Nano-engineered devices with potential for trace level detection of chemical or biological species are investigated. The sensor system is a ChemFET device based on micro- and nano-scale silicon wires. The sensor response to changes in pH reveals a significantly higher sensitivity of nano-scale devices compared to micro-scale devices. By immobilizing DNA probe molecules on the silicon wire surface, the ChemFET devices are rendered specific to this DNA sequence. Differential measurements minimize the effects of non-specific binding. At a concentration of C<sub>DNA</sub>=10μM, two different single stranded 24-base DNA oligonucleotides have been clearly distinguished in the sensor response. DNA hybridization on the silicon wire surface is further corroborated by fluorescence spectroscopy and analysis of characteristic time constants in the sensors response.