Label-free colorimetric biosensing of copper(II) ions with unimolecular self-cleaving deoxyribozymes and unmodified gold nanoparticle probes

Using unimolecular copper(II)-dependent self-cleaving deoxyribozymes (DNAzymes), a label-free colorimetric biosensor for copper(II) ions (Cu2+) has been developed based on the sequence-length-dependent adsorption of single-stranded deoxyribonucleic acid (ssDNA) on unmodified gold nanoparticles (AuNPs). In the presence of Cu2+, the Cu2+-dependent DNAzyme could be self-cleaved into short ssDNA fragments. The cleaved short ssDNA could adsorb rapidly onto the surface of the AuNPs. This enhanced the stability of the AuNPs against salt-induced aggregation, and thus the solution color remained red. In the absence of Cu2+, however, uncleaved long ssDNA adsorbed relatively slowly onto the AuNPs and upon the addition of salt, the electrostatic repulsion between the AuNPs was screened, resulting in aggregation of the AuNPs which produced a red-to-blue color change. Thus, Cu2+ detection could be realized by monitoring the color change of the AuNPs. The calibration curve showed that the absorption ratio values at 520 and 620 nm increased linearly over the Cu2+ concentration range of 0.625-15 μM, with a limit of detection of 290 nM. The other environmentally relevant metal ions did not interfere with the determination of Cu2+. Subsequently, the assay was employed to determine Cu2+ in several water samples, and the results were satisfactory. It is expected that the present colorimetric strategy will be possibly extended to the detection of cofactors of other in vitro-selected unimolecular self-cleaving DNAzymes, such as amino acids, nucleic acids, metal ions and small organic molecules.