The aim of this work is to develop a sensor for specific DNA sequences, using non-complex synthetic single-stranded oligonucleotides as a model system. A capacitance-based sensor for the direct detection of DNA sequences is described. Hybridisation of analyte DNA with immobilised DNA on the silicon surface induces charge effects, altering the dielectric properties of the biolayer, and can be detected by the associated change in the measured capacitance. DNA has been immobilised on a silicon electrode either by passive adsorption or covalent coupling via 4-aminobutyldimethylmethoxysilane (4-ABDMMS). The work presented here introduces a colourimetric immunodetection technique for the evaluation of the immobilisation process and describes the electrical characterisation and performance of three silicon-based sequence-specific DNA sensors. These sensors consisted of a standard electrolyte-insulator-semiconductor (EIS) structure with covalently bound probe DNA, a mechanically degraded structure with passively adsorbed probe DNA and a mechanically degraded structure with covalently bound probe DNA. The last device had an improved signal to noise ratio and was, therefore, used to construct a standard curve, revealing a detection limit of 100 pmol DNA. On addition of analyte DNA, there was a decrease in measured capacitance. This response was fast, specific and required no addition of mediators to enhance or amplify the signal. This device can be optimised for the detection of complex sequences.