Introducing longitudinal oxidation reaction for ultrarapid and single-step synthesis of thiol-mediated end-to-end assembly of gold nanorods

The end-to-end (EE) assembly of gold nanorods (Au-NRs) is of immense interest as it offers great electro-optical properties. This assembly is mainly produced in solution via replacing the cetyltrimethylammonium bromide (CTAB) molecules from as-synthesized Au-NR tips with thiolated linker molecules. However, this replacement demands a centrifugation-driven two-step ligand exchange reaction, till date, which is cumbersome, demands a few hours to even a day to occur, and often hampers the stability of Au-NRs during the process. Herein, we present a simple and fast one-step strategy that relies on the ferric chloride–mediated longitudinal oxidation of Au-NRs as an innovative mode to facilitate the in situ exchange of CTAB with thiols at the Au-NR tips to produce the Au-NR EE-assembly in an aqueous medium. We find that the temperature has a strong impact on our approach and it does not require any prior washing of as-synthesized Au-NRs via conventional centrifugation or any other modes. Our principle observation is that incubation of as-synthesized Au-NRs and thiolated linkers (e.g., cysteine, glutathione) in an acidic solution (pH = 1.3) in the presence of FeCl₃ for just 10 min at 75 °C is sufficient to achieve EE-nanoassemblies. We also notice that it is easy to control the extent of assembly by modulating either the FeCl₃ amount or temperature. To assess this assembly, we have used Vis–NIR absorption spectroscopy, dynamic light scattering study, and transmission electron microscopy. Such rapid and one-step fabrication of EE-assembly of Au-NRs is indeed rare in the literature and establishes the novelty of this work.