Aminooxy-PEG4-t-butyl ester is widely used in bioconjugation and chemical biology research. Its primary application involves linking biomolecules, such as proteins, peptides, or antibodies, to other molecules or surfaces. The aminooxy group reacts selectively with aldehydes or ketones, forming stable oxime bonds, while the PEG4 spacer enhances solubility and reduces steric hindrance. This makes it valuable for creating targeted drug delivery systems, diagnostic probes, and functionalized biomaterials. The t-butyl ester group provides protection during synthesis, which can be removed under specific conditions to expose a carboxylic acid for further modifications. Its versatility and efficiency make it a key reagent in developing advanced bioconjugates for therapeutic and diagnostic applications.