We have projected and fabricated a microfluidic platform for DNA sensing that makes use of an optical colorimetric detection method based on gold nanoparticles. The platform was fabricated using replica moulding technology in PDMS patterned by high-aspect-ratio SU-8 moulds. Biochips of various geometries were tested and evaluated in order to find out the most efficient architecture, and the rational for design, microfabrication and detection performance is presented. The best biochip configuration has been successfully applied to the DNA detection of Mycobacterium tuberculosis using only 3 mu l on DNA solution (i.e. 90 ng of target DNA), therefore a 20-fold reduction of reagents volume is obtained when compared with the actual state of the art.