Active Plasmonics with Responsive Hydrogels

This project embraces the fields of materials research and nanophotonics. It focuses on new hybrid metal - hydrogel materials which can rapidly collapse and swell upon an external stimulus and can be functionalized with biomolecular recognition elements. These materials are hypothesized to open new routes in plasmonics of which potential is locked in many fields due to the lack of efficient actuating of (mostly) passive plasmonic structures and components. In particular, development of new tunable integrated plasmonic elements for actuating of surface plasmon beams will be pursued and active control of plasmon-mediated fluorescence will be investigated. Moreover, responsive functionalized hydrogels carrying biomolecular recognition elements will be explored in order to efficiently manipulate and collect of molecular analytes on metallic nanostructures with plasmon characteristics. Combining of these will be carried out in order to provide biosensor platforms with improved sensitivity and analysis time for detection of molecular analytes relevant to important areas such as medical diagnostics.

Plasmonics refers to research that concerns tight sub-wavelength confinement of light by its coupling to surface plasmons at metallic films and metallic nanoparticles. It originates from collective oscillations of charge density and associated electromagnetic field. The probing of biomolecules with the highly intense surface plasmon field enables their sensitive analysis based on various optical formats. These include direct measurements of molecular binding-induced refractive index changes as well as plasmonically amplified optical spectroscopy including fluorescence, Raman spectroscopy or infrared absorption spectroscopy. Through the course of the project ACTIPLAS, there were developed and investigated new plasmonic materials with actively tunable properties. They were based on responsive hydrogels tethered to the surface of plasmonic metallic structures. The swelling and collapsing of these hydrogels can be reversibly triggered by an external stimulus. Such changes were accompanied with modulation of wavelength at which the resonant plasmonic interaction occurs and it alters properties of surface plasmon waves. These materials can be post-modified with selected biomolecules in order to provide were specific biological function such as recognition of target molecular species present in analyzed liquid samples. The detection of the specific capture of target molecules on the surface with a responsive hydrogel was pursued by using surface plasmon-enhanced fluorescence and limit of detection at femtomolar concentrations were achieved. In addition, new means for preparation and observation of actively tunable hybrid metallic / hydrogel materials were developed. Copolymers based on isoproprylacrylamide or oxazoline backbones were synthesized and their thermoresponsive properties characterized. These polymers were employed for the preparation of thin films that were structured by newly developed variants of laser interference lithography and nano-imprint lithography. Novel adaptive materials combing highly swollen responsive hydrogels and metallic nanostructures were prepared. These include responsive hydrogel nanogratings that can be reversibly erased and recovered, diffractive structures for improved extraction of fluorescence light from the sensor surface, and free standing plasmonic membrane with arrays of nanoholes actuated by a responsive hydrogel cushion. These novel structures were functionalized for sensing applications and employed in new plasmonic biosensor concepts. The performance of pursued biosensor concepts was demonstrated by the analysis of minute amounts of biomolecules that can serve as biomarkers of diseases. The project supported two PhD theses and one master thesis. Its results were published in 12 papers in peer reviewed journals, 18 conference contributions, and 4 book chapters.