Fluorescence enhancement mechanism

The periodic arrangement adds two additional fluorescence enhancement mechanisms - increased excitation intensity due to SPP excitation (34) and modification of the radiation emission pattern. The latter effect has been referred to as "beaming" (56), or more appropriately for fluorescence, surface plasmon coupled emission (SPCE) (57), and results from the coupling of multiple nanocavities via the interstitial surface regions. 

BPLP HD 8 CUBPLP fluorescent, enhanced mechanical properties vascular graft, bioimaging [48]... 

It is well known that flnorescence from an RP-18 phase is much brighter than from a silica gel plate, because the coating of RP-18 material blocks nomadiative deactivation of the activated sample molecules. By spraying a TLC plate with a viscous liquid, e g., paraffin oil dissolved in hexane (20 to 67%), the fluorescence of a sample can be tremendously enhanced. The mechanism behind fluorescence enhancement is to keep molecules at a distance either from the stationary layer or from other sample molecules [14]. Therefore, not only paraffin oil, but a number of different molecules show this enhancement effect. 

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. 

Intercalators associate with dsDNA by insertion between the stacked base pairs of DNA [52], EtBr binds to dsDNA with little to no sequence specificity, with one dye molecule inserting for every 4-5 base pairs [53]. It also binds weakly via a non-intercalative binding mechanism only after the intercalative sites have been saturated [54], Propidium iodide (PRO) is structurally similar to ethidium bromide, and both dyes show a fluorescence enhancement of approximately 20-30 fold upon binding to dsDNA [41]. As well, their excitation maxima shift 30-40 nm upon binding due to the environment change associated with intercalation into the more rigid and hydrophobic interior of the double-stranded nucleic acid structure relative to aqueous solution [41]. 

The positive response of 82 offers evidence for the luminescent PET signaling of irreversible interactions. More specifically, it underpins the previously known, but empirically designed, lumophore-spacer-maleimide systems 83 [158] and 84 [159] operating at shorter communication wavelengths. In addition, the present results help to explain the larger thiol-induced fluorescence enhancements found in 85 [160] with unassigned mechanism of action. The latter are twisted lumophore-maleimide systems. Their virtual Co spacers ensure rapid PET rates and, subsequently, low fluorescence quantum yields in the absence of thiol. 

A simple, rapid, sensitive, and selective spectrofluorimetric method (2ex/ lem = 345/455nm) has been developed for the determination of zaleplon. Tang et al. have studied the influence of micellar medium on the absorption, fluorescent excitation, and emission spectra character of zaleplon The nonionic surfactant of Triton X-100 showed a strong sensitizing effect for the fluorescence of zaleplon in a pH 5.0 buffer. The possible enhancement mechanism was discussed. Based on the optimum conditions, the linear range was 1.32 x 10 8-1.00 x 10 mol/1. The detection limit was 4.0 x 10 mol/1 with a relative standard deviation (RSD) of 0.06%. The proposed method was successfully applied to the determination of zaleplon in tablets, serum, and urine. 

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... 

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]. 

Of the two different types of plasmonic enhancement which were described in section 6.1, the emphasis here is on the excitation enhancement mechanism. There is a linear dependence of the excitation rate of a fluorescent dye on the intensity of the excitation light in the direction of the electric dipole, e, of the molecule. When the dye molecule is located near the NP, the electric field acting on the dipole changes from Ei to Ej + E,. In this case, the excitation enhancement factor, fjoh. for one dye molecule is defined as a ratio of intensities ... 

So, for all three dyes, most fluorescence intensity is observed when the dye emission peak is red-shifted from the SPR peak. In addition, it would seem that the optimal SPR location is between the absorption and emission maxima of the dyes, since for two of the three dyes studied, the maximum brightness occurs when the SPR peak is in between the dye absorption and emission maxima. This could be explained if both the dye excitation and emission rates are being enhanced. This is not unexpected as enhancement of fluorescence by increasing the excitation of nearby fluorophores would be the main enhancement mechanism for fluorophores that have a high intrinsic quantum yield. 

The exact mechanism leading to enhanced fluorescence signal on ZnO NRs is not clearly understood yet and requires further study. This section provides discussion on some plausible mechanisms for the aforementioned ZnO NR-enabled fluorescence enhancement effect. 

Additional topics covered in this chapter are possible mechanisms leading to the fluorescence enhancement effect observed on ZnO NRs and key advantages of ZnO NR platforms in biomolecular detection. 

The two SERS bands of eosin at 292 and 500 cm" are developed in spectrum 1 on Fig. 2b. It indicates the major plasmon-dependent mechanism of fluorescence enhancement. Besides the plasmon enhancement, the interference effect between two silver surfaces is possible. We suggest that both these effects are responsible for an observable phenomenon. But more sensitive parameters for the secondary emission enhancement can be tuned through engineering of LP band position and optical density. The low and disadvantageous spectral overlap of LP and molecular absorption bands, as well as the silver deposition excess lead to the significant quenching of analyte fluorescence. 

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