A homogenous “signal-on” aptasensor for antibiotics based on a single stranded DNA binding protein-quantum dot aptamer probe coupling exonuclease-assisted target recycling for signal amplification

Abstract

A novel fluorescent “signal-on” switch aptasensor based on a single stranded DNA binding protein (SSB) labeled quantum and exonuclease-assisted target recycling was designed for detecting antibiotics in homogeneous media. In this assay, streptomycin (STR) was employed as a model and the fluorescent probe was synthesized by labeling SSB on quantum dots (QDs–SSB). The SSB can specifically bind an aptamer. When an aptamer serving as a bridging ligand is added, it can hybrid with SSB. Meanwhile, the quantum dots dispersed in the solution were aggregated, which resulted in self-quenching of the QDs' fluorescence and its intensity decreased sharply and switched from “on” to “off”. When the fluorescent probe coexisted with STR and Exo I, the aptamer preferentially bound with a target, and the aptamer-target was digested into mononucleotides by Exo I. Then the liberated target could be further involved in the reaction cycling to produce a strong fluorescence signal. Consequently, the distance of QDs increased and the fluorescence intensity was recovered. Thus, the switch changed from the “off” state to “on”. Under optimized conditions, the assay indicates good linear relationship in a range from 0.1 to 100 ng mL⁻¹ and the detection limit of STR was 0.03 ng mL⁻¹ (S/N = 3). And after 8 times of detecting streptomycin, the fluorescence intensity of the system was still able to reach the initial 90.19% efficiency. In addition, this type of switch fluorescent probe provides a simple and specific approach for antibiotics detection especially suitable for a homogeneous system.