Solvothermal synthesis of Au@Fe3O4 nanoparticles for antibacterial applications

We present Au@Fe3O4 nanoparticles obtained from Fe nanoparticles and HAuCl4 using a simple solvothermal method. Trisodium citrate (C6H5Na3O7*2H2O) served as a reducing agent for Au. X-ray diffraction analysis, electronic microscopes and energy-dispersive X-ray spectroscopy revealed cubic structure, elemental composition (Au, Fe and O) and spherical shape of nanoparticles. Antibacterial activity of the sample was tested against E. coli bacteria and obtained results were discussed.


Introduction
Bimetallic nanoparticles have recently attracted much attention, because of their unique catalytic, electronic, optical, magnetic and other novel dual (depending on metals) properties and wide range of possible applications [1].In particular, binary 3d5d nanoparticles formed by transition metals such as Fe or Co together with 5d noble metals such as Au or Pt allow the possibility to tune the magnetic properties based on an indepth knowledge of their geometrical and magnetic behaviour [23].The bimetallic nanoparticles are generally divided into two types in the structure: the alloyed and the core/shell structured bimetallic nanoparticles [4].
Concerning the synthesis of Au@Fe 3 O 4 nanoparticles, Seino et al. [5] synthesized magnetic carriers consisting of nano iron oxide and gold coreshell structure in an aqueous solution by using -ray irradiation.Lyon et al. [6] prepared coreshell structures with gold γ coating layers on the surface of either Fe 2 O 3 or partially oxidized Fe 3 O 4 via iteration hydroxylamine seeding.Yu et al. [7] also synthesized the socalled dumbbelllike AuFe 3 O 4 bifunctional nanoparticles with interesting novel properties.
Among existed techniques [89], solvothermal synthesis can be one of the simplest and environmentally friendly ways for preparation of bimetallic nanomaterials [10].The method is based on chemical reaction between initial substances and solvent mixture, and being heated at temperatures from 100 to 1000 °C, and pressure starting from 1 MPa.Solvothermal reaction takes place inside the stainless steel autoclave, covered by stainless steel lid, and fixed tightly by screws.Experimental conditions of this method can be varied and allow to synthesize different types of materials.In this work, Au@Fe 3 O 4 bimetallic nanoparticles were synthesized from the mixture of Fe nanoparticles with HAuCl 4 solution using solvothermal reaction and their properties were studied.For biomedical applications, synthesized Au@Fe 3 O 4 nanoparticles were tested against Escherichia coli bacteria.E. coli, a gram-negative bacterium can enter human intestines and cause urinary tract infection, cholecystitis, or septicaemia [11].

Experimental
Experimental setup presented in Figure 1a was used for Au@Fe3O4 nano-sample preparation.Fe nanoparticles with an average diameter of 5 nm, synthesized by pulsed plasma in liquid method [12], were used as Fe sources.Teflon cup (total volume of 60 mL) was filled with Fe nanoparticles, 1.0 mM tetracloroauric acid (HAuCl4) as Au source, Trisodium Citrate Dehydrate (TCD: C 6H5Na3O7*2H2O) as a reducing agent for Au nanoparticles and deionized water.Experimental conditions were as following the temperature T = 150 °C, pressure P =1.8 MPa and reaction time t = 2 hours.After the reaction, the temperature of autoclave cooled to room temperature and the liquid was taken out of the autoclave.The colour of the liquid changed from transparent to bright brown, sample was separated from the liquid, dried and then characterized.The starting materials, experimental conditions and process of the nanoparticles' formation are schematically represented in Figure 1b.
The sample was characterized by X-ray diffraction (XRD) performed on a Rigaku RINT-2500 HV diffractometer (Cu-Kα radiation, λ= 0.15406 nm; voltage of 40 kV, and current of 200 mA).Surface view and elemental composition analysis were examined by JEOL JSM-7600F Field-Emission Scanning Electron Microscope (FE-SEM) equipped with an Energy-Dispersive X-ray Spectroscopy (EDS) operated at 200 keV.High-resolution transmission electronic microscopy (HR-TEM) with a Philips Tecnai F20S-Twin instrument was used to determine the size and morphology of the nanoparticles at 200 keV.For analysis the particles dispersed in ethanol were drop cast onto a carbon-film-coated copper grid, followed by evaporation and drying.Further, the evidence for the presence of Au and Fe3O4 in the sample was proved by EDS, as shown in Figure 3 (left), where the elements gold (Au), iron (Fe), and oxygen (O) have been detected.The composition of the magnetite phase (Fe3O4) is 43 at% of iron and 57 at% of oxygen.In addition, peaks belonging to sodium (Na), chlorine (Cl) and carbon (C) were detected due to the reducing agent (TCD) and sample holding carbon tape.Surface view of the Au@Fe3O4 nanoparticles taken by FE-SEM is presented on microphotography on the right (Figure 3).Image was captured at resolution of ×13000, and the agglomerates are clearly displayed.Because of the high surface energy of the nanoparticles and chemical bonds between the particle's surfaces, their tendency to form agglomerates is quite strong.Magnetic dipole forces also potentially contribute to inter-particulate forces [13].As it can be seen from Figure 4 the size of obtained nanoparticles ranged between 10-13 nm.Bacterial infections were one of the major causes of mortality in the nineteenth century.Nowadays, antimicrobial effects are intensively studied due to an enormously increasing bacterial resistance against excessively and repeatedly used classical antibiotics.Magnetic iron oxide nanoparticles are of particular interest as antibacterial agents, as they can be prepared with extremely high surface areas and unusual crystalline morphologies with a high number of edges and corners, and other potentially reactive sites [14].Also, inorganic antibacterial agents such as metal and metal oxides are advantageous compared to organic compounds due to their stability.Ismail et al. studied that iron oxide nanoparticles with antibacterial properties and superparamagnetic nature are very prominent in biomedicine [15].Antibacterial activity of the Au@Fe 3O4 sample prepared by solvothermal synthesis was tested against E coli bacteria.Inhibition effect of the nanoparticles against the target microorganism was evaluated using 3M Petrifilm E coli / Coliform Count plates and inhibition time was 24 hours.Number of colonies were counted using the technique called TNTC (too numerous to count) and the results are represented in Table 1.Following results were obtained after an average E. coli bacteria colonies calculation: as for control sheet there are 350 counts ; at 0.1 % concentration of the nanoparticles in medium -202 counts and at 1.0 % concentration of the nanoparticles in media the value was 100.As it can be seen from obtained results, the bacteria count number decreased with increase of the nanoparticles concentration in medium.This means that Au@Fe 3O4 nanoparticles synthesized by solvothermal synthesis can inhibit bacteria growth indicating on their good antibacterial activity.

Conclusions
Magnetic Au@Fe3O4 nanoparticles were synthesized from Fe nanoparticles and HAuCl 4, using TCD as reducing agent by solvothermal synthesis method.XRD and EDS analysis indicated the presence of Fe3O4 and Au phases in the sample.The Au@Fe3O4 nanoparticles have potent antibacterial activity against E coli bacteria and the inhibitory effect increases with increasing the nanoparticles concentration.Obtained results highlighte possible biomedical applications of Au@Fe3O4 nanoparticles as biocompatible components of antibiotics as well as medicaments.
This research was funded by startup research acceleration fund (Funding number: 53511535016) of the Kumamoto University) and by the International Innovative center of Nanotechnology, CIS (Funding number: 038/110).

Table 1 .
Antibacterial activity of Au@Fe 3 O 4 nanoparticles synthesized by solvothermal method