Femtomolar\nElectrochemical Detection of 4 nm Diameter\nCitrate-Stabilized Gold Nanoparticles by Electrophoretic and Electrochemical\nAmplification

The detection of metal nanoparticles (NPs) in solution\nis essential\nfor environmental monitoring and indirect detection of chemical and\nbiological analytes when NPs are used as labels. Here we detect 4\nnm diameter citrate-stabilized (cit) Au NPs using indium tin oxide-coated\nglass electrodes (glass/ITO) by (1) electrophoretic deposition (EPD)\nof varying concentrations of 4 nm cit Au NPs and varying EPD time\n(30 s to 5 h), (2) seeded electrochemical deposition (ECD) of Au for\n1 to 10 min to selectively grow the 4 nm cit Au NPs into larger structures,\nand (3) anodic stripping voltammetry (ASV) of Au in 0.010 M KBr plus\n0.1 M KClO<sub>4</sub>. For varying concentrations from 170 nM down\nto 1.7 nM (in terms of Au) and a constant 1 min ECD time, the EPD\ntime required to achieve a maximum ASV signal increases with decreasing\nAu NP concentration. With 0.17 nM and 0.017 nM concentrations and\n1 min ECD times, the Au NPs could not be distinguished from a blank\nsolution even for EPD times up to 5 h. Using EPD times of 30 min and\nincreasing the ECD time to 5–10 min allowed reliable detection\nwith a linear response from 0 nM to 0.2 nM with a sensitivity of 371\nμA/nM and limit of detection (LOD) of 0.01 nM in terms of Au\nand 5 fM in terms of Au NPs, which is competitive with the lowest\nreported values in the literature. Our method is fast, simple, and\nlow cost with very low LOD that can likely be pushed even lower with\nincreasing ECD time further. The method is selective against 9 nm\ndiameter cit Ag NPs with a signal for Au 10 times greater than that\nfor Ag.