Biogenic synthesis of gold nanoparticles and their application in photocatalytic degradation of toxic dyes
Graphical Abstract
Introduction
Over the last two decades there has been an increased interest in the synthesis of different metal nanoparticles. The noble metals are of particular interest because of the interesting changes in their properties at the nano level [1]. Gold in particular has become the focus of intense research owing to their fascinating optical, chemical and electronic properties and their wide range of applications in pharmacology, drug delivery, cosmetics, lubricants and particularly interesting use in catalysis [2,3]. However these properties vary with the morphology and size of the gold nanoparticles. Li et al. demonstrated the synthesis of MXene@AuNPs where MXene acted as both the template and reductant, and the size of the AuNPs varied with increase in reaction time [4]. There are also several reports which describe the synthesis of various nanostructures like spherical, hexagonal, rod like, cubic and planar nanoparticles. The various shapes are achieved by modulating different experimental parameters like temperature, pH, concentration, nanoparticle precursor, time and so on [[5], [6], [7]]. Hexagonal and pentagonal gold nanoparticles are considered to be more catalytically active when compared to other shapes because of the presence of more number of reactive facets [6].
The waste effluents from textile, pharmaceuticals, paper industries consist of organic dyes which leads to environmental pollution. These industrial effluents generally contaminate the nearest water bodies which in turn has toxic effects on the aquatic life [8]. Toxic dyes can be removed from the contaminated water using various processes such as photo-electric degradation, photocatalytic reduction, microwave assisted degradation and adsorption [[9], [10], [11], [12]]. Since these processes use organic solvents and are energy consuming, there has been a search for alternate green and simple approach which has shifted the focus towards the use of nanoparticles. Researchers are now developing different types of nanomaterials for removal of toxic dyes. Guo et al. [13] used a sandwiched composite absorbent nanomaterial (Fe2O3/Carboxylated Graphene oxide), Liu et al [14] used AgNPs on electrospun fibres, Nguyen and Doong [15] prepared visible light responsive catalyst by coupling low loading mass of ZnFe2O4 with anatase TiO2 for adsorption and degradation of dyes. Furthermore, plant mediated nanoparticles such as Fe-NP [16] and ZnO NPs [17] have been synthesized which degrades dyes and also exhibit antimicrobial activities. Nanoparticles with their relatively large surface area to volume ratio exhibit enhanced activity.
The chemical synthesis of nanoparticles uses chemicals which are toxic to the environment and are biological hazards. To combat the negative impact of these toxic agents researchers are now integrating the tenets of Green Chemistry by using environment friendly solvents and methods in the synthesis of nanoparticles. Different plants, bacteria and fungi have been explored for green synthesis of nanoparticles [18,19]. However the use of microbes requires proper handling and maintenance of cells, which makes it a more costly method. Plants on the other hand have become the popular choice for green synthesis of nanoparticles as they are environmentally benign and there are no added cost and labour involved in their maintenance. The different phytochemicals such as phenols, flavonoids, terpenoids present in the plant extracts act as reducing and stabilizing agents in the synthesis of nanoparticles. There have been numerous reports of plants being used as reducing and stabilizing agents. Recently, Mussaenda glabrata [20], Galaxaura elongate [21], Plumeria alba [22] were used in the synthesis of gold nanoparticles.
Alpinia nigra (A. nigra) is a biennial shrub belonging to the Zingiberaceae family. It grows in marshy land and is endemic to South East Asia including China, Sri Lanka, Bangladesh and India. The different parts of the plants are used in traditional medicine for treatment of jaundice, rheumatism, bronchitis, gastric ulcers, intestinal parasitic infections and irregular menstruation [23]. This plant is also used as vegetable and a food flavoring agent by the people of North East India. Studies have reported the presence of many important phytoconstituents such as alkaloids, flavonoids, terpenoids, phenols, glycosides etc. in the leaves and seeds [24,25]. Additionally, the leaves have been reported to have antioxidant and antimicrobial activity [25,26]. Based on these reports, we have postulated that the leaves of A. nigra would act as an effective reducing and stabilizing agent in the synthesis of gold nanoparticles.
This study reports a facile green method for A. nigra leaves mediated synthesis of gold nanoparticles and its photocatalytic, antioxidant and antimicrobial activities.
Section snippets
Materials and Methods
The leaves of A. nigra were collected from CSIR-NEIST campus, Jorhat, Assam, India. Chloroauric acid was bought from Sigma-Aldrich. Methyl orange, Rhodamine B, other chemicals and reagents and the media used in antimicrobial assay were purchased from Merck India Ltd. All the reagents were of analytical grade and used without any further purification.
Total Phenolic Content and Total Flavonoid Content
The polyphenolic compounds present in plants act as natural reducing agents in the synthesis of nanoparticles. The total flavonoid and phenolic content in ALE is 491mgRE/g extract and 85.25mgGAE/g extract respectively. The presence of polyphenolic compounds in the leaves suggest that its extract could act as a potent bio reductant in the synthesis of gold nanoparticles.
The preliminary indication of the nanoparticles formation was from the change in colour of the colloidal solution. The colour
Conclusion
The uses of natural resources have opened a wide avenue for the synthesis of nanoparticles via green routes. Our study demonstrates the use of Alpinia nigra leaves for rapid synthesis of gold nanoparticles. Colorimetric assay for flavonoids and HPLC analysis of the extract shows that the leaves are rich sources of polyphenolic compounds such as Catechin. These polyphenolic compounds result in the rapid reduction of gold salts to gold nanoparticles. The characterization of the gold nanoparticles
Acknowledgement
The authors would like to thank the Director, CSIR-NEIST for providing laboratory facilities. The authors express their sincere gratitude to SAIF, NEHU, Shillong for TEM analysis of the samples. The authors thank Dr. G. Krishnamoorthy for his insights that have helped in the improvement of this manuscript. Debjani Baruah would like to thank University Grants Commission, New Delhi for fellowship.
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