Digital One-Step Competitive Detection of a Small\nMolecule in Synthetic and Environmental Waters

Optical methods for single-molecule\nanalysis hold the promise of\naccurate, sensitive, and rapid detection of target molecules. Here,\nwe demonstrate the efficiency of such an approach for the competitive\ndetection of small molecules in water. Our biosensing method is based\non a combination of a single-DNA biochip for the parallelization of\ntethered particle motion real-time measurements with antibodies and\nmodified targets as molecular competitors. The antibodies are coupled\nto the particles tethered to the surface by a long DNA bearing in\nits middle the molecular competitor bound to the antibodies. Competitive\ntarget binding leads to a detectable conformational change of the\nDNA tethers from looped to unlooped in proportions related to the\ntarget concentration. We thus managed to detect fluorescein, chosen\nas a model of a target molecule, in freshwater of various qualities,\nfrom solutions prepared with ultrapure water to more complex matrices\nsuch as river water and wastewater treatment plant effluent samples.\nSimilar dose–response curves were obtained under these various\nconditions in a wide range of concentrations from nanomolar to micromolar\nwith a limit of detection around 2 nM.