Nanoplasmonic Sensors based on Periodic Arrays of Gold Nanoparticles

Nanoplasmonic sensors use the localized surface plasmon resonance (LSPR) of metal nanostructures to sense the refractive index change in a surface-bound layer caused by biomolecular interactions or changing chemical environment. In this thesis, four types of sensor configurations based on gold nanoparticle arrays are thoroughly investigated. The first configuration is a periodic array of gold nanoparticles excited by the evanescent field of an optical waveguide mode. Since light carried by a waveguide mode propagates along the same plane of the periodicities, the coupling of nanoparticles are strongly affected by the photonic crystal lattice. This configuration is investigated both in simulations and in experiments, focusing on the sensing aspects of various spectral features attributed to different types of LSPRs. In simulation, it was found that, the LSPR modes of the gold nanoparticle array excited by the waveguide modes demonstrated similar trends as the array being excited by normal transmission. However, under waveguide excitation, the coherent interactions of the periodic array are through the grating order carried by the waveguide mode. Selective suppression of LSPR and grating-induced mode were also found from the waveguide excitation, which extended the existing knowledge of the optical properties of the periodic array. Under waveguide excitation, both quadrupolar and dipolar resonance peaks are sensitive to the surface-bound layer, however, the grating-induced mode is not sensitive. Preliminary experiments were conducted on ion-exchanged channel waveguides on BK7 glass and the trends of the dipolar resonance peak has been proved. The second configuration is a biosensor based on gold nanodisk arrays under normal transmission. By varying the lattice constant, the refractive index resolution was found to depend on the lattice constant, as a result of the figure of merit and signal/noise ratio together. The best refractive index resolution achieved is better than 1.5x10-4 RIU, when lattice constant equals to 550 nm. The sensor structure was used in detecting the binding of antigen (human IgG) and antibody (anti-human IgG) and a limit of detection better than 1 ng/mL (equivalent to 8 pM) was achieved. The third configuration is a chemical sensor based on a gold nanocrescent array combined with hydrogel. Under changing chemical environment, the hydrogel thin film can swell or shrink, depending on the pH of the solution. The swelling or shrinking of hydrogel thin film changes the refractive index, which can be detected by the near-infrared LSPR peak shift of the gold nanocrescent array. The sensor was proved to function in the range 4.5 pH - 6.4 pH and the detection resolution is better than 0.045 pH. At the most sensitive point, pH = pKa = 5.45, the peak-shift sensitivity is 11.1 nm/pH and transmission-shift sensitivity is 1.16 /pH. The fourth configuration is periodic array of gold nanorings under normal transmission. The effects of coherent interactions on the sensing characteristics of periodic arrays of gold nanorings were investigated in detail. In simulations, it was found that, the sensitivity, figure of merit both significantly depend on the lattice constant. The structure with highest figure of merit was found to be the one with lattice constant smaller than and close to the resonant lattice constant. The periodic array can improve the figure of merit by more than 2.5 times, compared to a single nanoring, which demonstrates the great sensing capabilities of periodic arrays. The simulated trend was proved by the experiments of the gold nanoring arrays patterned on top of pyrex substrate. A method was also demonstrated on how to tune the sensor structure to function in a desired spectrum window, with high figure of merit and high signal/noise ratio, at the same time. The highest figure of merit achieved from experiment is around 5.1, which is among the highest in literature