Visual detection of copper(II) by azide- and alkyne-functionalized gold nanoparticles using click chemistry.

We report a method for the detection of Cu ions by azideand terminal alkyne-functionalized gold nanoparticles (Au NPs) in aqueous solutions using click chemistry. The catalyst, Cu(I), was conveniently derived from the reduction of Cu(II) in the presence of sodium ascorbate. This method allows the naked eye, without the aid of any advanced instrument, to assay for the presence of Cu ions by the aggregation of Au NPs as a result of the Cu(I)-catalyzed conjugation between the two functional groups. Copper is a transition metal essential for life but also highly toxic to organisms, such as certain algae, fungi and many bacteria and viruses. In recent years, copper has been suspected of causing liver damage in children. The analysis and measurement of copper in environmental and biological samples have become increasingly important. Several methods exist for the detection of Cu ions, for example, those based on organic fluorophores or chromogenic sensors, quantum dots, atomic absorption spectroscopy, inductively coupled plasma mass spectroscopy, absorbance spectro-photometry, peptides and voltammetry. The color changes associated with the aggregation of metal nanoparticles has led to the development of a number of assays for a variety of target species. Colorimetric methods can be convenient and attractive in many applications because they can be easily monitored with the naked eye, without the aid of any advanced instruments. The extinction coefficient of 13 nm-diameter gold nanoparticles is 2.7 4 10m 1 cm , several orders of magnitude more than those of traditional organic chromophores. As a result, colors arising from nanoparticles at nanomolar concentrations can be observed by the naked eye, allowing sensitive detection of small amounts of analytes. Since Cu(I) is used as a catalyst in the cycloaddition reaction between azides and alkynes in click chemistry based on Huisgen6s reaction, the amount of copper needed for its completion is typically small. Therefore, a method that can visualize the progress of the reaction using the aggregation of Au NPs might also be useful for the detection of trace amounts of Cu(II) (by detection of Cu(I)). Because the azide/alkyne functional groups and their conjugation are highly selective and are essentially inert to most biological molecules, oxygen, water, and the majority of common reaction conditions in chemical synthesis, and are tolerant of a wide range of solvents, temperatures, and pH values, we reasoned that an assay based on such chemistry may find myriad uses. Our method for the detection of Cu ions relies on the Cu(I)-catalyzed 1,3-dipolar cycloaddition of alkynes and azides on the surface of functionalized Au NPs, that results in the aggregation of Au NPs (Scheme 1). We synthesized azideand terminal alkyne-functionalized thiols, 1 and 2, and prepared gold NPs coated with these