A large number of analytes, including non-fluorescent ones, can be sensitively detected by fluorescence scanning densitometry using silica gel HPTLC plates impregnated with a solution of coralyne cation. This is carried out by the variation, increase or decrease, that the corresponding analyte induces on native coralyne emission at a given excitation wavelength. A similar phenomenon was previously described for berberine cation, and Reichardt's dye probes. However, the sensitivity of coralyne in HPTLC detection of non-fluorescent, structurally different analytes (e.g., long-chain alkanes, alcohols, alkylbromides, neutral lipids) is superior to that of the above-mentioned probes. In this work, the analytical viability of this phenomenon for HPTLC detection using coralyne as a probe is explored, and fluorescent responses of a number of analytes on the coralyne system are rationalized in the light of a previously proposed model. This establishes that the resulting intensity for a probe in the presence of a given compound can be explained as a balance between radiative (contribution of non-specific interactions) and non-radiative processes (specific interactions), the latter producing fluorescence quenching. Experimental results and proposed model suggest that this phenomenon may be general for practically all kinds of analytes.

Changes in fluorescence emission due to non-covalent analyte-fluorophore interactions in silica gel plates are studied and used as a general detection procedure for thin-layer chromatography (TLC). The presence of the analyte modifies the microenvironment of the fluorophore and thus changes the balance between radiative (k(r)) and non-radiative (k(nr)) emission constants. A model is proposed for analyte-fluorophore induced electrostatic interactions, which depend on analyte polarizability and are responsible for fluorescence enhancements. As consequence of these induced interactions, the analyte creates an apolar environment that prevents non-fluorescent decay mechanisms, decreasing k(nr). On the other hand, the effect of an increase in refractive index on k(r) is investigated, as it contributes to some extent to fluorescence enhancements in silica gel medium. Changes in fluorescence emission should be regarded as a general property of fluorophores in the presence of analytes, and criteria that fluorophores should meet to be used as sensitive TLC probes are discussed here.

In this paper a mathematical model describing the non-specific interactions of the medium surrounding a fluorophore on its fluorescence intensity is proposed. The model, which has been developed for quantitative analytical applications, is based on the following general ideas: (1) the medium affects the fluorescence quantum yield across the non-radiative decay constant (k(nr)); (2) the k(nr) can be simplified to the singlet-to-triplet intersystem crossing (k(ISC)) constants; (3) k(ISC) follows the energy gap law and then depends on the singlet and triplet energy difference, and (4) the medium, due to solvation, changes the energy of both excited levels (singlet and triplet), then the constants and finally the fluorescence intensity. In our model, the strength of the fluorophore solvation by the solvent (represented by its refraction index, n, dielectric constant, epsilon, and electric charge) changes the singlet (excited)-to-fundamental and the singlet-to-triplet energy gaps, thus the k(ISC) and k(IC) (internal conversion constant) values and in consequence the fluorescence quantum yield. The final model relates the fluorescence intensity (F) with the solvent dielectric constant and refraction index. Finally, the model is particularized for the case of a medium composed of a solvent and a solute, obtaining an F-to-solute concentration relationship and enabling this fact to be used for analytical applications. The very first experimental data are shown demonstrating the fulfilment of this model.

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