Aptamers are nucleic acid based binding molecules that are obtained through a combinatorial selection process known as systematic evolution of ligands by exponential enrichment (SELEX). [1,2] They are emerging as a new class of molecules that can rival antibodies in terms of the broad range of molecules they can selectively bind. [3,4] In comparison with antibodies, aptamers, particularly DNA aptamers, are relatively easy to obtain, more stable to biodegradation, and less vulnerable to denaturation. Therefore they are prime candidates as sensors in a number of applications, such as environmental monitoring and medical diagnostics. The key challenge to their successful application is transforming the aptamer-binding events into physically detectable signals. To meet the challenge, a number of methods have been developed, most of which involve fluorescence-based detection. [5][6][7][8][9][10][11][12][13] Simple colorimetric sensors can eliminate the use of analytical instruments and have attracted much attention recently. [14][15][16][17] For example, organic-dye replacement was employed to design a colorimetric cocaine sensor. [14] However, an appropriate dye has to be found for a designated aptamer, and a waiting time of 12 hours is needed to observe a color change. Cationic conjugated polymers form complexes of different color with aptamers in the presence or absence of a target analyte. A number of colorimetric sensors were made with this method. [15] The high extinction coefficients and distance-dependent optical properties have made metallic nanoparticles very attractive in DNA-related colorimetric assays, [18][19][20] such as the detection of DNA with high sequence selectivity, [21,22] and the detection of metal ions [23,24] and other analytes. [16,17] Recently, aptamer-functionalized gold nanoparticles were employed to detect thrombin. This system took advantage of the fact that each thrombin molecule binds two aptamers, so nanoparticles [