Polyamide fluorescence emission

Figure 7. Oxidative polyamide fluorescence emission spectrum from oxidized soy lecithin liposomes (excitation wavelength, 360 nm).

Figure 8. Oxidative polyamide fluorescence emission spectra in packaged foods.

Figure 6. Fluorescence emission spectrum for polyamide on plastic facing oxidizing linoleic acid. Conditions 69 °C for 20 h.

Hydrophilic liquids can also cause stabilization and amplification of fluorescence Thus, Dunphy et al employed water or ethanol vapor to intensify the emissions of their chromatograms after treatment with 2, 7 dichlorofluorescein [260] Some groups of workers have pointed out that the layer matenal itself can affect the yield of fluorescent energy [261 —263] Thus, polyamide and cellulose layers were employed m addition to silica gel ones [245] The fluorescence yield was generally increased by a factor of 5 to 10 [264], but the increase can reach 100-fold [234, 265]... 

The similarities are obvious. In all cases, excitation maximum is at 356 nm and emission at about 422 nm. The blank shown is the spectrum from a polyamide plate exposed in a similar situation but without lipid. It shows the residual of the scatter peak at 360 nm which is not removed by the 39 filter. There is also a pattern of diffraction peaks produced by the polyamide (and, indeed, by any fine powder-coated surface like silica TLC plates). The wavelengths of this diffraction pattern are excitation wave-length-dependent, unlike the situation in normal fluorescence, a given peak of which is conservative in wavelength with changes in... 

Amino acid derivatives are best extracted from silica gel layers with methanol or with a mixture of methanol/ 25% NH3 (95 5) (7,10,17] or chloroform/methanol/ acetic acid (7 2 2) [77]. Peptides are extractable with acetone/water (1 1) [78], and amine derivatives are extractable with less polar solvents, e.g. ethyl acetate, benzene/acetic acid (99 1), benzene/triethylamine (95 5) [10,17], or with dioxane if, as well as fluorescence, radioactivity is to be measured by liquid scintillation counting [80]. Chloroform is suitable for the extraction of Dns-amino adds from polyamide sheets [40]. Exdt-ation and emission wavelengths are, if possible, adjusted to those of the individual Dns-deiivatives however, for almost every compound, exdtation can generally be achieved using the 365 nm mercury line. 

Bns-Cl is an analogue of Dns-Cl. It is somewhat more stable in storage, but its reaction with amines is slightly slower. The fluorescence excitation maxima of its derivatives are shifted a little towards longer wavelengths and emission maxima towards shorter wavelenghts, and the fluorescence eflfidency is about 10% higher compared with Dns-derivatives. Its uses are the same as those of Dns-Cl [ 165). A suitable solvent system has been reported for the complete separation on polyamide sheets of the Bns derivatives of all the amino adds normally found in... 

A different approach to dendritic sensors involves modification of a sensor core unit with dendritic substituents to confer beneficial solubility properties. An example of a sensor core unit is the porphyrin macrocycle, a heterocycle that has been employed extensively in prototypical photochemical sensor systems. Vinogradov and co-workers have exploited the versatile photoactive porphyrin sensor unit as a fluorescence-based pH indicator for use in biological assays [73], by attaching acid terminated polyamide-ether dendrons as substituents (Figure 8.12). The two imino nitrogen atoms present in the free-base porphyrin are susceptible to stepwise protonation to afford initially a cation and then a dication, respectively. Upon protonation, both the emission and absorption fluorescence spectroscopic characteristics of the porphyrin core are subject to dramatic hypochromic shifts. This spectroscopic phenomenon formed the basis for an accurate pH indicator with potential applications in proton gradient determination studies in biological systems. 

An interesting result was also obtained (10) in a study of polyamides with stilbene residues in the chain backbone. In that case,an increasing polymer concentration led to a decreasing fluorescence intensity. This effect can be understood since the trans-cis isomerization and the emission from the excited stilbene moiety compete with each other. Thus, an increasing concentration of chain molecules which hinders the photoisomerization favors the fluorescence. This effect is observed long before the systems becomes glassy and it is, therefore, distinct from the enhancement of fluorescence in rigid media in dyes whose emission is quenched by internal motions of the excited molecules (11). 

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