Surface plasmon field-enhanced fluorescence

Liebermann T, Knoll W (2003) Parallel multispot detection of target hybridization to surface-bound probe oligonucleotides of different base mismatch by surface-plasmon field-enhanced fluorescence microscopy. Langmuir 9 1567-1572... 

Alternatively, various analytical methods based on SPR phenomenon have been developed, including surface plasmon field-enhanced Raman scattering (SERS) [7], surface plasmon field-enhanced fluorescence spectroscopy (SPFS) [8-11], surface enhanced second harmonic generation (SHG) [12], surface enhanced infrared absorption (SEIRA) [13], surface plasmon field-enhanced diffraction spectroscopy (SPDS) [14-18], Most of these methods take advantage of the greatly enhanced electromagnetic field of surface plasmon waves, in order to excite a chromophoric molecule, e.g., a Raman molecule or a fluorescent dye. Therefore, a better sensitivity is expected. 

Surface Plasmon Field-Enhanced Fluorescence Spectroscopy... 

Fig. 13. (A) Schematic representation of the interaction between neighboring residues of the His-tag (6 consecutive Histidine residues) and an NTA-complexed Ni2+ ion. (B) The performance of LHCII immobilization via chelating interaction. SPR kinetic curve of LHCII immobilization and regeneration cycles, monitored with an Nd YAG DPSS laser (X = 473 nm). (C) Surface plasmon field-enhanced fluorescence emission spectrum of surface attached LHCII compared with the fluorescence emission from free LHCII in solution, excited by an Nd YAG DPSS laser (X — 473 nm).

Liebermann, T., and Knoll, W. (2000). Surface-plasmon field-enhanced fluorescence spectroscopy. Colloid and Surfaces A. Physicochemical and Engineering Aspects 171 115-130. 

Ekgasit, S., Thammacharoen, C., Yu, F., and Knoll, W. (2004). Evanescent Field in Surface Plasmon Resonance and Surface Plasmon Field-Enhanced Fluorescence Spectroscopies./Ina/. Chem. 76 2210-2219. 

PRINCIPLES AND APPLICATIONS OF SURFACE-PLASMON FIELD-ENHANCED FLUORESCENCE... 

In an attempt to overcome this limit of detection we recently introduced surface plasmon field-enhanced fluorescence spectroscopy (SPFS) following an earlier report liy Attridge et al. The basic principle of this approach combines the excitation of a surface plasmon mode as an interfacial light source with the well-established detection schemes of fluorescence spectroscopy the resonantly excited surface plasmon waves excite chromophores that are attached to the analyte either chemically or by genetic engineering techniques. The emitted fluorescence photons are then monitored and analyzed in the usual way to give information about the behavior of the analyte itself. 

Figure 17. Schematic experimental setup for surface-plasmon and surface-plasmon field-enhanced fluorescence microscopy in the Kretschmann configuration.

Yu YM, Feng CL, Caminade AM, Majoral JP, Knoll W (2009) The detection of DNA hybridization on phosphorus dendrimer multilayer films by surface plasmon field enhanced-fluorescence spectroscopy. Langmuir 25 13680-13684... 

The electromagnetic field enhancement provided by nanostructure plasmonics is the key factor to manipulate the quantum efficiency. However, as it is illustrated in the unified theory of enhancement, since both the radiative and non-radiative rates of the molecular systems are affected by proximity of the nanostructure, the tuning has to be done on a case by case basis. In addition, there are factors due to molecule-metal interactions and molecular orientation at the surface causing effects that are much more molecule dependent and as are much more difficult to predict. Given the fact that fluorescence cross sections are the one of the highest in optical spectroscopy the analytical horizon of SEF or MEF is enormous, in particular in the expanding field of nano-bio science. 

The detected fluorescence can be significantly enhanced, however, by exploiting the plasmonic enhancement which can occur when a metal nanoparticle (NP) is placed in the vicinity of a fluorescent label or dye [1-3]. This effect is due to the localised surface plasmon resonance (LSPR) associated with the metal NP, which modifies the intensity of the electromagnetic (EM) field around the dye and which, under certain conditions, increases the emitted fluorescence signal. The effect is dependent on a number of parameters such as metal type, NP size and shape, NP-fluorophore separation and fluorophore quantum efficiency. There are two principal enhancement mechanisms an increase in the excitation rate of the fluorophore and an increase in the fluorophore quantum efficiency. The first effect occurs because the excitation rate is directly proportional to the square of the electric field amplitude, and the maximum enhancement occurs when the LSPR wavelength, coincides with the peak of the fluorophore absorption band [4, 5]. The second effect involves an increase in the quantum efficiency and is maximised when the coincides with the peak of the fluorophore emission band [6]. 

Shape The radiative emission from molecules confined within metallic nanocavities and on the surface of nanoparticles is of great relevance to biotechnology. In 1986, it has been suggested that fluorescence enhancement and reduced observation volumes could be obtained from small metal apertures (85). Nanocavities of different shapes could induce different surface plasmon (SP) fields. More recently, some studies has been done for different shapes, such as circular (86-90), elliptical (91), coaxial (92), or rectangular (93, 94) metallic nanocavity(95). In 2003, single-molecule detection from a nanocavity was demonstrated (86). However, it might be difficult to position the biospecies in the nanocavities. 

Metal nanoparticles have attracted considerable interest due to their properties and applications related to size effects, which can be appropriately studied in the framework of nanophotonics [1]. Metal nanoparticles such as silver, gold and copper can scatter light elastically with remarkable efficiency because of a collective resonance of the conduction electrons in the metal (i.e., the Dipole Plasmon Resonance or Localized Surface Plasmon Resonance). Plasmonics is quickly becoming a dominant science-based technology for the twenty-first century, with enormous potential in the fields of optical computing, novel optical devices, and more recently, biological and medical research [2]. In particular, silver nanoparticles have attracted particular interest due to their applications in fluorescence enhancement [3-5]. 

Understanding the field enhancement of radiative rates is insufficient to predict how molecular photophysical properties such as enhancement of fluorescence quantum yield will be affected by interactions of the molecule with plasmons. A more detailed model of the photophysics that accounts for non-radiative rates is necessary to deduce effects on photoluminescence (PL) yields. Such a model must include decay pathways present in the absence of metal nanoparticles as well as additional pathtvays such as charge transfer quenching that are associated with the introduction of the metal particles. Schematically, we depict the simplest conceivable model in Figure 19. IB. Note that both the contributions of radiative rate enhancement and the excited state quenching by proximity to the metal surface will depend on distance of the chromophore from the metal assembly. In most circumstances, one expects the optimal distance of the chromophores from the surface to be dictated by the competition between quenching when it is too close and reduction of enhancement when it is too far. The amount of PL will be increased both due to absorption enhancement and emissive rate enhancement. Hence, it is possible to increase PL substantially even for molecules with 100 % fluorescence yield in the absence of metal nanoparticles. 

The field reaches its maximum at the surface plasmon resonance frequency when e = -2 Co where Co is the dielectric constant of the medium surrounding the particle surface. This induced field of the metallic nanoparticies provides an external field for the fluorescence excitation of the molecules in addition to the electric field of the incident light and thus increases the absorption rate which is responsible for the enhanced fluorescence intensity. 

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