PVA Blend Films with Curcumin ... - ACS Publications

Jun 14, 2018 - The curcumin-loaded cellulose nanocrystals were incorporated in a tara gum/PVA blend film to prepare antioxidant and antibacterial film...
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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Active Tara Gum/PVA Blend Films with Curcumin-Loaded CTAC Brush-TEMPO-Oxidized Cellulose Nanocrystals Qianyun Ma,†,‡ Lele Cao,†,‡ Tieqiang Liang,†,‡ Jian Li,†,‡ Lucian A. Lucia,§ and Lijuan Wang*,†,‡

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Key Laboratory of Bio-based Materials Science and Technology of Ministry of Education, Northeast Forestry University, 26th Hexing Road, Xiangfang District, Harbin 150040, P. R. China ‡ Research Center of Wood Bionic Intelligent Science, Northeast Forestry University, 51th Hexing Road, Xiangfang District, Harbin 150040, P. R. China § The Laboratory of Soft Materials & Green Chemistry, Department of Chemistry, College of Natural Resources, North Carolina State University, 2820 Faucette Drive, Raleigh, North Carolina 27606, United States ABSTRACT: Active films containing curcumin exhibit outstanding antioxidant and antibacterial properties. Because of curcumin’s poor solubility in water, cetyltrimethylammonium chloride (CTAC) brush-TEMPO-oxidized cellulose nanocrystal (TCN) colloidal systems were prepared to be used as a delivery excipient to modulate the hosting of curcumin. The curcuminloaded cellulose nanocrystals were incorporated in a tara gum/ PVA blend film to prepare antioxidant and antibacterial films. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) measurements, transmission electron microscope (TEM), and solid-state 13C NMR spectroscopy were performed to characterize the products. The results indicate the synthesis of TCN with a carboxyl content of 1.1878 mmol/g and 0.71 degree of substitution of CTAC based on carboxyl groups. There was 26.57% of the curcumin bound in the brush. The mechanical properties and barrier properties of the films were characterized. DPPH and ABTS+ assays were used to measure the antioxidant properties. The activities against Gram-negative (E. coli) and Gram-positive (S. aureus) bacteria were also evaluated. The release of curcumin from the films into food simulants were also characterized to determine whether the antioxidants could provide intermediate protection from lipid oxidation. The results show that the mechanical and barrier properties of the films improved although water the vapor permeability slightly decreased. The film possessed desirable antioxidant and antibacterial properties. The release test revealed that curcumin was initially released rapidly into 50% ethanol solution and then released more slowly into the bulk. This suggests that the film could provide short-term protection from food oxidation. The films can be used for prolonging the shelf life of packed fat-rich foods. KEYWORDS: Active films, Tara gum, TEMPO, Cellulose nanocrystals, Curcumin

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INTRODUCTION Recently, a series of new packaging technologies, including active packaging and intelligent packaging, have appealed to scientists due to consumers’ demand for healthy and safe lifestyles.1 Active packaging innovates by releasing substances into food and the environment for specific functions. Oxygen can facilitate microbial growth and lipid oxidation, consequently leading to off flavors, color changes, and also nutritional losses in food, which necessitates the inclusion of antioxidants in active packaging.2 Synthetic antioxidants have already been introduced into active packaging, which has caused increasing toxicological concerns; thus, natural antioxidants have gained increasing interests.3 Spices and herbs contain many phytochemicals such as catechols, flavonoids, tannins, phenolic ditherpenes, and phenolic acids that may be used as potential antioxidants due to their safeties.4 Curcumin is a natural polyphenolic compound with desirable antioxidant, antitumor, and anti© XXXX American Chemical Society

inflammatory activities but very poor solubility in water. In a previous study in our lab, curcumin was used to prepare antioxidant films after being dissolved in alkaline solution5 that showed good antioxidant character despite phenolic hydroxyl converting to phenolic oxygen anion. However, the alkali dissolved curcumin was unstable and had no antibacterial properties. The nanocarrier can be used as a host complex to load curcumin whose solubility problem will be solved. Hydrophobically modified cellulose nanocrystals (CNC) have been used as a carrier for hydrophobes due to their high aspect ratio, specific surface area, and biodegradability of CNC.6 Cetyltrimethylammonium bromine (CTAB) was reported to modify the sulfuric acid hydrolyzed CNC whose sulfate groups provided sites for modification.7 As it is known Received: March 21, 2018 Revised: May 13, 2018 Published: June 14, 2018 A

DOI: 10.1021/acssuschemeng.8b01281 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of Curcumin-Loaded Products (CQTCN)

was previously adjusted to the same pH as the buffer solution (10.24) by using HCl. The slurry was stirred for 2 h, and its pH was monitored by a pH meter. A total of 30 mL of ethanol was added into the final slurry to quench the reaction. The TEMPO-oxidized MCC suspension was thoroughly washed with 1000 mL of deionized water and centrifuged at 10 000 rpm for three times to remove excess TEMPO, inorganic salts, and acids to obtain an oxidized MCC suspension. The obtained suspension was further sonicated at 1000 W for 20 min in an ice bath to obtain TEMPO-oxidized cellulose nanocrystals (TCN). The resultant TCN was stored in a refrigerator around 4 °C before further use. A total of 10 mL of TCN suspension was frozen at −18 °C for 5 h and freeze-dried at −46 °C for 36 h. The dry weight of the sample was measured to calculate the concentration of oxidized MCC. Three samples were tested to obtain an average concentration. Modification of TCN. Around 60 mL of CTAC aqueous solution (0.05 g/mL) was slowly added to 250 mL of TCN solution (0.0197 g/mL). The mixture was first heated at 60 °C for 3 h and kept stirring at ambient temperature overnight. Then, it was centrifuged at 10,000 rpm for 10 min to remove the excess CTAC. The resulting quaternized product was denoted as QTCN. Afterward, 1.05 g of QTCN and 20 mL of curcumin/DMSO solution (20 μg/mL) were shaken in a conical flask at 120 rpm for 12 h. The DMSO solvent and unbound curcumin were removed by centrifugation. The absorbance at 432 nm of the solution with unbound curcumin was measured to calculate the binding efficiency. Curcumin/DMSO solutions (1, 2, 3, 4, and 5 μg/mL) were previously prepared to obtain a standard curve. The amount of unbounded curcumin could be calculated according to the standard curve and the binding efficiency was further calculated by using the eq 1. The QTCN products loaded with curcumin were denoted as CQTCN as shown in Scheme 1.

that more carboxyl groups can be introduced to cellulose molecules in CNC prepared by TEMPO-oxidation, indicating more sites could be offered for the surface modification. Cetyltrimethylammonium chloride (CTAC) is an antimicrobial and hydrophobic quaternary ammonium surfactant with a C16 alkyl chain often used to modify the hydrophobic properties of CNC.8 In this study, TEMPO-oxidized CNC modified by CTAC was used as a carrier to bind quantities of curcumin. The resulting products were incorporated into the tara gum/PVA matrix to prepare antioxidant/antibacterial films. The morphology as well as the chemical structure of the obtained products were characterized by TEM, 13C NMR, FTIR, and XRD. The binding efficiency of curcumin was also studied. In addition, because of the characteristic properties of nanocarriers, such as high Young’s modulus and high surface area,9 the effects of TCN bound with curcumin on the films’ mechanical and barrier properties were investigated. The antioxidant and antibacterial properties were also studied. To examine the extent and rate curcumin could release into food, a release test was also conducted.

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MATERIAL AND METHODS

Materials. Tara gum (TG) of ∼1000 kDa MW was obtained from Dymatic Fine Chemical Co., Ltd. (Guangzhou, China). PVA (average Mw = 84 000−89 000 g/mol; degree of polymerization (DP) = 1700− 1800; 88% alcoholysis) was purchased from Sinopec Shanghai Petrochemical Co., Ltd. (Shanghai, China). Microcrystalline cellulose (MCC) with a DP of 200 was purchased from Shanghai Shenmei Pharmaceutical Technology Co., Ltd. (Shanghai, China). TEMPO (analytical grade) was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd. NaClO with a 10% available amount of chlorine was purchased from Tianjin Tianli Chemical Reagent Co., Ltd. NaBr and cetyltrimethylammonium chloride (CTAC) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Preparation of TEMPO-Oxidized Cellulose Nanocrystals. With continuous stirring in a three-neck flask, MCC (5 g) was placed in carbonate buffer solution (500 mL) with sodium carbonate and sodium bicarbonate at 8/2 ratio and at pH 10.24.10 TEMPO (0.100 g) with a final concentration of 1.28 mmol/L and sodium bromide (1.000 g) were added, after which 16 mL of NaClO solution was added dropwise by a constant-pressure-drop funnel. NaClO solution

binding efficiency(%) = 1 − Cunbounded/Cadded

(1)

Preparation of CQTCN-Incorporated Films. Around 4.2 g of TG and 1.8 g of PVA were thoroughly dissolved in distilled water according to this lab’s previous study.11 Afterward, 1%, 3% and 5% of CQTCN (based on 6 g) and 1.5 mL of glycerol were added to the above mixed solution, kept stirring for 30 min at 80 °C. The final solution after removing bubbles was cast onto the Plexiglas plate with a size of 26 × 26 × 4 cm and dried at 60 °C for 24 h in a vacuum oven. All the CQTCN incorporated films were conditioned at 43% RH before other tests. Characterization Methods. Carboxylate Contents. Titration method was used to measure the carboxylate contents (C (COO−)) of TCN.12 The sample (0.5 g) was treated with 50 mL of HCl B

DOI: 10.1021/acssuschemeng.8b01281 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) TEM image of the resulting TCN, (b) FTIR spectra of MCC and TCN, and (c) XRD patterns of MCC and TCN. film was elevated by Folin-Ciocalteu method.14 A sample (0.2 g) conditioned in the desiccator with silica was immerged in 10 mL of ethanol/water in conical flask at 25 °C. Then 1 mL of the obtained solution was reacted with 4 mL of Foline-Ciocalteu reagent for 3 min and then reacted with 5 mL of sodium carbonate solution (7.5%, w/v) for 2 h. The absorbance (765 nm) was measured by a UV spectroscope. The result of TPC was expressed as mg of gallic acid/g of dried sample. DPPH free-radical and ABTS assay were conducted to investigate the antioxidant properties of film. The sample solution (1 mL) reacted with 50 mg/L of DPPH ethanol solution (3 mL) in a dark environment for 30 min. The absorbance (517 nm) was tested to calculate the radical scavenging activity of DPPH as shown in eq 3.

solution (0.1 mol/L) for 120 min. The mixture was washed completely and filtered using deionized water for several times. The wet specimen after washing was fully dispersed in sodium bicarbonate/sodium chloride solution (50 mL, 1/1) for 60 min. A total of 25 mL of the solution after filtration was titrated with 0.010 mol/L HCl solution with methyl red as an indicator. The C (COO−) of TCN was calculated as follows: C(COO− , mmol/g) = (B − A − AC /50)2N /w

(2)

where A and B are the volumes of 0.010 mol/L HCl solution consumed in the titration of 25 mL of the filtrate and the sodium bicarbonate−sodium chloride solution, respectively (mL), C is the water content in the pulp pad (g), and N is the normality of the HCl solution used in titration. Morphological Analysis. The surface morphologies of TCN and CQTCN were measured by TEM (JEM-2100, JEOL, Tokyo, Japan). Drops of the suspension were deposited on carbon-coated electron microscope grids, and excess liquid was absorbed by filter paper. The suspension was further stained with phosphotungstic acid before drying. Size distributions of TCN were conducted by image analysis software (Nano Measurer software, version 1.2.0). The morphologies of CQTCN incorporated films were investigated by SEM (Philips-FEI Co., AMS, The Netherlands). Before observation of cross section, the samples were first fractured in liquid nitrogen. All samples were measured after being sprayed with gold under an accelerating voltage of 10 kV. Solid-State 13C NMR Analysis. Solid-state 13C NMR spectra of MCC and TCN were analyzed on an Avance 400 Bruker spectrometer (Bruker Inc., Switzerland). XPS Analysis. The valence states of the samples were determined by using an XPS analyzer (Thermo Fisher Scientific Co., USA). The high-resolution N 1s spectra were decomposed into subcomponents by using the software XPS-Peak 4.0 with an 80% Gaussian−20% Lorentzian function. It was acceptable that the full width at halfmaximum varied from each peak. However, the fitting deviation of the curve must be as low as possible. In order to determine whether all of the carboxyl groups of TCN were modified by CTAC, the degree of substitution (DS) was obtained according to the N+ content from XPS analysis. FTIR and XRD Snalysis. The FTIR spectra of specimens were analyzed by Thermo Nicolette 6700 spectrophotometer (Thermo Fisher Scientific Co., Ltd., MA, U.S.A.) in the range of 500−4000 cm−1 at a resolution of 4 cm−1. The XRD patterns of specimens were analyzed in the 2θ range of 5−40° at a scanning rate of 5°/min in a diffractometer (D/max-2200, Rigaku, Japan) using Cu Kα radiation (40 kV, 30 mA). Physical Properties of the Films. An auto tensile tester (PARAM XLW-PC, Jinan, China) were used to measure the tensile strength (TS) and elongation at break (E%) of films at a cross-head speed of 300 mm/s. Each specimen was tested five times. Oxygen permeability (OP) of films was tested by a Perme OX2/230 (Labthink, Jinan, China). Water vapor permeability (WVP) of films was measured by gravimetric method at 75% RH.13 Antioxidant Properties and Total Phenolic Content Measurement. Total phenolic content (TPC) of the CQTCN incorporated

scavenging effect (%) = (Abscontrol − Abs film extract )/Abscontrol (3) The ABTS assay is on the basis of removal of 2,2′-azinobis (3ethylbenzothiazoline-6-sulfonate) radical cation (ABTS•+).15 The ABTS•+ solution was obtained by mixing 10 mL of potassium persulfate (2.45 mmol/L) with 7 mol/L of ABTS. The absorbance (751 nm) of ABTS•+ solution was controlled in the range of 0.70 ± 0.02 by diluting with ethanol. A total of 0.2 mL of the sample solution reacted with 3.9 mL of the above ABTS•+ for 10 min in a dark. The absorbance (751 nm) of this mixture was further measured. The radical scavenging activity was calculated as shown as eq 3. Curcumin Release Test. A total of 300 mg of antioxidant film was placed in 50 mL of fatty food simulants (50% ethanol/water, v/v), recommended by the U.S. Food and Drug Administration (USFDA, 2007). Then 3 mL of the sample solution was periodically withdrawn, and its absorbance at 425 nm was assayed by a spectrophotometer. The tests were conducted at 25, 35, and 45 °C. The released curcumin concentration at different time intervals up to equilibrium was calculated by the absorbance. To investigate the mechanism of curcumin release from the films, the Korsmeyer−Peppas equation was used: Ct /C∞ = kt n

(4)

where Ct/C∞ represents the fraction of curcumin released at different time t, Ct is the released concentration at time t, and C∞ is the maximum concentration. In this study, C∞ was the concentration of curcumin at 10 h. n and C were respectively calculated from the slope and intercept of the graph of ln(Ct/C∞) versus ln t, as shown in eq 5. ln(Ct /C∞) = n ln t + ln k

(5)

Antibacterial Properties. Gram-negative (E. coli) and Grampositive (S. aureus) bacteria were selected as representative microorganisms to evaluate the CQTCN loaded films’ antibacterial properties. Film samples were aseptically cut into discs with a diameter of 0.8 cm and then placed on agar Petri dishes previously smeared with 700 μL of diluted bacterial suspension (105 CFU/mL). The plates were incubated at 37 °C for 12 h. The diameter of the inhibition zone was recorded. Statistical Analysis. The results of multiple samples were expressed are average values ± standard deviation. The differences C

DOI: 10.1021/acssuschemeng.8b01281 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 2. (a) 13C NMR spectra of MCC and TCN and (b) oxidation of C6 primary hydroxyls to carboxylates.

Figure 3. (a) TEM image of the resulting QTCN, (b) FTIR spectra of QTCN and CQTCN, and (c) XRD patterns of QTCN and CQTCN. among the data were processed by the Duncan’s multiple range tests (SPSS software). Significance was defined at P < 0.05.

TCN slightly decreased, but those of cellulose I crystal was not altered. This indicates that oxidation mainly happened in the disordered regions without affecting the internal crystalline region because of its high crystallinity and low accessibility to the reagents.18 We found that the crystallinity slightly decreased from 59% to 56% after oxidation. A possible reason for this decrease is that the original MCC structure was maintained during the oxidation process, but it was destroyed during ultrasonication during preparation of the nanosized product because oxidation can occur inside the cellulose I crystalline region at much higher temperatures.12,19 Hydroxyls in the MCC were not fully oxidized, as determined by FTIR spectroscopy. The selectivity of the oxidization on hydroxyl groups was further investigated by 13C NMR (Figure 2a). Typical signals of MCC appeared at 104.8 ppm (C1), 89.2 ppm (crystalline C4), 84.2 ppm (noncrystalline C4), 74.8 and 72.3 ppm (C2, C3, and C5), 64.9 ppm (crystalline C6), and 62.7 ppm (noncrystalline C6).20 The same signals were observed in the spectrum of TCN. We noticed that a new peak at 174.5 ppm appeared after oxidation, which was due to the introduction of carboxyl. In addition, the peak intensity of noncrystalline C6 carbons at 62.7 ppm decreased compared with that of MCC, demonstrating the selective oxidation was happened at C6 position, as shown in Figure 2b. Characterization of QTCN and CQTCN. The QTCN product was not rod-like individualized cellulose crystals, in contrast to TCN (Figure 3a). It aggregated because of electrostatic attraction between −N+(CH3)3 groups in CTAC molecules and −COO− groups in TCN. The products and

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RESULTS AND DISCUSSION Characterization of TCN. The morphology of the resulting TCN is shown in Figure 1a. The width of the TCN was approximately 5 nm, and the length was ∼100−200 nm. The resultant TCN was thinner than CNC previously obtained by acid hydrolysis.16 Cao et.al also reported the nanocellulose prepared by TEMPO was thinner than those obtained by acid hydrolysis.17 We also found that there were agglomerated cellulose rods because of their high specific areas and strong hydrogen interaction established between the nanocellulose, although most of them were separated well. FTIR spectra were used to investigate the changes in chemical structure after oxidation. Ostensible bands at 3400 (−OH stretching vibrations), 2893 (−CH2 stretching vibrations), and 1600 cm−1 (C = O vibrations) were observed in the spectrum of MCC. The same peaks were found in the spectrum of TCN. A new peak at 1745 cm−1 is direct proof of the existence of carboxyl. The sharper peak at 1600 cm−1 and the substantially weakened peak at 2893 cm−1 also signify the introduction of carboxyl into the TCN backbone.12 The carboxylate content of the prepared TCN further as determined by titration was 1.1878 mmol/g. Figure 1c shows the XRD patterns of MCC and TCN. The diffraction peaks of MCC at 15.1°, 20.3°, 22.8°, and 34.6° are indexed to the (110), (110), (012), and (040) planes, respectively, which suggest a characteristic cellulose I crystal. Relative to the intensity of these diffraction peaks, those of D

DOI: 10.1021/acssuschemeng.8b01281 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. XPS spectra of the resulting products: (a) full-scan spectra of TCN, (a′) deconvoluted N 1s peaks of TCN, (b) full-scan spectra of QTCN, (b′) deconvoluted N 1s peaks of QTCN, (c) full-scan spectra of CQTCN, and (c′) deconvoluted N 1s peaks of CQTCN.

Figure 5. UV−vis spectra (a) and FTIR spectra (b) of films with incorporated CQTCN.

QTCN. The FTIR spectrum of CQTCN was similar to that of QTCN due to overlapped peaks although 26.57% of curcumin was bound onto the surface of QTCN according to the binding efficiency results. The XRD patterns of QTCN and CQTCN were observed in Figure 3c. The modification of TCN with CTAC did not lead to new peaks, indicating that the crystalline I structure did not

CTAC were also studied by FTIR, as shown in Figure 3b. CTAC had obvious symmetric −CH2 (2851 cm−1) and asymmetric −CH2 (2921 cm−1) vibrations of the alkyl chain, also shown in the spectrum of the QTCN samples. In addition, there was a new peak at 1470 cm−1 corresponding to the trimethyl groups of quaternary ammoniums.21 These peaks proved the installation of quaternary ammonium groups into E

DOI: 10.1021/acssuschemeng.8b01281 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Mechanical and Barrier Properties of QTCN Incorporated Filmsa film

thickness (mm)

TC0 TC1 TC3 TC5

0.072 0.077 0.097 0.098

± ± ± ±

0.002a 0.003a 0.026b 0.055b

OP (cm3 mmm−2 atm−1 day−1)

TS (MPa)

E (%)

± ± ± ±

21.6 ± 3.67ab 19.49 ± 0.91a 22.60 ± 3.74ab 24.03 ± 4.24b

20.71 19.99 26.03 27.17

2.84a 2.49a 2.68b 2.44b

2.12 2.47 1.58 1.69

± ± ± ±

0.01b 0.55b 0.27a 0.39a

WVP (gs−1 m−1 Pa−1 × 10−10) 1.71 3.76 3.37 3.22

± ± ± ±

0.18a 0.032c 0.056b 0.012a

a

Different letters in the same column indicate significant differences (P < 0.05).

Figure 6. Antioxidant properties (a) and antibacterial properties (b) of films with incorporated CQTCN.

The FTIR spectra of films with CQTCN showed differences with the control tara gum film in Figure 5. The spectrum of the control film had peaks at 3301 (−OH stretching vibration), 2924 (C−H stretching), and 1600 cm−1 (CO vibrations). After incorporation of CQTCN, the peak intensity of the blend films at 1728, 1626, 1430, and 1272 cm−1 became increasingly stronger. Apparently, the peak at 1728 cm−1 resulted from CQTCN (carboxyl). We also observed peaks at 1626, 1430, and 1272 cm−1 in the spectrum of curcumin, which directly indicated that the increasing CQTCN addition resulted in the stronger peak intensity. The peaks at 3310 cm−1 in the spectra of the films became sharper and also shifted to lower wavenumber, demonstrating that the hydrogen-bond interactions in tara gum, PVA, and glycerol molecules were destroyed because of the introduction of CQTCN.25 Physical Properties of Film. The physical properties (TS, E%, OP, and WVP) of films were investigated. The TS of the film slightly decreased with 1% of CQTCN (p > 0.05), which was related to diminished hydrogen interactions among the remaining molecules after addition of QCTCN as shown in FTIR spectra of the blend films. Yet, the TS increased significantly (p < 0.05) to 27.17 MPa as the CQTCN content increased up to 5% due to the higher crystallinity of CQTCN from the XRD results. It should be noted that E% also slightly increased. The good interfacial interaction among tara gum, PVA, QTCN (CTA+ and anionic carbonate group electrostatic interaction), and hydrophobic interactions between QTCN and curcumin may have resulted in the increasing trend. The results showed that the film was reinforced by CQTCN. WVP values of the films with 1% CQTCN first increased to 3.75 × 10−10 g s−1 m−1 Pa−1, as shown in Table 1. This may be caused by the decrease in number of hydrogen bonds between the tara gum/PVA network and CQTCN, as indicated by the FTIR results. The decrease in hydrogen-bond interactions increased the availability of hydroxyl groups for forming hydrophilic bonds, generating an increasing affinity toward water and thus a higher WVP of the films after addition of CQTCN. However, the WVP value of the film containing 5% of CQTCN slightly decreased to 3.21 × 10−10 g s−1 m−1 Pa−1.

change. The noncrystal peak of QTNC at 15.1° became a shoulder, which demonstrates that the cellulose crystalline structure became disordered during modification. The attachment of the bulky structure of CTAC with a long alkyl chain may slightly increase the amorphous area of QTCN, thus reducing its crystallinity.22 In addition, introduction of curcumin did not alter the crystalline structure of CQTCN as compared with QTCN (Figure 3c), but peak intensities at 15.1° and 22.8° both increased. The crystalline index of CQTCN was ∼62%, which was slightly higher than that of TCN (56%) because of the high crystallinity of curcumin. XPS analysis was further used to analyze for the existence of quaternary ammonium group. The elements carbon, oxygen, and nitrogen were observed in the XPS spectra, as shown in Figure 4. The high-resolution nitrogen spectra further highlight the differences between TCN, QTCN, and CQTCN (Figure 4). All of the spectra of TCN, QTCN, and CQTCN had a peak at 399.5 eV due to the nitrogen atom linked to a carbon atom by a single bond. An obvious peak observed at 402.0 eV in the spectra of QTCN was assigned to −N+(CH3)3;23 it suggests that CTAC was introduced into the skeleton of TCN. The results based on carboxyl groups show that the DS of CTAC was 0.71. We also note that the peak intensity of − N+(CH3)3 in the spectra of CQTCN was not affected after incorporation of curcumin. Analysis of Films Incorporated with CQTCN. The UV− vis spectra of the film after incorporating CQTCN are shown in Figure 5a. It was shown that the film with CQTCN possessed better UV barrier properties (200−280 nm) than the control film, implying its potential use in protecting food from oxidative deterioration induced by light.24 Along with the increasing CQTCN content, their UV barrier properties improved as shown by the increasingly yellowish color. In the visual range, the T% of film with 5% CQTCN was slightly higher than that of the film with 3% CQTCN, with T% of all films lower than that of the control film. Lower transparency is required in certain packaging applications in which the products should be protected from light. F

DOI: 10.1021/acssuschemeng.8b01281 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 7. Release of curcumin into 50% ethanol/water and linear fit of Ritger−Peppas model at (a) 25, (b) 35, and (c) 45 °C.

teichoic acid (phosphate groups) in the cell wall of Grampositive bacteria.28 Determination of Curcumin Release. The release rate and extent of curcumin from the film into food simulants were crucial for their application. We expected it could release in a certain period to prolong the shelf life of certain product. The release of curcumin from the film was shown in Figure 7. The film displayed a burst of release in the first 100 min, after which the release of curcumin slowed down. Total amount of curcumin released was only 60% at 25 °C, while it reached 70% and 95% at 35 and 45 °C, respectively. To better study the release of curcumin, the data were fitted with the Ritger− Peppas model. According to the fitting results (Table 2), the

This could be related to the increasing content of the hydrophobic curcumin and CTAC. The OP value of the control film was 2.12 cm3 mm m−2 atm−1 day−1. After addition of 1% CQTCN, the OP value of the film increased to 2.47 cm3 mm m−2 atm−1 day−1, but it decreased to 1.69 cm3 mm m−2 atm−1 day−1 with 5% of CQTCN, lower than the value of control film. This means that the oxygen barrier property was enhanced by further additions of CQTCN. The increasing availability of hydroxyl as shown by FTIR spectroscopy also made the film a strong barrier to nonpolar substances such as oxygen.26 Antioxidant and Antibacterial Properties. Curcumin is antioxidant due to the existence of phenolic hydroxyls and the binding efficiency test has been proved that there was 26.57% of curcumin incorporated onto CQTCN. To test the antioxidant properties of CQTCN incorporated films, the TPC and scavenging activity to DPPH and ABTS of film were measured. As shown in Figure 6a, the TPC increased from 0.76 to 1.97 mg of gallic acid/g of sample as the CQTCN content increased from 1% to 5%. Generally, there was a linear correlation between TPC content and antioxidant activity because the antioxidant characteristics of curcumin rose largely from phenolic hydroxyl. The DPPH and ABTS scavenging activity results indeed showed enhancement in antioxidant activity. The DPPH radical scavenging activity increased from 5.8% to 30.84% while ABTS scavenging activity increased from 21.46% to 49.61%. As known, oxidative deterioration could cause off-flavors of packaged food and further decreased nutritional qualities. Therefore, the enhanced antioxidant properties were crucial to keep the sensory perceptions and prolong the shelf life of food. The film also possessed antibacterial properties as shown in Figure 6b. Antibacterial activity was quantified by measuring the diameter of inhibition against S. aureus and E. coli. A distinct inhibition zone against S. aureus was observed. Also, the diameter of the inhibition zone increased from 2.25 to 2.90 cm with increasing content of CQTCN. It has been reported that curcumin enhanced the antibacterial effect against S. aureus.27 Certainly, antibacterial ability may also come from CTAC, but no inhibition zone was observed when tested against E. coli, especially with higher content of CQTCN. Additionally, there was no E. coli observed on the film surface with higher additions of CQTCN as shown in Figure 6b, indicating that higher content of CQTCN may inhibit the growth of E. coli. Thus, the resulting films showed stronger inhibitory effects against Gram-positive bacteria versus Gramnegative bacteria. The possible reason was that the electrostatic interactions occurred between CTAC (−N+(CH3)3) and

Table 2. Ritger−Peppas Model Fitting Results of CQTCN Incorporated Films temperature (°C)

release stage

fitted equation

R2

25

first stage second stage first stage second stage first stage second stage

ln(Ct/C∞) = 0.43lnt−2.56 ln(Ct/C∞)=0.19lnt−1.52 ln(Ct/C∞)=0.55lnt−2.48 ln(Ct/C∞)=0.06lnt−0.69 ln(Ct/C∞)=0.60lnt−1.23 ln(Ct/C∞)=0.01lnt−0.27

0.9147 0.9283 0.9761 0.9049 0.8395 0.8115

35 45

release can be divided into two stages. In the first stage, the diffusional exponent (n), specified the mechanism of active component, was higher than 0.45 at 35 and 45 °C, indicating the release of curcumin displayed non-Fickian diffusion behavior.29 This was attributed to the truth that the film was not fully swollen. Therefore, the diffusion was restricted.30 In the second stage, all values of n were