Surface plasmon resonance: signal amplification using colloidal gold nanoparticles for enhanced sensitivity

Among all the detection methods developed for bioassays, optical-based approaches are the most popular techniques. A widely used and accepted bioanalytical technique for routine characterization of molecular recognition events at a solid interface is surface plasmon resonance (SPR) spectroscopy. Compared with other techniques, SPR offers a series of advantages including rapid and real-time analysis, in situ detection, and the possibility to determine kinetic as well as thermodynamic parameters of interacting partners without the requirement of tagging one of the two partners. This method is nowadays used in academic and industrial research laboratories for drug discovery processes, safety, and quality control applications as well as for understanding bioaffinity reactions. Past studies have shown that SPR can detect approximately 100 ng/l of proteins. Currently, much effort is still invested to further improve SPR with respect to the sensitivity in the detection of low molecular weight analytes as well as in the detection limit for different analyte/ligand reactions. This review will focus on the use of gold nanoparticles as an amplification strategy in SPR sensing. Compared to other review articles highlighting the several synthetic routes and properties of gold nanoparticles and research utilizing gold nanoparticles for various sensing strategies, this review will exclusively look at the impact of gold nanostructures for ultrasensitive SPR sensing. Signal enhancement by gold nanoparticles is caused by several effects such as surface mass increase due to enhanced surface area, larger refractive index changes by the particle mass, themselves, and electromagnetic field coupling between the plasmonic properties of the particles (localized surface plasmon resonance) and propagating plasmons. The unique optical, electronic, and catalytic properties of gold nanoparticles put them into the forefront of interest for signal amplification as will be outlined here.