Biosynthesis of Silver and Gold Crystals Using Grapefruit Extract

In this paper, biological synthesis of silver and gold crystals using grapefruit extract is reported. On treatment of aqueous solutions of silver nitrate and chloroauric acid with grapefruit extract, the formation of stable silver and gold particles at high concentrations is observed to occur. The silver particles formed are quasi-spherical or irregular with sizes ranging from several hundred nanometers to several microns. The gold quasi-spheres with holes on surfaces and with diameters ranging from 1 to 3 microns are obtained. The formation mechanism of silver and gold crystals is discussed, indicating that the soluble biomolecules such as protein(s) and vitamin C in grapefruit extract may play a crucial role in defining the morphology and/or crystal phase of silver and gold crystals.


Introduction
Silver, gold and other metal common nanoparticles, nanoclusters, nanowires and related nanostructures have received tremendous attention owing to their unique catalytic, electrical, magnetic and thermal properties [1,2].Silver nanoparticles have been used in sensor technology [3], biological leveling [4], and many other biomedical applications [5,6].
Gold nanoparticles have been widely employed in medicine [7,8], disease diagnostic and drug delivery systems [9,10].Currently, there is a growing need to develop environmentally benign material synthesis processes that do not use toxic chemicals in the synthesis protocol.
As a result, biological systems are used by many researchers in the field of metal nano-materials synthesis [11].
Plant-mediated synthesis of silver and gold nanoparticles has emerged in recent years.
For example, silver and gold ions could be reduced to nanoparticles using a leaf extract of Cinnamomum camphora [12].Synthesis of silver nanoparticles using a leaf extract of Moringa oleifera was reported by Prasad and Elumalai [13].Reduction of silver ions to stable and spherically shaped nanoparticles with average size about 10nm using extract of Desmodium trifolium was ascribed to the presence of H + ions, NAD + and ascorbic acid in the extract [14].Huang et al. reported the formation of gold nanoparticles from the sun-dried leaves of Cinnamomum camphora [15].The extracellular synthesis of gold nanoparticles using the leaf extract of Coriandrum sativum has been attempted by Narayanan and Sakthivel [16].
In this paper, we report on the synthesis of silver and gold crystals with sizes of several hundred nanometers to several microns by the reduction of aqueous Ag + and AuCl 4 -ions with the grapefruit extract.Grapefruit is one of the most common fruits.This study can provide a new way for green synthesis of functional materials.

Materials and methods
Materials used for the synthesis of silver and gold crystals are silver nitrate (AgNO 3 ), chloroauric acid (HAuCl 4 ), and grapefruit extract solution prepared by taking 30 g of grapefruit flesh squeezed and blending the juice obtained with 100 mL of sterile distilled water in a 200-mL beaker.
For reduction of Ag + ions, 25 mL of grapefruit extract solution was added to 25 mL of 10 í2 mol/L aqueous AgNO 3 solution.Similarly, 25 mL of grapefruit extract solution was added to 25 mL of 10 í2 mol/L HAuCl 4 solution for reduction of AuCl 4 í ions.The reduction of pure Ag + and Au 3+ ions was monitored by measuring the UV-vis spectra of the solution at regular intervals after diluting a small aliquot (0.2 mL) of the sample 20 times.UV-vis spectra were recorded as a function of time of reaction on a UV-1800 double beam spectrophotometer (Shanghai Mapada Co., Ltd, China).
The silver and gold particles synthesized after 24 hours of reaction of the different salt solutions with the grapefruit extract were centrifuged at 10,000 rpm for 15 min.The precipitates obtained were washed three times with double-distilled water and ethanol, respectively, and then vacuum dried.X-ray diffraction (XRD) measurements of the silver and gold particles were carried out using dry powders on a D8 Advance instrument with CuKD radiation (Bruker, German).The morphologies of the samples were observed by scanning electron microscopy (SEM) (Hitachi S-3400, Japan) with an accelerating voltage of 20 kV.

Results and discussion
Formation of the silver and gold particles by reduction of the aqueous metal ions during exposure to the grapefruit extract may be easily followed by UV-vis spectroscopy.It is well known that silver and gold particles exhibit yellowish-brown and ruby red colors, respectively, in water, these colors arising due to excitation of surface plasmon vibrations in the metal particles.2. The reduction of the two noble metal ions occurs fairly rapidly; more than 90% of reduction of Ag + and Au 3+ ions is complete within 5 hours, respectively, after addition of the grapefruit extract to the metal ion solutions (Figs. 1 and 2).To our best knowledge, grapefruit extract also contains reductive biomolecules such as vitamin C, etc.Here, we speculate that the soluble biomolecules such as protein(s) and vitamin C in grapefruit extract play a crucial role in defining the morphology and/or crystal phase of silver and gold crystals.

Summary
In summary, a process for the biological synthesis of stable silver and gold crystals using grapefruit extract was demonstrated.Quasi-spherical or irregular silver particles with sizes ranging from several hundred nanometers to several microns, as well as quasi-spherical gold particles in the range of 1 to 3 microns were produced in aqueous solution containing grapefruit extract and AgNO

Figs. 1
Figs.1 and 2show the UV-vis spectra recorded from the aqueous solutions AgNO 3 -grapefruit extract and HAuCl 4 -grapefruit extract reaction medium as a function of time of reaction.It is observed from Fig.1that the silver surface plasmon resonance band occurs at about 450 nm and steadily increases in intensity as a function of time of reaction without any shift in the peak wavelength.In the case of gold ion reduction, the band corresponding to the surface plasmon resonance occurred at about 550 nm as shown in Fig.

Fig. 3a and
Fig. 3a and b show the XRD patterns obtained for silver and gold particles synthesized using grapefruit extract.A number of Bragg reflections corresponding to the (111), (200), (220), (311), and (222) sets of lattice planes are observed which may be indexed based on the fcc structures of silver and gold.The XRD pattern thus clearly shows that the silver and gold particles formed by the reduction of Ag + ions and AuCl -by grapefruit extract are crystalline in nature.

Fig. 4 Fig. 1 .Fig. 2 .Fig. 3
Fig.4shows representative SEM images of the silver and gold particles synthesized by treating AgNO 3 or HAuCl 4 solutions with grapefruit extract for 24 hours.From Fig.4a, it can be seen that the silver particles formed were quasi-spherical or irregular with sizes ranging from several hundred nanometers to several microns.We speculate the large particles are formed by aggregation of small particles.Fig.4bindicates that the gold particles formed were quasi-spherical with diameters in the range of 1 to 3 microns.If seen clearly, there are holes on surfaces of the particles can be identified.
3or HAuCl 4 .The soluble biomolecules such as protein(s) and vitamin C in grapefruit extract may play a crucial role in defining the morphology and/or crystal phase of silver and gold crystals.This study can provide a new way for green synthesis of functional materials.