Metal-enhanced fluorescence absorption

Figure 1.2. (A) A schematic diagram depicting the processes in close proximity to metals (< 10 nm) involved in Metal-Enhanced Fluorescence enhanced absorption and coupling to surface piasmons. (B) Emission spectra of FITC deposited onto SIFs and glass. The inset shows the real-color photographs of FITC emission from these surfaces. (C) Intensity decays for FITC on both glass and SiFs. IRF Instrument Response Function.

The EM theory of Metal Enhanced Fluorescence (MEF) was studied and developed extensively in the 70-80 s [2,3,4,5,6], All the EM mechanims involved in MEF can be understood within classical EM theory [3,4,5,6] as confirmed in the simplest cases by quantum studies [2,12,17,18], In most of these models, the emitter is depicted as a simple two- (or three-) level system, i.e. only one emission wavelength is considered. This is appropriate in general to understand modifications of absorption or emission rates, but it entirely ignores the spectral profile of the fluorescence emission. We will first review... 

Enhanced fluorescence, or MEF, is a result of both a net system absorption and plasmon coupling and subsequently efficient emission, but to date, it has not been possible to quantify the relative contributions of enhanced emission and net increase in the system absorption to the MEF phenomena.(23) Due to the increase in the population of the singlet excited state or net system absorption, the very presence of MEP has also suggests an increase in the population of the triplet state.(23) The presence of Metal-Enhanced Fluorescence, Phosphorescence, Metal-Enhanced singlet oxygen and superoxide anion radical generation in the same system is an effect of the enhanced absorption and emission effects of the fluorophores near-to silver, although these processes are effectively competitive and ultimately provide a route for deactivation of electronic excited states. 

It is possible that surface enhancement effects, similar to the observations made earlier in metal-fluorophore systems [11, 83-85] may occur. Metal surfaces are known to have effects on fluorophores such as increasing or decreasing rates of radiative decay or resonance energy transfer. A similar effect may take place in ZnO nanomaterial platforms. However, decay lengths of fluorescence enhancement observed in the semiconducting ZnO NRs are not commensurate with the length scale seen on metals such as Au or Ag. For effective metal enhanced fluorescence, fluorophores should be placed approximately between 5-20 nm away from the metal surface. However, fluorescence enhancement effect on ZnO NRs is observed even when fluorophores are located well beyond 20 nm away from the NR surface. At the same time, no quenching effec en when they are placed directly onto ZnO NR surfaces. In addition, there overlap between the absorption and emission... 

Figure 1.8. (A) Current interpretation Metal-Enhanced unstructured (Top) and structured emission (Bottom). F-Fluorophore. MEF- Metal Enhanced Fluorescence. (B) Absorption spectra of Perylene sandwiched between two silvered and unsilvered slides respectively (Sandwich experimental geometry (Top Insert). (C) Fluorescence emission spectra of Perylene sandwiched between two silvered, unsilvered slides, 50 run thick continuous Ag films, respectively.

Figure 16.3 Modified Jablonski diagram ows the energy absorption effects of near metal surface enhanced fluorescence. The process involves o eating an excited electronic singlet state by optical absorption and subsequent emission of fluorescence with different decay paths.

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. 

For 2,7-dmnapy complexes of the same rare earth, the recorded intensities of the nitrate complexes are always greater than those in which acac is the anion. Conductivity measurements on nitromethane solutions of M(acac)3(2,7-dmnapy) give A values of 6-9 which are typical of nonelectrolytes (II, 12). IR spectra of the acac complexes have vCO absorptions at approximately 1600 and 915 cm" and i/M-0 bands at 405 and 313 cm" These absorptions and the lack of a 1700-cm band indicate that both oxygens of each of the acac units are coordinated to the metal (II, 30). The more intense fluorescence of the nitrate complex may result from the presence of a second 2,7-dmnapy ligand which would increase the coordination number of the rare earth from 8 to 10. The triplet state of acac is reported at 25,300 cm (31). The triplet state of napy at 22,210 cm" is closer to the rare earth resonance levels and may contribute to a more efficient energy transfer which in turn would enhance fluorescence intensity. 

Due to the surface sensitivity surface enhanced fluorescence has become particularly popular in the characterisation of thin molecular films, such as Langmuir-Blodgett films and self-assembled biomembranes. Two surface enhanced spectroscopic techniques (surface enhanced IR absorption, SEIRA, and surface enhanced fluorescence, SEF) were recently applied to the study of biomembrane systems by the group of Reiner Salzer [323]. With SEIRA, specific fingerprints of biomolecules could be obtained with a tenfold IR intensity enhancement With SEF signal enhancement factors greater than 100 were obtained. The enhancement factor was very dependent on the properties and structure of the metal clusters used. With the two techniques biomembranes formed from vesicles with embedded nicotinic acetylchoHne receptors were spectroscopically characterized. 

In this chapter, a brief theoretical overview is provided that discusses, among other things, EM enhancement of emission, enhanced absorption, quenching to metal surfaces, the distance, coverage, and temperature dependence of SEF, and the effects of quantum efficiencies on enhancement. Also discussed, is the preparation and characteristics of several different nanoparticle metal substrates that have been employed in the collection of SEF, and the surface-enhanced fluorescence of Langmuir-Blodgett (LB) monolayers. Finally, a summary of these concepts is presented, and the future of SEF is discussed. 

As with enhanced fluorescence, the enhancement of the absorption of dyes is able to function at a small distance of separation between the metal nanoparticle and the... 

An additional advantage of increasing the excitation intensity of LSCs is the incorporation of noble metal or copper NPs into the fluorescent species [61], which enhances the absorption of the fluorescent complexes [103—106]. 

A nano-light-source generated on the metallic nano-tip induces a variety of optical phenomena in a nano-volume. Hence, nano-analysis, nano-identification and nanoimaging are achieved by combining the near-field technique with many kinds of spectroscopy. The use of a metallic nano-tip applied to nanoscale spectroscopy, for example, Raman spectroscopy [9], two-photon fluorescence spectroscopy [13] and infrared absorption spectroscopy [14], was reported in 1999. We have incorporated Raman spectroscopy with tip-enhanced near-field microscopy for the direct observation of molecules. In this section, we will give a brief introduction to Raman spectroscopy and demonstrate our experimental nano-Raman spectroscopy and imaging results. Furthermore, we will describe the improvement of spatial resolution... 

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