Homogeneous time-resolved fluorescence assay for the detection of ricin using an aptamer immobilized on europium-doped KGdF4 nanoparticles and graphene oxide as a quencher

Abstract

We have developed an aptamer-based assay for the food toxin ricin. It is based on a competitive quenching strategy along with time-resolved fluorescence detection. The assay involves the following steps: (a) The aptamer is immobilized on the europium-doped KGdF4 nanoparticles (NPs); (b) these NPs are added to the sample where ricin binds to the aptamer; (c) graphene oxide (GO) is added and competitively binds to the aptamer on the NPs that are not blocked by ricin; this causes the quenching of the fluorescence of the NPs; (d) fluorescence is detected at 593 nm in a microplate reader in the time-resolved mode at an excitation wavelength of 273 nm, a delay time of 100 μs, and a gating time of 1 s. Under optimal conditions, the calibration plot is linearly related to the concentration of ricin in the 50 pg·mL−1 to 50 ng·mL−1 range (R2 = 0.9975), and the limit of detection is 8 pg·mL−1. The method was compared to a standard ELISA, and correlation was excellent. The assay presented here provides a sensitive, dependable and convenient platform that is expected to have promising applications for the homogeneous assay of various other target analytes.