Coupled surface plasmon modes
Surface plasmon propagating along thin metallic films or along periodically modulated metallic surfaces can become coupled giving rise to new SP modes, see Fig.1.
Fig. 1: Coupled surface plasmon modes a) propagating along a thin metallic film and b) formed by the Bragg scattering on periodically modulated surface.
Characteristics of coupled SPs can be tailored for specific applications. For instance, a thin metallic film embedded between dielectrics with similar refractive indices supports a SP mode with an anti-symmetric component of electric intensity that is parallel to the metallic surface. This coupled mode is referred to as long range surface plasmon (LRSP) and can propagate along the surface with lower orders of damping than conventional surface plasmons.
Implementation of SPFS biosensors
The excitation of LRSP leads to an improvement of biosensors based on surface plasmon–enhanced fluorescence spectroscopy (SPFS). In these devices, target molecules contained in the sample of interest are captured by bio-recognition elements immobilized on a metallic surface. The binding of fluorophore-labeled molecules to the surface is observed via the induced fluorescence signal, which is greatly increased by the strong enhancement of the electromagnetic field on the sensor surface upon the resonant excitation of SPs. The excitation of LRSPs provides an even greater enhancement of the electromagnetic field (|E/E0|2 larger than 100 is possible) and thus enabling a further increase in the sensitivity of the SPFS method.
Detection of aflatoxin M1 in milk
LRSP-enhanced fluorescence spectroscopy was implemented in a highly sensitive biosensor for detection of aflatoxin M1 in milk. These biosensors employed an optimized layer structure for the excitation of LRSPs on the sensor surface and an inhibition immunoassay, see Fig.2. The sensor allowed for fast detection of AFM1 at sub pg/mL levels.
Fig. 2: Scheme of the SPFS-based biosensor exploiting inhibition competitive immunoassay and the excitation of LRSPs.
Fig. 3:Calibration curve of a LRSP-FS biosensor for detection of AFM1 in milk (red curve) and a buffer (blue curve).