Gold nanoparticles based colorimetric aptasensor for theophylline
A simple colorimetric aptasensor for the detection of theophylline was investigated using stable RNA aptamer and gold nanoparticles (AuNPs). Theophylline, a purine derivative, structurally similar to caffeine is broadly used in the treatment of respiratory diseases and bronchial asthma. A highly stable RNA aptamer that selectively binds to the theophylline molecule was prepared by structural modification of the 2′-hydroxyl group of ribose to 2′-fluoro (5′GGC GAU ACC AGC CGA AAA GGC CCU UGG CAG CGU C3′) in all the pyrimidines in a 34-mer length aptamer. The RNA aptamer binds to AuNPs by simple electrostatic interactions thereby inhibiting salt induced aggregation. AuNPs are aggregated due to detachment of the theophylline specific RNA aptamer from the surface of the AuNPs as a result of interaction with the target leading to a color change from red to purple. By successfully optimizing the concentration of aptamer, AuNPs, salts and stability of the aptamer, a detection limit of 50 ng mL−1 theophylline was achieved. Analyses of theophylline in samples were carried out, and the observed recovery was 97–98.7% with a relative standard deviation in the range of 2.69–19.4%. The results obtained were compared with those of an HPLC method. This colorimetric aptasensor is advantageous for onsite visual detection.