Bimetallic Hollow Nanostructures for Colorimetric Detection of Picomolar Level of Mercury

In this paper, we report the use of bimetallic hollow nanostructures (BHNS), consisting of gold and silver metals, for colorimetric detection of mercury. The sodium dodecyl sulphate (SDS)-capped BHNS were prepared by galvanic etching of silver nanoparticles (AgNPs) using gold chloride resulting in a partially hollow AgNPs with the gold layer at its surface. These BHNS were interacted with an aqueous solution of mercury ions (Hg²⁺) in the concentration range of 10 pM-10 mM. Interestingly, at higher concentration range (10 μM-10 mM), a noticeable change in the solution color was observed with a prominent decrease in the absorption intensity and blue-shift in the peak plasmonic wavelength. This could be attributed to (i) complexation reaction between the anionic BHNS (due to the negatively charged SDS capping) and cationic Hg²⁺ and (ii) oxidative etching of silver from BHNS causing its depletion and resulting into Ag-Hg amalgam and/or aggregation of the nanostructures. In contrast, at lower concentration range (i.e., 10 pM-10 nM), an increase in the absorption intensity was observed, which was possibly due to the oxidative etching of silver from BHNS without aggregation of the nanostructures. The low amount of Hg²⁺ was not sufficient enough to interact with SDS capping layer present on the BHNS surface, unlike the higher concentrations of mercury and therefore, did not cause any aggregation. The developed colorimetric sensor showed high sensitivity and selectivity towards Hg²⁺ detection with a limit of detection of 10 pM and good linearity (R² = 0.97) in the concentration range of 10 pM-10 nM.