1 Department of Micro- and Nanotechnology, Technical University of Denmark2 Department of Mechanical Engineering, Technical University of Denmark3 Center for Nanostructured Graphene, Center, Technical University of Denmark
To confine light in a liquid and thereby form a liquid core waveguide, the surrounding cladding materials must have a lower refractive index than the liquid core. In the context of biosensing, it is a challenge to obtain the right cladding material, as most of the relevant liquids are aqueous and have refractive index around 1.33 i.e. close to water, the earth’s most abundant liquid. The objective of this Ph.D study is to design and develop a liquid core waveguide platform from nanoporous polymer materials rendering refractive indices below 1.33, and to investigate the optical waveguiding in these devices. Nanoporous materials with refractive index below 1.33 are obtained from self assembling polymers called block copolymers by selectively etching one of the polymer blocks from the self assembly. The effective index of the porous material is a weighted average of the refractive indices of the polymer and air in the self assembly. Using UV-assisted surface modification, the hydrophobic nanoporous materials are selectively made hydrophilic. On infiltration with water, the pores in the UV exposed region are filled, thereby increasing the effective index in these regions by a value of n=0.17. The index contrast is exploited to confine light in the exposed infiltrated regions. As light and liquid are confined in the same volume and the scale of heterogeneity in these regions are lower than the guided wavelength, they form solid-liquid core waveguides. Two different surface modification methods have been tried: photo-oxidation and thiol-ene click chemistry. The former uses UV irradiation in presence of oxygen to hydrophilize the polymer surfaces, while the latter grafts functional groups onto the polymer. To perfom thiol-ene chemistry, two thiols: mercaptosuccinic acid (MSA) and sodium mercaptoethanesulfonate (MESNA) are used. The prepared devices are characterized by measuring their propagation loss using substitution method. A propagation loss of 0.62±0.03 dB/mm are obtained in the photo-oxidation modified waveguides. The MSA and MESNA modified waveguides yield a propagation loss of 0.26±0.05 dB/mm and 0.54±0.05 dB/mm respectively. Nanofiltering via integrated liquid core waveguides is also demonstrated and described in this PhD thesis. With fluorescence spectroscopy and microscopy it is proved that large particle 22 nm fluorescing beads are restricted from entering the waveguides, while small molecules (Rhodamine B) uniformly penetrate. Nanoporous liquid core waveguides can thus exclude scattering particles, making them an ideal integrated platform for analysis of turbid solutions like blood or milk. We explore the example of filtering large fatty particles (2 μm) from milk. These developed waveguiding particle filters can be a promising platform for optofluidic and biosensing applications.