Comparative evaluation of antibacterial activity of silver nanoparticles synthesized using Rhizophora apiculata and glucose

https://doi.org/10.1016/j.colsurfb.2011.06.022Get rights and content

Abstract

The focus of the study is to compare the antibacterial efficacy of silver nanoparticles (AgNPs) fabricated by exploiting biological (a mangrove plant, Rhizophora apiculata) and chemical means (Glucose). The synthesized nanoparticles were characterised using UV–visible absorption spectrophotometry (UV–vis), Fourier transform Infra-red Spectroscopy (FTIR) and Transmission electron microscopy (TEM). Biologically synthesized silver nanoparticles (BAgNPs) were observed at 423 nm with particle sizes of 19–42 nm. The chemically synthesized silver nanoparticles (CAgNPs) showed a maximum peak at 422 nm with particle sizes of 13–19 nm. An obvious superiority of the antibacterial potency of BAgNPs compared to the CAgNPs as denoted by the zone of inhibition (ZoI) was noted when the nanoparticles were treated against seven different Microbial Type Culture Collection (MTCC) strains. The current study therefore elucidates that the synthesized AgNPs were efficient against the bacterial strains tested.

Highlights

• Synthesis of silver nanoparticles using Rhizophora apiculata and glucose. • Characterization of nanoparticles by UV, FTIR and TEM. • Comparison of antimicrobial efficacy of the silver nanoparticles of biological and chemical origin.

Introduction

Particles of nanorange have been synthesized without aggregation using chemical methods for its simplicity and the added advantage of high yield for large scale production. The chemical methods follow electrochemical, thermal, laser, microwave, polyol, radiolytic, sonochemical and various other techniques [1], [2]. Hazardous reducing agents used for chemical procedures mounts a bias for an eco-friendly and feasible approach for the synthesis of nanoparticles. Hence, plants are used as an alternative trigger for the green synthesis of nanomaterials [3], [4]. Major parameters for synthesis of nanoparticles are the selection of solvent, reducing agent and non-toxic substances for synthesis [5]. Hence, biological means of synthesizing nanoparticles provides an edge over chemical means as it is cost effective, does not involve physical barriers with regard to reducing agents and eliminates the toxic effects of chemicals used for the synthesis [6], [7].

Microbial resistance towards the available antimicrobial agents has been a major factor for the development of novel microbe-inhibitory agents. Silver has been known to possess antimicrobial effects [3], [8], [9], [10] with distinctive properties of conductivity, stability and activity [11]. Therefore, the antimicrobial endurance of AgNPs is slowly finding its avenue in various health related applications [12]. The reason is that, the possibility of the microbes becoming resistant to AgNPs is slim as the nano-formulation acts on a broad range of targets in the microorganism [13]. AgNPs known to possess inhibitory and bactericidal effects have a high surface area to volume ratio along with high fraction of surface atoms that elicits elevated antimicrobial activity compared to the silver metal as a whole [14]. Though the bactericidal activity of AgNPs has been established by past studies, the mode of action still stays unclear. Researchers predict that silver species have an effect at the molecular, metabolic or membrane level of the microorganism [15]. Taking these studies as an initiative, we have synthesized AgNPs for analyzing their antibacterial activity. Mangroves are plants of the coastal ecosystem used in traditional medicine for their antibacterial, antiviral and anti-ulcer properties [16]. This attempt is hence an effort to revitalize the use of ethical knowledge of plants for ailments that can be treated with traditional medicine applying modern techniques at nano-scale.

To our knowledge this is the first report considering three different aspects. This is with regard to exploration of the biological reduction performed by the leaf extract of Rhizophora apiculata, chemical synthesis mediated by glucose using silver oxide as the precursor, and the comparison of antibacterial efficacy of the biologically and chemically synthesized nanoparticles. The synthesized nanoparticles were characterized and their antimicrobial activity against seven different pathogens, two of Gram positive and five of Gram negative origin was determined. The CAgNPs are used as comparative materials, a positive control to implicit the reliability and efficacy of BAgNPs.

Section snippets

Preparation of silver organosol

Shade dried leaves of R. apiculata collected from the Mangroves of Pichavaram, India, were powdered and used for the study. Briefly, 1% aqueous R. apiculata extract was prepared with Millipore water and incubated at 60 °C for 5 min and used for the bioreduction.

Synthesis of silver nanoparticles

The procedure followed for the preparation of the nanoparticles was the protocol used by Song and Kim [7]. 5 ml of the organosol was added to 95 ml of 1 mM aqueous Silver nitrate (Qualigens – 99.8%) solution and the mixture was gradually

Characterization of BAgNPs

Blending silver nitrate with R. apiculata extract changed the colour of the solution to dark brown at varying temperatures with 90 °C indicating a prominent colour change (Fig. 1A). Before thermal reduction, the UV–visible spectrum indicated no absorption in the range of 260–800 nm. SPR peaks corresponding to AgNPs were not observed when silver nitrate was incubated at varying temperatures without any reductant. The silver nitrate solution alone was not thermally reduced without the presence of

Discussion

The colour change observed indicates the transition of silver nitrate to AgNPs [19]. The colour change that was not observed in the silver nitrate solution when incubated alone indicates that the change has occurred by the reductants involved in the procedures followed. Surface Plasmon peak at 423 nm of BAgNPs and 422 nm of CAgNPs corresponds to the SPR of AgNPs. An intense spectrum in the range of 335 and 560 nm for BAgNPs was not observed as they denote nanoparticle aggregation [12], [20].

Conclusions

To conclude, the antimicrobial activity of AgNPs fabricated using a biosynthetic method was evaluated and compared with chemically synthesized AgNPs. In this analysis, the AgNPs displayed antimicrobial activity against all seven cultures tested. BAgNPs revealed higher microbicidal activity compared to CAgNPs indicating that they are not alone eco-friendly, but also yield an enriched turnover compared to chemically synthesized particles. Consequently, this is a future prospect for novel

Acknowledgements

This study has been supported by grants of University Grant Commission (UGC), Council for Scientific and Industrial Research (CSIR) and Department of Science and Technology (DST), New Delhi, Government of India. The author gratefully acknowledges the agencies for their financial support.

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