Femtomolar electrochemical detection of DNA hybridization using hollow polyelectrolyte shells bearing silver nanoparticles

The preparation, and use as electrochemical labels, of polyelectrolyte shells bearing Ag nanoparticles is described. Their potential for highly sensitive detection is demonstrated. The shells are prepared by layer-by-layer self-assembly around templates (500 mn diameter) which are then dissolved. The shells can be opened and closed by adjustment of solution pH, and this process is utilized to encapsulate Ag nanoparticles, chiefly by adsorption to the inner walls of the capsules. Based on absorbance, TEM and voltammetric measurements, the highest loading achieved is approximately 78 Ag particles per capsule. The Ag capsules are used via biotin-avidin binding as labels for the detection of DNA hybridization, following acid dissolution and then detection of the Ag+ by ASV. A 30-mer sequence specific to Escherichia coli is measured at DNA-modified screen-printed electrodes with a detection limit of similar to 25 fM, which corresponds to the detection of 4.6 fg (similar to 3 x 10(5) molecules) in the 20 mu L analyte sample. A 200 fM target containing a single mismatch gives a significantly (<74%) lower response than 200 fM of complementary target; 60 pM of noncomplementary target gives a negligible response.