A colorimetric gold nanoparticle aggregation assay for malathion based on target-induced hairpin structure assembly of complementary strands of aptamer

The authors describe a method for the colorimetric determination of the pesticide malathion. It is based on the use of a hairpin structure consisting of a complementary strand of aptamer and a double-stranded DNA (dsDNA) structure to protect gold nanoparticles (AuNPs) against salt-induced aggregation. In the absence of malathion, the dsDNA structure is preserved on the surface of AuNPs and the color of the AuNPs in solutions containing NaCl remains red. However, in the presence of malathion, a hairpin structure of complementary strand is formed. The Aptamer/Malathion complex and the complementary strand are released from the surface of the AuNPs. As a result, the AuNPs undergo salt-induced aggregation which is accompanied by a color change to blue. The assay allows malathion to be quantified within 35 min (A(650)/A(520) was measured). The detection limit is 1 pM, and response is linear in the 5 pM to 10 nM malathion concentration range. The method is specific and was successfully applied to the determination of malathion in spiked human serum samples.