Morphological tuning of plasmonic silver nanostars by controlling the nanoparticle growth mechanism: Application in the SERS detection of the amyloid marker Congo Red

Abstract

Silver nanostars (AgNS) have promising applications in spectroscopic, theranostic and sensing applications. In this work we have tuned the morphology of AgNS by the modification of the experimental fabrication conditions. Silver nanostars were fabricated by following a two-step process that involves the use of two different chemical reducers: neutral hydroxylamine (HA), employed as primary chemical reducer, and citrate (CIT), in a second reduction step. The key experimental parameters which were modified in this work were the [HA]/[Ag+] ratio (R1), the [HA]/[CIT] ratio (R2) and the time between reductions or elapsed time (T), which also determines a factor of the highest importance: the ratio between the Ag+ concentrations existing at the beginning of each reduction process (RAg). The growth mechanisms followed by Ag nanostars to reach their final size and shape determined different growth pathways which were identified in this work as well, as deduced by the analysis derived from the transmission electron micoroscopic (TEM) images and the extinction spectra from the resulting plasmons. The SERS effectiveness of the resulting substrates was analyzed by using thiophenol (TP) as molecular probe, and a correlation between morphology and SERS intensification factor was deduced. The experimental parameters leading to the highest SERS enhancement were identified. Finally, these Ag nanostars were probed in the sensitive detection of Congo Red, an amyloid marker dye usually employed in the detection of amyloid fibrils related to Alzheimer diseases.