A non-aggregation colorimetric method for trace lead(II) ions based on the leaching of gold nanorods

We report a simple and fast colorimetric method using gold nanorods (Au NRs) as sensing probes to detect Pb²⁺ ions in complex real samples. The method is based on Pb²⁺ ions increasing the leaching rate of Au NRs induced by thiosulfate (S₂O₃^2-). A positively charged bilayer of the surfactant cetyltrimethylammonium bromide on the Au NR surfaces facilitates the electrostatic adsorption of S₂O₃^2-, and the longitudinal surface plasmon resonance absorption of S₂O₃^2-/Au NRs that are produced is slightly decreased. The formation of Pb-Au alloys on the surfaces of Au NRs leads to a significant decrease in the electrode potential of gold; thus, increasing the dissolution rate of the gold, which is dependent on the Pb²⁺ ion concentration. Using matrix-assisted laser desorption/ionization mass spectrometry, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, we proved that Pb-Au alloys were formed on the Au NR surfaces in the presence of Pb²⁺ ions and S₂O₃^2-. The reaction temperature and time, the buffer pH, and the S₂O₃^2- concentration are important parameters in determining the sensitivity and selectivity of the method for sensing Pb²⁺ ions. Under the optimum conditions (3.0 mM S₂O₃^2-, 51 pM Au NRs, 10 mM Tris-HCl buffer, and pH 8.0), the limit of detection for Pb²⁺ ions, giving a signal-to-noise ratio of 3, was 4.3 nM. We used this nanosensor system to determine Pb²⁺ ions in complex real samples (lake, pond, seawater, urine, and soil samples), and found that this approach offers advantages in terms of its simplicity, accuracy, and precision (the relative standard deviations of triplicate analyses of each sample were below 3%).