Development of an automatic high-throughput assay for tetracycline determination by using Eu2O3 nanoparticles and dry-reagent technology

The usefulness of europium oxide nanoparticles (Eu2O3 NPs) as analytical reagent for the direct determination of organic compounds is described for the first time. Tetracycline, which forms a luminescent chelate with europium, has been chosen as a model analyte. Dry reagent chemistry is used in a 96-well format, which considerably speeds up the determination and contributes to its automation. The NPs are immobilized onto polystyrene wells by adding a volume of a Eu2O3 NP dispersion in 2-propanol to each well and drying in an oven until they dry completely. At the moment of analysis, a standard or sample volume (200 μL) in the appropriate medium is added, and the mixture shaken for 15 min at 37 °C. The method allows the determination of tetracycline in the range 20-1000 ng mL−1, with a detection limit of 8 ng mL−1. The inter-assay and intra-assay precision, which were assayed at two different tetracycline concentrations and expressed as relative standard deviation, were in the ranges of 6.5-8.2% and 9.2-12.7%, respectively. The study of the selectivity of the system showed that the method is adequate for tetracycline determination in agri-food samples, since most of antibiotics assayed did not interfere the determination. Only other tetracycline antibiotics provided luminescent signal when reacting to Eu2O3 NPs. The method has been applied to the determination of tetracycline in calf urine and in honey samples obtaining recovery values in the ranges of 85.0-110.0% and 99.7-116.7%, respectively.