Controlling the end-to-end assembly of gold nanorods to enhance the plasmonic response in near infrared

In the current paper, we report an efficient approach to produce in a controllable manner electromagnetic hot-spots localized in linear assemblies of gold nanorods (AuNRs) that can be further exploited as highly active surface-enhanced Raman scattering (SERS) sites. The process of the AuNRs self-assembling was initiated in an aqueous solution at pH = 3.1 in the presence of L-cysteine (Cys) molecules and was monitored in real-time for minutes up to hours. The recorded sequential extinction spectra featuring a well-defined isosbestic point at 834 nm is consistent with the first-order like reaction between individual and end-to-end assembled AuNRs as final products. Moreover, the dynamic of the self-assembling process was examined by correlating the extinction spectra with the average number of AuNRs in the chains as provided from the transmission electron microscopy (TEM) pictures. Noteworthy, the average number of connected AuNRs can be fixed at any moment in solution, depending of the desired plasmonic response, by blocking the assembling process with a negative layer of poly (sodium-p-styrenesulfonate) (PSS) polyelectrolyte. The high electric field in the hot-spots in between the linked nanoparticles was proved by SERS experiments using as target analyte para-aminothiophenol (p-ATP) and conducted to clear the demonstration of a higher enhancement factor relative to the individual AuNRs. Finally, all experimental findings and, especially, the far and near-field optical properties of such fabricated plasmonic nanoassemblies were supported by finite-difference time-domain (FDTD) simulations. (paper)