Femtomolar Electrochemical Detection of 4 nm Diameter Citrate-Stabilized Gold Nanoparticles by Electrophoretic and Electrochemical Amplification

The detection of metal nanoparticles (NPs) in solution is essential for environmental monitoring and indirect detection of chemical and biological analytes when NPs are used as labels. Here we detect 4 nm diameter citrate-stabilized (cit) Au NPs using indium tin oxide-coated glass electrodes (glass/ITO) by (1) electrophoretic deposition (EPD) of varying concentrations of 4 nm cit Au NPs and varying EPD time (30 s to 5 h), (2) seeded electrochemical deposition (ECD) of Au for 1 to 10 min to selectively grow the 4 nm cit Au NPs into larger structures, and (3) anodic stripping voltammetry (ASV) of Au in 0.010 M KBr plus 0.1 M KClO₄. For varying concentrations from 170 nM down to 1.7 nM (in terms of Au) and a constant 1 min ECD time, the EPD time required to achieve a maximum ASV signal increases with decreasing Au NP concentration. With 0.17 nM and 0.017 nM concentrations and 1 min ECD times, the Au NPs could not be distinguished from a blank solution even for EPD times up to 5 h. Using EPD times of 30 min and increasing the ECD time to 5-10 min allowed reliable detection with a linear response from 0 nM to 0.2 nM with a sensitivity of 371 μA/nM and limit of detection (LOD) of 0.01 nM in terms of Au and 5 fM in terms of Au NPs, which is competitive with the lowest reported values in the literature. Our method is fast, simple, and low cost with very low LOD that can likely be pushed even lower with increasing ECD time further. The method is selective against 9 nm diameter cit Ag NPs with a signal for Au 10 times greater than that for Ag.