Fluorescent diazapyrenium films and their response to dopamine

Experimental protocols for the preparation of 2,7-diazapyrenium films on glass, quartz, and silica in one or two steps have been developed. The one-step procedures involve the adsorption of preformed 2,7-diazapyrenium dications with trimethoxysilane appendages to the hydroxylated substrates. The two-step procedures consist in the formation of interfacial polysiloxanes with pendent chloromethyl groups and their subsequent coupling to monoalkylated 2,7-diazapyrene derivatives. For the modification of the glass slides, the silane building blocks have been copolymerized with Si(OEt)4. The transmission absorption spectra of the coated glass and quartz slides all reveal the characteristic bands of the 2,7-diazapyrenium chromophores. Combustion analyses confirm the adsorption of the 2,7-diazapyrenium dications on the silica particles. A comparison of the surface coverages of all films indicates that the one-step procedures are significantly more efficient than their two-step counterparts. Furthermore, the copolymerization of the silane building blocks with Si(OEt)4 translates into an increase in 2,7-diazapyrenium surface coverage of approximately 1 order of magnitude. The emission and excitation spectra of all modified substrates reveal the characteristic bands of the 2,7-diazapyrenium fluorophores. The fluorescence quantum yield, however, decreases as the surface coverage increases. Presumably, interactions between adjacent fluorophores encourage nonradiative deactivation pathways. With the exception of the glass slides modified in two steps, all films respond to the presence of dopamine, in aqueous environments at neutral pH, with pronounced decreases in emission intensity. The association of the 2,7-diazapyrenium acceptors and dopamine donors at the solid/liquid interface is responsible for fluorescence quenching. The glass slides and silica particles modified in one step are the most sensitive substrates and respond to sub-millimolar concentrations of dopamine with large changes in emission intensity. Furthermore, their fluorescence is not affected by relatively large concentrations of ascorbic acid, which is the main interferent in conventional dopamine detection protocols. Thus, these results demonstrate that the supramolecular association of 2,7-diazapyrenium dications and pi-electron rich substrates can be reproduced successfully at solid/liquid interfaces and suggest that the unique properties of 2,7-diazapyrenium films might lead to dopamine-sensing schemes based on fluorescence measurements.