Tuneable Plasmonic and Catalytic Gold Nanostars for Surface-Enhanced Resonance Raman Scattering (SERRS): Synthesis, Characterization and Optimization

Surface-Enhanced Raman Spectroscopy (SERS) is an excellent approach to identify vibrational information from target molecules which adsorb on or in the vicinity of metallic nanoparticle surface. It enables excellent sensitivity down to single molecule level due to a significant electromagnetic field and chemical enhancements while having a non-destructive nature. However, SERS intensity is highly dependent on proximity and orientation of target analytes, e.g. macromolecules such as DNA or proteins, with respect to the metal surface. Therefore, SERS-based biosensing usually suffers from a wide distribution of enhancement factors, thus lacking reliably quantifiable signals for macromolecular analytes. To overcome this limitation, we propose to use anisotropic gold nanostars (AuNSt) that can be exploited for surface enhanced resonance Raman scattering (SERRS). SERRS arises when there is a chromophore close in energy to the frequency of the laser excitation. Thus, both enhancement from the plasmon resonance and molecular resonance from the chromophore contribute to give very intense SERRS signal. In this work, the AuNSts were synthesized through a bottom-up approach with tuneable optical response ranging from visible to near IR. The AuNSts also exhibit peroxidase-mimicking activity which can catalyse for the oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) into an oxidized chromogenic form (oxTMB), resulting in an increase in absorbance at 650 nm that matches the frequency of the laser wavelength (633 nm) used to excite the plasmon of AuNSts to generate very strong SERRS signal. We found that the plasmonic and catalytic properties of the AuNSt are highly dependent on the AuNSt sizes and coatings. The findings derived from our study, combining with capability to use AuNSt as label within SERRS configuration that translate the fingerprint signature of the small oxTMB into a diagnostic signal, will provide useful insights into applications such as detections of macromolecular analytes (i.e. DNA/RNA or proteins) in ultrasensitive and reproducible manners.