Metal-enhanced fluorescence quenching

Fabbrizzi L., Lichelli M., Paliavicini P., Sacchi D., Taglietti A. (1996) Sensing of Transition Metals Through Fluorescence Quenching or Enhancement, Analyst 121, 1763-8. 

Several other studies (150-153) reported that metal surfaces were able to either enhance or suppress the radiative decay rates of fluorophores. Furthermore, an increase in the extent of resonance energy transfer was also observed. These effects might be due to the interactions of excited-state fluorophores with SPs, which in turn produce constructive effects on the fluorophore. The effects of metallic surfaces include fluorophore quenching at short distances ( 0-5 nm), spatial variation of the incident light field (-0-15 nm), and changes in the radiative decay rates (-0-20 nm) (64). The term of metal-enhanced fluorescence could be referred to the appplication of fluorophore and metal interactions in biomedical diagnosis (64). 

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... 

Fig. 3 Typical ICT probes (left) and representative spectroscopic responses toward selected metal ions (right). Color code (left) coordinating atoms in blue, bridgehead atom of the fluorophore that takes part in complexation in orange, formal donor fragment in red, formal acceptor fragment in green (right) hypsochromic shifts in red, bathochromic shifts in green, fluorescence enhancement in violet, fluorescence quenching in blue. Symbols in table Aabs, 7em,

Figure 5 shows two typical core-shell structures (a) contains a metal core and a dye doped silica shell [30, 32, 33, 78-85] and (b) has a dye doped silica core and a metal shell [31, 34]. There is a spacer between the core and the shell to maintain the distance between the fluorophores and the metal to avoid fluorescence quenching [30, 32, 33, 78-80, 83]. Usually, the spacer is a silica layer in this type of nanostructures. Various Ag and Au nanomaterials in different shapes have been used for fluorescence enhancement. Occasionally, Pt and Au-Ag alloys are selected as the metal. A few fluorophores have been studied in these two core-shell structures including Cy3 [30], cascade yellow [78], carboxyfluorescein [78], Ru(bpy)32+ [31, 34], R6G [34], fluorescein isothiocyanate [79], Rhodamine 800 [32, 33], Alexa Fluor 647 [32], NIR 797 [82], dansylamide [84], oxazin 725 [85], and Eu3+ complexes [33, 83]. 

Calixarene containing a dioxotetraaza unit, PET-18, is responsive to transition metal ions like Zn2+ and Ni2+. Interaction of Zn2+ with the amino groups induces a fluorescence enhancement according to the PET principle. In contrast, some fluorescence quenching is observed in the case of Ni2+. PET from the fluorophore to the metal ion is a reasonable explanation but energy transfer by electron exchange (Dexter mechanism) cannot be excluded. 

Since the pioneering work by Cotton et al. on heme proteins (Cotton et al., 1980), surface enhanced resonance Raman spectroscopy (SERRS), Sec. 6.1, has been used to study a large variety of biomolecules, such as retinal proteins (Nabiev et al., 1985), flavoproteins (Coperland et al., 1984 Holt and Cotton, 1987), chlorophylls (Cotton and Van Duyne, 1982 Hildebrandt and Spiro, 1988), and oxyhemoglobins (de Groot and Hesters, 1987). The advantages of this technique include low sample concentration and fluorescence quenching. The main question is whether or not the native structure and function of the molecule is preserved on the metal surface. 

Metal nanostructures can act as small antennas that aid in the reception and broadcasting (absorption and emission) of light from nearby fluorophores. Whether fluorescence enhancement or quenching is observed in a given system is determined by the relative extent of excitation enhancement (increased light absorption), emission enhancement (increased radiative decay), and quenching (increased non-... 

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