title: Material-selective surface chemistry for nanoplasmonic sensors: optimizing sensitivity and controlling binding to local hot spots.
abstract: Optical sensors utilizing the principle of localized surface plasmon resonance (LSPR) offer the advantage of a simple label-free mode of operation, but the sensitivity is typically limited to a very thin region close to the surface. In bioanalytical sensing applications, this can be a significant drawback, in particular since the surface needs to be coated with a recognition layer in order to ensure specific detection of target molecules. We show that the signal upon protein binding decreases dramatically with increasing thickness of the recognition layer, highlighting the need for thin high quality recognition layers compatible with LSPR sensors. The effect is particularly strong for structures that provide local hot spots with highly confined fields, such as in the gap between pairs of gold disks. While our results show a significant improvement in sensor response for pairs over single gold disks upon binding directly to the gold surface, disk pairs did not provide larger signal upon binding of proteins to a recognition layer (already for around 3 nm thin layers) located on the gold. Local plasmonic hot spots are however shown advantageous in combination with directed binding to the hot spots. This was demonstrated using a structure consisting of three surface materials (gold, titanium dioxide, and silicon dioxide) and a new protocol for material-selective surface chemistry of these three materials, which allows for controlled binding only in the gap between pairs of disks. Such a design increased the signal obtained per bound molecule by a factor of around four compared to binding to single disks.
authors.affiliation: Department of Applied Physics, Chalmers University of Technology, SE-41296 Gothenburg, Sweden.
authors.forename: Laurent
authors.initials: L
authors.lastname: Feuz
authors.forename: Magnus P
authors.initials: MP
authors.lastname: Jonsson
authors.forename: Fredrik
authors.initials: F
authors.lastname: Höök
mesh_terms: D001665:Binding Sites; D001710:Biotin / Q000737:chemistry*; D006046:Gold / Q000737:chemistry*; D049329:Nanostructures / Q000737:chemistry*; D036103:Nanotechnology; D010316:Particle Size; D011092:Polyethylene Glycols / Q000737:chemistry; D020349:Surface Plasmon Resonance; D013499:Surface Properties
chemical_list: D011092:Polyethylene Glycols; D001710:Biotin; D006046:Gold