Development of biodetection techniques has become the matter of intense research in the field of bioassays. Modern DNA microarrays now target in vitro diagnostics and allow simultaneous identification of several hundred biomarkers. Thus, targeting complex diseases due to the deregulation of several genes can be reached. However some technological bottlenecks slow down the blooming of this technology. Our research focuses on microarray improvement for in vitro diagnostic by implementing nanotechnology processes in order to reduce the cost of multiplexed analysis. For immobilizing the probe molecules at the surface of the biochip, we selected a modified microcontact printing technique which enables us to generate patterns of probe DNA fragments of arbitrary shape and dimensions while preserving the capability of multiplexing in one printing step. Due to its submicrometric resolution, this biopatterning method has been used to generate periodic arrays of DNA probe molecules (1micron pitch). We demonstrated that these molecular gratings efficiently diffracted light from a laser beam. We exploited the changes in the diffracted intensity of these gratings to perform a label-free optical biodetection. In this contribution, using a modified scanner system capable of collecting at high speed the diffracted intensity, we could perform detection of specific DNA fragments (from 25pb to 320pb). Based on this principle, we show that we are able to detect a change of signature due to DNA hybridization. This biodetection method should allow detection of specific gene targets from an analytical solution and will solve the technological bottleneck of target labeling required for fluorescence read out. Test validation of this technology focuses on a dedicated DNA microarray used for screening validated genetic signatures for breast cancer diagnostic as a new medical tool helping the orientation of therapies.