Assembly of gold nanorods for highly sensitive detection of heavy metals

A simple and low cost strategy for heavy metal sensing based on an easy and reliable colorimetric detection was investigated by using water soluble gold nanorods (Au NRs), functionalized with L-cysteine. Detection tests have been performed on several heavy metal ions, namely Zn²⁺, Cd²⁺, Hg²⁺, Cu²⁺, Pb²⁺ and As³⁺ ions and monitored by UV-Vis absorption spectroscopy and transmission electron microscopy (TEM). The system has demonstrated a remarkable sensitivity for Hg²⁺ with limit of detection (LOD) at ppt level. A red-shift in the maximum of the typical longitudinal plasmon band of Au NRs has been observed and recognized related to aggregation phenomena occurring among Au NRs only in presence of Hg²⁺ ions. Interestingly, a significantly different response is recorded for the other tested heavy metal ions. The results highlight that the functionalization of Au NRs with L-cysteine is an excellent route to implement a reliable colorimetric sensing device, able to push further down the detection limit recorded for similar strategies based on spherical Au NPs.