Multipath colourimetric assay for copper(II) ions utilizing MarR functionalized gold nanoparticles.

We use the multiple antibiotic resistance regulator (MarR), as a highly selective biorecognition elements in a multipath colourimetric sensing strategy for the fast detection of Cu in water samples. The colourimetric assay is based on the aggregation of MarR-coated gold nanoparticles in the presence of Cu ions, which induces a red-to-purple colour change of the solution. The colour variation in the gold nanoparticle aggregation process can be used for qualitative and quantitative detection of Cu by the naked eye, and with UV-vis and smartphone-based approaches. The three analysis techniques used in the multipath colourimetric assay complement each other and provide greater flexibility for differing requirements and conditions, making the assay highly applicable for Cu detection. Under optimal conditions, the Cu concentration was quantified in less than 5 min with limits of detection for the naked eye, UV-vis and smartphone-based approaches of 1 μM, 405 nM and 61 nM, respectively. Moreover, the sensing system exhibited excellent selectivity and practical application for Cu detection in real water samples. Thus, our strategy has great potential for application in on-site monitoring of Cu, and the unique response of MarR towards copper ions may provide a new approach to Cu sensing.