Fluorescence biosensing strategy based on mercury ion-mediated DNA conformational switch and nicking enzyme-assisted cycling amplification for highly sensitive detection of carbamate pesticide. 
Pesticides are of great importance in agricultural and biological fields, but pesticide residues may harm the environment and human health.  A highly sensitive fluorescent biosensor for the detection of carbamate pesticide has been developed based on acetylcholinesterase (AChE)-catalyzed hydrolysis product triggered Hg(2+) release coupled with subsequent nicking enzyme-induced cleavage of a duplex DNA for cycling amplification.  In this protocol, two DNA probes, an unmodified single-stranded helper DNA probe 1 (HP1) and a quencher-fluorophore probe (QFP) are ingeniously designed.  HP1 can be folded into hairpin configuration through T-Hg(2+)-T base pair formation.  QFP, labeled with FAM and BHQ1 at its two terminals, contains the recognition sequence and the cleavage site of the nicking enzyme.  In the presence of carbamate pesticide, the activity of AChE is inhibited, and the amount of the product containing the thiol group generated by the hydrolysis reaction of acetylthiocholine chloride (ACh) decreases, resulting in the release of a low concentration of Hg(2+).  The number of HP1 that can be selectively unfolded would be reduced and the subsequent nicking enzyme-assisted cleavage processes would be affected, resulting in decreased fluorescence signals.  The fluorescence intensity further decreases with the increase of the pesticide concentration.  Therefore, the pesticide content can be easily obtained by monitoring the fluorescence signal change, which is inversely proportional to the logarithm of the pesticide concentration.  The detection limit of aldicarb, the model analyte, is 3.3 μgL(-1), which is much lower than the Chinese National Standards or those previously reported.  The as-proposed method has also been applied to detect carbamate pesticide residues in fresh ginger and artificial lake water samples with satisfactory results, which demonstrates that the method has great potential for practical application in biological or food safety field.