Design and characterization of colorimetric plasmonic nanostructures for imaging and sensing

Increasing demand for early disease diagnostic techniques has attracted huge interest in plasmon-based optical sensors, which can detect small concentrations of chemical and bio-analytes that are not detectable by the conventional analytic optical tools. Advances in nanofabrication techniques have driven in-depth understanding of plasmons, which result from the interactions between nano-materials and the electromagnetic fields. Precisely designing and controlling unique optical properties of plasmons have shown better sensing limits than the conventional ones by amplifying optical signals as well as detecting sensitive plasmon resonance shift upon dielectric property change on the sensing surface. In this dissertation, a series of experiments are undertaken using a colorimetric plasmonic nanocup array substrate with a single extraordinary optical transmission peak in the visible light range. Sensitive colorimetric sensing is demonstrated by detecting transmission peak shift upon the molecular adsorption or the dielectric property change on the surface. The surface modification of the plasmonic substrate using plasmonic metallic NPs is attempted in order to maximize the plasmonic sensitivity to the refractive index change through heterogeneous plasmon coupling. The plasmon coupling between the plasmons of NPs and nanostructure results in strong localized electric field and denser hot-spot formation; hence, the sensitivity is enhanced. Sensitive detections of specific bioanalytes that undergo antigen-antibody binding as well as bulk refractive index change are detected through a plasmonic dark mode shift, resulted from the heterogeneous plasmon coupling. Optical near-field interactions among plasmons, fluorophores, chromophores, and molecules are studied in order to amplify weak fluorescence, absorbance, and Raman signals from a small number of target molecules. Strong scattering field and large scattering cross-section at the plasmon resonance wavelength are the main factors for amplifying fluorescence, absorbance, and Raman scattering. Tuning plasmon resonance to target molecular optical characteristic wavelengths is critical in each application. The amplification of fluorescence is achieved by matching the plasmon resonance with the fluorescence emission band. The absorbance from chromophores, which are involved in conventional immunoassays, is enhanced by matching the chromophores’ absorbance peak with the plasmon resonance wavelength. The improved surface enhanced Raman scattering is accomplished by tuning the plasmon resonance to be close to the laser excitation wavelength. Understanding the signal amplification mechanisms from these results achieves two orders of magnitude lower limit-of-detection as well as improved sensitivity and signal-to-noise ratio. The colorimetric sensor, which is capable of enhancing fluorescence, absorbance and Raman signals from the nearby molecules, provides a versatile multifunctional sensing platform for chemical, biomedical, and environmental monitoring