Dual Mode Electro photonic Silicon Biosensors Introduction Fundamentals of Label Free Biosensing Fabrication and Experimental Techniques The Electro photonic Silicon Biosensor Study and Application of Electrografted Layers of Diazonium Ions Tailoring Light matter Interaction for Quantiﬁcation of Biological and Molecular Layers Conclusions and Outlook. This highly interdisciplinary thesis reports ontwo innovative photonic biosensors that combine multiple simultaneous measurements to provide unique insights into the activity and structure of surface immobilized biological molecules. In addition it presents a new silicon photonic biosensor that exploits two cascaded resonant sensors to provide two independent measurements of a biological layer immobilized on the surface. By combining these two measurements it is possible to unambiguously quantify the density and thickness of the molecular layer; here the approach’s ability to study molecular conformation and conformational changes in real timeis demonstrated. The electrophotonic biosensor integrates silicon photonics with electrochemistry into a single technology. This multi modal biosensor provides a number of unique capabilities that extend the functionality of conventional silicon photonics. For example by combining the complementary information revealed by simultaneous electrochemical and photonic measurements it is possible to provide unique insights into on surface electrochemical processes. Furthermore the ability to create electrochemical reactions directly on the silicon surface provides a novel approach for engineering the chemical functionality of the photonic sensors. The electrophotonic biosensor thus represents a critical advance towards the development of very high density photonic sensor arrays for multiplexed diagnostics.