We will present surface enhanced Raman spectroscopy (SERS) and localized surface plasmon resonance (LSPR) detection of hydrogen peroxide (H2O2) using plasmonic gels. It has been known that reactive oxygen species (ROS), such as hydrogen peroxide, are involved in various biological processes, including metabolism, cell signaling, protein folding, biosynthesis, and host defense. Therefore, developing a simple and sensitive method for monitoring ROS levels is very important for live cell studies. Nevertheless, a challenge of utilizing SERS-based or LSPR-based method for molecular detection in complex fluids, such as cell culture media, is that a variety of molecules in sample solutions could interfere with detection results. In addition, when using SERS-based methods, the chance of having the target molecules at the SERS hot spots is reduced when other molecules are present in the solution. To enable detection of H2O2 in cell culture media, we have developed SERS and LSPR detection methods based on gels containing plasmonic nanoparticles. Since gels are filter-like materials, H2O2 can penetrate through the gels, but cells and large molecules such as proteins are blocked. We have successfully utilized these two methods to detect H2O2 in cell culture media without any sample pretreatment.