In situ kinetic studies and derivatives synthesis of the latent fingermark reagent DFO

Abstract

1,8-diazafluoren-9-one (DFO) is a commonly used latent fingermark reagent that reacts with the amino acids from eccrine sweat deposited on porous substrates. The mechanism of reaction of DFO is known with two molecules of DFO reacting with one molecule of an amino acid to form a faintly red product that is high fluorescent. However the optimal conditions still need to be investigated. The reaction of DFO with amino acids in an ethanol-acetic acid solution was investigated in the temperature range 40-70°C. The reaction showed a lag time prior to the colour development that ranged from 2 min to 15 min. After this, colour development occurred over 60-120 min for reagent concentrations in the range 3-7x10⁻⁴ M. DFO imine reacted much more rapidly than DFO and showed no lag time before reaction. Its reaction with phenylalanine proceeded via an intermediate that absorbed strongly at 400 nm. Preheating DFO in the solution can increase the speed of reaction of DFO presumably because it generates the more reactive hemiketal form. An optimised protocol was developed to measure the reaction of DFO with amino acids on the surface of cellulose plates. The reaction of DFO with alanine and phenylalanine gave a constant absorbance within 10 min at 100°C, although the application of additional solvent allowed the reaction to continue a little further. Placing a hot glass cover on top of the surface of the cellulose plate resulted in higher final absorbances at 100°C, compared to the absence of the glass cover, However, experiments without the glass cover were more reproducible. Decreasing the temperature decreased both the rate and final intensity of the colour development as measured at 557nm. Attempts to prepare a fluorinated DFO derivative via ether formation with a perfluoro alcohol were unsuccessful, even with an ethoxylated perfluoro alcohol. Instead, a fluorinated amide derivative was prepared via 2-amino DFO acetal. However, due to the low yield of the product full characterisation could not be performed.