Probing nanoscale phenomena with nanoplasmonic sensors

Doctoral thesis, 2014

The development of the modern society is strongly influenced by the improved possibility to integrate sensing technology into traditional devices. The development of smaller, more efficient and integrated sensors have already transformed traditional business areas, such as healthcare with the introduction of e-health and portable diagnostics. In the near future one might envision healthcare practice to be heavily supported by electronic processes and communication where each individual is able to self-monitor his/her health by making use of portable devices. In this work, the potential of using metal nanoparticles to provide miniaturized, integrated and cost-efficient sensors for a broad range of applications is explored. The underlying principle of the investigated sensor concept is based on the optical properties of gold nanoparticles, which are sensitive to changes in refractive index at the interface between the nanoparticles and the surrounding medium. By designing the interface such that it is reactive only to desired events, reaction-induced changes in interfacial refractive index can be probed as a change in the optical properties of the gold nanoparticles. The thesis work focuses on evaluating the performance of nanoplasmonic sensors for applications in biosensing and deactivation of catalysts. In biosensing, the integration challenge was tackled by combining nanoplasmonic sensors with electrical readout on the very same chip. This was accomplished by fabricating gold nanoplasmonic structures onto photodiodes that acted as detectors. This sensor was used for detection of ligand-protein binding reactions, where probe ligands were anchored on the gold nanostructures and target proteins were brought in contact with the surface using a custom made flow cell. The efficiency of nanoplasmonic sensors was addressed by investigating the sensitivity to biomolecular binding with a focus on the dependence on the decay length of the sensitivity, i.e. the distance within which the sensitivity field is confined. Further, gold nanoplasmonic sensor elements were integrated in a nanofluidic network to provide a novel platform for the detection of target proteins present at low concentration in small volumes. This work also explores the application of nanoplasmonic sensors in the field of catalyst deactivation. Among the deactivation mechanisms, this thesis work focused on sintering, which refers to the decrease in total catalytically active metal surface area caused by an increase in the size of the catalyst nanoparticle. For this purpose, nanoplasmonic sensors were exploited to follow the sintering of platinum nanocatalysts in oxidizing atmospheres on alumina and silica supports in real time at high temperatures.