Improvement of bacterial cellulose nonwoven fabrics by physical entrapment of lauryl gallate oligomers

Abstract

The present study aimed to improve the properties of bacterial cellulose nonwoven fabrics by physical entrapment of lauryl gallate oligomers. The lauryl gallate oligomerization process was conducted by laccase-mediated oligomerization. Lauryl gallate was chemically confirmed by matrix-assisted laser desorption/ionization with time-of-flight analyses. The oligomerization conditions were controlled considering the surface properties (water contact angle, surface energy, and water absorption time) of bacterial cellulose nonwoven fabrics. The controlled oligomerization conditions were 160 U/mL of laccase and 20 mM lauryl gallate. After bacterial cellulose was treated by the physical entrapment of lauryl gallate oligomers, X-ray photoelectron spectroscopy analysis showed that the N1 atomic composition (%) of bacterial cellulose increased from 0.78% to 4.32%. This indicates that the lauryl gallate oligomer molecules were introduced into the bacterial cellulose nanofiber structure. In addition, the water contact angle was measured after washing the bacterial cellulose nonwoven fabric treated by the physical entrapment of lauryl gallate oligomers for 180 minutes, and it was found to maintain a water contact angle of 88°. The durability of bacterial cellulose nonwoven fabric treated by the physical entrapment of lauryl gallate oligomers was confirmed by measuring the tensile strength after wetting and dimensional stability. As a result, the tensile strength after wetting was about five times higher and the dimensional stability was three times higher than that of untreated bacterial cellulose nonwoven fabric.

Keywords

bacterial cellulose lauryl gallate laccase nonwoven physical entrapment

Bacterial cellulose (BC) is a renewable bio-nanomaterial with unique characteristics^1,2 that include high purity, high degree of polymerization, high crystallinity, and high water-holding capability.³ Altering the fermentation conditions can produce BC with excellent biodegradability, biocompatibility, and moldability.^3,4 These physicochemical properties have attracted significant research and industrial interest. The diverse applications of BC include biomedical, cosmetic, papermaking, electronic devices, and textile applications as nonwoven fabric.^3,5–9 BC nonwoven fabrics have the advantages of good mechanical properties and hydrophilicity. However, the hydrophilicity of BC nonwoven fabric has several drawbacks; its hydrophilicity and high moisture uptake cause the poor rehydration¹⁰ and durability of fabrics. When BC nonwoven fabric is exposed to moist or wet conditions, it loses its original shape and it is difficult to recover its shape and strength.^1,11–14

To overcome these aforementioned drawbacks, various physical and chemical modification methods have been explored. In previous studies, BC was modified chemically using various functional materials, such as polyethylene glycol,¹⁵ silver nanoparticles,¹⁶ and zinc oxide.^17,18 These materials have been incorporated into three-dimensional BC matrices to provide increased crystallinity, porosity, hydrophobicity, and mechanical properties.^10,17,18

Lauryl gallate, the n-dococyl ester of gallic acid, which belongs to the gallate homologue series, is a useful functional polymer^19–25 for lignocellulose fiber modification, such as hard wood kraft pulp,²¹ softwood cellulose pulp,²² and jute fiber,²⁶ and protein fibers like wool,²⁷ but it has not been applied to BC nonwoven fabric. Several studies have used lauryl gallate enzymatically oxidized in conjunction with laccase (EC 1.10.3.2. p-diphenol dioxygen oxidoreductase).^21–24 Laccases are copper-containing oxidoreductases that act on phenolic compounds, such as substituted phenols, and convert these to radical species via radical cation intermediates. In laccase-catalyzed oxidation, a phenolic hydroxyl group loses a single electron and a phenoxyl radical is formed.²⁸²⁹ Due to the high reactivity of these radicals further reactions, such as oligomerization, can occur.^29,30 Laccase is able to oxidize phenolic substrates to their radical cations, which rapidly lose a proton to form radical species that further react to oligomers. Its ability for enzymatic oligomerization improves the fiber properties, such as hydrophobicity, antioxidation activity, ultraviolet (UV) protection, and altered physical and mechanical properties, depending on the phenolic compound properties.^22–25

Therefore, this study aims to improve the durability of BC nonwoven fabric by physical entrapment of lauryl gallate oligomers. For this, firstly, lauryl gallate oligomers were produced through laccase-mediated oligomerization. Secondly, the lauryl gallate oligomer entrapment conditions for BC nonwoven fabric, involving the concentrations of lauryl gallate and laccase, were controlled by evaluating the surface properties, such as the water contact angle (WCA), surface energy, and water absorption time value of BC nonwoven fabric, respectively. The nanostructure of BC nonwoven fabric was physically modified through a swelling process followed by the entrapment of lauryl gallate oligomers. Thirdly, alterations in the chemical structure and surface morphology of BC nonwoven fabric were then investigated. Finally, the durability of BC nonwoven fabric in terms of its dimensional stability and tensile strength was evaluated.

Materials and methods

Materials

Glucose and peptone were obtained from Merck Co. Ltd (Darmstadt, Germany). Yeast extract was obtained from Sigma-Aldrich (St. Louis, Mo, USA). Analytical grade sodium hydroxide hydrate pellet and sodium chloride were also purchased from Sigma-Aldrich. Analytical grade glacial acetic acid was purchased from Fisher Chemical (Fair Lawn, NJ. USA). Acetate buffer solution (pH 4.0) was prepared using sodium acetate (C₂H₃NaO₂) and acetic acid (CH₃COOH) acquired from Sigma-Aldrich. Lauryl gallate (3,4,5-trihydroxybenzoate) was purchased from Sigma-Aldrich. Ethyl alcohol (99.5%) was purchased from Duksan Pure Chemicals Co. Ltd (Seoul, Korea) and was used to dissolve the lauryl gallate. Commercial laccase (Novozym 51003) (EC 1.10.3.2) from Thermobifida fusca was obtained from Novozymes (Bagsveard, Denmark). The wetting agent (sodium dodecylbenzenesulfonate (C₁₈H₂₉NaO₃S)) was obtained from Sigma-Aldrich.

Methods

Oligomerization of lauryl gallate

Lauryl gallate oligomers were prepared as follows (see Figure 1).³¹ The lauryl gallate monomers was dissolved (1:2, organic: aqueous) in 8% v/v ethanol in acetate buffer (pH 5.0, 0.1 M) by magnetic agitation. Laccase (245.5 U/mL) was added to catalyze lauryl gallate monomers solution for oligomerization. The oligomerization of lauryl gallate by laccase-mediated were conducted by previous methods of Reynaud et al.²¹ Here in after, prior to oligomerization, the lauryl gallate is named as lauryl gallate monomer, and the lauryl gallate after oligomerization

Figure 1.

Schematic of the preparation process of lauryl gallate oligomers by laccase-mediated oligomerization.

The effect of ethanol on laccase activity during the oligomerization process to prepare the lauryl gallate oligomers was measured spectrophotometrically by monitoring the enzymatic oxidation rate of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) to its cation radical (ABTS^.+) at 420 nm (ɛ₄₂₀ =36,000 M⁻¹cm⁻¹) in 0.1 M acetate buffer (pH 4.0) at 25℃. One unit (U) of activity is defined as the amount of enzyme forming 1 μmol/min of ABTS^.+. All spectrophotometric measurements were carried out by UV-visible spectrophotometry (Multimode Microplate Reader Synergy™ Mx and Gen5™; BioTek Instruments, Winooski, VT, USA).

Confirmation of lauryl gallate oligomerization

The new lauryl gallate oligomers were confirmed by matrix-assisted laser desorption/ionization with time-of-flight (MALDI-TOF) spectrometry. MALDI-TOF spectrometry was conducted by using 2,5-dihydroxy benzoic acid (DHB) as the matrix (≥99.5%). The mass spectra were acquired on an Ultra-flex MALDI-TOF mass spectrophotometer (Bruker Daltonics) equipped with a 337 nm nitrogen laser. For this, the samples were dissolved in a TA30 (30% acetonitrile/70% trifluoroacetic acid) solution and mixed with a 20 mg/mL solution of DHB (1:1). A volume of 2 μL was placed in the ground steel plate (Bruker part no. 209519) until dry. The mass spectra were acquired and analyzed in the linear positive mode.

Preparation of BC nonwoven fabric

BC gel was cultured according to the previous methods of Han et al.³² The BC nonwoven fabrics that were produced and used for the subsequently described experiments are of similar size (2 cm × 2 cm), with an average thickness of 1 ± 0.5 cm and average weight of 2 ± 0.8 g, respectively. As shown in Figure 2, two types of BC nonwoven fabrics were produced.

Figure 2.

The schematic preparation process of (a) untreated bacterial cellulose (BC) and (b) treated BC nonwoven fabric. *The lauryl gallate oligomers were prepared as described in the Oligomerization of lauryl gallate section.

Physical entrapment of lauryl gallate oligomers on BC nonwoven fabric

For the physical entrapment of lauryl gallate oligomer BC nonwoven fabric, BC nonwoven fabric firstly was treated by swelling, as shown in Figure 2(b). The swelling of BC was conducted as described by Song et al.^33,34 After swelling, the lauryl gallate oligomers, which were prepared as described in the Oligomerization of lauryl gallate section, were physically entrapped as follows. Swelled BC nonwoven fabric was incubated in 10 mL of lauryl gallate oligomers solution at 50℃ for 12 h in a shaking water bath (BS-31; JEIO TECH Co.) at 135 rpm. After the physical entrapment of lauryl gallate oligomers on BC nonwoven fabric for 12 h, supernatant was collected then BC entrapped lauryl gallate oligomer nonwoven fabric was washed with distilled water to remove the free monomers and oligomers. BC entrapped lauryl gallate oligomer nonwoven fabric was then dried in an oven dryer (OF-22G; JEIO TECH Co.) at 35℃ for 5 h. Hereinafter, it was named the “treated BC nonwoven fabric,” as indicated in Figure 2(b). The same conditions were used for the control and treated samples of laccase or lauryl gallate.

Figure 3.

Schematic of the preparation process for the evaluation of the tensile strength of untreated and treated bacterial cellulose (BC) nonwoven fabrics before and after wetting.

To control the entrapment conditions of lauryl gallate oligomers for BC nonwoven fabric, the optimum concentrations of laccase and lauryl gallate were evaluated, respectively. Firstly, treated BC nonwoven fabrics were prepared by different laccase concentrations (8, 40, 160, and 240 U/mL) in 10 mM of lauryl gallate monomers solution at 50℃ for 12 h. To optimize the lauryl gallate concentration, the tested concentrations of 1, 5, 10, 20, and 30 mM lauryl gallate monomers were used with the optimal laccase concentration that had been determined.³⁵

The entrapment conditions of lauryl gallate oligomers were determined by the WCA and water absorption time.³⁵ The WCA was determined using a contact angle measurement system (DSA100; KRÜSS Inc., Hamburg, Germany). A water droplet of 3 μL dosing volume was placed in each sample using a Hamilton 500 μL syringe at a suitable distance from the testing platform. Each measurement was performed on a different spot of the sample and the results were based on the average of at least three measurements. The apparatus was used in conjunction with specialized software for determination of surface tension. The Girifalco–Good–Fowkes–Young model was used to calculate the average surface energy. The water absorption time was recorded until the specular reflectance of a droplet completely disappeared. Each sample was tested in at least three spots and the results were prepared as an average.

Effects of the physical entrapment of lauryl gallate oligomers on BC nonwoven fabric

Fourier transform infrared analysis

The chemical structures of untreated and treated BC nonwoven fabrics were analyzed through Fourier transform infrared (FTIR) analysis using a FT/IR-670Plus spectrophotometer (Jasco International Co. Ltd, Tokyo, Japan). Scans were completed between 4000 and 450 cm⁻¹ at a resolution of 4 cm⁻¹. Baselines for each sample spectrum were normalized using spectrum software.

X-ray photoelectron spectroscopy analysis

The changes in the surface chemistry of untreated and treated BC nonwoven fabrics were determined using X-ray photoelectron spectroscopy (XPS) analysis using a PHI 5000 VersaProbe device (ULVAC PHI, Kanagawa, Japan) equipped with a monochromator AI Ka source (1486.6 eV). A narrow scan was completed at pass energy of 40 eV. This was followed by high-resolution scanning for C1s (284.6 eV). For quantitative analysis, a subtraction method in the linear line background was applied to all the main spectral bands and the respective areas were calculated. These data and the respective X-ray cross-sectional values were used to calculate the percentage atomic concentration for each element present. Component analysis of the spectral regions was carried out by peak fitting using pure Gaussian line shapes.

X-ray diffraction analysis

The structure of untreated and treated BC nonwoven fabrics were investigated by X-ray diffraction (XRD) using a model D/MAX-2500PC multi-purpose diffractometer (Rigaku, Tokyo, Japan). XRD patterns were recorded at the CuKα radiation wavelength of 1.54180 Å generated at a voltage of 40 kV and a filament emission of 40 mA. Untreated and treated BC nonwoven fabrics were scanned from 5° to 40° 2θ ranges at a scan speed of 1°/min. The degree of crystallinity was calculated by the following formula (1)^36–38

CrI = (I_{200} - I_{am}) / I_{200}

(1)

where I₂₀₀ is the overall intensity of the peak at 2θ and I_am is the intensity of the baseline at 2θ.

Scanning electron microscopy analysis

The surface morphology of untreated and treated BC nonwoven fabrics were examined by scanning electron microscopy (SEM) using a JSM-7600F microscope (JEOL Korea Ltd, Japan). For SEM analysis, the samples were coated with an ultrathin layer of gold in an ion sputter and scanned at different points using an electron EICA S360 at a magnification of 5000×.

Washing fastness of BC nonwoven fabric

The fastness of treated BC nonwoven fabrics was evaluated by washing. After treatment, BC nonwoven fabrics were washed for 0, 30, 60, and 180 min., then dried for 1 h. After drying, the WCA of treated BC nonwoven fabrics was evaluated.

Evaluation of the durability of BC nonwoven fabric

The durability properties of untreated and treated BC nonwoven fabrics were evaluated by measurement of the tensile strength and dimensional stability. The tensile strength of untreated and treated BC nonwoven fabrics was evaluated before and after wetting. As shown in Figure 3, for measurement of the tensile strength after wetting, untreated and treated BC nonwoven fabrics were immersed in water for 5 min then dried for 3 h at 25℃. After drying, untreated and treated nonwoven fabrics were evaluated with a load cell of 100 N at a crosshead speed of 0.17 mm/s. They were evaluated three times using a tensile testing machine (ASA-211-1; Digital Tensility Strength, Seoul, Korea) according to the ISO 13934-1:2013 method.

The dimensional stability was evaluated using the ISO 7771:2012 method. Untreated and treated BC nonwoven fabrics (70 mm × 250 mm) were marked at four locations to measure the dimensional change. These were immersed in distilled water with added wetting agent (sodium dodecylbenzenesulfonate, C₁₈H₂₉NaO₃S) for 60, 120, and 180 min. After immersion, the wet untreated and treated BC nonwoven fabrics were dried in an oven dryer (OF-22G; JEIO TECH Co.) at 35℃ for 5 h and then the dimensional change was measured in terms of dimensional stability. The percentage of dimensional stability was calculated according to Equation (2)

Dimensionalstability (%) = \frac{A_{mm}}{B_{mm}} \times 100

(2)

where B_mm is the size in millimeters of the BC samples before immersion and A_mm is the size in millimeters of BC nonwoven fabrics after immersion.

Results and discussion

Effect of ethanol on the oligomerization of lauryl gallate

During the lauryl gallate oligomerization process, the addition of ethanol (8%, v/v) in the mixture solution is inevitable because the water-insoluble lauryl gallate is not dissolved without the presence of a solvent like ethanol.³⁸ The effect of ethanol on laccase activity during lauryl gallate oligomerization was determined before and after the oligomerization reaction depending on the use of ethanol. Figure 4 indicates laccase activity before and after oligomerization in lauryl gallate monomer solution with ethanol (a) and without ethanol (b). No remarkable decrease of laccase activity was apparent in the solution compared with the native laccase (245.5 U/mL). The activity of laccase in lauryl gallate solution with ethanol changed from 198.5 ± 2 to 156.4 ± 28 U/mL after the oligomerization reaction (Figure 4(a)). For laccase in lauryl gallate solution without ethanol the activity changed from 188.5 ± 3.5 to 145.5 ± 2.5 U/mL (Figure 4(b)). Muñiz-Mouro et al.³⁸ evaluated the effect of ethanol on laccase stability and reported that the deactivation of laccase that occurred only at ethanol concentrations over 50% (v/v) led to a rapid inactivation of the enzyme. For lower ethanol concentrations, the effect was less dramatic. Rodakiewicz-Nowak et al.³⁹ also reported that laccases respond differently to the presence of ethanol. No evidence for the denaturation of laccase at moderate ethanol concentrations was observed.

Figure 4.

Laccase activity (U/mL) in lauryl gallate solution with ethanol (a) and without ethanol (b) before and after laccase-mediated oligomerization of lauryl gallate.

Presently, low concentrations of ethanol, such as 8% (v/v), were used in preparing lauryl gallate oligomers by laccase-mediated oligomerization. We assumed that the addition of ethanol does not affect laccase activity during the lauryl gallate oligomerization process. Therefore, for BC nonwoven fabric, lauryl gallate oligomers were prepared using ethanol to dissolve the lauryl gallate monomer during the laccase-mediated oligomerization process.

Confirmation of lauryl gallate oligomerization

After laccase-mediated oligomerization, the produced lauryl gallate oligomers solution was confirmed by MALDI-TOF spectrometry.

As shown in Figure 5, the oligomerization reaction yielded products with a broad molecular weight distribution from 434.248 Da to 1609.335 m/z (Da). After oligomerization by laccase-mediated oligomerization, lauryl gallate was detected at 682.891, 1057.299, 1432.475, and 1609.335 m/z (Da), which is in a similar range to that of previous research.²² In particular, the peak at 682.891 m/z indicates that lauryl gallate oligomers occurred. The oligomeric structures indicated in the spectra resulted from different degrees of oligomerization. The peaks combined in multiplets, which seemed to correspond to different end groups of the oligomers.²²

Figure 5.

Matrix-assisted laser desorption/ionization with time-of-flight mass spectrometry of (a) lauryl gallate and (b) lauryl gallate oligomers produced by laccase-mediated oligomerization.

The mass spectrum indicated that the maximum degree of oligomerization achieved for the lauryl gallate was at 1609.335 m/z (Da) (Figure 5(b)). This heterogeneity might be related to the different oligomer sizes obtained and the ionization events that occurred during the analysis. Despite the heterogeneity, the data are valuable as they confirm that oligomerization really occurred and because they reveal the medium degree of oligomerization for each polymer obtained. The data indicate that the oxidation of lauryl gallate generated longer oligomers.^22,31,35

Physical entrapment of lauryl gallate oligomers on BC nonwoven fabric

To control the entrapment conditions of lauryl gallate oligomers for BC nonwoven fabric, the optimum laccase concentration (U/mL) and lauryl gallate concentration (mM) were evaluated, respectively. BC nonwoven fabrics were treated by different solutions, including different laccase and lauryl gallate concentrations, respectively. The optimum conditions were evaluated by the surface change, namely the WCA, surface energy, and water absorption time of BC nonwoven fabrics.

Table 1 indicates the surface change of treated BC nonwoven fabrics that were prepared by different laccase concentrations ranging from 8 to 240 (U/mL). Generally, untreated BC nonwoven fabric has a low WCA value of 48.1 ± 1.5° with high surface energy (56.15 ± 0.4 mN/m) due to the numerous hydroxyl groups. After BC nonwoven fabric was entrapped lauryl gallate oligomers, its surface became more hydrophobic with the increase of the WCA. This is because of the decline in the number of hydroxyl groups and moisture uptake, since lauryl gallate oligomers convert hydroxyl groups of BC nonwoven fabric to hydrophobic groups. As shown in Table 1, the more laccase concentration was added during lauryl gallate oligomerization, the more hydrophobic surface was indicated on treated BC nonwoven fabrics. The highest value of the WCA (118 ± 1.4°) with the longest water absorption time (over 7 min) was obtained when laccase of 160 U/mL was used. This laccase concentration provided sufficient enzyme to catalyze lauryl gallate oligomerization, which conferred hydrophobic groups to treated BC nonwoven fabrics.²³ However, with the highest laccase concentration, the value of the WCA was decreased. This trend is similar to that of the previous study of Reynaud et al.²¹ This phenomenon occurred because the oxidation reaction of phenolic moiety in lauryl gallate by excess concentration of laccase is continuously increased and the number of anionic groups is increased.⁴⁰⁴¹

Table 1.

Water contact angle (WCA), surface energy, and water absorption time of untreated bacterial cellulose (BC) and treated BC nonwoven fabrics by different laccase concentrations (treatment conditions: laccase concentrations of 8, 40, 80, 160, and 240 U/mL, 10 mM lauryl gallate at 50℃ for 12 h)

	Untreated BC nonwoven fabric	Treated BC nonwoven fabrics
		Laccase concentration (U/mL)
		8	40	80	160	240
WCA (°)	48.1 ± 1.5	72 ± 1.5	92 ± 1.4	104 ± 1.5	118 ± 1.4	75 ± 3.0
Surface energy (mN/m)	56.15 ± 0.4	45 ± 1.5	34 ± 1.8	30 ± 1.0	17 ± 1.2	38 ± 2.5
Water absorption time	2 min, 40 s ± 5 s	2 min ± 40 s	3 min ± 55 s	5 min ± 30 s	7 min ± 15 s	2 min ± 50 s

The effect of lauryl gallate concentration during the laccase-mediated oligomerization was evaluated using 1–50 mM lauryl gallate. As indicated in Table 2, the WCAs of treated BC nonwoven fabrics were gradually increased with the increase of lauryl gallate concentration. The highest value of the WCA (118 ± 1.4°) with the lowest surface energy (17 ± 1.2 mN/m) was obtained at 20 mM lauryl gallate monomer. On the other hand, higher monomer concentrations (>20 mM) caused a decrease of the WCA. When an excessive concentration of monomer was added, laccase is likely to reach a limited state of catalysis ability for oligomerization.^36,42

Table 2.

Water contact angle (WCA), surface energy, and water absorption time of treated bacterial cellulose (BC) nonwoven fabrics using different concentrations of lauryl gallate (treatment conditions: 1, 5, 10, 20, and 30 mM lauryl gallate, laccase concentration of 160 U/mL at 50℃ for 12 h)

	Treated BC nonwoven fabrics
	Lauryl gallate monomer concentration (mM)
	1	5	10	20	30
WCA (°)	80 ± 0.5	85 ± 1.5	92 ± 2.0	118 ± 1.4	88 ± 1.5
Surface energy (mN/m)	28 ± 1.0	26 ± 1.2	33 ± 0.5	17 ± 1.2	25 ± 2.0
Water absorption time	3 min ± 10 s	3 min ± 45 s	4 min ± 55 s	7 min ± 15 s	4 min ± 10 s

The collective results demonstrated that the optimum oligomerization conditions of lauryl gallate oligomers for BC nonwoven fabric were concluded as laccase of 160 U/mL and 20 mM lauryl gallate.

Effects of the physical entrapment of lauryl gallate oligomers on BC nonwoven fabric

FTIR analysis

To determine the chemical change of untreated and treated BC nonwoven fabrics by the physical entrapment of lauryl gallate, FTIR analysis was conducted. In the FTIR spectra (Figure 6(a)), untreated BC nonwoven fabric presented characteristic cellulose peaks at approximately 3300, 2940, and 2893 cm⁻¹. The broad and strong bond peak at 3300 cm⁻¹ corresponds to the stretching vibration of the hydroxyl groups. The peaks at 2940, 2893, and 1640 cm⁻¹ are attributed to aliphatic C-H stretching vibrations.²⁰ After the physical entrapment of lauryl gallate oligomers, the change of peak in the range of 2940–2893 cm⁻¹ was indicated because of the long alkyl and methylene chain from lauryl gallate.²⁰ The spectrum of treated BC nonwoven fabric displayed marked differences in the 1500–1800 cm⁻¹ region (Figure 6(b)). The peak at 1710 cm⁻¹ is consistent with the addition of a carbonyl stretching band from the ester of the lauryl gallate ester.⁴³ Furthermore, there was a change in absorbance in the primary amine banding region at 1590 cm⁻¹, which would be expected from the incorporation of amino groups from lauryl gallate.^43,44 The decrease of the water peak at 3300 cm⁻¹ was also observed owing to the reduced number of hydroxyl groups converted to hydrophobic groups.

Figure 6.

Fourier transform infrared spectra of (a) untreated and (b) treated bacterial cellulose nonwoven fabrics, treated at pH 5.0 using a laccase concentration of 160 U/mL and 20 mM lauryl gallate at 50℃ for 12 h.

The observed changes in the FTIR spectra are consistent with an interpretation that lauryl gallate oligomers were successfully entrapped into BC nonwoven fabric.

XPS analysis

The change of surface composition was determined by XPS analysis. Untreated BC nonwoven fabric is mainly comprised of C1s and O1s (Table 3(a)). To confirm the effect of laccase on the oligomerization of lauryl gallate, BC nonwoven fabrics were also treated by only laccase and lauryl gallate monomers, respectively. When BC nonwoven fabric was treated by only laccase (Table 3(b)), it does not show a remarkable change. As shown in Table 3(c), BC nonwoven fabric treated by only lauryl gallate monomers indicated the increase of N1s (%), which is attributed to the effect of the adsorption of tiny amounts of lauryl gallate monomer. After the entrapment of lauryl gallate oligomers, treated BC nonwoven fabric displayed a remarkable alteration in the surface composition. The percentage of N1s was increased by over five times, along with the increase of C1s (Table 3(d)). These results clearly demonstrate that laccase effectively catalyzes lauryl gallate oligomerization and also that the entrapment of lauryl gallate oligomers in BC nonwoven fabric did occur.^26,33

Table 3.

Atomic composition (%) of bacterial cellulose nonwoven fabrics: (a) untreated; (b) treated by only laccase; (c) treated by only lauryl gallate monomer; (d) treated by the physical entrapment of lauryl gallate oligomers (treatment conditions: pH 5.0; laccase concentration of 160 U/mL and 20 mM lauryl gallate at 50℃ for 12 h)

	Surface atomic composition (%)
	C1s	N1s	O1s
(a)	61.07	0.78	36.60
(b)	70.47	0.39	29.14
(c)	72.72	2.37	24.91
(d)	74.59	4.32	21.40

XRD analysis

XRD analysis was conducted to examine the crystallinity and microstructure of untreated and treated BC nonwoven fabrics. Three diffraction peaks at 2θ = 14.7°, 16.5°, and 22.5° evident in BC nonwoven fabric were attributed to the cellulose structure (Figure 7(a)).⁴⁵ These peaks correspond to the (1 $\bar{1}$ 0), (110), and (200) diffraction planes, respectively. In the case of treated BC nonwoven fabric, the changes in peaks occurred with a greater intensity (Figure 7(b)). This different diagram showed a pattern of (1 $\bar{1}$ 0) and (200) reflections, suggesting an improved in the alignment of the crystallites. In addition, the degree of crystallinity was increased from 73.8 ± 3.2 to 79.2 ± 1.2% (Table 4). This increased crystallinity contributed to a decrease in moisture uptake, due to the long hydrophobic aliphatic groups entrapped in BC nonwoven fabric.^46,47 Das⁴⁸ reported that crystallinity provides strength to fibers and, thus, plays a significant role in fiber mechanical properties. Therefore, the increase of crystallinity by the physical entrapment of lauryl gallate oligomers can improve durability of BC nonwoven fabric.⁴⁹

Figure 7.

X-ray diffraction spectra of (a) untreated and (b) treated bacterial cellulose nonwoven fabrics, treated at pH 5.0 using a laccase concentration of 160 U/mL and 20 mM lauryl gallate at 50℃ for 12 h.

Table 4.

Structural values of untreated and treated bacterial cellulose (BC) nonwoven fabrics, treated at pH 5.0 using a laccase concentration of 160 U/mL and 20 mM lauryl gallate at 50℃ for 12 h

	2θ (°)	Crystallinity (%)
Untreated BC nonwoven fabric	19.173 ± 0.1	73.8 ± 3.2
Treated BC nonwoven fabric	23.012 ± 0.1	79.2 ± 1.2

SEM analysis

SEM analysis revealed a remarkable difference between untreated and treated BC nonwoven fabrics. As shown in Figure 8(a), untreated BC nonwoven fabric displayed a characteristic nanofiber network structure. After the entrapment of lauryl gallate oligomers, the BC nonwoven fabric surface appeared to be covered between the nanofiber structures (Figure 8(b)). This suggests that the lauryl gallate oligomer molecules are evenly distributed among the nanofiber structures of the BC nonwoven fabric.¹²

Figure 8.

Scanning electron micrographs of (a) untreated and (b) treated bacterial cellulose nonwoven fabrics, treated at pH 5.0 using a laccase concentration of 160 U/mL and 20 mM lauryl gallate at 50℃ for 12 h.

Washing fastness of BC nonwoven fabric

To evaluate the fastness of treated BC nonwoven fabric, treated BC nonwoven fabrics were washed in distilled water for 0, 30, 60, and 180 min. As shown in Figure 9, after washing for 30 min, the WCA was decreased from 118 ± 1.4° to 91.5 ± 1.5°. However, treated BC nonwoven fabric kept its WCA value over 88° after washing for 180 min. These results indicate that lauryl gallate oligomers are stably entrapped in BC nonwoven fabric, suggesting that BC nonwoven fabric can be used repeatedly.

Figure 9.

Water contact angle (WCA) of treated bacterial cellulose (BC) nonwoven fabric after washing for 0, 30, 60, and 180 min (treatment conditions: pH 5.0; laccase concentration of 160 U/mL and 20 mM lauryl gallate at 50℃ for 12 h).

Durability of BC nonwoven fabric

By the physical entrapment of lauryl gallate oligomers on BC nonwoven fabric, the change of durability properties in terms of tensile strength and dimensional stability was evaluated, respectively. Firstly, the tensile strength was evaluated before and after wetting, respectively. As shown in Figure 10(a), the tensile strength of untreated BC nonwoven fabric was decreased from 6.8 ± 0.5 to 1.8 ± 0.7 N/mm² after wetting. This indicates that approximately 70% of the original tensile strength was lost. On the other hand, treated BC nonwoven fabric maintained its tensile strength over 88% after wetting (Figure 10(b)). These results are qualitatively similar to prior studies. Herrero Acero et al.⁵⁰ reported that coconut and flax fibers exhibited increased mechanical properties with laccase oxidation of phenols, which they explained by the enzymatic coupling of hydrophobic molecules that formed the adhesion between fibers. Consequently, increases in tensile strength and tensile modules of the composite were noted. Similarly, Kudanga et al.⁵¹ reported that the treatment of BC with laccase in the presence of a hydrophobic substrate, such as methyl syringate or dodecyl gallate, led to a greater increase in wet strength. Thus, lauryl gallate appears to have accelerated the laccase-mediated oligomerization, resulting in absorption within BC nonwoven fabric. The removal of adsorbed water caused an increase in the strength of interactions between BC nonwoven fabrics.

Figure 10.

Tensile strength of (a) untreated and (b) treated bacterial cellulose nonwoven fabrics, treated at pH 5.0 using a laccase concentration of 160 U/mL and 20 mM lauryl gallate at 50℃ for 12 h.

The dimensional stability of untreated BC nonwoven fabric significantly declined from 40.5% to 19% after immersion for 180 min (Figure 11). It underwent significant shrinkage with the deformation of its shape and loss of its initial color (Table 5), which was attributed to the poor rehydration after immersion. These drawbacks are important for textile end uses of BC nonwoven fabric. On the other hand, treated BC nonwoven fabric retained its initial dimensional stability over 50% even with prolonged immersion time. Its appearance did not change appreciably. The incorporation of lauryl gallate oligomers into BC nonwoven fabric promoted the strong adhesion between nanofibers of BC nonwoven fabric with the decrease of absorbed water, which could have prevented the deformation of the fiber structure.^52,53

Figure 11.

The dimensional stability (%) of (a) untreated and (b) treated bacterial cellulose nonwoven fabrics, treated at pH 5.0 using a laccase concentration of 160 U/mL and 20 mM lauryl gallate at 50℃ for 12 h.

Table 5.

The appearance of untreated and treated bacterial cellulose (BC) nonwoven fabrics, treated at pH 5.0 using a laccase concentration of 160 U/mL and 20 mM lauryl gallate at 50℃ for 12 h

	Immersion time (min)
	60	120	180
Untreated BC nonwoven fabric
Treated BC nonwoven fabric

Conclusions

The purpose of this study was to improve the durability of BC nonwoven fabrics. For this, the physical entrapment of lauryl gallate oligomers was conducted on BC nonwoven fabrics. The oligomerization conditions of lauryl gallate oligomers were controlled to a laccase concentration of 160 U/mL and 20 mM lauryl gallate monomer concentration. Changes in the surface properties of BC nonwoven fabric by the entrapment of lauryl gallate oligomers were evaluated by measuring the WCA, surface energy, and absorption time. As a result, the hydrophobicity of the BC nonwoven fabric surface was increased. In addition, FTIR, XPS, and XRD analyses showed that the chemical properties of BC nonwoven fabric were changed due to the entrapped lauryl gallate oligomers between the BC nanofiber structures. Further, after the treated BC nonwoven fabric was washed for 180 min, the WCA was measured, and it was found that it was 88°or more.

The durability (tensile strength and dimensional stability) was improved as the hydrophobicity of the BC nonwoven fabric was increased by the physical entrapment of lauryl gallate oligomers. In particular, BC nonwoven fabrics containing lauryl gallate oligomers exhibited dimensional stability three times higher than that of untreated BC nonwoven fabric after immersing in the wetting agent solution for 180 min.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03031959) and the Korea government (MSIT) (No. NRF-2019R1A2C1009217).

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