Adsorption of tetracycline antibiotics to gold nanoparticles and feasibility of aptamer-based label-free colorimetric detection

Tetracyclines are a group of very important antibiotics that are still in use to date. To extract, detect and remove tetracyclines from the environment, various nanomaterials have been employed. Although gold nanoparticles (AuNPs) are a commonly cited material for these purposes, fundamental understanding of these tetracycline-AuNP systems are still limited. In this work, the adsorption of tetracycline, oxytetracycline and doxycycline to AuNPs was studied. The effect on the colloidal stability of AuNPs, adsorption kinetics, and the resulting adsorption isotherms were measured. While millimolar concentrations of the tetracyclines can cause aggregation of AuNPs, saturated monolayer adsorption was achieved with low micromolar concentrations of the tetracyclines. Adsorption was instantaneous, and adsorption to AuNPs enhanced their intrinsic fluorescence instead of quenching. With the assumption of aptamer/target complexes cannot be easily adsorbed by AuNPs compared to free aptamers, a label-free colorimetric detection method was tested. While the label-free sensor showed target-dependent aggregation of AuNPs, a non-binding mutant aptamer showed the same trend, suggesting that the color change did not reflect aptamer adsorption but other events such as target adsorption. This study indicates the importance of fundamental understanding of target/AuNP interactions to correctly design aptamer and AuNP-based label-free biosensors.