[en] We report on a sensitive and specific method for the visual and colorimetric detection of 8-hydroxy-2′-deoxyguanosine (8-OHdG) in human urine by using the 8-OHdG-aptamer (Apt) as the recognition element. The aptamer was adsorbed on the surface of gold nanoparticles which enhances their stability. On addition of 8-OHdG, the conformation of the aptamer changes to form a G-quadruplex structure, thereby losing the ability to protect the nanoparticles and causing a color change from red to blue. The conformational changes were also studied by circular dichroism. The response is linearly dependent on the concentration of 8-OHdG in the range from 5.6 to 282 nM, the limit of detection is 1.7 nM, the relative standard deviation ranges from 1.1 to 4.2 % (for n = 6), and the recoveries range from 95.9 to 104.9 %. The method paves the way for visual analysis of samples without the use of costly instruments. (author)