Nanotechnology is a growing field that has drawn the attention of researchers in different disciplines. Metal nanoparticles with controlled size and shape form the basis   for  advanced  functional  materials  for  electronic, sensor, optical devices (Gurunathan et al., 2009). Metal nanoparticles synthesis is achieved majorly using physical, chemical and biological approaches.  Laser ablation (Tsuji et al., 2003), Arc discharged method  (Tien et al., 2008), irradiation (Yong et al., 1999), polyol process (Sun and Xia, 2002), polymers (Hebeish et al., 2011; Chengcai et al., 2005; Narendra and Khanna, 2007), co-precipitation (Yang et al., 2003), sol-gel (Lu et al., 2002), bacteria (Korbekandi et al., 2012), fungi (Ahmad et al., 2003; Balaji et al., 2009) have been used in the synthesis of nanoparticles. The use of plant extracts, enzymes, microorganisms, polymers, sugars and vitamins in nanoparticles formation has made nanotechnology a promising area of research (Oxana et al., 2013). Among the noble metals (e.g., Ag, Pt, Au and Pd), silver (Ag) is the metal of choice for potential applications in the field of biological systems, living organisms and medicine (Jain et al., 2009). The use of plant extract in synthesizing silver nanoparticles is eco-friendly, energy saving and can be easily scaled up. Biologically synthesized silver nanoparticles using medicinal plants or plant extracts include locust  bean  gum (Chandrakant et al., 2013), Andrographis paniculata (Venkata et al., 2014), lingonberry and cranberry juices (Judita et al., 2014), Morinda citrifolia (Gnanasekar et al., 2012), Alternanthera dentata (Deenadayalan et al., 2014), guava leaves extract (Shinde et al., 2014), Gardenia jasminoides (Lü et al., 2014), Jatropha seedcake (Anjali et al., 2013), Mimusops elengi (Hoskote et al., 2014), Abutilon indicum (Ashokkumar et al., 2015), Ziziphora tenuior (Babak and Gholamhoseinpoor, 2015), Ficus carica (Bulent et al., 2015), Acacia auriculiformis (Pradnya et al., 2014), Cocos nucifera (Mariselvama et al., 2014). Pineapple leave extract has also been used for the green synthesis of silver nanoparticles and its high antibacterial activities have been reported (Elemike et al., 2014). Recently our research group studied the growth kinetic of silver nanoparticles under the influence of plant biodiversity (Dare et al., 2015) for pharmaceutical and biological applications have been reported. G. kola Heckel (Clusiaceae), commonly known as bitter kola (English) and orogbo (Yoruba) is a widespread tree of evergreen forest valued in Nigeria for its medicinal nuts which has led to its exploitation in the natural forests in recent times (Ibikunle and Emmanuel, 2011). The phytochemicals obtained from  G. kola as documented in literature include biflavonoids  such as kolaflavonone, and 2 hydroxyflavonoids, xanthones, kolanone, ameakoflavone, 2,4-methylenecyclartenol, coumarine and prenylatebenzophenones, oleoresin, the chromanols, garcioic and garcinal (Adesuyi et al., 2012).  Others are tannin, saponins, alkaloids, and cardiac glycoside. The proximate analyses showed that the sample has high level of carbohydrate, little amount of crude fibre and protein respectively and negligible amount of ash content and crude fat. This composition shows that the sample could be a good source of carbohydrate, dietary fibre and protein (He et al., 2007). Biomolecules with carbonyl, hydroxyl, and amine functional groups have the potential for metal ion reduction and capping the newly formed particles during their  growth  processes  (Adesuyi  et  al., 2012). In this work, we report a simple, nontoxic, and eco-friendly green synthesis of silver nanoparticles using aqueous seed extract of G. kola and no toxic chemicals are used as reducing and stabilizing agent during the synthesis. 

XRD analysis

Figure 2 shows XRD patterns for Ag nanoparticles synthesized by G. kola seed extract. Four peaks at 2θ, 38.4, 44.5, 64.8, and 77.7 corresponding to (111), (200), (220) and (311) reflect planes of face-centred cubic (fcc) structure of Ag. These peaks are consistent with the standard data file JCPDS No 04–0783 JCPD. The broadening of X-ray peaks observed is primarily due to the small particle size. The mean size of silver nanoparticles was calculated using the Debye-Scherrer’s equation (Sheny et al., 2011). The average mean size of AgNPs was 10.40 nm. No peaks from any other phase were observed showing that single phase Ag with cubic structure nanoparticles has been obtained directly.

FTIR spectra analysis

FTIR measurements were carried out on both the extract and the silver nanoparticles from G. kola. This was done to identify the biomolecules for reduction, and efficient stabilization of the metal nanoparticles synthesized by G. kola seed extract. The FTIR spectrum of silver nanoparticles is shown in Figure 3b. The presence of peaks at 3434, 2346, 2096, 1643 and 1028 cm⁻¹ was observed for the crude extract of G. kola (Figure 3a), which indicates that the silver nanoparticles may be surrounded by hydroxyl group, because the peaks indicate  –OH  symmetric  stretching  and  C–O  bonds  in aliphatic or carboxylic compounds (Ghosh et al., 2012). The band at 3434 cm^-1 corresponds to O-H stretching H-bonded alcohols. The peak at 1643 cm^-1 corresponds to C=O of ketone or aldehyde group and was shifted to lower frequency of 1635 cm⁻¹ in the spectra of the silver nanoparticles from G. kola. The peak at 1028 cm^-1 was assigned to the stretch of the C−O bond (Renquan et al., 2012). Therefore, the synthesized nanoparticles were surrounded by proteins and metabolites such as terpenoids, biflavonoids having functional groups of alcohols, ketones, aldehydes and carboxylic acids. This suggests that the biological molecules could possibly perform dual functions of formation and stabilization of silver nanoparticles in the aqueous medium (Mukherjee et al., 2001). This shows that the aqueous seed extract of G. kola is a very good and robust bioreductant for the synthesis of silver nanoparticles.

TEM and EDX analysis

The TEM image gives the morphology and size of the synthesized silver nanoparticles. A spherical nanoparticle which conforms with the SPR band in the UV- Vis spectrophotometer and to that of the XRD analysis was obtained with average particle size of 10.40 nm (Figure 4). Table 2 shows the EDX profile of the synthesized silver nanoparticle from G. kola extract. The selected area electron diffraction SAED pattern of G. kola silver nanoparticles   was   recorded   at  different  scales. The observed spot   resulting into a circular ring confirmed the polycrystalline nature and face centered cubic structure of GSNP. The circular spot corresponds to the (111), (200), (220) and (311) Plane of Ag°.

Antibacterial assay

The bio-reduced silver nanoparticles from G. kola show a clear and high zone of inhibition as a result of the attachment of the silver ion to the negative charged cell wall. This led to structural changes in the cell wall, permeabilility and subsequently death of the bacteria (Sondi and Salopek-Sondi, 2004) which prevent the multipilication of Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae and Bacillus subtilis; likewise on fungal pathogen namely Aspergillus niger , Rhizophus stolonifer , and Candida albican. S. aureus shows the highest zone of inhibition at 500mg/L compared to Gentamycin while K. pneumoniae shows the lowest. For fungal pathogen, both A. niger and Candida albican show same  efficacy   compared to   Tioconazole;  R. stolonifer shows no zone of inhibition (Table 1).

Also due to high zone of inhibition exhibited by S. aureus and A. niger followed by C.  albican in Figure 5, the silver nanoparticles from Garcinia kola possess high antimicrobial activity which can be used in household appliances like clothes washer and refrigerator which can kill bacteria and reduce odour

In this paper, we have reported a green method to synthesize silver nanoparticles using the aqueous seed extract of G. kola. The aqueous seed extract of G. kola helped in reducing and stabilizing the silver nanoparticles with remarkable growth onset of 10 min. This makes our green approach superior to conventional methods. The average  size   was   found   to   be   10.4 nm   with   high susceptibility against tested pathogenic bacteria and fungi.

The authors have not declared any conflict of interest.

The authors acknowledge the Vice Chancellor of Federal University Lafia, Prof. Ekanem Ikpi Braide, Dean Faculty of Science, Prof. Martin Ogbe and the entire staff of Chemistry and  Physics  Department  for  their  support  during the  course  of  this research  work.