Colorimetric biosensing of mercury(II) ion using unmodified gold nanoparticle probes and thrombin-binding aptamer

A colorimetric assay for the determination of mercury(II) (Hg2+) in the presence of lead(II) (Pb2+) was demonstrated with unmodified gold nanoparticles (AuNPs) as probes and 15-mer thrombin-binding aptamer (TBA, 5'-GGTTGGTGTGGTTGG-3') as sensing elements. Upon the addition of Hg2+ or Pb2+, TBA consisting of six thymidine units and nine guanosine units interacted specifically with both ions to form a hairpin-like or a quadruplex structure, respectively. As a result, these conformation changes facilitated the salt-induced AuNP aggregation. Subsequently, to eliminate Pb2+ interference in the determination of Hg2+, a novel technique by the use of a characteristic wavelength of aggregated AuNPs instead of the universal masking agent of Pb2+ (2,6-pyridinedicarboxylic acid, PDCA) was herein proposed. A comparison of the absorption spectra of the aggregated AuNPs in the presence of Hg2+ and Pb2+ showed that the characteristic wavelength of the aggregated AuNPs (800 nm) facilitated the determination of Hg2+ in the presence of Pb2+. The calibration curve showed that the absorbance value at 800 nm increased linearly over the Hg2+ concentration range of 0.39-8.89 microM with a limit of detection of 200 nM. Then, the assay was successfully employed to determine Hg2+ in several water samples.