Fluorometric trace methanol detection in ethanol and isopropanol in a water medium for application in alcoholic beverages and hand sanitizers

Detection of methanol (MeOH) in an ethanol (EtOH)/isopropanol ((PrOH)-Pr-i) medium containing water is crucial to recognize MeOH poisoning in alcoholic beverages and hand sanitizers. Although chemical sensing methods are very sensitive and easy to perform, the chemical similarities between the alcohols make MeOH detection very challenging particularly in the presence of water. Herein, the fluorometric detection of a trace amount of MeOH in EtOH/(PrOH)-Pr-i in the presence of water using alcohol coordinated Al(iii)-complexes of an aldehydic phenol ligand containing a dangling pyrazole unit is described. The presence of MeOH in the EtOH/(PrOH)-Pr-i causes a change of the complex geometry from tetrahedral (Td) to octahedral (Oh) due to the replacement of the coordinated EtOH/(PrOH)-Pr-i by MeOH molecules. The Td-complex exhibited fluorescence but the Oh-species did not, because of the intramolecular photo-induced electron transfer (PET). By interacting the Oh species with water, its one MeOH coordination is replaced by a water molecule followed by the proton transfer from the water to pyrazole-N which generates strong fluorescence by inhibiting the PET. In contrast, the water interaction dissociates the Td-complex to exhibit fluorescence quenching. The water induced reversal of the fluorescence response from the decrease to increase between the absence and presence of MeOH is utilized to detect MeOH in an EtOH/(PrOH)-Pr-i medium containing water with a sensitivity of similar to 0.03-0.06% (v/v). The presence of water effected the MeOH detection and allows the estimation of the MeOH contamination in alcoholic beverages and hand sanitizers containing large amounts of water.

Automated summary

The study presents a fluorescence method for detecting trace amounts of methanol in ethanol/isopropanol medium containing water, using Al(iii) complexes and aldehydic phenol ligands. The presence of methanol changes the complex geometry and fluorescence characteristics, allowing for sensitive detection in the presence of water.