Abstract: A method for determining the presence of a compound in a liquid solution, by admixing the liquid solution with a plurality of nanoparticles; providing conditions effective to cause aggregation of the nanoparticles in the liquid solution in the absence of said compound in the liquid solution; and observing a detectable signal reflecting the amount of aggregation of nanoparticles in the liquid solution, wherein the presence of the compound in the liquid solution results in a detectable signal reflecting a reduced amount of aggregation of nanoparticles in the liquid solution, in comparison to the amount of aggregation of nanoparticles obtained in the liquid solution in the absence of the compound therein. A nanoparticle, a composition, a kit and a for multi-well plate for use in the method are also disclosed. In some embodiments the association is cation-, anion and/or PH induced e.g. by using helix-loop-helix polypeptides as first and second molecules attached to the nanoparticles.