Magnetic relaxation switch and colorimetric detection of thrombin using aptamer-functionalized gold-coated iron oxide nanoparticles.

We describe a sensitive biosensing system combining magnetic relaxation switch diagnosis and colorimetric detection of human α-thrombin, based on the aptamer-protein interaction induced aggregation of Fe(3)O(4)@Au nanoparticles. To demonstrate the concept, gold-coated iron oxide nanoparticle was synthesized by iterative reduction of HAuCl(4) onto the dextran-coated Fe(3)O(4) nanoparticles. The resulting core-shell structure had a flowerlike shape with pretty narrow size distribution (referred to as "nanorose"). The two aptamers corresponding to human α-thrombin were conjugated separately to two distinct nanorose populations. Once a solution containing human α-thrombin was introduced, the nanoroses switched from a well dispersed state to an aggregated one, leading to a change in the spin-spin relaxation time (T(2)) as well as the UV-Vis absorption spectra of the solution. Thus the qualitative and quantitative detection method for human α-thrombin was established. The dual-mode detection is clearly advantageous in obtaining a more reliable result; the detection range is widened as well. By using the dual-mode detection method, a detectable T(2) change is observed with 1.0 nM human α-thrombin, and the detection range is from 1.6 nM to 30.4 nM.