A simple “clickable” biosensor for colorimetric detection of copper(II) ions based on unmodified gold nanoparticles

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A novel colorimetric copper(II) biosensor has been developed based on the high specificity of alkyne–azide click reaction to the catalysis of copper ions and unmodified gold nanoparticles (AuNPs) as the signal reporter. The clickable DNA probe consists of two parts: an azide group-modified double-stranded DNA (dsDNA) hybrid with an elongated tail and a short alkyne-modified single-stranded DNA (ssDNA). Because of low melting temperature of the short ssDNA, these two parts are separated in the absence of Cu2+. Copper ion-induced azide–alkyne click ligation caused a structural change of probe from the separated form to entire dsDNA form. This structural change of probe can be monitored by the unmodified AuNPs via mediating their aggregation with a red-to-blue colorimetric read-out because of the differential ability of ssDNA and dsDNA to protect AuNPs against salt-induced aggregation. Under the optimum conditions, this biosensor can sensitively and specifically detect Cu2+ with a low detection limit of 250nM and a linear range of 0.5–10μM. The method is simple and economic without dual-labeling DNA and AuNPs modification. It is also highly selective for Cu2+ in the presence of high concentrations of other environmentally relevant metal ions because of the great specificity of the copper-caused alkyne–azide click reaction, which potentially meets the requirement of the detection in real samples.