A colorimetric assay for measuring iodide using Au@Ag core–shell nanoparticles coupled with Cu2+

• Au@Ag core–shell NPs were synthesized and coupled with Cu2+ for the colorimetric I− sensing.
• This assay is simple, rapid and selective.
• Au@Ag core–shell NPs-Cu2+ were embedded into agarose gels as test strips.

Au@Ag core–shell nanoparticles (NPs) were synthesized and coupled with copper ion (Cu2+) for the colorimetric sensing of iodide ion (I−). This assay relies on the fact that the absorption spectra and the color of metallic core–shell NPs are sensitive to their chemical ingredient and dimensional core-to-shell ratio. When I− was added to the Au@Ag core–shell NPs-Cu2+ system/solution, Cu2+ can oxidize I− into iodine (I2), which can further oxidize silver shells to form silver iodide (AgI). The generated Au@AgI core–shell NPs led to color changes from yellow to purple, which was utilized for the colorimetric sensing of I−. The assay only took 10 min with a lowest detectable concentration of 0.5 μM, and it exhibited excellent selectivity for I− over other common anions tested. Furthermore, Au@Ag core–shell NPs-Cu2+ was embedded into agarose gels as inexpensive and portable “test strips”, which were successfully used for the semi-quantitation of I− in dried kelps.

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