Virus shaped gold nanoparticles with tunable near infrared plasmon as SERS substrates

A facile seed mediated strategy for the synthesis of virus shaped gold nanoparticles (AuNPs) with plasmon band tunable from 650 to 1200 nm is reported. Compared to other works, a very low concentration of 0.01 mM of the precursor HAuCl4 is used in this work. Amount of the weak reducing agent modifies both the size and shape of the nanoparticles. This is a single-root synthesis for the growth and formation of gold polyhedral like nanoparticles, spiky nanostars and nanoflowers, resembling the shape of viruses. The longitudinal plasmon was found to shift towards infrared with increase in the length of spikes in the nanostars. The maximum length of these spikes obtained in this synthesis was ~65 nm. A substantial red-shift was observed in the plasmon characteristics of nanoflowers compared to that of spherical AuNPs of the same size, indicating plasmon hybridization. The use of nanoflowers and spiky nanostars were tested with three different probe molecules viz., azure a chloride, nile blue chloride (NBC) and crystal violet. The Raman enhancement was of the order of ~1012 with NBC molecules, when nanoflowers were used as substrates and this towering of Raman signal is attributed to the closeness to 'pre-resonance'. A re-branching of spikes due to twinning of spike edges in the nanostars grown under optimum conditions were observed and are described as hotspots causing high amplification of Raman signals in the surface enhanced Raman spectroscopy spectra. Investigations on the influence of incubation time on the Raman enhancement of the molecules varies with incubation time.