Fig. 1.

Here we demonstrate a sensitive colorimetric assay for the detection of DNA. We find that, whereas a mixture of a positively charged, water-soluble, conjugated polyelectrolyte and single-stranded DNA leads to the aggregation of gold nanoparticles (and, consequently, a readily detectable change in their color), admixtures of this polymer with double-stranded DNA does not similarly produce a color change. (A) Here we have used this effect as the basis of a ready colorimetric assay for the detection of specific oligonucleotides. The assay is rapid and sensitive: (< 125 attomoles of target in 10 μL solution differentiates a color change observable with the naked eye in less than 10 min. (B) According to the scheme, the solution only contains a single-stranded probe that makes the gold nanoparticles aggregate (blue), while the probe–target duplex keeps the gold nanoparticles stable (red). (C) The ratio A₅₂₀/A₇₀₀ for a perfect match target is much larger than no target indicating the specificity of our assay. (D) By calculating the ratio A₅₂₀/A₇₀₀ form the UV-Vis test we could measure the target concentration, which is also crucial for detection. Shown is our ability to specifically detect target DNA molecules at both E, high (125 nM), and F, (1.25 pM) concentrations.