Ultrasensitive covalently-linked Aptasensor for cocaine detection based on electrolytes-induced repulsion/attraction of colloids.

A quick and easy colorimetric sensor based on gold nanoparticles (GNPs) and aptamers for the detection of cocaine was developed. The sensor was named as 'GAPTA' and showed extremely interesting results regarding cocaine detection with a sensitivity to doses of 0.2 nM. The experimental approach consisted of creating a conjugate between GNPs (10 nm size) and aptamers as a sensing base with the addition of an electrolyte (NaCl) that plays the role of aggregation inducer. In the absence of the aptamer, the electrolyte was able to induce aggregation of the GNPs turning the color of the solution from red to blue while the presence of the aptamer is able to hinder the charges attraction and protects the GNPs from aggregating. The optimization of the aptamer and electrolyte concentration was determined to be 118 nM and 55 mM, respectively, and the resultant GAPTA sensor had a detection limit of 0.97 nM. Furthermore, the selectivity of the platform was tested in the presence of different interferents and showed a specific response towards cocaine while interference ranged between 20 and 40%. The applicability of the GAPTA biosensor was tested on synthetic saliva and demonstrated a sensitivity range between 0.2 and 25 nM. These results suggest the potential of the current colorimetric sensor in abuse drugs screening and creates a stable base for new routine platforms for biomedical and toxicology applications. Graphical abstract.