Thermally addressed immunoassay is developed to detect multiple proteins using phase change nanoparticles as thermal barcodes. The solid to liquid phase changes of nanoparticles absorb heat energy and generate sharp melting peaks, which are used as thermal signatures to determine the existence and concentration of proteins. Multiple proteins can be detected by using different types of nanoparticles in order to create a one-to-one correspondence between one type of nanoparticle and one type of protein. The fusion enthalpy that is proportional to the amount of phase change materials has been used to derive the amount of protein. The melting temperatures of nanoparticles are designed to be higher than 100 degrees C to avoid interference from species contained in the fluid. Thus, the use of thermal nanoparticles allows the detection of multiple low concentration proteins in a complex fluid such as cell lysate regardless of the color, salt concentration, and conductivity of the sample.