Synthesis and characterization of gum arabic stabilized zinc oxide nanoparticles: green method and microbial activity
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https://www.eduzhai.net American Journal of M aterials Science 2013, 3(5): 169-177 DOI: 10.5923/j.materials.20130305.09 Synthesis and Characterization of Gum Acacia-Stabilized Zinc Oxide Nanoparticles: A Green Approach and Microbial Activity M. Sulochana1, Ch. Satya Vani2, D. Keerthi Devi3, N. V. Subba Naidu1, B. Sreedhar3,* 1Department of Chemistry, Sri Venkateswara University, Tirupati, Andhra Pradesh, 517 502, India 2Department of Chemistry, SR International Institute of Technology, Rampally, Keesara (M ), RR District, Andhra Pradesh, 501301, India 3Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (Council of Scientific and Industrial Research), Hyderabad, Andhra Pradesh, 500607, India Abstract A novel crystal-growth system, morphology and size control of zinc o xide nano composites is achieved by using gum acacia, a natural polysaccharide, in a new role. The morphologies and dimensions of the ZnO nanocomposites can be easily tuned by varying the molar ratios of the Gu m Acacia (GA ). The temp late-assisted approach concurs with ‘‘green’’ chemistry as it is simple and environmentally friendly, the results revealed that the GA directed process is essential to obtain ZnO nanocomposites in low temperature precipitation experiments. On the basis of the experimental results, we discuss and propose a plausible mechanis m to elucidate the format ion of ZnO nanocomposites. The formed products are well characterized by XRD, TEM , UV-DRS, PL-spectrum, Raman, FT-IR, XPS, spectra and show good microbial activity towards different strains of bacteria. Keywords Zn O Nanoco mposites, Gu m Acacia, Screw Capped Technique, TEM, Microbial Activity 1. Introduction Zinc o xide (Zn O) a lo w cost, industrial important material, has been extensively investigated with good potential applications in lu minescence, gas sensors, solar cells, light emitt ing diodes, surface acoustic wave filters, piezoelectric transducers, actuators, optical devices, transparent UV-protection films, antibacterial agents, chemical sensors, and as photo catalysts. Different methods are adopted for preparing ZnO structures especially at lower temperatures (<100°C) fro m water soluble Zn precursors, using various structure directing agents and zinc comp lexes [1-5]. A wide variety of techn iques such as wet chemical method , che mica l vapo r depos it ion , the rmal decomposition, spray pyrolysis, high-temperature physical or chemical methods[7-15] etc have been employed for the synthesis of nanomaterials and structures. The conventional processes require h igh temperature (> 500°C), low or high pressure which may require sophisticated systems. On the other hand precipitation techniques for the synthesis of ZnO nanoparticles have been found to be simp le and economical, * Corresponding author: email@example.com (B. Sreedhar) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved compared to other traditional methods for large scale production. Tuning of different chemical synthetic conditions such as concentration, variation in time, pH, pressure and temperature lead to the formation of nano range materials with different mo rphological growth, size and properties[16-19 ]. Diversity of ZnO aggregate structures has been obtained with different additives such as ethylene diamine, ascorbate, malate, Tween-85, and various polymers[24-27]. In general, poly mers are selected due to negatively charged moiet ies that preferably interact with Zn2+ ions and induce a directional growth of the formed structure. A special case is the use of naturally occurring additives such as amino acids, peptides[28-30], or gelatin. Recently, Feng et al. has reported peanut-like ZnO structures obtained in the presence of naturally occurring apple pectin, through hydrothermal approach. Due to the co mplexity and the demand of ZnO in ext remely mild experimental conditions, proteins or b io molecules are rarely employed for chemical synthesis to preserve their physical and chemical properties[33-35]. Instead, many researchers have used synthetic polymers as substitutes to prepare ZnO nanostructures with different shapes, sizes and crystal phases[36-40]. Recently, bio-nanocomposites have attracted great attention by many researchers as they exhib it signiﬁcant improvements in mechanical properties, dimensional stability, and thermal properties. In addition 170 M . Sulochana et al.: Synthesis and Characterization of Gum Acacia-Stabilized Zinc Oxide Nanoparticles: A Green Approach and M icrobial Activity bio-nanocomposites offer other beneﬁts like low density, transparency, good ﬂow, better surface properties, and recyclability. In early days of colloid science, plant extracts and gums have been used routinely to stabilize colloidal metal dispersions. However, rev iew of literature, confirmed that the synthesis of GA – ZnO nanocomposite has been unexplored, which aroused our interest in the present investigation. Gu m Acacia is a natural, highly branched, neutral or slightly acid ic arabinogalactan polysaccharide with high mo lecular weight obtained naturally fro m the stems and branches of the Acacia Senegal tree. GA consists of three main fractions: The majo r one is a highly branched polysaccharide consisting of a β-(1-3)-galactose backbone with linked branches of arabinose and rhamnose, which terminate in g lucuronic acid. A smaller fract ion (~10 wt % of the total) is arabinogalactan–protein complex[GA GP (gum acacia-g lycoprotein)], in which arab inogalactan chains are covalently lin ked to a protein chain through serine and hydroxyproline groups. The attached arabinogalactan in the co mp lex contains about 13 mo l % glucuronic acid. The s mallest fraction (~1% of the total), with the highest protein content (~50 wt %), is a glycoprotein that differs in its amino acids co mposition fro m that of the GA GP co mplex. Here, the functional group (-OH) present in arabinose and rhamnose and -COOH of glucuronic acids play a crucial role in the formation of nanoparticles, whereas the proteinaceous core with amino acids stabilizes the formed nanoparticles. The non-toxic and biocompatible propert ies of GA have made it widely used in the food and pharmaceutical industries as additives or emu lsifying agents. Moreover, it has also been increasingly used as a stabilize r for various novel nanomateria ls, such as carbon nanotubes, nanogold, nanosilver, nanop allad iu m, nanoplatinum and o xide nanoparticles . GA also shows a superb dispersing ability in the preparation of quantum dot nanocolloids. In our prev ious study, we have used GA both as a reducing agent for the stabilization of silver, palladiu m and platinum nanoparticles and as crystal growth modifier for metal carbonates[50-51]. In the present study, we have investigated the influence of GA as a structure directing agent of ZnO deposition. Depending on the concentration of gum acacia we have prepared zincite composite structure with diffe rent habiti of the prima ry zinc ite nanoparticles. 2. Experimental 2.1. Materials All the reagents were of analytical grade and used without further purification. Zinc acetate di-hydrate and sodium hydroxide (NaOH) were obtained fro m SD Fine chemicals, India. All the solutions were prepared with de-ionized water. 2.2. Preparati on of Zinc Oxi de Nanoparticles In a typical experiment, different proportions of homogenized GA (0.25%, 0.5% and 1.0%) solutions were mixed with Zn(CH3 COO)2.2H2O (1mmol) under vigorous stirring for 15 minutes at room temperature. The mixed solution was transferred into 100ml round bottomed flask and reflu xed for 24 h by maintaining 100°C. After co mplete dissolution, 10ml of NaOH (0.1M) is added dropwise to an above precursor solution with continuous stirring. Similar precursor solutions were placed into glass bottles with tight fitting screw caps. The bottles were p laced in an oil bath for reflu xion at a constant temperature of 100°C for 5h. Subsequently, the products were filtered, washed with ethanol and dried in hot air oven at 80°C. The resultant products were examined in terms of their structural compositional morphological and optical properties UV-DRS absorption and photoluminescence studies. The ZnO nanoparticles were prepared by precip itation fro m solution using Zn(CH3COO)2 and NaOH. The overall reaction for the synthesis of ZnO fro m Zn(II) acetate can be written as Zn(CH3COO)2 + 2NaOH → ZnO + 2Na(CH3COO)2 + H2O 2.3. Characterization Methods The structural properties of the obtained products were recorded using a Rigaku X-ray powder diffractometer (Cu radiation, λ = 0.1546 n m) running at 40 kV and 40 mA (Tokyo, Japan). TEM images were observed on TECNAI FE12 TEM instrument operating at 120 kV using SIS imaging software. The particles were d ispersed in methanol and a drop of it was placed on formvar-coated copper grid followed by air dry ing. UV-Vis-DRS spectra were recorded on a Perkin Elmer Lambda 750 spectrophotometer. Photoluminescence measurements were done using Jobin–Yvon Fluorolog-3 spectrofluorimeter using the 285 nm excitation line of a xenon lamp (450 W). The samp les for PL studies were prepared by dispersing small amounts of ZnO powder in water by ultrasonication. The micro Raman spectra of the synthesized samp les were recorded using a confocal microscope (Oly mpus Fluoview 1000). FT-IR spectra were recorded on Thermo Nicolet Nexus (Washington, USA) 670 spectrophotometer. XPS measurements were obtained on a KRATOS-AXIS 165 instrument equipped with dual aluminiu m– magnesium anodes using Mg Kα radiation (hν = 1253.6 eV) operated at 5 kV and 15 mA with pass energy 80 eV and an increment of 0.1 eV. The samp les were degassed out for several hours in the XPS chamber to min imize air contamination to the sample surface. To overcome the charging problem, a charge neutralizer of 2 eV was applied and the binding energy of C 1s core level (BE = 284.6 eV) of adventitious hydrocarbon was used as a standard. The XPS spectra were fitted using a nonlinear square method with the convolution of Lorentzian and Gaussian functions after a polynomial background was subtracted from the raw spectra. American Journal of M aterials Science 2013, 3(5): 169-177 171 2.4. Anti microbi al Acti vi ty To examine the antibacterial effect of the as-synthesized ZnO nanoparticles, five different samples i.e., ZnO synthesized without GA, Zn O prepared with 0.25%, 0.5% and 1% GA in oil bath and ZnO prepared with 1% GA in screw capped method were loaded on a nutrient agar media. The agar med iu m was prepared by mixing 0.5 g of peptone, 3.0 g of beef ext ract, and 5.0 g of sodium ch loride (NaCl) in 1000 mL distilled water, and the pH was adjusted to 7.0. Lastly, 15.0 g of agar was added to the solution. The agar med iu m was sterilized in a conical flask at a pressure of 15 lbs for 30 min. This agar was transferred into sterilized petri dishes in a laminar air flow. After solidification of the media, Gram positive and Gram negative cultures were streaked on the solid surface of the media. To this inoculated petri dish, one drop (75 lL) of synthesized ZnO nanoparticles solutions (20 mg/10 mL distilled water) was added using 50 lL tip into the wells bored on the agar plate, and the plates were incubated for 48 h at 37°C. 3. Results and Discussion 3.1. Structural Characterizati on of ZnO Nanoparticles Figure 1. XRDpatternsof ZnO nanocrystals formed (a) in double distilled water without GA, (b) with 1% GA solution using oil bath reaction, and (c) with 1% GA solution using screw capped method Figure 1 shows the X-ray d iffraction patterns of ZnO nanoparticles formed in double distilled water without GA, with 1% GA using oil bath reaction and screw capped method. Based on the XRD pro files, crystallin ity and phase of the products are examined. The overall crystalline structure shows the hexagonal phase with preferred (101) orientation at 2θ = 36.2. A ll other less intense diffraction peaks at (100), (002), (102), (110), (103), (200), (112), (201) orientations at 2θ = 31.7, 34.4, 47.5, 56.6, 62.8, 66.3, 67.9, 69.1 (Fig. 1) are assigned well to the pure hexagonal phase of ZnO with wurt zite structure (JCPDS card No. 36–1451) and are in good agreement with the reported data. The peak corresponding to (002) reflect ion indicates the polycrystalline nature of the product with a preferred c-axis orientation. All the as synthesized samples show similar XRD patterns, except a difference in peak intensities, and this may be attributed due to the random orientation of the nanoparticles. The strong and narrow diffraction peaks indicate that the product has a good crystallin ity. It a lso suggests that the adsorption of ZnO nanoparticles on GA does not alter the crystallin ity. Absence of any other peaks clearly indicates that no impurities are present. The diameter of the nanoparticles is calculated by Debye-Sherrer equation: D = Kλ/ β cos θ where K is Sherrer constant, λ is the X-ray wavelength, β is the peak width at half maximu m, and θ is the Bragg diffraction angle. The average crystallite size D was estimated to be around 21.6 n m using the Debye–Scherrer for mula . 3.2. Morpholog y Control of ZnO Nanoparticles To access the size and morphology of the as-synthesized samples, we performed TEM and the images are presented in figure 2. As can be seen from Fig. 2a, ZnO part icles synthesized without GA are very large and they are also of various shapes such as rods (~ diameter = 40 n m and length in few microns) and cubes (~ in the range 30 - 210 n m, Fig. 2d). In the presence of 1% GA , both in o il bath and screw capped methods, the produced globular structures consisting of ZnO nanoparticles (Fig. 2b, c), are of nearly uniform size distribution between 5 – 23 n m (Fig. 2e, f), that compares well in accordance with the size – 21.6 n m estimated with XRD studies. Particle size distribution shown in Fig. 2 d, e and f clearly indicate a remarkable reduction in the average size of ZnO nanoparticles fro m 130 n m to 14 n m in the presence of 1% GA. A small increase in the average particle size was observed when the reaction was carried out in screw capped method that can be assumed due to the reagglomeration of the nanosized particles in the pressure med iu m. The influence of GA on the formation of ZnO nanoparticles was studied using three mo lar concentrations of GA – 0.25%, 0.5%, 1% in both oil bath and screw capped method. At 0.25% GA concentration, ZnO nanoparticles are not distinct (Fig. 2g) and appear to be less in number on GA globules. Increasing the concentration of GA to 0.5% (Fig. 2h) and 1.0% showed similar features but covered with a thick layer of ZnO c rystals. Figure 2i shows TEM images of the ZnO structures corresponding to the sample calcined at 400°C for 3 h. As can be seen, on calcination, ho llo w and porous ZnO spheres are formed due to the decomposition of GA g lobules and agglomeration of crystalline subcrystals ranging fro m 3-4 n m forming a thick and continuous shell[52-53]. Fro m the above it can be inferred that soluble GA plays an important role in the format ion of sphere-shaped particles. It is likely that init ially Zn2+ ions form bonds with the high number of coordinating functional groups of the GA, leading to nucleation and preferentially 172 M . Sulochana et al.: Synthesis and Characterization of Gum Acacia-Stabilized Zinc Oxide Nanoparticles: A Green Approach and M icrobial Activity crystal growth. In most cases, the vander Waals interactions between the surface mo lecules of the formed nanoparticles form the driving force for self-assembly, and then ZnO nanocrystals can be assembled to form large ZnO spheres. These microscopic results suggest that natural polysaccharide – GA can be used as a temp late fo r the fabrication of metal o xide hollow spheres as compared to other alternative methods. The selected-area electron diffraction (SAED) patterns taken fro m the TEM images for all the samples showed a similar pattern. As can be seen, the observed SAED patterns show distinct spots indexed to (100), (002), (101), (102) and (110) corresponding to wurtzite ZnO structure, wh ich is consistent with the XRD res u lts . nanoparticles in the present study. The observed band energies are slightly lesser when co mpared to that of commercial ZnO (3.37eV). The difference in the band gap values ranging fro m 3.0 eV to 3.3 eV is considered as one of the curious feature of ZnO material. This demonstrates that the synthesized Zn O part icles are pure, showing band gap in the range 3.10 eV - 3.22 eV, and has good optical property. Figure 3. UV-DRS spectra of ZnO nanocomposites synthesized (a) in double distilled water without GA, (b, c, d) in 0.25%, 0.5% and 1% GA in oil bath reaction, and (e) in 1% GA in screw capped method. Inset showing the band energies value of ZnO nanocomposites synthesized (a) in double distilled water without GA, (b, c, d) in 0.25%, 0.5% and 1% GA in oil bath reaction, and (e) in 1% GA in screw capped method 3.4. Photoluminescence Spectra Fi gure 2. TEM images of ZnO nanocryst als formed (a) in double dist illed water without GA, (b) with 1% GA in oil bath, (c) with 1% GA in screw capped method, (d) Particle size distribution of ZnO nanocrystals without GA, (e) with 1% GA in oil bath, (f) with 1% GA in screw capped method, (g) TEM images of ZnOnanocrystals with 0.25% GA in oil bath, (h) with 0.5% GA in oil bath, and (i) with 1% GA in oil bath calcined at 400°C for 3h 3.3. UV-Vis-DRS Spectra The ZnO products synthesized in this study were examined by ultravio let-visible-diffused reflectance spectroscopy (UV-Vis-DRS) for the optical property. Figure 3 shows the UV-Vis-DRS spectra of the as-synthesized ZnO nanoparticles without GA and with different concentrations (0.25, 0.5 and 1.0%) of GA using oil bath and screw cap method. Usually zinc o xide exh ibits an ultraviolet emission (at about 380 n m at room temperature) due to near band edge electron transition[55-56]. As displayed in figure 3, a strong absorption at about 400, 395, 392, 390 and 385 n m is observed for ZnO nanoparticles formed without GA, with 0.25, 0.5, 1% GA in oil bath and screw cap methods, respectively, which is the characteristic peak of wu rtzite hexagonal Zn O. Surprisingly, the absorption edge of ZnO nanoparticles shows an obvious blue shift due to the quantum confinement effect. The corresponding band gap energies were determined and found to be 3.10, 3.14, 3.16, 3.18 and 3.22 eV (Fig. 3 Inset) for the synthesized ZnO Figure 4 shows the room temperature photoluminescence spectra of the as-synthesized ZnO nanoparticles. The PL spectra of all the samp les are composed of two main emissions - a strong UV band located at around ~ 385 n m, and a broad visible emission band spreading fro m 420-680 nm. The strong UV emission is attributed to the recomb ination of free excitons in ZnO and the broad visible emission band is due to native defects of ZnO. To analyze the broad v isible emission band in detail, the PL spectra obtained for ZnO synthesized without GA (Fig. 4a) and with 1% GA screw capped method (Fig. 4e) are deconvoluted into several Gaussian functions. On deconvolution, a blue band (471n m), a green band (519 n m), and a yellow band (580 n m) are revealed for ZnO synthesized without GA. The positions and relative intensities of the observed bands are similar for ZnO synthesized with 0.25% GA, 0.5% GA and 1% o il bath (Fig. 4b-d). The blue band is due to VO native defects and the green and yellow bands originate fro m reco mb ination of electrons in singly occupied oxygen vacancies with photoexcited holes. ZnO synthesized using 1% GA in screw capped method (Fig. 4e) showed a shift in these band positions - blue band (492 n m), green – yellow band (560 nm), and red band (640 n m). Green yellow co mbination emissions in screw capped grown ZnO are co mmon ly observed in thermal methods and that may be due to the defects such as oxygen vacancies, Zn interstitials or impurities. Finally the red emission band is believed to American Journal of M aterials Science 2013, 3(5): 169-177 173 be associated with the shallow Vo levels. Raman-active mode of wurtzite hexagonal Zn O. The strong peak observed at 436 cm-1 due to E2H mode shifts to 440 cm-1 may be attributed to the ZnO nanoparticles trapped in GA resulting in the format ion of organic-inorganic hybrid n an o co mp os ites . 3.6. FT-IR S pectra Figure 6 represents the typical FT-IR spectra of as-synthesized ZnO without GA, GA-Zn O in oil bath and screw cap methods. In general, the FT-IR spectrum of the main absorption band is due to Zn-O stretching of ZnO in the range of 600-400 cm-1. It is observed that the existence of distinct absorption peak at 439 cm-1 corresponds to the E2 mode of he xagonal ZnO (Ra man active) and the peaks at 445 and 472 cm-1 (Fig. 6a) which are characteristic absorption peaks of Zn-O stretching modes. Figure 4. Luminescence spectra of ZnO nanocomposites synthesized (a) in double distilled water without GA (with deconvoluted spectra), (b, c, d) in 0.25%, 0.5% and 1% GA in oil bath method, and (e) in 1% GA in screw capped method (with deconvoluted spectra) 3.5. Confocal Micro Raman Spectra Figure 6. FT-IR spectra of ZnO nanocomposites synthesized (a) in double distilled water without GA, (b) in 1% GA in oil bath method, and (c) in 1% GA in screw cap method Figure 5. Micro Raman spectra of ZnO nanocomposites (a) in double distilled water without GA (b, c, d) 0.25%, 0.5% and 1% GA in oil bath reaction, and (e) in 1% GA in screw capped method The confocal micro Raman spectra of the as-synthesized ZnO nanoparticles in the wavenumber range fro m 200 – 800 cm-1 at roo m temperature are shown in figure 5. ZnO has a wurtzite structure with C6V point group symmetry with two formula units per primit ive cell, and all the atoms occupy the C3v symmetry. There are six Raman active modes: two E2 vibrations at 101 and 437 cm‒1; one transverse A1 at 381 cm‒1 and one transverse E1 at 407 cm‒1; one longitudinal A1 at 574 cm‒1 and one longitudinal E1 at 583 cm‒1[61-62]. The modes A1, E1 and E2 are Raman active for ZnO single crystal materials. Fro m figure 5 the peaks at 329 cm-1 and 380 cm-1 correspond to E2H-E2L (mu lti-phonon process) and A1T modes, respectively. A sharp strong peak located at 436 cm-1 due to E2H mode, is the intrinsic characteristic of the The broad peak in the higher energy region, 3400 – 3600 cm-1 is due to the stretching vibration of -OH and -NH2 groups on the surface of ZnO nanoparticles. FT-IR spectra of as-synthesized ZnO without GA in the present investigation are similar to that of GA -ZnO nanoparticles which are in good agreement with the reported values. The characteristic absorption peak for the bridging coordination modes of acetate group with Zn appears in the range 1500-1650 cm-1 resulting fro m a residual acetate group of zinc acetate used for the synthesis of ZnO nanoparticles. The peaks at 1000 to 1100 cm-1 and 2928 cm-1 can be assigned to the symmetric methylene stretching. The stabilization of on ZnO nanoparticles by using GA caused slight changes in the intensities of absorption band in the range of 600-400 cm-1 wh ich is attributed to the Zn-O stretching. These differences in IR spectra can be explained on the basis of constrained growth of the formed n an o p articles . 3.7. X-ray Photoelectronic S pectra XPS is an important surface sensitive analytical technique 174 M . Sulochana et al.: Synthesis and Characterization of Gum Acacia-Stabilized Zinc Oxide Nanoparticles: A Green Approach and M icrobial Activity useful for the identification of the surface characteristics of metal o xide nanoparticles. Fig. 7a shows the survey scan of ZnO nanostructure obtained from 1% GA using oil bath method. As can be seen from the survey scan, peaks characteristic of C 1s (284.6 e V), O 1s (531.5 e V) and Zn 2p (1022.2 eV) are observed. The high resolution narrow scan of ZnO nanoparticles results fro m t wo pairs of spin-orbit components namely, Zn 2p3/2 and 2p1/2 peaks at 1022.2 and 1045.2 eV, respectively and can be assigned due to Zn2+ in ZnO nanocrystals (Fig. 7b). The deconvolution of the high resolution scan of C 1s peak indicated four co mponents of carbon. The binding energy peak at 284.6 eV is due to the presence of –C-C-/-C-H- bond, the peaks at 285.7 eV and 286.8 eV are attributed to the presence of –C-O-, -O-C-Oand the last peak at 288.5 eV is assigned due to the presence of carboxy late group (-O=C-O-) adsorbed on the surface of the formed Zn O nanoparticles. These carboxylate groups are either fro m GA or zinc precursor used for the synthesis of ZnO nanocomposites. result was in agreement with the XRD patterns. The antibacterial act ivity of the as-synthesized ZnO nanoparticles was investigated against various pathogenic bacteria of Gram positive (Bacillus subtilis and Staphylococcus aureus,) and Gram negative strains (Escherichia co li, Pseudomonas aueroginosa) using agar diffusion technique. In order to investigate this, we examined the antibactericidal property of five samples i.e., ZnO synthesized without GA (a) ZnO prepared with 0.25%, 0.5% and 1% GA in o il bath (b-d) and ZnO prepared with 1% GA in screw capped method (e) under identical conditions. The diameter of the zone of inhibit ion showed a significant dependence on the different type of bacteria, E.Co li, Bacillus, Pseudomonas and Staphylococcus and also on the synthetic protocol used for the synthesis of ZnO and GA-ZnO nanoparticles. Among the four bacteria used in the present work, pseudomonas and staphylococcus showed better inhibitory activ ity, when co mpared to E. Co li and Bacillus. Figure 8. Photographs showing antimicrobial activity of ZnO nanoparticles stabilized with GA on different bacteria (a) E.Coli, (b) Bacillus, (c) Pseudomonas, and (d) Staphylococcus Figure 7. Typical XPS spectra (a) survey scan, (b) high-resolut ion narrow Zn 2p scan, (c) C 1s scan, and (d) O 1s scan of ZnO nanocomposite This observation is in consonance with the FTIR studies, the absorption band observed at 1500-1650 cm-1 due to carboxy late group. The presence of C 1s peaks characteristic of GA in the as-synthesized ZnO nanoparticles, reiterates that the surface of the formed ZnO nanoparticles is coated with GA leading to the format ion of inorganic–organic hybrid material. Fro m the high resolution narrow scan for O 1s, the lower b inding energy peak centered at 531.5 eV can be attributed to the O2- ions at the interstitial sites (surface and near surface region lattice oxygen) in the wurt zite structure and also due to some contribution originating fro m O2- ions in the o xygen deficient regions within the ZnO mat rix wh ile the higher binding energy peak at 533.3 eV is associated with –O-Hgroups on the surface of the ZnO nanoparticles[65-66]. No obvious peaks for other elements were observed and the As can be seen from figure 8 the diameter of inhibit ion zones of ZnO synthesized without GA (a) show less inhibitory activity when co mpared to GA-ZnO nanoparticles (b-e) against all the bacteria - E.Coli, Bacillus, Pseudomonas and Staphylococcus. This can be attributed to the decrease in the average size of particles fro m 130 n m to 14 n m when GA was used to stabilize ZnO nanoparticles as was observed fro m TEM studies. The antibacterial efficacy increased with increase in concentration of ZnO particles. As the concentration of GA was increased - 0.25%, 0.5% and 1.0%, increase in the inhibitory activity is observed. This can be attributed due to fact that the number of ZnO nanoparticles formed with 0.25% GA is less in number as is evident fro m TEM studies, thus shows moderate activity. When 0.5% and 1.0% GA was used, a sufficient nu mber o f Zn O-GA nanoparticles are formed resulting in greater efficacy of inhibit ion against the bacteria studied. Smaller the particle size, easier to adhere to the cell wall of the microorganisms causes it’s destruction - leading to the death of the cell. American Journal of M aterials Science 2013, 3(5): 169-177 175 To understand the growth mechanism of Zn O structures, the growth process was analyzed based on the observations of the products at different reaction conditions. An oriented aggregation mechanism is proposed and this is regarded as one of the non-classical crystallizat ion mechanism. According to TEM observations a possible growth p rocess for the ZnO spheres is illustrated in Scheme 1. Gu m acacia is a natural polysaccharide constituting of negatively charged ions (-COOH, NH2 and -OH-) that slowly interacts with Zn2+ in the presence of a strong base NaOH and influences the morphological changes resulting the format ion of ZnO nanoparticles. Several factors such as crystal face interactions, electrostatic interactions, vander waals forces, hydrophobic interactions and hydrogen bond format ion may have various effects during the formation of n an o co mp os ites . ZnO nanocomposites have been successfully synthesized using zinc acetate, gum acacia and NaOH at 100°C in oil bath and screw capped methods. The observations show that screw capped method is more e ffective than oil bath method for obtaining nanosized particles in the range 5 - 23 n m. The antibacterial activity of thus prepared GA-ZnO nanocomposites against the bacteria - E.Coli, Bacillus, Pseudomonas and Staphylococcus is reported. 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