Fast colorimetric sensing of adenosine and cocaine based on a general sensor design involving aptamers and nanoparticles.

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). 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. 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. Simple colorimetric sensors can eliminate the use of analytical instruments and have attracted much attention recently. For example, organic-dye replacement was employed to design a colorimetric cocaine sensor. 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. The high extinction coefficients and distance-dependent optical properties have made metallic nanoparticles very attractive in DNA-related colorimetric assays, such as the detection of DNAwith high sequence selectivity, 22] and the detection of metal ions 24] and other analytes. 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