Sub-femtomolar electrochemical detection of DNA hybridization based on latex/gold nanoparticle-assisted signal amplification

We report a relatively simple electrostatic method for modifying submicrometer-size latex spheres with gold nanoparticles (AuNPs) based on layer-by-layer modification of the latex by polyelectrolytes. The AuNP coverages for 343- and 501-nm-diameter spheres were 4.0 x 10(10) +/- 1.3 x 10(10) and 8.2 x 10(10) +/- 2.7 x 10(10) particles cm(-2), respectively, which is an increase of 1 order of magnitude on the previously reported coverage at latex-AuNPs using streptavidin-biotin binding (Kawde, A.N.; Wang, J. Electroanalysis 2004, 16, 101-107). Due to the fact that the AuNPs used here are also of a larger size (mean diameter 15.5 +/- 1.6 nm, cf. 5 nm), this represents an increase of 2 orders of magnitude in the number of Au atoms delivered per sphere. The spheres were attached to DNA probes specific to E. coli and used to detect probe hybridization by dissolution of the AuNPs, followed by measurement of AU(3+) ions by anodic stripping voltammetry (ASV). Use of differential pulse voltammetry for the stripping step, along with optimization of the ASV conditions, enabled a detection limit of 0.5 fM, which is, to the best of our knowledge, equal or lower than previous voltammetric nanoparticle methods for detection of DNA hybridization.