Label-free Colorimetric Detection Of Certain Targets Based On DNA Oligonucleotides And Nanoparticles

In this dissertation, the work is focused on developing label-free colorimetric assay methods based on DNA/nanoparticles. (1) Taking advantage of T-Hg²⁺-T coordination chemistry as well as the different interactions between ssDNA/AuNPs and dsDNA/AuNPs, a colorimetric detection method for Hg²⁺ was developed. Complementary DNA stands with certain number of T-T mismatches could not hybridize thus retained random-coil single-stranded state, which could effectively protect AuNPs from salt-induced aggregation. Hg²⁺-mediated T-T base pairs lead to the formation of duplexes and the rigid structures lost the protecting ability for AuNPs, thus AuNPs aggregated after salt addition, accompanying by a color change from red to blue. The assay region, linear range and detection limit could be tuned by rationally varying the number of T-T mismatches in DNA duplexes. A low proportion of T-T mismatches accounted for a sensitive detection range and a low detection limit, while a high proportion brought an enlarged detection range and a high upper detection limit. This strategy might be potentially enlarged to distinguish other metal ions, ligands or drugs interacting specially with certain bases in nucleotides. (2) Based on an investigation into unmodified DNA adsorption onto AgNPs, unmodified DNA and AgNPs were utilized to develop a novel colorimetric method for distinguishing ligands binding to homoadenine (here coralyne was used). Adenine deoxynucleoside showed a high affinity to AgNPs and the homoadenine sequence stabilized nanoparticles effectively. The presence of ligands binding to homoadenine could bind to the adsorbed sequences and take them away from the surface of AgNPs, thus the AgNPs were less well protected and aggregated after salt addition, accompanying by a color change from yellow to brown. The results demonstrated that a label-free colorimetric assay based on DNA/AgNPs could be achieved. Meanwhile, the targets could be small molecules rather than oligonucleotides in DNA/AgNPs systems.