Date Published: May 16, 2015
Publisher: Springer Berlin Heidelberg
Author(s): Pulicherla Yugandhar, Reddla Haribabu, Nataru Savithramma.
Today green synthesis of silver nanoparticles (SNPs) from plants is an utmost emerging filed in nanotechnology. In the present study, we have reported a green method for synthesis of SNPs from aqueous stem bark extract of Syzygium alternifolium, an endemic medicinal plant of South Eastern Ghats. These green-synthesised nanoparticles are characterised by colour change pattern, and the broad peak obtained at 448 nm with UV–Vis surface plasmon resonance studies confirm that the synthesised nanoparticles are SNPs. FT-IR spectroscopic studies confirm that phenols and proteins of stem bark extract is mainly responsible for capping and stabilisation of synthesised SNPs. Crystallographic studies from XRD indicates, the SNPs are crystalline in nature owing to 44 nm size. EDAX analysis shows 19.28 weight percentage of Ag metal in the sample indicates the purity of sample. AFM, SEM and TEM microscopic studies reveal that the nanoparticles are spherical in shape with sizes ranging from 4 to 48 nm. Antimicrobial studies of the synthesised SNPs on clinically isolated microbes showed very toxic effects. It indicates that stem bark extract of S. alternifolium is suitable for synthesising stable silver nanoparticles which act as excellent antimicrobial agents.
Nanotechnology is one of the most fascinating research areas in modern material science. Nanoparticles are gaining importance in the fields of biology, medicine and electronics owing to their unique physical and biological properties (Morones et al. 2005). Recent studies are focused towards synthesis of nanoparticles using plant materials like, iron, copper, calcium, gold, palladium, zinc and silver. Silver has been recognised of its importance in chemistry, physics and biology due to its unique properties. Conventional methods to synthesise silver nanoparticles are mainly by different chemical, physical and microbial approaches. The most common approach for synthesis of SNPs in chemical approach is by the use of sodium borohydride (NaBH4) and citrate as reducing agents. Topical exposure of NaBH4 severely irritates skin and eye, breathing NaBH4 irritates nose and throat, higher exposures can cause pulmonary edema, and very higher exposure may affect nervous system. Citrate causes hypocalcaemia, fatigue, paresthesia and muscle spasms. Common methods for the synthesis of SNPs by physical approach are laser ablation and evaporation/condensation methods. Evaporation/condensation method which could be carried out by using a tube furnace at atmospheric pressure has some disadvantages: occupies large space and consumes a great amount of power. Laser ablation method is also not a cost effective method. These chemical and physical approaches are complicated, expensive and cause potential environmental and biological hazards. In recent times, 50–120 nm-sized silver nanoparticles are synthesised from Bacillus species which acts as a good reducing agent (Vithiyav et al. 2014) but, a significant drawback of microbe-mediated synthesis is that it is not industrially feasible due to its lab maintenance. Therefore, the biosynthesis of SNPs using plant materials is easy, efficient and eco-friendly in comparison to chemical-mediated or microbe-mediated synthesis of SNPs (Anamika et al. 2012). Silver has long been known to have strong inhibitory and bactericidal effects as well as broad spectrum of antimicrobial activity even at low concentrations (Morones et al. 2005). Hence, among the metal nanoparticles, SNPs synthesised from medicinal plants have received much attention for their various biological properties such as anthelmintic (Seema and Amrish 2012), antilarvicidic (Sundaravadivelan et al. 2013), antioxidant (Kumara Swamy et al. 2014), anticancer (Vasanth et al. 2014), anti-inflammatory (Rafie and Hamed 2014), hepatoprotective (Bhuvaneswari et al. 2014), wound healing (Seema et al. 2014) and antimicrobial (Marutikesavakumar et al. 2014).
When the aqueous stem bark extract of S. alternifolium was mixed with 1 mM Ag(NO3)2 solution, the colour changed from brown to grey which is the primary method to confirm that the synthesised nanoparticles are silver (Fig. 1). The colour change is due to the reduction of silver ions with the help of bio molecules present in the sample (Sankar et al. 2014). NAD and ascorbic acid present at higher levels in all plant parts act as strong reducing agents by donating electrons to Ag+ ions and reduced to form Ag0 nanoparticles (Ahmad et al. 2011). This may be the main reason behind the reduction and colour change pattern of SNPs. Reduction of these silver ions was monitored by using UV–Vis spectroscopy from 190 to 750 nm scan range. The peak obtained at 448 nm is a typical absorption peak for metallic nanoparticles which further confirms the reduced nanoparticles are silver (Fig. 2). Same type of results was found in leaf-mediated synthesis of silver nanoparticles from Albizia adianthifolia (Gengan et al. 2013). Here, the nanoparticles in reaction mixture absorb light at different wavelengths and get excited due to charge density at the interface between conductor and insulator of UV–Vis spectroscope to give a respective peak. This mechanism is known as surface plasmon resonance (SPR). FT-IR spectrum of synthesised SNPs was carried out to know the possible bio-molecules responsible for the capping and stabilisation of nanoparticles. For this, the sample was analysed in the scan range from 4000 to 500 cm−1 of near IR spectra by FT-IR. The broad peaks obtained at 3323 cm−1 and 1636 cm−1 were assigned for O–H bond of phenols and N–H bond of primary amines, respectively(Fig. 3). This suggests that the hydroxyl groups of phenols and amide groups of proteins forming a layer of the nanoparticles, act as capping agents to prevent agglomeration and provide stability to the reaction medium. Same type of results was found in Myristica fragrans seed extract-mediated synthesis of silver nanoparticles (Sharma et al. 2014).Fig. 1Colour change pattern of synthesised SNPs: a brown, b greyFig. 2Surface plasmon resonance analysis of synthesised SNPs with UV–Vis spectroscopy shows a typical broad peak at 448 nmFig. 3FT-IR analysis of synthesised SNPs shows broad peaks at 3323 cm−1 of phenols and 1636 cm−1 of primary amines of proteins
The present study is aimed to develop a fast, eco-friendly and cost effective method for the synthesis of silver nanoparticles from S. alternifolium. Due to significant drawbacks with physical, chemical- and microbe-mediated methods of silver nanoparticles, green synthesis is the best method. These green-synthesised SNPs are polydispersed, without any agglomeration and have sizes ranging from 4 to 48 nm with spherical shape show broad spectrum of antimicrobial activity against different clinically isolated bacteria and fungi by acting as a potential antimicrobial agent. High amount of small-sized nanoparticles produced with little amount of plant extract is beneficial because it is an endemic and endangered medicinal plant. Based on these results, we conclude that S. alternifolium stem bark is an efficient and effective source for the synthesis of silver nanoparticles.