Aiming the prediction of ionic liquids' (ILs) human toxicity, an automated carboxylesterase activity assay was developed. The method was implemented on a sequential injection analysis (SIA) system and relied on the hydrolysis of 4-methylumbelliferyl acetate by the enzyme, to produce the fluorescent compound 4-methylumbelliferone (λ(exc)=365 nm; λ(em)=460 nm). The adopted approach was based on activity/inhibition assays in which the reduction of enzyme activity was denoted by a decrease of fluorescence. The activity assays were performed in aqueous media and in the presence of increasing concentrations of seven commercially available ILs and sodium bis(p-nitrophenyl) phosphate, a strong inhibitor of carboxylesterase. The inhibitory effects were quantified by means of the calculation of half maximal inhibitory effective concentration (EC(50)) values that were between 38 and 134 μmol L(-1). This allowed us to perform some considerations about the toxicity of the compounds. The less and more toxic ILs found in this study were emim [Ms] and tbph [Ms], respectively. The developed SIA methodology is robust and exhibits good repeatability in all the tested conditions (rsd<5.0%, n=10). At the same time a reduction of costs was attained by reduced consumption of reagents and minimal effluent production (2 mL per cycle).
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