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

•A novel optical DNA biosensor for sensitive determination of Hg2+ was reported.•The biosensor showed good sensitivity, very high selectivity, simple, and cheap.•The absorption peak was linear to Hg2+ concentration from 10 nM to 10 μM.

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+.

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