pH controlled synthesis of end to end linked Au nanorod dimer in an aqueous solution for plasmon enhanced spectroscopic applications

Abstract

End-to-end linked nanorod dimer nanogap antennas exhibit superior plasmonic enhancement compared to monomers due to the coupling of localized surface plasmon resonances (LSPR) of individual nanorods. However, controlling the assembly to stop at the dimer stage is challenging. Here, we report a pH-controlled synthesis of Au nanorod dimer nanogap antennas in an aqueous solution using 1,4-dithiothreitol (DTT) as a linker. Neutral to acidic pH (4.0 to 7.0) favors dimer formation, while higher pH decreases dimer yield, stopping completely at pH 11.0. The reaction can also be halted in neutral and acidic solutions by abruptly increasing the pH to 11.0 or higher. At basic pH, both thiol groups of DTT deprotonate and acquire a negative charge, causing both thiolate ends to adsorb onto the positively charged cetyltrimethylammonium bromide (CTAB) micellar layer on the transverse surface of the Au nanorod, preventing dimer formation. TEM images confirm nanorod dimers, showing a good conversion yield (∼80%) from monomers to dimers. Overall, out of all the DTT induced NR assemblies, 70% are found to be dimers. The majority of these dimers (>90%) are end-to-end linked dimers, with a gap distance of ∼1 nm, exhibiting exceptional stability and remaining intact for over two weeks. FDTD simulations demonstrate a significant enhancement of the light E field in the nanogap, ∼80 times higher than in a homogeneous water environment and 11 times higher than in nanorod monomers. Simulations also show that E field enhancement varies with the angular separation of monomeric nanorods, being highest for end-to-end dimers (180°) and lower for side-to-side dimers (0°). Overall, we present an inexpensive method to design and control nanorod dimer nanogap antennas in aqueous solution, useful for plasmon-enhanced spectroscopic applications such as biosensing, chemical sensing, and biomedical devices.