Front-face fluorescence spectroscopy of tryptophan and fluorescein using laser induced fluorescence and excitation emission matrix fluorescence

Herein we study the responses of two standard molecules, tryptophan and fluorescein, over a large concentration range in boric acid via two front-face fluorescence spectroscopies: excitation emission matrix fluorescence and laser-induced fluorescence. The responses in relation to the tryptophan content were measured and were found to depend on the measurement technique. For fluorescein, no response was found for the excitation emission matrix, and an inverse relationship with the solid content was observed for the measured laser-induced fluorescence. Aiming to enhance the very weak fluorescence response of fluorescein, a novel protocol using a gel interface was established. These methods represent a future premise to investigate molecules with low fluorescence efficiency on surfaces using a logarithmic response.