tannin bio-nanofibers

tannin bio-nanofibers

Industrial Crops and Products 52 (2014) 298–304 Contents lists available at ScienceDirect Industrial Crops and Products journal homepage: www.elsevi...

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Industrial Crops and Products 52 (2014) 298–304

Contents lists available at ScienceDirect

Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop

Electrospinning of zein/tannin bio-nanofibers Cláudia L.S. de Oliveira Mori c,∗ , Nathália Almeida dos Passos a , Juliano Elvis Oliveira b , Luiz Henrique Capparelli Mattoso d , Fábio Akira Mori c , Amélia Guimarães Carvalho c , Alessandra de Souza Fonseca c , Gustavo Henrique Denzin Tonoli c a

Department of Food Science, Federal University of Lavras, C. P. 3037, 37200-000 Lavras, MG, Brazil DEMAT, Federal University of Paraíba, Brazil Department of Forest Science, Federal University of Lavras, C. P. 3037, 37200-000 Lavras, MG, Brazil d Lab. Nac. de Nanotecnologia para o Agronegócio (LNNA), Embrapa Instrumentac¸ão (CNPDIA), C. P. 741, CEP 13560-970 São Carlos, SP, Brazil b c

a r t i c l e

i n f o

Article history: Received 18 June 2013 Received in revised form 25 September 2013 Accepted 28 October 2013 Keywords: Stryphnodendron adstringens Polyphenol Nanotechnology

a b s t r a c t In order to investigate the incorporation of tannin from barbatimão bark in zein nanofibers obtained by electrospinning, it was studied the effect of addition of different contents of tannin in the properties of zein nanostructured membranes and their relationship with fiber morphology. It was confirmed the interaction occurring between the tannin and zein by thermal and microscopy analysis. Addition of tannins increased the glass transition temperature of the nanofibers, suggesting higher energy input for processing for example. SEM micrographs provided evidence of a homogeneous structure for the nanostructured membranes. X-ray analysis showed the presence of zein crystals in the nanofibers. This ongoing research confirms the possibility of incorporation of barbatimão tannin in the production of bio-nanofibers that will be studied for multi-purpose applications. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction Nanostructured materials obtained from renewable resources have been used for healthcare (Lee et al., 2009), production and processing of food (Kriegel et al., 2008), agriculture (Bansal et al., 2012), environmental protection (Gavrilescu and Chisti, 2005) and forestry (Spasova et al., 2011). The application of nanobiomaterials across various sectors led to both environmental and economic benefits, including enhanced product quality and sustainable technologies (Gavrilescu and Chisti, 2005). Nanofibers obtained from biopolymers are of particular interest for potential applications in medicine, drug delivery and agriculture (Neo et al., 2012). Biomedical applications of such non-wovens nanofibers comprise wound dressing and release of drugs like antibiotics and anti-inflammatory (Chen et al., 2013). Applications of nanofibers in agriculture could include water treatment (Feng et al., 2013), the estrous control of livestock animals (Oliveira et al., 2013) and crop protection (Sun et al., 2010). The actual stage of several technologies requires the development of entirely new approaches for the construction of two and three-dimensional nano-architectures (Sun et al., 2010). In many cases, these nanostructures can be obtained by electrospinning (Lowe, 2000). The

∗ Corresponding author. Tel.: +55 35 2142 2032; fax: +55 35 2142 2032. E-mail address: [email protected] (C.L.S. de Oliveira Mori).

electrospinning process is based on the application of a high voltage across a conductive needle attached to a syringe containing the polymer solution and a conductive collector (Kumar et al., 2012). The majority of scientific papers related with electrospinning biodegradable polymers have been focused on synthetic materials, mostly on polylactic acid, polyglycolic acid and polycaprolactone and their copolymers (Ashammakhi et al., 2009). In comparison with synthetic counterparts, biopolymers generally have better biocompatibility and are ecofriendly (Cha and Chinnan, 2004; Lawton, 2002; Shukla and Cheryan, 2001). High added-value nanocomposites have been reported in literature (Wang et al., 2013; Fernandez et al., 2009) using vegetable extracts for different applications. Wang et al. (2013) evaluated nanocomposite fiber mats of the soy protein isolate using low concentrations of poly(ethylene oxide) containing higher levels of red raspberry extract, rich in anthocyanins (Rubus strigosus) extract and obtained by electrospinning. The addition of the plant extract after denaturation formed soy protein isolate solutions with a significantly higher level of bioactive anthocyanin and greater antibacterial activity against Staphylococcus epidermidis. Therefore, plant extracts rich in anthocyanin can serve as new ingredients to create new biological active functionalized nanomaterials for based food systems. Fernandez et al. (2009) evaluated successfully the encapsulation of a food additive with antioxidant properties in an ultrafine fiber zein prolamins edible biopolymer through electrospinning. According to the authors the encapsulated component

0926-6690/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.10.047

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Table 1 Blend-design for the bio-nanofibers studied. Formulations

Blend ratio (zein:tannin)

Mass fraction of zein (wt%)

Neat zein ZT1 ZT2 ZT3

1.0:0.0 0.90:0.10 0.85:0.15 0.80:0.20

100 90 85 80

2. Materials and methods Fig. 1. Basic structure of flavonoids precursor units of condensed tannins.

showed a remarkably good protection against oxidation when exposed to UV–vis. Zein is the prolamine in corn, and is an abundant biopolymer in corn gluten meal, which is a co-product of corn wet milling (Shukla and Cheryan, 2001; Lawton, 2002). Because zein is hydrophobic, with good elasticity and with film forming capabilities, it has been used in the food industry as a coating material for several products (Lawton, 2002). Zein has also been used in the pharmaceutical industry as an excipient in pharmaceutics, and controlled release (Karthikeyan et al., 2012). Zein possesses the additional benefits of being renewable and biodegradable (Shukla and Cheryan, 2001). The production of zein nanofibers by electrospinning was reported previously (Neo et al., 2012; Karthikeyan et al., 2012), however no information was found in literature about the possibility of incorporating tannin in its structure for different applications. Among several interesting species, the barbatimão (Stryphnodendron adstringens) is a species of Cerrado biome that presents 30–35% (in mass) of tannin in the tree bark (Siqueira, 2005). Condensed tannin from bark extracts of S. adstringens and Stryphnodendron polyphyllum contains about the same amount of prodelphinidins units in both species (Santos et al., 2002). However, the bark tannin of S. polyphyllum contains more esterified gallic acid in its structure, which increases the phenolic hydroxyl groups in the molecules and consequently modifies the spatial arrangement of the polymers. The available hydroxyl groups in the condensed tannins (Fig. 1) permit their reaction with other chemical species for development of new materials. Tannins extracted from barbatimão species has attracted the interest of the scientific community due to their chemical activities, such as anti-inflammatory, antibacterial, antiseptic and antimicrobial, and for the treatment of leucorrhea, gonorrhea, vulvo-vaginal candidiasis, gastritis, sore throat, diarrhea, bleeding, and antiulcer (Mendonc¸a et al., 2012; Costa et al., 2010). Some of these properties already exhibit scientifically validated activity (Ishida et al., 2006). Besides the use in folk medicine as an anti-inflammatory, healing and anti-hemorrhagic to treat venereal and stomach diseases, recent studies have shown that tannins can be used as feedstock for the production of adhesives, due to the reactivity of their condensed tannins present in the bark and leaves (Almeida et al., 2010). Then, there is the interest to use the tannins for multi-purpose applications, since it is a natural material with a renewable character, which can be extracted through sustainable techniques and from non-destructive removal of the bark. The electrospinning process allows different applications that are derived from materials such as zein and tannin, with their proper use as polymer precursors for processing of new materials. The present study sought to contribute for the need of obtaining biomaterials that can replace synthetic polymers commercially used in several industrial sectors. The objective of this study was to investigate the effect of adding bark tannin obtained from barbatimão (S. adstringens), at different concentrations (10, 15 and 20% by mass), in the properties of bio-nanostructured zein membranes.

2.1. Bark collection and extraction of the barbatimão tannin The barbatimão (S. adstringens) bark was collected in an area of Cerrado biome fragment, located in Lavras, state of Minas Gerais, Brazil, at 919 m height, 21◦ 15 56 , 97 S latitude and 44◦ 58 34 , 65 O longitude. The climate is type Cwb, according to the Köppen classification (mesothermal with mild summers and mild and dry winter). In order to permits the natural recovering of its bark by the barbatimão tree, it was collected approximately 25% of the outer bark, relative to the total volume of the bark. Samples were collected from 1 m above the ground level. The bark material was air dried after collection. After drying, the material was ground in a chopped hammer mill in order to obtain a uniform and finer material and selected in an 18-mesh sieve. Then, the tannin was obtained by extraction of the milled bark in hot water for 3 h. After extraction the tannin extract solution was oven dried to evaporate the water and the resultant dry material was milled with a pistil. 2.2. Production of the bio-nanofibers Zein from maize (CAS 9010-66-6) was obtained from Sigma–Aldrich (USA) and used as the main polymer of the nanofibers to be produced. Ethanol (CH3 CH2 OH, CAS 64-17-5) was purchased from Synth Chemical and was used as solvent. Four bio-nanofiber formulations were tested, as presented in Table 1. The polymer solutions were prepared by dissolving the zein and the corresponding tannin content in ethanol:water (8:2, v/v). The formulations were prepared using a polymer concentration of 20% (by mass). The formulations were rigorously stirred for several hours (up to 24 h) to ensure the complete dissolution of the constituents. The polymer solutions were spun into nanofibers by electrospinning according the procedures described in Oliveira et al. (2012). A syringe pump (KD scientific, model 781100) was used to feed the polymer solution (20 ␮L/min) through a needle. High voltage was applied between the needle and the collector, at a constant value (20 kV). The electrospinning parameters were kept constant for all experiments, and the nanofibers were collected on a rotating drum with a working distance of 12 cm. The nanofiber mats of each formulation were stored in a desiccator until the characterization tests. 2.3. Scanning electron microscopy (SEM) The morphology of the electrospun nanofibers was analyzed using scanning electron microscopy (SEM, Zeiss, model DSM960). Samples were prepared by cutting the nanofiber mats with a razor blade and mounting them on aluminum stubs using double-side adhesive tape. Samples were then gold sputtering coated (Balzers model SCD 050, Balzer Union AG, Balzers). The nanofibers diameters were measured with the aid of an image analyses software (Image J, National Institutes of Health). The average nanofiber diameter and diameter distribution were determined from approximately 100 random measurements using representative micrographs.

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Table 2 Average diameters and standard deviations of the electrospun bio-nanofibers and bead content in the bio-nanofiber mats. Electrospun nanofibers

Diameter (nm)

Zein neat ZT1 ZT2 ZT3

146 111 86 120

± ± ± ±

33 27 22 44

Bead content (beads/␮m2 )

Formulation

˚ d1 (A)

˚ d2 (A)

˚ d3 (A)

D1 (nm)

D2 (nm)

D3 (nm)

0.1 0.5 0.3 0.8

Neat zein ZT1 ZT2 ZT3

12 12 12 12

9 9 9 9

14 14 14 14

4 4 4 2

4 3 3 3

1 1 1 1

Subscripted: peak number 1 (at 14◦ ); 2 (at 17◦ ) and 3 (at 25◦ ).

ZT1: zein + 10% of tannin; ZT2: zein + 15% of tannin; ZT3: zein + 20% of tannin.

2.4. X-ray diffraction (XRD) XRD patterns of the electrospun nanofibers were recorded using a Shimadzu (XRD-6000) X-ray diffractometer. Scans were carried out from 3◦ to 35◦ (2) at a scan rate of 5◦ /min using Ni filtered Cu-K␣ radiation (wavelength of 0.154 nm) at 50 kV and 20 mA. The full-width at half-maximum height (FWHM) of the diffraction peaks was calculated by fitting the X-ray diffraction patterns with a Gaussian–Lorentzian function (Origin 7.5 software, Origin Lab, USA). The d-spacing for a given scattering angle, 2, was calculated by application of the Bragg equation (Eq. (1)): d=

 2 sin 

(1)

where  is the wavelength of the Cu-K␣ radiation. The crystallite size, D, was estimated by calculating the broadening of the diffraction peaks according to the Scherrer equation (Eq. (2)): D=

k ˇ cos 

Table 3 Interplanar distances (d) and crystallite diameters (D) of the crystalline parts of the electrospun nanofibers.

(2)

where k is the Scherrer constant that is dependent upon the lattice direction and crystallite morphology, and ˇ is the full-width at halfmaximum height given in radians. A k value of 0.9 was used in this study, which is based on values found in the literature for crystals of biopolymers (Huang et al., 2007; Marega et al., 1992). 2.5. Thermal analysis Differential scanning calorimetry (DSC, TA Instruments calorimetric analyzer, Q100 model) was performed under nitrogen atmosphere, at a flow rate of 20 mL/min and with a heating rate of 10 ◦ C/min. Samples were sealed in aluminum pans and heated from −85 ◦ C to 230 ◦ C for all nanofibers samples. 3. Results and discussion 3.1. Scanning electron microscopy (SEM) The images obtained by SEM shows that the interaction between tannin and zein has occurred (Fig. 2), leading to nanofibers with homogeneous morphology. The miscibility of zein and tannin is the result of the interactions between the amino and carboxyl groups of zein and hydroxyl groups of the tannin (Fig. 1). Table 2 presents the average diameters and the beads content formed during electrospinning, while Fig. 3 depicts the diameter distribution of the electrospun nanofibers of the different formulations. It is observed a decrease of the average diameter (Table 2) of the nanofibers with the addition of tannin. This decrease in the diameter of the nanofibers can be attributed to decrease in the viscosity of the solution, which influences the electrospinning processing due to the changes promoted on the tip of the Taylor Cone. Upon increasing the voltage to a critical value, charges on the surface of the solution drop lead to a new geometry, i.e. the Taylor Cone (Khadka and Haynie, 2012). Two forces play as antagonist role: electric forces and surface tension. Beyond a critical voltage,

the electric forces overcome the surface tension of the solution and an electrically charged jet erupts from the tip of the Taylor Cone. As the charged jet accelerates toward regions of lower potential, the solvent evaporates while the viscoelastic behavior of the polymer chains prevents the jet from breaking up (Reneker et al., 2007). This behavior results in the nanofiber formation. The diameter of the electrospun nanofibers generally ranged between 40 and 260 nm (Fig. 3). Zein (neat) mats have just around 5% of their nanofibers lower than 100 nm in diameter (Fig. 3a), while at least around 40% of the nanofibers obtained with addition of tannins were lower than 100 nm in diameter (Fig. 3b–d). The diameter distribution was narrower for the formulation with 15% of tannin, whose around 70% of their nanofibers were lower than 100 nm in diameter (Fig. 3c). This behavior is an indicative that 15% is the content of tannin that implies in the optimum viscosity of the electrospinning solution for obtaining nanofibers with lower diameters. Costa et al. (2013) reported the increase in the diameter of electrospun nanofibers produced with PVA and pineapple nanofibers when adding 1% (w/w) of S. adstringens tannin extracted in alcoholic solution. It is noteworthy that the extraction method and the solvent type have influence on the quality and quantity of the tannic extract, which can be confirmed in studies of Mori et al. (2001, 2002, 2003). The molecular weight and structure of the phenolic compounds in the tannin can vary significantly and they may contain different contents of hydroxyl groups able react with peptide carbonyl groups of proteins (Damodaran, 1996). Torres-Giner et al. (2008) obtained cylindrical electrospun zein nanofibers with average cross-section below 100 nm and using very specific concentrations of ethanol-aqueous solutions. Torres-Giner et al. (2008) reported the production of zein nanofibers by eletrospinning, which led to white colored zein fiber networks with fiber diameters ranging from less than 100 nm to above 1 mm. In that study, the nanofibers produced exhibit tubularlike shapes and complex fiber morphologies, such as nanobeads, were also observed. According to the authors (Torres-Giner et al., 2008), acidifying the alcohol zein solution yielded ribbon like morphologies. In the present work, the addition of tannin appears to have changed the morphology of the nanofibers from a cylindrical format (Fig. 2a) to a ribbon-like format (Fig. 2b–d). 3.2. X-ray diffraction (XRD) The X-ray diffractograms (XRD) showed the presence of crystals in the electrospun nanofibers (Fig. 4). Similar diffraction patterns were observed for all the bio-nanofiber formulations, with three broad peaks having maxima at 2 = 14◦ , 17◦ and 25◦ . Table 3 shows the average interplanar spacing (d) and crystallite diameters (D) calculated from the diffractograms shown in Fig. 4. It is observed that adding tannin lead to the decrease of the crystallite diameter, however no differences were observed for the interplanar distances. It is reported that the larger d-spacing around 14 A˚ is associated with the mean distance of approach of neighboring helices (the spacing of the inter-helix packing of zein chains) whereas the shorter d-spacing at around 9 A˚ is related to

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Fig. 2. Scanning electron microscopy (SEM) images of the bio-nanofibers: (a) zein (neat); (b) ZT1 (10% of tannin and 90% of zein); (c) ZT2 (15% of tannin and 85% of zein); and (d) ZT3 (20% of tannin and 80% of zein).

the average backbone distance within ␣-helix structure of the zein. Literature reports (Yao et al., 2009) indicate values between 10 and 4.5 A˚ for broad peaks of zein electrospun nanofibers. Difference between literature and experimental XRD parameters can be

attributed to variations in the secondary and tertiary zein structure with different solvents used in the electrospinning. The crystal structure and orientation of the biopolymer chain depends strongly on the characteristics of the polymer (molecular weight, tacticity

Fig. 3. Diameter distribution histograms of the bio-nanofibers: (a) neat zein; (b) ZT1 (10% of tannin and 90% of zein); (c) ZT2 (15% of tannin and 85% of zein); and (d) ZT3 (20% of tannin and 80% of zein).

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Fig. 4. X-ray diffractograms (XRD) of the bio-nanofibers: (a) neat zein; (b) ZT1 (10% of tannin and 90% of zein); (c) ZT2 (15% of tannin and 85% of zein); and (d) ZT3 (20% of tannin and 80% of zein).

and glass transition temperature), the solvent medium used (evaporation rate and polymer–solvent interaction) and the process variables (voltage applied and collector speed) (Lee et al., 2008; Pant et al., 2011; Cozza et al., 2013). Structural changes for zein chain packing were reported by Kayaci and Uyar (2012), whose the spacing of the inter-helix packing of zein chains was disturbed with the addition of cyclodextrins. In addition, those authors (Kayaci and Uyar, 2012) found that cyclodextrins were mostly distributed in the fiber matrix without forming crystalline aggregates when lower weight percentages of cyclodextrins (␣-CD) were used (10% and 25% of ␤-CD and ␥-CD and 10% of ␣-CD), however, incorporation of 50% (w/w) of all three types of CDs and 25% of ␣-CD yielded crystalline aggregates in the zein fiber matrix. The XRD of as-received CDs have shown diffraction patterns for ‘cage-type’ crystalline structures as reported in the literature (Harata, 1998; Rusa et al., 2002; Saenger et al., 1998). Fig. 5. DSC curves showing the effect of tannin addition on the melting temperature of water retained in the electrospun bio-nanofibers.

3.3. Thermal analysis Figs. 5 and 6 show the DSC curves of the electrospun bionanofibers. The addition of tannin increased the glass transition temperature of the nanofibers, suggesting an antiplasticizing effect (Kalogeras et al., 2009; Delcambre et al., 2010) of the tannin on the zein (arrows in Fig. 6). The tannin extract used in the present study is mainly formed by condensed tannins. The high reactivity of the condensed tannins is due to hydroxyl groups in the phenol species (Fig. 1). When the breaking of these bonds occurs (in acidic medium) the reactivity of the tannin is reduced. The tannins have the property of precipitating proteins, and zein is a protein with a high degree of polymerization. Then, when the tannin is added to the zein matrix, the precipitation reaction occurs, which can form a compatible blend, decreasing the zein chain mobility and influencing the polymer crystallization. This lower mobility of

the chains of zein can be associated with the strong interactions between the hydroxyl groups of tannin and amine groups present in zein. The increase in melting temperature of the zein crystals can be associated with the formation of more homogeneous crystals in zein + tannin blends. Table 4 depicts the average glass transition (Tg ) and melting temperatures (Tm ), and enthalpies for each electrospun formulation. It is observed a change in the glass transition and melting (water evaporation) temperatures of the samples. The melting temperature is related with the crystallinity of the bio-nanofiber studied. It is observed that the use of 20% of tannin increased the melting temperature of the nanofibers to 120 ◦ C (Fig. 5). The increase of water evaporation enthalpy with the presence of tannin can also be associated with the antiplasticizing effect commented

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these nanofibers, allied with the chemical activities of the tannins and their compatibility with hydrophobic polymer matrices, opens up new perspectives for the development of new polymer based materials for several applications, such as adhesives, self-cleaning materials, air purification filters, antibacterial applications, cosmetics and controlled release of medicines. Acknowledgments

Fig. 6. DSC curves showing the effect of tannin addition on the glass transition temperature of electrospun bio-nanofibers.

The authors acknowledge the support of the Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo – FAPESP, Fundac¸ão de Amparo à Pesquisa do Estado de Minas Gerais – FAPEMIG, Coordenac¸ão de Aperfeic¸oamento de Pessoa de Nível Superior – CAPES, Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq, Embrapa Instrumentac¸ão and Brazilian Research Network in Lignocellulosic Composites and Nanocomposites – RELIGAR.

Table 4 Thermal properties of the electrospun bio-nanofibers.

References

Electrospun nanofibers

Tg (◦ C)

Tm (◦ C)

Hm (J/g)

Neat zein ZT1 ZT2 ZT3

130 149 149 144

83 98 98 120

111 119 128 119

Tg (◦ C), glass transition temperature; Tm (◦ C), melting temperature; Hm (J/g), melting enthalpy.

above. The lower zein chain mobility when tannins are included led to lower water diffusion in the bio-nanofibers, and hydroxyl and carboxyl groups of the tannins into the nanofibers increased water evaporation (melting) temperature and the enthalpy of reaction (activation energy to evaporate the water molecules). This result also agree with that found by Costa et al. (2013), whose the addition of barbatimão tannin increased the thermal properties of the electrospun PVA + pineapple nanofibers. The authors (Costa et al., 2013) attributed the increase of the thermal properties to the crystallization changes promoted by the addition of S. adstringens bark extract. Torres-Giner et al. (2008) reported that acidifying the alcohol zein solution led to nanofibers that exhibited higher glass transition temperature than zein nanofibers obtained from pure alcohol solutions. On the contrary, alkaline solutions of zein yielded low solution viscosity and hence a faulty electrospinning experience. Nanofiber networks have presented increased thermal properties when compared to solvent cast films due probably to their particular molecular structure and high solvent removing efficiency. Kayaci and Uyar (2012) reported the increase of the glass transition temperature and degradation temperature of electrospun zein nanofibers with the increase of cyclodextrin content, when compared with the pristine zein nanofibers. 4. Conclusion The thermal analysis and morphological characterization of the electrospun bio-nanofibers showed that the interaction between the barbatimão tannin and the zein occurred. The images obtained by scanning electron microscopy show nanofibers with ribbonlike shape and homogeneous morphology on the nanostructured membranes. X-ray diffraction showed that the crystallization of the polymers occurs. The addition of tannin increased the glass transition temperature of the nanofibers, showing the effect of antiplasticizer of the tannin on the zein matrix. The melting temperature increased with the addition of tannins. This work confirms the possibility of incorporating the barbatimão tannin for producing electrospun nanofibers. The improved thermal properties of

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