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An efficient visual detection method of Ba2+ using tiopronin (Tio)-modified gold nanoparticles (GNPs) in aqueous solution is reported. Tio could bind onto the GNP surfaces through the sulphur atom and the cross-linking of Tio-functionalised GNPs could be quickly accomplished by chelation upon the addition of Ba2+. Colour changes were observed from red to blue (or purple) within minutes with the increased concentration of Ba2+, in response to surface plasmon absorption of dispersed and aggregated nanoparticles. Using this colourimetric sensor, the concentration of Ba2+ was determined with a limit of detection lower than 1.5 × 10-6 M by the naked eye and could further be easily monitored by a UV-vis spectrometry. The possible mechanism of the aggregated Tio-GNPs was discussed by the addition of ethylenediaminetetraacetate and Ba2+ in different orders.