Preparation of Pectin–ZnO Nanocomposite
© to the authors 2008
Received: 5 August 2008
Accepted: 2 October 2008
Published: 21 October 2008
Pectin–ZnO nanocomposite was prepared in the aqueous solution condition at room temperature. The Fourier transform infrared, X-ray diffraction, and transmission electron microscope (TEM) measurements confirmed the nanoscaled structure of pectin–ZnO composite. According to the TEM observation, the average composite granules size was about 150 nm and the embedded ZnO nanoparticles were uniform with an average diameter of 70 nm.
Pectin is a natural, non-toxic, and amorphous carbohydrate present in cell walls of all plant tissues, which functions as an intercellular and intracellular cementing material. As a secondary product of fruit juice, sunflower oil, and sugar manufacture industries, pectin is both inexpensive and abundantly available. Therefore, pectin is an excellent candidate for eco-friendly biodegradable applications. Pectin is commonly used in the food industry as a gelling and stabilizing agent. Pectin macromolecules are able to bind with some organic or inorganic substances via molecular interactions. So, pectin can be used to construct matrices to absorb desired materials and deliver them in a controlled manner . Indeed, pectin has been used to fabricate delivery systems for controlled drug release , implantable cell carriers , and so on.
Zinc (Zn) is an essential micronutrient critical for human health, and its deficiency cause serious and sometimes even disastrous health problems [4, 5]. It has been estimated that more than 50% of poor children and 30% of non-poor children get <70% of the recommended dietary allowance of Zn [6–9]. The main reason may be the presence of phytate in staple foods such as cereals and pulses; phytate has a strong negative effect on Zn absorption from composite meals. If suitable Zn fortificants can be developed to fortify staple foods, it will go a long way in alleviating Zn deficiency. Zinc oxide (ZnO), a safe source for Zn supplementation and fortification, will decompose into Zn ions after consumption . Therefore, ZnO is commonly used to fortify foodstuff in the food industry. Wheat products fortified with ZnO have been shown to possess good Zn absorption .
Currently, hybrid inorganic–organic nanocomposite materials are of great interest because of their multifunctionality owing to a combination of different compounds incorporated . We have recently reported preparation of ZnO-whey protein isolate nanocomposite . Nanocomposite of ZnO wrapped in pectin will survive the gastric environment and become available in the intestine and readily absorbed due to their nanoscale size. The incorporation of nanocrystalline ZnO into pectin to form nanocomposite may impart unique functionalities to the nanocomposite prepared.
Herein we report the preparation of pectin-coated nanocrystalline ZnO particles with a facile solution approach at room temperature. This approach may find potential application in the food industry.
All reagents used were of analytical grade and were used without further purification. In a typical procedure, 0.2 g pectin, 1.2 g Zn(NO3)2 · 6H2O, and 40 mL distilled water were added into a 100-mL beaker. After full dissolution, 40 mL of 0.125 M NaOH solution was added dropwise under constant stirring. The reaction was allowed to proceed at room temperature (~20 °C) for 24 h. Then, the obtained white precipitate was centrifuged at 10,000 rpm for 10 min and collected and washed with distilled water several times to remove the byproducts. After drying in vacuum at 30 °C for 4 h, the final product was obtained as white powder.
Fourier transform infrared (FTIR) spectra of the sample were obtained with a Shimadzu IR-400 spectrometer with the KBr pressed disks. The overall crystallinity of the product was examined by a powder X-ray diffraction (XRD) unit (Scintag Pad V with a Ge solid-state detector; Cu Kα radiation) with the solid specimens mounted on a low background quartz holder. Detailed microstructure analysis was carried out using a transmission electron microscope (TEM, PhilipsCM120). The UV–Vis spectrum of the product dispersed in distilled water was recorded in a UV–Vis spectrophotometer (UV-1601PC, Shimadzu Corporation). A particle size analyzer (90Plus, Brookhaven Instruments Corporation, New York, USA) was used to determine the granular average size distribution of pectin–ZnO nsnocomposite. Thermogravimetric analysis (TGA), differential thermogravimetric analysis (DGA), and differential thermal analysis (DTA) profiles were performed with a Shimadzu-50 thermoanalyzer apparatus under airflow with a heating rate of 10 °C/min.
Results and Discussion
The two oxygen atoms of zinc hydroxide are highly repulsive since they all have lone pair of electrons. This makes the dehydration of zinc hydroxide quite quickly at low temperature and facilitates the production of ZnO nanoparticles. Due to the high polarity of water, ZnO nanoparticles usually agglomerate immediately and form larger particles. In our approach, pectin is added into the solution and bind with ZnO molecules with COO−and CH2groups in a hydrogen bond system to restrain the formed ZnO nanoparticles from further agglomeration by the action of steric hindrance. After a long time reaction, the pectin-wrapped ZnO nanocomposites are formed.
We developed a novel yet simple approach to prepare pectin–ZnO nanocomposite in aqueous solution at room temperature. The structural properties, morphology, thermal decomposition process, and optical absorption of the nanocomposite were studied. The experimental results confirm the true pectin–ZnO composite structure and the existence of strong interaction between pectin molecules and ZnO. This method may be extended to prepare other hybrid inorganic–organic nanocomposite materials.
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