Green chemistry approach for the synthesis of gold nanoparticles with gum kondagogu: characterization, catalytic and antibacterial activity
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Gold nanoparticles (AuNPs) were prepared from HAuCl4 using gum kondagogu, by adopting green synthesis, which is a simple, low cost and ecofriendly technique. The gum kondagogu (Cochlospermum gossypium) serves as both reducing agent and stabilizer. The formation of the AuNPs was identified through the change in the color of the solution from yellow to red. The synthesized AuNPs were characterized by various techniques. The green synthesized AuNPs were found to be stable in the pH range pH 2–12 and up to the concentration of 5 M NaCl. The stabilized AuNPs demonstrated the excellent catalytic activity in reducing p-nitrophenol to p-aminophenol in the presence of a reducing agent, NaBH4. The effects of catalyst dose and temperature were studied. The synthesized, new gum-based catalyst was very efficient, easy to prepare, stable, cost-effective and ecofriendly. The synthesized AuNPs showed good antibacterial activity.
KeywordsGold nanoparticles Gum kondagogu Catalytic activity Antibacterial activity Nanomaterials
In recent years, nanomaterials play a substantial role in science and technology due to their unique shape, size, peculiar properties and wide range of potential applications. Metal nanoparticles possessing considerable electronic, chemical and optical [1, 2] properties, are different from the bulk materials. Among the several metal nanoparticles, AuNPs have attracted the attention of scientists due to their numerous applications in catalysis, sensing, imaging, and diagnostics [3, 4, 5, 6]. ‘AuNPs’ is the area of research interest due to their unique properties such as tunable surface plasmon resonance (SPR), surface-enhanced Raman scattering, electrical, magnetic, thermal conductivity, antibacterial activity, chemical and biostability [7, 8, 9, 10, 11, 12, 13]. Besides this, the use of AuNPs as potential materials in the field of drug delivery and DNA delivery systems is noteworthy [14, 15]. Most of the available methods for the synthesis of AuNPs involve photochemical reduction, chemical reduction, and electrochemical reduction [16, 17, 18]. The reagents used in these methods are NaBH4, hydrazine, ascorbic acid and amino acids which are capable of being oxidized. However, the uses of such chemical reagents that are toxic and ecologically injurious are to be avoided in the synthesis of AuNPs [19, 20]. Hence, the above-mentioned chemical methods are non-ecofriendly in nature and have inherent drawbacks. The replacement of non-ecofriendly synthesis methods with clean, non-toxic and globally acceptable green chemistry methods  is the current need in the synthesis of AuNPs. Several biological systems such as bacteria, fungi, fruit extract and plants can actively reduce metal ions to form metal nanoparticles in an ecofriendly manner [22, 23, 24]. Among these, gums obtained from plants such as gum gellan, chitosan and katira gum, (natural polymers), etc., act as reducing agents and stabilizing agents [25, 26, 27]. They are also known to be the best and suitable for large-scale green synthesis.
Gum kondagogu is a naturally available polysaccharide component. This gum is basically cheap, easily available, and non-toxic and has a potential application as a food additive . It is extracted from the bark of Cochlospermum gossypium (Bixaceae family) and is largely collected by tribes. The primary structure of gum kondagogu is made up of sugars such as galactose, arabinose, mannose, glucose, gluconic acid, rhamnose and galacturonic acid with sugar linkage of (1 → 2) β-d-Gal p, (1 → 4) β-d-Glc p, (1 → 6) β-d-Gal p, 4-O-Me-α-D-Glc p, (1 → 2) α-l-Rha . Gum kondagogu is an acidic gum and carboxylic acid, acetyl, hydroxyl and carbonyl groups are identified as major functional groups in the gum. There were some reports on the synthesis of silver nanoparticles using gum kondagogu as a reducing and stabilizing agent .
Gold as a bulk material has usually been regarded to be inactive as a catalyst. But gold in the form of nanoparticles shows excellent catalytic activity towards the reduction of ferrocyanate(III), deoxygenation of epoxides into alkenes, reduction of nitroarenes and cyanosilylation of aldehydes by TMSCNf [31, 32]. The p-nitrophenol (4-NP), as with other nitrophenols and derivatives, is a common by-product from the production of herbicides, pesticides, and synthetic dyes . Nitrophenols (NP) are environmental poisons due to their toxicity and are inhibitory in nature . In addition, nitrophenols have high solubility and stability in water. These NPs tend to get accumulated in deep soil and stay indefinitely. 4-NP poses a hazard to living beings due to their carcinogenic and recalcitrant properties . Due to this reason, the reduction of 4-NP into p-amino phenol (4-AP) is prominent. Sodium borohydride (NaBH4) is a strong reducing agent. However, it has no ability to reduce the nitrophenol. Hence, NaBH4 is not effective in this reaction except provided with some catalyst to reduce the kinetic barrier of the reaction. A variety of catalysts were used in the past and recently, Pt, Ag [36, 37] nanoparticles have been used for the same purpose. Antimicrobial agents are important weapons in fighting bacterial infections and have significantly benefited the health-related quality of human life . Organic compounds used as disinfectants have some disadvantages, including toxicity to the human body. For that reason, the interest in inorganic disinfectants such as metal nanoparticles (NPs) is increasing. In addition, AuNPs with smaller particle size have been reported to show good antimicrobial activity .
The present study reports the green synthesis of AuNPs using the gum kondagogu as an essential material. This gum serves as both reducing and stabilizing agent. The characterization of AuNPs was carried out by UV–Visible spectrophotometer, FTIR spectroscopy, XRD analysis and TEM. The AuNPs were explored with respect to their prospective catalytic applications. The antibacterial activity of AuNPs was tested against Gram-positive and Gram-negative strains of bacteria.
Results and discussion
Stability of gold nanoparticles against pH and NaCl
A potential application of metal nanoparticles is the catalysis of certain reactions, which would not otherwise occur. In the present study, we have taken a model reduction reaction of 4-NP to 4-AP by NaBH4. The substrates and products of this reaction are detected by spectroscopic methods and there is no appreciable formation of byproducts. The reaction took place after mixing 1.7 mL of 0.2 mM 4NP with 1 mL of 15 mM NaBH4 in the quartz cell leading to the change of color from light yellow to deep yellow color. 4-NP solution exhibits a strong absorption peak at 317 nm. After the addition of NaBH4, the reaction mixture showed a strong absorption peak at 400 nm which is due to the formation of p-nitrophenolate ion . It is also observed that the pH of the solution has changed from acidic to highly basic nature.
Rate constants at different temperatures for AuNPs-catalyzed reduction of 4-NP by NaBH4
Rate constant (min−1)
0.0761 ± 0.01
0.0984 ± 0.013
0.1343 ± 0.014
0.2107 ± 0.018
0.3358 ± 0.012
Gum kondagogu is an efficient source for the synthesis of AuNPs. The gum kondagogu acts both as a reductant and as a stabilizer. The synthesized AuNPs were characterized by various techniques. The XRD pattern showed that the synthesized AuNPs were essentially crystalline. It is found that both hydroxyl and carbonyl groups of gum kondagogu are involved in the synthesis and stabilization of AuNPs. The morphology by TEM showed that the synthesized AuNPs were spherical in shape and crystalline in nature with the average size distribution of 12 ± 2 nm. The catalytic activity of green synthesized AuNPs was examined by the hydrogenation of 4-NP reaction. The detailed kinetic aspects for catalytic hydrogenation were evaluated by changing the process parameters. It is as well observed that with an increase in AuNPs concentration and temperature, the total reaction time decreased and the rate of the reaction has increased. The synthesized AuNPs showed significant antibacterial action on both the gram classes of bacteria (E. coli and B. subtilis).
The starting materials for the synthesis of AuNPs were gum kondagogu (obtained from Girijan Co-operative Corporation Limited, Hyderabad), tetrachloroauric(III) acid trihydrate—HAuCl4 3H2O (99.9 %, Aldrich), sodium borohydride—NaBH4 (98 %, S-D Fine Chemicals), nitric acid—HNO3(S-D Fine Chemicals), hydrochloric acid—HCl (S-D Fine Chemicals) and p-nitrophenol (S-D Fine chemicals).
Synthesis of AuNPs
All the solutions were prepared in Milli-Q water. 0.5 %(w/v) of homogeneous gum stock solution was prepared by adding a calculated quantity of gum kondagogu powder into the reagent bottle containing milli-q water and stirring the same for 1 h at room temperature. 1 mL of 1 mM chloroauric acid solution and 3 mL of gum kondagogu solution were mixed in a boiling tube. This mixture was kept in an autoclave at 15 psi pressure and 120 °C for 10 min. The resulting solution was red colored, indicating the formation of AuNPs. The solution of synthesized gold nanoparticles was centrifuged at high speed. The pellet and supernatant liquid were separated. The pellet was again dispersed in double-distilled water.
In order to confirm the formation of AuNPs, the UV–Visible absorption spectra of the prepared colloidal solution was recorded using a UV–Vis–NIR spectrophotometer (UV-3600, Shimadzu) having a scanning range of 200–700 nm against blank autoclaved gum.
FTIR analysis was carried out in order to determine the possible functional groups of gum kondagogu, which helps in the reduction and stabilization agent of synthesized nanoparticles. The colloidal solution of AuNPs was first lyophilized and the sample was used for FTIR analysis in the form of a thin transparent pellet with KBr. A pure KBr pellet was used as a background and this was subtracted from the FTIR spectra of the gum kondagogu and AuNPs sample. FTIR spectra were recorded with an instrument IR Affinity-1 (Shimadzu) in the scanning range of 650–4000 cm−1.
The crystallinity of the AuNPs was studied by XRD (Rigaku, Miniflex) method with Cukα radiation.
The morphology and size of synthesized AuNPs was examined by TEM. The sample gird for TEM measurement was prepared by placing a drop of aqueous AuNPs dispersion on the carbon-coated copper grid and subsequently evaporating the water naturally overnight at ambient conditions. The measurements were done on JEOL 2000 FX-II TEM.
Catalytic reduction of p-nitrophenol
As a sample reaction, the reduction of p-nitrophenol to p-aminophenol by sodium borohydride has been selected. The reduction took place in aqueous solution in a standard quartz cell with 1 cm path length. In the reaction process, 1.7 mL of 0.2 mM p-nitrophenol was mixed with 1.0 mL of 0.015 M NaBH4 in the cell for UV–visible measurements. Immediately, the color changed from light yellow to deep yellow. Varying concentrations of (50–150 µL) of AuNPs solution was added to the above mixture. The UV–visible absorption spectra were recorded with a time interval of 1 min, in a scanning range of 200–650 nm.
Antibacterial property of AuNPs
Antibacterial properties of the synthesized AuNPs were carried out using the disc diffusion method. Gram-positive and Gram-negative bacteria, Bacillus subtilis and Escherichia coli, respectively, were used as model test strains. Luria–Bertani (LB) agar medium was prepared and transferred to sterilized petri dishes. The medium was allowed to solidify and then the petri plates were spread with Bacillus subtilis and Escherichia coli separately in a laminar air flow hood. Using micropipette, 5 and 10 µL of the AuNPs solution and 5 µL of gum kondagogu solutions added to each well on both plates. The discs were air dried in laminar hood and incubated at 37 °C for 24 h. Then, zone of inhibition of bacteria was measured. The assays were performed in triplicate.
One of the authors, Bhagavanth Reddy G, gratefully acknowledges CSIR, New Delhi, for providing Senior Research Fellowship. The authors wish to thank Centre for Nanotechnology, University of Hyderabad, for allowing the use of their TEM facility.
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