Abroma augusta Linn bark extract-mediated green synthesis of gold nanoparticles and its application in catalytic reduction

Subhajit Das 0 1 Braja Gopal Bag 0 1 Ranadhir Basu 0 1 0 R. Basu Central Research Facility, Indian Institute of Technology , Kharagpur 721302, West Bengal , India 1 S. Das B. G. Bag (&) Department of Chemistry and Chemical Technology, Vidyasagar University , Midnapore 721102, West Bengal , India The bark extract of Abroma augusta Linn is rich in medicinally important phytochemicals including antioxidants and polyphenols. First one step green synthesis of gold nanoparticles (AuNPs) has been described utilizing the bark extract of Abroma augusta L. and chloroauric acid under very mild reaction conditions. The phytochemicals present in the bark extract acted both as a reducing as well as a stabilizing agent, and no additional stabilizing and capping agents were needed. Detailed characterizations of the stabilized AuNPs were carried out by surface plasmon resonance spectroscopy, high resolution transmission electron microscopy, and X-ray diffraction studies. The catalytic activity of the freshly synthesized gold nanoparticles has been demonstrated for the sodium borohydride reduction of 4-nitrophenol to 4-aminophenol, and the kinetics of the reduction reaction have been studied spectrophotometrically. - Gold nanoparticles (AuNPs) with unique optical, electronic, and magnetic properties have drawn tremendous research interests during the last two decades (Alkilany et al. 2013, Zhang et al. 2012) because of their applications in diversified areas such as catalysis (Wittstock and Baumer 2014; Liu et al. 2014) drug delivery, biodiagnostics (Murphy et al. 2008), medical imaging (Huang et al. 2009), plasmonics (Pelton et al. 2008), and chemical sensing. AuNPs dispersed in water and stabilized with non-toxic biomolecules are required for many of such applications. Among various methods reported for the synthesis of AuNPs, the plant extract-mediated reductive method, involving the reduction of Au(III) to Au(0) by the phytochemicals, has gained profound significance in recent years due to the renewable and non-toxic nature of the phytochemicals, mild reaction conditions, eco-friendly aqueous medium, etc. The method is advantageous over other synthetic methods because the phytochemicals present in the plant extract act both as a reducing agent as well as a stabilizer, and no additional stabilizers or capping agents are needed. The green synthesis of AuNPs from the extracts of Macrotyloma uniflorum (Aromal et al. 2012), Trigonella foenum-graecum (Aromal and Philip 2012), Aloe vera (Chandran et al. 2006), Acacia nilotica leaf (Majumdar et al. 2013), Saraca indica bark (Dash et al. 2014), Punica granatum (Dash and Bag 2014), Green coconut shell (Paul et al. 2014), etc., has been reported (Mittal et al. 2013). Due to rapid emergence of newer applications of nanoparticles and nanomaterials, there is an ever growing need for the development of newer methods for the synthesis of metal nanoparticles utilizing plant resources as renewables. Abroma augusta Linn is a small, ever-green plant growing up-to 34 m in height with velvety branches and found in tropical Asia, South and Eastern Africa, and Australia. Usually found as a wild plant or as an ornamental plant in garden, Abroma augusta L. belongs to the family of Sterculiaceae, and it is also an important Ayurvedic medicinal plant. Various parts of the plant have been used for the treatment of diabetes, rheumatic pain of joints, uterine disorders, headache with sinusitis, nervous dysmenorrhoea, amenorrhoea, sterility, menstrual disorder, etc. (Gupta et al. 2011). During our investigations on the utilization of triterpenoids (C30s) as renewable functional nano-entities (Bag and Dash 2011; Bag and Paul 2012; Bag et al. 2012, 2013; Bag and Majumdar 2012, 2014), it occurred to us that the medicinally important bark extract of Abroma augusta L., rich in polyphenolic compounds, can be utilized for the synthesis of AuNPs from HAuCl4 (Mittal et al. 2013). Herein, we report the experimental evidence for the presence of antioxidants including polyphenols in the bark extract of Abroma augusta L. and the use of the bark extract for a very mild, environment friendly and efficient synthesis of AuNPs without any additional capping or stabilizing agents. The stabilized colloidal AuNPs were characterized by high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), surface plasmon resonance (SPR) spectroscopy, and X-ray diffraction studies. Catalytic activity of the freshly synthesized colloidal AuNPs has been demonstrated for the sodium borohydride reduction of 4-nitrophenol to 4-aminophenol in water at room temperature as a model reaction, and the kinetics of the reduction reaction have been investigated spectrophotometrically. Materials and methods Au(III) solution: HAuCl4 was purchased from SRL and used without purification. HAuCl4 (36.5 mg) was dissolved in distilled water (10 mL) to obtain a Au(III) stock solution (10.74 mM). Preparation of the Abroma augusta bark extract The bark of Abroma augusta L. was collected from the local area of Midnapore, West Bengal, India and identified at the Department of Botany and Forestry, Vidyasagar University, Midnapore. The bark was cut into small pieces, dried in air, and then powdered using a grinder. Finely powdered Abroma augusta bark (20 g) was suspended in ethanol (120 mL), refluxed for 5 h, and filtered. Volatiles of the filtrate were removed under reduced pressure to afford a greenish black sticky solid (0.44 g). The crude greenish black sticky solid was dissolved in ethanol (30 mL) and filtered through a Celite bed. The volatiles of the filtrate were removed under reduced pressure to afford a solid (0.33 g). Purified Abroma augusta bark extract (0.02 g) was suspended in a mixture of distilled water and ethanol (10 mL, 4:1) and sonicated using an ultrasonicator bath for 45 min to obtain a semi-transparent solution (2,000 mg L-1). The most extensively used stable DPPH radical was employed for the study of antioxidant property of bark extract of Abroma augusta. Antioxidants present in the extract react with DPPH radical and convert it to 1,1diphenyl-2-picryl-hydrazine. The color change from violet to yellowish within 30 min upon addition of the bark extract to DPPH solution indicated the antioxidant property of the extract. The reduction in the absorbance intensity at 517 nm was studied by UVvisible spectrophotometry. Identification of polyphenolic compounds The presence of phenolic compounds in the bark extract of Abroma augusta was examined qualitatively by ferric chloride test. The extract of the bark of Abroma augusta (1 mL) was mixed with an aliquot of freshly prepared concentrated FeCl3 solution, and the mixtures was shaken vigorously. Appearance of greenish color almost instantly indicated the presence of phenolic compounds in the bark extract. Synthesis of gold nanoparticles Aliquots of Au(III) solution (0.2 mL, 10.74 mM) were added dropwise to the (...truncated)