Probe accessibility effects on the performance of electrochemical biosensors employing DNA monolayers

Surface-confined DNA probes are increasingly used as recognition elements (or presentation scaffolds) for detection of proteins, enzymes, and other macromolecules. Here we demonstrate that the density of the DNA probe monolayer on the gold electrode is a crucial determinant of the final signalling of such devices. We do so using redox modified single-stranded and double-stranded DNA probes attached to the surface of a gold electrode and measuring the rate of digestion in the presence of a non-specific nuclease enzyme. We demonstrate that accessibility of DNA probes for binding to their macromolecular target is, as expected, improved at lower probe densities. However, with double-stranded DNA probes, even at the lowest densities investigated, a significant fraction of the immobilized probe is inaccessible to nuclease digestion. These results stress the importance of the accessibility issue and of probe density effects when DNA-based sensors are used for detection of macromolecular targets.