A novel optical DNA biosensor for detection of trace amounts of mercuric ions using gold nanoparticles introduced onto modified glass surface.

In this work we report a DNA spectrophotometric biosensor for detection of Hg2+ ions in which a pair of oligonucleotides with four thymine-thymine (T-T) mismatched bases was immobilized onto modified glass surface. Firstly, glass surface modified with 3-(mercaptopropyl) trimethoxysilane (MSPT) and gold nano-particles respectively and then one oligonucleotide (P1) modified with hexanthiol at 5-terminal was immobilized on gold nano-particles via self-assembly and inserted in methylene blue. Methylene blue can intercalate on single strand DNA (ss-DNA) and its absorption peak can measure spectrophotometrically. Then the other oligonucleotide was able to hybridize with P1 by forming thymine-Hg2+-thymine (T-Hg2+-T) complexes in the presence of Hg2+, and absorption signal of methylene blue reduced upon Hg2+ increasing concentration because inaccessibility of guanine base in DNA duplex. However, when Hg2+ was absent, the two oligonucleotides could not hybridize due to the T-T mismatched bases, and P2 could not be fixed on the modified glass surface and any change in absorption peak of methylene blue takes place. The UV-Vis spectrum showed a linear correlation between the absorption peak of methylene blue and the concentration of Hg2+ over the range from 10 nM to 10 μM (R2=0.9985) with a detection limit of 6 nM. This spectrophotometric biosensor could be widely used for selective detection of Hg2+.