Quenching metal-enhanced fluorescence, metallic

In calixarene-based compound M-8 (Figure 10.28), bearing four anthracene moieties on the lower rim, some changes in fluorescence intensity were observed on binding of alkali metal ions but no excimer emission was detected. Quenching of the fluorescence by Na+ may arise from interaction of four anthracene residues brought in closer proximity to one another enhancement of fluorescence by K+ is difficult to explain. 

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

Water-soluble cyclophane 86145 exhibited a well-defined fluorescence band at 290 nm with a 210 nm excitation. The emission intensity was markedly increased by complexation with Zn2+ which forms a 2 1 (metal-ligand) complex. The fluorescence emission is pH-independent to pH 2. The fluorescence enhancement factor is 5.0 at pH 6 and 50 at pH 8.6 (due to the pH dependence of the free ligand). Ni2+ and Cu2+ ions quenched the ligand fluorescence via a PET mechanism. Furthermore, when cyclophane 86 was coordinated to Cu2+, the molar absorptivity of the transition band observed at 260 nm was increased by a factor of about 10. Such a large spectral change was not observed for the Zn2+ and Ni2+ complexes. In the Cu2+ complex, the two phenyl rings of the cyclophane are expected to be... 

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). 

Recently, there has been a flurry of experimental work investigating the phenomenon of MEF with a range of metal nanoparticle shapes and sizes. In this Section, we provide a brief overview of some recent experiments that have shown quenching and enhancement effects, and demonstrate a correlation between observed effects on fluorescence and the morphology of the nanoparticles employed. 

There are essentially two models that describe the interaction between an excited fluorophore and the SPR of the metal to account for quenching and enhancement of the fluorescence. They both depend on coupling of the fluorophore excited state to the SPR and this is dependent of the spectral overlap of the emission of the fluorophore and the SPR, and the distance between the fluorophore and the metal nanoparticle surface. 

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... 

One common misunderstanding about the underlying physics of the plasmon-enhanced fluorescence is made clear by the distance dependence data. Researchers often cite Fbrster transfer from the molecular chromophore to the electrcxi plasma in the metal as a quenching mechanism for the excited state. If this... 

The presence of at least two fluorophores, and possibly a third, associated with metal ion binding in fulvic acid strongly suggests the need for multiple binding site models. Existing linear and nonlinear models will be reviewed for both fluorescence quenching and enhancement. A new modified 1 1 Stem - Volmer model will be introduced as well as two site and multiple site models. Application of the models to Cu binding by fulvic acid and certain well defined model systems are discussed. 

Fig. 4.6 Relative fluorescence intensity of monolayers TM0-TM4 and L0-L4 in the presence of 10 M solutions of Pb2+, Ca2+, Co2+, and Cu2 as perchlorate salts in acetonitrile. These data have been normalized in the absence of metal cations the maximum fluorescence emission of each layer is set to 0. Positive values correspond to an enhancement in the fluorescence emission intensity of the layer while negative values represent a quenching of the fluorescence emission intensity of the layer.35 Reprinted with permission from35. Copyright 2004 American Chemical Society...

When comparing the patterns produced by the printing of different metal ions, it can be seen that Pb2+ and Ca2+ induced a fluorescence intensity enhancement of the native monolayer, creating a pattern with brighter dots while Co2+ and Cu2+ quenched the initial fluorescence intensity, resulting in a pattern with darker dots (Fig. 4.21a). Ca2+ produced higher fluorescence intensity enhancement than Pb2+. 

---