Surfaces of metallic films and metallic nanoparticles can strongly confine electromagnetic field through its coupling to propagating or localized surface plasmons. This interaction is associated with large enhancement of the field intensity and local optical density of states which provides means to increase excitation rate, raise quantum yield, and control far field angular distribution of fluorescence light emitted by organic dyes and quantum dots. Such emitters are commonly used as labels in assays for detection of chemical and biological species. Their interaction with surface plasmons allows amplifying fluorescence signal (brightness) that accompanies molecular binding events by several orders of magnitude. In conjunction with interfacial architectures for the specific capture of target analyte on a metallic surface, plasmon-enhanced fluorescence (PEF) that is also referred to as metal-enhanced fluorescence (MEF) represents an attractive method for shortening detection times and increasing sensitivity of various fluorescence-based analytical technologies. This review provides an introduction to fundamentals of PEF, illustrates current developments in design of metallic nanostructures for efficient fluorescence signal amplification that utilizes propagating and localized surface plasmons, and summarizes current implementations to biosensors for detection of trace amounts of biomarkers, toxins, and pathogens that are relevant to medical diagnostics and food control.

The use of thermoresponsive poly(N-isopropylacrylamide)-based hydrogel (pNIPAAm) for rapid tuning of surface plasmon resonance (SPR) is reported. This approach is implemented by using an SPR layer architecture with an embedded indium tin oxide microheater and pNIPAAm film on its top. It takes advantage of rapid thermally induced swelling and collapse of pNIPAAm that is accompanied by large refractive index changes and leads to high thermo-optical coefficient of dn/dT = 2 × 10–2 RIU/K. We show that this material is excellently suited for efficient control of refractive index-sensitive SPR and that it can serve simultaneously as a 3D binding matrix in biosensor applications (if modified with biomolecular recognition elements for a specific capture of target analyte). We demonstrate that this approach enables modulating of the output signal in surface plasmon-enhanced fluorescence spectroscopy biosensors and holds potential for simple time-multiplexing of sensing channels for parallelized readout of fluorescence assays.

This paper describes unique plasmonic characteristics of two dimensional (2D) crystalline sheets composed of homogeneous Ag nanoparticles (AgNPs) fabricated by the Langmuir-Schaefer method at an air-water interface. The localized surface plasmon resonance (LSPR) band of the Ag nanosheet was tuned by changing the interparticle distance of AgNPs via the length of the organic capping molecules. Red shift of the LSPR band of the AgNPs sheet followed an exponential law against the interparticle distance in a similar manner to the previous reports of metal nanodisc pairs. However, the shift was much larger and less dependent on the interparticle separation gap. This phenomenon is reasonably interpreted as the long-range interaction of LSPR in the 2D sheet ('delocalized' LSPR) confirmed by simulation using the finite difference time domain (FDTD) method. The FDTD simulation also revealed additional enhancement of local electric fields on the 2D sheet compared to those on the single or paired particles.