Intracellular redox potential is a delicately balanced property in cells. It plays an important role in the regulation of cellular processes and dysfunction of the redox state is believed to get involved in initiation of many kinds of diseases. However, lack of suitable techniques for quantitatively monitoring the redox potential in cells with a wide range is a significant challenge. My project aims to develop SERS nanosensors for measuring intracellular redox state quantitatively and applying them to quantify hypoxia, which is generally described by the cell having a reducing environment and defined as a form of “reductive stress”. Four redox active probe molecules have been synthesised and characterised. Their Raman spectra all change as a function of local redox potential. Since these probe molecules can be assembled on gold nanoshells which are able to enhance the Raman Effect significantly, we can calculate the redox potential from simple optical SERS measurements. Transmission Electron Microscopy was used to investigate the cellular delivery of nanosensors. TEM images confirmed that either single nanosensor or small aggregates can be taken up controllably by cells and translocated in the cytoplasm. The introduction of nanosensors was also found not to be toxic to the cells. The nanosensor has been used to monitor the redox potential in resting cells as well as the redox changes when cells responded to pharmacologically induced hypoxia and oxidative stress. These measurements demonstrated that the SERS based nanosensor developed is able to monitor the intracellular redox change in a reversible, noninvasive way and respond to cellular hypoxia quantitatively.