Benzaldehyde-PEG4-azide is widely used in bioconjugation and click chemistry applications due to its reactive azide group and PEG spacer. The azide group enables efficient coupling with alkyne-containing molecules via copper-catalyzed azide-alkyne cycloaddition (CuAAC), making it valuable for labeling, modifying, and crosslinking biomolecules such as proteins, peptides, and nucleic acids. The PEG4 linker enhances solubility and reduces steric hindrance, improving reaction efficiency in aqueous environments. Additionally, the benzaldehyde moiety allows for further functionalization through Schiff base formation or reductive amination, expanding its utility in designing complex molecular architectures for drug delivery, diagnostics, and materials science. Its versatility makes it a key reagent in developing targeted therapies, imaging probes, and advanced biomaterials.