Morphological Transition during Reversible Aqueous and Organic Phase Transfer of Gold Nanostructures Synthesized by Tyrosine-Based Amphiphiles

We describe the successful synthesis of multi-pod-shaped gold (Au) nanostructures by an in situ reduction approach using a series of tyrosine-based redox active amphiphiles (conjugates of stearic/palmitic/myristic/lauric/capric acid with tyrosine) as reducing-cum-structure directing agents under alkaline condition. High-resolution transmission electron microscopy (HRTEM) confirms that the oriented attachment of the initially formed spherical Au nanoparticles (NPs) through {111) planes give rise to the formation of multi-pod-shaped morphology. We also demonstrate that the successful phase transfer of amphiphile-capped Au nanomultipods from water to nonpolar organics can be achieved by simple HCl treatment. We discover that there is a complete morphological transition in the Au nanostructures from multipod to spherical upon transfer to nonpolar organics. The water-to-organic phase transfer efficiency decreases with a decrease in the length of alkyl chain of the amphiphiles. The reversibility of the phase transfer is probed by the addition of aqueous NaOH solution to organic phase containing spherical Au NPs. This toluene-to-water transfer is also associated with a reversible shape change from spherical to multi-pod-shaped Au nanostructures. The reversible phase transfer and the associated shape transitions are monitored visually as well as via UV-vis spectroscopy and TEM analysis. HRTEM results show that the Au nanomultipods and the spherical Au NPs obtained after phase transfer are bounded by {111} planes. The mechanism of formation of Au nanostructures and their reversible phase transfer with associated shape transition is proposed and discussed.