs, although the dispersion was not satisfactory, the final weight loss was similar to samples incorporated by well-dispersed MB HNTs. This could be explained by taking into account the bird nest structure of PT HNTs. PT long tubes with a high surface area provided better barrier properties than MB HNTs because the gaseous products had to take a longer pass through the sample. Furthermore, it is believed that HNTs enhance the thermal properties by entrapping the gaseous products in the lumen, which at the same time acts as an insulator. In case of PT HNTs, in addition to lumen, gaseous products may get trapped between the closely packed tubes. Moreover, the presence of interlayer water in HNTs will reduce the flammability and lead to improvement in thermal stability, while the ceramic nature of HNTs produces more char residue in comparison to polymers. Figure 7 illustrates the passage of gaseous products through pectin samples reinforced by PT and MB HNTs. Temperature at 5% weight loss, the maximum weight loss (%), and the temperature at maximum weight loss rate have been presented in Table 3. In case of samples reinforced by MB HNTs, it can be seen that the temperature at 5% weight loss increased at 5 wt % loading to 77.61 °C but decreased with further increase in HNTs content to 75.68 °C. However, the temperature at 5% weight loss for samples reinforced by PT HNTs was higher than pure pectin samples by 4 and 13 °C due to incorporation of 5 and 30 wt % HNTs, respectively. Furthermore, it can be seen that incorporation of 5 and 30 wt % of PT HNTs decreased the maximum weight losses of bionanocomposites by 72% and 61%, respectively. However, addition of 5 wt % MB HNTs did not enhance the thermal stability of the bionanocomposite. Conversely, further increase in MB HNTs content to 30 wt % greatly decreased the value of maximum weight losses. Consequently, except for the sample

Figure 7. Schematic of heat transfer across the pectin bionanocomposite samples containing (a) PT and (b) MB HNTs.

Table 3. Thermal Stability Parameters and Average Activation Energy of Pectin Bionanocomposites Containing Different wt % of MB and PT HNTs sample

temperature at 5% weight loss (°C)

residual matter at 700 °C (%)

temperature at maximum weight loss rate (°C)

activation energy (kJ mol−1)

AP APMB5 APMB30 APPT5 APPT30

59.03 77.61 75.68 63.20 72.59

26.03 26.13 39.47 28.13 39.16

241.55 243.36 238.10 243.25 246.11

150 ± 5 162 ± 13 206 ± 20

containing 5 wt % MB HNTs, char residue of reinforced bionanocomposites was increased as compared to pure pectin samples. It has been reported that the thermal stability is enhanced by increasing the char residue as the formation of char hinders the diffusion of the volatile decomposition products.42 In addition, it must be mentioned that solid reside of HNTs contributed to the increase in value of char residue. Furthermore, increase in the temperature at maximum weight loss rate was observed for bionanocomposites containing 5 wt % MB HNTs and samples reinforced by PT HNTs, indicating the enhancement in thermal stability of the bionanocomposite as compared to pure pectin sample, while this value was lower for samples containing 30 wt % MB HNTs. Enhancement in thermal properties of pectin films by incorporation of nanoclays has been studied previously. For instance, Cavallaro et al.2 reported a sharp improvement in thermal stability of pectin films reinforced by HNTs, kaolinite, and laponite. In another study, Cavallaro et al.14 reported enhancement in thermal stability of pectin/polyethylene glycol films reinforced by HNTs. TG experiments at different β were conducted to investigate the effect of HNTs on the kinetics of pectin degradation. Figure 8 shows that Ea is not influenced by α (in the range between 0.2 and 0.6) for the pristine polymer and the bionanocomposites at larger filler content (Cf = 30 wt %). On the basis of the Ea versus α trends, we estimated the average values for the activation energy (Table 3). PT HNTs generated a strong enhancement of the energetic barrier to the degradation process of pectin. On the contrary, the presence of MB HNTs did not induce a significant variation of the Ea value. These results are in agreement with the thermal stability properties of the bionanocomposites. Mechanical Analysis. The tensile properties of the pectin bionanocomposite films reinforced with MB and PT HNTs are illustrated in Figure 9. It can be seen that addition of MB HNTs 17482


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nanocomposites containing high HNTs loadings have been reported previously. For instance, Pasbakhsh et al.56 observed a significant enhancement in mechanical properties of ethylene propylene diene monomer (EPDM) nanocomposites by addition of 30 and 50 phr of HNTs. Furthermore, for pectin films reinforced with PT HNTs, a halloysite content of 5 wt % leads to an increase of tensile fracture from 41.4 MPa in the pure pectin film to 62.1 MPa in the bionanocomposite, while significantly enhancing the elastic modulus from 2072 to 4298 MPa. This improvement is a clear indication of the role of the patchy and bird nest structure of PT HNTs on the strength of the pectin films and their uniform distribution in pectin bionanocomposites, which enhanced the affinity and the adhesion of the polymers to the filler surface as reported for other polymer/clay composites.19,43,60,63 Further enhancement in the PT halloysite content up to 15 wt % leads to a reduction in the value of tensile fracture, while the improvement was observed in the value of elastic modulus to 5412 MPa. Furthermore, a downtrend was observed in tensile fracture and elastic modulus of bionanocomposites reinforced by 30 wt % PT HNTs, indicating that PT HNTs are no longer a reinforcement to the polymer at high loadings due to the formation of HNTs agglomerations, while the value of elastic modulus recovers back to the value in the range of films reinforced with 5 wt % HNTs. It must be mentioned that formation of agglomerations of nests resulted in a reduction of the effective surface area, which decreases the matrix−filler interfacial regions and impairs the effective load transfer from the polymer matrix to the fillers, leading to an overall reduction in mechanical properties of the bionanocomposite. In means of values for strain at break, it can be seen that incorporation of both MB and PT HNTs into pectin bionanocomposites gradually reduces the magnitude of strain in comparison with pristine pectin. This indicates that addition of HNTs increased the stiffness of the bionanocomposite, which is attributed to the corresponding increase in the structural rigidity of the pectin polymer chains, arising from the interaction between adjacent HNTs and pectin. In other words, the presence of HNTs restricts the movement of pectin polymer chains located at the matrix−filler interface, and hence the overall stiffness of the bionanocomposite increases. These results are in accordance with the literature.54,63,64 Overall, it could be said that incorporation of HNTs into pectin bionanocomposite films enhanced the values of tensile fracture and elastic modulus, while the magnitude of strain had a reducing trend. Furthermore, bionanocomposite films with 5 wt % PT HNTs showed the best mechanical properties, while the biggest improvement in films reinforced with MB HNTs was observed with addition of 30 wt % of MB HNTs. This variation of the obtained results between both types of HNTs could be attributed to their dissimilarity in physical characteristics. Stress−strain curves associated with the bionanocomposite films have been provided in Figure S3. Functional Biofilms Based on Pectin and HNTs Loaded with Salicylic Acid. Loading of the Salicylic Acid into the HNTs Lumen. The HNTs/SA hybrids were investigated by thermogravimetry to estimate the loading efficiency of both types of halloysite nanotubes (PT HNTs and MB HNTs). In particular, the amount of SA entrapped into the HNTs cavity (Table 4) was determined by using the rule of mixtures for the residual matters at 800 °C.65 To this purpose, TG curves of both the hybrid materials and the corresponding

Figure 8. Activation energy as a function of the conversion degree for pristine pectin and pectin/HNTs bionanocomposites with a filler content of 30% w/w.

Figure 9. Mechanical properties of the pectin bionanocomposite films reinforced with MB and PT HNTs.

up to 5 wt % slightly improves the values of tensile fracture and elastic modulus, while further enhancement in HNTs content to 15 wt % does not provide a significant alteration in these properties. Interestingly, by addition of MB HNTs up to 30 wt %, significant improvement was observed in the value of tensile fracture, which increased from 41.4 MPa in the pristine pectin film to 49 MPa in the bionanocomposite. Additionally, major enhancement was observed in the value of elastic modulus of the same sample by means of an increase from 2072 to 4617 MPa, which indicates excellent dispersion of HNTs in the bionanocomposite. Improvements of mechanical properties of 17483


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where R%(t) is the percent of SA released at time t, while k and n are the characteristic kinetics constant and the release exponent, respectively. It should be noted that n depends on the release mechanism as well as on the specific geometry of the device. As evidenced in Figure 10, the proposed model was successful for all of the investigated films. Table 5 highlights that the biocomposite films possess smaller k values with respect to that of pristine pectin.

Table 4. Amounts of SA Entrapped into HNTs sample

loading (wt %)

PT HNTs/SA MB HNTs/SA

6.6 ± 0.4 9.4 ± 0.3

pristine components (HNTs and SA) were collected (see Figure S2). The loading values (Table 4) evidenced that MB HNTs possess a better ability for the encapsulation of SA into their cavity. These results could be attributed to the larger inner diameter of MB HNTs with respect to that of PT HNTs. In addition, the utilized loading procedure might not be appropriate for a complete filling of the longer nanotubes (PT HNTs). Moreover, due to the patchy and bird nest structure of PT HNTs, encapsulation was relatively difficult, and it might make it impossible for biocide to access all of the tubes. Antimicrobial Pectin Films: Kinetics Release of SA in Food Simulant Solvent. The modified HNTs were dispersed into the pectin matrix to obtain bionanocomposite films with antimicrobial properties, which can be tuned over time. Within this, the kinetics release of SA from the prepared biofilms was investigated. These experiments were conducted in 50% v/v ethanol, which simulated the exposure of the films to a food solvent. Figure 10 shows the kinetics release for the

Table 5. Kinetics Parameters for the SA Release sample

K (s−n)

n

R2

AP APPT10 APMB10

21.1 ± 1.2 15.3 ± 0.3 9.0 ± 0.4

0.248 ± 0.011 0.287 ± 0.003 0.375 ± 0.008

0.9671 0.9960 0.9853

This effect indicates that the confinement of SA into the HNTs lumen can be used to control the release of the antimicrobial agent into the food simulant solvent. Interestingly, k is significantly smaller for the bionanocomposites filled with MB HNTs. Accordingly, with the loading results, the faster kinetics for the APPT10 could be ascribed to the presence of SA mostly on the edge of the long nanotubes, which cannot be fully loaded inside the HNTs lumen. As concerns n data, we determined low values for AP and APPT10. On the contrary, AP MB10 showed larger n values, highlighting a peculiar release mechanism for the bionanocomposite filled with MB HNTs. The power fit equation was successful in the release data analysis as evidenced by the values of the coefficient of determination (R2) (Table 5). Antimicrobial Activity. Antimicrobial activities of bionanocomposite films reinforced by modified MB and PT HNTs against various Gram-positive and Gram-negative bacteria were evaluated by the disk diffusion method (Table 6). It can be seen Table 6. Antimicrobial Activity of Functional Bionanocomposite Films diameter of zone of inhibition (mm) bacteria E. coli P. aeruginosa S. aureus Salmonella

Figure 10. Percent of the SA released on time from the prepared biofilms in 50% v/v ethanol. Dashed lines represent the best fitting according to eq 2.

19.0 16.0 23.0 19.0

± ± ± ±

1.0 1.5 2.0 1.5

APPT 17.0 13.0 21.0 16.0

± ± ± ±

1.5 2.0 1.0 1.0

that SA encapsulated MB HNTs possessed better antimicrobial properties, especially against Gram-positive S. aureus, while a smaller zone of inhibition was observed in samples reinforced by modified PT HNTs. This could be attributed to the lower content of encapsulated SA in PT HNTs in comparison to MB HNTs, as revealed by TGA analysis, in conjunction with their dissimilar release kinetics. Overall, the bionanocomposite films reinforced by modified HNTs displayed effective activity against both Gram-positive and Gram-negative bacteria, as illustrated in Figure 11. Comparative Discussion on MB and PT HNTs as Fillers and Carriers. In this study, lengthy PT HNTs with thin fibrous morphology, high surface area (81.6 m2/g), and bird nest-like structure were compared to MB HNTs, which consist of short and stubby tubes with a low surface area (22.10 m2/g), to evaluate their enhancing properties in improvement of mechanical, thermal, and physical properties of the pectin bionanocomposite films. Tensile properties were enhanced by

bionanocomposites with both types of HNTs. These data were compared to those of films based on pristine pectin containing SA (0.24 wt %). It can be seen that in the first 3 h, samples reinforced by modified MB HNTs released a lower amount of SA in comparison to samples reinforced by modified PT HNTs, while subsequently the release rate became similar for both samples. The faster kinetics for the APPT10 in the first 3 h could be ascribed to the presence of SA on the edge and between the long nanotubes. To study the release mechanism, the release data were analyzed according to the power fit equation: R %(t ) = kt n

APMB

(2) 17484


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Figure 11. Zones of inhibitions of APMB (above) and APPT (below) bionanocomposite films against various Gram-positive and Gram-negative bacteria.

Table 7. Assessment of the Enhancing Performance of the Studied HNTs in Relation to Their Suitability for Use as Nanofillers and Carriers suitability for

a

HNTs sample

mechanical propertiesa

hydrophilicity

thermal properties

loading efficiency

release rate

additives

carriers

MB PT

M H

H L

M H

H M

M M

M H

H M

H = high, M = moderate, L = low.

enhancement of mechanical and thermal strength of polymers, was grouped as additives, while utilization of HNTs as vehicles for the supply and controlled release of active agents was grouped into carriers. MB HNTs provided adequate enhancement in thermal stability and hydrophilicity of the films, in addition to their excellent loading efficiency. However, their attribution in improvement of mechanical and thermal properties of the bionanocomposite films was inferior in comparison to PT HNTs. This could be enhanced by the chemical surface treatment of HNTs, which leads to the formation of chemical bonds between the polymer matrix and HNTs. In case of PT HNTs, significant improvement in thermal properties was observed along with an enhancement in tensile properties and notable loading efficiency. Loading efficiency could be significantly enhanced by enlargement of the HNTs lumen with alumina etching method, as described by Abdullayev et al.67 In addition, a negligible effect of PT HNTs on hydrophilicity of the bionanocomposites is of importance in coatings and self-cleaning surfaces. It can be concluded that MB HNTs have high potential as a carrier, while PT HNTs provide mechanical and thermal enhancement to the bionanocomposite as an additive. The contrast between the properties of these HNTs could be explained by their dissimilarity in size and morphology. It must be mentioned that further study on PT HNTs consisting of methods for chemical exfoliation of these closely packet nanotubes is required, as we believe that separated PT HNTs have the capability to be utilized as both carrier and additive.

incorporation of PT HNTs as a clear indication of the role of the patchy and bird nest structure on the strength of the pectin films and their uniform distribution in pectin bionanocomposites, which enhanced the affinity and the adhesion of the polymers to the filler. In addition, it can be concluded that incorporation of longer tubes (PT HNTs) leads to further enhancement in mechanical properties, in comparison to the addition of shorter tubes (MB HNTs). Similar results were observed by Erpek et al.66 where incorporation of long carbon nanotubes (CNTs) led to much stronger poly lactic acid (PLA) composites, in comparison to addition of HNTs, which were shorter in length. In means of water contact angle, incorporation of MB HNTs greatly enhanced surface hydrophilicity of the bionanocomposite films, while addition of PT HNTs did not lead to significant alteration, even at high HNTs content (30 wt %). This could be attributed to the difference in dispersion of PT HNTs, which were not dispersed as individual nanotubes like MB HNTs in the bionanocomposite matrix. In means of thermal properties, PT HNTs with a high surface area provided better barrier properties than MB HNTs, because the gaseous products have to take longer to pass through the sample. Furthermore, PT HNTs generated a strong enhancement of the energetic barrier to the degradation process of pectin. On the contrary, the presence of MB HNTs did not induce a significant variation of the Ea value. In means of loading of antibacterial agent, MB HNTs possess better encapsulation ability as well as have a slower release mechanism in comparison to PT HNTs. Higher encapsulation of antibacterial agent led to better performance of functional pectin bionanocomposite pectin films reinforced by MB HNTs against various Gram-positive and Gram-negative bacteria. The performance of both studied HNTs in enhancing mechanical, thermal, and physical properties of pectin bionanocomposite has been summarized in Table 7. The applications of HNTs as nanomaterials, such as components for

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CONCLUSION In this study, we have used 2.5, 5, 10, 15, and 30 wt % of two different types of halloysite nanotubes (HNTs) to improve the properties of apple pectin bionanocomposites as potential films to be used in food packaging applications: patch (PT) from 17485


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ACS Applied Materials & Interfaces western Australia with long, thin nanotubes and patchy structure versus a usual type of halloysite from New Zealand (Matauri Bay) (MB) with shorter and stubbier tubes. Morphological study revealed that patchy PT HNTs created a bird nest structure inside the pectin bionanocomposite films, while MB HNTs were dispersed uniformly in the bionanocomposites in all of the loadings. Tensile properties indicated an exceptional improvement in tensile strength of the pectin films reinforced with 5 wt % PT HNTs and 30 wt % MB HNTs. Pectin bionanocomposite films with 30 wt % PT HNTs exhibited exceptional hydrophobicity properties as compared to films with MB HNTs. Thermal properties analysis also revealed that pectin films reinforced by PT HNTs possess higher thermal stability, due to their unique structure. This is due to the fact that during thermal degradation it will take much more energy and time for volatile compounds to pass through to the patchy and bird nest structure of PT HNTs inside the pectin matrix. The release kinetics of bionanocomposites was studied by encapsulating a biocide into the lumen of both HNTs. Results indicated that longer tubes are able to release the SA in a rapid manner, while shorter tubes represent a peculiar release mechanism. Furthermore, antimicrobial activity of bionanocomposite films against various Gram-positive and Gramnegative bacteria indicated the effective antimicrobial properties of these functionalized films. These results signified the potential of functionalized pectin/HNTs bionanocomposite films to be used for food packaging applications, although further studies are required to fabricate these films on an industrial scale.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04297. Water contact angle evaluation over time (Figure S1); TG curves for HNTs/salicylic acid hybrids and their corresponding pristine components (Figure S2); and DMA stress−strain curves (Figure S3) (PDF)

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AP = apple pectin DM = degree of methylation β = heating rates Ea = activation energy α = conversion degree E = elastic modulus σr = dracture stress ε = elongation at break K = kinetics constant n = release exponent R2 = coefficient of determination

AUTHOR INFORMATION

Corresponding Author

*Tel.: +60-3-55146211. E-mail: pooria.pasbakhsh@monash. edu. ORCID

Maziyar Makaremi: 0000-0002-5610-1834 Pooria Pasbakhsh: 0000-0003-4771-9453 Giuseppe Lazzara: 0000-0003-1953-5817 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to express our gratitude to the Advanced Engineering Platform at Monash University Malaysia. This work was partially funded by the University of Palermo, FIRB 2012 (prot. RBFR12ETL5).

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ABBREVIATIONS HNTs = halloysite nanotubes SA = salicylic acid MB = Matauri Bay PT = patch 17486


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