Nanoparticle contrast agents for targeted imaging have widespread diagnostic applications with cellular resolution, specificity and selectivity for visualization and assessment of various disease processes. Of particular interest are gold nanoparticles owing to the tunability of the localised surface plasmon resonance (LSPR) and its relative inertness. Synthesizing gold nanoprobes in the near infrared (NIR) region is of particular interest in developing nanosensors due to the minimal light attenuation from biomolecules. The ability of plasmonic gold nanostars (GNS), with novel shape-dependent dual LSPR, to elicit signal contrast at NIR wavelengths is described here for multiple biomedical modalities. First, the surface enhanced Raman scattering (SERS) capability of these dual plasmonic GNS has been demonstrated to elicit high SERS enhancement factor (EF) of 2 x 10e7 with 785 nm excitation and the potential to elicit the highest SERS EF ever reported for gold nanoparticles, with further longer wavelength excitations at and beyond 1064 nm.
We have also demonstrated the longer wavelength contrast imaging capability of GNS with photoacoustic imaging (PAI) and for photothermal therapy (PTT). GNS possess unique structural characteristics that impart superior optical properties resulting in higher photothermal efficiency. The photothermal capability of GNS was demonstrated in vivo with localized temperature rise of 9℃ in tumors when irradiated with a 1064 nm CW laser that resulted in significant tumor cell death. Since photothermal conversion is the optical process responsible for eliciting PA contrast and for PTT, this development represents a novel theranostic substrate to be used at 1064 nm excitation, a longer wavelength than the conventional clinical range. The ability of GNS to elicit signal contrast at NIR wavelengths has also been demonstrated for photothermal optical coherence tomography (PT-OCT). When irradiated with a 1064 nm continuous wave laser, GNS elicited photothermal contrast well beyond 2 mm, displaying great potential for deep tissue imaging. We have also recently obtained a European Commission grant worth €5.98M on developing, demonstrating and validating a novel GNS enhanced photoacoustic imaging platform which will be capable of tracking mesenchymal stem cells (MSC) and MSC-derived exosomes, at unprecedented depth and sensitivity.