Aptamer based fluorometric acetamiprid assay using three kinds of nanoparticles for powerful signal amplification

The authors describe an aptamer-based fluorometric assay for the insecticide acetamiprid. It is based on target-induced release of the fluorescein-labeled complementary strand of the aptamer (CS) from the aptamer/CS conjugate (dsDNA). Three kinds of nanoparticles with opposite effects on the fluorophore (FAM) were used. These include gold nanoparticles (AuNPs), single-walled carbon nanotubes (SWNTs) and silica nanoparticles (SiNPs) coated with streptavidin. In the presence of acetamiprid, FAM-labeled CS is released from the dsDNA-modified SNP-streptavidin complex and accumulates in the supernatant (phase I) after centrifugation. Fluorescence intensity decreases on addition of the supernatant to the SWNTs and AuNPs because they act as quenchers (phase II). In the absence of acetamiprid, the dsDNA-modified SiNP-streptavidin complex remains intact and no labeled CS is present in the supernatant containing the AuNPs and SWNTs. So, the relative fluorescence intensity is quite low. The assay is highly selective for acetamiprid and has a limit of detection (LOD) as low as 127 pM. The method was successfully applied to the determination of acetamiprid in spiked serum and water where it gave LODs of 198 and 130 pM, respectively.