Facile Synthesis of High Aspect Ratio Gold Coated Magnetic Nanowires

Magnetic nanoparticles are of immense importance in various biological applications such as bio-imaging techniques, drug delivery, hematologic disease detection, biomolecules separation (Figure 1). However, one of the limitations faced while using magnetic nanoparticles is to synthesize small size, high surface to volume ratio particles via simple approach. Magnetic nanoparticles in general are synthesized using hydrothermal method, template based method or electrochemical methods. These conventional synthesis approaches are not only complex to carry out but are also time consuming. Herein, we report a wet chemical method to synthesize high aspect ratio magnetite (Fe3O4) nanowires. The benefit of this method is facile, rapid and cost-effective process. The method involves a chemical reduction of ferrous sulphate and ferric chloride. The resultant product was obtained in two portions – the precipitate and the supernatant. Physical appearance of both the precipitate and supernatant was observed as brown in colour. These synthesized magnetic nanoparticles in supernatant were light in weight and completely dispersible in water for more than 6h. Surface morphology analysis of these nanoparticles demonstrated that the precipitate was aggregate of nanoparticles whereas the particles present in the supernatant were observed to be thin nanowires of dimension approximately ranged from 200 to 275nm with calculated aspect ratio of approximately eleven (Figure 2). Magnetic property was observed in these nanowires using a simple bar magnet.  Further, to enhance its biocompatibility, a thin gold coating was carried out on its surface. A reaction was performed by citrate capping followed by reduction using sodium borohydride in presence of chloroauric acid[1]. Gold coating was confirmed by a UV-vis absorbance spectrum that showed a distinct plasmonic peak at 540 nm (Figure 3). The prospective application planned in this study is for efficient separation of exosomes from bodily fluids. Conventionally exosome isolation is challenging in terms of process complexity, cost and accuracy[2]. Gold coated magnetic nanowires will allow direct attachment of a large number of exosomes by affinity interactions which can significantly enhance the capture efficiency. The magnetic activity allows magnetic nanowire based mixing, separation and purification via magnetic isolation and purification steps[3]. Alongside the light weight and dispersible nature of these nanowires prevents their agglomeration in the cell suspension allowing easy separation after exosome attachment using the external magnetic field. Overall these as synthesized high aspect ratio magnetic nanowires can be used as highly efficient nanocarriers for direct isolation of bulk exosomes from bodily fluids via magnetic separation method and thus can avoid the use of expensive commercial kit based multi-step pre-extraction procedures.                  Figure 1. Applications of magnetic nanoparticles                Figure 2. SEM image of nanowires      Figure 3. Absorption spectra of gold coated nanowires