Ultrasensitive Detection of D-glucose and TNT molecules using Vertically Aligned Self-assembled Gold Nanorods

Abstract

Nanoscience is a rapidly growing discipline related with Materials Science which deals with the study of structures and materials on the scale of nanometers. Devising ingenious methods of working with nanomaterials for real-world applications is known as Nanotechnology. The properties of materials change remarkably as one transitions from the bulk to the nanoscale. This happens mainly due to the increasing spatial confinement of electrons (which determine many material properties) and the increasing surface area to volume ratio. Nanomaterials occupy a somewhat special position on the size-scale, situated between the atoms (ruled completely by quantum effects) and bulk objects. Also, unlike their bulk counterparts, the properties of nanomaterials are a function of their sizes. Thus, development of experimental methods of obtaining any given nanomaterial of a precise size enables control over their properties. It is this versatility which makes nanomaterials specially suited for a variety of applications. Nanoparticles of the noble metals such as gold, silver, and copper have the ability to confine and resonantly enhance incident electromagnetic fields due to localized surface plasmon resonances. These are collective oscillations of the conduction electrons of metals in response to external electromagnetic fields. Nanomaterials supporting LSPRs are referred to as plasmonic nanomaterials. Noble metal nanostructures can exhibit these optical resonances from the visible to the near-infrared region of the electromagnetic spectrum, which makes them viable candidates for the possibility of controlling the interaction, confinement and flow of light at the nanoscale. Since this is much faster than controlling electric current, Plasmonics can spread its metaphorical technological wings and leap off like electronics did, in the 1960’s. In short, plasmonic nanomaterials offer excellent potential for applications that might very well constitute the next technological revolution. This work focuses on nanoparticles of gold - specifically, gold nanorods. Anisotropic nanoparticles such as rods offer more interesting properties than isotropic ones - in this case, plasmonic properties. The synthesis of colloidal gold nanorods via wet chemical methods and their characterization by absorption spectroscopy and scanning electron microscopy was carried out. Further, gold nanorods were self-assembled on clean silicon substrates into a variety of novel superstructures using a simple, solvent evaporation strategy, on large (few tens of μm2) scales. The most exquisite self-assembly of all, in the form of hexagonally close-packed large arrays of vertically aligned gold nanorods was studied. Finally, it was shown that self-assembled vertically aligned gold nanorods (VA-GNRs) can serve as probes or substrates for ultra-high sensitive detection of molecules such as Dglucose and TNT (2,4,6-trinitrotoluene) via Raman spectroscopy. These molecules were chosen as model systems due to their very low Raman cross sections as well as to show that the vertical assemblies of gold nanorods can offer even as low as yoctomole sensitivity. The advantage of these vertical assemblies is their extremely high reproducible morphology accompanied by ultrahigh sensitivity which would be useful in general in many fields where very small amount of analyte is available. Moreover the assembly can be reused number of times after removing the already present molecules. The method of obtaining VA-GNRs is simple, inexpensive and reproducible., and hence has the potential to be designed and developed for technological applications requiring ultra-sensitive detection.