Category: Advances in Bioanalytics and Biomarkers
Colloidal gold nanorods have attracted considerable interest as biosensors and imaging platforms due to their anisotropy, and strong spectral characteristics. The change in the localized surface plasmon resonance (LSPR) of the rods as a function of dielectric environment has been studied extensively and been utilized to construct high-sensitivity, label-free, low cost biosensors. However, most studies related to the construction of such biosensors neglect to discuss the role of the surfactant in the shift in the LSPR resonance wavelength. Those that do discuss surfactant properties tend to focus on the synthesis of the rods themselves rather than their function as biosensors. It is well known that the adsorption of bio-thiols on nanoparticles, for instance, is sensitive to several parameters, such as pH and capping agent. However, the magnitude of the LSPR shift induced by the monolayer created by bio-thiols is underestimated by Mie theory under certain circumstances.
Furthermore, it is well known that the nature of LSPR is to automatically multiplex all the individual and varied refractive index contributions from the local environment. The result of this is that LSPR biosensors do not extend well to more complex solutions of multiple biomolecules, and they often require significant a-priori information such as TEM or other high resolution imaging methods. This limits the usefulness of label-free LSPR sensors to relatively simple solutions, or, in the case of enantiomeric selection sensors, solutions with only one molecule with significant chirality.
This work studies the interfacial relationships in the gold-biomolecule conjugate. The effect on the giant optical response of the nanorod-biomolecule system was probed using UV-Vis-IR absorption spectroscopy and circular dichroic spectroscopy. CTAB-capped gold nanorods are synthesized and used as illustrative examples. By altering the relative concentrations of capping molecule with respect to an introduced thiol-linker, as well as the nanorods themselves, we induced different binding and agglomeration modalities in solution. From this, we infer the presence of chiral oligomers that occur before general disorder.
We found that the change in LSPR bands as a result of nanorod-protein conjugation, while being reproducible, are also indicative of particular aggregation forms arising from the surface binding of adsorbates and the agglomeration of the rods themselves due to adsorbate-mediated electrostatic forces. We then present the chiral response of the nanorod aggregates and demonstrate that the same forces also govern the magnitude of that response. Therefore, we have demonstrated using simply the giant optical response of the rods, examples of controllable self-assembly even during agglomeration events. We verify our findings through the use of typical TEM and numerical simulations. This can be leveraged to create controllable aggregants for biosensors that provide an unprecedented degree of quantitative estimation of molecular interaction and binding kinetics of ever-complex solutions.
Arun Nagpal– Bionanoplasmonics Researcher, Johns Hopkins University, Kalamazoo, MI
Johns Hopkins University
Arun is an undergraduate pursuing a degree in Electrical Engineering at the University of Michigan. He has conducted research in plasmonics and medical technology at the University of Michigan, California Institute of Technology, and most recently at Johns Hopkins University. His research interests include van der Waals, plasmonic materials, high-throughput label-free medical sensors, and spectroscopy.