A technique relates to determining a presence of a known nucleic acid sequence of a known molecule in a sample. A sample surface is coated with a select segment of the known molecule. Beads are coated with a first molecule. A targeted nucleic acid sequence is attached to a second molecule that binds to the first molecule, such that the targeted nucleic acid sequence is attached to the beads via first and second molecules. The sample is placed on the sample surface. The sample includes the liquid medium, beads, and targeted nucleic acid sequence being tested. Brownian motion of beads is monitored to determine whether the Brownian motion of the beads is restricted or not restricted. When the Brownian motion of beads is restricted, the presence of the known nucleic acid sequence is in the sample, thus indicating that the targeted nucleic acid sequence is the known nucleic acid sequence.