Preparation of Zinc Oxide-Starch Nanocomposite and Its Application on Coating
© Ma et al. 2016
Received: 15 January 2016
Accepted: 4 April 2016
Published: 14 April 2016
A new production method of zinc oxide (ZnO)-starch nanocomposite was invented in this study. Starch was dissolved in zinc chloride (ZnCl2) solution (65 wt%) at 80 °C. Then, ZnO-starch nanocomposite was achieved when the pH of the solution was adjusted to 8.4 by NaOH solution (15 wt%). ZnO nanoparticles were also obtained when the generated ZnO-starch nanocomposite was calcined at 575 °C. The properties of ZnO-starch nanocomposite and ZnO nanoparticle were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicated that the sizes of ZnO-starch composite and ZnO particle were 40–60 nm. UV blocking effect was observed from both ZnO-starch nanocomposite and ZnO nanoparticle. The ZnO-starch nanocomposite was used to directly coat the surface of plain paper with a laboratory paper coater. The surface strength and smoothness of paper were improved by the coating of ZnO-starch nanocomposite. The antibacterial property was also identified from the coated paper.
KeywordsNano-ZnO ZnO-starch nanocomposite Starch Zinc chloride
Microbial contamination is a serious health issue in the food industries and hospital and medical settings. The development of agents or surface coatings with antimicrobial activity has gained increasing interest in recent years. Compared with organic antimicrobial agents such as quaternary ammonium salt and chlorine disinfectant, inorganic oxides have the advantages of robustness, long shelf life, high stability at higher temperatures or pressures, and the ability to withstand harsh processes. This has attracted much more focus on developing alternative inorganic oxides to substitute for the conventional organic compounds. Coatings are expected to be both stable and safe. Inorganic materials with antibacterial properties have been used as antimicrobial coatings on various devices to eliminate microorganisms on surfaces in the environment, community, and health care settings to help stop the spread of the diseases [1–4]. Among the inorganic oxides, zinc oxide (ZnO), titanium dioxide (TiO2), magnesium oxide (MgO), and calcium oxide (CaO) are not only stable under harsh conditions but also regarded as safe materials for humans and animals [3, 5–7]. Moreover, TiO2 and ZnO have been used in the formulations of various personal care products. Interestingly, ZnO shows antimicrobial activity without photoactivation, in contrast to TiO2, which requires photoactivation [5, 8–12]. Due to their small size (1–100 nm) and novel structures, ZnO nanoparticles exhibit significantly improved physical, chemical, and biological properties compared with bulk ZnO. ZnO nanoparticles are a multifunctional material with good catalytic, electrical, photochemical, and optical properties. They are suitable for a broad range of applications such as semiconductors, piezoelectric devices, field emission displays, gas sensors, biosensors, UV-shielding materials, photocatalytic degradation of pollutants, and antimicrobial treatments [1–4].
The methods for nanoscale ZnO production include vapor deposition, precipitation, the microemulsion method, the sol-gel process, and hydrothermal synthesis [1, 4, 7, 13]. The variety of methods enables obtaining particles with a variety of shapes, sizes, and spatial structures. Precipitation is a widely used method to obtain ZnO nanoparticles. The simplest route is an acid-base precipitation method for either coatings or biological applications. A zinc precursor solution such as zinc nitrate (Zn(NO3)2), zinc acetate (Zn(CH3COO)2), or zinc sulfate (ZnSO4) and an alkaline aqueous solution such as sodium hydroxide (NaOH), potassium hydroxide (KOH), or ammonium hydroxide (NH4OH) are prepared in deionized water. The acid solution is mixed with the base in varying proportions to achieve the desired molar hydrolysis ratio of Zn2+/OH−, and the precipitate is harvested, washed, and dried in a static air oven at 80–90 °C for several hours. Adding organic compounds such as polyvinyl alcohol (PVA) , polyethylene glycol (PEG) [15–17], starch [18, 19], sodium dodecyl sulfate (SDS), and cetyltrimethyl ammonium bromide (CTAB)  is increasingly common to control the growth of precipitated particles. These compounds affect not only nucleation and particle growth but also coagulation and flocculation of the particles. Therefore, the process of precipitation is controlled by parameters such as pH, temperature, precipitation time, and the addition of macromolecules. Wang et al. used aqueous solutions of ammonium bicarbonate (NH4HCO3) and zinc sulfate (ZnSO4·7H2O) to synthesize ZnO nanoparticles (ca. 40 nm) . These authors also precipitated ZnO nanoparticles from zinc chloride (ZnCl2) and NH4OH in the presence of the cationic surfactant CTAB. The process was carried out at room temperature, and the resulting powder was calcined at 500 °C to remove residues of the surfactant. The product was small, well-dispersed 50-nm spherical nanoparticles composed of highly crystalline ZnO with a wurtzite structure . Lanje et al. obtained ZnO nanoparticles from low-cost precursors such as ZnNO3 and NaOH . To reduce agglomeration among the smaller particles, a starch was used, which contains many O-H functional groups and can bind to the surface of the nanoparticles at the initial nucleation stage.
ZnO nanoparticles can be used as part of organic/inorganic nanocomposite coatings in numerous industrial sectors, including food, textiles, packaging, environmental, health care, and medical care. ZnO nanoparticles have been incorporated with methyl cellulose (MC) , starch [24–26], acrylic binder , alginate , chitosan , and polyimide  to show strong antibacterial activity, dielectric properties, and UV absorbance. At present, there are some problems in the synthesis of ZnO nanoparticles and ZnO-starch composite materials such as the very low concentration of ZnO precursors. Most precursor concentrations are mmol/L (mM), and the highest concentration is 2 mol/L (M). Therefore, the process requires a large quantity of water or organic solvent. In addition, ZnO nanoparticles and ZnO-starch composites cannot be directly used as coatings but must be incorporated into polymer binders.
In this work, a ZnO-starch nanocomposite was synthesized using a facile process. The aqueous solution of 65 wt% zinc chloride (ZnCl2) is not only a precursor solution for ZnO nanoparticles but also a solvent for starch. The method involves first dissolving the starch by the ZnCl2 aqueous solution then adding NaOH. ZnCl2 is well known to be an effective swelling agent or solvent for cellulose [31–35]. To date, little information is available on ZnCl2 aqueous solutions as a starch solvent. Furthermore, few studies have focused on a route to synthesize a ZnO-starch nanocomposite with a ZnCl2 aqueous solution as both the precursor solution of ZnO nanoparticles and starch solvent. Moreover, the process to prepare the ZnO-starch nanocomposite requires less water. Therefore, this work aimed to develop a simple route to prepare a ZnO-starch nanocomposite and to characterize its morphology. The ZnO-starch nanocomposite was directly coated onto the surface of art base paper as a functional coating, and the antibacterial, surface strength, and optical properties of the coated paper were studied. In addition, the UV-vis absorbance and fluorescence behavior of the ZnO-starch nanocomposite and the ZnO nanoparticles were investigated to explore their potential applications in the field of UV-shielding materials.
All of the reagents are of analytical grade (i.e., purity higher than 99.9 %) and used without further purification. ZnCl2 and NaOH were analytical grade from Nanjing Chemical Reagent Factory, China. Starch and plain papers (art base papers) with base weight of 90 g/m2 were supplied by the Gold East Paper Company, China.
Preparation of the ZnO-Starch Nanocomposite and ZnO Nanoparticle
First, 15 g of starch was completely dissolved in 100 g of 65 wt% ZnCl2 aqueous solution at 80 °C with 500 r/min constant stirring. Then, a 15 wt% NaOH aqueous solution was added drop-wise to the starch-zinc chloride aqueous solution with 500 r/min constant stirring to achieve a final pH value of 8.4. After the composite was aged for 30 min with constant stirring at 80 °C, the ZnO-starch nanocomposite was obtained. The ZnO nanoparticles were easily obtained by calcining the dried ZnO-starch nanocomposite at 575 °C for one hour.
Characterization of the ZnO-Starch Nanocomposite and ZnO Nanoparticles
The X-ray diffraction patterns (XRD) were recorded on an X-ray diffractometer (Ultima IV, Japan) with nickel-filtered Cu Kα (λ = 0.1542 nm) radiation. The diffracted intensities were recorded at 2θ angles from 10° to 70°. The crystallite grain size of the ZnO particles was estimated using the Scherrer equation, D = kλ/βcosθ, where k is a constant, generally considered 0.89 for ZnO, λ is the wavelength of the Cu Kα radiation, 0.1542 nm, β is the full width at half maximum of the XRD peaks, and θ is the diffraction angle. The morphology of the ZnO-starch nanocomposite and ZnO nanoparticles was studied using a transmission electron microscope (TEM) (JEM-2100, JEOL, Japan) and a scanning electron microscope (SEM) (JSM-7600 F; JEOL, Tokyo, Japan). The UV blocking test of the ZnO-starch nanocomposite and ZnO nanoparticles was recorded with a UV-visible spectrophotometer (Lambda 950, Perkin Elmer, USA), and all scans were in the range from 200 to 800 nm. The fluorescence behavior of the ZnO-starch nanocomposite and ZnO nanoparticles was recorded with a spectrofluorimeter (LS55, Perkin Elmer, USA). The excitation maximum wavelength was 310 nm, and the emission scan was carried out in the range from 350 to 550 nm.
Paper Coating by ZnO-Starch Nanocomposite
A K-type (K-303) coater was used to coat the ZnO-starch nanocomposite on art base papers. The concentration of the ZnO-starch composite was 17.2 wt%. The coating weight was controlled by adjusting the power and speed used in the coating process. All sheets were dried on a standard drier set to 105 °C and then stored in a conditioned environment (23 °C and 50 % relative humidity (RH)) for 48 hours until further analysis. The smoothness was measured in accordance with GB/T 456-2002 using a smoothness tester (DCP-BKP10K, Changjiang Papermaking Equipment Company, China). Brightness and gloss were measured in accordance with GB/T 7974-2002 and GB/T 8941-2007 using a brightness tester (YQ-Z-48A, Hangzhou Qingtong Instrument Company, China) and a gloss tester (WZL-300H, Hangzhou Qingtong Instrument Company, China), respectively. The IGT dry picking velocity was measured using AIC2-5 printability tester (Holland) with medium viscosity oil. To ensure the reliability of measurements, five parallel tests were performed for each sample.
Antibacterial Assessment of Coated Papers
The inhibition effects of papers coated with the ZnO-starch nanocomposite were measured according to the GB/T 20944.1-2007 disk diffusion method. Escherichia coli (a type of gram-negative bacteria) and Staphylococcus aureus (a type of gram-positive bacteria) were used in the experimentation. The culture medium for the aforementioned microorganism was a mixture of 17 g of agar, 15 g of beef extracts, 5 g of peptone, and 5 g of NaCl in 1000 mL of water. The pH value was adjusted to 8.0 using 1 mol/L HCl or 1 mol/L NaOH. Then, 0.1 mL of the bacterial suspension (approximately 106 CFU/mL) was spread on the agar plates, and circular samples of the coated papers (diameter 15 mm) were placed on the surface of agar. Then, the dishes were placed in an incubator at 37 °C for 24 h. The antibacterial activity was evaluated by measuring the diameter of the inhibition zones. Three replicates were carried out for each sample.
Results and Discussion
Preparation and Characterization of the ZnO-Starch Nanocomposite
Characterizations and Properties of the ZnO-Starch Nanocomposite and ZnO Nanoparticle
Effect of Coating on Paper Properties
Effect of coating on paper properties
Types of coating
Coating weight (g/m2)
Picking velocity (cm/s)
ZnO nanoparticle mixture with cooked starch
Table 1 indicates that the coating made the paper brightness and gloss decrease, while the picking velocity and smoothness increased. The presence of starch on the paper surface or in the art base paper affected the brightness and gloss due to an increase in the paper transparency. Moreover, ZnO nanoparticles could reduce the reflectance of incident light on the papers. The increased smoothness and picking velocity were also caused by the starch and ZnO nanoparticles. The highest smoothness and picking velocity was achieved by the paper coated with the ZnO-starch nanocomposite, while the ZnO nanoparticle mixture with cooked starch had lower smoothness and picking velocity than the ZnO-starch nanocomposite. This revealed that the ZnO nanoparticles had a high specific surface area and could not be completely and uniformly dispersed in the cooked starch. For the ZnO-starch nanocomposite, the ZnO nanoparticles were dispersed uniformly in the starch network, and the dissolved starch could uniformly bind with the ZnO nanoparticles and fibers in the paper. This binding resulted in the best surface strength and smoothness.
The ZnO-starch nanocomposites were prepared by a facile process in which NaOH was added to a starch-zinc chloride solution at 80 °C to adjust the pH value to 8.4. The size of the ZnO-starch nanocomposites and ZnO nanoparticles was approximately 40–60 nm. The ZnO-starch nanocomposite and ZnO nanoparticles exhibited a UV blocking property, strong visible fluorescence and an efficient antibacterial capability. Moreover, The ZnO-starch nanocomposite could be directly coated onto the surface of plain paper. The picking velocity and smoothness of papers coated with the ZnO-starch nanocomposite were superior to those of papers coated by a mixture of ZnO nanoparticles with cooked starch and with cooked starch alone.
The authors are grateful for the support of the National Natural Science Foundation of China (Grant No. 31570576 and No. 31200444), the Innovation Training Program for Jiangsu College Students (China) (No. 201410298056Z), and Jiangsu Co‑Innovation Center for Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University.
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