Abstract
Biopolymers such as PLA, ZnO nanoparticles and chitosan are materials that have some properties in common, such as oxidizing action, antimicrobial activity, biocompatibility and formation of a mechanical barrier, which make them promising for application in food packaging, as they ensure safety in contact with food. In this study, the antibacterial activity of pure PLA films and those incorporating chitosan, ZnO nanoparticles and both was analyzed for four strains (two Gram-positive and two Gram-negative bacteria) by the agar diffusion method, aiming to verify the action of each substance and their combination. The characterization of ZnO NPs was performed by XRD, FTIR, Raman spectroscopy, UV-Vis, SEM and Zeta potential (ZP) methods. The results showed that the ZnO nanoparticles synthesized by the Pechinhi method, with calcination temperatures of 600 and 800ºC, presented a predominant morphology of nanorods and nanospheres, according to SEM images, with high purity and crystallinity. The PLA-based films presented a rough structure, with small pores, and the incorporation of chitosan and ZnO NPs was effective, which was proven by the SEM/EDS technique. The results showed that the films incorporated with ZnO NPs presented inhibition for Gram-positive bacteria in the range of 12.5 to 14 mm.
Keywords: Zinc oxide; nanoparticles; chitosan; PLA films; food packaging; antimicrobial activity
1. Introduction
The function of packaging is to protect and maintain the quality of products, mainly against external factors. However, to fulfill this function, toxic compounds derived from petroleum are often used, which can cause harm to the environment and human health. If packaging is not adequate, food may deteriorate due to the growth of microorganisms that can cause serious infectious diseases and food poisoning1,2.
The most common packaging plastics are poly(ethylene), poly(ethylene terephthalate), poly(propylene), poly(vinyl chloride) and poly(styrene). As these materials are not biodegradable, they take hundreds or even thousands of years to decompose, contributing significantly to environmental pollution1,3,4.
Aiming to reduce these impacts, research is being carried out focused on the development and promotion of the use of biopolymers, which offer benefits such as thermal stability, flexibility, barrier against gases and water, biodegradability, resistance to chemicals and biocompatibility3. These renewable biopolymers have stood out as sustainable solutions in several applications, including smart food packaging, biomedical and drug distribution, biomembranes, the automotive sector and industrial composting5.
Nanotechnology can help combat resistant bacteria through nanocomposites with antimicrobial properties. These compounds act on bacterial receptors through a complexation reaction, preventing their growth or division and, thus, preventing the spread of pathogens6. Zinc oxide nanoparticles are especially interesting due to this characteristic, offering great conservation potential. Therefore, to improve the quality of polymers used in food packaging, the incorporation of nanostructured zinc oxide in these matrices is a significant option, as it is effective as an antibacterial agent against several types of bacteria, both Gram-positive and Gram-negative7,8.
Chitosan is a polysaccharide considered safe and sustainable by the United States Environmental Protection Agency, due to its notable advantages, such as non-toxicity, biodegradability, biocompatibility, antioxidant properties, and its natural production and regeneration, contrasting with petroleum-derived materials and coal. Because it is an abundant by-product, resulting from discarded fishing, and because it is renewable, it has a reduced cost, giving it significant economic and environmental relevance9-11. Furthermore, this biopolymer has effective antimicrobial properties against Gram-positive and Gram-negative bacteria, as well as fungi and yeast. This makes it attractive for a wide variety of applications in different sectors, both in its pure form and when incorporated into other polymers11-14.
Poly (lactic acid), PLA, is a thermoplastic, biodegradable and biocompatible aliphatic polyester, which can be semi-crystalline or amorphous. It is of natural origin, composed of lactic acid molecules, obtained from renewable sources such as corn, cassava, sugar cane, beetroot, potatoes and other starchy vegetables15-17.
Among the main advantages of PLA biopolymer are: biodegradability, which can be decomposed by microorganisms under appropriate composting conditions; biocompatibility and biological absorption, making it safe for applications in the medical, dental, pharmaceutical and food industries; good mechanical properties of rigidity and resistance, enabling a wide range of uses; simple processing techniques such as extrusion, injection and thermal molding; thermal stability and transparency, useful for packaging that displays the product, such as food packaging, in addition to being dyeable for greater protection against UV rays; CO2 consumption during synthesis; and low energy consumption in production. These characteristics make PLA a viable substitute for conventional petroleum-derived polymers, reducing environmental impact. The main disadvantage remains its high cost compared to conventional commercial plastics18,19.
The Pechini method involves the formation of metal citrates by the interaction between a hydrocarboxylic acid and a metal cation. Then, a polyhydroxyalcohol, such as ethylene glycol, is added to the chelate, and the increase in temperature promotes esterification and polyesterification reactions until the formation of a polymeric resin. This resulting gel is calcined to remove the organic matter and obtain the desired metallic oxide, forming a homogeneous polyester with the cations distributed in its structure20-22. This technique allows precise control of the stoichiometry of atoms and generates particles of high purity, homogeneity and surface area. However, due to the several steps involved, the process can be slow, complex and have high costs22,23.
Considering the characteristics of these materials and compounds, this work aims to investigate the influence of the incorporation of chitosan and ZnO nanoparticles into the PLA polymer matrix, regarding antimicrobial properties in contact with strains of Gram-positive and Gram-negative bacteria, for application in food packaging.
2. Materials and Methods
The reagents used for the synthesis of ZnO nanoparticles were citric acid monohydrate (99-102%), zinc chloride (99%) and ethylene glycol (96%), all from the Dinâmica brand.
To prepare the films, PLA filaments, dichloromethane (ECIBRA – 99,8%), double-distilled glycerin (Dinâmica – 99,5%), chitosan (Sigma-Aldrich – 99,999%) and ZnO nanoparticles obtained by the Pechini method were used. PLA, from the Voolt 3D brand, used as a base polymer, was obtained in the form of filament for a 3D printer, in natural color (transparent).
2.1. Synthesis of ZnO nanoparticles
Zinc oxide nanoparticles were synthesized using the polymeric precursor method (Pechini), by mixing solutions of citric acid monohydrate and zinc chloride, in a molar ratio of 3:1 (citric acid:metal cation)24. The solutions were kept under constant stirring for 2 hours at 80ºC. Then, ethylene glycol was added, at a ratio of 40/60 (%w/w) in relation to citric acid. The temperature was raised to 120ºC, for total evaporation of the water and formation of the polymeric resin. For the pyrolysis stage, the material was subjected to heating in a muffle furnace, at a temperature of 400ºC (heating rate of 10°C/min), for 3 hours, until the formation of polyester, in the form of a characteristic foam, colored dark and shiny, known as puff. The resulting polyester went through a deagglomeration process, until it reached a fine powder, which was separated into two porcelain boats. In the final stage, to obtain the ZnO NPs, calcination was carried out in a muffle furnace, for one of the samples at a temperature of 600ºC and, for the other, at 800ºC (heating rate of 10°C/min), for 3 hours.
2.1.1. X-Ray diffraction (XRD)
The diffractometer used was Rigaku, model RINT 2000, operated under conditions of 40kV and 150 mA, with CuKα radiation (λ = 1.5406 Å), scanning speed of 0.02º per minute, and interval (2θ) of 10° to 80°, at a speed of 0.5° min-1.
2.1.2. Fourier transform infrared spectroscopic (FTIR)
The spectrometer used to obtain the data was a Vertex 70, from Bruker Instruments, with ATR diamond and using potassium bromide (KBr) tablets, using the total reflectance method, with a scan from 4000 to 400 cm-1.
2.1.3. Raman spectroscopy
Raman measurements, referring to the vibrational aspects of molecular groups, were carried out on a Metrohm Raman Spectrometer equipment, model i-Raman Plus 532H/USA, applying a wavelength of 532 nm and 100% laser level power, in a Raman shift range of 200 to 800 cm-1. The resulting spectrum was obtained by fusing ten spectra, collected in 60 s each, totaling 6 min, for each of the ZnO NPs samples.
2.1.4. UV–Vis spectroscopy
For these measurements, the LAMBDA 1050 UV/Vis/NIR spectrometer from PerkinElmer was used, with 150 mm fitting spheres and dual Si and InGaAs detectors. The scanning range was fixed in the range of 250 to 1000 nm.
2.1.5. Scanning electron microscopy (SEM)
The ZnO NPs were subjected to scanning electron microscopy to evaluate their morphologies, using Zeiss equipment, model EVO LS-15.
2.1.6. Zeta potential
For zeta potentials measurements, the Zetasizer Nano ZS equipment from the company Malvern Instruments was used.
2.2. Preparation of PLA-based films incorporated with chitosan and ZnO NPs
PLA-based films were obtained by dissolving 200 mg of PLA in 5 ml of dichloromethane (DCM) for approximately 48 hours. The mixture was kept under constant stirring in a hood, at 1000 rpm, with the addition of 2 drops of glycerol, for 30 minutes. The solution was placed in a 7 cm diameter petri dish and left in the hood to dry, at room temperature, for 24 hours, to eliminate the solvent. The incorporation of chitosan was carried out by adding 80 mg of the substance. To add the ZnO NPs, 10 mg was used, and the mixture of chitosan and ZnO NPs was used 80 and 10 mg, respectively.
2.2.1. Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDX)
In the films, a scanning electron microscope (SEM) was used, which uses a field emission gun (FEG) as an electron source, coupled to an energy dispersive X-ray spectrometry (EDS) system, from the brand Jeol, model JSM IT500-HR.
2.2.2. Antibacterial activity: assay by the agar diffusion (AD) method
Bacterial cultures used in the present studies were obtained from the American Type Culture Collection (ATCC). Four microbial strains were tested: Gram-positive - Staphylococcus aureus (ATCC 6538) and Bacillus cereus (ATCC 14579), and Gram-negative - Escherichia coli (ATCC 25922) and Salmonella enterica subsp. enterica (ATCC 14028).
The antibacterial activity of the samples was determined by the agar diffusion (AD) method, using the disc technique against the test bacteria25. Initially, the Mueller-Hinton agar (MHa) culture medium was prepared. After cooling to around 50 °C, a volume of 25 mL of the liquefied medium was transferred to sterile 15 × 90 mm diameter Petri dishes, until it reached a thickness of 4 mm. Using a sterile swab, the inoculum (1.5 × 108 CFU/mL) was distributed evenly over the surface of the agar and left to stand at room temperature for 3 min. Disks from PLA and PLA incorporated with ZnO NPs, chitosan and the combination of these, were placed on the agar surface. After an interval of 15 min, the plates were incubated in a bacteriological oven at 36°C for 18 h. Chloramphenicol (30 µg) and imipenem (10 µg) discs were used as positive controls. Bacterial growth inhibition was determined as the diameter of the inhibition zones around the discs and were measured in millimeters using a millimeter ruler. All tests were performed in triplicate.
3. Results and Discussion
3.1. Synthesis of ZnO nanoparticles
The synthesis of ZnO NPs was carried out using the Pechini method, with citric acid and zinc chloride as precursors. After the metal citrate pyrolysis step, the sample was divided into two parts, one subjected to calcination at 600º and the other at 800ºC, as specified in the characterization graphics below.
3.1.1. X-Ray diffraction (XRD)
The Figure 1 shows the X-ray diffraction patterns of ZnO NPs, obtained for the two calcination temperatures (600ºC and 800ºC), with zinc chloride as the precursor. The Bragg diffraction peaks are characteristic of the hexagonal structure, wurzite, for zinc oxide crystals.
By analyzing the diffractograms, it is possible to observe, for the two samples, well-defined peaks and the absence of additional impurity peaks, meaning that the zinc oxide nanoparticles, synthesized by Pechini, have high purity and crystallinity. The intensities were proportionally greater with the increase in the calcination temperature, which is possible to verify through the elongation and narrowing of the peaks in the sample calcined at 800ºC, compared to that subjected to calcination at 600ºC.
The peaks at 2θ = 31,77076, 34,44688, 36,26843, 47,56715, 56,61587, 62,90208, 65,96185, 67,98429 and 69,11140°, for the sample calcined at 600ºC and 31,75711, 34,43128, 36,25133, 47,55214, 56,60258, 62,89478, 65,98518, 67,97532, 69,10466º for calcined at 800ºC were assigned to (100), (002), (101), (102), (110), (103), (200), (112) and (201), respectively, corresponding to the reflection planes of the hexagonal wurtzite structure of ZnO26.
3.1.2. Fourier transform infrared spectroscopic (FTIR)
FTIR was performed to detect the functional groups present in the ZnO NPs samples, in the range of 500 to 4000 cm-1, as shown in Figure 2.
The two samples presented main absorption peaks in the same regions (496 cm-1, 1224 cm-1, 1383 cm-1, 1729 cm-1, 2366 cm-1, 2930 cm-1, 3092 cm-1, 3463 cm-1). The spectrum range between 400 and 500 cm-1 is related to the vibrational mode of metal-oxygen bonds. Thus, the sharp absorption peak at 495 cm-1 refers to the stretching of the Zn-O bond, confirming the formation of the substance27.
The intense peak at 1224 cm-1 comprises a region related to the stretching vibration of the C-O single bond and the following peak, also of high intensity, at 1383 cm-1, to the C-H stretching. The most intense peak in the sample, located at 1729 cm-1, represents the stretching vibration of the carbonyl group (C=O). The region of discrete peaks at 2366 cm-1 can be related to the absorption of CO2, present in the atmosphere, from metallic cations. The peaks formed in the region around 3000 cm-1 have lower intensity and are characteristic of the C-H group. More discreet, the peak around 3463 cm-1 refers to the characteristic absorption of hydroxyl groups, it corresponds to the O-H stretching of the intramolecular hydrogen bond28,29.
3.1.3. Raman spectroscopy
The graph in Figure 3 demonstrates, for both samples of ZnO NPs, that the main peak was formed at 438 cm-1, which corresponds to the E2 (high) mode, associated with the vibration of O, for the wurzite structure of ZnO, and which is related to the degree of crystallinity30. These data corroborate the data obtained from FTIR for the formation of zinc oxide.
3.1.4. UV–Vis spectroscopy
From the observation spectra in the UV-Vis region, of ZnO NPs, shown in Figure 4, for both samples it is possible to note that the maximum observation occurred at a wavelength of 324 nm, in the UV region, a characteristic attributed to the electronic transition of ZnO, confirming the formation of the inorganic compound. In the visible area, the spectra show low absorption31. The ZnO NPs calcined at 800ºC showed a higher absorption peak compared to 600ºC.
It is possible to identify that the absorption values of ZnO NPs are high for lower wavelengths (less than 400 nm), but lower for longer wavelengths (above 450 nm). High absorbance in the shorter wavelength range is a property of opaque materials32. This may be since the analysis was carried out directly on the compacted powder, making it difficult for light to pass through.
3.1.5. Scanning electron microscopy (SEM)
As evidenced by the micrographs obtained in the SEM, shown in Figure 5, the calcination temperature had an influence on the final properties of the ZnO nanoparticles. In samples calcined at a temperature of 800ºC, it appears that the crystallites had a larger average size compared to those treated at 600ºC, also presenting a clearer crystalline structure. These observations confirm the results obtained in the X-ray diffractograms.
Regarding morphology, the nanoparticles appeared, for the most part, as nanorods or nanospheres, in some regions of agglomerates and with an average size about 25-150 nm, for the sample calcined at 600ºC and 40-200 nm for the one calcined at 800ºC.
3.1.6. Zeta potential
For information regarding the stability of oxide nanoparticles of zinc synthesized, in colloidal suspension, due to their tendency to repel each other or if they aggregate, zeta potential measurements were carried out for both samples of ZnO NPs, depending on the pH of the solution, starting at pH 12 and adding HCl until pH 2, as shown in Figure 6 below.
Due to the differences in zeta potential between the samples, it appears that the calcination temperature interferes with the final surface structure and chemistry of ZnO NPs. The distinction between the values denotes the variation in the interactions between the particles and in the net density of the surface charge, which can also cause divergence in the properties of these nanoparticles.
With the increase in pH, the surface of the nanoparticles is negatively charged, due to the hydroxyl ions. As a result, the repulsion of the particles becomes more accentuated, increasing the colloidal property33.
The ZnO NPs sample calcined at 800ºC reached a higher zeta potential value, which means that it has greater colloidal stability. The SEM micrographs and XRD patterns for the ZnO NPs also demonstrated that the material calcined at a higher temperature presented a higher percentage of crystallinity and better-defined morphology, despite its larger size, in agreement with the zeta potential results.
The isoelectric point (pI), which corresponds to the balance between positive and negative charges, of the ZnO NPs samples was 4.06 for the sample calcined at 600ºC and 3.35 for the one calcined at 800ºC.
3.2. Preparation of PLA-based films incorporated with chitosan and ZnO NPs
Six PLA-based films obtained [PLA; PLA + ZnO (600ºC); PLA + ZnO (800ºC); PLA + CS; PLA + CS + ZnO (600ºC); PLA + CS + ZnO (800ºC)], are shown in Figure 7.
From image 7, it is possible to observe that the films have a slightly rough surface and that the ZnO NPs and chitosan are dispersed throughout the PLA, but there are some small deposition sites, as they are not soluble in dichloromethane, remaining suspended in the solution.
The addition of glycerin, as a plasticizing agent, made the PLA more resistant, making it possible to bend it without causing cracks or deformations.
3.2.1. Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDS)
The cross-sectional morphology of PLA-based biofilms and the distribution of ZnO and chitosan (CS) nanocomposites were analyzed by scanning electron microscopy, and representative images are shown in Figure 8.
Micrographs obtained by SEM of PLA, PLA+ZnO (600ºC and 800ºC), PLA+CS, and PLA+CS+ZnO (600ºC and 800ºC) films.
The poly (lactic acid) film, PLA, has a rough and heterogeneous texture, with the presence of cavities and pores throughout the structure being identified, but without cracks or fractures. The addition of glycerin, as a plasticizing agent, can be a significant factor in obtaining the final morphology, as when it penetrates the molecular structure, it causes a reduction in the attractive forces of the polymer chains, which also leads to increased flexibility, making the polymer more flexible and resistant to breakage. Another factor that can also generate these changes is the drying process, in which the evaporation of the solvent can create empty spaces, causing these holes34.
When incorporating ZnO NPs into the biopolymer, the nanoparticles appear as nanorods or nanospheres, in a more pronounced shape, in brighter areas. They also had a homogeneous distribution throughout the polymer matrix, highlighting some regions with association of small clusters. In nanocomposite films, the existence of a good dispersion of inorganic filler, through the polymer matrix, can result in improvements in the physical properties of the material34.
Although the dispersion of chitosan directly in the PLA film is visible, it is not possible to distinguish the materials in the SEM images, since the addition of CS did not cause changes in the morphology of the biopolymer. However, through EDS analysis, it was possible to identify the chitosan particles, as indicated by the arrows in Figure 9.
EDS spectra of PLA, PLA+ZnO (600ºC and 800ºC), PLA+CS, and PLA+CS+ZnO (600ºC and 800ºC) films.
The films that had the two substances added, ZnO and chitosan, generated irregular morphologies, but with good dispersion. Even in a smaller proportion, ZnO NPs appear predominantly in the micrographs, and with good incorporation into PLA, in mostly nanorod structures, standing out in relation to chitosan.
Energy dispersive X-ray spectroscopy (EDS) was used to identify the elements that make up the samples and analyze the presence of possible contaminants. The EDS patterns are represented by Figure 9, with the captured X-ray images.
The percentage by weight of the chemical composition, identified in the samples using the EDS technique, revealed that carbon and oxygen are the most abundant elements in all the films analyzed. Considering that hydrogen hardly produces X-rays, due to its electronic structure and energetic properties, and is rarely identified in EDS, the results are consistent with the materials used. The base of the films is the polymer PLA (C3H4O2)n, composed of lactic acid units, with 50% weight percentage carbon and about 44% oxygen.
In all films that had the incorporation of ZnO NPs, zinc was identified, however in the PLA + ZnO (600ºC) sample an indicator of the presence of sodium (Na) also appeared, in red, which could be contamination in the handling of the product, material or spectral failure (interferences or overlaps).
Chitosan has the chemical formula (C6H11NO4)n, and is therefore also represented mainly by the elements C and O, since nitrogen also requires special conditions for identification by EDS, presenting low sensitivity to the technique. Another significant factor, presented by EDS, is that the films that had incorporated ZnO NPs calcined at 600ºC, both alone and together with chitosan, showed signs of the element chlorine (Cl), originating from the ZnCl2 precursor, demonstrating that this temperature was not sufficient to eliminate it. With the samples involving ZnO NPs calcined at 800ºC, the presence of intermediate synthesis compounds or other contaminants was not identified.
3.2.2. Antibacterial activity: assay by the agar diffusion (AD) method
The results obtained for PLA-based biofilms with the addition of ZnO NPs and chitosan, after carrying out the agar disk diffusion tests, demonstrated that there was inhibition in the biofilm tests that had the incorporation of ZnO nanoparticles, both calcined and 600ºC and 800ºC, due to the appearance of inhibition halos around the discs of deposited biofilms, for Gram-positive bacteria (Staphylococcus aureus and Bacillus cereus). For Gram-negative bacteria, no growth inhibition zone was formed, proving ineffective as an antibacterial for Escherichia coli and Salmonella enterica. The structure of bacterial cell membranes may be related to the divergence in these responses, as Gram-positive bacteria are composed of teichoic acids and layers of peptidoglycan, a substance responsible for the rigidity of the cell wall, while Gram-negative bacteria have one or a few layers of peptidoglycan in addition to another outer membrane composed of lipopolysaccharides (LPS) and proteins, which makes it more resistant35.
In the samples that contained only PLA and chitosan incorporated into the biopolymer, no significant inhibitory areas were detected for any of the types of bacteria, Gram-negative and Gram-positive. In PLA discs that had chitosan and ZnO NPs integrated into the polymeric matrix, the response was like those in which only ZnO NPs were added, demonstrating that chitosan did not influence the antimicrobial property.
According to the literature, chitosan has high antimicrobial properties. However, when incorporated into PLA, the antibacterial efficacy of chitosan may be affected due to some factors, such as the proportion of chitosan added to the polymer matrix, the processing method and the interaction of these materials36-38. Another hypothesis regarding the inactivation of the antimicrobial action of chitosan may be the occurrence of encapsulation of these particles by PLA, making it difficult for them to encounter bacteria.
The test was also applied, for comparative purposes, to two samples of antibiotics, chloramphenicol and imipenem, drugs aimed at treating bacterial infections, that is, with great antimicrobial potential, as shown in Table 1.
4. Conclusions
The synthesis of ZnO NPs using the Pechini method proved to be effective in obtaining a material with high crystallinity and purity, with a predominant form of nanorods. Through analysis, it was demonstrated that calcination temperatures are important factors in the product obtained, interfering in some properties and structural characteristics. The materials subjected to a temperature of 800ºC showed greater crystallinity and diameter, compared to those calcined at 600ºC, which resulted in smaller crystallites, but slightly less crystalline.
PLA-based biofilms exhibited a microstructure with roughness and the presence of small pores, but flexible and resistant.
The incorporation of ZnO NPs and chitosan into the PLA polymer matrix was carried out with the aim of offering antimicrobial properties, which were tested by the agar disk diffusion method. The results confirmed the antimicrobial activity of the discs that had the addition of ZnO NPs, for Gram-positive bacteria (Staphylococcus aureus and Bacillus cereus), through the inhibition halo formed, with no significant distinction between the applied calcination temperatures (600 and 800ºC). Therefore, the size of the nanoparticles was not a relevant factor in the biocidal action. Efficiency has not been proven for more resistant Gram-negative bacteria. Chitosan did not demonstrate antimicrobial potential to any of the types of bacteria.
Considering the characteristics presented, the aggregation of ZnO NPs to biopolymers shows promise, as they are safe for health, easily inserted (by simple and economical methods), and improve mechanical and antimicrobial properties. This makes it a significant option in the area of food packaging, as it is capable of extending its shelf life, as well as helping to prevent contamination.
5. Acknowledgments
The authors would like to thank POSMAT/UNESP and FAPESP (process: 2013/07296-2).
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Publication Dates
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Publication in this collection
02 Dec 2024 -
Date of issue
2024
History
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Received
11 June 2024 -
Reviewed
02 Aug 2024 -
Accepted
01 Sept 2024