Gel electrophoresis separation and origins of light emission in fluorophores prepared from citric acid and ethylenediamine

We investigated light emission of hydrothermally treated citric acid and ethylenediamine (EDA) with various precursor ratios using gel-electrophoresis. We show that this relatively simple approach can deliver significant insights into the origins of photoluminescence. We found that products of the synthesis consist of both positively and negatively charged species and exhibit large dispersion in electrophoretic mobility (i.e. charge-to-size ratio). We observed that despite the large dispersion of the reaction products the blue light emission is confined to discrete bands clearly identifiable in the gel. We demonstrate clear evidence that this emission originates from the negatively charged light molecular fraction with the highest mobility which shows no excitation-dependent light emission. This molecular fluorophore exhibits spectral characteristics similar to previously reported 1,2,3,5-tetrahydro-5-oxo-imidazo[1,2-a]pyridine-7-carboxylic acid (IPCA). Secondary gel electrophoresis run performed on the bands extracted from the first run indicates that no further separation takes place. On the basis of our experimental results, we conclude that relatively stable binding exists between IPCA and EDA-derived product. Thus, the products of the reaction contain IPCA both in molecular form and in complexes with EDA-derived products. We conclude that excitation-dependent emission is related to the fluorophore binding to the positively charged EDA-derived products with a positive charge.