Pb²⁺ Ion Sensors Employing Gold Etching Process: Comparative Investigation on Au Nanorods and Au Nanotriangles

The leaching phenomenon of gold (Au) nanomaterials by Pb²⁺ ions in the presence of 2-mercaptoethanol (2-ME) and thiosulfate (S₂O₃^2- ion) has been systematically applied to a Pb²⁺ ion sensor. To further investigate the role of Pb²⁺ ions in sensors containing Au nanomaterials, we revisited the leaching conditions for Au nanorods and compared them with the results for Au nanotriangles. By monitoring the etching rate, it was revealed that Pb²⁺ ions were important for the acceleration of the etching rate mainly driven by 2-ME and S₂O₃^2- pairs, and nanomolar detection of Pb²⁺ ions were shown to be promoted through this catalytic effect. Using the etchant, the overall size of the Au nanorods decreased but showed an unusual red-shift in UV-Vis spectrum indicating increase of aspect ratio. Indeed, the length of Au nanorods decreased by 9.4% with the width decreasing by 17.4% over a 30-min reaction time. On the other hand, the Au nanotriangles with both flat sides surrounded mostly by dense Au{111} planes showed ordinary blue-shift in UV-Vis spectrum as the length of one side was reduced by 21.3%. By observing the changes in the two types of Au nanomaterials, we inferred that there was facet-dependent alloy formation with lead, and this difference resulted in Au nanotriangles showing good sensitivity, but lower detection limits compared to the Au nanorods.