Propargyl-PEG4-thiol is widely used in bioconjugation and surface modification due to its unique reactive groups. The propargyl moiety allows for click chemistry, particularly copper-catalyzed azide-alkyne cycloaddition (CuAAC), enabling efficient and selective attachment to azide-containing molecules. This makes it valuable in drug delivery systems, where it can link therapeutic agents to targeting molecules or carriers. The thiol group facilitates binding to gold surfaces or other thiol-reactive substrates, making it suitable for functionalizing nanoparticles, biosensors, and electrodes. Its PEG4 spacer enhances solubility and reduces steric hindrance, improving the efficiency of conjugation processes. Additionally, it is utilized in the development of biomaterials, such as hydrogels and coatings, where precise molecular assembly is required. Its dual functionality makes it a versatile tool in nanotechnology, diagnostics, and therapeutic applications.