Multiplexing Nanoplasmonic Sensors
Complex biological samples such as serum contain thousands of proteins and other molecules spanning up to 13 orders of magnitude in concentration. Present measurement techniques such as protein microarrays and quantum dot FRET do not permit the analysis of all pair-wise interactions between the components of such a complex mixture to a given target molecule. In this thesis, we explore the use of nanoparticle tags which encode the identity of the molecule to obtain the statistical distribution of pair-wise interactions using their Localized Surface Plasmon Resonance (LSPR) signals. The encoding is done through choice of shape, size and material of the nanoparticles. Protein interactions produce plasmonic coupling between pairs of such particles which are well resolved in the scattering spectra. In this thesis, we analyze the multiplexing capacity of such an approach and compare it with a FRET based method. It is observed that multiplexing capacity is significantly higher for plasmonic coupling based method proposed in this thesis compared to the use of commonly available FRET pairs. One of the practical disadvantages of single nano-particle resolved plasmonic sensing is the low scattering intensity causing a deteriorated signal to noise ratio of detection. As part of this work, we investigated the effect of substrate on light scattering from the nanoparticle and showed that high reflectivity substrates with field reflection coefficients approaching unity result in nearly an order of magnitude enhancement of scattered light intensity. Going further on the practical realizations of plasmonic sensors, we analyzed Grating Coupled Surface Plasmon Resonance (GC-SPR) sensors based on commercially available DVD gratings using Rigorous Coupled Wave Analysis (RCWA) technique implemented in the numerical package R-Soft. We showed that by using a multi-layer dielectric coating the linewidth of the plasmonic response can be reduced by more than an order of magnitude, consequently enhancing the limit of detection of the DVD based sensor.