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Here we have developed a facile and rapid colorimetric method for the sensitive and selective detection of Ag+ based on the non-aggregation of gold nanoparticles (Au NPs) capped with hyperbranched polyethylenimine derivatives. In the detection process, an unusual colour change from red to brown was observed due to the formation of Au–Ag core–shell nanoparticles, which was more sensitive than that of the usual colorimetric assays (red to blue) based on the aggregation of Au NPs. After the colour changed, the non-aggregation-based Au–Ag core–shell nanomaterials did not aggregate further and could remain stable for a long time, which was convenient to record, detect and observe. The sensing probe exhibited a drastically long observing time for detecting Ag+ owing to the stability of the Au–Ag core–shell non-aggregates, high sensitivity with a low detection limit of 8.76 nM by the naked eye and 1.09 nM by using a UV-vis spectrophotometer and a good linear relationship within the range from 1.09 to 109 nM. The colour change of the system is very fast, occurring within 1 to 2 minutes. Moreover, the proposed method also showed a remarkably high selectivity toward Ag+ and was successfully used in tap water and drinking water samples. Therefore, this unusual colorimetric assay based on the non-aggregation of Au NPs has a great potential as a simple, rapid, sensitive and selective detection method for the detection of Ag+. [ABSTRACT FROM AUTHOR]