Palladium (Pd) is a widely applied transition metal, for example in dental restorations, chemical catalysts, jewellery, electronic equipment, automobile industry, among others. As a consequence of its wide application, high levels of palladium in the environment have been reported, representing a risk to human health. So far, a variety of small fluorescence probes have been successfully developed for palladium detection, but there is an interest to design improved water-soluble probes for recognition of this metal in biological and environmental systems.

Considering the research group´s experience on fluorescent chemosensors based on unnatural amino acid derivatives containing heterocyclic fluorescent and/or coordination units at the side chain, we report herein the evaluation of two benzoxazolyl-alanine derivatives bearing a crown ether moiety as potential fluorimetric chemosensors for palladium detection in aqueous media. Preliminary chemosensory studies for these unnatural amino acids in the presence of selected metal cations, with biological and environmental relevance, were performed in acetonitrile solution and in aqueous mixtures of sodium dodecyl sulfate (SDS, 20 mM, pH 7.5) solution with acetonitrile, 90:10 v/v. As expected, these probes had a fluorescence response for different cations in acetonitrile solutions but, most importantly, in aqueous solutions both crown ether benzoxazolyl-alanine derivatives displayed a selective fluorescence response in the presence of palladium.