We have developed a fluorescence turn-on assay using DNA-templated silver nanoclusters (Ag NCs) (i.e., 12 polycytosine-templated silver nanoclusters, dC12-Ag NCs), which is amenable to rapid, ultrasensitive assay of acetylcholinesterase (AChE). The detection mechanism is based on the concept, that is, AChE hydrolyzes the acetylthiocholine (ATCh) chloride to produce thiocholine (TCh). Subsequently, TCh sensitively and rapidly reacts with dC12-Ag NCs via Ag-S bond forming and enhances the fluorescence of dC12-Ag NCs. Using dC12-Ag NCs, detection of TCh has a linear concentration range of 2.0 nM to 16.0 nM and a detection limit of 0.3 nM. Due to the sensitive and rapid fluorescence turn-on response of dC12-Ag NCs to TCh, AChE with activity as low as 0.5 × 10(-4) U/mL (signal/noise = 3) can be analyzed with a dynamic range of 0.1 to 1.25 × 10(-3) U/mL. The promising application of the proposed method in AChE inhibitor screening was demonstrated. AChE concentrations were determined in human blood red cell (RBC) membranes from clinical specimens using dC12-Ag NCs, and the quantitative results were validated with Ellman's method. Aside from the ease of manufacture, reduction of matrix effect, and low background noise, the continuous detection format and detection sensitivity can open up to wider applications to AChE activity assay in neurobiology, toxicology, and pharmacology, among other fields.

The magnitude of fluorescence enhancement was found to depend strongly on the distance between fluorophores and metal nanostructures in metal-enhanced fluorescence (MEF). However, the precise placement of the particle in front of the molecule with nanometer accuracy and distance control is a great challenge. We describe a method using acetylcholinesterase (AChE) to modulate the distance between a gold nanoparticle (AuNP) and the fluorophore 7-hydroxy-9H-(1,3-dichloro-9,9-dimethylacridin-2-one) (DDAO). We found that DDAO is a reversible mixed type-I AChE inhibitor. DDAO binds to the peripheral anionic site and penetrates into the active gorge site of AChE via inhibition kinetics test and molecular docking study. The affinity ligand DDAO bound to AChE which was immobilized onto AuNPs, and its fluorescence was sharply enhanced due to MEF. The fluorescence was reduced by distance variations between the AuNP and DDAO, which resulted from other inhibitors competitively binding with AChE and partly or completely displacing DDAO. Experimental results show that changes in fluorescence intensity are related to the concentration of inhibitors present in the solution. In addition, the nanobiosensor has high sensitivity, with detection limits as low as 0.4 μM for paraoxon and 10 nM for tacrine, and also exhibits different reduction efficiencies for the two types of inhibitor. Thus, instead of an inhibition test, a new type of affinity binding-guided fluorescent nanobiosensor was fabricated to detect AChE inhibitors, determine AChE inhibitor binding mode, and screen more potent AChE inhibitors. The proposed strategy may be applied to other proteins or protein domains via changes in the affinity ligand.

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